PKCγ-mediated Phosphorylation of Mtss1 Regulates the Dendritic Outgrowth and Spine Development of Cerebellar Purkinje Cells

Protein kinase C gamma (PKCγ) is a signalling protein expressed in Purkinje cells (PCs) of the cerebellum. It is involved in regulating synapse formation and PC dendritic development through phosphorylation of target proteins that control cytoskeleton dynamics and PC dendritic morphology. Mutations in the protein kinase C gamma gene cause spinocerebellar ataxia 14 (SCA14), an autosomal dominant neurodegenerative disease leading to motor deficits and cognitive decline. The molecular targets of PKCγ and its interaction with cytoskeletal regulators are not yet well understood. Using a PKCγ mutant mouse line (PKCγ-A24E), which expresses a constitutively active PKCγ causing an SCA14-like phenotype, we identified Metastasis suppressor protein 1 (Mtss1) as a PKCγ molecular target involved in PC dendritic development. The two Mtss1 phospho-sites (S265 and S266) showed an increased phosphorylation in the cerebellum of PKCγ-A24E mice compared to wildtypes. Altering Mtss1-S265 and Mtss1-S266 with phospho-mimetic and phospho-defective mutations led to a loss of PC dendritic tree complexity in dissociated cerebellar cultures, with a developmental delay, a decreased dendritic branching, reduced spine density, and fewer synaptic processes, partially mimicking the PKCγ-A24E phenotype. Moreover, we found that these Mtss1 effects are partially regulated via the Arp2/3 complex. Our results demonstrate the importance of PKCγ-mediated phosphorylation of Mtss1 at S265 and S266 for PC dendritic outgrowth and suggest its contribution to PKCγ cytoskeleton signalling and the SCA14 phenotype.