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The Reelin (Reln)-deficient mouse (reeler) and the Dab1-deficient mouse (yotari) are autosomal recessive mutant mice characterized by cerebellar ataxia. Previously, we reported that Reelin and Dab1 proteins have slightly different functions during the development of the cerebral cortex. To analyze the functional roles of Reelin and Dab1 proteins in detail, we attempted to generate a reelin/Dab1 compound-mutant mouse by breeding heterozygote reeler and yotari mice. We examined the cytoarchitecture of the cerebral and cerebellar cortices and the hippocampus of wild-type (Reln ( +/+ ); Dab1 ( +/+ )), double-heterozygote (Reln ( rl/+ ); Dab1 ( yot/+ )), reeler (Reln ( rl/rl ); Dab1 ( +/+ ), Reln ( rl/rl ); Dab1 ( yot/+ )), yotari (Reln ( +/+ ); Dab1 ( yot/yot ), Reln ( rl/+ ); Dab1 ( yot/yot )), and double-compound-deficient (Reln ( rl/rl ); Dab1 ( yot/yot )) mice. Nissl staining demonstrated that no abnormality was recognized in the mice of reelin/Dab1 double-heterozygote (Reln ( rl/+ ); Dab1 ( yot/+ )). The reelin/Dab1-compound mutant mouse (Reln ( rl/rl ); Dab1 ( yot/yot )) showed histological abnormalities in the cerebral and cerebellar cortices and the hippocampus, in addition to those of reeler and yotari mice. We injected HRP into the lumbar cord of these animals with various gene compositions to examine the distribution pattern of corticospinal tract (CST) neurons. CST neurons of the reelin/Dab1-compound mutant mice were not confined to layer V, but scattered throughout the motor cortex. This quantitative and statistical analysis shows that the distribution pattern of CST neurons of the reelin/Dab1-compound mutant mouse differs from those of either of the reeler or yotari counterparts. Taken together, although Reelin/Dab1 signal transduction is a primary cascade in neurons during developmental periods, other signaling cascades (e.g., the Cdk-5/Dab1 pathway) may lie in a parallel fashion to Reelin/Dab1 signal transduction.

To explore the functional roles of Reelin in the adult olfactory system, we examined changes in the expression of reelin mRNA and Reelin protein in the olfactory bulb (OB) of adult mice after a chemical lesion to the olfactory epithelium. Following intranasal irrigation with 2% zinc sulphate solution, animals were perfused at various times between 5 and 40 days post-lesion. The expression of reelin mRNA in mitral cells in the OB was slightly reduced at 5 days post-lesion, completely abolished by 20 days, but restored almost to the normal level at 40 days post-lesion. Similarly, the expression of Reelin protein in mitral cells of the deafferented OB also recovered, although not to the normal level. No recovery of either reelin mRNA or Reelin immunoreactivity was seen in the periglomerular cells and external tufted cells. The expression profile of reelin mRNA and Reelin protein in the OB coincided with the time course of degeneration and regeneration of olfactory nerves, as indicated by anterograde labeling of olfactory nerves with WGA-HRP. These results suggest that expression of reelin mRNA in the adult OB is regulated by olfactory inputs.

In the development of the olfactory system, olfactory receptor neurons (ORNs) project their axons from the olfactory epithelium (OE) to the olfactory bulb (OB). The surface of the OB is covered by the central nervous system (CNS) basal lamina. To establish this connection, pioneer axons of the ORNs penetrate the CNS basal lamina at embryonic day 12.5 in mice. The importance of this penetration is highlighted by the Kallmann syndrome. However, little has been known about the molecular mechanism underlying this penetration process. Fezf1 (also called as Fez, Zfp312-like, and 3110069A13Rik) is a C2H2-type zinc-finger gene expressed in the OE and hypothalamic region in mice. In Fezf1-deficient mice, ORN axons (olfactory axons) do not reach the OB. Here we demonstrate that Fezf1-deficient olfactory axons do not penetrate the CNS basal lamina in vivo, and the penetration activity of the axons in Matrigel is impaired in vitro. Coculture experiments using the OE and OB reveal that axonal projection of ORNs is rescued in Fezf1-deficient mice in which the meninges including the CNS basal lamina are removed from the mutant OB. These data indicate that Fezf1 is required for the penetration of olfactory axons through the CNS basal lamina before they innervate the OB.