Regulation of hippocampal neuronal differentiation by the basic helix-loop-helix transcription factors HES-1 and MASH-1.

HES-1 is a vertebrate homologue of the Drosophila basic helix-loop-helix (bHLH) protein Hairy, a transcriptional repressor that negatively regulates neuronal differentiation. HES-1 expression in neuronal precursors precedes and represses the expression of the neuronal commitment gene MASH-1, a bHLH activator homologous to the proneural Achaete-Scute genes in Drosophila. Down-regulation of HES-1 expression in developing neuroblasts may be necessary for the induction of a regulatory cascade of bHLH activator proteins that controls the commitment and progression of neuronal differentiation. Here we show that the differentiation of embryonic day-17 rat hippocampal neurons in culture was coincident with a decline in HES-1 expression and DNA binding. Therefore, we examined the effect of forced expression of HES-1 and MASH-1 upon nerve growth factor (NGF) -induced differentiation in TrkA transfected hippocampal neurons. Expression of HES-1 inhibited both the intrinsic and NGF-induced neurite outgrowth, whereas MASH-1 expression increased neurite outgrowth. Strikingly, the increased hippocampal differentiation observed with MASH-1 expression is completely blocked by coexpression of HES-1. Furthermore, both wild-type HES-1 and a non-DNA binding mutant of HES-1 repressed MASH-1-dependent transcription activation. These results suggest that down-regulation of HES-1 is necessary for autonomous, growth factor-induced and MASH-1-activated hippocampal differentiation.