| Summary: | The mechanism that controls adult stem cell commitment is not fully elucidated, although it is known that the transcriptional control plays a major role in this process. Since the discovery of stem cells in the Drosophila melanogaster posterior midgut, many transcription factors have been identified to control whether intestinal stem cells (ISC) remain in the stem compartment, commit into the absorptive fate (enterocytes, EC) or differentiate into enteroendocrine (EE) cells. However, the molecular mechanisms governing these cell fate decisions remain to be fully elucidated. We have identified a network of basic helix-loop-helix (bHLH) transcription factors to be key for ISC fate and the maintenance of the tissue homeostasis. The class I bHLH factor Daughterless (Da) impedes terminal differentiation by forming Da:Da homodimers, accumulating cells as ISCs or as absorptive committed cells (enteroblasts, EB). However, the class V HLH protein Extramacrochaete (Emc) inhibits the formation of Da:Da by forming dimers which lack transcriptional activity. We confirmed that emc is expressed in the posterior midgut and we showed that emc over-expression produces terminal differentiation of ISCs into ECs. Moreover, we found that Emc function is downstream of the Notch-Delta pathway. Importantly, we identified that Emc is also important in EBs to block de-differentiation of EBs into ISCs. This blocking is due to Emc inhibition over a bHLH class II, Scute (Sc). We showed that Sc has a dual role; it drives the expression of stem cells genes, such as Dl, while when being expressed at high levels, Sc initiates secretory differentiation. Finally, we studied the function of a bHLH-leucine zipper called Cropped (Crp), whose expression produces the complete arrest of cell division and differentiation in ISCs. Our results indicate that a simple Sc/Da/Emc network of bHLH factors act as a three-position toggle switch, choosing between the stem, secretory and absorptive fates by swapping dimerization partners. This is the first time that such a mechanism can account for all cell fate transitions in the fly gut, and it has direct implications for the maintenance of the mammalian intestine.
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