Cramer et al . (Reports, 23 March 2012, p. 1503; published online 9 February 2012) demonstrates short-term bexarotene treatment clearing preexisting β-amyloid deposits from the brains of APP/PS1ΔE9 mice with low amyloid burden, providing a rationale for repurposing this anticancer agent as an Alzheimer’s disease (AD) therapeutic. Using a nearly identical treatment regimen, we were unable to detect any evidence of drug efficacy despite demonstration of target engagement.

Inspired by marine cyanobacterial natural products, we synthesized modified peptides with a central statine-core unit, characteristic for aspartic protease inhibition. A series of tasiamide B analogues inhibited BACE1, a therapeutic target in Alzheimer's disease. We probed the stereospecificity of target engagement and determined additional structure-activity relationships with respect to BACE1 and related aspartic proteases, cathepsins D and E. We cocrystallized selected inhibitors with BACE1 to reveal the structural basis for the activity. Hybrid molecules that combine features of tasiamide B and an isophthalic acid moiety-containing sulfonamide showed nanomolar cellular activity. Compounds were screened in a series of rigorous complementary cell-based assays. We measured secreted APP ectodomain (sAPPβ), membrane bound carboxyl terminal fragment (CTF), levels of β-amyloid (Aβ) peptides and selectivity for β-secretase (BACE1) over γ-secretase. Prioritized compounds showed reasonable stability in vitro and in vivo, and our most potent inhibitor showed efficacy in reducing Aβ levels in the rodent brain.

Transgenic mice that express mutant amyloid precursor protein (APPsi) using tet-Off vector systems provide an alternative model for assessing short- and long-term effects of Aβ-targeting therapies on phenotypes related to the deposition of Alzheimer-type amyloid. Here we use such a model, termed APPsi:tTA, to determine what phenotypes persist in mice with high amyloid burden after new production of APP/Aβ has been suppressed. We find that 12-13 month old APPsi:tTA mice are impaired in cognitive tasks that assess short- and long-term memories. Acutely suppressing new APPsi/Aβ production produced highly significant improvements in performance short-term spatial memory tasks; which upon continued suppression translated to superior performance in more demanding tasks that assess long-term spatial memory and working memory. Deficits in episodic-like memory and cognitive flexibility, however, were more persistent. Arresting mutant APPsi production caused a rapid decline in the brain levels of soluble APP ectodomains, full-length APP, and APP C-terminal fragments. As expected, amyloid deposits persisted after new APP/Aβ production was inhibited whereas, unexpectedly, we detected persistent pools of solubilizable, relatively mobile, Aβ42. Additionally, we observed persistent levels of Aβ immunoreactive entities that were of a size consistent with SDS-resistant oligomeric assemblies. Thus, in this model with significant amyloid pathology, a rapid amelioration of cognitive deficits was observed despite persistent levels of oligomeric Aβ assemblies and low, but detectable solubilizable Aβ42 peptides. These findings implicate complex relationships between accumulating Aβ and activities of APP, soluble APP ectodomains, and/or APP CTFs in mediating cognitive deficits in this model of amyloidosis.

The biological underpinnings linking stress to Alzheimer's disease (AD) risk are poorly understood. We investigated how corticotrophin releasing factor (CRF), a critical stress response mediator, influences amyloid-b (Ab) production. In cells, CRF treatment increases Ab production and triggers CRF receptor 1 (CRFR1) and c-secretase internalization. Co-immunoprecipitation studies establish that c-secretase associates with CRFR1; this is mediated by b-arrestin binding motifs. Additionally, CRFR1 and c-secretase co-localize in lipid raft fractions, with increased c-secretase accumulation upon CRF treatment. CRF treatment also increases c-secretase activity in vitro, revealing a second, receptorindependent mechanism of action. CRF is the first endogenous neuropeptide that can be shown to directly modulate c-secretase activity. Unexpectedly, CRFR1 antagonists also increased Ab. These data collectively link CRF to increased Ab through c-secretase and provide mechanistic insight into how stress may increase AD risk. They also suggest that direct targeting of CRF might be necessary to effectively modulate this pathway for therapeutic benefit in AD, as CRFR1 antagonists increase Ab and in some cases preferentially increase Ab42 via complex effects on c-secretase.