α-Synuclein in synaptic function and dysfunction

alpha-Synuclein is a neuronal protein that is enriched in presynaptic terminals. Under physiological conditions, it binds to synaptic vesicle membranes and functions in neurotransmitter release, although the molecular details remain unclear, and it is controversial whether alpha-synuclein inhibits or facilitates neurotransmitter re-lease. Pathologically, in synucleinopathies including Parkinson's disease (PD), alpha-synuclein forms aggregates that recruit monomeric alpha-synuclein and spread throughout the brain, which triggers neuronal dysfunction at molecular, cellular, and organ levels. Here, we present an overview of the effects of alpha-synuclein on SNARE-complex assembly, neurotransmitter release, and synaptic vesicle pool homeostasis, and discuss how the observed divergent effects of alpha-synuclein on neurotransmitter release can be reconciled. We also discuss how gain-of -function versus loss-of-function of alpha-synuclein may contribute to pathogenesis in synucleinopathies.