We also performed experiments in which ISCs were

We also performed experiments in which ISCs were completely ablated, and observed no recovery of the ISC pool over the lifespan of flies. This is consistent with another recent report in which ISCs were completely ablated (Lu and Li, 2015). In both of these cases the esg-Gal4 driver was used for depletion, allowing the conclusion that there are no esg− ISC precursors in the adult fly. Interestingly, in contrast to a recent report that found that flies lacking ISCs had almost normal lifespans (Resende et al., 2017), we found that complete ISC ablation reduced fly survival. Our data suggest that while flies can survive partial ISC loss for at least 4 weeks, complete ISC depletion results in a loss of midgut homeostasis and reduced survival. The dynamics of ISC pool maintenance in the fly midgut have significant differences from those in the mammalian intestine. Symmetric ISC lineages are very often observed in the mouse intestine, while only 10% of ISC lineages are typically symmetric in the fly midgut (Figure 3) (de Navascues et al., 2012; O\'Brien et al., 2011). Strikingly, murine ISC pools readily recover after stem cell depletion whereas in the fly ISC depletion appears to be irreversible. Furthermore, in the murine intestine, selection of stem opioid receptor based on niche occupancy is important (Barker, 2014), and partially differentiated TA cells can revert into ISCs if they can access the niche. These phenomena are not observed in the fly midgut, which has a dispersed niche, no TA cells, and a fixed number of ISCs. The different behavior of ISCs in these two species could be due to differing requirements for stem cell capability. The mouse\'s lifespan is more than ten times longer than the fly\'s, so murine ISCs need to maintain gut homeostasis for much longer. Mammalian ISCs have to renew themselves many more times during the host\'s lifespan and also accumulate more genomic damage from DNA replication, which could alternatively drive cell death or transformation. Perhaps because of these pressures, the mammalian intestine evolved a more flexible system for stem cell pool control, which allows both better recovery from injury and the capability to select defective ISCs while maintaining a normal-sized stem cell pool.
Experimental Procedures
Author Contributions Y.J. designed and performed experiments shown in Figures 1, 2, 3, S2, and S3, and wrote the paper. P.H.P. designed and performed experiments shown in Figures 4, S1, and S4 and wrote the paper. B.A.E. conceived the project and wrote the paper. A.K. contributed to early stages of the work. C.Z. and B.P. helped with experiments in Figures 1, 2, and 3.
Acknowledgments We acknowledge DFGSFB873 and ERCAdG 268515 to B.A.E., and the Helmholtz Gemeinschaft (DKFZ A220) for support.
Introduction To maintain their cellular identity, embryonic stem cells (ESCs) utilize a network of core transcription factors and chromatin remodeling enzymes that bind and regulate pluripotency genes and differentiation genes in response to developmental signaling (Kim et al., 2008). The Nucleosome Remodeling and Deacetylase (NuRD) complex is unique among chromatin regulators because it couples ATP-dependent nucleosome remodeling activity with histone modification (deacetylase) activity (Tong et al., 1998; Wade et al., 1998; Xue et al., 1998; Zhang et al., 1998). NuRD alters nucleosome occupancy to block the binding of transcriptional machinery at gene promoters, thus functioning primarily as a co-repressor (Denslow and Wade, 2007; Yildirim et al., 2011). In addition, mice deleted for the Mbd3 gene, which encodes a NuRD subunit important for NuRD targeting and assembly, are nonviable (Hendrich et al., 2001). ESCs derived from Mbd3-null mouse embryos do not differentiate and are capable of self-renewal in culture in the absence of leukemia inhibitory factor (LIF) (Kaji et al., 2006). Mbd3 was subsequently shown to be important for differentiation and development through silencing of pluripotency genes (Reynolds et al., 2012), functioning in part by deacetylation of H3K27 (Reynolds et al., 2011).