Synapsin III (SynIII) is a neuron-specific phosphoprotein that plays a unique 
role in neuronal development. SynIII is phosphorylated by cAMP-dependent protein 
kinase (PKA) at a highly conserved phosphorylation site and by cyclin-dependent 
kinase-5 (Cdk5) at a newly described site. Although SynIII is known to be 
involved in axon elongation in vitro, the role of its phosphorylation by PKA and 
Cdk5 in the modulation of this process is unknown. We expressed either wild-type 
(WT) or phosphorylation-site mutants of SynIII in primary SynIII knock-out (KO) 
mouse neurons at early stages of in vitro development. Whereas the neurite 
elongation phenotype of SynIII KO neurons was fully rescued by the expression of 
WT SynIII, the expression of nonphosphorylatable and pseudo-phosphorylated PKA 
mutants was ineffective. Also, the nonphosphorylatable Cdk5 mutant was unable to 
rescue the neurite elongation phenotype of SynIII KO neurons. By contrast, the 
pseudo-phosphorylated mutant rescued the delay in neuronal maturation and axonal 
elongation, revealing a Cdk5-dependent regulation of SynIII function. 
Interestingly, SynIII KO neurons also exhibited decreased survival that was 
fully rescued by the expression of WT SynIII, but not by its phosphorylation 
mutants, and was associated with increased activated caspase3 and altered 
tropomyosin receptor kinase B isoform expression. These results indicate that 
PKA and Cdk5 phosphorylation is required for the physiological action of SynIII 
on axon specification and neurite outgrowth and that the expression of a 
functional SynIII is crucial for cell survival. Significance statement: Synapsin 
III is an atypical member of the synapsin family of synaptic vesicle-associated 
phosphoproteins that is precociously expressed in neurons and is downregulated 
afterward. Although experimental evidence suggests a specific role for Synapsin 
III in neuronal development, the molecular mechanisms are still largely unknown. 
We found that Synapsin III plays a central role in early stages of neuronal 
development involving neuronal survival, polarization, and neuritic growth and 
that these effects are dependent on phosphorylation by cAMP-dependent protein 
kinase and cyclin-dependent protein kinase-5. These results explain the recently 
described neurodevelopmental defects in the migration and orientation of 
Synapsin III-depleted cortical neurons and support the potential association of 
Synapsin III with neurodevelopmental disorders such as schizophrenia.