# Patent

## Title
Methods for modulating transcriptional activation using mint proteins

## Abstract

The present invention is directed to isolated nucleic acids encoding Mint protein variants having enhanced abilities to modulate the transcriptional activation mediated by the cytoplasmic tail of the amyloid precursor protein (APP) relative to wild-type Mint proteins. The present invention is further directed toward purified Mint protein variants having enhanced abilities to modulate the transcriptional activation mediated by the cytoplasmic tail of APP relative to wild-type Mint proteins. The present invention also encompasses methods of modulating transcriptional activation and methods of identifying compounds that modulate transcriptional activation, and vectors, as well as transfected cells and kits useful for modulating transcriptional activation or for the identification of compounds that can modulate transcriptional activation. The present invention further encompasses transgenic knockout mice with little or no expression of Mint 1, Mint 2 or Mint 3 proteins. Such reagents may be useful as candidate therapeutics for Alzheimer's disease (AD), or as models for the rational design of drugs useful for the treatment of AD.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a divisional of U.S. patent application Ser. No. 10/227,490, filed Aug. 23, 2002, now U.S. Pat. No. 7,081,337 which is a continuation-in-part of U.S. patent application Ser. No. 09/821,861, filed Mar. 30, 2001 and issued as U.S. Pat. No. 6,649,346, to each of which priority is claimed, and each of which is incorporated by reference in its entirety herein.


The subject matter described herein was supported, at least in part, by funds provided by the United States Government. Accordingly, the United States Government may have certain rights to this invention.


INTRODUCTION
The present invention relates to methods for modulating transcriptional activation and hence to methods for the treatment of Alzheimer's disease (AD). The invention is based, at least in part, on the observation that, while all three MsX2-interacting nuclear target (Mint) proteins (Mints 1-3) can bind to the cytoplasmic tail of APP, only Mints 1 and 2 modulate the transcriptional activation mediated by the cytoplasmic tail of APP.
BACKGROUND OF THE INVENTION
Alzheimer's disease (AD) is a degenerative brain disorder that is characterized clinically by progressive loss of memory and cognitive impairment. Pathologically, the disease is characterized by lesions comprising neurofibrillary tangles, cerebrovascular amyloid deposits, and neuritic plaques. The cerebrovascular amyloid deposits and neuritic plaques contain amyloid-β peptide. The aggregation of amyloid-β peptide may be instrumental in the pathogenesis of AD.
Amyloid-β peptide is derived from amyloid-β precursor protein (APP). APP is a ubiquitous type 1 membrane protein (Kang et al., 1987, Nature 325:733-736; Kitaguchi et al., 1988, Nature 331:530-532; Tanzi et al., 1988, Nature 331:528-30) that is physiologically processed by proteolytic cleavage. (Selkoe, 1998, Trends Cell Biol 8: 447-453; Bayer et al., 1999, Mol Psychiatry 4:524-528; Haass and De Strooper, 1999, Science 286:916-919; Wolfe and Haass, 2001, J Biol Chem 276:5413-5416). First, cleavage by α- or β-secretases releases the large extracellular portion of APP. Subsequently, the remaining sequences of APP composed of a small extracellular stub, the transmembrane region (TMR), and the cytoplasmic tail are digested by γ-secretase at multiple positions. See Sastre et al., 2001, EMBO Rep 2:835-841; Yu et al., 2001, J Biol Chem 276:43756-43760. γ-Cleavage liberates an intracellular cytoplasmic fragment that may be translocated to the nucleus (Cupers et al., 2001, J Neurochem 78:1168-1178; Kimberly et al., 2001, J Biol Chem 276:40288-40292) and may function as a transcriptional activator (Cao and Südhof, 2001, Science 293:115-120; Gao and Pimplikar, 2001, Proc Natl Acad Sci USA 98:14979-14984). In addition, γ-cleavage generates small peptides derived from the TMR and adjacent extracellular sequences that include Aβ40 and Aβ42 which form the amyloid fibrils in Alzheimer's disease. See Glenner and Wong, 1984, Biochem Biophys Res Commun 122:1131-1135; Masters et al., 1985, EMBO J 4:2757-2763; reviewed in Selkoe, 1998, Trends Cell Biol 8: 447-453; Haass and De Strooper, 1999, Science 286:916-919.
The γ-cleavage of APP is mediated by presenilins, intrinsic membrane proteins that may correspond to γ-secretase and that are mutated in some cases of familial AD. See, e.g., Esler et al., 2000, Nat Cell Biol 2:428-434. Also, γ-cleavage occurs in APP homologs that are not implicated in AD. For example, Notch proteins are membrane proteins that are also cleaved in the middle of the TMR in a presenilin-dependent reaction. See, e.g., Yea et al., 1999, Nature 398:525-529; De Strooper et al., 1999, Nature 398:518-522; Struhl et al., 1999, Nature 398:522-525. Notch proteins are cell-surface proteins involved in intercellular signaling in which presenilin-dependent cleavage liberates a cytoplasmic fragment that functions in nuclear transcription. Struhl et al., 2000, Mol Cell 6:625-636. Sterol regulatory element binding proteins (SREPPs) are also cleaved to generate nuclear transcription factors. Brown et al., 2000, Cell 100:391-398. These observations suggested that the short cytoplasmic tail fragment of APP also may function as a transcriptional activator, and that feedback loops may exist between the nucleus and the cytoplasm or cell membrane whereby the rate of APP proteolysis is regulated.
This first of these hypotheses was confirmed by the findings that the short cytoplasmic tail of APP contains an NPTY (SEQ ID NO: 17) sequence that binds to phosphotyrosine binding (PTB) domains in multiple proteins, including Fe65 and Mints/X11a. See Fiore et al., 1995, J Biol Chem 270:30853-30856; Borg et al., 1996, Mol Cell Biol 16:6229-6241; Guenette et al., 1996, Proc Natl Acad Sci USA 93:10832-10837; McLoughlin and Miller, 1996, FEBS Lett 397:197-200; Zhang et al., 1997, EMBO J. 16: 6141-6150. Fe65 is an adaptor protein that forms a transcriptionally active complex with the released APP tail and a nuclear histone acetyltransferase, Tip60. See Cao and Südhof, 2001, Science 293:115-120.
In published United States patent application 20010034884, Peraus modified APP to create an APP fusion protein that incorporated a Gal4 binding domain and a VP16 transactivating domain, so that the rate of formation of the cytoplasmic tail of APP could be monitored by measuring the level of expression of a reporter gene whose transcription was controlled by a regulatable promoter containing Gal 4 binding sites. However, in contrast to the study of Cao and Südhof (2001), Peraus expressed no appreciation that the cytoplasmic tail of APP, without modification, acted as a transcription factor under physiological conditions, through an interaction with Fe65 and Tip60.
As introduced above, the NPTY sequence in the cytoplasmic tail fragment of APP can facilitate the binding of this protein to the PTB domains that are present in proteins of the Mints/X11 family. Mints 1 and 2 are genes initially identified as candidates for Friedreich's ataxia. Based on partial sequence analysis, these genes were thought to be orthologs. See Duclos and Koenig, 1995, Mamm Genome 6: 57-58. However, the sequencing of full-length cDNAs showed that the encoded proteins were products of distinct genes. See Okamoto and Südhof, 1997, J Biol Chem 272:31459-31464. To prevent confusion among different types of X11s, these proteins were named Mints 1 and 2. A third isoform was dubbed Mint 3. See Okamoto and Südhof, 1997, J Biol Chem 272:31459-31464; Okamoto and Südhof, 1998, Eur J Cell Biol 77:161-165. Subsequent recloning of the same proteins led to further renaming, and they are now also variably referred to as X11α/β/γ, mLin-10s, X11a/b/c, or X11L1/L2.
Mints/X11 proteins are composed of a long isoform-specific N-terminal sequence, a central PTB domain, and two C-terminal PSD-95, Drosophila disc large, zona occludens (PDZ) domains. Mint proteins interact with several other proteins in addition to APP. Mint 1 (but not Mints 2 or 3) binds to calcium/calmodulin-dependent serine protein kinase (CASK) (Butz et al., 1998, Cell 94:773-782), another adaptor protein (Hata et al., 1996, J Neurosci 16: 2488-2494). In C. elegans, CASK and Mint 1 homologs are encoded by the Lin-2 and Lin-10 genes whose mutation causes similar vulvaless phenotypes, suggesting that the Mint 1/CASK complex is evolutionarily conserved. See Butz et al., 1998, Cell 94:773-782; Kaech et al., 1998, Cell 94:761-771; Borg et al., 1998b, J Biol Chem 273:31633-31636; and Borg et al., 1999, J Neurosci 19:1307-1316. Mints 1 and 2 also bind to Munc18-1, an essential fusion protein at the synapse (Okamoto and Südhof, 1997, J Biol Chem 272:31459-31464; Biederer and Südhof, 2000, J Biol Chem. 275:39803-39806; Verhage et al., 2000, Science 287:864-869), and to presenilins which are intrinsic components of the γ-secretase (Lau et al., 2000, Mol Cell Neurosci 16:557-565).
The functions of the Mint proteins remain obscure. In C. elegans, Lin-10 (Mint 1) mediates the correct targeting of EGF-like receptors to the basolateral membrane of vulval precursor cells (Whitfield et al., 1999, Mol Biol Cell 10:2087-2100), and is necessary for delivery of AMPA-like glutamate receptors to synapses (Rongo et al., 1998, Cell 94:751-759). These data suggest that Lin-10/Mint 1 functions in membrane traffic of proteins to specific plasma membrane domains. In vertebrates, however, a variety of somewhat contradictory functions for Mints have been proposed. Transfection experiments revealed that Mints alter production of Aβ peptides, indicating a role in APP cleavage. See Borg et al., 1998a, J Biol Chem 273:14761-14766; Sastre et al., 1998, J Biol Chem 273:22351-22357; Mueller et al., 2000, J Biol Chem 275:39302-39306. In contrast, an interaction of Mint 1 with KIF17 in vitro led to the proposal that Mint 1 functions in trafficking neuronal NMDA-, but not AMPA-type glutamate receptors in vertebrates. See Setou et al., 2000, Science 288:1796-1802. This study renamed Mint 1 “mLin-10” in analogy to the C. elegans gene, but did not reference the previous finding that in nematodes Lin-10 only affects AMPA- but not NMDA-receptors (Rongo et al., 1998, Cell 94:751-759).
The small size of the APP cytodomain and the overlapping of its regions involved in the binding of Fe65 and the Mints proteins suggest that the latter may be involved in the competitive regulation of the intracellular signaling events mediated by the cytoplasmic tail of APP. Because it is likely that the rate of APP proteolysis is directly or indirectly regulated through feedback mechanisms that operate between the nucleus and the cytoplasm or cell membrane, a better understanding of the interactions between the cytoplasmic tail of APP and the Mints proteins will expand our knowledge of the mechanisms whereby the rate of APP proteolysis is regulated, thereby leading to greater insight into the pathophysiology of AD.
These studies may also facilitate the development of new methods of treatment for this disease. At present, the only medications approved by the U.S. Food and Drug Administration for the treatment of AD are cholinesterase inhibitors. Unfortunately, these drugs provide only limited clinical benefit in controlling the symptoms of AD, and do little to actually intervene in the disease process. Thus, there is a strong need for the development of better treatments for AD.
In accordance with the present invention, it has been discovered that, while all three Mint proteins (Mints 1-3) bind to the cytoplasmic tail of APP, only Mint 1 and 2 can modulate the transcriptional activation mediated by a fusion protein consisting of the cytoplasmic tail of APP coupled to the transcription factor Gal4/VP16. These findings suggest that Mints 1 and 2, variants of these proteins that display enhanced abilities to modulate the transcriptional activation mediated by the cytoplasmic tail of APP relative to wild-type Mints 1 and 2, or small molecules which either mimic, inhibit or potentiate the effects of these proteins in modulating the transcriptional activation mediated by the cytoplasmic tail of APP, may be useful in regulating the rate of APP proteolysis and hence in the treatment of AD.
SUMMARY OF THE INVENTION
The present invention is directed to isolated nucleic acids encoding Mint protein variants having enhanced abilities to modulate the transcriptional activation mediated by the cytoplasmic tail of the amyloid precursor protein (APP) relative to wild-type Mint proteins. In preferred embodiments, the nucleic acids are those having the nucleotide sequences of SEQ ID NOS:1-8.
The present invention is further directed to purified Mint protein variants having enhanced abilities to modulate the transcriptional activation mediated by the cytoplasmic tail of APP relative to wild-type Mint proteins. In preferred embodiments, these proteins are those having the amino acid sequences of SEQ ID NOS:9-16.
The present invention further provides a method of modulating transcriptional activation. In one embodiment, this method comprises introducing into a target cell a nucleic acid encoding a wild-type Mint protein or a Mint protein variant having enhanced abilities to modulate transcriptional activation relative to wild-type Mint proteins. In preferred embodiments, the nucleic acids are those having the nucleotide sequences of SEQ ID NOS:1-8. In another embodiment, this method comprises introducing into a target cell a wild-type Mint protein or a Mint protein variant having enhanced abilities to modulate transcriptional activation relative to wild-type Mint proteins. In preferred embodiments, these proteins are those having the amino acid sequences of SEQ ID NOS:9-16. The Mint proteins and Mint protein variants of the present invention modulate transcriptional activation mediated by the cytoplasmic tail of APP.
The present invention also provides a method of identifying compounds that modulate transcriptional activation comprising contacting a cell containing an APP molecule modified in the C-terminal cytoplasmic tail to permit the specific transcriptional activation of a reporter gene and measuring the levels of reporter gene transcription in the presence and absence of the compound, wherein increased or decreased levels of reporter gene transcription in the presence of the compound indicate that the compound is capable of modulating transcriptional activation. Such compounds may be useful as candidate therapeutics for AD, or as models for the rational design of drugs useful for the treatment of AD.
The present invention further provides for transgenic knockout mice for Mint 1, Mint 2 and Mint 3. These animals may be useful for elucidating the pathophysiology of AD and for developing improved treatments for this disease.
In other embodiments, the present invention is directed to vectors, transfected cells and kits useful for modulating transcriptional activation or for the identification of compounds that can modulate APP-mediated transactivation.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A-B shows the similarly tight binding of Mints 1, 2, and 3 to the cytoplasmic tail of APP. A. Immunoblot analysis of Mint binding to the immobilized cytoplasmic tail of APP. B. Quantitation of the amount of Mints bound to the immobilized cytoplasmic tail of APP as percent of Mints in the starting brain extract.
FIG. 2A-B show that co-transfection of Mints 1, 2, or 3 increases the steady-state levels of APP. A. Immunoblot analysis of HEK293 cells co-transfected with expression plasmids encoding APP695, Mints 1-3, and Fe65 as indicated. B. Quantitation of the levels of APP695 in transfected HEK293 cells shown in A.
FIG. 3A-C show the binding of both PDZ domains of Mints to the cytoplasmic C-terminal sequence of presenilins. A. Binding of Mints 1 and 3 to presenilin 1. B. Percent of Mints solubilized from a rat forebrain membrane preparation that bound to an affinity column containing a peptide corresponding to the C-terminus of presenilin 1 or a control peptide derived from gp41 (n=3). C. Binding of various Mint 1 mutants prepared from transfected COS cells to the immobilized cytoplasmic C-terminal sequence of presenilin 1. Mint 1 mutants containing inactivating point mutations in either the first (Mint 1 PDZ1*), the second (Mint 1 PDZ2*), or both PDZ domains (Mint 1 PDZ1/2*), or a truncation mutant of Mint 1 lacking the two PDZ domains (Mint 1 ΔPDZ) were analyzed by affinity chromatography on immobilized presenilin 1 or gp41 control peptides.
FIG. 4A-F shows the immunoreactivity of Mint 1, Mint 2 and APP in mouse hippocampus. Vibratome sections (35 μm) from adult mouse hippocampus were stained for Mint 1 (A and D), Mint 2 (B and E), and APP (C and F) using the avidin-biotin peroxidase method.
FIG. 5A-L shows the immunofluorescence localization of Mints 1 and 2 in cultured hippocampal neurons. The left (red) and center (green) pictures show the separate fluorescence channels from double immunofluorescence labeling experiments, whereas the right pictures show the merged images. Neurons were labeled with the following antibodies: A-C, antibodies to Mint 1 (A) and Mint 2 (B). Note that the pictures show a low-magnification overview with a high-magnification inset. D-F, antibodies to synapsins (D) and Mint 2 (E). G-I, antibodies to APP (G) and Mint 1 (H). J-L, antibodies to APP (J) and Mint 2 (K). Scale bar in panel (applies to the low-magnification views in A-C)=50 μm; scale bar in panel 1 (applies to the insets in A-C, and the full images in all other pictures)=20 μm.
FIG. 6A-L shows that Mints 1 and 2 are concentrated in the trans-Golgi complex. Subcellular distribution of Mints has been further investigated with a group of well-characterized markers to Golgi apparatus, endoplasmic reticulum and early endosome. Cultured hippocampal neurons were double-labeled with antibodies to the following proteins: A-C, Mint 2 (A), TGN38 (B), and merged images (C); D-F, Mint 1 (D), the Golgi 58K protein (E), and merged images (F); G-I, Mint 2 (G), calnexin (H), and merged images (I); J-L, Mint 2 (J), EEA1 (K), and merged images (L). Scale bar in L (applies to all images)=15 μm.
FIG. 7A-B shows the use of APP-Gal4/VP16 fusion proteins to measure γ-cleavage of APP. A. Relative luciferase activity in PC12 cells following transfection with plasmids containing the constructs indicated in B.
FIG. 8A-B shows the sequence requirements of APP γ-cleavage measured by Gal4/VP16-dependent transactivation. A. Requirements of extra- and intracellular APP sequences for transactivation. B. Test of the specific functions of extra- vs. intracellular APP sequences in γ-cleavage, and of the effect of a dominant-negative mutant of presenilin 2 (PS2DA; Steiner et al., 1999, J Biol Chem 274:28669-28673).
FIG. 9A-C shows that Mints 1 and 2 but not Mint 3 inhibit transactivation by APP-Gal4/VP16 fusion proteins. A. Dose-dependent inhibition by Mint 1 of wild-type APP-Gal4/VP16 transactivation but not of APP-Gal4/VP16 carrying a point mutation in the cytoplasmic NPTY binding sequence for Mints (APP*-GV). B. Effects of Mints 1, 2, and 3 on APP-Gal4/VP16 dependent transactivation. C. Quantitated immunoblot analysis of the Mints expressed in the experiment shown in B.
FIG. 10A-C shows the structure-function analysis of Mint 1 indicating the role of PDZ domains in inhibiting APP-Gal4/VP16 dependent transactivation. A. Transactivation as a function of protein concentration. B. Transactivation observed in HEK293 cells co-transfected with APP-Gal4/VP16 and a control plasmid, or wild-type Mint 1, a Mint 1 mutant lacking the PDZ-domains, wild-type Mint 3, or a Mint 3 mutant lacking both PDZ domains. C. Immunoblot analysis of the Mint mutants analyzed in B to control expression of the constructs in the experiments shown.
FIG. 11A-C show that the PTB and PDZ domains of Mint 1 cooperate in inhibiting transactivation by APP-Gal4/VP16. A. Effect of co-transfecting Mint 1 and various Mint 1 point-mutants in the PTB-domain and the first PDZ domain with APP-Gal4/VP16. B. Effect of deleting the N-terminal isoform-specific Mint 1 sequences on inhibition of transactivation. C. Quantitated immunoblot analysis of Mint expression in the experiment shown in B.
FIG. 12 shows the binding-dependent effect of Mints on transactivation mediated by a fusion protein of the intracellular fragment of APP with Gal4/VP16.
FIG. 13A-P shows the sequences of the various Mint 1 and 2 nucleic acids and peptides. A. Mint 1 cDNA (SEQ ID NO:1). B. Mint 1 PDZ1* cDNA Sequence (SEQ ID NO:2). C. Mint 1 ΔPDZ cDNA Sequence (SEQ ID NO:3). D. Mint 1 Δ N-term cDNA Sequence (SEQ ID NO:4). E. Mint 2 cDNA Sequence (SEQ ID NO:5). F. Mint 2 PDZ1* cDNA Sequence (SEQ ID NO:6). G. Mint 2 Δ PDZ cDNA Sequence (SEQ ID NO:7). H. Mint 2 Δ N-term cDNA Sequence (SEQ ID NO:8). I. MINT 1 Peptide Sequence (SEQ ID NO:9). J. MINT 1 PDZ1* Peptide Sequence (SEQ ID NO:10). K. MINT 1 Δ PDZ Peptide Sequence (SEQ ID NO:11). L. MINT 1 Δ N-term Peptide Sequence (SEQ ID NO:12). M. MINT 2 Peptide Sequence (SEQ ID NO:13). N. MINT 2 PDZ1 *Peptide Sequence (SEQ ID NO:14). O. MINT 2 Δ PDZ Peptide Sequence (SEQ ID NO:15). P. MINT 2 Δ N-term Peptide Sequence (SEQ ID NO:16).
FIG. 14A-C depicts the strategies used for the production of transgenic mice in which the function of the Mint 1 (A), Mint 2 (B) or Mint 3 (C) gene has been eliminated. In each case, schematic representations of the wild-type gene, the targeting vector and the mutant gene are provided.
FIG. 15A-B shows Southern blot analysis of Mint 1 knockouts. A. Genomic tail DNA from offspring of heterozygous interbreedings was digested with SpeI and hybridized with the outside probe shown in FIG. 14A. The upper band corresponds to wild-type allele and the lower to the mutant allele. B. Southern blot analysis of Mint 1 knockouts. Genomic tail DNA from offspring of heterozygous interbreedings was digested with SpeI and hybridized with a probe to the first exon shown in FIG. 14A.



DETAILED DESCRIPTION OF THE INVENTION
Proteolytic processing of the amyloid precursor protein (APP) produces, inter alia, amyloid-β peptide, which may contribute to the pathogenesis of Alzheimer's disease (AD), and a C-terminal cytoplasmic tail, which may act as a transcriptional activator. In accordance with the present invention, it has been discovered that the proteins Mint 1, Mint 2 and Mint 3 can strongly bind to the PDZ domain of the C-terminal cytoplasmic tail of APP (see Example #1). Moreover, it has been discovered that the binding of Mint 1 and Mint 2 to the cytoplasmic tail of APP strongly inhibits the transcriptional activation produced by this protein (see Example #6). In particular, it has been discovered that the transcriptional activation of a reporter gene whose transcription is regulated by the binding of the GAL4/VP-16 transcription factor by an APP-Gal4/VP16 fusion protein can be strongly inhibited by Mint 1 and Mint 2 (see Example #6). This observation suggests that Mint proteins, especially Mint 1 and 2, variants of these proteins with enhanced abilities to modulate transcriptional activation relative to wild-type Mint proteins, or other molecules that may mimic the effects of the Mints or inhibit or potentiate the interaction between Mint proteins and the cytoplasmic tail of APP may be useful either as candidate therapeutics for the treatment of AD, as reagents for the identification of candidate therapeutics for the treatment of AD, or as models or targets for rational drug design.
The present invention relates to isolated nucleic acids encoding Mint proteins or Mint protein variants having enhanced abilities to modulate transcriptional activation relative to wild-type Mint proteins. In preferred embodiments, the nucleic acids are those having the nucleotide sequences of SEQ ID NOS:1-8. In other preferred embodiments, the nucleic acids are nucleic acids encoding the polypeptides having the amino acid sequences of SEQ ID NOS:9-16. In other embodiments, the nucleic acids are nucleic acids comprising the nucleic acids having the nucleotide sequences of SEQ ID NOS:1-8 or the nucleic acids encoding the polypeptides having the amino acid sequences of SEQ ID NOS:9-16. In still further embodiments, the nucleic acids are those substantially identical to the nucleic acids of SEQ ID NOS:1-8 or nucleic acids comprising nucleic acids substantially identical to the nucleic acids of SEQ ID NOS:1-8, wherein substantial identity at the nucleotide level occurs when at least about 60% to 75% or preferably at least about 80% or most preferably at least about 90% of the nucleotides comprising the nucleic acid molecule are identical over a defined length of the molecule, while the substantially identical nucleic acids retain the ability to encode for proteins having the biological function of the polypeptides having the amino acid sequences of SEQ ID NOS:9-16.
Substantially identical nucleic acid molecules may be identified by hybridization under suitably stringent hybridization conditions. Defining appropriate hybridization conditions is within the skill of the art. See e.g. Current Protocols in Molecular Biology, Volume I. Ausubel et al., eds. John Wiley: New York N.Y., pp. 2.10.1-2.10.16, first published in 1989 but with annual updating, wherein maximum hybridization specificity for DNA samples immobilized on nitrocellulose filters may be achieved through the use of repeated washings in a solution comprising 0.1-2×SSC (15-30 mM NaCl, 1.5-3 mM sodium citrate, pH 7.0) and 0.1% SDS (sodium dodecylsulfate) at temperatures of 65-68° C. or greater. For DNA samples immobilized on nylon filters, a stringent hybridization washing solution may be comprised of 40 mM NaPO4, pH 7.2, 1-2% SDS and 1 mM EDTA. Again, a washing temperature of at least 65-68° C. is recommended, but the optimal temperature required for a truly stringent wash will depend on the length of the nucleic acid probe, its GC content, the concentration of monovalent cations and the percentage of formamide, if any, that was contained in the hybridization solution (Current Protocols in Molecular Biology, Volume I. Ausubel et al., eds. John Wiley: New York N.Y., pp. 2.10.1-2.10.16. 1989 with annual updating), all of which can be determined by the skilled artisan.
The present invention is further directed to purified Mint proteins or Mint protein variants having enhanced abilities to modulate transcriptional activation, and particularly the transcriptional activation mediated by the cytoplasmic tail of APP, relative to wild-type Mint proteins. In preferred embodiments, these proteins are those having the amino acid sequences of SEQ ID NOS:9-16. In further embodiments, the proteins are polypeptides encoded by the nucleic acids having the nucleotide sequences of SEQ ID NOS:1-8. In other embodiments, the proteins are polypeptides encoded by nucleic acids that are substantially identical to the nucleic acids of SEQ ID NOS:1-8, wherein substantial identity at the nucleotide level occurs when at least about 60% to 75% or preferably at least about 80% or most preferably at least about 90% of the nucleotides comprising the nucleic acid molecule are identical over a defined length of the molecule.
The present invention further provides a method of modulating the transcriptional activation mediated by the cytoplasmic tail of APP. In one embodiment, this method comprises introducing into a target cell a nucleic acid encoding a Mint protein or a Mint protein variant having enhanced abilities to modulate the transcriptional activation mediated by the cytoplasmic tail of APP relative to wild-type Mint proteins.
In preferred embodiments, the nucleic acids are those having the nucleotide sequences of SEQ ID NOS:1-8. In other preferred embodiments, the nucleic acids are nucleic acids encoding the polypeptides having the amino acid sequences of SEQ ID NOS:9-16. In other embodiments, the nucleic acids are nucleic acids comprising the nucleic acids having the nucleotide sequences of SEQ ID NOS:1-8 or the nucleic acids encoding the polypeptides having the amino acid sequences of SEQ ID NOS:9-16. In still further embodiments, the nucleic acids are those substantially identical to the nucleic acids of SEQ ID NOS:1-8 or nucleic acids comprising nucleic acids substantially identical to the nucleic acids of SEQ ID NOS:1-8, wherein substantial identity at the nucleotide level occurs when at least about 65% to 70% or preferably at least about 80% or most preferably at least about 90% of the nucleotides comprising the nucleic acid molecule are identical over a defined length of the molecule.
The nucleic acids described above may be introduced into target cells using a variety of vectors. These vectors include virus-based vectors and non-virus based DNA or RNA delivery systems. Examples of potential virus-based gene transfer vectors include, but are not limited to, those derived from the following nonlimiting virus types: Adenoviridae; Birnaviridae; Bunyaviridae; Caliciviridae, the Capillovirus group; the Carlavirus group; the Carmovirus group; the Caulimovirus group; the Closterovirus group; the Commelina Yellow Mottle Virus group; the Comovirus virus group; Coronaviridae; the PM2 Phage group; Corcicoviridae; the Cryptic Virus group; the Cryptovirus group; the Cucumovirus Virus group; the Family Φ6 Phage group; Cysioviridae; the Carnation Ringspot group; the Dianthovirus group; the Broad Bean Wilt virus group; the Fabavirus virus group; Filoviridae; Flaviviridae; the Furovirus group; the Germinivirus group; the Giardiavirus group; Hepadnaviridae; Herpesviridae; the Hordeivirus virus group; the Illarvirus group; Inoviridae; Iridoviridae; Leviviridae; Lipothrixviridae; the Luteovirus group; the Marafivirus group; the Maize Chlorotic Dwarf Virus group; Icroviridae; Myoviridae; the Necrovirus group; the Nepovirus group; Nodaviridae; Orthomyxoviridae; Papovaviridae; Paramyxoviridae; the Parsnip Yellow Fleck Virus group; Partitiviridae; Parvoviridae; the Pea Enation Mosaic Virus group; Phycodnaviridae; Picomaviridae; Plasmaviridae; Prodoviridae; Polydnaviridae; the Potexvirus group; Potyvirus; Poxviridae; Reoviridae; Retroviridae; Rhabdovindae; the Rhizidiovirus group; Siphoviridae; the Sobemovirus group; SSV1-Type Phages; Tectiviridae; Tenuivirus; Tetraviridae; the Tobamovirus group; the Tobravirus group; Togaviridae; the Tombusvirus group; the Torovirus group; Totiviridae; the Tymovirus group; and Plant virus satellites.
In preferred embodiments, the viral vectors are those derived from retroviruses, for example Moloney murine leukemia-virus based vectors such as LX, LNSX, LNCX or LXSN (Miller and Rosman, Biotechniques 1989;7:980-989); lentiviruses, for example human immunodeficiency virus (“HIV”), feline leukemia virus (“FIV”) or equine infectious anemia virus (“EIAV”)-based vectors (Case et al., 1999, Proc Natl Acad Sci USA 96:2988-2993; Curran et al., 2000, Molecular Ther 1:31-38; Olsen, 1998, Gene Ther 5:1481-1487); adenoviruses (Zhang, 1999, Cancer Gene Ther 6:113-138; Connelly, 1999, Curr Opin Mol Ther 1:565-572), for example Ad5/CMV-based E1-deleted vectors (Li et al., 1993, Human Gene Ther 4:403-409); adeno-associated viruses, for example pSub201-based AAV2-derived vectors (Walsh et al., 1992, Proc Natl Acad Sci USA 89:7257-7261); herpes simplex viruses, for example vectors based on HSV-1 (Geller and Freese, 1990, Proc Natl Acad Sci USA 87:1149-1153); baculoviruses, for example AcMNPV-based vectors (Boyce and Bucher, 1996, Proc Natl Acad Sci USA 93:2348-2352); SV40, for example SVluc (Strayer and Milano, 1996, Gene Ther 3:581-587); Epstein-Barr viruses, for example EBV-based replicon vectors (Hambor et al., 1988, Proc Natl Acad Sci USA 85:4010-4014); alphaviruses, for example Semliki Forest virus- or Sindbis virus-based vectors (Polo et al., 1999, Proc Natl Acad Sci USA 96:4598-4603); vaccinia viruses, for example modified vaccinia virus (MVA)-based vectors (Sutter and Moss, 1992, Proc Natl Acad Sci USA 89:10847-10851) or any other class of viruses that can efficiently transduce human tumor cells and that can accommodate the nucleic acid sequences required for physiologic efficacy.
Non-limiting examples of non-virus-based delivery systems that may be used according to the invention include, but are not limited to, so-called naked nucleic acids (Wolff et al., 1990, Science 247:1465-1468), nucleic acids encapsulated in liposomes (Nicolau et al., 1987, Meth Enzymology 149:157-176), nucleic acid/lipid complexes (Legendre and Szoka, 1992, Pharmaceutical Res 9:1235-1242), and nucleic acid/protein complexes (Wu and Wu, 1991, Biother 3:87-95). The vectors comprising these nucleic acids may include, but are not limited to, plasmids, cosmids, phagemids, bacmids, artificial chromosomes or replicons.
In another embodiment, the method of modulating transcriptional activation comprises introducing into a target cell a Mint protein or a Mint protein variant having enhanced abilities to modulate transcriptional activation relative to wild-type Mint proteins. In preferred embodiments, these proteins are those having the amino acid sequences of SEQ ID NOS:9-16. In other embodiments, the proteins are polypeptides encoded by the nucleic acids having the nucleotide sequences of SEQ ID NOS:1-8. In other embodiments, the proteins are polypeptides encoded by nucleic acids that are substantially identical to the nucleic acids of SEQ ID NOS:1-8, wherein substantial identity at the nucleotide level occurs when at least about 60% to 75% or preferably at least about 80% or most preferably at least about 90% of the nucleotides comprising the nucleic acid molecule are identical over a defined length of the molecule.
The amount of a Mint protein within a cell may be increased by introducing the protein directly into the target cell. For example, for introduction into a cell, the Mint protein could be incorporated into a microparticle for uptake by pinocytosis or phagocytosis. In other embodiments, the Mint protein could be incorporated into a liposome. Other protein-stabilizing formulations useful for the delivery of proteins to target cells are known in the art.
The present invention also provides a method of identifying compounds that modulate transcriptional activation comprising contacting a cell containing an APP molecule modified in the C-terminal cytoplasmic tail to permit the specific transcriptional activation of a reporter gene and measuring the levels of reporter gene transcription in the presence and absence of the compound, wherein increased or decreased levels of reporter gene transcription in the presence of the compound indicate that the compound is capable of modulating transcriptional activation.
The term APP as used herein includes naturally occurring mammalian APP and also APP that has been modified, for example in a way to facilitate measurement of the transcriptional activation produced by the cytoplasmic tail of APP. Naturally occurring human APP is a 695 amino acid protein, in which the C-terminal 47 residues are designated the cytoplasmic tail. The gene encoding APP, its splice variants, and resulting nucleotide and amino acid sequences are known in the art and disclosed for example by Kang et al., supra; Selkoe, supra; Bayer et al., 1999, supra; Haass et al., supra; and Price et al., supra, the disclosures of which are incorporated herein by reference.
Further, APP as defined herein may include other modifications such as insertions, deletions and substitutions provided that the functions of ability of the cytoplasmic tail or part thereof to be cleaved from the remainder of APP and translocated to the nucleus are retained.
The C-terminal cytoplasmic tail of APP may be modified to allow detection of nuclear localization. The modification may be in any region of the cytoplasmic tail. The modification may be at the C-terminal or N-terminal end of the tail, for example at the junction of the transmembrane and cytoplasmic domains. In one embodiment, the cytoplasmic tail of APP is modified to include the DNA binding domain and the activation domain of the same or different heterologous transcription factors. Heterologous as used herein means not derived from a gene encoding APP. In this embodiment, nuclear localization is measured by determining activation of transcription of a reporter gene that is under the transcriptional control of a binding site for the DNA binding domain. Transcription factors and their component DNA-binding and activation domains are well known in the art.
In a preferred embodiment, the cytoplasmic tail is modified to include a heterologous DNA-binding domain such as the DNA-binding domain of the yeast transcription factor Gal4, or the bacterial LexA DNA binding domain. The Gal4 and LexA DNA binding domains are known in the art and disclosed for example by Giniger et al., 1985, Cell 40:767-774 and Hurstel et al., 1986, EMBO J 5:793-798. The modification may further contain the transcriptional activation domain of Gal4, or another activator such as the viral VP16 activator, which is disclosed for example by Stringer et al., 1990, Nature 345:783-786. In a preferred embodiment, the cytoplasmic tail of APP is modified to include Gal4 and VP16. A transcription factor module of Gal4-VP16 is described by Sadowski et al., 1988, Nature 335:563-564. Accordingly, the modification of the cytoplasmic tail may consist of a module consisting of a DNA-binding domain and a transcriptional activation domain, which may be from the same or different sources.
The reporter gene is operably linked to a binding site for the DNA-binding protein. For example, the reporter gene may be provided in the form of a Gal4- or LexA-dependent reporter plasmid containing a reporter gene such as luciferase or chloramphenicol acetyl transferase under the control of a Gal4 or LexA regulatory element, respectively, such as an upstream activating sequence. Translocation of the cytoplasmic tail of APP to the nucleus results in translocation of the transcription factor as well, resulting in activation of transcription of the reporter gene. Accordingly, detection of the reporter gene product provides an assay for nuclear localization of the cytoplasmic tail of APP, and hence measures cleavage of APP. Transcriptional activation assays are described by Fields et al., 1989, Nature 340:245-246, the disclosure of which is incorporated herein by reference. Gal4 and LexA reporter plasmids are described by Lillie et al., 1989, Nature 338:38-44 and Hollenberg et al., 1995, Mol Cell Biol 15:3813-3822.
Candidate compounds that may be tested by the assays of the present invention include proteins, peptides, non-peptide small molecules, and any other source of therapeutic candidate compounds. The compounds may be naturally occurring or synthetic, and may be a single substance or a mixture. Screening may be performed in high throughput format using combinatorial libraries, expression libraries and the like. Compounds identified as inhibitors of the transcriptional activation mediated by the cytoplasmic tail of APP may be subsequently tested for biological activity and used as therapeutics or as models for rational drug design.
Modulation in this context is defined as little as a 5% increase or decrease in transcriptional activation of the cytoplasmic tail of APP in the presence of the compound relative to the level of transcriptional activation in the absence of the compound. In preferred embodiments, the level of increase or decrease greater than 10% or more preferably greater than 20%.
Cells useful for the assays of the present invention include eukaryotic cells in which the cytoplasmic tail of APP can be translocated to the nucleus. Suitable cells include, for example, insect and mammalian cells. Preferred cells include Schneider, PC12, COS, HeLa and HEK293 cells.
Cells containing APP may be cells stably or transiently transfected with a construct encoding APP as described above using methods known to those of ordinary skill in the art. Constructs containing chimeric genes comprising a promoter operably linked to nucleic acid encoding an APP modified to include a module comprising the DNA-binding domains and transcriptional activation domain of the same or different transcription factor, are constructed using well-known recombinant DNA methods. These constructs are co-transfected into cells with the corresponding reporter constructs described above.
The transfected cells are contacted with the compound to be tested for its ability to modulate the transactivation mediated by the cytoplasmic tail of APP. A detectable increase or decrease in transcriptional activation of the reporter gene is indicative of a compound that modulates APP-mediated transcriptional activation. In one embodiment, the cells may be contacted with the candidate compound before expression of the modified APP is induced from an inducible promoter.
In a preferred embodiment of the present invention, human APP is modified to include Gal4-VP16 within the cytoplasmic tail. In particular, Gal4-VP16 is inserted between residues 651 and 652 of APP. The modified APP is generated by means of a mammalian expression plasmid containing a chimeric gene encoding residues 1-651 of APP, Gal4, VP16, and residues 652-695 of APP (i.e. the cytoplasmic tail of APP) under the control of a promoter (see Example #4). The plasmid may further comprise regulatory sequences, linkers, and other elements to facilitate cloning, replication, transfection and expression.
A cell comprising the modified APP is provided by transfecting a cell, preferably a mammalian cell, and most preferably a human cell, with the expression plasmid. The cell is co-transfected with a Gal4 reporter plasmid in which luciferase mRNA is driven by multiple copies of the Gal4 upstream activating sequence (UAS). When the modified APP is cleaved by γ-secretase, the cleavage product containing Gal4-VP16 enters the nucleus and activates transcription from the Gal4 reporter plasmid. The transfected cells are contacted with a candidate compound, and luciferase expression is measured in the presence and absence of the compound. Expression of luciferase is measured by standard assays, for example by measuring luciferase activity using a commercially available kit. Luciferase expression is a measure of transactivation. The compound that increases or decreases luciferase expression is the compound that modulates transcriptional activation.
In another preferred embodiment of the present invention, a cell, preferably a mammalian cell, and most preferably a human cell, is co-transfected with plasmids expressing the APP/Gal4-VP16 fusion protein, a Gal4 reporter plasmid in which luciferase mRNA is driven by multiple copies of the Gal4 upstream activating sequence (UAS), and a plasmid expressing one of the Mint proteins encoded by the nucleic acids of SEQ ID NOS:1-8, the nucleic acids encoding the polypeptides having the amino acid sequences of SEQ ID NOS:9-16, nucleic acids comprising the nucleic acids having the nucleotide sequences of SEQ ID NOS:1-8 or the nucleic acids encoding the polypeptides having the amino acid sequences of SEQ ID NOS:9-16, or nucleic acids substantially identical to the nucleic acids of SEQ ID NOS:1-8 or nucleic acids comprising nucleic acids substantially identical to the nucleic acids of SEQ ID NOS:1-8. The Mint protein expressed from one of these nucleic acids will interact with the cytoplasmic tail of the APP/Gal4-VP16 fusion protein, preventing it from activating transcription of the luciferase gene present on the reporter plasmid (see Example #7). Compounds which potentiate the modulatory affects of Mint on transcriptional activation may be identified by comparing the level of expression of the luciferase or any other suitable reporter gene from the reporter plasmid in the presence and absence of the compound. The compound that increases or decreases luciferase expression is the compound that potentiates the modulatory affects of Mint on transcriptional activation. Other reporter genes suitable for use in this assay are known to those of ordinary skill in the art.
In another embodiment, the present invention provides vectors that contain nucleic acids encoding the Mints proteins. In preferred embodiments, the vector comprises one of the nucleic acids having the nucleotide sequences of SEQ ID NOS:1-8 operably linked to expression control sequences, e.g. a promoter. In other preferred embodiments, the vector comprises a nucleic acid encoding one of the polypeptides having the amino acid sequences of SEQ ID NOS:9-16 operably linked to a promoter. In other embodiments, the vector comprises nucleic acids that are substantially identical to the nucleic acids of SEQ ID NOS:1-8 operably linked to a promoter, wherein substantial identity at the nucleotide level occurs when at least about 60% to 75% or preferably at least about 80% or most preferably at least about 90% of the nucleotides comprising the nucleic acid molecule are identical over a defined length of the molecule, and wherein the nucleic acids still encode peptides having the biological function of the polypeptides having the amino acid sequences of SEQ ID NOS:9-16.
In another embodiment, the present invention provides vectors that contain nucleic acids encoding the modified APP protein. In preferred embodiments, the vector comprises a nucleic acid encoding APP operably linked to a promoter wherein a nucleic acid module encoding a heterologous DNA binding domain of a transcription factor and a transcriptional activator of the same or a different transcription factor is contained within the portion of the nucleic acid that encodes the C-terminal cytoplasmic tail of APP. A module “within” the tail includes embodiments in which the module is at the 5′-end or 3′-end of the region encoding the cytoplasmic tail. In a preferred embodiment the module is Gal4-VP16. The vectors may further comprise regulatory sequences, linkers, and other elements to facilitate cloning, replication, transfection and expression.
The present invention further provides cells containing the foregoing vectors. The cells are eukaryotic, preferably mammalian, and most preferably human. Cells containing the vectors of the invention may be obtained by methods known in the art, and may be transiently or stably transfected. The cells may also further contain a corresponding reporter plasmid as described hereinabove.
The present invention further provides kits useful for identifying a compound that modulates transcriptional activation. The kits may comprise vectors encoding modified APP, modified Mint proteins, and the reporter gene. Alternatively, the kits may contain cells transformed by one or more of these vectors or cells suitable for transfection by these vectors and a means for transfecting these cells. The kits may also comprise a means for measuring expression of the reporter gene contained in the reporter plasmid.
The present invention also provides for transgenic knockout mice for Mint 1, Mint 2 and Mint 3. These animals may be useful for elucidating the pathophysiology of AD and for developing improved treatments for this disease.
The following nonlimiting examples serve to further illustrate the present invention.
EXAMPLES
Materials and Methods
Plasmid construction. 1. Gal4-containing transactivation plasmids. Most of the plasmids used for the transactivation experiments described herein were reported previously (Cao and Südhof, 2001, Science 293:115-120). All eukaryotic expression vectors containing Gal4 or Gal4/VP16 were based on pMst (Gal4) and pMst-GV (Gal4/VP16) which are derived from the SV40 promoter-based mammalian expression vector pM (Clontech; Cao and Südhof, 2001, Science 293:115-120). In addition to the previously described vectors, the following vectors were constructed: PMSt-GV-APPICF (APPICF-Gal4/VP16), generated by cloning the intracellular fragment of human APP695 (APPICF, residues 652-695) into the BamHI/SalI sites of pMst-GV; pMst-GV-APP (APP-Gal4/VP16), by cloning the extracellular and TMR fragments of human APP695 (APPe, residues 1-651) into the NheI site of pMst-GV-APPICF; pMst-GV-APPγ (APPγ-Gal4/VP16), by cloning residues 639-651 of human APP695 preceded by a methionine into the BglII/NheI sites of PMst-GV-APPICF. PMst-GV-APPICF* (APPICF*-Gal4/VP16), pMst-GV-APP* (APP*-Gal4/VP16), and pMst-GV-APPγ* (APPγ*-Gal4/VP16) were generated from their respective parent plasmid by site-specific mutagenesis replacing NPTY(684-687) with NATA(684-687) using the QUIKCHANGE® mutagenesis kit (Stratagene, La Jolla). pMst-GV-NRX (NRX-Gal4/VP16) was generated by cloning the intracellular fragment of rat Neurexin 1β (NRXICF, residues 414-468) into the BamHI/SalI sites of pMst-GV, followed by cloning the extracellular and TMR fragments (NRXe, residues 1-417) into the NheI site. pMst-GV-NA (NRXe-Gal4/VP16-APPICF) was generated by cloning NRXe into the NheI site of pMst-GV-APPICF, and pMst-GV-AN (APPe-Gal4/VP16-NRXICF) by cloning the intracellular fragment of Neurexin 1β (NRXICF) into the BamHI/SalI-sites of pMst-GV, followed by cloning of APPe into the NheI site. 2. Mint plasmids. The eukaryotic pCMV5 expression vectors for full-length rat Mints 1, 2 and 3 have been described previously (Okamoto and Südhof, 1997, J Biol Chem 272:31459-31464; Okamoto and Südhof, 1998, Eur J Cell Biol 77:161-165). Mutations were inserted in these parent vectors by site-specific mutagenesis as described above. pCMV-Mint 1-PDZ1* was constructed by mutating GV(670,671) to AA(670,671) in the first PDZ domain's carboxylate binding loop, and pCMV-Mint 1PDZ2* was created by mutating GF(762,763) to AA(762,763) in the carboxylate binding loop of the second PDZ domain. pCMV-Mint 1-ΔPDZ was generated by introducing a stop codon after residue 659 in pCMV-Mint 1. A hydrophobic pocket of the PTB domain of Mint 1 was altered at positions YQEF(613,616) to SQES(613,616) to generate the pCMV-Mint 1-PTB* construct. pCMVmyc-Mint 1 was generated by cloning the full-length Mint 1 coding sequence from pEGFP-Mint 1 (a gift from Dr. Anton Maximov, UT Southwestern, Dallas) into the EcoRI/KpnI sites of pCMVmyc. To construct pCMVmyc-Mint 1-ΔNterm, the C-terminal part of Mint 1 encoding residues 451-839 was cloned into the KpnI/XbaI sites of pCMVmyc. pCMV-Mint 2-ΔPDZ was generated by introducing a stop codon after residue 570 in pCMV-Mint 2, and pCMV-Mint 3-ΔPDZ by introducing a stop codon after residue 391 in pCMV-Mint 3. 3. Other plasmids. The eukaryotic expression vectors for CASK (Hata et al., 1996, J Neurosci 16: 2488-2494) and Fe65 (Cao and Südhof, 2001, Science 293:115-120) were described previously.
Antibodies. Some antibodies were described previously (Biederer and Südhof, 2000, J Biol Chem 275:39803-39806; Biederer and Südhof, 2001, J Biol Chem 276: 47869-47876; Cao and Südhof, 2001, Science 293:115-120). The APP antibody used was a polyclonal rabbit serum (U955) raised against the cytosolic, extreme C-terminal 15 residues of APP coupled to keyhole-limpet hemocyanin. Monoclonal antibodies to Mints 1, 2 and 3, TGN 38, and EEA1 were obtained from Transduction Laboratories (Lexington Ky.); monoclonal antibodies to calnexin were obtained from Chemicon (Temecula Calif.); and monoclonal antibodies to Golgi 58K protein were obtained from Sigma (St. Louis Mo.). Polyclonal Mint 1 antibodies were described previously (P932; Okamoto and Südhof, 1997, J Biol Chem 272:31459-31464), as were antibodies against Velis (T813; Butz et al., 1998, Cell 94:773-782). Polyclonal anti-myc antibodies were obtained from Santa Cruz Biotechnology (Santa Cruz Calif.), and polyclonal antibodies directed against Mint 3 were obtained from Affinity Bioreagents (Denver Colo.). For all Mint antibodies, specificity was confirmed using preparations from COS cells transfected with expression vectors for Mints 1-3 (data not shown).
Biochemical preparations. All steps were carried out on ice or at 4° C. Rat forebrains (Pel Freez, Rogers AK) were homogenized in a pestle tissue grinder using a slow speed stirrer at a tissue:buffer ratio of 10% (w/v) in buffer RMP (20 mM Hepes-KOH (pH 7.4), 125 mM K-acetate, 5 mM MgCl2, 320 mM sucrose) adjusted to 1.0% TRITON® X-100 in the presence of protease inhibitors. For preparation of membrane proteins, rat forebrains were homogenized in buffer RMP, the samples were centrifuged in an Eppendorf (Hamburg, Germany) microcentrifuge at 600 g for 10 mm to obtain the postnuclear supernatant, and membranes were pelleted in a Sorvall (Kendro Laboratory Products, Newtown Conn.) S80-AT3 rotor at 280,000 g for 20 mm. The membrane pellet was extracted in buffer RMP adjusted to 1.0% TRITON® X-100 using a pestle tissue grinder, and centrifuged again at 280,000 g for 20 mm to yield the solubilized membrane proteins.
Peptide bead affinity chromatography. Peptides were synthesized on an ABI (Applied Biosystems, Foster City Calif.) synthesizer with an added N-terminal cysteine for coupling to SulfoLink® Beads (a 6% cross-linked beaded agarose matrix that has been derivatized with a 12-atom spacer arm that ends in an iodoacetyl group; Pierce, Rockford Ill.) according to the manufacturer's instructions at 1.0 mg peptide/ml beads. For binding to the APP NPTY motif, a peptide corresponding to the APP-derived sequence CGYENPTYKFFEQMQN (human APP, residues 398-412), was immobilized on SulfoLink® (a 6% cross-linked beaded agarose matrix that has been derivatized with a 12-atom spacer arm that ends in an iodoacetyl group; Pierce, Rockford Ill.) beads. For binding to Presenilin C-terminal sequences, peptides corresponding to the extreme C-termini of human Presenilin 1 (sequence CMDQLAFHQFYI; SEQ ID NO:18), human Presenilin 2 (sequence CMDTLASHQLYI; SEQ ID NO:19), or Drosophila Presenilin (sequence CMEDLSAKQVFI; SEQ ID NO:20) were immobilized. As negative controls, peptides corresponding to the extreme C-terminus of human HPV2 (poliovirus receptor-related protein 2; sequence CGSLISRRAVYV; SEQ ID NO:21) and a peptide derived from the gp41 glycoprotein (sequence CWFSITNWLWYI; SEQ ID NO:22) were utilized. Extracts of transfected eukaryotic cells or proteins solubilized from rat brain were incubated with 20 μg peptide immobilized on SulfoLink® beads for 12-16 h at 4° C. under mild agitation. Binding was performed in buffer RMP adjusted to 1.0% TRITON® X-100 and 600 mM potassium acetate. For APP competition experiments, the soluble peptides QNGYENPTYKFFEQ (SEQ ID NO:23) or QNGYENATAKFFEQ (SEQ ID NO:24), corresponding to the native or mutated APP NPTY (SEQ ID NO:17) motif, were added during the binding incubation at the concentrations of 0.1 mg/ml and 1.0 mg/ml. Bound proteins were eluted with 2% SDS.
Immunoprecipitations. HEK293 were co-transfected for APP and the individual Mints 1, 2 or 3, respectively, and after 2 days collected in IP buffer (25 mM Hepes-KOH (pH 7.4), 125 mM K-acetate, 5 mM MgCl2, 1.0% IGEPAL CA-630, Sigma), 10% glycerol) in the presence of protease inhibitors. After passing the cell suspension through a 28 gauge syringe, the lysate was centrifuged in an Eppendorf microcentrifuge at 21,000 g for 20 min and the detergent-extracted material was subjected for 2 h to immunoprecipitation using antibodies directed against APP (U955) or the respective pre-immune serum.
Miscellaneous biochemical procedures. SDS-PAGE and immunoblotting was performed as described (Laemmli, 1970, Nature 277:680-685; Towbin et al., 1979, Proc Natl Acad Sci USA 76:4350-4354). For standard immunodetection on Western blots, enhanced chemiluminiscence (ECL; Amersham) was applied. Quantitative immunoblotting was performed using radiolabeled 125I secondary antibodies (Amersham Biosciences, Piscataway N.J.), and the signals were quantitated on a PhosphorImager (Molecular Dynamics, Sunnyvale Calif.) using ImageQuant software. To determine levels of expressed Mint 1 protein, signals of cell lysates were compared to those from known amounts of purified GST-Mint 1. Protein concentrations were determined using the BCA protein assay (Pierce, Rockford Ill.).
Immunocytochemistry. 1. Immunoperoxidase staining: Adult mice were perfusion-fixed in 4% paraformaldehyde in phosphate-buffered saline (PBS) pH 7.4. 35 μm vibratome sections from brain were blocked in 10% normal goat serum containing 0.1% TRITON® X-100 (octyl phenol ethoxylate) for one hour, incubated with the various primary antibodies overnight at 4° C. and with the biotinylated secondary antibody for 1 hr. Sections were processed using a VECTASTAIN® ABC Elite Kit, which contain avidin DH and biotinylated enzyme (Vector, Burlingame, Calif.) according to the manufacturer's instruction. The final immunosignal was developed using 3′3-diaminobenzidine tetrahydrochloride (DAB). For immunofluorescence labeling, primary hippocampal cells on cover slips were fixed in situ for 10 min with absolute methanol at −20° C., permeabilized in 0.1% saponin/PBS, and blocked in 3% milk/PBS. The primary incubation was carried out in blocking buffer for one hour at room temperature. After washing with PBS, the cells were incubated with goat anti-rabbit or goat anti-mouse secondary antibodies that were coupled with ALEXA FLUOR® 488 and ALEXA FLUOR® 546 (fluorescent dyes; Molecular Probes, Eugene Oreg.). Labeled cells were viewed with a Leica (Leica Microsystems, Bannockburn Ill.) TCS SP2 confocal microscope or a Zeiss (Carl Zeiss Optical Inc., Chester Va.) fluorescent microscope with a Hamamatsu (Bridgewater N.J.) ORCA-100 digital camera. Final images were processed by METAMORPH® (an integrated software and hardware system for the capture and analysis of microscopic or macroscopic images from scientific grade digital and video CCD cameras; Universal Imaging, Downingtown Pa.) and ADOBE® PHOTOSHOP® imaging analysis software.
Transfections were performed at 50-80% confluency in 6-well plates using FUGENE6TM transfection reagent (Roche, Basel, Switzerland).
Transactivation assays. PC12, COS, HeLa, and HEK293 cells were co-transfected with three or four plasmids: a. pG5E1B-luc (HEK293 cells, HeLa cells, and COS cells=0.2-0.5 μg DNA; PC12 cells=1.0 μg); b. pCMV-LacZ (HEK293 cells, HeLa cells, and COS cells=0.05 μg DNA; PC12 cells=0.5 μg DNA); c. pMst (Gal4), pMst-GV-APP (APP-GV), pMst-GV (GV), pMst-GV-APPct (APPct-GV), pMst-APPct (APPct-Gal4), pMst-GV-APP* (APP*-GV), pMst-GV-APPct* (APPct*-GV), pMst-APPct* (APPct* -Gal4), pMst-GV-APP (APP-GV), pMst-GV-NRX (NRX-GV), pMst-G V-NA (NRXe-GV-APPc), pMst-GV-AN (APPe-GV-NRXc)(HEK293, HeLa, and COS cells=0.1-0.3 μg DNA; PC12 cells 1.0 μg DNA). Where indicated, a fourth plasmid was co-transfected: pcDNA3.1-PS2D366A (kind gift of Dr. C. Haass, Munich); pCMV-Mint1; pCMV-Mint 1-PDZ1*; pCMV-Mint 1-PDZ2*; pCMV-Mint 1-PTB*; pCMVmyc-Mint 1; pCMVmyc-Mint 1-ΔNterm; pCMV-Mint 1-ΔPDZ; pCMV-Mint 2; pCMV-Mint 2-ΔPDZ; pCMV-Mint 3; pCMV-Mint 3-ΔPDZ; pCMV5-Fe65; pCMV-CASK (HEK293, HeLa, and COS cells=0.1-0.3 g DNA or as specified in the discussion of each individual experiment; PC12 cells=0.5 g DNA). For negative controls, the expression vector pCMV5 was used without insert. Cells were harvested 48 hr post-transfection in 0.2 ml/well reporter lysis buffer (Promega, Madison Wis.), and their luciferase and β-galactosidase activities were determined with the Promega luciferase assay kit using a chemiluminescence reader (LUCY2, Anthos Labtec, Austria) and the standard O-nitrophenyl-D-galacto-pyranoside (Sigma, St. Louis Mo.) method, respectively. The luciferase activity was standardized by the β-galactosidase activity to control for transfection efficiency and general effects on transcription, and further normalized for the transactivation observed in cells expressing Gal4 alone where indicated. Values shown are averages of transactivation assays carried out in duplicate, and repeated at least three times for each cell type and constructs. All constructs were assayed in three or four cell lines, but usually only representative results for one cell line are shown.
Example 1
Comparison of the binding of Mints/X11 1, 2, and 3 to APP. Previous results have separately examined the ability of Mints to bind to APP or to presenilins, and to stabilize APP in co-transfected cultured cells (Borg et al., 1996, Mol Cell Biol 16:6229-6241; McLoughlin and Miller, 1996, FEBS Lett 397:197-200; Zhang et al., 1997, EMBO J. 16: 6141-6150; Borg et al., 1998a, J Biol Chem 273:14761-14766; Sastre et al., 1998, J Biol Chem 273:22351-22357; Lau et al., 2000, Mol Cell Neurosci 16:557-565; Okamoto et al., 2001, Eur J Neurosci 12:3067-3072). These experiments established a potentially important connection between Mints, presenilins, and APP, but the relative activities of the three Mints and the generality of these putative targets were not analyzed.
As an initial step towards understanding the common versus unique properties of Mint isoforms, a comparison of the binding of different Mints to APP and presenilin, and their effect on APP in transfected cells, was performed. Since these and subsequent experiments critically depended on the specificity of the Mint antibodies used, a study validating the specificity of these antibodies was first performed using transfected COS cells that express individual Mints. As shown in FIG. 1, antibodies used for the respective Mint isoforms bound specifically to protein extracts prepared from COS cells transfected with a control vector (lane 1) or Mint 1-3 expression vectors (lanes 2-4). Immunoblotting was performed using monoclonal antibodies against Mints 1 and 2, and polyclonal antibodies against Mint 3. Fractions were also analyzed by inmmunoblotting for the negative control proteins Rab GDP-dissociation inhibitor protein (GDI), a soluble protein, and for synaptophysin 1 (Syp), a membrane protein of synaptic vesicles. This study confirmed the mono-specificity of the antibodies, which could thus be applied to detect each Mint isoform separately in complex solutions containing multiple Mints, such as brain homogenates.
Affinity chromatography of rat brain proteins on the immobilized cytoplasmic tail of APP was then used to examine the binding of endogenous Mints to APP. Proteins from rat forebrain homogenates (lane 5) were bound to an immobilized cytoplasmic peptide derived from APP in the absence of added soluble peptide (lane 6), or in the presence of 0.1 or 1.0 mg/ml of a soluble peptide containing the wild-type sequence of the APP cytoplasmic tail (lanes 7 and 8), or of a APP tail peptide point-mutated in the NPTY binding sequence for Mints (lane 9). As a further control, binding of Mints to a control column with an immobilized peptide derived from gp41 was examined (lane 10). Binding was performed at 600 mM salt. The samples tested in lanes 6-10 are proteins bound to beads after the respective binding and peptide competition experiments. Recovery of the three Mints from the brain homogenates was quantitated with 125I-labeled secondary antibodies, as shown in FIG. 1A, lanes 5-10. All three Mints tightly bound to APP (FIG. 1A, lane 6), and binding was inhibited by high concentrations of the corresponding wild-type but not mutant APP tail peptide (FIG. 1A, lanes 7-9). As shown in FIG. 1B, quantitation revealed that Mints from the brain homogenate were recovered efficiently on the affinity column with yields of 25% to 90%; for example, the immobilized APP tail extracted almost all of the brain Mint 3, and even 1 mg/ml of competing peptide was unable to completely block it from binding to the column. In contrast, only 25% of brain Mint 1 were bound. No Mint binding to a control peptide was observed (FIG. 1A, lane 10).
To test if Mints interact with APP in vivo, the levels of the transfected APP were measured by quantitative immunoblotting using 125-labeled secondary antibodies (FIGS. 2A and 2B). As a control, the gene for Fe65, a protein that also binds to the cytoplasmic tail of APP (Fiore et al., 1995, J Biol Chem 270:30853-30856), was transfected. Under control conditions (lane 1), little APP695 is expressed in the cells because the predominant splice variants in peripheral tissues include the Kunitz domain (Kitaguchi et al., 1988, Nature 331:530-532; Tanzi et al., 1988, Nature 331:528-30). The migration position of the endogenous APP containing the Kunitz domain and the transfected APP695 lacking this domain are indicated on the right of the panel. The quantities of loaded lysates were normalized to equal amounts of transfected cells based on the activity of co-transfected β-galactosidase.
All three Mints dramatically increased APP levels (up to three-fold; FIG. 2, lanes 3-5), extending previous observations that Mint 1 stabilizes transfected APP and increases its steady-state levels (Borg et al., 1998a, J Biol Chem 273:14761-14766; Sastre et al., 1998, J Biol Chem 273:22351-22357). As a control, Fe65 (which also binds to APP) slightly decreased the steady-state levels of transfected APP, although this was not necessarily a specific effect since co-transfection of any protein usually decreases expression because it dilutes the transcription/translation machinery. The stabilization of APP in the transfected cells could be due to a direct or indirect interaction of Mints with APP. The levels of endogenous APP do not change significantly because most of the cells are not transfected, and thus are not exposed to the transfected Mints.
Although the affinity chromatography experiments already suggested a direct interaction (FIG. 1), this question was further examined by the co-immunoprecipitation of Mints with APP from the transfected cells (FIG. 2C). In these studies, APP695 and Mints 1, 2, or 3 were co-expressed in transfected HEK293 cells (“Start”; lane 1). Cellular proteins were then immunoprecipitated with an antibody to the C-terminus of APP (“IP APP”; lane 2), or a control antibody (“IP Control”; lane 3). Samples were analyzed by immunoblotting with monoclonal antibodies to the indicated proteins. The double band for Mint 2 is probably due to a hypersensitive proteolytic site in the N-terminus of Mint 2. For detection of APP, the quantity loaded in lanes 2 and 3 was 5-fold less than for detection of Mints. Indeed, Mints 1, 2 and 3 were co-immunoprecipitated with antibodies to APP, but not with control antibodies (FIG. 2C), supporting the notion that Mints directly bind to APP.
Example 2 
Binding of Mints to presenilins. The potential interaction of Mints with presenilins was examined using the same affinity chromatography approach employed for APP binding. These studies showed that all three Mints bound to the cytoplasmic C-terminal sequence of presenilin 1 (FIG. 3A and data not shown), in agreement with previous observations (Lau et al., 2000, Mol Cell Neurosci 16:557-565). Binding was specific because Mints were not bound to control beads, and control proteins such as GDI and synaptophysin were not retained on the presenilin 1 column (data not shown). However, quantitations revealed that presenilin binding differed among Mints in (FIG. 3B, and data not shown). 14-16% of endogenous Mints 1 and 3 from brain were recovered on the presenilin column, but only 3% of Mint 2 was bound.
Mints bind to the cytoplasmic tail of APP via an interaction of the Mint PTB domain with the NPTY sequence in APP (Borg et al., 1996, Mol Cell Biol 16:6229-6241; McLoughlin and Miller, 1996, FEBS Lett 397:197-200; Zhang et al., 1997, EMBO J. 16: 6141-6150). Presenilin 1 does not include an NPxY sequence but feature a C-terminal sequence that resembles that of neurexins which bind to the PDZ domains of Mints (Biederer and Südhof, 2000, J Biol Chem. 275:39803-39806), suggesting that the binding of Mints to presenilin 1 may be mediated by one or both Mint PDZ domains.
To test if other PDZ domain proteins also bind to the C-terminal sequence of presenilin 1, the presenilin binding of PSD95, an abundant component of the postsynaptic density that contains three PDZ domains (Cho et al., 1992, Neuron 9: 929-942; Kistner et al., 1993, J Biol Chem 268: 4580-4583), and of Velis, a family of three proteins that contain a single PDZ domain (Butz et al., 1998, Cell 94:773-782), was examined. Both PDZ domain proteins were bound; whereas binding of PSD95 was weak, Velis were captured to the same extent as Mints 1 and 3 (data not shown).
The fact that multiple unrelated PDZ domain proteins bind to the C-terminal sequence of presenilin 1, but various Mints exhibit large differences in binding, raised concerns about the specificity of the interaction of Mints with presenilins, prompting a further examination of Mints binding specificity to presenilin.
The C-terminal presenilin 1 sequence is conserved in vertebrate presenilin 2 and in Drosophila presenilin. C-terminal peptides from all of these presenilins captured Mint 1, whereas control peptides did not, suggesting that all of these presenilins potentially interact with PDZ-domain proteins (FIG. 3B).
To identify which of the two PDZ domains in Mints binds to presenilins, we mutated the PDZ1 and PDZ2 domains of Mint 1 separately or together, or deleted them both. Presenilin binding assays with wild-type and mutant Mint proteins produced in transfected COS cells revealed that mutations in each of the two Mint 1 PDZ domains resulted in an approximately two-fold decrease in binding (FIG. 3C). Mutations in both PDZ domains or deletion of both PDZ domains almost completely abolished binding. Together these data suggest that each of the two Mint PDZ domains individually binds to presenilins in vitro.
Example 3 
Localization of endogenous Mints in neurons. In vertebrates, Mints 1 and 2 are only detectable in brain, whereas Mint 3 is ubiquitously expressed (Okamoto and Südhof, 1997, J Biol Chem 272:31459-31464; Okamoto and Südhof, 1998, Eur J Cell Biol 77:161-165). To study the localization of Mints, rat brain sections were stained with antibodies to Mints 1 and 2. Mint 3 could not be investigated because the available antibodies were not suitable for immunocytochemistry.
Abundant labeling of the neuronal cell bodies, with less staining of the dendrites, was observed (FIG. 4, A, B, D, and E; see also McLoughlin et al., 1999, Neurosci 11:1988-1994). APP exhibited a very similar distribution (FIG. 4, C and F). Notably, nuclei were not labeled. Staining throughout the neuropil was detected that was weaker than the cell body staining, indicating that low levels of Mints may be present at synapses as suggested by Okamoto and colleagues. See Okamoto et al., 2000, Eur J Neurosci 12:3067-3072. FIG. 4, D, E and F are enlarged images of CA3 regions from FIG. 4, A, B and C. Antibodies against Mint 1 were polyclonal, and monoclonal against Mint 2. The polyclonal APP antibody used is directed against a C-terminal epitope in the cytoplasmic APP tail which is liberated upon γ-cleavage. Scale bars in FIG. 4C (for FIG. 4, A-C)=0.5 mm; in FIG. 4F (for FIG. 4, D-F)=0.1 mm.
To examine the subcellular distribution of Mints, immunofluorescence staining of cultured hippocampal neurons was performed (FIGS. 5 and 6). Mints 1 and 2 were predominantly localized in a perinuclear compartment where they almost completely overlapped (FIG. 5, A-C). Double labeling with antibodies to synapsins as a presynaptic marker failed to detect high levels of Mints in synapses (FIG. 5, D-F, and data not shown). APP was largely co-localized with Mints (FIG. 5, G-L). Compared to Mints, more APP appeared to be present in neurites, suggesting that at steady state, Mints are more concentrated in the perinuclear compartments than APP.
In the next set of experiments, cultured hippocampal neurons were double-labeled with antibodies to marker proteins to identify the perinuclear compartment containing Mints (FIG. 6). These studies demonstrated that Mints co-localize best with TGN38 (FIG. 6, A-C, and data not shown), a marker of the trans-Golgi complex (Luzio et al., 1990, Biochem J 270:97-102). A similar localization, but not quite as precise, was observed with the 58K Golgi protein (FIG. 6, D-F, and data not shown), a peripheral membrane protein that is enriched in, but also found outside of, the trans-Golgi complex (Bloom and Brashear, 1989, J Biol Chem 264:16083-16092).
By contrast, double labeling of neurons with antibodies to Mint 1 or 2 and the endoplasmic reticulum protein calnexin (Wada et al., 1991, J Biol Chem 266:19599-19610) failed to detect an overlap in localization (panels G-I). Similarly, the early endosomal protein EEA1 (Mu et al., 1995, J Biol Chem 270:13503-13511) exhibited a different staining pattern (panels J-L). Together these data support the conclusion that in mature neurons in situ (FIG. 4) and in culture (FIGS. 5 and 6), Mints 1 and 2 are co-localized in the trans-Golgi complex, a localization consistent with a role in APP trafficking (Borg et al., 1998a, J Biol Chem 273:14761-14766; Sastre et al., 1998, J Biol Chem 273:22351-22357) and in directing proteins out of the trans-Golgi apparatus towards defined plasma membrane domains (Rongo et al., 1998, Cell 94:751-759; Whitfield et al., 1999, Mol Biol Cell 10:2087-2100). Thus at steady state, Mints exhibit a localization that is similar to that of APP but notably distinct from that of either CASK or Munc18-1.
Example 4 
A transactivation assay of APP cleavage. A function for the cytoplasmic tail of APP in activating transcription has recently been described, wherein this protein forms a protein complex with Fe65 (Cao and Südhof, 2001, Science 293:115-120). The transcriptional activation observed in these assays could potentially be used to measure APP cleavage, but depends on Fe65 which binds to the same NPTY sequence of APP as Mints (which, however, do not activate transcription; Cao and Südhof, 2001, Science 293:115-120).
In order to test the potential function of Mints in APP cleavage and signaling, a variant of the assay was employed that allows monitoring APP cleavage independent of Fe65 binding. For this purpose, both Gal4 and VP16 were introduced into the cytoplasmic tail of APP695 at the cytoplasmic boundary of the TMR. This assay differs from the original assay (Cao and Südhof, 2001, Science 293:115-120) in that the powerful viral transcriptional activator VP16 (Sadowski et al., 1988, Nature 335:563-564) is introduced together with the yeast DNA binding protein Gal4 into APP. Thus transactivation by APP-Gal4/VP16 is independent of the binding of cellular transcriptional activators, but can only occur after APP is cleaved by γ-secretase and the released cytoplasmic tail fragment moves into the nucleus.
DNA encoding the APP-Gal4/VP16 fusion protein was transfected into PC12, HEK293, COS, or HeLa cells, and transactivation of transcription from a co-transfected Gal4-dependent reporter plasmid encoding luciferase was measured. As a negative control, Gal4 was used alone without VP16 or APP (see Cao and Südhof, 2001, Science 293:115-120), and as a positive control, Gal4/VP16 without APP was used. In all experiments, cells were co-transfected with a constitutive β-galactosidase expression vector in order to control for transfection efficiency and to rule out direct effects of transfected proteins on transcription. Furthermore, in all cases expression of transfected proteins was verified by immunoblotting.
The bar diagram in FIG. 7A shows results from Gal4-transactivation assays in PC12 cells that were co-transfected with a Gal4-dependent luciferase reporter plasmid (to measure transactivation using luciferase expression), a β-galactosidase control plasmid (to normalize for transfection efficiency), and the test plasmids identified by numbers below the bars. The domain structures of the proteins encoded by the test plasmids are shown schematically in FIG. 7B (APPe=extracellular sequences of APP; APPICF=cytoplasmic sequences of APP). Constructs marked with an asterisk (APP*-GV, APPICF*-GV, and APPICF*-Gal4) contain a point mutation in which the NPTY sequence in the cytoplasmic tail of APP is replaced by NATA. Transfected cells were harvested two days after transfection, luciferase and β-galactosidase activities were determined, and the luciferase activity was normalized for the β-galactosidase activity to control for transfection efficiency as described in the Materials and Methods. The β-galactosidase-normalized luciferase activity is expressed in relation to the activity of cells co-transfected with Gal4 alone. Data shown are from a representative experiment repeated multiple times in PC12 cells and in COS, HEK293, and HeLa cells with similar results. Abbreviations: GV, Gal4/VP16 module; APPICF, intracellular fragment of APP; APPγ, γ-secretase cleavage product of APP.
In all cell types tested, full-length APP-Gal4//VP16 (APP-GV) transactivated Gal4-dependent transcription almost as strongly as Gal4/VP16 alone ( ̃500-2,000 fold activation over Gal4 alone, depending on cell type), suggesting that cleavage of APP to release the intracellular fragment was not the major rate-limiting step (FIG. 7A, Constructs 1-3). A chimeric protein in which Gal4/VP16 was fused to the isolated cytoplasmic tail of APP (APPICF-GV) was an even more potent transactivator than Gal4/VP16 alone or full-length APP-Gal4/VP16 ( ̃4,000 vs. ̃500-2,000 fold activation; FIG. 7A, Construct 5). Addition of the 12 hydrophobic residues from the TMR that are present in the initial γ-cleavage product had no inhibitory effect on transactivation, but induced a moderate stimulation (FIG. 7A, Construct 6). In contrast, the cytoplasmic APP tail containing only Gal4 without VP16 (APPICF-Gal4) was inactive when Fe65 was not co-transfected (<5 fold activation; FIG. 7A Construct 7; Cao and Südhof, 2001, Science 293:115-120). Cleavage of APP does not appear to require binding of endogenous cellular proteins to the NPTY tail sequence (residues 684-687 of APP695) since mutation of NPTY to NATA had no effect on transactivation. Specifically, no effect of this mutation was observed with either full-length APP or the cytoplasmic APP tail fused to Gal4/VP16 (FIG. 7A, Constructs 4 and 7).
Example 5 
APP sequences required for cleavage. The assay described above utilizes transfected cells that overexpress the respective test proteins, raising the concern that transactivation by APP-Gal4/VP16 may be caused by non-specific proteolysis of the APP-Gal4/VP16 fusion protein instead of specific α-/β- and γ-cleavage. To address this concern, assays were performed to examine determine if specific sequences of APP were required for transactivation by the embedded Gal4/VP16 module (FIG. 8).
The bar diagram in FIG. 8A shows results of Gal4/VP16-transactivation assays obtained with the constructs that are schematically displayed and identified by numbers below the diagrams. APP-Gal4/VP16 proteins that contain all APP sequences (construct 3) or lack the intracellular (construct 4) or the extracellular and transmembrane sequences (construct 5) were analyzed as described in the previous study above. Gal4 (construct 1) and Gal4/VP16 (construct 2) were used as standardization controls to establish the background and maximal response, respectively. Transactivation was measured by constructs in which Gal4/VP16 is placed in all possible combinations into the context of the extra- and intracellular sequences of APP (APPe and APPICF=extracellular and cytoplasmic sequences of APP, respectively) or neurexin 1β (NRXe and NRXICF=extracellular and cytoplasmic sequences of neurexin, respectively). Gal4/VP16 constructs were transfected without (−) and with (+) the dominant negative presenilin 2 expression vector. All bar diagrams exhibit representative experiments in the cell types identified in A. Experiments were carried out with test plasmids co-transfected with a Gal4 luciferase reporter plasmid and a β-galactosidase control plasmid as described in the previous experiment. In A, transactivation as measured by β-galactosidase-normalized luciferase activity is expressed relative to the activity of Gal4 alone, whereas in B, transactivation as measured by luciferase activity is shown in arbitrary units only normalized to β-galactosidase activities.
In these studies, the cytoplasmic tail of APP was first removed to generate a “tailless” APP-Gal4/VP16 fusion protein in which the extracellular sequences and the TMR of APP were linked to intracellular Gal4/VP16 followed by a stop codon (APPe-GV). The tailless APP-Gal4/VP16 protein was almost completely inactive in transactivation assays compared to either Gal4/VP16 alone or to APP-Gal4/VP16 or to APPγ-GV which represents the initial γ-secretase cleavage product (FIG. 8A, Constructs 2-5). This suggests that Gal4/VP16 is not released from APP-Gal4/VP16 by non-specific degradation, and that the tail of APP is required either for recognition by the APP cleavage enzymes, or for trafficking of APP to the cleavage compartments.
To differentiate between these two possibilities, we inserted Gal4/VP16 into the cytoplasmic tail of neurexin 1β (NRX-GV). Neurexin 1β is expressed on the neuronal cell-surface similar to APP, but is not known to be processed by proteolytic cleavage (Ushkaryov et al., 1992, Science 257:50-56). Neurexin 1β-Gal4/VP16 was nearly inactive in transactivation assays in contrast to APP-Gal4/VP16 (FIG. 8B, Construct 3 vs. Construct 6), suggesting that Gal4/VP16-mediated transactivation requires specific sequences in the APP molecule. To identify these sequences, chimeric Gal4/VP16-fusion proteins containing either extracellular APP- and intracellular neurexin 1β-sequences, or vice versa, were constructed. A fusion protein composed of the extracellular sequences and TMR of APP coupled to intracellular Gal4/VP16 and the cytoplasmic tail of neurexin 1 (APPe-GV-NRXICF) strongly transactivated transcription (FIG. 8B, Construct 8). In contrast, the reverse fusion protein of the extracellular neurexin sequences and the neurexin TMR with the cytoplasmic APP sequences (NRXe-GV-APPICF) was inactive (FIG. 8B, Construct 7). These experiments demonstrate that the extracellular sequences of APP are essential for proper cleavage, in agreement with results of Struhl and Adachi, 2000, Mol Cell 6:625-36.
Although the intracellular sequences of APP are also essential for its processing (see “tailless” mutant, FIG. 8A), they can be functionally replaced by the intracellular fragment of a plasma membrane protein like neurexin that exhibits no sequence similarity with APP, and in particular does not contain an NPxY sequence (Ushkaryov et al., 1992, Science 257:50-56). This may indicate a role of the APP tail in trafficking to a cleavage compartment at the plasma membrane, or derived from the plasma membrane.
Finally, to test if presenilins are involved in transactivation by APP-Gal4/VP16, a plasmid encoding a dominant negative mutant of presenilin 2 (Steiner et al., 1999, J Biol Chem 274:28669-28673) was co-transfected along with a plasmid encoding APP-Gal4/VP16 (FIG. 8B). Transactivation of Gal4-dependent transcription by full-length APP-Gal4/VP16 was inhibited by the presenilin 2 mutant, whereas the small amount of residual transactivation observed with neurexin 1β-Gal4/VP16 was insensitive to presenilin 2 (FIG. 8B, Construct 3 vs. Construct 6). As expected, mutant presenilin 2 also potently inhibited transactivation by the fusion protein of the extracellular domain of APP with intracellular neurexin sequences (FIG. 8B, Construct 8), but had no effect on the residual transactivation observed with the reverse fusion protein containing the extracellular domain of neurexin 1β coupled to the cytoplasmic tail of APP (FIG. 8B, Construct 7).
Example 6 
Mint 1 inhibits transactivation mediated by APP-Gal4/VP16. The transactivation assay was next used to study the effects of Mints on APP. A constant amount of wild-type or mutant APP-Gal4/VP16 plasmid (100 ng DNA/well) was co-transfected with the indicated amounts of Mint 1 expression vector into HEK293 cells. Transactivation and Mints levels in the cells were quantified in the same samples as described in the Materials and Methods. Co-transfection of Mint 1 strongly inhibited transactivation mediated by APP-Gal4/VP16 (FIG. 9A). This inhibition was abolished by mutation of the NPTY sequence in the cytoplasmic tail of APP-Gal4/VP16, consistent with the notion that direct binding of Mint 1 to APP is required.
Comparison of the activity of the three Mint isoforms in the transactivation assay demonstrated that Mints 1 and 2 potently inhibited transactivation by APP-Gal4/VP16, whereas Mint 3 had no significant effect (FIG. 9B). In these studies, Mint amounts in transactivation assay samples were quantified on immunoblots and are expressed as a fraction of the amount observed with the maximal amount of DNA transfected to control for the non-linearity of the relation between transfected DNA and expressed protein. Immunoblotting confirmed that all three Mints were expressed at high levels in the co-transfected cells (FIG. 9C).
Since all three Mints bind to APP in vitro and in transfected cells (FIGS. 1 and 2), the selective inability of Mint 3 to inhibit transactivation was surprising. To gain insight into the mechanism by which Mint 1 inhibits APP-Gal4/VP16 mediated transactivation, and to understand why Mint 3 has no effect, a series of Mint mutants were examined. In these studies, HEK293 cells were co-transfected with a constant amount of APP-Gal4/VP16 expression vector and reporter plasmids, and increasing amounts of expression vectors expressing wild-type Mint 1 or mutants of Mint 1 carrying point mutations in the first or second PDZ domains (Mint 1 PDZ1* or PDZ2*, respectively) or lacking both C-terminal PDZ domains (Mint 1 ΔPDZ). Transactivation and Mint 1 amounts in the cells were then quantified in the same samples as described in the Materials and Methods. All transactivation levels in the experiments reported in FIGS. 10A and 10B are normalized for the amount of transactivation observed under control conditions.
These studies showed that an inactivating point mutation in the first PDZ domain of Mint 1 or deletion of both PDZ domains that were studied above in the presenilin-binding experiments (FIG. 3) dramatically increased the inhibition of transactivation by Mint 1 (FIG. 10A). In contrast, the second PDZ domain point mutation did not alter the inhibitory effect of Mint 1.
Since the various Mint 1 mutants exhibited different expression levels, the relative amounts of expressed protein were quantified using 125I-labeled secondary antibodies. These quantitations showed that the PDZ-domain deletion mutant was approximately 10 times more potent than wild-type Mint 1 (FIG. 10A). A Mint 3 mutant with a PDZ-domain deletion was also tested; this mutant was completely inactive in the assay similar to wild-type Mint 3 (FIGS. 10B, C). Control transfections showed that the various Mint 1 proteins did not inhibit general transcription, but specifically impaired transactivation by APP-Gal4/VP16 (data not shown).
The results of FIG. 10 suggest that Mint 1 binding to APP may couple it to another protein which binds to the first PDZ domain, implying among others that the two PDZ domains of Mint 1 are not equivalent. To examine this further, the effect of mutating the PTB domain of Mint 1 on its inhibitory activity was examined. Based on the available structural information (Zhang et al., 1997, EMBO J. 16:6141-6150), a mutant Mint 1 PTB* was designed in which critical residues involved in binding to the NPTY sequence of APP were altered. Specifically, YQEF(613-616) was mutagenized to SQES(613-616). In this study, the same DNA amount of control vector or Mint 1 vector or the vectors encoding the indicated Mint 1 mutants was co-transfected into HEK293 cells with the APP-Gal4/VP16 and reporter plasmids, and transactivation was determined as described in the Materials and Methods. As shown in FIG. 11A, the PTB domain mutation decreased inhibition of transactivation, but did not abolish it, possibly because the mutant PTB domain retained residual binding activity for APP. Only the combination of the PTB domain mutation with the point mutation in the first PDZ domain of Mint1 or with the deletion of both PDZ domains abolished its inhibitory effect on transactivation (FIG. 11A).
Conversely, after deletion of the isoform-specific N-terminal residues of Mint 1 (the sequences that are N-terminal to the PTB and PDZ domains and account for 451 of the 839 residues of Mint 1; Okamoto and Südhof, 1997, J Biol Chem 272:31459-31464), the specific inhibitory activity of Mint 1 was also increased significantly (FIG. 11B). In these studies, the same amounts of myc-tagged full-length Mint 1 or an N-terminally truncated Mint 1 mutant containing only the PTB- and PDZ-domains were co-transfected with APP-Gal4/VP16 and reporter constructs into HEK293 cells, and their specific inhibitory activity on transactivation was determined. To exclude an effect of the myc-epitope, the specific activity of wild-type and myc-tagged Mint1 was compared.
Expressed proteins were detected using antibodies against the myc epitope. The expression level of the N-terminal Mint 1 deletion mutant was low, suggesting that it may be partially cytotoxic (FIG. 11C).
Example 7 
Effect of Mints on transactivation by the intracellular fragment of APP. A possible explanation for the effects of Mints on transactivation by APP-Gal4/VP16, based on the binding of Mints to APP via their PTB domain and to presenilins via their PDZ domains (FIGS. 1-3) would be that the Mints 1 and 2 interfere with γ-cleavage of APP. However, the facts that Mint 3 also binds to APP and presenilins better than Mint 2 but does not inhibit transactivation, and that both Mint 1 PDZ domains bind to presenilin 1 in vitro but only the first PDZ-domain is involved in the inhibition of transactivation argue against this hypothesis. An alternative explanation for the transactivational inhibition is that Mints 1 and 2 act on the APP cytoplasmic tail after it has been released by γ-cleavage.
To differentiate between these explanations, we measured the effect of Mints on transactivation by the “precleaved” cytoplasmic tail of APP fused to Gal4/VP16 (APPICF-GV). To distinguish specific, i.e. binding-dependent, effects from non-specific effects, a mutant cytoplasmic tail of APP that was unable to bind to Mints (APPICF*-GV) was also examined in the same experiments. In addition, Mints were compared to Fe65 as another APP-binding protein, and to CASK as an unrelated control (FIG. 12). In these studies, constant amounts of plasmids encoding APPICF-GV or the NPTY(684-687) to NATA(684-687) mutant APPICF*-GV were co-transfected into HEK293 cells together with reporter plasmids and expression vectors for the indicated proteins.
Mint 1 expression significantly inhibited transactivation mediated by the cytoplasmic tail of APP fused to Gal4/VP16; this inhibition was only observed when the APP tail contained a normal Mint binding sequence (FIG. 12). A similar inhibition was detected with Mint 2 (data not shown). Mint 3, by contrast, had no significant effect, in agreement with the results obtained with full-length APP-Gal4/VP16 (FIGS. 9B and 10B). Fe65 enhanced transactivation, again only when the NPTY sequence in the cytoplasmic tail was intact (FIG. 12), consistent with the overall function of Fe65 in stimulating transcription (Cao and Südhof, 2001, Science 293:115-120). CASK used as a negative control had no effect on transactivation. Identical effects of both Mints 1 and 3, their PDZ truncation mutants, and Fe65 were observed when transactivation was assayed by a construct that mimicked the initial γ-cleavage product of APP, i.e. that contained 12 hydrophobic residues from the TMR preceding the cytoplasmic tail of APP (data not shown). These results demonstrate that Mint 1 inhibits transactivation downstream of APP cleavage, and that this effect is highly specific for neuronal vs. ubiquitous Mints.
Example 8 
Generation of Mint 1, Mint 2 and Mint 3 KO mice. The strategies used for the creation of the Mint 1, Mint 2 and Mint 3 KO mice are shown in FIG. 14, panels A-C, respectively. Briefly, genomic clones encoding exons of the Mint 1, 2 and 3 genes were mapped and inducible targeting vectors were constructed. In these inducible targeting vectors, an exon of the Mint gene was flanked by loxP sites, and a neomycin gene cassette was introduced which was flanked by flp sites for positive selection. A negative selection marker diphtheria toxin (DT) was placed at the end of the genomic sequences for negative selection. Embryonic stem (ES) cells were electroporated with Mint 1, 2 and 3 inducible targeting vectors and subjected to positive and negative selection. Double-resistant clones were screened by Southern analysis to verify proper targeting.
All three inducible targeting vectors could be successfully employed to generate recombinant ES cells. Injection of these cells into pseudopregnant female hosts led to the production of chimeric offspring carrying the mutant allele, which was subsequently shown to be transmitted through the germline. Crossing of the heterozygous animals led to the production of null mutant knockout mice. As shown for Mint 1 KO mice in FIG. 15, Southern blot analysis was employed to confirm the presence of the wild-type or mutant allele.
Mint 1 null mutant knockout mice are viable and fertile. Although incompletely characterized at present, they show no obvious abnormalities other than that they weight comparatively less than their age-matched, wild-type littermates. No changes in protein expression in Mint 1-associated proteins such as munc18, CASK, velis, neurexins and APP were observed. There were no abnormalities in excitatory synaptic transmission or plasticity, but an increase in the magnitude of paired-pulse depression was noted at inhibitory synapses in the Mint 1 null mutant knockout mice, as compared to wild-type littermates, suggest that these animals may exhibit some change in GABA release from inhibitory terminals. A second group has also recently reported the creation and characterization of a transgenic Mint-1 knockout mouse. See Mori et al., Neurosci Res. 2002; 43:251-257.
Null mutant knockout mice for Mint 2 and Mint 3 are also viable and fertile. Phenotypic characterization of these animals is in progress.
The foregoing data provide a model for the interaction between proteins of the Mint family and the cytoplasmic tail of the APP wherein binding of Mint 1 or 2 to the cytoplasmic tail of APP inhibits the transcriptional activation otherwise mediated by this protein.
All references cited herein are incorporated herein in their entirety.

## Claims

1. A method of identifying compounds that modulate transcriptional activation in a target cell comprising:
a contacting a target cell with a test compound, wherein the target cell comprises
(i) a first nucleic acid encoding an amyloid-β precursor protein (APP) fusion protein comprising a modified cytoplasmic tail of the AAP, protein wherein the cytoplasmic tail is modified to include a heterologous DNA-binding domain of a transcription factor and a transcriptional activator of the same or a different transcription factor, and
(ii) a second nucleic acid encoding a reporter gene whose transcription is regulated by the DNA-binding domain of a transcription factor contained in the APP fusion protein; and
(iii) a third nucleic acid encoding a Mint 2 protein (SEQ ID NO:13);

(b) measuring the levels of reporter gene transcription that occurs in the presence and absence of the test compound; and
(c) comparing the levels of reporter gene transcription measured in step (b) wherein increased or decreased levels of reporter gene transcription in the presence of the test compound indicate that the compound modulates transcriptional activation.

2. The method of claim 1 wherein the DNA-binding domain is from Gal4 or LexA.

3. The method of claim 1 wherein the transcriptional activation domain is from VP 16.

4. The method of claim 1 wherein the reporter gene transcription levels are measured by measuring expression of the reporter gene.

5. The method of claim 1 wherein the cell is a eukaryotic cell.

6. The method of claim 5 wherein the cell is a mammalian cell.

7. The method of claim 6 wherein the cell is a human cell.