The Role of Intestinal Stem Cells in Epithelial Regeneration Following Radiation-Induced Gut Injury

Inhibiting Glycogen Synthase Kinase-3 Mitigates the Hematopoietic Acute Radiation Syndrome in a Sex- and Strain-dependent Manner in Mice

The Radiation and Nuclear Countermeasures Program at the National Institute of Allergy and Infectious Diseases (NIAID) mandated that medical countermeasures for treating Acute Radiation Syndrome (ARS) must have efficacy when administered at least 24 h after radiation exposure. At this time point, many cells within key target tissues, such as the hematopoietic system and the gastrointestinal (GI) tract, will already be dead. Therefore, drugs that promote the regeneration of surviving cells may improve outcomes. The serine/threonine kinase glycogen synthase kinase-3 (GSK-3) regulates stem and progenitor cell self-renewal and regeneration in the hematopoietic and GI compartments. We tested inhibition of GSK-3β by SB216763 24 h after total body irradiation (TBI) and sub-total body irradiation (SBI). Here, we show that subcutaneous administration of SB216763 promotes the regeneration of surviving hematopoietic stem/progenitor cells (HSPCs), including myeloid progenitor cells, and improves survival of C57Bl/6 male mice when administered 24 h after TBI. However, these results were not recapitulated in female C57Bl/6 animals, suggesting a sex difference in GSK-3β signaling in HSPCs. Subcutaneous administration of SB216763 in male mice stimulated activation of Sox2 transcription but failed to induce Sox2 transcription in female C57Bl/6 mice. Using TCF/lef-GFP reporter mice, we examined Wnt signaling in HSPCs of irradiated male and female mice treated with SB216763. GSK-3 inhibition elevated Wnt reporter activity in HSPCs isolated from male but not female mice. SB216763 did not mitigate hematopoietic ARS in males or females of a second strain of wild-type mice, C3H. In addition, administration of SB216763 did not mitigate hematopoietic ARS beyond the currently available standard approved therapy of ciprofloxacin and granulocyte-colony stimulating factor (G-CSF) in male C57Bl/6 mice. Further, SB216763 did not mitigate GI-ARS after SBI in C57Bl/6 male mice. The lack of efficacy in both sexes and multiple strains of mice indicate that SB216763 is not suitable for further drug development as a mitigator of ARS. Our studies demonstrate that activation of Wnt signaling in HSPCs promotes hematopoietic regeneration following radiation exposure, and targeting this pathway downstream of GSK-3β may mitigate ARS in a sex- and strain-independent manner.

Evaluation of the radiation response and regenerative effects of mesenchymal stem cell-conditioned medium in an intestinal organoid system

Intestinal organoids have recently emerged as an in vitro model relevant to the gut system owing to their recapitulation of the native intestinal epithelium with crypt-villus architecture. However, it is unclear whether intestinal organoids reflect the physiology of the in vivo stress response. Here, we systemically investigated the radiation response in organoids and animal models using mesenchymal stem cell-conditioned medium (MSC-CM), which contains secreted paracrine factors. Irradiated organoids exhibited sequential induction of viability loss and regrowth after irradiation (within 12 days), similar to the response of the native intestinal epithelium. Notably, treatment with MSC-CM facilitated the reproliferation of intestinal stem cells (ISCs) and restoration of damaged crypt-villus structures in both models. Furthermore, Wnt/Notch signaling pathways were commonly upregulated by MSC-CM, but not radiation, and pharmacologically selective inhibition of Wnt or Notch signaling attenuated the enhanced recovery of irradiated organoids, with increases in ISCs, following MSC-CM treatment. Interestingly, the expression of Wnt4, Wnt7a, and active β-catenin was increased, but not notch family members, in MSC-CM-treated organoid after irradiation. Treatment of recombinant mouse Wnt4 and Wnt7a after irradiation improved to some extent intestinal epithelial regeneration both in vitro and in vivo. Overall, these results suggested that intestinal organoids recapitulated the physiological stress response of the intestinal epithelium in vivo. Thus, our findings provided important insights into the physiology of intestinal organoids and may contribute to the development of strategies to enhance the functional maturation of engineered organoids.

Elevating EGFR-MAPK program by a nonconventional Cdc42 enhances intestinal epithelial survival and regeneration

The regulatory mechanisms enabling the intestinal epithelium to maintain a high degree of regenerative capacity during mucosal injury remain unclear. Ex vivo survival and clonogenicity of intestinal stem cells (ISCs) strictly required growth response mediated by cell division control 42 (Cdc42) and Cdc42-deficient enteroids to undergo rapid apoptosis. Mechanistically, Cdc42 engaging with EGFR was required for EGF-stimulated, receptor-mediated endocytosis and sufficient to promote MAPK signaling. Proteomics and kinase analysis revealed that a physiologically, but nonconventionally, spliced Cdc42 variant 2 (V2) exhibited stronger MAPK-activating capability. Human CDC42-V2 is transcriptionally elevated in some colon tumor tissues. Accordingly, mice engineered to overexpress Cdc42-V2 in intestinal epithelium showed elevated MAPK signaling, enhanced regeneration, and reduced mucosal damage in response to irradiation. Overproducing Cdc42-V2 specifically in mouse ISCs enhanced intestinal regeneration following injury. Thus, the intrinsic Cdc42-MAPK program is required for intestinal epithelial regeneration, and elevating this signaling cascade is capable of initiating protection from genotoxic injury.

LIF is essential for ISC function and protects against radiation-induced gastrointestinal syndrome

Leukemia inhibitory factor (LIF) is a cytokine essential for maintaining pluripotency of mouse embryonic stem cells. However, its role in adult intestinal stem cells (ISCs) is unclear. The adult intestinal epithelium has a high self-renewal rate driven by ISCs in crypts. Here, we find that LIF is present in the ISC niche in crypts and critical for the function of ISCs in maintaining the intestinal epithelial homeostasis and regeneration. Mechanistically, LIF maintains β-catenin activity through the AKT/GSK3β signaling to regulate ISC functions. LIF deficiency in mice impairs the renewal of the intestinal epithelium under the physiological condition. Further, LIF deficiency in mice impairs the regeneration of intestinal epithelium in response to radiation and shortens the lifespan of mice after high doses of radiation due to gastrointestinal (GI) syndrome, which can be rescued by administering recombinant LIF (rLIF). Importantly, LIF exhibits a radioprotective role in wild-type (WT) mice by protecting mice from lethal radiation-induced GI syndrome; administering rLIF promotes intestinal epithelial regeneration and prolongs survival in WT mice after radiation. These results reveal a previously unidentified and a crucial role of LIF in ensuring ISC function, promoting regeneration of the intestinal epithelium in response to radiation and protecting against radiation-induced GI syndrome.

Intestinal stem cells acquire premature senescence and senescence associated secretory phenotype concurrent with persistent DNA damage after heavy ion radiation in mice

Heavy ion radiation, prevalent in outer space and relevant for radiotherapy, is densely ionizing and poses risk to stem cells that are key to intestinal homeostasis. Currently, the molecular spectrum of heavy ion radiation-induced perturbations in intestinal stem cells (ISCs), that could trigger intestinal pathologies, remains largely unexplored. The Lgr5-EGFP-IRES-creERT mice were exposed to 50 cGy of iron radiation. Mice were euthanized 60 d after exposure and ISCs were sorted using fluorescence activated cell sorting. Reactive oxygen species (ROS) and mitochondrial superoxide were measured using fluorescent probes. Since DNA damage is linked to senescence and senescent cells acquire senescence-associated secretory phenotype (SASP), we stained ISCs for both senescence markers p16, p21, and p19 as well as SASP markers IL6, IL8, and VEGF. Due to potential positive effects of SASP on proliferation, we also stained for PCNA. Data show increased ROS and ongoing DNA damage, by staining for γH2AX, and 53BP1, along with accumulation of senescence markers. Results also showed increased SASP markers in senescent cells. Collectively, our data suggest that heavy-ion-induced chronic stress and ongoing DNA damage is promoting SASP in a fraction of the ISCs, which has implications for gastrointestinal function, inflammation, and carcinogenesis in astronauts and patients.

Utility Evaluation of Porcine Enteroids as PDCoV Infection Model in vitro

Porcine deltacoronavirus (PDCoV) is a novel emerging enteric coronavirus found in pigs. Intestinal enteroids, which partially recreate the structure and function of intestinal villi-crypts, have many physiological similarities to the intestinal tissues in vivo. Enteroids exhibit advantages in studying the interactions between intestines and enteric pathogens. To create a novel infection model for PDCoV, we developed an in vitro system to generate porcine intestinal enteroids from crypts of duodenum, jejunum, and ileum of pigs. Enterocytes, enteroendocrine cells, Paneth cells, stem cells, proliferating cells, and goblet cells were found in the differentiated enteroids. Replication of PDCoV was detected in the cultured enteroids by immunofluorescence and quantitative RT-PCR. Double immunofluorescence labeling demonstrated that PDCoV was present in Sox9-positive intestinal cells and Villin1-positive enterocytes. There were multiple cellular responses shown as changes of transcription of genes related to mucosal immunity, antiviral genes, and marker genes of stem cells and other cells in the enteroids infected with PDCoV. We conclude that the 2-D enteroids derived from porcine jejunum can be used as an in vitro multicellular model for the investigation of pathogenesis and host immune responses to porcine enteric pathogens, such as PDCoV.

Radiation-induced tissue damage and response

Normal tissue responses to ionizing radiation have been a major subject for study since the discovery of X-rays at the end of the 19th century. Shortly thereafter, time-dose relationships were established for some normal tissue endpoints that led to investigations into how the size of dose per fraction and the quality of radiation affected outcome. The assessment of the radiosensitivity of bone marrow stem cells using colony-forming assays by Till and McCulloch prompted the establishment of in situ clonogenic assays for other tissues that added to the radiobiology toolbox. These clonogenic and functional endpoints enabled mathematical modeling to be performed that elucidated how tissue structure, and in particular turnover time, impacted clinically relevant fractionated radiation schedules. More recently, lineage tracing technology, advanced imaging and single cell sequencing have shed further light on the behavior of cells within stem, and other, cellular compartments, both in homeostasis and after radiation damage. The discovery of heterogeneity within the stem cell compartment and plasticity in response to injury have added new dimensions to the consideration of radiation-induced tissue damage. Clinically, radiobiology of the 20th century garnered wisdom relevant to photon treatments delivered to a fairly wide field at around 2 Gy per fraction, 5 days per week, for 5-7 weeks. Recently, the scope of radiobiology has been extended by advances in technology, imaging and computing, as well as by the use of charged particles. These allow radiation to be delivered more precisely to tumors while minimizing the amount of normal tissue receiving high doses. One result has been an increase in the use of schedules with higher doses per fraction given in a shorter time frame (hypofractionation). We are unable to cover these new technologies in detail in this review, just as we must omit low-dose stochastic effects, and many aspects of dose, dose rate and radiation quality. We argue that structural diversity and plasticity within tissue compartments provides a general context for discussion of most radiation responses, while acknowledging many omissions. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Stem and Progenitor Cells in the Pathogenesis and Treatment of Digestive Diseases

The global epidemic of chronic degenerative diseases expands rapidly. The pathogenesis of these noncommunicable disorders revolves around innate immunity, microbiome, and stem cell alterations. Understanding the mechanisms behind stem cell biology and their regulatory pathways is a key to understanding the origin of human disease. Stem cells are involved in tissue and organ damage and regeneration. The evidence is mounting that not only eukaryotic cells but also gut microbiota may release extracellular microvesicles that are absorbed from the gut into the portal and systemic circulation. Linking the fields of stem cells, innate immunity and microbiome research opens up new avenues to develop novel diagnostic (e.g., biomarkers), therapeutic (e.g., microbiome modulation, stem cell-based medicines), and prognostic (personalized diets) tools. In this chapter, we present the short overview of various stem and progenitor cells of adult tissues circulating in peripheral blood and their role in the pathogenesis and treatment of digestive diseases. We also briefly discuss the role of host-stem cell-microbial interactions as a new frontier of research in gastroenterology.

Sub-acute Toxicity in Non-cancerous Tissue and Immune-Related Adverse Events of a Novel Combination Therapy for Cancer

Brain, lung, and colon tissue experience deleterious immune-related adverse events when immune-oncological agents or radiation are administered. However, there is a paucity of information regarding whether the addition of radiation to immuno-oncological regimens exacerbates the tissue inflammatory response. We used a murine model to evaluate sub-acute tissue damage and the systemic immune response in C57Bl/6 mice when administered systemic anti-programmed cell death protein 1 (αPD-1) immunotherapy alone or in combination with stereotactic fractionated 10 gray/5 X-ray radiation to normal brain, lung or colon tissue. The model indicated that combinatorial αPD-1 immunotherapy and radiation may alter normal colon cell proliferation and cerebral blood vasculature, and induce systemic thrombocytopenia, lymphopenia, immune suppression, and altered immune repertoire (including interleukin-1β). Therein our data supports close monitoring of hematological and immune-related adverse events in patients receiving combination therapy.

Krüppel-like Factor 5 Regulates Stemness, Lineage Specification, and Regeneration of Intestinal Epithelial Stem Cells

BACKGROUND & AIMS

Self-renewal and multipotent differentiation are cardinal properties of intestinal stem cells (ISCs), mediated in part by WNT and NOTCH signaling. Although these pathways are well characterized, the molecular mechanisms that control the 'stemness' of ISCs are still not well defined. Here, we investigated the role of Krüppel-like factor 5 (KLF5) in regulating ISC functions.

METHODS

We performed studies in adult Lgr5^{EGFP-IRES-creERT2};Rosa26^LSLtdTomato (Lgr5^Ctrl) and Lgr5^{EGFP-IRES-creERT2};Klf5^fl/fl;Rosa26^LSLtdTomato (Lgr5^ΔKlf5) mice. Mice were injected with tamoxifen to activate Cre recombinase, which deletes Klf5 from the intestinal epithelium in Lgr5^ΔKlf5 but not Lgr5^Crtl mice. In experiments involving irradiation, mice were subjected to 12 Gy total body irradiation (TBI). Tissues were collected for immunofluorescence (IF) analysis and next generation sequencing. Oganoids were derived from fluoresecence activated cell sorted- (FACS-) single cells from tamoxifen-treated Lgr5^ΔKlf5 or Lgr5^Crtl mice and examined by immunofluorescence stain.

RESULTS

Lgr5⁺ ISCs lacking KLF5 proliferate faster than control ISCs but fail to self-renew, resulting in a depleted ISC compartment. Transcriptome analysis revealed that Klf5-null Lgr5⁺ cells lose ISC identity and prematurely differentiate. Following irradiation injury, which depletes Lgr5⁺ ISCs, reserve Klf5-null progenitor cells fail to dedifferentiate and regenerate the epithelium. Absence of KLF5 inactivates numerous selected enhancer elements and direct transcriptional targets including canonical WNT- and NOTCH-responsive genes. Analysis of human intestinal tissues showed increased levels of KLF5 in the regenerating epithelium as compared to those of healthy controls.

CONCLUSION

We conclude that ISC self-renewal, lineage specification, and precursor dedifferentiation require KLF5, by its ability to regulate epigenetic and transcriptional activities of ISC-specific gene sets. These findings have the potential for modulating ISC functions by targeting KLF5 in the intestinal epithelium.

Radiation-induced intestinal damage: latest molecular and clinical developments

Aim: To systematically review the prophylactic and therapeutic interventions for reducing the incidence or severity of intestinal symptoms among cancer patients receiving radiotherapy. Materials & methods: A literature search was conducted in the PubMed database using various search terms, including 'radiation enteritis', 'radiation enteropathy', 'radiation-induced intestinal disease', 'radiation-induced intestinal damage' and 'radiation mucositis'. The search was limited to in vivo studies, clinical trials and meta-analyses published in English with no limitation on publication date. Other relevant literature was identified based on the reference lists of selected studies. Results: The pathogenesis of acute and chronic radiation-induced intestinal damage as well as the prevention and treatment approaches were reviewed. Conclusion: There is inadequate evidence to strongly support the use of a particular strategy to reduce radiation-induced intestinal damage. More high-quality randomized controlled trials are required for interventions with limited evidence suggestive of potential benefits.

Macrophages as an Emerging Source of Wnt Ligands: Relevance in Mucosal Integrity

The Wnt signaling pathway is a conserved pathway involved in important cellular processes such as the control of embryonic development, cellular polarity, cellular migration, and cell proliferation. In addition to playing a central role during embryogenesis, this pathway is also an essential part of adult homeostasis. Indeed, it controls the proliferation of epithelial cells in different organs such as intestine, lung, and kidney, and guarantees the maintenance of the mucosa in physiological conditions. The origin of this molecular pathway is the binding between Wnt ligands (belonging to a family of 19 different homologous secreted glycoproteins) and their specific membrane receptors, from the Frizzled receptor family. This specific interaction triggers the activation of the signaling cascade, which in turn activates or suppresses the expression of different genes in order to change the behavior of the cell. On the other hand, alterations of this pathway have been described in pathological conditions such as inflammation, fibrosis, and cancer. In recent years, macrophages-among other cell types-have emerged as a potential source of Wnt ligands. Due to their high plasticity, macrophages, which are central to the innate immune response, are capable of adopting different phenotypes depending on their microenvironment. In the past, two different phenotypes were described: a proinflammatory phenotype-M1 macrophages-and an anti-inflammatory phenotype-M2 macrophages-and a selective expression of Wnt ligands has been associated with said phenotypes. However, nowadays it is assumed that macrophages in vivo move through a continual spectrum of functional phenotypes. In both physiological and pathological (inflammation, fibrosis and cancer) conditions, the accumulation and polarization of macrophages conditions the future of the tissue, facilitating various scenarios, such as resolution of inflammation, activation of fibrosis, and cancer development due to the modulation of the Wnt signaling pathway, in autocrine and paracrine manner. In this work, we provide an overview of studies that have explored the role of macrophages and how they act as a source of Wnt ligands and as mediators of mucosal integrity.

Response of Human Small Intestinal Epithelium to Fractionated Irradiation: Dynamical Modeling Approach

A biologically motivated mathematical model of the dynamics of the small intestinal epithelium in humans treated with fractionated radiotherapy has been developed and is further investigated here. This model, originating from our previous work, is implemented as a system of nonlinear ordinary differential equations, in which the variables and parameters have a clear biological meaning. The model also includes, as input, the key parameters of fractionated irradiation. The modeling results on the dynamical response of the human normal small intestinal epithelium to fractionated radiation therapy regimens were in agreement with the corresponding empirical data, which, in turn, demonstrates the capability of the developed model for predicting the dynamics of this vital body system in humans receiving fractionated radiotherapy. It is also revealed that the cumulative damage effects of hypofractionated radiation therapy regimens on the human normal small intestinal epithelium are somewhat less pronounced than those of conventional fractionated radiation therapy regimens with the same total doses.

Comparison of Proton and Photon Beam Irradiation in Radiation-Induced Intestinal Injury Using a Mouse Model

When radiotherapy is applied to the abdomen or pelvis, normal tissue toxicity in the gastrointestinal (GI) tract is considered a major dose-limiting factor. Proton beam therapy has a specific advantage in terms of reduced doses to normal tissues. This study investigated the fundamental differences between proton- and X-ray-induced intestinal injuries in mouse models. C57BL/6J mice were irradiated with 6-MV X-rays or 230-MeV protons and were sacrificed after 84 h. The number of surviving crypts per circumference of the jejunum was identified using Hematoxylin and Eosin staining. Diverse intestinal stem cell (ISC) populations and apoptotic cells were analyzed using immunohistochemistry (IHC) and a terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling (TUNEL) assay, respectively. The crypt microcolony assay revealed a radiation-dose-dependent decrease in the number of regenerative crypts in the mouse jejunum; proton irradiation was more effective than X-ray irradiation with a relative biological effectiveness of 1.14. The jejunum is the most sensitive to radiations, followed by the ileum and the colon. Both types of radiation therapy decreased the number of radiosensitive, active cycling ISC populations. However, a higher number of radioresistant, reserve ISC populations and Paneth cells were eradicated by proton irradiation than X-ray irradiation, as shown in the IHC analyses. The TUNEL assay revealed that proton irradiation was more effective in enhancing apoptotic cell death than X-ray irradiation. This study conducted a detailed analysis on the effects of proton irradiation versus X-ray irradiation on intestinal crypt regeneration in mouse models. Our findings revealed that proton irradiation has a direct effect on ISC populations, which may result in an increase in the risk of GI toxicity during proton beam therapy.

Food Supplements to Mitigate Detrimental Effects of Pelvic Radiotherapy

Pelvic radiotherapy has been frequently reported to cause acute and late onset gastrointestinal (GI) toxicities associated with significant morbidity and mortality. Although the underlying mechanisms of pelvic radiation-induced GI toxicity are poorly understood, they are known to involve a complex interplay between all cell types comprising the intestinal wall. Furthermore, increasing evidence states that the human gut microbiome plays a role in the development of radiation-induced health damaging effects. Gut microbial dysbiosis leads to diarrhea and fatigue in half of the patients. As a result, reinforcement of the microbiome has become a hot topic in various medical disciplines. To counteract GI radiotoxicities, apart from traditional pharmacological compounds, adjuvant therapies are being developed including food supplements like vitamins, prebiotics, and probiotics. Despite the easy, cheap, safe, and feasible approach to protect patients against acute radiation-induced toxicity, clinical trials have yielded contradictory results. In this review, a detailed overview is given of the various clinical, intestinal manifestations after pelvic irradiation as well as the role of the gut microbiome herein. Furthermore, whilst discussing possible strategies to prevent these symptoms, food supplements are presented as auspicious, prophylactic, and therapeutic options to mitigate acute pelvic radiation-induced GI injury by exploring their molecular mechanisms of action.

Radiotherapy toxicity

Radiotherapy is used in >50% of patients with cancer, both for curative and palliative purposes. Radiotherapy uses ionizing radiation to target and kill tumour tissue, but normal tissue can also be damaged, leading to toxicity. Modern and precise radiotherapy techniques, such as intensity-modulated radiotherapy, may prevent toxicity, but some patients still experience adverse effects. The physiopathology of toxicity is dependent on many parameters, such as the location of irradiation or the functional status of organs at risk. Knowledge of the mechanisms leads to a more rational approach for controlling radiotherapy toxicity, which may result in improved symptom control and quality of life for patients. This improved quality of life is particularly important in paediatric patients, who may live for many years with the long-term effects of radiotherapy. Notably, signs and symptoms occurring after radiotherapy may not be due to the treatment but to an exacerbation of existing conditions or to the development of new diseases. Although differential diagnosis may be difficult, it has important consequences for patients.

Atoh1+ secretory progenitors possess renewal capacity independent of Lgr5+ cells during colonic regeneration

During homeostasis, the colonic epithelium is replenished every 3-5 days by rapidly cycling Lgr5 ⁺ stem cells. However, various insults can lead to depletion of Lgr5 ⁺ stem cells, and colonic epithelium can be regenerated from Lgr5-negative cells. While studies in the small intestine have addressed the lineage identity of the Lgr5-negative regenerative cell population, in the colon this question has remained unanswered. Here, we set out to identify which cell(s) contribute to colonic regeneration by performing genetic fate-mapping studies of progenitor populations in mice. First, using keratin-19 (Krt19) to mark a heterogeneous population of cells, we found that Lgr5-negative cells can regenerate colonic crypts and give rise to Lgr5 ⁺ stem cells. Notch1 ⁺ absorptive progenitor cells did not contribute to epithelial repair after injury, whereas Atoh1 ⁺ secretory progenitors did contribute to this process. Additionally, while colonic Atoh1 ⁺ cells contributed minimally to other lineages during homeostasis, they displayed plasticity and contributed to epithelial repair during injury, independent of Lgr5 ⁺ cells. Our findings suggest that promotion of secretory progenitor plasticity could enable gut healing in colitis.

Genome Toxicity and Impaired Stem Cell Function after Conditional Activation of CreERT2 in the Intestine

With the tamoxifen-inducible CreER^T2 system, genetic recombination can be temporally controlled in a cell-type-specific manner in intact animals, permitting dissection of the molecular underpinnings of mammalian physiology. Here we present a significant drawback to CreER^T2 technology for analysis of intestinal stem cells. Using the intestine-specific Villin-CreER^T2 mouse strain, we observed delayed intestinal regeneration post irradiation. Villin-CreER^T2 activation was associated with DNA damage and cryptic loxP site cleavage. Analysis of stem cell-specific CreER^T2 strains showed that the genome toxicity impairs function of crypt base columnar stem cells, resulting in loss of organoid initiating activity. Importantly, the stem cell impairment is short-lived, with return to normal by 7 days post tamoxifen treatment. Our findings demonstrate that mouse genetic experiments that utilize CreER^T2 should consider the confounding effects of enhanced stem cell sensitivity to genome toxicity resulting from CreER^T2 activation.

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Intestinal stem cell (ISC) toxicity induced in mice by CreER^T2 activation

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Impaired organoid formation after activation of ISC-specific CreER^T2 strains

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Genotoxicity and impaired crypt regeneration in Villin-CreER^T2 mice

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Impaired ISC function and genotoxicity repaired by 7 days after activation

The Microbiome and Radiation Induced-Bowel Injury: Evidence for Potential Mechanistic Role in Disease Pathogenesis

Radiotherapy has played a major role in both the curative and palliative treatment of cancer patients for decades. However, its toxic effect to the surrounding normal healthy tissue remains a major drawback. In cases of intra-abdominal and/or pelvic malignancy, healthy bowel is inevitably included in the radiation field, causing undesirable consequences that subsequently manifest as radiation-induced bowel injury, which is associated with significant morbidity and mortality. The pathophysiology of radiation-induced bowel injury is poorly understood, although we now know that it derives from a complex interplay of epithelial injury and alterations in the enteric immune, nervous, and vascular systems in genetically predisposed individuals. Furthermore, evidence supporting a pivotal role for the gut microbiota in the development of radiation-induced bowel injury has been growing. In this review, we aim to appraise our current understanding of radiation-induced bowel injury and the role of the microbiome in its pathogenesis as well as prevention and treatment. Greater understanding of the relationship between the disease mechanism of radiation-induced bowel injury and gut microbiome might shed light on potential future prevention and treatment strategies through the modification of a patient's gut microbiome.