Amyloid-lowering isocoumarins are not direct inhibitors of gamma-secretase

Generation of monoclonal antibody fragments binding the native γ-secretase complex for use in structural studies

A detailed understanding of γ-secretase structure is crucially needed to elucidate its unique properties of intramembrane protein cleavage and to design therapeutic compounds for the safe treatment of Alzheimer's disease. γ-Secretase is an enzyme complex composed of four membrane proteins, and the scarcity of its supply associated with the challenges of crystallizing membrane proteins is a major hurdle for the determination of its high-resolution structure. This study addresses some of these issues, first by adapting CHO cells overexpressing γ-secretase to growth in suspension, thus yielding multiliter cultures and milligram quantities of highly purified, active γ-secretase. Next, the amounts of γ-secretase were sufficient for immunization of mice and allowed generation of Nicastrin- and Aph-1-specific monoclonal antibodies, from which Fab fragments were proteolytically prepared and subsequently purified. The amounts of γ-secretase produced are compatible with robot-assisted crystallogenesis using nanoliter technologies. In addition, our Fab fragments bind exposed regions of native γ-secretase in a dose-dependent manner without interfering with its catalytic properties and can therefore be used as specific tools to facilitate crystal formation.

γ-Secretase as a target for Alzheimer's disease

γ-Secretase is a protease complex responsible for cutting the transmembrane domain of the amyloid β-protein precursor (APP) to form the amyloid β-protein (Aβ), an aggregation-prone product that accumulates in the brain in Alzheimer's disease. As evidence suggests that Aβ is critical to Alzheimer pathogenesis, γ-secretase is considered a key target for the development of disease-modifying therapeutics. The protease complex cuts many other substrates, and some of these proteolytic events are part of signaling pathways or other important cellular functions. Among these, proteolysis of the Notch receptor is essential for signaling that is involved in a number of cell-fate determinations. Many inhibitors of γ-secretase have been identified, but it is clear that drug candidates for Alzheimer's disease should have minimal effects on the Notch signaling pathway, as serious safety issues have arisen with nonselective inhibitors. Two types of promising candidates that target this protease complex have emerged: the so-called "Notch-sparing" γ-secretase inhibitors, which block cleavage of APP selectively over that of Notch, and γ-secretase modulators, which shift the proportion of Aβ peptides produced in favor of shorter, less aggregation-prone species. The current status and prospects for these two general types of candidates will be discussed.

γ-Secretase inhibitors and modulators for Alzheimer's disease

γ-Secretase is a membrane embedded aspartyl protease complex with presenilin as the catalytic component. Along with β-secretase, this enzyme produces the amyloid β-protein of Alzheimer's disease (AD) from the amyloid β-protein precursor. Because of its key role in the pathogenesis of AD, γ-secretase has been a prime target for drug discovery, and many inhibitors of this protease have been developed. The therapeutic potential of these inhibitors is virtually negated by the fact that γ-secretase is an essential part of the Notch signaling pathway, rendering the compounds unacceptably toxic upon chronic exposure. However, these compounds have served as useful chemical tools for biological investigations. In contrast, γ-secretase modulators continue to be of keen interest as possible AD therapeutics. These modulators either shift amyloid β-protein production to shorter, less pathogenic peptides or inhibit the proteolysis of amyloid β-protein precursor selectively compared to that of Notch. The various chemical types of inhibitors and modulators will be discussed, along with their use as probes for basic biology and their potential as AD therapeutics.

The physiology of the β-amyloid precursor protein intracellular domain AICD

The amyloid-β precursor protein (βAPP) undergoes several cleavages by enzymatic activities called secretases. Numerous studies aimed at studying the biogenesis and catabolic fate of Aβ peptides, the proteinaceous component of the senile plaques that accumulate in Alzheimer's disease-affected brains. Relatively recently, another secretase-mediated β-APP-derived catabolite called APP IntraCellular Domain (AICD) entered the game. Whether AICD corresponded to a biologically inert by-pass product of βAPP processing or whether it could harbor its own function remained questionable. In this study, we review the mechanisms by which AICD is generated and how its production is regulated. Furthermore, we discuss the degradation mechanism underlying its rapid catabolic fate. Finally, we review putative AICD-related functions and more particularly, the numerous studies indicating that AICD could translocate to the nucleus and control at a transcriptional level, the expression of a series of proteins involved in various functions including the control of cell death and Aβ degradation.

Alzheimer's disease-linked presenilin mutation (PS1M146L) induces filamin expression and γ-secretase independent redistribution

Presenilin mutations are linked to the early onset familial Alzheimer's disease (FAD) and lead to a range of neuronal changes, indicating that presenilins interact with multiple cellular pathways to regulate neuronal functions. In this report, we demonstrate the effects of FAD-linked presenilin 1 mutation (PS1M146L) on the expression and distribution of filamin, an actin cross-linking protein that interacts with PS1 both physically and genetically. By using immunohistochemical methods, we evaluated hippocampal dentate gyrus for alterations of proteins involved in synaptic plasticity. Among many proteins expressed in the hippocampus, calretinin, glutamic acid decarboxylase (GAD67), parvalbumin, and filamin displayed distinct changes in their expression and/or distribution patterns. Striking anti-filamin immunoreactivity was associated with the polymorphic cells of hilar region only in transgenic mice expressing PS1M146L. In over 20% of the PS1M146L mice, the hippocampus of the left hemisphere displayed more pronounced upregulation of filamin than that of the right hemisphere. Anti-filamin labeled the hilar neurons only after the PS1M146L mice reached after four months of age. Double labeling immunohistochemical analyses showed that anti-filamin labeled neurons partially overlapped with cholecystokinin (CCK), somatostatin, GAD67, parvalbumin, and calretinin immunoreactive neurons. In cultured HEK293 cells, PS1 overexpression resulted in filamin redistribution from near cell peripheries to cytoplasm. Treatment of CHO cells stably expressing PS1 with WPE-III-31C or DAPT, selective γ-secretase inhibitors, did not suppress the effects of PS1 overexpression on filamin. These studies support a γ-secretase-independent role of PS1 in modulation of filamin-mediated actin cytoskeleton.

BACE and gamma-secretase characterization and their sorting as therapeutic targets to reduce amyloidogenesis

Secretases are named for enzymes processing amyloid precursor protein (APP), a prototypic type-1 membrane protein. This led directly to discovery of novel Aspartyl proteases (beta-secretases or BACE), a tetramer complex gamma-secretase (gamma-SC) containing presenilins, nicastrin, aph-1 and pen-2, and a new role for metalloprotease(s) of the ADAM family as a alpha-secretases. Recent advances in defining pathways that mediate endosomal-lysosomal-autophagic-exosomal trafficking now provide targets for new drugs to attenuate abnormal production of fibril forming products characteristic of AD. A key to success includes not only characterization of relevant secretases but mechanisms for sorting and transport of key metabolites to abnormal vesicles or sites for assembly of fibrils. New developments we highlight include an important role for an 'early recycling endosome' coated in retromer complex containing lipoprotein receptor LRP-II (SorLA) for switching APP to a non-amyloidogenic pathway for alpha-secretases processing, or to shuttle APP to a 'late endosome compartment' to form Abeta or AICD. LRP11 (SorLA) is of particular importance since it decreases in sporadic AD whose etiology otherwise is unknown.

Inhibition of gamma-secretase as a therapeutic intervention for Alzheimer's disease: prospects, limitations and strategies

Genetic and experimental evidence points to amyloid-beta (Abeta) peptide as the culprit in Alzheimer's disease pathogenesis. This protein fragment abnormally accumulates in the brain cortex and hippocampus of patients with Alzheimer's disease, and self-aggregates to form toxic oligomers causing neurodegeneration.Abeta is heterogeneous and produced from a precursor protein (amyloid precursor protein [APP]) by two sequential proteolytic cleavages that involve beta- and gamma-secretases. This latter enzyme represents a potentially attractive drug target since it dictates the solubility of the generated Abeta fragment by creating peptides of various lengths, namely Abeta(40) and Abeta(42), the longest being the most aggregating. gamma-Secretase comprises a molecular complex of four integral membrane proteins - presenilin, nicastrin, APH-1 and PEN-2 - and its molecular mechanism remains under extensive scrutiny. The ratio of Abeta(42) over Abeta(40) is increased by familial Alzheimer's disease mutations occurring in the presenilin genes or in APP, near the gamma-secretase cleavage site. Potent gamma-secretase inhibitors have been identified by screening drug libraries or by designing aspartyl protease transition-state analogues based on the APP substrate cleavage site. Most of these compounds are not specific for gamma-secretase cleavage of APP, and equally inhibit the processing of other gamma-secretase substrates, such as Notch and a subset of cell-surface receptors and proteins involved in embryonic development, haematopoiesis, cell adhesion and cell/cell contacts. Therefore, current research aims at finding compounds that show selectivity for APP cleavage, and particularly that inhibit the formation of the aggregating form, Abeta(42). Compounds that target the substrate docking site rather than the enzyme active site are also being investigated as an alternative strategy. The finding that some NSAID analogues preferentially inhibit the formation of Abeta(42) over Abeta(40) and do not affect Notch processing has opened a new therapeutic window. The progress in design of selective inhibitors as well as recent results obtained in animal studies prove that gamma-secretase remains among the best targets for the therapeutic control of amyloid build-up in Alzheimer's disease. The full understanding of gamma-secretase regulation may yet uncover new therapeutic leads.

Electron microscopic structure of purified, active gamma-secretase reveals an aqueous intramembrane chamber and two pores

Gamma-secretase is an intramembrane-cleaving aspartyl protease required for the normal development of metazoans because it processes Notch within cellular membranes to release its signaling domain. More than two dozen additional substrates of diverse functions have been reported, including the Notch ligands Delta and Jagged, N- and E-cadherins, and a sodium channel subunit. The protease is causally implicated in Alzheimer's disease because it releases the neurotoxic amyloid beta-peptide (Abeta) from its precursor, APP. Gamma-secretase occurs as a large complex containing presenilin (bearing the active site aspartates), nicastrin, Aph-1, and Pen-2. Because the complex contains at least 18 transmembrane domains, crystallographic approaches to its structure are difficult and remote. We recently purified the human complex essentially to homogeneity from stably expressing mammalian cells. Here, we use EM and single-particle image analysis on the purified enzyme, which produces physiological ratios of Abeta40 and Abeta42, to obtain structural information on an intramembrane protease. The 3D EM structure revealed a large, cylindrical interior chamber of approximately 20-40 A in length, consistent with a proteinaceous proteolytic site that is occluded from the hydrophobic environment of the lipid bilayer. Lectin tagging of the nicastrin ectodomain enabled proper orientation of the globular, approximately 120-A-long complex within the membrane and revealed approximately 20-A pores at the top and bottom that provide potential exit ports for cleavage products to the extra- and intracellular compartments. Our reconstructed 3D map provides a physical basis for hydrolysis of transmembrane substrates within a lipid bilayer and release of the products into distinct subcellular compartments.

APPepsilon, the epsilon-secretase-derived N-terminal product of the beta-amyloid precursor protein, behaves as a type I protein and undergoes alpha-, beta-, and gamma-secretase cleavages

beta-Amyloid peptide accumulates in the brain of patients affected by sporadic or familial forms of Alzheimer's disease. It derives from the proteolytic attacks of the beta-amyloid precursor protein (betaAPP) by beta- and gamma-secretase activities. An additional epsilon cleavage taking place a few residues C-terminal to the gamma-site has been reported, leading to the formation of an intracellular fragment referred to as APP intracellular domain C50. This epsilon cleavage received particular attention because it resembles the S3 Notch cleavage generating Notch intracellular domain. Indeed, APP intracellular domain, like its Notch counterpart, appears to mediate important physiological functions. gamma and epsilon cleavages on betaAPP appear spatio-temporally linked but pharmacologically distinct and discriminable by mutagenesis approaches. As these cleavages could be seen as either deleterious (gamma-site) or beneficial (epsilon-site), it appears of most interest to set up models aimed at studying these activities separately, particularly to design specific and bioavailable inhibitors. On the other hand, it is important to respect the topology of the substrates in order to examine physiologically relevant cleavages. Here we describe the obtention of cells overexpressing APPepsilon, the epsilon-secretase-derived N-terminal fragment of betaAPP. Interestingly, this N-terminal fragment of betaAPP was shown by biochemical and immunohistochemical approaches to behave as a genuine membrane-bound protein. APPepsilon undergoes constitutive and protein kinase C-regulated alpha-secretase cleavages. Furthermore, APPepsilon is targeted by the beta-secretase beta-site APP-cleaving enzyme and is subsequently cleaved by gamma-secretase. The resulting beta-amyloid peptide production is fully prevented by various gamma-secretase inhibitors. Altogether, our study shows that APPepsilon is a relevant betaAPP derivative to study gamma-secretase activities and to design specific inhibitors without facing any rate-limiting effect of epsilon-secretase-derived cleavage.

Presenilin function and gamma-secretase activity

Alzheimer's disease (AD) is the most common form of dementia and is characterized pathologically by the accumulation of beta-amyloid (Abeta) plaques and neurofibrillary tangles in the brain. Genetic studies of AD first highlighted the importance of the presenilins (PS). Subsequent functional studies have demonstrated that PS form the catalytic subunit of the gamma-secretase complex that produces the Abeta peptide, confirming the central role of PS in AD biology. Here, we review the studies that have characterized PS function in the gamma-secretase complex in Caenorhabditis elegans, mice and in in vitro cell culture systems, including studies of PS structure, PS interactions with substrates and other gamma-secretase complex members, and the evidence supporting the hypothesis that PS are aspartyl proteases that are active in intramembranous proteolysis. A thorough knowledge of the mechanism of PS cleavage in the context of the gamma-secretase complex will further our understanding of the molecular mechanisms that cause AD, and may allow the development of therapeutics that can alter Abeta production and modify the risk for AD.

Notch1 co-localizes with CD4 on activated T cells and Notch signaling is required for IL-10 production

The effector function of activated CD4(+) T cells and secretion of cytokines are important in the establishment of productive immune responses and tolerance. We identified expression by CD4(+) T cells of Notch receptors and ligands and enhanced Notch signaling upon activation. Notch1 expression was up regulated and co-localized with CD4 upon T cell stimulation. Disruption of Notch signaling did not affect proliferation, but attenuated cytokine secretion following CD3 ligation in the absence of anti-CD28 antibody. Notch signaling was absolutely necessary for transcription of IL-10 by stimulated CD4(+) T cells. CD4(+) T cells transfected with constitutively active Notch1 failed to proliferate, but exhibited enhanced cytokine secretion upon stimulation. Our data indicates that Notch receptor signaling can influence both proliferative and cytokine responses of CD4(+) T cells. In addition, the finding that Notch signaling is required for production of IL-10 may allude to a role in immune regulation.

Signal peptide peptidase forms a homodimer that is labeled by an active site-directed gamma-secretase inhibitor

Presenilin (PS) is the presumptive catalytic component of the intramembrane aspartyl protease gamma-secretase complex. Recently a family of presenilin homologs was identified. One member of this family, signal peptide peptidase (SPP), has been shown to be a protease, which supports the hypothesis that PS and presenilin homologs are related intramembrane-cleaving aspartyl proteases. SPP has been reported as a glycoprotein of approximately 45 kDa. Our initial characterization of SPP isolated from human brain and cell lines demonstrated that SPP is primarily present as an SDS-stable approximately 95-kDa protein on Western blots. Upon heating or treatment of this approximately 95-kDa SPP band with acid, a approximately 45-kDa band could be resolved. Co-purification of two different epitope-tagged forms of SPP from a stably transfected cell line expressing both tagged versions demonstrated that the approximately 95-kDa band is a homodimer of SPP. Pulse-chase metabolic labeling studies demonstrated that the SPP homodimer assembles rapidly and is metabolically stable. In a glycerol velocity gradient, SPP sedimented from approximately 100-200 kDa. Significantly the SPP homodimer was specifically labeled by an active site-directed photoaffinity probe (III-63) for PS, indicating that the active sites of SPP and PS/gamma-secretase are similar and providing strong evidence that the homodimer is functionally active. Collectively these data suggest that SPP exists in vivo as a functional dimer.

Identity and function of ?-secretase

gamma-Secretase catalyzes intramembrane proteolysis of various type I membrane proteins, including the amyloid-beta precursor protein and the Notch receptor. Despite its importance in the pathogenesis of Alzheimer's disease and to normal development, this protease has eluded identification until only very recently. Four membrane proteins are now known to be members of the protease complex: presenilin, nicastrin, aph-1, and pen-2. Recent findings suggest that these four proteins are sufficient to reconstitute the active gamma-secretase complex and that together they mediate the cell surface signaling of a variety of receptors via intramembrane proteolysis.

Generation of the beta-amyloid peptide and the amyloid precursor protein C-terminal fragment gamma are potentiated by FE65L1

Members of the FE65 family of adaptor proteins, FE65, FE65L1, and FE65L2, bind the C-terminal region of the amyloid precursor protein (APP). Overexpression of FE65 and FE65L1 was previously reported to increase the levels of alpha-secretase-derived APP (APPs alpha). Increased beta-amyloid (A beta) generation was also observed in cells showing the FE65-dependent increase in APPs alpha. To understand the mechanism for the observed increase in both A beta and APPs alpha given that alpha-secretase cleavage of a single APP molecule precludes A beta generation, we examined the effects of FE65L1 overexpression on APP C-terminal fragments (APP CTFs). Our data show that FE65L1 potentiates gamma-secretase processing of APP CTFs, including the amyloidogenic CTF C99, accounting for the ability of FE65L1 to increase generation of APP C-terminal domain and A beta 40. The FE65L1 modulation of these processing events requires binding of FE65L1 to APP and APP CTFs and is not because of a direct effect on gamma-secretase activity, because Notch intracellular domain generation is not altered by FE65L1. Furthermore, enhanced APP CTF processing can be detected in early endosome vesicles but not in endoplasmic reticulum or Golgi membranes, suggesting that the effects of FE65L1 occur at or near the plasma membrane. Finally, although FE65L1 increases APP C-terminal domain production, it does not mediate the APP-dependent transcriptional activation observed with FE65.

Evidence that nonsteroidal anti-inflammatory drugs decrease amyloid beta 42 production by direct modulation of gamma-secretase activity

Chronic use of nonsteroidal anti-inflammatory drugs (NSAIDs) is associated with a lower risk of developing Alzheimer's disease. Recent evidence indicates that some NSAIDs specifically inhibit secretion of the amyloidogenic A beta 42 peptide in cultured cells and mouse models of Alzheimer's disease. The reduction of A beta 42 peptides is not mediated by inhibition of cyclooxygenases (COX) but the molecular mechanism underlying this novel activity of NSAIDs has not been further defined. We now demonstrate that NSAIDs efficiently reduce the intracellular pool of A beta 42 in cell-based studies and selectively decrease A beta 42 production in a cell-free assay of gamma-secretase activity. Moreover, we find that presenilin-1 (PS1) mutations, which affect gamma-secretase activity, differentially modulate the cellular A beta 42 response to NSAID treatment. Overexpression of the PS1-M146L mutation enhances the cellular drug response to A beta 42 lowering NSAIDs as compared with cells expressing wild-type PS1. In contrast, expression of the PS1-Delta Exon9 mutation strongly diminishes the A beta 42 response, showing that PS1 mutations can modulate the cellular drug response to NSAID treatment both positively and negatively. Enhancement of the NSAID drug response was also observed with overexpression of the APP V717F mutation but not with Swedish mutant APP, which affects beta-secretase cleavage. In sum, these results strongly suggest that NSAIDs represent a founding group of compounds that lower A beta 42 production by direct modulation of gamma-secretase activity or its substrate.

Synthesis of new 3-alkoxy-7-amino-4-chloro-isocoumarin derivatives as new beta-amyloid peptide production inhibitors and their activities on various classes of protease

A series of new 7-substituted-4-chloro-3-alkoxy isocoumarin derivatives were synthesized and evaluated as inhibitors of representative classes of proteases: serine protease (alpha-chymotrypsin, trypsin), cysteine protease (Caspase-3), and aspartyl protease (HIV-protease), 20S proteasome and also as inhibitors of amyloid peptide gamma-secretase-mediated production. Protease inhibition selectivity is directly related to the structure of the substituent at the 7-position of the isocoumarin nucleus. 7-Nitro-isocoumarin derivatives (4c, 4d, 4f) are potent alpha-chymotrypsin inhibitors but slightly active or inactive on HIV-protease, as well as on cysteine protease. In contrast, only derivatives bearing a free amino (5d, 5f) or a substituted amino group (6f) at the 7-position of the isocoumarin nucleus, were found weakly active or inactive on alpha-chymotrypsin, trypsin, Caspase-3 and HIV-protease, but prevent gamma-secretase-mediated production of Abeta 40/42 amyloid peptides, which is known to be involved in Alzheimer's disease. Moreover, the most active compounds on beta-amyloid peptide production [JLK6 (5d), JLK2 (5f) and JLK7 (6f)] show only weak or moderate inhibitory activity on the 20S proteasome. The obtained results suggest that the described new isocoumarin analogues could be of interest, since compounds like JLK6 (5d), JLK2 (5f) and JLK7 (6f) can be considered as possible hits for the development of new agents directed towards Alzheimer's disease.

Peptide-based, irreversible inhibitors of gamma-secretase activity

The characterization of the enzymes responsible for amyloid beta-peptide (Abeta) production is considered to be a primary goal towards the development of future therapeutics for the treatment of Alzheimer's disease. Inhibitors of gamma-secretase activity were critical in demonstrating that the presenilins (PSs) likely comprised at least part of the active site of the gamma-secretase enzyme complex, with two highly conserved membrane aspartates presumably acting as catalytic residues. However, whether or not these aspartates are actually the catalytic residues of the enzyme complex or are merely essential for normal PS function and/or maturation is still unknown. In this paper, we report the development of reactive inhibitors of gamma-secretase activity that are functionally irreversible. Since such inhibitors have been shown to bind catalytic residues in other aspartyl proteases (e.g., HIV protease), they might be used to determine if the transmembrane aspartates of PSs are involved directly in substrate cleavage.

Sulindac sulfide is a noncompetitive gamma-secretase inhibitor that preferentially reduces Abeta 42 generation

Nonsteroidal anti-inflammatory drugs (NSAIDs) have been known to reduce risk for Alzheimer's disease. In addition to the anti-inflammatory effects of NSAIDs to block cylooxygenase, it has been shown recently that a subset of NSAIDs selectively inhibits the secretion of highly amyloidogenic Abeta42 from cultured cells, although the molecular target(s) of NSAIDs in reducing the activity of gamma-secretase for Abeta42 generation (gamma(42)-secretase) still remain unknown. Here we show that sulindac sulfide (SSide) directly acts on gamma-secretase and preferentially inhibits the gamma(42)-secretase activity derived from the 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate-solubilized membrane fractions of HeLa cells, in an in vitro gamma-secretase assay using recombinant amyloid beta precursor protein C100 as a substrate. SSide also inhibits activities for the generation of Abeta40 as well as for Notch intracellular domain at higher concentrations. Notably, SSide displayed linear noncompetitive inhibition profiles for gamma(42)-secretase in vitro. Our data suggest that SSide is a direct inhibitor of gamma-secretase that preferentially affects the gamma(42)-secretase activity.

Differential effects of inhibitors on the gamma-secretase complex. Mechanistic implications

Gamma-secretase is a protease complex of four integral membrane proteins, with presenilin (PS) as the apparent catalytic component, and this enzyme processes the transmembrane domains of a variety of substrates, including the amyloid beta-protein precursor and the Notch receptor. Here we explore the mechanisms of structurally diverse gamma-secretase inhibitors by examining their ability to displace an active site-directed photoprobe from PS heterodimers. Most gamma-secretase inhibitors, including a potent inhibitor of the PS-like signal peptide peptidase, blocked the photoprobe from binding to PS1, indicating that these compounds either bind directly to the active site or alter it through an allosteric interaction. Conversely, some reported inhibitors failed to displace this interaction, demonstrating that these compounds do not interfere with the protease by affecting its active site. Differential effects of the inhibitors with respect to photoprobe displacement and in cell-based and cell-free assays suggest that these compounds are important mechanistic tools for deciphering the workings of this intramembrane-cleaving protease complex and its similarity to other polytopic aspartyl proteases.

Aspartic proteases involved in Alzheimer's disease

Alzheimer's disease afflicts every tenth human aged over 65. Despite the dramatic progress that has been made in understanding the disease, the exact cause of Alzheimer's disease is still unknown. Most gene mutations associated with Alzheimer's disease point at the same culprits: amyloid precursor protein and ultimately amyloid beta. The enigmatic proteases alpha-,beta-, and gamma-secretase are the three executioners of amyloid precursor protein processing, and disruption of their delicate balance is suspected to result in Alzheimer's disease. Significant progress has been made in the selective control of these proteases, regardless of the availability of structural information. Not even the absence of a robust cell-free assay for gamma-secretase could hamper the identification of nonpeptidic inhibitors of this enzyme for long. Within five years, four distinctly different structural moieties were developed and the first drug candidates are in clinical trials. Unfortunately, selective inhibition of amyloid beta formation remains a crucial issue because fundamental fragments of the gamma-secretase complex are important for other signaling events. This problem makes beta-secretase inhibition and alpha-secretase induction even more appealing.

Presenilins mediate a dual intramembranous gamma-secretase cleavage of Notch-1

Following ectodomain shedding, Notch-1 undergoes presenilin (PS)-dependent constitutive intramembranous endoproteolysis at site-3. This cleavage is similar to the PS-dependent gamma-secretase cleavage of the beta-amyloid precursor protein (betaAPP). However, topological differences in cleavage resulting in amyloid beta-peptide (Abeta) or the Notch-1 intracellular domain (NICD) indicated independent mechanisms of proteolytic cleavage. We now demonstrate the secretion of an N-terminal Notch-1 Abeta-like fragment (Nbeta). Analysis of Nbeta by MALDI-TOF MS revealed that Nbeta is cleaved at a novel site (site-4, S4) near the middle of the transmembrane domain. Like the corresponding cleavage of betaAPP at position 40 and 42 of the Abeta domain, S4 cleavage is PS dependent. The precision of this cleavage is affected by familial Alzheimer's disease-associated PS1 mutations similar to the pathological endoproteolysis of betaAPP. Considering these similarities between intramembranous processing of Notch and betaAPP, we conclude that these proteins are cleaved by a common mechanism utilizing the same protease, i.e. PS/gamma-secretase.

Gamma-secretase-mediated proteolysis in cell-surface-receptor signalling

Many cell-surface receptors transmit signals to the nucleus through complex protein cascades. By contrast, the Notch signalling pathway uses a relatively direct mechanism, in which the intracellular domain of the receptor is liberated by intramembrane cleavage and translocates to the nucleus. This critical cleavage is mediated by the gamma-secretase complex, and new findings reveal that this mechanism is used by various receptors, although many questions remain about the biochemical details.

Novel therapeutic strategies provide the real test for the amyloid hypothesis of Alzheimer's disease

The amyloid and tangle cascade hypothesis is the dominant explanation for the pathogenesis of Alzheimer's disease (AD). A complete knowledge of the metabolic pathways leading to beta-amyloid (A beta) production and clearance in vivo and of the pathological events that lead to fibril formation and deposition into plaques is crucial for the development of an 'anti-amyloid' therapeutic strategy. Important advances in this respect have been achieved recently, revealing new candidate drug targets. Among the most promising potential treatments are beta- and gamma-secretase inhibitors, A beta vaccination, Cu-Zn chelators, cholesterol-lowering drugs and non-steroidal anti-inflammatory drugs. Now, the major question is whether these drugs will work in the clinic.