Synapsin III Acts Downstream of Semaphorin 3A/CDK5 Signaling to Regulate Radial Migration and Orientation of Pyramidal Neurons In Vivo

• Precise regulation of SynIII expression is essential during brain development
• SynIII regulates neuronal migration, orientation, and morphological maturation
• SynIII acts downstream of the Sema3A pathway, which involves NP1 and kinase CDK5
• Phosphorylation of SynIII by CDK5 on Ser404 is essential for SynIII function

SummarySynapsin III (SynIII) is a phosphoprotein that is highly expressed at early stages of neuronal development. Whereas in vitro evidence suggests a role for SynIII in neuronal differentiation, in vivo evidence is lacking. Here, we demonstrate that in vivo downregulation of SynIII expression affects neuronal migration and orientation. By contrast, SynIII overexpression affects neuronal migration, but not orientation. We identify a cyclin-dependent kinase-5 (CDK5) phosphorylation site on SynIII and use phosphomutant rescue experiments to demonstrate its role in SynIII function. Finally, we show that SynIII phosphorylation at the CDK5 site is induced by activation of the semaphorin-3A (Sema3A) pathway, which is implicated in migration and orientation of cortical pyramidal neurons (PNs) and is known to activate CDK5. Thus, fine-tuning of SynIII expression and phosphorylation by CDK5 activation through Sema3A activity is essential for proper neuronal migration and orientation.

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