Intestinal epithelial SCs IESCs are
Intestinal epithelial SCs (IESCs) are roughly categorized as either quiescent or active IESCs based in part on the expression of specific markers, including LGR5, Olfm4, Ascl2, BMI1, MTERT, and LRIG1 (Barker et al., 2007; Montgomery et al., 2011; Powell et al., 2012; Sangiorgi and Capecchi, 2008). They are believed to dynamically switch from one type to the other in response to inhibitory and stimulatory signals caused by cytokines, hormones, or growth factors (Li and Clevers, 2010). Active IESCs, the majority of which are LGR5+ crypt soluble guanylate cyclase columnar cells (CBCs), maintain intestinal lineage development and self-renewal with rapid cycling (Barker et al., 2007), and are highly sensitive to intestinal injury (Tian et al., 2011). In contrast, slow-cycling IESCs (label-retaining cells [LRCs]), which are present at the ‘‘+4 crypt position,’’ contribute to homeostatic regenerative capacity, particularly during recovery from injury (Takeda et al., 2011). These LRCs express markers such as BMI1, HOPX, LRIG1, and/or DCLK1, and can convert to rapidly cycling IESCs in response to injury (Yan et al., 2012). Signal transduction pathways, including WNT, NOTCH, TGF-β/BMP, Hedgehog, nuclear hormone receptor, and JAK-STAT, temporally and spatially regulate IESC homeostasis in cell-based tissue self-renewal and regeneration (Crosnier et al., 2006). Recent studies indicated that IESCs can regulate the intestinal homeostatic response to infection and inflammation (Buczacki et al., 2013). However, the mechanisms underlying this cellular regulation remain largely unknown.
JAK-STAT signaling was recently found to mediate IESC self-renewal and differentiation in response to bacterial infection and tissue impairment in Drosophila (Jiang et al., 2009). Compromised JAK-STAT signaling caused loss of IESC quiescence (Buchon et al., 2009), whereas JAK-STAT activation produced extra IESC-like and progenitor cells (Lin et al., 2010). However, the subsequent molecular events by which STAT signaling regulates adult IESCs are poorly defined in mammals. STAT5 activity, as well as its target genes, was predominantly associated with long-term self-renewal and maintenance of hematopoietic (Kato et al., 2005), mammary (Vafaizadeh et al., 2010), and embryonic SC (ESC) phenotypes (Kyba et al., 2003). Temporally controlled STAT5 expression and activation increased mammary SC proliferation, thereby contributing to the functional tissue formation upon chronic inflammatory injury (Vafaizadeh et al., 2010). We previously reported that epithelial STAT5 signaling is required for intestinal epithelial cell (IEC) integrity and homeostatic response to gut injury (Gilbert et al., 2012). Growth hormone (GH) and granulocyte macrophage-colony stimulating factor (GM-CSF) can protect IECs against inflammatory injury through activation of STAT5 (Han et al., 2007, 2010). These findings suggest that STAT5 signaling mediates IEC repopulation through regulation of somatic IESC proliferation or differentiation. Here, utilizing Stat5-modified transgenic mouse models and mouse or human SCs, we characterized the role of STAT5 in IESC homeostasis and response to injury, and deciphered the molecular machineries of STAT5 activation in protecting gut injury. Furthermore, our findings suggest that STAT5 activation could be used as a functional marker for IESC intervention of gut injury.
Discussion IESCs and intestinal progenitor cells maintain intestinal homeostasis and regeneration in response to gut injury (Zhang et al., 2014). LGR5+ IESCs play a critical role in intestinal homeostasis and regeneration (Metcalfe et al., 2014; Van Landeghem et al., 2012). Interestingly, BMI1+ IESCs are able to replenish LGR5+ IESC upon small-intestinal injury or regeneration (Yan et al., 2012). However, the molecular mechanisms that regulate these two IESC populations remain largely unexplored. Mucosal cytokines regulate IESC responses to inflammation, in part by JAK-STAT signaling (Farin et al., 2014; Jiang et al., 2009). In this study, we investigated whether cytokine-STAT5 signaling plays a role in modulation of these two IESC populations during IEC regeneration. Based on the combined results of our LOF and GOF studies of STAT5 in murine models with cultured mouse or human SCs, we propose a model in which, first, loss of STAT5 impairs rapidly cycling IESCs (Figure 7E-I), and second, genetic activation of Stat5 promotes CBC proliferation and regeneration (Figure 7E-II). However, our current data cannot exclude the potential effects of STAT5 signaling on intestinal progenitors or mucosal cytokine secretion. Interestingly, ChIP analyses identified STAT5 binding to the Bmi1 locus, suggesting that activated STAT5 could directly regulate key genes involved in IESC identity. Collectively, STAT5 controls adult IESC activity upon intestinal injury. PY-STAT5 could be developed as a biomarker for IESC regeneration of inflamed epithelia.