Relative importance of the tyrosine phosphorylation sites of Disabled-1 to the transmission of Reelin signaling.

Reelin regulates radial migration of the projection neurons in the developing cerebral cortex by inducing tyrosine phosphorylation of an intracellular adaptor protein, Disabled-1 (Dab1), through activation of Src family tyrosine kinases (SFKs). Five tyrosine residues of Dab1 (Y185, Y198, Y200, Y220, and Y232) are capable of being phosphorylated by SFKs. Among them, phosphorylation of Y198, Y220, and Y232 has been demonstrated after Reelin stimulation, and Y185 has been suggested to be an additional Reelin-induced phosphorylation site. In this study we established a reconstitution system in which a migratory defect in the cortex of Dab1-deficient mice is rescued by transfection with a wild-type Dab1 gene. The transfected neurons in the mutant cortex migrated radially and split the superficial preplate into the marginal zone and subplate by a mechanism that depended on interaction between Dab1 and Reelin receptors. Although this migration rescue was also observed in the mutant cortex transfected with a Dab1 gene containing a single substitution at Y198 by phenylalanine (Y198F), Y220F, Y232F, both of the Y185F and Y200F (Y185F/Y200F), Y185F/Y220F, Y185F/Y232F, Y198F/Y220F, or Y198F/Y232F, it was never observed in the mutant cortex transfected with a Dab1 gene containing the Y185F/Y198F or Y220F/Y232F. These findings suggest that Reelin induces phosphorylation at Y185 of Dab1, and that there are two Reelin signaling pathways, one mediated by the Y185/Y198 phosphorylation of Dab1 and the other mediated by the Y220/Y232 phosphorylation of Dab1. The results also suggest that phosphorylation of either one of the residues in each pair is sufficient for the transmission of Reelin signaling.