Synapsin isoforms differentially modulate presynaptic voltage-gated Ca2+ channels in excitatory and inhibitory neurons

Synapsins are a family of phosphoproteins essential for controlling neurotransmitter release by clustering synaptic vesicles and tethering them to the actin cytoskeleton, thereby supporting synaptic plasticity. Beyond these roles, Synapsins are crucial for maintaining the balance between excitatory and inhibitory synaptic transmission. Mutations in Synapsin genes have been associated with epilepsy and autism spectrum disorder. Furthermore, silencing Synapsin in invertebrates causes an increase in calcium current density. To investigate the specific contributions of Synapsin I and Synapsin II to neuronal function, we developed in vitro models in which the expression of each isoform was selectively silenced using RNA interference. We examined the effects of Synapsin I and Synapsin II knockdown on excitatory and inhibitory synaptic transmission, calcium influx, and intrinsic neuronal excitability. Silencing Synapsin I increased post-tetanic potentiation and the colocalization of N-type calcium channels in excitatory terminals. In inhibitory synapses, knockdown of Synapsin I decreased synchronous GABA release while enhancing asynchronous GABA release through an increase in N-type calcium currents. N-type calcium current density also increased in excitatory neurons, accompanied by elevated somatic and presynaptic N-type calcium signals. In contrast, silencing of Synapsin II in excitatory synapses enhanced evoked excitatory transmission through P/Q-type calcium channels, increased P/Q-type calcium signals at both soma and presynaptic terminals, and promoted clustering of P/Q-type channels. In inhibitory synapses, knockdown of Synapsin II also increased P/Q-type calcium current density, increasing the synchronous and decreasing asynchronous GABA release. Overall, our findings demonstrate that Synapsin I and Synapsin II differentially regulate excitatory and inhibitory synaptic transmission by modulating the organization and function of presynaptic calcium channels. This study offers new insights into the mechanisms underlying the excitation-inhibition balance and presynaptic terminal organization, with implications for neurological disorders caused by Synapsin gene mutations.