The bHLH gene Hes1 as a repressor of the neuronal commitment of CNS stem cells

Hes1 is one of the basic helix-loop-helix transcription factors that regula te mammalian CNS development, and its loss- and gain-of-function phenotypes indicate that it negatively regulates neuronal differentiation. Here we report that Hes1(-/-) mice expressed both early (TuJ1 and Hu) and l ate (MAP2 and Neurofilament) neuronal markers prematurely, and that there w ere approximately twice the normal number of neurons in the Hes1(-/-) brain during early neural development. However, immunochemical analyses of secti ons and dissociated cells using neural progenitor markers, including nestin , failed to detect any changes in Hes1(-/-) progenitor population. Therefor e, further characterization of neural progenitor cells that discriminated b etween multipotent and monopotent cells was performed using two culture met hods, low-density culture, and a neurosphere assay. We demonstrate that the self-renewal activity of multipotent progenitor cells was reduced in the H es1(-/-) brain, and that their subsequent commitment to the neuronal lineag e was accelerated. The Hes1(-/-) neuronal progenitor cells were functionall y abnormal, in that they divided, on average, only once, and then generated two neurons, (instead of one progenitor cell and one neuron), whereas wild -type progenitor cells divided more. In addition, some Hes1(-/-) progenitor s followed an apoptotic fate. The overproduction of neurons in the early He s1(-/-) brains may reflect this premature and immediate generation of neuro ns as well as a net increase in the number of neuronal progenitor cells. Taken together, we conclude that Hes1 is important for maintaining the self -renewing ability of progenitors and for repressing the commitment of multi potent progenitor cells to a neuronal fate, which is critical for the corre ct number of neurons to be produced and for the establishment of normal neu ronal function.