Promotion of the development of enteric neurons and glia by neuropoietic cytokines: Interactions with neurotrophin-3

BAP1 is required prenatally for differentiation and maintenance of postnatal murine enteric nervous system

Epigenetic regulatory mechanisms are underappreciated, yet are critical for enteric nervous system (ENS) development and maintenance. We discovered that fetal loss of the epigenetic regulator Bap1 in the ENS lineage caused severe postnatal bowel dysfunction and early death in Tyrosinase-Cre Bap1fl/fl mice. Bap1-depleted ENS appeared normal in neonates; however, by P15, Bap1-deficient enteric neurons were largely absent from the small and large intestine of Tyrosinase-Cre Bap1fl/fl mice. Bowel motility became markedly abnormal with disproportionate loss of cholinergic neurons. Single-cell RNA sequencing at P5 showed that fetal Bap1 loss in Tyrosinase-Cre Bap1fl/fl mice markedly altered the composition and relative proportions of enteric neuron subtypes. In contrast, postnatal deletion of Bap1 did not cause enteric neuron loss or impaired bowel motility. These findings suggest that BAP1 is critical for postnatal enteric neuron differentiation and for early enteric neuron survival, a finding that may be relevant to the recently described human BAP1-associated neurodevelopmental disorder.

Complement factor B is essential for the proper function of the peripheral auditory system

Sensorineural hearing loss is associated with dysfunction of cochlear cells. Although immune cells play a critical role in maintaining the inner ear microenvironment, the precise immune-related molecular mechanisms underlying the pathophysiology of hearing loss remain unclear. The complement cascade contributes to the regulation of immune cell activity. Additionally, activation of the complement cascade can lead to the cellular opsonization of cells and pathogens, resulting in their engulfment and elimination by phagocytes. Complement factor B (fB) is an essential activator protein in the alternative complement pathway, and variations in the fB gene are associated with age-related macular degeneration. Here we show that mice of both sexes deficient in fB functional alleles (fB-/-) demonstrate progressive hearing impairment. Transcriptomic analysis of auditory nerves from adult mice detected 706 genes that were significantly differentially expressed between fB-/- and wild-type control animals, including genes related to the extracellular matrix and neural development processes. Additionally, a subset of differentially expressed genes was related to myelin function and neural crest development. Histological and immunohistochemical investigations revealed pathological alterations in auditory nerve myelin sheathes of fB-/- mice. Pathological alterations were also seen in the stria vascularis of the cochlear lateral wall in these mice. Our results implicate fB as an integral regulator of myelin maintenance and stria vascularis integrity, underscoring the importance of understanding the involvement of immune signaling pathways in sensorineural hearing loss.

Enteric Nervous System in Neonatal Necrotizing Enterocolitis

BACKGROUND

The pathophysiology of necrotizing enterocolitis (NEC) is not clear, but increasing information suggests that the risk and severity of NEC may be influenced by abnormalities in the enteric nervous system (ENS).

OBJECTIVE

The purpose of this review was to scope and examine the research related to ENS-associated abnormalities that have either been identified in NEC or have been noted in other inflammatory bowel disorders (IBDs) with histopathological abnormalities similar to NEC. The aim was to summarize the research findings, identify research gaps in existing literature, and disseminate them to key knowledge end-users to collaborate and address the same in future studies.

METHODS

Articles that met the objectives of the study were identified through an extensive literature search in the databases PubMed, EMBASE, and Scopus.

RESULTS

The sources identified through the literature search revealed that: (1) ENS may be involved in NEC development and post-NEC complications, (2) NEC development is associated with changes in the ENS, and (3) NEC-associated changes could be modulated by the ENS.

CONCLUSION

The findings from this review identify the enteric nervous as a target in the development and progression of NEC. Thus, factors that can protect the ENS can potentially prevent and treat NEC and post-NEC complications. This review serves to summarize the existing literature and highlights a need for further research on the involvement of ENS in NEC.

Development of the Autonomic Nervous System: Clinical Implications

Investigations of the cellular and molecular mechanisms that mediate the development of the autonomic nervous system have identified critical genes and signaling pathways that, when disrupted, cause disorders of the autonomic nervous system. This review summarizes our current understanding of how the autonomic nervous system emerges from the organized spatial and temporal patterning of precursor cell migration, proliferation, communication, and differentiation, and discusses potential clinical implications for developmental disorders of the autonomic nervous system, including familial dysautonomia, Hirschsprung disease, Rett syndrome, and congenital central hypoventilation syndrome.

Cerebral dopamine neurotrophic factor is essential for enteric neuronal development, maintenance, and regulation of gastrointestinal transit

Cerebral dopamine neurotrophic factor (CDNF) is expressed in the brain and is neuroprotective. We have previously shown that CDNF is also expressed in the bowel and that its absence leads to degeneration and autophagy in the enteric nervous system (ENS), particularly in the submucosal plexus. We now demonstrate that enteric CDNF immunoreactivity is restricted to neurons (submucosal > myenteric) and is not seen in glia, interstitial cells of Cajal, or smooth muscle. Expression of CDNF, moreover, is essential for the normal development and survival of enteric dopaminergic neurons; thus, expression of the dopaminergic neuronal markers, dopamine, tyrosine hydroxylase, and dopamine transporter are deficient in the ileum of Cdnf ^-/- mice. The normal age-related decline in proportions of submucosal dopaminergic neurons is exacerbated in Cdnf ^-/- animals. The defect in Cdnf ^-/- animals is not dopamine-restricted; proportions of other submucosal neurons (NOS-, GABA-, and CGRP-expressing), are also deficient. The deficits in submucosal neurons are reflected functionally in delayed gastric emptying, slowed colonic motility, and prolonged total gastrointestinal transit. CDNF is expressed selectively in isolated enteric neural crest-derived cells (ENCDC), which also express the dopamine-related transcription factor Foxa2. Addition of CDNF to ENCDC promotes development of dopaminergic neurons; moreover, survival of these neurons becomes CDNF-dependent after exposure to bone morphogenetic protein 4. The effects of neither glial cell-derived neurotrophic factor (GDNF) nor serotonin are additive with CDNF. We suggest that CDNF plays a critical role in development and long-term maintenance of dopaminergic and other sets of submucosal neurons.

BMP2 Is Related to Hirschsprung's Disease and Required for Enteric Nervous System Development

The enteric nervous system (ENS) is derived from neural crest cells (NCCs). Defects in ENS NCCs colonizing in the intestines lead to an absence of enteric ganglia in the colon and results in Hirschsprung’s disease (HSCR). Bone morphogenetic proteins (BMPs) play diverse roles in the proliferation, migration and survival of ENS NCCs; however, whether BMPs are involved in HSCR and the underlying mechanism remains largely unknown. In this study, we found that BMP2 expression is significantly decreased in HSCR patients. Further experiments demonstrated that BMP2 is involved in the regulation of NCC proliferation, migration and differentiation. In a detailed analysis of the role of BMP2 in HSCR development in vivo, we demonstrated that BMP2b regulates the proliferation, migration and differentiation of vagal NCCs in zebrafish and that BMP2b is required for intestinal smooth muscle development. In addition, we showed that BMP2b is involved in regulating the expression of glial cell line-derived neurotrophic factor (GDNF) in the intestine, which mediates the regulation of ENS development by BMP2b in zebrafish. These results highlight a central role of the BMP-GDNF cascade in intestinal patterning and ENS development. Our results further demonstrate the key role of BMP2 in the etiology of HSCR in vitro and in vivo.

Activation of Wnt Signaling Increases Numbers of Enteric Neurons Derived From Neonatal Mouse and Human Progenitor Cells

BACKGROUND & AIMS

Neural stem and progenitor cells from the enteric nervous system (ENS) might serve as a source of cells for treatment of neurogastrointestinal disorders. Before we can use these cells, we must increase our understanding of the signaling mechanisms that regulate proliferation and differentiation. We systematically evaluated the effects of canonical Wnt signaling on proliferation and differentiation of cultured ENS progenitor cells from neonatal mice and humans.

METHODS

We isolated ENS progenitors from tunica muscularis of the small intestine of newborn (postnatal day 0) wild-type C57BL/6 mice as well as from Wnt1-Cre2 reporter mice. We also obtained intestinal tissue samples from infants (2 and 7 months old) undergoing surgery for imperforate anus or focal intestinal perforation and isolated ENS cells. ENS cells were cultured under proliferation conditions leading to formation of 3-dimensional spheres, which we activated with Wnt3a and SB216763 in order to activate the β-catenin-dependent canonical Wnt pathway. We used immunoblot and quantitative polymerase chain reaction to evaluate the molecular response to Wnt stimuli and immunohistochemistry, proliferation, and cell death assays to identify new neurons.

RESULTS

In proliferating enterospheres derived from ENS progenitor cells, we verified the expression of Wnt receptors frizzled 1-10 and the co-receptors low-density lipoprotein receptor-related proteins 5 and 6. Pharmacologic stimulation with Wnt agonists led to intracellular accumulation of Wnt-dependent β-catenin and up-regulated expression of known Wnt target genes axin2, lef1, and lgr5. Activation of the canonical Wnt pathway promoted growth of ENS cell spheres during cell expansion and increased the number of newborn neurons derived from mouse and human progenitor cells.

CONCLUSIONS

In studies of human and mouse ENS progenitors, we found activation of the Wnt signaling pathway to promote neurogenesis of the ENS in vitro. The neurogenic effect of Wnt agonists on ENS progenitors supports their use in generation of cell pools for autologous cell replacement therapies.

Genetic variants of IL-11 associated with risk of Hirschsprung disease

BACKGROUND

Hirschsprung disease (HSCR) is a congenital and heterogeneous disorder characterized by the absence of enteric ganglia during enteric nervous system (ENS) development. Our recent genome-wide association study has identified a variant (rs6509940) of interleukin-11 (IL-11) as a potential susceptible locus for HSCR. As interleukins play important roles in the ENS, we further studied associations with HSCR of nine common single nucleotide polymorphisms (SNPs) on IL-11.

METHODS

Biopsy specimens or surgical materials from all patients that were tested for histological examination based on the absence of the enteric ganglia were collected. A total of nine SNPs on IL-11 were genotyped in 187 HSCR patients and 283 unaffected controls using TaqMan genotyping assay.

KEY RESULTS

Combined analysis revealed that several SNPs (minimum p = 1.57 × 10(-7) ) showed statistically significant associations with HSCR, even after Bonferroni correction (pcorr  = 1.73 × 10(-6) for the SNP). Moreover, the most common haplotype was strongly associated with HSCR (pcorr  = 2.20 × 10(-6) ). In further analysis among three HSCR subtypes (short segment, S-HSCR; long segment, L-HSCR; total colonic aganglionosis, TCA) based on the extent of aganglionic segment, the result showed a different association pattern depending on the subtypes (minimum pcorr  = 6.12 × 10(-5) for rs6509940 in S-HSCR; but no significant SNP in L-HSCR and TCA).

CONCLUSIONS & INFERENCES

Although further replication in a larger cohort and functional evaluations are needed, our findings suggest that genetic variations of IL-11 may be associated with the risk of HSCR and/or the mechanisms related to ENS development.

Neuroenteric Staining as a Tool in the Evaluation of Pediatric Motility Disorders

The diagnosis of enteric neuromuscular disorders has come a long way since the first description of an enteric neuropathic disorder by the Danish physician Harald Hirschsprung in 1886. Advances in specialized enteric histopathological staining techniques have made it possible to identify subtle neuropathies and myopathies that cause intestinal motility disorders, from the common and now better understood and relatively easily diagnosed Hirschsprung's disease to the less common and more severe and not well-characterized chronic idiopathic intestinal pseudoobstruction, which continues to present a diagnostic challenge to the gastroenterologist and histopathologist alike. This article will discuss the common gastrointestinal motility disorders and some of the specialized histological stains, such as the relatively common enzyme stain, acetylcholinesterase, used to diagnose Hirschsprung's disease; advanced tinctorial stains, such as Masson trichrome, which may aid in diagnosis of enteric myopathies causing pseudoobstruction; and immunohistochemical stains such as C-Kit or PG 9.5, which may aid in the diagnosis of enteric neuropathies causing pseudoobstruction.

Anti-inflammatory activity of Wnt signaling in enteric nervous system: in vitro preliminary evidences in rat primary cultures

Background

In the last years, Wnt signaling was demonstrated to regulate inflammatory processes. In particular, an increased expression of Wnts and Frizzled receptors was reported in inflammatory bowel disease (IBD) and ulcerative colitis to exert both anti- and pro-inflammatory functions regulating the intestinal activated nuclear factor κB (NF-кB), TNFa release, and IL10 expression.

Methods

To investigate the role of Wnt pathway in the response of the enteric nervous system (ENS) to inflammation, neurons and glial cells from rat myenteric plexus were treated with exogenous Wnt3a and/or LPS with or without supporting neurotrophic factors such as basic fibroblast growth factor (bFGF), epithelial growth factor (EGF), and glial cell-derived neurotrophic factor (GDNF). The immunophenotypical characterization by flow cytometry and the protein and gene expression analysis by qPCR and Western blotting were carried out.

Results

Flow cytometry and immunofluorescence staining evidenced that enteric neurons coexpressed Frizzled 9 and toll-like receptor 4 (TLR4) while glial cells were immunoreactive to TLR4 and Wnt3a suggesting that canonical Wnt signaling is active in ENS.

Under in vitro LPS treatment, Western blot analysis demonstrated an active cross talk between canonical Wnt signaling and NF-кB pathway that is essential to negatively control enteric neuronal response to inflammatory stimuli. Upon costimulation with LPS and Wnt3a, a significant anti-inflammatory activity was detected by RT-PCR based on an increased IL10 expression and a downregulation of pro-inflammatory cytokines TNFa, IL1B, and interleukin 6 (IL6). When the availability of neurotrophic factors in ENS cultures was abolished, a changed cell reactivity by Wnt signaling was observed at basal conditions and after LPS treatment.

Conclusions

The results of this study suggested the existence of neuronal surveillance through FZD9 and Wnt3a in enteric myenteric plexus. Moreover, experimental evidences were provided to clarify the correlation among soluble trophic factors, Wnt signaling, and anti-inflammatory protection of ENS.

Site-specific gene expression and localization of growth factor ligand receptors RET, GFRα1 and GFRα2 in human adult colon

Two of the glial-cell-line-derived neurotrophic factor (GDNF) family ligands (GFLs), namely GDNF and neurturin (NRTN), are essential neurotropic factors for enteric nerve cells. Signal transduction is mediated by a receptor complex composed of GDNF family receptor alpha 1 (GFRα1) for GDNF or GFRα2 for NRTN, together with the tyrosine kinase receptor RET (rearranged during transfection). As both factors and their receptors are crucial for enteric neuron survival, we assess the site-specific gene expression of these GFLs and their corresponding receptors in human adult colon. Full-thickness colonic specimens were obtained after partial colectomy for non-obstructing colorectal carcinoma. Samples were processed for immunohistochemistry and co-localization studies. Site-specific gene expression was determined by real-time quantitative polymerase chain reaction in enteric ganglia and in circular and longitudinal muscle harvested by microdissection. Protein expression of the receptors was mainly localized in the myenteric and submucosal plexus. Dual-label immunohistochemistry with PGP 9.5 as a pan-neuronal marker detected immunoreactivity of the receptors in neuronal somata and ganglionic neuropil. RET immunoreactivity co-localized with neuronal GFRα1 and GFRα2 signals. The dominant source of receptor mRNA expression was in myenteric ganglia, whereas both GFLs showed higher expression in smooth muscle layers. The distribution and expression pattern of GDNF and NRTN and their corresponding receptors in the human adult enteric nervous system indicate a role of both GFLs not only in development but also in the maintenance of neurons in adulthood. The data also provide a basis for the assessment of disturbed signaling components of the GDNF and NRTN system in enteric neuropathies underlying disorders of gastrointestinal motility.

Pleiotropic effects of the bone morphogenetic proteins on development of the enteric nervous system

Formation of the enteric nervous system (ENS) from migratory neural crest-derived cells that colonize the primordial gut involves a complex interplay among different signaling molecules. The bone morphogenetic proteins (BMPs), specifically BMP2 and BMP4, play a particularly important role in virtually every stage of gut and ENS development. BMP signaling helps to pattern both the anterior-posterior axis and the radial axis of the gut prior to colonization by migratory crest progenitor cells. BMP signaling then helps regulate the migration of enteric neural crest-derived precursors as they colonize the fetal gut and form ganglia. BMP2 and -4 promote differentiation of enteric neurons in early fetal ENS development and glia at later stages. A major role for BMP signaling in the ENS is regulation of responses to other growth factors. Thus BMP signaling first regulates neurogenesis by modulating responses to GDNF and later gliogenesis through its effects on GGF-2 responses. Furthermore, BMPs promote growth factor dependency for survival of ENS neurons (on NT-3) and glia (on GGF-2) by inducing TrkC (neurons) and ErbB3 (glia). BMP signaling limits total neuron numbers, favoring the differentiation of later born neuronal phenotypes at the expense of earlier born ones thus influencing the neuronal composition of the ENS and the glia/neuron ratio. BMP2 and -4 also promote gangliogenesis via modification of neural cell adhesion molecules and promote differentiation of the circular and then longitudinal smooth muscles. Disruption of BMP signaling leads to defects in the gut and in ENS function commensurate with these complex developmental roles.

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.

Bone morphogenetic proteins regulate enteric gliogenesis by modulating ErbB3 signaling

The neural crest-derived cell population that colonizes the bowel (ENCDC) contains proliferating neural/glial progenitors. We tested the hypothesis that bone morphogenetic proteins (BMPs 2 and 4), which are known to promote enteric neuronal differentiation at the expense of proliferation, function similarly in gliogenesis. Enteric gliogenesis was analyzed in mice that overexpress the BMP antagonist, noggin, or BMP4 in the primordial ENS. Noggin-induced loss-of-function decreased, while BMP4-induced gain-of-function increased the glial density and glia/neuron ratio. When added to immunoisolated ENCDC, BMPs provoked nuclear translocation of phosphorylated SMAD proteins and enhanced both glial differentiation and expression of the neuregulin receptor ErbB3. ErbB3 transcripts were detected in E12 rat gut, before glial markers are expressed; moreover, expression of the ErbB3 ligand, glial growth factor 2 (GGF2) escalated rapidly after its first detection at E14. ErbB3-immunoreactive cells were located in the ENS of fetal and adult mice. GGF2 stimulated gliogenesis and proliferation and inhibited glial cell derived neurotrophic factor (GDNF)-promoted neurogenesis. Enhanced glial apoptosis occurred following GGF2 withdrawal; BMPs intensified this GGF2-dependence and reduced GGF2-stimulated proliferation. These observations support the hypotheses that BMPs are required for enteric gliogenesis and act by promoting responsiveness of ENCDC to ErbB3 ligands such as GGF2.

Bone morphogenetic protein regulation of enteric neuronal phenotypic diversity: relationship to timing of cell cycle exit

The effects of bone morphogenetic protein (BMP) signaling on enteric neuron development were examined in transgenic mice overexpressing either the BMP inhibitor, noggin, or BMP4 under control of the neuron specific enolase (NSE) promoter. Noggin antagonism of BMP signaling increased total numbers of enteric neurons and those of subpopulations derived from precursors that exit the cell cycle early in neurogenesis (serotonin, calretinin, calbindin). In contrast, noggin overexpression decreased numbers of neurons derived from precursors that exit the cell cycle late (gamma-aminobutyric acid, tyrosine hydroxylase [TH], dopamine transporter, calcitonin gene-related peptide, TrkC). The numbers of TH- and TrkC-expressing neurons were increased by overexpression of BMP4. These observations are consistent with the idea that phenotypic expression in the enteric nervous system (ENS) is determined, in part, by the number of proliferative divisions neuronal precursors undergo before their terminal mitosis. BMP signaling may thus regulate enteric neuronal phenotypic diversity by promoting the exit of precursors from the cell cycle. BMP2 increased the numbers of TH- and TrkC-expressing neurons developing in vitro from immunoselected enteric crest-derived precursors; BMP signaling may thus also specify or promote the development of dopaminergic TrkC/NT-3-dependent neurons. The developmental defects in the ENS of noggin-overexpressing mice caused a relatively mild disturbance of motility (irregular rapid transit and increased stool frequency, weight, and water content). Although the function of the gut thus displays a remarkable tolerance for ENS defects, subtle functional abnormalities in motility or secretion may arise when ENS defects short of aganglionosis occur during development.

Expression and developmental regulation of oxytocin (OT) and oxytocin receptors (OTR) in the enteric nervous system (ENS) and intestinal epithelium

Although oxytocin (OT) and oxytocin receptor (OTR) are known for roles in parturition and milk let-down, they are not hypothalamus-restricted. OT is important in nurturing and opposition to stress. Transcripts encoding OT and OTR have been reported in adult human gut, and OT affects intestinal motility. We tested the hypotheses that OT is endogenous to the enteric nervous system (ENS) and that OTR signaling may participate in enteric neurophysiology. Reverse transcriptase polymerase chain reaction confirmed OT and OTR transcripts in adult mouse and rat gut and in precursors of enteric neurons immunoselected from fetal rats. Enteric OT and OTR expression continued through adulthood but was developmentally regulated, peaking at postnatal day 7. Coincidence of the immunoreactivities of OTR and the neural marker Hu was 100% in the P3 and 71% in the adult myenteric plexus, when submucosal neurons were also OTR-immunoreactive. Co-localization with NeuN established that intrinsic primary afferent neurons are OTR-expressing. Because OTR transcripts and protein were detected in the nodose ganglia, OT signaling might also affect extrinsic primary afferent neurons. Although OT immunoreactivity was found only in approximately 1% of myenteric neurons, extensive OT-immunoreactive varicosities surrounded many others. Villus enterocytes were OTR-immunoreactive through postnatal day 17; however, by postnatal day 19, immunoreactivity waned to become restricted to crypts and concentrated at crypt-villus junctions. Immunoelectron microscopy revealed plasmalemmal OTR at enterocyte adherens junctions. We suggest that OT and OTR signaling might be important in ENS development and function and might play roles in visceral sensory perception and neural modulation of epithelial biology.

Endoderm-derived Sonic hedgehog and mesoderm Hand2 expression are required for enteric nervous system development in zebrafish

The zebrafish enteric nervous system (ENS), like those of all other vertebrate species, is principally derived from the vagal neural crest cells (NCC). The developmental controls that govern the migration, proliferation and patterning of the ENS precursors are not well understood. We have investigated the roles of endoderm and Sonic hedgehog (SHH) in the development of the ENS. We show that endoderm is required for the migration of ENS NCC from the vagal region to the anterior end of the intestine. We show that the expression of shh and its receptor ptc-1 correlate with the development of the ENS and demonstrate that hedgehog (HH) signaling is required in two phases, a pre-enteric and an enteric phase, for normal ENS development. We show that HH signaling regulates the proliferation of vagal NCC and ENS precursors in vivo. We also show the zebrafish hand2 is required for the normal development of the intestinal smooth muscle and the ENS. Furthermore we show that endoderm and HH signaling, but not hand2, regulate gdnf expression in the intestine, highlighting a central role of endoderm and SHH in patterning the intestine and the ENS.

A model of lytic, latent, and reactivating varicella-zoster virus infections in isolated enteric neurons

Because human primary afferent neurons are not readily obtained, we sought to develop a model in which the lytic, latent, and reactivating phases of varicella-zoster virus (VZV) infection were recapitulated in neurons from an animal source. Enteric neurons were obtained from the small intestine of adult guinea pigs and from the bowel of fetal mice. Latency was established when these neurons were infected by cell-free VZV in the absence of fibroblasts or other cells of mesodermal origin. In contrast, lytic infection ensued when fibroblasts were present or when the enteric neurons were infected by cell-associated VZV. Latency was associated with the expression of a limited subset of viral genes, the products of which were restricted to the cytoplasm. Lysis was associated with the expression of viral glycoproteins, nuclear translocation of latency-associated gene products, and rapid cell death. Reactivation was accomplished by expressing VZV open reading frame (ORF) 61p or herpes simplex virus ICP0 in latently infected neurons. Isolated enteric neurons from guinea pigs and mice recapitulate latent gene expression in human cranial nerve and dorsal root ganglia. Expression of latency-associated VZV gene products was detected in 88% of samples of adult human intestine, suggesting that VZV not only infects enteric neurons but also is latent in the human enteric nervous system. This in vitro model should facilitate further understanding of latency and reactivation of VZV.

Neural progenitors from isolated postnatal rat myenteric ganglia: expansion as neurospheres and differentiation in vitro

Identification of the stem cell niche is crucial for understanding the factors that regulate these cells. Rodent enteric neural crest-derived stem cells have previously been isolated by flow cytometry and culture of cell suspensions from the outer smooth muscle layers or the entire gut wall from postnatal and adult animals. Such cell suspensions contain a mixture of cell types, including smooth muscle, fibroblasts and cells associated with the vasculature and extrinsic innervation. Thus these preparations may be contaminated by stem cells associated with extrinsic sensory and autonomic nerves and by other types of stem cell that reside in the gut. Here we describe a different approach, similar to that recently used for infant human gut, to obtain enteric ganglion-derived cells, with properties of neural progenitor cells, using isolated myenteric ganglia from postnatal rat ileum. Myenteric ganglia were separated from the gut wall, dispersed and resulting cell dissociates were plated in non-adherent culture conditions with EGF and FGF-2. Under these conditions neurosphere-like bodies (NLB) developed. Cells in NLB incorporated BrdU and expressed the stem cell marker nestin but not the pan-neuronal marker PGP 9.5. Upon growth factor withdrawal some BrdU-immunopositive cells assumed the morphology of neurons and expressed PGP 9.5; others were flattened and expressed the glial cell marker GFAP. This work therefore provides evidence that neural crest-derived progenitors in the postnatal rat gut are located in the myenteric plexus, and shows that these cells can be expanded and differentiated in NLB in vitro.

Effect of Gdnf haploinsufficiency on rate of migration and number of enteric neural crest-derived cells

The enteric nervous system arises predominantly from vagal level neural crest cells that migrate into the foregut and then colonize the entire length of the gastrointestinal tract. Previous studies have demonstrated that glial cell line-derived neurotrophic factor (GDNF) promotes the migration of enteric neural crest-derived cells (ENCs) in vitro, but a role for GDNF in the migration of ENCs in vivo has yet to be demonstrated. In this study, the effects of Gdnf haploinsufficiency on ENC rate of migration and number during mid embryonic development were examined. Although the entire gut of embryonic Gdnf(+/-) mice was colonized, a significant delay in the migration of ENCs along the embryonic hindgut was found. However, significant effects of Gdnf haploinsufficiency on ENC number were detected before the stage at which migration defects were first evident. As previous studies have shown a relationship between ENC number and migration, the effects of Gdnf haploinsufficiency on migration may be due to an indirect effect on cell number and/or a direct effect of GDNF on ENC migration. Gdnf haploinsufficiency did not cause any detectable change in the rate of neuronal differentiation of ENCs.

lessen encodes a zebrafish trap100 required for enteric nervous system development

The zebrafish enteric nervous system (ENS), like those of all other vertebrate species, is principally derived from the vagal neural crest. The developmental controls that govern the specification and patterning of the ENS are not well understood. To identify genes required for the formation of the vertebrate ENS, we preformed a genetic screen in zebrafish. We isolated the lessen (lsn) mutation that has a significant reduction in the number of ENS neurons as well as defects in other cranial neural crest derived structures. We show that the lsn gene encodes a zebrafish orthologue of Trap100, one of the subunits of the TRAP/mediator transcriptional regulation complex. A point mutation in trap100 causes a premature stop codon that truncates the protein, causing a loss of function. Antisense-mediated knockdown of trap100 causes an identical phenotype to lsn. During development trap100 is expressed in a dynamic tissue-specific expression pattern consistent with its function in ENS and jaw cartilage development. Analysis of neural crest markers revealed that the initial specification and migration of the neural crest is unaffected in lsn mutants. Phosphohistone H3 immunocytochemistry revealed that there is a significant reduction in proliferation of ENS precursors in lsn mutants. Using cell transplantation studies, we demonstrate that lsn/trap100 acts cell autonomously in the pharyngeal mesendoderm and influences the development of neural crest derived cartilages secondarily. Furthermore, we show that endoderm is essential for ENS development. These studies demonstrate that lsn/trap100 is not required for initial steps of cranial neural crest development and migration, but is essential for later proliferation of ENS precursors in the intestine.

Gangliogenesis in the enteric nervous system: Roles of the polysialylation of the neural cell adhesion molecule and its regulation by bone morphogenetic protein-4

The neural crest-derived cells that colonize the fetal bowel become patterned into two ganglionated plexuses. The hypothesis that bone morphogenetic proteins (BMPs) promote ganglionation by regulating neural cell adhesion molecule (NCAM) polysialylation was tested. Transcripts encoding the sialyltransferases, ST8Sia IV (PST) and ST8Sia II (STX), which polysialylate NCAM, were detectable in fetal rat gut by E12 but were downregulated postnatally. PSA-NCAM-immunoreactive neuron numbers, but not those of NCAM, were developmentally regulated similarly. Circular smooth muscle was transiently (E16-20) PSA-NCAM-immunoreactive when it is traversed by migrating precursors of submucosal neurons. Neurons developing in vitro from crest-derived cells immunoselected at E12 with antibodies to p75(NTR) expressed NCAM and PSA-NCAM. BMP-4 promoted neuronal NCAM polysialylation and clustering. N-butanoylmannosamine, which blocks NCAM polysialylation, but not N-propanoylmannosamine, which does not, interfered with BMP-4-induced neuronal clustering. Observations suggest that BMP signaling enhances NCAM polysialylation, which allows precursors to migrate and form ganglionic aggregates during the remodeling of the developing ENS.

Evidence for the involvement of neurotrophins in muscle transdifferentiation and acetylcholine receptor transformation in the esophagus ofMyf5−/−:MyoD−/−andNT-3−/−embryos

The primary aim of our study was to determine whether the esophageal innervation (i.e., vagal and enteric) and the skeletal muscle-secreted neurotrophins have a role in smooth-to-skeletal muscle transdifferentiation and in the muscarinic-to-nicotinic acetylcholine receptor type transition. To that end, we used genetically engineered embryos and immunohistochemistry. We found that, in the absence of Myf5 and MyoD, the esophageal muscle cells failed to develop the striated phenotype of acetylcholine receptors. In addition, the development of vagal and enteric innervation was delayed in Myf5(-/-):MyoD(-/-) and NT-3(-/-) mutants, but it was reestablished 2 days before the end of gestation. The smooth muscle cells in the esophagus appeared to be a distinct subpopulation of cells and their ability to transdifferentiate was based on their competence to express neurotrophins and their receptors. Finally, our data suggest a role for NT-3 in the esophageal muscle transdifferentiation.

Neurotrophin-3 in the development of the enteric nervous system

To date, the only neurotrophin that has been shown to influence the development of the enteric nervous system (ENS) is neurotrophin-3 (NT-3). NT-3 plays an essential role in the development of both the neural-crest-derived peripheral nervous system and the central nervous system (i.e., Chalazonitis, 1996, Mol. Neurobiol., 12: 39-53; Sieber-Blum, 1999, Neurotrophins and the Neural Crest, CRC Press, Boca Raton). This review integrates data obtained from our laboratory and from our collaboration with other investigators that demonstrate a late-acting role for NT-3 in the development of enteric neurons in vitro and in vivo. Studies of the biological actions of NT-3 on enteric neuronal precursors in vitro demonstrate that NT-3 acts directly on the precursor cells and that it also acts in combination with other neurotrophic factors such as glial cell line-derived neurotrophic factor and a ciliary neurotrophic factor-like molecule, to promote the survival and differentiation of enteric neurons and glia. Importantly, bone morphogenetic protein-2 (BMP-2) and BMP-4, members of the transforming growth factor-beta (TGF-beta) superfamily, regulate the onset of action of NT-3 during fetal gut development. Analyzes performed on mice deficient in the genes encoding NT-3 or its transducing tyrosine kinase receptor, TrkC, and conversely on transgenic mice that overexpress NT-3 substantiate a physiological role for NT-3 in the development and maintenance of a subset of enteric neurons. There is loss of neurons in both the myenteric and submucosal plexuses of mice lacking NT-3/TrkC signaling and selective hyperplasia in the myenteric plexus of mice overexpressing NT-3. Analyzes performed on transgenic mice that overexpress noggin, a specific BMP-4 antagonist, show significant decreases in the density of TrkC-expressing neurons but significant increase in overall neuronal density of both plexuses. Conversely, overexpression of BMP-4 is sufficient to produce, an increase in the proportion of TrkC-expressing neurons in both plexuses. Overall, our data point to a regulatory role of BMP-4 in the responses of subsets of myenteric and submucosal neurons to NT-3. NT-3 is required for the differentiation, maintenance and proper physiological function of late-developing enteric neurons that are important for the control of gut peristalsis.

Increased expression of vasoactive intestinal polypeptide in cultured myenteric neurons from adult rat small intestine

Adult neurons possess the ability to adapt to a changing environment. Loss of target-derived neurotrophic factors due to axotomy or isolation by culturing is known to induce changes in neuropeptide expression in several types of peripheral neurons. The aim of the present study was to investigate changes in the expression of vasoactive intestinal polypeptide (VIP) and nitric oxide synthase (NOS) in cultured myenteric ganglia and dissociated neurons. Myenteric ganglia and neurons from rat small intestine were dissociated and cultured for up to 21 days. Immunocytochemistry was used to determine the total number of neurons and the proportions of subpopulations containing VIP or NOS or both in preparations of whole mounts (controls used to determine the conditions in vivo), myenteric ganglion culture and dissociated myenteric neuronal culture. In situ hybridization was used to determine changes in the expressions of NOS and VIP mRNA. The relative number of VIP-expressing neurons increased significantly during culturing. The percentage of all neurons expressing VIP was 3.6+/-0.3% in whole mounts, 22-24% in cultured myenteric ganglia, and up to 35% in cultured dissociated neurons. NOS-expressing neurons constituted approximately 30-40% of all neurons in whole mounts as well as in cultured ganglia or dissociated neurons. A dramatic increase in NOS/VIP-containing neurons were detected in cultured neurons irrespective of whether they were arranged in ganglia or dissociated, as compared to whole mount preparations. This suggests that the NOS-containing neurons are the ones that increase their VIP expression. The induced expression of VIP in cultured adult myenteric neurons indicates that VIP is important for neuronal adaptation, maintenance and survival.

GDNF availability determines enteric neuron number by controlling precursor proliferation

To clarify the role of Ret signaling components in enteric nervous system (ENS) development, we evaluated ENS anatomy and intestinal contractility in mice heterozygous for Ret, GFRalpha1 and Ret ligands. These analyses demonstrate that glial cell line-derived neurotrophic factor (GDNF) and neurturin are important for different aspects of ENS development. Neurturin is essential for maintaining the size of mature enteric neurons and the extent of neuronal projections, but does not influence enteric neuron number. GDNF availability determines enteric neuron number by controlling ENS precursor proliferation. However, we were unable to find evidence of programmed cell death in the wild type ENS by immunohistochemistry for activated caspase 3. In addition, enteric neuron number is normal in Bax(-/-) and Bid(-/-) mice, suggesting that, in contrast to most of the rest of the nervous system, programmed cell death is not important for determining enteric neuron numbers. Only mild reductions in neuron size and neuronal fiber counts occur in Ret(+/-) and Gfra1(+/-) mice. All of these heterozygous mice, however, have striking problems with intestinal contractility and neurotransmitter release, demonstrating that Ret signaling is critical for both ENS structure and function.

Enteric nervous system: development and developmental disturbances--part 1

This review, which is presented in two parts, summarizes and synthesizes current views on the genetic, molecular, and cell biological underpinnings of the early embryonic phases of enteric nervous system (ENS) formation and its defects. In the first part, we describe the critical features of two principal abnormalities of ENS development: Hirschsprung's disease (HSCR) and intestinal neuronal dysplasia type B (INDB) in humans, and the similar abnormalities in animals. These represent the extremes of the diagnostic spectrum: HSCR has agreed and unequivocal diagnostic criteria, whereas the diagnosis and even existence of INDB as a clinical entity is highly controversial. The difficulties in diagnosis and treatment of both these conditions are discussed. We then review the genes now known which, when mutated or deleted, may cause defects of ENS development. Many of these genetic abnormalities in animal models give a phenotype similar or identical to HSCR, and were discovered by studies of humans and of mouse mutants with similar defects. The most important of these genes are those coding for molecules in the GDNF intercellular signaling system, and those coding for molecules in the ET-3 signaling system. However, a range of other genes for different signaling systems and for transcription factors also disturb ENS formation when they are deleted or mutated. In addition, a large proportion of HSCR cases have not been ascribed to the currently known genes, suggesting that additional genes for ENS development await discovery.

Development of the enteric nervous system

Development of the ENS requires the function of a diverse set of genes encoding transcription factors, signaling molecules, and their receptors. Mutations of these genes result in altered ENS function in animals and humans. In particular, such mutations have been shown to contribute to many cases of Hirschsprung's disease. Elucidation of the mechanisms of ENS development and function allow the development of new approaches to the diagnosis, therapy, and prevention of human disorders of gastrointestinal motility.

Effects of IL-1beta, IL-6 or LIF on rat sensory neurons co-cultured with fibroblast-like cells

Inflammation may affect the local presence of sensory nerve fibers in situ and inflammatory mediators influence sensory neurons in vitro. In the present study we have investigated effects of the cytokines interleukin-1beta (IL-1beta, interleukin-6 (IL-6), and leukemia inhibitory factor (LIF) on survival of and neurite growth from neonatal rat sensory neurons co-cultured with fibroblast-like cells prepared from neonatal rat skin (sFLCs) or perichondrium (pFLCs). The results showed that both FLC types expressed receptors for all three cytokines. Five ng/ml of either cytokine, but not lower or higher concentrations, supported survival of DRG neurons co-cultured with sFLCs. Neuronal survival was also enhanced by addition of the soluble IL-6 receptor (rsIL-6R) with or without IL-6. In co-cultures with pFLCs neuronal survival was promoted by IL-6, increasing with cytokine concentration. Addition of rsIL-6R without IL-6 did also stimulate neuronal survival. The growth of neurites from DRG neurons co-cultured with sFLCs was stimulated by 0.5 ng/ml LIF, unaffected by 5 ng/ml LIF and inhibited by 50 ng/ml LIF. Considering DRG neurons co-cultured with pFLCs, 50 ng/ml of either of the three cytokines, as well as rsIL-6R conditioned medium, stimulated neurite outgrowth. Some of the cytokine effects observed were reduced by application of antibodies against nerve growth factor (NGF). We conclude that that the cytokines examined affect DRG neurons in terms of survival or neuritogenesis, that the effects are influenced by cytokine concentration and the origin of the FLCs and that some of the effects are indirect, probably being mediated by factors released from FLCs.

Neuropathology of paediatric chronic intestinal pseudo-obstruction and related animal models

Chronic intestinal pseudo-obstruction (CIP) in paediatric patients is due to heterogeneous aetiologies that include primary disorders of the enteric nervous system. These conditions are poorly delineated by contemporary diagnostic approaches, in part because the complex nature of the enteric nervous system may shelter significant physiological defects behind subtle or quantitative anatomical changes. Until recently, relatively few experimental animal models existed for paediatric CIP. However, the availability of rodent models, particularly novel mutants created in the last few years by genetic manipulations, has brought unprecedented opportunities to investigate molecular, cellular, physiological, and histological details of enteric neuropathology. Information gleaned from studies of these animals is likely to change diagnostic and therapeutic approaches to paediatric CIP and related conditions.

Decreased tyrosine kinase C expression may reflect developmental abnormalities in Hirschsprung's disease and idiopathic slow-transit constipation

BACKGROUND

Some patients with Hirschsprung's disease have refractory constipation following excision of aganglionic bowel, as do patients with idiopathic slow-transit constipation (STC). Gut motility depends on enteric neuronal development in response to expression of trophic factors and their receptors. Recent studies indicate the importance of neurotrophin 3 (NT-3) and its high-affinity receptor tyrosine kinase C (trk C) in enteric neuronal development.

METHODS

Blinded quantitative immunohistochemical analysis of colon from patients with Hirschsprung's disease (aganglionic, hypoganglionic and normoganglionic) (n = 5), STC (n = 6) and appropriate age-matched control tissues (n = 5) was performed for NT-3 and trk C. Sural nerve morphometry and immunostaining were undertaken in three patients with STC who had abnormalities on limb autonomic and sensory testing.

RESULTS

A significantly higher proportion of submucous plexus neurones was trk C immunoreactive in control infant than adult colon (mean(s.e.m.) 73(9) versus 16(3) per cent of the total; P < 0.001), in accord with a role in development. The proportion of submucous plexus trk C-immunoreactive neurones was reduced in colon from patients with Hirschsprung's disease (28(7) per cent of total in normoganglionic Hirschsprung's disease; P < 0.007 versus infant controls) and STC (10(1) per cent of total; P = 0.053 versus adult controls). No abnormalities of STC sural nerves were detected by morphometry or immunostaining.

CONCLUSION

Decreased trk C expression may reflect developmental abnormalities in Hirschsprung's disease and idiopathic STC. Trk C activation by NT-3 or drugs may provide novel treatments. Presented in abstract form to the Pacific Association of Pediatric Surgeons, Las Vegas, Nevada, USA, May 2000

Hirschsprung's disease: a more generalised neuropathy?

BACKGROUND/PURPOSE

Aberrant neural crest migration is implicated in the pathogenesis of hindgut aganglionosis in Hirschsprung's disease (HSCR). Neural crest progenitors also give rise to a variety of cell types outside the gut, including nerves of the inner ear and peripheral nervous system. This study aims to determine whether such nerves are also affected in HSCR.

METHODS

Pure tone audiometry and oto-acoustic emission tests were performed on 53 patients with Hirschsprung's disease and 28 age-matched controls. Forty-two children with Hirschsprung's disease and 30 age-matched controls also underwent neurologic evaluation, including quantitative peripheral tests of sensory function.

RESULTS

Moderate to severe sensorineural hearing loss (1 profound [Waardenburg syndrome] and 3 high frequency) was detected in 4 children (8%) from the Hirschsprung's group. Abnormal (absent or reduced) oto-acoustic emission was detected in 77 of 102 ears (41 of 51 patients) in the HSCR group and 8 out of 48 ears (6 of 24 children) in the control group (75% v 17%; P<.0001). Two HSCR children had marked abnormalities on tests of peripheral nerve function. Significantly higher thresholds for vibration sensation were detected in HSCR versus controls (P<.05).

CONCLUSION

Hirschsprung's disease may have a more generalised neuropathy than just that involving the affected bowel.

Postmigratory enteric and sympathetic neural precursors share common, developmentally regulated, responses to BMP2

The development of enteric and sympathetic neurons from neural crest precursor cells is regulated by signals produced by the embryonic environments to which the cells migrate. Bone morphogenetic proteins (BMPs) are present in the developing embryo and act to induce neuronal differentiation and noradrenergic properties of neural crest cells. We have investigated the role of BMP2 in regulating the appearance of distinct populations of autonomic neurons from postmigratory, HNK-1-positive neural crest precursor cells. BMP2 promotes neuronal differentiation of sympathetic and enteric precursor cells isolated from E14.5 rat. The effects of BMP2 change over time, resulting in a decrease in neuron number that can be attributed to apoptotic cell death. BMP2-dependent neuron death is rescued by gut-derived factors that provide trophic support to maturing neurons, indicating that BMP2 regulates the acquisition of trophic dependence of developing peripheral neurons. In addition to regulating neuron number, BMP2 promotes both panneuronal maturation and the acquisition of an enteric phenotype, as measured by lineage-specific changes in the expression of tyrosine hydroxylase and MASH-1. While BMP2 is sufficient to induce neuronal differentiation and panneuronal development, these results suggest that additional factors in the environment must collaborate with BMP2 to promote the final noradrenergic phenotype of sympathetic neurons.

Regulation of Hoxb3 expression in the hindbrain and pharyngeal arches by rae28, a member of the mammalian Polycomb group of genes

During animal development, Hox genes are expressed in characteristic, spatially restricted patterns and specify regional identities along the anterior-posterior (A-P) axis. Polycomb group (PcG) proteins in Drosophila repress Hox expression and maintain the expression patterns during development. Mice deficient for homologues of the Drosophila PcG genes, such as M33, bmi1, mel18, rae28 and eed, show altered Hox expression patterns. In this study, we examined the time course of Hoxb3 expression during late gastrulation and early segmentation of rae28-deficient mice. Hoxb3 was expressed ectopically in pharyngeal arch and hindbrain from embryonic day (E) 9.5 and 10.5, respectively. The anterior boundary of ectopic expression in the hindbrain extended gradually in the rostral direction as development proceeded from E10.5 to E12.5. Expression of kreisler and Krox20, which function as positive regulators of Hoxb3 expression, was not affected in rae28-deficient embryos. Analysis of a neural crest marker, p75, in rae28-deficient mice revealed that the neural crest cells begin to ectopically express Hoxb3 after leaving the hindbrain. Our results suggest that rae28 is not required for the establishment but maintenance of Hoxb3 expression.

Developmental regulation of GDNF response and receptor expression in the enteric nervous system

The development of the enteric nervous system is dependent upon the actions of glial cell line-derived neurotrophic factor (GDNF) on neural crest-derived precursor cells in the embryonic gut. GDNF treatment of cultured enteric precursor cells leads to an increase in the number of neurons that develop and/or survive. Here we demonstrate that, although GDNF promoted an increase in neuron number at all embryonic ages examined, there was a developmental shift from a mitogenic to a trophic response by the developing enteric neurons. The timing of this shift corresponded to developmental changes in gut expression of GFR alpha-1, a co-receptor in the GDNF-Ret signaling complex. GFR alpha-1 was broadly expressed in the gut at early developmental stages, at which times soluble GFR alpha-1 was released into the medium by cultured gut cells. At later times, GFR alpha-1 became restricted to neural crest-derived cells. GFR alpha-1 could participate in GDNF signaling when expressed in cis on the surface of enteric precursor cells, or as a soluble protein. The GDNF-mediated response was greater when cell surface, compared with soluble, GFR alpha-1 was present, with the maximal response seen the presence of both cis and trans forms of GFR alpha-1. In addition to contributing to GDNF signaling, cell-surface GFR alpha-1 modulated the specificity of interactions between GDNF and soluble GFR alphas. These experiments demonstrate that complex, developmentally regulated, signaling interactions contribute to the GDNF-dependent development of enteric neurons.

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.

Factors controlling lineage specification in the neural crest

The neural crest is a transitory tissue of the vertebrate embryo that originates in the neural folds, populates the embryo, and gives rise to many different cell types and tissues of the adult organism. When neural crest cells initiate their migration, a large fraction of them are still pluripotent, that is, capable of generating progeny that consists of two or more distinct phenotypes. To elucidate the cellular and molecular mechanisms by which neural crest cells become committed to a particular lineage is therefore crucial to the understanding of neural crest development and represents a major challenge in current neural crest research. This chapter discusses selected aspects of neural crest cell differentiation into components of the peripheral nervous system. Topics include sympathetic neurons, the adrenal medulla, primary sensory neurons of the spinal ganglia, some of their mechanoreceptive and proprioceptive end organs, and the enteric nervous system.

Idiopathic slow-transit constipation is not associated with mutations of the RET proto-oncogene or GDNF

PURPOSE

Idiopathic slow-transit constipation is a severe disorder of unknown cause. The onset in early childhood and history of constipation or Hirschsprung's disease in close family relatives suggest that slow-transit constipation could have a genetic basis. Several germline mutations have been described in Hirschsprung's disease, including mutations of RET and the gene encoding its ligand glial cell-derived neurotrophic factor. The aim of this study was to screen a panel of 16 cases of familial idiopathic slow-transit constipation, including 4 families in which there were relatives with Hirschsprung's disease, for RET and glial cell-derived neurotrophic factor mutations previously identified in Hirschsprung's disease.

METHODS

Genomic DNA from 16 patients with slow-transit constipation and four relatives with Hirschsprung's disease was analyzed using single strand and heteroduplex conformation polymorphism analysis at two conditions and by direct DNA sequencing using the fluorescent dideoxy terminator method.

RESULTS

Although common sequence polymorphisms were demonstrated with a frequency comparable with published data, no published or new mutation was seen in any of the exons of RET or glial cell-derived neurotrophic factor.

CONCLUSIONS

Mutation of RET or glial cell-derived neurotrophic factor is not a frequent cause of idiopathic slow-transit constipation.

Development of the interstitial cell of Cajal: origin, kit dependence and neuronal and nonneuronal sources of kit ligand

Kit is a marker for interstitial cells of Cajal (ICC). ICCs interact with enteric neurons and are essential for gastrointestinal motility. The roles of neural crest-derived cells, neurons, Kit, and Kit ligand (KL) in ICC development were analyzed. ICC development lagged behind that of neurons and smooth muscle. Although mRNA encoding Kit and KL was detected at E11, Kit-immunoreactive ICCs did not appear until E12 in foregut and E14 in terminal hindgut. Transcripts of Kit and KL and Kit-immunoreactive cells were found in aganglionic gut from ls/ls and c-ret -/- mice. ICCs also developed in crest-free cultures of ls/ls terminal colon. ICCs appeared in cultures of noncrest- but not those of crest-derived cells isolated from the fetal bowel by immunoselection with antibodies to p75(NTR). KL immunoreactivity was coincident in cells with neuronal or smooth muscle markers. The development of ICCs in cultures of mixed cells dissociated from the fetal gut was dependent on plating density. No ICCs appeared at </=80,000 cells/ml, but many cells, including filamentous ICCs, appeared at >/=200,000 cells/ml. Exogenous KL partially substituted for a high plating density. These data support the ideas that mammalian ICCs are neither derived from the neural crest nor developmentally dependent on neurons. ICC differentiation/survival requires KL, which can be provided by neurons or cells in a smooth muscle lineage. Neurons may be needed for development of myenteric ICCs and the mature ICC network.

Development of the interstitial cell of Cajal: origin, kit dependence and neuronal and nonneuronal sources of kit ligand

Kit is a marker for interstitial cells of Cajal (ICC). ICCs interact with enteric neurons and are essential for gastrointestinal motility. The roles of neural crest-derived cells, neurons, Kit, and Kit ligand (KL) in ICC development were analyzed. ICC development lagged behind that of neurons and smooth muscle. Although mRNA encoding Kit and KL was detected at E11, Kit-immunoreactive ICCs did not appear until E12 in foregut and E14 in terminal hindgut. Transcripts of Kit and KL and Kit-immunoreactive cells were found in aganglionic gut from ls/ls and c-ret -/- mice. ICCs also developed in crest-free cultures of ls/ls terminal colon. ICCs appeared in cultures of noncrest- but not those of crest-derived cells isolated from the fetal bowel by immunoselection with antibodies to p75(NTR). KL immunoreactivity was coincident in cells with neuronal or smooth muscle markers. The development of ICCs in cultures of mixed cells dissociated from the fetal gut was dependent on plating density. No ICCs appeared at </=80,000 cells/ml, but many cells, including filamentous ICCs, appeared at >/=200,000 cells/ml. Exogenous KL partially substituted for a high plating density. These data support the ideas that mammalian ICCs are neither derived from the neural crest nor developmentally dependent on neurons. ICC differentiation/survival requires KL, which can be provided by neurons or cells in a smooth muscle lineage. Neurons may be needed for development of myenteric ICCs and the mature ICC network.

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.

Hirschsprung disease and other enteric dysganglionoses

Hirschsprung disease has become a paradigm for multigene disorders because the same basic phenotype is associated with mutations in at least seven distinct genes. As such, the condition poses distinct challenges for clinicians, patients, diagnostic pathologists, and basic scientists, who must cope with the implications of this genetic complexity to comprehend the pathogenesis of the disorder and effectively manage patients. This review focuses on the anatomic pathology, genetics, and pathogenesis of Hirschsprung disease and related conditions. The nature and functions of "Hirschsprung disease genes" are examined in detail and emphasis is placed on the importance of animal models to this field. Where possible, potential uses and limitations of new data concerning molecular genetics and pathogenesis are discussed as they relate to contemporary medical practices.

Neurotrophins and other growth factors in the generation of retinal neurons

The generation of neurons in the vertebrate retina, as in other areas of the developing nervous system, largely depends on extracellular signals. Of the known signaling molecules, neurotrophins play decisive, defined, and distinct roles. The three neurotrophins identified in the chick, namely, neurotrophin-3 (NT-3), brain-derived neurotrophic factor (BDNF), and nerve growth factor (NGF), are expressed in either the pigment epithelium (NT-3 and BDNF) or in the neural retina (NGF) at the onset of neuron birth. In addition, trkC and trkB, receptors for NT-3 and BDNF, respectively, together with p75, the low-affinity neurotrophin receptor, are expressed in the retina at the same developmental period. The role of these three neurotrophins in the differentiation of neurons in the chick retina has been elucidated by a combination of in vitro and in vivo experiments. Thus, NT-3 promotes the conversion of neuroepithelial cells into neurons, whereas BDNF and NGF control the programmed cell death (apoptosis) that affects early postmitotic neuroblasts. BDNF, acting via its trkB receptor, is a survival factor for these cells, whereas NGF, binding to p75 receptor, acts as a killing factor, thereby controlling the provisional number of newly generated neurons.

Inhibition of in vitro enteric neuronal development by endothelin-3: mediation by endothelin B receptors

The terminal colon is aganglionic in mice lacking endothelin-3 or its receptor, endothelin B. To analyze the effects of endothelin-3/endothelin B on the differentiation of enteric neurons, E11-13 mouse gut was dissociated, and positive and negative immunoselection with antibodies to p75(NTR)were used to isolate neural crest- and non-crest-derived cells. mRNA encoding endothelin B was present in both the crest-and non-crest-derived cells, but that encoding preproendothelin-3 was detected only in the non-crest-derived population. The crest- and non-crest-derived cells were exposed in vitro to endothelin-3, IRL 1620 (an endothelin B agonist), and/or BQ 788 (an endothelin B antagonist). Neurons and glia developed only in cultures of crest-derived cells, and did so even when endothelin-3 was absent and BQ 788 was present. Endothelin-3 inhibited neuronal development, an effect that was mimicked by IRL 1620 and blocked by BQ 788. Endothelin-3 failed to stimulate the incorporation of [3H]thymidine or bromodeoxyuridine. Smooth muscle development in non-crest-derived cell cultures was promoted by endothelin-3 and inhibited by BQ 788. In contrast, transcription of laminin alpha1, a smooth muscle-derived promoter of neuronal development, was inhibited by endothelin-3, but promoted by BQ 788. Neurons did not develop in explants of the terminal bowel of E12 ls/ls (endothelin-3-deficient) mice, but could be induced to do so by endothelin-3 if a source of neural precursors was present. We suggest that endothelin-3/endothelin B normally prevents the premature differentiation of crest-derived precursors migrating to and within the fetal bowel, enabling the precursor population to persist long enough to finish colonizing the bowel.

Age-dependent differences in the effects of GDNF and NT-3 on the development of neurons and glia from neural crest-derived precursors immunoselected from the fetal rat gut: expression of GFRalpha-1 in vitro and in vivo

No enteric neurons or glia develop in the gut below the rostral foregut in mice lacking glial cell line-derived neurotrophic factor (GDNF) or Ret. We analyzed the nature and age dependence of the effects of GDNF and, for comparison, those of NT-3, on the in vitro development of the precursors of enteric neurons and glia. Positive and negative immunoselection with antibodies to p75(NTR) were used to isolate crest-derived and crest-depleted populations of cells from the fetal rat bowel at E12, 14, and 16. Cells were typed immunocytochemically. GDNF stimulated the proliferation of nestin-expressing precursor cells isolated at E12, but not at E14-16. GDNF promoted the development of peripherin-expressing neurons (E12 >> E14-16) and expression of TrkC. GDNF inhibited expression of S-100-expressing glia at E14-16. NT-3 did not affect cells isolated at E12, never stimulated precursors to proliferate, and promoted glial as well as neuronal development at E14-16. GFRalpha-1 was expressed both by crest- and non-crest-derived cells, although only crest-derived cells anchored GFRalpha-1 and GFRalpha-2 (GFRalpha-1 >> GFRalpha-2). GDNF increased the number of neurons anchoring GFRalpha-1. GFRalpha-1 is immunocytochemically detectable in neurons of the E13 intestine and persists in adult neurons of both plexuses. We suggest that GDNF stimulates the proliferation of an early (E12) NT-3-insensitive precursor common to enteric neurons and glia; by E14, this common precursor is replaced by specified NT-3-responsive neuronal and glial progenitors. GDNF exerts a neurotrophic, but not a mitogenic, effect on the neuronal progenitor. The glial progenitor is not maintained by GDNF.