γ-Secretase substrates and methods of use
Polypeptide substrates based on modifications or fragments of the various APP isoforms, assay methods based on the use of these substrates, and screening methods directed toward identifying inhibitors of γ-secretase activity. The assay methods and the screening methods are adapted for use in high throughput multi-well plate assay apparatuses. In many embodiments the substrate polypeptides are labeled for ease of detection, and/or may bind specific ligands that themselves are labeled. Generally the labels promote high specificity as well as high sensitivity of detection. These features render the assay and screening methods that employ the labeled substrates especially suited for use in high throughput assay formats. This disclosure further identifies small polypeptides based on a subsequence motif of Aβ that are shown herein to be potent inhibitors of the activity of γ-secretase.
1. A method of screening for an inhibitor of γ-secretase activity comprising the steps of:
a) providing a container comprising a composition comprising γ-secretase activity;
b) contacting the composition with a mixture comprising a candidate compound and a polypeptide substrate for γ-secretase, wherein the polypeptide substrate comprises modified APP polypeptide, wherein the modified APP polypeptide comprises at least one amino acid substitution, deletion, or insertion, in the γ-secretase inhibitory domain Leu-Val-Phe-Phe-Ala (SEQ ID NO:19) and a detectable label bound thereto, wherein the detectable label is a chimeric peptide sequence attached to the amino terminus of the modified APP, wherein cleavage of the modified APP polypeptide by γ-secretase provides a labeled product that is detectable;
c) incubating the candidate compound, the polypeptide substrate, and the γ-secretase; and
d) determining whether the candidate compound inhibits formation of the labeled product.
2. The method described in claim 1 wherein the chimeric peptide sequence comprises an avidin, a maltose binding protein, a FLAG epitope, a glutathione dehydrogenase, or a horse radish peroxidase.
3. The method described in claim 1 wherein the determining comprises assaying for the labeled product, wherein assaying for the labeled product comprises assaying for a detectable complex comprising the labeled product and at least one detectable probe.
4. The method described in claim 3 wherein the detectable complex comprises a specific binding member that comprises the detectable probe, wherein the specific binding member specifically binds the labeled product to form a binary complex.
5. The method described in claim 4 wherein the detectable probe comprises ruthenium.
6. The method described in claim 1 wherein the determining further comprises
d) contacting the labeled product with
1) a first ligand bearing a first tag wherein the first ligand specifically binds the detectable label, and
2) a second ligand bearing a second tag wherein the second ligand specifically binds the labeled product;
3) a third ligand bearing a third tag wherein the third ligand specifically binds the second ligand; and
e) determining the presence or the amount of the labeled product bound to the first ligand, the second ligand, and the third ligand.
7. The method described in claim 6 wherein the first ligand comprises a biotin and the first tag comprises a detectable fluorescence acceptor.
8. The method described in claim 7 wherein the second ligand comprises a first antibody that specifically binds a C-terminus of the labeled product, the first antibody being bound to a second antibody bearing a fluorescence donor that excites the fluorescence acceptor tag bound to the first ligand.
9. The method described in claim 1 wherein the container is a well in a multi-well assay plate, thereby providing a high throughput method of screening.
10. The method described in claim 9 wherein the plate contains at least 96 wells, or at least 384 wells, or at least 1536 wells.
11. The method described in claim 1 wherein the modified APP polypeptide comprises a deletion of the γ-secretase inhibitory domain.
12. The method described in claim 1 wherein the modified APP polypeptide comprises an amino acid sequence chosen from SEQ ID NO:9, 10, 13, 14, 25, or 6.
13. The method described in claim 1 wherein the γ-secretase inhibitory domain comprises an amino acid residue that is not phenylalanine at position 3 of SEQ ID NO:4, position 4 of SEQ ID NO:4, or both position 3 and position 4 of SEQ ID NO:4.
14. The method described in claim 1 wherein each residue of the γ-secretase inhibitory domain comprises a substitution.
15. The method described in claim 1 wherein the modified APP polypeptide comprises amino acid residues 620-695 of APP695 (SEQ ID NO:1).
16. The method described in claim 4 wherein the specific binding member comprises an anti-Aβ antibody.
17. The method described in claim 16 wherein the anti-Aβ antibody binds the C-terminus of the modified APP that is exposed after (i) γ-secretase mediated cleavage at the Aβ38 γ-secretase cleavage and does not bind an uncleaved modified APP, (ii) γ-secretase mediated cleavage at the Aβ40 γ-secretase cleavage and does not bind an uncleaved modified APP, or (iii) γ-secretase mediated cleavage at the Aβ42 γ-secretase cleavage and does not bind an uncleaved modified APP.
18. The method described in claim 6 wherein the second ligand comprises an anti-Aβ antibody, wherein the anti-Aβ antibody binds the C-terminus of the modified APP that is exposed after (i) gamma-secretase mediated cleavage at the Aβ38 gamma-secretase cleavage and does not bind an uncleaved modified APP, (ii) gamma-secretase mediated cleavage at the Aβ40 gamma-secretase cleavage and does not bind an uncleaved modified APP, or (iii) gamma-secretase mediated cleavage at the Aβ42 gamma-secretase cleavage and does not bind an uncleaved modified APP.
19. The method described in claim 6 wherein the third ligand comprises a rare earth metal ion.