Csmd2 interacts with Dab1 and is Required in Reelin-Mediated Neuronal Maturation

Reelin is a glycoprotein secreted by Cajal-Retzius cells to regulate development of the cerebral cortex. Reelin binding to its receptors on immature neurons initiates a signaling cascade through the downstream adaptor protein, Dab1. Defects in this signaling mechanism result in perturbed neuronal migration, reductions in dendrite complexity, and deficits in synapse development and function. How Reelin controls neuronal migration and brain lamination have been extensively investigated over the years, but the pathways that regulate dendrite and spine development downstream of Reelin and Dab1 have yet to be fully elucidated. Here, we have identified a novel interaction between Dab1 and Csmd2, a synaptic transmembrane protein required for dendrite and dendritic spine development in forebrain excitatory neurons. We demonstrate that Csmd2 contains an NPxY motif on its intracellular region, through which Dab1 interacts with Csmd2. Interestingly, we find that this NPxY consensus motif is not required for Csmd2 to localize at the postsynaptic densities of spiny neurons. Rather, the introduction of an NPxY mutant form of Csmd2 results in a significant overproduction of immature, filopodia-like dendritic spines in maturing neurons. Moreover, we show that knockdown of mRNA expression in immature developing neurons abolishes the ability of Reelin to promote dendrite elaboration and dendritic spine maturation. This suggests that the Csmd2-Dab1 interaction may be a requirement of Reelin/Dab1 signaling to mediate the structural maturation of neurons. Together, these results point toward a role of Csmd2 in the Reelin/Dab1 signaling axis that promotes the development of dendrites and dendritic spines in maturing neurons.