Homo- and heterodimerization of bHLH transcription factors balance stemness and bipotential differentiation

Abstract

Multipotent adult stem cells must balance self-renewal with differentiation into various mature cell types. How this activity is molecularly regulated is poorly understood. By using genetic and molecular analyses in vivo, we show that a small network of basic Helix-Loop-Helix (bHLH) transcription factors controls both stemness and bi-potential differentiation in the Drosophila adult intestine. We find that homodimers of Daughterless (Da, homolog to mammalian E proteins) maintain the self-renewal of intestinal stem cells and antagonise the activity of heterodimers of Da and Scute (Sc, homolog to ASCL and known to promote intestinal secretory differentiation). We find a novel role for the HLH factor Extramacrochaetae (Emc, homolog to Id proteins), titrating Da and Sc to promote absorptive differentiation. We further show that Emc prevents committed absorptive progenitors from de-differentiating, revealing the plasticity of these cells. This mechanism of interaction partner-switching enables the active maintenance of stemness, but primes stem cells for differentiation along two alternative fates. Such regulatory logic could be recapitulated in other bipotent stem cell systems.