Dab1 tyrosine phosphorylation sites relay positional signals during mouse brain development.

The extracellular protein Reln controls neuronal migrations in parts of the cortex, hippocampus and cerebellum. In vivo, absence of Reln correlates with up-regulation of the docking protein Dab1 and decreased Dab1 tyrosine phosphorylation. Loss of the Reln receptor proteins, apolipoprotein receptor 2 and very low density lipoprotein receptor, results in a Reln-like phenotype accompanied by increased Dab1 protein expression. Complete loss of Dab1, however, recapitulates the Reln phenotype. To determine whether Dab1 tyrosine phosphorylation affects Dab1 protein expression and positioning of embryonic neurons, we have identified Dab1 tyrosine phosphorylation sites. We then generated mice in which the Dab1 protein had all the potential tyrosine phosphorylation sites mutated. This mutant protein is not tyrosine phosphorylated during brain development and is not upregulated to the extent observed in the Reln or the apoER2 and VLDLR receptor mutants. Animals expressing the non-phosphorylated Dab1 protein have a phenotype similar to the dab1-null mutant. Dab1 is downregulated by the Reln signal in neurons in the absence of tyrosine phosphorylation. Dab1 tyrosine phosphorylation sites and not downregulation of Dab1 protein are required for Reln signaling.