Sphingosine kinases (SphK) 1 and 2 phosphorylate sphingosine to generate sphingosine-1-phosphate (S1P), a pluripotent lipophilic mediator implicated in a variety of cellular events. Here we show that the activity of β-site APP cleaving enzyme-1 (BACE1), the rate limiting enzyme for amyloid-β peptide (Aβ) production, is modulated by S1P in neurons. Treatment by SphK inhibitor, RNAi knockdown of SphK or overexpression of S1P degrading enzymes decreased BACE1 activity to reduce Aβ production. S1P specifically bound to full-length BACE1 and increased its proteolytic activity, suggesting that the cellular S1P directly modulates BACE1 activity. Notably, the relative activity of SphK2 was upregulated in the brains of patients with Alzheimer disease. The unique modulatory effect of cellular S1P on BACE1 activity is a novel potential therapeutic target for Alzheimer disease.

Amyloid-b peptide ending at the 42nd residue (Ab42) is implicated in the pathogenesis of Alzheimer's disease (AD). Small compounds that exhibit selective lowering effects on Ab42 production are termed c-secretase modulators (GSMs) and are deemed as promising therapeutic agents against AD, although the molecular target as well as the mechanism of action remains controversial. Here, we show that a phenylpiperidine-type compound GSM-1 directly targets the transmembrane domain (TMD) 1 of presenilin 1 (PS1) by photoaffinity labelling experiments combined with limited digestion. Binding of GSM-1 affected the structure of the initial substrate binding and the catalytic sites of the c-secretase, thereby decreasing production of Ab42, possibly by enhancing its conversion to Ab38. These data indicate an allosteric action of GSM-1 by directly binding to the TMD1 of PS1, pinpointing the target structure of the phenylpiperidine-type GSMs.

BackgroundAmyloid-β peptide ending at 42nd residue (Aβ42) is believed as a pathogenic peptide for Alzheimer disease. Although γ-secretase is a responsible protease to generate Aβ through a processive cleavage, the proteolytic mechanism of γ-secretase at molecular level is poorly understood.ResultsWe found that the transmembrane domain (TMD) 1 of presenilin (PS) 1, a catalytic subunit for the γ-secretase, as a key modulatory domain for Aβ42 production. Aβ42-lowering and -raising γ-secretase modulators (GSMs) directly targeted TMD1 of PS1 and affected its structure. A point mutation in TMD1 caused an aberrant secretion of longer Aβ species including Aβ45 that are the precursor of Aβ42. We further found that the helical surface of TMD1 is involved in the binding of Aβ45/48 and that the binding was altered by GSMs as well as TMD1 mutation.ConclusionsBinding between PS1 TMD1 and longer Aβ is critical for Aβ42 production.

Substrate-selective inhibition or modulation of the activity of γ-secretase, which is responsible for the generation of amyloid-β peptides, might be an effective strategy for prevention and treatment of Alzheimer's disease. We have shown that helical β-peptide foldamers are potent and specific inhibitors of γ-secretase. Here we report identification of target site of the foldamers by using a photoaffinity probe. The photoprobe directly and specifically labeled the N-terminal fragment of presenilin 1, in which the initial substrate docking site is predicted to be located. We also optimized the foldamer structure by preparing a variety of derivatives and obtained two highly potent foldamers by incorporation of a hydrophilic and neutral functional group into the parent structure. The class of side chain functional group and the position of incorporation were both important for γ-secretase-inhibitory activity. The substrate selectivity of the inhibitory activity was also quite sensitive to the class of side chain group incorporated.

An enantioselective route to the tetracyclic skeleton of sarain A has been developed. Asymmetric reduction of an ynone introduced a chiral center which was transferred to the contiguous tertiary stereogenic centers through an Ireland-Claisen rearrangement. The 2-azabicyclo[3.3.1]nonane framework was constructed by an unprecedented intramolecular cycloaddition of an eight-membered cyclic nitrone. Using the steric bias of the bicyclic system, the quaternary carbon atom was constructed by a stereoselective aldol reaction. Further ring formations were performed by ring-closing metathesis for the 13-membered ring and an iodoamidation reaction for the pyrrolidine ring. The present synthesis has successfully provided an alternative route to the late-stage intermediate of Overman's synthesis.