α-Synuclein (αSyn) Preformed Fibrils Induce Endogenous αSyn Aggregation, Compromise Synaptic Activity and Enhance Synapse Loss in Cultured Excitatory Hippocampal Neurons

Synucleinopathies are characterized by the accumulation of insoluble alpha-synuclein (alpha Syn). To test whether alpha Syn aggregates modulate synaptic activity, we used a recently developed model in primary neurons for inducing alpha Syn pathology. Wedemonstrated that preformed fibrils (PFFs) generated with recombinant human alpha Syn compromised synaptic activity in a time-and dose-dependent manner and that the magnitude of these deficits correlated with the formation of alpha Syn pathology in cultured excitatory hippocampal neurons from both sexes of mice. Remarkably, acute passive infusion of alpha Syn PFFs from whole-cell patch-clamp pipette decreased mEPSC frequency within 10 min followed by induction of alpha Syn pathology within 1 d. Moreover, by direct addition of alpha Syn PFFs into culture medium, the formation of misfolded alpha Syn inclusions dramatically compromised the colocalization of synaptic markers and altered dynamic changes of dendritic spines, but the viability of neurons was not affected up to 7 d post-treatment with alpha Syn PFFs. Our data indicate that intraneuronal alpha Syn fibrils impaired the initiation of synaptogenesis and their physiological functions, thereby suggesting that targeting synaptic dysfunction in synucleinopathies may provide a promising therapeutic direction.