XNGNR1-dependent neurogenesis mediates early neural cell death.

Early neural cell death is programmed cell death occurring within proliferating and undifferentiated neural progenitors. Little is known about the regulation and role of early neural cell death. In Xenopus embryos, primary neurogenesis is disrupted following the inhibition of early neural cell death, indicating that it is required for normal primary neurogenesis. Here we show that early neural cell death is dependent on primary neurogenesis. Overexpression of XSoxD concomitantly reduced N-Tubulin expression and early neural cell death, as seen by reduced TUNEL staining in stage 15 embryos. Conversely, overexpression of XNgnr1 led to ectopic N-Tubulin expression and TUNEL staining. However, XNeuroD overexpression, which induces ectopic N-Tubulin expression downstream of XNgnr1, had no effect on early neural cell death. E1A12S differentially inhibits the differentiation pathway induced by XNGNR1 protein. E1A12S-mediated inhibition of XNGNR1 neurogenic activity resulted in the reduction of N-Tubulin expression and TUNEL staining. Taken together, our data establish that primary neurogenesis induced by XNGNR1 promotes early neural cell death. This indicates that XNgnr1 positively regulates early neural cell death. We propose that early neural cell death might eliminate cells with abnormally high levels of XNGNR1, which can result in pre-mature neuronal differentiation.