Modulation of the Activity of the Insulin-Degrading Enzyme by Aß Peptides.

The insulin-degrading enzyme (IDE) is an evolutionarily conserved protease implicated in the degradation of insulin and amyloidogenic peptides. Most of the biochemical and biophysical characterization of IDE's catalytic activity has been conducted using solutions containing a single substrate, i.e., insulin or Aß(1-40). IDE's activity toward a particular substrate, however, is likely to be influenced by the presence of other substrates. Here, we show by a kinetic assay based on insulin's helical circular dichroic signal and MALDI TOF mass spectrometry that Aß peptides modulate IDE's activity toward insulin in opposing ways. Aß(1-40) enhances IDE-dependent degradation of insulin, whereas Aß(pyroE3-42), the most pathogenic pyroglutamate-modified Aß peptide in AD, inhibits IDE's activity. Intriguingly, Aß(pyroE3-42) also inhibits IDE's ability to degrade Aß(1-40). Together, our results implicate Aß peptides in the abnormal catabolism of IDE's key substrates.

Exchange Broadening Underlies the Enhancement of IDE-Dependent Degradation of Insulin by Anionic Membranes.

Insulin-degrading enzyme (IDE) is an evolutionarily conserved ubiquitous zinc metalloprotease implicated in the efficient degradation of insulin monomer. However, IDE also degrades monomers of amyloidogenic peptides associated with disease, complicating the development of IDE inhibitors. In this work, we investigated the effects of the lipid composition of membranes on the IDE-dependent degradation of insulin. Kinetic analysis based on chromatography and insulin's helical circular dichroic signal showed that the presence of anionic lipids in membranes enhances IDE's activity toward insulin. Using NMR spectroscopy, we discovered that exchange broadening underlies the enhancement of IDE's activity. These findings, together with the adverse effects of anionic membranes in the self-assembly of IDE's amyloidogenic substrates, suggest that the lipid composition of membranes is a key determinant of IDE's ability to balance the levels of its physiologically and pathologically relevant substrates and achieve proteostasis.

Differential Effects of Polyphenols on Insulin Proteolysis by the Insulin-Degrading Enzyme.

The insulin-degrading enzyme (IDE) possesses a strong ability to degrade insulin and Aß42 that has been linked to the neurodegeneration in Alzheimer's disease (AD). Given this, an attractive IDE-centric strategy for the development of therapeutics for AD is to boost IDE's activity for the clearance of Aß42 without offsetting insulin proteostasis. Recently, we showed that resveratrol enhances IDE's activity toward Aß42. In this work, we used a combination of chromatographic and spectroscopic techniques to investigate the effects of resveratrol on IDE's activity toward insulin. For comparison, we also studied epigallocatechin-3-gallate (EGCG). Our results show that the two polyphenols affect the IDE-dependent degradation of insulin in different ways: EGCG inhibits IDE while resveratrol has no effect. These findings suggest that polyphenols provide a path for developing therapeutic strategies that can selectively target IDE substrate specificity.

Inhibition of the Self-Assembly of Aß and of Tau by Polyphenols: Mechanistic Studies.

The amyloid-ß (Aß) peptide and tau protein are thought to play key neuropathogenic roles in Alzheimer's disease (AD). Both Aß and tau self-assemble to form the two major pathological hallmarks of AD: amyloid plaques and neurofibrillary tangles, respectively. In this review, we show that naturally occurring polyphenols abundant in fruits, vegetables, red wine, and tea possess the ability to target pathways associated with the formation of assemblies of Aß and tau. Polyphenols modulate the enzymatic processing of the amyloid-ß precursor protein and inhibit toxic Aß oligomerization by enhancing the clearance of Aß42 monomer, modulating monomer-monomer interactions and remodeling oligomers to non-toxic forms. Additionally, polyphenols modulate tau hyperphosphorylation and inhibit tau ß-sheet formation. The anti-Aß-self-assembly and anti-tau-self-assembly effects of polyphenols increase their potential as preventive or therapeutic agents against AD, a complex disease that involves many pathological mechanisms.