Fibroblasts in intestinal homeostasis, damage, and repair

Current understanding of the interplay between extracellular matrix remodelling and gut permeability in health and disease

The intestinal wall represents an interactive network regulated by the intestinal epithelium, extracellular matrix (ECM) and mesenchymal compartment. Under healthy physiological conditions, the epithelium undergoes constant renewal and forms an integral and selective barrier. Following damage, the healthy epithelium is restored via a series of signalling pathways that result in remodelling of the scaffolding tissue through finely-regulated proteolysis of the ECM by proteases such as matrix metalloproteinases (MMPs). However, chronic inflammation of the gastrointestinal tract, as occurs in Inflammatory Bowel Disease (IBD), is associated with prolonged disruption of the epithelial barrier and persistent damage to the intestinal mucosa. Increased barrier permeability exhibits distinctive signatures of inflammatory, immunological and ECM components, accompanied by increased ECM proteolytic activity. This narrative review aims to bring together the current knowledge of the interplay between gut barrier, immune and ECM features in health and disease, discussing the role of barrier permeability as a discriminant between homoeostasis and IBD.

Characterizing the human intestinal chondroitin sulfate glycosaminoglycan sulfation signature in inflammatory bowel disease

The intestinal extracellular matrix (ECM) helps maintain appropriate tissue barrier function and regulate host-microbial interactions. Chondroitin sulfate- and dermatan sulfate-glycosaminoglycans (CS/DS-GAGs) are integral components of the intestinal ECM, and alterations in CS/DS-GAGs have been shown to significantly influence biological functions. Although pathologic ECM remodeling is implicated in inflammatory bowel disease (IBD), it is unknown whether changes in the intestinal CS/DS-GAG composition are also linked to IBD in humans. Our aim was to characterize changes in the intestinal ECM CS/DS-GAG composition in intestinal biopsy samples from patients with IBD using mass spectrometry. We characterized intestinal CS/DS-GAGs in 69 pediatric and young adult patients (n = 13 control, n = 32 active IBD, n = 24 IBD in remission) and 6 adult patients. Here, we report that patients with active IBD exhibit a significant decrease in the relative abundance of CS/DS isomers associated with matrix stability (CS-A and DS) compared to controls, while isomers implicated in matrix instability and inflammation (CS-C and CS-E) were significantly increased. This imbalance of intestinal CS/DS isomers was restored among patients in clinical remission. Moreover, the abundance of pro-stabilizing CS/DS isomers negatively correlated with clinical disease activity scores, whereas both pro-inflammatory CS-C and CS-E content positively correlated with disease activity scores. Thus, pediatric patients with active IBD exhibited increased pro-inflammatory and decreased pro-stabilizing CS/DS isomer composition, and future studies are needed to determine whether changes in the CS/DS-GAG composition play a pathogenic role in IBD.

Ultrasensitive Optical Detection and Elimination of Residual Microtumors with a Postoperative Implantable Hydrogel Sensor for Preventing Cancer Recurrence

In vivo optical imaging of trace biomarkers in residual microtumors holds significant promise for cancer prognosis but poses a formidable challenge. Here, a novel hydrogel sensor is designed for ultrasensitive and specific imaging of the elusive biomarker. This hydrogel sensor seamlessly integrates a molecular beacon nanoprobe with fibroblasts, offering both high tissue retention capability and an impressive signal-to-noise ratio for imaging. Signal amplification is accomplished through exonuclease I-mediated biomarker recycling. The resulting hydrogel sensor sensitively detects the biomarker carcinoembryonic antigen with a detection limit of 1.8 pg mL-1 in test tubes. Moreover, it successfully identifies residual cancer nodules with a median diameter of less than 2 mm in mice bearing partially removed primary triple-negative breast carcinomas (4T1). Notably, this hydrogel sensor is proven effective for the sensitive diagnosis of invasive tumors in post-surgical mice with infiltrating 4T1 cells, leveraging the role of fibroblasts in locally enriching tumor cells. Furthermore, the residual microtumor is rapidly photothermal ablation by polydopamine-based nanoprobe under the guidance of visualization, achieving ≈100% suppression of tumor recurrence and lung metastasis. This work offers a promising alternative strategy for visually detecting residual microtumors, potentially enhancing the prognosis of cancer patients following surgical interventions.

Chronic arsenic exposure affects stromal cells and signaling in the small intestine in a sex-specific manner

Arsenic is a toxicant that is ingested through drinking water and food, exposing nearly 140 million people to levels above the 10 ppb guideline concentration. Studies have shown that arsenic affects intestinal stem cells (ISCs), but the mechanisms by which arsenic alters the formation of adult cells in the small intestine are not well understood. Signals derived from intestinal stromal cells initiate and maintain differentiation. The goal of this study is to evaluate arsenic's effect on intestinal stromal cells, including PdgfrαLo trophocytes, located proximal to the ISCs, and PdgfrαHi telocytes, located proximal to the transit-amplifying region and up the villi. Adult Sox9tm2Crm-EGFP mice were exposed to 0, 33, and 100 ppb sodium arsenite in their drinking water for 13 weeks, and sections of duodenum were examined. Flow cytometry indicated that arsenic exposure dose-responsively reduced Sox9+ epithelial cells and trended toward increased Pdgfrα+ cells. The trophocyte marker, CD81, was reduced by 10-fold and 9.0-fold in the 100 ppb exposure group in male and female mice, respectively. Additionally, a significant 2.2- to 3.1-fold increase in PdgfrαLo expression was found in male mice in trophocytes and Igfbp5+ cells. PdgfrαHi protein expression, a telocyte marker, was more prevalent along the villus/crypt structure in females, whereas Gli1 expression (telocytes) was reduced in male mice exposed to arsenic. Principle coordinate analysis confirmed the sex-dependent response to arsenic exposure, with an increase in trophocyte and decrease in telocyte marker expression observed in male mice. These results imply that arsenic alters intestinal mesenchymal cells in a sex-dependent manner.

Role reversals: non-canonical roles for immune and non-immune cells in the gut

The intestine is home to an intertwined network of epithelial, immune, and neuronal cells as well as the microbiome, with implications for immunity, systemic metabolism, and behavior. While the complexity of this microenvironment has long since been acknowledged, recent technological advances have propelled our understanding to an unprecedented level. Notably, the microbiota and non-immune or structural cells have emerged as important conductors of intestinal immunity, and by contrast, cells of both the innate and adaptive immune systems have demonstrated non-canonical roles in tissue repair and metabolism. This review highlights recent works in the following two streams: non-immune cells of the intestine performing immunological functions; and traditional immune cells exhibiting non-immune functions in the gut.

Stromal Cell Regulation of Intestinal Inflammatory Fibrosis

Intestinal inflammatory fibrosis is a severe consequence of inflammatory bowel diseases (IBDs). There is currently no cure for the treatment of intestinal fibrosis in IBD. Although inflammation is necessary for triggering fibrosis, the anti-inflammatory agents used to treat IBD are ineffective in preventing the progression of intestinal fibrosis and stricture formation once initiated, suggesting that inflammatory signals are not the sole drivers of fibrosis progression once it is established. Among multiple mechanisms involved in the initiation and progression of intestinal fibrosis in IBD, stromal cells play critical roles in mediating the process. In this review, we summarize recent progress on how stromal cells regulate intestinal fibrosis in IBD and how they are regulated by focusing on immune regulation and gut microbiota. We also outline the challenges moving forward in the field.

Cardiac and intestinal tissue conduct developmental and reparative processes in response to lymphangiocrine signaling

Lymphatic vessels conduct a diverse range of activities to sustain the integrity of surrounding tissue. Besides facilitating the movement of lymph and its associated factors, lymphatic vessels are capable of producing tissue-specific responses to changes within their microenvironment. Lymphatic endothelial cells (LECs) secrete paracrine signals that bind to neighboring cell-receptors, commencing an intracellular signaling cascade that preludes modifications to the organ tissue's structure and function. While the lymphangiocrine factors and the molecular and cellular mechanisms themselves are specific to the organ tissue, the crosstalk action between LECs and adjacent cells has been highlighted as a commonality in augmenting tissue regeneration within animal models of cardiac and intestinal disease. Lymphangiocrine secretions have been owed for subsequent improvements in organ function by optimizing the clearance of excess tissue fluid and immune cells and stimulating favorable tissue growth, whereas perturbations in lymphatic performance bring about the opposite. Newly published landmark studies have filled gaps in our understanding of cardiac and intestinal maintenance by revealing key players for lymphangiocrine processes. Here, we will expand upon those findings and review the nature of lymphangiocrine factors in the heart and intestine, emphasizing its involvement within an interconnected network that supports daily homeostasis and self-renewal following injury.

Activation and Functions of Col6a1+ Fibroblasts in Colitis-Associated Cancer

Cancer-associated fibroblasts (CAFs) comprise a group of heterogeneous subpopulations with distinct identities indicative of their diverse origins, activation patterns, and pro-tumorigenic functions. CAFs originate mainly from resident fibroblasts, which are activated upon different stimuli, including growth factors and inflammatory mediators, but the extent to which they also maintain some of their homeostatic properties, at least at the earlier stages of carcinogenesis, is not clear. In response to cytokines, such as interleukin 1 (IL-1) and tumor necrosis factor (TNF), as well as microbial products, CAFs acquire an immunoregulatory phenotype, but its specificity and pathophysiological significance in individual CAF subsets is yet to be determined. In this study, we analyzed the properties of Col6a1-positive fibroblasts in colitis-associated cancer. We found that Col6a1+ cells partly maintain their homeostatic features during adenoma development, while their activation is characterized by the acquisition of a distinct proangiogenic signature associated with their initial perivascular location. In vitro and in vivo experiments showed that Col6a1+ cells respond to innate immune stimuli and exert pro-tumorigenic functions. However, Col6a1+-specific inhibition of TNF receptor 1 (TNFR1) or IL-1 receptor (IL-1R) signaling does not significantly affect tumorigenesis, suggesting that activation of other subsets acts in a compensatory way or that multiple immune stimuli are necessary to drive the proinflammatory activation of this subset. In conclusion, our results show that adenoma-associated CAF subsets can partly maintain the properties of homeostatic fibroblasts while they become activated to support tumor growth through distinct and compensatory mechanisms.

Understanding Fibroblast Heterogeneity in Form and Function

Historically believed to be a homogeneous cell type that is often overlooked, fibroblasts are more and more understood to be heterogeneous in nature. Though the mechanisms behind how fibroblasts participate in homeostasis and pathology are just beginning to be understood, these cells are believed to be highly dynamic and play key roles in fibrosis and remodeling. Focusing primarily on fibroblasts within the skin and during wound healing, we describe the field's current understanding of fibroblast heterogeneity in form and function. From differences due to embryonic origins to anatomical variations, we explore the diverse contributions that fibroblasts have in fibrosis and plasticity. Following this, we describe molecular techniques used in the field to provide deeper insights into subpopulations of fibroblasts and their varied roles in complex processes such as wound healing. Limitations to current work are also discussed, with a focus on future directions that investigators are recommended to take in order to gain a deeper understanding of fibroblast biology and to develop potential targets for translational applications in a clinical setting.

Primary Intestinal Fibroblasts: Isolation, Cultivation, and Maintenance

Intestinal fibroblasts maintain homeostasis and contribute to inflammatory responses and the development of cancer. Intestinal fibroblasts express pattern recognition receptors which can mount an immune response. Since intestinal fibroblasts interact with diverse immune and nonimmune cells, further insights into the biology of intestinal fibroblasts could expand our knowledge of the development, homeostasis, and pathophysiology of the intestine. Here, we describe a simple protocol for the isolation, cultivation, and maintenance of primary fibroblasts from the mouse colon. These cells express α-smooth muscle actin, a characteristic of specialized contractile fibroblasts called myofibroblasts. We also outline the use of these colonic fibroblasts for immunoblotting and immunofluorescence assays with or without stimulation with a growth factor.