Function and regulation of distal-less-related homeobox genes during visceral arch development in zebrafish.

Abstract

Vertebrate distal-less homeobox genes are known to play a role in embryonic pattern formation. Six zebrafish distal-less homeobox genes have been isolated; dlx1, dlx2, dlx3, dlx4, dlx6, and dlx7. Their combinatorial embryonic expression patterns seen in the forebrain, visceral arches, and fins are suggestive of a new "homeobox code". Various doses of all-trans retinoic acid were used to disrupt distal-less expression during different visceral arch developmental time points. The abnormal visceral arch distal-less expression was further correlated with craniofacial cartilage development. Zebrafish embryos at various stages, treated with 10$\sp{-6}$ M RA, displayed a loss of all distal-less expression in the mandibular and hyoid arches. The embryos exhibiting a loss of distal-less expression, also developed with a loss of mandibular and hyoid arch derived craniofacial cartilage components. Embryos treated with 10$\sp{-7}$M RA displayed a stage dependent loss of distal-less expression, mainly in the branchial arches. The visceral arch craniofacial cartilage differentiated abnormally in embryos displaying a loss of distal-less expression. When distal-less expression was lost in the branchial arches, then only the branchial arch cartilage was lost. However, abnormal distal-less expression in any of the visceral arches could also be correlated with abnormal visceral arch cartilage development. These experiments suggest that dlx genes are part of the multi-step process leading to visceral arch cartilage development. Zebrafish genes involved in this process are sensitive to retinoic acid (10$\sp{-7}$M) until 24hpf, where RA does not seem to affect the process of craniofacial cartilage development. Accordingly, a loss in visceral arch dlx expression during any stage (up to 24hpf) of this process leads to abnormal craniofacial cartilage development. (Abstract shortened by UMI.)