The role of gastrulation brain homeobox 2(gbx2)in the development of the ventral telencephalon in zebrafish embryos

During vertebrate brain development, the gastrulation brain homeobox 2gene (gbx2) is expressed in the forebrain, but its precise roles are still unknown. In this study, we addressed this issue in zebrafish (Danio rerio) first by carefully examining gbx2expression in the developing forebrain. We showed that gbx2was expressed in the telencephalon during late somitogenesis, from 18h post-fertilization (hpf) to 24 hpf, and in the thalamic primordium after 26 hpf. In contrast, another gbxgene, gbx1, was expressed in the anterior-most ventral telencephalon after 36 hpf. Thus, the expression patterns of these two gbxgenes did not overlap, arguing against their redundant function in the forebrain. Two-color fluorescence in situ hybridization (FISH) showed close relationships between the telencephalic expression of gbx2and other forebrain-forming genes, suggesting that their interactions contribute to the regionalization of the telencephalon. FISH further revealed that gbx2is expressed in the ventricular region of the telencephalon. By using transgenic fish in which gbx2can be induced by heat shock, we found that gbx2induction at 16 hpf repressed the expression of emx3, dlx2a, and six3bin the ventral telencephalon. Among secreted factor genes, bmp2band wnt1were repressed in the vicinity of the gbx2domain in the telencephalon. The expression of forebrain-forming genes was examined in mutant embryos lacking gbx2, showing emx3and dlx2ato be upregulated in the subpallium at 24 hpf. Taken together, these findings indicate that gbx2contributes to the development of the subpallium through its repressive activities against other telencephalon-forming genes. We further showed that inhibiting FGF signaling and activating Wnt signaling repressed gbx2and affected the regionalization of the telencephalon, supporting a functional link between gbx2, intracellular signaling, and telencephalon development.