A C-terminal motif containing a PKC phosphorylation site regulates γ-Protocadherin-mediated dendrite arborization in the cerebral cortex in vivo

The Pcdhg gene cluster encodes 22 {gamma}-Protocadherin ({gamma}-Pcdh) cell adhesion molecules that critically regulate multiple aspects of neural development, including neuronal survival, dendritic and axonal arborization, and synapse formation and maturation. Each {gamma}-Pcdh isoform has unique protein domains-a homophilically-interacting extracellular domain and a juxtamembrane cytoplasmic domain- as well as a C-terminal cytoplasmic domain shared by all isoforms. The extent to which isoform-specific vs. shared domains regulate distinct {gamma}-Pcdh functions remains incompletely understood. Our previous in vitro studies identified PKC phosphorylation of a serine residue within a shared C-terminal motif as a mechanism through which {gamma}-Pcdh promotion of dendrite arborization via MARCKS is abrogated. Here, we used CRISPR/Cas9 genome editing to generate two new mouse lines expressing only non-phosphorylatable {gamma}-Pcdhs, due either to a serine-to-alanine mutation (PcdhgS/A) or to a 15-amino acid C-terminal deletion resulting from insertion of an early stop codon (PcdhgCTD). Both lines are viable and fertile, and the density and maturation of dendritic spines remains unchanged in both PcdhgS/A and PcdhgCTDcortex. Dendrite arborization of cortical pyramidal neurons, however, is significantly increased in both lines, as are levels of active MARCKS. Intriguingly, despite having significantly reduced levels of {gamma}-Pcdh proteins, the PcdhgCTD mutation yields the strongest phenotype, with even heterozygous mutants exhibiting increased arborization. The present study confirms that phosphorylation of a shared C-terminal motif is a key {gamma}-Pcdh negative regulation point, and contributes to a converging understanding of {gamma}-Pcdh family function in which distinct roles are played by both individual isoforms and discrete protein domains.