The homeobox gene, Xanf-1, can control both neural differentiation and patterning in the presumptive anterior neurectoderm of the Xenopus laevis embryo

From the onset of neurectoderm differentiation, homeobox genes of the Anf class are expressed within a region corresponding to the presumptive telencephalic and rostral diencephalic primordia. Here we investigate functions of the Xenopus member of Anf, Xanf-1, in the differentiation of the anterior neurectoderm. We demonstrate that ectopic Xanf-1 can expand the neural plate at expense of adjacent non-neural ectoderm. In tadpoles, the expanded regions of the plate developed into abnormal brain outgrowths. At the same time, Xanf-1 can inhibit terminal differentiation of primary neurones. We also show that, during gastrula/neurula stages, the exogenous Xanf-1 can downregulate four transcription regulators, XBF-1, Otx-2, Pax-6 and the endogenous Xanf-1, that are expressed in the anterior neurectoderm. However, during further development, when the exogenous Xanf-1 was presumably degraded, reactivation of XBF-1, Otx-2 and Pax-6 was observed in the abnormal outgrowths developed from blastomeres microinjected with Xanf-1 mRNA. Other effects of the ectopic Xanf-1 include cyclopic phenotype and inhibition of the cement gland, both by Otx-2-dependent and independent mechanisms. Using fusions of Xanf-1 with the repressor domain of Drosophila engrailed or activator domain of herpes virus VP16 protein, we showed that most of the observed effects of Xanf-1 were probably elicited by its functioning as a transcription repressor. Altogether, our data indicate that the repressor function of Xanf-1 may be necessary for regulation of both neural differentiation and patterning in the presumptive anterior neurectoderm.