# Patent

## Title
Aβ-Peptide screening assay

## Abstract

The invention relates to a process for the determination of the γ-secretase activity, individual components of the process and the use of the process.
The present invention relates to a novel process for the determination of the γ-secretase activity and for the detection of γ-secretase; particular embodiments of the process relate on the one hand to processes for the identification of a γ-secretase or of a cDNA which codes for a γ-secretase and on the other hand to processes for the identification of substances which can inhibit the activity of a γ-secretase. Such substances have particular importance, as they can be used, for example, as pharmaceutical active compounds, e.g. for the treatment of Alzheimer's disease.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. application Ser. No. 09/455,367, filed Dec. 3, 1999, now abandoned, and claims priority under 35 U.S.C. 119 to German application number 189 56 261.6, dated Dec. 7, 1998. All of the above-mentioned applications and all documents cited therein, as well as documents referenced or cited in documents cited herein, are hereby incorporated herein by reference.


FIELD OF THE INVENTION
The invention relates to processes for the determination of the activity of a γ-secretase, individual components of the processes and the use of the processes.
BACKGROUND OF THE INVENTION
Several publications are referenced in this application. These references describe the state of the art to which this invention pertains, and are incorporated herein by reference.
Alzheimer's disease is a neurodegenerative disorder of the brain which is accompanied at the cellular level by a massive loss of neurons in the limbic system and in the cerebral cortex. In the brain areas affected, at the molecular level protein deposits, so-called plaques, can be detected at the molecular level, which are an essential characteristic of Alzheimer's disease. The protein occurring most frequently in these plaques is a peptide 40 to 42 amino acids in size, which is designated as Aβ-peptide. This peptide is a cleavage product of a significantly larger protein of 695 to 770 amino acids, the so-called amyloid precursor protein (APP).
APP is an integral transmembrane protein which firstly traverses the lipid bilayer. By far the largest part of the protein is extracellular, while the shorter C-terminal domain is directed into the cytosol (FIG. 1). The Aβ-peptide is shown dark-gray in FIG. 1. About two thirds of the Aβ-peptide originates from the extracellular domain and about one third from the transmembrane domain of APP.
Beside the membrane-based APP, a secreted form of the amyloid precursor protein can be detected which consists of the large ectodomain of the APP and is designated as APPsec (“secreted APP”). APPsec is formed from APP by proteolytic cleavage, which is effected by the α-secretase. The proteolytic cleavage takes place in a site of the amino acid sequence of APP which is within the amino acid sequence of the Aβ-peptide (after amino acid residue 16 of the Aβ-peptide). Proteolysis of APP by the α-secretase thus excludes the formation of the Aβ-peptide.
The Aβ-peptide can thus only be formed from APP in an alternative processing route. It is postulated that two further proteases are involved in this processing route, one protease, which is designated as β-secretase, cleaving at the N-terminus of the Aβ-peptide in the APP and the second protease, which is designated as γ-secretase, releasing the C-terminus of the Aβ-peptide (Kang, J. et al., Nature, 325, 733) (FIG. 1).
Up to now, it has not been possible to identify any of the three secretases or proteases (α-secretase, β-secretase, γ-secretase). Knowledge of the secretases, however, is of great interest, in particular in the context of investigations on Alzheimer's disease and for the identification of the proteins involved, which can then in turn be employed as targets in continuing studies. On the one hand, the inhibition of the β-secretase and in particular of the γ-secretase could lead to a reduction in the Aβ-production, on the other hand an activation of the α-secretase could increase the processing of APP in APPsec and would thus simultaneously reduce the formation of the Aβ-peptide. A transgenic C. elegans which is found in the course of such investigations is described in the unpublished German Patent Application having the reference 198 49 073.9.
There are many indications of the fact that the Aβ-peptide is a crucial factor in the occurrence of Alzheimer's disease. Inter alia, neurotoxicity of Aβ-fibrils in cell culture is postulated (Yankner, B. A. et al., (1990) Proc Natl Acad Sci USA, 87, 9020). In patients with Down's syndrome, in which APP occurs in an additional copy, the neuropathology characteristic of Alzheimer's disease also occurs even at an age of 30 years. Here, it is assumed that the overexpression of APP follows an increased conversion into the Aβ-peptide (Rumble, B. et al., (1989), N. Engl. J. Med., 320, 1446).
Perhaps the strongest indication of the central role of the Aβ-peptide are the familial forms of Alzheimer's disease. Here, mutations are found in the APP gene around the area of the β and γ-secretase cleavage sites or in two further AD-associated genes (presenilins), which in cell culture lead to a significant increase in Aβproduction (Scheuner, D. et al., (1996), Nature Medicine, 2, 864).
There are a number of indications of the fact that APP is first cleaved into the Aβ-peptide by the β-secretase during its processing in order, following this, to serve as a substrate for γ-secretase (Maruyama, K. Y. et al., (1994) Biochem. Biophys Res Commun, 202, 1517; Estus, S. et al., (1992), Science, 255, 726). The γ-secretase therefore has a crucial role in the formation of the Aβ-peptide. A demonstration of the activity of the γ-secretase which is customarily used is the detection of the Aβ-peptide, which, however, frequently turns out to be difficult.
An important reason for this is that only a small part of APP is converted into the Aβ-peptide (Simons M, et al., Neurosci (1996) 1;16(3):899–908). Moreover, the Aβ-peptide is an only very small breakage fragment of about 4 kDa and, on account of its hydrophobic character, has a great tendency to self-aggregation so that it easily precipitates under physiological conditions (Hilbich, C. et al., (1991) J. Mol. Biol., 218, 149).
The detection of the Aβ-peptide in eukaryotic cells is carried out by means of immunobiological methods such as, for example, ELISA, immunoprecipitation and Western blofting (Suzuki, N. et al., Science 1994, 27, 264(5163) 1336; Haass, C. et al., (1992) Nature, 359, 322). These processes are relatively laborious, as they involve incubation with appropriate antibodies and necessitate destruction of the cells used, which are obtained from cell culture or model organisms (inter alia C. elegans).
SUMMARY OF THE INVENTION
The present invention relates to a novel process for the determination of the γ-secretase activity and for the detection of γ-secretase; particular embodiments of the process relate on the one hand to processes for the identification of a γ-secretase or of a cDNA which codes for a γ-secretase and on the other hand to processes for the identification of substances which can inhibit the activity of a γ-secretase. Such substances have particular importance, as they can be used, for example, as pharmaceutical active compounds, e.g for the treatment of Alzheimer's disease.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a process for the detection of γ-secretase, where

    
    
        1. a transgene is used which encodes a fusion protein and contains the following constituents:
        
            a) a first nucleotide sequence which codes for a protein which contains the amino acid sequence GAIIGLMVGGVVIATVIVITLVML (SEQ ID NO. 1),
            b) at the 5′ end of the first nucleotide sequence, a second nucleotide sequence which codes for a signal peptide,
            c) a promoter and,
            d) if appropriate, further coding and/or noncoding nucleotide sequences;
        
        
        2. this transgene is incorporated into a cell and the fusion protein is expressed;
        3. the fusion protein is cleaved within the amino acid sequence SEQ ID NO. 1 by γ-secretase present in the cell, whereby a first partial protein, which contains the amino acid sequence GAIIGLMVGGVV (SEQ ID NO. 2), and a second partial protein, which contains the amino acid sequence VIVITLVML (SEQ ID NO. 3), are formed; and
        4. the first partial protein and/or the second partial protein are detected.
    
    


The invention also relates to a process for the detection of the activity of γ-secretase, where

    
    
        1. a transgene is prepared/used which encodes a fusion protein and contains the following constituents:
        
            a) a first nucleotide sequence which codes for a protein which contains the amino acid sequence GAIIGLMVGGVVIATVIVITLVML (SEQ ID NO. 1),
            b) at the 5′ end of the first nucleotide sequence, a second nucleotide sequence which codes for a signal peptide,
            c) a promoter and,
            d) if appropriate, further coding and/or noncoding nucleotide sequences;
        
        
        2. this transgene is incorporated into a cell and the fusion protein is expressed;
        3. the fusion protein is cleaved within the amino acid sequence SEQ ID NO. 1 by γ-secretase present in the cell, whereby a first partial protein, which contains the amino acid sequence GAIIGLMVGGVV (SEQ ID NO. 2), and a second partial protein, which contains the amino acid sequence VIVITLVML (SEQ ID NO. 3), are formed; and
        4. the amount of second partial protein is determined and the activity of the γ-secretase is determined from the amount of second partial protein formed.
    
    


The processes (“Aβ-peptide screening assay”, “γ-secretase assay”) are suitable for the in vivo detection of a γ-secretase or of the activity of a γ-secretase, it being possible to employ the processes universally, even, for example, in high throughput screening (“HTS”). The processes do not have the above mentioned disadvantages of conventional detection processes, in particular, laborious isolation and detection steps are not necessary. The basis of the processes is that the C-terminal APP fragment is cleaved by the γ-secretase into two fragments—a first partial protein which contains the amino acid sequence GAIIGLMVGGVV (SEQ ID NO. 2) and a second partial protein which contains the amino acid sequence VIVITLVML (SEQ ID NO. 3), and the second partial protein containing the amino acid sequence VIVITLVML (SEQ ID NO. 3) diffuses into the cytosol of the cell (FIG. 2). This second partial protein can be easily detected in the cytosol of a cell, for example, with the aid of a reporter gene, and it serves as a detection for a γ-secretase or the activity of a γ-secretase. The γ-secretase cleavage site is located in the transmembrane domain of the APP (Kang, J. et al., (1987) Nature, 325, 733). The APP transmembrane domain has the amino acid sequence GAIIGLMVGGVV40 IA42 TVIVITLVML (SEQ ID NO: 1). The γ-secretase cleaves after V40, A42 or T43. In contrast to this, the Aβ-peptide, which is produced by eukaryotic cells in cell culture, is secreted into the medium supernatant.
With the aid of a suitable reporter system, the release of the second partial protein can activate the expression of a reporter protein which can be detected in eukaryotic cells. By means of the detection of the reporter protein, it can be demonstrated that a γ-secretase cleavage has taken place in the APP. As a result, the γ-secretase or the activity of the γ-secretase can be determined qualitatively and/or quantitatively.
The constituents of the process can be characterized in greater detail as follows:
The first nucleotide sequence codes for an amyloid precursor protein (APP) or a part thereof. Preferably, the first nucleotide sequence codes for a protein which contains the amino acid sequence SEQ ID NO. 4 (SEQ ID NO. 4 contains SEQ ID NO. 1).
The second nucleotide sequence codes for a signal peptide, preferably for the signal peptide of APP (subsequently abbreviated “SP”). The signal peptide contains, for example, the amino acid sequence SEQ ID NO. 5.
As a promoter, it is possible to use a regulatable or a constitutive promoter. The promoter can be suitable, for example, for expression in mammalian cells, in C. elegans, in yeast or in Drosophila. Suitable promoters for mammalian cells are, for example, CMV (e.g: Clontech, Heidelberg, Germany), HSV TK (e.g. Clontech), RSV (e.g. Invitrogen, NV Leek, Netherlands), SV40 (e.g. Clontech) and LTR (e.g. Clontech). Promoters which can be used for C. elegans are, for example, unc119, unc54, hspl 16-2, G0A1 and sel-12. For expression in yeast, the promoters ADH1 (constitutive) (Vlckova et al. (1994) Gene, 25(5), 472–4), Gal 1 (conditionally inducible) (Selleck et al. (1987) Nature 325, 173–7), MET3 (conditional) (Cherest et al. (1987) Mol Gen Genet 210, 307–13) and Met 25 are suitable. In Drosophila, it is possible to use, for example, the promoters MT (metallothionine) (e.g. Invitrogen), Ac5 (Invitrogen) or Ds47 (Invitrogen).
Preferably, a eukaryotic cell is employed in the process, for example a human cell or a nonhuman cell, e.g. monkey, hamster, mouse, Drosophila, Zebrafish or yeast. For example, an HeLa, 293, H4, SH-SY5Y, H9, Cos, CHO, N2A, SL-2 or Saccharomyces cerevisiae cell can be employed. In a particular embodiment of the invention a C. elegans cell is employed. The cell can be a constituent of a transgenic, nonhuman animal. In a particular embodiment, the transgenic cell can be a constituent of a transgenic C. elegans. In particular, the invention relates to processes in which yeast cells, e.g. from the strain MAV203 (Life Technologies, Rockville, Md., USA) or EGY 48 (OriGene Technologies, Inc. Rockville, Md., USA), are used.
The transgene codes for a fusion protein; this is composed of the partial proteins which are encoded by the first and the second nucleotide sequence and, if appropriate, further nucleotide sequences. The fusion protein thus contains the first partial protein and the second partial protein and, if appropriate, a further partial protein. The fusion protein, for example, has the amino acid sequence SEQ ID NO. 6.
In particular, a transgene which has the nucleotide sequence SEQ ID NO. 8 can be employed in the process. In particularly preferred embodiments of the process, the transgene is present in a vector. The recombinant vector can have the nucleotide sequence SEQ ID NO. 9. This special embodiment of the invention is also designated as a SP-C100-Gal 4-VP16 system. In this case, a fusion protein consisting of the signal peptide of APP, the C100 fragment of APP, Gal4 and VP16 is expressed. This protein located in the transmembrane domain is cleaved within the C100 fragment and the second partial protein, i.e. the part of the fusion protein which contains one part of the C 100 fragment, Gal4 and VP16, is detected with the aid of a reporter plasmid.
Beside the transgene construct SPC100-Gal4-VP16, other reporter constructs are also conceivable in which, for example, the transcription-activating domain could be inserted between the transmembrane domain and cytosolic domain of SPC100 or a Tag (e.g. MYC, FLAG) on the N- and C-terminus and between the transmembrane and the cytosolic domain of SPC100.
The further coding nucleotide sequence can code, for example, for a protein which can be used for the detection of the second partial protein. Preferably, the further coding nucleotide sequence is therefore located at the 3′ end of the first nucleotide sequence. The further coding nucleotide sequence codes, for example, for a chimeric protein or another protein which is constructed from a number of domains, e.g. a protein which contains a DNA-binding domain and a transcription-activating domain. In a particular embodiment of the invention, the further coding nucleotide sequence codes for a protein which consists of a Gal4-binding domain and of the transcription-activating domain of VP16 (Gal4-VP 16), and the further partial protein preferably then has the amino acid sequence SEQ ID NO. 7. In yeast cells, the further partial protein can also contain a LexA-binding domain (e.g. Lex A-VP16). This further partial protein is particularly suitable for processes in which cells of the yeast strain EGY48 are used.
In particular, the invention relates to processes in which cells are used which are cotransfected with a reporter plasmid. The reporter plasmid contains a reporter gene under the control of a regulatable promoter. For example, the reporter gene can code for GFP and its derivatives, e.g. EGFP (Enhanced Green Fluorescent Protein), EBFP, EYFP, d2EGFP, GFPuv or Luciferase (e.g. Promega, Mannheim, Germany), CAT (e.g. Promega), SEAP (e.g. Clontech), βGal (e.g. Clontech) or apoptosis-inducing factors, e.g. Fas, TNF-R1, death domain and homologs (Tartaglia et al. (1993) Cell 74, 845–53), ced3, ced4, ced9. As a regulatable promoter, the reporter plasmid can contain, for example, a minimal promoter of HIV, of the CD4 promoter or the mec7 promoter. The choice of the suitable regulatable promoter depends on the transcription-activating domain used.
A particular embodiment of the invention relates to the implementation of the process, where the cells used are yeast cells. As an alternative to the yeast expression vector pDBTRP (Life Technologies Inc., Rockville, Md, USA) (SEQ ID NO.: 11) into which in a special embodiment of the invention a MET-25 promotor is integrated (e.g. SEQ ID NO. 12), a large number of other expression vectors with different promoters (e.g. the inducible Gal1-promoter, the constitutively active ADH1 promoter) and with different selection markers (ADE, LEU, TRP, HIS, LYS, PHE) can be selected.
A particular embodiment of the invention relates to the use of yeast cells which contain Gal4- or LexA-inducible reporter genes either stable integrated in their genome or extrachromosomal. In this embodiments preferably the yeast strains MaV203 (Life Technologies Inc.) or EGY48 (OriGene Technologies, Inc., Rockville, Md. USA) are used.
A particular embodiment of the processes relates to the use of a cell which was additionally transfected with a further recombinant vector. Preferably, the cell which is used for these embodiments normally has no or hardly any endogenous γ-secretase or endogenous γ-secretase activity and is not detectable using the abovementioned processes. This cell can be employed transformed with a further vector in which a nucleotide sequence—preferably a cDNA—is contained which codes for a γ-secretase. For example, a cDNA bank can be employed. This embodiment of the process can then be used, inter alia, to identify a γ-secretase or a cDNA which codes for a γ-secretase. cDNA banks which can be searched for a γ-secretase can be prepared from cells or tissues, e.g. B cells, neurons, glia cells, hippocampus whole brain, placenta, kidney. Preferably, the cDNA is prepared from human cells or human tissues but also from other organisms (e.g. hamster, rat, mouse, dog, monkey).
In the case of cells which without transfection exhibit no γ-secretase activity, but after transfection with a cDNA bank exhibit γ-secretase activity, the cDNA present in the cell codes for a γ-secretase. This cDNA can be isolated by known processes from cells which exhibit this behavior and further be analyzed by known methods.
The invention also relates to a transgene which codes for a fusion protein and contains the following constituents:

    
    
        a) a first nucleotide sequence which codes for a protein which contains the amino acid sequence GAIIGLMVGGVVIATVIVITLVML (SEQ ID NO. 1),
        b) at the 5′ end of the first nucleotide sequence, a second nucleotide sequence which codes for a signal peptide,
        c) a promoter and
        d) at least one further nucleotide sequence at the 3′ end of the first nucleotide sequence, which codes for a DNA-binding domain and for a transcription-activating domain.
    
    


Preferably, the first nucleotide sequence codes for APP or a part of APP. The transgene can, for example, have the nucleotide sequence SEQ ID NO. 8. The transgene can be present in a vector. This can, for example, have the nucleotide sequence SEQ ID NO. 9.
The process relates to the use of a transgene and/or of a vector for the production of a transgenic cell, it being possible for the cell to be a constituent of a nonhuman organism. For example, the transgene and/or the vector can be used for the production of a transgenic C. elegans. In another particular embodiment, the transgene and/or the vector is used for the production of transgenic yeast cells, e.g. S. cerevisiae cells.
The invention also relates to a process for the production of a nonhuman organism, e.g. of a transgenic C. elegans, where a transgene and/or a vector which contains a transgene is microinjected into the gonads of the organism, i.e., for example, of a C. elegans. The invention also relates to a cell which contains a transgene according to the invention and a transgenic C. elegans which contains a transgene according to the invention. The invention also relates to a cell, in particular a yeast cell, which contains a transgene according to the invention, preferably present in a suitable vector. The invention relates in particular to cells, preferably yeast cells, which contain the transgene according to the invention and additionally a cDNA bank.
The invention relates to the use of transgenic or recombinant cells, preferably yeast cells, or of a transgenic C. elegans in a process for the determination of γ-secretase, or the activity of γ-secretase, the use of these cells or of a transgenic C. elegans in a process for the identification of inhibitors of the activity of the γ-secretase, and the process itself.
In particular, the invention relates to processes for the identification of substances which inhibit the activity of a γ-secretase, the process containing the following process steps:

    
    
        1. Production of a transgenic nonhuman organism, e.g. of a transgenic C. elegans or Saccharomyces cerevisiae or of a transgenic cell, the transgenic nonhuman organism or the transgenic cell containing a transgene which has the following constituents:
        
            a) a first nucleotide sequence which codes for a protein which contains the amino acid sequence GAIIGLMVGGVVIATVIVITLVML (SEQ ID NO. 1),
            b) at the 5′ end of the first nucleotide sequence, a second nucleotide sequence which codes for a signal peptide and
            c) a promoter and
        
        
        the transgenic nonhuman organism or the transgenic cell moreover contains a reporter plasmid, the reporter plasmid carrying a protein binding site, a minimal promoter and a reporter gene and,
        if appropriate, a cDNA which encodes a γ-secretase, where the transgenic nonhuman organism or the transgenic cell expresses the transgene and, if appropriate, the γ-secretase encoded by the cDNA;
        2. the transgenic nonhuman organism or the transgenic cell is incubated with a substance to be investigated; and
        3. the amount of second partial protein is detected.
    
    


The invention also relates to a process for the identification of substances which inhibit the activity of the γ-secretase, where

    
    
        1. a transgene is prepared/used which contains the following constituents:
        
            a) a first nucleotide sequence which codes for a protein which contains the amino acid sequence GAIIGLMVGGVVIATVIVITLVML (SEQ ID NO. 1),
            b) at the 5′ end of the first nucleotide sequence, a second nucleotide sequence which codes for a signal peptide and
            c) a promoter and,
            d) if appropriate, further coding and/or noncoding nucleotide sequences;
        
        
        2. this transgene and a reporter plasmid and, if appropriate, a cDNA which codes for a γ-secretase are incorporated into a cell and the fusion protein encoded by the transgene and, if appropriate, the γ-secretase encoded by the cDNA are expressed in the presence of a substance to be investigated,
        3. the fusion protein is
        
            a) cleaved or
            b) not cleaved within the amino acid sequence SEQ ID NO. 1 by γ-secretase present in the cell, as a result of which either
            c) a first partial protein which contains the amino acid sequence GAIIGLMVGGVV (SEQ ID NO. 2) and a second partial protein which contains the amino acid sequence VIVITLVML (SEQ ID NO. 3) are formed, or
            d) no detectable amount of first and/or second partial protein is formed,
        
        
        4. it is determined whether a second partial protein was formed.
    
    


The invention also relates to processes for the identification of substances which inhibit the activity of a γ-secretase, where a transgene which codes for a protein which contains a signal peptide and the SEQ ID NO. 1 is expressed in the presence of a substance to be investigated and of a reporter plasmid and the effect of the substance to be investigated on the amount of second partial protein formed is determined, the second partial protein containing the amino acid sequence VIVITLVML (SEQ ID NO. 3).
The invention also relates to inhibitors of a γ-secretase which are identified by the processes.
Inter alia, the processes can be used, for example, in conjunction with the C100-Gal 4-VP16 system (i.e. a fusion protein consisting of C100, Gal4 and VP16 or using a nucleic acid which codes for a corresponding fusion protein) for:

    1. Identification and determination (qualitative and/or quantitative) of the activity of a γ-secretase.
    2. Identification of γ-secretases in different tissues, cells and organisms or species. Identification and isolation of the cDNAs concerned which code for this γ-secretase and the further use of the cDNAs.
    3. Screening in vivo, e.g. in yeast cells (e.g. Saccharomyces cerevisiae) or in C. elegans, it being possible to determine the activity of the γ-secretase without use of immunobiological methods.
    4. Use of the process for the identification and characterization of substances, e.g. pharmacological active compounds, which modulate the activity of the γ-secretase, e.g. inhibitors of the γ-secretase. In particular, this process can be employed in an HTS (High Throughput Screening). For example, substances can be identified which can be employed for the treatment of Alzheimer's disease and/or for preventive treatment.
    5. Investigations in the context of Alzheimer's disease, e.g. with mutated APP or C100.
    6. The described fusion proteins/transgenes, e.g. C100 in SP-C100-Gal 4-VP16, can be replaced by whole APP and the γ-secretase, its activity and regulation can likewise be investigated with the aid of the processes.





DETAILED DESCRIPTION OF THE FIGURES
FIG. 1: FIG. 1 shows the amyloid precursor protein (Isoform APP695 and Isoforms APP770 or APP751) and secretase cleavage products.
FIG. 2: Shows schematically the principle on which the processes are based:
β-secretase cleavage site at the N-terminus; γ-secretase cleavage site in the transmembrane domain; C100=C100 fragment of APP; Gal4-VP16=DNA-binding domain, transcription-activating domain (consisting of DNA-binding domain and transcription activator), which binds to the protein-binding domain on the DNA of the reporter plasmid.
FIG. 3: Construction of the expression plasmids SP-C100-Gal4-VP16:
aa=amino acids; restriction cleavage sites Sac I, Hind III and Kpn I indicating the position of the cleavage site on the plasmid.
FIG. 4: Expression plasmid pDBTRP-MET25-SP-C100-Gal4-VP16:
Construction of the expression plasmid for the expression of the transgene in yeast.



EXAMPLES
The following examples are illustrative of some of the products and compositions and methods of making and using the same falling within the scope of the present invention.
Example 1
Construction of the Expression Plasmid SP-C100-Gal4-VP16
The plasmid encodes the APP signal peptide (SP) which is fused to the C-terminal 100 amino acid residues of APP (C100). C100 begins with the N-terminus of the Aβ-peptide and ends with the C-terminus of APP. It must additionally be cleaved by the Aγ-secretase in order to release the Aβ-peptide.
Gal4-VP16 was fused to the C-terminus of SP-C100. Gal4-VP16 is composed of the first 147 amino acid residues of the yeast transcription activator Gal4 and the 78 C-terminal amino acid residues of VP 16, a transcription activator from the herpes simplex virus. As a fusion protein, the Gal 4 fragment takes over the function of the DNA binding while the VP16 fragment activates the transcription (Sadowski et al., (1988) Science 335, 563). pcDNA3.1+from Invitrogen, Netherlands, serves as a vector plasmid.
Example 2
Construction of the Reporter Plasmid pGL2 MRG5 EGFP
The reporter plasmid pGL2 MRG5 has five Gal 4 binding sites ahead of the HIV-TATA box. For easier detection in cell culture, the luciferase gene was exchanged for the gene for EGFP (Enhanced Green Fluorescent Protein) from the vector pEGFP N1 from Clontech, Heidelberg.
Example 3
Human neuroblastoma cells (SH-SY5Y cells) were cotransfected with both plasmids and then microscopically analyzed under irradiation with light of wavelength 480 nm, by means of which EGFP is excited. In some cases, it was possible to detect cells luminescing a strongly green color.
Since this effect could also be based on expression of the EGFP by the reporter plasmid without specific activation, SH-SY5Y cells were transfected only with reporter plasmid. In these cells, no green fluorescence was detectable. The expression must therefore be activated by Gal4-VP16, which presupposes a proteolytic release of the APP-C terminus. Until now, apart from γ-secretase, no further proteolytic activities had been described which proteolytically process APP within the transmembrane domain or in the cytoplasmic part. It is therefore assumed that the release of the APP-C terminus, fused to Gal 4-VP16, is based on the activity of the γ-secretase.
Example 4
Use of the C 100-Gal4-VP16 system for the detection of a cDNA coding for a γ-secretase activity in cDNA banks:
SPC 100-Gal4-VP 16 was cloned in the yeast expression vector pDBTRP (Life Technologies Rockville, Md, U.S.A.) under control of the MET25 promoter and the yeast strain MaV203 (Life Technologies) was transformed using these constructs. The yeast strain MaV203 is genetically modified and contains three GAL4-inducible reporter genes (URA3, HIS3, LacZ), which are stably integrated into the genome. The expression of the SPC 100-Gal4-VP16 cDNA in MaV203 afforded only a small activity of the reporters, such that this system is suitable for a search for a γ-secretase in a cDNA bank.
Example 5
The recombinant MaV203 cells from Example 4 can be used, for example, for the identification of γ-secretases or screening of a human B cell cDNA bank (American Type Culture Collection, Manassas, Va., U.S.A.). Analogously, a human hippocampal cDNA bank, integrated into the yeast expression vectors p415-MET25 (ATCC, Nucleic Acid Research, 1994, Vol. 22, No. 25, 5767) or p415-ADH1 (ATCC, GENE, 1995,158: 119–122), could also be employed for screening for a cDNA which codes for a γ-secretase or a protein having γ-secretase activity.
The above description of the invention is intended to be illustrative and not limiting. Various changes or modifications in the embodiments described may occur to those skilled in the art. these can be made without departing from the spirit or scope of the invention.

## Claims

1. A process for detecting the presence of γ-secretase in a cell in vitro, comprising:
a) transforming the cell with a nucleic acid encoding a fusion protein having (i) a first amino acid sequence comprising GAIIGLMVGGVVIATVIVITLVML (SEQ ID NO. 1); wherein said first amino acid sequence comprises a transmembrane domain; (ii) a signal peptide fused to the N-terminal end of the first amino acid sequence, and (iii) a protein located at the C-terminal end of the first amino acid sequence and containing a DNA-binding domain and a transcription-activating domain, wherein the fusion protein is expressed in the cell, and whereby if γ-secretase is present in the cell, the fusion protein is cleaved into a first partial protein containing amino acid sequence GAIIGLMVGGVV (SEQ ID NO. 2), and a second partial protein containing amino acid sequence VIVITLVML (SEQ ID NO. 3);
b) transforming the cell with a reporter gene under the control of a regulatable promoter, wherein protein of (iii) regulates the promoter; and
c) detecting the presence of the first partial protein and/or the second partial protein by detecting expression of the reporter gene, thereby detecting γ-secretase activity, and thus, the presence of γ-secretase in the cell.

2. A process for detecting γ-secretase activity in vitro in a cell containing γ-secretase, comprising:
a) transforming the cell with a nucleic acid encoding a fusion protein having (i) a first amino acid sequence comprising GAIIGLMVGGVVIATVIVITLVML (SEQ ID NO. 1); wherein said first amino acid sequence comprises a transmembrane domain; (ii) a signal peptide fused to the N-terminal end of the first amino acid sequence, and (iii) a protein located at the C-terminal end of the first amino acid sequence and containing a DNA-binding domain and a transcription-activating domain, wherein the fusion protein is expressed in the cell, and whereby the fusion protein is cleaved into a first partial protein containing amino acid sequence GAIIGLMVGGVV (SEQ ID NO. 2), and a second partial protein containing amino acid sequence VIVITLVML (SEQ lD NO. 3);
b) transforming the cell with a reporter gene under the control of a regulatable promoter, wherein the protein of (iii) regulates the promoter; and
c) determining the amount of second partial protein in the cell by detecting expression of the reporter gene, thereby detecting γ-secretase activity.

3. The process as claimed in claim 1, wherein the first amino acid sequence is an amyloid precursor protein (APP) or a part thereof.

4. The process as claimed in claim 2, wherein the first amino acid sequence is an amyloid precursor protein (APP) or a part thereof.

5. The process as claimed in claim 1, wherein the first amino acid sequence has the amino acid sequence of SEQ ID NO. 4.

6. The process as claimed in claim 2, wherein the first amino acid sequence has the amino acid sequence of SEQ ID NO. 4.

7. The process as claimed in claim 1, wherein the signal peptide is the signal peptide of APP (SP).

8. The process as claimed in claim 2, wherein the signal peptide is the signal peptide of APP (SP).

9. The process as claimed in claim 1, wherein the signal peptide has the amino acid sequence of SEQ ID NO. 5.

10. The process as claimed in claim 2, wherein the signal peptide has the amino acid sequence of SEQ ID NO. 5.

11. The process as claimed in claim 1, wherein the fusion protein is under the control of a promoter for expression in mammalian cells, in C. elegans, in yeast or in Drosophila.

12. The process as claimed in claim 2, wherein the fusion protein is under the control of a promoter for expression in mammalian cells, in C. elegans, in yeast or in Drosophila.

13. The process as claimed in claim 1, wherein the fusion protein is under the control of a promoter selected from the group consisting of CMV, HSV TK, RSV, SV40, LTR, unc119, unc54, hsp16-2, G0A1, sel-12, ADH1, Gal1, MET3, MET25, MT, Ac5 and Ds47.

14. The process as claimed in claim 2, wherein the fusion protein is under the control of a promoter selected from the group consisting of CMV, HSV TK, RSV, SV40, LTR, unc119, unc54, hsp16-2, G0A1, sel-12, ADH1, Gal1, MET3, MET25, MT, Ac5 and Ds47.

15. The process as claimed in claim 1, wherein the cell is a eukaryotic cell.

16. The process as claimed in claim 2, wherein the cell is a eukaryotic cell.

17. The process as claimed in claim 1, wherein the cell is a human cell.

18. The process as claimed in claim 2, wherein the cell is a human cell.

19. The process as claimed in claim 1, wherein the cell is a nonhuman cell.

20. The process as claimed in claim 2, wherein the cell is a nonhuman cell.

21. The process as claimed in claim 15, wherein the cell is selected from the group consisting of a HeLa, 293, H4, SH-SY5Y, H9, Cos, CHO, N2A, SL-2 and Saccharomyces cerevisiae.

22. The process as claimed in claim 16, wherein the cell is selected from the group consisting of a HeLa, 293, H4, SH-SY5Y, H9, Cos, CHO, N2A, SL-2 and Saccharomyces cerevisiae.

23. The process as claimed in claim 19, wherein the cell is a C. elegans cell.

24. The process as claimed in claim 20, wherein the cell is a C. elegans cell.

25. The process as claimed in claim 19, wherein the cell is a yeast cell.

26. The process as claimed in claim 1, wherein the fusion protein contains the amino acid sequence of SEQ ID NO. 6.

27. The process as claimed in claim 2, wherein the fusion protein contains the amino acid sequence of SEQ ID NO. 6.

28. The process as claimed in claim 3, wherein the DNA-binding domain is a Gal4-binding domain and the transcription-activating domain is a of VP16 transcription-activating domain (Gal4-VP16).

29. The process as claimed in claim 1, wherein the reporter gene encodes EGFP (Enhanced Green Fluorescent Protein), Ura 3, His 3 or Lac Z, and wherein the regulatable promoter contains Gal4 binding sites and a minimal promoter of HIV.

30. The process as claimed in claim 1, wherein the first amino acid sequence is encoded by the nucleotide sequence of SEQ ID NO. 8.

31. The process as claimed in claim 2, wherein the first amino acid sequence is encoded by the nucleotide sequence of SEQ ID NO. 8.

32. The process as claimed in claim 1, wherein the cell is transformed with a recombinant vector comprising a nucleic acid molecule encoding the fusion protein.

33. The process as claimed in claim 2, wherein the cell is transformed with a recombinant vector comprising a nucleic acid molecule encoding the fusion protein.

34. The process as claimed in claim 32, wherein the recombinant vector has the nucleotide sequence of SEQ ID NO. 9.

35. The process as claimed in claim 33, wherein the recombinant vector has the nucleotide sequence of SEQ ID NO. 9.

36. A process for detecting the absence of γ-secretase in a cell in vitro, comprising:
a) transforming the cell with a nucleic acid encoding a fusion protein having (i) a first amino acid sequence comprising GAIIGLMVGGVVIATVIVITLVML (SEQ ID NO. 1); wherein said first amino acid sequence comprises a transmembrane domain; (ii) a signal peptide fused to the N-terminal end of the first amino acid sequence, and (iii) a protein located at the C-terminal end of the first amino acid sequence and containing a DNA-binding domain and a transcription-activating domain, wherein the fusion protein is expressed in the cell, and whereby if γ-secretase is present in the cell, the fusion protein is cleaved into a first partial protein containing amino acid sequence GAIIGLMVGGVV (SEQ ID NO. 2), and a second partial protein containing amino acid sequence VIVITLVML (SEQ ID NO. 3);
b) transforming the cell with a reporter gene under the control of a regulatable promoter, wherein protein of (iii) regulates the promoter; and
c) detecting whether the reporter gene is expressed, thereby detecting whether the first partial protein or the second partial protein is present, whereby the absence of reporter gene expression indicates the absence of the first partial protein or the second partial protein, which indicates the absence of γ-secretase activity, and thus, the absence of γ-secretase in the cell.

37. The process as claimed in claim 1, wherein the cell is cotransfected with a cDNA bank.

38. The process as claimed in claim 37, where cDNA prepared from human or non-human tissue or human or non-human cells is present in the cDNA bank.

39. The process as claimed in claim 1, wherein the DNA-binding domain is a Gal4-binding domain and the transcription-activating domain is a VP16 transcription-activating domain (Gal4-VP 16).