Defects in cortical migration are a major cause of epilepsy and cognitive problems in humans. Although the mechanisms that govern neuronal migration are largely unknown, targeted gene disruptions have previously shown that the unconventional cyclin-dependent kinase 5 (Cdk5) is required for the migration of post-mitotic neurons. In this study in Nature Cell Biology, Kawauchi et al. find that p27^kip1, a Cdk inhibitor that prevents cell-cycle progression, regulates the actin cytoskeleton downstream of Cdk5 during cortical migration.

Kawauchi et al. find that Cdk5 and p27^kip1 colocalize in growth cones. Moreover, they show that Cdk5 protects p27^kip1 from proteasomal degradation both in vitro and in vivo by directly phosphorylating p27^kip1 at Ser10. Despite the fact that p27^kip1-null mice do not show any migration-related abnormalities, short-hairpin (sh)-RNA-mediated depletion of p27^kip1 in cerebral cortices leads to a migration defect that is very similar to that observed in brains electroporated with dominant-negative Cdk5: at post natal day 0 (5 days after electroporation), p27^kip1-depleted cells fail to reach the superficial layer of the cortex and remain in the intermediate zone. From this, the authors propose that p27^kip1 and Cdk5 may act together to allow normal neuronal migration but that, in transgenic mice, the loss of p27^kip1 can be compensated for.

To determine whether this phenotype is related to p27^kip1's effect on differentiation or the proliferation of neuronal precursors, the authors examined the expression of neuronal markers and BrdU incorporation in cortical neurons. At this stage of development, no significant differences between p27^kip1-depleted cells and controls were found probably due to the delayed RNA interference effects. These results suggest that p27^kip1 functions not only as a Cdk inhibitor but also as a downstream effector of Cdk5 in post-mitotic neurons exiting the ventricular zone.

The fact that p27^kip1 colocalizes with F-actin in growth cones and that the levels of F-actin were reduced in neuronal cells depleted of p27^kip1 suggests that p27^kip1 could be regulating the actin cytoskeleton. Indeed, both Cdk5 and p27^kip1 were found to suppress the phosphorylation, and thus prevent the inactivation of, the actin-severing protein cofilin through effects on the RhoA-ROCK pathway.

Together, this study reveals that one way Cdk5 regulates migration is through control of p27^kip1 and perhaps its effects on the F-actin cytoskeleton. Moreover, it demonstrates that acute protein inactivation using in utero electroporation is a powerful technique for revealing protein functions that may be compensated for in transgenic animals.

Author: Monica Hoyos-Flight