Synapsin knockdown is associated with decreased neurite outgrowth, functional synaptogenesis impairment, and fast high-frequency neurotransmitter release

Synapsins are an evolutionarily-conserved family of synaptic vesicle-associated proteins, related to fine-tuning of synaptic transmission. Studies in mammals have partially clarified the different roles of synapsins, however the presence of different genes and isoforms and the development of compensatory mechanisms hinder accurate data interpretation. Here, we used a simple in vitro monosynaptic Helix-neurons connection, reproducing an in vivo physiological connection, as a reliable experimental model, in order to investigate the effects of synapsin knock-down. Cells overexpressing an antisense construct against Helix synapsin showed a time-dependent decrease of synapsin immunostaining, confirming protein loss. At the morphological level, synapsin-silenced cells showed a reduction in neurite linear outgrowth and branching, and in the size and number of synaptic varicosities. Functionally, synapsin-silenced cells presented a reduced ability to form synaptic connections; however functional chemical synapses showed similar basal excitatory postsynaptic potentials and similar short-term plasticity paradigms. In addition, synapsin-silenced cells presented faster neurotransmitter release, and decreased postsynaptic response toward the end of long tetanic presynaptic stimulations, probably related to an impairment of the synaptic vesicles trafficking due to a different vesicle handling, with an increased readily releasable pool and a compromised reserve pool.