γ-Secretase is a large enzyme complex comprising presenilin, nicastrin, presenilin enhancer 2 and anterior pharynx-defective 1 that mediates the intramembrane proteolysis of a large number of proteins including amyloid precursor protein and Notch. Recently, a novel γ-secretase activating protein, GSAP was identified that interacts with γ-secretase and the C-terminal fragment of amyloid precursor protein to selectively increase amyloid-β production. In this study we have further characterised the role of endogenous and exogenous GSAP in the regulation of γ-secretase activity and amyloid-β production in vitro. Knockdown of GSAP expression in N2a cells decreased amyloid-β levels. In contrast, overexpression of GSAP in HEK cells expressing amyloid precursor protein or in N2a cells had no overt effect on amyloid-β generation. Likewise, purified recombinant GSAP had no effect on amyloid-β generation in two distinct in vitro γ-secretase assays. In subsequent cellular studies with imatinib, a kinase inhibitor that reportedly prevents the interaction of GSAP with the C-terminal fragment of amyloid precursor protein, a concentration-dependent decrease in amyloid-β levels was observed. However, no interaction between GSAP and the C-terminal fragment of amyloid precursor protein was evident in co-immunoprecipitation studies. In addition, sub-chronic administration of imatinib to rats had no effect on brain amyloid-β levels. In summary, these findings suggest the roles of GSAP and imatinib in the regulation of γ-secretase activity and amyloid-β generation are uncertain.
[Show abstract][Hide abstract] ABSTRACT: γ-Secretase is an aspartyl intramembranal protease composed of presenilin, Nicastrin, Aph1 and Pen2 with 19 transmembrane domains. γ-Secretase cleaves the amyloid precursor proteins (APP) to release Aβ peptides that likely play a causative role in the pathogenesis of Alzheimer disease (AD). In addition, γ-secretase cleaves Notch and other type I membrane proteins. γ-Secretase inhibitors (GSIs) have been developed and used for clinical studies. However, clinical trials have shown adverse effects of GSIs that are potentially linked with non-discriminatory inhibition of Notch signaling, overall APP processing and possible other substrates. Therefore, these findings call for the development of disease modifying agents that target γ-secretase activity to lower Aβ42 production without blocking the overall processing of γ-secretase substrates. γ-Secretase modulators (GSMs) originally derived from non-steroidal anti-inflammatory drugs (NSAIDs) display such characteristics and are the focus of this review. However, first generation GSMs have limited potential due to low potency and undesired neuropharmacokinetic properties. This generation of GSMs has been suggested to interact with APP substrate, γ-secretase or both. To improve the potency and brain availability, second generation GSMs including NSAID-derived carboxylic acid and non-NSAID-derived heterocyclic chemotypes as well as natural product-derived GSMs have been developed. Animal studies of this generation of GSMs have shown encouraging preclinical profiles. Moreover, using potent GSM photoaffinity probes, multiple studies unambiguously have showed that both carboxylic acid and heterocyclic GSMs specifically target presenilin, the catalytic subunit of γ-secretase. In addition, two types of GSMs have distinct binding sites within the γ-secretase complex and exhibit different Aβ profiles. GSMs induce a conformational change of γ-secretase to achieve modulation. Various models are proposed and discussed. Despite the progress of GSM research, many outstanding issues remain to be investigated to achieve the ultimate goal of developing GSMs as effective AD therapies.
[Show abstract][Hide abstract] ABSTRACT: γ-Secretase is a four subunit, 19-pass transmembrane enzyme that cleaves amyloid precursor protein (APP), catalyzing the formation of amyloid beta (Aβ) peptides that form amyloid plaques, which contribute to Alzheimer's disease (AD) pathogenesis. γ-Secretase also cleaves Notch, among many other type I transmembrane substrates. Despite its seemingly promiscuous enzymatic capacity, γ-secretase activity is tightly regulated. This regulation is a function of many cellular entities, including but not limited to the essential γ-secretase subunits, nonessential (modulatory) subunits, and γ-secretase substrates. Regulation is also accomplished by an array of cellular events, such as presenilin (active subunit of γ-secretase) endoproteolysis and hypoxia. In this review we discuss how γ-secretase is regulated with the hope that an advanced understanding of these mechanisms will aid in the development of effective therapeutics for γ-secretase-associated diseases like AD and Notch-addicted cancer.
[Show abstract][Hide abstract] ABSTRACT: Amyloid-β peptide (Aβ), the cerebral accumulation of which is thought to cause Alzheimer's disease (AD), is produced throughout life. The level of insoluble Aβ rises with age and is further increased in AD. In contrast, we showed previously that in mid-frontal cortex in a cohort without neurological disease, soluble Aβ declined progressively between 16 and 95 y. We speculated that the divergent changes in the levels of soluble and insoluble Aβ with age might reflect an increasing tendency to favor the production or retention within the brain of Aβ42 over Aβ40, leading to elevation of Aβ42 : 40 even as total soluble Aβ decreased. We have now measured Aβ40 and Aβ42 in soluble and insoluble (guanidine-extractable) fractions of human postmortem brain tissue from the same cohort studied previously. Although in normal brains the absolute level of Aβ40 in both soluble and insoluble fractions and that of Aβ42 in the soluble fraction declined with age, those declines were predominantly before about 50 y, after which Aβ42 : 40 tended to increase in both the soluble and insoluble fractions. Insoluble Aβ42 increased progressively with age. Differential production or retention of Aβ40 and Aβ42 in the over-50 s is likely to contribute to the influence of age on the risk of sporadic AD.
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