The Role of Presenilins in γ-Secretase Activity (2001)

Alzheimer's disease amyloid-β peptide amyloid-β precursor protein familial AD presenilin transmembrane domain tumor necrosis factor-α converting enzyme Notch intracellular domain endoplasmic reticulum It is an aphorism that basic biological research lays the groundwork for solving important biomedical problems. However, the reverse is true as well: focusing on problems of human disease can lead to important new insights into the intricacies of nature. Research on Alzheimer's disease (AD)1provides a case study for the latter, where the race to identify molecular players involved in pathogenesis has unveiled some unexpected and fascinating aspects of protease biochemistry and proteolysis-dependent signal transduction. The 40–42-residue amyloid-β peptide (Aβ) is implicated in the early pathogenic events that lead to AD, and this peptide is carved out of the amyloid-β precursor protein (APP) by β- and γ-secretases (Fig. 1 A) (1Selkoe D.J. Nature. 1999; 399: A23-31Crossref PubMed Scopus (1520) Google Scholar). These two proteases are considered such important drug targets that the search for inhibitors of Aβ production within pharmaceutical companies has gone on for years even though both β- and γ-secretases remained unknown until very recently. Last year, β-secretase was identified as a new membrane-tethered member of the aspartyl protease family (2Vassar R. Bennett B.D. Babu-Khan S. Kahn S. Mendiaz E.A. Denis P. Teplow D.B. Ross S. Amarante P. Loeloff R. et al.Science. 1999; 286: 735-741Crossref PubMed Scopus (3271) Google Scholar, 3Sinha S. Anderson J.P. Barbour R. Basi G.S. Caccavello R. Davis D. Doan M. Dovey H.F. Frigon N. Hong J. et al.Nature. 1999; 402: 537-540Crossref PubMed Scopus (1474) Google Scholar, 4Yan R. Bienkowski M.J. Shuck M.E. Miao H. Tory M.C. Pauley A.M. Brashier J.R. Stratman N.C. Mathews W.R. Buhl A.E. et al.Nature. 1999; 402: 533-537Crossref PubMed Scopus (1329) Google Scholar, 5Hussain I. Powell D. Howlett D.R. Tew D.G. Meek T.D. Chapman C. Gloger I.S. Murphy K.E. Southa C.D. Ryan D.M. et al.Mol. Cell. Neurosci. 1999; 14: 419-427Crossref PubMed Scopus (997) Google Scholar, 6Lin X. Koelsch G. Wu S. Downs D. Dashti A. Tang J. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 1456-1460Crossref PubMed Scopus (738) Google Scholar), a finding that should greatly facilitate drug discovery. α-Secretases, which prevent Aβ formation by cleaving in the middle of the amyloid domain (7Esch F.S. Keim P.S. Beattie E.C. Blacher R.W. Culwell A.R. Oltersdorf T. McClure D. Ward P.J. Science. 1990; 248: 1122-1124Crossref PubMed Scopus (1202) Google Scholar), have now been shown to be members of the ADAM family (adisintegrin andmetalloprotease) (8Lammich S. Kojro E. Postina R. Gilbert S. Pfeiffer R. Jasionowski M. Haass C. Fahrenholz F. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 3922-3927Crossref PubMed Scopus (977) Google Scholar, 9Buxbaum J.D. Liu K.N. Luo Y. Slack J.L. Stocking K.L. Peschon J.J. Johnson R.S. Castner B.J. Cerretti D.P. Black R.A. J. Biol. Chem. 1998; 273: 27765-27767Abstract Full Text Full Text PDF PubMed Scopus (834) Google Scholar). At the same time, the mysterious γ-secretase is giving up its secrets as well. Genetic studies of families susceptible to autosomal dominant, early onset (<60 years) AD underscore the importance of Aβ formation to the disease process (10Hardy J. Trends Neurosci. 1997; 20: 154-159Abstract Full Text Full Text PDF PubMed Scopus (1267) Google Scholar, 11Selkoe D.J. Science. 1997; 275: 630-631Crossref PubMed Scopus (844) Google Scholar). Several different missense mutations in the APP gene on chromosome 21 cause familial AD (FAD), and these mutations are found immediately near the α-, β-, or γ-secretase processing sites. A Lys → Asn/Met → Leu double mutation near the β-secretase cleavage site found in a Swedish pedigree leads to increased Aβ production (both Aβ40 and Aβ42) (12Citron M. Oltersdorf T. Haass C. McConlogue L. Hung A.Y. Seubert P. Vigo-Pelfrey C. Lieberburg I. Selkoe D.J. Nature. 1992; 360: 672-674Crossref PubMed Scopus (1527) Google Scholar), and indeed purified β-secretase cuts peptide substrates with the "Swedish mutation" better than peptides based on the wild-type sequence (2Vassar R. Bennett B.D. Babu-Khan S. Kahn S. Mendiaz E.A. Denis P. Teplow D.B. Ross S. Amarante P. Loeloff R. et al.Science. 1999; 286: 735-741Crossref PubMed Scopus (3271) Google Scholar, 4Yan R. Bienkowski M.J. Shuck M.E. Miao H. Tory M.C. Pauley A.M. Brashier J.R. Stratman N.C. Mathews W.R. Buhl A.E. et al.Nature. 1999; 402: 533-537Crossref PubMed Scopus (1329) Google Scholar, 6Lin X. Koelsch G. Wu S. Downs D. Dashti A. Tang J. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 1456-1460Crossref PubMed Scopus (738) Google Scholar). The several FAD-causing mutations in APP near the γ-secretase processing site also affect Aβ production, but in these cases the mutations alter the specificity of the protease, leading to increases in the 42-residue Aβ variant (13Suzuki N. Cheung T.T. Cai X.D. Odaka A. Otvos Jr., L. Eckman C. Golde T.E. Younkin S.G. Science. 1994; 264: 1336-1340Crossref PubMed Scopus (1343) Google Scholar, 14Tamaoka A. Odaka A. Ishibashi Y. Usami M. Sahara N. Suzuki N. Nukina N. Mizusawa H. Shoji S. Kanazawa I. Mori H. J. Biol. Chem. 1994; 269: 32721-32724Abstract Full Text PDF PubMed Google Scholar). These findings bolstered the "amyloid hypothesis" of AD pathogenesis, because Aβ42 is particularly prone to fibril formation (15Jarrett J.T. Berger E.P. Lansbury Jr., P.T. Biochemistry. 1993; 32: 4693-4697Crossref PubMed Scopus (1747) Google Scholar) and is the protein initially deposited in the neuritic plaques that characterize the disease (16Iwatsubo T. Odaka A. Suzuki N. Mizusawa H. Nukina N. Ihara Y. Neuron. 1994; 13: 45-53Abstract Full Text PDF PubMed Scopus (1540) Google Scholar). Genes encoding the polytopic presenilin-1 and presenilin-2 (PS1 and PS2), found on chromosomes 14 and 1, respectively, were later identified as major loci for FAD (17Sherrington R. Rogaev E.I. Liang Y. Rogaeva E.A. Levesque G. Ikeda M. Chi H. Lin C. Li G. Holman K. et al.Nature. 1995; 375: 754-760Crossref PubMed Scopus (3565) Google Scholar, 18Levy-Lahad E. Wasco W. Poorkaj P. Romano D.M. Oshima J. Pettingell W.H., Yu, C.E. Jondro P.D. Schmidt S.D. Wang K. et al.Science. 1995; 269: 973-977Crossref PubMed Scopus (2220) Google Scholar), and mutations in the presenilins were likewise found to increase Aβ42 production in transfected cell lines (19Borchelt D.R. Thinakaran G. Eckman C.B. Lee M.K. Davenport F. Ratovitsky T. Prada C.M. Kim G. Seekins S. Yager D. et al.Neuron. 1996; 17: 1005-1013Abstract Full Text Full Text PDF PubMed Scopus (1329) Google Scholar, 20Citron M. Westaway D. Xia W. Carlson G. Diehl T. Levesque G. Johnson-Wood K. Lee M. Seubert P. Davis A. et al.Nat. Med. 1997; 3: 67-72Crossref PubMed Scopus (1157) Google Scholar, 21Tomita T. Maruyama K. Saido T.C. Kume H. Shinozaki K. Tokuhiro S. Capell A. Walter J. Grunberg J. Haass C. Iwatsubo T. Obata K. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 2025-2030Crossref PubMed Scopus (349) Google Scholar), transgenic animals (19Borchelt D.R. Thinakaran G. Eckman C.B. Lee M.K. Davenport F. Ratovitsky T. Prada C.M. Kim G. Seekins S. Yager D. et al.Neuron. 1996; 17: 1005-1013Abstract Full Text Full Text PDF PubMed Scopus (1329) Google Scholar, 20Citron M. Westaway D. Xia W. Carlson G. Diehl T. Levesque G. Johnson-Wood K. Lee M. Seubert P. Davis A. et al.Nat. Med. 1997; 3: 67-72Crossref PubMed Scopus (1157) Google Scholar, 22Duff K. Eckman C. Zehr C., Yu, X. Prada C.M. Perez-tur J. Hutton M. Buee L. Harigaya Y. Yager D. Morgan D. Gordon M.N. Holcomb L. Refolo L. Zenk B. Hardy J. Younkin S. Nature. 1996; 383: 710-713Crossref PubMed Scopus (1310) Google Scholar), and human plasma (23Scheuner D. Eckman C. Jensen M. Song X. Citron M. Suzuki N. Bird T.D. Hardy J. Hutton M. Kukull W. et al.Nat. Med. 1996; 2: 864-870Crossref PubMed Scopus (2253) Google Scholar). Thus, presenilins somehow modulate γ-secretase activity. In fact, deletion of PS1 in mice dramatically reduced γ-secretase activity (24De Strooper B. Saftig P. Craessaerts K. Vanderstichele H. Guhde G. Annaert W. Von Figura K. Van Leuven F. Nature. 1998; 391: 387-390Crossref PubMed Scopus (1544) Google Scholar), and knockout of both PS1 and PS2 resulted in complete abolition of this APP processing event (25Herreman A. Serneels L. Annaert W. Collen D. Schoonjans L. De Strooper B. Nat. Cell Biol. 2000; 2: 461-462Crossref PubMed Scopus (450) Google Scholar, 26Zhang Z. Nadeau P. Song W. Donoviel D. Yuan M. Bernstein A. Yankner B.A. Nat. Cell Biol. 2000; 2: 463-465Crossref PubMed Scopus (358) Google Scholar), demonstrating that presenilins are absolutely required for proteolysis by γ-secretase. γ-Secretase carries out an unusual proteolysis within the middle of the predicted transmembrane domain of its substrates, C99, generated by β-secretase, and C83, generated through alternative processing by α-secretase (Fig. 1 A). Indeed, a helical conformation of the γ-secretase cleavage site in APP can explain why nearby FAD-causing mutations specifically increase Aβ42production; these mutations are immediately adjacent to the amide bond processed to give Aβ42 (27Wolfe M.S. Xia W. Moore C.L. Leatherwood D.D. Ostaszewski B. Donkor I.O. Selkoe D.J. Biochemistry. 1999; 38: 4720-4727Crossref PubMed Scopus (306) Google Scholar). A phenylalanine-scanning study near the γ-secretase cleavage site and the resulting effects on Aβ40 and Aβ42 production provided important experimental support for the idea that the substrate is in a helical conformation upon initial interaction with the protease (28Lichtenthaler S.F. Wang R. Grimm H. Uljon S.N. Masters C.L. Beyreuther K. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 3053-3058Crossref PubMed Scopus (192) Google Scholar). Other mutagenesis studies likewise indicated that γ-secretase has loose sequence specificity (29Tischer E. Cordell B. J. Biol. Chem. 1996; 271: 21914-21919Abstract Full Text Full Text PDF PubMed Scopus (74) Google Scholar, 30Maruyama K. Tomita T. Shinozaki K. Kume H. Asada H. Saido T.C. Ishiura S. Iwatsubo T. Obata K. Biochem. Biophys. Res. Commun. 1996; 227: 730-735Crossref PubMed Scopus (49) Google Scholar, 31Lichtenthaler S.F. Ida N. Multhaup G. Masters C.L. Beyreuther K. Biochemistry. 1997; 36: 15396-15403Crossref PubMed Scopus (80) Google Scholar). Despite the loose sequence specificity, γ-secretase nevertheless appears to process its substrate APP at specific sites. This apparent paradox can be resolved by considering distance within the membrane as a key determinant of the site of proteolysis, a concept that together with the helical model suggests that γ-secretase might catalyze an intramembranous proteolysis. The development of substrate-based inhibitors led to further indirect characterization of γ-secretase; inhibition by peptidomimetic transition-state analogues suggested an aspartyl protease mechanism (27Wolfe M.S. Xia W. Moore C.L. Leatherwood D.D. Ostaszewski B. Donkor I.O. Selkoe D.J. Biochemistry. 1999; 38: 4720-4727Crossref PubMed Scopus (306) Google Scholar). Moreover, γ-secretase appears to have loose sequence specificity for its peptidomimetic inhibitors as it displays for its substrates (27Wolfe M.S. Xia W. Moore C.L. Leatherwood D.D. Ostaszewski B. Donkor I.O. Selkoe D.J. Biochemistry. 1999; 38: 4720-4727Crossref PubMed Scopus (306) Google Scholar). Distinctions are observed for the ability of a given compound to block Aβ40 vis-à-vis Aβ42, leading to the suggestion that these two Aβ species are generated by different γ-secretases (32Citron M. Diehl T.S. Gordon G. Biere A.L. Seubert P. Selkoe D.J. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 13170-13175Crossref PubMed Scopus (273) Google Scholar, 33Klafki H. Abramowski D. Swoboda R. Paganetti P.A. Staufenbiel M. J. Biol. Chem. 1996; 271: 28655-28659Abstract Full Text Full Text PDF PubMed Scopus (156) Google Scholar). The findings that PS mutations affect Aβ42generation and that PS knockouts prevent Aβ production raised the question if presenilins are identical with the γ-secretases. Presenilins appear to bear no obvious sequence homology with known proteases, and their homology to Spe-4, a Caenorhabditis elegans protein apparently involved in vesicle transport in spermatozoa, initially suggested a less direct role in γ-secretase processing of APP (17Sherrington R. Rogaev E.I. Liang Y. Rogaeva E.A. Levesque G. Ikeda M. Chi H. Lin C. Li G. Holman K. et al.Nature. 1995; 375: 754-760Crossref PubMed Scopus (3565) Google Scholar). Furthermore, overexpression of presenilins does not lead to increased γ-secretase activity (20Citron M. Westaway D. Xia W. Carlson G. Diehl T. Levesque G. Johnson-Wood K. Lee M. Seubert P. Davis A. et al.Nat. Med. 1997; 3: 67-72Crossref PubMed Scopus (1157) Google Scholar). However, presenilins themselves undergo proteolytic processing within the hydrophobic region of the large cytosolic loop between transmembrane domain (TM) 6 and TM 7 (Fig. 1 B) to form stable heterodimeric complexes (39Thinakaran G. Borchelt D.R. Lee M.K. Slunt H.H. Spitzer L. Kim G. Ratovitsky T. Davenport F. Nordstedt C. Seeger M. et al.Neuron. 1996; 17: 181-190Abstract Full Text Full Text PDF PubMed Scopus (937) Google Scholar, 40Capell A. Grunberg J. Pesold B. Diehlmann A. Citron M. Nixon R. Beyreuther K. Selkoe D.J. Haass C. J. Biol. Chem. 1998; 273: 3205-3211Abstract Full Text Full Text PDF PubMed Scopus (298) Google Scholar). (Most studies support an eight-TM topology for the presenilins (34Doan A. Thinakaran G. Borchelt D.R. Slunt H.H. Ratovitsky T. Podlisny M. Selkoe D.J. Seeger M. Gandy S.E. Price D.L. Sisodia S.S. Neuron. 1996; 17: 1023-1030Abstract Full Text Full Text PDF PubMed Scopus (355) Google Scholar, 35Li X. Greenwald I. Neuron. 1996; 17: 1015-1021Abstract Full Text Full Text PDF PubMed Scopus (184) Google Scholar, 36Li X. Greenwald I. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 7109-7114Crossref PubMed Scopus (142) Google Scholar), although six- and seven-TM topologies have also been suggested (37Lehmann S. Chiesa R. Harris D.A. J. Biol. Chem. 1997; 272: 12047-12051Abstract Full Text Full Text PDF PubMed Scopus (135) Google Scholar, 38Dewji N.N. Singer S.J. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 14025-14030Crossref PubMed Scopus (48) Google Scholar)). These heterodimers are only produced to limited levels even upon overexpression of the holoprotein (39Thinakaran G. Borchelt D.R. Lee M.K. Slunt H.H. Spitzer L. Kim G. Ratovitsky T. Davenport F. Nordstedt C. Seeger M. et al.Neuron. 1996; 17: 181-190Abstract Full Text Full Text PDF PubMed Scopus (937) Google Scholar, 41Ratovitski T. Slunt H.H. Thinakaran G. Price D.L. Sisodia S.S. Borchelt D.R. J. Biol. Chem. 1997; 272: 24536-24541Abstract Full Text Full Text PDF PubMed Scopus (190) Google Scholar, 42Podlisny M.B. Citron M. Amarante P. Sherrington R. Xia W. Zhang J. Diehl T. Levesque G. Fraser P. Haass C. et al.Neurobiol. Dis. 1997; 3: 325-337Crossref PubMed Scopus (272) Google Scholar, 43Steiner H. Capell A. Pesold B. Citron M. Kloetzel P.M. Selkoe D.J. Romig H. Mendla K. Haass C. J. Biol. Chem. 1998; 273: 32322-32331Abstract Full Text Full Text PDF PubMed Scopus (181) Google Scholar) and may be found at the cell surface (44Ray W.J. Yao M. Mumm J. Schroeter E.H. Saftig P. Wolfe M. Selkoe D.J. Kopan R. Goate A.M. J. Biol. Chem. 1999; 274: 36801-36807Abstract Full Text Full Text PDF PubMed Scopus (238) Google Scholar). Expression of exogenous presenilins leads to replacement of endogenous presenilin heterodimers with the corresponding exogenous heterodimers, indicating competition for limiting cellular factors needed for stabilization and endoproteolysis (45Thinakaran G. Harris C.L. Ratovitski T. Davenport F. Slunt H.H. Price D.L. Borchelt D.R. Sisodia S.S. J. Biol. Chem. 1997; 272: 28415-28422Abstract Full Text Full Text PDF PubMed Scopus (286) Google Scholar). FAD-causing presenilin mutants are likewise processed to stable heterodimers with one exception, a missense mutation in PS1 that leads to the aberrant splicing out of exon 9, a region that encodes the endoproteolytic cleavage site (39Thinakaran G. Borchelt D.R. Lee M.K. Slunt H.H. Spitzer L. Kim G. Ratovitsky T. Davenport F. Nordstedt C. Seeger M. et al.Neuron. 1996; 17: 181-190Abstract Full Text Full Text PDF PubMed Scopus (937) Google Scholar, 41Ratovitski T. Slunt H.H. Thinakaran G. Price D.L. Sisodia S.S. Borchelt D.R. J. Biol. Chem. 1997; 272: 24536-24541Abstract Full Text Full Text PDF PubMed Scopus (190) Google Scholar). This PS1 ΔE9 variant is an active presenilin, like other FAD-causing presenilin mutants, causes increased Aβ42 production (19Borchelt D.R. Thinakaran G. Eckman C.B. Lee M.K. Davenport F. Ratovitsky T. Prada C.M. Kim G. Seekins S. Yager D. et al.Neuron. 1996; 17: 1005-1013Abstract Full Text Full Text PDF PubMed Scopus (1329) Google Scholar, 46Steiner H. Romig H. Grim M.G. Philipp U. Pesold B. Citron M. Baumeister R. Haass C. J. Biol. Chem. 1999; 274: 7615-7618Abstract Full Text Full Text PDF PubMed Scopus (114) Google Scholar). Upon overexpression, most PS1 ΔE9 is rapidly degraded similar to unprocessed wild-type presenilins; however, a small portion of this PS1 variant is stabilized in cells (41Ratovitski T. Slunt H.H. Thinakaran G. Price D.L. Sisodia S.S. Borchelt D.R. J. Biol. Chem. 1997; 272: 24536-24541Abstract Full Text Full Text PDF PubMed Scopus (190) Google Scholar, 47Zhang J. Kang D.E. Xia W. Okochi M. Mori H. Selkoe D.J. Koo E.H. J. Biol. Chem. 1998; 273: 12436-12442Abstract Full Text Full Text PDF PubMed Scopus (134) Google Scholar) and forms a high molecular weight complex like the N- and C-terminal fragments (40Capell A. Grunberg J. Pesold B. Diehlmann A. Citron M. Nixon R. Beyreuther K. Selkoe D.J. Haass C. J. Biol. Chem. 1998; 273: 3205-3211Abstract Full Text Full Text PDF PubMed Scopus (298) Google Scholar, 48Yu G. Chen F. Nishimura M. Steiner H. Tandon A. Kawarai T. Arawaka S. Supala A. Song Y.Q. Rogaeva E. et al.J. Biol. Chem. 2000; 275: 27348-27353Abstract Full Text Full Text PDF PubMed Google Scholar), suggesting that it can interact with the same limiting cellular factors as wild-type presenilins. These observations are consistent with the idea that the bioactive form of presenilin is the heterodimer and that the hydrophobic region is an inhibitory domain. However, several artificial PS variants have been generated which are biologically active/inactive independent of their ability to undergo endoproteolysis (49Steiner H. Romig H. Pesold B. Philipp U. Baader M. Citron M. Loetscher H. Jacobsen H. Haass C. Biochemistry. 1999; 38: 14600-14605Crossref PubMed Scopus (86) Google Scholar, 50Steiner H. Kostka M. Romig H. Basset G. Pesold B. Hardy J.A. Capell A. Meyn L. Grim M.G. Baumeister R. Fechteler K. Haass C. Nat. Cell Biol. 2000; 2: 848-851Crossref PubMed Scopus (251) Google Scholar). Therefore, further evidence is required to finally demonstrate the role of PS endoproteolysis for their biological and pathological the characterization of γ-secretase as an aspartyl protease that might catalyze an unusual intramembranous proteolysis, the of presenilins were and found to two predicted to within TM 6 and TM the large cytosolic loop M.S. Xia W. Ostaszewski Diehl T.S. Selkoe D.J. Nature. 1999; PubMed Scopus Google Scholar). These appear to with other within the membrane and with the γ-secretase cleavage in APP (Fig. 1 of to PS1 and these → PS1 mutants in a with to γ-secretase processing of effects on APP processing were observed even mutations to M.S. Xia W. Ostaszewski Diehl T.S. Selkoe D.J. Nature. 1999; PubMed Scopus Google Scholar) or (49Steiner H. Romig H. Pesold B. Philipp U. Baader M. Citron M. Loetscher H. Jacobsen H. Haass C. Biochemistry. 1999; 38: 14600-14605Crossref PubMed Scopus (86) Google Scholar, U. K. Romig H. Walter J. Grunberg J. M. Haass C. Biochemistry. 1999; 38: PubMed Scopus Google Scholar) were indicating the of these two as and suggesting that the effects are not because of These effects have been by several and have been for both PS1 and PS2 G. Chen F. Nishimura M. Steiner H. Tandon A. Kawarai T. Arawaka S. Supala A. Song Y.Q. Rogaeva E. et al.J. Biol. Chem. 2000; 275: 27348-27353Abstract Full Text Full Text PDF PubMed Google Scholar, H. Romig H. Pesold B. Philipp U. Baader M. Citron M. Loetscher H. Jacobsen H. Haass C. Biochemistry. 1999; 38: 14600-14605Crossref PubMed Scopus (86) Google M.S. Xia W. Ostaszewski Diehl T.S. Selkoe D.J. Nature. 1999; PubMed Scopus Google Scholar, U. K. Romig H. Walter J. Grunberg J. M. Haass C. Biochemistry. 1999; 38: PubMed Scopus Google Scholar, Xia W. T. Wolfe M.S. Selkoe D.J. J. Biol. Chem. 2000; 275: Full Text Full Text PDF PubMed Scopus Google Scholar, H. K. Capell A. Romig H. Grim M.G. S. Hardy Yu, X. M. Fechteler K. et al.J. Biol. Chem. 1999; 274: Full Text Full Text PDF PubMed Scopus (272) Google Scholar), although a study that the TM 7 may be less of mutagenesis with the TM 6 A. Steiner H. Romig H. S. Baader M. Grim M.G. Baumeister R. Haass C. Nat. Cell Biol. 2000; 2: PubMed Scopus Google Scholar). The are for γ-secretase independent of their role in presenilin mutation in the PS1 ΔE9 variant γ-secretase even though endoproteolysis is not required of this presenilin variant M.S. Xia W. Ostaszewski Diehl T.S. Selkoe D.J. Nature. 1999; PubMed Scopus Google Scholar). these suggested that presenilins might be the of γ-secretase; upon interaction with as limiting cellular presenilin the two and the two presenilin one to the active site of γ-secretase. The of however, further because of a PS in human cells leads to a PS variant that is not M. S. A. Baumeister R. Romig H. Walter J. Capell A. Steiner H. Haass C. J. Biol. Chem. 2000; 275: Full Text Full Text PDF PubMed Scopus Google Scholar), indicating that a may at be required to The presenilins are also for processing of the Notch a for S. Science. 1999; PubMed Scopus Google Scholar). in the Notch is processed by a protease, resulting in a heterodimeric that is to the cell surface (Fig. 1 A). Upon interaction with a the of Notch is by a apparently identical to tumor necrosis factor-α converting enzyme C. F. N. C. J.R. A. P. Black R.A. A. Cell. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar, Schroeter E.H. A. X. D.J. W.J. Kopan R. Cell. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar). such as and are the identified that the APP (8Lammich S. Kojro E. Postina R. Gilbert S. Pfeiffer R. Jasionowski M. Haass C. Fahrenholz F. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 3922-3927Crossref PubMed Scopus (977) Google Scholar, 9Buxbaum J.D. Liu K.N. Luo Y. Slack J.L. Stocking K.L. Peschon J.J. Johnson R.S. Castner B.J. Cerretti D.P. Black R.A. J. Biol. Chem. 1998; 273: 27765-27767Abstract Full Text Full Text PDF PubMed Scopus (834) Google Scholar). The is within the transmembrane domain to the Notch intracellular domain which to the where it with and the family of factors E.H. J.A. Kopan R. Nature. 1998; PubMed Scopus Google Scholar). formation is absolutely required for the Notch of a mutation near the transmembrane cleavage site in in an in mice identical to that observed upon knockout of the protein S.S. A. Schroeter E.H. Mumm Kopan R. Nature. 2000; PubMed Scopus Google Scholar). The between APP and Notch processing are only are both by but also the transmembrane of both are processed by a protease that presenilins. of PS1 in mice is with a similar to that observed upon knockout of H. Chen H. D.J. M.E. Chen Price D.L. Van Sisodia S.S. Nature. 1997; PubMed Scopus Google Scholar, J. Chen Xia W. Selkoe D.J. S. Cell. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar), and the double knockout is even similar A. D. Annaert W. Saftig P. Craessaerts K. Serneels L. L. F. Vanderstichele H. et Natl. Acad. Sci. U. S. A. 1999; 96: PubMed Scopus Google D.B. Ikeda M. H. P.S. Bernstein A. Genes 1999; 13: PubMed Scopus Google Scholar). in PS1 dramatically formation Strooper B. Annaert W. P. Saftig P. Craessaerts K. Mumm Schroeter E.H. Wolfe M.S. W.J. Goate A. Kopan R. Nature. 1999; PubMed Scopus Google Scholar), and the complete of presenilins in abolition of production (25Herreman A. Serneels L. Annaert W. Collen D. Schoonjans L. De Strooper B. Nat. Cell Biol. 2000; 2: 461-462Crossref PubMed Scopus (450) Google Scholar, 26Zhang Z. Nadeau P. Song W. Donoviel D. Yuan M. Bernstein A. Yankner B.A. Nat. Cell Biol. 2000; 2: 463-465Crossref PubMed Scopus (358) Google Scholar). of cells with γ-secretase inhibitors the transmembrane cleavage site within APP likewise production Strooper B. Annaert W. P. Saftig P. Craessaerts K. Mumm Schroeter E.H. Wolfe M.S. W.J. Goate A. Kopan R. Nature. 1999; PubMed Scopus Google Scholar) and C. P. C. Xia W. Wolfe M.S. G. Selkoe D.J. J. 2000; PubMed Scopus (114) Google Scholar) and Notch a gene C. P. C. Xia W. Wolfe M.S. G. Selkoe D.J. J. 2000; PubMed Scopus (114) Google Scholar). Moreover, the two TM in presenilins are required for cleavage of the Notch TM as with γ-secretase of PS1 or PS2 in of and (44Ray W.J. Yao M. Mumm J. Schroeter E.H. Saftig P. Wolfe M. Selkoe D.J. Kopan R. Goate A.M. J. Biol. Chem. 1999; 274: 36801-36807Abstract Full Text Full Text PDF PubMed Scopus (238) Google Scholar, H. K. Capell A. Romig H. Grim M.G. S. Hardy Yu, X. M. Fechteler K. et al.J. Biol. Chem. 1999; 274: Full Text Full Text PDF PubMed Scopus (272) Google Scholar, A. Steiner H. Romig H. S. Baader M. Grim M.G. Baumeister R. Haass C. Nat. Cell Biol. 2000; 2: PubMed Scopus Google Scholar, C. P. C. Xia W. Wolfe M.S. G. Selkoe D.J. J. 2000; PubMed Scopus (114) Google Scholar). Thus, if presenilins are the of the γ-secretases that process are also the of the proteases that the transmembrane region of a for the idea that presenilins be presenilins are found in the and early Levesque L. Craessaerts K. I. G. Westaway D. P.S. Cordell B. Fraser P. De Strooper B. J. Cell Biol. 1999; PubMed Scopus Google Scholar), γ-secretase activity apparently on the cell surface and in the and C. Koo E.H. A. Hung A.Y. Selkoe D.J. Nature. 1992; PubMed Scopus