Lessons from genetically engineered animal models. II. Disorders of enteric neuronal development: insights from transgenic mice

Who's talking to whom: microbiome-enteric nervous system interactions in early life

The enteric nervous system (ENS) is the intrinsic nervous system of the gastrointestinal tract (GI) and regulates important GI functions, including motility, nutrient uptake, and immune response. The development of the ENS begins during early organogenesis and continues to develop once feeding begins, with ongoing plasticity into adulthood. There has been increasing recognition that the intestinal microbiota and ENS interact during critical periods, with implications for normal development and potential disease pathogenesis. In this review, we focus on insights from mouse and zebrafish model systems to compare and contrast how each model can serve in elucidating the bidirectional communication between the ENS and the microbiome. At the end of this review, we further outline implications for human disease and highlight research innovations that can lead the field forward.

The pro-inflammatory cytokines IL-1β and TNFα are neurotrophic for enteric neurons

Intestinal inflammation causes initial axonal degeneration and neuronal death but subsequent axon outgrowth from surviving neurons restores innervation density to the target smooth muscle cells. Elsewhere, the pro-inflammatory cytokines TNFα and IL-1β cause neurotoxicity, leading us to test their role in promoting enteric neuron death. In a rat coculture model, TNFα or IL-1β did not affect neuron number but did promote significant neurite outgrowth to twofold that of control by 48 h, while other cytokines (e.g., IL-4, TGFβ) were without effect. TNFα or IL-1β activated the NFκB signaling pathway, and inhibition of NFκB signaling blocked the stimulation of neurite growth. However, nuclear translocation of NFκB in smooth muscle cells but not in adjacent neurons suggested a dominant role for smooth muscle cells. TNFα or IL-1β sharply increased both mRNA and protein for GDNF, while the neurotrophic effects of TNFα or IL-1β were blocked by the RET-receptor blocker vandetanib. Conditioned medium from cytokine-treated smooth muscle cells mimicked the neurotrophic effect, inferring that TNFα and IL-1β promote neurite growth through NFκB-dependent induction of glial cell line-derived neurotrophic factor (GDNF) expression in intestinal smooth muscle cells. In vivo, TNBS-colitis caused early nuclear translocation of NFκB in smooth muscle cells. Conditioned medium from the intact smooth muscle of the inflamed colon caused a 2.5-fold increase in neurite number in cocultures, while Western blotting showed a substantial increase in GDNF protein. Pro-inflammatory cytokines promote neurite growth through upregulation of GDNF, a novel process that may facilitate re-innervation of smooth muscle cells and a return to homeostasis following initial damage.

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.

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.

Recent advances in studies of spontaneous activity in smooth muscle: ubiquitous pacemaker cells

The general and specific properties of pacemaker cells, including Kit-negative cells, that are distributed in gastrointestinal, urethral and uterine smooth muscle tissues, are discussed herein. In intestinal tissues, interstitial cells of Cajal (ICC) are heterogeneous in both their forms and roles. ICC distributed in the myenteric layer (ICC-MY) act as primary pacemaker cells for intestinal mechanical and electrical activity. ICC distributed in muscle bundles play a role as mediators of signals from autonomic nerves to smooth muscle cells. A group of ICC also appears to act as a stretch sensor. Intracellular Ca2+ dynamics play a crucial role in ICC-MY pacemaking; intracellular Ca2+ ([Ca2+](i)) oscillations periodically activate plasmalemmal Ca2+-activated ion channels, such as Ca2+-activated Cl(-) channels and/or non-selective cation channels, although the relative contributions of these channels are not defined. With respect to gut motility, both the ICC network and enteric nervous system, including excitatory and inhibitory enteric neurons, play an essential role in producing highly coordinated peristalsis.

A bird's eye view of enteric nervous system development: lessons from the avian embryo

The avian embryo has been an important model system for studying enteric nervous system (ENS) development for over 50 y. Since the initial demonstration in chick embryos that the ENS is derived from the neural crest, investigators have used the avian model to reveal the cellular origins and migratory pathways of enteric neural crest-derived cells, with more recent work focusing on the molecular mechanisms regulating ENS development. Seminal contributions have been made in this field by researchers who have taken advantage of the strengths of the avian model system. These strengths include in vivo accessibility throughout development, ability to generate quail-chick chimeras, and the capacity to modulate gene expression in vivo in a spatially and temporally targeted manner. The recent availability of the chicken genome further enhances this model system, allowing investigators to combine classic embryologic methods with current genetic techniques. The strengths and versatility of the avian embryo continue to make it a valuable experimental system for studying the development of the ENS.

Role of glial cell-line derived neurotropic factor family receptor alpha2 in the actions of the glucagon-like peptides on the murine intestine

The intestinal glucagon-like peptides GLP-1 and GLP-2 inhibit intestinal motility, whereas GLP-2 also stimulates growth of the intestinal mucosa. However, the mechanisms of action of these peptides in the intestine remain poorly characterized. To determine the role of the enteric nervous system in the actions of GLP-1 and GLP-2 on the intestine, the glial cell line-derived neurotropic factor family receptor alpha(2) (GFRalpha2) knockout (KO) mouse was employed. The mice exhibited decreased cholinergic staining, as well as reduced mRNA transcripts for substance P-ergic excitatory motoneurons in the enteric nervous system (ENS) (P < 0.05). Examination of parameters of intestinal growth (including small and large intestinal weight and small intestinal villus height, crypt depth, and crypt cell proliferation) demonstrated no differences between wild-type and KO mice in either basal or GLP-2-stimulated mucosal growth. Nonetheless, KO mice exhibited reduced numbers of synaptophysin-positive enteroendocrine cells (P < 0.05), as well as a markedly impaired basal gastrointestinal (GI) transit rate (P < 0.05). Furthermore, acute administration of GLP-1 and GLP-2 significantly inhibited transit rates in wild-type mice (P < 0.05-0.01) but had no effect in GFRalpha2 KO mice. Despite these changes, expression of mRNA transcripts for the GLP receptors was not reduced in the ENS of KO animals, suggesting that GLP-1 and -2 modulate intestinal transit through enhancement of inhibitory input to cholinergic/substance P-ergic excitatory motoneurons. Together, these findings demonstrate a role for GFRalpha2-expressing enteric neurons in the downstream signaling of the glucagon-like peptides to inhibit GI motility, but not in intestinal growth.

Characterization of Myenteric Sensory Neurons in the Mouse Small Intestine

We recorded from myenteric AH/Dogiel type II cells, demonstrated mechanosensitive responses, and characterized their basic properties. Recordings were obtained using the mouse longitudinal muscle myenteric plexus preparation with patch-clamp and sharp intracellular electrodes. The neurons had an action potential hump and a slow afterhyperpolarization (AHP) current. The slow AHP was carried by intermediate conductance Ca2+-dependent K+-channel currents sensitive to charybdotoxin and clotrimazole. All possessed a hyperpolarization-activated current that was blocked by extracellular cesium. They also expressed a TTX-resistant Na+ current with an onset near the resting potential. Pressing on the ganglion containing the patched neuron evoked depolarizing potentials in 17/18 cells. The potentials persisted after synaptic transmission was blocked. Volleys of presynaptic electrical stimuli evoked slow excitatory postsynaptic potentials (EPSPs) in 9/11 sensory neurons, but 0/29 cells received fast EPSP input. The slow EPSP was generated by removal of a voltage-insensitive K+ current. Patch-clamp recording with a KMeSO4-containing, but not a conventional KCl-rich, intracellular solution reproduced the single-spike slow AHPs and low input resistances seen with sharp intracellular recording. Cell-attached recording of intermediate conductance potassium channels supported the conclusion that the single-spike slow AHP is an intrinsic property of intestinal AH/sensory neurons. Unitary current recordings also suggested that the slow AHP current probably does not contribute significantly to the high resting background conductance seen in these cells. The characterization of mouse myenteric sensory neurons opens the way for the study of their roles in normal and pathological physiology.

The Hlx homeobox transcription factor is required early in enteric nervous system development

Background

Development of the enteric nervous system (ENS) requires interactions between migrating neural crest cells and the nascent gastrointestinal tract that are dependent upon genes expressed by both cell compartments. Hlx, a homeobox transcription factor gene that is expressed in mouse intestinal and hepatic mesenchyme, is required for normal embryonic growth of intestine and liver, and the Hlx^-/- genotype is embryonic lethal. We hypothesized that Hlx is required for ENS development.

Results

Enteric neurons were identified in Hlx^+/+ and Hlx^-/- mouse embryos by immunostaining of embryo sections for the neural markers PGP9.5 and Phox2b, or by staining for β-galactosidase in whole-mount embryos containing the dopamine β-hydroxylase-nLacZ transgene. In Hlx^+/+ embryos, neural crest cells/enteric neurons have moved from the stomach into the intestine by E10.5. By contrast, neural crest cells/enteric neurons remain largely restricted to the lateral stomach mesenchyme of Hlx^-/- embryos, with only a few scattered neural crest cells/enteric neurons in the intestine between E10.5–16.5.

Conclusion

The Hlx homeobox transcription factor is required for early aspects of ENS development.

In vitro formation of enteric neural network structure in a gut-like organ differentiated from mouse embryonic stem cells

Using an embryoid body (EB) culture system, we developed a functional organ-like cluster--a "gut"--from mouse embryonic stem (ES) cells (ES gut). Each ES gut exhibited spontaneous contractions but did not exhibit distinct peristalsis-like movements. In these spontaneously contracting ES guts, dense distributions of interstitial cells of Cajal (c-kit [a transmembrane receptor that has tyrosine kinase activity]-positive cells; gut pacemaker cells) and smooth muscle cells were discernibly identified; however, enteric neural ganglia were absent in the spontaneously differentiated ES gut. By adding brain-derived neurotrophic factor (BDNF) only during EB formation, we for the first time succeeded in in vitro formation of enteric neural ganglia with connecting nerve fiber tracts (enteric nervous system [ENS]) in the ES gut. The ES gut with ENS exhibited strong peristalsis-like movements. During EB culture in BDNF(+) medium, we detected each immunoreactivity associated with the trk proto-oncogenes (trkB; BDNF receptors) and neural crest marker, proto-oncogene tyrosine-protein kinase receptor ret precursor (c-ret), p75, or sox9. These results indicated that the present ENS is differentiated from enteric neural crest-derived cells. Moreover, focal stimulation of ES guts with ENS elicited propagated increases in intracellular Ca(2+) concentration ([Ca(2+)](i)) at single or multiple sites that were attenuated by atropine or abolished by tetrodotoxin. These results suggest in vitro formation of physiologically functioning enteric cholinergic excitatory neurons. We for the first time succeeded in the differentiation of functional neurons in ENS by exogenously adding BDNF in the ES gut, resulting in generation of distinct peristalsis-like movements.

Polymerase chain reaction-single strand conformational polymorphism analysis of rearranged during transfection proto-oncogene in Chinese familial hirschsprung’s disease

AIM: To investigate the relationship between mutations of rearranged during transfection (RET) proto-oncogene and Chinese patients with Hirschsprung’s disease (HD), and to elucidate the genetic mechanism of familial HD patient at the molecular level.

METHODS: Genomic DNA was extracted from venous blood of probands and their relatives in two genealogies. Polymerase chain reaction (PCR) products, which were amplified using specific primers (RET, exons 11, 13, 15 and 17), were electrophoresed to analyze the single-strand conformational polymorphism (SSCP) patterns. The positive amplified products were sequenced. Forty-eight sporadic HD patients and 30 normal children were screened for mutations of RET proto-oncogene simultaneously.

RESULTS: Three cases with HD in one family were found to have a G heterozygous insertion at nucleotide 18974 in exon 13 of RET cDNA (18974insG), which resulted in a frameshift mutation. In another family, a heterozygosity for T to G transition at nucleotide 18888 in the same exon which resulted in a synonymous mutation of Leu at codon 745 was detected in the proband and his father. Eight RET mutations were confirmed in 48 sporadic HD patients.

CONCLUSION: Mutations of RET proto-oncogene may play an important role in the pathogenesis of Chinese patients with HD. Detection of mutated RET proto-oncogene carriers may be used for genetic counseling of potential risk for HD in the affected families.

Interstitial cells of Cajal and electrical activity in ganglionic and aganglionic colons of mice

An antibody directed against Kit protein was used to investigate the distribution of interstitial cells of Cajal (ICC) within the murine colon. The ICC density was greatest in the proximal colon and decreased along its length. The distribution of the different classes of ICC in the aganglionic colons of lethal spotted (ls/ls) mice was found to be similar in age-matched wild-type controls. There were marked differences in the electrical activities of the colons from ls/ls mutants compared with wild-type controls. In ls/ls aganglionic colons, the circular muscle was electrically quiescent compared with the spontaneous spiking electrical activity of wild-type tissues. In ls/ls aganglionic colons, postjunctional neural responses were greatly affected. Inhibitory junction potentials were absent or excitatory junction potentials inhibited by atropine were observed. In conclusion, the distribution of ICC in the ganglionic and aganglionic regions of the colons from ls/ls mutants appeared similar to that of wild-type controls. The electrical activity and neural responses of the circular layer are significantly different in aganglionic segments of ls/ls mutants.

Transcripts encoding HAND genes are differentially expressed and regulated by BMP4 and GDNF in developing avian gut

Growth and transcription factors provide important developmental cues to neural crest-derived precursors of enteric neurons. The basic helix-loop-helix transcription factors, HAND2 and HAND1, are expressed in the gastrointestinal tract, but neither the growth factors that induce their expression nor the cell types that express them in the gut are known. We show that transcripts encoding HAND2 are expressed in all segments of the developing gut while those encoding HAND1 are confined to the small intestine and colon. Using in situ hybridization combined with immunostaining using cell type-specific antigens, we demonstrate that transcripts encoding HAND2 are expressed in neurons of both the myenteric and submucosal ganglia. Transcripts encoding HAND1 are expressed by cells in the epithelial lining of the small intestine and colon. The differential localization of HAND2 and HAND1 is reflected in nonoverlapping patterns of regulation by gut-derived factors. The expression of transcripts encoding HAND2 is increased in neural crest-derived cells when cocultured with E4 gut, suggesting a gut-derived factor regulates expression of HAND genes. Exposure of gut-derived neural crest-derived cells to BMP4 significantly increased the expression of HAND2 in all gut segments. In the esophagus and gizzard, where HAND1 is not normally expressed, treatment with BMP4 induced the expression of transcripts encoding HAND1 in nonneural crest-derived cells. GDNF failed to induce consistent expression of transcripts encoding HAND2 in neural crest cells but did support a modest increase in HAND2 expression in gut-derived crest cells obtained from the esophagus and colon. GDNF had no detectable effect on the expression of transcripts encoding HAND1. These results suggest; 1) that HAND2 has a function in the development of enteric neurons, and 2) that BMP and GDNF differentially regulate HAND2 and HAND1 gene expression in the developing gastrointestinal tract.

Pax3 is required for enteric ganglia formation and functions with Sox10 to modulate expression of c-ret

Hirschsprung disease and Waardenburg syndrome are human genetic diseases characterized by distinct neural crest defects. Patients with Hirschsprung disease suffer from gastrointestinal motility disorders, whereas Waardenburg syndrome consists of defective melanocyte function, deafness, and craniofacial abnormalities. Mutations responsible for Hirschsprung disease and Waardenburg syndrome have been identified, and some patients have been described with characteristics of both disorders. Here, we demonstrate that PAX3, which is often mutated in Waardenburg syndrome, is required for normal enteric ganglia formation. Pax3 can bind to and activate expression of the c-RET gene, which is often mutated in Hirschsprung disease. Pax3 functions with Sox10 to activate transcription of c-RET, and SOX10 mutations result in Waardenburg-Hirschsprung syndrome. Thus, Pax3, Sox10, and c-Ret are components of a neural crest development pathway, and interruption of this pathway at various stages results in neural crest-related human genetic syndromes.