Epithelial WNT Ligands Are Essential Drivers of Intestinal Stem Cell Activation

Loss of mucin 2 and MHC II molecules causes rare resistance to murine RV infection

Enteric pathogen rotavirus (RV) primarily infects mature enterocytes at the tips of the intestinal villi; however, the role of secretory Paneth and goblet cells in RV pathogenesis remains unappreciated. Atoh1 knockout mice (Atoh1cKO) were used to conditionally delete Paneth, goblet, and enteroendocrine cells in the epithelium to investigate the role of secretory cells in RV infection. Unexpectedly, the number of infected enterocytes and the amount of RV shedding in the stool were greatly decreased following secretory cell deletion. Resistance to RV infection persisted for 7 days after virus inoculation, and Atoh1 knockout mice co-housed with infected wild-type mice were uninfected, based on lack of shedding virus, despite the highly infectious nature of RV. This response was directly proportional to the extent of secretory cell deletion, with infection predominantly occurring in areas containing intact secretory cells. RV infection of Muc2 knockout mice recapitulated the secretory cell deletion phenotype, indicating that goblet cell loss is responsible for attenuated infection. Transcriptome analysis of Atoh1cKO intestine via single-cell RNA sequencing revealed downregulation of MHC II molecules specifically in tip enterocytes, and MHC II-/- mice were likewise resistant to RV infection. These data suggest a previously unknown role for both MUC2 and MHC II expression in susceptibility to RV infection.IMPORTANCERotavirus (RV) is a highly contagious pathogen that primarily infects mature intestinal enterocytes. Murine rotavirus readily infects infant and adult mice, enabling evaluation of RV infection and immunity. We report that mice lacking secretory cells are one of the few genetically modified mouse lines not susceptible to murine rotavirus. Further investigation revealed loss of mucin 2 (MUC2) expression or major histocompatibility complex II (MCH II) expression recapitulated this rare resistance to rotavirus infection, suggesting a previously unrecognized link between secretory cell products and major histocompatibility complex II expression. Furthermore, these mouse models provide a platform to investigate rotavirus pathogenesis.

Paneth Cells: Dispensable yet Irreplaceable for the Intestinal Stem Cell Niche

Intestinal stem cells replenish the epithelium throughout life by continuously generating intestinal epithelial cell types, including absorptive enterocytes, and secretory goblet, endocrine, and Paneth cells. This process is orchestrated by a symphony of niche factors required to maintain intestinal stem cells and to direct their proliferation and differentiation. Among the various mature intestinal epithelial cell types, Paneth cells are unique in their location in the stem cell zone, directly adjacent to intestinal stem cells. Although Paneth cells were first described as an epithelial cell component of the innate immune system due to their expression of anti-microbial peptides, they have been proposed to be niche cells due to their close proximity to intestinal stem cells and expression of niche factors. However, function as a niche cell has been debated since mice lacking Paneth cells retain functional stem cells that continue to replenish the intestinal epithelium. In this review, we summarize the intestinal stem cell niche, including the Notch, Wnt, growth factor, mechanical, and metabolic niche, and discuss how Paneth cells might contribute to these various components. We also present a nuanced view of the Paneth cell as a niche cell. Although not required, Paneth cells enhance stem cell function, particularly during intestinal development and regeneration. Furthermore, we suggest that Paneth cell loss induces intestinal stem cell remodeling to adjust their niche demands.

Dynamic RNA polymerase II occupancy drives differentiation of the intestine under the direction of HNF4

Terminal differentiation requires massive restructuring of the transcriptome. During intestinal differentiation, the expression patterns of nearly 4,000 genes are altered as cells transition from progenitor cells in crypts to differentiated cells in villi. We identify dynamic occupancy of RNA polymerase II (Pol II) to gene promoters as the primary driver of transcriptomic shifts during intestinal differentiation in vivo. Changes in enhancer-promoter looping interactions accompany dynamic Pol II occupancy and are dependent upon HNF4, a pro-differentiation transcription factor. Using genetic loss-of-function, chromatin immunoprecipitation sequencing (ChIP-seq), and immunoprecipitation (IP) mass spectrometry, we demonstrate that HNF4 collaborates with chromatin remodelers and loop-stabilizing proteins and facilitates Pol II occupancy at hundreds of genes pivotal to differentiation. We also explore alternate mechanisms that drive differentiation gene expression and find that pause-release of Pol II and post-transcriptional mRNA stability regulate smaller subsets of differentially expressed genes. These studies provide insights into the mechanisms of differentiation in renewing adult tissue.

Matrine reduced intestinal stem cell damage in eimeria necatrix-infected chicks via blocking hyperactivation of Wnt signaling

BACKGROUND

Coccidiosis is a rapidly spreading and acute parasitic disease that seriously threatening the intestinal health of poultry. Matrine from leguminous plants has anthelmintic and anti-inflammatory properties.

PURPOSE

This assay was conducted to explore the protective effects of Matrine and the AntiC (a Matrine compound) on Eimeria necatrix (EN)-infected chick small intestines and to provide a nutritional intervention strategy for EN injury.

STUDY DESIGN

The in vivo (chick) experiment: A total of 392 one-day-old yellow-feathered broilers were randomly assigned to six groups in a 21-day study: control group, 350 mg/kg Matrine group, 500 mg/kg AntiC group, EN group, and EN + 350 mg/kg Matrine group, EN + 500 mg/kg AntiC group. The in vitro (chick intestinal organoids, IOs): The IOs were treated with PBS, Matrine, AntiC, 3 μM CHIR99021, EN (15,000 EN sporozoites), EN + Matrine, EN + AntiC, EN + Matrine + CHIR99021, EN + AntiC + CHIR99021.

METHODS

The structural integrity of chicks jejunal crypt-villus axis was evaluated by hematoxylin and eosin (H&E) staining and transmission electron microscopy (TEM). And the activity of intestinal stem cells (ISCs) located in crypts was assessed by in vitro expansion advantages of a primary in IOs model. Then, the changes of Wnt/β-catenin signaling in jejunal tissues and IOs were detected by Real-Time qPCR,Western blotting and immunohistochemistry.

RESULTS

The results showed that dietary supplementation with Matrine or AntiC rescued the jejunal injury caused by EN, as indicated by increased villus height, reduced crypt hyperplasia, and enhanced expression of tight junction proteins. Moreover, there was less budding efficiency of the IOs expanded from jejunal crypts of chicks in the EN group than that in the Matrine and AntiC group, respectively. Further investigation showed that AntiC and Matrine inhibited EN-stimulated Wnt/β-catenin signaling. The fact that Wnt/β-catenin activation via CHIR99021 led to the failure of Matrine and AntiC to rescue damaged ISCs confirmed the dominance of this signaling.

CONCLUSION

Our results suggest that Matrine and AntiC inhibit ISC proliferation and promote ISC differentiation into absorptive cells by preventing the hyperactivation of Wnt/β-catenin signaling, thereby standardizing the function of ISC proliferation and differentiation, which provides new insights into mitigating EN injury by Matrine and AntiC.

Mulberry Leaf-Derived Morin Activates β-Catenin by Binding to Frizzled7 to Promote Intestinal Stem Cell Expansion upon Heat-Stable Enterotoxin b Injury

Intestinal stem cells (ISCs) sustain epithelial renewal by dynamically altering behaviors of proliferation and differentiation in response to various nutrition and stress inputs. However, how ISCs integrate bioactive substance morin cues to protect against heat-stable enterotoxin b (STb) produced by Escherichia coli remains an uncertain question with implications for treating bacterial diarrhea. Our recent work showed that oral mulberry leaf-derived morin improved the growth performance in STb-challenged mice. Furthermore, morin supplementation reinstated the impaired small-intestinal epithelial structure and barrier function by stimulating ISC proliferation and differentiation as well as supporting intestinal organoid expansion ex vivo. Importantly, the Wnt/β-catenin pathway, an ISC fate commitment signal, was reactivated by morin to restore the jejunal crypt-villus architecture in response to STb stimulation. Mechanically, the extracellular morin-initiated β-catenin axis is dependent or partially dependent on the Wnt membrane receptor Frizzled7 (FZD7). Our data reveal an unexpected role of leaf-derived morin, which represents molecular signaling targeting the FZD7 platform instrumental for controlling ISC regeneration upon STb injury.

The evolving roles of Wnt signaling in stem cell proliferation and differentiation, the development of human diseases, and therapeutic opportunities

The evolutionarily conserved Wnt signaling pathway plays a central role in development and adult tissue homeostasis across species. Wnt proteins are secreted, lipid-modified signaling molecules that activate the canonical (β-catenin dependent) and non-canonical (β-catenin independent) Wnt signaling pathways. Cellular behaviors such as proliferation, differentiation, maturation, and proper body-axis specification are carried out by the canonical pathway, which is the best characterized of the known Wnt signaling paths. Wnt signaling has emerged as an important factor in stem cell biology and is known to affect the self-renewal of stem cells in various tissues. This includes but is not limited to embryonic, hematopoietic, mesenchymal, gut, neural, and epidermal stem cells. Wnt signaling has also been implicated in tumor cells that exhibit stem cell-like properties. Wnt signaling is crucial for bone formation and presents a potential target for the development of therapeutics for bone disorders. Not surprisingly, aberrant Wnt signaling is also associated with a wide variety of diseases, including cancer. Mutations of Wnt pathway members in cancer can lead to unchecked cell proliferation, epithelial-mesenchymal transition, and metastasis. Altogether, advances in the understanding of dysregulated Wnt signaling in disease have paved the way for the development of novel therapeutics that target components of the Wnt pathway. Beginning with a brief overview of the mechanisms of canonical and non-canonical Wnt, this review aims to summarize the current knowledge of Wnt signaling in stem cells, aberrations to the Wnt pathway associated with diseases, and novel therapeutics targeting the Wnt pathway in preclinical and clinical studies.

The secreted protein Amuc_1409 from Akkermansia muciniphila improves gut health through intestinal stem cell regulation

Akkermansia muciniphila has received great attention because of its beneficial roles in gut health by regulating gut immunity, promoting intestinal epithelial development, and improving barrier integrity. However, A. muciniphila-derived functional molecules regulating gut health are not well understood. Microbiome-secreted proteins act as key arbitrators of host-microbiome crosstalk through interactions with host cells in the gut and are important for understanding host-microbiome relationships. Herein, we report the biological function of Amuc_1409, a previously uncharacterised A. muciniphila-secreted protein. Amuc_1409 increased intestinal stem cell (ISC) proliferation and regeneration in ex vivo intestinal organoids and in vivo models of radiation- or chemotherapeutic drug-induced intestinal injury and natural aging with male mice. Mechanistically, Amuc_1409 promoted E-cadherin/β-catenin complex dissociation via interaction with E-cadherin, resulting in the activation of Wnt/β-catenin signaling. Our results demonstrate that Amuc_1409 plays a crucial role in intestinal homeostasis by regulating ISC activity in an E-cadherin-dependent manner and is a promising biomolecule for improving and maintaining gut health.

Glutamine Promotes Porcine Intestinal Epithelial Cell Proliferation through the Wnt/β-Catenin Pathway

Glutamine (Gln) is a critical nutrient required by neonatal mammals for intestinal growth, especially for newborn piglets. However, the mechanisms underlying the role of Gln in porcine intestinal epithelium development are not fully understood. The objective of the current study was to explore the possible signaling pathway involved in the promotion of porcine intestinal epithelial cell (IPEC-J2) proliferation by Gln. The results showed that 1 mM Gln promoted IPEC-J2 cell proliferation, and tandem mass tag proteomics revealed 973 differentially expressed proteins in Gln-treated IPEC-J2 cells, 824 of which were upregulated and 149 of which were downregulated. Moreover, gene set enrichment analysis indicated that the Wnt signaling pathway is activated by Gln treatment. Western blotting analysis further confirmed that Gln activated the Wnt/β-catenin signaling pathway. In addition, Gln increased not only cytosolic β-catenin but also nuclear β-catenin protein expression. LF3 (a β-catenin/TCF4 interaction inhibitor) assay and β-catenin knockdown demonstrated that Gln-mediated promotion of Wnt/β-catenin signaling and cell proliferation were blocked. Furthermore, the inhibition of TCF4 expression suppressed Gln-induced cell proliferation. These findings further confirmed that Wnt/β-catenin signaling is involved in the promotion of IPEC-J2 cell proliferation by Gln. Collectively, these findings demonstrated that Gln positively regulated IPEC-J2 cell proliferation through the Wnt/β-catenin pathway. These data greatly enhance the current understanding of the mechanism by which Gln regulates intestinal development.

Dynamic RNA Polymerase II Recruitment Drives Differentiation of the Intestine under the direction of HNF4

Terminal differentiation requires a massive restructuring of the transcriptome. During intestinal differentiation, the expression patterns of nearly 4000 genes are altered as cells transition from progenitor cells in crypts to differentiated cells in villi. We identified dynamic recruitment of RNA Polymerase II (Pol II) to gene promoters as the primary driver of transcriptomic shifts during intestinal differentiation in vivo. Changes in enhancer-promoter looping interactions accompany dynamic Pol II recruitment and are dependent upon HNF4, a pro-differentiation transcription factor. Using genetic loss-of- function, ChIP-seq and IP mass spectrometry, we demonstrate that HNF4 collaborates with chromatin remodelers and loop-stabilizing proteins and facilitates Pol II recruitment at hundreds of genes pivotal to differentiation. We also explore alternate mechanisms which drive differentiation gene expression and find pause-release of Pol II and post- transcriptional mRNA stability regulate smaller subsets of differentially expressed genes. These studies provide insights into the mechanisms of differentiation in a renewing adult tissue.

Human intestinal organoids from Cronkhite-Canada syndrome patients reveal link between serotonin and proliferation

Cronkhite-Canada Syndrome (CCS) is a rare, noninherited polyposis syndrome affecting 1 in every million individuals. Despite over 50 years of CCS cases, the etiopathogenesis and optimal treatment for CCS remains unknown due to the rarity of the disease and lack of model systems. To better understand the etiology of CCS, we generated human intestinal organoids (HIOs) from intestinal stem cells isolated from 2 patients. We discovered that CCS HIOs are highly proliferative and have increased numbers of enteroendocrine cells producing serotonin (also known as 5-hydroxytryptamine or 5HT). These features were also confirmed in patient tissue biopsies. Recombinant 5HT increased proliferation of non-CCS donor HIOs and inhibition of 5HT production in the CCS HIOs resulted in decreased proliferation, suggesting a link between local epithelial 5HT production and control of epithelial stem cell proliferation. This link was confirmed in genetically engineered HIOs with an increased number of enteroendocrine cells. This work provides a new mechanism to explain the pathogenesis of CCS and illustrates the important contribution of HIO cultures to understanding disease etiology and in the identification of novel therapies. Our work demonstrates the principle of using organoids for personalized medicine and sheds light on how intestinal hormones can play a role in intestinal epithelial proliferation.

The potential mechanism of qinghua quyu jianpi decoction in the treatment of ulcerative colitis based on network pharmacology and experimental validation

ETHNOPHARMACOLOGICAL RELEVANCE

Ulcerative colitis (UC) is a chronic and recurrent inflammation of the gastrointestinal tract. Following the idea of herbal property and compatibility, a traditional Chinese medicine (TCM) formula consists of a number of TCM herbs. Qinghua Quyu Jianpi Decoction (QQJD) has been clinically proven to be effective in treating UC, however, its therapeutic mechanism has not been fully elucidated.

AIM OF STUDY

Here, we used network pharmacology analysis and ultra-performance liquid chromatography-tandem mass spectrometry to predict the mechanism of action of QQJD, and then validated our predictions through in vivo and in vitro experiments.

MATERIALS AND METHODS

First, based on a number of datasets, relationship network diagrams between QQJD and UC were created. The target network for the QQJD-UC intersection genes was then built, and KEGG analysis was carried out to identify a potential pharmacological mechanism. Finally, the results of the previous prediction were validated in dextran sulfate sodium salt (DSS) induced UC mice and a cellular inflammatory model.

RESULTS

Network pharmacology results suggested that QQJD may play a role in repairing intestinal mucosa by activating Wnt pathway. In vivo experiments have shown that QQJD can significantly reduce weight loss, disease activity index (DAI) score, improve colon length, and effectively repair the tissue morphology of UC mice. In addition, we also found that QQJD can activate the Wnt pathway to promote epithelial cell renewal, reduce apoptosis, and repair the mucosal barrier. To further understand how QQJD promotes cell proliferation in DSS-induced Caco-2 cells, we performed a study in vitro experiment. We were surprised to find that QQJD activated the Wnt pathway by inducing nuclear translocation of β-catenin, accelerating the cell cycle and promoting cell proliferation in vitro.

CONCLUSION

Taken together, network pharmacology and experiments showed that QQJD achieves mucosal healing and restores the colonic epithelium barrier by activating Wnt/β-catenin signaling, regulating cell cycle progression, and promoting the proliferation of epithelial cells.

Modulation of stem cell fate in intestinal homeostasis, injury and repair

The mammalian intestinal epithelium constitutes the largest barrier against the external environment and makes flexible responses to various types of stimuli. Epithelial cells are fast-renewed to counteract constant damage and disrupted barrier function to maintain their integrity. The homeostatic repair and regeneration of the intestinal epithelium are governed by the Lgr5⁺ intestinal stem cells (ISCs) located at the base of crypts, which fuel rapid renewal and give rise to the different epithelial cell types. Protracted biological and physicochemical stress may challenge epithelial integrity and the function of ISCs. The field of ISCs is thus of interest for complete mucosal healing, given its relevance to diseases of intestinal injury and inflammation such as inflammatory bowel diseases. Here, we review the current understanding of the signals and mechanisms that control homeostasis and regeneration of the intestinal epithelium. We focus on recent insights into the intrinsic and extrinsic elements involved in the process of intestinal homeostasis, injury, and repair, which fine-tune the balance between self-renewal and cell fate specification in ISCs. Deciphering the regulatory machinery that modulates stem cell fate would aid in the development of novel therapeutics that facilitate mucosal healing and restore epithelial barrier function.

Paneth cell development in the neonatal gut: pathway regulation, development, and relevance to necrotizing enterocolitis

Paneth cells (PCs) are intestinal epithelial cells (IECs) that contain eosinophilic granules, which are located in Lieberkühn crypts. An increasing number of animal and human experiments have indicated that PCs are involved in the progression of a variety of intestinal as well as systemic inflammatory responses including necrotizing enterocolitis (NEC). NEC is an enteric acquired disease with high mortality that usually occurs in premature infants and neonates, however the underlying mechanisms remain unclear. In this review, we summarize the features of PCs, including their immune function, association with gut microbiota and intestinal stem cells, and their mechanism of regulating IEC death to explore the possible mechanisms by which PCs affect NEC.

Early weaning causes small intestinal atrophy by inhibiting the activity of intestinal stem cells: involvement of Wnt/β-catenin signaling

BACKGROUND

Early weaning and shorter breastfeeding duration are applied by a proportion of young mothers, especially in the social spheres of poverty-stricken areas. Early childhood is a critical period for intestinal development, which is driven by intestinal stem cells (ISCs). However, how early weaning practice affects the function of ISCs to mediate intestinal development remains unclear.

METHODS

We established an excellent early weaning mice model that has significant intestinal atrophy and growth arrest symptoms to explore the responses of ISCs to early weaning. The primary and passaged intestinal organoids from the suckling or early weaning mice were cultured to explore the underlying mechanism of early weaning affecting the ISCs.

RESULTS

Early weaning depressed the self-renewal of ISCs and attenuated the activity of ISCs-driven intestinal epithelial regeneration and crypt expansion in vivo and ex-vivo. Further results showed that early weaning retarded the differentiation of ISCs into transit-amplifying cells and Paneth cells, and accelerated the apoptosis of villous epithelial cells, jointly leading to intestinal epithelial atrophy. Mechanistically, early weaning inhibited Wnt signaling in ISCs, while an exogenous Wnt amplifier restored ISCs' function in ex-vivo.

CONCLUSION

Our findings indicate that early weaning depresses the activity of ISCs via attenuating Wnt/β-catenin signaling and triggers the proinflammatory cytokines TNF-α, IL-1β, IL-6, and IL-17 in jejunum, thereby impeding ISCs-driven epithelial regeneration and intestinal growth, which may provide a basal theory for the development of infant nutrients targeting stem cells to alleviate early weaning-induced intestinal problems.

A DLG1-ARHGAP31-CDC42 axis is essential for the intestinal stem cell response to fluctuating niche Wnt signaling

A central factor in the maintenance of tissue integrity is the response of stem cells to variations in the levels of niche signals. In the gut, intestinal stem cells (ISCs) depend on Wnt ligands for self-renewal and proliferation. Transient increases in Wnt signaling promote regeneration after injury or in inflammatory bowel diseases, whereas constitutive activation of this pathway leads to colorectal cancer. Here, we report that Discs large 1 (Dlg1), although dispensable for polarity and cellular turnover during intestinal homeostasis, is required for ISC survival in the context of increased Wnt signaling. RNA sequencing (RNA-seq) and genetic mouse models demonstrated that DLG1 regulates the cellular response to increased canonical Wnt ligands. This occurs via the transcriptional regulation of Arhgap31, a GTPase-activating protein that deactivates CDC42, an effector of the non-canonical Wnt pathway. These findings reveal a DLG1-ARHGAP31-CDC42 axis that is essential for the ISC response to increased niche Wnt signaling.

Differential Response to the Course of Cryptosporidium parvum Infection and Its Impact on Epithelial Integrity in Differentiated versus Undifferentiated Human Intestinal Enteroids

Cryptosporidium is a leading cause of diarrhea and death in young children and untreated AIDS patients and causes waterborne outbreaks. Pathogenic mechanisms underlying diarrhea and intestinal dysfunction are poorly understood. We previously developed stem-cell derived human intestinal enteroid (HIE) models for Cryptosporidium parvum which we used in this study to investigate the course of infection and its effect on intestinal epithelial integrity. By immunofluorescence and confocal microscopy, there was robust infection of undifferentiated and differentiated HIEs in two and three-dimensional (2D, 3D) models. Infection of differentiated HIEs in the 2D model was greater than that of undifferentiated HIEs but lasted only for 3 days, whereas infection persisted for 21 days and resulted in completion of the life cycle in undifferentiated HIEs. Infection of undifferentiated HIE monolayers suggest that C. parvum infects LGR5+ stem cells. Transepithelial electrical resistance measurement of HIEs in the 2D model revealed that infection resulted in decreased epithelial integrity which persisted in differentiated HIEs but recovered in undifferentiated HIEs. Compromised epithelial integrity was reflected in disorganization of the tight and adherens junctions as visualized using the markers ZO-1 and E-cadherin, respectively. Quantitation using the image analysis tools Tight Junction Organizational Rate and Intercellular Junction Organization Quantification, measurement of monolayer height, and RNA transcripts of both proteins by quantitative reverse transcription PCR confirmed that disruption persisted in differentiated HIEs but recovered in undifferentiated HIEs. These models, which more accurately recapitulate human infection, will be useful tools to dissect pathogenic mechanisms underlying diarrhea and intestinal dysfunction in cryptosporidiosis.

Transit-amplifying cells control R-spondins in the mouse crypt to modulate intestinal stem cell proliferation

Intestinal epithelium regenerates rapidly through proliferation of intestinal stem cells (ISCs), orchestrated by potent mitogens secreted within the crypt niche. However, mechanisms regulating these mitogenic factors remain largely unknown. Here, we demonstrate that transit-amplifying (TA) cells, marked by unconventional prefoldin RPB5 interactor (URI), control R-spondin production to guide ISC proliferation. Genetic intestinal URI ablation in mice injures TA cells, reducing their survival capacity, leading to an inflamed tissue and subsequently decreasing R-spondin levels, thereby causing ISC quiescence and disruption of intestinal structure. R-spondin supplementation or restoration of R-spondin levels via cell death inhibition by c-MYC elimination or the suppression of inflammation reinstates ISC proliferation in URI-depleted mice. However, selective c-MYC and p53 suppression are required to fully restore TA cell survival and differentiation capacity and preserve complete intestinal architecture. Our data reveal an unexpected role of TA cells, which represent a signaling platform instrumental for controlling inflammatory cues and R-spondin production, essential for maintaining ISC proliferation and tissue regeneration.

Zymosan-A promotes the regeneration of intestinal stem cells by upregulating ASCL2

Intestinal stem cells (ISCs) are responsible for intestinal tissue homeostasis and are important for the regeneration of the damaged intestinal epithelia. Through the establishment of ionizing radiation (IR) induced intestinal injury model, we found that a TLR2 agonist, Zymosan-A, promoted the regeneration of ISCs in vivo and in vitro. Zymosan-A improved the survival of abdominal irradiated mice (81.82% of mice in the treated group vs. 30% of mice in the PBS group), inhibited the radiation damage of intestinal tissue, increased the survival rate of intestinal crypts and the number of ISCs after lethal IR in vivo. Through organoid experiments, we found that Zymosan-A promoted the proliferation and differentiation of ISCs after IR. Remarkably, the results of RNA sequencing and Western Blot (WB) showed that Zymosan-A reduced IR-induced intestinal injury via TLR2 signaling pathway and Wnt signaling pathway and Zymosan-A had no radioprotection on TLR2 KO mice, suggesting that Zymosan-A may play a radioprotective role by targeting TLR2. Moreover, our results revealed that Zymosan-A increased ASCL2, a transcription factor of ISCs, playing a core role in the process of Zymosan-A against IR-induced intestinal injury and likely contributing to the survival of intestinal organoids post-radiation. In conclusion, we demonstrated that Zymosan-A promotes the regeneration of ISCs by upregulating ASCL2.

Investigating local and systemic intestinal signalling in health and disease with Drosophila

Whole-body health relies on complex inter-organ signalling networks that enable organisms to adapt to environmental perturbations and to changes in tissue homeostasis. The intestine plays a major role as a signalling centre by producing local and systemic signals that are relayed to the body and that maintain intestinal and organismal homeostasis. Consequently, disruption of intestinal homeostasis and signalling are associated with systemic diseases and multi-organ dysfunction. In recent years, the fruit fly Drosophila melanogaster has emerged as a prime model organism to study tissue-intrinsic and systemic signalling networks of the adult intestine due to its genetic tractability and functional conservation with mammals. In this Review, we highlight Drosophila research that has contributed to our understanding of how the adult intestine interacts with its microenvironment and with distant organs. We discuss the implications of these findings for understanding intestinal and whole-body pathophysiology, and how future Drosophila studies might advance our knowledge of the complex interplay between the intestine and the rest of the body in health and disease.

Summary: We outline work in the fruit fly Drosophila melanogaster that has contributed knowledge on local and whole-body signalling coordinated by the adult intestine, and discuss its implications in intestinal pathophysiology and associated systemic dysfunction.

Colonic healing requires Wnt produced by epithelium as well as Tagln+ and Acta2+ stromal cells

Although Wnt signaling is clearly important for the intestinal epithelial homeostasis, the relevance of various sources of Wnt ligands themselves remains incompletely understood. Blocking the release of Wnt in distinct stromal cell types suggests obligatory functions of several stromal cell sources and yields different observations. The physiological contribution of epithelial Wnt to tissue homeostasis remains unclear. We show here that blocking epithelial Wnts affects colonic Reg4+ epithelial cell differentiation and impairs colonic epithelial regeneration after injury in mice. Single-cell RNA analysis of intestinal stroma showed that the majority of Wnt-producing cells were contained in transgelin (Tagln+) and smooth muscle actin α2 (Acta2+) expressing populations. We genetically attenuated Wnt production from these stromal cells using Tagln-Cre and Acta2-CreER drivers, and found that blockage of Wnt release from either epithelium or Tagln+ and Acta2+ stromal cells impaired colonic epithelial healing after chemical-induced injury. Aggregated blockage of Wnt release from both epithelium and Tagln+ or Acta2+ stromal cells drastically diminished epithelial repair, increasing morbidity and mortality. These results from two uncharacterized stromal populations suggested that colonic recovery from colitis-like injury depends on multiple Wnt-producing sources.

REGγ drives Lgr5+ stem cells to potentiate radiation induced intestinal regeneration

Leucine-rich repeat containing G protein-coupled receptor 5 (Lgr5), a marker of intestinal stem cells (ISCs), is considered to play key roles in tissue homoeostasis and regeneration after acute radiation injury. However, the activation of Lgr5 by integrated signaling pathways upon radiation remains poorly understood. Here, we show that irradiation of mice with whole-body depletion or conditional ablation of REGγ in Lgr5⁺ stem cell impairs proliferation of intestinal crypts, delaying regeneration of intestine epithelial cells. Mechanistically, REGγ enhances transcriptional activation of Lgr5 via the potentiation of both Wnt and Hippo signal pathways. TEAD4 alone or cooperates with TCF4, a transcription factor mediating Wnt signaling, to enhance the expression of Lgr5. Silencing TEAD4 drastically attenuated β-catenin/TCF4 dependent expression of Lgr5. Together, our study reveals how REGγ controls Lgr5 expression and expansion of Lgr5⁺ stem cells in the regeneration of intestinal epithelial cells. Thus, REGγ proteasome appears to be a potential therapeutic target for radiation-induced gastrointestinal disorders.

Single-cell sequencing of rotavirus-infected intestinal epithelium reveals cell-type specific epithelial repair and tuft cell infection

Intestinal epithelial damage is associated with most digestive diseases and results in detrimental effects on nutrient absorption and production of hormones and antimicrobial defense molecules. Thus, understanding epithelial repair and regeneration following damage is essential in developing therapeutics that assist in rapid healing and restoration of normal intestinal function. Here we used a well-characterized enteric virus (rotavirus) that damages the epithelium at the villus tip but does not directly damage the intestinal stem cell, to explore the regenerative transcriptional response of the intestinal epithelium at the single-cell level. We found that there are specific Lgr5+ cell subsets that exhibit increased cycling frequency associated with significant expansion of the epithelial crypt. This was accompanied by an increase in the number of immature enterocytes. Unexpectedly, we found rotavirus infects tuft cells. Transcriptional profiling indicates tuft cells respond to viral infection through interferon-related pathways. Together these data provide insights as to how the intestinal epithelium responds to insults by providing evidence of stimulation of a repair program driven by stem cells with involvement of tuft cells that results in the production of immature enterocytes that repair the damaged epithelium.

Taking a Step Back: Insights into the Mechanisms Regulating Gut Epithelial Dedifferentiation

Despite the environmental constraints imposed upon the intestinal epithelium, this tissue must perform essential functions such as nutrient absorption and hormonal regulation, while also acting as a critical barrier to the outside world. These functions depend on a variety of specialized cell types that are constantly renewed by a rapidly proliferating population of intestinal stem cells (ISCs) residing at the base of the crypts of Lieberkühn. The niche components and signals regulating crypt morphogenesis and maintenance of homeostatic ISCs have been intensely studied over the last decades. Increasingly, however, researchers are turning their attention to unraveling the mechanisms driving gut epithelial regeneration due to physical damage or infection. It is now well established that injury to the gut barrier triggers major cell fate changes, demonstrating the highly plastic nature of the gut epithelium. In particular, lineage tracing and transcriptional profiling experiments have uncovered several injury-induced stem-cell populations and molecular markers of the regenerative state. Despite the progress achieved in recent years, several questions remain unresolved, particularly regarding the mechanisms driving dedifferentiation of the gut epithelium. In this review, we summarize the latest studies, primarily from murine models, that define the regenerative processes governing the gut epithelium and discuss areas that will require more in-depth investigation.

Microbial recognition regulates intestinal epithelial growth in homeostasis and disease

The intestine is constantly exposed to a dynamic community of microbes. Intestinal epithelial cells respond to microbes through evolutionarily conserved recognition pathways, such as the immune deficiency (IMD) pathway of Drosophila, the Toll-like receptor (TLR) response of flies and vertebrates, and the vertebrate nucleotide-binding oligomerization domain (NOD) pathway. Microbial recognition pathways are tightly controlled to respond effectively to pathogens, tolerate the microbiome, and limit intestinal disease. In this review, we focus on contributions of different model organisms to our understanding of how epithelial microbe recognition impacts intestinal proliferation and differentiation in homeostasis and disease. In particular, we compare how microbes and subsequent recognition by the intestine influences barrier integrity, intestinal repair and tumorigenesis in Drosophila, zebrafish, mice, and organoids. In addition, we discuss the importance of microbial recognition in homeostatic intestinal growth and discuss how immune pathways directly impact stem cell and crypt dynamics.

Subversion of Niche-Signalling Pathways in Colorectal Cancer: What Makes and Breaks the Intestinal Stem Cell

The intestinal epithelium fulfils pleiotropic functions in nutrient uptake, waste elimination, and immune surveillance while also forming a barrier against luminal toxins and gut-resident microbiota. Incessantly barraged by extraneous stresses, the intestine must continuously replenish its epithelial lining and regenerate the full gamut of specialized cell types that underpin its functions. Homeostatic remodelling is orchestrated by the intestinal stem cell (ISC) niche: a convergence of epithelial- and stromal-derived cues, which maintains ISCs in a multipotent state. Following demise of homeostatic ISCs post injury, plasticity is pervasive among multiple populations of reserve stem-like cells, lineage-committed progenitors, and/or fully differentiated cell types, all of which can contribute to regeneration and repair. Failure to restore the epithelial barrier risks seepage of toxic luminal contents, resulting in inflammation and likely predisposing to tumour formation. Here, we explore how homeostatic niche-signalling pathways are subverted in tumorigenesis, enabling ISCs to gain autonomy from niche restraints ("ISC emancipation") and transform into cancer stem cells capable of driving tumour initiation, progression, and therapy resistance. We further consider the implications of the pervasive plasticity of the intestinal epithelium for the trajectory of colorectal cancer, the emergence of distinct molecular subtypes, the propensity to metastasize, and the development of effective therapeutic strategies.

Defence and adaptation mechanisms of the intestinal epithelium upon infection

The intestinal epithelium is a monolayer of polarized columnar cells that act as a border between the host and its environment and are the first line of defence against the luminal microbes. In addition to providing a physical barrier, the epithelium possesses a multitude of active mechanisms to fight invading pathogens and regulate the composition and spatial distribution of commensals. The different epithelial cell types have unique functions in this context, and crosstalk with the immune system further modulates their intricate antimicrobial responses. The epithelium is organized into clonal crypt units with a high cellular turnover that is driven by stem cells located at the base. There is increasing evidence that this anatomical organization, the stem cell turnover, and the lineage determination processes are essential for barrier maintenance. These processes can be modulated by microbes directly or by the immune responses to enteric pathogens, resulting in a rapid and efficient adaptation of the epithelium to environmental perturbations, injuries, and infections. Here we discuss the complex host-microbial interactions that shape the mucosa and how the epithelium maintains and re-establishes homeostasis after infection.

Mutational inactivation of Apc in the intestinal epithelia compromises cellular organisation

The adenomatous polyposis coli (Apc) protein regulates diverse effector pathways essential for tissue homeostasis. Truncating oncogenic mutations in Apc removing its Wnt pathway and microtubule regulatory domains drives intestinal epithelia tumorigenesis. Exuberant cell proliferation is one well-established consequence of oncogenic Wnt pathway activity; however, the contribution of other deregulated molecular circuits to tumorigenesis has not been fully examined. Using in vivo and organoid models of intestinal epithelial tumorigenesis we found that Wnt pathway activity controls intestinal epithelial villi and crypt structure, morphological features lost upon Apc inactivation. Although the Wnt pathway target gene c-Myc (also known as Myc) has critical roles in regulating cell proliferation and tumorigenesis, Apc specification of intestinal epithelial morphology is independent of the Wnt-responsive Myc-335 (also known as Rr21) regulatory element. We further demonstrate that Apc inactivation disrupts the microtubule cytoskeleton and consequently localisation of organelles without affecting the distribution of the actin cytoskeleton and associated components. Our data indicates the direct control over microtubule dynamics by Apc through an independent molecular circuit. Our study stratifies three independent Apc effector pathways in the intestinal epithelial controlling: (1) proliferation, (2) microtubule dynamics and (3) epithelial morphology.This article has an associated First Person interview with the first author of the paper.

CREPT is required for murine stem cell maintenance during intestinal regeneration

Intestinal stem cells (ISCs) residing in the crypts are critical for the continual self-renewal and rapid recovery of the intestinal epithelium. The regulatory mechanism of ISCs is not fully understood. Here we report that CREPT, a recently identified tumor-promoting protein, is required for the maintenance of murine ISCs. CREPT is preferably expressed in the crypts but not in the villi. Deletion of CREPT in the intestinal epithelium of mice (Vil-CREPTKO) results in lower body weight and slow migration of epithelial cells in the intestine. Vil-CREPTKO intestine fails to regenerate after X-ray irradiation and dextran sulfate sodium (DSS) treatment. Accordingly, the deletion of CREPT decreases the expression of genes related to the proliferation and differentiation of ISCs and reduces Lgr5+ cell numbers at homeostasis. We identify that CREPT deficiency downregulates Wnt signaling by impairing β-catenin accumulation in the nucleus of the crypt cells during regeneration. Our study provides a previously undefined regulator of ISCs.

CREPT is required for murine stem cell maintenance during intestinal regeneration

Intestinal stem cells (ISCs) residing in the crypts are critical for the continual self-renewal and rapid recovery of the intestinal epithelium. The regulatory mechanism of ISCs is not fully understood. Here we report that CREPT, a recently identified tumor-promoting protein, is required for the maintenance of murine ISCs. CREPT is preferably expressed in the crypts but not in the villi. Deletion of CREPT in the intestinal epithelium of mice (Vil-CREPTKO) results in lower body weight and slow migration of epithelial cells in the intestine. Vil-CREPTKO intestine fails to regenerate after X-ray irradiation and dextran sulfate sodium (DSS) treatment. Accordingly, the deletion of CREPT decreases the expression of genes related to the proliferation and differentiation of ISCs and reduces Lgr5+ cell numbers at homeostasis. We identify that CREPT deficiency downregulates Wnt signaling by impairing β-catenin accumulation in the nucleus of the crypt cells during regeneration. Our study provides a previously undefined regulator of ISCs.

An adverse outcome pathway for small intestinal tumors in mice involving chronic cytotoxicity and regenerative hyperplasia: a case study with hexavalent chromium, captan, and folpet

Small intestinal (SI) tumors are relatively uncommon outcomes in rodent cancer bioassays, and limited information regarding chemical-induced SI tumorigenesis has been reported in the published literature. Herein, we propose a cytotoxicity-mediated adverse outcome pathway (AOP) for SI tumors by leveraging extensive target species- and site-specific molecular, cellular, and histological mode of action (MOA) research for three reference chemicals, the fungicides captan and folpet and the transition metal hexavalent chromium (Cr(VI)). The gut barrier functions through highly efficient homeostatic regulation of SI epithelial cell sloughing, regenerative proliferation, and repair, which involves the replacement of up to 10¹¹ cells per day. This dynamic turnover in the SI provides a unique local environment for a cytotoxicity mediated AOP/MOA. Upon entering the duodenum, cytotoxicity to the villous epithelium is the molecular initiating event, as indicated by crypt elongation, villous atrophy/blunting, and other morphologic changes. Over time, the regenerative capacity of the gut epithelium to compensate declines as epithelial loss accelerates, especially at higher exposures. The first key event (KE), sustained regenerative crypt proliferation/hyperplasia, requires sufficient durations, likely exceeding 6 or 12 months, due to extensive repair capacity, to create more opportunities for the second KE, spontaneous mutation/transformation, ultimately leading to proximal SI tumors. Per OECD guidance, biological plausibility, essentiality, and empirical support were assessed using modified Bradford Hill considerations. The weight-of-evidence also included a lack of induced mutations in the duodenum after up to 90 days of Cr(VI) or captan exposure. The extensive evidence for this AOP, along with the knowledge that human exposures are orders of magnitude below those associated with KEs in this AOP, supports its use for regulatory applications, including hazard identification and risk assessment.

Epithelial WNT2B and Desert Hedgehog Are Necessary for Human Colonoid Regeneration after Bacterial Cytotoxin Injury

Intestinal regeneration and crypt hyperplasia after radiation or pathogen injury relies on Wnt signaling to stimulate stem cell proliferation. Mesenchymal Wnts are essential for homeostasis and regeneration in mice, but the role of epithelial Wnts remains largely uncharacterized. Using the enterohemorrhagic E. coli-secreted cytotoxin EspP to induce injury to human colonoids, we evaluated a simplified, epithelial regeneration model that lacks mesenchymal Wnts. Here, we demonstrate that epithelial-produced WNT2B is upregulated following injury and essential for regeneration. Hedgehog signaling, specifically activation via the ligand Desert Hedgehog (DHH), but not Indian or Sonic Hedgehog, is another driver of regeneration and modulates WNT2B expression. These findings highlight the importance of epithelial WNT2B and DHH in regulating human colonic regeneration after injury.

IL-22-induced cell extrusion and IL-18-induced cell death prevent and cure rotavirus infection

Bacterial flagellin can elicit production of TLR5-mediated IL-22 and NLRC4-mediated IL-18 cytokines that act in concert to cure and prevent rotavirus (RV) infection. This study investigated the mechanism by which these cytokines act to impede RV. Although IL-18 and IL-22 induce each other's expression, we found that IL-18 and IL-22 both impeded RV independently of one another and did so by distinct mechanisms that involved activation of their cognate receptors in intestinal epithelial cells (IEC). IL-22 drove IEC proliferation and migration toward villus tips, which resulted in increased extrusion of highly differentiated IEC that serve as the site of RV replication. In contrast, IL-18 induced cell death of RV-infected IEC thus directly interrupting the RV replication cycle, resulting in spewing of incompetent virus into the intestinal lumen and causing a rapid drop in the number of RV-infected IEC. Together, these actions resulted in rapid and complete expulsion of RV, even in hosts with severely compromised immune systems. These results suggest that a cocktail of IL-18 and IL-22 might be a means of treating viral infections that preferentially target short-lived epithelial cells.

Senescence-accelerated mouse prone 8 mice exhibit specific morphological changes in the small intestine during senescence and after pectin supplemented diet

Impairment of gastrointestinal function and reduction of nutrient absorption associated with aging contribute to increased risk of malnutrition in the elderly population, resulting in physical weakness and vulnerability to disease. The present study was performed to examine the relationships between aging-associated morphological changes of the small intestine and nutrient malabsorption using senescence-accelerated mouse prone 8 (SAMP8) mice. Comparison of the morphology of the small intestine of young (22-week-old) and senescent (43-week-old) SAMP8 mice showed no significant changes in villus length, while the mRNA expression levels of secretory cell marker genes were significantly reduced in senescent mice. In addition, crypts recovered from the small intestine of senescent mice showed a good capacity to form intestinal organoids ex vivo, suggesting that the regenerative capacity of intestinal stem cells (ISCs) was unaffected by accelerated senescence. These results indicated that changes induced by accelerated senescence in the small intestine of SAMP8 mice are different from changes reported previously in normal aging mouse models. Biochemical analyses of serum before and during senescence also indicated that senescent SAMP8 mice are not in a malabsorption state. Furthermore, a diet supplemented with persimmon pectin had a mild effect on the small intestine of senescent SAMP8 mice. Intestinal villus length was slightly increased in the medial part of the small intestine of pectin-fed mice. In contrast, intestinal crypt formation capacity was enhanced by the pectin diet. Organoid culture derived from the small intestine of mice fed pectin exhibited a greater number of lobes per organoid compared with those from mice fed a control diet, and Lyz1 and Olfm4 mRNA levels were significantly increased. In conclusion, accelerated senescence induced exclusive changes in the small intestine, which were not related to nutrient malabsorption. Therefore, the SAMP8 strain may not be a suitable model to evaluate the effects of aging on intestinal homeostasis and nutrient absorption impairment.

Targeting STAT3 and STAT5 in Cancer

Insights into the mutational landscape of the human cancer genome coding regions defined about 140 distinct cancer driver genes in 2013, which approximately doubled to 300 in 2018 following advances in systems cancer biology studies [...].

Replicates in stem cell models-How complicated!

Current complexities in human pluripotent stem cell (hPSC)-based studies. hPSC studies begin with the recruitment of patients harboring disease-associated gene variant(s) that may increase susceptibility to disease development. Somatic reprogramming is then performed to derive patient-specific human induced pluripotent stem cells (hiPSCs), followed by step-wise directed differentiation to a relevant cell type before qualitative/quantitative assays are performed to assess for phenotypic or gene expression differences between the healthy and diseased hiPSCs.

Hdc-expressing myeloid-derived suppressor cells promote basal-like transition and metastasis of breast cancer

Metastases are the greatest contributors to death from breast cancer. Here, we identified a distinct subpopulation of luminal breast cancer characterized by cytokeratin 14 (CK14) expression in secondary colonies rather than primary tumors. This entity possessed a poorer prognosis compared to their CK14^- counterparts. Immunohistochemical analysis showed that myeloid-derived suppressor cells (MDSCs) were recruited into the tumor microenvironment and exhibited a close spatial relationship with CK14⁺ cancer cells. We demonstrated that histidine decarboxylase (Hdc) is capable of labeling myeloid-biased hematopoietic stem cell/progenitor cell (HSC/HSPC) and immature myeloid cells infiltrating in tumor tissues. FACS data obtained from Hdc-CreER^T2; eGFP; MMTV-PyVT female mice revealed an increased percentage of Hdc⁺ PMN-MDSCs in metastatic masses. Hdc⁺ PMN-MDSCs expressed high levels of canonical Wnts, including Wnt2, Wnt4, Wnt5a, and Wnt7b, to aberrantly activate Wnt/β-catenin signaling in CK14⁺ malignant cells. β-catenin translocated from the membrane into the cytoplasm and nucleus. Targeted ablation of Hdc⁺ PMN-MDSCs-derived Wnts through porcupine^flox/flox and iDTR transgenic models hampered the metastatic cascade, making Hdc⁺ immature myeloid cells an attractive candidate for tailed immunotherapies.

Aging induces aberrant state transition kinetics in murine muscle stem cells

Murine muscle stem cells (MuSCs) experience a transition from quiescence to activation that is required for regeneration, but it remains unknown if the trajectory and dynamics of activation change with age. Here, we use time-lapse imaging and single cell RNA-seq to measure activation trajectories and rates in young and aged MuSCs. We find that the activation trajectory is conserved in aged cells, and we develop effective machine-learning classifiers for cell age. Using cell-behavior analysis and RNA velocity, we find that activation kinetics are delayed in aged MuSCs, suggesting that changes in stem cell dynamics may contribute to impaired stem cell function with age. Intriguingly, we also find that stem cell activation appears to be a random walk-like process, with frequent reversals, rather than a continuous linear progression. These results support a view of the aged stem cell phenotype as a combination of differences in the location of stable cell states and differences in transition rates between them.

Histological differentiation of mucus cell subtypes suggests functional compartmentation in the eel esophagus

We investigated the morphological and histological changes in eel esophagus during the course of freshwater (FW) to seawater (SW) transfer and identified multiple types of mucus cells from tissues that were fixed using Carnoy's solution to retain the mucus structure. The FW esophageal epithelium is stratified and composed of superficial cells, mucus cells, club cells (exocrine cells with a large vacuole), and basal cells. Two types of periodic acid-Schiff (PAS)-positive mucus cells were identified, and they can be further distinguished by the periodic acid-thionin Schiff/KOH/PAS (PAT) method, indicating that C7/9- and C8-sialic acids were produced. Isolectin B4-positive mucus cells were found among the C8-sialic acid-producing cells and located at the tips of the villi at mid-posterior regions of the FW esophagus. The two different muci were immiscible and may form separate layers to protect the tissues from the high osmolality of imbibed SW during early SW acclimation. The densities of club cells and isolectin B4-positive cells decreased after SW acclimation, and cuboidal/columnar epithelial cells subsequently developed for active Na⁺ and Cl^- absorption. Cuboidal/columnar epithelial cells proliferated in scattered array rather than at the bases of the villi, thereby retaining the characteristic of the stratified epithelium. Prominent leukocyte invasion was found at the base of the stratified epithelium at early SW transfer, indicating that the immune system was also activated in response to antigen exposure from imbibed SW. The mucus composition in FW is more complicated than that in SW, fueling further studies for their functions to form unstirred layers as osmoregulatory barriers.

Use of organoids to study regenerative responses to intestinal damage

Intestinal organoid cultures provide an in vitro model system for studying pathways and mechanisms involved in epithelial damage and repair. Derived from either embryonic or induced pluripotent stem cells or adult intestinal stem cells or tissues, these self-organizing, multicellular structures contain polarized mature cells that recapitulate both the physiology and heterogeneity of the intestinal epithelium. These cultures provide a cutting-edge technology for defining regenerative pathways that are induced following radiation or chemical damage, which directly target the cycling intestinal stem cell, or damage resulting from viral, bacterial, or parasitic infection of the epithelium. Novel signaling pathways or biological mechanisms identified from organoid studies that mediate regeneration of the epithelium following damage are likely to be important targets of preventive or therapeutic modalities to mitigate intestinal injury. The evolution of these cultures to include more components of the intestinal wall and the ability to genetically modify them are key components for defining the mechanisms that modulate epithelial regeneration.

Impaired Wnt/β-catenin pathway leads to dysfunction of intestinal regeneration during necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is a devastating neonatal disease characterized by acute intestinal injury. Intestinal stem cell (ISC) renewal is required for gut regeneration in response to acute injury. The Wnt/β-catenin pathway is essential for intestinal renewal and ISC maintenance. We found that ISC expression, Wnt activity and intestinal regeneration were all decreased in both mice with experimental NEC and in infants with acute active NEC. Moreover, intestinal organoids derived from NEC-injured intestine of both mice and humans failed to maintain proliferation and presented more differentiation. Administration of Wnt7b reversed these changes and promoted growth of intestinal organoids. Additionally, administration of exogenous Wnt7b rescued intestinal injury, restored ISC, and reestablished intestinal epithelial homeostasis in mice with NEC. Our findings demonstrate that during NEC, Wnt/β-catenin signaling is decreased, ISC activity is impaired, and intestinal regeneration is defective. Administration of Wnt resulted in the maintenance of intestinal epithelial homeostasis and avoidance of NEC intestinal injury.

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.

The Hippo and Wnt signalling pathways: crosstalk during neoplastic progression in gastrointestinal tissue

The Hippo and Wnt signalling pathways play crucial roles in maintaining tissue homeostasis and organ size by orchestrating cell proliferation, differentiation and apoptosis. These pathways have been frequently found to be dysregulated in human cancers. While the canonical signal transduction of Hippo and Wnt has been well studied, emerging evidence shows that these two signalling pathways contribute to and exhibit overlapping functions in gastrointestinal (GI) tumorigenesis. In fact, the core effectors YAP/TAZ in Hippo signalling pathway cooperate with β-catenin in Wnt signalling pathway to promote GI neoplasia. Here, we provide a brief review to summarize the molecular mechanisms underlying the crosstalk between these two pathways and elucidate their involvement in GI tumorigenesis, particularly focusing on the intestine, stomach and liver.

Notum produced by Paneth cells attenuates regeneration of aged intestinal epithelium

A decline in stem cell function impairs tissue regeneration during ageing, but the role of the stem-cell-supporting niche in ageing is not well understood. The small intestine is maintained by actively cycling intestinal stem cells that are regulated by the Paneth cell niche^1,2. Here we show that the regenerative potential of human and mouse intestinal epithelium diminishes with age owing to defects in both stem cells and their niche. The functional decline was caused by a decrease in stemness-maintaining Wnt signalling due to production of Notum, an extracellular Wnt inhibitor, in aged Paneth cells. Mechanistically, high activity of mammalian target of rapamycin complex 1 (mTORC1) in aged Paneth cells inhibits activity of peroxisome proliferator activated receptor α (PPAR-α)³, and lowered PPAR-α activity increased Notum expression. Genetic targeting of Notum or Wnt supplementation restored function of aged intestinal organoids. Moreover, pharmacological inhibition of Notum in mice enhanced the regenerative capacity of aged stem cells and promoted recovery from chemotherapy-induced damage. Our results reveal a role of the stem cell niche in ageing and demonstrate that targeting of Notum can promote regeneration of aged tissues.

RAL GTPases Drive Intestinal Stem Cell Function and Regeneration through Internalization of WNT Signalosomes

Ral GTPases are RAS effector molecules and by implication a potential therapeutic target for RAS mutant cancer. However, very little is known about their roles in stem cells and tissue homeostasis. Using Drosophila, we identified expression of RalA in intestinal stem cells (ISCs) and progenitor cells of the fly midgut. RalA was required within ISCs for efficient regeneration downstream of Wnt signaling. Within the murine intestine, genetic deletion of either mammalian ortholog, Rala or Ralb, reduced ISC function and Lgr5 positivity, drove hypersensitivity to Wnt inhibition, and impaired tissue regeneration following damage. Ablation of both genes resulted in rapid crypt death. Mechanistically, RALA and RALB were required for efficient internalization of the Wnt receptor Frizzled-7. Together, we identify a conserved role for RAL GTPases in the promotion of optimal Wnt signaling, which defines ISC number and regenerative potential.

Stem cell proliferation is induced by apoptotic bodies from dying cells during epithelial tissue maintenance

Epithelial tissues require the removal and replacement of damaged cells to sustain a functional barrier. Dying cells provide instructive cues that can influence surrounding cells to proliferate, but how these signals are transmitted to their healthy neighbors to control cellular behaviors during tissue homeostasis remains poorly understood. Here we show that dying stem cells facilitate communication with adjacent stem cells by caspase-dependent production of Wnt8a-containing apoptotic bodies to drive cellular turnover in living epithelia. Basal stem cells engulf apoptotic bodies, activate Wnt signaling, and are stimulated to divide to maintain tissue-wide cell numbers. Inhibition of either cell death or Wnt signaling eliminated the apoptosis-induced cell division, while overexpression of Wnt8a signaling combined with induced cell death led to an expansion of the stem cell population. We conclude that ingestion of apoptotic bodies represents a regulatory mechanism linking death and division to maintain overall stem cell numbers and epithelial tissue homeostasis.

Damaged epithelial tissues are known to compensate for cell death through compensatory cell divisions to maintain epithelial integrity. Here, the authors show in living epithelia that dying cells stimulate adjacent stem cells to divide through caspase-dependent production of Wnt8a-containing apoptotic bodies.

Intestinal renewal across the animal kingdom: comparing stem cell activity in mouse and Drosophila

The gastrointestinal (GI) tract renews frequently to sustain nutrient digestion and absorption in the face of consistent tissue stress. In many species, proliferative intestinal stem cells (ISCs) are responsible for the repair of the damage arising from chemical and mechanical aspects of food breakdown and exposure to pathogens. As the cellular source of all mature cell types of the intestinal epithelium throughout adulthood, ISCs hold tremendous therapeutic potential for understanding and treating GI disease in humans. This review focuses on recent advances in our understanding of ISC identity, behavior, and regulation during homeostasis and injury-induced repair, as revealed by two major animal models used to study regeneration of the small intestine: Drosophila melanogaster and Mus musculus. We emphasize recent findings from Drosophila that are likely to translate to the mammalian GI system, as well as challenging topics in mouse ISC biology that may be ideally suited for investigation in flies. For context, we begin by reviewing major physiological similarities and distinctions between the Drosophila midgut and mouse small intestine.

Cellular Plasticity in Intestinal Homeostasis and Disease

The intestinal epithelium is one the fastest renewing tissues in mammals and is endowed with extensive adaptability. The more traditional view of a hierarchical organization of the gut has recently given way to a more dynamic model in which various cell types within the intestinal epithelium can de-differentiate and function as an alternative source of stem cells upon tissue damage and stress conditions such as inflammation and tumorigenesis. Here, we will review the mechanistic principles and key players involved in intestinal plasticity and discuss potential therapeutic implications of cellular plasticity in regenerative medicine and cancer.