Protocadherin clusters and cell adhesion kinase regulate dendrite complexity through Rho GTPase

Dendritic patterning and spine morphogenesis are crucial for the assembly of neuronal circuitry to ensure normal brain development and synaptic connectivity as well as for understanding underlying mechanisms of neuropsychiatric diseases and cognitive impairments. The Rho GTPase family is essential for neuronal morphogenesis and synaptic plasticity by modulating and reorganizing the cytoskeleton. Here, we report that protocadherin (Pcdh) clusters and cell adhesion kinases (CAKs) play important roles in dendritic development and spine elaboration. The knockout of the entire Pcdh cluster results in the dendritic simplification and spine loss in CA1 pyramidal neurons in vivo and in cultured primary hippocampal neurons in vitro. The knockdown of the whole Pcdh cluster or in combination with the Pcdh knockout results in similar dendritic and spine defects in vitro. The overexpression of proline-rich tyrosine kinase 2 (Pyk2, also known as CAK, RAFTK, FAK2, and CADTK) recapitulates these defects and its knockdown rescues the phenotype. Moreover, the genetic deletion of the Pcdh cluster results in phosphorylation and activation of Pyk2 and focal adhesion kinase (Fak) and the inhibition of Rho GTPases in vivo. Finally, the overexpression of Pyk2 leads to inactivation of Rac1 and, conversely, the constitutive active Rac1 rescues the dendritic and spine morphogenesis defects caused by the knockout of the Pcdh cluster and the knockdown of the Pcdh cluster. Thus, the involvement of the Pcdh-CAK-Rho GTPase pathway in the dendritic development and spine morphogenesis has interesting implications for proper assembly of neuronal connections in the brain.