Tyrosine phosphorylation of Disabled-1 is essential for Reelin-stimulated activation of Akt and Src family kinases.
Reelin is a large secreted signaling protein that is essential for proper positioning of migratory neurons during mammalian brain development. The Reelin signal is transduced into the cell by the lipoprotein receptors VLDLR and ApoER2, leading to tyrosine phosphorylation of the associated intracellular adaptor protein Disabled-1 (Dab1). Tyrosine phosphorylation of Dab1 is essential for responding to Reelin, as knock-in mice expressing a form of Dab1 that cannot be phosphorylated on tyrosine are indistinguishable from mice lacking Reelin, Reelin-receptors or Dab1. Molecular events dependent on Dab1 tyrosine phosphorylation are unknown. However, Reelin has recently been shown to activate the phosphoinositide-3-kinase (PI 3-K)-dependent kinase, Akt, as well as Src family kinases in wild type but not Dab1-/- primary embryonic neuronal cultures. Using pharmacological inhibitors and mice harboring mutant alleles of Dab1, we show here that tyrosine phosphorylation, but not the carboxyl-terminal region, of Dab1 is required for Reelin-induced activation of Akt and Src family kinases. Additionally, although Fyn is an important regulator of Dab1, Fyn deficiency does not prevent acute Reelin-induced Akt activation. Finally, whereas a number of growth factors propagate signals simultaneously through PI 3-K and mitogen-activated protein kinase (MAPK) cascades, we find Reelin does not engage the canonical MAPK cascade. These results define the first molecular events strictly dependent on Reelin-induced Dab1 tyrosine phosphorylation, and suggest that propagation of the Reelin signal is mediated by Akt, substrates of Src family kinases and/or unidentified molecules that share with these a common molecular link to phosphorylated Dab1.