| Cyclin-dependent kinase 5 (Cdk5) constitutes a proline-directed serine-threonine kinase, which is preferentially active in the nervous system due to the restricted expression of its co-activator p35. A deficiency in p35/Cdk5 function results in an aberrantly organized neocortex, where the individual neuronal layers are inverted. Investigations over recent years have provided molecular clues as to how p35/Cdk5 may regulate this neocortical process. However, as the molecular insights were primarily obtained from cell culture experiments, their applicability to the environment of a developing neocortex remains doubtful. In particular, cultured neurons do not possess the migratory properties of neurons that establish the neocortical layers, which include migration by specified modes, such as somal translocation and glia-guided locomotion.; To elucidate p35/Cdk5-dependent mechanisms of neocortical layer formation in a physiological context, we conducted a comparative series of time-lapse recordings in p35 null and wildtype cortical slices. We find that in the p35-/- neocortex somal translocation and locomotion are largely replaced by a distinct mode of migration that is characterized by excessively branched neuronal processes. Thus, for the first time, the formation of cortical layers is functionally linked to the proper execution of migration mode. Furthermore, this "branched migration" in the p35-/- neocortex is associated with impaired neuronal-glial interaction and not observed in another mouse mutant with neocortical layer inversion, namely a mouse with a defective Reelin signaling pathway. It therefore appears that layer formation requires p35/Cdk5-mediated neuronal-glial interaction, which by itself is insufficient for this process and independent of Reelin signaling. Finally, studies aimed at a molecular understanding of branched migration provide initial evidence that it is cell-autonomous in nature and potentially regulated by p35/Cdk5-dependent phosphorylation of Pak1. Taken together, these results support a refined but also revised model of neocortical layer formation, which is discussed in detail. |
