CUBILIN PROTEIN, DNA SEQUENCES ENCODING CUBILIN AND USES THEREOF

Abstract: The present invention provides novel renal receptors for ligands. Cubilin and megalin are representative examples of such renal receptors. Also provided are potential uses of these renal receptors for treating toxicity in various tissues and for detecting renal damage.

Description: CUBILIN PROTEIN, DNA SEQUENCES ENCODING CUBILIN AND USES THEREOF BACKGROUND OF THE INVENTION Cross-reference to Related Application This patent application claims benefit of provisional patent application U.S. Serial number 60/072, 197, filed January 22, 1998, now abandoned. Federal Funding Notice The present invention was funded in part by NIH grant DK46117. Consequently, the United States government has certain rights in this invention. Field of the Invention The present invention relates generally to the fields of molecular biology, biochemistry and medical therapy. More specifically, the present invention relates to major renal receptors for low molecular weight proteins and potential uses of th e receptors for therapy to prevent renal toxicity. Description of the Related Art Countless proteins of small or intermediate molecular weight, filtered freely or partially through the renal glomerulus, are bound by scavenger pathway receptors on the luminal surface of proximal tubular cells for reuptake (Batuman et al., 1990; Birn et al., 1997; Christensen et al., 1995; Saito et al., 1994). These proteins are then transcytosed back into the circulation, or degraded, releasing amino acids for fresh protein synthesis. The scavenger pathway receptors of the proximal tubular are a n essential physiological defense against the urinary loss of a diverse array of plasma proteins essential to homeostatic functions from coagulation to lipid metabolism. Unfortunately, exposure of the scavenger receptors to unusually high concentrations of ligands due to overproduction of a ligand such a s myeloma light chains, introduction of a freely filtered drug such a s gentamicin, or increased glomerular permeability can disrupt th e physiological balance, resulting in severe nephrotoxicity. The relative contribution of proximal and distal elements to the development of protein nephrotoxicity remains controversial and ill-defined. For light chain nephrotoxicity if they fail to be reabsorbed proximally, the ligands are delivered into the distal tubular segments of the nephron, where they precipitate as casts in combination with Tamm-Horsfall protein (Huang et al., 1997; Weiss et al., 1981 ; Winearls, 1995) .the severity of the renal dysfunctional correlates with the degree of (distal) cast formation (Myatt, 1994; Winearls, 1995). However, some light chains are associated with a pure (proximal) Fanconi syndrome. Myoglobin on the other hand is associated with little (distal) cast formation, but marked proximal tubular damage, with clinical acute tubular necrosis (Paller, 1988; Zager, 1991). Immunoglobulin light chains are filtered at the glomerulus and endocytosed in the proximal tubule (Batuman e t al., 1990; Batuman et al., 1997). In overproduction states, such a s multiple myeloma, light chains, also known as Bence-Jones proteins, may produce nephrotoxicity. It was shown previously that free -and λ-light chain isotypes bind to a single class of renal proximal tubular receptors which facilitate internalization and degradation (Batuman et al., 1997). To date, however, the receptor(s) which mediate endocytosis of light chains in th e proximal tubule have not been characterized. It has long been postulated that glycoproteins expressed at the apical pole of proximal tubule cells of the kidney acted as scavenger pathway receptors. The only known a n d cloned receptor until now is megalin, a fairly abundant proximal tubule protein, also known as gp330 or the "Heymann anti gen" . Megalin is a classic single transmembrane domain giant glycoprotein receptor (Saito et al., 1994), which belongs to th e LDLR family (Yamamoto, 1984), and is closely related to the 2 - macroglobulin receptor, which is not expressed in the ki dney (Moestrup, 1994). Characterization of megalin revealed that, like the α 2 M receptor, it was a multiligand receptor. Of particular interest for the renal pathology, megalin binds tPA and urokinase in complex with the corresponding inhibitor, but is also a poly basic drug receptor, binding ligands such as th e aminoglycoside antibiotics (Moestrup et al., 1995). Thus the prior art is deficient in the lack of renal receptors for toxic, physiological, and pathological proteins and drugs (such as myeloma light chains) and more generally, components that may gain access to the proximal tubule fluid. Further, the prior art is deficient in the lack of effective means of preventing renal toxicity by utilizing renal binding proteins o r 3 fragments thereof for such components. The present invention fulfills these long-standing needs and desires in the art. SUMMARY OF THE INVENTION The present invention discloses renal binding proteins for ligands. Also disclosed are the potential uses of these proteins for therapy to prevent renal toxicity or other types of toxicity. In one embodiment of the present invention, there is provided a DNA encoding a cubilin protein selected from the group consisting of: (a) isolated DNA which encodes a cubilin protein; (b) isolated DNA which hybridizes to isolated DNA of (a) and which encodes a cubilin protein; and (c) isolated DNA differing from the isolated DNAs of (a) and (b) in codon sequence due to th e degeneracy of the genetic code, and which encodes a cubilin protein. Preferably, the DNA has the sequence shown in SEQ ID No. 1, and cubilin protein has the amino acid sequence shown in SEQ ID No. 2. Still preferably, the DNA is expressed in the tissues like kidney, spleen, brain, liver, heart and thyroid. In one embodiment of the present invention, there is a vector capable of expressing the DNA adapted for expression in a recombinant cell and regulatory elements necessary for expression of the DNA in the cell. Specifically, the DNA encodes a cubilin protein. In another embodiment of the present invention, there is a host cell transfected with the vector expressing a cubilin protein. Specifically, the host cell is selected from the group consisting of bacterial cells, mammalian cells, plant cells and insect cells. More specifically, the bacterial cell is E. coli. In another embodiment of the present invention, there is provided isolated and purified cubilin protein or fragment coded for by DNA selected from the group consisting of: (a) isolated DNA which encodes a cubilin protein or fragment; (b) isolated DNA which hybridizes to isolated DNA of (a) and which encodes a cubilin protein or fragment; and (c) isolated DNA differing from the isolated DNAs of (a) and (b) in codon sequence due to the degeneracy of the genetic code, and which encodes a cubilin protein or fragment. Preferably, the protein has the amino acid sequence shown in SEQ ID No. 2, and the fragment has amino acid sequence consisting of one or more of the sequences selected from the group consisting of SEQ ID Nos. 21-27. In another embodiment of the present invention, there is provided a method of detecting expression of the cubilin protein or fragment in a sample, comprising the steps of: (a) contacting mRNA obtained from the sample with a labeled hybridization probe; and (b) detecting hybridization of the probe with the mRNA. In another preferred embodiment of the present invention, there is provided a pharmaceutical composition comprising the cubilin protein or fragment and a pharmaceutically acceptable carrier. Such composition can be used for treating or reducing nephrotoxicity or other types of toxicity in an in-need individual. In still another embodiment of the present invention, there is provided a receptor for a variety of ligands, comprising a cluster of EGF repeats and a cluster of CUB domains. Specifically, the receptor is cubilin and the ligand is selected from the group consisting of immunoglobulin light chain, myoglobin, intrinsic factor- vitamin B l2 , metallothionein, β-2-microglobulin, amyloid, hemoglobin, haptoglobin, interferon, insulin, cytochrome c, lysozyme, transferrin, transthyretin, polybasic drugs, apolipoprotein Al, high density lipoprotein and receptor related protein. More specifically, a representative example of polybasic drug is gentamicin. Representative examples of immunoglobulin light chain include -light chain and λ-light chain. In still yet another embodiment of the present invention, there is provided a method of detecting renal damage by measuring the level of cubilin in the urine of an individual suspected to have such damage. If the urinary cubilin level is lower than that of a normal individual, the test individual might have chronic renal damage. On the other hand, the test individual might have renal damage of acute origin if the urinary cubilin level is higher than that of a normal individual. Other and further aspects, features, and advantages of the present invention will be apparent from the following description of the presently preferred embodiments of th e invention given for the purpose of disclosure. BRIEF DESCRIPTION OF THE DRAWINGS So that the matter in which the above-recited features, advantages and objects of the invention, as well as others which will become clear, are attained and can be understood in detail, more particular descriptions of the invention briefly summarized above may be had by reference to certain embodiments thereof which are illustrated in the appended drawings. These drawings form a part of the specification. It is to be noted, however, that the appended drawings illustrate preferred embodiments of th e invention and therefore are not to be considered limiting in their scope. Figure 1 shows cDNA cloning of the rat yolk sac cubilin. The 11.6 kb cDNA sequence was encoded by three clones (b-d) from a rat yolk sac cell cDNA library. The ends of the cDNA were confirmed by sequencing 3'- and 5'-RACE products from a rat kidney cDNA library with end-ligated adaptors. A polyadenylation signal followed by a poly(A) tail is present 0.4 k b downstream the open reading frame (0.4-11.2 kb). The position of the initially identified clone is indicated (a). Figure 2 shows a Northern blot of yolk sac RNA using a cubilin RNA probe. Figure 3 shows amino acid sequence of rat cubilin (SEQ ID NO. 2) as deduced by cDNA cloning. The predicted 20 amino acid endoplasmatic import signal sequence is shown in italic type. Potential glycosylation sites are indicated by asterisks. The sequence verified by protein sequencing of tryptic peptides are shown in boldface type. The estimated mass of the peptide backbone is 396,953 kDa and pi is 5.6. Figure 4 shows deglycosylation of cubilin purified b y IF-B 12 affinity chromatography of rabbit renal cortex membranes . Reducing SDS-PAGE shows a reduction from 460 kDa (left lane) to approximately 400 kDa (right lane) after treatment with peptide N-glycosidase F (PNGase F). Figure 5 shows the extracellular modules of cubilin. Figure 5 A shows schematic representation of the 460 kDa receptor (designated cubilin) and related developmental control 7 proteins, human bone morphogenic protein-1 (BMP-1), human tumor necrosis factor stimulating gene 6 (TSG-6), pig spermadhesin aqn3, and the Drosophila protein tolloid. The EGF repeats and CUB domains encode the whole protein except the 1 1 0 residues after the signal peptide. Figure 5B shows a dot plot display of the high internal homology of the CUB domains in cubilin. Figure 6 A shows alignment of the EGF repeats in cubilin and homologous repeats in CUB domain-containing proteins (Bmp-1 , tolloid protein, Cls) and in human fibrillin-1 (SEQ ID Nos. 6-20). The consensus residues for calcium binding are indicated at the bottom of the figure. Figure 6B shows alignment of cubilin CUB domains and CUB domains in the development control proteins shown in Figure 5, Xenopus laevis Uvs-2 and human Cls (SEQ ID Nos. 21-36). Figure 7 shows release of membrane-associated cubilin by non-enzymatic and non-solubilizing procedures . Western blot analysis with anti-cubilin antibody (top panel) an d anti-megalin antibody (bottom panel). Lane 1 : yolk sac BN cells; lane 2: ileal mucosa; lane 3: membrane phase after mechanical grinding of renal cortex; lane 4: fluid phase after mechanical grinding of renal cortex membranes, lanes 5-11 : fluid phase of renal cortex membranes after a 1-h incubation in PBS (lane 5), PBS, phosphorylethanolamine, and heparin (lane 6), PBS, phosphorylethalolamine, heparine, and EDTA (lane 7), PBS and heparin (lane 8), PBS and EDTA (lane 9), PBS, heparin, and EDTA (lane 10), PBS and phosphorylethanolamine (lane 11). Figure 8 shows immunocytochemical localization of megalin (10 nm gold) and cubilin (5 nm gold) in the apical part of rat renal proximal tubule cell (Figure 8 A ) and epithelial cell of rat yolk sac (Figure 8B). The two proteins are colocalized in apical endocytic invaginations (Al), small (SE) and large (LE) endosomes, the small gold-particles (cubilin) being indicated b y large arrowheads. Colocalization is also seen in dense apical tubules (cubilin, small arrowheads). Micro villi (MV) of th e proximal tubule are labeled for both proteins, whereas very little labeling is seen on yolk sac microvilli. Bars, 0.1 μm . Figure 9 shows binding of cubilin to megalin a s determined by affinity chromatography. 125 I-cubilin was applied to a megalin-Sepharose-4B column (•) or a blank Sepharose-4B column (o). The inset demonstrates autoradiography of SDS-PAGE of 125 I-cubilin and the eluted fraction 13 (#13). Bound radioactivity was eluted by the addition (arrow) of 10 mM EDTA to the running buffer. Figure 10 shows characterization of the cubilin- megalin interaction by surface plasmon reasonance analysis . Rabbit megalin was immobilized to a sensor chip and the on rates and off rates for the binding of cubilin was recorded by flow of 20 nM purified cubilin along the chip surface. For control, cubilin was subjected to a blank chip. The values displayed are the recordings from the megalin-chip subtracted from the recordings from the blank chip. Figure 10A shows sensorgram of the binding of cubilin to megalin. The binding curves in the presence of 10 mM EDTA or after prebinding of RAP to megalin are also shown. Figure 10B demonstrates the formation of an IF- B 12 -cubilin megalin complex by subsequent flow with cubilin, running buffer and IF-B, 2 . Evaluation of the binding data suggests a complex binding. By fitting the binding data to a one-binding- site model a K d of 7 nM was measured. Figure 11 shows multiple lines of evidence suggesting that cubilin binds light chains: Figure 1 1 A shows Western blot analysis with a holo-gp280 polyclonal antiserum (left gel) and SDS-PAGE result (right gel). Cubilin at 460 kDa is accompanied by a prominent band at the region of 56 kDa. Figure 11 B shows coomassie-stained two-dimensional gel with pH gradient from 4-8 on abscissa and molecular weight on th e ordinate which demonstrates relative protein abundance. Figure 12 shows direct binding analysis of cubilin and myeloma light chains by surface plasmon resonance. Figure 12A shows binding of cubilin to immobilized κ-light chains is dose dependent with rapid low affinity association and dissociation kinetics. Figure 12B shows competition experiment which further demonstrates the specificity of binding of cubilin to immobilized κ-light chains. A sample of cubilin (100 nM) w as incubated with -light chains (10 or 490 μM), or κ-light chains ( 10 or 490 μM) prior to injecting the sample over the κ-light chain surface. The binding of cubilin to the immobilized surfaces w as reduced in the presence of κ-or λ-light chains in a dose-response fashion. Data are representative of experiments with 4 light chains on 3 chips. Figure 12C shows effect of temperature on binding of cubilin (110 nM) to λ-light chains surface. LC, light chains: RU, response units. Figure 13 shows displacement of light chain binding to rat renal cortical brush-border membranes by anti-cubilin and megalin antisera. Figure 13 A shows that anti-cubilin polyclonal 1 0 antiserum (•) inhibits ^ ^^I-labe\&d λ-light chain binding to rat renal brush-border membrane vesicles, but megalin antiserum (o) had no effect. Figure 13B shows vesicle-by- vesicle analysis of FITC-light chain binding by flow cytometry. Each panel depicts 2,000 vesicles, and each dot represents one vesicle. FITC- fluorescence on the abscissa is displayed against vesicle size on the ordinate. Representative of n=8. Note most but not all vesicles bind FITC-light chains in the left panel, and anti-cubilin antiserum reduces binding (right panel). Figure 14 shows effects of anti-cubilin antibody on light chain endocytosis. Figure 14A shows FITC-K-light chain endocytosis by yolk sac epithelial cells (BN/MSV) at 30 min. Figure 14B shows time course of the endocytosis of FITC-λ-light chain in BN/MSV cells in the presence and absence of anti-cubilin antibody over 40 min. LC, light chain; Ab, antibody. Figure 15 shows direct effects of light chains on endosomal fusion reconstituted in vitro. The λ_light chains w ere loaded into rat renal cortical intermicrovillar cleft at 400 _M b y addition to the homogenization buffer. Fusion reconstituted in vitro in light chain-loaded membranes was inhibited compared with albumin-loaded control membranes. Values are mean + standard error for n=8, p < 0.05 by unpaired t-test. Figure 16 shows fluorescein-light chain binding to renal brush-border membrane vehicles in the presence of gentamicin by flow cytometry, demonstrating megalin is a light chain receptor. Figure 17 shows extra renal expression of cubilin. Figure 17A shows RT-PCR with 2 sets of primers. Lanes 1, 3, 5, 7 1 1 and 9 used a primer (tgcctaccacagcccaaatga, SEQ ID No. 37) located in one of the 3' CUB domains; and lanes 2, 4, 6, 8 and 10 used another primer (agagccacaatgactgcag, SEQ ID No. 38) located in th e end of the EGF regions. Lanes 1-2: RNA from spleen; lanes 3-4 : RNA from brain; lanes 5-6: RNA from liver; lanes 7-8: RNA from heart; lanes 9-10: RNA from Brown Norway (BN) rat immortalized yolk sac epithelial cells; lane 11 : RNA from immortalized opossum kidney (OK) cells with megalin primers (SEQ ID Nos. 39-40) . Figure 17B shows RT-PCR with cubilin primers (SEQ ID Nos. 37 - 38). Lane 1 : kidney; lane 2: thyroid. Figure 18 shows urine cubilin. Normal rat urine (lane 1 ) and 2 separate preparations of rat renal brush border (lanes 2 - 3) were separated by PAGE-SDS, transferred onto nitrocellulose and probed with anti-cubilin antibodies. DETAILED DESCRIPTION OF THE INVENTION The present invention discloses the molecular characterization of the 460 kDa epithelial glycoprotein that functions as the receptor facilitating uptake of intrinsic factor- vitamin B 12 complexes in the intestine and kidney. The 3603 amino acid rat sequence has one cluster of 8 EGF-type domains followed by a cluster of 27 CUB domains accounting for 88% of the protein mass. The receptor, cubilin, has no similarity to known endocytic receptors. Instead, it displays homology to EGF and CUB domain-containing proteins involved in fetal development. Cubilin is a peripheral membrane protein which can be released from renal cortex membranes by non-enzymatic and non- solubilizing procedures. Electron microscopic immuno-gold 1 2 labeling of rat yolk sac and renal proximal tubules revealed that the endocytic receptor megalin and cubilin strictly colocalize in th e endocytic apparatus. Megalin-affinity chromatography an d surface plasmon analysis demonstrated a calcium-dependent high affinity binding of cubilin to the extracellular part of megalin which thereby may assist the intracellular trafficking of this novel type of receptor. Myeloma light chains are known to undergo receptor- mediated endocytosis in the kidney, however, the molecular identity of the receptor has not been characterized. The present studies provide several lines of evidence to identify cubilin (gp280), a giant glycoprotein receptor, which is preferentially expressed in endocytic scavenger pathways and which has potent effects on endosomal trafficking, as an endocytic receptor for immunoglobulin light chains. Binding showed dose and time- dependent saturability with low-affinity, high-capacity equilibrium binding parameters. The data demonstrate that cubilin plays a role in the endocytosis and trafficking of light chains in renal proximal tubule cells. Agents that inhibit binding of light chains are ligands of cubilin. More generally, the proteins present in the urine of patients or dogs deficient in cubilin contain a variety of proteins including albumin which constitute ligands. Independent evidence also suggests that light chains are ligands for megalin. These studies are important, both to understand the complex interactions of toxic and physiological ligands on proximal tubule scavenger pathway receptors, as well as the eventual development of clinical protective agents for 1 3 nephrotoxic damage mediated by ligands for cubilin and/or megalin. If appearing herein, the following terms shall have th e definitions set out below. As used herein, the term "cDNA" shall refer to the DNA copy of the mRNA transcript of a gene. As used herein, the term "derived amino acid sequence" shall mean the amino acid sequence determined b y reading the triplet sequence of nucleotide bases in the cDNA. As used herein the term "screening a library" shall refer to the process of using a labeled probe to check whether, under the appropriate conditions, there is a sequence complementary to the probe present in a particular DNA library. In addition, "screening a library" could be performed by PCR. As used herein, the term "PCR" refers to th e polymerase chain reaction that is the subject of U.S. Patent Nos. 4,683, 195 and 4,683,202 to Mullis, as well as improvements now known in the art. In accordance with the present invention there may b e employed conventional molecular biology, microbiology, and recombinant DNA techniques within the skill of the art. Such techniques are explained fully in the literature. See, e.g., Maniatis, Fritsch & Sambrook, "Molecular Cloning: A Laboratory Manual (1982); "DNA Cloning: A Practical Approach," Volumes I and I I (D.N. Glover ed. 1985); "Oligonucleotide Synthesis" (MJ. Gait ed. 1984); "Nucleic Acid Hybridization" [B.D. Hames & S.J. Higgins eds. (1985)] ; "Transcription and Translation" [B.D. Hames & S.J. Higgins eds. (1984)] ; "Animal Cell Culture" [R.I. Freshney, ed. ( 1986)] ; 1 4 "Immobilized Cells And Enzymes" [IRL Press, (1986)]; B. Perbal, "A Practical Guide To Molecular Cloning" (1984). A "vector" is a replicon, such as plasmid, phage or cosmid, to which another DNA segment may be attached so as to bring about the replication of the attached segment. A "DNA molecule" refers to the polymeric form of deoxyribonucleotides (adenine, guanine, thymine, or cytosine) in its either single stranded form, or a double-stranded helix. This term refers only to the primary and secondary structure of th e molecule, and does not limit it to any particular tertiary forms . Thus, this term includes double-stranded DNA found, inter alia, in linear DNA molecules (e.g., restriction fragments), viruses, plasmids, and chromosomes. In discussing the structure herein according to the normal convention of giving only the sequence in the 5' to 3' direction along the nontranscribed strand of DNA (i.e., the strand having a sequence homologous to the mRNA). A DNA "coding sequence" is a double-stranded DNA sequence which is transcribed and translated into a polypeptide in vivo when placed under the control of appropriate regulatory sequences. The boundaries of the coding sequence are determined by a start codon at the 5' (amino) terminus and a translation stop codon at the 3' (carboxyl) terminus. A coding sequence can include, but is not limited to, prokaryotic sequences, cDNA from eukaryotic mRNA, genomic DNA sequences from eukaryotic (e.g., mammalian) DNA, and even synthetic DNA sequences. A polyadenylation signal and transcription termination sequence will usually be located 3' to the coding sequence. Transcriptional and translational control sequences are DNA regulatory sequences, such as promoters, enhancers, 1 5 polyadenylation signals, terminators, and the like, that provide for the expression of a coding sequence in a host cell. An "expression control sequence" is a DNA sequence that controls and regulates the transcription and translation of another DNA sequence. A coding sequence is "under the control" of transcriptional and translational control sequences in a cell when RNA polymerase transcribes the coding sequence into mRNA, which is then translated into the protein encoded by th e coding sequence. A "signal sequence" can be included near the coding sequence. This sequence encodes a signal peptide, N-terminal to the polypeptide, that communicates to the host cell to direct th e polypeptide to the cell surface or secrete the polypeptide into th e media, and this signal peptide is clipped off by the host cell before the protein leaves the cell. Signal sequences can be found associated with a variety of proteins native to prokaryotes an d eukaryotes. The term "oligonucleotide", as used herein in referring to the probe of the present invention, is defined as a molecule comprised of two or more ribonucleotides, preferably more th an three. Its exact size will depend upon many factors which, in turn, depend upon the ultimate function and use of the oligonucleotide. The term "primer" as used herein refers to a n oligonucleotide, whether occurring naturally as in a purified restriction digest or produced synthetically, which is capable of acting as a point of initiation of synthesis when placed under conditions in which synthesis of a primer extension product, which is complementary to a nucleic acid strand, is induced, i.e., in th e presence of nucleotides and an inducing agent such as a DNA 1 6 polymerase and at a suitable temperature and pH. The primer may be either single-stranded or double-stranded and must b e sufficiently long to prime the synthesis of the desired extension product in the presence of the inducing agent. The exact length of the primer will depend upon many factors, including temperature, source of primer and use the method. For example, for diagnostic applications, depending on the complexity of the target sequence, the oligonucleotide primer typically contains 15-25 or more nucleotides, although it may contain fewer nucleotides. The primers herein are selected to be "substantially" complementary to different strands of a particular target DNA sequence. This means that the primers must be sufficiently complementary to hybridize with their respective strands . Therefore, the primer sequence need not reflect the exact sequence of the template. For example, a non-complementary nucleotide fragment may be attached to the 5' end of the primer, with the remainder of the primer sequence being complementary to the strand. Alternatively, non-complementary bases or longer sequences can be interspersed into the primer, provided that th e primer sequence has sufficient complementarity with th e sequence or hybridize therewith and thereby form the template for the synthesis of the extension product. A cell has been "transformed" by exogenous or heterologous DNA when such DNA has been introduced inside the cell. The transforming DNA may or may not be integrated (covalently linked) into the genome of the cell. In prokaryotes, yeast, and mammalian cells for example, the transforming DNA may be maintained on an episomal element such as a plasmid. With respect to eukaryotic cells, a stably transformed cell is one in 1 7 which the transforming DNA has become integrated into a chromosome so that it is inherited by daughter cells through chromosome replication. This stability is demonstrated by the ability of the eukaryotic cell to establish cell lines or clones comprised of a population of daughter cells containing th e transforming DNA. A "clone" is a population of cells derived from a single cell or ancestor by mitosis. A "cell line" is a clone of a primary cell that is capable of stable growth in vitro for m any generations. Two DNA sequences are "substantially homologous" when at least about 75% (preferably at least about 80%, and most preferably at least about 90% or 95%) of the nucleotides match over the defined length of the DNA sequences. Sequences that are substantially homologous can be identified by comparing the sequences using standard software available in sequence data banks, or in a Southern hybridization experiment under, for example, stringent conditions as defined for that particular system. Defining appropriate hybridization conditions is within the skill of the art. See, e.g., Maniatis et al., supra; DNA Cloning, Vols. I & II, supra; Nucleic Acid Hybridization, supra. A "heterologous' region of the DNA construct is a n identifiable segment of DNA within a larger DNA molecule that is not found in association with the larger molecule in nature. Thus, when the heterologous region encodes a mammalian gene, th e gene will usually be flanked by DNA that does not flank th e mammalian genomic DNA in the genome of the source organism. In another example, coding sequence is a construct where the coding sequence itself is not found in nature (e.g., a cDNA where the genomic coding sequence contains introns, or synthetic 1 8 sequences having codons different than the native gene). Allelic variations or naturally-occurring mutational events do not give rise to a heterologous region of DNA as defined herein. The labels most commonly employed for these studies are radioactive elements, enzymes, chemicals which fluoresce when exposed to untraviolet light, and others. A number of fluorescent materials are known and can be utilized as labels. These include, for example, fluorescein, rhodamine, auramine, Texas Red, AMCA blue and Lucifer Yellow. A particular detecting material is anti-rabbit antibody prepared in goats and conjugated with fluorescein through an isothiocyanate. Proteins can also be labeled with a radioactive element or with an enzyme. The radioactive label can be detected by any of the currently available counting procedures. The preferred isotope may be selected from 3H, 1 C, 3 2 P, 35 S , 36 C 1, 51 cr, 57 Co, 58 Cθ , 59 Fe , 90γ, 125 L 131 and 186 Re . Enzyme labels are likewise useful, and can be detected by any of the presently utilized colorimetric, spectrophotometric, fluorospectrophotometric, amperometric or gasometric techniques. The enzyme is conjugated to the selected particle by reaction with bridging molecules such as carbodiimides, diisocyanates, glutaraldehyde and the like. Many enzymes which can be used in these procedures are known and can be utilized. The preferred are peroxidase, β-glucuronidase, β-D-glucosidase, β-D- galactosidase, urease, glucose oxidase plus peroxidase and alkaline phosphatase. U.S. Patent Nos. 3,654,090, 3,850,752, and 4,016,043 are referred to by way of example for their disclosure of alternate labeling material and methods. 1 9 A particular assay system developed and utilized in the art is known as a receptor assay. In a receptor assay, th e material to be assayed is appropriately labeled and then certain cellular test colonies are inoculated with a quantitiy of both th e label after which binding studies are conducted to determine th e extent to which the labeled material binds to the cell receptors. I n this way, differences in affinity between materials can b e ascertained. An assay useful in the art is known as a "cis/trans " assay. Briefly, this assay employs two genetic constructs, one of which is typically a plasmid that continually expresses a particular receptor of interest when transfected into an appropriate cell line, and the second of which is a plasmid that expresses a reporter such as luciferase, under the control of a receptor/ligand complex. Thus, for example, if it is desired to evaluate a compound as a ligand for a particular receptor, one of the plasmids would be a construct that results in expression of the receptor in the chosen cell line, while the second plasmid would possess a promoter linked to the luciferase gene in which the response element to th e particular receptor is inserted. If the compound under test is a n agonist for the receptor, the ligand will complex with the receptor, and the resulting complex will bind the response element and initiate transcription of the luciferase gene. The resulting chemiluminescence is then measured photometrically, and dose response curves are obtained and compared to those of known ligands. The foregoing protocol is described in detail in U.S. Patent No. 4,981 ,784. As used herein, the term "host" is meant to include not only prokaryotes but also eukaryotes such as yeast, plant and 20 animal cells. A recombinant DNA molecule or gene which encodes cubilin protein of the present invention can be used to transform a host using any of the techniques commonly known to those of ordinary skill in the art. Especially preferred is the use of a vector containing coding sequences for the gene which encodes cubilin protein of the present invention for purposes of prokaryote transformation. Prokaryotic hosts may include E. coli, S. typhimurium, Serratia marcescens and Bacillus subtilis. Eukaryotic hosts include yeasts such as Pichia pastoris, mammalian cells and insect cells. In general, expression vectors containing promoter sequences which facilitate the efficient transcription of the inserted DNA fragment are used in connection with the host. The expression vector typically contains an origin of replication, promoter(s), terminator(s), as well as specific genes which are capable of providing phenotypic selection in transformed cells. The transformed hosts can be fermented and cultured according to means known in the art to achieve optimal cell growth. The invention includes a substantially pure DNA encoding a cubilin protein, a strand of which DNA will hybridize at high stringency to a probe containing a sequence of at least 1 5 consecutive nucleotides of SEQ ID NO: 1. The protein encoded b y the DNA of this invention may share at least 80% sequence identity (preferably 85%, more preferably 90%, and mo st preferably 95%) with the amino acids listed in SEQ ID NO. 2. More preferably, the DNA includes the coding sequence of th e nucleotides of SEQ ID NO:l, or a degenerate variant of such a sequence. 2 1 The probe to which the DNA of the invention hybridizes preferably consists of a sequence of at least 20 consecutive nucleotides, more preferably 40 nucleotides, even more preferably 50 nucleotides, and most preferably 1 00 nucleotides or more (up to 100%) of the coding sequence of the nucleotides listed in SEQ ID NO. 1 or the complement thereof. Such a probe is useful for detecting expression of cubilin in a human cell by a method such as a method including the steps of (a) contacting mRNA obtained from the cell with the labeled hybridization probe; and (b) detecting hybridization of the probe with the mRNA. This invention also includes a substantially pure DNA containing a sequence of at least 15 consecutive nucleotides (preferably 20, more preferably 30, even more preferably 50, and most preferably all) of the nucleotides listed in SEQ ID NO. 1. By "high stringency" is meant DNA hybridization and wash conditions characterized by high temperature and low salt concentration, e.g., wash conditions of 65°C at a salt concentration of approximately 0.1 x SSC, or the functional equivalent thereof. For example, high stringency conditions may include hybridization at about 42°C in the presence of about 50% formamide; a first wash at about 65 °C with about 2 x SSC containing 1% SDS; followed by a second wash at about 65°C with about 0.1 x SSC. By "substantially pure DNA" is meant DNA that is not part of a milieu in which the DNA naturally occurs, by virtue of separation (partial or total purification) of some or all of th e molecules of that milieu, or by virtue of alteration of sequences that flank the claimed DNA. The term therefore includes, for 2 2 example, a recombinant DNA which is incorporated into a vector, into an autonomously replicating plasmid or virus, or into th e genomic DNA of a prokaryote or eukaryote; or which exists as a separate molecule (e.g., a cDNA or a genomic or cDNA fragment produced by polymerase chain reaction (PCR) or restriction endonuclease digestion) independent of other sequences. It also includes a recombinant DNA which is part of a hybrid gene encoding additional polypeptide sequence, e.g., a fusion protein. Also included is a recombinant DNA which includes a portion of the nucleotides listed in SEQ ID NO. 1 encoding an alternative splice variant of cubilin. The DNA may have at least about 70% sequence identity to the coding sequence of the nucleotides listed in SEQ ID NO: l, preferably at least 75% (e.g. at least 80%); and most preferably at least 90%. The identity between two sequences is a direct function of the number of matching or identical positions. When a subunit position in both of the two sequences is occupied by the same monomeric subunit, e.g., if a given position is occupied by an adenine in each of two DNA molecules, then they are identical at that position. For example, if 7 positions in a sequence of 10 nucleotides in length are identical to th e corresponding positions in a second 10-nucleotide sequence, then the two sequences have 70% sequence identity. The length of comparison sequences will generally be at least 50 nucleotides, preferably at least 60 nucleotides, more preferably at least 7 5 nucleotides, and most preferably 100 nucleotides. Sequence identity is typically measured using sequence analysis software (e.g., Sequence Analysis Software Package of the Genetics 2 3 Computer Group, University of Wisconsin Biotechnology Center, 1710 University Avenue, Madison, WI 53705). The present invention comprises a vector comprising a DNA sequence coding for a human cubilin protein and said vector is capable of replication in a host which comprises, in operable linkage: a) an origin of replication; b) a promoter; and c) a DNA sequence coding for said protein. Preferably, the vector of the present invention contains a portion of the DNA sequence shown in SEQ ID No. 1. A "vector" may be defined as a replicable nucleic acid construct, e.g., a plasmid or viral nucleic acid. Vectors may be used to amplify and/or express nucleic acid encoding cubilin protein. An expression vector is a replicable construct in which a nucleic acid sequence encoding a polypeptide is operably linked to suitable control sequences capable of effecting expression of the polypeptide in a cell. The need for such control sequences will vary depending upon the cell selected and the transformation method chosen. Generally, control sequences include a transcriptional promoter and/or enhancer, suitable mRNA ribosomal binding sites, and sequences which control the termination of transcription and translation. Methods which are well known to those skilled in the art can be used to construct expression vectors containing appropriate transcriptional and translational control signals. See for example, the techniques described in Sambrook et al., 1989, Molecular Cloning: A Laboratory Manual (2nd Ed.), Cold Spring Harbor Press, N.Y. A gene and its transcription control sequences are defined a s being "operably linked" if the transcription control sequences effectively control the transcription of the gene. Vectors of th e invention include, but are not limited to, plasmid vectors an d 24 viral vectors. Preferred viral vectors of the invention are those derived from retroviruses, adenovirus, adeno-associated virus, SV40 virus, or herpes viruses. By a "substantially pure protein" is meant a protein which has been separated from at least some of those components which naturally accompany it. Typically, the protein is substantially pure when it is at least 60%, by weight, free from the proteins and other naturally-occurring organic molecules with which it is naturally associated in vivo. Preferably, the purity of the preparation is at least 75%, more preferably at least 90%, and most preferably at least 99%, by weight. A substantially pure cubilin protein may be obtained, for example, by extraction from a natural source; by expression of a recombinant nucleic acid encoding an cubilin polypeptide; or by chemically synthesizing the protein. Purity can be measured by any appropriate method, e.g., column chromatography such as immunoaffinity chromatography using an antibody specific for cubilin, polyacrylamide gel electrophoresis, or HPLC analysis. A protein is substantially free of naturally associated components when it is separated from at least some of those contaminants which accompany it in its natural state. Thus, a protein which is chemically synthesized or produced in a cellular system different from the cell from which it naturally originates will be, b y definition, substantially free from its naturally associated components. Accordingly, substantially pure proteins include eukaryotic proteins synthesized in E. coli, other prokaryotes, or any other organism in which they do not naturally occur. In addition to substantially full-length proteins, the invention also includes fragments (e.g., antigenic fragments) of 2 5 the cubilin protein (SEQ ID No. 2). As used herein, "fragment," a s applied to a polypeptide, will ordinarily be at least 10 residues, more typically at least 20 residues, and preferably at least 3 0 (e.g., 50) residues in length, but less than the entire, intact sequence. Fragments of the cubilin protein can be generated b y methods known to those skilled in the art, e.g., by enzymatic digestion of naturally occurring or recombinant cubilin protein, by recombinant DNA techniques using an expression vector th at encodes a defined fragment of cubilin, or by chemical synthesis . The ability of a candidate fragment to exhibit a characteristic of cubilin (e.g., binding to an antibody specific for cubilin, or binding to a known ligand of cubilin) can be assessed by methods described herein. Purified cubilin or antigenic fragments of cubilin can be used to generate new antibodies or to test existing antibodies (e.g., as positive controls in a diagnostic assay) b y employing standard protocols known to those skilled in the art. Included in this invention are polyclonal antisera generated b y using cubilin or a fragment of cubilin as the immunogen in, e.g., rabbits. Standard protocols for monoclonal and polyclonal antibody production known to those skilled in this art are employed. The monoclonal antibodies generated by this procedure can be screened for the ability to identify recombinant cubilin cDNA clones, and to distinguish them from known cDNA clones. Further included in this invention are cubilin proteins or fragments which are encoded at least in part by portions of SEQ ID NO. 2, e.g., products of alternative mRNA splicing or alternative protein processing events, or in which a section of cubilin sequence has been deleted. The fragment, or the intact 2 6 cubilin polypeptide, may be covalently linked to another polypeptide, e.g. which acts as a label, a ligand or a means to increase antigenicity. The lack of cross inhibition of a number of ligands suggests that specific therapeutic components can b e produced . Also within the invention is a method of detecting cubilin protein or fragment in a biological sample, which includes the steps of contacting the sample with the labelled antibody, e.g., radioactively tagged antibody specific for cubilin, an d determining whether the antibody binds to a component of th e sample. A standard Northern blot assay can be used to ascertain the relative amounts of cubilin mRNA in a cell or tissue obtained from a patient, in accordance with conventional Northern hybridization techniques known to those persons of ordinary skill in the art. This Northern assay uses a hybridization probe, e.g. radiolabelled cubilin cDNA, either containing the full- length, single stranded DNA having a sequence complementary to SEQ ID NO. 1, or a fragment of that DNA sequence at least 2 0 (preferably at least 30, more preferably at least 50, and most preferably at least 100 consecutive nucleotides in length). The DNA hybridization probe can be labelled by any of the m any different methods known to those skilled in this art. RNA probes can also be similarly utilized. The present invention is also directed to a vector capable of expressing the DNA of the present invention adapted for expression in a recombinant cell and regulatory elements necessary for expression of the DNA in the cell. Preferably, th e 27 vector contains DNA encoding a cubilin protein having the amino acid sequence shown in SEQ ID No. 2. The present invention is also directed to a host cell transfected with the vector described herein, said vector expressing a cubilin protein. Representative host cells include consisting of bacterial cells, mammalian cells and insect cells. The present invention is also directed to a isolated and purified cubilin protein or fragment coded for by DNA selected from the group consisting of: (a) isolated DNA which encodes a cubilin protein or fragment; (b) isolated DNA which hybridizes to isolated DNA of (a) and which encodes a cubilin protein or fragment; and (c) isolated DNA differing from the isolated DNAs of (a) and (b) in codon sequence due to the degeneracy of the genetic code, and which encodes a cubilin protein or fragment. Preferably, the isolated and purified cubilin protein has the amino acid sequence shown in SEQ ID No. 2, and the fragment has amino acid sequence consisting of one or more of the sequences selected from the group consisting of SEQ ID Nos. 21-27. The present invention is also directed to a method of detecting expression of the cubilin protein or fragment, comprising the steps of: (a) contacting mRNA obtained from a sample with a labeled hybridization probe; and (b) detecting hybridization of the probe with the mRNA. The present invention is further directed to a pharmaceutical composition comprising the cubilin protein or fragment and a pharmaceutically acceptable carrier. Such composition can be used for treating or reducing nephrotoxicity or other types of toxicity in an in-need individual. 2 8 In an additional embodiment, the present invention is directed to a receptor for a variety of ligands, comprising a cluster of EGF repeats and a cluster of CUB domains. Specifically, th e receptor is cubilin and ligand selected from the group consisting of immunoglobulin light chain, myoglobin, intrinsic factor-vitamin B 12 , metallothionein, β-2-microglobulin, amyloid, hemoglobin, haptoglobin, interferon, insulin, cytochrome c, lysozyme, transferrin, transthyretin, polybasic drugs, low density lipoprotein, high density lipoprotein and receptor related protein. A representative example of a polybasic drug is gentamicin. Representative examples of immunoglobulin light chain include K- light chain and λ-light chain. In still yet another embodiment of the present invention, there is provided a method of detecting renal damage by measuring the level of cubilin in the urine of an individual suspected to have such damage. If the urinary cubilin level is lower than that of a normal individual, the test individual might have chronic renal damage, on the other hand, the test individual might have renal damage of acute origin if the urinary cubilin level is higher than that of a normal individual. The following examples are given for the purpose of illustrating various embodiments of the invention and are not meant to limit the present invention in any fashion. EXAMPLE 1 Animals. Reagents and Antibodies Male Sprague Dawley rats (200-250 gm) were from Sasco, Omaha, NE, and all other reagents were from Sigma Chemical company (St. Louis MO) unless otherwise stated. 2 9 Polyclonal antibodies were raised against proteins purified b y immuno-affinity chromatography using reported monoclonal antibodies coupled to Sepharose 4B (Baricault et al., 1995; Sahali et al., 1988; Sahali et al., 1993). These antibodies are monospecific by immunoblotting on whole brush border preparations and b y immuno precipitation of biosynthetically labeled yolk sac epithelial cells in culture (Sahali et al., 1993), and bind the cytosolic domains of the protein (Hammond et al., 1993). Control antisera included normal rabbit antiserum, and polyclonal rabbit antiserum to the neurokinin- 1/substance P, NKl, receptor (from Professor Jean- Yves Courard, Gif.-Sur-Yvette, France). EXAMPLE 2 Sequencing and Estimation of N-Linked Carbohydrate of Cubilin CNBr fragments and tryptic digests of a 100 kDa CNBr fragment of purified rat cubilin were purified by reverse phase HPLC and seven isolated peptides subjected to Edmann degradation using an Applied Biosystems 477 A sequencer equipped with a 120 A on-line chromatograph. A cross-flow reaction and the Doublot reaction and conversion cycles w ere used. Deglycosylation with peptide N-glycosidase F (PGNase F) of 5 μg of purified rabbit intrinsic factor was carried out as described (Jensen et al. 1992). EXAMPLE 3 cDNA Cloning, Sequencing and Northern Blotting Total RNA was extracted from renal cortex and BN cells using Trizol (Gibco, Life Sciences) as described by th e manufacturer. mRNA required for library construction w as 3 0 isolated using the Qiagen Oligotex kit. Northern blots were m ade with 1 μg of mRNA and revealed with 32 P labeled riboprobes (bp 1205 to 1645 and bp 1702 to 2175) . Four libraries were used. Two conventional libraries were constructed in the laboratory using cDNAs synthesized b y oligo dT and random priming of polyA-selected RNA from yolk sac derived BN/MSV epithelial cells using the superscript Kit (Gibco Life Sciences). After ligation to EcoRl adaptors and size fractionation, they were introduced in λZap or λgtl l EcoRl site. Subsequently screening was performed on a commercial λZap cDNA library (Stratagene) prepared from yolk sac derived L2 epithelial cells. Finally to identify the 5' end, a library w as constructed in λgtl l using the 5' Cap Finder library from Clontech. Immunoscreening was carried out on the λZap-BN library using previously reported polyclonal antibodies to gp280. cDNA probes were constructed from known sequences by PCR using a 1 / 1 9 mixture of digoxygenin labeled nucleotide (Boehringer) and u s ed to identify overlapping clones. RACE was carried out using Marathon ready cDNA prepared from rat renal cortex (Clontech). Specific primers were from bp 838-859 (SEQ ID NO. 3) for 5' RACE and bp 6872-689 1 (SEQ ID NO. 4) and bp 7152-7172 (SEQ ID NO. 5) for 3' RACE. Inserts were prepared by the ex vivo excision system for λzap clones (Stratagene). cDNAs from λgtl l clones were isolated b y EcoRl digestion and inserted in Bluescript. Sequencing w as carried out by cycle sequencing in both directions with IRD-41 labeled primers and the sequence reaction were analyzed on a LICOR 4000 automatic sequencer. 3 1 EXAMPLE 4 Release of Cubilin from Renal Cortex Membranes Rat renal cortex (0.6 g) was suspended in 3 ml PBS, pH 7.4, containing 0.1 mM phenylmethylsulfonylfluoride and Pefablock (Boehringer) and homogenized on ice using a n ultrathorax homogenizer (23,000 rpm/min) for 20 sec. The homogenate was centrifuged at 20,800 x g for 20 min. The saline soluble and saline insoluble samples were analyzed b y immunoblotting with anti-cubilin and anti-megalin monoclonal antibodies (Birn et al. 1997). The amounts loaded on the gels were adjusted so that both fractions were derived from 20 μg of original cortex. IF-B 12 affinity chromatography of the fluid phase was performed as described (Birn et al. 1997) except that the buffer contained no detergent. Rabbit renal membranes were prepared as described (Moestrup et al. 1993). For release of cubilin, 2 mg of membranes were incubated in 525 μl of PBS, 250 units/ml heparin (LEO, Denmark), 20 mM EDTA or 5 mM phosphatidylethanolamine (Sigma) for 1 h at 22°C followed by centrifugation at 20,800 x g for 20 min. EXAMPLE 5 Immunocytochemistry Rat kidneys were fixed by retrograde perfusion through the abdominal aorta with 8% paraformaldehyde in 0.1 M sodium carcodylate buffer, pH 7.2. The tissue was trimmed into small blocks, further fixed by immersion for 1 hour in the same fixative, infiltrated with 2.3 M sucrose containing 2% 3 2 paraformaldehyde for 30 minutes and frozen in liquid nitrogen. Rat embryos at day 12 of gestation were dissected free of th e decidua and parietal layer to expose yolk sac epithelial cells. The tissue was then fixed by immersion and further processed as described above. For electron microscopy, 70 to 90 n m cryosections were obtained at - 100°C with an FCS Reichert Ultracut S cryoultramicrotome as described (Christensen et al. 1995). For double immunolabeling, the sections were incubated with the two primary antibodies overnight at 4°C after preincubation in PBS containing 0.05 M glycine and 1% bovine serum albumin. Sheep anti-rat megalin serum (Moestrup et al. 1993) was diluted 1 :200,000 and mouse monoclonal MAB75 ( 2 μg/ml) against cubilin (Sahali et al 1988). The sections were th en incubated for 30 minutes with rabbit anti-sheep serum 1 : 20,000 (Dako A/S, Glostrup, Denmark), and finally incubated with 10 n m goat anti-rabbit gold particles and 5 nm goat anti-mouse gold particles (BioCell, Cardiff, UK). The sections were embedded in methylcellulose and studied in a Philips CM100 electron microscope. As controls, sections were incubated with secondary antibodies alone or with non-specific monoclonal antibodies or sheep antiserum. EXAMPLE 6 Binding of 125 I-Cubilin to Megalin Megalin was immobilized to CNBr-Sepharose 4B (Pharmacia, Uppsala, Sweden) at a density of 0.5 mg megalin/ml gel. Cubilin was iodinated ( 10 6 Bq/μg) by the iodogen method (Pierce). The 125 I-labeled cubilin was purified by S -300 (Pharmacia) gel filtration and 10 6 cpm was loaded on the megalin- 3 3 column. After wash with binding buffer (20 mM Hepes, 150 m M NaCl, 2 mM CaCl 2 , pH 7.8), bound radioactivity was eluted with th e same buffer supplemented with 10 mM EDTA, counted and analyzed by SDS-PAGE. EXAMPLE 7 Megalin-Cubilin Interaction Analysis by Surface Plasmon Resonance Surface plasmon resonance measurements w ere performed on a BIAcore 2000 instrument (Pharmacia, Sweden). BIAcore sensor chips (type CM5, Pharmacia) were activated with 1:1 mixture of 0.2 M N-ethyl-N' -(3-dimethylaminopropyl) carbodiimide and 0.05 M N-hydroxysuccimine in water. Rabbit megalin was immobilized as described (Moestrup et al., 1996) at a concentration of 40 μg/ml in 10 mM sodium acetate, pH 4.5 and the remaining binding sites were blocked with 1 M ethanolamine, pH 8.5. The flow buffer was 10 mM Hepes, 150 mM NaCl and 1 .5 M CaCl 2 , 1 mM EDTA, pH 7.4. The binding data were analyzed using the BIAevaluation program. EXAMPLE 8 Preparation of Light Chains Four species of light chains, two K and two λ, w ere isolated and purified from the urine of four different patients with myeloma, as described (Batuman et al., 1990; Batuman et al., 1997). The purity and the immunologic identity of light chains were confirmed by SDS-PAGE and Western blotting. One of the λ- light chains and the κ-light chain used here were the same light 3 4 chains used to demonstrate receptor-mediated endocytosis b y radioisotope techniques. Competition experiments were initially conducted using radioiodinated λ-light chain, iodinated by th e Iodobead method as reported (Batuman et al., 1990; Batuman e t al., 1997). It was later switched to competition experiments with fluorescein isothiocyanate (FITC) conjugated κ-light chain. FITC conjugation was performed using FluoroTag FITC Conjugation Kit (Sigma ImmunoChemicals, St. Louis, MO). EXAMPLE 9 Preparation of Renal Brush-Border Membrane Vesicles an d Cortical Intermicrovillar Clefts Rat renal cortical brush border membrane vesicles, inside/in, were isolated by magnesium precipitation technique a s described (Batuman et al., 1990; Hammond et al., 1985). Rat renal cortical intermicrovillar clefts were prepared from kidneys harvested from anesthetized rats, utilizing differential Percoll gradient centrifugation and magnesium precipitation. It was also shown that the intermicrovillar clefts form vesicles "oriented inside out" in vitro during homogenization and can capture internally components added to the homogenization buffer. EXAMPLE 10 Preparation of Cubilin Intermicrovillar clefts prepared from renal cortices were biotinylated on the cytosolic facade using NHS-biotin (Winearls, 1995). Cubilin and the associated proteins were 3 5 purified by immunoaffinity chromatography MAb 75 was coupled to CNBr-activated Sepharose 4B (Pharmacia, Saint Quentin e n Yvelines, France) as previously described (Sahali et al., 1988 ; 1993). Protease inhibitors were added at all steps. EXAMPLE 11 Competition Between Light chains and Anti-Cubilin and Megalin Anti-Sera for Rat Renal Brush Border Membrane Binding 125 Binding of either [ I]-labeled or FITC conjugated light chain was investigated in the presence of up to 100,000 fold serial dilutions of anti-cubilin antibodies (Baricault et al., 1995). Equal dilutions of bovine serum albumin served as controls. With th e radio-labeled light chain, binding was assayed in a gamma counter as described (Batuman et al., 1990; 1997). Binding of FITC-conjugated light chain was assayed by flow cytometry using small particle techniques on a Becton-Dickinson FACStar flow cytometry with a Consort 30 computer and WinMidi software (Hammond et al., 1994; Sahali et al., 1988; 1993). The analog-to- digital conversion of fluorescence measurements on each particle passes through a logarithmic amplifier such that florescence is expressed on a log scale. EXAMPLE 12 Surface Plasmon Resonance Analysis of Light Chain/Cubilin Interaction K- or λ-light chains were immobilized via free amine groups to the dextran matrix of CM5 sensor chips activated by a 3 6 1/1 mixture of NHS and EDC. Unreacted sites were blocked with 1M ethanolamine, pH 8.5, (Jonsson et al., 1991 ; Sanders et al., 1988). The immobilization was conducted at 25°C using 10 mM HEPES, 2 mM CaCl 2 , 150 mM NaCl, 0.005% NP-40, pH 7.4 as the flow buffer. Then 10 mM acetate pH 4.8 was used for electrostatic pre-concentration of the protein. Different densities of K- or λ- light chains were immobilized to three of the four flow cells; the remaining flow cell was activated and blocked with no light chains immobilized for use as a control surface. Binding experiments were carried out using a BIACORE 2000 instrument. EXAMPLE 13 Identification by Two-dimensional Gel Electrophoresis and Microsequencing of Proteins Associated with Immunopurified Cubilin The approach used involved three steps: 1 ) biotinylation of intermicrovillar membranes, 2) immunoisolation of cubilin, and 3) identification of bound proteins b y microsequencing. Two-dimensional electrophoresis w as performed according to the method of O'Farrell (O'Farrell, 1975 ) by Kendrick Labs, Inc. (Madison, WI). Proteins other than cubilin observed on two-dimensional gels prepared from the eluate of detergent solubilization of intermicrovillar clefts were identified by microsequencing. For this purpose, three gels were run in parallel, and stained with Coomassie D. The two most abundant spots at MW 56 and 24 kDa from each gel were cut out and th e material pooled. The peptides derived from the eluted proteins 3 7 by C-leu digestion were separated by HPLC, and internal peptides sequenced (Ferrara et al., 1993). EXAMPLE 14 Effect of Light Chains on Endosomal Fusion To determine if light chains had a direct effect on membrane fusion, rat renal cortical intermicrovillar clefts w ere prepared as described (Hammond et al., 1997; 1994) and loaded with 400 mM light chains by addition of the light chains to th e homogenization buffer. Fusion of these light chain loaded membranes was compared to control membranes loaded with th e same concentration of albumin. All fluorescence measurements were corrected per mg of protein, and fusion reconstituted in vitro in cuvettes (Hammond et al., 1994). Data are expressed as mean ± standard error of the mean throughout the application. Statistical analysis was performed by analysis of variance and Bonferroni or Scheffe's post hoc comparison. EXAMPLE 15 Culture of Rat Visceral Yolk Sac Cells and Internalization Experiments The yolk sac epithelial cell line (BN/MSV) was derived from yolk sac teratocarcinoma induced by fetectomy and placental injection of mouse sarcoma virus (Sahali et al., 1988). When grown under conventional conditions in modified Eagle's medium, supplemented with 2.5 mM L-glutamine, 10% fetal calf serum, an d an antibiotic cocktail (penicillin, streptomycin, and Fungizone), th e 3 8 cells form a domed monolayer and express abundant cubilin (Sahali et al., 1988). EXAMPLE 16 Effect of Anti-Cubilin Antiserum on Endocytosis of Light Chains Internalization experiments were conducted b y exposing confluent yolk sac cells in 24-cell plates to 50 μm FITC- conjugated light chain. These cells were selected for endocytosis experiments because cubilin expression is 100-fold greater th an cultured proximal tubule cells (Sahali et al., 1988). Cells were allowed to endocytose FITC-light chain at various intervals for u p to 40 minutes at 37°C with and without polyclonal anticubilin antibody at 1 : 1 ,000 dilution (added at time 0). This concentration is selected because it is 10-fold higher than the half-maximal inhibitory concentration of the antibody determined from th e brush border binding inhibition experiments. Endocytosis is stopped by washing twice with PBS and removing light chain from medium. Cells are then trypsininzed, fixed in 1% formaldehyde, and suspended in PBS, and FITC incorporated into each cell is re ad in a Becton-Dickinson flow cytometer as described previously. Endocytosis curves are generated by plotting fluorescence units corrected for background against time. Excess unlabeled light chain was used to test for specificity, and bovine serum albumin was used as nonspecific protein control. EXAMPLE 17 cDNA Cloning of Cubilin By immunoscreening of the λ-Zap cDNA library from rat yolk sac BN cells (Le Panse et al. 1995), an initial 0.7 kb clone 3 9 encoding a portion of cubilin was identified. The 5' sequence of this clone was used to design two nested primers to perform 5' RACE on kidney cDNA allowing identification of the 5' end of cubilin. Using PCR-generated probes for further screening of yolk sac libraries a number of clones were identified. Figure 1 schematizes three overlapping clones completely sequenced and used to construct the final cDNA. The last clone contained a polyadenylation signal and a poly A tail. The 3' and 5' ends of the 11.8 kb sequence were further confirmed respectively b y sequencing a 3' end RACE product and a λgtll clone selected from a Cap Finder library. Northern blot analysis of yolk sac mRNA (Figure 2) identified a mRNA of the size as the cDNA. EXAMPLE 18 Primary Structure of Cubilin The assembled cDNA (SEQ ID no. 1) revealed a n uninterrupted open reading frame of 10.8 kb encoding a 20 amino acid signal peptide (in italic type) and a 3603 amino acid protein (SEQ ID NO. 2) with 42 potential N-glycosylation sites (Figure 3). The molecular size of the protein backbone was calculated to 397 kDa. The seven amino acid sequences determined by N-terminal microsequencing of tryptic and CNBr peptides were all identified in the translated sequence (bolded letters in Figure 3). The size of the protein was confirmed by SDS-PAGE (Figure 4). Deglycosylation of the receptor by PNGase F increased its electrophoretic mobility corresponding to a size of 400 kDa. Compared to the 460 kDa size of the untreated protein this indicates a carbohydrate content of -13% of the receptor mass . Figure 5 A shows the predicted domain organization of the 40 receptor. A stretch of approximately 110 amino acids with no apparent homology to known proteins is followed by a cluster of 8 EGF type B repeats which precedes 27 contiguous CUB domains accounting for 88% of the protein mass. The high degree of internal homology (overall similarity of 45%) between the CUB domains is evident from the dot plot display shown in Figure 5B. A total of 76 disulfide bridges is predicted if all the extracellular modules fold normally. The only cysteine outside the CUB domains and EGF repeats, is located in the 110 amino acid N- terminal sequence. This cysteine might account for the partial, disulfide bond dependent dimerization of a minor part of purified receptor (Le Panse et al., 1995; Birn et al. 1997). Figure 6 shows alignment of the EGF repeats and CUB domains of some of the most homologous regions of other proteins . Two of the EGF repeats (Nos. 2 and 4) contain the consensus sequence for Ca 2+ binding and β-hydroxylation of Asp/Asn (Selander-Sunnerhagen et al. 1992). The 110 amino acids CUB domains contain 4 cysteines except for CUB domain 13 which is missing the first two cysteins suggested to form the upstream disulfide bond (Bork and Beckmann 1993). The high homology of the CUB domains of bone morphogenic factor, the Drosophila dorsal-ventral patterning gene product tolloid, the embryonic protein Uvs2 in Xenopus Laevis, tumor necrosis factor stimulating gene 6 (Tsg6), Clr/Cls and spermadhesin is seen in the two lower panels (Figure 6). Save for the leader peptide, no sequence compatible with a transmembrane domain could be identified. This excludes the protein as a type 1 membrane protein or a glycosyl- phosphatidylinositol-anchored protein, which is synthesized with 4 1 a cleavable hydrophobic C-terminal. Furthermore, since almost the entire protein sequence consists of extracellular modules it is very unlikely that the protein is a type II or III protein with a non-cleaved hydrophobic signal peptide inserted in the membrane (Levy 1996 ). EXAMPLE 19 Cubilin is a Peripheral Membrane Protein In order to verify that cubilin is a peripheral membrane protein, as predicted by the lack of a transmembrane segment and cytoplasmic tail, its release from renal cortex membranes by procedures which do not involve solubilization of the membranes or enzymatic treatment was investigated. Figure 7 shows the identical size of the renal receptor and the receptor in yolk sac and intestinal mucosa (lanes 1-3). As seen in lane 4 vs. lane 3, approximately 50% of cubilin was released into the fluid phase by mechanical grinding of renal cortex in PBS, whereas megalin, the 600 kDa transmembrane protein expressed in th e same tissues (Saito et al. 1994), was released in minimal amounts . Cubilin, which remained membrane-associated, was tightly bound but could be released partly by EDTA, heparin and, to a low extent phosphorylethanolamine (Figure 7, lanes 5-11). Heparin and phosphorylethanolamine have been reported to bind to the spermadhesin CUB domains (Calvete et al. 1996; Dostolova et al., 1995). The same treatments released virtually no megalin (Figure 7). The size of the released cubilin, as estimated by SDS-PAGE, was not different from the membrane associated cubilin. 42 EXAMPLE 20 Cubilin Traffics with and Binds Megalin Previous studies have demonstrated megalin an d cubilin in endocytic vesicles of the same absorptive epithelia in the intestine, kidney and yolk sac. Figure 8 shows electron microscopic examination of rat yolk sac and kidney section subjected to double gold-labeling of megalin and cubilin using a sheep anti-megalin polyclonal antibody and a mouse anti-cubilin monoclonal antibody. The large gold particles label megalin antibody and the small particles label cubilin antibody. An almost identical localization of the two sizes of gold particles was seen. Formation of cubilin/megalin complexes was tested next. Figure 9 shows 125 I cubilin binds megalin covalently linked to Sepharose 4B. Bound radiolabel was released from the column by EDTA. Surface plasmon analysis (Fig. 10A) confirmed this binding. No difference in the dissociation of cubilin to megalin was seen in the pH interval 4-8. Binding of cubilin to megalin w as reduced (75%) when RAP was prebound to megalin indicating that cubilin binds to the extracellular domain of megalin. Megalin- bound cubilin was still capable of binding IF-B 12 as shown b y subjecting the megalin-chip to flow with IF-B 12 after the binding of cubilin (Fig. 10B). Thus, the response after adding IF-B I 2 represents the formation of a megalin-cubilin-IF-B ι 2 complex. Control experiments showed no binding of IF-B 12 to megalin. EXAMPLE 21 Light Chains are Ligands for Cubilin To identify candidate ligands with which cubilin interacts, a detergent extract of rat renal apical intermicrovillar 43 clefts biotinylated on their cytosolic facade to affinity chromatography was subjected. The extract was passed through an immunoaffinity raised against the whole molecule. Western blot analysis of the eluate using the same antibody showed a single band at the region of 460-540 kDa, consistent with cubilin (Figure 11 A, left lane). Coomassie staining of a parallel gel revealed several additional bands (Figure 11 A, right lane). For further characterization, the proteins eluted from the column w ere separated by two-dimensional gel electrophoresis and transferred, and the spots were cut of the gels (Figure 11B). Pooled material representing the same spot from multiple gels was C-leu digested, fragments separated by HPLC and microsequenced (Ferrara, et al., 1993). Proteins eluted from the column included cubilin (Figure 11 A, left at top of gel), a 56-kDa protein identified as the β- subunit of the H + -ATPase by the sequence VVDLLAPYA (Figure 11B, #1), a 24-kDa protein identified as -light chains by the sequence (I/S)PQLLVYNA (Figure 11B, #2), and an internal tropomycin control protein added exogenously to the gel (Figure 11B, solid arrow). The 56 kDa protein was biotinylated suggesting cytosolic residence, and hence was not pursued as a ligand. The 24-kDa protein was not biotinylated, suggesting exofacial residence (Table 1). 44 TABLE 1 Analysis of Anti-gp280 Affinity Column Eluate Protein Commassie Anti-gp280 Cytosolic W.M. (kDa) Stain Western Biotinylation 540 + + + 5 6 + - + 24 + - - Whether light chains are a ligand for cubilin, or were merely eluting from the antibody on the column remained uncertain. Analysis of cubilin binding to K and λ-light chains using surface plasma resonance techniques provides direct evidence that cubilin binds light chains. A stock solution of cubilin w as diluted serially with flow buffer and passed over the immobilized _-light chain surfaces for 5 minutes (50 μl at 10 μl/min., 25°C), followed by monitoring the dissociation phase induced b y introduction of cubilin free-flow buffer for 4 minutes (Figure 12A). After 4 minutes, the cubilin bound to the surface h ad dissociated completely, so it was not necessary to regenerate the surface prior to the next injection. The sensorgrams were corrected for bulk refractive index changes by subtracting the response on the blank flowcell from the other flowcells. Cubilin bound to -light chains in a dose-dependent fashion (Figure 12A). To further demonstrate the binding specificity of th e cubilin to the immobilized κ-light chains, a competition 45 experiment was conducted. A sample of cubilin (100 nM) was incubated with κ-light chains (10 or 490 μM), or λ-light chains ( 10 μM) prior to injecting the sample over the κ-light chain surface. The binding of cubilin to the immobilized surfaces was reduced in the presence of κ-light chains in a dose-response fashion (Figure 12B). Inhibition of cubilin binding to immobilized κ-light chains with 10 μM λ-light chains suggests K and λ light chains share a common binding site on cubilin. This series of experiments w as repeated with immobilized_λ-light chains, and four different light chains competing (two λ and two ) with similar results (data not shown). These studies showed that cubilin bound λ-light chains in a dose-dependent fashion, and that binding was interfered with in a dose-response fashion by both free λ- and κ-light chains. I n these studies, bovine serum albumin neither competed with light chains, nor bound to cubilin. Binding of cubilin to κ-light chains was much greater at 37°C than 25°C (Figure 12C), consistent with known thermal behavior of receptor-ligand interactions (Batuman et al., 1990). Hence, BIACORE surface plasmon resonance analysis allows for direct real time assay of the binding of myeloma light chains to cubilin, providing direct evidence that cubilin is a renal light chain receptor. To determine whether light chains bind to cubilin present in brush-border membranes in its native membrane- bound form, antibody interference with light chain binding to rat kidney brush-border membrane vesicles, which are known to express cubilin (Sahali et al., 1988), was tested. Binding of [l ^I] - 4 6 labeled human λ-light chain to rat renal brush-border membrane vehicles is displaced by polyclonal antibodies to cubilin. The half- maximal inhibitory concentration of anti-cubilin antibody w as observed at approximately 10,000 dilution (Figure 13A, solid circles). In contrast, antiserum to megalin, which is known to bind these membranes (Moestrup et al., 1995), had no effect on th e binding of this light chain (Figure 13 A, open circles), suggesting that this λ-light chain binds exclusively to cubilin. At the maximal inhibitory concentration, the anti- cubilin antiserum displacement of λ-light chain approached 90%, confirming near exclusive binding of this light chain to cubilin. 1 1 was also observed that binding of human FITC-conjugated κ-light chain to rat renal brush-border membrane vesicles was displaced by polyclonal antibodies to cubilin as assayed by flow cytometry (Figure 13B). Light chain binding (45.5 + 4.3 arbitrary fluorescent units, n=8) increased compared to unstained membranes (5.1 + 1 .2 units, n=8, p < 0.05), and was displaced by anti-cubilin (30.2 +_1.0 units, n=8, p < 0.05). There was no effect on light chain binding b y normal rabbit serum (42.9 ± 1.7 units, n=8), or antiserum to the neurokinin- 1/substance-P receptor (40.0 ± 1.2 units, n=4), a n irrelevant antibody which binds these membranes. This provides additional evidence that the competitive effect of cubilin antiserum on the binding of light chain is specific. Flow cytometry histograms of light chain binding on a vesicle-by-vesicle basis illustrate the effects of cubilin antisera on rat renal brush border binding of FITC-K-light chains. Each histogram (Figure 13B) displays 2000 vesicles as individual dots, with FITC fluorescence plotted against vesicle size. FITC-light chains bind most but not all 47 brush borders (Figure 13B, left panel). Cubilin antiserum displaced FITC light chain binding (Figure 13B, right panel). To examine the role of cubilin in light chain endocytosis, yolk sac cells were allowed to endocytose FITC-light chain in the absence and presence of anti-cubilin antiserum. These endocytosis experiments revealed a significant inhibitory effect but not total elimination of endocytosis (Figure 14). Excess unlabeled light chain and anti-cubilin antibody reduced FITC-light chain endocytosis significantly (n=4, p<0.002, Mann-Whitney-U test), whereas albumin had no effect (Figure 14A). Furthermore, a time course study showed that anti-cubilin antiserum inhibited light chain endocytosis significantly at all time intervals studied (Figure 14B, n = 3 each time period, p< 0.0001). This time course experiment also showed that anti-cubilin antiserum eliminated the saturable pattern of endocytosis with apparent linearization of the uptake curve (Figure 14B). This observation further supports that cubilin mediates light chain endocytosis in yolk sac cells. Less than complete inhibition of light chain endocytosis in the presence of anti-cubilin antiserum also indicates that, when this pathway is blocked, some light chain endocytosis occurs through alternate pathways, and that the cubilin-facilitated path is not exclusive endocytic pathway for light chains. EXAMPLE 22 Function of Light Chains on Endosomal Fusion To test whether myeloma light chains are functionally important in membrane trafficking and fusion events, intermicrovillar clefts were loaded with light chains by adding it 4 8 to the homogenization buffer (Hammond et al., 1994). Fusion reconstituted in vitro in cuvettes was assayed by energy transfer, and results were normalized per milligram protein (Hammond e t al., 1994; Jo et al., 1995). Fusion was significantly inhibited in membranes treated directly with light chains (111 ± 89 arbitrary fluorescence units/mg protein, n=8) compared with albumin entrapped controls (1584 ± 314, n=8, p < 0.0003 by unpaired t- test, Figure 15). EXAMPLE 23 Myeloma Light Chains Bind Megalin Given the abundance of megalin on the renal brush border membrane, if megalin is a light chain receptor this would predict that small polybasic drugs, such as gentamicin, which are known ligands for megalin, should compete for light chain binding to renal brush border membrane vesicles. To test this, rat renal brush border membrane vesicles were incubated in fluorescein- conjugated light chains with various concentrations of gentamicin. After washing, fluorescein-light chain binding to the membranes was analyzed by flow cytometry (Figure 16). Estimate of half maximal binding concentration of gentamicin between 60 and 7 0 μM was placed in the middle of the curve. Gentamicin competes with fluorescent light chain binding to renal brush border membranes in a dose-dependent manner (control FITC-light chain binding 39.6 ± 4.2 arbitrary fluorescence units, 10 μM gentamicin 45.1+3.1 , 100 μM gentamicin 8.9 ± 33.8*, 1000 μM gentamicin 6.3+1.9*, n=4, *p< 0.05). This demonstrates that gentamicin competes with light chains for brush border membrane binding. 4 9 EXAMPLE 24 Extra Renal Expressipon of Cubilin Cubilin was demonstrated to be expressed in extra- renal tissues as well (Figures 17A and 17B). Spleen, brain, liver, heart, kidney and thyroid are the possible sites where cubilin is expressed. Administering cubilin in a pharmaceutically acceptable carrier might lead to the reduction of toxicity and therefore protecting those sites. EXAMPLE 25 Urine Cubilin Cubilin was also detected in the urine (Figure 18), which indicates that cubilin is released into the urine and the assay of urinary cubilin might be an excellent marker for detecting renal damage. Several conditions can be considered: 1 ) renal damage of acute origin may increase the excretion of a tubular protein such as cubilin and constitute a more sensitive and specific marker than adenosine deaminase (Iglesias et al., 1994 ; Parvez et al., 1990; Tolkoff-Rubin et al., 1987); 2) chronic renal damage with tubular atrophy may be associated on the other h and with a reduced expression of cubilin, the assessment/follow up of which may be useful to monitor the evolution of renal fibrosis; 3 ) because Imerslund Grasbeck (IG) patients do not all have proteinuria, it is likely that the syndrome is associated with various mutations of cubilin (for comparison one or two hundred have been described in familial hypercholesterolemia) which m ay be associated with variable levels of cubilin excretion. In fact the assay of IF-B 12 binding activity of the urine of IG patients has 5 0 been studied (Dugue et al., 1998; Gueant et al., 1995); and 4) some diseases characterized by proteinuria of unknown cause may b e due to a defect in cubilin, which may be absent and therefore not detectable in the urine, or excreted in large amounts if it is not gathered adequately to the membrane. More generally it can be presumed that mutations of key receptors of trophoblastic cells, such as cubilin may account for a variety of pathologies. For instance, the ulk of fetal malformations, not accounted for by the known or suspected hereditary abnormalities, might be related to cubilin defect. Similarly, most cases of poor fetal development or fetal loss, which are of unknown origin up to date, might be caused by cubilin mutations. The present study provides novel molecular information on cubilin, previously known as the yolk sac target antigen of teratogenic antibodies and the intestinal receptor for IF-B 12 . The primary structure predicts 35 extracellular modules uniquely organized in a cluster of 8 EGF repeats followed by, from a molecular point of view, an huge cluster of 27 CUB domains which account for 88% of the mass of the protein. Northern and western blotting of kidney, yolk sac and intestine indicate no difference in size of the receptor in these organs. The EGF type B repeats are similar to the carboxyl- terminal extracellular modules of megalin and low density lipoprotein receptor-related protein. Cubilin has otherwise very little homology to these two giant receptors, which also bind RAP and mediate endocytosis of a variety of ligands. Also cubilin does not display homology to sortilin, the 95 kDa putative vesicular sorting receptor, which also binds RAP (Petersen et al., 1997). The 5 1 CUB domains conform to the description of Bork and Beckmann (1993) based on the analysis of 31 copies of a module initially identified in Clr and Cls components of complement and subsequently in a variety of proteins associated with fetal development. They consist of 110 amino acids defining a characteristic hydrophobicity pattern predicted to form antiparallel beta barrels (Dias et al. 1997). The four conserved cysteines, generally thought to form two S-S bridges (1-2, 3-4) , are found in all but domain 13 of cubilin which lacks the first 2 cysteines as already described in the first CUB domains of C l r/s and the homologues MASP1/2. When analyzed individually, th e CUB domains of cubilin are more closely related to those seen in developmental control proteins. On the functional level there is compelling evidence that the CUB domains are involved in the binding of proteins, a s described for the Ca 2+ -dependent formation of the Cl complex (Bosby and Ingham 1990), as well as for binding of phospholipids and carbohydrates, as demonstrated for spermadhesins (Calvete e t al. 1996a and 1996b, Dostalova et al. 1995). In addition to th e CUB domains, the EGF repeats might also account for some of th e binding properties of cubilin. EGF repeats are widely expressed and involved in a number of receptor-ligand interactions (Davis 1990). Two of the EGF repeats in cubilin have the consensus sequence for calcium binding (Rao et al. 1995) and may b e involved in the calcium-dependent binding of e.g. RAP or IF-B 12 . The lack of a transmembrane segment was surprising since in previous studies isolation of cubilin relied on the use of detergent solubilized membranes. Furthermore the previous studies showed that cubilin was internalized through clathrin 5 2 coated organelles and recycled via dense apical tubules (Le Panse et al. 1995). This first suggested to reassess membrane tethering of cubilin. Early results was thus confirmed indicating that an intrinsic factor-B 12 binding protein (Cotter and Rothenberg 1976) and the target of teratogenic antibodies (Leung 1982) could b e released at least in part from intestinal or renal tissue using mechanical dissociation in the absence of detergents. It w as further showed that whereas membrane association was stable between pH 4 and 8 cubilin could be released by heparin, phosphorylethanolamine and EDTA. These observations, which indicate nonionic interactions with sugars and phospholipids, are in line with the membrane binding properties of spermadhesins which consist of a single CUB domain, lack a transmembrane segment, but are yet tightly bound to the surface of sperm cells via phospholipids (Dostalova et al. 1995). Another region of th e same CUB domain binds to carbohydrates of zona pellucida, th e extracellular investment surrounding the mammalian egg. The lectin binding characteristics of the spermadhesins are not fully characterized but include heparin and a variety of carbohydrates including Gal beta (l-4)-GlcNac and Gal beta (l-3)-GlcNac (Calvete et al. 1996, 1997). In view of the 27 CUB domains present in cubilin this receptor may have multiple membrane attachments which may account for the inability to release all the membrane associated cubilin. The identification of the components which link cubilin to the membrane is also essential for understanding its internalization and recycling. The present study suggested th at the binding of cubilin to megalin is crucial for this process. Co- internalization of a receptor which lacks internalization signal(s) 5 3 by means of another receptor has previously been shown. The GPI linked urokinase receptor can thus be endocytosed b y coupling of urokinase receptor-bound urokinase/inhibitor complex to the LDL receptor-related protein (Nykjaer et al. 1997, Conese e t al. 1996). It is likely that a similar process can be mediated b y megalin which can also bind the urokinase-inhibitor complex (Moestrup et al. 1993). Based on the strict colocalization of cubilin and megalin at the subcellular level and on the ability of megalin to bind cubilin in vitro, it was proposed that megalin is crucial for the internalization of cubilin and cubilin ligand complexes. After internalization, the ligand IF-B 12 is segregated from the receptor and directed to lysosomes for degradation of IF (Dan and Cutler 1994, Birn et al. 1997) whereas cubilin is recycled to th e membrane. Since the cubilin/megalin complex is stable at pH 5 , the two receptors might remain in complex during the entire recycling pathway at variance from the urokinase receptor which recycles to the plasma membrane without being linked to the LDL receptor-related protein (Nykjaer et al. 1997). Upon analysis of the effect of polyclonal megalin antibodies and RAP on the endocytosis of 125 I-IF-B 12 in cultured yolk sac cell, only a 10 to 15% reduction was found in uptake . This modest effect might be accounted for a short cell surface expression of megalin and cubilin due to rapid recycling of the two proteins and thereby a too short time for the cubilin-megalin to dissociate, a prerequisite for RAP to block binding. Furthermore, a continuous incubation with RAP will probably have no effect on intracellular receptors, since externally receptor-bound RAP is transported to lysosomes for degradation (Iadonato et al., 1993). 5 4 In order to further characterize the partnership of these two giant receptors, studies have been initiated to investigate cubilin trafficking in megalin deficient or megalin- mutated cells expressing cubilin. However, such analysis might b e complex since recent data on megalin deficient mice indicate a key role of megalin for normal development of the endocytic apparatus in the proximal kidney tubules and for survival of th e mice in general (Willnow et al. 1996). The observation that the target of teratogenic antibodies contains CUB domains is of particular interest in view of the fact that these domains are often observed in developmentally regulated proteins. The mode of action of th e teratogenic antibodies is not known but have been shown to inhibit endocytosis, thus reducing the amount of maternal proteins internalized and consequently the amount of protein derived amino acid which can be incorporated into embryonic tissue (Beckman et al., 1997; Lloyd, 1990, Le Panse et al., 1994) . However, there is no direct evidence that a decreased amino acid supply is responsible for foetal malformations. Alternatively, th e teratogenic effect might relate to a more specific disturbance of the materno-foetal barrier such as an impaired transfer of B 12 or of other nutrients. Interestingly, the pattern of antibody induced fetal malformations which includes abnormal cranio-facial development, in particular of the eyes and hypophysis (Sahali e t al., 1988) resembles to some extent the holoprosencephalic syndrome induced by anti-cholesterol agents (Llirbat et al. 1997), knock out of the cholesterol-depedent Sonic hedgehog (shh) gene (Porter et al., 1996) or of the megalin gene (Willnow et al., 1996) . It has been proposed (Herz et al., 1997) that the defective 5 5 development of the central nervous system in megalin-deficient mice was related to a decreased megalin-mediated uptake of cholesterol-containing lipoproteins which in turn altered th e addition of cholesterol to the shh protein. It is therefore possible that anti-cubilin antibodies could interfere with cholesterol uptake either directly or indirectly via binding of cubilin to megalin in the yolk sac. The present study establishes cubilin as a novel type of peripheral membrane receptor with multiple potential sites for interaction with other proteins and membrane components. Cubilin can bind IF-B 12 , RAP, megalin, and most likely calcium, phospholipids and carbohydrates (Table 2). However a number of ligands may remain to be identified in order to explain the role of the receptor in kidney function and its importance in fetal development. 5 6 TABLE 2 Ligands for Cubilin competition Ligand BN cell surface antibody with known uptake plasmon interference ligands resonance light chains* X X X X myoglobin X X X metallothionein X X X haptoglobin X X polybasic drugs X X X Intrinsic factor X X X vitamin B 12 LDL X X HDL X X transferrin X RAP X X X albumin X X X X * as light chains compete with renal brush border membrane binding with multiple other medically and physiologically important proteins, this suggests that these proteins are also ligands for cubilin. This group of proteins include β2 microglobulin, amyloid, insulin, cytochrome c and interferon. LDL: Low density lipoprotein; HDL: High density lipoprotein; RAP: Receptor related protein. 5 7 Solution structure of spermadhesin PSP-I/PSP (Romero et al. Nat. Struct Biol 1997 10:78-788), a dimer which consists exclusively of two single CUB domains has recently been obtained. It reveals that CUB domains are characterized by 2 layers of 5 beta sheets, the top layer of one of the CUB domains contacting th e lower layer of the other CUB domain in a manner that leaves preferentially exposed the less conserved beta turns which carry the ligand binding sites. If such an arrangement prevails in cubilin, it is likely to account for and one would predict a wide variety of ligands binding to distinct CUB domains. Indeed the fact that RAP binds cubilin but does not inhibit binding of IF-B 12 complexes indicates that these 2 ligands bind to different sites and probably to different CUB domains . Similarly haptoglobin and light chains both bind cubulin but do not compete for binding. The modular structure of cubilin thu s strongly suggests that it may be feasible to produce fragments of small size corresponding to one or a few CUB domains which can be used therapeutically: this type of fragments will bind selected ligands but preserve many /most other functions of cubilin. The present studies also showed that cubilin, a giant receptor which participates in the endocytic scavenger pathway of the renal proximal tubule cells, binds and facilitates endocytosis of immunoglobulin light chains isolated from the urine of myeloma patients. Evidence that cubilin is a light chain receptor came from the analysis of eluates from an affinity column prepared with anti-cubilin antiserum in which cubilin coeluted with κ-light chain. The κ-light chain was definitively identified by microsequencing 5 8 after isolation by two-dimensional electrophoresis. Several additional lines of evidence add weight to the hypothesis that cubilin is a light chain receptor. Competition experiments by anti- cubilin antiserum and surface plasmon resonance experiments both showed that all tested light chains bind to cubilin. Surface plasmon resonance technology allowed direct analysis of the binding of light chains to cubilin. Several characteristics of the observed sensorgrams suggest that light chains bind cubilin specifically. First, cubilin bound to light chains in a temperature- and dose-dependent manner whether K or λ- light chain is immobilized. Second, four species of non- immobilized light chains all interfered with binding in a dose- dependent manner. Third, the kinetics of binding and displacement were very similar to values reported using radioactive membrane binding techniques (Batuman et al., 1997 ; Driesbach et al, 1994; Marchalonis et al., 1992). Lastly, λ-light chains interfere with κ-light chain binding to cubilin and vice versa. This data revalidates the use of surface plasmon resonance technology to quantitate low affinity binding (Jonsson et al., 1991 ; Sanders et al., 1988). As κ-light chains are 100-fold more abundant than λ- light chains in healthy animals and humans (Riedel et al., 1991), it is not surprising to observe κ-light chains eluting from the cubilin affinity column but not λ-light chains. The current surface plasmon resonance data provides direct evidence confirming and extending the observation made by membrane binding of light chains: both K- and λ-light chains are ligands for cubilin. 5 9 Studies of classic binding kinetics utilizing Scatchard analysis demonstrate several ligands competing with light chain for brush border membrane binding. These ligands include lysozyme, insulin, cytochrome c, myoglobin and β2-microglobulin (Batuman et al, 1990; 1997; Driesbach et al., 1994). Competition by low molecular weight proteins raise the probability that cubilin is a multi-ligand receptor responsible for the endocytosis and cellular trafficking of a number of proteins normally filtered in the glomerulus and catabolized in the kidney, extending the role of this scavenger pathway receptor to such diverse phenomena a s rhabdomyolysis and insulin metabolism. The multiple putative ligands for cubilin reflect the precedent set by other giant glycoprotein receptors such as the low-density-lipoprotein receptor, megalin, and the α2-macroglobulin receptor, which bind many ligands with a spectrum of affinities at multiple binding sites (Moestrup et al., 1994). Cloning data that reveal multiple EGF repeats and CUB domains strengthens this expectation. Receptor kinetic studies have demonstrated that light chain binding to receptors in cultured proximal tubule cells is followed by endocytosis and ultimate lysosomal degradation. The present observations suggest that cubilin is a receptor that can mediate endocytosis of light chains in renal proximal tubular cells. Nearly 90% of the λ-light chain binding was displaced by anti- cubilin antibody. In contrast, anti-megalin antibody did not compete with the brush border binding of this light chain at all. This suggests that cubilin is the quantitatively major receptor for this λ-light chain. However, at maximal inhibitory concentration of the anti-cubilin antibody, 10% of light chain remained bound to 6 0 brush border membranes, suggesting presence of additional binding sites for this light chain. Anti-cubilin antiserum also inhibited endocytosis of light chain significantly. This further confirms that cubilin binding is followed by endocytosis of light chain. However, less than total inhibition of light chain endocytosis by anti-cubilin antibody indicates that this pathway may not be the exclusive endocytic pathway for light chains and that there may be alternate pathways which can compensate partially when the cubilin- mediated pathway is blocked. The antibodies used in the present study may be less than blocking functionally, and incomplete inhibition of endocytosis may be on this basis. Importantly, binding of light chains to scavenger pathway receptors is not just a structural observation, as light chains had potent direct effects on endosomal fusion reconstituted in vitro. This raises the novel hypothesis that ligand binding m ay affect fusion properties of membranes, mediated by the receptors they bind. Select ligands are known to induce endocytosis of th e ligand-receptor complex by binding, and the protein components of the final common pathway of fusion have largely been identified and cloned. These mechanisms may provide new insights into nephrotoxicity of myeloma light chains and other nephrotoxic low molecular weight proteins. There are 13,500 new cases of myeloma annually in the U.S. and 1-4 new cases/100,000 of population worldwide. Although the precipitation of light chains with Tamm-Horsfall protein to form casts in renal distal nephron segments has been defined down to specific peptide sequences, the molecular 6 1 characteristics of receptors that mediate the endocytosis of light chains in the proximal tubule have not been defined. Identification of the proximal tubular receptor for light chains extends and compliments these observations. The proximal tubule determines the distal delivery of low molecular weight proteins by reabsorbing the bulk of filtered proteins including light chains. Many low-molecular weight proteins induce injury to the proximal tubule, while others precipitate in the distal nephron. Both these mechanisms contribute to the pathogenesis of tubulointerstitial nephropathies associated with low-molecular-weight proteins, such as multiple myeloma. Proximal reabsorption of light chains is associated with tubular atrophy, necrosis and Fanconi syndrome. Taken together with understanding of distal tubular cast formation, identification of major renal binding proteins for myeloma light chains in the proximal tubules will allow detailed characterization of the binding site between cubilin and light chains, as well as other nephrotoxic low-molecular weight proteins. This completes the necessary mechanistic data of all affected nephron sites for the rational design of agents to protect from nephrotoxicity caused by myeloma light chains as well a s other low-molecular weight proteins. The present studies also demonstrate several lines of evidence suggesting that light chains are a ligand for megalin: anti-megalin antiserum partially displaces brush border light chain binding, and gentamicin displaces brush border light chain binding. Independent evidence suggests that cubilin is a receptor for polybasic drugs as gentamicin directly interferes with light chain binding to cubilin in vitro. These observations are important, both to understand the complex interactions of toxic 6 2 and physiological ligands on proximal tubule scavenger pathway receptors, as well as the eventual development of clinical protective agents for nephrotoxic damage mediated by ligands for cubilin and/or megalin. The following references were cited herein. Baricault et al., Biochem Biophys Res Commun 212:353-359, 1995. Batuman et al., Am J Physiol 258 (Renal Fluid Electrolyte Physiol 27): F1259-F1265, 1990. Batuman et al., Am J Physiol. 272 (Renal Fluid Electrolyte Physiol 7 ):F521 -F530, 1997. 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Gueant et al., Gastroenterology 108, 1622-1628, 1995. 6 3 Hishida et al., EMBO J. 15, 4111-22, 1996. Hammond et al., Kidney Int 42:997-1005, 1992. Hammond et al., Am J Physiol 272 (Renal Fluid Electrolyte Physiol 41) :F1 17-F123, 1997. Hammond et al., Cytometry 14: 41 1-420, 1993. Hammond et al., Am J Physiol 267 (Renal Fluid Electrolyte Physiol 3(5j :F1021-F1033, 1994. Hammond et al.,Λm J Physiol 267:F516-F527, 1994. Hammond et al., J Clin Invest 75: 1983-1989, 1985. Huang et al., / Clin Invest 99(4):732-736, 1997. Iglesias et al., Transplant Proc. 26(l):75-6, 1994. Imerslund, Acta Paediatr. Scand. 49 (Suppl), 1-115, 1960. Jensen et al., FEBS Lett. 29, 129-32, 1992. Jo et al., Proc Natl Acad Sci USA 92: 1876- 1880, 1995 Jonsson et al., Biotechniques 11 :620-627, 1991. Krieger et al., Ann. Rev. Biochem. 63, 601-637, 1994. Le Panse et al., Am. J. Pathol. 145, 1526-36, 1994. Le Panse et al., Eur. J. Cell Biol. 67, 120-9, 1995 Levi, Essays in Biochemistry 31 , 49-59, 1996. Llirbat et al., /. Lipid Res. 38: 1 22-34, 1997. Leung et al., /. Exp. Med. 156, 372-84, 1982. Marchalonis et al., J Prot Chem 11(2): 129-137, 1992. Moestrup et al., J. Biol. Chem. 268,16564-16570, 1993. Moestrup et al., Biochimica et Biophys. Acta 1 197: 197-213, 1994 Moestrup et al., / Clin Invest 96: 1404- 1413, 1995. Nykjaer et al., EMBO J. 16, 2610-2620, 1997. O'Farrel P.H, /. Biol Chem 250:4007-4021 , 1975. Parvez et al., Clin Chim Acta 190(1-2): 111-3, 1990. Rao et al., Cell 82, 131-141 , 1995. 64 Riedel et al., From: "Neoplastic diseases of the blood" Second Ed. Eds: Wiernik et al., Churchill Livingtone, 1991 , Chap. 23:347-372. Rothman et al., Science 272:227-234, 1996. Rothman et al., Current Biology 4(3):220-233, 1994. Sahali et al., J Exp Med 167:213-218, 1988. Sahali et al., Am J Pathol 142: 1654- 1667, 1993. Saito et al., Proc Natl Acad Sci USA 91 :9725-9729, 1994. Sanders et al., J. Clin. Invest. 82:2086-2096, 1988. Tolkoff-Rubin et al., Nephrol Dial Transplant 2(3): 143-8, 1987. van der Merwe et al., Curr Opin in Immunol 8:257-261 , 1996. Yamamoto et al., Cell 39:27-38, 1984. Willnow et al., EMBO J. 15, 2632-2639, 1996. Winearls, Kidney International 48: 1347-1360, 1995. Zeidel et al., Am J Physiol 265:F822-F833, 1993. Any patents or publications mentioned in this specification are indicative of the levels of those skilled in the art to which the invention pertains. These patents and publications are herein incorporated by reference to the same extent as if each individual publication was specifically incorporated by reference. One skilled in the art will readily appreciate that th e present invention is well adapted to carry out the objects and obtain the ends and advantages mentioned, as well as those inherent therein. The present examples along with the methods, procedures, treatments, molecules, and specific compounds described herein are presently representative of preferred embodiments, are exemplary, and are not intended as limitations on the scope of the invention. Changes therein and other uses will occur to those skilled in the art which are encompassed within th e spirit of the invention as defined by the scope of the claims. 6 5

Claims: WHAT IS CLAIMED IS: 1 . DNA encoding a cubilin protein selected from th e group consisting of: (a) isolated DNA which encodes a cubilin protein; (b) isolated DNA which hybridizes to isolated DNA of (a) above and which encodes a cubilin protein; and (c) isolated DNA differing from the isolated DNAs of (a) and (b) above in codon sequence due to the degeneracy of th e genetic code, and which encodes a cubilin protein. 2. The DNA of claim 1, wherein said DNA has th e sequence shown in SEQ ID No. 1. 3. The DNA of claim 1, wherein said cubilin protein has the amino acid sequence shown in SEQ ID No. 2. 4. The DNA of claim 1, wherein said DNA is expressed in the tissues selected from the group consisting of kidney, spleen, brain, liver, heart and thyroid. 5. A vector capable of expressing the DNA of claim 1 adapted for expression in a recombinant cell and regulatory elements necessary for expression of the DNA in the cell. 6. The vector of claim 5, wherein said DNA encodes a cubilin protein having the amino acid sequence shown in SEQ ID No. 2. 6 6 7. A host cell transfected with the vector of claim 5, said vector expressing a cubilin protein. 8. The host cell of claim 7, wherein said cell is selected from the group consisting of bacterial cells, mammalian cells, plant cells and insect cells. 9. The host cell of claim 8, wherein said bacterial cell is E. coli. 10. Isolated and purified cubilin protein or fragment coded for by DNA selected from the group consisting of: (a) isolated DNA which encodes a cubilin protein or fragment; (b) isolated DNA which hybridizes to isolated DNA of (a) above and which encodes a cubilin protein or fragment; and (c) isolated DNA differing from the isolated DNAs of (a) and (b) above in codon sequence due to the degeneracy of the genetic code, and which encodes a cubilin protein or fragment. 1 1 . The isolated and purified cubilin protein or fragment of claim 10, wherein said cubilin protein having the amino acid sequence shown in SEQ ID No. 2, wherein said fragment having amino acid sequence consisting of one or more of th e sequences selected from the group consisting of SEQ ID Nos. 21-27. 12. A method of detecting expression of the protein or fragment of claim 10 in a sample, comprising the steps of: 6 7 (a) contacting mRNA obtained from said sample with a labeled hybridization probe; and (b) detecting hybridization of said probe with said mRNA. 13. A pharmaceutical composition comprising th e protein or fragment of claim 10 and a pharmaceutically acceptable carrier. 14. A method of treating or reducing toxicity in a n individual in need of such treatment, comprising the step of: contacting said individual with the pharmaceutical composition of claim 13. 15. The method of claim 14, wherein said toxicity occurs in the tissues selected from the group consisting of kidney, spleen, brain, liver, heart and thyroid. 16. A receptor for ligands, wherein said receptor comprising a cluster of EGF repeats and a cluster of CUB domains. 17. The receptor of claim 16, wherein said receptor is cubilin. 1 8. The receptor of claim 16, wherein said ligand is selected from the group consisting of immunoglobulin light chain, myoglobin, intrinsic factor-vitamin B 12 , metallothionein, β- 2 - microglobulin, amyloid, hemoglobin, haptoglobin, interferon, insulin, cytochrome c, lysozyme, transferrin, transthyretin, 6 8 polybasic drugs, low density lipoprotein, high density lipoprotein and receptor related protein. 19. The receptor of claim 18, wherein said immunoglobulin light chain is selected from the group consisting of -light chain and λ-light chain. 20. A method of detecting renal damage in a n individual suspected to have renal damage, comprising the steps of: extracting a urine sample from said suspected individual and a urine sample from a normal individual; measuring level of cubilin in said urine samples; comparing the level of cubilin in said suspected individual with the level of cubilin in said normal individual, wherein if said level of cubilin in said suspected individual is lower than said level of cubilin in said normal individual, said suspected individual has chronic renal damage; wherein if said level of cubilin in said suspected individual is higher than said level of cubilin in said normal individual, said suspected individual has renal damage of acute origin. 6 9

Inventors: HAMMOND TIMOTHY G, VERROUST PIERRE J, HAMMOND, TIMOTHY, G., VERROUST, PIERRE, J.
Applicants: HAMMOND TIMOTHY G, INST NAT SANTE RECH MED, UNIV TULANE, VERROUST PIERRE J, ADMINISTRATORS OF THE TULANE EDUCATIONAL FUND, HAMMOND, TIMOTHY, G., INSTITUT NATIONAL DE LA SANTE ET DE LA RECHERCHE MEDICALE, VERROUST, PIERRE, J.
Application year: 1999
Main classification: C12N15
IPC codes: A61K38, C07K14