Dietary polyphenols as novel protective agents against membrane damage in α-synucleinopathies

Abstract

BACKGROUND/AIM: Cumulative evidence now suggests that the abnormal aggregation of the neuronal protein α-synuclein (as) is critically involved in the pathogenesis of synucleinopathies, of which Parkinson's disease (PO) is the most prevalent. Development of neuropathology appears to be linked to events that accelerate the rate of aggregation of as from monomers, via soluble oligomeric intermediates, into amyloid fibrils. Although increasing data suggest that oligomeric aggregates, not amyloid fibrils, disrupt or permeabilise cellular membranes, the nature of the neurotoxic species and its precise molecular mechanism still remain largely unknown, hampering the development of an effective treatment for the disease. The aim of the study was to screen a select group of dietary polyphenolic compounds that can interfere with as oligomeri8Hlion and protect phospholipid membranes against damage by aggregated oB.

METHODOLOGY: An aggregation protocol that enabled the generation of stable low- molecular-weight ollgomers (LMWO), and not fibrils, was developed and characterised. By

using a range of in vitro assays, 15 polyphenolic compound! and black tea extract were

tested (i) for their ability to inhibit and disaggregate as oligomerisation, using confocal single- molecule fluorescence spectroscopy (ii) for the inhibition of as-induced liposome

permeabilisation using fluorophore-Ioaded synthetic small unilamellar vesicles and, (iii) for their ability to attenuate cytochrome c release from respiring mitochondria, isolated from a mammalian neuronal cell line. Mechanistic insights into vesicle disruption by the as oligomers were also attempted, including the use of a planar lipid bilayer technique and related elflr.trophysiology. PRINCIPAL FINDINGS: It was first established that a select group of small-molecule polyphenolic compounds, notably black tea extract, tannic acid, morin, baicalein, nordihydroguaiaretic acid and (-)-epigallocathecingallate, can efficiently hinder or completely abolish 'early as aggregiiltes' by a mechanism most likely involving aromatic intcractions. Although a specific increase in the conductivity of planar lipid bilayers was not clear in this study, it was evident that LMW as oligomers are key elements in disrupting membrane integrity. Results also showed that several polyphenolic compounds were able to inhibit disruption of synthetic phospholipid membranes by oligomeric as. Interestingly, the select group of polyphenols also efficiently suppressed cytochrome c release induced by as from isolated mitochondria. Our results are consistent with the interpretation that polyphenols protect oligomer-damaged membranes and/or act as anti-aggregators of LMW oligomers, thereby preventing the uncontrolled flux of ions and cellular contents. CONCLUSIONS: Currently, there is no approved therapeutic agent directed toward preventing as aggregation and only symptomatic therapies are available with a limited time frame of utility. Certainly, the conclusions from this study put forward a strong case for testing the efficacy of a select group of lead polyphenols in vivo. Further investigations into the structure-activity relationships of diet-derived phenolic products may guide the design of novel therapeutic drugs that block early stages of amyloid self-assembly in PO and related synucleinopathies. Keywords: Parkinson's disease, α-synuclein , oligomers, membrane, polyphenols, mitochondria