High affinity binding of Dab1 to Reelin receptors promotes normal positioning of upper layer cortical plate neurons.
The positions of neurons in the neocortex, hippocampus, cerebellum and various other laminated brain regions are regulated by a signaling pathway initiated by the secreted protein Reelin and requiring the intracellular adaptor protein Dab1. Dab1 and the Reelin receptors VLDLR and ApoER2 are expressed by neurons whose migrations are coordinated by Reelin. In vitro, Dab1 binds with high affinity to the cytoplasmic tails of VLDLR and ApoER2 via its PTB domain. To test the importance of Dab1 binding to VLDLR and ApoER2, we replaced the Dab1 gene with a cDNA cassette encoding a point mutant allele, Dab1(F158V). This mutation strongly decreases Dab1 binding in vitro to peptides containing the ApoER2 or VLDLR cytoplasmic regions. Surprisingly, Dab1(F158V/F158V) homozygotes have no discernable phenotype. However, Dab1(F158V/-) hemizygous animals have a subtle phenotype in which late-generated cortical plate neurons migrate excessively into the marginal zone. Early cortical plate neurons, subplate neurons, hippocampal pyramidal cells and cerebellar Purkinje cells are positioned normally. Thus Dab(F158V) is a weak loss-of-function (hypomorphic) allele that has no detectable effect when homozygous. The phenotype of Dab1(F158V/-) hemizygotes shows that late cortical plate neurons of layers 2-3 require efficient Reelin-Dab1 signaling to prevent them entering the marginal zone. The Dab1(F158V) allele adds to a series of Dab1 alleles that demonstrates cell type-specific variation in the Reelin-Dab1 pathway.