The mammalian cerebral cortex is an orderly laminated structure containing six distinct layers of neurons. This structure is constructed during cortical development involving programmed cell migration and positioning. Disruption of brain cytoarchitecture can result in severe consequences including epilepsy, mental retardation and early lethality. A small protein ser/thr kinase Cdk5 in conjunction with its regulatory partners, p35 and p39, plays an indispensable role in these processes. We and others showed that in the absence of Cdk5, neuronal migration is impaired resulting in cell positioning defects in the cerebral cortex, hippocampus, cerebellum and other hind brain structures. We have evidence to suggest that Cdk5 regulates actin and microtubule dynamics as well as cadherin mediated cell adhesion. These functions of Cdk5 may contribute to its role in neuronal positioning. Recently, we have obtained experimental evidence to suggest interactions of cdk5 with other known pathways regulating neuronal migration. Haplo-insufficiency of Lis 1 is responsible for type I lissencephaly, a disastrous neurological disorder in humans featuring abnormal neuronal positioning. We and others showed that mammalian Lis 1 interacts with cytoplasmic dynein and modulates dynein activity. Nudel is a novel protein that interacts with both Lis 1 and dynein. Interestingly, Nudel is a physiological substrate of cdk5 as phosphorylation of Nudel is diminished in brain extract lacking cdk5 kinase activity. The integrin receptors are implicated in migration of many different cell types. We found that cdk5 down regulates focal adhesions. Furthermore, the focal adhesion kinase (FAK) is phosphorylated on serine732 by cdk5. Phosphorylation of S732 is developmentally regulated and abolished in the p35/p39 compound mutant brain extract indicating that FAK is an endogenous substrate of cdk5. We hypothesize that phosphorylation of Nudel and FAK by cdk5 plays a regulatory role in neuronal migration and positioning and propose experiments to decipher the biological consequences of these phosphorylation events. Finally, we will make use of an in vitro system established in my laboratory to study and compare cdk5- and Lis1-dependent migration defects in living brain slices. These experiments should bring novel insight into the mechanism by which neuronal migration and positioning is regulated during cortical development.
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