Membrane cholesterol regulates the binding of alpha-synuclein to synaptic vesicles, and its subsequent functional and pathogenic behavior

Abstract

α-Synuclein (A-Syn) is a presynaptic protein, the aggregation of which underlies the pathogenesis of Parkinson's disease (PD). Although the physiological function of A-Syn is still unclear, numerous evidences indicate that it plays a role in the trafficking of synaptic vesicles (SVs) during neurotransmitter release, a task associated with its ability to bind SVs and promote their ‘clustering’. Partitioning of A-Syn between its membrane-bound and unbound states regulates the balance between its functional and pathogenic forms in the cell; which makes it crucial to identify the cellular factors regulating the binding of A-Syn to SV membranes. SV membranes contain cholesterol as a key component, which also happens to mediate the binding of A-Syn to these vesicles. Here we present the first systematic investigation of how the binding of A-Syn to SV-like membranes, and consequently its functional and pathogenic behaviour, varies with the amount of cholesterol present in the membranes. Using SV-like liposomes containing increasing percentages of membrane cholesterol (from 0 to 40%, mol/mol), we show that the binding of A-Syn with SV-like membranes reaches a maximum in presence of an optimum cholesterol level (10% in our study); and also devise a mathematical model to fit and explain the observed trend of membrane-binding. Our study demonstrates how the strongest membrane-binding of A-Syn in presence of the optimum cholesterol level, correlates to its maximum function (i.e. vesicle clustering) and minimum pathogenic aggregation; which explains why both depletion and enhancement of cholesterol levels has been linked to an increased risk of PD. Also, we show how SV-like membranes with higher cholesterol content are less prone to changes in membrane properties in course of their co-aggregation with A-Syn, as well as under the action of its pre-formed aggregates.