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.

Defects in reciprocal projections between the thalamus and cerebral cortex in the early development of Fezl-deficient mice.

Fez-like (Fezl), the forebrain embryonic zinc finger-like protein, is a transcriptional repressor selectively expressed in the deep layers of the developing cortex. We examined the thalamocortical and corticofugal pathways in Fezl-deficient fetal mice by using immunohistochemistry and by axonal labeling with the lipophilic dyes DiI and DiA, with special attention to the spatiotemporal relation between thalamocortical and corticofugal axons. In normal mice, thalamic and cortical axons meet in the internal capsule between embryonic day (E) 13.5 and E14.5 and fasciculate with each other as they extend to their targets, the cortex and thalamus, respectively. In Fezl-deficient mice, most of the thalamic and cortical axons stop in the internal capsule and at the pallial-subpallial boundary at E14.5, respectively. This abnormality is transient, and the thalamic and cortical axons reach their targets at E15.5, although the number of thalamic axons is remarkably reduced in the cortical anlage. Double labeling with DiI and DiA demonstrated close apposition of the thalamic and cortical axons in the subpallium and pallium as well as in the external capsule of this mutant after E15.5. Because the expression of genes that define the pallial-subpallial boundary and guidance molecules of thalamocortical axons did not show remarkable changes in Fezl-deficient mice, abnormal formation of thalamocortical pathway in this mutant may be caused by the defect of axons of cortical efferent neurons that express Fezl.

Zinc finger gene fez-like functions in the formation of subplate neurons and thalamocortical axons.

fez-like (fezl) is a forebrain-expressed zinc finger gene required for the formation of the hypothalamic dopaminergic and serotonergic (monoaminergic) neurons in zebrafish. To reveal its function in mammals, we analyzed the expression of the mouse orthologue of fezl and generated fezl-deficient mice by homologous recombination. Mouse fezl was expressed specifically in the forebrain from embryonic day 8.5. At mid-gestation, fezl expression was detected in subdomains of the forebrain, including the dorsal telencephalon and ventral diencephalon. Unlike the zebrafish fezl mutant too few, the fezl-deficient mice displayed normal development of hypothalamic monoaminergic neurons, but showed abnormal "hyperactive" behavior. In fezl(-/-) mice, the thalamocortical axons (TCA) were reduced in number and aberrantly projected to the cortex. These mutants had a reduced number of subplate neurons, which are involved in guiding the TCA from the dorsal thalamus, although the subplate neurons were born normally. These results suggest that fezl is required for differentiation or survival of the subplate neurons, and reduction of the subplate neurons in fezl-deficient mice leads to abnormal development of the TCA, providing a possible link between the transcriptional regulation of forebrain development and hyperactive behavior.