Functional roles of reelin signaling cascades In formation of cerebral neocortex

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The shaking rat Kawasaki (SRK) is an autosomal recessive mutant which exhibits reeler-like abnormal locomotor behaviors. The murine reeler mutants arise from several mutations in the specific gene called reelin, which result in defects of Reelin expression or secretion in the cerebral cortex and other regions of CNS. To address the issue whether the SRK mutation also arises from a mutation in reelin, we analyzed the reelin gene in SRK. Northern analysis of reelin mRNA from normal rats showed that rat reelin was expressed as a 〜12-kb transcript in both the cerebrum and cerebellum, while reelin expression was markedly reduced in SRK brains. In situ hybridization analysis showed that reelin mRNA in SRK brains was expressed in Cajal-Retzius cells in the marginal zone of the cerebral cortex and outer granular cells in the cerebellar cortex in similar manners to normal controls, but its expression was considerably reduced. The immunohistochemical analysis using the CR-50 monoclonal antibody, no immunoproduct was recognized in the cerebral and cerebellar cortices. From the cDNA sequences, we found a 61-base insertion in SRK reelin, which contains a termination codon in the reading frame. Furthermore, genomic DNA analysis revealed that a 10-base deletion, which contains a predicted exon-intron boundary, occurred in the SRK genomic reelin gene, resulting in incorporation of the insertion sequence originated from the following intron into SRK reelin mRNA. Thus, the SRK mutation is another type of mutations which lacks expression of the functional Reelin protein and, therefore, causes the reeler phenotype.