Nat Prod. Author manuscript, available in PMC 2011 July 23. NIH PA Author Manuscript NIH PA Author Manuscript NIH PA Author Manuscript A.24 Down,s syndrome patients who produce significantly higher amounts of A from birth and deposit A plaques in their brains as early as age 12, consistently develop AD by age the of 50.25 This further AZD8330 ARRY-424704 emphasizes the central role of Ain the pathogenesis of AD. Thus, a major focus of current AD drug discovery efforts is on developing novel therapeutics that may effectively decrease A production and deposition in the AD brain.21 The proteolytic processing of APP takes place by sequential cleavage by various proteases named , , and γ secretase. Secretase is a member of the ADAM family, such as ADAM17 or TACE, ADAM 9, ADAM10, MDC9, and an aspartyl protease, BACE2.
26 The secretase enzyme cleaves APP within the A domain between residues Lys16 and Leu17, thus avoiding the generation of intact A peptides. This leads to the formation BCR-ABL Signaling of a soluble domain, released into extracellular space, and a 10 kDa C terminal fragment, which remains within the cellular membrane and serves as substrate for further cleavage byγ secretase.27 Both sAPP and C83 have been shown not to contribute directly to Aplaques observed in AD brains.9 In fact, both secretase and sAPP have been shown to be reduced in AD patients as compared to healthy controls.28 On the contrary, secretase is up regulated significantly in the AD brain.29 BACE1 is a major secretase involved in the amyloidogenic processing of APP in neurons.
30 BACE1 cleaves APP at the Asp1 residue of the A region and leads to the generation of a secreted soluble fragment and a membrane bound C terminal fragment. The γ secretase cleavage of C99 constitutes an amyloidogenic pathway, leading to the generation of a spectrum of A peptides. The A peptides containing 40 or 42 amino acids are the two most common amyloidogenic A peptides and are involved in the formation of mature, neuritic plaques observed in the AD brain.31 In the present study, it was found that both withanolide A and asiatic acid dose dependently and significantly downregulated BACE1 levels in primary rat cortical neurons. BACE1 is a rate limiting enzyme in the production of A, our group and others have shown previously that a slight increase in BACE1 levels leads to a dramatic increase in the production of A 40/42.
32,33 A corollary to this is that even a slight decrease in BACE1 levels may lead to a considerable decrease in the production of A. Thus, 1 and 2, with their significant activity against BACE1, represent potentially effective lead compounds for AD aimed at decreasing A generation and deposition. Furthermore, it has been established recently that BACE1 and secretase compete for APP processing, whereby BACE1 cleavage of APP precludes its processing by secretase and vice versa.34,35 Thus, BACE1 down regulation induced by 1 and 2, in itself, may indirectly lead to the increased processing of APP by secretase. In the current work, it is encouraging that both 1 and 2 also had direct effects on secretase activity, which was evident by significantly enhanced ADAM10 maturation.
This increased secretase activity further affects non amyloidogenic processing of APP. It was found that levels of both C83 and sAPP, non amyloidogenic products of APP, were elevated by treatment with both 1 and 2 as compared to the respective controls. The secreted, secretase product of APP has been shown to protect neurons against various insults such as excitotoxic, metabolic, and oxidative.36 38 Thus, 1 and 2, with their dual activities against BACE1 and ADAM10, may prove highly beneficial against AD in terms of lowering A levels directly and also increasing sAPP levels, thus being neuroprotective indirectly. Compounds 1 and 2 are c