Crk and Crk-Like Play Essential Overlapping Roles Downstream of Disabled-1 in the Reelin Pathway

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Abstract
Reelin controls neuronal positioning in the developing brain by binding to the two lipoprotein receptors, very-low-density lipoprotein receptor and apolipoprotein E receptor 2, to stimulate phosphorylation of Disabled-1 (Dab1) by the Fyn and Src tyrosine kinases. Crk and Crk-like (CrkL) have been proposed to interact with tyrosine phosphorylated Dab1 to mediate downstream events in the Reelin pathway. However, these adaptor proteins are widely expressed, and they fulfill essential functions during embryonic development. To address their specific roles in Reelin-mediated neuronal migration, we generated mutant mice, by Cre-loxP recombination, lackingCrkandCrkLin most neurons. These animals displayed the major anatomic features ofreelerincluding, cerebellar hypofoliation, failure of Purkinje cell migration, absence of preplate splitting, impaired dendritic development, and disruption of layer formation in the hippocampus and cerebral cortex. However, proximal signaling involving tyrosine phosphorylation and turnover of Dab1 occurred normally in the mutant mouse brain and in primary cortical neurons treated with Reelin. In contrast, two downstream signaling events, Reelin-induced phosphorylation of C3G and Akt, were not observed in the absence of Crk and CrkL in mouse embryonic cortical neurons. These findings place C3G and Akt phosphorylation downstream of Crk and CrkL, which play essential overlapping functions in the Reelin signaling pathway.