Identification of γ-Secretase Inhibitor Potency Determinants on Presenilin

Abstract

Production of amyloid ␤ peptides (A␤), followed by their deposition in the brain as amyloid plaques, contributes to the hallmark pathology of Alzheimer disease. The enzymes responsible for production of A␤, BACE1 and ␥-secretase, are therapeutic targets for treatment of Alzheimer disease. Two presenilin (PS) homologues, referred to as PS1 and PS2, comprise the catalytic core of ␥-secretase. In comparing presenilin selectivity of several classes of ␥-secretase inhibitors, we observed that sulfonamides in general tend to be more selective for inhibition of PS1-comprising ␥-secretase, as exemplified by ELN318463 and BMS299897. We employed a combination of chimeric constructs and point mutants to identify structural determinants for PS1-selective inhibition by ELN318463. Our studies identified amino acid residues Leu 172 , Thr 281 , and Leu 282 in PS1 as necessary for PS1-selective inhibition by ELN318463. These residues also contributed in part to the PS1-selective inhibition by BMS299897. Alanine scanning mutagenesis of areas flanking Leu 172 , Thr 281 , and Leu 282 identified additional amino acids that affect inhibitor potency of not only these sulfonamides but also nonsulfonamide inhibitors, without affecting A␤ production and presenilin endoproteolysis. Interestingly, many of these same residues have been identified previously to be important for ␥-secretase function. These findings implicate TM3 and a second region near the carboxyl terminus of PS1 aminoterminal fragment in mediating the activity of ␥-secretase inhibitors. Our observations demonstrate that PS-selective inhibitors of ␥-secretase are feasible, and such inhibitors may allow differential inhibition of A␤ peptide production and Notch signaling.

Alzheimer's Disease: Presenilin 2-Sparing γ-Secretase Inhibition Is a Tolerable Aβ Peptide-Lowering Strategy

“…MRK-560 also showed a differential interaction to PS1 compared with PS2, providing a molecular explanation to the observed preference for PS1-containing ␥-secretases. It has previously been proposed that GSIs of the sulfonamide type preferentially inhibit PS1-containing ␥-secretase (Zhao et al, 2008;Borgegard et al, 2011), but our data show that the preference for PS1 versus PS2 processing varies considerably among different sulfonamide GSIs: MRK-560 exhibited a considerably stronger preference for PS1 over PS2 processing, whereas avagacestat (BMS-708163) did not. This difference is of interest with regard to the results from recent clinical trials, where avagacestat caused Notch-related side effects in Phase 2 clinical trials (Imbimbo et al, 2011).…”

“…Gene targeting of ␥-secretase subunits in mice has revealed distinct physiological roles for specific ␥-secretase components in vivo (Saura et al, 2004;Serneels et al, 2005). In the context of AD, it is of particular interest that PS1 seems to catalyze the vast majority of CNS A␤ production (De Strooper et al, 1998;Borgegard et al, 2011) and that certain GSIs of the sulfonamide class appear to exhibit a preference for PS1 over PS2 ␥-secretase activities (Zhao et al, 2008;Borgegard et al, 2011), supporting the notion that development of ␥-secretase subcomplex selective GSIs may be feasible. In line with this, the sulfonamide GSI, MRK-560, inhibits A␤ production in an efficacious manner and reduces neural plaque formation without producing the Notchrelated off target effects observed in chronic studies in mice (Best et al, 2007).…”

Functional Analysis of the Transmembrane Domains of Presenilin 1

“…Further analysis on the role of TMD3 on substrates other than APP and Notch would be required. Intriguingly, some residues on TMD3 have been implicated in the determination of PS1/PS2 specificity of a subset of the ␥-secretase inhibitors (26). Nevertheless, these data support the idea that TMD3 is involved in the acquisition of proper cleavage activity of the ␥-secretase after the assembly of the complex.…”

Evidence that Enzyme Processivity Mediates Differential Aβ Production by PS1 and PS2

“…Consequently, we suggest that γ-secretase inhibitors that preferentially inhibit PS1-containing γ-secretase complexes (e.g., sulfonamides [30, 40]) represent a promising avenue for reducing Aβ production while leaving the generation of the physiologically critical AICD and NICD products relatively unaffected. …”