title: Phosphorylation by PKA and Cdk5 Mediates the Early Effects of Synapsin III in Neuronal Morphological Maturation.
abstract: 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.
authors.affiliation: Department of Experimental Medicine, University of Genova, 16132 Genova, Italy, and.
authors.forename: Alessandra
authors.initials: A
authors.lastname: Piccini
authors.affiliation: Department of Neuroscience and Brain Technologies, Istituto Italiano di Tecnologia, 16163 Genova, Italy.
authors.forename: Laura E
authors.initials: LE
authors.lastname: Perlini
authors.affiliation: Department of Neuroscience and Brain Technologies, Istituto Italiano di Tecnologia, 16163 Genova, Italy.
authors.forename: Laura
authors.initials: L
authors.lastname: Cancedda
authors.affiliation: Department of Experimental Medicine, University of Genova, 16132 Genova, Italy, and Department of Neuroscience and Brain Technologies, Istituto Italiano di Tecnologia, 16163 Genova, Italy fabio.benfenati@iit.it.
authors.forename: Fabio
authors.initials: F
authors.lastname: Benfenati
authors.affiliation: Department of Experimental Medicine, University of Genova, 16132 Genova, Italy, and.
authors.forename: Silvia
authors.initials: S
authors.lastname: Giovedì
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D002478:Cells, Cultured; D002540:Cerebral Cortex / Q000166:cytology / Q000254:growth & development; D017868:Cyclic AMP-Dependent Protein Kinases / Q000235:genetics / Q000378:metabolism*; D051360:Cyclin-Dependent Kinase 5 / Q000235:genetics / Q000378:metabolism*; D004622:Embryo, Mammalian; D005260:Female; D018507:Gene Expression Regulation, Developmental / Q000235:genetics*; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009419:Nerve Tissue Proteins / Q000378:metabolism; D009474:Neurons / Q000502:physiology*; D011247:Pregnancy; D051381:Rats; D017207:Rats, Sprague-Dawley; D055647:Rats, Transgenic; D015398:Signal Transduction / Q000187:drug effects / Q000235:genetics; D016704:Synapsins / Q000235:genetics / Q000378:metabolism*; D014404:Tubulin / Q000378:metabolism
chemical_list: D009419:Nerve Tissue Proteins; D016704:Synapsins; D014404:Tubulin; C462941:beta3 tubulin, mouse; D051360:Cyclin-Dependent Kinase 5; D017868:Cyclic AMP-Dependent Protein Kinases
keywords: knock-out mice; neurite outgrowth; neuron survival; protein phosphorylation; synapsin III