Filopodia formation and Disabled degradation downstream of Reelin.

During Drosophila embryogenesis, Abl (Abelson tyrosine kinase) is localized in the axons of the CNS (central nervous system). Mutations in Abl have a subtle effect on the morphology of the embryonic CNS, and the mutant animals survive to the pupal and adult stages. However, genetic screens have identified several genes that, when mutated along with the Abl gene, modified the phenotypes. Two prominent genes that arose from these screens were enabled (Ena) and disabled (Dab). It has been known for some time that Enabled and its mammalian homologues are involved in the regulation of actin dynamics, and promote actin polymerization at the leading edge of motile cells. It was a defect in actin polymerization in migrating neurons in particular that resulted in the identification of Enabled as an important regulator of neuronal migration. Defects in Disabled, in both Drosophila and mammals, also gave rise to neuronal defects which, in mice, were indistinguishable from phenotypes observed in the reeler mouse. These observations suggested that mDab1 (mammalian Disabled homologue 1) acted in a pathway downstream of Reelin, the product of the reelin gene found to be defective in reeler mice. Now, in this issue of the Biochemical Journal, Takenawa and colleagues have demonstrated that Disabled also acts in a pathway to regulate actin dynamics through the direct activation of N-WASP (neuronal Wiskott-Aldrich syndrome protein). Furthermore, they were also able to link several lines of investigation from other groups to show that the ability of mDab1 to regulate actin dynamics during cell motility was under the negative control of tyrosine phosphorylation, leading to ubiquitin-mediated degradation of mDab1.