Receptor Clustering Is Involved in Reelin Signaling

Abstract

The Reelin signaling cascade plays a crucial role in the correct positioning of neurons during embryonic brain development. Reelin binding to apolipoprotein E receptor 2 (ApoER2) and very-low-density-lipoprotein receptor (VLDLR) leads to phosphorylation of disabled 1 (Dab1), an adaptor protein which associates with the intracellular domains of both receptors. Coreceptors for Reelin have been postulated to be necessary for Dab1 phosphorylation. We show that bivalent agents specifically binding to ApoER2 or VLDLR are sufficient to mimic the Reelin signal. These agents induce Dab1 phosphorylation, activate members of the Src family of nonreceptor tyrosine kinases, modulate protein kinase B/Akt phosphorylation, and increase long-term potentiation in hippocampal slices. Induced dimerization of Dab1 in HEK293 cells leads to its phosphorylation even in the absence of Reelin receptors. The mechanism for and the sites of these phosphorylations are identical to those effected by Reelin in primary neurons. These results suggest that binding of Reelin, which exists as a homodimer in vivo, to ApoER2 and VLDLR induces clustering of ApoER2 and VLDLR. As a consequence, Dab1 becomes dimerized or oligomerized on the cytosolic side of the plasma membrane, constituting the active substrate for the kinase; this process seems to be sufficient to transmit the signal and does not appear to require any coreceptor.

This work was supported by Austrian Science Foundation grants P13931-MOB, F606, and F608 and the Herzfelder'sche Familienstiftung. D.F. was supported by the Austrian Academy of Science (DOC-FFORT/21282). H.H.B. was supported by a postdoctoral fellowship from the Deutsche Forschungsgemeinschaft. J.H. is supported by grants from the National Institutes of Health, the Alzheimer Association, and the Humboldt Foundation.

Antibodies against Reelin and Dab1 were generous gifts from Andre Goffinet (Medical School, University of Louvain). The expression plasmid for Reelin and the phosphorylation site-specific antibodies against Dab1 (anti-P198/200 and anti-P220) were generously provided by Tom Curran (Department of Developmental Neurobiology, St. Jude's Children's Research Hospital, Memphis, Tenn.).