Synaptic Vesicle Disruption in Parkinson’s Disease: Dual Roles of α-Synuclein and Emerging Therapeutic Targets

Evidence increasingly indicates that synaptic vesicle dysfunction emerges early in Parkinson's disease (PD), preceding overt dopaminergic neuron loss rather than arising solely as a downstream consequence of neurodegeneration. α-Synuclein (αSyn), a presynaptic protein that regulates vesicle clustering, trafficking, and neurotransmitter release under physiological conditions, exhibits dose-, conformation-, and context-dependent actions that distinguish its normal regulatory roles from pathological effects observed in disease models. This narrative review synthesizes findings from a structured search of PubMed and Scopus, with emphasis on α-syn-knockout (αSynKO) and BAC transgenic (αSynBAC) mouse models, which do not recapitulate the full human PD trajectory but provide complementary insights into αSyn physiological function and dosage-sensitive vulnerability. Priority was given to studies integrating ultrastructural approaches-such as cryo-electron tomography, high-pressure freezing/freeze-substitution TEM, and super-resolution microscopy-with proteomic and lipidomic analyses. Across these methodologies, several convergent presynaptic alterations are consistently observed. In vivo and ex vivo studies associate αSyn perturbation with impaired vesicle acidification, consistent with altered expression or composition of vacuolar-type H⁺-ATPase subunits. Lipidomic analyses reveal age- and genotype-dependent remodeling of vesicle membrane lipids, particularly curvature- and charge-sensitive phospholipids, which may destabilize αSyn-membrane interactions. Complementary biochemical and cell-based studies support disruption of SNARE complex assembly and nanoscale release-site organization, while ultrastructural analyses demonstrate reduced vesicle docking, altered active zone geometry, and vesicle pool disorganization, collectively indicating compromised presynaptic efficiency. These findings support a synapse-centered framework in which presynaptic dysfunction represents an early and mechanistically relevant feature of PD. Rather than advocating αSyn elimination, emerging therapeutic concepts emphasize preservation of physiological vesicle function-through modulation of vesicle acidification, SNARE interactions, or membrane lipid homeostasis. Although such strategies remain exploratory, they identify the presynaptic terminal as a potential window for early intervention aimed at maintaining synaptic resilience and delaying functional decline in PD.