Reserve Intestinal Stem Cells Research Articles

Tuft cells act as regenerative stem cells in the human intestine.

In mice, intestinal tuft cells have been described as a long-lived, postmitotic cell type. Two distinct subsets have been identified: tuft-1 and tuft-2 (ref. 1). By combining analysis of primary human intestinal resection material and intestinal organoids, we identify four distinct human tuft cell states, two of which overlap with their murine counterparts. We show that tuft cell development depends on the presence of Wnt ligands, and that tuft cell numbers rapidly increase on interleukin-4 (IL-4) and IL-13 exposure, as reported previously in mice2-4. This occurs through proliferation of pre-existing tuft cells, rather than through increased de novo generation from stem cells. Indeed, proliferative tuft cells occur in vivo both in fetal and in adult human intestine. Single mature proliferating tuft cells can form organoids that contain all intestinal epithelial cell types. Unlike stem and progenitor cells, human tuft cells survive irradiation damage and retain the ability to generate all other epithelial cell types. Accordingly, organoids engineered to lack tuft cells fail to recover from radiation-induced damage. Thus, tuft cells represent a damage-induced reserve intestinal stem cell pool in humans.

Krüppel-like factor 5 activates chick intestinal stem cell and promotes mucosal repair after impairment

ABSTRACTThe mucosal renewal, which depends on the intestinal stem cell (ISC) activity, is the foundation of mucosal repairment. Importantly, activation of reserve ISCs (rISCs) plays a vital role in initiating mucosal repair after injury. However, the underlying regulatory mechanism of rISCs activation in chickens remains unclear. In this study, immediately after lipopolysaccharide (LPS) challenge, mitochondrial morphological destruction and dysfunction appeared in the crypt, accompanied by decreased epithelial secretion (decreased Muc2 mRNA abundance and LYSOZYME protein level). However, immediately after mucosal injury, the mucosal renewal accelerated, as indicated by the increased BrdU positive rate, proliferating cell nuclear antigen (PCNA) protein level and mRNA abundance of cell cycle markers (Ccnd1, Cdk2). Concerning the ISCs activity, during the early period of injury, there appeared a reduction of active ISCs (aISCs) marker Lgr5 mRNA and protein, and an increasing of rISCs marker Hopx mRNA and protein. Strikingly, upon LPS challenge, increased mRNA transcriptional level of Krüppel-like factor 5 (Klf5) was detected in the crypt. Moreover, under LPS treatment in organoids, the KLF5 inhibitor (ML264) would decrease the mRNA and protein levels of Stat5a and Hopx, the STAT5A inhibitor (AC-4-130) would suppress the Lgr5 mRNA and protein levels. Furthermore, the Dual-Luciferase Reporter assay confirmed that, KLF5 would bind to Hopx promoter and activate the rISCs, STAT5A would trigger Lgr5 promoter and activate the aISCs. Collectively, KLF5 was upregulated during the early period of injury, further activate the rISCs directly and activate aISCs via STAT5A indirectly, thus initiate mucosal repair after injury.

Intestinal Epithelial Regeneration in Response to Ionizing Irradiation.

The intestinal epithelium consists of a single layer of cells yet contains multiple types of terminally differentiated cells, which are generated by the active proliferation of intestinal stem cells located at the bottom of intestinal crypts. However, during events of acute intestinal injury, these active intestinal stem cells undergo cell death. Gamma irradiation is a widely used colorectal cancer treatment, which, while therapeutically efficacious, has the side effect of depleting the active stem cell pool. Indeed, patients frequently experience gastrointestinal radiation syndrome while undergoing radiotherapy, in part due to active stem cell depletion. The loss of active intestinal stem cells in intestinal crypts activates a pool of typically quiescent reserve intestinal stem cells and induces dedifferentiation of secretory and enterocyte precursor cells. If not for these cells, the intestinal epithelium would lack the ability to recover from radiotherapy and other such major tissue insults. New advances in lineage-tracing technologies allow tracking of the activation, differentiation, and migration of cells during regeneration and have been successfully employed for studying this in the gut. This study aims to depict a method for the analysis of cells within the mouse intestinal epithelium following radiation injury.

Aging of intestinal stem cells.

SummaryThe intestine is one of the organs that relies on stem cell function for maintaining tissue homeostasis. Recent findings on intestinal aging show that intestinal architecture, such as villus length, crypt size, and cell composition changes in the aged crypts. The correspondent decline in the regenerative capacity of the intestine is mainly due to a decline in intestinal stem cell function upon aging, as the underlying mechanisms of aging intestinal stem cells are beginning to unravel. This review summarizes our current knowledge on stem cell-intrinsic mechanisms of aging of intestinal stem cells and their connection to extrinsic factors, such as niche cells and microbiota and will introduce recent approaches to attenuate or even revert the aging of intestinal stem cells.

Mechanisms modulating the activities of intestinal stem cells upon radiation or chemical agent exposure.

Intestinal stem cells (ISCs) are essential for the regeneration of intestinal cells upon radiation or chemical agent damage. As for radiation-induced damage, the expression of AIM2, YAP, TLR3, PUMA or BVES can aggravate ISCs depletion, while the stimulation of TLR5, HGF/MET signaling, Ass1 gene, Slit/Robo signaling facilitate the radio-resistance of ISCs. Upon chemical agent treatment, the activation of TRAIL or p53/PUMA pathway exacerbate injury on ISCs, while the increased levels of IL-22, β-arrestin1 can ease the damage. The transformation between reserve ISCs (rISCs) maintaining quiescent states and active ISCs (aISCs) that are highly proliferative has obtained much attention in recent years, in which ISCs expressing high levels of Hopx, Bmi1, mTert, Krt19 or Lrig1 are resistant to radiation injury, and SOX9, MSI2, clusterin, URI are essential for rISCs maintenance. The differentiated cells like Paneth cells and enteroendocrine cells can also obtain stemness driven by radiation injury mediated by Wnt or Notch signaling. Besides, Mex3a-expressed ISCs can survive and then proliferate into intestinal epithelial cells upon chemical agent damage. In addition, the modulation of symbiotic microbes harboring gastrointestinal (GI) tract is also a promising strategy to protect ISCs against radiation damage. Overall, the strategies targeting mechanisms modulating ISCs activities are conducive to alleviating GI injury of patients receiving chemoradiotherapy or victims of nuclear or chemical accident.

TNFα Induces LGR5+ Stem Cell Dysfunction In Patients With Crohn's Disease.

Background & AimsTumor necrosis factor alpha (TNFα) is considered a major tissue damage-promoting effector in Crohn’s disease (CD) pathogenesis. Patient-derived intestinal organoid (enteroid) recapitulates the disease-specific characteristics of the intestinal epithelium. This study aimed to evaluate the intestinal epithelial responses to TNFα in enteroids derived from healthy controls and compare them with those of CD patient-derived enteroids.MethodsHuman enteroids derived from patients with CD and controls were treated with TNFα (30 ng/mL), and cell viability and gene expression patterns were evaluated.ResultsTNFα induced MLKL-mediated necroptotic cell death, which was more pronounced in CD patient-derived enteroids than in control enteroids. Immunohistochemistry and RNA sequencing revealed that treatment with TNFα caused expansion of the intestinal stem cell (ISC) populations. However, expanded ISC subpopulations differed in control and CD patient-derived enteroids, with LGR5+ active ISCs in control enteroids and reserve ISCs, such as BMI1+ cells, in CD patient-derived enteroids. In single-cell RNA sequencing, LGR5+ ISC-enriched cell cluster showed strong expression of TNFRSF1B (TNFR2) and cyclooxygenase-prostaglandin E2 (PGE2) activation. In TNFα-treated CD patient-derived enteroids, exogenous PGE2 (10 nmol/L) induced the expansion of the LGR5+ ISC population and improved organoid-forming efficiency, viability, and wound healing.ConclusionsTNFα increases necroptosis of differentiated cells and induces the expansion of LGR5+ ISCs. In CD patient-derived enteroids, TNFα causes LGR5+ stem cell dysfunction (expansion failure), and exogenous PGE2 treatment restored the functions of LGR5+ stem cells. Therefore, PGE2 can be used to promote mucosal healing in patients with CD.

Interplay among p21Waf1/Cip1, MUSASHI-1 and Kr\xfcppel-like factor 4 in activation of Bmi1-CreER reserve intestinal stem cells after gamma radiation-induced injury

Gamma radiation is a commonly used adjuvant treatment for abdominally localized cancer. Since its therapeutic potential is limited due to gastrointestinal (GI) syndrome, elucidation of the regenerative response following radiation-induced gut injury is needed to develop a preventive treatment. Previously, we showed that Krüppel-like factor 4 (KLF4) activates certain quiescent intestinal stem cells (ISCs) marked by Bmi1-CreER to give rise to regenerating crypts following γ irradiation. In the current study, we showed that γ radiation-induced expression of p21Waf1/Cip1 in Bmi1-CreER cells is likely mitigated by MUSASHI-1 (MSI1) acting as a negative regulator of p21Waf1/Cip1 mRNA translation, which promotes exit of the Bmi1-CreER cells from a quiescent state. Additionally, Bmi1-specific Klf4 deletion resulted in decreased numbers of MSI1+ cells in regenerating crypts compared to those of control mice. We showed that KLF4 binds to the Msi1 promoter and activates its expression in vitro. Since MSI1 has been shown to be crucial for crypt regeneration, this finding elucidates a pro-proliferative role of KLF4 during the postirradiation regenerative response. Taken together, our data suggest that the interplay among p21Waf1/Cip1, MSI1 and KLF4 regulates Bmi1-CreER cell survival, exit from quiescence and regenerative potential upon γ radiation-induced injury.

KLF4 Regulates Goblet Cell Differentiation in BMI1＋ Reserve Intestinal Stem Cell Lineage during Homeostasis.

Krüppel-like factor 4 (KLF4) is a zinc-finger transcription factor, expressed in villus cells of the intestinal epithelium, that promotes cellular differentiation and tissue homeostasis. Previous studies suggest that BMI1+ cells represent secretory progenitors with reserve intestinal stem cell (rISC) activity. However, it has not been elucidated how KLF4 contributes to crypt regeneration originated from BMI1+ rISC lineage during homeostasis. In this study, Bmi1-CreER;Rosa26eYFP (Bmi1Ctrl) and Bmi1-CreER;Rosa26eYFP;Klf4fl/fl (Bmi1ΔKlf4) mice were injected with tamoxifen to label BMI1+ cells and their lineage and to delete Klf4. During homeostasis, MUC2+ goblet cells appeared in the BMI1+ cell lineage 2, 3 and 7 days after tamoxifen administration. After Klf4 deletion in BMI1+ cells, the number of KLF4+ and MUC2+ cells in eYFP+ cells decreased in Bmi1ΔKlf4 mice compared with Bmi1Ctrl mice. Thus, KLF4 was positively correlated with goblet cell differentiation in BMI1+ cell derived lineage. In ex-vivo analysis, organoids derived from single eYFP+ cells of Bmi1Ctrl mice contained MUC2-expressing cells that co-expressed KLF4. On the other hand, organoids derived from Klf4-deleted eYFP+ cells from Bmi1ΔKlf4 mice showed reduced number of MUC2-expressing cells. In conclusion, these results suggest that KLF4 regulates goblet cell differentiation in BMI1+ ISC-derived lineage during homeostasis.

Role of an active reserve stem cell subset of enteroendocrine cells in intestinal stem cell dynamics and the genesis of small intestinal neuroendocrine tumors.

Small intestinal neuroendocrine tumors (SI-NET) are serotonin-secreting well-differentiated neuroendocrine tumors of putative enterochromaffin (EC) cell origin. Recent studies recognize a subset of EC cells that is label-retaining at the +4 position in the crypt and functions as a reserve intestinal stem cell. Importantly, this +4 reserve EC cell subset not only contributes to regeneration of the intestinal epithelium during injury and inflammation but also to basal crypt homeostasis at a constant rate. The latter function suggests that the +4 EC cell subset serves as an active reserve stem cell via a constant rate of dedifferentiation. Characterization of early tumor formation of SI-NET, observed as crypt-based EC cell clusters in many cases of familial SI-NETs, suggests that the +4 active reserve EC cell subset is the cell of origin. This newly discovered active reserve stem cell property of EC cells can account for unique biological mechanisms and processes associated with the genesis and development of SI-NETs. The recognition of this property of the +4 active reserve EC cell subset may provide novel opportunities to explore NETs in the gastrointestinal tract and other organs.

TIGAR/AP-1 axis accelerates the division of Lgr5\u2212 reserve intestinal stem cells to reestablish intestinal architecture after lethal radiation

During radiologic or nuclear accidents, high-dose ionizing radiation (IR) can cause gastrointestinal syndrome (GIS), a deadly disorder that urgently needs effective therapy. Unfortunately, current treatments based on natural products and antioxidants have shown very limited effects in alleviating deadly GIS. Reserve intestinal stem cells (ISCs) and secretory progenitor cells are both reported to replenish damaged cells and contribute to crypt regeneration. However, the suppressed β-catenin/c-MYC axis within these slow-cycling cells leads to limited regenerative response to restore intestinal integrity during fatal accidental injury. Current study demonstrates that post-IR overexpression of TIGAR, a critical downstream target of c-MYC in mouse intestine, mounts a hyperplastic response in Bmi1-creERT+ reserve ISCs, and thus rescues mice from lethal IR exposure. Critically, by eliminating damaging reactive oxygen species (ROS) yet retaining the proliferative ROS signals, TIGAR-overexpression enhances the activity of activator protein 1, which is indispensable for initiating reserve-ISC division after lethal radiation. In addition, it is identified that TIGAR-induction exclusively gears the Lgr5− subpopulation of reserve ISCs to regenerate crypts, and intestinal TIGAR-overexpression displays equivalent intestinal reconstruction to reserve-ISC-restricted TIGAR-induction. Our findings imply that precise administrations toward Lgr5− reserve ISCs are promising strategies for unpredictable lethal injury, and TIGAR can be employed as a therapeutic target for unexpected radiation-induced GIS.

Ascl2-Dependent Cell Dedifferentiation Drives Regeneration of Ablated Intestinal Stem Cells.

Ablation of LGR5+ intestinal stem cells (ISCs) is associated with rapid restoration of the ISC compartment. Different intestinal crypt populations dedifferentiate to provide new ISCs, but the transcriptional and signaling trajectories that guide this process are unclear, and a large body of work suggests that quiescent "reserve" ISCs contribute to regeneration. By timing the interval between LGR5+ lineage tracing and lethal injury, we show that ISC regeneration is explained nearly completely by dedifferentiation, with contributions from absorptive and secretory progenitors. The ISC-restricted transcription factor ASCL2 confers measurable competitive advantage to resting ISCs and is essential to restore the ISC compartment. Regenerating cells re-express Ascl2 days before Lgr5, and single-cell RNA sequencing (scRNA-seq) analyses reveal transcriptional paths underlying dedifferentiation. ASCL2 target genes include the interleukin-11 (IL-11) receptor Il11ra1, and recombinant IL-11 enhances crypt cell regenerative potential. These findings reveal cell dedifferentiation as the principal means for ISC restoration and highlight an ASCL2-regulated signal that enables this adaptive response.

774 – Role of Cell Cycle Inhibitor P21WAF1/CIP1, RNA Binding Protein Musashi-1, and KrüPPEL-Like Factor 4 in Regulation of Mouse Bmi1+ Reserve Intestinal Stem Cells Function After γ Radiation-Induced Injury In Vivo

774 – Role of Cell Cycle Inhibitor P21WAF1/CIP1, RNA Binding Protein Musashi-1, and KrüPPEL-Like Factor 4 in Regulation of Mouse Bmi1+ Reserve Intestinal Stem Cells Function After γ Radiation-Induced Injury In Vivo

Preservation of reserve intestinal epithelial stem cells following severe ischemic injury.

Intestinal ischemia is an abdominal emergency with a mortality rate >50%, leading to epithelial barrier loss and subsequent sepsis. Epithelial renewal and repair after injury depend on intestinal epithelial stem cells (ISC) that reside within the crypts of Lieberkühn. Two ISC populations critical to epithelial repair have been described: 1) active ISC (aISC; highly proliferative; leucine-rich-repeat-containing G protein-coupled receptor 5 positive, sex determining region Y-box 9 positive) and 2) reserve ISC [rISC; less proliferative; homeodomain only protein X (Hopx)+]. Yorkshire crossbred pigs (8-10 wk old) were subjected to 1-4 h of ischemia and 1 h of reperfusion or recovery by reversible mesenteric vascular occlusion. This study was designed to evaluate whether ISC-expressing biomarkers of aISCs or rISCs show differential resistance to ischemic injury and different contributions to the subsequent repair and regenerative responses. Our data demonstrate that, following 3-4 h ischemic injury, aISC undergo apoptosis, whereas rISC are preserved. Furthermore, these rISC are retained ex vivo in spheroids in which cell populations are enriched in the rISC biomarker Hopx. These cells appear to go on to provide a proliferative pool of cells during the recovery period. Taken together, these data indicate that Hopx+ cells are resistant to injury and are the likely source of epithelial renewal following prolonged ischemic injury. It is therefore possible that targeting reserve stem cells will lead to new therapies for patients with severe intestinal injury. NEW & NOTEWORTHY The population of reserve less-proliferative intestinal epithelial stem cells appears resistant to injury despite severe epithelial cell loss, including that of the active stem cell population, which results from prolonged mesenteric ischemia. These cells can change to an activated state and are likely indispensable to regenerative processes. Reserve stem cell targeted therapies may improve treatment and outcome of patients with ischemic disease.

The Intestinal Stem Cell Niche: Homeostasis and Adaptations.

The intestinal epithelium is a rapidly renewing cellular compartment. This constant regeneration is a hallmark of intestinal homeostasis and requires a tightly regulated balance between intestinal stem cell (ISC) proliferation and differentiation. Since intestinal epithelial cells directly contact pathogenic environmental factors that continuously challenge their integrity, ISCs must also actively divide to facilitate regeneration and repair. Understanding niche adaptations that maintain ISC activity during homeostatic renewal and injury-induced intestinal regeneration is therefore a major and ongoing focus for stem cell biology. Here, we review recent concepts and propose an active interconversion of the ISC niche between homeostasis and injury-adaptive states that is superimposed upon an equally dynamic equilibrium between active and reserve ISC populations.

Mature enteroendocrine cells contribute to basal and pathological stem cell dynamics in the small intestine.

Lgr5-expressing intestinal stem cells (ISCs) maintain continuous and rapid generation of the intestinal epithelium. Here, we present evidence that dedifferentiation of committed enteroendocrine cells (EECs) contributes to maintenance of the epithelium under both basal conditions and in response to injury. Lineage-tracing studies identified a subset of EECs that reside at +4 position for more than 2 wk, most of which were BrdU-label-retaining cells. Under basal conditions, cells derived from these EECs grow from the bottom of the crypt to generate intestinal epithelium according to neutral drift kinetics that is consistent with dedifferentiation of mature EECs to ISCs. The lineage tracing of EECs demonstrated reserve stem cell properties in response to radiation-induced injury with the generation of reparative EEC-derived epithelial patches. Finally, the enterochromaffin (EC) cell was the predominant EEC type participating in these stem cell dynamics. These results provide novel insights into the +4 reserve ISC hypothesis, stem cell dynamics of the intestinal epithelium, and in the development of EC-derived small intestinal tumors. NEW & NOTEWORTHY The current manuscript demonstrating that a subset of mature enteroendocrine cells (EECs), predominantly enterochromaffin cells, dedifferentiates to fully functional intestinal stem cells (ISCs) is novel, timely, and important. These cells dedifferentiate to ISCs not only in response to injury but also under basal homeostatic conditions. These novel findings provide a mechanism in which a specified cell can dedifferentiate and contribute to normal tissue plasticity as well as the development of EEC-derived intestinal tumors under pathologic conditions.

Mo1065 - P21Waf1/Cip1 and RNA Binding Protein Musashi-1 Regulate small Intestinal Epithelium Regeneration after Γ Radiation-Induced Injury by Activation of the Subpopulation of Reserve Intestinal Stem Cells In Vivo

Mo1065 - P21Waf1/Cip1 and RNA Binding Protein Musashi-1 Regulate small Intestinal Epithelium Regeneration after Γ Radiation-Induced Injury by Activation of the Subpopulation of Reserve Intestinal Stem Cells In Vivo

Calorie Restriction Governs Intestinal Epithelial Regeneration through Cell-Autonomous Regulation of mTORC1 in Reserve Stem Cells.

SummaryAging is a complex process associated with a decline in functionality of adult stem cells affecting tissue homeostasis and regeneration. Calorie restriction (CR) is the only experimental manipulation known to extend lifespan and reduce the incidence of age-related disorders across numerous species. These benefits are likely mediated, at least in part, through the preservation of stem cell function. Here, we show that CR enhances the regenerative capacity of the intestinal epithelium through preservation of an injury-resistant reserve intestinal stem cell (ISC) pool. Cell-autonomous activity of mechanistic target of rapamycin complex 1 (mTORC1) governs the sensitivity of reserve ISCs to injury. CR inhibits mTORC1 in these cells, protecting them against DNA damage, while mTORC1 stimulation, either genetically or through nutrient sensing, sensitizes reserve ISCs to injury, thus compromising regeneration of the epithelium. These data delineate a critical role for mTORC1 in epithelial regeneration and inform clinical strategies based on nutrient modulation.

Epithelial WNT Ligands Are Essential Drivers of Intestinal Stem Cell Activation

Intestinal stem cells (ISCs) maintain and repair the intestinal epithelium. While regeneration after ISC-targeted damage is increasingly understood, injury-repair mechanisms that direct regeneration following injuries to differentiated cells remain uncharacterized. The enteric pathogen, rotavirus, infects and damages differentiated cells while sparing all ISC populations, thus allowing the unique examination of the response of intact ISC compartments during injury-repair. Upon rotavirus infection in mice, ISC compartments robustly expand and proliferating cells rapidly migrate. Infection results specifically in stimulation of the active crypt-based columnar ISCs, but not alternative reserve ISC populations, as is observed after ISC-targeted damage. Conditional ablation of epithelial WNT secretion diminishes crypt expansion and ISC activation, demonstrating a previously unknown function of epithelial-secreted WNT during injury-repair. These findings indicate a hierarchical preference of crypt-based columnar cells (CBCs) over other potential ISC populations during epithelial restitution and the importance of epithelial-derived signals in regulating ISC behavior.

JAK/STAT-1 Signaling Is Required for Reserve Intestinal Stem Cell Activation during Intestinal Regeneration Following Acute Inflammation.

SummaryThe intestinal epithelium serves as an essential barrier to the outside world and is maintained by functionally distinct populations of rapidly cycling intestinal stem cells (CBC ISCs) and slowly cycling, reserve ISCs (r-ISCs). Because disruptions in the epithelial barrier can result from pathological activation of the immune system, we sought to investigate the impact of inflammation on ISC behavior during the regenerative response. In a murine model of αCD3 antibody-induced small-intestinal inflammation, r-ISCs proved highly resistant to injury, while CBC ISCs underwent apoptosis. Moreover, r-ISCs were induced to proliferate and functionally contribute to intestinal regeneration. Further analysis revealed that the inflammatory cytokines interferon gamma and tumor necrosis factor alpha led to r-ISC activation in enteroid culture, which could be blocked by the JAK/STAT inhibitor, tofacitinib. These results highlight an important role for r-ISCs in response to acute intestinal inflammation and show that JAK/STAT-1 signaling is required for the r-ISC regenerative response.

The histone variant macroH2A confers functional robustness to the intestinal stem cell compartment.

A stem cell’s epigenome directs cell fate during development, homeostasis, and regeneration. Epigenetic dysregulation can lead to inappropriate cell fate decisions, aberrant cell function, and even cancer. The histone variant macroH2A has been shown to influence gene expression, guide cell fate, and safeguard against genotoxic stress. Interestingly, mice lacking functional macroH2A histones (hereafter referred to as macroH2A DKO) are viable and fertile; yet suffer from increased perinatal death and reduced weight and size compared to wildtype (WT). Here, we ask whether the ostensible reduced vigor of macroH2A DKO mice extends to intestinal stem cell (ISC) function during homeostasis, regeneration, and oncogenesis. Lgr5-eGFP-IRES-CreERT2 or Hopx-CreERT2::Rosa26-LSL-tdTomato ISC reporter mice or the C57BL/6J-Apcmin/J murine intestinal adenoma model were bred into a macroH2A DKO or strain-matched WT background and assessed for ISC functionality, regeneration and tumorigenesis. High-dose (12Gy) whole-body γ-irradiation was used as an injury model. We show that macroH2A is dispensable for intestinal homeostasis and macroH2A DKO mice have similar numbers of active crypt-base columnar ISCs (CBCs). MacroH2A DKO intestine exhibits impaired regeneration following injury, despite having significantly more putative reserve ISCs. DKO reserve ISCs disproportionately undergo apoptosis compared to WT after DNA damage infliction. Interestingly, a macroH2A DKO background does not significantly increase tumorigenesis in the Apcmin model of intestinal adenoma. We conclude that macroH2A influences reserve ISC number and function during homeostasis and regeneration. These data suggest macroH2A enhances reserve ISC survival after DNA damage and thus confers functional robustness to the intestinal epithelium.