The homeodomain-containing transcription factor X-nkx-5.1 inhibits expression of the homeobox gene Xanf-1 during the Xenopus laevis forebrain development

Expression of the homeobox gene Xanf-1 starts within the presumptive forebrain primordium of the Xenopus embryo at the midgastrula stage and is inhibited by the late neurula. Such stage-specific inhibition is essential for the normal development as the experimental prolongation of the Xanf-1 expression elicits severe brain abnormalities. To identify transcriptional regulators that are responsible for the Xanf-1 inhibition, we have used the yeast one-hybrid system and identified a novel Xenopus homeobox gene X-nkx-5.1 that belongs to a family of Nkx-5.1 transcription factors. In terms of gene expression, X-nkx-5.1 shares many common features with its orthologs in other species, including expression in the embryonic brain and in the ciliated cells of the otic and lateral line placodes. However, we have also observed several features specific for X-nkx-5.1, such as expression in precursors of the epidermal ciliated cells that may indicate a possible common evolutionary origin of all ciliated cells derived from the embryonic ectoderm. Another specific feature is that the X-nkx-5.1 expression in the anterior neural plate starts early, within the area overlapping the Xanf-1 expression territory at the midneurula stage, and it correlates with the beginning of the Xanf-1 inhibition. Using various loss and gain-of-function techniques, including microinjections of antisense morpholino oligonucleotides and mRNA encoding for the X-nkx-5.1 and its dominant repressor and activator versions, we have shown that X-nkx-5.1 can indeed play a role of stage-specific inhibitor of Xanf-1 in the anterior neural plate during the Xenopus development.