Mi-2/NuRD complex protects stem cell progeny from mitogenic Notch signalling
To progress towards differentiation, the progeny of stem cells need to extinguish expression of stem cell maintenance genes. Failures in these mechanisms that suppress stem cell programmes can lead to supernumerary stem cells and drive tumorigenesis. In Drosophila neural stem cell lineages, excessive Notch signalling results in supernumerary stem cells causing hyperplasia. But the onset of hyperplasia is considerably delayed implying there are mechanisms that resist the mitogenic signal. Monitoring live the expression of an early NSC marker, the Notch target gene E(spl)m{gamma}, revealed that the normal process of NSC fate attenuation is still initiated even in the presence of excess Notch activity so that the re-emergence of stem cell properties occurs only in older progeny. Screening for factors responsible, we found that depletion of Mi-2 and other members of the NuRD ATP remodeling complex dramatically enhanced the Notch-induced hyperplasia. Under these conditions, E(spl)m{gamma} was no longer extinguished in the stem cell progeny, but instead remained at high levels. We propose that Mi-2 is required for decommissioning stem cell enhancers in their progeny, enabling the switch towards a more differentiated fate and rendering them insensitive to mitogenic factors such as Notch.