Activation of a Dab1/CrkL/C3G/Rap1 pathway in reelin-stimulated neurons

During brain development, many neurons migrate long distances before settling and differentiating. These migrations are coordinated to ensure normal development. The secreted protein Reelin controls the locations of manytypes of neurons, and its absence causes the classic "Reeler" phenotype [1, 2]. Reelin action requires tyrosine phosphorylation of the intracellular protein Dab1 by Src-family kinases [3-5]. However, little is known about signaling pathways downstream of Dab1. Here, we identify several proteins in embryonic brain extract that bind to tyrosine-phosphorylated, but not non-phosphorylated, Dab1. Of these, the Crk-family proteins (CrkL, CrkI, and CrkII [6]), bind significant quantities of Dabl when embryonic cortical neurons are exposed to Reelin. CrkL binding to Dab1 involves two tyrosine phosphorylation sites, Y220 and 232, that are critical for proper positioning of migrating cortical plate neurons. CrkL also binds C3G, an exchange factor (GEF) for the small GTPase Rap1 that is activated in other systems by tyrosine phosphorylation [7-10]. We report that Reelin stimulates tyrosine phosphorylation of C3G and activates Rap1. C3G and Rap1 regulate adhesion of fibroblasts and other cell types [11-14]. Regulation of Crk/CrkL, C3G, and Rap1 by Reelin may be involved in coordinating neuron migrations during brain development.