CHL1 cooperates with PAK1–3 to regulate morphological differentiation of embryonic cortical neurons

The cell adhesion molecule close homologue of L1 (CHL1) is important for apical dendritic projection and laminar positioning of pyramidal neurons in caudal regions of the cerebral cortex. The p21-activated kinase (PAK1–3) subfamily of serine/threonine kinases has also been implicated in regulating cell adhesion, migration, and morphology. Immunofluorescence staining in mouse embryonic brain showed that PAK1–3 was expressed in embryonic cortex and colocalized with CHL1 during neuronal migration and differentiation. To investigate a cooperative function for CHL1 and PAK in pyramidal cell differentiation or migration, a dominant-negative PAK mutant (PAK1 AID) that inhibits PAK1-3 kinase activity while coexpressing a green fluorescent protein (GFP) reporter was electroporated into the lateral ventricles of wild type (WT) and CHL1 null mutant mouse embryos (E14.5), then brain slices were cultured and neurons analyzed for laminar positioning and morphology by confocal microscopy after 3 days in vitro. Expression of PAK1 AID in CHL1 mutant cortex inactivated PAK and caused embryonic cortical neurons to branch profusely in the intermediate zone (IZ) and cortical plate (CP). The number of nodes, terminals and length of leading processes/apical dendrites of CHL1 mutant embryos expressing PAK1 AID increased dramatically, compared to CHL1 mutants without PAK1 AID, or WT embryos with or without PAK1 AID. These findings suggest that CHL1 and PAK1-3 kinase cooperate, most likely in independent pathways, in regulating morphological development of the leading process/apical dendrite of embryonic cortical neurons.