Muscularis macrophage development in the absence of an enteric nervous system

Efficient enzyme-free method to assess the development and maturation of the innate and adaptive immune systems in the mouse colon

Researchers who aim to globally analyze the gastrointestinal immune system via flow cytometry have many protocol options to choose from, with specifics generally tied to gut wall layers of interest. To get a clearer idea of the approach we should use on full-thickness colon samples from mice, we first undertook a systematic comparison of three tissue dissociation techniques: two based on enzymatic cocktails and the other one based on manual crushing. Using flow cytometry panels of general markers of lymphoid and myeloid cells, we found that the presence of cell-surface markers and relative cell population frequencies were more stable with the mechanical method. Both enzymatic approaches were associated with a marked decrease of several cell-surface markers. Using mechanical dissociation, we then developed two minimally overlapping panels, consisting of a total of 26 antibodies, for serial profiling of lymphoid and myeloid lineages from the mouse colon in greater detail. Here, we highlight how we accurately delineate these populations by manual gating, as well as the reproducibility of our panels on mouse spleen and whole blood. As a proof-of-principle of the usefulness of our general approach, we also report segment- and life stage-specific patterns of immune cell profiles in the colon. Overall, our data indicate that mechanical dissociation is more suitable and efficient than enzymatic methods for recovering immune cells from all colon layers at once. Additionally, our panels will provide researchers with a relatively simple tool for detailed immune cell profiling in the murine gastrointestinal tract, regardless of life stage or experimental conditions.

Muscularis macrophages controlled by NLRP3 maintain the homeostasis of excitatory neurons

Peristaltic movements in gut are essential to propel ingested materials through the gastrointestinal tract. Intestinal resident macrophages play an important role in this physiological function through protecting enteric neurons. However, it is incompletely clear how individuals maintain the homeostasis of gut motility. Here we found that NLRP3 is a critical factor in controlling loss of muscularis resident macrophages (MMs), and demonstrate that MMs are involved in the homeostasis of excitatory neurons such as choline acetyltransferase (ChAT)+ and vesicular glutamate transporter 2 (VGLUT2)+ but not inhibitory neuronal nitric oxide synthase (nNOS)+ neurons. NLRP3 knockout (KO) mice had enhanced gut motility and increased neurons, especially excitatory ChAT+ and VGLUT2+ neurons. Single cell analyses showed that there had increased resident macrophages, especially MMs in NLRP3 KO mice. The MM proportion in the resident macrophages was markedly higher than those in wild-type (WT) or caspase 1/11 KO mice. Deletion of the MMs and transplantation of the NLRP3 KO bone marrow cells showed that survival of the gut excitatory ChAT+ and VGLUT2+ neurons was dependent on the MMs. Gut microbiota metabolites β-hydroxybutyrate (BHB) could promote gut motility through protecting MMs from pyroptosis. Thus, our data suggest that MMs regulated by NLRP3 maintain the homeostasis of excitatory neurons.

Developmental role of macrophages modeled in human pluripotent stem cell-derived intestinal tissue

Macrophages populate the embryo early in gestation, but their role in development is not well defined. In particular, specification and function of macrophages in intestinal development remain little explored. To study this event in the human developmental context, we derived and combined human intestinal organoid and macrophages from pluripotent stem cells. Macrophages migrate into the organoid, proliferate, and occupy the emerging microanatomical niches of epithelial crypts and ganglia. They also acquire a transcriptomic profile similar to that of fetal intestinal macrophages and display tissue macrophage behaviors, such as recruitment to tissue injury. Using this model, we show that macrophages reduce glycolysis in mesenchymal cells and limit tissue growth without affecting tissue architecture, in contrast to the pro-growth effect of enteric neurons. In short, we engineered an intestinal tissue model populated with macrophages, and we suggest that resident macrophages contribute to the regulation of metabolism and growth of the developing intestine.

Sequencing Reveals miRNAs Enriched in the Developing Mouse Enteric Nervous System

The enteric nervous system (ENS) is an essential network of neurons and glia in the bowel wall. Defects in ENS development can result in Hirschsprung disease (HSCR), a life-threatening condition characterized by severe constipation, abdominal distention, bilious vomiting, and failure to thrive. A growing body of literature connects HSCR to alterations in miRNA expression, but there are limited data on the normal miRNA landscape in the developing ENS. We sequenced small RNAs (smRNA-seq) and messenger RNAs (mRNA-seq) from ENS precursor cells of mid-gestation Ednrb-EGFP mice and compared them to aggregated RNA from all other cells in the developing bowel. Our smRNA-seq results identified 73 miRNAs that were significantly enriched and highly expressed in the developing ENS, with miR-9, miR-27b, miR-124, miR-137, and miR-488 as our top 5 miRNAs that are conserved in humans. However, contrary to prior reports, our follow-up analyses of miR-137 showed that loss of Mir137 in Nestin-cre, Wnt1-cre, Sox10-cre, or Baf53b-cre lineage cells had no effect on mouse survival or ENS development. Our data provide important context for future studies of miRNAs in HSCR and other ENS diseases and highlight open questions about facility-specific factors in development.

New insights into muscularis macrophages in the gut: from their origin to therapeutic targeting

Muscularis macrophages, as the most abundant immune cells in the intestinal muscularis externa, exhibit tissue protective phenotype in the steady state. Owing to tremendous advances in technology, we now know the fact that muscularis macrophages are a heterogeneous population of cells which could be divided into different functional subsets depending on their anatomic niches. There is emerging evidence showing that these subsets, through molecular interactions with their neighbours, take part in a wide range of physiological and pathophysiological processes in the gut. In this review, we summarize recent progress (particularly over the past 4 years) on distribution, morphology, origin and functions of muscularis macrophages and, where possible, the characteristics of specific subsets in response to the microenvironment they occupy, with particular emphasis on their role in muscular inflammation. Furthermore, we also integrate their role in inflammation-related gastrointestinal disorders, such as post-operative ileus and diabetic gastroparesis, in order to propose future therapeutic strategies.

Enteric neuro-immune interactions in intestinal health and disease

The enteric nervous system is an autonomous neuronal circuit that regulates many processes far beyond the peristalsis in the gastro-intestinal tract. This circuit, consisting of enteric neurons and enteric glial cells, can engage in many intercellular interactions shaping the homeostatic microenvironment in the gut. Perhaps the most well documented interactions taking place, are the intestinal neuro-immune interactions which are essential for the fine-tuning of oral tolerance. In the context of intestinal disease, compelling evidence demonstrates both protective and detrimental roles for this bidirectional neuro-immune signaling. This review discusses the different immune cell types that are recognized to engage in neuronal crosstalk during intestinal health and disease. Highlighting the molecular pathways involved in the neuro-immune interactions might inspire novel strategies to target intestinal disease.

Endometriosis-Related Chronic Pelvic Pain

Endometriosis, which is the presence of endometrial stroma and glands outside the uterus, is one of the most frequently diagnosed gynecologic diseases in reproductive women. Patients with endometriosis suffer from various pain symptoms such as dysmenorrhea, dyspareunia, and chronic pelvic pain. The pathophysiology for chronic pain in patients with endometriosis has not been fully understood. Altered inflammatory responses have been shown to contribute to pain symptoms. Increased secretion of cytokines, angiogenic factors, and nerve growth factors has been suggested to increase pain. Also, altered distribution of nerve fibers may also contribute to chronic pain. Aside from local contributing factors, sensitization of the nervous system is also important in understanding persistent pain in endometriosis. Peripheral sensitization as well as central sensitization have been identified in patients with endometriosis. These sensitizations of the nervous system can also explain increased incidence of comorbidities related to pain such as irritable bowel disease, bladder pain syndrome, and vulvodynia in patients with endometriosis. In conclusion, there are various possible mechanisms behind pain in patients with endometriosis, and understanding these mechanisms can help clinicians understand the nature of the pain symptoms and decide on treatments for endometriosis-related pain symptoms.

Vancomycin-induced gut microbial dysbiosis alters enteric neuron-macrophage interactions during a critical period of postnatal development

Vancomycin is a broad-spectrum antibiotic widely used in cases of suspected sepsis in premature neonates. While appropriate and potentially lifesaving in this setting, early-life antibiotic exposure alters the developing microbiome and is associated with an increased risk of deadly complications, including late-onset sepsis (LOS) and necrotizing enterocolitis (NEC). Recent studies show that neonatal vancomycin treatment disrupts postnatal enteric nervous system (ENS) development in mouse pups, which is in part dependent upon neuroimmune interactions. This suggests that early-life antibiotic exposure could disrupt these interactions in the neonatal gut. Notably, a subset of tissue-resident intestinal macrophages, muscularis macrophages, has been identified as important contributors to the development of postnatal ENS. We hypothesized that vancomycin-induced neonatal dysbiosis impacts postnatal ENS development through its effects on macrophages. Using a mouse model, we found that exposure to vancomycin in the first 10 days of life, but not in adult mice, resulted in an expansion of pro-inflammatory colonic macrophages by increasing the recruitment of bone-marrow-derived macrophages. Single-cell RNA sequencing of neonatal colonic macrophages revealed that early-life vancomycin exposure was associated with an increase in immature and inflammatory macrophages, consistent with an influx of circulating monocytes differentiating into macrophages. Lineage tracing confirmed that vancomycin significantly increased the non-yolk-sac-derived macrophage population. Consistent with these results, early-life vancomycin exposure did not expand the colonic macrophage population nor decrease enteric neuron density in CCR2-deficient mice. Collectively, these findings demonstrate that early-life vancomycin exposure alters macrophage number and phenotypes in distinct ways compared with vancomycin exposure in adult mice and results in altered ENS development.

New insights into intestinal macrophages in necrotizing enterocolitis: the multi-functional role and promising therapeutic application

Necrotizing enterocolitis (NEC) is an inflammatory intestinal disease that profoundly affects preterm infants. Currently, the pathogenesis of NEC remains controversial, resulting in limited treatment strategies. The preterm infants are thought to be susceptible to gut inflammatory disorders because of their immature immune system. In early life, intestinal macrophages (IMφs), crucial components of innate immunity, demonstrate functional plasticity and diversity in intestinal development, resistance to pathogens, maintenance of the intestinal barrier, and regulation of gut microbiota. When the stimulations of environmental, dietary, and bacterial factors interrupt the homeostatic processes of IMφs, they will lead to intestinal disease, such as NEC. This review focuses on the IMφs related pathogenesis in NEC, discusses the multi-functional roles and relevant molecular mechanisms of IMφs in preterm infants, and explores promising therapeutic application for NEC.

Neonatal development of intestinal neuroimmune interactions

Interactions between the enteric nervous system (ENS), immune system, and gut microbiota regulate intestinal homeostasis in adults, but their development and role(s) in early life are relatively underexplored. In early life, these interactions are dynamic, because the mucosal immune system, microbiota, and the ENS are developing and influencing each other. Moreover, disrupting gut microbiota and gut immune system development, and potentially ENS development, by early-life antibiotic exposure increases the risk of diseases affecting the gut. Here, we review the development of the ENS and immune/epithelial cells, and identify potential critical periods for their interactions and development. We also highlight knowledge gaps that, when addressed, may help promote intestinal homeostasis, including in the settings of early-life antibiotic exposure.

Trans-cardiac perfusion of neonatal mice and immunofluorescence of the whole body as a method to study nervous system development

Whole animal perfusion is a well-established method that has been used for the past decades in multiple research fields. Particularly, it has been very important for the study of the brain. The rapid and uniform fixation of tissue is essential for the preservation of its integrity and the study of complex structures. For small tissue pieces submerging in formaldehyde solution oftentimes is sufficient to get a good fixation, larger tissues or organs with a more complicated structure present a greater difficulty. Here, we report the precise parameters to successfully perform trans-cardiac perfusion of neonatal mouse pups that allows a uniform fixation of the whole body for subsequent structural analysis and immunohistochemistry. In comparison to standard perfusion procedures of adult mice, changes in the pump velocity, the buffer volume and in the needle size lead to high quality fixation of neonatal mice pups. Further, we present a whole-body section staining, which results in a highly specific immunofluorescence signal suited for detailed analysis of multiple tissues or systems at the same time. Thus, our protocol provides a reproducible and reliable method for neonatal perfusion and staining that can rapidly be applied in any laboratory. It allows a high quality analysis of cellular structures and expression profiles at early developmental stages.

Environmental perception and control of gastrointestinal immunity by the enteric nervous system

The enteric nervous system (ENS) forms a versatile sensory system along the gastrointestinal tract that interacts with most cell types in the bowel. Herein, we portray host-environment interactions at the intestinal mucosal surface through the lens of the enteric nervous system. We describe local cellular interactions as well as long-range circuits between the enteric, central, and peripheral nervous systems. Additionally, we discuss recently discovered mechanisms by which enteric neurons and glia respond to biotic and abiotic environmental changes and how they regulate intestinal immunity and inflammation. The enteric nervous system emerges as an integrative sensory system with manifold immunoregulatory functions under both homeostatic and pathophysiological conditions.

Macrophages in intestinal fibrosis and regression

Intestinal macrophages are heterogenous cell populations with different developmental ontogeny and tissue anatomy. The concerted actions of intestinal macrophage subsets are critical to maintaining tissue homeostasis. However, the dysregulation of macrophages following tissue injury or chronic inflammation could also lead to intestinal fibrosis, with few treatment options in the clinic. In this review, we will characterize the features of intestinal macrophages in light of the latest advances in lineage tracing and single-cell sequencing technology. The roles of macrophages in distinct stages of intestinal fibrosis would be also elaborated. Finally, based on the reciprocal interaction between macrophages and intestinal fibrosis, we will propose the potential macrophage targeting anti-intestinal fibrosis therapies.

Telocytes and Macrophages in the Gut: From Morphology to Function. Do the Two Cell Types Interact with Each Other? Which Helps Which?

Telocytes and macrophages are ubiquitous cells located in loose connective tissues and share the same mesenchymal origin. Despite these common elements, depending on where they reside, these two cell types are profoundly different in terms of their morphology and functions. The purpose of this review is to provide an update on the knowledge regarding telocytes and macrophages in the gut, where their presence and significance have long been underestimated or misunderstood. The focus will be on the possibility that these two cell types interact with each other and on the potential meaning of these interactions. Based on the complexity of the topic, the variety of possible methodological approaches and the expertise of the author, the point of view in the discussion of the literature data will be mainly morphological. Furthermore, considering the relatively recent period in which these cell types have acquired a primary role in gastrointestinal functions, the attention will be greatly confined to those articles published in the last decade. The microbiota, another main protagonist in this context, will be mentioned only in passing. It is hoped that this review, although not exhaustive, will highlight the importance of macrophages and telocytes in the complex mechanisms that ensure intestinal functions.

Role of Macrophages and Mast Cells as Key Players in the Maintenance of Gastrointestinal Smooth Muscle Homeostasis and Disease

The gut contains the largest macrophage pool in the body, with populations of macrophages residing in the mucosa and muscularis propria of the gastrointestinal (GI) tract. Muscularis macrophages (MMs), which are located within the muscularis propria, interact with cells essential for GI function, such as interstitial cells of Cajal, enteric neurons, smooth muscle cells, enteric glia, and fibroblast-like cells, suggesting that these immune cells contribute to several aspects of GI function. This review focuses on the latest insights on the factors contributing to MM heterogeneity and the functional interaction of MMs with other cell types essential for GI function. This review integrates the latest findings on macrophages in other organs with increasing knowledge of MMs to better understand their role in a healthy and diseased gut. We describe the factors that contribute to (muscularis macrophage) MM heterogeneity, and the nature of MM interactions with cells regulating GI function. Finally, we also describe the increasing evidence suggesting a critical role of another immune cell type, the mast cell, in normal and diseased GI physiology.

CCR2 promotes monocyte recruitment and intestinal inflammation in mice lacking the interleukin-10 receptor

Macrophages are a heterogeneous population of mononuclear phagocytes abundantly distributed throughout the intestinal compartments that adapt to microenvironmental specific cues. In adult mice, the majority of intestinal macrophages exhibit a mature phenotype and are derived from blood monocytes. In the steady-state, replenishment of these cells is reduced in the absence of the chemokine receptor CCR2. Within the intestine of mice with colitis, there is a marked increase in the accumulation of immature macrophages that demonstrate an inflammatory phenotype. Here, we asked whether CCR2 is necessary for the development of colitis in mice lacking the receptor for IL10. We compared the development of intestinal inflammation in mice lacking IL10RA or both IL10RA and CCR2. The absence of CCR2 interfered with the accumulation of immature macrophages in IL10R-deficient mice, including a novel population of rounded submucosal Iba1+ cells, and reduced the severity of colitis in these mice. In contrast, the absence of CCR2 did not reduce the augmented inflammatory gene expression observed in mature intestinal macrophages isolated from mice lacking IL10RA. These data suggest that both newly recruited CCR2-dependent immature macrophages and CCR2-independent residual mature macrophages contribute to the development of intestinal inflammation observed in IL10R-deficient mice.

Neurons, Macrophages, and Glia: The Role of Intercellular Communication in the Enteric Nervous System

Neurons of the enteric nervous system (ENS) are the primary controllers of gastrointestinal functions. Although the ENS has been the central focus of research areas such as motility, this has now expanded to include the modulatory roles that non-neuronal cells have on neuronal function. This review discusses how enteric glia (EGC) and resident muscularis macrophages (mMacs) influence ENS communication. It highlights how the understanding of neuroglia interactions has extended beyond EGCs responding to exogenously applied neurotransmitters. Proposed mechanisms for neuron-EGC and glio-glia communication are discussed. The significance of these interactions is evidenced by gut functions that rely on these processes. mMacs are commonly known for their roles as immune cells which sample and respond to changes in the tissue environment. However, a more recent theory suggests that mMacs and enteric neurons are mutually dependent for their maintenance and function. This review summarizes the supportive and contradictory evidence for this theory, including potential mechanisms for mMac-neuron interaction. The need for a more thorough classification scheme to define how the "state" of mMacs relates to neuron loss or impaired function in disease is discussed. Despite the growing literature suggesting EGCs and mMacs have supportive or modulatory roles in ENS communication and gut function, conflicting evidence from different groups suggests more investigation is required. A broader understanding of why enteric neurons may need assistance from EGCs and mMacs in neurotransmission is still missing.

Macrophage Activation in the Dorsal Root Ganglion in Rats Developing Autotomy after Peripheral Nerve Injury

Autotomy, self-mutilation of a denervated limb, is common in animals after peripheral nerve injury (PNI) and is a reliable proxy for neuropathic pain in humans. Understanding the occurrence and treatment of autotomy remains challenging. The objective of this study was to investigate the occurrence of autotomy in nude and Wistar rats and evaluate the differences in macrophage activation and fiber sensitization contributing to the understanding of autotomy behavior. Autotomy in nude and Wistar rats was observed and evaluated 6 and 12 weeks after sciatic nerve repair surgery. The numbers of macrophages and the types of neurons in the dorsal root ganglion (DRG) between the two groups were compared by immunofluorescence studies. Immunostaining of T cells in the DRG was also assessed. Nude rats engaged in autotomy with less frequency than Wistar rats. Autotomy symptoms were also relatively less severe in nude rats. Immunofluorescence studies revealed increased macrophage accumulation and activation in the DRG of Wistar rats. The percentage of NF200+ neurons was higher at 6 and 12 weeks in Wistar rats compared to nude rats, but the percentage of CGRP+ neurons did not differ between two groups. Additionally, macrophages were concentrated around NF200-labeled A fibers. At 6 and 12 weeks following PNI, CD4+ T cells were not found in the DRG of the two groups. The accumulation and activation of macrophages in the DRG may account for the increased frequency and severity of autotomy in Wistar rats. Our results also suggest that A fiber neurons in the DRG play an important role in autotomy.

Beyond Immunity: Underappreciated Functions of Intestinal Macrophages

The gastrointestinal tract hosts the largest compartment of macrophages in the body, where they serve as mediators of host defense and immunity. Seeded in the complex tissue-environment of the gut, an array of both hematopoietic and non-hematopoietic cells forms their immediate neighborhood. Emerging data demonstrate that the functional diversity of intestinal macrophages reaches beyond classical immunity and includes underappreciated non-immune functions. In this review, we discuss recent advances in research on intestinal macrophage heterogeneity, with a particular focus on how non-immune functions of macrophages impact tissue homeostasis and function. We delve into the strategic localization of distinct gut macrophage populations, describe the potential factors that regulate their identity and functional heterogeneity within these locations, and provide open questions that we hope will inspire research dedicated to elucidating a holistic view on macrophage-tissue cell interactions in the body's largest mucosal organ.

Unique Neural Circuit Connectivity of Mouse Proximal, Middle, and Distal Colon Defines Regional Colonic Motor Patterns

BACKGROUND & AIMS

Colonic motor patterns have been described by a number of different groups, but the neural connectivity and ganglion architecture supporting patterned motor activity have not been elucidated. Our goals were to describe quantitatively, by region, the structural architecture of the mouse enteric nervous system and use functional calcium imaging, pharmacology, and electrical stimulation to show regional underpinnings of different motor patterns.

METHODS

Excised colon segments from mice expressing the calcium indicator GCaMP6f or GCaMP6s were used to examine spontaneous and evoked (pharmacologic or electrical) changes in GCaMP-mediated fluorescence and coupled with assessment of colonic motor activity, immunohistochemistry, and confocal imaging. Three-dimensional image reconstruction and statistical methods were used to describe quantitatively mouse colon myenteric ganglion structure, neural and vascular network patterning, and neural connectivity.

RESULTS

In intact colon, regionally specific myenteric ganglion size, architecture, and neural circuit connectivity patterns along with neurotransmitter-receptor expression underlie colonic motor patterns that define functional differences along the colon. Region-specific effects on spontaneous, evoked, and chemically induced neural activity contribute to regional motor patterns, as does intraganglionic functional connectivity. We provide direct evidence of neural circuit structural and functional regional differences that have only been inferred in previous investigations. We include regional comparisons between quantitative measures in mouse and human colon that represent an important advance in showing the usefulness and relevance of the mouse system for translation to the human colon.

CONCLUSIONS

There are several neural mechanisms dependent on myenteric ganglion architecture and functional connectivity that underlie neurogenic control of patterned motor function in the mouse colon.

Targeting Enteric Neurons and Plexitis for the Management of Inflammatory Bowel Disease

Ulcerative colitis (UC) and Crohn's disease (CD) are pathological conditions with an unknown aetiology that are characterised by severe inflammation of the intestinal tract and collectively referred to as inflammatory bowel disease (IBD). Current treatments are mostly ineffective due to their limited efficacy or toxicity, necessitating surgical resection of the affected bowel. The management of IBD is hindered by a lack of prognostic markers for clinical inflammatory relapse. Intestinal inflammation associates with the infiltration of immune cells (leukocytes) into, or surrounding the neuronal ganglia of the enteric nervous system (ENS) termed plexitis or ganglionitis. Histological observation of plexitis in unaffected intestinal regions is emerging as a vital predictive marker for IBD relapses. Plexitis associates with alterations to the structure, cellular composition, molecular expression and electrophysiological function of enteric neurons. Moreover, plexitis often occurs before the onset of gross clinical inflammation, which may indicate that plexitis can contribute to the progression of intestinal inflammation. In this review, the bilateral relationships between the ENS and inflammation are discussed. These include the effects and mechanisms of inflammation-induced enteric neuronal loss and plasticity. Additionally, the role of enteric neurons in preventing antigenic/pathogenic insult and immunomodulation is explored. While all current treatments target the inflammatory pathology of IBD, interventions that protect the ENS may offer an alternative avenue for therapeutic intervention.

Evidence of a Myenteric Plexus Barrier and Its Macrophage-Dependent Degradation During Murine Colitis: Implications in Enteric Neuroinflammation

BACKGROUND & AIMS

Neuroinflammation in the gut is associated with many gastrointestinal (GI) diseases, including inflammatory bowel disease. In the brain, neuroinflammatory conditions are associated with blood-brain barrier (BBB) disruption and subsequent neuronal injury. We sought to determine whether the enteric nervous system is similarly protected by a physical barrier and whether that barrier is disrupted in colitis.

METHODS

Confocal and electron microscopy were used to characterize myenteric plexus structure, and FITC-dextran assays were used to assess for presence of a barrier. Colitis was induced with dextran sulfate sodium, with co-administration of liposome-encapsulated clodronate to deplete macrophages.

RESULTS

We identified a blood-myenteric barrier (BMB) consisting of extracellular matrix proteins (agrin and collagen-4) and glial end-feet, reminiscent of the BBB, surrounded by a collagen-rich periganglionic space. The BMB is impermeable to the passive movement of 4 kDa FITC-dextran particles. A population of macrophages is present within enteric ganglia (intraganglionic macrophages [IGMs]) and exhibits a distinct morphology from muscularis macrophages, with extensive cytoplasmic vacuolization and mitochondrial swelling but without signs of apoptosis. IGMs can penetrate the BMB in physiological conditions and establish direct contact with neurons and glia. Dextran sulfate sodium-induced colitis leads to BMB disruption, loss of its barrier integrity, and increased numbers of IGMs in a macrophage-dependent process.

CONCLUSIONS

In intestinal inflammation, macrophage-mediated degradation of the BMB disrupts its physiological barrier function, eliminates the separation of the intra- and extra-ganglionic compartments, and allows inflammatory stimuli to access the myenteric plexus. This suggests a potential mechanism for the onset of neuroinflammation in colitis and other GI pathologies with acquired enteric neuronal dysfunction.

Niche-specific functional heterogeneity of intestinal resident macrophages

Intestinal resident macrophages are at the front line of host defence at the mucosal barrier within the gastrointestinal tract and have long been known to play a crucial role in the response to food antigens and bacteria that are able to penetrate the mucosal barrier. However, recent advances in single-cell RNA sequencing technology have revealed that resident macrophages throughout the gut are functionally specialised to carry out specific roles in the niche they occupy, leading to an unprecedented understanding of the heterogeneity and potential biological functions of these cells. This review aims to integrate these novel findings with long-standing knowledge, to provide an updated overview on our understanding of macrophage function in the gastrointestinal tract and to speculate on the role of specialised subsets in the context of homoeostasis and disease.

The Interplay between Nutrition, Innate Immunity, and the Commensal Microbiota in Adaptive Intestinal Morphogenesis

The gastrointestinal tract is a functionally and anatomically segmented organ that is colonized by microbial communities from birth. While the genetics of mouse gut development is increasingly understood, how nutritional factors and the commensal gut microbiota act in concert to shape tissue organization and morphology of this rapidly renewing organ remains enigmatic. Here, we provide an overview of embryonic mouse gut development, with a focus on the intestinal vasculature and the enteric nervous system. We review how nutrition and the gut microbiota affect the adaptation of cellular and morphologic properties of the intestine, and how these processes are interconnected with innate immunity. Furthermore, we discuss how nutritional and microbial factors impact the renewal and differentiation of the epithelial lineage, influence the adaptation of capillary networks organized in villus structures, and shape the enteric nervous system and the intestinal smooth muscle layers. Intriguingly, the anatomy of the gut shows remarkable flexibility to nutritional and microbial challenges in the adult organism.

Zebrafish harbor diverse intestinal macrophage populations including a subset intimately associated with enteric neural processes

Intestinal macrophages are essential for gut health but remain understudied outside of human and mouse systems. Here, we establish zebrafish as a powerful model that provides superior imaging capabilities for whole-gut analysis along all dimensions (anterior-posterior and center-outer axes) for dissecting macrophage biology in gastrointestinal health and disease. We utilized high-resolution imaging to show that the zebrafish gut contains bona fide muscularis and mucosal macrophages, as well as surprisingly large subsets intimately associated with enteric neural processes. Interestingly, most muscularis macrophages span multiple gut layers in stark contrast to their mammalian counterparts typically restricted to a single layer. Using macrophage-deficient irf8 zebrafish, we found a depletion of muscularis but not mucosal macrophages, and that they may be dispensable for gross intestinal transit in adults but not during development. These characterizations provide first insights into intestinal macrophages and their association with the enteric nervous system from development to adulthood in teleosts.

The enteric nervous system in gastrointestinal disease etiology

A highly conserved but convoluted network of neurons and glial cells, the enteric nervous system (ENS), is positioned along the wall of the gut to coordinate digestive processes and gastrointestinal homeostasis. Because ENS components are in charge of the autonomous regulation of gut function, it is inevitable that their dysfunction is central to the pathophysiology and symptom generation of gastrointestinal disease. While for neurodevelopmental disorders such as Hirschsprung, ENS pathogenesis appears to be clear-cut, the role for impaired ENS activity in the etiology of other gastrointestinal disorders is less established and is often deemed secondary to other insults like intestinal inflammation. However, mounting experimental evidence in recent years indicates that gastrointestinal homeostasis hinges on multifaceted connections between the ENS, and other cellular networks such as the intestinal epithelium, the immune system, and the intestinal microbiome. Derangement of these interactions could underlie gastrointestinal disease onset and elicit variable degrees of abnormal gut function, pinpointing, perhaps unexpectedly, the ENS as a diligent participant in idiopathic but also in inflammatory and cancerous diseases of the gut. In this review, we discuss the latest evidence on the role of the ENS in the pathogenesis of enteric neuropathies, disorders of gut-brain interaction, inflammatory bowel diseases, and colorectal cancer.

Lack of Mucosal Cholinergic Innervation Is Associated With Increased Risk of Enterocolitis in Hirschsprung's Disease

BACKGROUND & AIMS

Hirschsprung's disease (HSCR) is a congenital intestinal motility disorder defined by the absence of enteric neuronal cells (ganglia) in the distal gut. The development of HSCR-associated enterocolitis remains a life-threatening complication. Absence of enteric ganglia implicates innervation of acetylcholine-secreting (cholinergic) nerve fibers. Cholinergic signals have been reported to control excessive inflammation, but the impact on HSCR-associated enterocolitis is unknown.

METHODS

We enrolled 44 HSCR patients in a prospective multicenter study and grouped them according to their degree of colonic mucosal acetylcholinesterase-positive innervation into low-fiber and high-fiber patient groups. The fiber phenotype was correlated with the tissue cytokine profile as well as immune cell frequencies using Luminex analysis and fluorescence-activated cell sorting analysis of colonic tissue and immune cells. Using confocal immunofluorescence microscopy, macrophages were identified in close proximity to nerve fibers and characterized by RNA-seq analysis. Microbial dysbiosis was analyzed in colonic tissue using 16S-rDNA gene sequencing. Finally, the fiber phenotype was correlated with postoperative enterocolitis manifestation.

RESULTS

The presence of mucosal nerve fiber innervation correlated with reduced T-helper 17 cytokines and cell frequencies. In high-fiber tissue, macrophages co-localized with nerve fibers and expressed significantly less interleukin 23 than macrophages from low-fiber tissue. HSCR patients lacking mucosal nerve fibers showed microbial dysbiosis and had a higher incidence of postoperative enterocolitis.

CONCLUSIONS

The mucosal fiber phenotype might serve as a prognostic marker for enterocolitis development in HSCR patients and may offer an approach to personalized patient care and new therapeutic options.

Muscularis macrophages establish cell-to-cell contacts with telocytes/PDGFRα-positive cells and smooth muscle cells in the human and mouse gastrointestinal tract

BACKGROUND AND AIM

Muscularis macrophages (MMs) not only mediate the innate immunity, but also functionally interact with cells important for gastrointestinal motility. The aim of this study was to determine the spatial relationship and types of contacts between the MMs and neighboring cells in the muscularis propria of human and mouse stomach, small intestine, and large intestine.

METHODS

The distribution and morphology of MMs and their contacts with other cells were investigated by immunohistochemistry and transmission electron microscopy.

KEY RESULTS

Immunohistochemistry showed variable shape and number of MMs according to their location in different portions of the muscle coat. By double labeling, a close association between MMs and neighboring cells, that is, neurons, smooth muscle cells, interstitial cells of Cajal (ICCs), telocytes (TCs)/PDGFRα-positive cells, was seen. Electron microscopy demonstrated that in the muscle layers of both animal species, MMs have similar ultrastructural features and have specialized cell-to-cell contacts with smooth muscle cells and TCs/PDGFRα-positive cells but not with ICCs and enteric neurons.

CONCLUSION & INFERENCES

This study describes varying patterns of distribution of MMs between different regions of the gut, and reports the presence of distinct and extended cell-to-cell contacts between MMs and smooth muscle cells and between MMs and TCs/PDGFRα-positive cells. In contrast, MMs, although close to ICCs and nerve elements, did not make contact with them. These findings indicate specialized and variable roles for MMs in the modulation of gastrointestinal motility whose significance should be more closely investigated in normal and pathological conditions.

From Species to Regional and Local Specialization of Intestinal Macrophages

Initially intended for nutrient uptake, phagocytosis represents a central mechanism of debris removal and host defense against invading pathogens through the entire animal kingdom. In vertebrates and also many invertebrates, macrophages (MFs) and MF-like cells (e.g., coelomocytes and hemocytes) are professional phagocytic cells that seed tissues to maintain homeostasis through pathogen killing, efferocytosis and tissue shaping, repair, and remodeling. Some MF functions are common to all species and tissues, whereas others are specific to their homing tissue. Indeed, shaped by their microenvironment, MFs become adapted to perform particular functions, highlighting their great plasticity and giving rise to high population diversity. Interestingly, the gut displays several anatomic and functional compartments with large pools of strikingly diversified MF populations. This review focuses on recent advances on intestinal MFs in several species, which have allowed to infer their specificity and functions.

IL-34 and CSF-1, deciphering similarities and differences at steady state and in diseases

Although IL-34 and CSF-1 share actions as key mediators of monocytes/macrophages survival and differentiation, they also display differences that should be identified to better define their respective roles in health and diseases. IL-34 displays low sequence homology with CSF-1 but has a similar general structure and they both bind to a common receptor CSF-1R, although binding and subsequent intracellular signaling shows differences. CSF-1R expression has been until now mainly described at a steady state in monocytes/macrophages and myeloid dendritic cells, as well as in some cancers. IL-34 has also 2 other receptors, protein-tyrosine phosphatase zeta (PTPζ) and CD138 (Syndecan-1), expressed in some epithelium, cells of the central nervous system (CNS), as well as in numerous cancers. While most, if not all, of CSF-1 actions are mediated through monocyte/macrophages, IL-34 has also other potential actions through PTPζ and CD138. Additionally, IL-34 and CSF-1 are produced by different cells in different tissues. This review describes and discusses similarities and differences between IL-34 and CSF-1 at steady state and in pathological situations and identifies possible ways to target IL-34, CSF-1, and its receptors.

Gut innervation and enteric nervous system development: a spatial, temporal and molecular tour de force

During embryonic development, the gut is innervated by intrinsic (enteric) and extrinsic nerves. Focusing on mammalian ENS development, in this Review we highlight how important the different compartments of this innervation are to assure proper gut function. We specifically address the three-dimensional architecture of the innervation, paying special attention to the differences in development along the longitudinal and circumferential axes of the gut. We review recent information about the formation of both intrinsic innervation, which is fairly well-known, as well as the establishment of the extrinsic innervation, which, despite its importance in gut-brain signaling, has received much less attention. We further discuss how external microbial and nutritional cues or neuroimmune interactions may influence development of gut innervation. Finally, we provide summary tables, describing the location and function of several well-known molecules, along with some newer factors that have more recently been implicated in the development of gut innervation.

A Population of Radio-Resistant Macrophages in the Deep Myenteric Plexus Contributes to Postoperative Ileus Via Toll-Like Receptor 3 Signaling

Postoperative ileus (POI) is triggered by an innate immune response in the muscularis externa (ME) and is accompanied by bacterial translocation. Bacteria can trigger an innate immune response via toll-like receptor (TLR) activation, but the latter's contribution to POI has been disproved for several TLRs, including TLR2 and TLR4. Herein we investigated the role of double-stranded RNA detection via TLR3 and TIR-domain-containing adapter-inducing interferon-β (TRIF) signaling pathway in POI. POI was induced by small bowel intestinal manipulation in wt, TRIF^-/-, TLR3^-/-, type I interferon receptor^-/- and interferon-β reporter mice, all on C57BL/6 background, and POI severity was quantified by gene expression analysis, gastrointestinal transit and leukocyte extravasation into the ME. TRIF/TLR3 deficiency reduced postoperative ME inflammation and prevented POI. With bone marrow transplantation, RNA-sequencing, flow cytometry and immunohistochemistry we revealed a distinct TLR3-expressing radio-resistant MHCII^hiCX3CR1^- IBA-1⁺ resident macrophage population within the deep myenteric plexus. TLR3 deficiency in these cells, but not in MHCII^hiCX3CR1⁺ macrophages, reduced cytokine expression in POI. While this might not be an exclusive macrophage-privileged pathway, the TLR3/TRIF axis contributes to proinflammatory cytokine production in MHCII^hiCX3CR1^- IBA-1⁺ macrophages during POI. Deficiency in TLR3/TRIF protects mice from POI. These data suggest that TLR3 antagonism may prevent POI in humans.

Neuron-Glia Interaction in the Developing and Adult Enteric Nervous System

The enteric nervous system (ENS) constitutes the largest part of the peripheral nervous system. In recent years, ENS development and its neurogenetic capacity in homeostasis and allostasishave gained increasing attention. Developmentally, the neural precursors of the ENS are mainly derived from vagal and sacral neural crest cell portions. Furthermore, Schwann cell precursors, as well as endodermal pancreatic progenitors, participate in ENS formation. Neural precursorsenherite three subpopulations: a bipotent neuron-glia, a neuronal-fated and a glial-fated subpopulation. Typically, enteric neural precursors migrate along the entire bowel to the anal end, chemoattracted by glial cell-derived neurotrophic factor (GDNF) and endothelin 3 (EDN3) molecules. During migration, a fraction undergoes differentiation into neurons and glial cells. Differentiation is regulated by bone morphogenetic proteins (BMP), Hedgehog and Notch signalling. The fully formed adult ENS may react to injury and damage with neurogenesis and gliogenesis. Nevertheless, the origin of differentiating cells is currently under debate. Putative candidates are an embryonic-like enteric neural progenitor population, Schwann cell precursors and transdifferentiating glial cells. These cells can be isolated and propagated in culture as adult ENS progenitors and may be used for cell transplantation therapies for treating enteric aganglionosis in Chagas and Hirschsprung's diseases.

Origins and diversity of macrophages in health and disease

Macrophages are the first immune cells in the developing embryo and have a central role in organ development, homeostasis, immunity and repair. Over the last century, our understanding of these cells has evolved from being thought of as simple phagocytic cells to master regulators involved in governing a myriad of cellular processes. A better appreciation of macrophage biology has been matched with a clearer understanding of their diverse origins and the flexibility of their metabolic and transcriptional machinery. The understanding of the classical mononuclear phagocyte system in its original form has now been expanded to include the embryonic origin of tissue‐resident macrophages. A better knowledge of the intrinsic similarities and differences between macrophages of embryonic or monocyte origin has highlighted the importance of ontogeny in macrophage dysfunction in disease. In this review, we provide an update on origin and classification of tissue macrophages, the mechanisms of macrophage specialisation and their role in health and disease. The importance of the macrophage niche in providing trophic factors and a specialised environment for macrophage differentiation and specialisation is also discussed.

Microbial Modulation of the Development and Physiology of the Enteric Nervous System

The gastrointestinal tract harbors an intrinsic neuronal network, the enteric nervous system (ENS). The ENS controls motility, fluid homeostasis, and blood flow, but also interacts with other components of the intestine such as epithelial and immune cells. Recent studies indicate that gut microbiota diversification, which occurs alongside postnatal ENS maturation, could be critical for the development and function of the ENS. Here we discuss the possibility that this functional relationship starts in utero, whereby the maternal microbiota would prime the developing ENS and shape its physiology. We review ENS/microbiota interactions and their modulation in physiological and pathophysiological contexts. While microbial modulation of the ENS physiology is now well established, further studies are required to understand the contribution of the gut microbiota to the development and pathology of the ENS and to reveal the precise mechanisms underlying microbiota-to-ENS communications.

Enteric Glia Modulate Macrophage Phenotype and Visceral Sensitivity following Inflammation

Mechanisms resulting in abdominal pain include altered neuro-immune interactions in the gastrointestinal tract, but the signaling processes that link immune activation with visceral hypersensitivity are unresolved. We hypothesized that enteric glia link the neural and immune systems of the gut and that communication between enteric glia and immune cells modulates the development of visceral hypersensitivity. To this end, we manipulated a major mechanism of glial intercellular communication that requires connexin-43 and assessed the effects on acute and chronic inflammation, visceral hypersensitivity, and immune responses. Deleting connexin-43 in glia protected against the development of visceral hypersensitivity following chronic colitis. Mechanistically, the protective effects of glial manipulation were mediated by disrupting the glial-mediated activation of macrophages through the macrophage colony-stimulating factor. Collectively, our data identified enteric glia as a critical link between gastrointestinal neural and immune systems that could be harnessed by therapies to ameliorate abdominal pain.

Gastric Biopsies in Gastroparesis: Insights into Gastric Neuromuscular Disorders to Aid Treatment

The cellular and molecular understanding of human gastroparesis has markedly improved due to studies on full-thickness gastric biopsies. A decrease in the number of interstitial cells of Cajal (ICC) and functional changes in ICC constitutes the hallmark cellular feature of gastroparesis. More recently, in animal models, macrophages have also been identified to play a central role in development of delayed gastric emptying. Activation of macrophages leads to loss of ICC. In human gastroparesis, loss of anti-inflammatory macrophages in gastric muscle has been shown. Deeper molecular characterization using transcriptomics and proteomics has identified macrophage-based immune dysregulation in human gastroparesis.

Intestinal resident macrophages: Multitaskers of the gut

BACKGROUND

Intestinal resident macrophages play a crucial role in homeostasis and have been implicated in numerous gastrointestinal diseases. While historically believed to be largely of hematopoietic origin, recent advances in fate-mapping technology have unveiled the existence of long-lived, self-maintaining populations located in specific niches throughout the gut wall. Furthermore, the advent of single-cell technology has enabled an unprecedented characterization of the functional specialization of tissue-resident macrophages throughout the gastrointestinal tract.

PURPOSE

The purpose of this review was to provide a panorama on intestinal resident macrophages, with particular focus to the recent advances in the field. Here, we discuss the functions and phenotype of intestinal resident macrophages and, where possible, the functional specialization of these cells in response to the niche they occupy. Furthermore, we will discuss their role in gastrointestinal diseases.

Origin and Differentiation of Nerve-Associated Macrophages

The mature peripheral nervous system is a steady network structure yet shows remarkable regenerative properties. The interaction of peripheral nerves with myeloid cells has largely been investigated in the context of damage, following trauma or infection. Recently, specific macrophages dedicated to homeostatic peripheral nerves have come into focus. These macrophages are defined by tissue and nerve type, are seeded in part prenatally, and self-maintain via proliferation. Thus, they are markedly distinct from monocyte-derived macrophages invading after local disturbance of nerve integrity. The phenotypic and transcriptional adaptation of macrophages to the discrete nervous niche may exert axon guidance and nerve regeneration and thus contribute to the stability of the peripheral nervous network. Deciphering these conserved macrophage-nerve interactions offers new translational perspectives for chronic diseases of the peripheral nervous system, such as diabetic neuropathy and pain.

Immunohistochemical study of morphology and distribution of CD163+ve macrophages in the normal adult equine gastrointestinal tract

Intestinal macrophages are the largest group of mononuclear phagocytes in the body and play a role in intestinal innate immunity, neuroimmune interactions and maintaining intestinal homeostasis. Conversely, they also are implicated in numerous pathologies of the gastrointestinal tract, such as postoperative ileus and inflammatory bowel disease. As a result, macrophages could be potential therapeutic targets. To date, there are limited studies on the morphology and distribution of macrophages in the equine gastrointestinal tract (GIT). The aim of this study was to identify the location and abundance of resident macrophages in the equine GIT using CD163 as an immunohistochemical marker. Tissue samples were obtained post-mortem from 14 sites along the gastrointestinal tracts of 10 horses free from gastrointestinal disease; sample sites extended from the stomach to the small colon. CD163^+ve cells were present in all regions of the equine GIT from stomach to small colon. CD163^+ve cells were also identified in all tissue layers of the intestinal wall, namely, mucosa, submucosa, muscularis externa (ME), myenteric plexus and serosa. Consistent with a proposed function in regulation of intestinal motility, CD163^+ve cells were regularly distributed within the ME, with accumulations closely associated with the myenteric plexus and effector cells such as neurons and the interstitial cells of Cajal (ICC).

Intestinal Macrophages in Resolving Inflammation

Macrophages not only regulate intestinal homeostasis by recognizing pathogens to control enteric infections but also employ negative feedback mechanisms to prevent chronic inflammation. Hence, macrophages are intriguing targets for immune-mediated therapies, especially when barrier function in the gut is compromised to trigger aberrant inflammatory responses, most notably during inflammatory bowel diseases. Recently, there has been considerable progress in our understanding of human macrophage biology in different tissues, including the intestines. In this review, we discuss some new findings on the properties of distinct populations of intestinal macrophages, how resolution of inflammation and tissue repair by macrophages could be promoted by type 2 cytokines as well as other therapeutic interventions, and highlight some challenges for translating these findings into the future for this exciting area of immunology research.

Autonomic nervous system and inflammation interaction in endometriosis-associated pain

Endometriosis is a chronic inflammatory disease. Pain is the most common symptom in endometriosis. Endometriosis-associated pain is caused by inflammation, and is related to aberrant innervation. Although the specific mechanism between endometriosis-associated pain and the interaction of aberrant innervation and inflammation remains unclear, many studies have confirmed certain correlations between them. In addition, we found that some chronic inflammatory autoimmune diseases (AIDs) such as inflammatory bowel disease (IBD) and rheumatoid arthritis (RA) share similar characteristics: the changes in dysregulation of inflammatory factors as well as the function and innervation of the autonomic nervous system (ANS). The mechanisms underlying the interaction between the ANS and inflammation have provided new advances among these disorders. Therefore, the purpose of this review is to compare the changes in inflammation and ANS in endometriosis, IBD, and RA; and to explore the role and possible mechanism of sympathetic and parasympathetic nerves in endometriosis-associated inflammation by referring to IBD and RA studies to provide some reference for further endometriosis research and treatment.

Dlx1/2 mice have abnormal enteric nervous system function

Decades ago, investigators reported that mice lacking DLX1 and DLX2, transcription factors expressed in the enteric nervous system (ENS), die with possible bowel motility problems. These problems were never fully elucidated. We found that mice lacking DLX1 and DLX2 (Dlx1/2-/- mice) had slower small bowel transit and reduced or absent neurally mediated contraction complexes. In contrast, small bowel motility seemed normal in adult mice lacking DLX1 (Dlx1-/-). Even with detailed anatomic studies, we found no defects in ENS precursor migration, or neuronal and glial density in Dlx1/2-/- or Dlx1-/- mice. However, RNA sequencing of Dlx1/2-/- ENS revealed dysregulation of many genes, including vasoactive intestinal peptide (Vip). Using immunohistochemistry and reporter mice, we then found that Dlx1/2-/- mice have reduced VIP expression and fewer VIP-lineage neurons in their ENS. Our study reveals what we believe is a novel connection between Dlx genes and Vip and highlights the observation that dangerous bowel motility problems can occur in the absence of easily identifiable ENS structural defects. These findings may be relevant for disorders like chronic intestinal pseudo-obstruction (CIPO) syndrome.

Gut macrophages: key players in intestinal immunity and tissue physiology

The mammalian gastrointestinal tract harbors a large reservoir of tissue macrophages, which, in concert with other immune cells, help to maintain a delicate balance between tolerance to commensal microbes and food antigens, and resistance to potentially harmful microbes or toxins. Beyond their roles in resistance and tolerance, recent studies have uncovered novel roles played by tissue-resident, including intestinal-resident macrophages in organ physiology. Here, we will discuss recent advances in the understanding of the origin, phenotype and function of macrophages residing in the different layers of the intestine during homeostasis and under pathological conditions.

Macrophages in intestinal inflammation and resolution: a potential therapeutic target in IBD

Macrophages are the gatekeepers of intestinal immune homeostasis as they discriminate between innocuous antigens and potential pathogens to maintain oral tolerance. However, in individuals with a genetic and environmental predisposition, regulation of intestinal immunity is impaired, leading to chronic relapsing immune activation and pathologies of the gastrointestinal tract, such as IBD. As evidence suggests a causal link between defects in the resolution of intestinal inflammation and altered monocyte-macrophage differentiation in patients with IBD, macrophages have been considered as a novel potential target to develop new treatment approaches. This Review discusses the molecular and cellular mechanisms involved in the differentiation and function of intestinal macrophages in homeostasis and inflammation, and their role in resolving the inflammatory process. Understanding the molecular pathways involved in the specification of intestinal macrophages might lead to a new class of targets that promote remission in patients with IBD.

Enteric Glia: S100, GFAP, and Beyond

Since several years, the enteric nervous system (ENS) is getting more and more in the focus of gastrointestinal research. While the main interest was credited for years to the enteric neurons and their functional properties, less attention has been paid on the enteric glial cells (EGCs). Although the similarity of EGCs to central nervous system (CNS) astrocytes has been demonstrated a long time ago, EGCs were investigated in more detail only recently. Similar to the CNS, there is not "the" EGC, but also a broad range of diversity. Based on morphology and protein expression, such as glial fibrillary acidic protein (GFAP), S100, or Proteolipid-protein-1 (PLP1), several distinct glial types can be differentiated. Their heterogeneity in morphology, localization, and transcription as well as interaction with surrounding cells indicate versatile functional properties of these cells for gut function in health and disease. Although NG2 is found in a subset of CNS glial cells, it did not colocalize with the glial marker S100 or GFAP in the ENS. Instead, it in part colocalize with PDGFRα, as it does in the CNS, which do stain fibroblast-like cells in the gastrointestinal tract. Moreover, there seem to be species dependent differences. While GFAP is always found in the rodent ENS, this is completely different for the human gut. Only the compromised human ENS shows a significant amount of GFAP-positive glial cells. So, in general we can conclude that the EGC population is species specific and as complex as CNS glia. Anat Rec, 302:1333-1344, 2019. © 2019 Wiley Periodicals, Inc.

Unexpected Roles for the Second Brain: Enteric Nervous System as Master Regulator of Bowel Function

At the most fundamental level, the bowel facilitates absorption of small molecules, regulates fluid and electrolyte flux, and eliminates waste. To successfully coordinate this complex array of functions, the bowel relies on the enteric nervous system (ENS), an intricate network of more than 500 million neurons and supporting glia that are organized into distinct layers or plexi within the bowel wall. Neuron and glial diversity, as well as neurotransmitter and receptor expression in the ENS, resembles that of the central nervous system. The most carefully studied ENS functions include control of bowel motility, epithelial secretion, and blood flow, but the ENS also interacts with enteroendocrine cells, influences epithelial proliferation and repair, modulates the intestinal immune system, and mediates extrinsic nerve input. Here, we review the many different cell types that communicate with the ENS, integrating data about ENS function into a broader view of human health and disease. In particular, we focus on exciting new literature highlighting relationships between the ENS and its lesser-known interacting partners.