Identification of a novel basic helix-loop-helix gene, Heslike, and its role in GABAergic neurogenesis.

Neuronal subtype specification depends on multiple transcription factors such as basic helix-loop-helix (bHLH) factors. However, transcription factor codes for most neurons remain to be determined. Here, we report identification of a novel mouse bHLH factor, termed Heslike, that has Hes1-like bHLH domain and transcriptional repressor activity. Heslike is coexpressed with the bHLH factor Mash1 in brain regions that give rise to GABAergic neurons. In the mesencephalon and the caudal diencephalon, coexpression of Heslike and Mash1 is initially restricted to small regions but expanded dorsally from embryonic day 9.5 onward, and this expansion of coexpression is followed by GABAergic neurogenesis. Misexpression of Heslike in mouse embryos generates ectopic GABAergic neurons only from the Mash1(+) region. In contrast, in the mesencephalon and the caudal diencephalon of Mash1-null mice, GABAergic neurons are almost completely missing and, instead, other neurons are generated, although Heslike is still expressed. Furthermore, coexpression of Heslike and Mash1 significantly promotes formation of GABAergic neurons, compared with each gene alone, in neural precursor cell culture. Thus, Heslike or Mash1 alone is not sufficient, but their coexpression may be important for generation of GABAergic neurons. These results suggest that combinations of distinct bHLH factors promote formation of distinct neuronal subtypes, thereby increasing neuronal diversity.