Lgr5+ stem cells are indispensable for radiation-induced intestinal regeneration

BMP restricts stemness of intestinal Lgr5+ stem cells by directly suppressing their signature genes

The intestinal epithelium possesses a remarkable self-renewal ability, which is mediated by actively proliferating Lgr5⁺ stem cells. Bone morphogenetic protein (BMP) signalling represents one major counterforce that limits the hyperproliferation of intestinal epithelium, but the exact mechanism remains elusive. Here we demonstrate that epithelial BMP signalling plays an indispensable role in restricting Lgr5⁺ stem cell expansion to maintain intestinal homeostasis and prevent premalignant hyperproliferation on damage. Mechanistically, BMP inhibits stemness of Lgr5⁺ stem cells through Smad-mediated transcriptional repression of a large number of stem cell signature genes, including Lgr5, and this effect is independent of Wnt/β-catenin signalling. Smad1/Smad4 recruits histone deacetylase HDAC1 to the promoters to repress transcription, and knockout of Smad4 abolishes the negative effects of BMP on stem cells. Our findings therefore demonstrate that epithelial BMP constrains the Lgr5⁺ stem cell self-renewal via Smad-mediated repression of stem cell signature genes to ensure proper homeostatic renewal of intestinal epithelium.

Bone morphogenetic protein (BMP) maintains intestinal homeostasis by restricting its hyperproliferation but whether it directly regulates the stem cells is unknown. Here the authors show that BMP constrains the Lgr5⁺ stem cell expansion under both homeostatic and injury conditions through Smad-mediated repression of stem cell signature genes.

Determinants of metastatic competency in colorectal cancer

Colorectal cancer (CRC) is one of the most common cancer types and represents a major therapeutic challenge. Although initial events in colorectal carcinogenesis are relatively well characterized and treatment for early‐stage disease has significantly improved over the last decades, the mechanisms underlying metastasis – the main cause of death – remain poorly understood. Correspondingly, no effective therapy is currently available for advanced or metastatic disease. There is increasing evidence that colorectal cancer is hierarchically organized and sustained by cancer stem cells, in concert with various stromal cell types. Here, we review the interplay between cancer stem cells and their microenvironment in promoting metastasis and discuss recent insights relating to both patient prognosis and novel targeted treatment strategies. A better understanding of these topics may aid the prevention or reduction of metastatic burden.

An amino acid-based oral rehydration solution (AA-ORS) enhanced intestinal epithelial proliferation in mice exposed to radiation

Destruction of clonogenic cells in the crypt following irradiation are thought to cause altered gastrointestinal function. Previously, we found that an amino acid-based oral rehydration solution (AA-ORS) improved gastrointestinal function in irradiated mice. However, the exact mechanisms were unknown. Electrophysiology, immunohistochemistry, qPCR, and Western blot analysis were used to determine that AA-ORS increased proliferation, maturation, and differentiation and improved electrolyte and nutrient absorption in irradiated mice. A single-hit, multi-target crypt survival curve showed a significant increase in crypt progenitors in irradiated mice treated with AA-ORS for six days (8.8 ± 0.4) compared to the saline-treated group (6.1 ± 0.3; P < 0.001) without a change in D₀ (4.8 ± 0.1 Gy). The D_q values increased from 8.8 ± 0.4 Gy to 10.5 ± 0.5 Gy with AA-ORS treatment (P < 0.01), indicating an increased radiation tolerance of 1.7 Gy. We also found that AA-ORS treatment (1) increased Lgr5⁺, without altering Bmi1 positive cells; (2) increased levels of proliferation markers (Ki-67, p-Erk, p-Akt and PCNA); (3) decreased apoptosis markers, such as cleaved caspase-3 and Bcl-2; and (4) increased expression and protein levels of NHE3 and SGLT1 in the brush border membrane. This study shows that AA-ORS increased villus height and improved electrolyte and nutrient absorption.

Msi RNA-binding proteins control reserve intestinal stem cell quiescence

Regeneration of the intestinal epithelium is driven by multiple intestinal stem cell (ISC) types, including an active, radiosensitive Wnthigh ISC that fuels turnover during homeostasis and a reserve, radioresistant Wntlow/off ISC capable of generating active Wnthigh ISCs. We examined the role of the Msi family of oncoproteins in the ISC compartment. We demonstrated that Msi proteins are dispensable for normal homeostasis and self-renewal of the active ISC, despite their being highly expressed in these cells. In contrast, Msi proteins are required specifically for activation of reserve ISCs, where Msi activity is both necessary and sufficient to drive exit from quiescence and entry into the cell cycle. Ablation of Msi activity in reserve ISCs rendered the epithelium unable to regenerate in response to injury that ablates the active stem cell compartment. These findings delineate a molecular mechanism governing reserve ISC quiescence and demonstrate a necessity for the activity of this rare stem cell population in intestinal regeneration.

Regulation and plasticity of intestinal stem cells during homeostasis and regeneration

The intestinal epithelium is the fastest renewing tissue in mammals and has a large flexibility to adapt to different types of damage. Lgr5⁺ crypt base columnar (CBC) cells act as stem cells during homeostasis and are essential during regeneration. Upon perturbation, the activity of CBCs is dynamically regulated to maintain homeostasis and multiple dedicated progenitor cell populations can reverse to the stem cell state upon damage, adding another layer of compensatory mechanisms to facilitate regeneration. Here, we review our current understanding of how intestinal stem and progenitor cells contribute to homeostasis and regeneration, and the different signaling pathways that regulate their behavior. Nutritional state and inflammation have been recently identified as upstream regulators of stem cell activity in the mammalian intestine, and we explore how these systemic signals can influence homeostasis and regeneration.

Macrophage-derived extracellular vesicle-packaged WNTs rescue intestinal stem cells and enhance survival after radiation injury

WNT/β-catenin signalling is crucial for intestinal homoeostasis. The intestinal epithelium and stroma are the major source of WNT ligands but their origin and role in intestinal stem cell (ISC) and epithelial repair remains unknown. Macrophages are a major constituent of the intestinal stroma. Here, we analyse the role of macrophage-derived WNT in intestinal repair in mice by inhibiting their release using a macrophage-restricted ablation of Porcupine, a gene essential for WNT synthesis. Such Porcn-depleted mice have normal intestinal morphology but are hypersensitive to radiation injury in the intestine compared with wild-type (WT) littermates. Porcn-null mice are rescued from radiation lethality by treatment with WT but not Porcn-null bone marrow macrophage-conditioned medium (CM). Depletion of extracellular vesicles (EV) from the macrophage CM removes WNT function and its ability to rescue ISCs from radiation lethality. Therefore macrophage-derived EV-packaged WNTs are essential for regenerative response of intestine against radiation.

The intestinal stroma secretes WNT ligands but the role of WNT in intestinal repair is unclear. Here, the authors show that when WNT synthesis is ablated from stromal macrophages, the intestine morphology is normal but hypersensitive to radiation injury, implicating macrophage-derived WNT in intestinal repair.

Zinc Transporter SLC39A7/ZIP7 Promotes Intestinal Epithelial Self-Renewal by Resolving ER Stress

Zinc transporters play a critical role in spatiotemporal regulation of zinc homeostasis. Although disruption of zinc homeostasis has been implicated in disorders such as intestinal inflammation and aberrant epithelial morphology, it is largely unknown which zinc transporters are responsible for the intestinal epithelial homeostasis. Here, we show that Zrt-Irt-like protein (ZIP) transporter ZIP7, which is highly expressed in the intestinal crypt, is essential for intestinal epithelial proliferation. Mice lacking Zip7 in intestinal epithelium triggered endoplasmic reticulum (ER) stress in proliferative progenitor cells, leading to significant cell death of progenitor cells. Zip7 deficiency led to the loss of Olfm4⁺ intestinal stem cells and the degeneration of post-mitotic Paneth cells, indicating a fundamental requirement for Zip7 in homeostatic intestinal regeneration. Taken together, these findings provide evidence for the importance of ZIP7 in maintenance of intestinal epithelial homeostasis through the regulation of ER function in proliferative progenitor cells and maintenance of intestinal stem cells. Therapeutic targeting of ZIP7 could lead to effective treatment of gastrointestinal disorders.

Intestinal epithelium undergoes continuous self-renewal to maintain intestinal homeostasis. Given that dysregulation of zinc flux causes intestinal disorders, appropriate spatiotemporal regulation of zinc in the intracellular compartments should be a prerequisite for the intestinal epithelial self-renewal process. Zinc transporters such as Zrt-Irt-like proteins (ZIPs) are essential to fine-tune intracellular zinc flux. However, the link between specific zinc transporter(s) and intestinal epithelial self-renewal remains to be elucidated. Here, we found that ZIP7 is highly expressed in the intestinal crypts. The finding motivated us to further analyze the role of ZIP7 in intestinal homeostasis. ZIP7 deficiency greatly enhanced ER stress response in proliferative progenitor cells, which induced apoptotic cell death. This abnormality disrupted epithelial proliferation and intestinal stemness. Based on these observations, we reason that ZIP7-dependent zinc transport facilitates the vigorous epithelial proliferation in the intestine by ameliorating ER stress.

Single cell lineage tracing reveals a role for TgfβR2 in intestinal stem cell dynamics and differentiation

Intestinal stem cells (ISCs) are maintained by a niche mechanism, in which multiple ISCs undergo differential fates where a single ISC clone ultimately occupies the niche. Importantly, mutations continually accumulate within ISCs creating a potential competitive niche environment. Here we use single cell lineage tracing following stochastic transforming growth factor β receptor 2 (TgfβR2) mutation to show cell autonomous effects of TgfβR2 loss on ISC clonal dynamics and differentiation. Specifically, TgfβR2 mutation in ISCs increased clone survival while lengthening times to monoclonality, suggesting that Tgfβ signaling controls both ISC clone extinction and expansion, independent of proliferation. In addition, TgfβR2 loss in vivo reduced crypt fission, irradiation-induced crypt regeneration, and differentiation toward Paneth cells. Finally, altered Tgfβ signaling in cultured mouse and human enteroids supports further the in vivo data and reveals a critical role for Tgfβ signaling in generating precursor secretory cells. Overall, our data reveal a key role for Tgfβ signaling in regulating ISCs clonal dynamics and differentiation, with implications for cancer, tissue regeneration, and inflammation.

Inhibition of CDK4/6 protects against radiation-induced intestinal injury in mice

Radiotherapy causes dose-limiting toxicity and long-term complications in rapidly renewing tissues, including the gastrointestinal tract. Currently, there is no FDA-approved agent for the prevention or treatment of radiation-induced intestinal injury. In this study, we have shown that PD 0332991 (PD), an FDA-approved selective inhibitor of cyclin-dependent kinase 4/6 (CDK4/6), prevents radiation-induced lethal intestinal injury in mice. Treating mice with PD or a structurally distinct CDK4/6 inhibitor prior to radiation blocked proliferation and crypt apoptosis and improved crypt regeneration. PD treatment also enhanced LGR5+ stem cell survival and regeneration after radiation. PD was an on-target inhibitor of RB phosphorylation and blocked G1/S transition in the intestinal crypts. PD treatment strongly but reversibly inhibited radiation-induced p53 activation, which blocked p53-upregulated modulator of apoptosis-dependent (PUMA-dependent) apoptosis without affecting p21-dependent suppression of DNA damage accumulation, with a repair bias toward nonhomologous end joining. Further, deletion of PUMA synergized with PD treatment for even greater intestinal radioprotection. Our results demonstrate that the cell cycle critically regulates the DNA damage response and survival of intestinal stem cells and support the concept that pharmacological quiescence is a potentially highly effective and selective strategy for intestinal radioprotection.

Mesenchymal stem cells stimulate intestinal stem cells to repair radiation-induced intestinal injury

The loss of stem cells residing in the base of the intestinal crypt has a key role in radiation-induced intestinal injury. In particular, Lgr5⁺ intestinal stem cells (ISCs) are indispensable for intestinal regeneration following exposure to radiation. Mesenchymal stem cells (MSCs) have previously been shown to improve intestinal epithelial repair in a mouse model of radiation injury, and, therefore, it was hypothesized that this protective effect is related to Lgr5⁺ ISCs. In this study, it was found that, following exposure to radiation, transplantation of MSCs improved the survival of the mice, ameliorated intestinal injury and increased the number of regenerating crypts. Furthermore, there was a significant increase in Lgr5⁺ ISCs and their daughter cells, including Ki67⁺ transient amplifying cells, Vil1⁺ enterocytes and lysozyme⁺ Paneth cells, in response to treatment with MSCs. Crypts isolated from mice treated with MSCs formed a higher number of and larger enteroids than those from the PBS group. MSC transplantation also reduced the number of apoptotic cells within the small intestine at 6 h post-radiation. Interestingly, Wnt3a and active β-catenin protein levels were increased in the small intestines of MSC-treated mice. In addition, intravenous delivery of recombinant mouse Wnt3a after radiation reduced damage in the small intestine and was radioprotective, although not to the same degree as MSC treatment. Our results show that MSCs support the growth of endogenous Lgr5⁺ ISCs, thus promoting repair of the small intestine following exposure to radiation. The molecular mechanism of action mediating this was found to be related to increased activation of the Wnt/β-catenin signaling pathway.

Targeting LGR5+ cells with an antibody-drug conjugate for the treatment of colon cancer

Cancer stem cells (CSCs) are hypothesized to actively maintain tumors similarly to how their normal counterparts replenish differentiated cell types within tissues, making them an attractive therapeutic target for the treatment of cancer. Because most CSC markers also label normal tissue stem cells, it is unclear how to selectively target them without compromising normal tissue homeostasis. We evaluated a strategy that targets the cell surface leucine-rich repeat-containing G protein-coupled receptor 5 (LGR5), a well-characterized tissue stem cell and CSC marker, with an antibody conjugated to distinct cytotoxic drugs. One antibody-drug conjugate (ADC) demonstrated potent tumor efficacy and safety in vivo. Furthermore, the ADC decreased tumor size and proliferation, translating to improved survival in a genetically engineered model of intestinal tumorigenesis. These data demonstrate that ADCs can be leveraged to exploit differences between normal and cancer stem cells to successfully target gastrointestinal cancers.

Distal airway epithelial progenitor cells are radiosensitive to High-LET radiation

Exposure to high-linear energy transfer (LET) radiation occurs in a variety of situations, including charged particle radiotherapy, radiological accidents, and space travel. However, the extent of normal tissue injury in the lungs following high-LET radiation exposure is unknown. Here we show that exposure to high-LET radiation led to a prolonged loss of in vitro colony forming ability by airway epithelial progenitor cells. Furthermore, exposure to high-LET radiation induced clonal expansion of a subset of progenitor cells in the distal airway epithelium. Clonal expansion following high-LET radiation exposure was correlated with elevated progenitor cell apoptosis, persistent γ-H2AX foci, and defects in mitotic progression of distal airway progenitors. We discovered that the effects of high-LET radiation exposure on progenitor cells occur in a p53-dependent manner. These data show that high-LET radiation depletes the distal airway progenitor pool by inducing cell death and loss of progenitor function, leading to clonal expansion. Importantly, high-LET radiation induces greater long-term damage to normal lung tissue than the relative equivalent dose of low-LET γ-rays, which has implications in therapeutic development and risk assessment.

Intestinal Epithelial Cell-Intrinsic Deletion of Setd7 Identifies Role for Developmental Pathways in Immunity to Helminth Infection

The intestine is a common site for a variety of pathogenic infections. Helminth infections continue to be major causes of disease worldwide, and are a significant burden on health care systems. Lysine methyltransferases are part of a family of novel attractive targets for drug discovery. SETD7 is a member of the Suppressor of variegation 3-9-Enhancer of zeste-Trithorax (SET) domain-containing family of lysine methyltransferases, and has been shown to methylate and alter the function of a wide variety of proteins in vitro. A few of these putative methylation targets have been shown to be important in resistance against pathogens. We therefore sought to study the role of SETD7 during parasitic infections. We find that Setd7-/- mice display increased resistance to infection with the helminth Trichuris muris but not Heligmosomoides polygyrus bakeri. Resistance to T. muris relies on an appropriate type 2 immune response that in turn prompts intestinal epithelial cells (IECs) to alter differentiation and proliferation kinetics. Here we show that SETD7 does not affect immune cell responses during infection. Instead, we found that IEC-specific deletion of Setd7 renders mice resistant to T. muris by controlling IEC turnover, an important aspect of anti-helminth immune responses. We further show that SETD7 controls IEC turnover by modulating developmental signaling pathways such as Hippo/YAP and Wnt/β-Catenin. We show that the Hippo pathway specifically is relevant during T. muris infection as verteporfin (a YAP inhibitor) treated mice became susceptible to T. muris. We conclude that SETD7 plays an important role in IEC biology during infection.

PRC2 preserves intestinal progenitors and restricts secretory lineage commitment

Chromatin modifications shape cell heterogeneity by activating and repressing defined sets of genes involved in cell proliferation, differentiation and development. Polycomb-repressive complexes (PRCs) act synergistically during development and differentiation by maintaining transcriptional repression of common genes. PRC2 exerts this activity by catalysing H3K27 trimethylation. Here, we show that in the intestinal epithelium PRC2 is required to sustain progenitor cell proliferation and the correct balance between secretory and absorptive lineage differentiation programs. Using genetic models, we show that PRC2 activity is largely dispensable for intestinal stem cell maintenance but is strictly required for radiation-induced regeneration by preventing Cdkn2a transcription. Combining these models with genomewide molecular analysis, we further demonstrate that preferential accumulation of secretory cells does not result from impaired proliferation of progenitor cells induced by Cdkn2a activation but rather from direct regulation of transcription factors responsible for secretory lineage commitment. Overall, our data uncover a dual role of PRC2 in intestinal homeostasis highlighting the importance of this repressive layer in controlling cell plasticity and lineage choices in adult tissues.

The RNA polymerase III subunit Polr3b is required for the maintenance of small intestinal crypts in mice

BACKGROUND & AIMS

The continuously self-renewing mammalian intestinal epithelium, with high cellular turnover, depends on adequate protein synthesis for its proliferative capacity. RNA polymerase III activity is closely related to cellular growth and proliferation. Here, we studied the role of Polr3b, a large RNA polymerase III subunit, in the mammalian intestinal epithelium.

METHODS

We derived mice with an intestinal epithelium-specific hypomorphic mutation of the Polr3b gene, using VillinCre-mediated gene ablation. Phenotypic consequences of the Polr3b mutation on the intestinal epithelium in mice were assessed using histological and molecular methodologies, including genetic lineage tracing.

RESULTS

The Polr3b mutation severely reduced survival and growth in mice during the first postnatal week, the period when the expansion of the intestinal epithelium, and thus the requirement for protein synthesis, are highest. The neonatal intestinal epithelium of Polr3b^loxP/loxP;VillinCre mice was characterized by areas with reduced proliferation, abnormal epithelial architecture, loss of Wnt signaling and a dramatic increase in apoptotic cells in crypts. Genetic lineage tracing using Polr3b^LoxP/LoxP;Rosa26-lox-stop-lox-YFP;VillinCre mice demonstrated that in surviving mutant mice, Polr3b-deficient dying crypts were progressively replaced by 'Cre-escaper' cells that had retained wild type Polr3b function. In addition, enteroids cultured from Polr3b^loxP/loxP;VillinCre mice show reduced proliferative activity and increased apoptosis.

CONCLUSIONS

We provide evidence for an essential role of the Pol III subunit Polr3b in orchestrating the maintenance of the intestinal crypt during early postnatal development in mice.

Reducing radiation-induced gastrointestinal toxicity - the role of the PHD/HIF axis

Radiotherapy is an effective treatment strategy for cancer, but a significant proportion of patients experience radiation-induced toxicity due to damage to normal tissue in the irradiation field. The use of chemical or biological approaches aimed at reducing or preventing normal tissue toxicity induced by radiotherapy is a long-held goal. Hypoxia-inducible factors (HIFs) regulate the production of factors that may protect several cellular compartments affected by radiation-induced toxicity. Pharmacological inhibitors of prolyl hydroxylase domain-containing enzymes (PHDs), which result in stabilization of HIFs, have recently been proposed as a new class of radioprotectors. In this review, radiation-induced toxicity in the gastrointestinal (GI) tract and the main cellular compartments studied in this context will be discussed. The effects of PHD inhibition on GI radioprotection will be described in detail.

Current understanding concerning intestinal stem cells

In mammals, the intestinal epithelium is a tissue that contains two distinct pools of stem cells: active intestinal stem cells and reserve intestinal stem cells. The former are located in the crypt basement membrane and are responsible for maintaining epithelial homeostasis under intact conditions, whereas the latter exhibit the capacity to facilitate epithelial regeneration after injury. These two pools of cells can convert into each other, maintaining their quantitative balance. In terms of the active intestinal stem cells, their development into functional epithelium is precisely controlled by the following signaling pathways: Wnt/β-catenin, Ras/Raf/Mek/Erk/MAPK, Notch and BMP/Smad. However, mutations in some of the key regulator genes associated with these signaling pathways, such as APC, Kras and Smad4, are also highly associated with gut malformations. At this point, clarifying the biological characteristics of intestinal stem cells will increase the feasibility of preventing or treating some intestinal diseases, such as colorectal cancer. Moreover, as preclinical data demonstrate the therapeutic effects of colon stem cells on murine models of experimental colitis, the prospects of stem cell-based regenerative treatments for ulcerous lesions in the gastrointestinal tract will be improved all the same.

Mouse Label-Retaining Cells Are Molecularly and Functionally Distinct From Reserve Intestinal Stem Cells

BACKGROUND & AIMS

Intestinal homeostasis and regeneration after injury are controlled by 2 different types of cells: slow cycling, injury-resistant reserve intestinal stem cells (ISCs) and actively proliferative ISCs. Putative reserve ISCs have been identified using a variety of methods, including CreER insertions at Hopx or Bmi1 loci in mice and DNA label retention. Label-retaining cells (LRCs) include dormant stem cells in several tissues; in the intestine, LRCs appear to share some properties with reserve ISCs, which can be marked by reporter alleles. We investigated the relationships between these populations.

METHODS

Studies were performed in Lgr5-EGFP-IRESCreERT2, Bmi1-CreERT2, Hopx-CreERT2, and TRE-H2BGFP::Hopx-CreERT2::lox-stop-lox-tdTomato mice. Intestinal epithelial cell populations were purified; we compared reporter allele-marked reserve ISCs and several LRC populations (marked by H2B-GFP retention) using histologic flow cytometry and functional and single-cell gene expression assays.

RESULTS

LRCs were dynamic and their cellular composition changed with time. Short-term LRCs had properties of secretory progenitor cells undergoing commitment to the Paneth or enteroendocrine lineages, while retaining some stem cell activity. Long-term LRCs lost stem cell activity and were a homogenous population of terminally differentiated Paneth cells. Reserve ISCs marked with HopxCreER were primarily quiescent (in G0), with inactive Wnt signaling and robust stem cell activity. In contrast, most LRCs were in G1 arrest and expressed genes that are regulated by the Wnt pathway or are in the secretory lineage.

CONCLUSIONS

LRCs are molecularly and functionally distinct from reporter-marked reserve ISCs. This information provides an important basis for future studies of relationships among ISC populations.

Clonal analysis of stem cells in differentiation and disease

Tracking the fate of individual cells and their progeny by clonal analysis has redefined the concept of stem cells and their role in health and disease. The maintenance of cell turnover in adult tissues is achieved by the collective action of populations of stem cells with an equal likelihood of self-renewal or differentiation. Following injury stem cells exhibit striking plasticity, switching from homeostatic behavior in order to repair damaged tissues. The effects of disease states on stem cells are also being uncovered, with new insights into how somatic mutations trigger clonal expansion in early neoplasia.

The role of gene mutations and gene products in intestinal tissue reactions from ionising radiation

The response of the intestine to (low linear-energy-transfer) ionising radiation is reviewed regarding the cellular basis to the reactions, the regenerative processes which restore the tissue, and external agents which aid its recovery. In the steady-state, it is generally considered that the crypt cell lineages in both small and large intestine are maintained by a small number of stem cells, but there are differences for example in the composition of their niche residence and in the numbers of transit cell generations. Various cell surface markers are now available to indentify particular lineage cell types. Radiation doses up to 1Gy cause apoptotic stem-cell death in particular locations, at higher doses to >6Gy Lgr5⁺ stem cells are required for normal intestinal recovery, and at >8Gy some crypts are sterilised and the probability of animal death from intestinal injury increases with higher doses. Mutations in repair genes, tumour suppressor genes, and survival genes cause various degrees of stem cell and clonogenic cell radiosensitisation. Recent evidence is suggesting much plasticity in the crypt cell lineage, potentially contributing to flexibility in the hierarchical lineage, clonogen number variations and the sensitisation differences. Knockout mice for many different genes have been used to detect their role in both steady state and in irradiated conditions, expected to lead to further insight to the damage and restorative processes. Many different external agents have been used to ameliorate intestinal reactions, including prostaglandins, interleukins, angiogenic and epithelial growth factors, other cytokines, and intraluminal factors.

Stem Cells in the Intestine: Possible Roles in Pathogenesis of Irritable Bowel Syndrome

Irritable bowel syndrome is one of the most common functional gastrointestinal (GI) disorders that significantly impair quality of life in patients. Current available treatments are still not effective and the pathophysiology of this condition remains unclearly defined. Recently, research on intestinal stem cells has greatly advanced our understanding of various GI disorders. Alterations in conserved stem cell regulatory pathways such as Notch, Wnt, and bone morphogenic protein/TGF-β have been well documented in diseases such as inflammatory bowel diseases and cancer. Interaction between intestinal stem cells and various signals from their environment is important for the control of stem cell self-renewal, regulation of number and function of specific intestinal cell types, and maintenance of the mucosal barrier. Besides their roles in stem cell regulation, these signals are also known to have potent effects on immune cells, enteric nervous system and secretory cells in the gut, and may be responsible for various aspects of pathogenesis of functional GI disorders, including visceral hypersensitivity, altered gut motility and low grade gut inflammation. In this article, we briefly summarize the components of these signaling pathways, how they can be modified by extrinsic factors and novel treatments, and provide evidenced support of their roles in the inflammation processes. Furthermore, we propose how changes in these signals may contribute to the symptom development and pathogenesis of irritable bowel syndrome.

Paneth Cell-Rich Regions Separated by a Cluster of Lgr5+ Cells Initiate Crypt Fission in the Intestinal Stem Cell Niche

The crypts of the intestinal epithelium house the stem cells that ensure the continual renewal of the epithelial cells that line the intestinal tract. Crypt number increases by a process called crypt fission, the division of a single crypt into two daughter crypts. Fission drives normal tissue growth and maintenance. Correspondingly, it becomes less frequent in adulthood. Importantly, fission is reactivated to drive adenoma growth. The mechanisms governing fission are poorly understood. However, only by knowing how normal fission operates can cancer-associated changes be elucidated. We studied normal fission in tissue in three dimensions using high-resolution imaging and used intestinal organoids to identify underlying mechanisms. We discovered that both the number and relative position of Paneth cells and Lgr5+ cells are important for fission. Furthermore, the higher stiffness and increased adhesion of Paneth cells are involved in determining the site of fission. Formation of a cluster of Lgr5+ cells between at least two Paneth-cell-rich domains establishes the site for the upward invagination that initiates fission.

Crypt fission—a process responsible for normal intestinal growth and for the formation of adenomas —is governed by differential adhesion, stiffness, and proliferation of Lgr5+ cells and Paneth cells in the intestinal stem cell niche.

The intestinal tract undergoes many changes during development, and after birth it has to significantly elongate and widen in order to increase the surface area for absorption. Crypt fission is a key process in intestinal tissue expansion and is also involved in adenoma growth. Despite the importance of crypt fission, the mechanisms controlling it are poorly understood. Understanding how crypt fission is regulated in normal tissue can help us to determine how the process changes in cancer. Here, we describe cellular behaviour during crypt fission. We identify a specific cellular arrangement in the intestinal stem cell niche that is associated with crypt fission and reveals insights into the mechanisms controlling crypt fission. There are two different cell types at the crypt base, Lgr5+ and Paneth cells, which play distinct roles in this process. We find that both their location and differences between them in proliferation, stiffness, and adhesion are important for fission. Based on our data, we propose a model in which stiffer and more adhesive Paneth cells are necessary to shape the crypt base and establish where fission occurs, whereas softer Lgr5+ cells allow shape changes and proliferation to expand newly formed crypts. Our model is an important step in understanding how crypt fission is initiated in normal tissue and provides a framework to understand how the process changes in tumorigenesis.

Lgr5-positive cells in the lung and their clinical significance in patients with lung adenocarcinoma

Leucine-rich repeat-containing G-protein coupled receptor 5 (Lgr5) is well-characterized as a marker of intestinal stem cells and certain types of tumor stem cells, where positive cells may develop into intestinal epithelial cells or intestinal adenomas. However, the roles of Lgr5 in the lung and in lung tumors remain unclear. An immunohistochemistry (IHC) analysis was performed to detect the expression of Lgr5 in the lung from a normal mouse. Histopathological sections of the lungs from Lgr5 heterozygous knockout mice (Lgr5^+/−) were observed following with hematoxylin and eosin. Furthermore, tissue microarrays containing tumor cores from lung cancer patients were also analyzed by IHC. Lgr5-positive cells were present in the pulmonary alveoli and bronchi of normal mice, whereas the lungs of Lgr5^+/− mice lost their normal morphological structure compared with the lungs of the normal mice. Lgr5 was expressed in lung adenocarcinoma, however, not in squamous carcinoma, and Lgr5 expression was positively associated with tumor, node, metastasis stage. Lgr5 is expressed in normal murine lung and is associated with TNM stage in patients with lung adenocarcinoma.

An enduring role for quiescent stem cells

The intestine's ability to recover from catastrophic injury requires quiescent intestinal stem cells (q-ISCs). While rapidly cycling (Lgr5+) crypt base columnar (CBC) ISCs normally maintain the intestine, they are highly sensitive to pathological injuries (irradiation, inflammation) and must be restored by q-ISCs to sustain intestinal homeostasis. Despite clear relevance to human health, virtually nothing is known regarding the factors that regulate q-ISCs. A comprehensive understanding of these mechanisms would likely lead to targeted new therapies with profound therapeutic implications for patients with gastrointestinal conditions. We briefly review the current state of the literature, highlighting homeostatic mechanisms important for q-ISC regulation, listing key questions in the field, and offer strategies to address them. Developmental Dynamics 245:718-726, 2016. © 2016 Wiley Periodicals, Inc.

β-Arrestin1 inhibits chemotherapy-induced intestinal stem cell apoptosis and mucositis

The mechanism of chemotherapy-induced gastrointestinal (GI) syndrome (CIGIS) is still controversial, and it is unclear whether chemotherapy induces intestinal stem cell (ISC) apoptosis. β-Arrestins are regulators and mediators of G protein-coupled receptor signaling in cell apoptosis, division and growth. In this study, we aimed to investigate whether chemotherapy induces ISC apoptosis to contribute to mucositis in CIGIS and whether β-arrestin1 (β-arr1) is involved in this apoptosis. Different chemotherapeutic agents were used to generate a CIGIS model. Lgr5-EGFP-IRES-creERT2^+/− knock-in mice were used as a CIGIS model to investigate ISC apoptosis. β-arr1 knockout mice were used to determine whether β-arr1 is involved in the apoptosis in CIGIS. Intestinal histology was performed, the ISC apoptosis was analyzed and the mucosal barrier was examined. The effects of β-arr1 in apoptosis were investigated in the samples from humans and mice as well as in cell lines. Here, we demonstrate that chemotherapy induced intestinal mucositis by promoting crypt cell apoptosis, especially in Lgr5+ stem cells and Paneth cells but not in goblet cells, epithelial cells or vascular endothelial cells. Furthermore, β-arr1 deficiency exacerbated the Lgr5+ stem cell apoptosis, but not Paneth cell apoptosis, in CIGIS. In addition, the data showed that β-arr1 reduced the chemotherapy-induced Lgr5+ stem cell apoptosis by inhibiting endoplasmic reticulum stress-mediated mitochondrial apoptotic signaling. Our study indicates that β-arr1 inhibits chemotherapy-induced ISC apoptosis to alleviate intestinal mucositis in CIGIS.

Defining the cellular lineage hierarchy in the interfollicular epidermis of adult skin

The interfollicular epidermis regenerates from heterogeneous basal skin cell populations that divide at different rates. It has previously been presumed that infrequently dividing basal cells known as label-retaining cells (LRCs) are stem cells, whereas non-LRCs are short-lived progenitors. Here we employ the H2B-GFP pulse-chase system in adult mouse skin and find that epidermal LRCs and non-LRCs are molecularly distinct and can be differentiated by Dlx1(CreER) and Slc1a3(CreER) genetic marking, respectively. Long-term lineage tracing and mathematical modelling of H2B-GFP dilution data show that LRCs and non-LRCs constitute two distinct stem cell populations with different patterns of proliferation, differentiation and upward cellular transport. During homeostasis, these populations are enriched in spatially distinct skin territories and can preferentially produce unique differentiated lineages. On wounding or selective killing, they can temporarily replenish each other's territory. These two discrete interfollicular stem cell populations are functionally interchangeable and intrinsically well adapted to thrive in distinct skin environments.

Fbxw7-associated drug resistance is reversed by induction of terminal differentiation in murine intestinal organoid culture

Colorectal cancer (CRC) is one of the top three cancer-related causes of death worldwide. FBXW7 is a known tumor-suppressor gene, commonly mutated in CRC and in a variety of other epithelial tumors. Low expression of FBXW7 is also associated with poor prognosis. Loss of FBXW7 sensitizes cancer cells to certain drugs, while making them more resistant to other types of chemotherapies. However, is not fully understood how epithelial cells within normal gut and primary tumors respond to potential cancer therapeutics. We have studied genetically engineered mice in which the fbxw7 gene is conditionally knocked-out in the intestine (fbxw7^∆G). To further investigate the mechanism of Fbxw7-action, we grew intestinal crypts from floxed-fbxw7 (fbxw7^fl/fl) and fbxw7^ΔG mice, in a Matrigel-based organoid (mini-gut) culture. The fbxw7^ΔG organoids exhibited rapid budding events in the crypt region. Furthermore, to test organoids for drug response, we exposed day 3 intestinal organoids from fbxw7^fl/fl and fbxw7^∆G mice, to various concentrations of 5-fluorouracil (5-FU) for 72 hours. 5-FU triggers phenotypic differences in organoids including changing shape, survival, resistance, and death. 5-FU however, rescues the drug-resistance phenotype of fbxw7^ΔG through the induction of terminal differentiation. Our results support the hypothesis that a differentiating therapy successfully targets FBXW7-mutated CRC cells.

LIG4 mediates Wnt signalling-induced radioresistance

Despite the implication of Wnt signalling in radioresistance, the underlying mechanisms are unknown. Here we find that high Wnt signalling is associated with radioresistance in colorectal cancer (CRC) cells and intestinal stem cells (ISCs). We find that LIG4, a DNA ligase in DNA double-strand break repair, is a direct target of β-catenin. Wnt signalling enhances non-homologous end-joining repair in CRC, which is mediated by LIG4 transactivated by β-catenin. During radiation-induced intestinal regeneration, LIG4 mainly expressed in the crypts is conditionally upregulated in ISCs, accompanied by Wnt/β-catenin signalling activation. Importantly, among the DNA repair genes, LIG4 is highly upregulated in human CRC cells, in correlation with β-catenin hyperactivation. Furthermore, blocking LIG4 sensitizes CRC cells to radiation. Our results reveal the molecular mechanism of Wnt signalling-induced radioresistance in CRC and ISCs, and further unveils the unexpected convergence between Wnt signalling and DNA repair pathways in tumorigenesis and tissue regeneration.

The Wnt/β-catenin signalling pathway contributes to radio resistance in intestinal stem cells but the underlying mechanism is currently unknown. In this study, the authors demonstrate that LIG4, a DNA ligase involved in the DNA repair process, is a direct target of β-catenin and it specifically mediates non-homologous end joining repair in colorectal cancer cells.

Tissue-specific designs of stem cell hierarchies

Recent work in the field of stem cell biology suggests that there is no single design for an adult tissue stem cell hierarchy, and that different tissues employ distinct strategies to meet their self-renewal and repair requirements. Stem cells may be multipotent or unipotent, and can exist in quiescent or actively dividing states. 'Professional' stem cells may also co-exist with facultative stem cells, which are more specialized daughter cells that revert to a stem cell state under specific tissue damage conditions. Here, we discuss stem cell strategies as seen in three solid mammalian tissues: the intestine, mammary gland and skeletal muscle.

BVES Regulates Intestinal Stem Cell Programs and Intestinal Crypt Viability after Radiation

Blood vessel epicardial substance (BVES/Popdc1) is a junctional-associated transmembrane protein that is underexpressed in a number of malignancies and regulates epithelial-to-mesenchymal transition. We previously identified a role for BVES in regulation of the Wnt pathway, a modulator of intestinal stem cell programs, but its role in small intestinal (SI) biology remains unexplored. We hypothesized that BVES influences intestinal stem cell programs and is critical to SI homeostasis after radiation injury. At baseline, Bves(-/-) mice demonstrated increased crypt height, as well as elevated proliferation and expression of the stem cell marker Lgr5 compared to wild-type (WT) mice. Intercross with Lgr5-EGFP reporter mice confirmed expansion of the stem cell compartment in Bves(-/-) mice. To examine stem cell function after BVES deletion, we used ex vivo 3D-enteroid cultures. Bves(-/-) enteroids demonstrated increased stemness compared to WT, when examining parameters such as plating efficiency, stem spheroid formation, and retention of peripheral cystic structures. Furthermore, we observed increased proliferation, expression of crypt-base columnar "CBC" and "+4" stem cell markers, amplified Wnt signaling, and responsiveness to Wnt activation in the Bves(-/-) enteroids. Bves expression was downregulated after radiation in WT mice. Moreover, after radiation, Bves(-/-) mice demonstrated significantly greater SI crypt viability, proliferation, and amplified Wnt signaling in comparison to WT mice. Bves(-/-) mice also demonstrated elevations in Lgr5 and Ascl2 expression, and putative damage-responsive stem cell populations marked by Bmi1 and TERT. Therefore, BVES is a key regulator of intestinal stem cell programs and mucosal homeostasis. Stem Cells 2016;34:1626-1636.

Bcl-2 is a critical mediator of intestinal transformation

Intestinal tumour formation is generally thought to occur following mutational events in the stem cell pool. However, active NF-κB signalling additionally facilitates malignant transformation of differentiated cells. We hypothesized that genes shared between NF-κB and intestinal stem cell (ISCs) signatures might identify common pathways that are required for malignant growth. Here, we find that the NF-κB target Bcl-2, an anti-apoptotic gene, is specifically expressed in ISCs in both mice and humans. Bcl-2 is dispensable in homeostasis and, although involved in protecting ISCs from radiation-induced damage, it is non-essential in tissue regeneration. Bcl-2 is upregulated in adenomas, and its loss or inhibition impairs outgrowth of oncogenic clones, because Bcl-2 alleviates apoptotic priming in epithelial cells following Apc loss. Furthermore, Bcl-2 expression in differentiated epithelial cells renders these cells amenable to clonogenic outgrowth. Collectively, our results indicate that Bcl-2 is required for efficient intestinal transformation following Apc-loss and constitutes a potential chemoprevention target.

Replacement of Lost Lgr5-Positive Stem Cells through Plasticity of Their Enterocyte-Lineage Daughters

Intestinal crypts display robust regeneration upon injury. The relatively rare secretory precursors can replace lost stem cells, but it is unknown if the abundant enterocyte progenitors that express the Alkaline phosphate intestinal (Alpi) gene also have this capacity. We created an Alpi-IRES-CreERT2 (Alpi(CreER)) knockin allele for lineage tracing. Marked clones consist entirely of enterocytes and are all lost from villus tips within days. Genetic fate-mapping of Alpi(+) cells before or during targeted ablation of Lgr5-expressing stem cells generated numerous long-lived crypt-villus "ribbons," indicative of dedifferentiation of enterocyte precursors into Lgr5(+) stems. By single-cell analysis of dedifferentiating enterocytes, we observed the generation of Paneth-like cells and proliferative stem cells. We conclude that the highly proliferative, short-lived enterocyte precursors serve as a large reservoir of potential stem cells during crypt regeneration.

Human mesenchymal stem cells promote survival and prevent intestinal damage in a mouse model of radiation injury

In the present study, we examined the protective effects of human umbilical cord mesenchymal stem cells (hMSCs) against intestinal stem cell (ISC) death and intestinal damage in a mouse model of radiation injury.

Homeostatic responses of colonic LGR5+ stem cells following acute in vivo exposure to a genotoxic carcinogen

Perturbations in DNA damage, DNA repair, apoptosis and cell proliferation in the base of the crypt where stem cells reside are associated with colorectal cancer (CRC) initiation and progression. Although the transformation of leucine-rich repeat-containing G protein-coupled receptor 5 (Lgr5)(+) cells is an extremely efficient route towards initiating small intestinal adenomas, the role of Lgr5(+) cells in CRC pathogenesis has not been well investigated. Therefore, we further characterized the properties of colonic Lgr5(+) cells compared to differentiated cells in Lgr5-EGFP-IRES-creER(T2) knock-in mice at the initiation stage of carcinogen azoxymethane (AOM)-induced tumorigenesis using a quantitative immunofluorescence microscopy approach. At 12 and 24h post-AOM treatment, colonic Lgr5(+) stem cells (GFP(high)) were preferentially damaged by carcinogen, exhibiting a 4.7-fold induction of apoptosis compared to differentiated (GFP(neg)) cells. Furthermore, with respect to DNA repair, O(6)-methylguanine DNA methyltransferase (MGMT) expression was preferentially induced (by 18.5-fold) in GFP(high) cells at 24h post-AOM treatment compared to GFP(neg) differentiated cells. This corresponded with a 4.3-fold increase in cell proliferation in GFP(high) cells. These data suggest that Lgr5(+) stem cells uniquely respond to alkylation-induced DNA damage by upregulating DNA damage repair, apoptosis and cell proliferation compared to differentiated cells in order to maintain genomic integrity. These findings highlight the mechanisms by which colonic Lgr5(+) stem cells respond to cancer-causing environmental factors.

Interleukin-22 promotes intestinal-stem-cell-mediated epithelial regeneration

Epithelial regeneration is critical for barrier maintenance and organ function after intestinal injury. The intestinal stem cell (ISC) niche provides Wnt, Notch and epidermal growth factor (EGF) signals supporting Lgr5(+) crypt base columnar ISCs for normal epithelial maintenance. However, little is known about the regulation of the ISC compartment after tissue damage. Using ex vivo organoid cultures, here we show that innate lymphoid cells (ILCs), potent producers of interleukin-22 (IL-22) after intestinal injury, increase the growth of mouse small intestine organoids in an IL-22-dependent fashion. Recombinant IL-22 directly targeted ISCs, augmenting the growth of both mouse and human intestinal organoids, increasing proliferation and promoting ISC expansion. IL-22 induced STAT3 phosphorylation in Lgr5(+) ISCs, and STAT3 was crucial for both organoid formation and IL-22-mediated regeneration. Treatment with IL-22 in vivo after mouse allogeneic bone marrow transplantation enhanced the recovery of ISCs, increased epithelial regeneration and reduced intestinal pathology and mortality from graft-versus-host disease. ATOH1-deficient organoid culture demonstrated that IL-22 induced epithelial regeneration independently of the Paneth cell niche. Our findings reveal a fundamental mechanism by which the immune system is able to support the intestinal epithelium, activating ISCs to promote regeneration.

Dormant Intestinal Stem Cells Are Regulated by PTEN and Nutritional Status

The cellular and molecular mechanisms underlying adaptive changes to physiological stress within the intestinal epithelium remain poorly understood. Here, we show that PTEN, a negative regulator of the PI3K→AKT→mTORC1-signaling pathway, is an important regulator of dormant intestinal stem cells (d-ISCs). Acute nutrient deprivation leads to transient PTEN phosphorylation within d-ISCs and a corresponding increase in their number. This release of PTEN inhibition renders d-ISCs functionally poised to contribute to the regenerative response during re-feeding via cell-autonomous activation of the PI3K→AKT→mTORC1 pathway. Consistent with its role in mediating cell survival, PTEN is required for d-ISC maintenance at baseline, and intestines lacking PTEN have diminished regenerative capacity after irradiation. Our results highlight a PTEN-dependent mechanism for d-ISC maintenance and further demonstrate the role of d-ISCs in the intestinal response to stress.

Fasting protects mice from lethal DNA damage by promoting small intestinal epithelial stem cell survival

Short-term fasting protects mice from lethal doses of chemotherapy through undetermined mechanisms. Herein, we demonstrate that fasting preserves small intestinal (SI) architecture by maintaining SI stem cell viability and SI barrier function following exposure to high-dose etoposide. Nearly all SI stem cells were lost in fed mice, whereas fasting promoted sufficient SI stem cell survival to preserve SI integrity after etoposide treatment. Lineage tracing demonstrated that multiple SI stem cell populations, marked by Lgr5, Bmi1, or HopX expression, contributed to fasting-induced survival. DNA repair and DNA damage response genes were elevated in SI stem/progenitor cells of fasted etoposide-treated mice, which importantly correlated with faster resolution of DNA double-strand breaks and less apoptosis. Thus, fasting preserved SI stem cell viability as well as SI architecture and barrier function suggesting that fasting may reduce host toxicity in patients undergoing dose intensive chemotherapy.

Intestinal stem cells and inflammation

The intestinal epithelium is renewed every 3-5 days from at least two principal stem cell pools. Actively cycling crypt based columnar (CBC) Lgr5(+) cells and slower cycling Bmi1-expressing or Krt19-expressing cells maintain the small intestinal and colonic epithelium in homeostasis and injury. Following acute epithelial damage, Lgr5+ stem cells are susceptible to injury and a reserve stem cell or progenitor pool is responsible for regeneration of the epithelium. Current data suggests that intestinal stem cells respond to inflammatory signals to modulate their expansion during epithelial regeneration. Here, we review how inflammation and injury affect intestinal and colonic stem cells.

mTOR disruption causes intestinal epithelial cell defects and intestinal atrophy postinjury in mice

Intestinal stem cells (ISCs) drive small intestinal epithelial homeostasis and regeneration. Mechanistic target of rapamycin (mTOR) regulates stem and progenitor cell metabolism and is frequently dysregulated in human disease, but its physiologic functions in the mammalian small intestinal epithelium remain poorly defined. We disrupted the genes mTOR, Rptor, Rictor, or both Rptor and Rictor in mouse ISCs, progenitors, and differentiated intestinal epithelial cells (IECs) using Villin-Cre. Mutant tissues and wild-type or heterozygous littermate controls were analyzed by histologic immunostaining, immunoblots, and proliferation assays. A total of 10 Gy irradiation was used to injure the intestinal epithelium and induce subsequent crypt regeneration. We report that mTOR supports absorptive enterocytes and secretory Paneth and goblet cell function while negatively regulating chromogranin A-positive enteroendocrine cell number. Through additional Rptor, Rictor, and Rptor/Rictor mutant mouse models, we identify mechanistic target of rapamycin complex 1 as the major IEC regulatory pathway, but mechanistic target of rapamycin complex 2 also contributes to ileal villus maintenance and goblet cell size. Homeostatic adult small intestinal crypt cell proliferation, survival, and canonical wingless-int (WNT) activity are not mTOR dependent, but Olfm4(+) ISC/progenitor population maintenance and crypt regeneration postinjury require mTOR. Overall, we conclude that mTOR regulates multiple IEC lineages and promotes stem and progenitor cell activity during intestinal epithelium repair postinjury.

Mensenchymal stem cells can delay radiation-induced crypt death: impact on intestinal CD44(+) fragments

Intestinal stem cells are primitive cells found within the intestinal epithelium that play a central role in maintaining epithelial homeostasis through self-renewal and commitment into functional epithelial cells. Several markers are available to identify intestinal stem cells, such as Lgr5, CD24 and EphB2, which can be used to sort intestinal stem cells from mammalian gut. Here, we identify and isolate intestinal stem cells from C57BL/6 mice by using a cell surface antigen, CD44. In vitro, some CD44⁺ crypt cells are capable of forming “villus-crypt”–like structures (organoids). A subset strongly positive for CD44 expresses high levels of intestinal stem-cell-related genes, including Lgr5, Bmi1, Hopx, Lrig1, Ascl2, Smoc2 and Rnf43. Cells from this subset are more capable of developing into organoids in vitro, compared with the subset weakly positive for CD44. However, the organoids are sensitive to ionizing irradiation. We investigate the specific roles of mesenchymal stem cells in protecting organoids against radiation-induced crypt death. When co-cultured with mesenchymal stem cells, the crypt domains of irradiated organoids possess more proliferative cells and fewer apoptotic cells than those not co-cultured with mesenchymal stem cells. Cd44v6 continues to be expressed in the crypt domains of irradiated organoids co-cultured with mesenchymal stem cells. Our results indicate specific roles of mesenchymal stem cells in delaying radiation-induced crypt death in vitro.

Agonists of the TRAIL Death Receptor DR5 Sensitize Intestinal Stem Cells to Chemotherapy-Induced Cell Death and Trigger Gastrointestinal Toxicity

The combination of TRAIL death receptor agonists and radiochemotherapy to treat advanced cancers continues to be investigated in clinical trials. We previously showed that normal cells with a functional DNA damage response (DDR) upregulate the expression of death-inducing receptor DR5/TRAILR2/TNFRSF10B in a p53-dependent manner that sensitizes them to treatment with DR5 agonists. However, it is unclear if targeting DR5 selectively sensitizes cancer cells to agonist treatment following exposure to DNA-damaging chemotherapy, and to what extent normal tissues are targeted. Here, we show that the combined administration of the DR5 agonistic monoclonal antibody (mAb) and chemotherapy to wild-type mice triggered synergistic gastrointestinal toxicities (GIT) that were associated with the death of Lgr5(+) crypt base columnar stem cells in a p53- and DR5-dependent manner. Furthermore, we confirmed that normal human epithelial cells treated with the human DR5-agonistic mAb and chemotherapeutic agents were also greatly sensitized to cell death. Interestingly, our data also indicated that genetic or pharmacologic targeting of Chk2 may counteract GIT without negatively affecting the antitumor responses of combined DR5 agonist/chemotherapy treatment, further linking the DDR to TRAIL death receptor signaling in normal cells. In conclusion, the combination of DR5-targeting agonistic mAbs with DNA damaging chemotherapy may pose a risk of developing toxicity-induced conditions, and the effects of mAb-based strategies on the dose-limiting toxicity of chemotherapy must be considered when establishing new combination therapies.

Elp3 drives Wnt-dependent tumor initiation and regeneration in the intestine

Ladang et al. report that Elp3, a subunit of the Elongator complex, is induced by Wnt signaling and is required to initiate colon cancer development through the regulation of Sox9 translation. They also show that this mechanism is relevant in radiation-induced intestinal regeneration.

Tumor initiation in the intestine can rapidly occur from Lgr5⁺ crypt columnar stem cells. Dclk1 is a marker of differentiated Tuft cells and, when coexpressed with Lgr5, also marks intestinal cancer stem cells. Here, we show that Elp3, the catalytic subunit of the Elongator complex, is required for Wnt-driven intestinal tumor initiation and radiation-induced regeneration by maintaining a subpool of Lgr5⁺/Dclk1⁺/Sox9⁺ cells. Elp3 deficiency dramatically delayed tumor appearance in Apc-mutated intestinal epithelia and greatly prolonged mice survival without affecting the normal epithelium. Specific ablation of Elp3 in Lgr5⁺ cells resulted in marked reduction of polyp formation upon Apc inactivation, in part due to a decreased number of Lgr5⁺/Dclk1⁺/Sox9⁺ cells. Mechanistically, Elp3 is induced by Wnt signaling and promotes Sox9 translation, which is needed to maintain the subpool of Lgr5⁺/Dclk1⁺ cancer stem cells. Consequently, Elp3 or Sox9 depletion led to similar defects in Dclk1⁺ cancer stem cells in ex vivo organoids. Finally, Elp3 deficiency strongly impaired radiation-induced intestinal regeneration, in part because of decreased Sox9 protein levels. Together, our data demonstrate the crucial role of Elp3 in maintaining a subpopulation of Lgr5-derived and Sox9-expressing cells needed to trigger Wnt-driven tumor initiation in the intestine.

Novel regenerative peptide TP508 mitigates radiation-induced gastrointestinal damage by activating stem cells and preserving crypt integrity

In recent years, increasing threats of radiation exposure and nuclear disasters have become a significant concern for the United States and countries worldwide. Exposure to high doses of radiation triggers a number of potentially lethal effects. Among the most severe is the gastrointestinal (GI) toxicity syndrome caused by the destruction of the intestinal barrier, resulting in bacterial translocation, systemic bacteremia, sepsis, and death. The lack of effective radioprotective agents capable of mitigating radiation-induced damage has prompted a search for novel countermeasures that can mitigate the effects of radiation post exposure, accelerate tissue repair in radiation-exposed individuals, and prevent mortality. We report that a single injection of regenerative peptide TP508 (rusalatide acetate, Chrysalin) 24 h after lethal radiation exposure (9 Gy, LD100/15) appears to significantly increase survival and delay mortality by mitigating radiation-induced intestinal and colonic toxicity. TP508 treatment post exposure prevents the disintegration of GI crypts, stimulates the expression of adherens junction protein E-cadherin, activates crypt cell proliferation, and decreases apoptosis. TP508 post-exposure treatment also upregulates the expression of DCLK1 and LGR5 markers of stem cells that have been shown to be responsible for maintaining and regenerating intestinal crypts. Thus, TP508 appears to mitigate the effects of GI toxicity by activating radioresistant stem cells and increasing the stemness potential of crypts to maintain and restore intestinal integrity. These results suggest that TP508 may be an effective emergency nuclear countermeasure that could be delivered within 24 h post exposure to increase survival and delay mortality, giving victims time to reach clinical sites for advanced medical treatment.