Xebf3 is a regulator of neuronal differentiation during primary neurogenesis in Xenopus

During primary neurogenesis in Xenopus, a cascade of helix-loop-helix (HLH) transcription factors regulates neuronal determination and differentiation . While XNeuroD functions at a late step in this cascade to regulate neuron al differentiation, the factors that carry out terminal differentiation are still unknown. We have isolated a new Xenopus member of the Ebf/Olf-1 fami ly of HLH transcription factors, Xebf3, and provide evidence that, during p rimary neurogenesis, it regulates neuronal differentiation downstream of XN euroD. In developing Xenopus embryos, Xebf3 is turned on in the three strip es of primary neurons at stage 15.5, after XNeuroD. In vitro, XEBF3 binds t he EBF/OLF-1 binding site and functions as a transcriptional activator. Whe n overexpressed, Xebf3 is able to induce ectopic neurons at neural plate st ages and directly convert ectodermal cells into neurons in animal cap expla nts. Expression of Xebf3 can be activated by XNeuroD both in whole embryos and in animal caps, indicating that this new HLH factor might be regulated by XNeuroD. Furthermore, in animal caps, XNeuroD can activate Xebf3 in the absence of protein synthesis, suggesting that, in vitro, this regulation is direct. Similar to XNeuroD, but unlike Xebf2/Xcoe2, Xebf3 expression and f unction are insensitive to Delta/Notch-mediated lateral inhibition. In summ ary, we conclude that Xebf3 functions downstream of XNeuroD and is a regula tor of neuronal differentiation in Xenopus. (C) 2001 Academic Press.