PubMed1999
Reeler, an autosomal recessive mutant mouse, is characterized by ataxic gait and tremor. In this mutant, the cerebral and cerebellar cortices and hippocampus are cytoarchitectually disorganized: neuronal components are ectopically located in these laminated structures. Since reelin, the gene responsible for the reeler mutation, was discovered by D'Arcangelo et al. (Nature 374: 719-723, 1995), remarkable progress has occurred in this field. The reelin gene encodes an extracellular protein, Reelin, that is crucial for neuronal migration. During embryogenesis, reelin is expressed in the Cajal-Retzius cells in the cerebral cortex and in the outer granule cells in the cerebellar cortex. Although non-laminated structures such as facial nucleus, inferior olivary complex, and dorsal cochlear nucleus are also cytoarchitectually deranged in this mutant, only a few studies have been done to clarify the detailed abnormalities in these non-laminated structures. In this review, we focused on the cytoarchitectonic abnormality in the facial nucleus of the reeler mouse. The branchiomotor neurons in the facial nucleus are generated from the ventricular zone of the floor of the fourth ventricle, migrate ventrolaterally, and finally settle near the ventral surface of the hindbrain. Time schedules for the generation, axon formation and migration of facial motoneurons are similar both in the normal and reeler mice, but the reeler phenotype becomes identifiable at the end of neuronal migration. Although the reason why the facial nucleus is cytoarchitectually abnormal in the reeler mouse is still unknown, the long migration of the facial motoneurons seems to be susceptible to the absence of Reelin in the reeler mouse. In spite of the cytoarchitectual abnormality, retrograde horseradish peroxidase (HRP) study confirmed that the musculotopic arrangements within the facial nucleus of the reeler mouse are still preserved, suggesting that neuronal migration and target recognition are regulated independently. More recently, other reeler-like mutants have been reported. Among them, yotari and scrambler mice arise from mutations in mdab1, a mouse gene related to Drosophila gene disabled (dab). More than 10 years ago, an autosomal recessive rat mutant, shaking rat Kawasaki (SRK), was described that exhibits a phenotype identical to reeler, but the gene responsible for this rat mutation remains unknown. Interestingly, the facial nucleus is cytoarchitectually more deranged in yotari and SRK than their reeler counterpart. Although the reason why yotari exhibits a phenotype identical to reeler in the laminated structures but not in non-laminated structures such as the facial nucleus has remained obscure, mDab1 and Reelin proteins may function as signaling molecules in a different way between laminated and non-laminated structures. Phenotypes resembling that of reeler are seen with mutations in mdab1, cdk5 and p35. Cdk5 and p35 are respectively the catalytic and regulatory subunits of a serine/threonine kinase, that could potentially operate in a common signalling pathway with mDab and Reelin. These plausible partners for Reelin and mDab1 should help us to understand how the activities of these proteins coordinate neuronal migration and rearrangement.