Amyloid beta 42 peptide (Aβ42)-lowering compounds directly bind to Aβ and interfere with amyloid precursor protein (APP) transmembrane dimerization

Abstract

Following ectodomain shedding by β-secretase, successive proteolytic cleavages within the transmembrane sequence (TMS) of the amyloid precursor protein (APP) catalyzed by γ-secretase result in the release of amyloid-β (Aβ) peptides of variable length. Aβ peptides with 42 amino acids appear to be the key pathogenic species in Alzheimer's disease, as they are believed to initiate neuronal degeneration. Sulindac sulfide, which is known as a potent γ-secretase modulator (GSM), selectively reduces Aβ42 production in favor of shorter Aβ species, such as Aβ38. By studying APP-TMS dimerization we previously showed that an attenuated interaction similarly decreased Aβ42 levels and concomitantly increased Aβ38 levels. However, the precise molecular mechanism by which GSMs modulate Aβ production is still unclear. In this study, using a reporter gene-based dimerization assay, we found that APP-TMS dimers are destabilized by sulindac sulfide and related Aβ42-lowering compounds in a concentration-dependent manner. By surface plasmon resonance analysis and NMR spectroscopy, we show that sulindac sulfide and novel sulindac-derived compounds directly bind to the Aβ sequence. Strikingly, the attenuated APP-TMS interaction by GSMs correlated strongly with Aβ42-lowering activity and binding strength to the Aβ sequence. Molecular docking analyses suggest that certain GSMs bind to the GxxxG dimerization motif in the APP-TMS. We conclude that these GSMs decrease Aβ42 levels by modulating APP-TMS interactions. This effect specifically emphasizes the importance of the dimeric APP-TMS as a promising drug target in Alzheimer's disease.

Characterization of Intermediate Steps in Amyloid Beta (Aβ) Production under Near-native Conditions

“…differing in their affinities to multiple binding sites either on the target or the enzyme. We have earlier proposed a model suggesting that GSMs interfere with the dimerization of the APP homodimer, and thereby facilitate a more efficient processing of A␤40 and A␤42 into shorter A␤ peptides (12). The data presented here indicate a more complex situation, where GSMs appear to be associated not only with activation of cleavage reactions but also reduction of other reactions.…”

“…The majority of GSMs exhibit a slightly higher potency for A␤42 reduction compared with A␤40. The first generation GSMs appeared to target APP directly by affecting dimerization (11,12). More recently, and chemically very different, second generation GSMs instead appear to preferentially interact with ␥-secretase (10,13,14).…”

Lysine 624 of the Amyloid Precursor Protein (APP) Is a Critical Determinant of Amyloid β Peptide Length

“…The precise mechanism of GSM activity is still debated, and different mechanisms have been proposed, including allosteric modulation of PS1 (45), binding to Pen-2 (18), or binding to a portion of the A␤ domain within APP (16,17). Our current observation that mutation of Lys-28 can mimic the effect of GSMs (shifting the site of ␥ cleavage) supports but does not prove that these compounds work by binding to APP.…”

First and Second Generation γ-Secretase Modulators (GSMs) Modulate Amyloid-β (Aβ) Peptide Production through Different Mechanisms

“…Experiments with isotope-labeled, nontransition state ␥-secretase inhibitors have revealed a noncompetitive mechanism for NSAIDs (8). Pharmacological and biochemical experiments have suggested a direct interaction of certain NSAIDs with the APP-derived immediate substrate for ␥-secretase, ␤-C-terminal fragment, or C99, resulting in altered A␤ production (9,10). From a CNS drug discovery perspective, the GSMs of the NSAID class exhibit some generally less favorable features however, such as low potency and inefficient blood-brain barrier penetrance.…”