Telomerase expression marks transitional growth-associated skeletal progenitor/stem cells.

Skeletal progenitor/stem cells (SSCs) play a critical role in postnatal bone growth and maintenance. Telomerase (Tert) activity prevents cellular senescence and is required for maintenance of stem cells in self-renewing tissues. Here we investigated the role of mTert-expressing cells in postnatal mouse long bone and found that mTert expression is enriched at the time of adolescent bone growth. mTert-GFP+ cells were identified in regions known to house SSCs, including the metaphyseal stroma, growth plate, and the bone marrow. We also show that mTert-expressing cells are a distinct SSC population with enriched colony-forming capacity and contribute to multiple mesenchymal lineages, in vitro. In contrast, in vivo lineage-tracing studies identified mTert+ cells as osteochondral progenitors and contribute to the bone-forming cell pool during endochondral bone growth with a subset persisting into adulthood. Taken together, our results show that mTert expression is temporally regulated and marks SSCs during a discrete phase of transitional growth between rapid bone growth and maintenance.

Frizzled proteins are colonic epithelial receptors for C. difficile toxin B.

Clostridium difficile toxin B (TcdB) is a critical virulence factor that causes diseases associated with C. difficile infection. Here we carried out CRISPR-Cas9-mediated genome-wide screens and identified the members of the Wnt receptor frizzled family (FZDs) as TcdB receptors. TcdB binds to the conserved Wnt-binding site known as the cysteine-rich domain (CRD), with the highest affinity towards FZD1, 2 and 7. TcdB competes with Wnt for binding to FZDs, and its binding blocks Wnt signalling. FZD1/2/7 triple-knockout cells are highly resistant to TcdB, and recombinant FZD2-CRD prevented TcdB binding to the colonic epithelium. Colonic organoids cultured from FZD7-knockout mice, combined with knockdown of FZD1 and 2, showed increased resistance to TcdB. The colonic epithelium in FZD7-knockout mice was less susceptible to TcdB-induced tissue damage in vivo. These findings establish FZDs as physiologically relevant receptors for TcdB in the colonic epithelium.

The colonic epithelium plays an active role in promoting colitis by shaping the tissue cytokine profile.

Inflammatory bowel disease (IBD) is a chronic condition driven by loss of homeostasis between the mucosal immune system, the commensal gut microbiota, and the intestinal epithelium. Our goal is to understand how these components of the intestinal ecosystem cooperate to control homeostasis. By combining quantitative measures of epithelial hyperplasia and immune infiltration with multivariate analysis of inter- and intracellular signaling, we identified epithelial mammalian target of rapamycin (mTOR) signaling as a potential driver of inflammation in a mouse model of colitis. A kinetic analysis of mTOR inhibition revealed that the pathway regulates epithelial differentiation, which in turn controls the cytokine milieu of the colon. Consistent with our in vivo analysis, we found that cytokine expression of organoids grown ex vivo, in the absence of bacteria and immune cells, was dependent on differentiation state. Our study suggests that proper differentiation of epithelial cells is an important feature of colonic homeostasis because of its effect on the secretion of inflammatory cytokines.

An in silico Model of T Cell Infiltration Dynamics Based on an Advanced in vitro System to Enhance Preclinical Decision Making in Cancer Immunotherapy.

Cancer immunotherapy often involves the use of engineered molecules to selectively bind and activate T cells located within tumour tissue. Fundamental to the success of such treatments is the presence or recruitment of T cells localised within the tumour microenvironment. Advanced organ-on-a-chip systems provide an in vitro setting in which to investigate how novel molecules influence the spatiotemporal dynamics of T cell infiltration into tissue, both in the context of anti-tumour efficacy and off-tumour toxicity. While highly promising, the complexity of these systems is such that mathematical modelling plays a crucial role in the quantitative evaluation of experimental results and maximising the mechanistic insight derived. We develop a mechanistic, mathematical model of a novel microphysiological in vitro platform that recapitulates T cell infiltration into epithelial tissue, which may be normal or transformed. The mathematical model describes the spatiotemporal dynamics of infiltrating T cells in response to chemotactic cytokine signalling. We integrate the model with dynamic imaging data to optimise key model parameters. The mathematical model demonstrates a good fit to the observed experimental data and accurately describes the distribution of infiltrating T cells. This model is designed to complement the in vitro system; with the potential to elucidate complex biological mechanisms, including the mode of action of novel therapies and the drivers of safety events, and, ultimately, improve the efficacy-safety profile of T cell-targeted cancer immunotherapies.

Human Colon-on-a-Chip Enables Continuous In Vitro Analysis of Colon Mucus Layer Accumulation and Physiology.

BACKGROUND & AIMS: The mucus layer in the human colon protects against commensal bacteria and pathogens, and defects in its unique bilayered structure contribute to intestinal disorders, such as ulcerative colitis. However, our understanding of colon physiology is limited by the lack of in vitro models that replicate human colonic mucus layer structure and function. Here, we investigated if combining organ-on-a-chip and organoid technologies can be leveraged to develop a human-relevant in vitro model of colon mucus physiology. METHODS: A human colon-on-a-chip (Colon Chip) microfluidic device lined by primary patient-derived colonic epithelial cells was used to recapitulate mucus bilayer formation, and to visualize mucus accumulation in living cultures noninvasively. RESULTS: The Colon Chip supports spontaneous goblet cell differentiation and accumulation of a mucus bilayer with impenetrable and penetrable layers, and a thickness similar to that observed in the human colon, while maintaining a subpopulation of proliferative epithelial cells. Live imaging of the mucus layer formation on-chip showed that stimulation of the colonic epithelium with prostaglandin E2, which is increased during inflammation, causes rapid mucus volume expansion via an Na-K-Cl cotransporter 1 ion channel-dependent increase in its hydration state, but no increase in de novo mucus secretion. CONCLUSIONS: This study shows the production of colonic mucus with a physiologically relevant bilayer structure in vitro, which can be analyzed in real time noninvasively. The Colon Chip may offer a new preclinical tool to analyze the role of mucus in human intestinal homeostasis as well as diseases, such as ulcerative colitis and cancer.

Species-specific enhancement of enterohemorrhagic E. coli pathogenesis mediated by microbiome metabolites.

BACKGROUND: Species-specific differences in tolerance to infection are exemplified by the high susceptibility of humans to enterohemorrhagic Escherichia coli (EHEC) infection, whereas mice are relatively resistant to this pathogen. This intrinsic species-specific difference in EHEC infection limits the translation of murine research to human. Furthermore, studying the mechanisms underlying this differential susceptibility is a difficult problem due to complex in vivo interactions between the host, pathogen, and disparate commensal microbial communities. RESULTS: We utilize organ-on-a-chip (Organ Chip) microfluidic culture technology to model damage of the human colonic epithelium induced by EHEC infection, and show that epithelial injury is greater when exposed to metabolites derived from the human gut microbiome compared to mouse. Using a multi-omics approach, we discovered four human microbiome metabolites-4-methyl benzoic acid, 3,4-dimethylbenzoic acid, hexanoic acid, and heptanoic acid-that are sufficient to mediate this effect. The active human microbiome metabolites preferentially induce expression of flagellin, a bacterial protein associated with motility of EHEC and increased epithelial injury. Thus, the decreased tolerance to infection observed in humans versus other species may be due in part to the presence of compounds produced by the human intestinal microbiome that actively promote bacterial pathogenicity. CONCLUSION: Organ-on-chip technology allowed the identification of specific human microbiome metabolites modulating EHEC pathogenesis. These identified metabolites are sufficient to increase susceptibility to EHEC in our human Colon Chip model and they contribute to species-specific tolerance. This work suggests that higher concentrations of these metabolites could be the reason for higher susceptibility to EHEC infection in certain human populations, such as children. Furthermore, this research lays the foundation for therapeutic-modulation of microbe products in order to prevent and treat human bacterial infection.

A complex human gut microbiome cultured in an anaerobic intestine-on-a-chip.

The diverse bacterial populations that comprise the commensal microbiome of the human intestine play a central role in health and disease. A method that sustains complex microbial communities in direct contact with living human intestinal cells and their overlying mucus layer in vitro would thus enable the investigation of host-microbiome interactions. Here, we show the extended coculture of living human intestinal epithelium with stable communities of aerobic and anaerobic human gut microbiota, using a microfluidic intestine-on-a-chip that permits the control and real-time assessment of physiologically relevant oxygen gradients. When compared to aerobic coculture conditions, the establishment of a transluminal hypoxia gradient in the chip increased intestinal barrier function and sustained a physiologically relevant level of microbial diversity, consisting of over 200 unique operational taxonomic units from 11 different genera and an abundance of obligate anaerobic bacteria, with ratios of Firmicutes and Bacteroidetes similar to those observed in human faeces. The intestine-on-a-chip may serve as a discovery tool for the development of microbiome-related therapeutics, probiotics and nutraceuticals.

Author Correction: A complex human gut microbiome cultured in an anaerobic intestine-on-a-chip.

In the version of this Article originally published, the authors mistakenly cited Fig. 5d in the sentence beginning 'Importantly, the microbiome cultured in these primary Intestine Chips...'; the correct citation is Supplementary Table 2. This has now been amended.

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

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

Microfluidic Organ-on-a-Chip Models of Human Intestine.

Microfluidic organ-on-a-chip models of human intestine have been developed and used to study intestinal physiology and pathophysiology. In this article, we review this field and describe how microfluidic Intestine Chips offer new capabilities not possible with conventional culture systems or organoid cultures, including the ability to analyze contributions of individual cellular, chemical, and physical control parameters one-at-a-time; to coculture human intestinal cells with commensal microbiome for extended times; and to create human-relevant disease models. We also discuss potential future applications of human Intestine Chips, including how they might be used for drug development and personalized medicine.

Development of a primary human Small Intestine-on-a-Chip using biopsy-derived organoids.

Here we describe a method for fabricating a primary human Small Intestine-on-a-Chip (Intestine Chip) containing epithelial cells isolated from healthy regions of intestinal biopsies. The primary epithelial cells are expanded as 3D organoids, dissociated, and cultured on a porous membrane within a microfluidic device with human intestinal microvascular endothelium cultured in a parallel microchannel under flow and cyclic deformation. In the Intestine Chip, the epithelium forms villi-like projections lined by polarized epithelial cells that undergo multi-lineage differentiation similar to that of intestinal organoids, however, these cells expose their apical surfaces to an open lumen and interface with endothelium. Transcriptomic analysis also indicates that the Intestine Chip more closely mimics whole human duodenum in vivo when compared to the duodenal organoids used to create the chips. Because fluids flowing through the lumen of the Intestine Chip can be collected continuously, sequential analysis of fluid samples can be used to quantify nutrient digestion, mucus secretion and establishment of intestinal barrier function over a period of multiple days in vitro. The Intestine Chip therefore may be useful as a research tool for applications where normal intestinal function is crucial, including studies of metabolism, nutrition, infection, and drug pharmacokinetics, as well as personalized medicine.

Reprogrammed Stomach Tissue as a Renewable Source of Functional ß Cells for Blood Glucose Regulation.

The gastrointestinal (GI) epithelium is a highly regenerative tissue with the potential to provide a renewable source of insulin(+) cells after undergoing cellular reprogramming. Here, we show that cells of the antral stomach have a previously unappreciated propensity for conversion into functional insulin-secreting cells. Native antral endocrine cells share a surprising degree of transcriptional similarity with pancreatic ß cells, and expression of ß cell reprogramming factors in vivo converts antral cells efficiently into insulin(+) cells with close molecular and functional similarity to ß cells. Induced GI insulin(+) cells can suppress hyperglycemia in a diabetic mouse model for at least 6 months and regenerate rapidly after ablation. Reprogramming of antral stomach cells assembled into bioengineered mini-organs in vitro yielded transplantable units that also suppressed hyperglycemia in diabetic mice, highlighting the potential for development of engineered stomach tissues as a renewable source of functional ß cells for glycemic control.

Distinct Processes and Transcriptional Targets Underlie CDX2 Requirements in Intestinal Stem Cells and Differentiated Villus Cells.

Lgr5-expressing intestinal stem cells (ISCs) renew the adult gut epithelium by producing mature villus cells (VCs); the transcriptional basis for ISC functions remains unclear. RNA sequencing analysis identified transcripts modulated during differentiation of Lgr5(+) ISCs into VCs, with high expression of the intestine-restricted transcription factor (TF) gene Cdx2 in both populations. Cdx2-deleted mouse ISCs showed impaired proliferation and long-term inability to produce mature lineages, revealing essential ISC functions. Chromatin immunoprecipitation sequencing analysis of CDX2 in Lgr5(+) ISCs, coupled with mRNA profiling of control and Cdx2(-/-) ISCs, identified features of CDX2 regulation distinct from VCs. Most CDX2 binding in ISCs occurs in anticipation of future gene expression, but whereas CDX2 primarily activates VC genes, direct ISC targets are activated and repressed. Diverse CDX2 requirements in stem and differentiated cells may reflect the versatility of TFs that specify a tissue in development and control the same tissue in adults.

Dormant Intestinal Stem Cells Are Regulated by PTEN and Nutritional Status.

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