A neuronal subpopulation in the mammalian enteric nervous system expresses TrkA and TrkC neurotrophin receptor-like proteins

Gene-environment interactions and the enteric nervous system: Neural plasticity and Hirschsprung disease prevention

Intestinal function is primarily controlled by an intrinsic nervous system of the bowel called the enteric nervous system (ENS). The cells of the ENS are neural crest derivatives that migrate into and through the bowel during early stages of organogenesis before differentiating into a wide variety of neurons and glia. Although genetic factors critically underlie ENS development, it is now clear that many non-genetic factors may influence the number of enteric neurons, types of enteric neurons, and ratio of neurons to glia. These non-genetic influences include dietary nutrients and medicines that may impact ENS structure and function before or after birth. This review summarizes current data about gene-environment interactions that affect ENS development and suggests that these factors may contribute to human intestinal motility disorders like Hirschsprung disease or irritable bowel syndrome.

Brain-Derived Neurotrophic Factor Contributes to Colonic Hypermotility in a Chronic Stress Rat Model

BACKGROUND

Brain-derived neurotrophic factor (BDNF) has prokinetic effects on gut motility and is increased in the colonic mucosa of irritable bowel syndrome.

AIMS

We aimed to investigate the possible involvement of BDNF in stress-induced colonic hypermotility.

METHODS

Male Wistar rats were exposed to daily 1-h water avoidance stress (WAS) or sham WAS for 10 consecutive days. The presence of BDNF and substance P (SP) in the colonic mucosa was determined using enzyme immunoassay kits. Immunohistochemistry and western blotting were performed to assess the expression of BDNF and its receptor, TrkB. The contractions of muscle strips were studied in an organ bath system.

RESULTS

Repeated WAS increased the fecal pellet expulsion and spontaneous contractile activities of the colonic muscle strips. Both BDNF and SP in the colonic mucosa were elevated following WAS. Immunohistochemistry revealed the presence of BDNF and TrkB in the mucosa and myenteric plexus. BDNF and TrkB were both up-regulated in colon devoid of mucosa and submucosa from the stressed rats compared with the control. BDNF pretreatment caused an enhancement of the SP-induced contraction of the circular muscle (CM) strips. TrkB antibody significantly inhibited the contraction of the colonic muscle strips and attenuated the excitatory effects of SP on contractions of the CM strips. Repeated WAS increased the contractile activities of the CM strips induced by SP after BDNF pretreatment, and this effect was reversed by TrkB antibody.

CONCLUSIONS

The colonic hypermotility induced by repeated WAS may be associated with the increased expression of endogenous BDNF and TrkB. BDNF may have potential clinical therapeutic use in modulating gut motility.

NT-3 attenuates the growth of human neuron cells through the ERK pathway

Spinal cord injury is a devastating health problem that affects thousands of individuals each year. The neurons were destroyed. NT-3 is a recently discovered neurotrophin. This study sought to understand the potential involvement of MAPKs in NT-3-mediated growth inhibition of human neurons. We applied different concentrations of NT-3 and observed the growth rate of the cells and the changes in the phosphorylation state of the MAPKs ERK1/2, JNK and p38. This study discovered that NT-3-induced HNC growth was promoted primarily by phosphorylated ERK1/2, and that this phosphorylation, as well p90(rsk)phosphorylation, was mediated by TrkC. The ERK1/2 pathway is known to play an essential role in the NT-3-mediated growth of human neurons. In conclusion, our study suggests that NT-3 promotes the growth of human neurons cells primarily through the TrkC/ERK pathway.

Brain-derived neurotrophic factor enhances cholinergic contraction of longitudinal muscle of rabbit intestine via activation of phospholipase C

Brain-derived neurotrophic factor (BDNF) belongs to the neurotrophin family of proteins best known for its role in neuronal survival, differentiation, migration, and synaptic plasticity in central and peripheral neurons. BDNF is also widely expressed in nonneuronal tissues including the gastrointestinal tract. The role of BDNF in intestinal smooth muscle contractility is not well defined. The aim of this study was to identify the role of BDNF in carbachol (CCh)- and substance P (SP)-induced contraction of intestinal longitudinal smooth muscle. BDNF, selective tropomyosin-related kinase B (TrkB) receptor agonists, and pharmacological inhibitors of signaling pathways were examined for their effects on contraction of rabbit intestinal longitudinal muscle strips induced by CCh and SP. BDNF activation of intracellular signaling pathways was examined by Western blot in homogenates of muscle strips and isolated muscle cells. One-hour preincubation with BDNF enhanced intestinal muscle contraction induced by CCh but not by SP. The selective synthetic TrkB agonists LM 22A4 and 7,8-dihydroxyflavone produced similar effects to BDNF. The Trk antagonist K-252a, a TrkB antibody but not p75NTR antibody, blocked the effect of BDNF. The enhancement of CCh-induced contraction by BDNF was blocked by the phospholipase C (PLC) antagonist U73122, but not by ERK1/2 or Akt antagonists. Direct measurement in muscle strips and isolated muscle cells showed that BDNF caused phosphorylation of TrkB receptors and PLC-γ, but not ERK1/2 or Akt. We conclude that exogenous BDNF augments the CCh-induced contraction of longitudinal muscle from rabbit intestine by activating TrkB receptors and subsequent PLC activation.

Expansion and differentiation of neural progenitors derived from the human adult enteric nervous system

BACKGROUND & AIMS

Neural stem and progenitor cells from the enteric nervous system have been proposed for use in cell-based therapies against specific neurogastrointestinal disorders. Recently, enteric neural progenitors were generated from human neonatal and early postnatal (until 5 years after birth) gastrointestinal tract tissues. We investigated the proliferation and differentiation of enteric nervous system progenitors isolated from human adult gastrointestinal tract.

METHODS

Human enteric spheroids were generated from adult small and large intestine tissues and then expanded and differentiated, depending on the applied cell culture conditions. For implantation studies, spheres were grafted into fetal slice cultures and embryonic aganglionic hindgut explants from mice. Differentiating enteric neural progenitors were characterized by 5-bromo-2-deoxyuridine labeling, in situ hybridization, immunocytochemistry, quantitative real-time polymerase chain reaction, and electrophysiological studies.

RESULTS

The yield of human neurosphere-like bodies was increased by culture in conditional medium derived from fetal mouse enteric progenitors. We were able to generate proliferating enterospheres from adult human small or large intestine tissues; these enterospheres could be subcultured and maintained for several weeks in vitro. Spheroid-derived cells could be differentiated into a variety of neuronal subtypes and glial cells with characteristics of the enteric nervous system. Experiments involving implantation into organotypic intestinal cultures showed the differentiation capacity of neural progenitors in a 3-dimensional environment.

CONCLUSIONS

It is feasible to isolate and expand enteric progenitor cells from human adult tissue. These findings offer new strategies for enteric stem cell research and future cell-based therapies.

Enteric glial cells express full-length TrkB and depend on TrkB expression for normal development

The embryonic development of the enteric nervous system (ENS) from neural crest precursor cells requires neurotrophic signaling. Neurotrophins (NTs) are a family of growth factors that bind Trk receptors to signal diverse functions, including development and maintenance of different cell populations in the peripheral nervous system. In this study we investigated the expression and cell localization of TrkB, the high affinity receptor for brain-derived neurotrophic factor and NT-4, in the murine ENS using Western blot and immunohistochemistry. The results demonstrate that enteric glial cells within the ENS express full-length TrkB at all stages tested. The ENS of TrkB deficient mice have reduced expression of glial cell markers, and a disarrangement of glial cells and the plexular neuropil. These results strongly suggest TrkB has essential roles in the normal development and maintenance of glial cells in the ENS.

Brain-derived neurotrophic factor augments peristalsis by augmenting 5-HT and calcitonin gene-related peptide release

BACKGROUND & AIMS

Brain-derived neurotrophic factor (BDNF) acts rapidly to modulate synaptic neurotransmission in the brain. Although present in neurons, glial cells, and mucosal cells of the colon, and in higher concentrations than in brain, the action of BDNF in gut have not been characterized. The aim of this study was to identify the role of BDNF in mediating the peristaltic reflex.

METHODS

BDNF and a specific antiserum were examined for their effects on the peristaltic reflex and release of the sensory mediators serotonin and calcitonin gene-related peptide in rat colon. The peristaltic reflex and release of serotonin and calcitonin gene-related peptide were also examined in genetically modified mice (BDNF(+/-)) with reduced levels of BDNF.

RESULTS

Endogenous brain-derived neurotrophic factor was released into the sensory compartment in a stimulus-dependent manner during the peristaltic reflex induced by mucosal stimulation but not muscle stretch. BDNF stimulated and immunoneutralization of endogenous BDNF reduced ascending contraction and descending relaxation of circular muscle and release of serotonin and calcitonin gene-related peptide during the peristaltic reflex induced by mucosal stimulation but not muscle stretch. The peristaltic reflex and release of serotonin and calcitonin gene-related peptide during the peristaltic reflex induced by mucosal stimulation but not muscle stretch were significantly reduced in BDNF(+/-) mice.

CONCLUSIONS

Endogenous BDNF enhances the peristaltic reflex by augmenting the release of serotonin and calcitonin gene-related peptide that mediate the sensory limb of the reflex induced by mucosal stimulation.

BMPRIA is a promising marker for evaluating ganglion cells in the enteric nervous system--a pilot study

Congenital disorders of the enteric nervous system (ENS) comprise a large group of conditions characterized by abnormalities in the number, size, or location of enteric ganglia. Their diagnosis requires careful histological evaluation of intestinal biopsies to determine the presence and morphology of these cells. Based on the recently discovered role of bone morphogenetic proteins (BMPs) in ENS development, we examined the expression of the ligands, BMP2 and BMP4, and their receptors, BMPRIA, BMPRIB, and BMPRII, during formation of the human ENS. The spatiotemporal expression pattern of these proteins suggests a role for BMP signaling in human ENS formation. We find BMPRIA, in particular, strongly and specifically expressed in all ganglion cells of the ENS at every age examined, from fetus to adult. Moreover, BMPRIA immunohistochemistry consistently allowed the identification of ganglion cells in rectal biopsies from patients with Hirschsprung disease, intestinal neuronal dysplasia, and immature ganglion cells. We propose that BMPRIA immunohistochemistry may be a promising new tool for the identification of enteric ganglion cells in the evaluation of patients with neurointestinal disorders.

Development of the enteric nervous system, smooth muscle and interstitial cells of Cajal in the human gastrointestinal tract

The generation of functional neuromuscular activity within the pre-natal gastrointestinal tract requires the coordinated development of enteric neurons and glial cells, concentric layers of smooth muscle and interstitial cells of Cajal (ICC). We investigated the genesis of these different cell types in human embryonic and fetal gut material ranging from weeks 4-14. Neural crest cells (NCC), labelled with antibodies against the neurotrophin receptor p75NTR, entered the foregut at week 4, and migrated rostrocaudally to reach the terminal hindgut by week 7. Initially, these cells were loosely distributed throughout the gut mesenchyme but later coalesced to form ganglia along a rostrocaudal gradient of maturation; the myenteric plexus developed primarily in the foregut, then in the midgut, and finally in the hindgut. The submucosal plexus formed approximately 2-3 weeks after the myenteric plexus, arising from cells that migrated centripetally through the circular muscle layer from the myenteric region. Smooth muscle differentiation, as evidenced by the expression of alpha-smooth muscle actin, followed NCC colonization of the gut within a few weeks. Gut smooth muscle also matured in a rostrocaudal direction, with a large band of alpha-smooth muscle actin being present in the oesophagus at week 8 and in the hindgut by week 11. Circular muscle developed prior to longitudinal muscle in the intestine and colon. ICC emerged from the developing gut mesenchyme at week 9 to surround and closely appose the myenteric ganglia by week 11. By week 14, the intestine was invested with neural cells, longitudinal, circular and muscularis mucosae muscle layers, and an ICC network, giving the fetal gut a mature appearance.

Acceleration of blood-brain barrier formation after transplantation of enteric glia into spinal cords of rats

Enteric glia share morphological, biochemical, and functional properties with astrocytes. Thus, like astrocytes, transplantation of enteric glia into the central nervous system (CNS) might facilitate the development of the characteristics of the blood brain barrier (BBB) in endothelial cells. This study explored this possibility by examining barrier formation after implantation into the spinal cord of rats. Phaseolus vulgaris leucoagglutin (PHAL)-treated enteric glia suspensions were injected into the spinal cord at the T11-T12 level of adult Wistar female rats. Control animals were injected with either 3T3 fibroblast, glioma C6 cells, or culture medium. Evan's blue, a dye excluded by the BBB, was injected intravenously from 1 week to 2 months after implantation. Leakage of dye was determined macroscopically and the ultrastructure of the capillaries was examined. During the first week leakage of dye correlated ultrastructurally with predominantly non-overlapping endothelial cell junctions, even with clefts between adjacent cells. Tight junctions were fully formed by 2 months and no dye leaked. Electron microscopic analysis showed that enteric glia had end-feet in close contact with endothelial cells. In contrast, the injection sites in all control animals leaked dye until 2 months, and most of the tight junctions that did form were incomplete. Furthermore, most 3T3 or C6 control cells had died at 2 months and those that survived, unlike enteric glia, had no anatomical relationship to blood vessels. These data demonstrate that implantation of enteric glia accelerates the formation of the characteristics of the BBB in spinal cord capillaries.

Characterization of the neurotrophic response to acute pancreatitis

INTRODUCTION

Interesting preliminary data on changes in the neurotrophin system in various digestive diseases have recently begun to emerge.

AIMS

To measure changes in messenger RNA (mRNA) levels of neurotrophins and to identify cell types expressing neurotrophins in the pancreas of rats with L-arginine-induced pancreatitis.

METHODOLOGY

Rats were killed at time points from 2 hours to 4 weeks after the induction of pancreatitis, and responses were measured by assay.

RESULTS

By RNase protection assay, ciliary neurotrophic factor (CNTF) mRNA expression showed a rapid response (sixfold increase over control) in the inflamed pancreas at 2 hours. The levels of mRNA expression of brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4) in the inflamed pancreas reached a peak at 1 week (2.5-fold, twofold, fourfold, and fivefold increase, respectively). By immunohistochemistry, immunoreactivity for all neurotrophins examined was observed in the islets of Langerhans in the control pancreas at all time points, but it was markedly reduced in the islets in the inflamed pancreas at 2 and 6 hours. Acinar and ductal cells, inflammatory cells, and neural elements were immunoreactive for those neurotrophins in the inflamed pancreas from 2 hours to 2 weeks.

CONCLUSION

The temporal and spatial expression of neurotrophins in the course of experimental pancreatitis suggests that their upregulation is a critical component of the response of the pancreas to injury in this model.

Neurotrophin-4 deficient mice have a loss of vagal intraganglionic mechanoreceptors from the small intestine and a disruption of short-term satiety

Intraganglionic laminar endings (IGLEs) and intramuscular arrays (IMAs) are the two putative mechanoreceptors that the vagus nerve supplies to gastrointestinal smooth muscle. To examine whether neurotrophin-4 (NT-4)-deficient mice, which have only 45% of the normal number of nodose ganglion neurons, exhibit selective losses of these endings and potentially provide a model for assessing their functional roles, we inventoried IGLEs and IMAs in the gut wall. Vagal afferents were labeled by nodose ganglion injections of wheat germ agglutinin-horseradish peroxidase, and a standardized sampling protocol was used to map the terminals in the stomach, duodenum, and ileum. NT-4 mutants had a substantial organ-specific reduction of IGLEs; whereas the morphologies and densities of both IGLEs and IMAs in the stomach were similar to wild-type patterns, IGLEs were largely absent in the small intestine (90 and 81% losses in duodenum and ileum, respectively). Meal pattern analyses revealed that NT-4 mutants had increased meal durations with solid food and increased meal sizes with liquid food. However, daily total food intake and body weight remained normal because of compensatory changes in other meal parameters. These findings indicate that NT-4 knock-out mice have a selective vagal afferent loss and suggest that intestinal IGLEs (1) may participate in short-term satiety, probably by conveying feedback about intestinal distension or transit to the brain, (2) are not essential for long-term control of feeding and body weight, and (3) play different roles in regulation of solid and liquid diet intake.

TrkA and TrkB neurotrophin receptor immunoreactivity in the teleost (Scorpaena porcus) endocrine pancreas

Mammalian-like and specific neurotrophins, as well as their cognate tyrosine kinase Trk-like receptors have been identified in teleosts. They are mainly distributed in neuronal tissues, but evidence suggests that some non-neuronal tissues also express Trks. In this study we used immunohistochemistry to investigate the occurrence and cell distribution of Trks in the pancreas of teleosts (Scorpaena porcus). Immunoreactivity for TrkA and TrkB, but not for TrkC receptors, was found in the scorpionfish pancreas. TrkA-like positive cells were exclusively observed within pancreatic islets, among insulin- and glucagon-containing cells, but apparently did not co-localize with these hormones. TrkB immunostaining was found in islet cells, presumably colocalized with glucagon, as well as in some cells of the exocrine portion. These data provide a morphological basis for a role of the TrkA and TrkB ligands in the endocrine pancreas of teleosts.

Neurotrophin-3 and neurotrophin receptor immunoreactivity in peptidergic enteric neurons

In the rat small intestine, neurotrophin-3 immunoreactivity was identified in ganglion cells and in processes mostly innervating the mucosa and occasionally the muscle layer and vasculature. The vast majority of neurotrophin-3 immunoreactive neurons contained vasoactive intestinal polypeptide (VIP), but not substance P or related tachykinin (SP/TK). Neurotrophin receptors visualized by pan-trk immunoreactivity were found in numerous ganglion cells of both plexuses and in nerve processes in the intestinal wall. Pan-trk submucosal neurons contained VIP (36%) or SP/TK-IR (47%). Pan-trk myenteric neurons contained VIP-IR (57%) or SP/TK (27%). Our data suggest that neurotrophin-3 and neurotrophin receptors may be involved in the maintenance of enteric neuronal circuits, transmission and phenotypic expression.

Trophic actions of neurotrophin-3 on postnatal rat myenteric neurons in vitro

A number of neurotrophic factors have been implicated in the prenatal development of the enteric nervous system. Although several of these factors continue to be expressed in the gut during postnatal life, their actions on postnatal enteric neurons are not understood. One such factor is the neurotrophin, NT-3. Both NT-3 and its high affinity receptor, trk C, are expressed in the postnatal gut at a time when changes in the density of intestinal innervation are occurring. We have therefore examined the effects of NT-3 on postnatal myenteric neurons, using dissociated cell cultures of ganglia isolated from 6-8 day postnatal rat small intestine. Effects of NT-3 on neurite outgrowth and neuronal and glial cell numbers were measured after 2 days in vitro. The proportion of neurons was increased in NT-3 treated cultures, as was the proportion of neurons that extended processes. NT-3 treatment, at concentrations of between 0.1 ng and 10 ng/ml, also resulted in a significant increase in mean total neurite length. These results indicate that NT-3 may play a role in the postnatal development of the enteric nervous system.

Plasticity in the enteric nervous system

Enteric ganglia can maintain integrated functions, such as the peristaltic reflex, in the absence of input from the central nervous system, which has a modulatory role. Several clinical and experimental observations suggest that homeostatic control of gut function in a changing environment may be achieved through adaptive changes occurring in the enteric ganglia. A distinctive feature of enteric ganglia, which may be crucial during the development of adaptive responses, is the vicinity of the final effector cells, which are an important source of mediators regulating cell growth. The aim of this review is to focus on the possible mechanisms underlying neuronal plasticity in the enteric nervous system and to consider approaches to the study of plasticity in this model. These include investigations of neuronal connectivity during development, adaptive mechanisms that maintain function after suppression of a specific neural input, and the possible occurrence of activity-dependent modifications of synaptic efficacy, which are thought to be important in storage of information in the brain. One of the applied aspects of the study of plasticity in the enteric nervous system is that knowledge of the underlying mechanisms may eventually enable us to develop strategies to correct neuronal alterations described in several diseases.

Expression of Ret-, p75(NTR)-, Phox2a-, Phox2b-, and tyrosine hydroxylase-immunoreactivity by undifferentiated neural crest-derived cells and different classes of enteric neurons in the embryonic mouse gut

Cells of the enteric nervous system are derived from the neural crest. Probes to a number of molecules identify neural crest-derived cells within the gastrointestinal tract of embryonic mice prior to their differentiation into neurons and glial cells. However, it is unclear whether the different markers are identifying all neural crest-derived cells. In this study the distribution of p75(NTR)-immunoreactivity was compared with that of Ret-, Phox2a-, Phox2b-, and tyrosine hydroxylase (TH) in undifferentiated neural crest-derived cells in the E10.5-E13.5 mouse intestine. Neural crest-derived cells colonise the embryonic mouse gut in a rostral-to-caudal wave between E9.5-E14, and differentiation into enteric neurons also occurs in a rostral-to-caudal wave. Thus, the most caudal neural crest-derived cells within the gut are undifferentiated. These most caudal neural crest-derived cells co-expressed p75(NTR)-, Phox2b- and Ret-immunoreactivity; at E10.5 a sub-population was also TH-positive. The most caudal cells did not show Phox2a-immunoreactivity at any stage. However, a sub-population of cells, which was rostral to the undifferentiated neural crest-derived cells, was Phox2a-positive, and these are likely to be cells beginning to differentiate along a neuronal lineage. The expression of Ret-, Phox2a-, Phox2b- and p75(NTR)-immunoreactivity by two classes of enteric neurons that differentiate prior to birth was also examined. Nitric oxide synthase (NOS) neurons showed Phox2b and Ret immunoreactivity at all ages, and Phox2a and p75(NTR) immunoreactivity only transiently. Calcitonin gene-related peptide (CGRP) neurons showed Phox2b and Ret-immunoreactivity, but not Phox2a immunoreactivity. It is concluded that all undifferentiated neural crest-derived cells initially express Phox2b, Ret, and p75(NTR); a sub-population of these cells also expresses TH transiently. Those cells that are beginning to differentiate along a neuronal lineage maintain their expression of Phox2b and Ret, and they start to express Phox2a, but down-regulate p75(NTR); those cells that differentiate along a glial lineage down-regulate Ret and maintain their expression of p75(NTR). Dev Dyn 1999;216:137-152.