SoxD: an essential mediator of induction of anterior neural tissues in Xenopus embryos.

Vertebrate neurogenesis is initiated by the organizer factors that inhibit antineuralizing activities of bone morphogenetic proteins (BMPs) in the ectoderm. Here, we report a candidate mediator of neuralization, SoxD. Expression of SoxD starts at late blastula stages widely in the prospective ectoderm and becomes restricted to the dorsal ectoderm by mid-gastrula stages. SoxD expression is enhanced by the neural inducer Chordin and is suppressed by BMP4 and its downstream genes. Microinjection of SoxD mRNA causes ectopic formation of neural tissues in vivo and induces neural and neuronal markers in the isolated animal cap. Injection of a dominant-negative form of SoxD mRNA can block neuralization of ectoderm caused by attenuation of BMP signals and can strongly suppress formation of anterior neural tissues in vivo. These data show that SoxD functions as an essential mediator of downstream signaling of neural induction.