Mesenchymal cells are mesoderm-derived stromal cells that are best known for providing structural support to organs, synthesizing and remodeling the extracellular matrix (ECM) and regulating ...development, homeostasis and repair of tissues. Recent detailed mechanistic insights into the biology of fibroblastic mesenchymal cells have revealed they are also significantly involved in immune regulation, stem cell maintenance and blood vessel function. It is now becoming evident that these functions, when defective, drive the development of complex diseases, such as various immunopathologies, chronic inflammatory disease, tissue fibrosis and cancer. Here, we provide a concise overview of the contextual contribution of fibroblastic mesenchymal cells in physiology and disease and bring into focus emerging evidence for both their heterogeneity at the single-cell level and their tissue-specific, spatiotemporal functional diversity.
The mammalian intestine is a self-renewing tissue that ensures nutrient absorption while acting as a barrier against environmental insults. This is achieved by mature intestinal epithelial cells, the ...renewing capacity of intestinal stem cells at the base of the crypts, the development of immune tolerance, and the regulatory functions of stromal cells. Upon intestinal injury or inflammation, this tightly regulated mucosal homeostasis is disrupted and is followed by a series of events that lead to tissue repair and the restoration of organ function. It is now well established that fibroblasts play significant roles both in the maintenance of epithelial and immune homeostasis in the intestine and the response to tissue damage mainly through the secretion of a variety of soluble mediators and ligands and the remodeling of the extracellular matrix. In addition, recent advances in single-cell transcriptomics have revealed an unexpected heterogeneity of fibroblasts that comprise distinct cell subsets in normal and inflammatory conditions, indicative of diverse functions. However, there is still little consensus on the number, terminology, and functional properties of these subsets. Moreover, it is still unclear how individual fibroblast subsets can regulate intestinal repair processes and what is their impact on the pathogenesis of inflammatory bowel disease. In this mini-review, we aim to provide a concise overview of recent advances in the field, that we believe will help clarify current concepts on fibroblast heterogeneity and functions and advance our understanding of the contribution of fibroblasts in intestinal damage and repair.
The initiation of an intestinal tumour is a probabilistic process that depends on the competition between mutant and normal epithelial stem cells in crypts
. Intestinal stem cells are closely ...associated with a diverse but poorly characterized network of mesenchymal cell types
. However, whether the physiological mesenchymal microenvironment of mutant stem cells affects tumour initiation remains unknown. Here we provide in vivo evidence that the mesenchymal niche controls tumour initiation in trans. By characterizing the heterogeneity of the intestinal mesenchyme using single-cell RNA-sequencing analysis, we identified a population of rare pericryptal Ptgs2-expressing fibroblasts that constitutively process arachidonic acid into highly labile prostaglandin E
(PGE
). Specific ablation of Ptgs2 in fibroblasts was sufficient to prevent tumour initiation in two different models of sporadic, autochthonous tumorigenesis. Mechanistically, single-cell RNA-sequencing analyses of a mesenchymal niche model showed that fibroblast-derived PGE
drives the expansion οf a population of Sca-1
reserve-like stem cells. These express a strong regenerative/tumorigenic program, driven by the Hippo pathway effector Yap. In vivo, Yap is indispensable for Sca-1
cell expansion and early tumour initiation and displays a nuclear localization in both mouse and human adenomas. Using organoid experiments, we identified a molecular mechanism whereby PGE
promotes Yap dephosphorylation, nuclear translocation and transcriptional activity by signalling through the receptor Ptger4. Epithelial-specific ablation of Ptger4 misdirected the regenerative reprogramming of stem cells and prevented Sca-1
cell expansion and sporadic tumour initiation in mutant mice, thereby demonstrating the robust paracrine control of tumour-initiating stem cells by PGE
-Ptger4. Analyses of patient-derived organoids established that PGE
-PTGER4 also regulates stem-cell function in humans. Our study demonstrates that initiation of colorectal cancer is orchestrated by the mesenchymal niche and reveals a mechanism by which rare pericryptal Ptgs2-expressing fibroblasts exert paracrine control over tumour-initiating stem cells via the druggable PGE
-Ptger4-Yap signalling axis.
Fibroblastic reticular cells (FRCs) determine the organization of lymphoid organs and control immune cell interactions. While the cellular and molecular mechanisms underlying FRC differentiation in ...lymph nodes and the splenic white pulp have been elaborated to some extent, in Peyer's patches (PPs) they remain elusive. Using a combination of single-cell transcriptomics and cell fate mapping in advanced mouse models, we found that PP formation in the mouse embryo is initiated by an expansion of perivascular FRC precursors, followed by FRC differentiation from subepithelial progenitors. Single-cell transcriptomics and cell fate mapping confirmed the convergence of perivascular and subepithelial FRC lineages. Furthermore, lineage-specific loss- and gain-of-function approaches revealed that the two FRC lineages synergistically direct PP organization, maintain intestinal microbiome homeostasis and control anticoronavirus immune responses in the gut. Collectively, this study reveals a distinct mosaic patterning program that generates key stromal cell infrastructures for the control of intestinal immunity.
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.
Intestinal mesenchymal cells encompass multiple subsets, whose origins, functions, and pathophysiological importance are still not clear. Here, we used the
Col6a1
Cre
mouse, which targets distinct ...fibroblast subsets and perivascular cells that can be further distinguished by the combination of the CD201, PDGFRα and αSMA markers. Developmental studies revealed that the
Col6a1
Cre
mouse also targets mesenchymal aggregates that are crucial for intestinal morphogenesis and patterning, suggesting an ontogenic relationship between them and homeostatic PDGFRα
hi
telocytes. Cell depletion experiments in adulthood showed that
Col6a1
+
/CD201
+
mesenchymal cells regulate homeostatic enteroendocrine cell differentiation and epithelial proliferation. During acute colitis, they expressed an inflammatory and extracellular matrix remodelling gene signature, but they also retained their properties and topology. Notably, both in homeostasis and tissue regeneration, they were dispensable for normal organ architecture, while CD34
+
mesenchymal cells expanded, localised at the top of the crypts, and showed increased expression of villous-associated morphogenetic factors, providing thus evidence for the plasticity potential of intestinal mesenchymal cells. Our results provide a comprehensive analysis of the identities, origin, and functional significance of distinct mesenchymal populations in the intestine.
Stromal cells in secondary lymphoid organs (SLOs) are non-hematopoietic cells involved in the regulation of adaptive immune responses. Three major stromal populations have been identified in adult ...SLOs: fibroblastic reticular cells (FRCs), follicular dendritic cells (FDCs) and marginal reticular cells (MRCs). The properties of these individual populations are not clearly defined, mainly due to the lack of appropriate genetic tools, especially for MRCs. Here, we analyzed stromal cell targeting in SLOs from a transgenic mouse strain that expresses Cre recombinase under the CollagenVI promoter, using lineage tracing approaches. We show that these mice target specifically MRCs and FDCs, but not FRCs in Peyer's patches and isolated lymphoid follicles in the intestine. In contrast, stromal cells in lymph nodes and the spleen do not express the transgene, which renders ColVI-cre mice ideal for the specific targeting of stromal cells in the gut-associated lymphoid tissue (GALT). This funding further supports the hypothesis of organ-specific stromal precursors in SLOs. Interestingly, in all tissues analyzed, there was also high specificity for perivascular cells, which have been proposed to act as FDC precursors. Taken together, ColVI-Cre mice are a useful new tool for the dissection of MRC- and FDC-specific functions and plasticity in the GALT.
The infiltration and subsequent in situ subtype specification of monocytes to effector/inflammatory and repair macrophages is indispensable for tissue repair upon acute sterile injury. However, the ...chromatin-level mediators and regulatory events controlling this highly dynamic macrophage phenotype switch are not known. In this study, we used a murine acute muscle injury model to assess global chromatin accessibility and gene expression dynamics in infiltrating macrophages during sterile physiological inflammation and tissue regeneration. We identified a heme-binding transcriptional repressor, BACH1, as a novel regulator of this process.
knockout mice displayed impaired muscle regeneration, altered dynamics of the macrophage phenotype transition, and transcriptional deregulation of key inflammatory and repair-related genes. We also found that BACH1 directly binds to and regulates distal regulatory elements of these genes, suggesting a novel role for BACH1 in controlling a broad spectrum of the repair response genes in macrophages upon injury. Inactivation of heme oxygenase-1 (
), one of the most stringently deregulated genes in the
knockout in macrophages, impairs muscle regeneration by changing the dynamics of the macrophage phenotype switch. Collectively, our data suggest the existence of a heme-BACH1--HMOX1 regulatory axis, that controls the phenotype and function of the infiltrating myeloid cells upon tissue damage, shaping the overall tissue repair kinetics.
Tumor progression locus-2 (Tpl2) kinase is a major inflammatory mediator in immune cell types recently found to be genetically associated with inflammatory bowel diseases (IBDs). Here we show that ...Tpl2 may exert a dominant homeostatic rather than inflammatory function in the intestine mediated specifically by subepithelial intestinal myofibroblasts (IMFs). Mice with complete or IMF-specific Tpl2 ablation are highly susceptible to epithelial injury-induced colitis showing impaired compensatory proliferation in crypts and extensive ulcerations without significant changes in inflammatory responses. Following epithelial injury, IMFs sense innate or inflammatory signals and activate, via Tpl2, the cyclooxygenase-2 (Cox-2)-prostaglandin E ₂ (PGE ₂) pathway, which we show here to be essential for the epithelial homeostatic response. Exogenous PGE ₂ administration rescues mice with complete or IMF-specific Tpl2 ablation from defects in crypt function and susceptibility to colitis. We also show that Tpl2 expression is decreased in IMFs isolated from the inflamed ileum of IBD patients indicating that Tpl2 function in IMFs may be highly relevant to human disease. The IMF-mediated mechanism we propose also involves the IBD-associated genes IL1R1 , MAPK1 , and the PGE ₂ receptor-encoding PTGER4 . Our results establish a previously unidentified myofibroblast-specific innate pathway that regulates intestinal homeostasis and may underlie IBD susceptibility in humans.
Significance Tumor progression locus-2 (Tpl2) is a proinflammatory gene genetically associated with inflammatory bowel diseases. This study provides a mechanistic interpretation for this association showing a dominant Tpl2-mediated homeostatic mechanism protecting mice from epithelial injury-induced colitis. This function of Tpl2 is mediated specifically by subepithelial intestinal myofibroblasts, a cell type supporting crypt stem cells. Tpl2 in myofibroblasts is essential for the compensatory proliferative response of the epithelium by promoting arachidonic acid metabolism and cyclooxygenase-2 (Cox-2)/prostaglandin E ₂ activation. Notably, in Crohn’s Disease patients, Tpl2 is downregulated in myofibroblasts isolated from the inflamed ileum. These results challenge current concepts on a solely proinflammatory function of Tpl2 and highlight the dominant role of subepithelial myofibroblasts in sensing inflammation and tissue damage and promoting intestinal homeostasis through Tpl2-Cox-2-prostaglandin E ₂.
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