X-linked mental retardation gene CASK interacts with Bcl11A/CTIP1 and regulates axon branching and outgrowth

Calcium/calmodulin-dependent serine kinase (CASK), a causative gene in X-linked mental retardation, carries out multiple functions in neurons, including vesicle trafficking of ion channels, synapse formation, and gene transcription. From a yeast two-hybrid screen, Krüppel-like zinc finger protein B cell lymphoma/COUP-TF-interacting protein 1 (Bcl11A/CTIP1) was identified as a CASK binding protein. Through alternative splicing, a single Bcl11A gene encodes two major protein products in neurons, Bcl11A-S and Bcl11A-L. CASK interacted with both Bcl11A-S and Bcl11A-L in transfected COS cells and brain. Immunofluorescence staining further indicated the colocalization of CASK and Bcl11A in the nuclei of neurons. These studies supported an interaction between CASK and Bcl11A in vivo. Bcl11A-L has previously been shown to play a role in gene transcription as well as axon outgrowth and branching. Here, we further show that Bcl11A-L rearranges the distribution of nuclear actin, which may be related to the function of Bcl11A-L in gene expression. More importantly, using cultured hippocampal neurons as a model system, we show that CASK enhances the ability of Bcl11A-L to restrict axon outgrowth and branching. Interruption of the interaction between CASK and Bcl11A increased the outgrowth and branching of axons, suggesting that the interaction between CASK and Bcl11A controls axon arborization. In conclusion, our results suggest that, through the interaction with Bcl11A, CASK plays a role in axonogenesis, which may be related to brain anatomical characteristics in humans.