Piezo1 in Digestive System Function and Dysfunction

Piezo1, a non-selective cation channel directly activated by mechanical forces, is widely expressed in the digestive system and participates in biological functions physiologically and pathologically. In this review, we summarized the latest insights on Piezo1's cellular effect across the entire digestive system, and discussed the role of Piezo1 in various aspects including ingestion and digestion, material metabolism, enteric nervous system, intestinal barrier, and inflammatory response within digestive system. The goal of this comprehensive review is to provide a solid foundation for future research about Piezo1 in digestive system physiologically and pathologically.

Embryonic type 3 innate lymphoid cells sense maternal dietary cholesterol to control local Peyer's patch development

Lymphoid tissue inducer (LTi) cells develop during intrauterine life and rely on developmental programs to initiate the organogenesis of secondary lymphoid organs (SLOs). This evolutionary conserved process endows the fetus with the ability to orchestrate the immune response after birth and to react to the triggers present in the environment. While it is established that LTi function can be shaped by maternal-derived cues and is critical to prepare the neonate with a functional scaffold to mount immune response, the cellular mechanisms that control anatomically distinct SLO organogenesis remain unclear. We discovered that LTi cells forming Peyer's patches, gut-specific SLOs, require the coordinated action of two migratory G protein coupled receptors (GPCR) GPR183 and CCR6. These two GPCRs are uniformly expressed on LTi cells across SLOs, but their deficiency specifically impacts Peyer's patch formation, even when restricted to fetal window. The unique CCR6 ligand is CCL20, while the ligand for GPR183 is the cholesterol metabolite 7α,25-Dihydroxycholesterol (7α,25-HC), whose production is controlled by the enzyme cholesterol 25-hydroxylase (CH25H). We identified a fetal stromal cell subset that expresses CH25H and attracts LTi cells in the nascent Peyer's patch anlagen. GPR183 ligand concentration can be modulated by the cholesterol content in the maternal diet and impacts LTi cell maturation in vitro and in vivo, highlighting a link between maternal nutrients and intestinal SLO organogenesis. Our findings revealed that in the fetal intestine, cholesterol metabolite sensing by GPR183 in LTi cells for Peyer's patch formation is dominant in the duodenum, the site of cholesterol absorption in the adult. This anatomic requirement suggests that embryonic, long-lived non-hematopoietic cells might exploit adult metabolic functions to ensure highly specialized SLO development in utero.

Visualization and functional characterization of lymphoid organ fibroblasts

Fibroblastic reticular cells (FRCs) are specialized stromal cells of lymphoid organs that generate the structural foundation of the tissue and actively interact with immune cells. Distinct FRC subsets position lymphocytes and myeloid cells in specialized niches where they present processed or native antigen and provide essential growth factors and cytokines for immune cell activation and differentiation. Niche‐specific functions of FRC subpopulations have been defined using genetic targeting, high‐dimensional transcriptomic analyses, and advanced imaging methods. Here, we review recent findings on FRC‐immune cell interaction and the elaboration of FRC development and differentiation. We discuss how imaging approaches have not only shaped our understanding of FRC biology, but have critically advanced the niche concept of immune cell maintenance and control of immune reactivity.

Spatial and temporal key steps in early-life intestinal immune system development and education

Education of our intestinal immune system early in life strongly influences adult health. This education strongly relies on series of events that must occur in well-defined time windows. From initial colonization by maternal-derived microbiota during delivery to dietary changes from mother's milk to solid foods at weaning, these early-life events have indeed long-standing consequences on our immunity, facilitating tolerance to environmental exposures or, on the contrary, increasing the risk of developing noncommunicable diseases such as allergies, asthma, obesity, and inflammatory bowel diseases. In this review, we provide an outline of the recent advances in our understanding of these events and how they are mechanistically related to intestinal immunity development and education. First, we review the susceptibility of neonates to infections and inflammatory diseases, related to their immune system and microbiota changes. Then, we highlight the maternal factors involved in protection and education of the mucosal immune system of the offspring, the role of the microbiota, and the nature of neonatal immune system until weaning. We also present how the development of some immune responses is intertwined in temporal and spatial windows of opportunity. Finally, we discuss pending questions regarding the neonate particular immune status and the activation of the intestinal immune system at weaning.

Differentiation and activation of fibroblastic reticular cells

Secondary lymphoid organs (SLO) are underpinned by fibroblastic reticular cells (FRC) that form dedicated microenvironmental niches to secure induction and regulation of innate and adaptive immunity. Distinct FRC subsets are strategically positioned in SLOs to provide niche factors and govern efficient immune cell interaction. In recent years, the use of specialized mouse models in combination with single-cell transcriptomics has facilitated the elaboration of the molecular FRC landscape at an unprecedented resolution. While single-cell RNA-sequencing has advanced the resolution of FRC subset characterization and function, the high dimensionality of the generated data necessitates careful analysis and validation. Here, we reviewed novel findings from high-resolution transcriptomic analyses that refine our understanding of FRC differentiation and activation processes in the context of infection and inflammation. We further discuss concepts, strategies, and limitations for the analysis of single-cell transcriptome data from FRCs and the wide-ranging implications for our understanding of stromal cell biology.

Fibroblastic reticular cell lineage convergence in Peyer's patches governs intestinal immunity

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.

Intestinal Organoids Generated from Human Pluripotent Stem Cells

The gastrointestinal system is one of the most complex organ systems in the human body, and consists of numerous cell types originating from three germ layers. To understand intestinal development and homeostasis and elucidate the pathogenesis of intestinal disorders, including unidentified diseases, several in vitro models have been developed. Human pluripotent stem cells (PSCs), including embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), have remarkable developmental plasticity and possess the potential for a wide variety of applications. Three-dimensional organs, termed organoids and produced in vitro by PSCs, contain not only epithelium but also mesenchymal tissue and partially recapitulate intestinal functions. Such intestinal organoids have begun to be applied in disease models and drug development and have contributed to a detailed analysis of molecular interactions and findings in the synergistic development of biomedicine for human digestive organs. In this review, we describe gastrointestinal organoid technology derived from PSCs and consider its potential applications.

Peyer's patches: organizing B-cell responses at the intestinal frontier

Secondary lymphoid tissues share the important function of bringing together antigens and rare antigen-specific lymphocytes to foster induction of adaptive immune responses. Peyer's patches (PPs) are unique compared to other secondary lymphoid tissues in their continual exposure to an enormous diversity of microbiome- and food-derived antigens and in the types of pathogens they encounter. Antigens are delivered to PPs by specialized microfold (M) epithelial cells and they may be captured and presented by resident dendritic cells (DCs). In accord with their state of chronic microbial antigen exposure, PPs exhibit continual germinal center (GC) activity. These GCs not only contribute to the generation of B cells and plasma cells producing somatically mutated gut antigen-specific IgA antibodies but have also been suggested to support non-specific antigen diversification of the B-cell repertoire. Here, we review current understanding of how PPs foster B-cell encounters with antigen, how they favor isotype switching to the secretory IgA isotype, and how their GC responses may uniquely contribute to mucosal immunity.

The loss of Gnai2 and Gnai3 in B cells eliminates B lymphocyte compartments and leads to a hyper-IgM like syndrome

B lymphocytes are compartmentalized within lymphoid organs. The organization of these compartments depends upon signaling initiated by G-protein linked chemoattractant receptors. To address the importance of the G-proteins Gα_i2 and Gα_i3 in chemoattractant signaling we created mice lacking both proteins in their B lymphocytes. While bone marrow B cell development and egress is grossly intact; mucosal sites, splenic marginal zones, and lymph nodes essentially lack B cells. There is a partial block in splenic follicular B cell development and a 50-60% reduction in splenic B cells, yet normal numbers of splenic T cells. The absence of Gα_i2 and Gα_i3 in B cells profoundly disturbs the architecture of lymphoid organs with loss of B cell compartments in the spleen, thymus, lymph nodes, and gastrointestinal tract. This results in a severe disruption of B cell function and a hyper-IgM like syndrome. Beyond the pro-B cell stage, B cells are refractory to chemokine stimulation, and splenic B cells are poorly responsive to antigen receptor engagement. Gα_i2 and Gα_i3 are therefore critical for B cell chemoattractant receptor signaling and for normal B cell function. These mice provide a worst case scenario of the consequences of losing chemoattractant receptor signaling in B cells.

CXCR4 promotes B cell egress from Peyer's patches

Chemokine receptor CXCR4 promotes B cell localization to CXCL12⁺ perilymphatic zones and egress from Peyer’s patches.

Peyer’s patches (PPs) play a central role in supporting B cell responses against intestinal antigens, yet the factors controlling B cell passage through these mucosal lymphoid tissues are incompletely understood. We report that, in mixed chimeras, CXCR4-deficient B cells accumulate in PPs compared with their representation in other lymphoid tissues. CXCR4-deficient B cells egress from PPs more slowly than wild-type cells, whereas CXCR5-deficient cells egress more rapidly. The CXCR4 ligand, CXCL12, is expressed by cells adjacent to lymphatic endothelial cells in a zone that abuts but minimally overlaps with the CXCL13⁺ follicle. CXCR4-deficient B cells show reduced localization to these CXCL12⁺ perilymphatic zones, whereas CXCR5-deficient B cells preferentially localize in these regions. By photoconverting KikGR-expressing cells within surgically exposed PPs, we provide evidence that naive B cells transit PPs with an approximate residency half-life of 10 h. When CXCR4 is lacking, KikGR⁺ B cells show a delay in PP egress. In summary, we identify a CXCL12^hi perilymphatic zone in PPs that plays a role in overcoming CXCL13-mediated retention to promote B cell egress from these gut-associated lymphoid tissues.