A tailored tetravalent peptide displays dual functions to inhibit amyloid ß production and aggregation.

Inhibition of amyloid-ß peptide (Aß) accumulation in the brain is a promising approach for treatment of Alzheimer's disease (AD). Aß is produced by ß-secretase and γ-secretase in endosomes via sequential proteolysis of amyloid precursor protein (APP). Aß and APP have a common feature to readily cluster to form multimers. Here, using multivalent peptide library screens, we identified a tetravalent peptide, LME-tet, which binds APP and Aß via multivalent interactions. In cells, LME-tet-bound APP in the plasma membrane is transported to endosomes, blocking Aß production through specific inhibition of ß-cleavage, but not γ-cleavage. LME-tet further suppresses Aß aggregation by blocking formation of the ß-sheet conformation. Inhibitory effects are not observed with a monomeric peptide, emphasizing the significance of multivalent interactions for mediating these activities. Critically, LME-tet efficiently reduces Aß levels in the brain of AD model mice, suggesting it may hold promise for treatment of AD.

The senile plaque: Morphological differences in APP knock-in mice brains by fixatives.

The morphology of senile plaques depends on the APP knock-in mice brain fixative. Solid forms of senile plaques were detected in APP knock-in mice after formic acid treatment with Davidson's and Bouin's fluid fixative as the brain of AD patients. Aß42 was deposited as cored plaques and Aß38 accumulated around Aß42.

A nontoxigenic form of Shiga toxin 2 suppresses the production of amyloid ß by altering the intracellular transport of amyloid precursor protein through its receptor-binding B-subunit.

Accumulation of amyloid-ß peptide (Aß) in neuronal cells and in the extracellular regions in the brain is a major cause of Alzheimer's disease (AD); therefore, inhibition of Aß accumulation offers a promising approach for therapeutic strategies against AD. Aß is produced by sequential proteolysis of amyloid precursor protein (APP) in late/recycling endosomes after endocytosis of APP located in the plasma membrane. Aß is then released from cells in a free form or in an exosome-bound form. Shiga toxin (Stx) is a major virulence factor of enterohemorrhagic Escherichia coli. Recently, we found that one of the Stx subtypes, Stx2a, has a unique intracellular transport route after endocytosis through its receptor-binding B-subunit. A part of Stx2a can be transported to late/recycling endosomes and then degraded in a lysosomal acidic compartment, although in general Stx is transported to the Golgi and then to the endoplasmic reticulum in a retrograde manner. In this study, we found that treatment of APP-expressing cells with a mutant Stx2a (mStx2a), lacking cytotoxic activity because of mutations in the catalytic A-subunit, stimulated the transport of APP to the acidic compartment, which led to degradation of APP and a reduction in the amount of Aß. mStx2a-treatment also inhibited the extracellular release of Aß. Therefore, mStx2a may provide a new strategy to inhibit the production of Aß by modulating the intracellular transport of APP.

A potential defense mechanism against amyloid deposition in cerebellum.

Amyloid-ß (Aß) is the major component of senile plaques in Alzheimer's disease (AD) brains. Senile plaques are generally observed in cerebral cortex (CTX) rather than cerebellum (CBL) in AD patients. However, it is not clear why CBL has less Aß deposition than CTX. It is very important to elucidate the mechanism of suppressing Aß deposition in CBL, because it contributes to understanding of not only AD pathogenesis but also prevention and cure of AD. In this study, we explored to figure out the potential mechanism of reducing Aß deposition in CBL. We observed higher age-dependent elevation of Aß level in CTX rather than CBL of human APP knock-in AD model mice, although we detected no significant differences in the levels of interstitial fluid Aß in these brain tissues. These data imply that less Aß deposition in CBL is due to enhanced Aß clearance rather than altered Aß production in CBL. To gain insights into Aß clearance in CBL, we injected fluorescence-labeled Aß in brain tissues. Importantly diffusion area of fluorescent Aß in CBL was roughly six-times larger than that in CTX within 2 h of injection. In addition, injected Aß area in CBL decreased sharply after 24 h and CBL-injected Aß was robustly detected in deep cervical lymph nodes (DcLNs). In contrast, diffusion area of fluorescent Aß in CTX was consistent up to 72 h and CTX-injected Aß was faintly detected in DcLNs. Our data suggest that enhanced Aß drainage in association with meningeal lymphatic system is responsible for less Aß deposition in CBL.

Successive cleavage of ß-amyloid precursor protein by γ-secretase.

γ-Secretase is a multimeric aspartyl protease that cleaves the membrane-spanning region of the ß-carboxyl terminal fragment (ßCTF) generated from ß-amyloid precursor protein. γ-Secretase defines the generated molecular species of amyloid ß-protein (Aß), a critical molecule in the pathogenesis of Alzheimer's disease (AD). Many therapeutic trials for AD have targeted γ-secretase. However, in contrast to the great efforts in drug discovery, the enzymatic features and cleavage mechanism of γ-secretase are poorly understood. Here we review our protein-chemical analyses of the cleavage products generated from ßCTF by γ-secretase, which revealed that Aß was produced by γ-secretase through successive cleavages of ßCTF, mainly at three-residue intervals. Two representative product lines were identified. ε-Cleavages occur first at Leu49-Val50 and Thr48-Leu49 of ßCTF (in accordance with Aß numbering). Longer generated Aßs, Aß49 and Aß48, are precursors to the majority of Aß40 and Aß42, concomitantly releasing the tripeptides, ITL, VIV, and IAT; and VIT and TVI, respectively. A portion of Aß42 is processed further to Aß38, releasing a tetrapeptide, VVIA. The presence of additional multiple minor pathways may reflect labile cleavage activities derived from the conformational flexibility of γ-secretase through molecular interactions. Because these peptide byproducts are not secreted and remain within the cells, they may serve as an indicator that reflects γ-secretase activity more directly than secreted Aß.

γ-Secretase Activity Is Associated with Braak Senile Plaque Stages.

Amyloid ß-proteins (Aßs) Aß1-42 and Aß1-43 are converted via two product lines of γ-secretase to Aß1-38 and Aß1-40. This parallel stepwise processing model of γ-secretase predicts that Aß1-42 and Aß1-43, and Aß1-38 and Aß1-40 are proportional to each other, respectively. To obtain further insight into the mechanisms of parenchymal Aß deposition, these four Aß species were quantified in insoluble fractions of human brains (Brodmann areas 9 to 11) at various Braak senile plaque (SP) stages, using specific enzyme-linked immunosorbent assays. With advancing SP stages, the amounts of deposited Aß1-43 in the brain increased proportionally to those of Aß1-42. Similarly, the amounts of deposited Aß1-38 correlated with those of Aß1-40. Surprisingly, the ratios of deposited Aß1-38/Aß1-42 and Aß1-40/Aß1-43 were proportional and discriminated the Braak SP stages accurately. This result indicates that the generation of Aß1-38 and Aß1-40 decreased and the generation of Aß1-42 and Aß1-43 increased with advancing SP stages. Thus, Aßs deposition might depend on γ-secretase activity, as it does in the cerebrospinal fluid. Here, the extracted γ-secretase from Alzheimer disease brains generates an amount of Aß1-42 and Aß1-43 compared with cognitively normal brains. This refractory γ-secretase localized in detergent-solubilized fractions from brain cortices. But activity modulated γ-secretase, which decreases Aß1-42 and Aß1-43 in the cerebrospinal fluid, localized in detergent-insoluble fractions. These drastic alterations reflect Aß situation in Alzheimer disease brains.

Curcumin Derivative GT863 Inhibits Amyloid-Beta Production via Inhibition of Protein N-Glycosylation.

Amyloid-ß (Aß) peptides play a crucial role in the pathogenesis of Alzheimer's disease (AD). Aß production, aggregation, and clearance are thought to be important therapeutic targets for AD. Curcumin has been known to have an anti-amyloidogenic effect on AD. In the present study, we performed screening analysis using a curcumin derivative library with the aim of finding derivatives effective in suppressing Aß production with improved bioavailability of curcumin using CHO cells that stably express human amyloid-ß precursor protein and using human neuroblastoma SH-SY5Y cells. We found that the curcumin derivative GT863/PE859, which has been shown to have an inhibitory effect on Aß and tau aggregation in vivo, was more effective than curcumin itself in reducing Aß secretion. We further found that GT863 inhibited neither ß- nor γ-secretase activity, but did suppress γ-secretase-mediated cleavage in a substrate-dependent manner. We further found that GT863 suppressed N-linked glycosylation, including that of the γ-secretase subunit nicastrin. We also found that mannosidase inhibitors that block the mannose trimming step of N-glycosylation suppressed Aß production in a similar fashion, as was observed as a result of treatment with GT863. Collectively, these results suggest that GT863 downregulates N-glycosylation, resulting in suppression of Aß production without affecting secretase activity.

Glycosylation status of nicastrin influences catalytic activity and substrate preference of γ-secretase.

γ-Secretase complex, the assembly of nicastrin (NCT), Presenilin (PS), Presenilin Enhancer-2 (PEN-2) and Anterior pharynx defective 1 (Aph-1), catalyzes the cleavage of amyloid precursor protein to generate amyloid-ß protein (Aß), the main culprit of Alzheimer's disease. NCT becomes matured through complex glycosylation and play important role in γ-secretase activity by interacting with catalytic subunit PS. However, the role of NCT glycosylation on γ-secretase activity and substrate specificity is still unknown. The purpose of this study is to investigate the effect of NCT glycosylation on γ-secretase activity and substrate specificity in a group of glycosylation mutant lectin resistant CHO (Lec) cells. CHO Lec-1 cells lack glycosyltransferase-I, GnT-I, thus N-glycan on NCT are all oligomannose type, whereas CHO Lec-2 cells synthesize NCT containing sialic acid deficient oligosaccharides due to the impairment of cytidine 5'-monophosphate-sialic acid transporter. Here, we reported that mutant CHO Lec-1 and Lec-2 reduced γ-secretase activity in both cell-based and biochemical assays, and that CHO Lec-1 preferentially reduced Aß generation. Endogenous level of γ-secretase complex, subcellular distribution of γ-secretase subunits and the level of functional γ-secretase complex remained unchanged in mutants. Interestingly, Coimmunoprecipitation study revealed that mutant γ-secretase could recognize substrate as well as parental γ-secretase. Our data suggests that thorough glycosylation of NCT is critical for enzymatic activity and substrate preference of γ-secretase.

Pleckstrin homology domain of p210 BCR-ABL interacts with cardiolipin to regulate its mitochondrial translocation and subsequent mitophagy.

Chronic myeloid leukemia (CML) is caused by the chimeric protein p210 BCR-ABL encoded by a gene on the Philadelphia chromosome. Although the kinase domain of p210 BCR-ABL is an active driver of CML, the pathological role of its pleckstrin homology (PH) domain remains unclear. Here, we carried out phospholipid vesicle-binding assays to show that cardiolipin (CL), a characteristic mitochondrial phospholipid, is a unique ligand of the PH domain. Arg726, a basic amino acid in the ligand-binding region, was crucial for ligand recognition. A subset of wild-type p210 BCR-ABL that was transiently expressed in HEK293 cells was dramatically translocated from the cytosol to mitochondria in response to carbonyl cyanide m-chlorophenylhydrazone (CCCP) treatment, which induces mitochondrial depolarization and subsequent externalization of CL to the organelle's outer membrane, whereas an R726A mutant of the protein was not translocated. Furthermore, only wild-type p210 BCR-ABL, but not the R726A mutant, suppressed CCCP-induced mitophagy and subsequently enhanced reactive oxygen species production. Thus, p210 BCR-ABL can change its intracellular localization via interactions between the PH domain and CL to cope with mitochondrial damage. This suggests that p210 BCR-ABL could have beneficial effects for cancer proliferation, providing new insight into the PH domain's contribution to CML pathogenesis.

Alanine substitutions in the GXXXG motif alter C99 cleavage by γ-secretase but not its dimerization.

The amyloid ß (Aß) protein is a major component of senile plaques, one of the neuropathological hallmarks of Alzheimer's disease. Amyloidogenic processing of amyloid precursor protein (APP) by ß- and γ-secretases leads to production of Aß. APP contains tandem triple repeats of the GXXXG motif in its extracellular juxtamembrane and transmembrane regions. It is reported that the GXXXG motif is related to protein-protein interactions, but it remains controversial whether the GXXXG motif in APP is involved in substrate dimerization and whether dimerization affects γ-secretase-dependent cleavage. Therefore, the relationship between the GXXXG motifs, substrate dimerization, and γ-secretase-dependent cleavage sites remains unclear. Here, we applied blue native poly acrylamide gel electrophoresis to examine the effect of alanine substitutions within the GXXXG motifs of APP carboxyl terminal fragment (C99) on its dimerization and Aß production. Surprisingly, alanine substitutions in the motif failed to alter C99 dimerization in detergent soluble state. Cell-based and solubilized γ-secretase assays demonstrated that increasing alanine substitutions in the motif tended to decrease long Aß species such as Aß42 and Aß43 and to increase in short Aß species concomitantly. Our data suggest that the GXXXG motif is crucial for Aß production, but not for C99 dimerization.

Affinity-Based Screening of Tetravalent Peptides Identifies Subtype-Selective Neutralizers of Shiga Toxin 2d, a Highly Virulent Subtype, by Targeting a Unique Amino Acid Involved in Its Receptor Recognition.

Shiga toxin (Stx), a major virulence factor of enterohemorrhagic Escherichia coli (EHEC), can be classified into two subgroups, Stx1 and Stx2, each consisting of various closely related subtypes. Stx2 subtypes Stx2a and Stx2d are highly virulent and linked with serious human disorders, such as acute encephalopathy and hemolytic-uremic syndrome. Through affinity-based screening of a tetravalent peptide library, we previously developed peptide neutralizers of Stx2a in which the structure was optimized to bind to the B-subunit pentamer. In this study, we identified Stx2d-selective neutralizers by targeting Asn16 of the B subunit, an amino acid unique to Stx2d that plays an essential role in receptor binding. We synthesized a series of tetravalent peptides on a cellulose membrane in which the core structure was exactly the same as that of peptides in the tetravalent library. A total of nine candidate motifs were selected to synthesize tetravalent forms of the peptides by screening two series of the tetravalent peptides. Five of the tetravalent peptides effectively inhibited the cytotoxicity of Stx2a and Stx2d, and notably, two of the peptides selectively inhibited Stx2d. These two tetravalent peptides bound to the Stx2d B subunit with high affinity dependent on Asn16. The mechanism of binding to the Stx2d B subunit differed from that of binding to Stx2a in that the peptides covered a relatively wide region of the receptor-binding surface. Thus, this highly optimized screening technique enables the development of subtype-selective neutralizers, which may lead to more sophisticated treatments of infections by Stx-producing EHEC.

The A673T mutation in the amyloid precursor protein reduces the production of ß-amyloid protein from its ß-carboxyl terminal fragment in cells.

INTRODUCTION: The A673T mutation in the amyloid precursor protein (APP) protects against Alzheimer's disease by reducing ß-amyloid protein (Aß) production. This mutation reduced the release of the soluble APP fragment (sAPPß), which is processed by ß-secretase, suggesting a concomitant decrease in the ß-carboxyl fragment of APP (C99), which is a direct substrate of γ-secretase for Aß production. However, it remains controversial whether the level of C99 is significantly reduced in cells expressing APP that carry A673T as the cause of reduced Aß production. Here, we investigated the effect of the A673T mutation in C99 on γ-cleavage in cells. RESULTS: We found that the level of C99 in cells expressing APP A673T was indistinctive of that observed in cells expressing wild-type APP, although the release of sAPPß was significantly reduced in the APP A673T cells. In addition, our reconstituted ß-secretase assay demonstrated no significant difference in ß-cleavage on an APP fragment carrying the A673T mutation compared with the wild-type fragment. Importantly, cells expressing C99 containing the A673T mutation (C99 A2T; in accordance with the Aß numbering) produced roughly half the level of Aß compared with the wild-type C99, suggesting that the C99 A2T is an insufficient substrate of γ-secretase in cells. A cell-free γ-secretase assay revealed that Aß production from the microsomal fraction of cells expressing C99 A2T was diminished. A sucrose gradient centrifugation analysis indicated that the levels of the C99 A2T that was codistributed with γ-secretase components in the raft fractions were reduced significantly. CONCLUSIONS: Our data indicate that the A673T mutation in APP alters the release of sAPPß, but not the C99 level, and that the C99 A2T is an inefficient substrate for γ-secretase in cell-based assay.

γ-Secretase associated with lipid rafts: multiple interactive pathways in the stepwise processing of ß-carboxyl-terminal fragment.

γ-Secretase generates amyloid ß-protein (Aß), a pathogenic molecule in Alzheimer disease, through the intramembrane cleavage of the ß-carboxyl-terminal fragment (ßCTF) of ß-amyloid precursor protein. We previously showed the framework of the γ-secretase cleavage, i.e. the stepwise successive processing of ßCTF at every three (or four) amino acids. However, the membrane integrity of γ-secretase was not taken into consideration because of the use of the 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonic acid-solubilized reconstituted γ-secretase system. Here, we sought to address how the membrane-integrated γ-secretase cleaves ßCTF by using γ-secretase associated with lipid rafts. Quantitative analyses using liquid chromatography-tandem mass spectrometry of the ßCTF transmembrane domain-derived peptides released along with Aß generation revealed that the raft-associated γ-secretase cleaves ßCTF in a stepwise sequential manner, but novel penta- and hexapeptides as well as tri- and tetrapeptides are released. The cropping of these peptides links the two major tripeptide-cleaving pathways generating Aß40 and Aß42 at several points, implying that there are multiple interactive pathways for the stepwise cleavages of ßCTF. It should be noted that Aß38 and Aß43 are generated through three routes, and γ-secretase modulator 1 enhances all the three routes generating Aß38, which results in decreases in Aß42 and Aß43 and an increase in Aß38. These observations indicate that multiple interactive pathways for stepwise successive processing by γ-secretase define the species and quantity of Aß produced.

Substrate ectodomain is critical for substrate preference and inhibition of γ-secretase.

Understanding the substrate recognition mechanism of γ-secretase is a key step for establishing substrate-specific inhibition of amyloid ß-protein (Aß) production. However, it is widely believed that γ-secretase is a promiscuous protease and that its substrate-specific inhibition is elusive. Here we show that γ-secretase distinguishes the ectodomain length of substrates and preferentially captures and cleaves substrates containing a short ectodomain. We also show that a subset of peptides containing the CDCYCxxxxCxCxSC motif binds to the amino terminus of C99 and inhibits Aß production in a substrate-specific manner. Interestingly, these peptides suppress ß-secretase-dependent cleavage of APP, but not that of sialyltransferase 1. Most importantly, intraperitoneal administration of peptides into mice results in a significant reduction in cerebral Aß levels. This report provides direct evidence of the substrate preference of γ-secretase and its mechanism. Our results demonstrate that the ectodomain of C99 is a potent target for substrate-specific anti-Aß therapeutics to combat Alzheimer's disease.

Modulation of lipid kinase PI4KIIα activity and lipid raft association of presenilin 1 underlies γ-secretase inhibition by ginsenoside (20S)-Rg3.

Amyloid ß-peptide (Aß) pathology is an invariant feature of Alzheimer disease, preceding any detectable clinical symptoms by more than a decade. To this end, we seek to identify agents that can reduce Aß levels in the brain via novel mechanisms. We found that (20S)-Rg3, a triterpene natural compound known as ginsenoside, reduced Aß levels in cultured primary neurons and in the brains of a mouse model of Alzheimer disease. The (20S)-Rg3 treatment induced a decrease in the association of presenilin 1 (PS1) fragments with lipid rafts where catalytic components of the γ-secretase complex are enriched. The Aß-lowering activity of (20S)-Rg3 directly correlated with increased activity of phosphatidylinositol 4-kinase IIα (PI4KIIα), a lipid kinase that mediates the rate-limiting step in phosphatidylinositol 4,5-bisphosphate synthesis. PI4KIIα overexpression recapitulated the effects of (20S)-Rg3, whereas reduced expression of PI4KIIα abolished the Aß-reducing activity of (20S)-Rg3 in neurons. Our results substantiate an important role for PI4KIIα and phosphoinositide modulation in γ-secretase activity and Aß biogenesis.

γ-Secretase-Dependent Proteolysis of Transmembrane Domain of Amyloid Precursor Protein: Successive Tri- and Tetrapeptide Release in Amyloid ß-Protein Production.

γ-Secretase cleaves the carboxyl-terminal fragment (ßCTF) of APP not only in the middle of the transmembrane domain (γ-cleavage), but also at sites close to the membrane/cytoplasm boundary (ε-cleavage), to produce the amyloid ß protein (Aß) and the APP intracellular domain (AICD), respectively. The AICD49-99 and AICD50-99 species were identified as counterparts of the long Aß species Aß48 and Aß49, respectively. We found that Aß40 and AICD50-99 were the predominant species in cells expressing wild-type APP and presenilin, whereas the production of Aß42 and AICD49-99 was enhanced in cells expressing familial Alzheimer's disease mutants of APP and presenilin. These long Aß species were identified in cell lysates and mouse brain extracts, which suggests that ε-cleavage is the first cleavage of ßCTF to produce Aß by γ-secretase. Here, we review the progress of research on the mechanism underlying the proteolysis of the APP transmembrane domain based on tri- and tetrapeptide release.

Potent amyloidogenicity and pathogenicity of Aß43.

The amyloid-ß peptide Aß42 is known to be a primary amyloidogenic and pathogenic agent in Alzheimer's disease. However, the role of Aß43, which is found just as frequently in the brains of affected individuals, remains unresolved. We generated knock-in mice containing a pathogenic presenilin-1 R278I mutation that causes overproduction of Aß43. Homozygosity was embryonic lethal, indicating that the mutation involves a loss of function. Crossing amyloid precursor protein transgenic mice with heterozygous mutant mice resulted in elevated Aß43, impairment of short-term memory and acceleration of amyloid-ß pathology, which accompanied pronounced accumulation of Aß43 in plaque cores similar in biochemical composition to those observed in the brains of affected individuals. Consistently, Aß43 showed a higher propensity to aggregate and was more neurotoxic than Aß42. Other pathogenic presenilin mutations also caused overproduction of Aß43 in a manner correlating with Aß42 and with the age of disease onset. These findings indicate that Aß43, an overlooked species, is potently amyloidogenic, neurotoxic and abundant in vivo.

The production ratios of AICDε51 and Aß42 by intramembrane proteolysis of ßAPP do not always change in parallel.

BACKGROUND: During intramembrane proteolysis of ß-amyloid protein precursor (ßAPP) by presenilin (PS)/γ-secretase, ε-cleavages at the membrane-cytoplasmic border precede γ-cleavages at the middle of the transmembrane domain. Generation ratios of Aß42, a critical molecule for Alzheimer's disease (AD) pathogenesis, and the major Aß40 species might be associated with ε48 and ε49 cleavages, respectively. Medicines to downregulate Aß42 production have been investigated by many pharmaceutical companies. Therefore, the ε-cleavages, rather than the γ-cleavage, might be more effective upstream targets for decreasing the relative generation of Aß42. Thus, one might evaluate compounds by analyzing the generation ratio of the ßAPP intracellular domain (AICD) species (ε-cleavage-derived), instead of that of Aß42. METHODS: Cell-free γ-secretase assays were carried out to observe de novo AICD production. Immunoprecipitation/MALDI-TOF MS analysis was carried out to detect the N-termini of AICD species. Aß and AICD species were measured by ELISA and immunoblotting techniques. RESULTS: Effects on the ε-cleavage by AD-associated pathological mutations around the ε-cleavage sites (i.e., ßAPP V642I, L648P and K649N) were analyzed. The V642I and L648P mutations caused an increase in the relative ratio of ε48 cleavage, as expected from previous reports. Cells expressing the K649N mutant, however, underwent a major ε-cleavage at the ε51 site. These results suggest that ε51, as well as ε48 cleavage, is associated with Aß42 production. Only AICDε51, though, and not Aß42 production, dramatically changed with modifications to the cell-free assay conditions. Interestingly, the increase in the relative ratio of the ε51 cleavage by the K649N mutation was not cancelled by these changes. CONCLUSION: Our current data show that the generation ratio of AICDε51 and Aß42 do not always change in parallel. Thus, to identify compounds that decrease the relative ratio of Aß42 generation, measurement of the relative level of Aß42-related AICD species (i.e., AICDε48 and AICDε51) might not be useful. Further studies to reveal how the ε-cleavage precision is decided are necessary before it will be possible to develop drugs targeting ε-cleavage as a means for decreasing Aß42 production.

gamma-Secretase: successive tripeptide and tetrapeptide release from the transmembrane domain of beta-carboxyl terminal fragment.

Amyloid beta protein (Abeta), a pathogenic molecule associated with Alzheimer's disease, is produced by gamma-secretase, which cleaves the beta-carboxyl terminal fragment (betaCTF) of beta-amyloid precursor protein in the middle of its transmembrane domain. How the cleavage proceeds within the membrane has long been enigmatic. We hypothesized previously that betaCTF is cleaved first at the membrane-cytoplasm boundary, producing two long Abetas, Abeta(48) and Abeta(49), which are processed further by releasing three residues at each step to produce Abeta(42) and Abeta(40), respectively. To test this hypothesis, we used liquid chromatography tandem mass spectrometry (LC-MS/MS) to quantify the specific tripeptides that are postulated to be released. Using CHAPSO (3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxyl-1-propanesulfonate)-reconstituted gamma-secretase system, we confirmed that Abeta(49) is converted to Abeta(43/40) by successively releasing two or three tripeptides and that Abeta(48) is converted to Abeta(42/38) by successively releasing two tripeptides or these plus an additional tetrapeptide. Most unexpectedly, LC-MS/MS quantification revealed an induction period, 3-4 min, in the generation of peptides. When extrapolated, each time line for each tripeptide appears to intercept the same point on the x-axis. According to numerical simulation based on the successive reaction kinetics, the induction period exists. These results strongly suggest that Abeta is generated through the stepwise processing of betaCTF by gamma-secretase.

Phosphoinositides suppress gamma-secretase in both the detergent-soluble and -insoluble states.

gamma-Secretase is an aspartic protease that hydrolyzes type I membrane proteins within the hydrophobic environment of the lipid bilayer. Using the CHAPSO-solubilized gamma-secretase assay system, we previously found that gamma-secretase activity was sensitive to the concentrations of detergent and phosphatidylcholine. This strongly suggests that the composition of the lipid bilayer has a significant impact on the activity of gamma-secretase. Recently, level of secreted beta-amyloid protein was reported to be attenuated by increasing levels of phosphatidylinositol 4,5-diphosphate (PI(4,5)P2) in cultured cells. However, it is not clear whether PI(4,5)P2 has a direct effect on gamma-secretase activity. In this study, we found that phosphoinositides directly inhibited CHAPSO-solubilized gamma-secretase activity. Interestingly, neither phosphatidylinositol nor inositol triphosphate altered gamma-secretase activity. PI(4,5)P2 was also found to inhibit gamma-secretase activity in CHAPSO-insoluble membrane microdomains (rafts). Kinetic analysis of beta-amyloid protein production in the presence of PI(4,5)P2 suggested a competitive inhibition. Even though phosphoinositides are minor phospholipids of the membrane, the concentration of PI(4,5)P2 within the intact membrane has been reported to be in the range of 4-8 mm. The presence of PI(4,5)P2-rich rafts in the membrane has been reported in a range of cell types. Furthermore, gamma-secretase is enriched in rafts. Taking these data together, we propose that phosphoinositides potentially regulate gamma-secretase activity by suppressing its association with the substrate.