The intestinal epithelium tuft cells: specification and function

Decreased number of colonic tuft cells in quiescent ulcerative colitis patients

BACKGROUND

Colonic tuft cells are epithelial chemosensory cells involved in barrier integrity, modulation of inflammatory responses and gut homeostasis. Recent evidence indicates an involvement of tuft cells in ulcerative colitis pathogenesis, though mechanisms remain largely unknown.Here, we quantified the colonic tuft cell population in patients with quiescent ulcerative colitis as compared to patients without identified colonic disease (controls).

METHODS

In this retrospective study, we obtained endoscopic colonic sigmoid biopsies from 14 patients with quiescent ulcerative colitis and from 17 controls. In a blinded central-reading design, we identified tuft cells by immunohistochemistry using a cyclooxygenase-1 antibody as a marker and performed a simple counting by visual inspection. Poisson regression was employed for statistics and results were adjusted for gender, age and smoking status.

RESULTS

Ulcerative colitis patients demonstrated a 55% reduced tuft cell count in colonic mucosa compared with the control group (95% confidence limit: range 31-71%, P = 0.0002). Ulcerative colitis patients had a mean tuft cells count of 46 tuft cells/mm2 (95% CI, 36-59), while controls demonstrated a mean of 104 tuft cells/mm2 (95% CI, 79-136). No interactions of other covariates, such as age, smoking status, total duration of ulcerative colitis disease and duration of clinical remission prior to study inclusion were detected between ulcerative colitis patients and controls.

CONCLUSION

Quiescent ulcerative colitis patients have a relatively low number of colonic tuft cells. Further studies are warranted to explore the potential involvement of tuft cells in ulcerative colitis pathogenesis.

Clump sequencing exposes the spatial expression programs of intestinal secretory cells

Single-cell RNA sequencing combined with spatial information on landmark genes enables reconstruction of spatially-resolved tissue cell atlases. However, such approaches are challenging for rare cell types, since their mRNA contents are diluted in the spatial transcriptomics bulk measurements used for landmark gene detection. In the small intestine, enterocytes, the most common cell type, exhibit zonated expression programs along the crypt-villus axis, but zonation patterns of rare cell types such as goblet and tuft cells remain uncharacterized. Here, we present ClumpSeq, an approach for sequencing small clumps of attached cells. By inferring the crypt-villus location of each clump from enterocyte landmark genes, we establish spatial atlases for all epithelial cell types in the small intestine. We identify elevated expression of immune-modulatory genes in villus tip goblet and tuft cells and heterogeneous migration patterns of enteroendocrine cells. ClumpSeq can be applied for reconstructing spatial atlases of rare cell types in other tissues and tumors.

Adhesion receptor ADGRG2/GPR64 is in the GI-tract selectively expressed in mature intestinal tuft cells

Objective

GPR64/ADGRG2 is an orphan Adhesion G protein-coupled receptor (ADGR) known to be mainly expressed in the parathyroid gland and epididymis. This investigation aimed to delineate the cellular expression of GPR64 throughout the body with focus on the gastrointestinal (GI) tract.

Methods

Transgenic Gpr64^mCherry reporter mice were histologically examined throughout the body and reporter protein expression in intestinal tuft cells was confirmed by specific cell ablation. The GPCR repertoire of intestinal Gpr64^mCherry-positive tuft cells was analyzed by quantitative RT-PCR analysis and in situ hybridization. The Gpr64^mCherry was crossed into the general tuft cell reporter Trpm5^GFP to generate small intestinal organoids for time-lapse imaging. Intestinal tuft cells were isolated from small intestine, FACS-purified and transcriptionally compared using RNA-seq analysis.

Results

Expression of the Gpr64^mCherry reporter was identified in multiple organs and specifically in olfactory microvillous cells, enteric nerves, and importantly in respiratory and GI tuft cells. In the small intestine, cell ablation targeting Gpr64-expressing epithelial cells eliminated tuft cells. Transcriptional analysis of small intestinal Gpr64^mCherry -positive tuft cells confirmed expression of Gpr64 and the chemo-sensors Sucnr1, Gprc5c, Drd3, and Gpr41/Ffar3. Time-lapse studies of organoids from Trpm5^GFP:Gpr64^mCherry mice revealed sequential expression of initially Trpm5^GFP and subsequently also Gpr64^mCherry in maturing intestinal tuft cells. RNA-seq analysis of small intestinal tuft cells based on these two markers demonstrated a dynamic change in expression of transcription factors and GPCRs from young to mature tuft cells.

Conclusions

GPR64 is expressed in chemosensory epithelial cells across a broad range of tissues; however, in the GI tract, GPR64 is remarkably selectively expressed in mature versus young immunoregulatory tuft cells.

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GPR64-RFP is expressed mainly in chemosensory epithelial cells, including tuft cells.

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Maturing intestinal tuft cells initially express Trpm5 and subsequently Gpr64.

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Mature intestinal Gpr64+ tuft cells express Sucnr1, Gprc5c, Drd3, and Gpr41/Ffar3.

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RNAseq analysis reveals dynamic transcriptional change of mature vs. young tuft cells.

Crossing the barriers: Revisiting the gut feeling in rheumatoid arthritis

To avoid autoimmunity, it is essential to keep the balance between the defence against pathogens and the maintenance of tolerance to self-antigens. Mucosal inflammation may lead to breakdown of tolerance and activation of autoreactive cells. Growing evidence suggests a major contribution of gut microbiota to the onset of chronic, autoimmune inflammatory diseases including rheumatoid arthritis (RA). RA patients show significant differences in the composition of gut microbiota compared to healthy controls, and in murine arthritis models certain bacteria can induce inflammatory Th17 responses or autoantibody production. The gut microbiota plays an important role in regulating the balance between immunogenic and tolerogenic immune responses. The intestinal barrier is the site of the body where most host-microbiota interaction takes place. Certain microbiota or their metabolites can cause a break in homeostasis by affecting the intestinal barrier integrity and permeability. However, an intact intestinal barrier is essential to separate the intestinal epithelium from toxins, microorganisms, and antigens in the gut lumen. This review will focus on the correlation between a leaky gut and the onset of arthritis. Furthermore, it will be discussed how targeting the intestinal barrier function by dietary changes might provide an opportunity to modulate the development of RA.

Thymus and autoimmunity

The thymus prevents autoimmune diseases through mechanisms that operate in the cortex and medulla, comprising positive and negative selection and the generation of regulatory T-cells (Tregs). Egress from the thymus through the perivascular space (PVS) to the blood is another possible checkpoint, as shown by some autoimmune/immunodeficiency syndromes. In polygenic autoimmune diseases, subtle thymic dysfunctions may compound genetic, hormonal and environmental cues. Here, we cover (a) tolerance-inducing cell types, whether thymic epithelial or tuft cells, or dendritic, B- or thymic myoid cells; (b) tolerance-inducing mechanisms and their failure in relation to thymic anatomic compartments, and with special emphasis on human monogenic and polygenic autoimmune diseases and the related thymic pathologies, if known; (c) polymorphisms and mutations of tolerance-related genes with an impact on positive selection (e.g. the gene encoding the thymoproteasome-specific subunit, PSMB11), promiscuous gene expression (e.g. AIRE, PRKDC, FEZF2, CHD4), Treg development (e.g. SATB1, FOXP3), T-cell migration (e.g. TAGAP) and egress from the thymus (e.g. MTS1, CORO1A); (d) myasthenia gravis as the prototypic outcome of an inflamed or disordered neoplastic ‘sick thymus’.

In vitro models of intestinal epithelium: Toward bioengineered systems

The intestinal epithelium, the fastest renewing tissue in human, is a complex tissue hosting multiple cell types with a dynamic and multiparametric microenvironment, making it particularly challenging to recreate in vitro. Convergence of recent advances in cellular biology and microfabrication technologies have led to the development of various bioengineered systems to model and study the intestinal epithelium. Theses microfabricated in vitro models may constitute an alternative to current approaches for studying the fundamental mechanisms governing intestinal homeostasis and pathologies, as well as for in vitro drug screening and testing. Herein, we review the recent advances in bioengineered in vitro intestinal models.

Barrier therapies supporting the biology of the mucosal barrier-medical devices for common clinical mucosal disorders

Recently mucosal barrier therapies have been either CE marked or licensed by Food and Drug Administration (FDA) as medical devices. A barrier therapy (BT) uses a physical non-drug mode of action as its sole mechanism to manage a clinical syndrome. A BT is verified as technically or biologically safe having efficacy that has been proven by valid clinical trials. However, it remains unclear what anatomical portions of the mucosa are physically engaged by any given BT. Therefore, this article clarifies the physical basis for clinical efficacy of any given mucosal BT's. Current regulatory classification of medical devices is defined. More importantly, the biology of mucosal barrier is detailed by structure, compartmental elements and function. A live-function or cross-sectional anatomical perspective of the mucosa is provided. A cross-sectional anatomical perspective of the mucosa is provided in order to highlight the physical point of contact for any given mucosal BT's. Five traits of an effective mucosal BT are proposed to assess traits of fitness for any given BT. A BT is either classical, possessing four to five traits, or non-classical, possessing three or fewer traits. Among 16 commercially available mucosal BT's which share nine distinct formulations, most are non-classical BT while two (alginate and polymeric sucralfate) are classical mucosal BT's.

The cellular niche for intestinal stem cells: a team effort

The rapidly self-renewing epithelium in the mammalian intestine is maintained by multipotent intestinal stem cells (ISCs) located at the bottom of the intestinal crypt that are interspersed with Paneth cells in the small intestine and Paneth-like cells in the colon. The ISC compartment is also closely associated with a sub-epithelial compartment that contains multiple types of mesenchymal stromal cells. With the advances in single cell and gene editing technologies, rapid progress has been made for the identification and characterization of the cellular components of the niche microenvironment that is essential for self-renewal and differentiation of ISCs. It has become increasingly clear that a heterogeneous population of mesenchymal cells as well as the Paneth cells collectively provide multiple secreted niche signals to promote ISC self-renewal. Here we review and summarize recent advances in the regulation of ISCs with a main focus on the definition of niche cells that sustain ISCs.

Berberine in the treatment of ulcerative colitis: A possible pathway through Tuft cells

Ulcerative colitis (UC) is an inflammatory bowel disease with complex pathogenesis, which is affected by genetic factors, intestinal immune status and intestinal microbial homeostasis. Intestinal epithelial barrier defect is crucial to the development of UC. Berberine, extracted from Chinese medicine, can identify bitter taste receptor on intestinal Tuft cells and activate IL-25-ILC2-IL-13 immune pathway to impair damaged intestinal tract by promoting differentiation of intestinal stem cells, which might be a potential approach for the treatment of UC.

Farm Animal-derived Models of the Intestinal Epithelium: Recent Advances and Future Applications of Intestinal Organoids

Farm animals play an important role in translational research as large animal models of the gastrointestinal (GI) tract. The mechanistic investigation of zoonotic diseases of the GI tract, in which animals can act as asymptomatic carriers, could provide important information for therapeutic approaches. In veterinary medicine, farm animals are no less relevant, as they can serve as models for the development of diagnostic and therapeutic approaches of GI diseases in the target species. However, farm animal-derived cell lines of the intestinal epithelium are rarely available from standardised cell banks and, in addition, are not usually specific for certain sections of the intestine. Immortalised porcine or bovine enterocytic cell lines are more widely available, compared to goat or sheep-derived cell lines; no continuous cell lines are available from the chicken. Other epithelial cell types with intestinal section-specific distribution and function, such as goblet cells, enteroendocrine cells, Paneth cells and intestinal stem cells, are not represented in those cell line-based models. Therefore, intestinal organoid models of farm animal species, which are already widely used for mice and humans, are gaining importance. Crypt-derived or pluripotent stem cell-derived intestinal organoid models offer the possibility to investigate the mechanisms of inter-cell or host-pathogen interactions and to answer species-specific questions. This review is intended to give an overview of cell culture models of the intestinal epithelium of farm animals, discussing species-specific differences, culture techniques and some possible applications for intestinal organoid models. It also highlights the need for species-specific pluripotent stem cell-derived or crypt-derived intestinal organoid models for promotion of the Three Rs principles (replacement, reduction and refinement).

Teasaponin Ameliorates Murine Colitis by Regulating Gut Microbiota and Suppressing the Immune System Response

Background: Dietary intervention is an exciting topic in current research of inflammatory bowel disease (IBD). The effect of teasaponin (TS) on IBD has not been fully elucidated. Here, we aim to investigate the intestinal anti-inflammatory activity of TS in a dextran sodium sulfate (DSS)-induced colitis mouse model and identify potential mechanisms.

Methods: We applied TS to mice with DSS-induced colitis and then monitored the body weight, disease activity index (DAI) daily. When sacrificed, the intestinal permeability was measured. The analysis of mucin and tight junction proteins was conducted. We detected the inflammatory cytokines, the immune cells and related inflammatory signaling pathways. In addition, the gut microbiota were analyzed by 16S rRNA sequencing and we also performed fecal microbiota transplantation (FMT).

Results: It showed that TS ameliorated the colonic damage by lowering the DAI, prolonging the colon length, reducing inflammatory cytokines and improving the mucus barrier. Parallel to down-regulation of the inflammatory cytokines, the fecal lipocalin 2, p-P65, p-STAT3, and neutrophil accumulation were also decreased in TS-treated mice. Microbiota characterization showed that Campylobacteria, Proteobacteria, Helicobacter, and Enterobacteriaceae were the key bacteria associated with IBD. In addition, TS could reverse the Firmicutes/Bacteroidetes (F/B) ratio and increase the beneficial bacteria, including Akkermansia and Bacteroides. TS ameliorated DSS-induced colitis by regulating the gut microbiota, and the gut microbiota could regulate gut inflammation.

Conclusions: These studies demonstrated that TS ameliorated murine colitis through the modulation of immune response, mucus barrier and gut microbiota, thus improving gut dysbiosis. In addition, the gut microbiota may play an important role in regulating the host's innate immune system, and the two coexist and are mutually beneficial. We provide a promising perspective on the clinical treatment of IBD.

Group 2 Innate Lymphoid Cells Coordinate Damage Response in the Stomach

BACKGROUND & AIMS

Severe injury to the lining of the stomach leads to changes in the epithelium (reprogramming) that protect and promote repair of the tissue, including development of spasmolytic polypeptide-expressing metaplasia (SPEM) and tuft and foveolar cell hyperplasia. Acute gastric damage elicits a type-2 inflammatory response that includes production of type-2 cytokines and infiltration by eosinophils and alternatively activated macrophages. Stomachs of mice that lack interleukin 33 (IL33) or interleukin 13 (IL13) did not undergo epithelial reprogramming after drug-induced injury. We investigated the role of group 2 innate lymphoid cells (ILC2s) in gastric epithelial repair.

METHODS

Acute gastric injury was induced in C57BL/6J mice (wild-type and RAG1 knockout) by administration of L635. We isolated ILC2s by flow cytometry from stomachs of mice that were and were not given L635 and performed single-cell RNA sequencing. ILC2s were depleted from wild-type and RAG1-knockout mice by administration of anti-CD90.2. We assessed gastric cell lineages, markers of metaplasia, inflammation, and proliferation. Gastric tissue microarrays from patients with gastric adenocarcinoma were analyzed by immunostaining.

RESULTS

There was a significant increase in the number of GATA3-positive ILC2s in stomach tissues from wild-type mice after L635-induced damage, but not in stomach tissues from IL33-knockout mice. We characterized a marker signature of gastric mucosal ILC2s and identified a transcription profile of metaplasia-associated ILC2s, which included changes in expression of Il5, Il13, Csf2, Pd1, and Ramp3; these changes were validated by quantitative polymerase chain reaction and immunocytochemistry. Depletion of ILC2s from mice blocked development of metaplasia after L635-induced injury in wild-type and RAG1-knockout mice and prevented foveolar and tuft cell hyperplasia and infiltration or activation of macrophages after injury. Numbers of ILC2s were increased in stomach tissues from patients with SPEM compared with patients with normal corpus mucosa.

CONCLUSIONS

In analyses of stomach tissues from mice with gastric tissue damage and patients with SPEM, we found evidence of type 2 inflammation and increased numbers of ILC2s. Our results suggest that ILC2s coordinate the metaplastic response to severe gastric injury.

Expression of POU2F3 Transcription Factor Control Inflammation, Immunological Recruitment and Metastasis of Pancreatic Cancer in Mice

Simple Summary

The presence and the role of TUFT cells in pancreatic ductal adenocarcinoma (PDAC) is discussed. Therefore, we decided to inactivate the POU2F3 gene, which is essential for TUFT cells development, in an aggressive PDAC mice model known as PDX1-Cre;LSL-Kras^G12D;Ink4a^fl/fl. Morphological and molecular analysis of POU2F3-deleted PDAC show not significant changes in tumors growth and survival of animals although it promotes EMT. Remarkably, we observed that in POU2F3-deleted animals the lack of TUFT cells prevents metastasis formation and strongly modifies the immunological and inflammatory landscape.

Abstract

TUFT cells have been described as strong modulators of inflammatory cells in several tissues including pancreas. TUFT cells, also known as DCLK1⁺ cells, are dependent of the transcriptional factor POU2F3. Several works report DCLK1⁺ cells in early stages of PDAC development suggesting an important role of TUFT cells in PDAC development. Therefore, we developed a mice model (PDX1-Cre;Kras^G12D;Ink4a^fl/fl), known as PKI model, deficient or not of POU2F3. In this animal model, deficiency of POU2F3 results in the absence of TUFT cells in PDAC as expected. Although, tumor development and growth are not significantly influenced, the development of liver metastasis was almost completely inhibited in POU2F3-deficient mice. Surprisingly, the absence of metastasis was associated with a higher expression of epithelial-to-mesenchymal transition markers, but to a lower inflammatory microenvironment suggesting that inflammation influences metastasis production more than epithelial-to-mesenchymal transition in this animal model. We can conclude that POU2F3 could be a new therapeutic target for control PDAC progression.

Intrathymic Selection and Defects in the Thymic Epithelial Cell Development

Intimate interactions between thymic epithelial cells (TECs) and thymocytes (T) have been repeatedly reported as essential for performing intrathymic T-cell education. Nevertheless, it has been described that animals exhibiting defects in these interactions were capable of a proper positive and negative T-cell selection. In the current review, we first examined distinct types of TECs and their possible role in the immune surveillance. However, EphB-deficient thymi that exhibit profound thymic epithelial (TE) alterations do not exhibit important immunological defects. Eph and their ligands, the ephrins, are implicated in cell attachment/detachment and govern, therefore, TEC–T interactions. On this basis, we hypothesized that a few normal TE areas could be enough for a proper phenotypical and functional maturation of T lymphocytes. Then, we evaluated in vivo how many TECs would be necessary for supporting a normal T-cell differentiation, concluding that a significantly low number of TEC are still capable of supporting normal T lymphocyte maturation, whereas with fewer numbers, T-cell maturation is not possible.

Helminth Sensing at the Intestinal Epithelial Barrier-A Taste of Things to Come

Human intestinal helminth infection affects more than 1 billion people often in the world's most deprived communities. These parasites are one of the most prevalent neglected tropical diseases worldwide bringing huge morbidities to the host population. Effective treatments and vaccines for helminths are currently limited, and therefore, it is essential to understand the molecular sensors that the intestinal epithelium utilizes in detecting helminths and how the responding factors produced act as modulators of immunity. Defining the cellular and molecular mechanisms that enable helminth detection and expulsion will be critical in identifying potential therapeutic targets to alleviate disease. However, despite decades of research, we have only recently been able to identify the tuft cell as a key helminth sensor at the epithelial barrier. In this review, we will highlight the key intestinal epithelial chemosensory roles associated with the detection of intestinal helminths, summarizing the recent advances in tuft cell initiation of protective type 2 immunity. We will discuss other potential sensory roles of epithelial subsets and introduce enteroendocrine cells as potential key sensors of the microbial alterations that a helminth infection produces, which, given their direct communication to the nervous system via the recently described neuropod, have the potential to transfer the epithelial immune interface systemically.

Euglena Gracilis and β-Glucan Paramylon Induce Ca2+ Signaling in Intestinal Tract Epithelial, Immune, and Neural Cells

The intestinal tract contains over half of all immune cells and peripheral nerves and manages the beneficial interactions between food compounds and the host. Paramylon is a β-1,3-glucan storage polysaccharide from Euglena gracilis (Euglena) that exerts immunostimulatory activities by affecting cytokine production. This study investigated the signaling mechanisms that regulate the beneficial interactions between food compounds and the intestinal tract using cell type-specific calcium (Ca²⁺) imaging in vivo and in vitro. We successfully visualized Euglena- and paramylon-mediated Ca²⁺ signaling in vivo in intestinal epithelial cells from mice ubiquitously expressing the Yellow Cameleon 3.60 (YC3.60) Ca²⁺ biosensor. Moreover, in vivo Ca²⁺ imaging demonstrated that the intraperitoneal injection of both Euglena and paramylon stimulated dendritic cells (DCs) in Peyer’s patches, indicating that paramylon is an active component of Euglena that affects the immune system. In addition, in vitro Ca²⁺ imaging in dorsal root ganglia indicated that Euglena, but not paramylon, triggers Ca²⁺ signaling in the sensory nervous system innervating the intestine. Thus, this study is the first to successfully visualize the direct effect of β-1,3-glucan on DCs in vivo and will help elucidate the mechanisms via which Euglena and paramylon exert various effects in the intestinal tract.

Interactions of commensal and pathogenic microorganisms with the mucus layer in the colon

The intestinal mucosal barrier, which is composed of epithelial cells and mucus layers secreted by goblet cells and contains commensal bacteria, constitutes the first line of defense against pathogenic gut microbiota. However, homeostasis between the microbiota and mucus layer is easily disrupted by certain factors, resulting in alteration of the gut microbiota and entry of pathogens to the intestinal mucosal barrier. In this review, we describe the structures and functions of the mucus layer, expound several crucial influencing factors, including diet styles, medications and host genetics, and discuss how pathogenic microorganisms interact with the mucus layer and commensal microbiota, with the understanding that unraveling their complex interactions under homeostatic and dysbiosis conditions in the colon would help reveal some underlying pathogenic mechanisms and thus develop new strategies to prevent pathogenic microbiological colonization.

The Effect of Thiol Structure on Allyl Sulfide Photodegradable Hydrogels and their Application as a Degradable Scaffold for Organoid Passaging

Intestinal organoids are useful in vitro models for basic and translational studies aimed at understanding and treating disease. However, their routine culture relies on animal-derived matrices that limit translation to clinical applications. In fact, there are few fully defined, synthetic hydrogel systems that allow for the expansion of intestinal organoids. Here, an allyl sulfide photodegradable hydrogel is presented, achieving rapid degradation through radical addition-fragmentation chain transfer (AFCT) reactions, to support routine passaging of intestinal organoids. Shear rheology to first characterize the effect of thiol and allyl sulfide crosslink structures on degradation kinetics is used. Irradiation with 365 nm light (5 mW cm-2 ) in the presence of a soluble thiol (glutathione at 15 × 10-3 m), and a photoinitiator (lithium phenyl-2,4,6-trimethylbenzoylphosphinate at 1 × 10-3 m), leads to complete hydrogel degradation in less than 15 s. Allyl sulfide hydrogels are used to support the formation of epithelial colonies from single intestinal stem cells, and rapid photodegradation is used to achieve repetitive passaging of stem cell colonies without loss in morphology or organoid formation potential. This platform could support long-term culture of intestinal organoids, potentially replacing the need for animal-derived matrices, while also allowing systematic variations to the hydrogel properties tailored for the organoid of interest.

Cholinergic Activation of Primary Human Derived Intestinal Epithelium Does Not Ameliorate TNF-α Induced Injury

Introduction

The intestinal epithelium contains specialized cells including enterocytes, goblet, Paneth, enteroendocrine, and stem cells. Impaired barrier integrity in Inflammatory Bowel Disease is characterized by elevated levels of pro-inflammatory cytokines, including tumor necrosis factor-alpha (TNF-α). Prior studies in immortalized lines such as Caco-2, without native epithelial heterogeneity, demonstrate the amelioration of TNF-α compromised barrier integrity via nicotinic (nAChR) or muscarinic (mAChR) acetylcholine receptor activation.

Methods

A tissue-engineered model of primary human small intestinal epithelium was derived from dissociated organoids cultured on collagen-coated Transwells. Differentiation was accomplished with serum-containing media and compared to Caco-2 and HT-29 regarding alkaline phosphatase expression, transepithelial electrical resistance (TEER), and IL-8 secretion. Inflammation was modeled via basal stimulation with TNF-α (25 ng/mL) with or without nicotine (nAChR agonist) or bethanechol (mAChR agonist). Apoptosis, density (cells/cm²), TEER, lucifer yellow permeability, 70 kDa dextran transport, cell morphology, and IL-8 secretion were characterized.

Results

Primary intestinal epithelium demonstrates significant functional differences compared to immortalized cells, including increased barrier integrity, IL-8 expression, mucus production, and the presence of absorptive and secretory cells. Exposure to TNF-α impaired barrier integrity, increased apoptosis, altered morphology, and increased secretion of IL-8. Stimulation of nAChR with nicotine did not ameliorate TNF-α induced permeability nor alter 70 kDa dextran transport. However, stimulation of mAChR with bethanechol decreased transport of 70 kDa dextran but did not ameliorate TNF-α induced paracellular permeability.

Conclusions

A primary model of intestinal inflammation was evaluated, demonstrating nAChR or mAChR activation does not have the same protective effects compared to immortalized epithelium. Inclusion of other native stromal support cells are underway.

Exploration of Cell Development Pathways through High-Dimensional Single Cell Analysis in Trajectory Space

High-dimensional single cell profiling coupled with computational modeling is emerging as a powerful tool to elucidate developmental programs directing cell lineages. We introduce tSpace, an algorithm based on the concept of "trajectory space", in which cells are defined by their distance along nearest neighbor pathways to every other cell in a population. Graphical mapping of cells in trajectory space allows unsupervised reconstruction and exploration of complex developmental sequences. Applied to flow and mass cytometry data, the method faithfully reconstructs thymic T cell development and reveals development and trafficking regulation of tonsillar B cells. Applied to the single cell transcriptome of mouse intestine and C. elegans, the method recapitulates development from intestinal stem cells to specialized epithelial phenotypes more faithfully than existing algorithms and orders C. elegans cells concordantly to the associated embryonic time. tSpace profiling of complex populations is well suited for hypothesis generation in developing cell systems.

Mucosal Tuft Cell Density Is Increased in Diarrhea-Predominant Irritable Bowel Syndrome Colonic Biopsies

Tuft cells are rare chemosensory sentinels found in the gut epithelium. When triggered by helminth infection, tuft cells secrete interleukin-25 (IL-25) basolaterally and subsequently evoke an immune response. Irritable bowel syndrome (IBS) is a common and heterogeneous disorder characterized by bowel dysfunction and visceral pain sensitivity. Dysfunctional gut-brain communication and immune activation contribute to the pathophysiology of this disorder. The study aims were to investigate changes in tuft cell density in non-post-infectious IBS patients. Immunofluorescent labeling of DCLK1-positive tuft cells was carried out in mucosal biopsies from the distal colons of diarrhea and constipation-predominant IBS patients and healthy controls. Tuft cell numbers were also assessed in animal models. Concentrations of interleukin-25 (IL-25) secreted from colonic biopsies and in plasma samples were analyzed using an immunoassay. The density of tuft cells was increased in diarrhea-but not constipation-predominant IBS patient colonic biopsies. Biopsy secretions and plasma concentrations of IL-25 were elevated in diarrhea-but not constipation-predominant IBS participants. Tuft cell hyperplasia was detected in a rat model of IBS but not in mice exposed to chronic stress. Tuft cell hyperplasia is an innate immune response to helminth exposure. However, the patients with diarrhea-predominant IBS have not reported any incidents of enteric infection. Moreover, rats exhibiting IBS-like symptoms displayed increased tuft cell density but were not exposed to helminths. Our findings suggest that factors other than helminth exposure or chronic stress lead to tuft cell hyperplasia in IBS colonic biopsies.

Tight Junction Protein Claudin-7 Is Essential for Intestinal Epithelial Stem Cell Self-Renewal and Differentiation

BACKGROUND & AIMS

Claudin-7 (Cldn7) is a tight junction (TJ) membrane protein located at the apical TJ and basolateral side of intestinal epithelial cells. Deletion of Cldn7 by gene targeting leads to the inflammatory bowel disease-like phenotype in mice, which includes weight loss, diarrhea, mucosa ulceration, and severe intestinal epithelial damage. In this study, we test our hypothesis that Cldn7 plays a critical role in regulating intestinal crypt stem cell functions.

METHODS

Gene expression microarray, quantitative reverse-transcription polymerase chain reaction, in situ hybridization, histologic examinations, immunoblotting, 3-dimensional organoid culture, and various treatments to rescue Cldn7-deficient organoid defects were conducted using global Cldn7 knockout mice and inducible, conditional Cldn7 knockout mice.

RESULTS

Gene deletion of Cldn7 in intestines showed significant alteration of expression profiles with striking down-regulation of intestinal crypt stem cell markers such as Olfm4, dislocated proliferative cells, and disrupted epithelial cell differentiation. In addition, the isolated Cldn7-deficient crypts where the stem cells reside were either unable to survive at all or formed defective spheroids, highlighting the functional impairment of crypt stem cells in the absence of Cldn7. Remarkably, the Cldn7-expressing organoids with buddings underwent rapid cell degeneration within days after turning off Cldn7 expression in the culture. We identified that activation of Wnt/β-catenin signaling rescued the organoid defects caused by Cldn7 deletion.

CONCLUSIONS

In this study, we show that Cldn7 is indispensable in controlling Wnt/β-catenin signaling-dependent intestinal epithelial stem cell survival, self-renewal, and cell differentiation. This study could open a door to study roles of TJ proteins in stem cell regulations in other tissues and organs.

Organoids in immunological research

Much of our knowledge regarding the interactions between epithelial tissues and the immune system has been gathered from animal models and co-cultures with cell lines. However, unique features of human cells cannot be modelled in mice, and cell lines are often transformed or genetically immortalized. Organoid technology has emerged as a powerful tool to maintain epithelial cells in a near-native state. In this Review, we discuss how organoids are being used in immunological research to understand the role of epithelial cell-immune cell interactions in tissue development and homeostasis, as well as in diseases such as cancer.

Thymic epithelial cell heterogeneity: TEC by TEC

The generation of a functional T cell repertoire in the thymus is mainly orchestrated by thymic epithelial cells (TECs), which provide developing T cells with cues for their navigation, proliferation, differentiation and survival. The TEC compartment has been segregated historically into two major populations of medullary TECs and cortical TECs, which differ in their anatomical localization, molecular characteristics and functional roles. However, recent studies have shown that TECs are highly heterogeneous and comprise multiple subpopulations with distinct molecular and functional characteristics, including tuft cell-like or corneocyte-like phenotypes. Here, we review the most recent advances in our understanding of TEC heterogeneity from a molecular, functional and developmental perspective. In particular, we highlight the key insights that were recently provided by single-cell genomic technologies and in vivo fate mapping and discuss them in the context of previously published data.

Generating an Artificial Intestine for the Treatment of Short Bowel Syndrome

Intestinal failure is defined as the inability to maintain fluid, nutrition, energy, and micronutrient balance that leads to the inability to gain or maintain weight, resulting in malnutrition and dehydration. Causes of intestinal failure include short bowel syndrome (ie, the physical loss of intestinal surface area and severe intestinal dysmotility). For patients with intestinal failure who fail to achieve enteral autonomy through intestinal rehabilitation programs, the current treatment options are expensive and associated with severe complications. Therefore, the need persists for next-generation therapies, including cell-based therapy, to increase intestinal regeneration, and development of the tissue-engineered small intestine.

Differences in immune responses to Haemonchus contortus infection in the susceptible Ile de France and the resistant Santa Ines sheep under different anthelmintic treatments regimens

Understanding the immunological basis of resistance to gastrointestinal nematode infections in livestock is important in order to develop novel methods of parasite control such as vaccination or genetic selection for parasite resistance. The present study aimed to investigate differences in immune response between parasite resistant Santa Ines and susceptible Ile de France sheep breeds to natural Haemonchus contortus infection. Parasitological parameters, humoral immunity, local and circulating cellular immune responses were evaluated in 19 Santa Ines and 19 Ile de France lambs undergoing different anthelmintic treatments regimens: suppressive treatments (SUP) or targeted selective treatments (TST) over a 5-month grazing period. Santa Ines lambs had significantly lower Haemonchus faecal egg count and worm burden compared to Ile de France regardless of treatment regime. In addition, circulating blood eosinophils count and parasite-specific IgG levels were significantly higher and more rapidly induced in Santa Ines lambs. Abomasal immune responses were generally greater in the resistant breed, which had significantly higher levels of parasite-specific IgA in mucus, and elevated number of globule leukocytes and CD3+ T cells within the abomasal mucosal. Furthermore, numbers of POU2F3+ epithelial cells, a tuft-cell specific transcription factor, were also elevated in the Santa Ines breed, suggesting that this breed is better able to initiate T-helper type 2 immune responses within the abomasum. In conclusion, the differential immunological responses detailed here are relevant to understanding resistance to gastrointestinal nematodes in other host breeds, as well as to resistance breeding as a sustainable control approach for parasitic infections.

Distribution pattern and molecular signature of cholinergic tuft cells in human gastro-intestinal and pancreatic-biliary tract

Despite considerable recent insight into the molecular phenotypes and type 2 innate immune functions of tuft cells in rodents, there is sparse knowledge about the region-specific presence and molecular phenotypes of tuft cells in the human digestive tract. Here, we traced cholinergic tuft cells throughout the human alimentary tract with immunohistochemistry and deciphered their region-specific distribution and biomolecule coexistence patterns. While absent from the human stomach, cholinergic tuft cells localized to villi and crypts in the small and large intestines. In the biliary tract, they were present in the epithelium of extra-hepatic peribiliary glands, but not observed in the epithelia of the gall bladder and the common duct of the biliary tract. In the pancreas, solitary cholinergic tuft cells were frequently observed in the epithelia of small and medium-size intra- and inter-lobular ducts, while they were absent from acinar cells and from the main pancreatic duct. Double immunofluorescence revealed co-expression of choline acetyltransferase with structural (cytokeratin 18, villin, advillin) tuft cell markers and eicosanoid signaling (cyclooxygenase 1, hematopoietic prostaglandin D synthase, 5-lipoxygenase activating protein) biomolecules. Our results indicate that region-specific cholinergic signaling of tuft cells plays a role in mucosal immunity in health and disease, especially in infection and cancer.

Trickle infection and immunity to Trichuris muris

The majority of experiments investigating the immune response to gastrointestinal helminth infection use a single bolus infection. However, in situ individuals are repeatedly infected with low doses. Therefore, to model natural infection, mice were repeatedly infected (trickle infection) with low doses of Trichuris muris. Trickle infection resulted in the slow acquisition of immunity reflected by a gradual increase in worm burden followed by partial expulsion. Flow cytometry revealed that the CD4+ T cell response shifted from Th1 dominated to Th2 dominated, which coincided with an increase in Type 2 cytokines. The development of resistance following trickle infection was associated with increased worm expulsion effector mechanisms including goblet cell hyperplasia, Muc5ac production and increased epithelial cell turn over. Depletion of CD4+ T cells reversed resistance confirming their importance in protective immunity following trickle infection. In contrast, depletion of group 2 innate lymphoid cells did not alter protective immunity. T. muris trickle infection resulted in a dysbiotic mircrobiota which began to recover alpha diversity following the development of resistance. These data establish trickle infection as a robust and informative model for analysis of immunity to chronic intestinal helminth infection more akin to that observed under natural infection conditions and confirms the importance of CD4+ T cell adaptive immunity in host protection.

Infection with parasitic worms (helminths) is a considerable cause of morbidity in humans. Understanding how we respond to infection is crucial to developing novel therapies. Laboratory models of helminth infection have been a valuable tool in understanding fundamental immune responses to infection. However, typically an individual mouse will be infected with a large, single-dose of the parasite. This is in contrast to the natural scenario in which individuals will receive frequent low level exposures. However, it is unknown how repeated infection alters the development of immunity to infection. We have developed a laboratory model to tackle this question. We infected mice with the model helminth Trichuris muris on a weekly basis and assessed a range of responses in comparison with a more traditional infection regime. We found striking differences in the dynamics of the infection, the host immune response, and in changes to host gut microbial populations. Our study shows how resistance to helminth infection can develop over time in response to repeat infection, and provides a model system that better reflects human immunity to this parasite.

The Immune Function of Tuft Cells at Gut Mucosal Surfaces and Beyond

Tuft cells were first discovered in epithelial barriers decades ago, but their function remained unclear until recently. In the last 2 years, a series of studies has provided important advances that link tuft cells to infectious diseases and the host immune responses. Broadly, a model has emerged in which tuft cells use chemosensing to monitor their surroundings and translate environmental signals into effector functions that regulate immune responses in the underlying tissue. In this article, we review the current understanding of tuft cell immune function in the intestines, airways, and thymus. In particular, we discuss the role of tuft cells in type 2 immunity, norovirus infection, and thymocyte development. Despite recent advances, many fundamental questions about the function of tuft cells in immunity remain to be answered.

The transcriptome difference between colorectal tumor and normal tissues revealed by single-cell sequencing

The previous cancer studies were difficult to reproduce since the tumor tissues were analyzed directly. But the tumor tissues were actually a mixture of different cancer cells. The transcriptome of single-cell was much robust than the transcriptome of a mixed tissue. The single-cell transcriptome had much smaller variance. In this study, we analyzed the single-cell transcriptome of 272 colorectal cancer (CRC) epithelial cells and 160 normal epithelial cells and identified 342 discriminative transcripts using advanced machine learning methods. The most discriminative transcripts were LGALS4, PHGR1, C15orf48, HEPACAM2, PERP, FABP1, FCGBP, MT1G, TSPAN1 and CKB. We further clustered the 342 transcripts into two categories. The upregulated transcripts in CRC epithelial cells were significantly enriched in Ribosome, Protein processing in endoplasmic reticulum, Antigen processing and presentation and p53 signaling pathway. The downregulated transcripts in CRC epithelial cells were significantly enriched in Mineral absorption, Aldosterone-regulated sodium reabsorption and Oxidative phosphorylation pathways. The biological analysis of the discriminative transcripts revealed the possible mechanism of colorectal cancer.

Biphenotypic Differentiation of Pancreatic Cancer in 3-Dimensional Culture

OBJECTIVE

Pancreatic ductal adenocarcinoma (PDAC) is the third most common cause of cancer death in the United States. Improved characterized models of PDAC are needed for drug screening.

METHODS

We grew 4 established pancreatic cancer cell lines in hanging drop cultures to produce spheroids. We also grew organoids from explanted xenografted PDAC and surgically resected primary PDAC. We performed transmission and scanning electron microscopy and compared findings with those of the normal pancreatic duct. We also performed single-cell cloning to determine the potential options for differentiation.

RESULTS

Spheroids contained tight junctions and desmosomes but lacked zymogen granules, as expected. The former features were present in normal pancreatic duct but absent from PDAC cell lines grown in standard 2-dimensional culture. Spheroids functionally excluded macromolecules in whole mounts. Cells on the surface of PDAC spheroids were carpeted by microvilli except for rare cells with prominent stereocilia. Carpets of microvilli were also seen in low passage organoids produced from xenografts and surgically resected human PDAC, in addition to normal human pancreatic duct. We performed single-cell cloning and resulting spheroids produced both cell phenotypes at the same approximate ratios as those from bulk cultures.

CONCLUSIONS

Pancreatic cancer spheroids/organoids are capable of biphenotypic differentiation.

A robust human norovirus replication model in zebrafish larvae

Human noroviruses (HuNoVs) are the most common cause of foodborne illness, with a societal cost of $60 billion and 219,000 deaths/year. The lack of robust small animal models has significantly hindered the understanding of norovirus biology and the development of effective therapeutics. Here we report that HuNoV GI and GII replicate to high titers in zebrafish (Danio rerio) larvae; replication peaks at day 2 post infection and is detectable for at least 6 days. The virus (HuNoV GII.4) could be passaged from larva to larva two consecutive times. HuNoV is detected in cells of the hematopoietic lineage and the intestine, supporting the notion of a dual tropism. Antiviral treatment reduces HuNoV replication by >2 log10, showing that this model is suited for antiviral studies. Zebrafish larvae constitute a simple and robust replication model that will largely facilitate studies of HuNoV biology and the development of antiviral strategies.

Dclk1 in tuft cells promotes inflammation-driven epithelial restitution and mitigates chronic colitis

Ulcerative colitis (UC) is a chronic inflammatory bowel disease characterized by defective intestinal barrier integrity toward the microbiota and epithelial damage. Double cortin-like kinase 1 (Dclk1), a marker of intestinal tuft cells, can regulate tissue regenerative responses, but its role in epithelial repair during bacterial-dependent chronic colitis is unclear. We addressed this question using our recently developed mouse model of spontaneous microbiota-dependent colitis induced by mucin-type O-glycan deficiency (DKO), which recapitulates most features of human UC. We generated DKO mice lacking intestinal epithelial Dclk1 (DKO;Dclk1ΔIEC) and analyzed colitis onset and severity using clinical and histologic indices, immune responses by qPCR and immunostaining, and epithelial responses using proliferation markers and organoid culture. We found 3-4-week-old DKO;Dclk1ΔIEC mice developed worsened spontaneous colitis characterized by reduced body weight, loose stool, severe colon thickening, epithelial lesions, and inflammatory cell infiltrates compared with DKO mice. The primary defect was an impaired epithelial proliferative response during inflammation. Dclk1 deficiency also reduced inflammation-induced proliferation and growth of colon organoids ex vivo. Mechanistically, Dclk1 expression was important for inflammation-induced Cox2 expression and prostaglandin E2 (PGE2) production in vivo, and PGE2 rescued proliferative defects in Dclk1-deficient colonic organoids. Although tuft cells were expanded in both DKO and DKO;Dclk1ΔIEC relative to WT mice, loss of Dclk1 was associated with reduced tuft cell activation (i.e., proliferation) during inflammation. Similar results were found in DKO vs. DKO;Dclk1ΔIEC mice at 3-6 months of age. Our results support that tuft cells, via Dclk1, are important responders to bacterial-induced colitis by enhancing epithelial repair responses, which in turn limits bacterial infiltration into the mucosa.

Taste and the Gastrointestinal tract: from physiology to potential therapeutic target for obesity

Flavor is the combination of gustatory, olfactory and trigeminal sensations, representing the three main sensory pathways that allow detecting environmental chemical substances. Taste, in particular, is a complex chemosensory path that allows identification of substances present in ingested foods and beverages. In this manuscript, we propose a conceptual roadmap from aspects related to the evolution and the physiological role of taste, up to the current knowledge about its implication in the modulation of a healthy state, or obesity. More specifically, we focused on the role of stimulation of taste receptors in releasing gut hormones (also known as enterohormones), and their effects on the regulation of food intake, by inducing satiety, either by locally acting (in the gastrointestinal tract), or centrally (in the brain). Recent evidence demonstrated that some enterohormones are able to modulate gastrointestinal motility, thus affecting an orexigenic responses in the central nervous system. In keeping with this, we discuss the ability of the gustatory system to be a final checkpoint control for food intake regulation, and we speculate about taste perception manipulation in the management of obesity.

Extra-Adrenal Glucocorticoid Synthesis in the Intestinal Mucosa: Between Immune Homeostasis and Immune Escape

Glucocorticoids (GCs) are steroid hormones predominantly produced in the adrenal glands in response to physiological cues and stress. Adrenal GCs mediate potent anti-inflammatory and immunosuppressive functions. Accumulating evidence in the past two decades has demonstrated other extra-adrenal organs and tissues capable of synthesizing GCs. This review discusses the role and regulation of GC synthesis in the intestinal epithelium in the regulation of normal immune homeostasis, inflammatory diseases of the intestinal mucosa, and the development of intestinal tumors.

Innate lymphoid cells, mediators of tissue homeostasis, adaptation and disease tolerance

Innate lymphoid cells (ILC) are a recently identified group of tissue-resident innate lymphocytes. Available data support the view that ILC or their progenitors are deposited and retained in tissues early during ontogeny. Thereby, ILC become an integral cellular component of tissues and organs. Here, we will review the intriguing relationships between ILC and basic developmental and homeostatic processes within tissues. Studying ILC has already led to the appreciation of the integral roles of immune cells in tissue homeostasis, morphogenesis, metabolism, regeneration, and growth. This area of immunology has not yet been studied in-depth but is likely to reveal important networks contributing to disease tolerance and may be harnessed for future therapeutic approaches.

Therapeutic Potential of Hematopoietic Prostaglandin D2 Synthase in Allergic Inflammation

Worldwide, there is a rise in the prevalence of allergic diseases, and novel efficient therapeutic approaches are still needed to alleviate disease burden. Prostaglandin D₂ (PGD₂) has emerged as a central inflammatory lipid mediator associated with increased migration, activation and survival of leukocytes in various allergy-associated disorders. In the periphery, the hematopoietic PGD synthase (hPGDS) acts downstream of the arachidonic acid/COX pathway catalysing the isomerisation of PGH₂ to PGD₂, which makes it an interesting target to treat allergic inflammation. Although much effort has been put into developing efficient hPGDS inhibitors, no compound has made it to the market yet, which indicates that more light needs to be shed on potential PGD₂ sources and targets to determine which particular condition and patient will benefit most and thereby improve therapeutic efficacy. In this review, we want to revisit current knowledge about hPGDS function, expression in allergy-associated cell types and their contribution to PGD₂ levels as well as beneficial effects of hPGDS inhibition in allergic asthma, rhinitis, atopic dermatitis, food allergy, gastrointestinal allergic disorders and anaphylaxis.

Unique Regulation of Na-K-ATPase during Growth and Maturation of Intestinal Epithelial Cells

Na-K-ATPase on the basolateral membrane provides the favorable transcellular Na gradient for the proper functioning of Na-dependent nutrient co-transporters on the brush border membrane (BBM) of enterocytes. As cells mature from crypts to villus, Na-K-ATPase activity doubles, to accommodate for the increased BBM Na-dependent nutrient absorption. However, the mechanism of increased Na-K-ATPase activity during the maturation of enterocytes is not known. Therefore, this study aimed to determine the mechanisms involved in the functional transition of Na-K-ATPase during the maturation of crypts to villus cells. Na-K-ATPase activity gradually increased as IEC-18 cells matured in vitro from day 0 (crypts) through day 4 (villus) of post-confluence. mRNA abundance and Western blot studies showed no change in the levels of Na-K-ATPase subunits α1 and β1 from 0 to 4 days post-confluent cells. However, Na-K-ATPase α1 phosphorylation levels on serine and tyrosine, but not threonine, residues gradually increased. These data indicate that as enterocytes mature from crypt-like to villus-like in culture, the functional activity of Na-K-ATPase increases secondary to altered affinity of the α1 subunit to extracellular K⁺, in order to accommodate the functional preference of the intestinal cell type. This altered affinity is likely due to increased phosphorylation of the α1 subunit, specifically at serine and tyrosine residues.

"Candida Albicans Interactions With The Host: Crossing The Intestinal Epithelial Barrier"

Formerly a commensal organism of the mucosal surfaces of most healthy individuals, Candida albicans is an opportunistic pathogen that causes infections ranging from superficial to the more life-threatening disseminated infections, especially in the ever-growing population of vulnerable patients in the hospital setting. In these situations, the fungus takes advantage of its host following a disturbance in the host defense system and/or the mucosal microbiota. Overwhelming evidence suggests that the gastrointestinal tract is the main source of disseminated C. albicans infections. Major risk factors for disseminated candidiasis include damage to the mucosal intestinal barrier, immune dysfunction, and dysbiosis of the resident microbiota. A better understanding of C. albicans' interaction with the intestinal epithelial barrier will be useful for designing future therapies to avoid systemic candidiasis. In this review, we provide an overview of the current knowledge regarding the mechanisms of pathogenicity that allow the fungus to reach and translocate the gut barrier.

Hydrolysis and Transport of Egg White-Derived Peptides in Caco-2 Cell Monolayers and Everted Rat Sacs

The objective of this paper was to investigate the hydrolysis and transepithelial transport of egg white peptides in Caco-2 cell monolayers and everted rat sacs. Results showed that egg white peptides had higher permeability but lower degradation in Caco-2 cell monolayers than found for everted rat sacs. Peptides LGAKDSTRT, DGSRQPVDN, VNDLQGKTS, and GKKDPVLKD were identified from not only the basolateral (BL) side of Caco-2 cell monolayers but also the serous side of everted rat sacs, suggesting that these four peptides could be transported intact in both model systems. In addition, there were 24 peptides identified from the apical (AP) side of Caco-2 cell monolayers and the mucosal side of everted rat sacs, indicating potential resistance to hydrolysis by brush border membrane peptidases. Among these, peptides IRDLLER, YAEERYP, and IRNVLQPS were demonstrated as having dipeptidyl peptidase IV (DPP-IV) inhibitory activities with IC₅₀ values of 186.23 ± 15.25, 340.62 ± 4.73, and 598.28 ± 15.12 μM ( P < 0.05), respectively. Furthermore, molecular docking revealed that the DPP-IV inhibitory peptides were predicted to form hydrogen-bonds, π-π bonds, and charge interactions with the activity sites, especially the amino acid residues located in the S2 pocket of DPP-IV, potentially contributing to their DPP-IV inhibitory activities.

ILC2s-Trailblazers in the Host Response Against Intestinal Helminths

Group 2 innate lymphoid cells (ILC2s) were first discovered in experimental studies of intestinal helminth infection—and much of our current knowledge of ILC2 activation and function is based on the use of these models. It is perhaps not surprising therefore that these cells have also been found to play a key role in mediating protection against these large multicellular parasites. ILC2s have been intensively studied over the last decade, and are known to respond quickly and robustly to the presence of helminths—both by increasing in number and producing type 2 cytokines. These mediators function to activate and repair epithelial barriers, to recruit other innate cells such as eosinophils, and to help activate T helper 2 cells. More recent investigations have focused on the mechanisms by which the host senses helminth parasites to activate ILC2s. Such studies have identified novel stromal cell types as being involved in this process—including intestinal tuft cells and enteric neurons, which respond to the presence of helminths and activate ILC2s by producing IL-25 and Neuromedin, respectively. In the current review, we will outline the latest insights into ILC2 activation and discuss the requirement for—or redundancy of—ILC2s in providing protective immunity against intestinal helminth parasites.

Tuft cells: From the mucosa to the thymus

Tuft cells are epithelial chemosensory cells with unique morphological and molecular characteristics, the most noticeable of which is a tuft of long and thick microvilli on their apical side, as well as expression of a very distinct set of genes, including genes encoding various members of the taste transduction machinery and pro-inflammatory cyclooxygenases. Initially discovered in rat trachea, tuft cells were gradually identified in various mucosal tissues, and later also in non-mucosal tissues, most recent of which is the thymus. Although tuft cells were discovered more than 60 years ago, their functions in the various tissues remained a mystery until recent years. Today, tuft cells are thought to function as sensors of various types of chemical signals, to which they respond by secretion of diverse biological mediators such as IL25 or acetylcholine. Intestinal tuft cells were also shown to mediate type 2 immunity against parasites. Here, we review the current knowledge on tuft cell characteristics, development and heterogeneity, discuss their potential functions and explore the possible implications and significance of their discovery in the thymus.

Infection by the parasitic helminth Trichinella spiralis activates a Tas2r-mediated signaling pathway in intestinal tuft cells

Intestinal tuft cells are sentinels monitoring the luminal contents and play a critical role in type 2 immunity. In this work, Trichinella spiralis excretion–secretion and extract were shown to directly induce interleukin 25 (IL-25) release from the intestinal villi, evoke calcium responses in tuft cells, and activate Tas2r bitter-taste receptors, whereas the bitter compound salicin was shown to activate and induce tuft cells to release IL-25. Gα-gustducin/Gβ1γ13 and/or Gαo/Gβ1γ13, Plcβ2, Ip₃r2, and Trpm5 comprise the signal transduction pathways that tuft cells utilize to initiate type 2 immune responses. Potentiation of Trpm5 by a natural sweet compound, stevioside, can enhance the tuft cell–ILC2 circuit’s activity, indicating that modulating these signaling components can help devise new means of combating parasites.

The parasitic helminth Trichinella spiralis, which poses a serious health risk to animals and humans, can be found worldwide. Recent findings indicate that a rare type of gut epithelial cell, tuft cells, can detect the helminth, triggering type 2 immune responses. However, the underlying molecular mechanisms remain to be fully understood. Here we show that both excretory–secretory products (E–S) and extract of T. spiralis can stimulate the release of the cytokine interleukin 25 (IL-25) from the mouse small intestinal villi and evoke calcium responses from tuft cells in the intestinal organoids, which can be blocked by a bitter-taste receptor inhibitor, allyl isothiocyanate. Heterologously expressed mouse Tas2r bitter-taste receptors, the expression of which is augmented during tuft-cell hyperplasia, can respond to the E–S and extract as well as to the bitter compound salicin whereas salicin in turn can induce IL-25 release from tuft cells. Furthermore, abolishment of the G-protein γ13 subunit, application of the inhibitors for G-protein αo/i, Gβγ subunits, and phospholipase Cβ2 dramatically reduces the IL-25 release. Finally, tuft cells are found to utilize the inositol triphosphate receptor type 2 (Ip₃r2) to regulate cytosolic calcium and thus Trpm5 activity, while potentiation of Trpm5 by a sweet-tasting compound, stevioside, enhances tuft cell IL-25 release and hyperplasia in vivo. Taken together, T. spiralis infection activates a signaling pathway in intestinal tuft cells similar to that of taste-bud cells, but with some key differences, to initiate type 2 immunity.

Hyperlipidemias and Obesity

The organs require oxygen and other types of nutrients (amino acids, sugars, and lipids) to function, the heart consuming large amounts of fatty acids for oxidation and adenosine triphosphate (ATP) generation.

Tales from the crypt: new insights into intestinal stem cells

The intestinal epithelium withstands continuous mechanical, chemical and biological insults despite its single-layered, simple epithelial structure. The crypt-villus tissue architecture in combination with rapid cell turnover enables the intestine to act both as a barrier and as the primary site of nutrient uptake. Constant tissue replenishment is fuelled by continuously dividing stem cells that reside at the bottom of crypts. These cells are nurtured and protected by specialized epithelial and mesenchymal cells, and together constitute the intestinal stem cell niche. Intestinal stem cells and early progenitor cells compete for limited niche space and, therefore, the ability to retain or regain stemness. Those cells unable to do so differentiate to one of six different mature cell types and move upwards towards the villus, where they are shed into the intestinal lumen after 3-5 days. In this Review, we discuss the signals, cell types and mechanisms that control homeostasis and regeneration in the intestinal epithelium. We investigate how the niche protects and instructs intestinal stem cells, which processes drive differentiation of mature cells and how imbalance in key signalling pathways can cause human disease.

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.

Primary Cell-Derived Intestinal Models: Recapitulating Physiology

The development of physiologically relevant intestinal models fueled by breakthroughs in primary cell-culture methods has enabled successful recapitulation of key features of intestinal physiology. These advances, paired with engineering methods, for example incorporating chemical gradients or physical forces across the tissues, have yielded ever more sophisticated systems that enhance our understanding of the impact of the host microbiome on human physiology as well as on the genesis of intestinal diseases such as inflammatory bowel disease and colon cancer. In this review we highlight recent advances in the development and usage of primary cell-derived intestinal models incorporating monolayers, organoids, microengineered platforms, and macrostructured systems, and discuss the expected directions of the field.