PROGRAMMED CELL DEATH GENES AND PROTEINS

Abstract: This invention relates to genes involved in regulating programmed cell death, the proteins encoded by such genes and methods for controlling programmed cell death by regulating the activity of the cell death gene products.

Description: Programmed Cell Death Genes and Proteins Background of the Invention Statement as to Rights to Inventions Made Under Federally-Sponsored Research and Development Part of the work performed during development of this invention utilized U.S. Government funds. The U.S. Government has certain rights in this invention. Cross-Reference to Related Applications This application is a continuation-in-part application of U.S. Application No. 08/258,287, filed June 10, 1994, which is a continuation-in- part application of U.S. Application No. 08/080,850, filed June 24, 1993, now abandoned. Field of the Invention The invention is in the field of molecular biology as related to the control of programmed cell death. Description of the Background Art Programmed Cell Death Cell death occurs as a normal aspect of animal development as well as in tissue homeostasis and aging (Glucksmann, A., Biol. Rev. Cambridge Philos. Soc. 26:59-86 (1950); Ellis et al., Dev. 772:591-603 (1991); Vaux et al. , Cell 76:111-119 (1994)). Naturally occurring cell death acts to regulate cell number, to facilitate morphogenesis, to remove harmful or otherwise abnormal cells and to eliminate cells that have already performed their function. Such regulated cell death is achieved through an endogenous cellular mechanism of suicide, termed programmed cell death or apoptosis (Wyllie, A. H., in Cell Death in Biology and Pathology, Bowen and Lockshin, eds., Chapman and Hall (1981), pp. 9-34). Programmed cell death or apoptosis occurs when a cell activates this internally encoded suicide program as a result of either internal or external signals. The moφhological characteristics of apoptosis include plasma membrane blebbing, condensation of nucleoplasm and cytoplasm and degradation of chromosomal DNA at inter-nucleosomal intervals. (Wyllie, A. H., in Cell Death in Biology and Pathology, Bowen and Lockshin, eds., Chapman and Hall (1981), pp. 9-34). In many cases, gene expression appears to be required for programmed cell death, since death can be prevented by inhibitors of RNA or protein synthesis (Cohen et al., J. Immunol. 32:38-42 (1984); Stanisic et al, Invest. Urol. 76: 19-22 (1978); Martin et al., J. Cell Biol. 706:829-844 (1988)). The genetic control of programmed cell death has been well -elucidated by the work on programmed cell death in the nematode C. elegans. Programmed cell death is characteristic and widespread during C. elegans development. Of the 1090 somatic cells formed during the development of the hermaphrodite, 131 undergo programmed cell death. When observed with Nomarski microscopy, the moφhological changes of these dying cells follow a characteristic sequence. (Sulston et al, Dev. Biol. 52: 110-156 (1977); Sulston et al, Dev. Biol. 700:64-119 (1983)). Fourteen genes have been identified that function in different steps of the genetic pathway of programmed cell death in C. elegans (Hedgecock et al, Science 220: X211-X280 (1983); Ellis et al., Cell 44.817-829 (1986); Ellis et al, Dev. 772:591-603 (1991); Ellis et al., Genetics 772:591-603 (1991b); Hengartner et l, Nature 556:494-499 (1992); Ellis et al, Dev. 772:591-603 (1991)). Two of these genes, ced-3 and ced-4, play essential roles in either the initiation or execution of the cell death program. Recessive mutations in these genes prevent almost all of the cell deaths that normally occur during C. elegans development. The ced-4 gene encodes a novel protein that is expressed primarily during embryogenesis, the period during which most programmed cell deaths occur (Yuan et al., Dev. 776:309-320 (1992)). The 549 amino acid sequence of ced-4, deduced from cDNA and genomic clones, contain two regions that are similar to the calcium-binding domain known as the EF-hand (Kretsinger, 1987); however, it is still not clear at present whether calcium plays a role in regulating ced-4 or programmed cell death in C. elegans. A gain-of- function mutation in ced-9 prevents normal programmed cell death, while mutations that inactivate ced-9 are lethal. This suggests that ced-9 acts by suppressing programmed cell death genes in cells that normally do not undergo programmed cell death (Hengartner, M., et al , Nature 356:494-499 (1992)). The ced-9 gene encodes a protein product that shares sequence similarity with the mammalian proto-oncogene and cell death suppressor bcl-2 (Hengartner, M., et al , Cell 76:665-676 (1994)). The lethality of ced-9 loss-of-function mutations can be suppressed by mutations in ced-3 and ced-4 , indicating that ced-9 acts by suppressing the activity of ced-3 and ced-4. Genetic mosaic analyses indicate that ced-3 and ced-4 likely act in a cell-autonomous fashion within dying cells, suggesting that they might be cytotoxic proteins and/or control certain cytotoxic proteins in the process of programmed cell death (Yuan, J. , et al , Dev. Bio. 735:33-41 (1990)). neddl Cell death also occurs in mammals, neddl is a mouse gene which is preferentially expressed during early embryonic brain development (Kumar et al, Biochem. Biophys. Res. Commun. 785: 1155-1161 (1992)). Since many neurons die during early embryonic brain development, it is possible that nedd-2 is a cell death gene. Nedd-2 mRNA is down-regulated in the adult brain (Kumar et al. , Biochem. & Biophys. Res. Comm. 185: 1155-1161 (1992); Yuan, J., et al , Cell 75:641-752 (1993)). bcl-2 bcl-2 is an oncogene known to inhibit programmed cell death and to be overexpressed in many follicular and B cell lymphomas. Overexpression of bcl-2 as a result of chromosomal translocation occurs in 85 % of follicular and 20% of diffuse B cell lymphomas, Fukuhara et al , Cancer Res. 39:3119 (1979); Levine et al, Blood 66: 1414 (1985); Yunis et al , N. Engl. J. Med. 376:79-84 (1987). Overexpression of bcl-2 protects or delays the onset of apoptotic cell death in a variety of vertebrate cell types as well as in C. elegans (Vaux et al., Science 258: 1955-1957 (1992); Nunez et al, J. Immun. 144:3602-3610 (1990); Vaux et al., Science 258: 1955-1957 (1992); Sentman et al., Cell 67:879-888 (1992); Strasser et al, Cell 67:889-899 (1991)). Expression of bcl-2 in the nematode C. elegans has been shown to partially prevent programmed cell death. Thus, bcl-2 is functionally similar to the C. elegans ced-9 gene (Vaux et al, Science 255: 1955-1957 (1992); Hengartner et al., nature 356:494-499 (1992)). Interleukin-lβ Converting Enzyme Interleukin-13 converting enzyme (ICE) is a substrate-specific cysteine protease that cleaves the inactive 31 KD prointerleukin-13 at Asp ll6 -Ala ,π , releasing a carboxy-terminal 153 amino-acid peptide to produce the mature 17.5 kD interleukin-1 / 3 (IL-1/3) (Kostura et al, Proc. Natl. Acad. Sci., USA 86:5221-5231 (1989); Black et al, FEBS Lett. 247:386-390 (1989); Cerretti et al . Science 256:97-100 (1992); Thornberry et al . Nature 356:768-774 (1992)). IL-1/3 is a cytokine involved in mediating a wide range of biological responses including inflammation, septic shock, wound healing, hematopoiesis and growth of certain leukemias (Dinarello, C.A., Blood 77: 1627-1652 (1991); diGiovine et al.. Today 77: 13 (1990)). A specific inhibitor of ICE, the crmA gene product of cowpox virus, prevents the proteolytic activation of IL-13 (Ray et al., Cell 69:597-604 (1992)) and inhibits host inflammatory response (Ray et al., Cell 69:591-604 (1992)). Cowpox virus carrying a deleted crmA gene is unable to suppress the inflammatory response of chick embryos, resulting in a reduction in the number of virus-infected cells and less damage to the host (Palumbo et al, Virology 777:262-273 (1989)). This observation indicates the importance of ICE in bringing about the inflammatory response. Tumor Necrosis Factor Tumor necrosis factor-α (TNF-α) is a pleiotropic tumoricidal cytokine (Tracey, K.J. et al, Ann. Rev. Cell. Biol. 9:317-343 (1993)). One of the striking functions of TNF-α is to induce apoptosis of transformed cells. In the case of non-transformed cells, TNFα can also induce apoptosis in the presence of metabolic inhibitors (Tracey, K.J., et al , Ann. Rev. Cell. Biol. 9:317-343 (1993). Apoptosis induced by TNF-α is also suppressed by bcl-2. One of the most extensively studied functions of TNF-α is its cytotoxicity on a wide variety of tumor cell lines in vitro (Laster, S. M. et al. , J. Immunol. 141:2629-2634 (1988)). However, the mechanism of cell death induced by TNF has been largely unknown. HeLa cells express predominantly p55 TNF receptor which is thought to be responsible for cell death signaling (Englemann, H. et al , J. Biol. Chem. 265: 14497-14504 (1990); Thoma, B. et al , J. Exp. Med. 772: 1019-1023 (1990)). HeLa cells are readily killed by TNF-α in the presence of the metabolic inhibitor cycloheximide (CHX). The cell death induced by TNF-α/CHX shows DNA fragmentation and cytolysis, which are typical features of apoptosis (White, E. et al. , Mol. Cell. Biol. 72:2570-2580 (1992)). Expression of adenovirus E1B 19K protein, which is functionally similar to bcl-2, inhibits apoptosis induced by TNF in HeLa cells (White, E. et al. , Mol. Cell. Biol 72:2570-2580 (1992)). Summary of the Invention In the present invention, the ced-3 gene has been cloned and sequenced and the amino acid sequence of the protein encoded by this gene is disclosed. Structural analysis of the ced-3 gene revealed that it is similar to the enzyme interleukin-13 converting enzyme ("ICE") and that overexpression of the murine interleukin-1 - converting enzyme ("mICE") causes programmed cell death in vertebrate cells. Based upon these results, a novel method for controlling programmed cell dead, in vertebrates by regulating the activity of ICE is claimed. The amino acid sequence of ced-3 was also found to be similar to another murine protein, nedd-2, which is detected during early embryonic brain development, a period when many cells die. The results suggest that ced-3, ICE and nedd-2 are members of a gene family which function to cause programmed cell death. A new cell death gene, mIch-2 (murine ICE-ced-3 homolog), has been discovered which appears to be in the same family as ced-3, ICE, and nedd-2. mIch-2 is distinguished from other previously identified cell death genes in that it is preferentially expressed in the thymus and placenta of vertebrates. Thus, the invention is also directed to a newly discovered gene, mIch-2, which is preferentially expressed in thymus and placenta and which encodes a protein causing programmed cell death. Thus, the present invention is directed to both the mIch-2 gene sequence and the protein encoded by mIch-2. Also encompassed are vectors expressing mlch-2 and host cells transformed with such vectors. The invention also encompasses methods of regulating cell death by expressing mlch-2. A comparison of the nucleotide sequences of ced-3, mICE, human ICE "hlCE"), nedd-2 and mlch-2 indicates that they are members of a gene family that promotes programmed cell death. The identification of this family facilitated the isolation of the newly discovered cell deadi gene Ice-ced 3 homolog-1 (Ich-1). Ich-1 is homologous to the other cell death genes described above and particularly to neddl. Based upon its structure and the presence of DNA encoding the QACRG sequence characteristic of the active center of cell death genes, Ich-1 was identified as a new member of the ced-3IICE family. Thus, the present invention is directed to both the Ich-1 gene sequence and the protein encoded by Ich-1. Also encompassed are vectors expressing Ich-1 and host cells transformed with such vectors. Alternative splicing results in two distinct 7cΛ-7 mRNA species. Thus, the invention also encompasses these species, proteins produced from them, vectors containing and expressing the genes, and the uses described herein. The inventors have also identified a further member of the ICE/ced-3 family, Ich-3. Ich-3 has at least two alternative splicing products. A full length cDNA of one of them from a mouse thymus cDNA library has been identified. It encodes a protein of 418 amino acids that is 38% identical with mICE, 42% identical with mlch-2, 25% with Ich-1, and 24% identical with C. elegans ced-3. The invention is thus directed to genomic or cDNA nucleic acids having genetic sequences which encode ced-3, mlch-2, Ich-1, and Ich-3. The invention also provides for vectors and expression vectors containing such genetic sequences, the host cells transformed with such vectors and expression vectors, the recombinant nucleic acid or proteins made in such host/vectors systems and the functional derivatives of these recombinant nucleic acids or proteins. The use of the isolated genes or proteins for the puφose of regulating, and especially promoting cell death is also part of the invention. The invention is also directed to methods for controlling the programmed death of vertebrate cells by regulating the activity of interleukin- Xβ converting enzyme. Such regulation may take the form of inhibiting the enzymatic activity, e.g. through the use of specific antiproteases such as crmA, in order to prevent cell death. In this way, it may be possible to develop cell lines which remain viable in culture for an extended period of time or indefinitely. Certain cells can only be maintained in culture if they are grown in the presence of growth factors. By blocking cell death, it may be possible to make such cells growth factor independent. Alternatively, ICE activity may be increased in order to promote cell death. Such increased activity may be used in cancer cells to antagonize the effect of oncogenes such as bcl-2. The present invention is also based on the discovery that TNF-α activates endogenous ICE activity in HeLa cells and that TNF-α-induced apoptosis is suppressed by crmA, which can specifically inhibit ICE-mediated cell death. Thus, certain embodiments of the invention are based on the activation of the ICE! ced-3- mediated cell death pathway by TNF-α. Brief Description of the Figures Figure 1 and 1A: Genetic and Physical Maps of the ced-3 Region on Chromosome TV Figure 1 shows the genetic map of C. elegans in the region near ced-3 with the cosmid clones representing this region depicted below the map. nP33, nP34, nP35, nP36, and nP37 are restriction fragment length polymoφhisms (RFLP) between Bristol and Bergerac wild type C. elegans strains. C43C9, W07H6 and C48D1 are three cosmid clones tested for rescue of the ced phenotype of ced-3(n717) animals. The ability of each cosmid clone to rescue ced-3 mutants and the fraction of independently obtained transgenic lines that were rescued are indicated on the right of the figure (+ , rescue; -, no rescue; see text for data). The results indicate that ced-3 is contained in the cosmid C48D1. Figure 1A is a restriction map of C48D1 subclones. C48D 1 was digested with BamHl and self-ligated to generate subclone C48D 1 -28. C48D1-43, pJ40 and pJ107 were generated by partial digesting C48D1-28 with BglXX. pJ7.5 and pJ7.4 were generated by ExόXlX deletion of pJ107. These subclones were assayed for rescue of the ced phenotype of ced-3 (n? '17) animals (+ , rescue; -, no rescue, -/ + , weak rescue). The numbers in parentheses indicate the fraction of independently obtained transgenic lines that were rescued. The smallest fragment that fully rescued the ced-3 mutant phenotype was the 7.5 kb pJ7.5 subclone. Figure 2, 2A(i)-2A(v), 2B and 2C: Genomic Organization, Nucleotide Sequence, and Deduced Amino Acid Sequence of ced-3 Figure 2 shows the genomic sequence of the ced-3 region, as obtained from plasmid pJ107. The deduced amino acid sequence of ced-3 is based on the DNA sequence of ced-3 cDNA pJ87 and upon other experiments described in the text and in Experimental Procedures. The 5' end of pJ87 contains 25 bp of poly-A/T sequence (not shown), which is probably a cloning artifact since it is not present in the genomic sequence. The likely start site of translation is marked with an arrowhead. The SL1 splice acceptor site of the ced-3 transcript is boxed. The positions of 12 ced-3 mutations are indicated. Repetitive elements are indicated as arrows above the relevant sequences. Numbers on the left indicate nucleotide positions, beginning with the start of pJ107. Numbers below the amino acid sequence indicate codon positions. Five types of imperfect repeats were found: repeat 1, also found in fern- 1 (Spence et al., Cell 0:981-990 (1990)) and hlh-1 (Krause et al., Cell 63:901- 919 (1990)); repeat 2, novel; repeat 3, also found in lin-12 and fem-1; repeat 4, also found in lin-12; and repeat 5, novel. Numbers on the sides of the figure indicate nucleotide positions, beginning with the start of pJ107. Numbers under the amino acid sequence indicate codon positions. Figure 2A(i) - Figure 2A(iv) contain comparisons of the repetitive elements in ced-3 with the repetitive elements in the genes ced-3, fem-1 , hlh-1 , lin-12, glp-1, and the cosmids B0303 and ZK643 (see text for references). In the case of inverted repeats, each arm of a repeat ("for" or "rev" for "forward" or "reverse", respectively) was compared to both its partner and to individual arms of the other repeats. 2A(i): Repeat 1; 2A(ii): Repeat 2; 2A(iii): Repeat 3; 2A(iv): Repeat 4; and 2A(v): Repeat 5. The different ced-3 sequences which appear in the comparisons are different repeats of the same repetitive element. The numbers "la", "lb" etc. are different repeats of the same class of repetitive element. Figure 2B shows the locations of the introns (lines) and exons (open boxes) of the ced-3 gene as well as the positions of 12 ced-3 mutations analyzed. The serine-rich region, the trans-spliced leader (SLl), the possible start of translation (ATG) and polyadenlyation (AAA) site are also indicated. Figure 2C shows the cDNA sequence and deduced amino acid sequence of ced-3 as obtained from plasmid pJ87. Figure 3 and 3 A: Comparison of the Structure of the ced-3 Protein and hICE Protein Figure 3 shows a comparison of structural features of ced-3 with those of the hICE gene. The predicted proteins corresponding to the hICE proenzyme and ced-3 are represented. The active site in hICE and the predicted active site in ced-3 are indicated by die black rectangles. The four known cleavage sites in hICE flanking the processed hICE subunits (p24, which was detected in low quantities when hICE was purified (Thornberry et al. (1992), p20, and plO) and two conserved presumptive cleavage sites in ced-3 are indicated with solid lines and linked with dotted lines. Five other potential cleavage sites in ced-3 are indicated with dashed lines. The positions of the aspartate (D) residues at potential cleavage sites are indicated below each diagram. Figure 3A contains a comparison of the amino acid sequences of ced-3 from C. elegans, C. briggsae and C. vulgaris with hICE, mICE, and mouse nedd-2. Amino acids are numbered to the right of each protein. Dashes indicate gaps in the sequence made to allow optimal alignment. Residues that are conserved among more man half of the proteins are boxed. Missense ced- 3 mutations are indicated above the comparison blocks showing the residue in the mutant ced-3 and the allele name. Asterisks indicate potential aspartate self-cleavage sites in ced-3. Circles indicate known aspartate self-cleavage sites in hICE. Residues indicated in boldface correspond to the highly conserved pentapeptide containing the active cysteine. Figure 4: Construction of Expression Cassettes of mICE-lacZ and ced-3-lacZ Fusion Genes Figure 4 shows several expression cassettes used in studying die cellular effects of ICE and ced-3 expression. The cassettes are as follows: p JactMlOZ contains intact mICE fused to the E. coli lacZ gene (mICE-lacZ). pβactMUZ contains the P20 and PlO subunits of mICE fused to the E. coli lacZ gene (?20/PX0-lacZ). p3actM 19Z contains the P20 subunit of mICE fused to the E. coli lacZ gene (P20- lacZ). pjSactM 12Z contains the PlO subunit of mICE fused to the E. coli lacZ gene (PXO-lacZ). pβactced38Z contains the C. elegans ced-3 gene fused to the lacZ gene (ced-3-lacZ . pJ485 and p _actced37Z contain a Gly to Ser mutation at the active domain pentapeptide "QACRG" in mICE and ced-3 respectively. p/3actM17Z contains a Cys to Gly mutation at the active domain pentapeptide "QACRG" in mICE. pactβgal' is a control plasmid (Maekawa et al , Oncogene 6:627-632 (1991)). All plasmids use the . -actin promoter. Figure 5: Genetic Pathways of Programmed Cell Death in the Nematode C. elegans and in Vertebrates In vertebrates, bcl-2 blocks the activity of ICE thereby preventing programmed cell deadi. Enzymatically active ICE causes vertebrate cell death. In C. elegans, ced-9 blocks the action of ced-3/ced-4. Active ced-3 together with active ced-4 causes cell death. Figure 6: mlch-2 cDNA Sequence and Deduced Amino Acid Sequence Figure 6 shows the nucleotide sequence of the mlch-2 cDNA sequence and the amino acid sequence deduced therefrom. Figure 7 and 7 A: mlch-2 Amino Acid Sequence Figures 7 and 7A contain a comparison of the amino acid sequences of mICE, hICE, mlch-2 and ced-3. Figure 8: Potential QACRG Coding Region in the Mouse nedd2 cDNA The reading frame proposed by Kumar et al. (Biochem. & Biophys. Res. Comm. 785:1155-1161 (1992)) is b. In reading frame a, a potential QACRG coding region is underlined. Figure 9 and 9 A: Comparison of the Amino Acid Sequences of ced-3, hICE and Ich-1 Figure 9 contains a comparison of the amino acid sequences of ced-3 and Ich-1. There is a 52% similarity between the sequences and a 28% identity. Figure 9A contains a comparison of the amino acid sequences of hICE and Ich-1. There is a 52% similarity between the sequences and a 27% identity. Figure IOA: The cDNA Sequence of Ich-1 L and the Deduced Amino Acid Sequence of Ich-1 L Protein Product The putative active domain is underlined. Figure 10B: The cDNA Sequence of Ich-l s and the Deduced Amino Acid Sequence of Ich-l s Protein Product The intron sequence is underlined. Figure 11A: The Alternative Splicing of Ich-1 mRNA The exons are shown in bars. The intron is shown in a line. Nucleotides at the exon-intron borders are indicated. Figure UB: The Schematic Diagram of the Two Alternatively Spliced Ich-I Transcripts and Ich-1 L and Ich-l s Proteins The peptide marked with X may not be translated in vivo. The open reading frames and proteins are shown in bars. Untranslated regions and introns are shown in lines. The positions of Ich-l s and Ich-1 L stop codons are indicated on die transcript diagram as "stop". Amino acid sequences in Ich-l s that are different from Ich-1 L are shaded. Figure 12A: A Comparison of the Ich-1 Protein Sequence with the Mouse nedd-2 Protein, the hICE Protein, the mICE Protein, and C. elegans ced-3 Protein Amino acids are numbered to the right of each sequence. Any residues in nedd-2, hICE, mICE, and ced-3 that are identical with Ich-1 are highlighted. The two point mutations made by site-directed mutagenesis are marked on me top of the sequence. Figure 12B: A Schematic Comparison of Structural Features of Ich-1 L and hICE The active site of hICE and predicted active site of Ich-1 L are indicated. The four known cleavage sites of hICE and potential cleavage sites of Ich-1 L are marked. Figure 13: Stable Expression of Ich-l s Prevents Cell Death of Rat-1 Cells Induced by Serum Removal Stable transfectants of Rat-1 cells expressing bcl-2, crmA or lch-l s were prepared as described in Experimental Procedures. Independent 7cΛ-7 s -positive and 7c/z-7 5 -negative clones were used. At time 0, exponentially growing cells were washed with serum-free DMEM and dead cells were counted over time by trypan blue staining. Figure 14: The cDNA Sequence and Putative Ich-3 Protein Sequence The putative first Met is marked wim a dot. Figure 15: Comparison of Amino Acid Sequences of Ich-3 with hICE, mlch-2, lch-1 and ced-3 Figure 16: Suppression of TNF-lnduced Cytotoxicity by Overexpression of CrmA The results are from three separate experiments with each condition done in duplicate. Definitions In the description that follows, a number of terms used in recombinant DNA (rDNA) technology or in the research area of programmed cell deadi are extensively utilized. In order to provide a clear and consistent understanding of the specification and claims, including the scope to be given such terms, the following definitions are provided. Gene. A DNA sequence containing a template for a RNA polymerase. The RNA transcribed from a gene may or may not code for a protein. RNA that codes for a protein is termed messenger RNA (mRNA). It is understood, however, that a gene also encompasses non-transcribed regulatory sequences including, but not limited to, such sequences as enhancers, promoters, and poly-A addition sequences. A "complementary DNA" or "cDNA" gene includes recombinant genes synthesized by reverse transcription of mRNA and from which intervening -16- sequences (introns) have been removed. Of course, cDNA may also include any complementary part of any gene sequence. The complement could be synthesized, for example, and may not exclude DNA sequences not found in the naturally occurring mRNA. Cloning vector. A plasmid or phage DNA or other DNA sequence which is able to replicate autonomously in a host cell, and which is characterized by one or a small number of endonuclease recognition sites at which such DNA sequences may be cut in a determinable fashion without loss of an essential biological function of the vehicle, and into which DNA may be spliced in order to bring about its replication and cloning. The cloning vector may further contain a marker suitable for use in the identification of cells transformed with the cloning vehicle. Markers, for example, are tetracycline resistance or ampicillin resistance. The term "cloning vehicle" is sometimes used for "cloning vector. " Expression vector. A vector similar to a cloning vector but which is capable of expressing a gene which has been cloned into it, after transformation into a host. The cloned gene is usually placed under the control of (i.e. , operably linked to) certain control sequences such as promoter sequences. Control sequences will vary depending on whether the vector is designed to express the operably linked gene in a prokaryotic or eukaryotic host and may additionally contain transcriptional elements such as enhancer elements, termination sequences, tissue-specificity elements, and/or translational initiation and termination sites. Programmed cell death. The process in which cell deadi is genetically programmed. Programmed cell death allows organisms to eliminate cells mat have served a developmental puφose but which are no longer beneficial. Functional Derivative. A "functional derivative" of mlch-2, lch-1 (Ich-1 L and Ich-l s ), or Ich-3 is a protein, or DNA encoding a protein, which possesses a biological activity that is substantially similar to the biological activity of the non-recombinant. A functional derivative of may or may not contain post-translational modifications such as covalently linked carbohydrate, depending on the necessity of such modifications for the performance of a specific function. The term "functional derivative" is intended to include the "fragments, " "variants, " "analogues, " or "chemical derivatives" of a molecule. The derivative retains at least one of the naturally-occurring functions of the parent gene or protein. The function can be any of the regulatory gene functions or any of the function(s) of the finally processed protein. The degree of activity of die function need not be quantitatively identical as long as the qualitative function is substantially similar. Fragment. A "fragment" is meant to refer to any subgenetic sequence of the molecule, such as the peptide core, or a variant of the peptide core. Detailed Description of the Preferred Embodiments Description The present invention relates, inter alia, to isolated DNA encoding ced-3 of C. elegans, mlch-2, Ich-1 , and Ich-3. The invention also encompasses nucleic acids having the cDNA sequence of ced-3, mlch-2, Ich-1 , and Ich-3. The invention also encompasses related sequences in other species that can be isolated without undue experimentation. It will be appreciated that trivial variations in the claimed sequences and fragments derived from the full- length genomic and cDNA genes are encompassed by the invention as well. The invention also encompasses protein sequences encoded by ced-3, mlch-2, Ich-1, and lch-3. It should also be understood that Ich-1 encompasses both Ich-l s and lch-l L . ced-3 The genomic sequence of the claimed gene encoding ced-3 is shown in Figure 2. The gene is 7,656 base pairs in length and contains seven introns ranging in size from 54 base pairs to 1 , 195 base pairs. The four largest introns as well as sequences 5' to the START codon contain repetitive elements, some of which have been previously characterized in the non-coding regions of other C. elegans genes such asfem-l (Spence et al , Cell 60:981- 990 (1990)) and hlh-I (Krause et al. , Cell 63:907-919 (1990)). A comparison of the repetitive elements in ced-3 with previously characterized repetitive elements is shown in figures 2A(i) - 2A(v). The START codon of ced-3 is die methionine at position 2232 of the genomic sequence shown in Figure 2. The cDNA sequence of ced-3 shown in Figure 2C. The cDNA is 2,482 base pairs in length with an open reading frame encoding 503 amino acids and 953 base pairs of 3' untranslated sequence. The last 380 base pairs of the 3' sequence are not essential for the expression of ced-3. In addition to encompassing the genomic and cDNA sequences of ced-3 from C. elegans, the present invention also encompasses related sequences in other nematode species which can be isolated without undue experimentation. For example, the inventors have shown that ced-3 genes from C. briggsae and C. vulgaris may be isolated using me ced-3 cDNA from C. elegans as a probe (see Example 1). The invention also encompasses protein products from the ced-3 gene, gene variants, derivatives, and related sequences. As deduced from the DNA sequence, ced-3 is 503 amino acids in length and contains a serine-rich middle region of about 100 amino acids. The amino acid sequence comprising me claimed ced-3 is shown in Figure 2 and Figure 2C. A comparison of ced-3 of C. elegans with the inferred ced-3 sequences from the related nematode species C. briggsae and C. vulgaris indicates tiiat the non-serine-rich region is highly conserved and that the serine-rich region is more variable. The non- serine-rich portion of ced-3 is also homologous with hICE. The C-terminal portions of both ced-3 and hICE are similar to the mouse nedd-2. The results suggest that ced-3 acts as a cysteine protease in controlling the onset of programmed cell death in C. elegans and that members of the ced-3 IICE/nedd- 2 gene family function in programmed cell death in a wide variety of species. mIch-2 The cDNA sequence and deduced amino acid sequence of mlch-2 are shown in Figure 6. As expected, mlch-2 shows homology to both hICE and mICE as well as to C. elegans ced-3 (see Figure 7 and 7A). In contrast to other cell-death genes that have been identified, mlch-2 is preferentially expressed in the thymus and placenta. Example 3 describes how die gene was obtained by screening a mouse thymus cDNA library with a DNA probe derived from hICE under conditions of low stringency. Given die amino acid sequence and cDNA sequence shown in Figure 6, preferred methods of obtaining the mlch-2 gene (either genomic or cDNA) are described below. Ich-1 neddl, hICE, mlch-2 and ced-3 are all members of the same gene family. This suggested that new genes might be isolated based upon their homology to identified family members. lch-1 is 1456 base pairs in length and contains an open reading frame of 435 amino acids (Figure IOA). The C-terminal 130 amino acids of Ich-1 are over 87% identical to mouse nedd2. However, Ich-1 contains a much longer open reading frame and has the pentapeptide QACRG which is the active center of the proteins of the ced-3/ICE family. The results indicate tiiat the cDNA isolated by Kumar et al. may not have been synthesized from a fully processed mRNA and mat the 5' 1147 base pairs which Kumar et al. reported for neddl cDNA may actually represent the sequence of an intron. The sequence reported by Kumar et al. contains a region which could potentially code for QACRG but these amino acids are encoded in a different reading frame than that indicated by Kumar et al. (Figure 8). This suggests that Kumar et al. made an error in cloning. The coding regions of nedd2 and Ich-1 L are highly homologous. The amino acid sequence of die deduced Ich-1 L protein shares 28% identity widi ced-3 and 27% identity with hICE (Figures 9, 9A). Ich-1 mRNA is alternatively spliced into two different forms. One mRNA species encodes a protein product of 435 amino acids, designated Ich-1 L , which contains amino acid sequence homologous to both P20 and PlO subunits of hICE as well as entire ced-3 (28% identity). The other mRNA encodes a 312 amino-acid truncated version of Ich-1, designated Ich-l s , diat terminates 21 amino acid residues after the QACRG active domain of Ich-1. Expression of Ich-1 L and Ich-l s has opposite effects on cell dead . Overexpression of Ich-1 L induces Rat-1 fibroblast cells to die in culture, while overexpression of die Ich-l s suppresses Rat-1 cell deadi induced by serum deprivation. Results herein suggest that lch-1 may play an important role in both positive and negative regulation of programmed cell deadi in vertebrate animals. Ich-3 lch-3 was identified based on its sequence homology with hICE and other isolated 7C£ homologs. Since the lch-3 clone isolated by PCR only contains the coding region for the C-terminal half of lch-3, a mouse thymus cDNA library was screened using die lch-3 insert. Among 2 million clones screened, 9 positive clones were isolated. The sequence herein is from one clone that contains the complete coding region for lch-3 gene. Methods of Making ced-3 There are many standard procedures for cloning genes which are well- known in the art and which can be used to obtain the ced-3 gene (see e.g. , Sambrook et al. , Molecular Cloning, a Laboratory Manual, 2nd edition, vol. 1-3, Cold Spring Harbor Laboratory Press, 1989). In Example 1, a detailed description is provided of two preferred procedures. The first preferred procedure does not require die availability of ced-3 gene sequence information and is based upon a metiiod described by Ruvkun et al. (Molecular Genetics of Caenorhabditis Elegans Heterochromic Gene lin-14121: 501-516 (1988)). In brief, Bristol and Bergerac strains of nematode are crossed and restriction fragment length polymoφhism mapping is performed on the DNA of the resulting inbred strain. Restriction fragments closely linked to ced-3 are identified and tiien used as probes to screen cosmid libraries for cosmids carrying all or part of the ced-3 gene. Positive cosmids are injected into a nematode strain in which ced-3 has been mutated. Cosmids carrying active ced-3 genes are identified by dieir ability to rescue the ced-3 mutant phenotype A second mediod for cloning ced-3 genes relies upon the sequence information which has been disclosed herein. Specifically, DNA probes are constructed based upon die sequence of the ced-3 gene of C. elegans. These probes are labelled and used to screen DNA libraries from nematodes or odier species. Procedures for carrying out such cloning and screening are described more fully below in connection with the cloning and expression of mlch-2, 7cΛ-7, and lch-3, and are well-known in the art (see, e.g., Sambrook et al., Molecular Cloning, a Laboratory Manual, 2nd edition (1988)). When hybridizations are carried out under conditions of high stringency, genes are identified which contain sequences corresponding exactly to tiiat of the probe. In this way, the exact same sequence as described by die inventors herein may be obtained. Alternatively, hybridizations may be carried out under conditions of low stringency in order to identify genes in other species which are homologous to ced-3 but which contain structural variations (see Example 1 for a description of how such hybridizations may be used to obtain the ced-3 genes from C. briggsae and C. vulgaris). The results in Example 2 demonstrate that the products of cell-deadi genes may be tolerated by cells provided they are expressed at low levels. Therefore, ced-3 may be obtained by incoφorating the ced-3 cDNA described above into any of a number of expression vectors well-known in the art and transferring these vectors into appropriate hosts (see Sambrook et al. , Molecular Cloning, a Laboratory Manual, vol. 3 (1988)). As described below in connection witii die expression of mlch-2, lch-1, and lch-3, expression systems may be utilized in which cells are grown under conditions in which a recombinant gene is not expressed and, after cells reach a desired density, expression may be induced. In tiiis way, die tendency of cells which express ced-3 to die may be circumvented. mlch-2, lch-1, and lch-3 DNA encoding mlch-2, lch-1, and lch-3 may be obtained from either genomic DNA or from cDNA. Genomic DNA may include naturally occurring introns. Moreover, such genomic DNA may be obtained in association with the 5' promoter region of the sequences and/or witii the 3' transcriptional termination region. Further, such genomic DNA may be obtained in association witii the genetic sequences which encode die 5' non- translated region of the mlch-2, lch-1, and lch-3 mRNA and/or with d e genetic sequences which encode the 3' non-translated region. To die extent that a host cell can recognize the transcriptional and/or translational regulatory signals associated with die expression of die mRNA and protein, then the 5' and/or 3' non-transcribed regions of die native gene, and/or, the 5' and/or 3' non-translated regions of d e mRNA, may be retained and employed for transcriptional and translational regulation. Genomic DNA can be extracted and purified from any cell containing mouse chromosomes by means well known in the art (for example, see Guide to Molecular Cloning Techniques, S.L. Berger et al , eds., Academic Press (1987)). Alternatively, mRNA can be isolated from any cell which expresses the genes, and used to produce cDNA by means well known in the art (Id.). The preferred sources for mlch-2 are thymus or placental cells. The mRNA Λ _ , _, 6/20721 -23- coding for any of the proteins may be enriched by techniques commonly used to enrich mRNA preparations for specific sequences, such as sucrose gradient centrifugation, or both. For cloning into a vector, DNA prepared as described above (either human genomic DNA or preferably cDNA) is randomly sheared or enzyma¬ tically cleaved, and ligated into appropriate vectors to form a recombinant gene library. A DNA sequence encoding die protein or its functional derivatives may be inserted into a DNA vector in accordance with conventional techniques. Techniques for such manipulations are disclosed by Sambrook, et al, supra, and are well known in d e art. In a preferred metiiod, oligonucleotide probes specific for the gene are designed from the cDNA sequences shown in die Figures 6, IOA, 10B, and 14. The oligonucleotide may be syntiiesized by means well known in the art (see, for example, Synthesis and Application of DNA and RNA, S.A. Narang, ed., Academic Press, San Diego, CA (1987)) and employed as a probe to identify and isolate the cloned gene by techniques known in die art. Techniques of nucleic acid hybridization and clone identification are disclosed by Maniatis, T., et al. (In: Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Laboratories, Cold Spring Harbor, NY (1982)), and by Hames, B.D. , etal. (In: Nucleic Acid Hybridization, A Practical Approach, IRL Press, Washington, DC (1985)). Those members of the above-described gene library which are found to be capable of such hybridization are tiien analyzed to determine the extent and nature of the coding sequences which they contain. To facilitate the detection of the desired coding sequence, the above- described DNA probe is labeled with a detectable group. This group can be any material having a detectable physical or chemical property. Such materials are well-known in the field of nucleic acid hybridization and any label useful in such metiiods can be applied to the present invention. Particularly useful are radioactive labels, such as 32 P, 3 H, 14 C, 35 S, 125 I, or the like. Any radioactive label may be employed which provides for an adequate signal and has a sufficient half-life. The oligonucleotide may be radioactively labeled, for example, by "nick-translation" by well-known means, as described in, for example, Rigby, P.J.W., et al. , J. Mol. Biol. 113:231 (1977) or by T4 DNA polymerase replacement synthesis as described in, for example, Deen, K.C., et al., Anal. Biochem. 735:456 (1983). Alternatively, oligonucleotide probes may be labeled wi a non- radioactive marker such as biotin, an enzyme or a fluorescent group. See, for example, Leary, J.J., et al., Proc. Natl. Acad. Sci. USA 80:4045 (1983); Renz, M., et al, Nucl. Acids Res. 72:3435 (1984); and Renz, M., EMBO J. 6:817 (1983). For 7cΛ-7, the isolation shown in die Examples was as follows. Two primers were used in die polymerase chain reaction to amplify nedd2 cDNA from embryonic day 15 mouse brain cDNA (Sambrook et al., Molecular Cloning, a Laboratory Manual, vol. 3 (1988)). One primer had the sequence: ATGCTAACTGTCCAAGTCTA and the other primer had the sequence: TCCAACAGCAGGAATAGCA. The cDNA thus amplified was cloned using standard metiiodology. The cloned mouse nedd2 cDNA was used as a probe to screen a human fetal brain cDNA library purchased from Stratagene. Such methods of screening and isolating clones are well known in die art (Maniatis, T., et al., Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Laboratories, Cold Spring Harbor, NY (1982)); Hames, B.D. , et al.. Nucleic Acid Hybridization, A Practical Approach, IRL Press, Washington, DC (1985)). A human nedd-2 cDNA clone was isolated that encodes a protein much longer than the mouse nedd-2 and contains amino acid sequences homologous to the entire hICE and ced-3. The isolated clone was given the name [ce-ced 3 homolog or lch-1. The 7cΛ-7 cDNA may be obtained using the nucleic acid sequence information given in Figures 10A or 10B. DNA probes constructed from this sequence can be labeled and used to screen human gene libraries as described herein. Also as discussed herein, 7cΛ-7 may be cloned into expression vectors and expressed in systems in which host cells are grown under conditions in which recombinant genes are not expressed and, after cells reach a desired density, expression is induced. In d is way, a tendency of cells which express lch-1 to die may be circumvented. One method of making lch-3 is as follows. mRNA was isolated from embryonic day 14 mouse embryos using Invitrogen's microfast track mRNA isolation kit. The isolated mRNA was reverse transcribed to generate template for PCR amplification. The degenerate PCR primers were: clCEB {TG(ATCG)CC(ATCG)GGGAA(ATCG)AGGTAGAA} and clCEAs {ATCAT(ATC)ATCCAGGC(ATCG)TGCAG(AG)GG}. The PCR cycles were set up as follows: 1. 94°C, 3 min; 2. 94°C, 1 min; 3. 48°C, 2 min; 4. 72°C, 3 min; 5. return to "2" 4 cycles; 6. 94°C, 1 min; 7. 55°C, 2 min; 8. 72°C, 3 min; 9. return to "6" 34 cycles; 10. 72°C, 10 min; 11. end. Such PCR generated a band about 400bp, the predicted size of ICE homologs. The PCR products were cloned into T-tailed blunt-ended pBSKII plasmid vector (Stratagene). Plasmids tiiat contain an insert were analyzed by DNA sequencing. The 7cΛ-3 cDNA may also be obtained using die nucleic acid sequence information given in Figure 14. DNA probes constructed from this sequence can be labeled and used to screen human gene libraries as described herein. Also as discussed herein, lch-3 may be cloned into expression vectors and expressed in systems in which host cells are grown under conditions in which recombinant genes are not expressed and, after cells reach a desired density, expression is induced. The methods discussed herein are capable of identifying genetic sequences which encode mlch-2, lch-1 , and lch-3. In order to further charac- terize such genetic sequences, and, in order to produce the recombinant protein, it is desirable to express the proteins which these sequences encode. To express any of the genes herein or their derivatives, transcriptional and translational signals recognizable by an appropriate host are necessary. The cloned coding sequences, obtained through die methods described herein, may be operably linked to sequences controlling transcriptional expression in an expression vector and introduced into a host cell, either prokaryote or eukaryote, to produce recombinant protein or a functional derivative thereof. Depending upon which strand of die sequence is operably linked to the sequences controlling transcriptional expression, it is also possible to express antisense RNA or a functional derivative diereof. Expression of the protein in different hosts may result in different post- translational modifications which may alter the properties of die protein. Preferably, the present invention encompasses the expression of mlch-2, lch-1 , and 7cΛ-3 or a functional derivative diereof, in eukaryotic cells, and especially mammalian, insect and yeast cells. Especially preferred eukaryotic hosts are mammalian cells either in vivo, or in tissue culture. Mammalian cells provide post-translational modifications which should be similar or identical to those found in the native protein. Preferred mammalian host cells include rat-1 fibroblasts, mouse bone marrow derived mast cells, mouse mast cells immortalized with Kirsten sarcoma virus, or normal mouse mast cells that have been co-cultured with mouse fibroblasts (Razin et al , J. of Immun. 732: 1479 (1984); Levi-Schaffer et al., Proc. Natl. Acad. Sci. (USA) 83:6485 (1986) and Reynolds et al, J. Biol. Chem. 263:12783-12791 (1988)). A nucleic acid molecule, such as DNA, is said to be "capable of expressing" a polypeptide if it contains expression control sequences which contain transcriptional regulatory information and such sequences are "operably linked" to die nucleotide sequence which encodes die polypeptide. An operable linkage is a linkage in which a coding sequence is connected to a regulatory sequence (or sequences) in such a way as to place expression of the coding sequence under the influence or control of the regulatory sequence. Two DNA sequences (e.g. die coding sequence of protein and a promoter) are said to be operably linked if induction of promoter function results in the transcription of the coding sequence and if the nature of the linkage between the two DNA sequences does not (1) result in the introduction of a frame-shift mutation; (2) interfere with the ability of regulatory sequences to direct die expression of the coding sequence, antisense RNA, or protein; or (3) interfere with die ability of the coding sequence template to be transcribed by die promoter region sequence. Thus, a promoter region would be operably linked to a DNA sequence if the promoter were capable of effecting transcription of that DNA sequence. The precise nature of the regulatory regions needed for gene expression may vary between species or cell types, but shall in general include, as necessary, 5' non-transcribing and 5' non-translating (non-coding) sequences involved with initiation of transcription and translation respectively, such as d e TATA box, capping sequence, CAAT sequence, and die like. Especially, such 5' non-transcribing control sequences will include a region which contains a promoter for transcriptional control of the operably linked gene. Expression of proteins of die invention in eukaryotic hosts requires the use of regulatory regions functional in such hosts, and preferably eukaryotic regulatory systems. A wide variety of transcriptional and translational regu¬ latory sequences can be employed, depending upon die nature of the eukaryotic host. The transcriptional and translational regulatory signals can also be derived from the genomic sequences of viruses which infect eukaryotic cells, such as adenovirus, bovine papilloma virus, simian virus, heφesvirus, or the like. Preferably, these regulatory signals are associated witii a particular gene which is capable of a high level of expression in the host cell. In eukaryotes, where transcription is not linked to translation, control regions may or may not provide an initiator methionine (AUG) codon, depending on whether the cloned sequence contains such a methionine. Such regions will, in general, include a promoter region sufficient to direct the initiation of RNA synthesis in the host cell. Promoters from heterologous mammalian genes which encode mRNA capable of translation are preferred, and especially, strong promoters such as the promoter for actin, collagen, myosin, etc. , can be employed provided they also function as promoters in the host cell. Preferred eukaryotic promoters include the promoter of the mouse 6/20721 -28- metallothionein I gene (Hamer, D., et al., J. Mol. Appl. Gen. 7:273-288 (1982)); the TK promoter of heφesvirus (McKnight, S., Cell 37:355-365 (1982)); the SV40 early promoter (Benoist, C, et al, Nature (London) 290:304-310 (1981)); in yeast, the yeast gal4 gene promoter (Johnston, S.A., etal., Proc. Natl. Acad. Sci. (USA) 79:6971-6975 (1982); Silver, P. A. , et al. , Proc. Natl. Acad. Sci. (USA) 87:5951-5955 (1984)) or a glycolytic gene promoter may be used. It is known that translation of eukaryotic mRNA is initiated at die codon which encodes die first methionine. For this reason, it is preferable to ensure that the linkage between a eukaryotic promoter and a DNA sequence which encodes the proteins of the invention or functional derivatives diereof, does not contain any intervening codons which are capable of encoding a mediionine. The presence of such codons results eidier in the formation of a fusion protein or a frame-shift mutation. If desired, a fusion product of d e proteins may be constructed. For example, d e sequence coding for the proteins may be linked to a signal sequence which will allow secretion of the protein from, or the compartmentalization of the protein in, a particular host. Such signal sequences may be designed with or widiout specific protease sites such mat the signal peptide sequence is amenable to subsequent removal. Alternatively, die native signal sequence for this protein may be used. Transcriptional initiation regulatory signals can be selected which allow for repression or activation, so that expression of operably linked genes can be modulated. Of interest are regulatory signals which are temperature- sensitive so that by varying the temperature, expression can be repressed or initiated, or are subject to chemical regulation, e.g. , metabolite. If desired, the non-transcribed and/or non-translated regions 3' to the sequence coding for the proteins can be obtained by die above-described cloning methods. The 3 '-non-transcribed region may be retained for transcriptional termination regulatory sequence elements; the 3 '-non-translated region may be retained for translational termination regulatory sequence elements, or for those elements which direct polyadenylation in eukaryotic cells. Where native expression control signals do not function satisfactorily in a host cell, functional sequences may be substituted. The vectors of the invention may further comprise other operably linked regulatory elements such as enhancer sequences, or DNA elements which confer tissue or cell-type specific expression on an operably linked gene. To transform a mammalian cell with the DNA constructs of the invention many vector systems are available, depending upon whetiier it is desired to insert d e DNA construct into the host cell chromosomal DNA, or to allow it to exist in extrachromosomal form. If the protein encoding sequence and an operably linked promoter are introduced into a recipient eukaryotic cell as a non-replicating DNA (or RNA) molecule, the expression of the protein may occur through the transient expression of die introduced sequence. In a preferred embodiment, genetically stable transformants may be constructed with vector systems, or transformation systems, whereby mlch-2, lch-1, or lch-3 DNA is integrated into the host chromosome. Such integration may occur de novo within the cell or, in a most preferred embodiment, through the aid of a cotransformed vector which functionally inserts itself into the host chromosome, for example, retroviral vectors, transposons or other DNA elements which promote integration of DNA sequences in chromosomes. Cells which have stably integrated the introduced DNA into tiieir chromosomes are selected by also introducing one or more markers which allow for selection of host cells which contain the expression vector in the chromosome, for example the marker may provide biocide resistance, e.g., resistance to antibiotics, or heavy metals, such as copper, or the like. The selectable marker gene can either be directly linked to the DNA gene sequences to be expressed, or introduced into the same cell by co-transfection. In another embodiment, the introduced sequence is incoφorated into a plasmid or viral vector capable of autonomous replication in the recipient host. Any of a wide variety of vectors may be employed for this puφose. Factors of importance in selecting a particular plasmid or viral vector include: die ease with which recipient cells that contain the vector may be recognized and selected from those recipient cells which do not contain the vector; the number of copies of the vector which are desired in a particular host; and whedier it is desirable to be able to "shuttle" the vector between host cells of different species. Preferred eukaryotic plasmids include those derived from die bovine papilloma virus, vaccinia virus, SV40, and, in yeast, plasmids containing the 2-micron circle, etc., or their derivatives. Such plasmids are well known in the art (Botstein, D., et al , Miami Wntr. Symp. 79:265-274 (1982); Broach, J.R. , In: The Molecular Biology of the Yeast Saccharomyces: Life Cycle and Inheritance, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY, p. 445-470 (1981); Broach, J.R., Cell 8:203-204 (1982); Bollon, D.P. , et al , /. Clin. Hematol. Oncol. 70:39-48 (1980); Maniatis, T., In: Cell Biology: A Comprehensive Treatise, Vol.3, Gene Expression, Academic Press, NY, pp. 563-608 (1980)), and are commercially available. Once the vector or DNA sequence containing the construct(s) is prepared for expression, die DNA construct(s) is introduced into an appropriate host cell by any of a variety of suitable means, including transfection. After d e introduction of the vector, recipient cells are grown in a medium which selects for die growdi of vector-containing cells. Expression of the cloned gene sequence(s) results in the production of die protein, or in the production of a fragment of this protein. This expression can take place in a continuous manner in the transformed cells, or in a controlled manner, for example, expression which follows induction of differentiation of die transformed cells (for example, by administration of bromodeoxyuracil to neuroblastoma cells or the like). The latter is preferred for the expression of die proteins of the invention. By growing cells under conditions in which the proteins are not expressed, cell death may be avoided. When a high cell density is reached, expression of the proteins may be induced and die recombinant protein harvested immediately before deadi occurs. The expressed protein is isolated and purified in accordance witii conventional procedures, such as extraction, precipitation, gel filtration chromatography, affinity chromatography, electrophoresis, or the like. The mlch-2, lch-1, and lch-3 sequences, obtained through the methods above, will provide sequences which not only encode proteins but which also provide for transcription of mlch-2, lch-1, and lch-3 antisense RNA; the antisense DNA sequence will be that sequence found on die opposite strand of the strand transcribing the mRNA. The antisense DNA strand may also be operably linked to a promoter in an expression vector such that transformation with this vector results in a host capable of expression of the antisense RNA in the transformed cell. Antisense DNA and RNA may be used to interact with endogenous mlch-2, lch-1, or lch-3 DNA or RNA in a manner which inhibits or represses transcription or translation of the genes in a highly specific manner. Use of antisense nucleic acid to block gene expression is discussed in Lichtenstein, C, Nature 333:801-802 (1988). Methods of Using ced-3 The ced-3 gene (as well as ced-3 homologs and od er members of the ced-3 gene family) may be used for a number of distinct purposes. First, portions of the gene may be used as a probe for identifying genes homologous to ced-3 in other strains of nematode (see Example 1) as well as in other species (see Examples 2 and 3). Such probes may also be used to determine whe er the ced-3 gene or homologs of ced-3 are being expressed in cells. The cell death genes will be used in the development of therapeutic meti ods for diseases and conditions characterized by cell deadi. Among diseases and conditions which could potentially be treated are neural and 6/20721 -32- muscular degenerative diseases, myocardial infarction, stroke, virally induced cell deadi and aging. The discovery tiiat ced-3 is related to ICE suggests that cell deadi genes may play an important role in inflammation (IL-1/3 is known to be involved in the inflammatory response). Thus therapeutics based upon ced-3 and related cell death genes may also be developed. mlch-2, lch-1, and lch-3 mlch-2, lch-1, and lch-3 will have the same uses as those described in connection widi ced-3 (above) and ICE (see below). The gene sequences may be used to construct antisense DNA and RNA oligonucleotides, which, in turn, may be used to prevent programmed cell deadi in thymus or placental cells. Techniques for inhibiting the expression of genes using antisense DNA or RNA are well-known in the art (Lichtenstein, C, Nature 333: 801-802 (1988)). Portions of the claimed DNA sequence may also be used as probes for determining the level of expression. Similarly the protein may be used to generate antibodies that can be used in assaying cellular expression. Portions of the mlch-2, lch-1, and lch-3 genes described above may be used for determining the level of expression of the proteins (mlch-2 in thymus or placental cells as well as in other tissues and organs). Such metiiods may be useful in determining if tiiese cells have undergone a neoplastic transformation. Probes based upon d e gene sequences may be used to isolate similar genes involved in cell deadi. A portion of the gene may be used in homologous recombination experiments to repair defective genes in cells or, alternatively, to develop strains of mice that are deficient in the gene. Antisense constructs may be transfected into cells, according to the native cellular expression patterns of each gene (placental or thymus cells for mlch-2, for example) in order to develop cells which may be maintained in culture for an extended period of time or indefinitely. Alternatively antisense constructs may be used in cell culture or in vivo to block cell death. The protein may be used for the purpose of generating polyclonal or monoclonal antibodies using standard techniques well known in the art (Klein, J., Immunology: The Science of Cell-Noncell Discrimination, John Wiley & Sons, N.Y. (1982); Kennett et al., Monoclonal Antibodies, Hybridoma: A New Dimension in Biological Analyses, Plenum Press, N.Y. (1980); Campbell, A., "Monoclonal Antibody Technology," In: Laboratory Techniques in Biochemistry and Molecular Biology 13, Burdon et al. eds., Elseiver, Amsterdam (1984); Harlow and Lane, Antibodies, A Laboratory Manual, Cold Spring Harbor Laboratory, N.Y. (1988)). Such antibodies may be used in assays for determining the expression of die genes. Purified protein would serve as the standard in such assays. Based upon the sequences of Figures 6, probes may be used to determine whether the mlch-2 gene or homologs of mlch-2 are being expressed in cells. Such probes may be utilized in assays for correlating mlch-2 expression widi cellular conditions, e.g. neoplastic transformation, as well as for the puφose of isolating other genes which are homologous to mlch-2. mlch-2 will be used in die development of tiierapeutic methods for diseases and conditions characterized by cell deadi. The diseases and conditions which could potentially be treated include neural and muscular degenerative diseases, myocardial infarction, stroke, virally induced cell death and aging. Antisense nucleic acids based upon die sequences shown in Figure 6 may be used to inhibit mlch-2 expression. Such inhibition will be useful in blocking cell death in cultured cells. mlch-2 may be used to generate polyclonal or monoclonal antibodies using methods well known in the art (see above). The antibodies may be used in assays for determining the expression of mlch-2. Purified mlch-2 would serve as the standard in such assays. Based upon the sequences of Figures IOA and 10B, probes may be used to determine whether the lch-1 gene or homologs of lch-1 are being expressed in cells. Such probes may be utilized in assays for correlating lch-1 expression with cellular conditions, e.g. neoplastic transformation, as well as for the puφose of isolating other genes which are homologous to lch-1. lch-1 will be used in the development of tiierapeutic methods for diseases and conditions characterized by cell deadi. The diseases and conditions which could potentially be treated include neural and muscular degenerative diseases, myocardial infarction, stroke, virally induced cell death and aging. Antisense nucleic acids based upon die sequences shown in Figures IOA and 10B, may be used to inhibit lch-1 expression. Such inhibition will be useful in blocking cell death in cultured cells. Ich-1 protein may be used to generate polyclonal or monoclonal antibodies using methods well known in the art (see above). The antibodies may be used in assays for determining the expression of lch-1. Purified lch-1 would serve as the standard in such assays. Based upon the sequence of Figure 14, probes may be used to determine whedier the lch-3 gene or homologs of lch-3 are being expressed in cells. Such probes may be utilized in assays for correlating lch-3 expression with cellular conditions, e.g. neoplastic transformation, as well as for the puφose of isolating other genes which are homologous to 7cΛ-3. 7cΛ-3 will be used in the development of tiierapeutic methods for diseases and conditions characterized by cell deadi. The diseases and conditions which could potentially be treated include neural and muscular degenerative diseases, myocardial infarction, stroke, virally induced cell deadi and aging. Antisense nucleic acids based upon the sequence shown in Figure 14 may be used to inhibit lch-3 expression. Such inhibition will be useful in blocking cell death in cultured cells. Ich-3 protein may be used to generate polyclonal or monoclonal antibodies using methods well known in the art (see above). The antibodies may be used in assays for determining the expression of lch-3. Purified lch-3 would serve as the standard in such assays. Method for Preventing Programmed Cell Death in Vertebrate Cells by Inhibiting the Enzymatic Activity of ICE The present invention is directed to preventing the programmed death of vertebrate cells by inhibiting the action of ICE. The detailed structural analysis performed on the ced-3 gene from C. elegans revealed a homology to human and murine ICE which is especially strong at the QACRG active domain of hICE (see Figure 3A). In order to determine if ICE functions as a cell deadi gene in vertebrates, the mICE gene was cloned, inserted into an expression vector and then transfected into rat cells. A close correlation was found between ICE expression and cell deadi (see Example 2). Further support for the function of 7CE as a cell deadi gene was obtained from inhibition studies. In order to determine whedier cell death can be prevented by inhibiting die enzymatic action of ICE, cell lines were established which produced a high level of crmA. When ti ese cells were transfected witii mICE, it was found tiiat a large percentage of the cells expressing mICE maintained a healthy moφhology and did not undergo programmed cell deadi (Example 2 herein). Evidence tiiat ICE has a physiological role as a vertebrate cell death gene was also obtained by examining cells engineered to over-express bcl-2, an oncogene known to inhibit programmed cell death and to be overexpressed in many follicular and B cell lymphomas. It was found that cells expressing bcl-2 did not undergo cell death despite die high levels of 7CE expression (Example 2 herein). These results suggest that bcl-2 may promote malignancy by inhibiting the action of 7CE. Any method of specifically regulating the action of ICE in order to control programmed cell death in vertebrates is encompassed by the present invention. This would include using not only inhibitors specific to ICE, e.g. crmA, or the inhibitors described by Thornberry et al. (Nature 356:768-774 (1992)), but also any method which specifically prevented the expression of the 7CE gene. Thus, antisense RNA or DNA comprised of nucleotide sequences complementary to ICE and capable of inhibiting the transcription or translation of 7CE are within the scope of the invention (see Lichtenstein, C, Nature 333:801-802 (1988)). The ability to prevent vertebrate programmed cell death is of use in developing cells which can be maintained for an indefinite period of time in culture. For example, cells over-expressing crmA may be used as hosts for expressing recombinant proteins. The ability to prevent programmed cell death may allow cells to live independent of normally required growth factors. It has been found tiiat microinjecting crmA mRNA or a crm i-expressing nucleic acid construct into cells allows chicken sympathetic neurons to live in vitro after the removal of neural growth factor. Method for Promoting Programmed Cell Death in Vertebrate Cells by Increasing or Inducing the Activity of ICE The expression of ICE can be increased in order to cause programmed cell deadi. For example, homologous recombination can be used to replace a defective region of an 7CE gene with its normal counteφart. The level of regulation amenable to manipulation to either increase or decrease the expression of 7C£ include DNA, RNA, or protein. Genomic DNA, for example, can be mutated by the introduction of selected DNA sequences introduced into die genome by homologous recombination. Any desired mutation can be introduced in vitro and, through gene replacement, eitiier decoding or regulatory sequences of the gene can be manipulated. Εxtrachromosomal DNA with the appropriate gene sequence can also be introduced into cells to compete with the endogenous product. At the level of RNA, antisense RNA molecules can be introduced, as well as RNA having more or less affinity for the translational apparatus or greater or lesser tendency to be transcribed. At the level of protein, protein counteφarts can be designed having a higher or lower activity. In addition to direct regulation, and particularly an increase in gene expression of ICE, the possibility also exists for indirect regulation by regulating those cellular components that either induce or suppress the expression of ICE. The inventors have found, for example, that TNF-α induces a program of cell death via die activation of hICE genetic sequences. Thus, a further level of regulation is that of modulating the expression of TNF-α and its functional counteφarts. Thus, any cellular component regulating programmed cell deadi by means of the lCEIced-3 pathway can itself be regulated rather than directly regulating the ICE gene. Regulation can occur by any of the means discussed above, for example. The genes in the bcl-2 family, p53 and the genes that are regulated by p53 (such as p21), the proteins in the ras pathway (ras, raf 14-3-3), Fas and die proteins in the cytotoxic T cell granules (such as granzyme B) may all directly and indirectly influence die activity of the ICE family. Accordingly, die regulation can also occur by means of any of these genes and otiiers in such pathways. In diis way, it may be possible to prevent the uncontrolled growth of certain malignant cells. Methods of increasing ICE activity may be used to kill undesired organisms such as parasites. crmA is a viral protein diat is important for cowpox infection. This suggests that die prevention of cell deadi may be important for successful infection and tiiat the promotion of ICE expression may provide a means for blocking infection. Activation of ICE family genes may be used to eliminate cancerous cells or any other unwanted cells. Prevention of cell death by inactivating the ICE family of genes could prevent neuronal degenerative diseases, such as Alzheimer's disease, amylotrophic lateral sclerosis, and cell death associated with stroke, ischemic heart injury, and aging. Having now generally described this invention, the same will be further described by reference to certain specific examples which are provided herein for puφoses of illustration only and are not intended to be limiting unless otherwise specified. All references cited tiiroughout the specification are incorporated by reference in their entirety. Example I Experimental Procedures General methods and strains The techniques used for culturing C. elegans have been described by Brenner (Brenner, S., Genetics 77:71-94 (1974)). All strains were grown at 20°C. The wild-type parent strains were C. elegans variety Bristol strain N2, Bergerac strain EM 1002 (Emmons et al, Cell 32:55-65 (1983)), C. briggsae and C. vulgaris. The genetic markers used are described below. These markers have been previously described (Brenner, S., Genetics 77:71-94 (1974)); and Hodgkin et al., Genetics in the Nematode Caenorhabditis Elgens (Wood et al. eds.) pp.491-584, Cold Spring Harbor, New York (1988)). Genetic nomenclature follows the standard system (Horvitz et al, Mol. Gen. Genet. 775: 129-133 (1979)). LG I: ced-1 (ei 735); unc-54 (r323) LG VI: unc-31 (e928), unc-30 (e!91), ced-3 (n7I7, n718, nl040, nll29, nll634, nll64, nll65, nl286, nl949, n2426, n2430, n2433), unc-26 (e205), dpy-4 (ell66) LG V: eg-l(n986); unc-76 (e9U) LG X: dpy-3(e27) Isolation of additional alleles of ced-3 A non-complementation screen was designed to isolate new alleles of ced-3. Because animals heterozygous for ced3(n7I7) in trans to a deficiency are viable (Ellis et al, Cell 44:8X1-1,29 (1986)), it was expected that animals carrying a complete Ioss-of-function mutant ced-3 allele in trans to ced- 3(n717) would be viable even if homozygotes for the allele were inviable. EMS mutagenized egl-1 L4 males were mated witii ced-3 (n717) unc-26(e205); egl-l(n487) dpy-3(e27) hermaphrodites, egl-1 was used as a marker in this screen. Dominant mutations in egl-1 cause the two hermaphrodite-specific neurons, die HSNs, to undergo programmed cell deadi (Trent et al., Genetics 104:619-647 (1983)). The HSNs are required for normal egg-laying, and egl-1 (n986) hermaphrodites, which lack HSNs are egg-laying defective (Trent et al., Genetics 104:6X9-641)). The mutant phenotype of egl-1 is suppressed in a ced-3; egl-1 strain because mutations in ced-3 block programmed cell deaths, egl-1 males were mutagenized with EMS and crossed with ced3(n717) unc-26(e205); egl-l(n487); dpy-3(e27). Most cross progeny were egg-laying defective because diey were heterozygous for ced-3 and homozygous for egl-1. Rare egglaying competent animals were picked, those animals being candidates for carrying new alleles of ced-3. Four such animals were isolated from about 10,000 Fl cross progeny of EMS-mutagenized animals. These new mutations were made homozygous to confirm that they carried mutations of ced-3. RFLP mapping Two cosmid libraries were used extensively in this work - a Sau3A I partial digest genomic library of 7000 clones in the vector pHC79 and a Sau3A I partial digest genomic library of 6000 clones in the vector pJB8 (Coulson et al , Proc. Natl. Acad. Sci. U.S.A. 83:7821-7825 (1986). Bristol (N2) and Bergerac (EM 1002) DNA was digested witii various restriction enzymes and probed with different cosmids to look for RFLPs. nP33 is a HindXXX RFLP detected by the "right" end of Jc8. The "right" end of Jc8 was made by digesting Jc8 with EcoRI and self-ligating. nP34 is a HindXXX RFLP detected by the "left" end of Jc8. The "left" end of Jc8 was made by digesting Jc8 by Sail and self ligating. nP36 and nP37 are both HindXXX RFLPs detected by T10H5 and B0564, respectively. Germ line transformation The procedure used for microinjection basically follows that of A. Fire (Fire, A., EMBO J. 5:2673-2680 (1986)). Cosmid DNA was twice CsCl gradient purified. Miniprep DNA was used when deleted cosmids were injected and was prepared from 1.5 ml overnight bacterial culture in superbrotii. Superbroth was prepared by combining 12 g Bacto tryptone, 24 g yeast extract, 8 ml 50% glycerol and 900 ml H 2 O. The mixture was autoclaved and then 100 ml of 0.17 M KH 2 PQ, and 0.72 M K 2 HPO < were added. The bacterial culture was extracted by die alkaline lysis method as described in Maniatis et al. (Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Press, Cold Spring Harbor, NY (1983)). DNA was treated witii RNase A (37°C, 30 min) and then with protease K (55°C, 30 min). The preparation was phenol- and dien chloroform-extracted, precipitated twice (first in 0.3 M Na acetate and second in 0.1 M K acetate, pH 7.2), and resuspended in 5 1 injection buffer as described by A. Fire (Fire, A., EMBO J. 5:2673-2680 (1986)). The DNA concentration for injection was in the range of 100 / xg to 1 mg per ml. All transformation experiments used the ced-l(e!735); unc-31(e928) ced-3 (n717) strain, unc-31 was used as a marker for co-transformation (Kim et al , Genes & Dev. 4:351-311 (1990)). ced-1 was present to facilitate scoring of the ced-3 phenotype. The mutations in ced-1 block the engulfment process of cell deadi, which makes die coφses of the dead cells persist much longer than that in the wild-type (Hedgecock et al. Science 220: 1277-1280 (1983)). ced-3 phenotype was scored as die number of dead cells present in die head of young LI animals. The cosmid C10D8 or the plasmid subclones of C10D8 were mixed witii C14G10 (unc-31(+ J-containing) at a ratio of 2: 1 or 3: 1 to increase the chances that an Unc-31(+) transformant would contain the cosmid or plasmid being tested. Usually, 20-30 animals were injected in one experiment. Non-Unc Fl progeny of injected animals were isolated three to four days later. About 1/2 to 1/3 of the non-Unc progeny transmitted the non-Unc phenotype to F2 and established a line of transformants. The young LI progeny of such non-Unc transformants were checked for the number of dead cells present in die head using Nomarski optics. Determination of ced-3 transcript initiation site Two primers, Pex 1 : (5 'GTTGCACTGCTTTCACGATCTCCCGTCTCT3 ' ) and Pex2: (5 CATCGACTTTTAGATGACTAGAGAACATC3'), were used for primer extension. The primers for RT-PCR were: SLl (5'GTTTAATTACCCAAGTTTGAG3') and log-5 (5'CCGGTGACATTGGACACTC3'). The products were reamplified using the primers SLl and oligolO (5ΑCTATTCAACACTTG3'). A product of the expected length was cloned into die PCRIOOO vector (Invitrogen) and sequenced. Determination and analysis of DNA sequence For DNA sequencing, serial deletions were made according to a procedure developed by Henikoff (Heinkoff, S. , Gene 28:351-359 (1984)). DNA sequences were determined using Sequenase and protocols obtained from US Biochemicals with minor modifications. The ced-3 amino acid sequence was compared with amino acid sequences in the GenBank, PIR and SWISS-PROT databases at the National Center for Biotechnology Information (NCBI) using the blast network service. Cloning of ced-3 genes from other nematode species C. briggsae and C. vulgaris ced-3 genes were isolated from corresponding phage genomic libraries using the ced-3 cDNA subclone pJl 18 insert as a probe under low stringency conditions (5xSSPE, 20% formamide, 0.02% Ficoll, 0.02% BSA, 0.02% polyvinylpyrrolidone, and 1 % SDS) at 40°C overnight and washed in IxSSPE and 0.5% SDS twice at room temperature and twice at 42 °C for 20 min each time. Results ced-3 is not essential for viability All previously described ced-3 alleles were isolated in screens designed to detect viable mutants in which programmed cell death did not occur (Ellis et al, Cell 44:817-829 (1986)). Such screens might systematically have missed classes of ced-3 mutations that result in inviability. Since animals witii die genotype of c«/-3/deficiency are viable (Ellis et al, Cell 44: 817-829 (1986)), a noncomplementation-screening scheme was designed that would allow die isolation of recessive lethal alleles of ced-3. Four new ced-3 alleles (nll63, nll64, n!165, and nl286) were obtained which were viable as homozygotes. These new alleles were isolated at a frequency of about 1 in 2500 mutagenized haploid genomes, approximately the frequency expected for the generation of null mutations in an average C. elegans gene (Brenner, S., Genetics 77:71-94 (1974); Meneely et al, Genetics 92:99-105 (1990); Greenwald et al., Genetics 96: 147-160 (1980)). These results suggest that animals lacking ced-3 gene activity are viable. In support of this hypothesis, molecular analysis has revealed diat three ced-3 mutations are nonsense mutations that prematurely terminate ced-3 translation and one alters a highly conserved splice acceptor site (see below). These mutations would be expected to eliminate ced-3 activity completely. Based upon these considerations, it was concluded that ced-3 gene activity is not essential for viability. ced-3 is contained within a 7.5 kb genomic fragment The ced-3 gene was cloned using the approach of Ruvkun et al. (Molecular Genetics of the Caenorhabditis elegans Heterochronic Gene lin-14 121:501-516 (1988)). Briefly (for further details, see Experimental Procedures), die C. elegans Bristol strain N2 contains 30 dispersed copies of the transposable element Tel, whereas the Bergerac strain contains more than 400 copies (Emmons et al, Cell 32:55-65 (1983); Finney, M., Ph.D. Thesis, "The Genetics and Molecular Biology of unc-86, a Caenorhabditis elegans Cell Lineage Gene," Cambridge, MA (1987)). By crossing Bristol and Bergerac strains, a series of recombinant inbred strains were generated in which chromosomal material was mostly derived from die Bristol strain with varying amounts of Bergerac-specific chromosome IV-derived material in the region of die ced-3 gene. By probing DNA from these strains witii plasmid pCe2001 which contains Tel (Emmons et al , Cell 32:55-65 (1983), a 5.1 kb EcoRI Tel -containing restriction fragment specific to the Bristol strain (restriction fragment length polymoφhism nP35) and closely linked to ced-3 was identified. Cosmids that contained this 5.1 kb restriction fragment were identified and it was found tiiat these cosmids overlapped an existing cosmid contig that had been defined as part of the C. elegans genome project (Coulson et al, Proc. Natl. Acad. Sci. 83:7821-7825 (1986). Four other Bristol-Bergerac restriction fragment length polymoφhisms were defined by cosmids in this contig (nP33, np34, nP36, nP37). By mapping these restriction fragment length polymoφhisms with respect to the genes unc-30, ced-3 and unc-26, the physical contiguity was oriented with respect to the genetic map and the region containing the ced-3 gene was narrowed to an interval spanned by three cosmids (Fig. 1). On Southern blot, three of three + Berg unc-26 recombinants showed the Bristol nP33 pattern while two of two ced-3 4- Berg recombinants showed the Bergerac pattern (data not shown). Thus, nP33 maps very close or to the right of unc-26. For nP34, two of two ced-3 + Berg recombinants and two of three + Berg unc-26 recombinants showed the Bergerac pattern; one of the three + Berg unc-26 recombinant showed die Bristol pattern (data not shown). The genetic distance between ced-3 and unc-26 is about 0.2 mu. Thus, nP34 maps between ced-3 and unc-26, about 0.1 mu to die right of ced-3. Similar experiments mapped nP35, the 5.1 kb Bristol specific Tel element, to about 0.1 mu to the right of ced-3. In order to map n36 and n37, Bristol unc-30 ced-3 + + males were crossed with Bergerac hermaphrodites. From the progeny of heterozygotes of genotype unc-30 ced-3 (Bristol)/-. -!- (Bergerac), Unc-30/non-ced-3 and non- Unc-30/ ced-3 animals were picked and DNA was prepared from these strains. nP36 maps very close or to the right of unc-30 since two of two unc-30 + Berg recombinants showed Bristol pattern and two of two + Berg ced-3 recombinants showed die Bergerac pattern (data not shown). Similarly, nP37 maps very close or to die right of unc-30 since four of the four -. Berg ced-3 showed Bergerac pattern and six of six unc-30 + Berg recombinants showed die Bristol pattern (data not shown). These experiments narrowed die region containing the ced-3 gene to an interval spanned by the three cosmids (Fig. la). Cosmids tiiat were candidates for containing the ced-3 gene were microinjected (Fire, A., EMBO J. 5:2673-2680 (1986)) into ced-3 mutant animals to test for rescue of the mutant phenotype. Specifically, cosmid C14G10, which contains the wild-type unc-31 gene and a candidate cosmid were coinjected into ced-1 (el 375); unc-31(e928) ced-3 (n717) hermaphrodites. Non-unc progeny were isolated and observed to see if the non-Unc phenotype was transmitted to the next generation, thus establishing a line of transformed animals. Young LI progeny of such transformant lines were examined for the presence of cell deaths using Nomarski optics to see whether the ced-3 phenotype was complemented (see Experimental Procedures). Cosmid C 14G10 alone does not confer ced-3 activity when injected into a ced-3 mutant. unc-31 was used as a marker for co-transformation (Kim et al. , Genes & Devel. 4:357-371 (1990)). ced-1 was present to facilitate scoring of the ced-3 phenotype. Mutations in ced-1 block the engulfment process of programmed cell deadi, causing die coφses of dead cells to persist much longer tiian in the wild-type (Hedgecock et al, Science 220: 1277-1280 (1983)). Thus, the presence of a coφse indicates a cell that has undergone programmed cell death. The ced-3 phenotype was scored as the number of coφses present in the head of young LI animals. As indicated in Fig. 1, of die three cosmids injected (C43C9, W07H6 and C48D1), only C48D1 rescued the ced-3 mutant phenotype. Both non-Unc transformed lines obtained, nisi and nEx2, were rescued. Specifically, LI ced-1 animals contain an average of 23 cell coφses in the head, and LI ced-1; ced3 animals contain an average of 0.3 cell coφses in the head (Ellis et al, Cell 44:817-829 (1986)). By contrast, ced-1; unc-31 ced-3; nisi; and ced-1; unc-31 ced-3; nEx2 animals contained an average of 16.4 and 14.5 cell coφses in the head, respectively. From these results, it was concluded tiiat C48D1 contains the ced-3 gene. To locate ced-3 more precisely within the cosmid C48D1 , this cosmid was subcloned and d e subclones tested for their ability to rescue the ced-3 mutant phenotype (Fig. 1A). From these experiments, ced-3 was localized to a DNA fragment of 7.5 kb (pJ7.5). A 2.8 kb ced-3 transcript is expressed primarily during embryogenesis and independently of ced-4 function The 7.6 kb pJ107 subclone of C48D1 (Fig. 1A) was used as a probe in a northern blot of polyA + RNA derived from the wild-type C. elegans strain N2. This probe hybridized to a 2.8 kb transcript. Although this transcript is present in 11 different EMS-induced ced-3 mutant strains, subsequent analysis has shown that all 11 tested ced-3 mutant alleles contain mutations in the genomic DNA that encodes this mRNA (see below), thus establishing this RNA as a ced-3 transcript. The developmental expression pattern of ced-3 was determined by hybridizing a northern blot of RNA from animals at different stages of development with the ced-3 cDNA subclone pJ118 (see below). The ced-3 transcript was found to be most abundant during embryogenesis, when most programmed cell deaths occur, but was also detected during the LI through L4 larval stages. It is present in relatively high levels in young adults. Since ced-3 and ced-4 are both required for programmed cell deadi in C. elegans, and since both are highly expressed during embryonic development (Yuan et al, Dev. 776:309-320 (1992), the possibility existed tiiat one of the genes might regulate the mRNA level of the other. Previous studies have revealed tiiat ced-3 does not regulate ced-4 mRNA levels (Yuan et al, Dev. 116:309-320 (1992)). To determine if ced-4 regulates ced-3 mRNA levels, a northern blot of RNA prepared from ced-4 mutant embryos was probed witii die ced-3 cDNA subclone pJ118. It was found that the amount and size of the ced-3 transcript was normal in the ced-4 mutants Λ.7762, nl416, n!894 and nl920. Thus, ced-4 does not appear to affect die steady-state levels of ced-3 mRNA. ced-3 cDNA and genomic Sequences To isolate ced-3 cDNA clones, ced-3 genomic DNA pJ40 (Fig. 1A) was used as a probe to screen a cDNA library of die C. elegans wild-type strain N2 (Kim et al., Genes & Dev. 4:357-371 (1990)). The 2.5 kb cDNA clone pJ87 was isolated in this way. On northern blots, pJ87 hybridized to a 2.8 kb transcript and on Southern blots, it hybridized only to bands to which pJ40 hybridizes (data not shown). Thus, pJ87 represents an mRNA transcribed entirely from pJ40 which can rescue the ced-3 mutant phenotype when microinjected into ced-3 mutant animals. To confirm that pJ87 contains the ced-3 cDNA, a frameshift mutation in the Sail site of pJ40 was made corresponding to the SalX site in the pJ87 cDNA. Constructs containing the frameshift mutation failed to rescue the ced-3 phenotype when microinjected into ced-3 mutant animals (6 transformant lines; data not shown), suggesting tiiat ced-3 activity had been eliminated by mutagenizing a region of genomic DNA that corresponds to the pJ87 cDNA. The DNA sequence of pJ87 is shown in Figure 2C. pJ87 contains an insert of 2482 bp with an open reading frame of 503 amino acids. It has 953 bp of 3' untranslated sequence, not all of which is essential for ced-3 expression; genomic constructs that do not contain 380 bp of the 3 '-most region (pJ107 and its derivatives, see Fig. la) were capable of rescuing ced-3 mutant phenotype. The cDNA ends with a poly-A sequence, suggesting that the complete 3' end of die ced-3 transcript is present. To confirm the DNA sequence obtained from die ced-3 cDNA and to study the structure of the ced-3 gene, the genomic sequence of the ced-3 gene from the plasmid pJ107 was determined. The insert in pJ107 is 7656 bp in length (Fig. 2). To determine the location and nature of d e 5' end of the ced-3 transcript, a combination of primer extension and amplification using the polymerase chain reaction (PCR) was used. Two primers, Pexl and Pex2, were used for primer extension. The Pexl reaction yielded two major bands, whereas the Pex2 reaction gave one band. The Pex2 band corresponds in size to the smaller band from the Pexl reaction, and agrees in lengtii with a possible transcript that is trans-spliced to a C. elegans splice leader (Bektesh et al, Genes & Dev. 2: 1277-1283 (1988)) at a consensus splice acceptor at position 2166 of the genomic sequence. The nature of the larger Pexl band is unclear. To confirm these observations, wild-type total RNA was reverse- transcribed and then amplified using the primers SLl and log-5 followed by reamplification using the primers SLl and oligolO. A product of the expected length was cloned into the PCRIOOO vector (Invitrogen) and sequenced. The sequence obtained confirmed the presence of a ced-3 message trans-spliced to SLl at position 2166 of the genomic sequence. These experiments suggest that a ced-3 transcript is trans-spliced to the C. elegans splice leader SLl (Bektesh et al., Genes & Dev. 2: 1277-1283 (1988)) at a consensus splice acceptor at position 2166 of the genomic sequence. Based upon these observations, it is concluded that the start codon of ced-3 is the methionine encoded at position 2232 of the genomic sequence and tiiat ced-3 is 503 amino acids in length. The predicted ced-3 is hydrophilic (256/503 residues are charged or polar) and does not contain any obvious potential trans-membrane domains. One region of ced-3 is rich in serines: from amino acid 107 to amino acid 205, 32 of 99 amino acids are serine residues. The sequences of 12 EMS-induced ced-3 mutations (Table 1) were determined. Eight are missense mutations, three are nonsense mutations, and one alters a conserved G at the splice acceptor site of intron 6. Interestingly, nine of these 12 mutations alter residues within die last 100 amino acids of die protein, and none occurs witiiin the serine-rich region. Table 1. Sites of mutations in the ced-3 gene Allele Mutation Nucleotide Codon Consequence n717 G to A 6297 Altered splicing n718 G to A 2487 65 G to R nl040 C to T 2310 27 L to F n!129 & nl64 C to T 6434 449 A to V n!163 C to T 7020 486 S to F nll65 C to T 5940 403 Nonsense n!286 G to A 6371 428 Nonsense nl949 C to T 6222 412 Nonsense n2426 G to A 6535 483 E to K n2430 C to T 6485 466 A to V n2433 G to A 5757 360 G to S Nucleotide and codon positions correspond to the numbering in Fig. 2. To identify functionally important regions of ced-3, the genomic sequences of die ced-3 genes from the related nematode species C. briggsae and C. vulgaris were cloned and sequenced. Sequence comparison of the three ced-3 genes showed that die relatively non-serine-rich regions of the proteins are more conserved than are serine-rich regions (Fig. 3A). All 12 EMS-induced ced-3 mutations altered residues that are conserved among the three species. These results suggest diat the non-serine-rich region is important for ced-3 function and tiiat the serine rich region is either unimportant or that residues widiin it are functionally redundant. ced-3 protein is similar to the mammalian ICE and nedd-2 proteins A search of the GenBank, PIR and SWISS-PROT databases revealed tiiat the non-serine-rich regions of ced-3 are similar to hICE and mICE (Fig. 3A). The most highly conserved region among die proteins shown in Figure 3A consists of amino acids 246-360 of ced-3 and amino acids 166-287 of the hICE: 49 residues are identical (43% identity). The active site cysteine of hICE is located at cysteine 285 (Thornberry et al, Nature 356:768-774 (1992)). The five-amino-acid peptide (QACRG) around this active cysteine is the longest conserved peptide among mICE and hICE and ced-3. hICE is composed of two subunits (p20 and plO) that appear to be proteolytically cleaved from a single proenzyme to the mature enzyme (Thornberry et al, Nature 356:768-774 (1992)). Two cleavage sites in the proenzyme, Asp-Ser at positions 103 and 297 of hICE, are conserved in ced-3 (position 131 and 371 , respectively). The C-terminal portion of ced-3 and the plO subunit of hICE are similar to the protein product of the murine nedd-2 gene, ced-3, nedd-2 and hICE are 27% identical (Fig. 3A). nedd-2 does not contain the QACRG peptide at the active site of hICE and mICE (Fig. 3A). Seven of eight point mutations that were analyzed (n718, nlCHO, nil 29, n!164, n2430, n2426 & n2433) result in alterations of amino acids that are conserved or semi- conserved among d e three nematode ced-3 proteins, hICE and nedd-2. In particular, die mutation, n2433, introduces a Gly to Ser change near die putative active cysteine (Fig. 2, Table 1). Discussion The genes ced-3 and ced-4 are the only genes known to be required for programmed cell death to occur in C. elegans (Ellis et al., Cell 44:817-829 (1986)). Genetic and molecular studies have revealed tiiat the ced-3 gene shares a number of features with ced-4 (see Yuan et al., Dev. 776:309-320 (1992)). Like ced-4, ced-3 is not required for viability. It appears to contain the sequence for a single mRNA which is expressed mostly in die embryo, the stage during which most programmed cell death occurs. Furthermore, just as ced-3 gene function is not required for ced-4 gene expression (Yuan et al., Dev. 776:309-320 (1992)), ced-4 gene function is not required for ced-3 gene expression. Thus, these two genes do not appear to control the onset of programmed cell deadi by acting sequentially in a regulatory transcriptional cascade. Unlike ced-4 (Yuan et al , Dev. Biol 738:33-41 (1992)), ced-3 is expressed at a substantial level in young adults. This observation suggests that ced-3 expression is not limited to cells undergoing programmed cell death. The ced-4 amino acid sequence is novel. Two regions show similarity to the EF-hand motif, which binds calcium (Yuan et al., Dev. 116:309-320 (1992)). For this reason it has been suggested that ced-4 protein and hence, programmed cell death in C. elegans, might be regulated by calcium, ced-3 contains a region of 99 amino acids that contain 32 serines. Since serines are common phosphorylation sites (Edelman et al., Ann. Rev. Biochem. 56:561- 613 (1987)), ced-3 and hence, programmed cell deadi in C. elegans, may be regulated by phosphorylation. Phosphorylation has previously been suggested to function in cell death (McConkey et al, J. Immunol. 745: 1227-1230 (1990)). McConkey et al. have shown that several agents that elevate cytosolic cAMP level induce thymocyte death. This suggests that protein kinase A may mediate cell deadi by phosphorylating certain proteins. Although the precise sequence of the serine-rich region varies among the three Caenorhabditis species studied, the relatively high number of serines is conserved in C. elegans, C. briggsae and C. vulgaris. None of the mutations in ced-3 affect the serine-rich region. These observations are consistent with the hypothesis that the presence of serines is more important than die precise amino acid sequence within this region. Much more striking than the presence of the serine-rich region in ced-3 is die similarity between the non-serine-rich regions of ced-3 and hICE and mICE. The carboxy half of ced-3 is the region that is the most similar to ICE. A stretch of 115 residues (amino acids 246-360 of ced-3) is 43% identical between ced-3 and hICE. This region in ICE contains a conserved pentapeptide QACRG (positions 361-365 of ced-3), which surrounds die active cysteine. Specific modification of this cysteine in hICE results in complete loss of activity (Thornberry et al., Nature 356:768-774 (1992)). The ced-3 mutation n2433 alters the conserved glycine in this pentapeptide and eliminates ced-3 function. This suggests that this glycine is important for ced-3 activity and is an integral part of the active site of ICE. Interestingly, while the mutations n77 (position 67 of ced-3) and nl 040 (position 27 of ced-3) eliminate ced-3 function in vivo, they contain alterations in conserved residues which are outside of the mature P20 subunit of hICE (Thornberry et al., Nature 356:768-774 (1992)). These residues may have a non-catalytic role in both ced-3 and ICE function, e.g., they may maintain a proper conformation for proteolytic activation. The hICE precursor (p45) is proteolytically cleaved at 4 sites (Aspl03, Aspl l9, Asp297 and Asp316) to generate p24, p20, and plO (Thronberry et al. Nature 356:168-114 (1992)). At least two of the cleavage sites are conserved in ced-3. This indicates that the ced-3 protein is processed. The similarity between ced-3 and ICE suggests that ced-3 functions as a cysteine protease, controlling programmed cell death by proteolytically activating or inactivating a substrate protein. A substrate for ced-3 could be the product of die ced-4 gene which contains 6 Asp residues. These could be the target of ced-3 (Asp25, Aspl51, Aspl85, Aspl92, Asp459 and Asp541). Alternatively, ced-3 could directly cause cell deadi by proteolytically cleaving certain proteins or subcellular structures that are crucial for cell viability. ced-3 and ICE are part of a novel protein family. Thornberry et al. suggested that the sequence GDSPG at position 287 of hICE resembles a GX(S/C)XG motif found in serine and cysteine protease active sites (Nature 356:168-114 (1992)). However, in the three nematode ced-3 proteins examined, only me first glycine is conserved, and in mICE, the SIC is not present. This suggests that the ced-3/ICE family shares little sequence similarity with known protease families. The similarity between ced-3 and ICE suggests not only tiiat ced-3 functions as a cysteine protease, but also that ICE functions in programmed cell death in vertebrates. Thus, it has been observed that after murine peritoneal macrophages are stimulated with lipopolysaccharide (LPS) and induced to undergo programmed cell death by exposure to extracellular ATP, mature active IL-lβ is released into die culture supernatant. In contrast, when cells are injured by scraping, XL-Xβ is released exclusively as the inactive proform (Hogoquist et al., Proc. Natl. Acad. USA 88:8485-8489 (1991)). These results suggest that 7CE is activated upon induction of programmed cell deadi. 7CE transcript has been detected in cells tiiat do not make IL-13 (Cerretti et al, Science 256:97-100 (1992)), suggesting that other 7CE substrates exist. This suggests that ICE could mediate programmed cell death by cleaving a substrate other than XL-Xβ. The carboxy-terminal portions of both ced-3 and die plO subunit of hICΕ are similar to the protein encoded by nedd-2. Since nedd-2 lacks the QACRG active domain, it might function to regulate ICE or ICE-like p20 subunits. Interestingly, four ced-3 mutations alter residues conserved between nedd-2 and ced-3. Further, nedd-2 gene expression is high during embryonic brain development, when much programmed cell death occurs. These observations suggest that nedd-2 functions in programmed cell death. The C. elegans gene ced-9 protects cells from undergoing programmed cell death by directly or indirectly antagonizing the activities of ced-3 and ced- 4 (Hengartner et al., Nature 356:494-499 (1992)). bcl-2 also affects the onset of apoptotic cell death. Thus, if hICE or another ced-3IICE family member is involved in vertebrate programmed cell death, bcl-2 might act by modulating its activity. The fact that bcl-2 is a dominant oncogene suggests that hICE and other ced-3/lCE family members might be recessive oncogenes. The elimination of such cell death genes would prevent normal cell deadi and promote malignancy, just as overexpression of bcl-2 does. Example 2 The mouse ICE homolog (mICE) from a mouse thymus cDNA library (Stratagene) was cloned by low stringency hybridization using hICE as a probe. The clone is identical to the clone isolated by Nett et al. (J. Immun. 149:3245-3259 (1992)) except that base pair 166 is an A and encodes Asn rather than Asp. This may be a DNA polymoφhism because the clone was derived from a B6/CBAF1J (C57Black x CBA) strain cDNA library (Stratagene), while the Nett et al. clone was derived from a WEH13 cDNA library (Stratagene). Subsequent experiments have shown that this variation is not in a region essential for 7CE function (see below). A transient expression system was developed to determine if overexpression of mICE kills cells. mICE cDNA was fused with the E. coli lac-Z gene and placed under the control of the chicken 3-actin promoter (Fig. 4). To test the function of the subunits, P20 and PlO, which are processed from a precursor peptide, two additional fusion genes were made (P20/P10-lacZ and PlO-lacZ). The constructs, shown in Fig. 4, were transfected into Rat-1 cells by calcium phosphate precipitation. 24 hours after transfection, cells were fixed _.,_-_ « _- « __-,, _ WO 96/2072 -54- and X-gal was added. Healtiiy living rat cells are flat and well-attached to plates, while dying cells are round and often float into the medium. After 3 hours of color development, most blue cells transfected with intact mICE-lacZ or P20/P10-lacZ were round. However, most blue cells transfected with P10- lacZ or the control lac-Z construct were normal flat cells (Table 2). Similar results were obtained with NG108-15 neuronal cells (not shown). Table 2. Overexpression of mICE causes Rat-1 cells to undergo programmed cell death The constructs shown in Fig. 4 were transientiy transfected into Rat-1 cells, Rat-1 cells expressing bcl-2 (Rzt-l/bcl-2) or Rat-1 cells expressing crmA (Rat-1/ crmA). 24 hrs after transfection, cells were fixed and stained with X-gal for 3 hrs. The data shown are the percentage of round blue cells among total number of blue cells. The data were collected from at least three different experiments. Construct Rat-1 Rat-l/bcl-2 Rat-1/crmA pactøgaT 1.44 ± 0.18 2.22 ± 0.53 2.89 ± 0.79 pøactMIOZ 80.81 ± 2.33 9.91 ± 2.08 18.83 ± 2.86 p/SactMUZ 93.33 ± 2.68 13.83 ± 4.23 24.48 ± 2.78 p3actM19Z 2.18 ± 0.54 p/3actM12Z 2.44 ± 0.98 3.33 ± 1.45 2.55 ± 0.32 p3actl7Z 2.70 ± 1.07 pJ485 1.32 ± 0.78 p/Sactced38Z 46.73 ± 4.65 35.28 ± 1.36 34.40 ± 2.38 p/3actced37Z 3.67 ± 1.39 Methods: a: Construction of bcl-2 expression vector (pJ415): pJ415 was constructed by first inserting 5', the 400bp BglXX/BamHϊ crmA fragment into the BamHX site of the pBabe/puro vector and then inserting the remaining lkb BamHX crmA fragment into the 3' BamHX site in the sense direction, b: Construction of the bcl-2 expression vector (pJ436): pJ436 was constructed by inserting an EcoRXISalX bcl-2 fragment into the EcoRXISalX sites of the pBabe/puro vector. c: Establishing Rat-1 cell lines that overexpress crmA and bcl-2: pJ415 and pJ436 were electroporated into ΨCRE retroviral packaging cells (Danos et al, Proc. Natl. Acad. Sci. (USA) 85:6460-6464 (1988)) using a BioRad electroporating apparatus. Supernatant either from overnight transiently transfected ΨCRE cells or from stable lines of ΨCRE cells expressing either crmA or bcl-2 were used to infect Rat-1 cells overnight in the presence of 8 μg/ml of polybrene. Resistant cells were selected using 30 ug/ml puromycin for about 10 days. Resistant colonies were cloned and checked for expression using both Northern and Western blots. Bcl-2 antibodies were from S.J. Korsmeyer and from DAKO. crmA antiserum was made by immunizing rabbits with an E. coli-expτessed crmA fusion protein (pJ434). pJ434 was made by inserting an EcoRI/ Sail fragment of crmA cDNA into EcoRXISalX sites of pET21a (Novagen) and fusion protein was expressed in die E. coli BL21 (DE3) strain. Multiple lines that express either bcl-2 or crmA were checked for suppression of mICE induced cell death and all showed similar results. When cells were stained with rhodamine-coupled anti-/3 galactosidase antibody and Hoechst dye, it was found that jS-galactosidase-positive round cells had condensed, fragmented nuclei. Such nuclei are indicative of programmed cell death. When observed with an electron microscope, the X- gal reaction product was electron dense, allowing cells expressing mICE-lacZ to be distinguished from other cells (Snyder et al. , Cell 68:33-51 (1992)). The cells expressing the chimeric gene showed condensed chromatin and membrane blebbing. These are characteristics of cells undergoing programmed cell deadi (Wyllie, A.H., in Cell Death in Biology and Pathology, 9-34 (1981); Oberhammer et al , Proc. Natl. Acad. Sci. U.S.A. 89:5408-5412 (1992); Jacobson et al , Nature 367:365-369 (1993)). Thus, the results indicate that overexpression of mICE induces programmed cell death and that induction depends on both P20 and PlO subunits. When color development in Rat-1 cells transfected with mICE-lacZ or P20/P10-lacZ is allowed to proceed for 24 hours, a greater number of flat cells become blue. This result indicates that cells tolerate lower levels of ICE activity. If mICE is a vertebrate homolog of ced-3, then ced-3 might also be expected to cause cell death in vertebrates. This hypothesis was tested by making a ced-3-lacZ fusion construct and examining its ability to cause cell death using d e assay as described above. As expected, die expression of ced-3 caused die deadi of Rat-1 cells (Table 2). XfmlCE functions in a similar way to ced-3, mutations eliminating ced- 3 activity in C. elegans should also eliminate its activity in vertebrates. This hypothesis was tested by mutating the Gly residue in die pentapeptide active domain of hICE, QACRG, to Ser. It was found that this mutation eliminated the ability of both 7CE and ced-3 to cause rat cell death in Rat-1 cells (Table 2). crmA specifically inhibits 7CE activity (Ray et al , above). To demonstrate that cell death associated witii overexpression of 7CE is due to the enzymatic activity of mICΕ, Rat-1 cells were infected with a pBabe retroviral construct (Morgenstern et al , Nucl. Acids Res. 78:3587-3596 (1990)) expressing crmA and cell lines identified which produce high levels of crmA. When the mICE-lacZ construct was transfected into tiiese cell lines, it was found tiiat a large percentage of blue cells had a healthy, flat moφhology (Table 2). In addition, a point mutation that changes the Cys residue in d e active site pentapeptide, QACRG to a Gly eliminates the ability of mICE to cause cell deadi (construct p / 3actM17Z, Figure 4, Table 2). This result indicates that the proteolytic activity of ml CE is essential to its ability to kill cells. bcl-2 can also prevent or inhibit cell death (Vaux et al. , Nufiez et al , Strasser et al , Sentman et al. , above). Rat-1 cells were infected with the pBabe retroviral construct expressing bcl-2. Transfection of the mICE-lacZ fusion construct into the cells lines overexpressing bcl-2 showed that a high percentage of blue cells were now healthy (Table 2). Thus, cell death induced by overexpression of mICE can be suppressed by bcl-2. This result indicates that cell death induced by overexpression of mICE is probably caused by activation of a normal programmed cell death mechanism. Thus, taken together, the results all suggest that vertebrate animals have a genetic pathway of programmed cell death similar to that of C. elegans (Fig. 5). Example 3 As described above, the genes in the ICE/ced-3 family would be expected to function during die initiation of programmed cell death. In order to identify additional members of this gene family, cDNA encoding hICE was used to screen a mouse thymus cDNA library (Stratagene) under conditions of low stringency. Using this procedure, a new gene was identified and named "mlch-2" (see Figure 6 for the cDNA sequence and deduced amino acid sequence of mlch-2). Figures 7 and 7 A show that the protein encoded by mlch-2 contains significant homology to hICE, mICE, and ced-3. The sequence homology indicates that mlch-2, like mICE, is a vertebrate cell deadi gene. Northern blot analyses showed that the expression of mlch-2, unlike mICE, which is broadly expressed during embryonic development, is restricted to die thymus and placenta, areas in which cell death frequently occurs. In addition, it was found that the expression of mlch-2 in the diymus can be induced by dexamethasone, an agent which causes thymus regression. It is concluded tiiat mlch-2 is a thymus/placenta specific vertebrate cell death gene. Example 4 Extensive cell death occurs in the developing nervous system (Oppenheim, R. W., Ann. Rev. Neurosci. 745:453-501 (1991)). Many neurons die during the period of synapse formation. During this critical period, the survival of neurons depends on the availability of neural trophic factors. The survival of isolated primary neurons in vitro depends critically on the presence of such trophic factors (Davies, A. M., Development 100: 185- 208 (1987)). Removal of such factors induces neuronal cell death, usually within 48 hrs. The death of the sympathetic neurons and sensory neurons whose survival depends on one or more members of the nerve growth factor family (nerve growth factor, brain-derived neurotrophic factor, and neurotrophin-3) can be prevented by microinjection of a bcl-2 expression vector (Garcia, I., et al , Science 258:302-304 (1993); Allsopp et al , 1993). To examine if the genes in die lCE/ced-3 family are involved in neuronal cell death, die ability of crmA (which inhibits ICE) to inhibit the death of chicken dorsal root ganglionic neurons induced by NGF removal was examined. It was found that microinjection of an expression vector containing crmA inhibits the death of DRG neurons as effectively as that of a bcl-2 expression vector (Gagliardini, V. , et al , Science 263:826-828 (1994)). This result demonstrates that the genes in the lCEIced-3 family play a role in regulating neuronal cell death during development. Example 5 Results Cloning of lch-1 The protein product of the C. elegans cell death gene, ced-3, is homologous to the product of the mouse gene, nedd-2. The nedd-2 cDNA in the data bank has an open reading frame of 171 amino acids and has long 3' and 5' untranslated regions. This 171-amino acid nedd-2 protein does not contain the active domain, QACRG, of ICE and ced-3 proteins and is homologous only to the PlO subunit of mammalian ICE and the C-terminal part of ced-3. While analyzing nedd-2 cDNA, the inventors discovered that it contains a sequence that can potentially encode a QACRG pentapeptide, but that the sequence is in another reading frame. The inventors considered the possibility that the nedd-2 cDNA isolated by Kumar et al. contains cloning artifacts and that another nedd-2 transcript encodes a protein homologous to both the P20 and PlO subunits of ICE. A mouse nedd-2 probe was made by polymerase chain reaction (PCR). Using this probe, three cDNA libraries were screened: a mouse embryonic day 11.5 cDNA library from CLONTECH (one million clones screened), a human fetal brain cDNA library from James Gusella's laboratory (10 million clones screened), and a human fetal brain cDNA library from Stratagene (one million clones screened). The longest positive cDNA clones were obtained from d e Stratagene cDNA library. From the Stratagene library, two cDNA species (pBSH37 and pBSH30) were identified tiiat encode two closely related proteins homologous to the mouse nedd-2 (Figures 10-12). The insert of pBSH37 (2.5 kb) encodes a protein of 435 amino acids that contains amino acid sequence similarities to both the P20 and PlO subunits of hICE and the entire ced-3. The insert of pBSH30 (2.2 kb) has an open reading frame of 512 amino acids and contains an additional 61 bp one basepair after the sequence encoding QACRG. This causes an early termination of protein translation. The Northern blot analysis showed that expression of this human gene is different than expression of nedd-2 (Kumar et al); thus, the sequences were renamed lch-1 L (pBSH37) (Figure IOA) and Ich-l s (pBSH30) (Figure 10B). A comparison of cDNA sequences revealed that Ich-l s cDNA differs from lch-1 L at the 5' end, around the beginning of the initiation of translation, and by the presence of an additional intron in the middle of Ich-l s cDNA (Figure 1 IB). The first difference is at the beginning of the coding region. The putative first methionine of lch-1 s is 15 amino acids downstream from the first methionine of Ich-1 L ; the first 35 bp of 7cΛ-7 s are different from lch-1 L and include a stop codon (Figures 10A and 10B). PCR analysis using primers specific to the first 35 bp of lch-1 and the 7cΛ-7 s -specific intron (see below), and human placenta cDNA as template, amplified a DNA fragment of predicted size. This suggests that the 35 bp 7cΛ-7 s -specific sequence is not a cloning artifact and is present in the endogenous 7cΛ-7 5 mRNA. The second difference is distal to the active domain QACRG. lch-1 s begins to differ from Ich-1 L one basepair after the coding region of the active site QACRG. The difference is caused by a 61 bp insertion, which results in a termination codon 21 amino acids downstream from the insertion. The last two identical basepairs of lch-l s and lch-l L are AG, the general eukaryotic splicing donor consensus sequence (Mount, 1982). To eliminate the possibility that the 61 bp insertion in pBSH30 is a result of incomplete RNA processing, both forms of murine lch-1 were cloned from adult mouse brain mRNA by PCR using primers flanking the insertion site as described (Experimental Procedure). The resulting 233 and 172 bp fragments (Figure 1 IB) were cloned separately and sequenced. Three murine lch-l L clones and two murine Ich-l s clones were sequenced. Sequencing confirmed tiiat the murine Ich-l s contains the same 61 bp insertion as in human 7cΛ-7 s at the same position (Figure 11 A). Chicken 7cΛ-7 from an embryonic chicken cDNA library (Clontech) was also cloned using a chicken lch-1 probe obtained by PCR (see Experimental Procedures). Two clones were isolated. One encodes lch-1 L and the other encodes Ich-l s which contains a 62 bp insertion at the same position. The DNA sequence of the 62 bp insertion is 72% identical to that of human and murine Ich-l s and also caused premature termination of protein translation. The extra basepair in the intron of chicken lch-1 s causes the amino acid sequence of the last 41 amino acids of chicken Ich-l s to differ from human and murine Ich-l s ; however, truncation of the protein may be the important point. To examine the origin of the 61 bp insertion in murine and human Ich-l s , mouse genomic lch-1 DNA was cloned. Analysis showed that the 61 bp is from an intron whose sequence is identical in human and mouse lch-1. The difference between Ich-l s and Ich-1 L is caused by alternative splicing from the two different 5' splicing donor sequences. The first two basepairs of the 61 bp intron and the two basepairs after the 61 bp intron are GT (Figure 11A). This sequence is the 100% conserved general eukaryotic splicing donor consensus sequence (Mount, 1982). The DNA sequence at the 3' splicing acceptor site is AG. This sequence is the 100% conserved eukaryotic splicing acceptor sequence (Figure 11 A). Thus, the DNA sequences at the splicing junction are completely consistent with alternative splicing of Ich-l s . As the result of an insertion of an intron between coding regions, the open reading frame of lch-1 s is divided into two: die first encodes a 312 amino acid peptide homologous to the P20 subunit of hICE. The second encodes a 235 amino acid peptide homologous to a part of the P20 subunit and the PlO subunit of hICE. The second is nearly identical to mouse nedd-2 (Figure 10B). The data suggest that only the first open reading frame is translated in cells. A schematic diagram of lch-1 L and lch-1 s is shown in Figure 1 IB. Ich-1 L contains similarities to both ICE (27% identity and 52 % similarity) and ced-3 (28% identity and 52% similarity) (Figures 12A and 12B). Thus, the homology between lch-1 and ced-3, lch-1 and ICE is about equal. lch-1 is expressed in many tissues and THP-1 cells which express interleukin-lβ converting enzyme To characterize the function of lch-1, the expression pattern of lch-1 was examined. Northern blot analysis of human fetal heart, brain, lung, liver and kidney tissue was done using the insert of pBSH37 as a probe. The probe hybridizes to both Ich-l s and lch-1 L transcripts. The analysis showed that 4 kb lch-1 mRNA is expressed at the same low levels in all tissues examined. When the same Northern blot (completely stripped of the previous probe) was analyzed using the lch-1 s 61 bp intron as a probe (which hybridizes to Ich-l s transcript only), it showed that Ich-l s is expressed in a larger amount in the embryonic heart and brain than in the lung, liver, and kidney. This result demonstrates that in the embryonic lung, liver and kidney, lch-1 L is expressed to a greater extent than lch-1 s is. In Northern blot analysis of adult RNA with the pBSH37 probe, lch-1 is detected in all the tissues examined. The level is higher in placenta, lung, kidney and pancreas than in heart, brain, liver and skeletal muscle. To study the expression of lch-1 L and 7cΛ-7 s during mouse embryonic development, a quantitative RT-PCR analysis was developed using specific primers tiiat differentiate between lch-1 L and lch-1 s . Primers were synthesized tiiat flank the 61 bp intron sequence of lch-1 s . The two primers are located in separate exons separated by a 2.8 kb intron in genomic DNA. Thus, d e possibility of genomic DNA contamination was eliminated. Ich-l L and lch-1 s were amplified simultaneously to produce DNA fragments of 172 bp and 233 bp, respectively. The cDNA templates were reverse-transcribed from mRNA isolated from thymus, adult heart, adult kidney, embryonic 15d brain, and adult brain. Negative (no DNA template) and positive (Ich-l s and Ich-1 L ) controls were used. Actin primers were used on one set of each sample. Analyses showed that only expression of Ich-l L can be detected in thymus while the expression of both Ich-l L and Ich-l s can be detected in heart, kidney, and both embryonic and adult brain. The expression of Ich-l s was found to be highest in embryonic brain by this PCR analysis. The results are consistent with Northern blot analysis described above. The results were reproducible among multiple mRNA preparations. To examine whether Ich-l and hICE are expressed in die same cells, a Northern blot of THP-1 and U937 cells was analyzed with the Ich-l probe, pBSH37. hICE expression has been detected in these cells (Thornberry, N. A. , et al , Nature 356:768-774 (1992); Cerretti, D. P., et al , Science 256:91 '- 100 (1992)). The inventors found that lch-1 can be detected in THP. l and U937 cells. Thus, both Ich-l and hICE are expressed in THP.1 and U937 cells. Ich-l s , lch-1 L and Λ7CE expression were compared in human cell lines. Using a similar quantitative RT-PCR analysis, the expression of Ich-l L , Ich-l s and Λ7CE were compared in HeLa, Jurkat, THP.1 and U937 cells. Ic -l L and lch-1 s were amplified simultaneously to produce DNA fragments of 234 bp and 295 bp, respectively. hICE was amplified as a fragment of 191 bp. cDNA templates were reverse-transcribed from mRNA isolated from HeLa, Jurkat, THP. l, and U937 cells. Negative (no DNA template) and positive (hICE cDNA) controls were used. pBSH37 and pBSH30 were used as positive controls for lch-1 L and lch-1 s expression. Chicken actin cDNA was used as a positive control for actin. Expression of lch-1 s was detected in HeLa and Jurkat cells but not in THP.l and U937 cells. Both hICE and Ich-l transcripts are present in relatively high levels in HeLa cells. The level of Ich-l transcript is higher than that of hICE transcript in Jurkat cells. Both hi CE and 7cΛ-7 expression is detected in THP.1 and U937 cells. Using a quantitative RT-PCR analysis, the inventors examined the expression of Λ7CE and Ich-l in the normal living T-cell hybridoma DO 11.10 cells (Haskins, K., et al , Exp. Med. 757: 1149-1169 (1983)) and dying DO11.10 cells (serum-deprived). The expression of both hICE and Ich-l can be detected in DO11.10 cells. Interestingly, the expression levels of both Ich-l L and hICE appear to increase in dying DO 11.10 cells. Overexpression of Ich-l L induces rat-1 fibroblast death To examine the function of Ich-l L , the same transient expression system used for 7CE (Miura, M., et al , Cell 75:653-660 (1993)) was used to determine if overexpression of Ich-l induces programmed cell death. The human Ich-l L cDNA was fused with the E. coli lacZ gene and the fused gene was placed under the control of the chicken /-.-actin promotor (p/3actH37Z). This fusion gene was transfected into Rat-1 cells by lipofectamine-mediated gene transfer and the expression of the gene was examined using the X-gal reaction. Results showed that most of the blue (X-gal-positive) Rat-1 cells transfected with p 5actH37Z were round. These results are similar to those obtained with cells transfected with mICE-lacZ fusion sequence shown in Table 2. In contrast, most blue cells transfected with vector alone were flat and healthy. This result suggests that the expression of Ich-l L induces Rat-1 cells to die. „,_ n ^ nΛ . m , m WO 96/20721 -64- To examine whether the cell deadi induced by lch-1 has any cell type specificity and to compare its effect with that of 7CE, mICE-lacZ and lch-1 - lacZ fusion constructs were transfected to HeLa cells, NG108-15 cells, Rat-1 cells, and COS cells. These cells thus expressed mICE-lacZ (pβactMlOZ) lch-l L -LacZ (p/3actH37Z) and lch-lg-lac-Z (p 3actH30Zl) and lacZ control (pactβgal'). The cell killing effect was assayed as for Table 2. The results are shown in Table 3. TABLE 3 EFFECTS OF ICH-1 OVEREXPRESSION ON CELLS IN CULTURE Expression cassettes COS HeLa NG108-15 Rat-1 Rat-l bcI-2 R&t-l/crmA pactβgal' • 1.3+0.1(983) 2.9±0.2(1020) 4.2±0.2(1535) 2.9±0.2(1470) 3.4±0.2(1446) 3.7±0.1(1459) p/JactMIOZ • 93.9±0.3(1003) 80.2±0.5(1545) 94.2± 1.1(978) 28.8±0.5(691) 45.8± 1.6(233) 11.0±0.2(1080) p/3actH37Z 8.3±0.9(1053) 91.4 ±0.2(1076) 68.7 ± 1.5(1605) 92.1 ±0.3(1079) 21.5±3.2(1335) 80.7±0.9(1010) p/3actH37ZCS ND 5.6±0.1(1039) 5.9±0.9(707) 4.1 ±0.2(1477) ND ND p/?actH37ZAT ND 8.2±0.7(435) 5.2±0.2(640) 5.4±0.3(1356) ND ND p/3actH30Zl 1.3 ±0.2(676) 0.0±0.0(40) 0.0±0.0(61) 1.8±0.4(785) ND ND The /7i/CE--__2cZ(pj8actM10Z), /c/--7 r tocZ(p0actH37Z), /cΛ-7 t (S303C)-/_.cZ(p|3actH37ZCS), ch- l L (Υ352A)-lacZ (p0actH37ZAT), Ich-lg-lacZ (p3actH30Zl) and control vector alone (pactβgal') were transiently transfected into Rat-1 cells, Rat-1 cells expressing human bcl-2, Rat-1 cells expressing cowpox virus crmA gene, HeLa cells, NG108-15 cells and COS cells. Cells were fixed lightly 24 hr after transfection and stained with X-Gal for 3 hr. The data (mean±SEM) shown are the percentage of round blue cells among total number of blue cells counted. The numbers in the parendieses are the number of blue cells counted. The data were collected from at least three independent experiments. ND = not determined. Compared to controls, the cytotoxic effect of 7cΛ-7 and mICE exhibit certain cell type specificities. Expression of Ich-l or mICE kills Rat-1 cells and HeLa cells effectively (>90% dead). NG108 cells are more resistant to Ich-l and mICE expression than Rat-1 cells and HeLa cells (68-80% dead). Expression of Ich-l or mICE cannot kill COS cells. To confirm that the cell deadi caused by lch-1 L expression is apoptosis, the inventors examined die nuclear morphology of the cell death induced by Ich-l expression. Rat-1 cells were transiently transfected wid control pactjSgal' vector and 24 hours later, fixed and stained by anti-3-galactosidase antibody or by Hoechst dye 33258 using a protocol from Miura et al. (1993). The nuclear morphology in β-galactosidase-expressing cells is normal and non- condensed. Rat-1 cells were also transiently transfected wid p3actH37Z expressing lch-l L . The nuclei of round cells expressing the Ich-l L -lacZ chimeric gene were condensed and fragmented. This is one of the characteristics of cells undergoing apoptosis. Thus, the results suggest that overexpression of Ich-l L , like that of mICE, causes Rat-1 cells to undergo programmed cell death. To determine the structure and function of Ich-l L protein, two mutant Ich-1 L fusion proteins were made: me first is a Ser -* Cys 303 in the active site of Ich-1 L ; the second is a Thr → Ala 352 in the putative PlO subunit (Fig. 12A). The Ala 352 in PlO is an amino acid residue of ced-3 that is conserved in Ich-1 L but not in ICE. The mutant Ich-l L -lacZ fusion constructs were transfected into Rat-1 cells and expression was examined by me X-gal reaction. The analysis revealed that the S303C (p ?actH37ZCS) and T352A (p/3actH37ZAT) mutations eliminated the activity of Ich-l L completely (Table 3). These results suggest that the ability of Ich-1 L to cause cell death depends upon its enzymatic activity and that only some characteristics of ced-3 are conserved in Ich-l L . The cell death induced by overexpression of ICE can be inhibited by bcl-2 and crmA (Miura, M. , et al , Cell 75:653-660 (1993)). To examine if the cell deadi induced by expression of lch-1 L could also be inhibited by bcl-2 and crmA, lch-1 L -lacZ fusion construct (p3 actH37Z) was transfected into Rat-1 cells that overexpress either bcl-2 or crmA. Cell deadi was assayed as described for Table 3. The results showed tiiat the cell death induced by overexpression of lch-1 L could be inhibited effectively by bcl-2 but only marginally by crmA. .Expression of lch-1 s protects Rat-1 fibroblast death induced by serum removal Since lch-1 s contains two open reading frames, it was important to determine which reading frame is functionally translated (Figure 11 A). 7cΛ-7 s was translated in die presence of 35 S-methionine using in vitro transcribed RNA in a reticulocyte lysate as described in Experimental Procedures. The translated products were run on an SDS-polyacrylamide gel with molecular weight standards, lch-1 s antisense RNA was used as a negative control. Results showed diat only the first reading frame was translated. Second, E. coli lacZ gene was fused to the ends of the first (p/3actH30Zl) and second (p/3actH30Z2) open reading frames. The constructs were separately transfected into Rat-1 cells and the cells were assayed for color using the X-gal reaction. Results showed tiiat when the lacZ gene was fused to die end of the first open reading frame, blue cells could be detected. Blue cells were not detected when the lacZ gene was fused to die second open reading frame. Thus, it is likely that only the first open reading frame is used in vivo. To characterize the function of Ich-l s , the ability of p/3actH30Zl to cause cell death was examined. pj3actH30Zl was transfected into Rat- 1 cells, COS cells, HeLa cells and NG108-15 cells, and the X-gal reaction was developed as before. The analysis showed that the expression of p 3actH30Zl did not cause cell death (Table 3). To examine if Ich-l s has any protective effect against cell deadi, a stable Rat-1 cell line that expresses Ich-l s was established. The cDNA Ich-l s was cloned into pBabepuro retroviral expression vector (Morgenstern et al. , Nucl. Acids Res. 18:3587-3596 (1990)) and transfected into Rat-1 cells. The stable transfectants were selected in puromycin and individual clones were assayed for expression of Ich-l s by Northern blot analysis. The clones that expressed Ich-l s were used for analysis and the clones that did not express Ich-l s were used as negative controls together with untransfected Rat-1 cells. Nomarski micrographs were taken on days 0, 2, 3, and 4 of control Rat-1 cells, 7cΛ-7 5 non-expressing Rat-1 cells, crmA expressing Rat-1 cells, and bcl-2 expressing Rat-1 cells in serum-free medium. Trypan blue assay was also performed. When plated to non-confluent density and washed carefully, 90% of Rat-1 cells die in serum-free medium. However, under these conditions, Rat-1 cells expressing bcl-2 or crmA are resistant to deadi (Fig. 13). When the ability of the stable Rat-1 cell lines that express human Ich-l s was tested under serum-free conditions, it was found that tiiey are more resistant to serum deprivation than parental Rat-1 cells and negative control transfectants not expressing Ich-l s (Fig. 13). These experiments suggest that Ich-l s has the ability to prevent cell death. Ich-l s may prevent cell death by inhibiting Ich-l L . The inventors thus examined whedier Rat-1 cells express 7cΛ-7. Using mouse Ich-l cDNA as a probe, an mRNA species predictive of the Ich-l transcript was detected in Rat-1 cells under low stringency conditions. Discussion The isolation and characterization of lch-1, a mammalian gene belonging to the cell death gene family of ICE/ced-3, has been described. Two distinct Ich-l mRNA species have been identified (Ich-l L and Ich-I s ). These two cDNAs differ in the 5' region around the translation initiation and in the middle region. The difference in the middle is die result of alternative use of two different 5' splicing donor sites. The Ich-l gene is expressed at low levels in embryonic and adult tissues. Ich-l s is expressed at higher levels tiian Ich-l L in embryonic heart and brain. The converse is true in embryonic lung, liver and kidney. Expression of Ich-l s can be detected in all tissues examined except thymus. The expression of both hICEznά Ich-l can be detected in THP.1 cells, HeLa cells, Jurkat cells, U937 cells, and DO11.10 cells. The expression of both hICE and Ich-1 L appears to increase in dying cells under serum deprived conditions. Overexpression of Ich-l L in rat fibroblast cells caused programmed cell deadi prevented by bcl-2. This suggests that Ich-l is a programmed cell deadi gene. Overexpression of Ich-l s , however, did not cause cell death. Stable expression of Ich-l s prevented Rat-1 cell death induced by serum deprivation. The collective results show that 7cΛ-7 encodes protein products that regulate cell death positively and negatively. The mouse nedd-2 gene was originally isolated by Kumar et al. (Biochem. ά Biophy. Res. Comm. 185: 1155-1161 (1992)). The nedd-2 gene was identified as having a transcript of 3.7 kb that is abundantly expressed in embryonic day 10 mouse brain and almost undetectable in adult brain. The nedd-2 cDNA isolated contained an open reading frame of 171 amino acids and long 5' and 3' untranslated regions widi stop codons in all reading frames. The 171 -amino-acid open reading frame is homologous to PlO subunit of hICE and the C-terminal part of ced-3 (Yuan, J., et al , Cell 75:641-752 (1993)). The amino acid sequence of the C-terminal part of Ich-l L is 87% identical to the amino acid sequence of the mouse nedd-2 protein from residues 42 to 172 (the first 41 amino acids are different because of the presence of the 61 bp intron). In mouse, nedd-2 is a unique gene (L. Wang, unpublished data). Thus, human 7cΛ-7 and mouse nedd-2 must be the same gene. In the Northern blot analysis described herein, Ich-l expression in human fetal brain is not high compared to other tissues tested (heart, lung, liver and kidney) and does not appear to be significantly down-regulated in adult brain. Part of the difference could be explained by d e different developmental stages tested: mouse ElO versus human 20-26 week old fetuses. However, Ich-l expression can be detected in human and mouse adult tissues. In die studies herein, amplification of the 5 ' untranslated regions of die mouse nedd-2 cDNA that Kumar et al. reported was not achieved. It is possible tiiat the 5' untranslated region in die Kumar et al. clone was a product of incompletely processed nedd-2 mRNA. Both 7cΛ-7 mRNAs are about 4 kb; since the cDNA clones described herein are 2.5 kb and 2.2 kb for Ich-l L and Ich-l s , respectively, these cDNAs are incomplete. However, since they are fully functional in the assay reported herein, the complete coding regions should be encoded in tiiese two cDNAs. 7cΛ-7 is a new member of the ICE/ced-3 family of cell deadi genes. Thus, unlike C. elegans, mammals must have multiple members of ICE/ced-3. Ich-l is even slightly more homologous to ced-3 than mICE. The cell deadi induced by overexpression of Ich-l was poorly inhibited by crmA. This result is similar to that with ced-3 (Miura, M., et al, Cell 75:653-660 (1993)). The nucleotides corresponding to die two amino acid residues of ced-3 that are conserved in Ich-l, but not in ICE, were mutagenized. Results showed tiiat T352A completely eliminated die ability of Ich-l to cause cell deatii, despite the fact that the corresponding amino acid in ICE is a Ser. These data also suggest that lch-1 is mechanistically more similar to ced-3 than 7CE, and tiiat Ich-l and 7CE may have evolved independently from ced-3. The overexpression of 7CE and 7cΛ-7 can kill Rat-1 cells and HeLa cells effectively but NG108 cells only moderately. It is possible that NG108 cells express a higher level of 7CE and 7cΛ-7 inhibitors. COS cells are completely resistant to the cell killing activity of ICE and 7cΛ-7. COS cells may lack either the activator or the substrates of ICE and Ich-l . This result also suggests that the cytotoxic effects of ICE and Ich-l have certain specificity and are unlikely to be caused by random cleavage activities of proteases. Ich-1 encodes protein products tiiat prevent or cause cell deadi, depending on how die mRNA is processed. Similar regulation has been observed with bcl-x, a bcl-2 related gene (Boise et al , 1993). The bcl-x transcripts can also be processed in two different ways: the larger mRNA, bcl-x L , encodes a bcl-2 related protein product that can inhibit cell death. Alternative splicing of bcl-x transcript generates another smaller transcript, bcl-x s . This encodes an internal truncated version of bcl-x that inhibits the ability of bcl-2 to enhance the survival of growtii factor-deprived cells. Control of die RNA splicing may be an important regulatory point in programmed cell death. Ich-l s could act to prevent cell deadi by inactivating the activator of cell deadi or by directly inactivating Ich-l L . In the transient transfection assay, the expression of Ich-l L -XncZ fusion gene and the 7CE-lacZ fusion gene kill the stable 7cΛ-7 s -expressing cells as efficiently as the control Rat- 1 cells (L. Wang , unpublished data). Thus, unlike crmA or bcl-2, the inhibition of cell death by Ich-l s may be highly dosage-dependent. This could explain why die expression of Ich-l s provided only partial protection of serum deprived Rat-1 cells. Possibly, only cells expressing high levels of Ich-l s are protected. crmA has the ability to suppress cell death induced by overexpression of Ich-l L . The amino acid sequence of crmA is homologous to the members of the serpin superfamily (Pickup et al , 1986), which usually inhibit serine proteases by acting as pseudosubstrates. The nature of interaction of ICE and crmA protein is likely to be similar to die interaction of odier serpin and serine proteases. The inhibition of ICE family members by crmA may depend upon both the affinity and relative concentration of ICEs and crmA. The fact that crmA can suppress a certain percentage of cell deaths induced by overexpression of the Ich-l h suggests that crmA and Ich-l can bind to each other. It is possible that when the Ich-l concentration is lower, crmA may be able to suppress cell death induced by Ich-l to a greater extent. Microinjection of crmA expression construct can effectively suppress the death of dorsal root ganglia neurons induced by nerve growth factor deprivation (Gagliardini, V. , et al. , Science 263:826-828 (1994)). One or more ICEI ced-3 family members may be responsible for neuronal cell deadi. When crmA expression construct is microinjected into neurons, the transient concentration of crmA may be very high. Thus, it is possible that crmA may be able to suppress multiple members of ICEI ced-3 family under such conditions despite the fact that their affinity to crmA is not very high. Since the expression of lch-1 and ICE can be detected in the same cells, the results described herein suggest tiiat multiple members of lCE/ced-3 family may contribute to cell death induced by a single signal. There are tiiree possible ways tiiat 7ce and lch-1 may act to cause cell death. First, 7cΛ-7 may activate 7CE, directly or indirectly, to cause cell death. Second, 7CE may inactivate Ich-l, directly or indirectly, to cause cell death. Third, ICE and lch-1 may act in parallel to cause cell deadi. In die first scenario, die inhibitor of 7CE should inhibit cell deadi induced by Ich-l. In the second scenario, the inhibitor of Ich-l should inhibit die cell deadi induced by ICE. To test tiiis hypothesis, specific inhibitors for each member of 7CHare necessary. For the reasons discussed above, it seems likely that crmA can inhibit other members of ICEI ced-3 family as well. These models can be tested directly by "knock¬ out" mutant mice in which a specific member of the ICEIced-3 family is mutated. Experimental Procedures Cloning and construction of plasmids The mouse nedd-2 cDNA was isolated using embryonic mouse brain cDNA and the primer pairs specific for the 5' and 3' untranslated regions and the coding region. Primers nedd2/I (5'-CAACCCTGTAACTCTTGATT-3') and nedd2/2 (5'-ACCTCTTTGGAGCTACCAGAA-3') were used for amplifying the 5' untranslated region. Primers nedd2/3 (5'-CCAGATCTATGCTAACTGTCCAAGTCTA-3') and nedd2/4 -73- (5ΑAGAGCTCCTCCAACAGCAGGAATAGCA-3') were used for amplifying the nedd-2 coding region. Primer nedd2/5 (AGAAGCACTTGTCTCTGCTC) and nedd2/6 (5'TTGGCACCTGATGGCAATAC-3') were used for amplifying the 3' untranslated region. 0.5 kb PCR product of nedd-2 coding region was cloned into pBluescript plasmid vector to be used as a probe (Stratagene). Two human fetal brain cDNA libraries and one mouse embryonic 11.5d cDNA library were screened with murine nedd-2 cDN A probe at low stringency and one mouse embryonic 11.5d cDNA library. The filters were hybridized in 5x SSPE, 30% formamide, lx Denhardt's solution, 1 % SDS at 42°C overnight and washed in lx SSPE and 0.5% SDS, twice at room temperature and twice at 45 °C (20 min). The human Ich-l s (pBSH30) was isolated from the positive clones using a BamHX-SalX fragment of the murine nedd-2 cDNA, a 70 bp fragment which contains 52 bp of the 61 bp intron, as a probe under die same hybridization and washing conditions described above. The phage clones (pBSH37 for Ich-1 L , pBSH30 for Ich-l s ) were excised in vivo to obtain plasmids by an in vivo excision protocol (Stratagene). To construct expression constructs, PCR was performed using syndietic primers. HI (5'-GATATCCGCACAAGGAGCTGA-3') and H2 (5'-CTATAGGTGGGAGGGTGTCC-3') were used for lch-l L construction. H3 (5'-GATATCCAGAGGGAGGGAACGAT-3'), corresponding to sequences in the 5' region of Ich-l s cDNA and H4 (5'-GATATCAGAGCAAGAGAGGCGGT-3'), corresponding to the sequences in die 3' region of the first open reading frame (ORF) of Ich-l s were used for the first ORF of 7cΛ-7 s construction. H3 and H5 (5'-GATATCGTGGGAGGGTGTCCT-3'), corresponding to the sequences in the 3' region of the second ORF of Ich-l s were used for the second ORF of Ich-l s construction. pBSH37 and pBSH30 were used as templates where appropriate. The three PCR products were inserted into the EcoRV site of pBluescript II, and the inserts of these subclones, p _SIIh37, p3SIIh30.1 , and p -SIIh30.2, were isolated by digestion with Smal and Kpn\ and cloned into SmaX-Kp X sites of BSLacZ (Miura, M., et al. , Cell 75:653-660 (1993)). ΛtotI linkers were added to the KpήX site by digesting with Kpnl, blunt-ending by T4 polymerase and ligating in the presence of excess Notl linker. These constructs, BSh37Z, BSh30Zl, and BSh30Z2, were digested with Notl and individually cloned into pβactstneoB (which uses the chicken /3-actin promoter) (Miyawaki, A., et al., Neuron 5: 11-18 (1990)). The final plasmids were designated p;3actH37Z, p 3actH30Zl and p3actH30Z2, respectively. pBabeH30 plasmid, used for establishing stable Rat-1 cell lines carrying lch-1 s , was constructed by inserting d e full lengtii lch-1 s cDNA into the SalX site of pBabe/puro vector (Morgenstern, J. P. , et al , Nucl. Acids Res. 18:3581-3596 (1990)). To mutagenize Cys 303 to a Ser residue in die active domain of Ich-l L and Ala 352 to a Thr residue in die PlO subunit of Ich-l L , primers containing mutant sites were synthesized as follows: HM1 5'-ATCCAGGCCTCTAGAGGAGAT-3' HM2 5'-ATCTCCTCTAGAGGCCTGGAT-3' HM3 5'-TGCGGCTATACGTGCCTCAAA-3' HM4 5'-TTTGAGGCACGTATAGCCGCA-3' (HM1 corresponds with HM2 and HM3 corresponds with HM4. PCRs were performed in two steps. To make the Cys 303 to Ser mutation, in die first round of PCR, die fragments from the N-terminus to the mutation site of Ich-l L and from die mutant site to the C-terminus of Ich-l L were synthesized using two primer pairs, T3 and HM1, HM2 and T7, and PBSH37 as a template. In the second round of PCR, the two PCR fragments generated in the first reaction were used as templates and T7 and T3 were used as primers. Two such rounds of PCR generated a full length lch-l L mutant. The other mutation was generated in a similar way, using T3 and HM3, HM4 and T7, for the Ala 352 to Thr mutation, as primers for the first PCR. The PCR products were inserted into the EcσRV site of pBluescript II and sequenced. The mutant cDNA inserts were cloned into expression vectors as described above. The mutated clones were designated p3actH37ZCS and p/3actH37ZAT. Cell culture and functional studies COS cells, Rat-1 cells, HeLa cells, and NG108-15 cells were maintained in Dulbecco's modified Eagle's medium (DMEM) supplemented wid 10% fetal calf serum (FCS). The day before transfection, cells were seeded at a density of about 2.5x10 s in each of the 6-well dishes. For each well, 0.7-lμg of the lacZ chimeric construct and 10 g of lipofectamine reagent were used according to a protocol from GIBCO BRL (Gaithersburg, MD). The cells were incubated for 3 hr in serum-free medium containing DNA and lipofectamine. Then an equal volume of growth medium containing 20% serum was added widiout removing the transfection mixture and incubation was continued for 24 hr. The expression of die chimeric gene in cells in culture was detected as previously described (Miura, M., et al , Cell 75:653- 660 (1993)). To establish Rat-1 cell lines overexpressing lch-l s , pBabeH30 was transfected into Rat-1 cells using lipofectamine mediated gene transfer. Resistant cells were selected using 3μg/ml puromycin for about 10 days. Cells were assayed for expression of lch-1 s by Northern blot analysis. To examine whether Ich-l s can render Rat-1 cells resistant to apoptosis under conditions of serum deprivation, Rat-1 cells overexpressing Ich-l s , untransfected control Rat-1 cells, transfected negative control Rat-1 cells, and Rat-1 cells overexpressing bcl-2 or crmA, were seeded in 24-well dishes at 5X10 4 cells in 500 μX of DMEM containing 10% FCS for 24 hr, washed once with serum- free DMEM, and transferred into 500 μl of serum-free DMEM. The cells were harvested at daily intervals and stained with 0.4% trypan blue for 5 min. at room temperature. The numbers of dead and living cells were counted using a haemocytometer. Jurkat cells, THP. l cells, and U937 cells were cultured in RPMI 1640 medium (GIBCO) with 10% fetal calf serum. RNA analysis The Multiple Tissue Northern (MTN) blots membrane of human fetal and adult tissues (CLONTECH) were probed using human lch-1 L cDNA or the intron of lch-1 s cON A (for fetal tissue) in 5x SSPE, lOx Denhardt's solution, 50% formamide, 2% SDS, and lOO g/ml salmon sperm DNA at 42°C overnight. The blots were washed twice in 2x SSPE and 0.05% SDS at room temperature, and twice in O. lx SSPE, and 0.1 % SDS for 20 min. at 50°C. The Multiple Tissue Northern (MTN) blot membrane of human fetal tissue was first probed with a 1.3 kb fragment from the insert of pBSH37, which hybridizes to bodi Ich-l L and lch-1 s . The blot was exposed for two days and developed. Then the blot was stripped by boiling the filter in H 2 O twice for 20 min. After stripping, the filter was re-exposed for three days to ensure tiiat d e stripping was complete. Then the filter was re-hybridized widi the 70 bp BamHl-Sali fragment derived from mouse nedd-2 gene. This fragment contains 52 bp of the 61 bp intron (which is identical to the human 7cΛ-7 5 intron; the remaining 18 bp are from an exon and 5 out of the 18 bp are different from the human Ich-l sequence). The Northern blot of THP.1 cells and U936 cells was carried out under die same conditions. To detect 7cΛ-7 expression in Rat-1 cells, hybridization was carried out in 25% formamide and under otherwise identical conditions. Cloning of murine lch-1 A murine Ich-l cDNA was cloned in two steps by PCR. A 5' murine Ich-l cDNA fragment was amplified using a primer derived from the pBSH37 5' end sequence (5' ATTCCGCACAAGGAGCTGATGGCC 3') and a primer from the mouse nedd-2 after the active site sequence (5' GCTGGTCGACACCTCTATC 3') using a mouse embryo cDNA library as template (a gift from D. Nathan). The resulting 945bp fragment was cloned into d e EcoRV site of plasmid pBSKII (Stratagene) and sequenced. A 3' murine Ich-l cDNA fragment was amplified using a human 7cλ-7 primer further downstream (5 ' CAAGCTTTTGATGCCTTCTGTGA 3 ' and a nedd-2 primer downstream from d e coding region (5' CTCCAACAGCAGGAATAGCA 3'). The resulting fragment was also cloned in pBSKII and sequenced. The two fragments were then joined togedier using an unique SalX site at nucleotide 930 from the beginning of the coding region. Cloning of chicken lch-1 A chicken lch-1 cDNA fragment was obtained using murine Ich-l degenerate primers. The 5' primer was from murine Ich-l nucleotide 241 to 268 bp: 5 'GC(GATC)TT(TC)GA(TC)GC(GATC)TT(TC)TG(TCG)GA(AG)GC3 ' . The 3' primer was from murine 7cΛ-7 nucleotide 883 to 908 bp: 5 'CA(GATC)GC(CT)TG(TAG) AT(AG) AA(AG) AACAT(CT)TT(G ATQGG3 ' . The resulting DNA fragment was used as a probe to screen a chicken embryonic library (Clontech) by high stringency hybridization. Quantitative PCR analysis mRNA was isolated using the MicroFast mRNA isolation kit from Invitrogen. 1 to 2 μg of mRNA was used from reverse transcription by random priming (Invitrogen) using MMLV (Moloney Murine Leukemia Virus) reverse transcriptase (Invitrogen). The primers used to amplify murine Ich-l: 5' primer (5' ATGCTAACTGTCCAAGTCTA 3') and 3' primer (5' GTCTCATCTTCATCAACTCC 3'). The primers used to amplify human Ich-l: 5' primer (5' GTTACCTGCACACCGAGTCACG 3') and 3' primer (5' GCGTGGTTCTTTCCATCTTGTTGGTCA 3'). The primers used to amplify Λ7CE: 5' primer (5' ACCTTAATATGCAAGACTCTCAAGGAG 3') and 3' primer (5' GCGGCTTGACTTGTCCATTATTGGATA 3'). Mouse 3-actin primers were used as controls to amplify a 350 bp actin fragment from mouse and human tissue. 5' /3-actin primer: 5' GACCTGACAGACTACCTCAT 3'. 3' 3-actin primer: 5' AGACAGCACTGTGTTGGCAT 3'. The following conditions were used for the PCR reactions: 1 x reaction buffer (Promega), 1.5 mM MgCl 2 , 200 μM dNTP, 2 μM each primers, 1 unit of Taq DNA polymerase (Promega) in a total volume of 50 μl. The DNA was denatured for 4 min at 94°C prior to 26 PCR cycles (94°C, 1 min/55°C, 1 min/72°C, 2 min). In some of the experiments, Vent polymerase (Biolab) was used. In vitro transcription and translation of Ich-l s To determine which open reading frame of lch-l s was expressed, pBluescript plasmid containing lch-1 s (pBSH30) was linearized at die 3' multiple cloning site with Xhol, purified, and transcribed with T3 RNA polymerase for 2 hr at 37°c using a protocol from Stratagene. The plasmid was also linearized at the 5' multiple cloning site with NoiX, purified, and transcribed with T7 polymerase as an antisense control. The resulting runoff transcripts were extracted widi phenol-chloroform and edianol precipitated. In vitro translation was performed with rabbit reticulocyte lysate (Promega) in the presence of 35 S-methionine for 1 hr. at 30°C. 5μl lysate was mixed witii equal volume of 2xSDS gel loading buffer and subjected to SDS- polyacrylamide gel electrophoresis (12%). The gel was dried and exposed to X-ray film. Example 6 Experimental Procedures Cells and tissue culture HeLa cells were grown in Dulbecco's modified Eagle's medium (DMEM) with 10% fetal bovine serum (FBS). HeLa cells were transfected widi pHD1.2 crmA expression vector (Gagliardini, V. et al. , Science 263:826-828 (1994)) by calcium phosphate precipitation and two days after transfection, 600 μg/ml of G418 (Gibco) was added for selection. Resistant colonies were cloned by limiting dilution. Dosage response of HeLa and HeLa/crmA cells to TNF-α treatment was tested as follows. Cells were seeded in DMEM plus 10% fetal calf serum in a 24-well plate at a density of 4X10 cells per well. After overnight incubation, the cells were washed twice with serum-free DMEM. Drugs were then added to a total volume of 0.2 ml of serum-free DMEM and die cells were incubated for 24 hours. Cells were then trypsinized and dead cells scored on a hemocytometer by trypan blue exclusion (Sigma, St. Louis, MO). At least 200 cells were scored per well. Each concentration was tested in duplicate each time. Western blotting Cells were lysed in SDS sample buffer and cell lysates were subjected to 15% SDS-PAGE. After electroblotting the proteins to an Immobilon nylon membrane (Millipore), the membrane was blocked with 4% nonfat milk in 25 mM Tris-HCl pH7.5, 150 mM NaCl, 0.2 % Tween (TBST). The membrane was incubated with anti-crmA antibody (Gagliardini et al. , Science 263:826- 828 (1994)) (5 μg/ml) for 1 hour at room temperature and dien washed five times with TBST. The membrane was incubated with HRP-conjugated goat anti-rabbit IgG (1/1000 dilution, Amersham) for 30 minutes and washed five times widi TBST. crmA protein was detected with an ECL detection kit (Amersham). DNA transfection One day before transfection, cells were seeded at a density of about 2 xlO 5 per well in 6-well dishes. For each well, 1 μg of plasmid DNA and 10 μg of lipofectamine reagent was added according to a protocol from Gibco BRL. Cells were incubated for 3 hours in serum-free medium containing DNA and lipofectamine, and tiien medium was changed to DMEM containing 10% FBS and incubation was continued for 24 hours. The expression of chimeric gene was detected as previously described (Muira et al. , Cell 75:653- 660 (1993)). Detection of IL-lβ production from HeLa cells HeLa cells were grown overnight in medium containing 10% fetal calf serum and tiien the medium was changed to a serum-free DMEM with or without drugs. After 24 hours, cells were scraped off and precipitated. Conditioned medium was collected, dialyzed against distilled water at 4°C overnight, lyophilized, and d e residue dissolved in distilled water. Cell precipitates were extracted widi extraction buffer (20 mM HEPES-NaOH pH7.4, 10 mM KC1, 1.5 mM MgC12, 0.5 mM EDTA, 10 μg/ml PMSF, 10 μg/ml E64, 2 μg/ml pepstatin, 1 μg/ml leupeptin, 0.5 μg/ml aprotinin, 1 % NP-40). Insoluble materials were removed by centrifugation. Proteins were separated by 15 % SDS-PAGE and IL-13 was detected by immunoblotting using anti-human XL-Xβ antibody (1/300 dilution, Calbiochem). Results Establishment of crmA-expressing HeLa cells To test the hypothesis that activation of ICE is responsible for TNF-α induced apoptosis, the inventors first established HeLa cell lines constitutively expressing cowpox virus crmA protein. This protein is a viral serpin that can specifically inhibit ICE activity (Ray, CA. et al , Cell 69:591-604 (1992)). HeLa cell clones expressing crmA were analyzed by Western blot analysis with affinity-purified anti-crmA. HeLa cells were transfected with crmA expression vector and selected for G418 resistance as described in "Experimental Procedures" above. Six different G418 resistant HeLa cell clones were analyzed. As a positive control, cell lysate of a Rat-1 cell clone expressing crmA (Miura, H. et al. , Cell 75:653-660 (1993)) was applied to the gel. Several HeLa cell clones stably expressing crmA were established. Overexpression of lce/ced-3 family gene in HeLa/crmA cells The cell lines were tested for resistance to cell death induced by 7CE overexpression. The overexpression of crmA in HeLa cells suppressed 7CE- induced cell death. HeLa cells constitutively expressing crmA were transfected with a chimeric expression vector expressing both die lacZ and die mICE gene (pβactMlOZ). Transfection is described above in "Experimental Procedures". One of die HeLa/crmA clones expressing high levels of crmA could efficiently suppress 7CE induced cell death. A clone expressing crmA at low levels also could efficiently suppress the ICE induced cell death under the same conditions (viability 69.8 ± 1.2%, Table 4.) -82- Table 4. Prevention of apoptosis by CrmA % round blue cells Expression cassettes HeLa Hela/CrmA pact/SGal * 3.2 ± 0.8 1.1 ± 0.4 pøactMlOZ 85.5 ± 3.0 46.9 ± 7.0 p/SactH37Z 91.4 ± 4.9 87.5 ± 3.0 Drug treatments % dead cells control 1.5 ± 0.3 5.5 ± 1.8 CHX 2.9 ± 0.7 3.8 ± 0.5 TNF 2.7 ± 1.2 2.8 ± 1.0 CHX + TNF 68.2 ± 1.9 9.7 ± 1.2 Cells were transfected and stained as described herein. Plasmid pactøGaT is a control LacZ gene expression vector and plasmid p / 3actM10Z is a mouse Ice/lacZ chimeric gene expression vector (Muira, M. et al, Cell 75:653-66 (1993)). Plasmid p3actH37Z is a Ich-l /lacZ chimeric gene expression vector (Lin, W. et al, Cell 78:739-150 (1994). The data (mean ± SEM) shown are the percentage of round blue cells among total number of blue cells counted. To see the effects of CHX (20 μg/ml) and the TNF-α (5 ng/ml), cells were treated with drugs for 24 h and cell viabilities were measured by typan blue dye exclusion. The data (mean ± SEM) shown are die percentage of dead cells. The data were collected from at least three independent experiments. CrmA is a potent and highly specific serpin for ICE. However, cell death induced by overexpression of ced-3 is poorly suppressed by crmA (Miura, M. et al , Cell 75:653-660 (1993)). Ich-l (nedd-2) has been described above as third member of the ICE/Ced-3 gene family (see also, Kumar, S. et al. , Genes Dev. 5: 1613-1626 (1994)) . As with 7CE, overexpression of Ich- 1 induces Rat-1 and HeLa cell death efficiently. However, 7cΛ-7 induced cell death is weakly suppressed by overexpression of crmA in HeLa cells (Table 4) and Rat-1 cells. Thus, crmA does not appear to be a general inhibitor of ICE/Ced-3 family protease. Suppression of TNFa-induced apoptosis by crmA The effect of crmA expression on TNF-induced apoptosis was tested. TNF induced cytotoxicity was suppressed by overexpression of crmA. HeLa cells or HeLa/crmA cells were treated with cycloheximide (CHX) (20 μg/ml, Sigma), TNF-α (5ng/ml, Sigma), or a combination of both drugs. Cells were photographed 24 hours after drug treatment. Control HeLa and HeLa/crmA cells were tested for the ability to resist increasing amounts of TNF-α in the presence of 10 μg/ml CHX (Figure 18). In the presence of CHX, TNF-α efficiently induced HeLa cell death (White, E. et al., Mol. Cell. Biol. 72:2570-2580 (1992)). Under the same conditions, HeLa cells expressing crmA at high levels are resistant to the TNF-α cell death stimulus (Table 4). A clone of HeLa/crmA cells that expresses lower levels of crmA was also resistant under the same conditions (% dead cells = 30.2± 1.2%). The dose response of c/m i-expressing HeLa cells to increasing amounts of TNF-α in the presence of 10 μg/ml of CHX was tested. HeLa/crmA cells are resistant to O.Olpg/ml to lOOng/ml of TNF-α (Figure 16). After a 24 hour incubation in the presence of lOOng/ml TNF-α, 83% of the control HeLa cells died compared to 23% of HeLa/crmA cells. Activation of endogenous ICE after TNF stimulation The inventors have detected the expression of both ICE and /cΛ-7 in HeLa cells (Lin, W. et al. , Cell 78:139-750 (1994)). Since /cΛ-7-induced cell death is only weakly suppressed by crmA and crmA appears to be very effective in preventing cell death induced by TNF and CHX treatment, the ZCE-mediated cell death pathway may be activated by TNF stimulation and may play a role in HeLa cell death. If this is the case, TNF stimulation should activate endogenous /CE in HeLa cells. Pro-IL-1/3 is the only known endogenous substrate for 7CE. Active ICE is an oligometric enzyme with p20 and plO subunits (Thornberry, N.A. et al.. Nature 356:768-774 (1992); Cerretti, D.P. et al., Science 256:97-100 (1992)). These subunits are derived from a p45 precursor form of ICE (Thornberry, N.A. et al.. Nature 356:768-774 (1992); Cerretti, D.P. et al., Sάence 256:97-100 (1992)). If ICE is activated after TNF stimulation, the endogenous 33kd pro-IL-1 3 should be processed and mature 17.5kd IL-1 3 secreted into the medium. To detect mature II-- 1/3, conditioned medium was collected from HeLa cells with or without TNF stimulation. The processing of pro-IL-1/3 was analyzed by Western blot. Processing of IL-13 was observed only after induction of apoptosis by TNF-α/CHX in HeLa cells. The following samples were compared: purified mature human IL-l/J, cell lysates (10 μg protein/lane), conditioned medium (5 μg/lane), serum-free controls, LPS (10 μg/ml, Sigma) treatment, cycloheximide (20 μg/ml) and TNF (5 ng/ml). The procedure for detecting IL-l S is described above under "Experimental Procedures". Cell viabilities were measured by trypan blue dye exclusion (97.4 ± 1.5% for serum free control, 97.4 ± 0.2% for LPS treatment, 56.3 ± 2.2% for CHX/TNF-α treatment). The data showed that mature IL-1 3 was only observed after induction of apoptosis by TNF. These results strongly suggest that TNF stimulation induces apoptosis by activation of an /CE-dependent cell death pathway. Discussion The inventors have demonstrated herein tiiat overexpression of ICE induces Rat-1 cells to undergo apoptosis (Miura, M. et al , Cell 75:653-660 (1993)) and expression of crmA can prevent chicken DRG neurons from cell death induced by trophic factor deprivation (Gagliardini, V. et al , Science 263:826-828 (1994)). These results show that /CE has the ability to induce cell death and that inhibition of ICE activity can prevent programmed cell death. However, the results did not show that 7CE was, indeed, activated during programmed cell death. Using pro-IL-1 - processing as an indicator, the inventors have demonstrated that 7CE is activated when HeLa cells are induced to die with TNF-α and CHX. ICE has unique substrate specificity which requires an Asp in the PI position (Sleath et al , J. Biol. Chem 265: 14526-14528 (1990)). Only two eukaryotic proteases are reported to cleave after the Asp. The other is granzyme B, a serine protease in the cytotoxic granules of killer T lymphocytes (Odake et al , Biochemistry 50:2217-2227 (1991)). In THP.l cells and HeLa cells, expression of both /CE and lch-1 were detected (Wang et al , Cell 75:739-750 (1994)). However, affinity labeling of THP. l cell lysates with a competitive, irreversible ICE inhibitor, biotinylated tetrapeptide (acyloxy) methyl ketone, resulted in labeling only of ICE (Thornberry et al , Biochemistry 55:3934-3940 (1994)). This suggests that ICE is the only enzyme to cleave ptoXL-β in the human monocytic cell line, THP. l. The present studies show that crmA cannot prevent cell deadi induced by lch-1 in HeLa cells. How CHX potentiates the TNF cytotoxicity in non-transformed cells is unclear. Most of the cell lines, including HeLa cells, NIH3T3 cells, and TA1 cells, are not killed by TNF alone, but are killed by the combined action of TNF and CHX (Reid et al , J. Biol Chem. 264:4583-4589 (1991); Reid et al , J. Biol. Chem. 266: 16580-16586 (1991)). TNF-α is a pleiotrophic cytokine which may induce more than one cellular response in a single cell. The presence of CHX may inhibit the synthesis of certain signaling molecules and thus, potentiate the killing activity of TNF. Alternatively, CHX may simply inhibit the synthesis of a general cell survival factor(s) and thus, allow cells to become more sensitive to TNF cytotoxicity. HeLa cells express predominantly p55 TNF receptor, thought to be responsible for cell death signalling (Engelmann et al , J. Biol. Chem. 265: 14497-14504 (1990); Thoma et al , J. Exp. Med. 72: 1019- 1023 (1990)). TNF p55 receptor triggers the activation of phospholypase A2, protein kinase C, sphingomyelinase, phosphatidylcholine-specific phospholypase C, and NF- kB (Weigmann et al. . Biol. Chem. 267: 17997-18001 (1992); Schutze et al . Cell 77:765-776 (1992)). In TNF p55 receptor knockout mice, TNF-mediated induction of NK-kB is prevented in thymocytes (Pfeffer et al. , Cell 73:451-461 (1993)). TNF p55 receptor knockout mice were resistant to lethal doses of either lipopolysaccharides or S. aureus enterotoxin B. This suggests that TNF p55 receptor mediated hepatocyte necrosis (Pfeffer et al. , Cell 75:457-467 (1993)). After the stimulation of HeLa cells by TNF-α/CHX, not all the proIL-13 was converted into mature IL-1/3. ICE activity is likely to be tightly controlled within the cells. A small amount of active ICE may be sufficient for induction of apoptosis. Even in the mature IL-1 ((--producing monocytic cell line, THP. l, most of the ICE is the p45 inactive form. In non-LPS- stimulated THP. l cells, only 0.02 % of ICE is in the active form. In stimulated cells, die maximum amount of active ICE is less than 2% of total ICE (Ayala et al, J. Immunol 153:2592-2599 (1994)). THP. l cells may have some protective mechanism to prevent the activated ICE from inducing apoptosis. LPS induces the synthesis of a large amount of pro-IL-1/3, which may, in fact, confer protection on THP.1 cells, because substrates usually are good competitive inhibitors of enzymes. In addition to the secretion of mature XL-Xβ, there was a significant drop of the proIL-13 level in the cell lysates prepared from TNF-α/CHX- treated cells. This could be the result of secretion of mature XL-Xβ, inhibition of biosynthesis of proIL-1 3, or increase of proteolytic activity witiiin the dying cells. If, however, the inventors' hypothesis is correct (i.e., that pro-IL-1/3 does act as a competitive inhibitor of ICE), a reduction in the level of pro-IL- Xβ would in fact represent a further reduction in cellular defense against apoptosis and could be one of the reasons that CHX can increase the cytotoxicity of TNF-a. TNF is a central component in the mammalian host inflammatory response (Tracey, K.G. et al., Ann. Rev. Cell. Biol. 9:317-343 (1993)). In models of septic shock, injection of endotoxin (LPS) rapidly induces TNF, IL- 1 and IL-6 (Ford, Y. et al , J. Exp. Med. 170: 1627-1633 (1989)). Under these conditions, the secretion of IL-l _ appears to be dependent upon TNF because passive immunization with TNF monoclonal antibodies during endotoxemia in vivo attenuates the appearance of XL-Xβ (Fong, Y. et al. , J. Exp. Med. 170: 1621-1633 (1989)). The results herein show that TNF plays a role in activating ICE, the key enzyme in processing IL-13. Expression of mitochondrial manganese superoxide dismutase has been shown to promote the survival of tumor cells exposed to TNF (Hirose, K. et al , Mol. Cell. Biol. 75:3301-3310 (1993)). This suggests that the generation of free radicals plays a role in cell deadi induced by TNF. There are several reports that TNF cytotoxicity is related to the generation of free radicals and lipid peroxides (Hennet, T. et al. , Biochem. J. 289:581-592 (1993); Schulze-Osthoff, K. et al. , J. Biol. Chem. 267:53X1-5323 (1992)). If that is the case, /CE may be activated directly or indirectly by free radicals. The death of Hela cells induced by TNF is also suppressed by bcl-2 overexpression (the viability is 83.7 ± 2.3% under the experimental conditions described for Table 4). Bcl-2 has been suggested to have the ability to either inhibit the production of free radicals (Kane, D.J. et al , Science 262: 1274-1277 (1993)) or prevent free radicals from damaging cells (Hockenberry, D.M. et al , Cell 75:241-251 (1993)). Thus, in HeLa cells, bcl-2 and crmA may suppress cell death induced by TNF d rough a single biochemical pathway of programmed cell death. Fas/Apo (ref)-antigen is a member of TNF receptor family (Itoh, N. et al , Cell 66:233-243 (1991)). Apoptosis can be induced by stimulation of Fas-antigen by anti-Fas/anti-Apo antibody (Yonehara, S. et al. , J. Exp. Med. 769: 1747-1756 (1989)) or Fas-ligand (Suda, T. et al , Cell 75: 1169-1178 (1993)), a type II transmembrane protein homologous to TNF. Cell death signalling after the stimulation of Fas-antigen is largely unknown. However, Fas mediated cell death is protected by overexpression of Elb (Hashimoto, S. et al , Intern. Immun. 3:343-351 (1991)) or bcl-2 (Itoh, N. et al. , J. Immun. /5/:621-627 (1993)). The data presented herein suggest that stimulation of Fas-antigen may also activate ICE/ced-3 cell death pathway.

Claims: What Is Claimed Is: 1. A method of preventing programmed cell death in vertebrates comprising the step of inhibiting the enzymatic activity of interleukin-1/3 converting enzyme. 2. The method of claim 1, wherein said enzymatic activity is inhibited by an interleukin-13 converting enzyme-specific antiprotease. 3. The method of claim 2, wherein said antiprotease is encoded by the crmA gene. 4. A method of promoting programmed deadi in vertebrate cells by increasing the enzymatic activity of interleukin-1/3 converting enzyme in said cells. 5. The method of claim 4, wherein said vertebrate cells are cancer cells. 6. The method of claim 5, wherein said cancer cells overexpress the oncogene bcl-2. 1. A substantially pure gene which is preferentially expressed in thymus and placental cells and which encodes a protein causing programmed cell death. 8. The gene of claim 7, wherein said protein has the amino acid sequence shown in Figure 6. 9. The gene of claim 8, wherein said gene has the cDNA sequence shown in Figure 6. 10. An expression vector having the gene of eitiier claim 8 or claim 9. 11. A host cell transformed witii the vector of claim 10. 12. A substantially pure protein wherein said protein is preferentially expressed in thymus or placental cells and which causes the deadi of said cells. 13. The protein of claim 12, wherein said protein has the amino acid sequence shown in Figure 6. 14. A functional derivative of die protein of claim 13. 15. A method of promoting programmed cell death in thymus or placental cells comprising the step of increasing the activity of the protein of claim 7. 16. A substantially pure DNA molecule comprising a cDNA sequence selected from the group consisting of the cDNA sequence shown in Figures IOA and 10B. 17. An expression vector having the DNA of claim 16. 18. A host cell transformed with the vector of claim 17. 19. A substantially pure protein comprising an amino acid sequence selected from the group consisting of the amino acid sequence shown in Figures 10A and 10B. 20. A functional derivative of the protein of claim 19. 21. A substantially pure DNA molecule comprising the cDNA sequence shown in Figure 14. 22. An expression vector having the DNA of claim 21. 23. A host cell transformed with the vector of claim 22. 24. A substantially pure protein comprising the amino acid sequence shown in Figure 14. 25. A functional derivative of the protein of claim 24. 26. A method of regulating interleukin-l S converting enzyme by regulating the levels or activity of tumor necrosis factor.

Inventors: MIURA MASAYUKI, YUAN JUNYING, MIURA, MASAYUKI, YUAN, JUNYING
Applicants: GEN HOSPITAL CORP, THE GENERAL HOSPITAL CORPORATION
Application year: 1996
Main classification: A61K38
IPC codes: A61K38, A61K48, A61P35, A61P43, C07K14, C12N5, C12N15, C12P21, C12R1, F02B75