Binding of purified Reelin to ApoER2 and VLDLR mediates tyrosine phosphorylation of Disabled-1.

Reelin, Disabled-1 (Dab1), apolipoprotein E receptor 2 (ApoER2), and very low density lipoprotein receptor (VLDLR) participate in a signaling pathway required for layer formation during mammalian brain development. Binding of Reelin to ApoER2 and VLDLR induces a rapid increase in tyrosine phosphorylation of Dab1, an adaptor protein that associates with the cytoplasmic domain of the receptors. However, Reelin has also been proposed to signal through integrin and protocadherin. Here we compare the roles of ApoER2 and VLDLR in Reelin signaling. We used layer-specific markers to identify the final positions of early- and late-born neurons in the cortices of mice lacking ApoER2, VLDLR, or both ApoER2 and VLDLR. Subtle alterations were observed in mice lacking VLDLR, whereas more severe abnormalities were detected in the absence of ApoER2, and major disruptions were obvious in mice lacking both receptors. Purified Reelin associated more readily with ApoER2 than with VLDLR and no synergy was observed in the presence of both receptors. Consistent with the binding data, the level of Reelin-induced Dab1 phosphorylation was more severely reduced in neurons lacking ApoER2 than in neurons lacking VLDLR. However, similarly low levels of Dab1 tyrosine phosphorylation were observed in ApoER2(-/-) and VLDLR(-/-) mice in vivo. Finally, there was a complete absence of Reelin-induced tyrosine phosphorylation of Dab1 in cortical neurons from mice lacking both ApoER2 and VLDLR. These findings demonstrate that ApoER2 and VLDLR are essential for Reelin signaling and that no other receptor molecules can compensate for their role in mediating tyrosine phosphorylation of Dab1.