Synapsins are neuronal phosphoproteins that fine-tune synaptic transmission and suppress seizure activity. Synapsin II (SynII) deletion produces epileptic seizures and overexcitability in neuronal networks. Early studies in primary neuronal cultures have shown that SynII deletion results in a delay in synapse formation. More recent studies at hippocampal slices have revealed increased spontaneous activity in SynII knockout (SynII(−)) mice. To reconcile these observations, we systematically re-examined synaptic transmission, synapse formation, and neurite growth in primary hippocampal neuronal cultures. We find that spontaneous glutamatergic synaptic activity was suppressed in SynII(−) neurons during the initial developmental epoch (7 days in vitro, DIV) but was enhanced at later times (12 and18 DIV). The density of synapses, transmission between connected pairs of neurons, and the number of docked synaptic vesicles were not affected by SynII deletion. However, we found that neurite outgrowth in SynII(−) neurons was suppressed during the initial developmental epoch (7 DIV) but enhanced at subsequent developmental stages (12 and18 DIV). This finding can account for the observed effect of SynII deletion on synaptic activity. To test whether the observed phenotype resulted directly from the deletion of SynII we expressed SynII in SynII(−) cultures using an adeno-associated virus (AAV). Expression of SynII at 2 DIV rescued the SynII deletion-dependent alterations in both synaptic activity and neuronal growth. To test whether the increased neurite outgrowth in SynII(−) observed at DIV 12 and18 represents an overcompensation for the initial developmental delay or results directly from SynII deletion we performed “late expression” experiments, transfecting SynII(−) cultures with AAV at 7 DIV. The late SynII expression suppressed neurite outgrowth at 12 and 18 DIV to the levels observed in control neurons, suggesting that these phenotypes directly depend on SynII. These results reveal a novel developmentally regulated role for SynII function in the control of neurite growth.

突触蛋白是神经元磷酸化蛋白，可微调突触传递并抑制癫痫发作活动。Synapsin II（SynII）缺失会在神经元网络中引起癫痫发作和过度兴奋。在原代神经元文化中的早期研究表明，SynII缺失会导致突触形成的延迟。在海马切片上进行的最新研究表明，SynII基因敲除（SynII（-））小鼠的自发活动增加。为了调和这些意见，我们系统地重新检查了原代海马神经元培养物中的突触传递，突触形成和神经突生长。我们发现自发的谷氨酸能突触活性在最初的发育时期（体外7天，DIV）期间在SynII（-）神经元中被抑制，但在以后的时间（12和18 DIV）中得到增强。突触的密度，连接的神经元对之间的传递以及对接的突触小泡的数量不受SynII缺失的影响。但是，我们发现SynII（-）神经元的神经突生长在最初的发育时期（7 DIV）受到抑制，但在随后的发育阶段（12和18 DIV）得到增强。该发现可以解释观察到的SynII缺失对突触活性的影响。为了测试观察到的表型是否直接由SynII的缺失引起，我们使用腺相关病毒（AAV）在SynII（-）培养物中表达了SynII。SynII在2 DIV的表达挽救了突触活性和神经元生长中SynII缺失依赖性的改变。为了测试在DIV 12和18观察到的SynII（-）中增加的神经突向外生长是否代表对初始发育延迟的过度补偿或直接由SynII缺失导致的结果，我们进行了“后期表达”实验，以7 DIV的AAV转染了SynII（-）培养物。SynII的晚期表达抑制了在12和18 DIV时神经突向外生长到在对照神经元中观察到的水平，表明这些表型直接依赖于SynII。