Dispersion of the neurons expressing layer specific markers in the reeler brain.

Neurons with similar functions including neuronal connectivity and gene expression form discrete condensed structures within the vertebrate brain. This is exemplified within the circuitry formed by the cortical layers and the neuronal nuclei. It is well known that the Reelin protein is required for development of these neuronal structures in rodents and human, but the function of Reelin remains controversial. In this report, we used "layer-specific markers" of the cerebral cortex to carry out detailed observations of spatial distribution of the neuronal subpopulations in the brain of the Reelin deficient mouse, reeler. We observed a spatially dispersed expression of the markers in the reeler cerebral cortex. These markers are expressed also in other laminated and non-laminated structures of brain, in which we observed similar abnormal gene expression. Our observations suggest that neurons within the brain structures (such as the layers and the nuclei), which normally exhibit condensed distribution of marker expressions, loosen their segregation or scatter by a lack of Reelin.

Expression of zebrafish ROR alpha gene in cerebellar-like structures.

Mouse genetic studies have identified several genes involved in cerebellar development. The mouse mutants staggerer and lurcher are functionally deficient for the retinoid-related orphan receptor alpha (ROR alpha) and glutamate receptor delta2 (Grid2) genes, respectively, and they show similar functional and developmental abnormalities in the cerebellum. Here, we report the cloning and expression pattern of zebrafish ROR alpha orthologues rora1 and rora2, and compare their expression pattern with that of grid2. Expression of rora1 and rora2 is initiated at late gastrula and pharyngula stages, respectively. Both rora1 and rora2 are spatially expressed in the retina and tectum. Expression of rora2 was further observed in the cerebellum, as reported for mammalian ROR alpha. In the adult brain, rora2 and grid2 are coexpressed in brain regions, designated as cerebellar-like structures. These observations suggest an evolutionarily conserved function of ROR alpha orthologues in the vertebrate brain.

Anterograde labeling of the corticospinal tract in jimpy mutant mice by Di I injection into the motor cortex.

Carbocyanine fluorescent dye, DiI, is an excellent anterograde/retrograde neural tracer, but its efficacy for the anterograde labeling of neural circuits in the adult brain tends to decrease with ages. The present study shows that an injection of DiI into the motor cortex of the young adult jimpy mutant mice (Plp1(jp)/+) resulted in successful anterograde labeling of corticospinal tract fibers. Furthermore, an injection of Fast Blue into the lumbar spinal cord of the mutant mice resulted in retrograde labeling of layer 5 corticospinal tract neurons within the motor cortex. Since no abnormality except for myelin deficiency is known in the long descending and ascending tracts of jimpy mutant mouse, this mutant is suitable for neural tracing studies of long axonal trajectories with the use of carbocyanine dye, DiI, although these males die between 20 and 40 days of age.

A mouse homologue of Strawberry Notch is transcriptionally regulated by Reelin signal.

Reelin is a glycoprotein secreted by specific neuronal populations of the adult and developing nervous system of vertebrates. The morphological abnormalities in the brain of reeler, the Reelin deficient mutant mice, indicate that Reelin is essential for the brain morphogenesis. However, biochemical function of Reelin signal is not well understood. Here, we examined possible function of Reelin signal in regulation of gene expression by performing a microarray analysis. We found that expression level of a mouse homologue of Strawberry Notch (mSno1) is markedly reduced in the reeler embryos. In situ hybridization showed that mSno1 is expressed in the developing nervous system colocalizing with expression of ApoER2, a Reelin receptor. Treatment of P19 cells with Reelin protein enhanced mSno1 expression. Overexpression of ApoER2 with Reelin treatment gave a synergistic effect on mSno1 expression level. These observations suggest that Reelin signal is involved in embryonic expression of a novel vertebrate gene, mSno1.