Regulation of protein tyrosine kinase signaling by substrate degradation during brain development

Disabled-1 (Dab1) is a cytoplasmic adaptor protein that regulates neuronal migrations during mammalian brain development. Dahl function in vivo depends on tyrosine phosphorylation, which is stimulated by extracellular Reelin and requires Src family kinases. Reelin signaling also negatively regulates Dahl protein levels in vivo, and reduced Dahl levels may be part of the mechanism that regulates neuronal migration. We have made use of mouse embryo cortical neuron cultures in which Reelin induces Dahl tyrosine phosphorylation and Src family kinase activation. We have found that Dahl is normally stable, but in response to Reelin it becomes polyubiquitinated and degraded via the proteasome pathway. We have established that tyrosine phosphorylation of Dab1 is required for its degradation. Dab1 molecules lacking phosphotyrosine are not degraded in neurons in which the Dab1 degradation pathway is active. The requirements for Reelin-induced degradation of Dahl in vitro correctly predict Dab1 protein levels in vivo in different mutant mice. We also provide evidence that Dahl serine/threonine phosphorylation may be important for Dahl tyrosine phosphorylation. Our data provide the first evidence for how Reelin down-regulates Dahl protein expression in vivo. Dab1 degradation may be important for ensuring a transient Reelin response and may play a role in normal brain development.