Robust, 3-Dimensional Visualization of Human Colon Enteric Nervous System Without Tissue Sectioning

Three-Dimensional Imaging of the Enteric Nervous System in Human Pediatric Colon Reveals New Features of Hirschsprung's Disease

BACKGROUND & AIMS

Hirschsprung's disease is defined by the absence of the enteric nervous system (ENS) from the distal bowel. Primary treatment is "pull-through" surgery to remove bowel that lacks ENS, with reanastomosis of "normal" bowel near the anal verge. Problems after pull-through are common, and some may be due to retained hypoganglionic bowel (ie, low ENS density). Testing this hypothesis has been difficult because counting enteric neurons in tissue sections is unreliable, even for experts. Tissue clearing and 3-dimensional imaging provide better data about ENS structure than sectioning.

METHODS

Regions from 11 human colons and 1 ileal specimen resected during Hirschsprung's disease pull-through surgery were cleared, stained with antibodies to visualize the ENS, and imaged by confocal microscopy. Control distal colon from people with no known bowel problems were similarly cleared, stained, and imaged.

RESULTS

Quantitative analyses of human colon, ranging from 3 days to 60 years old, suggest age-dependent changes in the myenteric plexus area, ENS ganglion area, percentage of myenteric plexus occupied by ganglia, neurons/mm2, and neuron Feret's diameter. Neuron counting using 3-dimensional images was highly reproducible. High ENS density in neonatal colon allowed reliable neuron counts using 500-μm2 × 500-μm2 regions (36-fold smaller than in adults). Hirschsprung's samples varied 8-fold in proximal margin enteric neuron density and had diverse ENS architecture in resected bowel.

CONCLUSIONS

Tissue clearing and 3-dimensional imaging provide more reliable information about ENS structure than tissue sections. ENS structure changes during childhood. Three-dimensional ENS anatomy may provide new insight into human bowel motility disorders, including Hirschsprung's disease.

The crosstalk between enteric nervous system and immune system in intestinal development, homeostasis and diseases

The gut is the largest digestive and absorptive organ, which is essential for induction of mucosal and systemic immune responses, and maintenance of metabolic-immune homeostasis. The intestinal components contain the epithelium, stromal cells, immune cells, and enteric nervous system (ENS), as well as the outers, such as gut microbiota, metabolites, and nutrients. The dyshomeostasis of intestinal microenvironment induces abnormal intestinal development and functions, even colon diseases including dysplasia, inflammation and tumor. Several recent studies have identified that ENS plays a crucial role in maintaining the immune homeostasis of gastrointestinal (GI) microenvironment. The crosstalk between ENS and immune cells, mainly macrophages, T cells, and innate lymphoid cells (ILCs), has been found to exert important regulatory roles in intestinal tissue programming, homeostasis, function, and inflammation. In this review, we mainly summarize the critical roles of the interactions between ENS and immune cells in intestinal homeostasis during intestinal development and diseases progression, to provide theoretical bases and ideas for the exploration of immunotherapy for gastrointestinal diseases with the ENS as potential novel targets.

The impact of the recipient intestinal site on the differentiation of transplanted enteric neural crest cells

PURPOSE

It has long been established that the failure of enteric neural crest cells (ENCCs) to colonize the entire gut results in aganglionosis at the distal colon in Hirschsprung disease (HD). However, it is still unclear how the intestinal microenvironment of the distal aganglionic gut differs from that of the proximal ganglionic gut in HD versus normal gut. We have recently succeeded in transplanting ENCC into aganglionic gut in endothelin receptor B (Ednrb) knockout (KO) mice. to advance the development of cell therapy for HD, it is essential to determine if the transplanted ENCCs differentiate normally in aganglionic gut. Therefore, we designed this study to investigate the impact of the environment of the recipient intestinal tract, at various sites of aganglionic gut, on the differentiation of transplanted ENCCs.

METHODS

ENCCs were isolated from Sox10 Venus transgenic (Tg) mouse gut on embryonic day 18.5 (E18.5) and neurospheres (NS) were generated. Then, NS were transplanted into aganglionic KO and wildtype (WT) gut that had been transected just distal to the ENCC wavefront (KO-wf: n = 6, WT: n = 7), and into distal KO gut transected at a site equivalent to that of the WT (KO-d: n = 6) on E12.5. ENCC differentiation was evaluated using whole-mount immunohistochemistry with Tuj-1 (neuronal marker) and GFAP (glial marker) antibodies.

RESULTS

The transplanted ENCCs migrated to form the myenteric and submucosal plexus in all groups. The ratio of the area of Tuj-1-positive cells/GFAP-positive cells in migrated cells in the recipient gut was found to be significantly lower in KO-d compared to KO-wf and WT, while there was no significant difference between KO-wf and WT groups. This suggests that neuronal/glial differentiation was decreased in KO-d compared to that in KO-wf and WT groups.

CONCLUSION

Our study highlights the differences in ENCC differentiation depending on the site of transplantation. To further develop cell therapy for HD, it is important to consider the impact of the recipient intestinal environment on transplanted ENCCs.

Historical Cohort Study of Congenital Isolated Hypoganglionosis of the Intestine: Determining the Best Surgical Interventions

No standard diagnostic method or surgical treatment for congenital isolated hypoganglionosis (CIHG) has been established. This study aimed to analyze the clinical outcomes of patients with CIHG and identify the best surgical interventions provided thus far. Data on surgical interventions in 19 patients were collected between 1992 and 2020, including the type of enterostomy, type of revision, and length of the intestines. Ganglion cells in the myenteric plexus were enumerated using Hu C/D staining. The ratio of the length of the small intestine to its height was defined as the intestinal ratio (IR). The outcomes were assessed using the stoma output, growth parameters including the body mass index (BMI), and parenteral nutrition (PN) dependency. All patients required a diverting enterostomy. The IR ranged from 0.51 to 1.75 after multiple non-transplant surgeries. The stoma types were tube-stoma, end-stoma, Santulli-type, and Bishop-Koop (BK)-type. Patients with Santulli- or BK-type stomas had better BMIs and less PN dependency in terms of volume than those with end-stomas or tube-stomas. Two patients with BK-type stomas were off PN, and three who underwent an intestinal transplantation (Itx) achieved enteral autonomy. The management of CIHG involves a precise diagnosis using Hu C/D staining, neonatal enterostomy, and stoma revision using the adjusted IR and Itx if other treatments do not enable enteral autonomy.

Enteric neurospheres retain the capacity to assemble neural networks with motile and metamorphic gliocytes and ganglia

BACKGROUND

Neurosphere medium (NSM) and self-renewal medium (SRM) were widely used to isolate enteric neural stem cells (ENSCs) in the form of neurospheres. ENSCs or their neurosphere forms were neurogenic and gliogenic, but the compelling evidence for their capacity of assembling enteric neural networks remained lacking, raising the question of their aptitude for rebuilding the enteric nervous system (ENS) in ENSC therapeutics. It prompted us to explore an effective culture protocol or strategy for assembling ENS networks, which might also be employed as an in vitro model to simplify the biological complexity of ENS embedded in gut walls.

METHODS

NSM and SRM were examined for their capacity to generate neurospheres in mass culture of dispersed murine fetal enterocytes at serially diluted doses and assemble enteric neural networks in two- and three-dimensional cell culture systems and ex vivo on gut explants. Time-lapse microphotography was employed to capture cell activities of assembled neural networks. Neurosphere transplantation was performed via rectal submucosal injection.

RESULTS

In mass culture of dispersed enterocytes, NSM generated discrete units of neurospheres, whereas SRM promoted neural network assembly with neurospheres akin to enteric ganglia. Both were highly affected by seeding cell doses. SRM had similar ENSC mitosis-driving capacity to NSM, but was superior in driving ENSC differentiation in company with heightened ENSC apoptosis. Enteric neurospheres were motile, capable of merging together. It argued against their clonal entities. When nurtured in SRM, enteric neurospheres proved competent to assemble neural networks on two-dimensional coverslips, in three-dimensional hydrogels and on gut explants. In the course of neural network assembly from enteric neurospheres, neurite extension was preceded by migratory expansion of gliocytes. Assembled neural networks contained motile ganglia and gliocytes that constantly underwent shapeshift. Neurospheres transplanted into rectal submucosa might reconstitute myenteric plexuses of recipients' rectum.

CONCLUSION

Enteric neurospheres mass-produced in NSM might assemble neural networks in SRM-immersed two- or three-dimensional environments and on gut explants, and reconstitute myenteric plexuses of the colon after rectal submucosal transplantation. Our results also shed first light on the dynamic entity of ENS and open the experimental avenues to explore cellular activities of ENS and facilitate ENS demystification.

Enteric glial cells aggravate the intestinal epithelial barrier damage by secreting S100β under high-altitude conditions

Damage to the intestinal epithelial barrier (IEB) has been reported under high-altitude (HA) conditions and may be responsible for HA-associated gastrointestinal (GI) disorders. However, this pathogenetic mechanism does not fully explain the GI stress symptoms, such as flatulence and motility diarrhea, which accompany the IEB damage under HA conditions, especially for the people exposed to HA acutely. In the present study, we collected the blood samples from the people who lived at HA and found the concentration of enteric glial cells (EGCs)-associated biomarkers increased significantly. HA mouse model was then established and the results revealed that EGCs were involved in IEB damage. Zona occludens (ZO)-1, occludin, and claudin-1 expression was negatively correlated with that of glial fibrillary acidic protein (GFAP) and S100β under HA conditions. In order to learn more about how EGCs influence IEB, the in vitro EGC and MODE-K hypoxia experiments that used hypoxic stimulation for simulating in vivo exposure to HA was performed. We found that hypoxia increased S100β secretion in EGCs. And MODE-K cells cultured in medium conditioned by hypoxic EGCs showed low ZO-1, occludin, and claudin-1 levels of expression. Furthermore, treatment of MODE-K cells with recombinant mouse S100β resulted in diminished levels of ZO-1, occludin, and claudin-1 expression. Thus, HA exposure induces greater S100β secretion by EGCs, which aggravates the damage to the IEB. This study has revealed a novel mechanism of IEB damage under HA conditions, and suggest that EGCs may constitute a fresh avenue for the avoidance of GI disorders at HA.

Dichotomous colorectal cancer behaviour

Colorectal cancer (CRC) is the third most common malignant tumor and one of the deadliest cancers. At molecular level, CRC is a heterogeneous disease that could be divided in four Consensus Molecular Subtypes. Given the differences in the disease due to its anatomical location (proximal and distal colon), another classification should be considered. Here, we review the current knowledge on CRC dichotomic´s behaviour based on two different entities; right and left-sided tumors, their impact on clinical trial data, microbiota spatial composition and the interaction with the nervous system. We discuss recent advances in understanding how the spatial tumor heterogeneity influences the tumor growth, progression, and responses to current therapies.

Mini-review: Enteric glial cell heterogeneity: Is it all about the niche?

Enteric glial cells represent the enteric population of peripheral glia. According to their 'glial' nature, their principal function is to support enteric neurons in both structural and functional ways. Mounting evidence however demonstrates that enteric glial cells crucially contribute to the majority of enteric nervous system functions, thus acting as pivotal players in the maintenance of gut homeostasis. Various types of enteric glia are present within the gut wall, creating an intricate interaction network with other gastrointestinal cell types. Their distribution throughout the different layers of the gut wall translates in characteristic phenotypes that are tailored to the local tissue requirements of the digestive tract. This heterogeneity is assumed to be mirrored by functional specialization, but the extensive plasticity and versatility of enteric glial cells complicates a one on one phenotype/function definition. Moreover, the relative contribution of niche-specific signals versus lineage determinants for driving enteric glial heterogeneity is still uncertain. In this review we focus on the current understanding of phenotypic and functional enteric glial cell heterogeneity, from a microenvironmental and developmental perspective.

Volume imaging to interrogate cancer cell-tumor microenvironment interactions in space and time

Volume imaging visualizes the three-dimensional (3D) complexity of tumors to unravel the dynamic crosstalk between cancer cells and the heterogeneous landscape of the tumor microenvironment (TME). Tissue clearing and intravital microscopy (IVM) constitute rapidly progressing technologies to study the architectural context of such interactions. Tissue clearing enables high-resolution imaging of large samples, allowing for the characterization of entire tumors and even organs and organisms with tumors. With IVM, the dynamic engagement between cancer cells and the TME can be visualized in 3D over time, allowing for acquisition of 4D data. Together, tissue clearing and IVM have been critical in the examination of cancer-TME interactions and have drastically advanced our knowledge in fundamental cancer research and clinical oncology. This review provides an overview of the current technical repertoire of fluorescence volume imaging technologies to study cancer and the TME, and discusses how their recent applications have been utilized to advance our fundamental understanding of tumor architecture, stromal and immune infiltration, vascularization and innervation, and to explore avenues for immunotherapy and optimized chemotherapy delivery.

Anatomical and functional maturation of the mid-gestation human enteric nervous system

Immature gastrointestinal motility impedes preterm infant survival. The enteric nervous system controls gastrointestinal motility, yet it is unknown when the human enteric nervous system matures enough to carry out vital functions. Here we demonstrate that the second trimester human fetal enteric nervous system takes on a striped organization akin to the embryonic mouse. Further, we perform ex vivo functional assays of human fetal tissue and find that human fetal gastrointestinal motility matures in a similar progression to embryonic mouse gastrointestinal motility. Together, this provides critical knowledge, which facilitates comparisons with common animal models to advance translational disease investigations and testing of pharmacological agents to enhance gastrointestinal motility in prematurity.

Up-Regulation of microRNA-424 Causes an Imbalance in AKT Phosphorylation and Impairs Enteric Neural Crest Cell Migration in Hirschsprung Disease

Insights into the role of microRNAs (miRNAs) in disease pathogenesis have made them attractive therapeutic targets, and numerous miRNAs have been functionally linked to Hirschsprung disease (HSCR), a life-threatening genetic disorder due to defective migration, proliferation, and colonization of enteric neural crest cells (ENCCs) in the gut. Recent studies have demonstrated that miR-424 strongly inhibits migration in a variety of cell types and its potential target RICTOR is essential for neural crest cell development. We therefore sought to interrogate how miR-424 and RICTOR contribute to the pathogenesis of HSCR. We utilized HSCR cases and human neural cells to evaluate the miR-424-mediated regulation of RICTOR and the downstream AKT phosphorylation. We further developed an ex vivo model to assess the effects of miR-424 on ENCC migration and proliferation. Then, single-cell atlases of gene expression in both human and mouse fetal intestines were used to determine the characteristics of RICTOR and AKT expression in the developing gut. Our findings demonstrate that miR-424 levels are markedly increased in the colonic tissues of patients with HSCR and that it regulates human neural cell migration by directly targeting RICTOR. Up-regulation of miR-424 leads to decreased AKT phosphorylation levels in a RICTOR-dependent manner, and this, in turn, impairs ENCC proliferation and migration in the developing gut. Interestingly, we further identified prominent RICTOR and AKT expressions in the enteric neurons and other types of enteric neural cells in human and mouse fetal intestines. Our present study reveals the role of the miR-424/RICTOR axis in HSCR pathogenesis and indicates that miR-424 is a promising candidate for the development of targeted therapies against HSCR.

The enteric nervous system

Of all the organ systems in the body, the gastrointestinal tract is the most complicated in terms of the numbers of structures involved, each with different functions, and the numbers and types of signaling molecules utilized. The digestion of food and absorption of nutrients, electrolytes, and water occurs in a hostile luminal environment that contains a large and diverse microbiota. At the core of regulatory control of the digestive and defensive functions of the gastrointestinal tract is the enteric nervous system (ENS), a complex system of neurons and glia in the gut wall. In this review, we discuss 1) the intrinsic neural control of gut functions involved in digestion and 2) how the ENS interacts with the immune system, gut microbiota, and epithelium to maintain mucosal defense and barrier function. We highlight developments that have revolutionized our understanding of the physiology and pathophysiology of enteric neural control. These include a new understanding of the molecular architecture of the ENS, the organization and function of enteric motor circuits, and the roles of enteric glia. We explore the transduction of luminal stimuli by enteroendocrine cells, the regulation of intestinal barrier function by enteric neurons and glia, local immune control by the ENS, and the role of the gut microbiota in regulating the structure and function of the ENS. Multifunctional enteric neurons work together with enteric glial cells, macrophages, interstitial cells, and enteroendocrine cells integrating an array of signals to initiate outputs that are precisely regulated in space and time to control digestion and intestinal homeostasis.

Hirschsprung Disease for the Practicing Surgical Pathologist

OBJECTIVES

Hirschsprung disease (HD) is a congenital condition defined by the absence of ganglion cells in the distal-most portion of the gastrointestinal tract. Biopsies and resections for HD can be adrenaline inducing for the general surgical pathologist because specimens are infrequent; HD is 1 of only a few neuroanatomic diseases that general surgical pathologists diagnose; numerous preanalytic factors (eg, biopsy adequacy, surgeon sampling protocol, processing artifacts) can affect histologic interpretation; and most importantly, the diagnosis has high stakes.

METHODS

We provide a comprehensive overview of the background, relevant clinical procedures, and pathologic assessment of HD. Grossing and frozen section protocols, an algorithmic approach to diagnosis, and histologic pearls and pitfalls are also discussed.

RESULTS

Evaluation and recognition of the features of HD have evolved significantly in the past 2 decades with the discovery of the value of calretinin immunohistochemistry in the late 2000s and the recent development of straightforward and reproducible histologic criteria for identification of the HD transition zone.

CONCLUSIONS

These advancements have substantially improved the pathologist's ability to reliably evaluate for HD. Nonetheless, as with any high-stakes surgical pathology specimen, clear communication with the clinical team is essential.

A combinatorial panel for flow cytometry-based isolation of enteric nervous system cells from human intestine

Efficient isolation of neurons and glia from the human enteric nervous system (ENS) is challenging because of their rare and fragile nature. Here, we describe a staining panel to enrich ENS cells from the human intestine by fluorescence-activated cell sorting (FACS). We find that CD56/CD90/CD24 co-expression labels ENS cells with higher specificity and resolution than previous methods. Surprisingly, neuronal (CD24, TUBB3) and glial (SOX10) selective markers appear co-expressed by all ENS cells. We demonstrate that this contradictory staining pattern is mainly driven by neuronal fragments, either free or attached to glial cells, which are the most abundant cell types. Live neurons can be enriched by the highest CD24 and CD90 levels. By applying our protocol to isolate ENS cells for single-cell RNA sequencing, we show that these cells can be obtained with high quality, enabling interrogation of the human ENS transcriptome. Taken together, we present a selective FACS protocol that allows enrichment and discrimination of human ENS cells, opening up new avenues to study this complex system in health and disease.

Label-free, fast, 2-photon volume imaging of the organization of neurons and glia in the enteric nervous system

The enteric nervous system (ENS), sometimes referred to as a "second brain" is a quasi-autonomous nervous system, made up of interconnected plexuses organized in a mesh-like network lining the gastrointestinal tract. Originally described as an actor in the regulation of digestion, bowel contraction, and intestinal secretion, the implications of the ENS in various central neuropathologies has recently been demonstrated. However, with a few exceptions, the morphology and pathologic alterations of the ENS have mostly been studied on thin sections of the intestinal wall or, alternatively, in dissected explants. Precious information on the three-dimensional (3-D) architecture and connectivity is hence lost. Here, we propose the fast, label-free 3-D imaging of the ENS, based on intrinsic signals. We used a custom, fast tissue-clearing protocol based on a high refractive-index aqueous solution to increase the imaging depth and allow us the detection of faint signals and we characterized the autofluorescence (AF) from the various cellular and sub-cellular components of the ENS. Validation by immunofluorescence and spectral recordings complete this groundwork. Then, we demonstrate the rapid acquisition of detailed 3-D image stacks from unlabeled mouse ileum and colon, across the whole intestinal wall and including both the myenteric and submucosal enteric nervous plexuses using a new spinning-disk two-photon (2P) microscope. The combination of fast clearing (less than 15 min for 73% transparency), AF detection and rapid volume imaging [less than 1 min for the acquisition of a z-stack of 100 planes (150*150 μm) at sub-300-nm spatial resolution] opens up the possibility for new applications in fundamental and clinical research.

Isolation, Expansion, and Endoscopic Delivery of Autologous Enteric Neuronal Stem Cells in Swine

The enteric nervous system (ENS) is an extensive network of neurons and glia within the wall of the gastrointestinal (GI) tract that regulates many essential GI functions. Consequently, disorders of the ENS due to developmental defects, inflammation, infection, or age-associated neurodegeneration lead to serious neurointestinal diseases. Despite the prevalence and severity of these diseases, effective treatments are lacking as they fail to directly address the underlying pathology. Neuronal stem cell therapy represents a promising approach to treating diseases of the ENS by replacing the absent or injured neurons, and an autologous source of stem cells would be optimal by obviating the need for immunosuppression. We utilized the swine model to address key questions concerning cell isolation, delivery, engraftment, and fate in a large animal relevant to human therapy. We successfully isolated neural stem cells from a segment of small intestine resected from 1-month-old swine. Enteric neuronal stem cells (ENSCs) were expanded as neurospheres that grew optimally in low-oxygen (5%) culture conditions. Enteric neuronal stem cells were labeled by lentiviral green fluorescent protein (GFP) transduction, then transplanted into the same swine from which they had been harvested. Endoscopic ultrasound was then utilized to deliver the ENSCs (10,000-30,000 neurospheres per animal) into the rectal wall. At 10 and 28 days following injection, autologously derived ENSCs were found to have engrafted within rectal wall, with neuroglial differentiation and no evidence of ectopic spreading. These findings strongly support the feasibility of autologous cell isolation and delivery using a clinically useful and minimally invasive technique, bringing us closer to first-in-human ENSC therapy for neurointestinal diseases.

How big is the little brain in the gut? Neuronal numbers in the enteric nervous system of mice, Guinea pig, and human

BACKGROUND

Despite numerous studies on the enteric nervous system (ENS), we lack fundamental knowledge on neuronal densities or total neuron numbers in different species. There are more anecdotal than actual figures on nerve counts.

METHODS

We used standardized preparation techniques and immunohistochemistry with validated panneuronal markers (human or mouse anti-HuD/C) to determine neuronal densities in specimen from the entire gastrointestinal tract of mice, guinea pig, and humans. In parallel, we measured the dimensions of the gastrointestinal regions in mouse and guinea pig. For humans, we had to rely on literature data.

KEY RESULTS

The average neuronal densities along the gastrointestinal tract were 35,011 ± 25,017 1/cm² for the myenteric and 16,685 ± 9098 1/cm² for the submucous plexus in mice, 24,315 ± 16,627 and 11,850 ± 6122 1/cm² for guinea pig myenteric and submucous plexus, respectively, and 21,698 ± 9492 and 16,367 ± 5655 1/cm² for human myenteric and submucous plexus, respectively. The total number of neurons in the ENS was 2.6 million for mice, 14.6 million for guinea pig, and 168 million for human.

CONCLUSIONS & INFERENCES

This study reports the first comprehensive nerve cell count in mice, guinea pig, and human ENS. Neuronal densities were comparable between the three species and the differences in the total numbers of enteric neurons are likely due to body size and intestinal length. The number of enteric neurons is comparable to the number of neurons in the spinal cord for all three species.

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.

A Novel Method for Identifying the Transition Zone in Long-Segment Hirschsprung Disease: Investigating the Muscle Unit to Ganglion Ratio

Hirschsprung disease (HSCR) is characterised by the absence of enteric ganglia along variable lengths of the distal bowel. Current gold standard treatment involves the surgical resection of the defective, aganglionic bowel. Clear and reliable distinction of the normoganglionated bowel from the transition zone is key for successful resection of the entire defective bowel, and the avoidance of subsequent postoperative complications. However, the intraoperative nature of the tissue analysis and the variability of patient samples, sample preparation, and operator objectivity, make reproducible identification of the transition zone difficult. Here, we have described a novel method for using muscle units as a distinctive landmark for quantifying the density of enteric ganglia in resection specimens from HSCR patients. We show that the muscle unit to ganglion ratio is greater in the transition zone when compared with the proximal, normoganglionated region for long-segment HSCR patients. Patients with short-segment HSCR were also investigated, however, the muscle unit to ganglion ratio was not significantly different in these patients. Immunohistochemical examination of individual ganglia showed that there were no differences in the proportions of either enteric neurons or glial cells through the different regions of the resected colon. In addition, we identified that the size of enteric ganglia was smaller for patients that went on to develop HSCR associated enterocolitis; although the density of ganglia, as determined by the muscle unit to ganglia ratio, was not different when compared with patients that had no further complications. This suggests that subtle changes in the enteric nervous system, even in the “normoganglionated” colon, could be involved in changes in immune function and subsequent bacterial dysbiosis.

Structure of the myenteric plexus in normal and diseased human ileum analyzed by X-ray virtual histology slices

BACKGROUND

The enteric nervous system (ENS) is situated along the entire gastrointestinal tract and is divided into myenteric and submucosal plexuses in the small and large intestines. The ENS consists of neurons, glial cells, and nerves assembled into ganglia, surrounded by telocytes, interstitial cells of Cajal, and connective tissue. Owing to the complex spatial organization of several interconnections with nerve fascicles, the ENS is difficult to examine in conventional histological sections of 3-5 μm.

AIM

To examine human ileum full-thickness biopsies using X-ray phase-contrast nanotomography without prior staining to visualize the ENS.

METHODS

Six patients were diagnosed with gastrointestinal dysmotility and neuropathy based on routine clinical and histopathological examinations. As controls, full-thickness biopsies were collected from healthy resection ileal regions after hemicolectomy for right colon malignancy. From the paraffin blocks, 4-µm thick sections were prepared and stained with hematoxylin and eosin for localization of the myenteric ganglia under a light microscope. A 1-mm punch biopsy (up to 1 cm in length) centered on the myenteric plexus was taken and placed into a Kapton^® tube for mounting in the subsequent investigation. X-ray phase-contrast tomography was performed using two custom-designed laboratory setups with micrometer resolution for overview scanning. Subsequently, selected regions of interest were scanned at a synchrotron-based end-station, and high-resolution slices were reported. In total, more than 6000 virtual slices were analyzed from nine samples.

RESULTS

In the overview scans, the general architecture and quality of the samples were studied, and the myenteric plexus was localized. High-resolution scans revealed details, including the ganglia, interganglional nerve fascicles, and surrounding tissue. The ganglia were irregular in shape and contained neurons and glial cells. Spindle-shaped cells with very thin cellular projections could be observed on the surface of the ganglia, which appeared to build a network. In the patients, there were no alterations in the general architecture of the myenteric ganglia. Nevertheless, several pathological changes were observed, including vacuolar degeneration, autophagic activity, the appearance of sequestosomes, chromatolysis, and apoptosis. Furthermore, possible expulsion of pyknotic neurons and defects in the covering cellular network could be observed in serial slices. These changes partly corresponded to previous light microscopy findings.

CONCLUSION

The analysis of serial virtual slices could provide new information that cannot be obtained by classical light microscopy. The advantages, disadvantages, and future possibilities of this method are also discussed.

A Novel Three-Dimensional Imaging System Based on Polysaccharide Staining for Accurate Histopathological Diagnosis of Inflammatory Bowel Diseases

BACKGROUND & AIMS

Tissue-clearing and three-dimensional (3D) imaging techniques aid clinical histopathological evaluation; however, further methodological developments are required before use in clinical practice.

METHODS

We sought to develop a novel fluorescence staining method based on the classical periodic acid-Schiff stain. We further attempted to develop a 3D imaging system based on this staining method and evaluated whether the system can be used for quantitative 3D pathological evaluation and deep learning-based automatic diagnosis of inflammatory bowel diseases.

RESULTS

We successfully developed a novel periodic acid-FAM hydrazide (PAFhy) staining method for 3D imaging when combined with a tissue-clearing technique (PAFhy-3D). This strategy enabled clear and detailed imaging of the 3D architectures of crypts in human colorectal mucosa. PAFhy-3D imaging also revealed abnormal architectural changes in crypts in ulcerative colitis tissues and identified the distributions of neutrophils in cryptitis and crypt abscesses. PAFhy-3D revealed novel pathological findings including spiral staircase-like crypts specific to inflammatory bowel diseases. Quantitative analysis of crypts based on 3D morphologic changes enabled differential diagnosis of ulcerative colitis, Crohn's disease, and non-inflammatory bowel disease; such discrimination could not be achieved by pathologists. Furthermore, a deep learning-based system using PAFhy-3D images was used to distinguish these diseases The accuracies were excellent (macro-average area under the curve = 0.94; F1 scores = 0.875 for ulcerative colitis, 0.717 for Crohn's disease, and 0.819 for non-inflammatory bowel disease).

CONCLUSIONS

PAFhy staining and PAFhy-3D imaging are promising approaches for next-generation experimental and clinical histopathology.

The human enteric nervous system. Historical and modern advances. Collaboration between science and surgery

BACKGROUND

There are considerable advantages and opportunities for surgeons and trainee surgeons in conducting a period of research allied with basic scientists. Such clinicians are well placed to define relevant clinical questions, provide human material (tissue, biopsy and blood) and translate the techniques derived in experimental animals to human subjects.

METHODS

This small review explores research conducted on the nervous system of the intestines, with an emphasis on the translation of findings from animal to human.

RESULTS

This work shows that new techniques of immunohistochemistry and retrograde tracing, developed in animal tissue, have greatly expanded our knowledge of the structure of the human enteric nervous system.

CONCLUSIONS

Such findings have sparked therapeutic trials for the treatment of gastrointestinal disorders in patients.

Whole gut imaging allows quantification of all enteric neurons in the adult zebrafish intestine

BACKGROUND

A fundamental understanding of the enteric nervous system in normal and diseased states is limited by the lack of standard measures of total enteric neuron number. The adult zebrafish is a useful model in this context as it is amenable to in toto imaging of the intestine. We leveraged this to develop a technique to image and quantify all enteric neurons within the adult zebrafish intestine and applied this method to assess the relationship between intestinal length and total enteric neuron number.

METHODS

Dissected adult zebrafish intestines were immunostained in wholemount, optically cleared with refractive index-matched solution, and then imaged in tiles using light-sheet microscopy. Imaging software was used to stitch the tiles, and the full image underwent automated cell counting. Total enteric neuron number was assessed in relation to intestinal length using linear regression modeling.

KEY RESULTS

Whole gut imaging of the adult zebrafish intestine permits the visualization of endogenous and immunohistochemistry-derived fluorescence throughout the intestine. While enteric neuron distribution is heterogeneous between intestinal segments, total enteric neuron number positively correlates with intestinal length.

CONCLUSIONS & INFERENCES

Imaging of all enteric neurons within the adult vertebrate intestine is possible in models such as the zebrafish. In this study, we apply this to demonstrate a positive correlation between enteric neuron number and intestinal length. Quantifying total enteric numbers will facilitate future studies of enteric neuropathies and ENS structure in animal models and potentially in biopsied tissue samples.

Development, Diversity, and Neurogenic Capacity of Enteric Glia

Enteric glia are a fascinating population of cells. Initially identified in the gut wall as the "support" cells of the enteric nervous system, studies over the past 20 years have unveiled a vast array of functions carried out by enteric glia. They mediate enteric nervous system signalling and play a vital role in the local regulation of gut functions. Enteric glial cells interact with other gastrointestinal cell types such as those of the epithelium and immune system to preserve homeostasis, and are perceptive to luminal content. Their functional versatility and phenotypic heterogeneity are mirrored by an extensive level of plasticity, illustrated by their reactivity in conditions associated with enteric nervous system dysfunction and disease. As one of the hallmarks of their plasticity and extending their operative relationship with enteric neurons, enteric glia also display neurogenic potential. In this review, we focus on the development of enteric glial cells, and the mechanisms behind their heterogeneity in the adult gut. In addition, we discuss what is currently known about the role of enteric glia as neural precursors in the enteric nervous system.

Identifying Types of Neurons in the Human Colonic Enteric Nervous System

Distinguishing and characterising the different classes of neurons that make up a neural circuit has been a long-term goal for many neuroscientists. The enteric nervous system is a large but moderately simple part of the nervous system. Enteric neurons in laboratory animals have been extensively characterised morphologically, electrophysiologically, by projections and immunohistochemically. However, studies of human enteric nervous system are less advanced despite the potential availability of tissue from elective surgery (with appropriate ethics permits). Recent studies using single cell sequencing have confirmed and extended the classification of enteric neurons in mice and human, but it is not clear whether an encompassing classification has been achieved. We present preliminary data on a means to distinguish classes of myenteric neurons in specimens of human colon combining immunohistochemical, morphological, projection and size data on single cells. A method to apply multiple layers of antisera to specimens was developed, allowing up to 12 markers to be characterised in individual neurons. Applied to multi-axonal Dogiel type II neurons, this approach demonstrated that they constitute fewer than 5% of myenteric neurons, are nearly all immunoreactive for choline acetyltransferase and tachykinins. Many express the calcium-binding proteins calbindin and calretinin and they are larger than average myenteric cells. This methodology provides a complementary approach to single-cell mRNA profiling to provide a comprehensive account of the types of myenteric neurons in the human colon.

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.

Persistent bowel dysfunction after surgery for Hirschsprung’s disease: A neuropathological perspective

Hirschsprung’s disease (HD) is a congenital disorder, characterized by aganglionosis in the distal part of the gastrointestinal tract. Despite complete surgical resection of the aganglionic segment, both constipation and fecal incontinence persist in a considerable number of patients with limited treatment options. There is growing evidence for structural abnormalities in the ganglionic bowel proximal to the aganglionosis in both humans and animals with HD, which may play a role in persistent bowel dysfunction. These abnormalities include: (1) Histopathological abnormalities of enteric neural cells; (2) Imbalanced expression of neurotransmitters and neuroproteins; (3) Abnormal expression of enteric pacemaker cells; (4) Abnormalities of smooth muscle cells; and (5) Abnormalities within the extracellular matrix. Hence, a better understanding of these previously unrecognized neuropathological abnormalities may improve follow-up and treatment in patients with HD suffering from persistent bowel dysfunction following surgical correction. In the long term, further combination of clinical and neuropathological data will hopefully enable a translational step towards more individual treatment for HD.

Enteric glia in homeostasis and disease: From fundamental biology to human pathology

Glia, the non-neuronal cells of the nervous system, were long considered secondary cells only necessary for supporting the functions of their more important neuronal neighbors. Work by many groups over the past two decades has completely overturned this notion, revealing the myriad and vital functions of glia in nervous system development, plasticity, and health. The largest population of glia outside the brain is in the enteric nervous system, a division of the autonomic nervous system that constitutes a key node of the gut-brain axis. Here, we review the latest in the understanding of these enteric glia in mammals with a focus on their putative roles in human health and disease.

Pain in Inflammatory Bowel Disease: Optogenetic Strategies for Study of Neural-Epithelial Signaling

Abdominal pain is common in patients with active inflammation of the colon but can persist even in its absence, suggesting other mechanisms of pain signaling. Recent findings suggest colon epithelial cells are direct regulators of pain-sensing neurons. Optogenetic activation of epithelial cells evoked nerve firing and pain-like behaviors. Inhibition of epithelial cells caused the opposite effect, reducing responses to colon distension and inflammatory hypersensitivity. Thus, epithelial cells alone can regulate the activation of pain circuits. Future goals are to define the anatomical and cellular mechanisms that underlie epithelial-neural pain signaling and how it is altered in response to colon inflammation.

Regulation of Neurotransmitters by the Gut Microbiota and Effects on Cognition in Neurological Disorders

Emerging evidence indicates that gut microbiota is important in the regulation of brain activity and cognitive functions. Microbes mediate communication among the metabolic, peripheral immune, and central nervous systems via the microbiota–gut–brain axis. However, it is not well understood how the gut microbiome and neurons in the brain mutually interact or how these interactions affect normal brain functioning and cognition. We summarize the mechanisms whereby the gut microbiota regulate the production, transportation, and functioning of neurotransmitters. We also discuss how microbiome dysbiosis affects cognitive function, especially in neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease.

Sarm1-mediated neurodegeneration within the enteric nervous system protects against local inflammation of the colon

Axonal degeneration is one of the key features of neurodegenerative disorders. In the canonical view, axonal degeneration destructs neural connections and promotes detrimental disease defects. Here, we assessed the enteric nervous system (ENS) of the mouse, non-human primate, and human by advanced 3D imaging. We observed the profound neurodegeneration of catecholaminergic axons in human colons with ulcerative colitis, and similarly, in mouse colons during acute dextran sulfate sodium-induced colitis. However, we unexpectedly revealed that blockage of such axonal degeneration by the Sarm1 deletion in mice exacerbated the colitis condition. In contrast, pharmacologic ablation or chemogenetic inhibition of catecholaminergic axons suppressed the colon inflammation. We further showed that the catecholaminergic neurotransmitter norepinephrine exerted a pro-inflammatory function by enhancing the expression of IL-17 cytokines. Together, this study demonstrated that Sarm1-mediated neurodegeneration within the ENS mitigated local inflammation of the colon, uncovering a previously-unrecognized beneficial role of axonal degeneration in this disease context.

Optical clearing reveals TNBS-induced morphological changes of VGLUT2-positive nerve fibers in mouse colorectum

Colorectal hypersensitivity and sensitization of both mechanosensitive and mechanically insensitive afferents develop after intracolonic instillation of 2,4,6-trinitrobenzenesulfonic acid (TNBS) in the mouse, a model of postinfectious irritable bowel syndrome. In mice in which ∼80% of extrinsic colorectal afferents were labeled genetically using the promotor for vesicular glutamate transporter type 2 (VGLUT2), we systematically quantified the morphology of VGLUT2-positive axons in mouse colorectum 7-28 days following intracolonic TNBS treatment. After removal, the colorectum was distended (20 mmHg), fixed with paraformaldehyde, and optically cleared to image VGLUT2-positive axons throughout the colorectal wall thickness. We conducted vector path tracing of individual axons to allow systematic quantification of nerve fiber density and shape. Abundant VGLUT2-positive nerve fibers were present in most layers of the colorectum, except the serosal and longitudinal muscular layers. A small percentage of VGLUT2-positive myenteric plexus neurons was also detected. Intracolonic TNBS treatment significantly reduced the number of VGLUT2-positive nerve fibers in submucosal, myenteric plexus, and mucosal layers at day 7 post-TNBS, which mostly recovered by day 28. We also found that almost all fibers in the submucosa were meandering and curvy, with ∼10% showing pronounced curviness (quantified by the linearity index). TNBS treatment resulted in a significant reduction of the proportions of pronounced curvy fibers in the rectal region at 28 days post-TNBS. Altogether, the present morphological study reveals profound changes in the distribution of VGLUT2-positive fibers in mouse colorectum undergoing TNBS-induced colitis and draws attention to curvy fibers in the submucosa with potential roles in visceral nociception.NEW & NOTEWORTHY We conducted genetic labeling and optical clearing to visualize extrinsic sensory nerve fibers in whole-mount colorectum, which revealed widespread presence of axons in the submucosal layer. Remarkably, axons in the submucosa were meandering and curvy, in contrast to axons in other layers generally aligned with the basal tissues. Intracolonic TNBS treatment led to pronounced changes of nerve fiber density and curviness, suggesting nerve fiber morphologies as potentially contributing factors to sensory sensitization.

scRNA-Seq Reveals New Enteric Nervous System Roles for GDNF, NRTN, and TBX3

BACKGROUND AND AIMS

Bowel function requires coordinated activity of diverse enteric neuron subtypes. Our aim was to define gene expression in these neuron subtypes to facilitate development of novel therapeutic approaches to treat devastating enteric neuropathies, and to learn more about enteric nervous system function.

METHODS

To identify subtype-specific genes, we performed single-nucleus RNA-seq on adult mouse and human colon myenteric plexus, and single-cell RNA-seq on E17.5 mouse ENS cells from whole bowel. We used immunohistochemistry, select mutant mice, and calcium imaging to validate and extend results.

RESULTS

RNA-seq on 635 adult mouse colon myenteric neurons and 707 E17.5 neurons from whole bowel defined seven adult neuron subtypes, eight E17.5 neuron subtypes and hundreds of differentially expressed genes. Manually dissected human colon myenteric plexus yielded RNA-seq data from 48 neurons, 3798 glia, 5568 smooth muscle, 377 interstitial cells of Cajal, and 2153 macrophages. Immunohistochemistry demonstrated differential expression for BNC2, PBX3, SATB1, RBFOX1, TBX2, and TBX3 in enteric neuron subtypes. Conditional Tbx3 loss reduced NOS1-expressing myenteric neurons. Differential Gfra1 and Gfra2 expression coupled with calcium imaging revealed that GDNF and neurturin acutely and differentially regulate activity of ∼50% of myenteric neurons with distinct effects on smooth muscle contractions.

CONCLUSION

Single cell analyses defined genes differentially expressed in myenteric neuron subtypes and new roles for TBX3, GDNF and NRTN. These data facilitate molecular diagnostic studies and novel therapeutics for bowel motility disorders.

Pancreas Optical Clearing and 3-D Microscopy in Health and Diabetes

Although first described over a hundred years ago, tissue optical clearing is undergoing renewed interest due to numerous advances in optical clearing methods, microscopy systems, and three-dimensional (3-D) image analysis programs. These advances are advantageous for intact mouse tissues or pieces of human tissues because samples sized several millimeters can be studied. Optical clearing methods are particularly useful for studies of the neuroanatomy of the central and peripheral nervous systems and tissue vasculature or lymphatic system. Using examples from solvent- and aqueous-based optical clearing methods, the mouse and human pancreatic structures and networks will be reviewed in 3-D for neuro-insular complexes, parasympathetic ganglia, and adipocyte infiltration as well as lymphatics in diabetes. Optical clearing with multiplex immunofluorescence microscopy provides new opportunities to examine the role of the nervous and circulatory systems in pancreatic and islet functions by defining their neurovascular anatomy in health and diabetes.

3D analysis of the myenteric plexus of the human bowel by X-ray phase-contrast tomography - a future method?

OBJECTIVES

Light microscopical analysis in two dimensions, combined with immunohistochemistry, is presently the gold standard to describe the enteric nervous system (ENS). Our aim was to assess the usefulness of three-dimensional (3D) imaging by X-ray phase-contrast tomography in evaluating the ENS of the human bowel.

MATERIAL AND METHODS

Myenteric ganglia were identified in full-thickness biopsies of the ileum and colon by hematoxylin & eosin staining. A1-mm biopsy punch was taken from the paraffin blocks and placed into a Kapton^® tube for subsequent tomographic investigation. The samples were scanned, without further preparation, using phase-contrast tomography at two different scales: overview scans (performed with laboratory setups), which allowed localization of the nervous tissue (∼1µm effective voxel size); and high-resolution scans (performed with a synchrotron endstation), which imaged localized regions of 320x320x320 µm³ (176 nm effective voxel size).

RESULTS

The contrast allowed us to follow the shape and the size changes of the ganglia, as well as to study their cellular components together with the cells and cellular projections of the periganglional space. Furthermore, it was possible to show the 3D network of the myenteric plexus and to quantify its volume within the samples.

CONCLUSIONS

Phase-contrast X-ray tomography can be applied for volume analyses of the human ENS and to study tissue components in unstained paraffin-embedded tissue biopsies. This technique could potentially be used to study disease mechanisms, and to compare healthy and diseased tissues in clinical research.

Mycotoxins and the Enteric Nervous System

Mycotoxins are secondary metabolites produced by various fungal species. They are commonly found in a wide range of agricultural products. Mycotoxins contained in food enter living organisms and may have harmful effects on many internal organs and systems. The gastrointestinal tract, which first comes into contact with mycotoxins present in food, is particularly vulnerable to the harmful effects of these toxins. One of the lesser-known aspects of the impact of mycotoxins on the gastrointestinal tract is the influence of these substances on gastrointestinal innervation. Therefore, the present study is the first review of current knowledge concerning the influence of mycotoxins on the enteric nervous system, which plays an important role, not only in almost all regulatory processes within the gastrointestinal tract, but also in adaptive and protective reactions in response to pathological and toxic factors in food.

Clinically Actionable Strategies for Studying Neural Influences in Cancer

Neuro-glial activation is a recently identified hallmark of growing cancers. Targeting tumor hyperinnervation in preclinical and small clinical trials has yielded promising antitumor effects, highlighting the need of systematic analysis of neural influences in cancer (NIC). Here, we outline the strategies translating these findings from bench to the clinic.