Inhibition of cell death results in hyperganglionosis: implications for enteric nervous system development

Identification of signaling pathways that specify a subset of migrating enteric neural crest cells at the wavefront in mouse embryos

During enteric nervous system (ENS) development, pioneering wavefront enteric neural crest cells (ENCCs) initiate gut colonization. However, the molecular mechanisms guiding their specification and niche interaction are not fully understood. We used single-cell RNA sequencing and spatial transcriptomics to map the spatiotemporal dynamics and molecular landscape of wavefront ENCCs in mouse embryos. Our analysis shows a progressive decline in wavefront ENCC potency during migration and identifies transcription factors governing their specification and differentiation. We further delineate key signaling pathways (ephrin-Eph, Wnt-Frizzled, and Sema3a-Nrp1) utilized by wavefront ENCCs to interact with their surrounding cells. Disruptions in these pathways are observed in human Hirschsprung's disease gut tissue, linking them to ENS malformations. Additionally, we observed region-specific and cell-type-specific transcriptional changes in surrounding gut tissues upon wavefront ENCC arrival, suggesting their role in shaping the gut microenvironment. This work offers a roadmap of ENS development, with implications for understanding ENS disorders.

GFRA4 improves the neurogenic potential of enteric neural crest stem cells via hedgehog pathway

BACKGROUND

Hirschsprung disease (HSCR) is a congenital intestinal disease characterised by functional obstruction of the colon. Herein, we investigated the role and mechanism of the gene GFRA4 in HSCR.

METHODS

GFRA4 expression in the ganglionic and aganglionic segment tissues in patients with HSCR and healthy colon tissues were detected using qRT-PCR, western blot, and immunohistochemistry. Cell proliferation, cycle distribution, apoptosis, changes in mitochondrial membrane potential, and differentiation were assessed in mouse enteric neural crest stem cells (ENCSCs) using the CCK-8 assay, EdU staining, flow cytometry, JC-1 probe, and immunofluorescence, respectively. GSEA analysis was performed to screen the signaling pathways regulated by GFRA4.

RESULTS

GFRA4 was downregulated in aganglionic segment tissues compared to control and ganglionic segment tissues. GFRA4 overexpression promoted proliferation and differentiation, and inhibited apoptosis in ENCSCs, while GFRA4 down-regulation had the opposite result. GFRA4 activated the hedgehog pathway. GFRA4 overexpression enhanced the expression of key factors of the hedgehog pathway, including SMO, SHH, and GLI1. However, GFRA4 down-regulation reduced their expression. An antagonist of hedgehog pathway, cyclopamine, attenuated the effect of GFRA4 overexpression on proliferation, differentiation, and apoptosis of ENCSCs.

CONCLUSION

GFRA4 promotes proliferation and differentiation but inhibits apoptosis of ENCSCs via the hedgehog pathway in HSCR.

IMPACT

This study confirms that GFRA4 improves the proliferation and differentiation of ENCSCs via modulation of the hedgehog pathway. This study for the first time revealed the role and the mechanism of the action of GFRA4 in HSCR, which indicates that GFRA4 may play a role in the pathological development of HSCR. Our findings may lay the foundation for further investigation of the mechanisms underlying HSCR development and into targets of HSCR treatment.

Enteric Nervous System Striped Patterning and Disease: Unexplored Pathophysiology

The enteric nervous system (ENS) controls gastrointestinal (GI) motility, and defects in ENS development underlie pediatric GI motility disorders. In disorders such as Hirschsprung's disease (HSCR), pediatric intestinal pseudo-obstruction (PIPO), and intestinal neuronal dysplasia type B (INDB), ENS structure is altered with noted decreased neuronal density in HSCR and reports of increased neuronal density in PIPO and INDB. The developmental origin of these structural deficits is not fully understood. Here, we review the current understanding of ENS development and pediatric GI motility disorders incorporating new data on ENS structure. In particular, emerging evidence demonstrates that enteric neurons are patterned into circumferential stripes along the longitudinal axis of the intestine during mouse and human development. This novel understanding of ENS structure proposes new questions about the pathophysiology of pediatric GI motility disorders. If the ENS is organized into stripes, could the observed changes in enteric neuron density in HSCR, PIPO, and INDB represent differences in the distribution of enteric neuronal stripes? We review mechanisms of striped patterning from other biological systems and propose how defects in striped ENS patterning could explain structural deficits observed in pediatric GI motility disorders.

Regional cytoarchitecture of the adult and developing mouse enteric nervous system

The organization and cellular composition of tissues are key determinants of their biological function. In the mammalian gastrointestinal (GI) tract, the enteric nervous system (ENS) intercalates between muscular and epithelial layers of the gut wall and can control GI function independent of central nervous system (CNS) input.1 As in the CNS, distinct regions of the GI tract are highly specialized and support diverse functions, yet the regional and spatial organization of the ENS remains poorly characterized.2 Cellular arrangements,3,4 circuit connectivity patterns,5,6 and diverse cell types7-9 are known to underpin ENS functional complexity and GI function, but enteric neurons are most typically described only as a uniform meshwork of interconnected ganglia. Here, we present a bird's eye view of the mouse ENS, describing its previously underappreciated cytoarchitecture and regional variation. We visually and computationally demonstrate that enteric neurons are organized in circumferential neuronal stripes. This organization emerges gradually during the perinatal period, with neuronal stripe formation in the small intestine (SI) preceding that in the colon. The width of neuronal stripes varies throughout the length of the GI tract, and distinct neuronal subtypes differentially populate specific regions of the GI tract, with stark contrasts between SI and colon as well as within subregions of each. This characterization provides a blueprint for future understanding of region-specific GI function and identifying ENS structural correlates of diverse GI disorders.

Cell death in development, maintenance, and diseases of the nervous system

Cell death, be it of neurons or glial cells, marks the development of the nervous system. Albeit relatively less so than in tissues such as the gut, cell death is also a feature of nervous system homeostasis-especially in context of adult neurogenesis. Finally, cell death is commonplace in acute brain injuries, chronic neurodegenerative diseases, and in some central nervous system tumors such as glioblastoma. Recent studies are enumerating the various molecular modalities involved in the execution of cells. Intimately linked with cell death are mechanisms of disposal that remove the dead cell and bring about a tissue-level response. Heretofore, the association between these methods of dying and physiological or pathological responses has remained nebulous. It is envisioned that careful cartography of death and disposal may reveal novel understandings of disease states and chart new therapeutic strategies in the near future.

Influence of bacterial components on the developmental programming of enteric neurons

BACKGROUND

Intestinal bacteria have been increasingly shown to be involved in early postnatal development. Previous work has shown that intestinal bacteria are necessary for the structural development and intrinsic function of the enteric nervous system in early postnatal life. Furthermore, colonization with a limited number of bacteria appears to be sufficient for the formation of a normal enteric nervous system. We tested the hypothesis that common bacterial components could influence the programming of developing enteric neurons.

METHODS

The developmental programming of enteric neurons was studied by isolating enteric neural crest-derived cells from the fetal gut of C57Bl/6 mice at embryonic day 15.5. After the establishment of the cell line, cultured enteric neuronal precursors were exposed to increasing concentrations of a panel of bacterial components including lipopolysaccharide, flagellin, and components of peptidoglycan.

KEY RESULT

Exposure to bacterial components consistently affected proportions of enteric neuronal precursors that developed into nitrergic neurons. Furthermore, flagellin and D-gamma-Glu-mDAP were found to promote the development of serotonergic neurons. Proportions of dopaminergic neurons remained unchanged. Proliferation of neuronal precursor cells was significantly increased upon exposure to lipopolysaccharide and flagellin, while no significant changes were observed in the proportion of apoptotic neuronal precursors compared to baseline with exposure to any bacterial component.

CONCLUSIONS AND INTERFACES

These findings suggest that bacterial components may influence the development of enteric neurons.

Dnmt3b knock-down in enteric precursors reveals a possible mechanism by which this de novo methyltransferase is involved in the enteric nervous system development and the onset of Hirschsprung disease

Hirschsprung disease (HSCR, OMIM 142623) is a pathology that shows a lack of enteric ganglia along of the distal gastrointestinal tract. This aganglionosis is attributed to an abnormal proliferation, migration, differentiation and/or survival of enteric precursor cells (EPCs) derived from neural crest cells (NCCs) during the enteric nervous system (ENS) embryogenesis. DNMT3b de novo methyltransferase is associated with NCCs development and has been shown to be implicated in ENS formation as well as in HSCR. In this study we have aimed to elucidate the specific mechanism underlying the DNMT3b role in such processes. We have performed the knockdown of Dnmt3b expression (Dnmt3b-KD) in enteric precursor cells (EPCs) to clarify its role on these cells in vitro. Moreover, we have analyzed several signaling pathways to determine the mechanisms responsible for the effect caused by Dnmt3b-KD in EPCs. Our results seem to support that Dnmt3b-KD promotes an increase EPCs proliferation that may be mediated by P53 and P21 activity, since both proteins were observed to be down-regulated in our Dnmt3b-KD cultures. Moreover, we observed a down-regulation of P53 and P21 in HSCR patients. These results lead us to propose that DNMT3b could be involved in HSCR through P53 and P21 activity.

Enteric nervous system development: A crest cell's journey from neural tube to colon

The enteric nervous system (ENS) is comprised of a network of neurons and glial cells that are responsible for coordinating many aspects of gastrointestinal (GI) function. These cells arise from the neural crest, migrate to the gut, and then continue their journey to colonize the entire length of the GI tract. Our understanding of the molecular and cellular events that regulate these processes has advanced significantly over the past several decades, in large part facilitated by the use of rodents, avians, and zebrafish as model systems to dissect the signals and pathways involved. These studies have highlighted the highly dynamic nature of ENS development and the importance of carefully balancing migration, proliferation, and differentiation of enteric neural crest-derived cells (ENCCs). Proliferation, in particular, is critically important as it drives cell density and speed of migration, both of which are important for ensuring complete colonization of the gut. However, proliferation must be tempered by differentiation among cells that have reached their final destination and are ready to send axonal extensions, connect to effector cells, and begin to produce neurotransmitters or other signals. Abnormalities in the normal processes guiding ENCC development can lead to failure of ENS formation, as occurs in Hirschsprung disease, in which the distal intestine remains aganglionic. This review summarizes our current understanding of the factors involved in early development of the ENS and discusses areas in need of further investigation.

Combination of exogenous cell transplantation and 5-HT4 receptor agonism induce endogenous enteric neural crest-derived cells in a rat hypoganglionosis model

Enteric neural crest-derived cells (ENCCs) can migrate into endogenous ganglia and differentiate into progeny cells, and have even partially rescued bowel function; however, poor reliability and limited functional recovery after ENCC transplantation have yet to be addressed. Here, we investigated the induction of endogenous ENCCs by combining exogenous ENCC transplantation with a 5-HT₄ receptor agonist mosapride in a rat model of hypoganglionosis, established by benzalkonium chloride treatment. ENCCs, isolated from the gut of newborn rats, were labeled with a lentiviral eGFP reporter. ENCCs and rats were treated with the 5-HT₄ receptor agonist/antagonist. The labeled ENCCs were then transplanted into the muscular layer of benzalkonium chloride-treated colons. At given days post-intervention, colonic tissue samples were removed for histological analysis. ENCCs and neurons were detected by eGFP expression and immunoreactivity to p75^NTR and peripherin, respectively. eGFP-positive ENCCs and neurons could survive and maintain levels of fluorescence after transplantation. With longer times post-intervention, the number of peripherin-positive cells gradually increased in all groups. Significantly more peripherin-positive cells were found following ENCCs plus mosapride treatment, compared with the other groups. These results show that exogenous ENCCs combined with the 5-HT₄ receptor agonist effectively induced endogenous ENCCs proliferation and differentiation in a rat hypoganglionosis model.

Geminin prevents DNA damage in vagal neural crest cells to ensure normal enteric neurogenesis

Background

In vertebrate organisms, the neural crest (NC) gives rise to multipotential and highly migratory progenitors which are distributed throughout the embryo and generate, among other structures, the peripheral nervous system, including the intrinsic neuroglial networks of the gut, i.e. the enteric nervous system (ENS). The majority of enteric neurons and glia originate from vagal NC-derived progenitors which invade the foregut mesenchyme and migrate rostro-caudally to colonise the entire length of the gut. Although the migratory behaviour of NC cells has been studied extensively, it remains unclear how their properties and response to microenvironment change as they navigate through complex cellular terrains to reach their target embryonic sites.

Results

Using conditional gene inactivation in mice we demonstrate here that the cell cycle-dependent protein Geminin (Gem) is critical for the survival of ENS progenitors in a stage-dependent manner. Gem deletion in early ENS progenitors (prior to foregut invasion) resulted in cell-autonomous activation of DNA damage response and p53-dependent apoptosis, leading to severe intestinal aganglionosis. In contrast, ablation of Gem shortly after ENS progenitors had invaded the embryonic gut did not result in discernible survival or migratory deficits. In contrast to other developmental systems, we obtained no evidence for a role of Gem in commitment or differentiation of ENS lineages. The stage-dependent resistance of ENS progenitors to mutation-induced genotoxic stress was further supported by the enhanced survival of post gut invasion ENS lineages to γ-irradiation relative to their predecessors.

Conclusions

Our experiments demonstrate that, in mammals, NC-derived ENS lineages are sensitive to genotoxic stress in a stage-specific manner. Following gut invasion, ENS progenitors are distinctly resistant to Gem ablation and irradiation in comparison to their pre-enteric counterparts. These studies suggest that the microenvironment of the embryonic gut protects ENS progenitors and their progeny from genotoxic stress.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0314-x) contains supplementary material, which is available to authorized users.

The Impact of Prenatal Exposure to Dexamethasone on Gastrointestinal Function in Rats

Antenatal treatment with synthetic glucocorticoids is commonly used in pregnant women at risk of preterm delivery to accelerate tissue maturation. Exposure to glucocorticoids during development has been hypothesized to underlie different functional gastrointestinal (GI) and motility disorders. Herein, we investigated the impact of in utero exposure to synthetic glucocorticoids (iuGC) on GI function of adult rats. Wistar male rats, born from pregnant dams treated with dexamethasone (DEX), were studied at different ages. Length, histologic analysis, proliferation and apoptosis assays, GI transit, permeability and serotonin (5-HT) content of GI tract were measured. iuGC treatment decreased small intestine size and decreased gut transit. However, iuGC had no impact on intestinal permeability. iuGC differentially impacts the structure and function of the GI tract, which leads to long-lasting alterations in the small intestine that may predispose subjects prone to disorders of the GI tract.

Mouse models of Hirschsprung disease and other developmental disorders of the enteric nervous system: Old and new players

Hirschsprung disease (HSCR, intestinal aganglionosis) is a multigenic disorder with variable penetrance and severity that has a general population incidence of 1/5000 live births. Studies using animal models have contributed to our understanding of the developmental origins of HSCR and the genetic complexity of this disease. This review summarizes recent progress in understanding control of enteric nervous system (ENS) development through analyses in mouse models. An overview of signaling pathways that have long been known to control the migration, proliferation and differentiation of enteric neural progenitors into and along the developing gut is provided as a framework for the latest information on factors that influence enteric ganglia formation and maintenance. Newly identified genes and additional factors beyond discrete genes that contribute to ENS pathology including regulatory sequences, miRNAs and environmental factors are also introduced. Finally, because HSCR has become a paradigm for complex oligogenic diseases with non-Mendelian inheritance, the importance of gene interactions, modifier genes, and initial studies on genetic background effects are outlined.

Development of myenteric plexus in human foetuses: a quantitative study

Maturation of neurons of the myenteric plexus (MP) of human fetal sigmoid colon was studied at various weeks of gestation (WG). There is abundant literature on the development of MP in various segments of the gut but there are fewer reports on the development of MP in human sigmoid colon which is a site of various disorders. Sigmoid colonic segments from 12 aborted foetuses aged 14-23WG were processed for NADPH histochemistry. Stereologic evaluation of the neuronal cell profiles, numerical density, number of neurons per ganglion and myenteric fraction was conducted using using imageJ software. According to gestational age, foetuses were assigned into two groups (group 1 [n=7], less than <17WG and group 2 [n=5], more than >17WG). The overall size of neuronal cell profiles in the MP was significantly increased (P<0.05). The numerical density of neurons decreased in group 2 in comparison to group 1, the number of neurons per ganglion and myenteric fraction were increased in group 2 but all these were not statistically significant. This study revealed that the maturational event increases after 17WG and extensive innervations is established at 23WG. During prenatal life there is an increase in the neuronal cell size from 14-23WG signifying maturational process. Such studies are essential for clinicians and surgeons to correlate the normal and pathologic development of the enteric nervous system.

Why are enteric ganglia so small? Role of differential adhesion of enteric neurons and enteric neural crest cells

The avian enteric nervous system (ENS) consists of a vast number of unusually small ganglia compared to other peripheral ganglia. Each ENS ganglion at mid-gestation has a core of neurons and a shell of mesenchymal precursor/glia-like enteric neural crest (ENC) cells. To study ENS cell ganglionation we isolated midgut ENS cells by HNK-1 fluorescence-activated cell sorting (FACS) from E5 and E8 quail embryos, and from E9 chick embryos. We performed cell-cell aggregation assays which revealed a developmentally regulated functional increase in ENS cell adhesive function, requiring both Ca ²⁺ -dependent and independent adhesion. This was consistent with N-cadherin and NCAM labelling. Neurons sorted to the core of aggregates, surrounded by outer ENC cells, showing that neurons had higher adhesion than ENC cells. The outer surface of aggregates became relatively non-adhesive, correlating with low levels of NCAM and N-cadherin on this surface of the outer non-neuronal ENC cells. Aggregation assays showed that ENS cells FACS selected for NCAM-high and enriched for enteric neurons formed larger and more coherent aggregates than unsorted ENS cells. In contrast, ENS cells of the NCAM-low FACS fraction formed small, disorganised aggregates.  This suggests a novel mechanism for control of ENS ganglion morphogenesis where i) differential adhesion of ENS neurons and ENC cells controls the core/shell ganglionic structure and ii) the ratio of neurons to ENC cells dictates the equilibrium ganglion size by generation of an outer non-adhesive surface.

Age-related gene expression analysis in enteric ganglia of human colon after laser microdissection

The enteric nervous system (ENS) poses the intrinsic innervation of the gastrointestinal tract and plays a critical role for all stages of postnatal life. There is increasing scientific and clinical interest in acquired or age-related gastrointestinal dysfunctions that can be manifested in diseases such as gut constipation or fecal incontinence. In this study, we sought to analyze age-dependent changes in the gene expression profile of the human ENS, particularly in the myenteric plexus. Therefore, we used the laser microdissection technique which has been proven as a feasible tool to analyze distinct cell populations within heterogeneously composed tissues. Full biopsy gut samples were prepared from children (4-12 months), middle aged (48-58 years) and aged donors (70-95 years). Cryosections were histologically stained with H&E, the ganglia of the myenteric plexus identified and RNA isolated using laser microdissection technique. Quantitative PCR was performed for selected neural genes, neurotransmitters and receptors. Data were confirmed on protein level using NADPH-diaphorase staining and immunohistochemistry. As result, we demonstrate age-associated alterations in site-specific gene expression pattern of the ENS. Thus, in the adult and aged distal parts of the colon a marked decrease in relative gene expression of neural key genes like NGFR, RET, NOS1 and a concurrent increase of CHAT were observed. Further, we detected notable regional differences of RET, CHAT, TH, and S100B comparing gene expression in aged proximal and distal colon. Interestingly, markers indicating cellular senescence or oxidative stress (SNCA, CASP3, CAT, SOD2, and TERT) were largely unchanged within the ENS. For the first time, our study also describes the age-dependent expression pattern of all major sodium channels within the ENS. Our results are in line with previous studies showing spatio-temporal differences within the mammalian ENS.

Enteric neural progenitors are more efficient than brain-derived progenitors at generating neurons in the colon

Gut motility disorders can result from an absent, damaged, or dysfunctional enteric nervous system (ENS). Cell therapy is an exciting prospect to treat these enteric neuropathies and restore gut motility. Previous studies have examined a variety of sources of stem/progenitor cells, but the ability of different sources of cells to generate enteric neurons has not been directly compared. It is important to identify the source of stem/progenitor cells that is best at colonizing the bowel and generating neurons following transplantation. The aim of this study was to compare the ability of central nervous system (CNS) progenitors and ENS progenitors to colonize the colon and differentiate into neurons. Genetically labeled CNS- and ENS-derived progenitors were cocultured with aneural explants of embryonic mouse colon for 1 or 2.5 wk to assess their migratory, proliferative, and differentiation capacities, and survival, in the embryonic gut environment. Both progenitor cell populations were transplanted in the postnatal colon of mice in vivo for 4 wk before they were analyzed for migration and differentiation using immunohistochemistry. ENS-derived progenitors migrated further than CNS-derived cells in both embryonic and postnatal gut environments. ENS-derived progenitors also gave rise to more neurons than their CNS-derived counterparts. Furthermore, neurons derived from ENS progenitors clustered together in ganglia, whereas CNS-derived neurons were mostly solitary. We conclude that, within the gut environment, ENS-derived progenitors show superior migration, proliferation, and neuronal differentiation compared with CNS progenitors.

In vivo transplantation of neurosphere-like bodies derived from the human postnatal and adult enteric nervous system: a pilot study

Recent advances in the in vitro characterization of human adult enteric neural progenitor cells have opened new possibilities for cell-based therapies in gastrointestinal motility disorders. However, whether these cells are able to integrate within an in vivo gut environment is still unclear. In this study, we transplanted neural progenitor-containing neurosphere-like bodies (NLBs) in a mouse model of hypoganglionosis and analyzed cellular integration of NLB-derived cell types and functional improvement. NLBs were propagated from postnatal and adult human gut tissues. Cells were characterized by immunohistochemistry, quantitative PCR and subtelomere fluorescence in situ hybridization (FISH). For in vivo evaluation, the plexus of murine colon was damaged by the application of cationic surfactant benzalkonium chloride which was followed by the transplantation of NLBs in a fibrin matrix. After 4 weeks, grafted human cells were visualized by combined in situ hybridization (Alu) and immunohistochemistry (PGP9.5, GFAP, SMA). In addition, we determined nitric oxide synthase (NOS)-positive neurons and measured hypertrophic effects in the ENS and musculature. Contractility of treated guts was assessed in organ bath after electrical field stimulation. NLBs could be reproducibly generated without any signs of chromosomal alterations using subtelomere FISH. NLB-derived cells integrated within the host tissue and showed expected differentiated phenotypes i.e. enteric neurons, glia and smooth muscle-like cells following in vivo transplantation. Our data suggest biological effects of the transplanted NLB cells on tissue contractility, although robust statistical results could not be obtained due to the small sample size. Further, it is unclear, which of the NLB cell types including neural progenitors have direct restoring effects or, alternatively may act via 'bystander' mechanisms in vivo. Our findings provide further evidence that NLB transplantation can be considered as feasible tool to improve ENS function in a variety of gastrointestinal disorders.

Enteric nervous system development: migration, differentiation, and disease

The enteric nervous system (ENS) provides the intrinsic innervation of the bowel and is the most neurochemically diverse branch of the peripheral nervous system, consisting of two layers of ganglia and fibers encircling the gastrointestinal tract. The ENS is vital for life and is capable of autonomous regulation of motility and secretion. Developmental studies in model organisms and genetic studies of the most common congenital disease of the ENS, Hirschsprung disease, have provided a detailed understanding of ENS development. The ENS originates in the neural crest, mostly from the vagal levels of the neuraxis, which invades, proliferates, and migrates within the intestinal wall until the entire bowel is colonized with enteric neural crest-derived cells (ENCDCs). After initial migration, the ENS develops further by responding to guidance factors and morphogens that pattern the bowel concentrically, differentiating into glia and neuronal subtypes and wiring together to form a functional nervous system. Molecules controlling this process, including glial cell line-derived neurotrophic factor and its receptor RET, endothelin (ET)-3 and its receptor endothelin receptor type B, and transcription factors such as SOX10 and PHOX2B, are required for ENS development in humans. Important areas of active investigation include mechanisms that guide ENCDC migration, the role and signals downstream of endothelin receptor type B, and control of differentiation, neurochemical coding, and axonal targeting. Recent work also focuses on disease treatment by exploring the natural role of ENS stem cells and investigating potential therapeutic uses. Disease prevention may also be possible by modifying the fetal microenvironment to reduce the penetrance of Hirschsprung disease-causing mutations.

Tcof1 acts as a modifier of Pax3 during enteric nervous system development and in the pathogenesis of colonic aganglionosis

Hirschsprung disease (HSCR) is a human congenital disorder, defined by the absence of ganglia from variable lengths of the colon. These ganglia comprise the enteric nervous system (ENS) and are derived from migratory neural crest cells (NCCs). The inheritance of HSCR is complex, often non-Mendelian and characterized by variable penetrance. Although extensive research has identified many key players in the pathogenesis of Hirschsprung disease, a large number of cases remain genetically undefined. Therefore, additional unidentified genes or modifiers must contribute to the etiology and pathogenesis of Hirschsprung disease. We have discovered that Tcof1 may be one such modifier. Haploinsufficiency of Tcof1 in mice results in a reduction of vagal NCCs and their delayed migration along the length of the gut during early development. This alone, however, is not sufficient to cause colonic aganglionosis as alterations in the balance of NCC proliferation and differentiation ensures NCC colonize the entire length of the gut of Tcof1(+/-) mice by E18.5. In contrast, Tcof1 haploinsufficiency is able to sensitize Pax3(+/-) mice to colonic aganglionosis. Although, Pax3 heterozygous mice do not show ENS defects, compound Pax3;Tcof1 heterozygous mice exhibit cumulative apoptosis which severely reduces the NCC population that migrates into the foregut. In addition, the proliferative capacity of these NCC is also diminished. Taken together with the opposing effects of Pax3 and Tcof1 on NCC differentiation, the synergistic haploinsufficiency of Tcof1 and Pax3 results in colonic aganglionosis in mice and may contribute to the pathogenesis of Hirschsprung disease.

Total colonic aganglionosis in Hirschsprung disease

Total colonic aganglionosis (TCA) is a relatively uncommon form of Hirschsprung disease (HSCR), occurring in approximately 2%-13% of cases. It can probably be classified as TCA (defined as aganglionosis extending from the anus to at least the ileocecal valve, but not >50 cm proximal to the ileocecal valve) and total colonic and small bowel aganglionosis, which may involve a very long segment of aganglionosis. It is not yet clear whether TCA merely represents a long form of HSCR or a different expression of the disease. There are many differences between TCA and other forms of HSCR, which require explanation if its ubiquitous clinical features are to be understood. Clinically, TCA appears to represent a different spectrum of disease in terms of presentation and difficulties that may be experienced in diagnosis, suggesting a different pathophysiology from the more common forms of HSCR. There is also some evidence suggesting that instead of being purely congenital, it may represent certain different pathophysiologic mechanisms. This study, in addition to reviewing current understanding and differences between TCA and the more frequently encountered rectosigmoid (or short-segment) expression, correlates them with what is currently known about the genetic and molecular biological background. Moreover, it reviews current outcomes to find consensus on management.

Hirschsprung disease: a developmental disorder of the enteric nervous system

Hirschsprung disease (HSCR), which is also called congenital megacolon or intestinal aganglionosis, is characterized by an absence of enteric (intrinsic) neurons from variable lengths of the most distal bowel. Because enteric neurons are essential for propulsive intestinal motility, infants with HSCR suffer from severe constipation and have a distended abdomen. Currently the only treatment is surgical removal of the affected bowel. HSCR has an incidence of around 1:5,000 live births, with a 4:1 male:female gender bias. Most enteric neurons arise from neural crest cells that emigrate from the caudal hindbrain and then migrate caudally along the entire gut. The absence of enteric neurons from variable lengths of the bowel in HSCR results from a failure of neural crest-derived cells to colonize the affected gut regions. HSCR is therefore regarded as a neurocristopathy. HSCR is a multigenic disorder and has become a paradigm for understanding complex factorial disorders. The major HSCR susceptibility gene is RET. The penetrance of several mutations in HSCR susceptibility genes is sex-dependent. HSCR can occur as an isolated disorder or as part of syndromes; for example, Type IV Waardenburg syndrome is characterized by deafness and pigmentation defects as well as intestinal aganglionosis. Studies using animal models have shown that HSCR genes regulate multiple processes including survival, proliferation, differentiation, and migration. Research into HSCR and the development of enteric neurons is an excellent example of the cross fertilization of ideas that can occur between human molecular geneticists and researchers using animal models. WIREs Dev Biol 2013, 2:113-129. doi: 10.1002/wdev.57 For further resources related to this article, please visit the WIREs website.

Phactr4 regulates directional migration of enteric neural crest through PP1, integrin signaling, and cofilin activity

Hirschsprung disease (HSCR) is caused by a reduction of enteric neural crest cells (ENCCs) in the gut and gastrointestinal blockage. Knowledge of the genetics underlying HSCR is incomplete, particularly genes that control cellular behaviors of ENCC migration. Here we report a novel regulator of ENCC migration in mice. Disruption of the Phactr4 gene causes an embryonic gastrointestinal defect due to colon hypoganglionosis, which resembles human HSCR. Time-lapse imaging of ENCCs within the embryonic gut demonstrates a collective cell migration defect. Mutant ENCCs show undirected cellular protrusions and disrupted directional and chain migration. Phactr4 acts cell-autonomously in ENCCs and colocalizes with integrin and cofilin at cell protrusions. Mechanistically, we show that Phactr4 negatively regulates integrin signaling through the RHO/ROCK pathway and coordinates protein phosphatase 1 (PP1) with cofilin activity to regulate cytoskeletal dynamics. Strikingly, lamellipodia formation and in vivo ENCC chain migration defects are rescued by inhibition of ROCK or integrin function. Our results demonstrate a previously unknown pathway in ENCC collective migration in vivo and provide new candidate genes for human genetic studies of HSCR.

Cell death and the developing enteric nervous system

This review discusses current knowledge about cell death in the developing enteric nervous system (ENS). It also includes findings about the molecular mechanisms by which such death is mediated. Additional consideration is given to trophic factors that contribute to survival of the precursors and neurons and glia of the ENS, as well to genes that, when mutated or deleted, trigger their death. Although further confirmation is needed, present observations support the view that enteric neural crest-derived precursor cells en route to the gut undergo substantial levels of apoptotic death, but that once these cells colonize the gut, there is relatively little death of precursor cells or of neurons and glia during the fetal period. There are also indications that normal neuron loss occurs in the ENS, but at times beyond the perinatal stage. Taken together, these findings suggest that ENS development is similar is some ways, but different in others from extra-enteric areas of the vertebrate central and peripheral nervous systems, in which large-scale apoptotic death of precursor neurons and glia occurs during the fetal and perinatal periods. Potential reasons for these differences are discussed such as a fetal enteric microenvironment that is especially rich in trophic support. In addition to the cell death that occurs during normal ENS development, this review discusses mechanisms of experimentally-induced ENS cell death, such as those that are associated with defective glial cell-line derived neurotrophic factor/Ret signaling, which are an animal model of human congenital megacolon (aganglionosis; Hirschsprung's disease). Such considerations underscore the importance of understanding cell death in the developing ENS, not just from a curiosity-driven point of view, but also because the pathophysiology behind many disorders of human gastrointestinal function may originate in abnormalities of the mechanisms that govern cell survival and death during ENS development.

Homeodomain interacting protein kinase 2 regulates postnatal development of enteric dopaminergic neurons and glia via BMP signaling

Trophic factor signaling is important for the migration, differentiation, and survival of enteric neurons during development. The mechanisms that regulate the maturation of enteric neurons in postnatal life, however, are poorly understood. Here, we show that transcriptional cofactor HIPK2 (homeodomain interacting protein kinase 2) is required for the maturation of enteric neurons and for regulating gliogenesis during postnatal development. Mice lacking HIPK2 display a spectrum of gastrointestinal (GI) phenotypes, including distention of colon and slowed GI transit time. Although loss of HIPK2 does not affect the enteric neurons in prenatal development, a progressive loss of enteric neurons occurs during postnatal life in Hipk2(-/-) mutant mice that preferentially affects the dopaminergic population of neurons in the caudal region of the intestine. The mechanism by which HIPK2 regulates postnatal enteric neuron development appears to involve the response of enteric neurons to bone morphogenetic proteins (BMPs). Specifically, compared to wild type mice, a larger proportion of enteric neurons in Hipk2(-/-) mutants have an abnormally high level of phosphorylated Smad1/5/8. Consistent with the ability of BMP signaling to promote gliogenesis, Hipk2(-/-) mutants show a significant increase in glia in the enteric nervous system. In addition, numbers of autophagosomes are increased in enteric neurons in Hipk2(-/-) mutants, and synaptic maturation is arrested. These results reveal a new role for HIPK2 as an important transcriptional cofactor that regulates the BMP signaling pathway in the maintenance of enteric neurons and glia, and further suggest that HIPK2 and its associated signaling mechanisms may be therapeutically altered to promote postnatal neuronal maturation.

L1cam acts as a modifier gene for members of the endothelin signalling pathway during enteric nervous system development

BACKGROUND

The enteric nervous system originates from neural crest cells that migrate into the embryonic foregut and then sequentially colonize the midgut and hindgut. Defects in neural crest migration result in regions of the gut that lack enteric ganglia, a condition in humans called Hirschsprung's disease. The high degree of phenotypic variability reported in Hirschsprung's disease suggests the involvement of modifier genes.

METHODS

We used a two-locus complementation approach to screen for genetic interactions between L1cam and members of the endothelin signalling pathway. Immunohistochemistry was used to label PGP9.5(+) enteric neurons and Sox10(+) neural crest-derived cells in wholemount preparations of embryonic gut. Key Results  Loss or haploinsufficiency of L1cam significantly increased the severity of aganglionosis in Et-3 and Ednrb null mutant embryos. Furthermore, the colonization of the developing gut by neural crest-derived cells was significantly delayed in L1cam(-/y) ; Et-3(-/-) and L1cam(-/y) ;Ednrb(sl/sl) embryos.

CONCLUSIONS & INFERENCES

We have identified the X-linked gene, L1cam, as the first modifier gene for members of the endothelin signalling pathway during development of the enteric nervous system. Mutations in L1CAM may act to modulate the severity of aganglionosis in some cases of Hirschsprung's disease.

Isolation and live imaging of enteric progenitors based on Sox10-Histone2BVenus transgene expression

To facilitate dynamic imaging of neural crest (NC) lineages and discrimination of individual cells in the enteric nervous system (ENS) where close juxtaposition often complicates viewing, we generated a mouse BAC transgenic line that drives a Histone2BVenus (H2BVenus) reporter from Sox10 regulatory regions. This strategy does not alter the endogenous Sox10 locus and thus facilitates analysis of normal NC development. Our Sox10-H2BVenus BAC transgene exhibits temporal, spatial, and cell-type specific expression that reflects endogenous Sox10 patterns. Individual cells exhibiting nuclear-localized fluorescence of the H2BVenus reporter are readily visualized in both fixed and living tissue and are amenable to isolation by fluorescence activated cell sorting (FACS). FACS-isolated H2BVenus+ enteric NC-derived progenitors (ENPs) exhibit multipotency, readily form neurospheres, self-renew in vitro and express a variety of stem cell genes. Dynamic live imaging as H2BVenus+ ENPs migrate down the fetal gut reveals cell fragmentation suggesting that apoptosis occurs at a low frequency during normal development of the ENS. Confocal imaging both during population of the fetal intestine and in postnatal gut muscle strips revealed differential expression between individual cells consistent with down-regulation of the transgene as progression towards non-glial fates occurs. The expression of the Sox10-H2BVenus transgene in multiple regions of the peripheral nervous system will facilitate future studies of NC lineage segregation as this tool is expressed in early NC progenitors and maintained in enteric glia.

Hirschsprung's disease

Hirschsprung's disease (HSCR) is characterized by absence of the enteric nervous system in a variable portion of the distal gut. Affected infants usually present in the days after birth with bowel obstruction. Despite surgical advances, long-term outcomes remain variable. In the last 2 decades, great advances have been made in understanding the genes and molecular biological mechanisms that underlie the disease. In addition, our understanding of normal enteric nervous system development and how motility develops in the developing fetus and infant has also increased. This review aims to draw these strands together to explain the developmental and biological basis of HSCR, and how this knowledge may be used in the future to aid children with HSCR.

The timing and location of glial cell line-derived neurotrophic factor expression determine enteric nervous system structure and function

Ret signaling is critical for formation of the enteric nervous system (ENS) because Ret activation promotes ENS precursor survival, proliferation, and migration and provides trophic support for mature enteric neurons. Although these roles are well established, we now provide evidence that increasing levels of the Ret ligand glial cell line-derived neurotrophic factor (GDNF) in mice causes alterations in ENS structure and function that are critically dependent on the time and location of increased GDNF availability. This is demonstrated using two different strains of transgenic mice and by injecting newborn mice with GDNF. Furthermore, because different subclasses of ENS precursors withdraw from the cell cycle at different times during development, increases in GDNF at specific times alter the ratio of neuronal subclasses in the mature ENS. In addition, we confirm that esophageal neurons are GDNF responsive and demonstrate that the location of GDNF production influences neuronal process projection for NADPH diaphorase-expressing, but not acetylcholinesterase-, choline acetyltransferase-, or tryptophan hydroxylase-expressing, small bowel myenteric neurons. We further demonstrate that changes in GDNF availability influence intestinal function in vitro and in vivo. Thus, changes in GDNF expression can create a wide variety of alterations in ENS structure and function and may in part contribute to human motility disorders.

Development of the autonomic nervous system: a comparative view

In this review we summarize current understanding of the development of autonomic neurons in vertebrates. The mechanisms controlling the development of sympathetic and enteric neurons have been studied in considerable detail in laboratory mammals, chick and zebrafish, and there are also limited data about the development of sympathetic and enteric neurons in amphibians. Little is known about the development of parasympathetic neurons apart from the ciliary ganglion in chicks. Although there are considerable gaps in our knowledge, some of the mechanisms controlling sympathetic and enteric neuron development appear to be conserved between mammals, avians and zebrafish. For example, some of the transcriptional regulators involved in the development of sympathetic neurons are conserved between mammals, avians and zebrafish, and the requirement for Ret signalling in the development of enteric neurons is conserved between mammals (including humans), avians and zebrafish. However, there are also differences between species in the migratory pathways followed by sympathetic and enteric neuron precursors and in the requirements for some signalling pathways.

Enteric nervous system development: Recent progress and future challenges

The enteric nervous system is the largest subdivision of the peripheral nervous system that plays a critical role in digestive functions. Despite considerable progress over the last 15 years in understanding the molecular and cellular mechanisms that control the development of the enteric nervous system, several questions remain unanswered. The present review will focus on recent progress on understanding the development of the mammalian enteric nervous system and highlight interesting directions of future research.

Death comes early: apoptosis observed in ENS precursors

Cell death is a physiological and fundamental process in normal organogenesis. During the development of the nervous system, cell death or apoptosis occurs in early and late developmental time periods, affecting neural precursors and neurons respectively. In the development of the enteric nervous system (ENS), however, apoptosis of neurons has not been detected, a feature unique to enteric neurons. In this issue of Neurogastroenterology and Motility, Wallace et al. focused on an early phase of ENS development and identified apoptotic cell death in vagal neural crest cells, the primary cellular source for the ENS. Introduction of an antiapoptotic molecule in the vagal neural crest and its derivatives resulted in the overproduction of neurons in the foregut. Thus, unlike the neurons themselves, ENS precursors do undergo apoptosis, which may, by regulating the size of the ENS precursor pool, be a crucial factor in determining the final cell number in the ENS.