Effects of phosphorylation and neuronal activity on the control of synapse formation by synapsin I

Abstract

SummarySynapsins are synaptic vesicle (SV)-associated proteins that regulate synaptic transmission and neuronal differentiation. At early stages, Syn I and II phosphorylation at Ser9 by cAMP-dependent protein kinase (PKA) and Ca 2+ /calmodulin-dependent protein kinase I/IV modulates axon elongation and SV-precursor dynamics. We evaluated the requirement of Syn I for synapse formation by siRNAmediated knockdown as well as by overexpression of either its wild-type (WT) form or its phosphorylation mutants. Syn1 knockdown at 14 days in vitro caused a decrease in the number of synapses, accompanied by a reduction of SV recycling. Although overexpression of WT Syn I was ineffective, overexpression of its phosphorylation mutants resulted in a complex temporal regulation of synapse density. At early stages of synaptogenesis, phosphomimetic Syn I S9E significantly increased the number of synapses. Conversely, dephosphomimetic Syn I S9A decreased synapse number at more advanced stages. Overexpression of either WT Syn I or its phosphomimetic S9E mutant rescued the decrease in synapse number caused by chronic treatment with tetrodotoxin at early stages, suggesting that Syn I participates in an alternative PKA-dependent mechanism that can compensate for the impairment of the activitydependent synaptogenic pathway. Altogether these results indicate that Syn I is an important regulator of synapse formation, which adjusts synapse number in response to extracellular signals.

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

“…We observed a 40% decrease in the total number of varicosities when the helSynKD cells were plated with their original axons and a stronger decrease (84%) when cells were plated in soma configuration, which represents an extreme condition for cell outgrowth. These results could be consistent with the 70% decrease in synapse formation quantified by using synaptophysin-immunoreactive spots after SynII suppression with asRNA or the less intense decrease of 28% observed in glutamatergic synapses from cells treated with SynI smallinterference RNA (Perlini et al, 2011) and highlight the important role of Syn proteins in the establishment of synapses.…”

O-Linked β-N-Acetylglucosamine (O-GlcNAc) Site Thr-87 Regulates Synapsin I Localization to Synapses and Size of the Reserve Pool of Synaptic Vesicles

“…Synapsin I knock-out primary hippocampal neurons display delayed synaptogenic phenotypes, including reduced numbers of synapses (37). In contrast, overexpression of synapsin I leads to an increase in the number of synapses in primary hippocampal culture (38,39). Therefore, we considered if the increased synaptic targeting of synapsin I T87A might correlate with an increase in the number of SV clusters along axons, especially as we observed higher numbers of EGFP-synapsin I puncta in neurons expressing the synapsin I T87A mutant compared with neurons expressing wild type synapsin I while quantifying synaptic targeting.…”

Training-Dependent Associative Learning Induced Neocortical Structural Plasticity: A Trace Eyeblink Conditioning Analysis

“…Furthermore, recent findings from our laboratory demonstrated that WTEB conditioning increases synapsin I expression in conditioned barrels compared to control whisker barrels [23], suggesting that WTEB conditioning induces neocortical synaptic modification. Synapsin I is a phosphoprotein involved with regulating the release of neurotransmitters at the synapse [25], and has been reported to be correlated with synapse number [26]–[29]. Collectively, these studies demonstrate that WTEB conditioning is a neocortical-dependent task that also induces neocortical synaptic modifications making it a suitable paradigm for investigating the timing of learning-induced structural plasticity.…”