Sacral neural crest-derived cells enter the aganglionic colon of Ednrb-/- mice along extrinsic nerve fibers

Sacral Neural Crest-Independent Origin of the Enteric Nervous System in Mouse

BACKGROUND & AIMS

The enteric nervous system (ENS), the gut's intrinsic nervous system critical for gastrointestinal function and gut-brain communication, is believed to mainly originate from vagal neural crest cells (vNCCs) and partially from sacral NCCs (sNCCs). Resolving the exact origins of the ENS is critical for understanding congenital ENS diseases but has been confounded by the inability to distinguish between both NCC populations in situ. Here, we aimed to resolve the exact origins of the mammalian ENS.

METHODS

We genetically engineered mouse embryos facilitating comparative lineage-tracing of either all (pan-) NCCs including vNCCs or caudal trunk and sNCCs (s/tNCCs) excluding vNCCs. This was combined with dual-lineage tracing and 3-dimensional reconstruction of pelvic plexus and hindgut to precisely pinpoint sNCC and vNCC contributions. We further used coculture assays to determine the specificity of cell migration from different neural tissues into the hindgut.

RESULTS

Both pan-NCCs and s/tNCCs contributed to established NCC derivatives but only pan-NCCs contributed to the ENS. Dual-lineage tracing combined with 3-dimensional reconstruction revealed that s/tNCCs settle in complex patterns in pelvic plexus and hindgut-surrounding tissues, explaining previous confusion regarding their contributions. Coculture experiments revealed unspecific cell migration from autonomic, sensory, and neural tube explants into the hindgut. Lineage tracing of ENS precursors lastly provided complimentary evidence for an exclusive vNCC origin of the murine ENS.

CONCLUSIONS

sNCCs do not contribute to the murine ENS, suggesting that the mammalian ENS exclusively originates from vNCCs. These results have immediate implications for comprehending (and devising treatments for) congenital ENS disorders, including Hirschsprung's disease.

Endothelin signaling in development

Since the discovery of endothelin 1 (EDN1) in 1988, the role of endothelin ligands and their receptors in the regulation of blood pressure in normal and disease states has been extensively studied. However, endothelin signaling also plays crucial roles in the development of neural crest cell-derived tissues. Mechanisms of endothelin action during neural crest cell maturation have been deciphered using a variety of in vivo and in vitro approaches, with these studies elucidating the basis of human syndromes involving developmental differences resulting from altered endothelin signaling. In this Review, we describe the endothelin pathway and its functions during the development of neural crest-derived tissues. We also summarize how dysregulated endothelin signaling causes developmental differences and how this knowledge may lead to potential treatments for individuals with gene variants in the endothelin pathway.

The transition zone in Hirschsprung's bowel contains abnormal hybrid ganglia with characteristics of extrinsic nerves

The aganglionic bowel in short-segment Hirschsprung's disease is characterized both by the absence of enteric ganglia and the presence of extrinsic thickened nerve bundles (TNBs). The relationship between the TNBs and the loss of enteric ganglia is unknown. Previous studies have described decreasing numbers of ganglia with increasing density of TNBs within the transition zone (TZ) between ganglionic and aganglionic gut, and there is some evidence of spatial contact between them in this region. To determine the cellular interactions involved, we have analysed the expression of perineurial markers of TNBs and enteric ganglionic markers for both neural cells and their ensheathing telocytes across four cranio-caudal segments consisting of most proximal ganglionic to most distal aganglionic from pull-through resected colon. We show that in the TZ, enteric ganglia are abnormal, being surrounded by perineurium cells characteristic of TNBs. Furthermore, short processes of ganglionic neurons extend caudally towards the aganglionic region, where telocytes in the TNB are located between the perineurium and nerve fibres into which they project telopodes. Thus, enteric ganglia within the TZ have abnormal structural characteristics, the cellular relationships of which are shared by the TNBs. These findings will help towards elucidation of the cellular mechanisms involved in the aetiology of Hirschsprung's disease.

Schwann Cells in the Aganglionic Colon of Hirschsprung Disease Can Generate Neurons for Regenerative Therapy

Cell therapy offers the potential to replace the missing enteric nervous system (ENS) in patients with Hirschsprung disease (HSCR) and to restore gut function. The Schwann cell (SC) lineage has been shown to generate enteric neurons pre- and post-natally. Here, we aimed to isolate SCs from the aganglionic segment of HSCR and to determine their potential to restore motility in the aganglionic colon. Proteolipid protein 1 (PLP1) expressing SCs were isolated from the extrinsic nerve fibers present in the aganglionic segment of postnatal mice and patients with HSCR. Following 7-10 days of in vitro expansion, HSCR-derived SCs were transplanted into the aganglionic mouse colon ex vivo and in vivo. Successful engraftment and neuronal differentiation were confirmed immunohistochemically and calcium activity of transplanted cells was demonstrated by live cell imaging. Organ bath studies revealed the restoration of motor function in the recipient aganglionic smooth muscle. These results show that SCs isolated from the aganglionic segment of HSCR mouse can generate functional neurons within the aganglionic gut environment and restore the neuromuscular activity of recipient mouse colon. We conclude that HSCR-derived SCs represent a potential autologous source of neural progenitor cells for regenerative therapy in HSCR.

Spatiotemporal mapping of sensory and motor innervation of the embryonic and postnatal mouse urinary bladder

The primary function of the urinary bladder is to store urine (continence) until a suitable time for voiding (micturition). These distinct processes are determined by the coordinated activation of sensory and motor components of the nervous system, which matures to enable voluntary control at the time of weaning. Our aim was to define the development and maturation of the nerve-organ interface of the mouse urinary bladder by mapping the organ and tissue distribution of major classes of autonomic (motor) and sensory axons. Innervation of the bladder was evident from E13 and progressed dorsoventrally. Increasing defasciculation of axon bundles to single axons within the muscle occurred through the prenatal period, and in several classes of axons underwent further maturation until P7. Urothelial innervation occurred more slowly than muscle innervation and showed a clear regional difference, from E18 the bladder neck having the highest density of urothelial nerves. These features of innervation were similar in males and females but varied in timing and tissue density between different axon classes. We also analysed the pelvic ganglion, the major source of motor axons that innervate the lower urinary tract and other pelvic organs. Cholinergic, nitrergic (subset of cholinergic) and noradrenergic neuronal cell bodies were present prior to visualization of these axon classes within the bladder. Examination of cholinergic structures within the pelvic ganglion indicated that connections from spinal preganglionic neurons to pelvic ganglion neurons were already present by E12, a time at which these autonomic ganglion neurons had not yet innervated the bladder. These putative preganglionic inputs increased in density prior to birth as axon terminal fields continued to expand within the bladder tissues. Our studies also revealed in numerous pelvic ganglion neurons an unexpected transient expression of calcitonin gene-related peptide, a peptide commonly used to visualise the peptidergic class of visceral sensory axons. Together, our outcomes enhance our understanding of neural regulatory elements in the lower urinary tract during development and provide a foundation for studies of plasticity and regenerative capacity in the adult system.

Extrinsic innervation of the pelvic organs in the lesser pelvis of human embryos

Realistic models to understand the developmental appearance of the pelvic nervous system in mammals are scarce. We visualized the development of the inferior hypogastric plexus and its preganglionic connections in human embryos at 4-8 weeks post-fertilization, using Amira 3D reconstruction and Cinema 4D-remodelling software. We defined the embryonic lesser pelvis as the pelvic area caudal to both umbilical arteries and containing the hindgut. Neural crest cells (NCCs) appeared dorsolateral to the median sacral artery near vertebra S1 at ~5 weeks and had extended to vertebra S5 1 day later. Once para-arterial, NCCs either formed sympathetic ganglia or continued to migrate ventrally to the pre-arterial region, where they formed large bilateral inferior hypogastric ganglionic cell clusters (IHGCs). Unlike more cranial pre-aortic plexuses, both IHGCs did not merge because the 'pelvic pouch', a temporary caudal extension of the peritoneal cavity, interposed. Although NCCs in the sacral area started to migrate later, they reached their pre-arterial position simultaneously with the NCCs in the thoracolumbar regions. Accordingly, the superior hypogastric nerve, a caudal extension of the lumbar splanchnic nerves along the superior rectal artery, contacted the IHGCs only 1 day later than the lumbar splanchnic nerves contacted the inferior mesenteric ganglion. The superior hypogastric nerve subsequently splits to become the superior hypogastric plexus. The IHGCs had two additional sources of preganglionic innervation, of which the pelvic splanchnic nerves arrived at ~6.5 weeks and the sacral splanchnic nerves only at ~8 weeks. After all preganglionic connections had formed, separate parts of the inferior hypogastric plexus formed at the bladder neck and distal hindgut.

Selective neuronal expression of the SoxE factor, Sox8, in direct pathway striatal projection neurons of the developing mouse brain

The striatum is the major component of the basal ganglia and is well known to play a key role in the control of motor function via balanced output from the indirect (iSPNs) and direct pathway striatal projection neurons (dSPNs). Little is known, however, about the molecular genetic mechanisms that control the formation of the iSPNs versus dSPNs. We show here that the SoxE family member, Sox8, is co-expressed with the dSPN markers, Isl1 and Ebf1, in the developing striatum. Moreover, dSPNs, as marked by Isl1-cre fate map, express Sox8 in the embryonic striatum and Sox8-EGFP BAC transgenic mice specifically reveal the direct pathway axons during development. These EGFP⁺ axons are first observed to reach their midbrain target, the substantia nigra pars reticulata (SNr), at E14 in the mouse with a robust connection observed already at birth. The selective expression of EGFP in dSPNs of Sox8-EGFP BAC mice is maintained at postnatal timepoints. Sox8 is known to be expressed in oligodendrocyte precursor cells (OPCs) together with other SoxE factors and we show here that the EGFP signal co-localizes with the OPC markers throughout the brain. Finally, we show that Sox8-EGFP BAC mice can be used to interrogate the altered dSPN development in Isl1 conditional mutants including aberrant axonal projections detected already at embryonic timepoints. Thus, Sox8 represents an early and specific marker of embryonic dSPNs and the Sox8-EGFP BAC transgenic mice are an excellent tool to study the development of basal ganglia circuitry.

Spontaneous calcium waves in the developing enteric nervous system

The enteric nervous system (ENS) is an extensive network of neurons in the gut wall that arises from neural crest-derived cells. Like other populations of neural crest cells, it is known that enteric neural crest-derived cells (ENCCs) influence the behaviour of each other and therefore must communicate. However, little is known about how ENCCs communicate with each other. In this study, we used Ca²⁺ imaging to examine communication between ENCCs in the embryonic gut, using mice where ENCCs express a genetically-encoded calcium indicator. Spontaneous propagating calcium waves were observed between neighbouring ENCCs, through both neuronal and non-neuronal ENCCs. Pharmacological experiments showed wave propagation was not mediated by gap junctions, but by purinergic signalling via P2 receptors. The expression of several P2X and P2Y receptors was confirmed using RT-PCR. Furthermore, inhibition of P2 receptors altered the morphology of the ENCC network, without affecting neuronal differentiation or ENCC proliferation. It is well established that purines participate in synaptic transmission in the mature ENS. Our results describe, for the first time, purinergic signalling between ENCCs during pre-natal development, which plays roles in the propagation of Ca²⁺ waves between ENCCs and in ENCC network formation. One previous study has shown that calcium signalling plays a role in sympathetic ganglia formation; our results suggest that calcium waves are likely to be important for enteric ganglia development.

Neural Crest Cells Contribute an Astrocyte-like Glial Population to the Spleen

Neural crest cells (NCC) are multi-potent cells of ectodermal origin that colonize diverse organs, including the gastrointestinal tract to form the enteric nervous system (ENS) and hematopoietic organs (bone marrow, thymus) where they participate in lymphocyte trafficking. Recent studies have implicated the spleen as an anatomic site for integration of inflammatory signals from the intestine with efferent neural inputs. We have previously observed alterations in splenic lymphocyte subsets in animals with defective migration of NCC that model Hirschsprung's disease, leading us to hypothesize that there may be a direct cellular contribution of NCC to the spleen. Here, we demonstrate that NCC colonize the spleen during embryogenesis and persist into adulthood. Splenic NCC display markers indicating a glial lineage and are arranged anatomically adjacent to blood vessels, pericytes and nerves, suggesting an astrocyte-like phenotype. Finally, we identify similar neural-crest derived cells in both the avian and non-human primate spleen, showing evolutionary conservation of these cells.

Impaired Cellular Immunity in the Murine Neural Crest Conditional Deletion of Endothelin Receptor-B Model of Hirschsprung's Disease

Hirschsprung’s disease (HSCR) is characterized by aganglionosis from failure of neural crest cell (NCC) migration to the distal hindgut. Up to 40% of HSCR patients suffer Hirschsprung’s-associated enterocolitis (HAEC), with an incidence that is unchanged from the pre-operative to the post-operative state. Recent reports indicate that signaling pathways involved in NCC migration may also be involved in the development of secondary lymphoid organs. We hypothesize that gastrointestinal (GI) mucosal immune defects occur in HSCR that may contribute to enterocolitis. EdnrB was deleted from the neural crest (EdnrB^NCC-/-) resulting in mutants with defective NCC migration, distal colonic aganglionosis and the development of enterocolitis. The mucosal immune apparatus of these mice was interrogated at post-natal day (P) 21–24, prior to histological signs of enterocolitis. We found that EdnrB^NCC-/- display lymphopenia of their Peyer’s Patches, the major inductive site of GI mucosal immunity. EdnrB^NCC-/- Peyer’s Patches demonstrate decreased B-lymphocytes, specifically IgM⁺IgD^hi (Mature) B-lymphocytes, which are normally activated and produce IgA following antigen presentation. EdnrB^NCC-/- animals demonstrate decreased small intestinal secretory IgA, but unchanged nasal and bronchial airway secretory IgA, indicating a gut-specific defect in IgA production or secretion. In the spleen, which is the primary source of IgA-producing Mature B-lymphocytes, EdnrB^NCC-/- animals display decreased B-lymphocytes, but an increase in Mature B-lymphocytes. EdnrB^NCC-/- spleens are also small and show altered architecture, with decreased red pulp and a paucity of B-lymphocytes in the germinal centers and marginal zone. Taken together, these findings suggest impaired GI mucosal immunity in EdnrB^NCC-/- animals, with the spleen as a potential site of the defect. These findings build upon the growing body of literature that suggests that intestinal defects in HSCR are not restricted to the aganglionic colon but extend proximally, even into the ganglionated small intestine and immune cells.

Isolation of Enteric Nervous System Progenitor Cells from the Aganglionic Gut of Patients with Hirschsprung's Disease

Enteric nervous system progenitor cells isolated from postnatal human gut and cultured as neurospheres can then be transplanted into aganglionic gut to restore normal patterns of contractility. These progenitor cells may be of future use to treat patients with Hirschprung’s disease, a congenital condition characterized by hindgut dysmotility due to the lack of enteric nervous system ganglia. Here we demonstrate that progenitor cells can also be isolated from aganglionic gut removed during corrective surgery for Hirschsprung’s disease. Although the enteric nervous system marker calretinin is not expressed in the aganglionic gut region, de novo expression is initiated in cultured neurosphere cells isolated from aganglionic Hirschsprung bowel. Furthermore, expression of the neural markers NOS, VIP and GFAP also increased during culture of aganglionic gut neurospheres which we show can be transplantation into cultured embryonic mouse gut explants to restore a normal frequency of contractility. To determine the origin of the progenitor cells in aganglionic region, we used fluorescence-activated cell sorting to demonstrate that only p75-positive neural crest-derived cells present in the thickened nerve trunks characteristic of the aganglionic region of Hirschsprung gut gave rise to neurons in culture. The derivation of enteric nervous system progenitors in the aganglionic gut region of Hirschprung’s patients not only means that this tissue is a potential source of cells for future autologous transplantation, but it also raises the possibility of inducing the differentiation of these endogenous cells in situ to compensate for the aganglionosis.

Immunostaining to visualize murine enteric nervous system development

The enteric nervous system is formed by neural crest cells that proliferate, migrate and colonize the gut. Following colonization, neural crest cells must then differentiate into neurons with markers specific for their neurotransmitter phenotype. Cholinergic neurons, a major neurotransmitter phenotype in the enteric nervous system, are identified by staining for choline acetyltransferase (ChAT), the synthesizing enzyme for acetylcholine. Historical efforts to visualize cholinergic neurons have been hampered by antibodies with differing specificities to central nervous system versus peripheral nervous system ChAT. We and others have overcome this limitation by using an antibody against placental ChAT, which recognizes both central and peripheral ChAT, to successfully visualize embryonic enteric cholinergic neurons. Additionally, we have compared this antibody to genetic reporters for ChAT and shown that the antibody is more reliable during embryogenesis. This protocol describes a technique for dissecting, fixing and immunostaining of the murine embryonic gastrointestinal tract to visualize enteric nervous system neurotransmitter expression.

A chicken model of pharmacologically-induced Hirschsprung disease reveals an unexpected role of glucocorticoids in enteric aganglionosis

The enteric nervous system originates from neural crest cells that migrate in chains as they colonize the embryonic gut, eventually forming the myenteric and submucosal plexus. Failure of the neural crest cells to colonize the gut leads to aganglionosis in the terminal gut, a pathological condition called Hirschsprung disease (HSCR) in humans, also known as congenital megacolon or intestinal aganglionosis. One of the characteristics of the human HSCR is its variable penetrance, which may be attributable to the interaction between genetic factors, such as the endothelin-3/endothelin receptor B pathway, and non-genetic modulators, although the role of the latter has not well been established. We have created a novel HSCR model in the chick embryo allowing to test the ability of non-genetic modifiers to alter the HSCR phenotype. Chick embryos treated by phosphoramidon, which blocks the generation of endothelin-3, failed to develop enteric ganglia in the very distal bowel, characteristic of an HSCR-like phenotype. Administration of dexamethasone influenced the phenotype, suggesting that glucocorticoids may be environmental modulators of the penetrance of the aganglionosis in HSCR disease.

Cross-inhibition of NMBR and GRPR signaling maintains normal histaminergic itch transmission

We previously showed that gastrin-releasing peptide receptor (GRPR) in the spinal cord is important for mediating nonhistaminergic itch. Neuromedin B receptor (NMBR), the second member of the mammalian bombesin receptor family, is expressed in a largely nonoverlapping pattern with GRPR in the superficial spinal cord, and its role in itch transmission remains unclear. Here, we report that Nmbr knock-out (KO) mice exhibited normal scratching behavior in response to intradermal injection of pruritogens. However, mice lacking both Nmbr and Grpr (DKO mice) showed significant deficits in histaminergic itch. In contrast, the chloroquine (CQ)-evoked scratching behavior of DKO mice is not further reduced compared with Grpr KO mice. These results suggest that NMBR and GRPR could compensate for the loss of each other to maintain normal histamine-evoked itch, whereas GRPR is exclusively required for CQ-evoked scratching behavior. Interestingly, GRPR activity is enhanced in Nmbr KO mice despite the lack of upregulation of Grpr expression; so is NMBR in Grpr KO mice. We found that NMB acts exclusively through NMBR for itch transmission, whereas GRP can signal through both receptors, albeit to NMBR to a much lesser extent. Although NMBR and NMBR(+) neurons are dispensable for histaminergic itch, GRPR(+) neurons are likely to act downstream of NMBR(+) neurons to integrate NMB-NMBR-encoded histaminergic itch information in normal physiological conditions. Together, we define the respective function of NMBR and GRPR in itch transmission, and reveal an unexpected relationship not only between the two receptors but also between the two populations of interneurons in itch signaling.

Birthdating of myenteric neuron subtypes in the small intestine of the mouse

There are many different types of enteric neurons. Previous studies have identified the time at which some enteric neuron subtypes are born (exit the cell cycle) in the mouse, but the birthdates of some major enteric neuron subtypes are still incompletely characterized or unknown. We combined 5-ethynynl-2'-deoxyuridine (EdU) labeling with antibody markers that identify myenteric neuron subtypes to determine when neuron subtypes are born in the mouse small intestine. We found that different neurochemical classes of enteric neuron differed in their birthdates; serotonin neurons were born first with peak cell cycle exit at E11.5, followed by neurofilament-M neurons, calcitonin gene-related peptide neurons (peak cell cycle exit for both at embryonic day [E]12.5-E13.5), tyrosine hydroxylase neurons (E15.5), nitric oxide synthase 1 (NOS1) neurons (E15.5), and calretinin neurons (postnatal day [P]0). The vast majority of myenteric neurons had exited the cell cycle by P10. We did not observe any EdU+/NOS1+ myenteric neurons in the small intestine of adult mice following EdU injection at E10.5 or E11.5, which was unexpected, as previous studies have shown that NOS1 neurons are present in E11.5 mice. Studies using the proliferation marker Ki67 revealed that very few NOS1 neurons in the E11.5 and E12.5 gut were proliferating. However, Cre-lox-based genetic fate-mapping revealed a small subpopulation of myenteric neurons that appears to express NOS1 only transiently. Together, our results confirm a relationship between enteric neuron subtype and birthdate, and suggest that some enteric neurons exhibit neurochemical phenotypes during development that are different from their mature phenotype.

Intestinal dysbiosis and bacterial enteroinvasion in a murine model of Hirschsprung's disease

BACKGROUND/PURPOSE

Hirschsprung's disease (HSCR), characterized by the absence of ganglia in the distal colon, results in functional obstruction. Despite surgical resection of the aganglionic segment, around 40% of patients suffer recurrent life threatening Hirschsprung's-associated enterocolitis (HAEC). The aim of this study was to investigate whether gut microbiota and intestinal immunity changes contribute to the HAEC risk in an HSCR model.

METHODS

Mice with neural crest conditional deletion of Endothelin receptor B (EdnrB) and their littermate controls were used (EdnrB-null and EdnrB-het). Bacterial DNA was prepared from cecal contents of P16-18 and P21-24 animals and pyrosequencing employed for microbiome analysis. Ileal tissue was isolated and secretory phospholipase A2 (sPLA2) expression and activity determined. Enteroinvasion of Escherichia coli into ileal explants was measured using an ex vivo organ culture system.

RESULTS

EdnrB-het and EdnrB-nulls displayed similar flora, sPLA2 expression and activity at P16-18. However, by P21-24, EdnrB-hets demonstrated increased Lactobacillus and decreased Bacteroides and Clostridium, while EdnrB-nulls exhibited reciprocal changes. EdnrB-nulls also showed reduced sPLA2 expression and luminal activity at this stage. Functionally, EdnrB-nulls were more susceptible to enteroinvasion with E. coli ex vivo and released less sPLA2 than EdnrB-hets.

CONCLUSIONS

Initially, EdnrB-het and EdnrB-nulls contain similar cecal flora but then undergo reciprocal changes. EdnrB-nulls display dysbiosis, demonstrate impaired mucosal defense, decreased luminal sPLA2 and increased enteroinvasion of E. coli just prior to robust colonic inflammation and death. These findings suggest a role for the intestinal microbiome in the development of HAEC.

Vascularisation is not necessary for gut colonisation by enteric neural crest cells

The vasculature and nervous system share striking similarities in their networked, tree-like architecture and in the way they are super-imposed in mature organs. It has previously been suggested that the intestinal microvasculature network directs the migration of enteric neural crest cells (ENCC) along the gut to promote the formation of the enteric nervous system (ENS). To investigate the inter-relationship of migrating ENCC, ENS formation and gut vascular development we combined fate-mapping of ENCC with immunolabelling and intravascular dye injection to visualise nascent blood vessel networks. We found that the enteric and vascular networks initially had very distinct patterns of development. In the foregut, ENCC migrated through areas devoid of established vascular networks. In vessel-rich areas, such as the midgut and hindgut, the distribution of migrating ENCC did not support the idea that these cells followed a pre-established vascular network. Moreover, when gut vascular development was impaired, either genetically in Vegfa(120/120) or Tie2-Cre;Nrp1(fl/-) mice or using an in vitro Wnt1-Cre;Rosa26(Yfp/+) mouse model of ENS development, ENCC still colonised the entire length of the gut, including the terminal hindgut. These results demonstrate that blood vessel networks are not necessary to guide migrating ENCC during ENS development. Conversely, in miRet(51) mice, which lack ENS in the hindgut, the vascular network in this region appeared to be normal suggesting that in early development both networks form independently of each other.

Enteric nervous system development: migration, differentiation, and disease

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

Altered neuronal density and neurotransmitter expression in the ganglionated region of Ednrb null mice: implications for Hirschsprung's disease

BACKGROUND

Hirschsprung's disease (HSCR) is a congenital condition in which enteric ganglia, formed from neural crest cells (NCC), are absent from the terminal bowel. Dysmotility and constipation are common features of HSCR that persist following surgical intervention. This persistence suggests that the portion of the colon that remains postoperatively is not able to support normal bowel function. To elucidate the defects that underlie this condition, we utilized a murine model of HSCR.

METHODS

Mice with NCC-specific deletion of Ednrb were used to measure the neuronal density and neurotransmitter expression in ganglia.

KEY RESULTS

At the site located proximal to the aganglionic region of P21 Ednrb null mice, the neuronal density is significantly decreased and the expression of neurotransmitters is altered compared with het animals. The ganglia in this colonic region are smaller and more isolated while the size of neuronal cell bodies is increased. The percentage of neurons expressing neuronal nNOS and VIP is significantly increased in Ednrb nulls. Conversely, the percentage of choline acetyltransferase (ChAT) expressing neurons is decreased, while Substance P is unchanged between the two genotypes. These changes are limited to the colon and are not detected in the ileum.

CONCLUSIONS & INFERENCES

We demonstrate changes in neuronal density and alterations in the balance of expression of neurotransmitters in the colon proximal to the aganglionic region in Ednrb null mice. The reduced neuronal density and complementary changes in nNOS and ChAT expression may account for the dysmotility seen in HSCR.

Anorectal neural crest derived cell behavior after the migration of vagal neural crest derived cells is surgically disrupted: implications for the etiology of Hirschsprung's disease

BACKGROUND/PURPOSE

In Hirschsprung's disease (HD), thick extrinsic nerve fibers can be associated with the aganglionic segment in the anorectum. We surgically disrupted the migration of vagal neural crest cell-derived cells (vagal NCC) in embryos from transgenic mice we created previously (SOX10-VENUS Tg) which have the SOX10 gene labeled with Venus (V), a green fluorescent protein, to observe sacral NCC activity in the anorectum.

METHOD

Proximal colon harvested from SOX10-VENUS Tg embryos on day 10.5 (n = 10) was transected at the ascending colon. V-positive sacral NCC in the anorectum were observed during organ culture under fluorescence stereoscopic microscopy, and compared with non-transected control specimens (n = 10).

RESULTS

In transected specimens, no V-positive sacral NCC were identified initially in the anorectum. By day 2, there were thick beaded sacral NCC in the anorectum in 6/10 (60 %) that migrated steadily to the transected end over 3-4 days. In controls, thinner and shorter V-positive sacral NCC began migrating cranially on day 2, and were met by distally migrating vagal NCC.

CONCLUSION

Disruption of vagal NCC migration appears to induce sacral NCC activity in the anorectum, suggesting that thick extrinsic nerve fibers seen in HD may be a secondary phenomenon.