GDNF availability determines enteric neuron number by controlling precursor proliferation

Retinoic acid regulates murine enteric nervous system precursor proliferation, enhances neuronal precursor differentiation, and reduces neurite growth in vitro

Enteric nervous system (ENS) precursors undergo a complex process of cell migration, proliferation, and differentiation to form an integrated network of neurons and glia within the bowel wall. Although retinoids regulate ENS development, molecular and cellular mechanisms of retinoid effects on the ENS are not well understood. We hypothesized that retinoids might directly affect ENS precursor differentiation and proliferation, and tested that hypothesis using immunoselected fetal ENS precursors in primary culture. We now demonstrate that all retinoid receptors and many retinoid biosynthetic enzymes are present in the fetal bowel at about the time that migrating ENS precursors reach the distal bowel. We further demonstrate that retinoic acid (RA) enhances proliferation of subsets of ENS precursors in a time-dependent fashion and increases neuronal differentiation. Surprisingly, however, enteric neurons that develop in retinoid deficient media have dramatically longer neurites than those exposed to RA. This difference in neurite growth correlates with increased RhoA protein at the neurite tip, decreased Smurf1 (a protein that targets RhoA for degradation), and dramatically decreased Smurf1 mRNA in response to RA. Collectively these data demonstrate diverse effects of RA on ENS precursor development and suggest that altered fetal retinoid availability or metabolism could contribute to intestinal motility disorders.

GDNF and GFRalpha: a versatile molecular complex for developing neurons

The GDNF family ligands (GFLs) signal through the canonical signaling receptor Ret and a glycosyl-phosphatidylinositol-anchored co-receptor, GFRalpha. In recent years, signaling by GFLs has been shown to be more complex than originally assumed. The discrepant expression between GFRalphas and Ret has suggested the existence of additional signal-transducing GDNF receptors, such as NCAM. Here we summarize novel functions and Ret-independent signaling mechanisms for GDNF and GFRalpha, focusing on developing neurons. Emerging evidence indicates a prominent role of GDNF and GFRalpha in the control of neuroblast migration and chemoattraction and in the formation of neuronal synapses by a new mechanism of ligand-induced cell adhesion. Therefore, these data highlight the importance of this versatile molecular complex for nervous system development, function and regeneration.

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.

Critical numbers of neural crest cells are required in the pathways from the neural tube to the foregut to ensure complete enteric nervous system formation

The enteric nervous system (ENS) is mainly derived from vagal neural crest cells (NCC) that arise at the level of somites 1-7. To understand how the size and composition of the NCC progenitor pool affects ENS development, we reduced the number of NCC by ablating the neural tube adjacent to somites 3-6 to produce aganglionic gut. We then back-transplanted various somite lengths of quail neural tube into the ablated region to determine the `tipping point',whereby sufficient progenitors were available for complete ENS formation. The addition of one somite length of either vagal, sacral or trunk neural tube into embryos that had the neural tube ablated adjacent to somites 3-6,resulted in ENS formation along the entire gut. Although these additional cells contributed to the progenitor pool, the quail NCC from different axial levels retained their intrinsic identities with respect to their ability to form the ENS; vagal NCC formed most of the ENS, sacral NCC contributed a limited number of ENS cells, and trunk NCC did not contribute to the ENS. As one somite length of vagal NCC was found to comprise almost the entire ENS, we ablated all of the vagal neural crest and back-transplanted one somite length of vagal neural tube from the level of somite 1 or somite 3 into the vagal region at the position of somite 3. NCC from somite 3 formed the ENS along the entire gut, whereas NCC from somite 1 did not. Intrinsic differences, such as an increased capacity for proliferation, as demonstrated in vitro and in vivo,appear to underlie the ability of somite 3 NCC to form the entire ENS.

Functional development of the enteric nervous system--from migration to motility

The enteric nervous system (ENS) consists of many different types of enteric neurones forming complex reflex circuits that underlie or regulate many gut functions. Studies of humans with Hirschsprung's disease (distal aganglionosis), and of animal models of Hirschsprung's disease, have led to the identification of many of the genetic, molecular and cellular mechanisms responsible for the colonization of the gut by enteric neurone precursors. However, later events in the ENS development are still poorly understood, including the development of functioning ENS circuits. This article is a personal view of the current state of play in our understanding of the ENS development and of the future of the field.

Disturbances of colonic motility in mouse models of Hirschsprung's disease

Mutations in genes encoding members of the GDNF and endothelin-3 (Et-3) signaling pathways can cause Hirschsprung's disease, a congenital condition associated with an absence of enteric neurons in the distal gut. GDNF signals through Ret, a receptor tyrosine kinase, and Et-3 signals through endothelin receptor B (Ednrb). The effects of Gdnf, Ret, and ET-3 haploinsufficiency and a null mutation in ET-3 on spontaneous motility patterns in adult and developing mice were investigated. Video recordings were used to construct spatiotemporal maps of spontaneous contractile patterns in colon from postnatal and adult mice in vitro. In Ret(+/-) and ET-3(+/-) mice, which have normal numbers of enteric neurons, colonic migrating motor complexes (CMMCs) displayed similar properties under control conditions and following inhibition of nitric oxide synthase (NOS) activity to wild-type mice. In the colon of Gdnf(+/-) mice and in the ganglionic region of ET-3(-/-) mice, there was a 50-60% reduction in myenteric neuron number. In Gdnf(+/-) mice, CMMCs were present, but abnormal, and the proportion of myenteric neurons containing NOS was not different from that of wild-type mice. In the ganglionic region of postnatal ET-3(-/-) mice, CMMCs were absent, and the proportion of myenteric neurons containing NOS was over 100% higher than in wild-type mice. Thus impairments in spontaneous motility patterns in the colon of Gdnf(+/-) mice and in the ganglionic region of ET-3(-/-) mice are correlated with a reduction in myenteric neuron density.

Conditional ablation of GFRalpha1 in postmigratory enteric neurons triggers unconventional neuronal death in the colon and causes a Hirschsprung's disease phenotype

The regulation of neuronal survival and death by neurotrophic factors plays a central role in the sculpting of the nervous system, but the identity of survival signals for developing enteric neurons remains obscure. We demonstrate here that conditional ablation of GFRalpha1, the high affinity receptor for GDNF, in mice during late gestation induces rapid and widespread neuronal death in the colon, leading to colon aganglionosis reminiscent of Hirschsprung's disease. Enteric neuron death induced by GFRalpha1 inactivation is not associated with the activation of common cell death executors, caspase-3 or -7, and lacks the morphological hallmarks of apoptosis, such as chromatin compaction and mitochondrial pathology. Consistent with these in vivo observations, neither caspase inhibition nor Bax deficiency blocks death of colon-derived enteric neurons induced by GDNF deprivation. This study reveals an essential role for GFRalpha1 in the survival of enteric neurons and suggests that caspase-independent death can be triggered by abolition of neurotrophic signals.

The development of avian enteric nervous system: distribution of artemin immunoreactivity

Among the factors that control neural crest cell precursors within the enteric nervous system, the ligands of the glial cell line-derived neurotrophic factor family (GFL) seem to be the most influential. Artemin, a member of the GFLs, was previously described only in the oesophagus and stomach of mouse embryos. In this study, the presence and distribution of artemin is reported in duck embryos and adults. Artemin immunoreactivity was apparent in the intestinal tract at embryonic day 7 (d7), firstly in the myenteric plexus and then in the submucous plexus. Later, artemin immunoreactive nerve fibres were also seen in the longitudinal muscle plexus, the circular muscle plexus, the plexus of the muscularis mucosa and in the mucosal plexus. Furthermore, at d7, weak labeling of artemin was detected in neurons and glial cells in the oesophagus, gastric region and duodenum. Subsequently, artemin was also detected in all other intestinal segments. Moreover, during development of the gut in the domestic duck, artemin immunoreactivity decreased in neuronal cell bodies, whilst it increased in neuronal fibres and glial cells. These findings suggest an involvement of artemin in the development and biology of the gut of the domestic duck.

The contribution of the sonic hedgehog cascade in the development of the enteric nervous system in fetal rats with anorectal malformations

OBJECTIVE

This study was designed to determine the expression of Sonic hedgehog (Shh) and its downstream genes during development of the enteric nervous system (ENS) in ethylenethiourea (ETU)-exposed fetal rats with anorectal malformations (ARMs).

MATERIAL AND METHODS

Anorectal malformations were induced by 1% ETU (125 mg/kg) given on gestational day 10, and the litter was harvested at term. The fetal anorectum and rectosigmoid region, including any communication with the urinary tract, were collected for gene expression studies and immunofluorescence study of the ENS. Gene expression of Shh cascade was performed using reverse transcription and real-time polymerase chain reaction (PCR). The myenteric plexuses of the ENS in normal and ARM rats were visualized with fluorescent antibodies.

RESULTS

Reverse transcription-PCR confirmed expression of Shh and its target genes in all parts of the ARMs. Quantitative PCR demonstrated that levels of expression of the genes of the Shh cascade were low in the ARMs. The immunoreactivity of neuromarkers was markedly reduced in high ARMs and slightly reduced in low ARMs.

CONCLUSION

This study demonstrates reduced expression of Shh and its target genes in ARMs in ETU-exposed fetal rats. Neurons in the myenteric plexus were decreased in high and low types of ARMs. Our results support a role for the Shh cascade during development of the ENS during hindgut development.

Protein kinase Czeta and glycogen synthase kinase-3beta control neuronal polarity in developing rodent enteric neurons, whereas SMAD specific E3 ubiquitin protein ligase 1 promotes neurite growth but does not influence polarity

Enteric nervous system (ENS) precursors migrate extensively before differentiating to form uni-axonal or multi-axonal neurons. ENS precursor survival, neurite growth, and cell migration are all directed by Ret kinase, but downstream signaling pathways are incompletely understood. We now demonstrate that proteins regulating polarity in other cells including partitioning defective 3 (PAR3), PAR6, protein kinase Czeta (PKCzeta), and glycogen synthase kinase 3beta (GSK3beta) are expressed in developing enteric neurons with a polarized distribution. Blocking PKCzeta or GSK3beta reduces ENS precursor migration and induces the formation of multi-axonal neurons. Axon elongation also depends on SMURF1 (SMAD specific E3 ubiquitin protein ligase 1), which promotes RhoA degradation and associates with polarity proteins. SMURF1 inhibition, however, does not increase the number of multi-axonal neurons in ENS precursors. These data link cell surface Ret activation with molecular machinery controlling cytoskeletal dynamics and suggest that polymorphisms influencing PKCzeta or GSK3beta might alter Hirschsprung disease penetrance or expressivity by affecting ENS precursor migration.

Mathematical and experimental insights into the development of the enteric nervous system and Hirschsprung's disease

The vertebrate enteric nervous system is formed by a rostro-caudally directed invasion of the embryonic gastrointestinal mesenchyme by neural crest cells. Failure to complete this invasion results in the distal intestine lacking intrinsic neurons. This potentially fatal condition is called Hirschsprung's Disease. A mathematical model of cell invasion incorporating cell motility and proliferation of neural crest cells to a carrying capacity predicted invasion outcomes to imagined manipulations, and these manipulations were tested experimentally. Mathematical and experimental results agreed. The results show that the directional invasion is chiefly driven by neural crest cell proliferation. Moreover, this proliferation occurs in a small region at the wavefront of the invading population. These results provide an understanding of why many genes implicated in Hirschsprung's Disease influence neural crest population size. In addition, during in vivo development the underlying gut tissues are growing simultaneously as the neural crest cell invasion proceeds. The interactions between proliferation, motility and gut growth dictate whether or not complete colonization is successful. Mathematical modeling provides insights into the conditions required for complete colonization or a Hirschsprung's-like deficiency. Experimental evidence supports the hypotheses suggested by the modeling.

Neural crest and the development of the enteric nervous system

The formation of the enteric nervous system (ENS) is a particularly interesting example of the migratory ability of the neural crest and of the complexity of structures to which neural crest cells contribute. The distance that neural crest cells migrate to colonize the entire length of the gastrointestinal tract exceeds that of any other neural crest cell population. Furthermore, this migration takes a long time--over 25% of the gestation period for mice and around 3 weeks in humans. After colonizing the gut, neural crest-derived cells within the gut wall then differentiate into glial cells plus many different types of neurons, and generate the most complex part of the peripheral nervous system.

Simulating invasion with cellular automata: connecting cell-scale and population-scale properties

Interpretive and predictive tools are needed to assist in the understanding of cell invasion processes. Cell invasion involves cell motility and proliferation, and is central to many biological processes including developmental morphogenesis and tumor invasion. Experimental data can be collected across a wide range of scales, from the population scale to the individual cell scale. Standard continuum or discrete models used in isolation are insufficient to capture this wide range of data. We develop a discrete cellular automata model of invasion with experimentally motivated rules. The cellular automata algorithm is applied to a narrow two-dimensional lattice and simulations reveal the formation of invasion waves moving with constant speed. The simulation results are averaged in one dimension-these data are used to identify the time history of the leading edge to characterize the population-scale wave speed. This allows the relationship between the population-scale wave speed and the cell-scale parameters to be determined. This relationship is analogous to well-known continuum results for Fisher's equation. The cellular automata algorithm also produces individual cell trajectories within the invasion wave that are analogous to cell trajectories obtained with new experimental techniques. Our approach allows both the cell-scale and population-scale properties of invasion to be predicted in a way that is consistent with multiscale experimental data. Furthermore we suggest that the cellular automata algorithm can be used in conjunction with individual data to overcome limitations associated with identifying cell motility mechanisms using continuum models alone.

Coalescence of interacting cell populations

We analyse the coalescence of invasive cell populations by studying both the temporal and steady behaviour of a system of coupled reaction-diffusion equations. This problem is relevant to recent experimental observations of the dynamics of opposingly directed invasion waves of cells. Two cell types, u and v, are considered with the cell motility governed by linear or nonlinear diffusion. The cells proliferate logistically so that the long-term total cell density, u+v approaches a carrying capacity. The steady-state solutions for u and v are denoted u(s) and v(s). The steady solutions are spatially invariant and satisfy u(s)+v(s)=1. However, this expression is underdetermined so the relative proportion of each cell type u(s) and v(s) cannot be determined a priori. Various properties of this model are studied, such as how the relative proportion of u(s) and v(s) depends on the relative motility and relative proliferation rates. The model is analysed using a combination of numerical simulations and a comparison principle. This investigation unearths some novel outcomes regarding the role of overcrowding and cell death in this type of cell migration assay. These observations have relevance to experimental design and interpretation regarding the identification and parameterisation of mechanisms involved in cell invasion.

Enteric nervous system development and Hirschsprung's disease: advances in genetic and stem cell studies

The enteric nervous system (ENS) has been explored by developmental neurobiologists and medical researchers for decades. Whereas developmental biologists have been unravelling the molecular mechanisms underlying the migration, proliferation and differentiation of the neural crest derivatives that give rise to the ENS, human geneticists have been uncovering the genetic basis for diseases of the ENS, notably Hirschsprung's disease. Here we discuss the exciting recent advances, including novel transgenic and genetic tools, a broadening range of model organisms, and the pursuit of ENS stem cells as a therapeutic tool, that are bringing these fields closer together.

Hand2 is necessary for terminal differentiation of enteric neurons from crest-derived precursors but not for their migration into the gut or for formation of glia

Hand genes encode basic helix-loop-helix transcription factors that are expressed in the developing gut, where their function is unknown. We now report that enteric Hand2 expression is limited to crest-derived cells, whereas Hand1 expression is restricted to muscle and interstitial cells of Cajal. Hand2 is developmentally regulated and is intranuclear in precursors but cytoplasmic in neurons. Neurons develop in explants from wild-type but not Hand2(-/-) bowel, although, in both, crest-derived cells are present and glia arise. Similarly, small interfering RNA (siRNA) silencing of Hand2 in enteric crest-derived cells prevents neuronal development. Terminally differentiated enteric neurons do not develop after conditional inactivation of Hand2 in migrating crest-derived cells; nevertheless, conditional Hand2 inactivation does not prevent precursors from expressing early neural markers. We suggest that enteric neuronal development occurs in stages and that Hand2 expression is required for terminal differentiation but not for precursors to enter the neuronal lineage.

Biology of the adult enteric neural stem cell

An increasing body of evidence has accumulated in recent years supporting the existence of neural stem cells in the adult gut. There are at least three groups that have obtained them using different methodologies and have described them in vitro. There is a growing amount of knowledge on their biology, but many questions are yet unanswered. Among these questions is whether these cells are part of a permanent undifferentiated pool or are recruited in a regular basis; in addition, the factors and genes involved in their survival, proliferation, migration, and differentiation are largely unknown. Finally, with between 10 and 20% of adults suffering from diseases involving the enteric nervous system, most notably irritable bowel syndrome and gastroesophageal reflux, what is the possible role of enteric nervous stem cells in health and disease?

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.

The developmental stage and cell type dependent phosphorylation of eNOS in murine enteric mucosa and myenteric plexus

In order to clarify the developmental regulation of the eNOS activity in intestine by phosphorylation, we examined the immunohistochemical localizations of the eNOS phosphorylation sites at Ser(1177), Ser(116) and at Thr(495) in cells of the mouse enteric mucosa and myenteric plexus at E13.5, E14.5, E16.5, E18.5, E20.5 and P3. In addition, in cells of the E16.5 stage the protein levels of eNOS and the phosphorylation sites of eNOS at Ser(1177), Ser(116) and at Thr(495) were investigated by immunoblot. From E14.5 to P3, phosphorylation residues of eNOS at Ser(1177) and at Ser(116) were detected with different staining intensities in the enteric mucosa epithelium. In ganglion cells of the myenteric plexus Ser(116) was identified at E18.5 to P3. The absence of phosphorylated Thr(495) in cells of intestine during all developmental stages, was confirmed by immunoblot at E16.5. The immunoblot levels of eNOS and eNOS phosphorylated at Ser(1177) and at Ser(116) were comparable with the immunohistochemical results of E16.5 mouse intestine. It was concluded that development of epithelial cells of the enteric mucosa may be modulated by phosphorylation of eNOS at Ser(1177) and at Ser(116). The phosphorylation of eNOS in cells of the myenteric plexus is modulated at Ser(116). These data suggest that there is a developmental stage and cell type dependent phosphorylation of eNOS.

Increased apoptosis of parasympathetic but not enteric neurons in mice lacking GFRα2

Enteric neurons, unlike sympathetic and sensory neurons that require target-derived neurotrophins for survival, do not undergo classical caspase-3-mediated programmed cell death (PCD) during normal development. Whether parasympathetic neurons in the pancreas, which originate from a subpopulation of enteric nervous system (ENS) precursors, or other parasympathetic neurons undergo PCD during normal mammalian development is unknown. In GFRalpha2-deficient mice, many submandibular and intrapancreatic parasympathetic neurons are missing but whether this is due to increased neuronal death is unclear. Here we show that activated caspase-3 and PGP9.5 doubly positive neurons are present in wild-type mouse pancreas between embryonic day E15 and birth. Thus, in contrast to ENS neurons, intrapancreatic neurons undergo PCD via apoptosis during normal development. We also show that, in GFRalpha2-deficient mice, most intrapancreatic neurons are lost during this late fetal period, which coincides with a period of increased apoptosis of the neurons. Since the percentage of BrdU and Phox2b doubly positive cells in the fetal pancreas and the number of intrapancreatic neurons at E15 were similar between the genotypes, impaired precursor proliferation and migration are unlikely to contribute to the loss of intrapancreatic neurons in GFRalpha2-KO mice. Caspase-3-positive neurons were also found in GFRalpha2-deficient submandibular ganglia around birth, suggesting that parasympathetic neurons depend on limited supply of (presumably target-derived neurturin) signaling via GFRalpha2 for survival.

Comparison of three methods to assess enteric neuronal apoptosis in patients with slow transit constipation

BACKGROUND

The pathophysiological basis of slow transit constipation are scarcely understood. Some recent evidence suggests that increased apoptotic phenomena in the colonic enteric neurons may play a role. However, the best method to assess these phenomena has not been evaluated.

AIMS

To compare three different methods to detect enteric neuronal apoptosis in these patients.

METHODS

Serial colonic tissue sections obtained in 10 patients with intractable slow transit constipation were evaluated with immunohistochemical methods aimed at evaluating apoptotic phenomena: the formamide-MAb method, the TUNEL, and the caspase-3.

RESULTS

The highest yield of apoptotic neurons was obtained by means of the formamide-MAb method, compared to the other two, whereas the lowest yield was observed with the caspase-3.

CONCLUSIONS

The formamide-MAb method, which is able to distinguish apoptosis from necrosis and is not influenced by DNA breaks, may prove useful to assess neuronal apoptotic phenomena in the human enteric nervous system. This represents a relevant method to detect enteric neuronal apoptosis.

RET is dispensable for maintenance of midbrain dopaminergic neurons in adult mice

Glial cell-line derived neurotrophic factor (GDNF)-mediated RET tyrosine kinase signaling is implicated in the survival of several PNS and CNS neuronal populations that are important in the pathogenesis of several disorders including Parkinson's disease and drug addiction. However, it has been difficult to study these processes and the physiological importance of this pathway in adult mice because of the neonatal lethality of Gdnf and Ret null mice. We report successful creation of RET conditional reporter mice to investigate postnatal physiologic roles of RET and monitor the fate of RET-expressing cell types. To delete RET specifically in dopaminergic neurons and determine the physiologic requirement of RET in the maintenance of substantia nigra compacta (SNC) and ventral tegmental area (VTA), we bred the RET conditional mice with mice that specifically express Cre from the dopamine transporter (Dat) locus. A detailed morphometric and biochemical analysis including dopaminergic neuron number and size in SNC and VTA, and fiber density in the striatum and nucleus accumbens, and dopamine levels indicate that RET is not required for providing global trophic support to midbrain dopaminergic neurons in adult mice. Furthermore, RET deficiency in these neurons does not cause major sensorimotor abnormalities. Hence our results support the idea that RET signaling is not critical for the normal physiology of the SNC and VTA in adult mice.

Cell proliferation drives neural crest cell invasion of the intestine

A general mathematical model of cell invasion is developed and validated with an experimental system. The model incorporates two basic cell functions: non-directed (diffusive) motility and proliferation to a carrying capacity limit. The model is used here to investigate cell proliferation and motility differences along the axis of an invasion wave. Mathematical simulations yield surprising and counterintuitive predictions. In this general scenario, cells at the invasive front are proliferative and migrate into previously unoccupied tissues while those behind the front are essentially nonproliferative and do not directly migrate into unoccupied tissues. These differences are not innate to the cells, but are a function of proximity to uninvaded tissue. Therefore, proliferation at the invading front is the critical mechanism driving apparently directed invasion. An appropriate system to experimentally validate these predictions is the directional invasion and colonization of the gut by vagal neural crest cells that establish the enteric nervous system. An assay using gut organ culture with chick-quail grafting is used for this purpose. The experimental results are entirely concordant with the mathematical predictions. We conclude that proliferation at the wavefront is a key mechanism driving the invasive process. This has important implications not just for the neural crest, but for other invasion systems such as epidermal wound healing, carcinoma invasion and other developmental cell migrations.

Engineering the Recruitment of Phosphotyrosine Binding Domain-containing Adaptor Proteins Reveals Distinct Roles for RET Receptor-mediated Cell Survival

The RET receptor tyrosine kinase is important for several different biological functions during development. The recruitment at the phosphorylated Tyr(1062) site in RET of a number of different phosphotyrosine binding (PTB) domain-containing adaptor proteins, including Shc and Frs2, plays a dominant role for the multiple different biological functions of the RET receptor during development, including stimulation of cell survival. Here, we demonstrate that a competitive recruitment of Shc as opposed to Frs2 mediates the survival signaling arising from RET activation. Based on results from a peptide array, we have genetically engineered the PTB domain binding site of RET to rewire its recruitment of the PTB proteins Shc and Frs2. An engineered RET that has a competitive interaction with Shc at the expense of Frs2, but not a RET receptor that only recruits Frs2, activates cell survival signaling pathways and is protective from cell death in neuronal SK-N-MC cells. Thus, cell type-specific functions involve a competitive recruitment of different PTB adaptor molecules by RET that activate selective signaling pathways.

Advances in ontogeny of the enteric nervous system

The neurons and glia that comprise the enteric nervous system (ENS), the intrinsic innervation of the gastrointestinal tract, are derived from vagal and sacral regions of the neural crest. In order to form the ENS, neural crest-derived precursors undergo a number of processes including survival, migration and proliferation, prior to differentiation into neuronal subtypes, some of which form functional connections with the gut smooth muscle. Investigation of the developmental processes that underlie ENS formation has progressed dramatically in recent years, in no small part due to the attention of scientists from a range of disciplines on the genesis of Hirschsprung's disease (aganglionic megacolon), the major congenital abnormality of the ENS. This review summarizes recent advances in the field of early ENS ontogeny and focuses on: (i) the spatiotemporal migratory pathways followed by vagal and sacral neural crest-derived ENS precursors, including recent in vivo imaging of migrating crest cells within the gut, (ii) the roles of the RET and EDNRB signalling pathways and how these pathways interact to control ENS development, and (iii) how perpendicular migrations of neural crest cells within the gut lead to the formation of the myenteric and submucosal plexi located between the smooth muscle layers of the gut wall.

Glial-derived neurotrophic factor modulates enteric neuronal survival and proliferation through neuropeptide Y

BACKGROUND & AIMS

Glial-derived neurotrophic factor (GDNF) promotes the survival and proliferation of enteric neurons. Neuropeptide Y (NPY) is an important peptide regulating gastrointestinal motility. The role of NPY on the survival and proliferation of enteric neurons is not known. We examined the effects of GDNF on the expression and release of NPY from enteric neurons and the role of NPY in promoting enteric neuronal proliferation and survival.

METHODS

Studies were performed in primary enteric neuronal cultures and NPY knockout mice (NPY(-/-)). GDNF-induced expression of NPY was assessed by reverse-transcription polymerase chain reaction (RT-PCR), immunocytochemistry, and enzyme-linked immunosorbent assay. Using NPY-siRNA and NPY-Y1 receptor antagonist, we examined the role of NPY in mediating the survival and proliferation effects of GDNF. Gastrointestinal motility was assessed by measuring gastric emptying, intestinal transit, and isometric muscle recording from intestinal muscle strips.

RESULTS

GDNF induced a significant increase in NPY messenger RNA and protein expression in primary enteric neurons and the release of NPY into the culture medium. NPY (1 mumol/L) significantly increased proliferation of neurons and reduced apoptosis. In the presence of NPY-siRNA and NPY-Y1 receptor antagonist or in enteric neurons cultured from NPY(-/-) mice, GDNF-mediated neuronal proliferation and survival was reduced. NPY increased the phosphorylation of Akt, a downstream target of the PI-3-kinase pathway. In NPY(-/-) mice, there were significantly fewer nNOS-containing enteric neurons compared with wild-type (WT) mice. NPY(-/-) mice had accelerated gastric emptying and delayed intestinal transit compared with WT mice.

CONCLUSIONS

We demonstrate that NPY acts as an autocrine neurotrophic factor for enteric neurons.

Glial cell line-derived neurotrophic factor-mediated enteric neuronal survival involves glycogen synthase kinase-3beta phosphorylation and coupling with 14-3-3

Glial cell line-derived neurotrophic factor (GDNF) promotes the growth and survival of enteric neurons, but the mechanisms involved are poorly understood. GDNF is known to promote the survival of enteric neurons through activation of the PI3-Kinase/Akt signaling pathway. We investigated the role of glycogen synthase kinase-3beta (GSK-3beta) in enteric neuronal survival, and the ability of GDNF to regulate the activity of GSK-3beta using primary rat embryonic enteric neurons. GDNF, through activation of the PI3-kinase pathway enhanced the phosphorylation of GSK-3beta at its N-terminal serine-9 residue, and promoted the association of GSK-3beta with 14-3-3. Transfection of a constitutively active S9A-GSK-3beta mutant prevented the survival effects of GDNF, whereas a dominant negative GSK-3beta construct prevented GDNF withdrawal-induced cell death. Increased GSK-3beta activity was associated with an increase in tau phosphorylation. Thus, GDNF promotes enteric neuronal survival by modulating GSK-3beta and its downstream target tau. Inhibitors of GSK-3beta activity may have therapeutic potential in improving enteric neuronal survival.

The Ret(C620R) mutation affects renal and enteric development in a mouse model of Hirschsprung's disease

In rare families RET tyrosine kinase receptor substitutions located in exon 10 (especially at positions 609, 618, and 620) can concomitantly cause the MEN 2A (multiple endocrine neoplasia type 2A) or FMTC (familial medullary thyroid carcinoma) cancer syndromes, and Hirschsprung's disease (HSCR). No animal model mimicking the co-existence of the MEN 2 pathology and HSCR is available, and the association of these activating mutations with a developmental defect still represents an unresolved problem. The aim of this work was to investigate the significance of the RET(C620R) substitution in the pathogenesis of both gain- and loss-of-function RET-associated diseases. We report the generation of a line of mice carrying the C620R mutation in the Ret gene. Although Ret(C620R) homozygotes display severe defects in kidney organogenesis and enteric nervous system development leading to perinatal lethality. Ret(C620R) heterozygotes recapitulate features characteristic of HSCR including hypoganglionosis of the gastrointestinal tract. Surprisingly, heterozygotes do not show any defects in the thyroid that might be attributable to a gain-of-function mutation. The Ret(C620R) allele is responsible for HSCR and affects the development of kidneys and the enteric nervous system (ENS). These mice represent an interesting model for studying new therapeutic approaches for the treatment of HSCR disease.

Maintenance of mammalian enteric nervous system progenitors by SOX10 and endothelin 3 signalling

The transcriptional regulator SOX10 and the signalling molecule endothelin 3 have important roles in the development of the mammalian enteric nervous system (ENS). Using a clonal cell culture system, we show that SOX10 inhibits overt neuronal and glial differentiation of multilineage ENS progenitor cells (EPCs), without interfering with their neurogenic commitment. We also demonstrate that endothelin 3 inhibits reversibly the commitment and differentiation of EPCs along the neurogenic and gliogenic lineages, suggesting a role for this factor in the maintenance of multilineage ENS progenitors. Consistent with such a role, the proportion of Sox10-expressing progenitors in the total population of enteric neural crest cells is reduced in the gut of endothelin 3-deficient embryos. This reduction may be related to the requirement of endothelin signalling for the proliferation of ENS progenitors. The dependence of ENS progenitors on endothelin 3 is more pronounced at the migratory front of enteric neural crest cells, which is associated with relatively high levels of endothelin 3 mRNA. Our findings indicate that SOX10 and endothelin 3 have a crucial role in the maintenance of multilineage enteric nervous system progenitors.

Differential effects of glial cell line-derived neurotrophic factor and neurturin in RET/GFRalpha1-expressing cells

The c-ret protooncogene, RET, encodes a receptor tyrosine kinase. RET is activated by members of the glial cell line-derived neurotrophic factor (GDNF) family of ligands, which include GDNF, neurturin, artemin, and persephin. The ligands bind RET through GDNF family receptor alpha, termed GFRalpha1-4. Despite the importance of RET signaling in the development of the enteric nervous system and the kidney, the differential signaling mechanisms between RET ligands are poorly established. It has been suggested that signal specificity is achieved through binding of the ligand to its preferred GFRalpha. To compare the signaling profiles of GDNF and neurturin, we have identified a cell line, NG108-15, which endogenously expresses RET and GFRalpha1 but not GFRalpha2-4. Immunoblot data showed that GDNF caused a transient activation, whereas neurturin caused a sustained activation, of both p44/p42 MAP kinases and PLCgamma. Under serum starvation, NG108-15 cells differentiate and form neurites. Neurturin but not GDNF stimulated neurite outgrowth, which could be blocked by the selective PLC inhibitor U73122. On the other hand, GDNF but not neurturin promoted cell survival, and this could be blocked by the p44/p42 MAP kinase inhibitor PD98059. Our findings not only show the differential signaling of GDNF and neurturin but also suggest that this can be achieved through binding to the same GFRalpha subtype, leading to distinct biological responses.

Inhibition of protein kinase A in murine enteric neurons causes lethal intestinal pseudo-obstruction

A number of in vitro studies suggest that many important developmental and functional events in the enteric nervous system are regulated by the intracellular signaling enzyme cAMP protein kinase A (PKA). To evaluate the in vivo significance of these observations, a Cre-inducible, dominant-negative, mutant regulatory subunit (RIalphaB) of PKA was activated in enteric neurons by either a Proteolipid protein-Cre transgene or a Hox11L1-Cre "knock-in" allele. In both models, RIalphaB activation resulted consistently in profound distension of the proximal small intestine within 2 weeks after birth. Intestinal transit of radio-opaque tracers was severely retarded in the double-transgenic animals, which died shortly after weaning. In the enteric nervous system, recombination was restricted to neurons as demonstrated by histochemical analysis and confocal microscopic colocalization of a Cre recombinase-dependent reporter gene with the neuronal marker Hu(C/D), in contrast with the glial marker S100. Histochemical analysis of beta-galactosidase expression and acetylcholinesterase activity, as well as neuronal counts, demonstrated that intestinal dysmotility was not associated with obvious malformation of the myenteric plexus. However, inhibition of PKA activity in enteric neurons disrupted the major motor complexes of isolated intestinal segments in vitro. These results provide strong evidence that PKA activity plays a critical role in enteric neurotransmission in vivo, and highlight neuronal PKA or related signaling molecules as potential therapeutic targets in gastrointestinal motility disorders.

Reduced endothelin converting enzyme-1 and endothelin-3 mRNA in the developing bowel of male mice may increase expressivity and penetrance of Hirschsprung disease–like distal intestinal aganglionosis

Hirschsprung disease (distal intestinal aganglionosis, HSCR) is a multigenic disorder with incomplete penetrance, variable expressivity, and a strong male gender bias. Recent studies demonstrated that these genetic patterns arise because gene interactions determine whether enteric nervous system (ENS) precursors successfully proliferate and migrate into the distal bowel. We now demonstrate that male gender bias in the extent of distal intestinal aganglionosis occurs in mice with Ret dominant-negative mutations (RetDN) that mimic human HSCR. We hypothesized that male gender bias could result from reduced expression of a gene already known to be essential for ENS development. Using quantitative real-time polymerase chain reaction (PCR) we demonstrated reduced levels of endothelin converting enzyme-1 and endothelin-3 mRNA in the male mouse bowel at the time that ENS precursors migrate into the colon. Other HSCR-associated genes are expressed at comparable levels in male and female mice. Testosterone and Mullerian inhibiting substance had no deleterious effect on ENS precursor development, but adding EDN3 peptide to E11.5 male RetDN heterozygous mouse gut explants in organ culture significantly increased the rate of ENS precursor migration through the bowel.

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.

BMP signaling is necessary for neural crest cell migration and ganglion formation in the enteric nervous system

The enteric nervous system (ENS) is derived from neural crest cells that migrate along the gastrointestinal tract to form a network of neurons and glia that are essential for regulating intestinal motility. Despite the number of genes known to play essential roles in ENS development, the molecular etiology of congenital disorders affecting this process remains largely unknown. To determine the role of bone morphogenetic protein (BMP) signaling in ENS development, we first examined the expression of bmp2, bmp4, and bmprII during hindgut development and find these strongly expressed in the ENS. Moreover, functional BMP signaling, demonstrated by the expression of phosphorylated Smad1/5/8, is present in the enteric ganglia. Inhibition of BMP activity by noggin misexpression within the developing gut, both in ovo and in vitro, inhibits normal migration of enteric neural crest cells. BMP inhibition also leads to hypoganglionosis and failure of enteric ganglion formation, with crest cells unable to cluster into aggregates. Abnormalities of migration and ganglion formation are the hallmarks of two human intestinal disorders, Hirschsprung's disease and intestinal neuronal dysplasia. Our results support an essential role for BMP signaling in these aspects of ENS development and provide a basis for further investigation of these proteins in the etiology of neuro-intestinal disorders.

Glial cell-line derived neurotrophic factor-mediated RET signaling regulates spermatogonial stem cell fate

Normal spermatogenesis is essential for reproduction and depends on proper spermatogonial stem cell (SSC) function. Genes and signaling pathways that regulate SSC function have not been well defined. We report that glial cell-line-derived neurotrophic factor (GDNF) signaling through the RET tyrosine kinase/GFRA1 receptor complex is required for spermatogonial self-renewal in mice. GFRA1 and RET expression was identified in a subset of gonocytes at birth, was restricted to SSCs during normal spermatogenesis, and RET expressing cells were abundant in a cryptorchid model of SSC self-renewal. We used the whole-testis transplantation technique to overcome the limitation of neonatal lethality of Gdnf-, Gfra1-, and Ret-deficient mice and found that each of these genes is required for postnatal spermatogenesis and not for embryological testes development. Each mutant testis shows severe SSC depletion by Postnatal Day 7 during the first wave of spermatogenesis. These defects were due to lack of SSC proliferation and an inability of SSCs to maintain an undifferentiated state. Our results demonstrate that GDNF-mediated RET signaling is critical for the fate of undifferentiated spermatogonia and that abnormalities in this pathway may contribute to male infertility and testicular germ cell tumors.

GDNF family ligand immunoreactivity in the gut of teleostean fish

Glial-derived neurotrophic factor (GDNF), neurturin (NRTN), persephin (PSPN), and artemin (ARTN) are a group of proteins belonging to the GDNF family ligands (GFLs). GDNF, NRTN, and ARTN support the survival of central, peripheral, and autonomic neuron populations, while PSPN supports the survival of only several central neuron populations. A common receptor, RET, modulates the action of this family and a co-receptor, GFRalpha, determines RET ligand specificity. GDNF and NRTN appear to be essential for enteric nervous system (ENS) development in mammals, zebrafish, and other teleostean species. GFLs are also essential for the maintenance and plasticity of adult mammalian ENS. In this study, the distribution pattern of GFLs in the intestine of five adult fish (bass, gilt-head, scorpionfish, trout, and zebrafish) was evaluated by immunochemical and immunocytochemical analysis. The results demonstrated the presence of GDNF, NRTN, and ARTN in the gut of all species studied. They appeared to be spread in the ENS and/or endocrine cells of the intestine. These findings suggest that the presence of GFLs in fish gut is not only limited to developmental period, but could be also involved in the enteric physiology of adult species.

Intestinal growth and differentiation in zebrafish

Intestinal development in amniotes is driven by interactions between progenitor cells derived from the three primary germ layers. Genetic analyses and gene targeting experiments in zebrafish offer a novel approach to dissect such interactions at a molecular level. Here we show that intestinal anatomy and architecture in zebrafish closely resembles the anatomy and architecture of the mammalian small intestine. The zebrafish intestine is regionalized and the various segments can be identified by epithelial markers whose expression is already segregated at the onset of intestinal differentiation. Differentiation of cells derived from the three primary germ layers begins more or less contemporaneously, and is preceded by a stage in which there is rapid cell proliferation and maturation of epithelial cell polarization. Analysis of zebrafish mutants with altered epithelial survival reveals that seemingly related single gene defects have different effects on epithelial differentiation and smooth muscle and enteric nervous system development.

Enteric neuroblasts require the phosphatidylinositol 3-kinase/Akt/Forkhead pathway for GDNF-stimulated survival

Glial cell line-derived neurotrophic factor (GDNF)/Ret signaling is required for enteric neural crest survival, proliferation, migration and process extension, but signaling pathways that mediate enteric nervous system (ENS) precursor development are poorly understood. We therefore examined GDNF effects on immunoselected ENS precursor survival and neuronal process extension in the presence of phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathway inhibitors. These studies demonstrated that GDNF promotes ENS precursor survival through phosphatidylinositol-3-kinase. Specifically, GDNF induces phosphorylation of Akt and loss of the Akt substrates FOXO1 and FOXO3a from the nucleus of ENS precursors. Furthermore, dominant negative Akt or active FOXO1 constructs promote ENS precursor cell death while a dominant negative FOXO1 construct prevents cell death. In contrast, the MAPK kinase inhibitor PD98059 did not influence ENS precursor survival or neurite extension. These data demonstrate a critical role for PI-3 kinase/Akt/FOXO signaling, but not for MAPK in ENS precursor survival and neurite extension.

Regulation of neural development by glial cell line-derived neurotrophic factor family ligands

Glial cell line-derived neurotrophic factor (GDNF) and its three relatives constitute a novel family of neurotrophic factors, the GDNF family ligands. These factors signal through a multicomponent receptor complex comprising a glycosylphosphatidylinositol-anchored cell surface molecule (GDNF family receptor (GFR) alpha) and RET tyrosine kinase, triggering the activation of multiple signaling pathways in responsive cells. Recent gene-targeting studies have demonstrated that GDNF family ligands are essential for the development of a diverse set of neuronal populations and we have now started to understand how these ligands uniquely regulate the formation and sculpting of the nervous system. Recent studies have also revealed interactions by multiple extracellular signals during neural development. The deciphering of GDNF family ligand signaling in neural cells promises to provide vital new insights into the development and pathology of the nervous system.

A human yeast artificial chromosome containing the multiple endocrine neoplasia type 2B Ret mutation does not induce medullary thyroid carcinoma but does support the growth of kidneys and partially rescues enteric nervous system development in Ret-deficient mice

We generated a line of transgenic mice using a yeast artificial chromosome containing the Ret mutation responsible for the multiple endocrine neoplasia type 2B syndrome (MEN 2B). The resulting animals did not develop any of the expected neoplasms associated with MEN 2B. Transgenic animals were then bred with animals lacking murine Ret (Ret(M)) to further evaluate the function of human mutated Ret (Ret(H)(2B)) in the murine context. Whereas mice lacking Ret(M) exhibit intestinal aganglionosis and the absence of kidneys with other genitourinary anomalies, expression of the Ret(H)(2B) transgene in Ret(M)-deficient mice allowed significant renal development with a partial rescue of the enteric nervous system. These Ret(H)(2B)-positive/Ret(M)-deficient mice exhibit normal Ret expression and survive longer than Ret(M)-deficient mice, but still die at 3 to 5 days of age with evidence of enterocolitis. We conclude that the normal expression of a human Ret proto-oncogene with the MEN 2B mutation does not cause any features of MEN 2B in mice. Although the gene is normally expressed in the appropriate target tissues, there is incomplete phenotypic rescue in mice lacking murine Ret. These results suggest important interspecies differences between humans and mice in the function of the Ret oncogene.

Enteric nervous system

PURPOSE OF THE REVIEW

The purpose of this review is to provide a synopsis of how the field of enteric neurobiology has advanced during the past 2 years.

RECENT FINDINGS

With more than 500 studies from which to choose, the authors have focused on several themes that illustrate recent progress. There has been an explosion of interest in the development of the enteric nervous system driven by the need to understand development abnormalities, particularly in Hirschsprung disease, and fueled by technical advances for investigating how neural crest-derived cells migrate, proliferate, and differentiate into enteric neurons and glia. The use of neural stem cells as a therapeutic strategy aimed at repopulating regions of bowel, where enteric neurones are reduced or absent, is on the horizon. Enteric reflexes involve interactions between sensory neurons, interneurons, and motor neurons. Recent findings suggest this distinction may be blurred, with neurons having multifunctional properties, perhaps because enteric neurons, unlike their central nervous system counterparts, are directly exposed to mechanical forces that they regulate. Another topic the authors have highlighted is pharmacology, with new tools for investigating ion channels, receptors, and transporters, leading to an expanding list of molecular mechanisms that regulate neuronal excitability. Long-term alterations in the expression of these molecules during disease or injury may underlie many gastrointestinal disorders that currently have unknown etiology. The authors finish with a look to the future and what may be the subject of this review next time.

SUMMARY

Basic science information gathered during the past 2 years provides insight into pathophysiologic processes and will pave the wave for improved understanding of both organic and 'functional' gastrointestinal disorders.

The location and phenotype of proliferating neural-crest-derived cells in the developing mouse gut

Neural crest cells that originate in the caudal hindbrain migrate into and along the developing gastrointestinal tract to form the enteric nervous system. While they are migrating, neural-crest-derived cells are also proliferating. Previous studies have shown that the expression of glial-derived neurotrophic factor (GDNF) and endothelin-3 is highest in the embryonic caecum, and that GDNF alone or in combination with endothelin-3 promotes the proliferation of enteric neural-crest-derived cells in vitro. However, whether neural proliferative zones, like those in the central nervous system, are found along the developing gut is unknown. We used a fluorescent nucleic acid stain to identify dividing cells or BrdU labelling (2 h after administration of BrdU to the mother), combined with antibodies specific to neural crest cells to determine the percentage of proliferating crest-derived cells in various gut regions of embryonic day 11.5 (E11.5) and E12.5 mice. The rate of proliferation of crest-derived cells did not vary significantly in different regions of the gut (including the caecum) or at different distances from the migratory wavefront of vagal crest-derived cells. The phenotype of mitotic enteric crest-derived cells was also examined. Cells expressing the pan-neuronal markers, neurofilament-M and Hu, or the glial marker, S100b, were observed undergoing mitosis. However, no evidence was found for proliferation of cells expressing neuron-type-specific markers, such as nitric oxide synthase (at E12.5) or calcitonin gene-related peptide (at E18.5). Thus, for enteric neurons, exit from the cell cycle appears to occur after the expression of pan-neuronal proteins but prior to the expression of markers of terminally differentiated neurons.

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

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

GFRalpha1 expression in cells lacking RET is dispensable for organogenesis and nerve regeneration

The GDNF family ligands signal through a receptor complex composed of a ligand binding subunit, GFRalpha, and a signaling subunit, the RET tyrosine kinase. GFRalphas are expressed not only in RET-expressing cells, but also in cells lacking RET. A body of evidence suggests that RET-independent GFRalphas are important for (1) modulation of RET signaling in a non-cell-autonomous fashion (trans-signaling) and (2) regulation of NCAM function. To address the physiological significance of these roles, we generated mice specifically lacking RET-independent GFRalpha1. These mice exhibited no deficits in regions where trans-signaling has been implicated in vitro, including enteric neurons, motor neurons, kidney, and regenerating nerves. Furthermore, no abnormalities were found in the olfactory bulb, which requires proper NCAM function for its formation and is putatively a site of GDNF-GFRalpha-NCAM signaling. Thus RET-independent GFRalpha1 is dispensable for organogenesis and nerve regeneration in vivo, indicating that trans-signaling and GFRalpha-dependent NCAM signaling play a minor role physiologically.

Sonic hedgehog regulates the proliferation, differentiation, and migration of enteric neural crest cells in gut

Enteric neural crest cells (NCCs) migrate and colonize the entire gut and proliferate and differentiate into neurons and glia of the enteric nervous system in vertebrate embryos. We have investigated the mitogenic and morphogenic functions of Sonic hedgehog (Shh) on enteric NCCs in cell and organ culture. Enteric NCCs expressed Shh receptor Patched and transcripts encoding the Shh signal transducer (Gli1). Shh promoted the proliferation and inhibited the differentiation of NCCs. The pro-neurogenic effect of glial cell line–derived neurotrophic factor (GDNF) on NCCs was abolished by Shh. In gut explants, NCCs migrated from the explants onto the adjacent substratum if GDNF was added, whereas addition of Shh abolished this migration. Neuronal differentiation and coalescence of neural crest–derived cells into myenteric plexuses in explants was repressed by the addition of Shh. Our data suggest that Shh controls the proliferation and differentiation of NCCs and modulates the responsiveness of NCCs toward GDNF inductions.

Mice expressing a dominant-negative Ret mutation phenocopy human Hirschsprung disease and delineate a direct role of Ret in spermatogenesis

The Ret receptor tyrosine kinase mediates physiological signals of glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) and is essential for postnatal survival in mice. It is implicated in a number of human diseases and developmental abnormalities. Here, we describe our analyses of mice expressing a Ret mutant (RetDN) with diminished kinase activity that inhibits wild-type Ret activity, including its activation of AKT. All RetDN/+ mice died by 1 month of age and had distal intestinal aganglionosis reminiscent of Hirschsprung disease (HSCR) in humans. The RetDN/+ proximal small intestine also had severe hypoganglionosis and reduction in nerve fiber density, suggesting a potential mechanism for the continued gastric dysmotility in postsurgical HSCR patients. Unlike Ret-null mice, which have abnormalities in the parasympathetic and sympathetic nervous systems, the RetDN/+ mice only had defects in the parasympathetic nervous system. A small proportion of RetDN/+ mice had renal agenesis, and the remainder had hypoplastic kidneys and developed tubulocystic abnormalities postnatally. Postnatal analyses of the testes revealed a decreased number of germ cells, degenerating seminiferous tubules,maturation arrest and apoptosis, indicating a crucial role for Ret in early spermatogenesis.

A targeting mutation of tyrosine 1062 in Ret causes a marked decrease of enteric neurons and renal hypoplasia

The Ret receptor tyrosine kinase plays a crucial role in the development of the enteric nervous system and the kidney. Tyrosine 1062 in Ret represents a binding site for the phosphotyrosine-binding domains of several adaptor and effector proteins that are important for the activation of intracellular signaling pathways, such as the RAS/ERK, phosphatidylinositol 3-kinase/AKT, and Jun-associated N-terminal kinase pathways. To investigate the importance of tyrosine 1062 for organogenesis in vivo, knock-in mice in which tyrosine 1062 in Ret was replaced with phenylalanine were generated. Although homozygous knock-in mice were born normally, they died by day 27 after birth and showed growth retardation. The development of the enteric nervous system was severely impaired in homozygous mutant mice, about 40% of which lacked enteric neurons in the whole intestinal tract, as observed in Ret-deficient mice. The rest of the mutant mice developed enteric neurons in the intestine to various extents, although the size and number of ganglion cells were significantly reduced. Unlike Ret-deficient mice, a small kidney developed in all knock-in mice, accompanying a slight histological change. The reduction of kidney size was due to a decrease of ureteric bud branching during embryogenesis. Thus, these findings demonstrated that the signal via tyrosine 1062 plays an important role in histogenesis of the enteric nervous system and nephrogenesis.

Guidance cues involved in the development of the peripheral autonomic nervous system

All peripheral autonomic neurons arise from neural crest cells that migrate away from the neural tube and navigate to the location where ganglia will form. After differentiating into neurons, their axons then navigate to a variety of targets. During the development of the enteric nervous system, GDNF appears to play a role in inducing vagal neural crest cells to enter the gut, in retaining neural crest cells within the gut and in promoting the migration of neural crest cells along the gut. Sema3A regulates the entry of extrinsic axons into the distal hindgut, netrin-DCC signaling is responsible for the centripetal migration of cells to form the submucosal ganglia within the gut, Slit-Robo signaling prevents trunk level neural crest cells from entering the gut, and neurturin plays a role in the innervation of the circular muscle layer. During the development of the sympathetic nervous system, the migration of trunk neural crest cells through the somites is influenced by ephrin-Bs, Sema3A and F-spondin. The migration of neural crest cells ventrally beyond the somites requires neuregulin signaling and the clumping of cells into columns adjacent to the dorsal aorta is regulated by Sema3A. The rostral migration of cells to form the superior cervical ganglion (SCG) and the extension of axons along blood vessels involves artemin signaling through Ret and GFRalpha3, and the entry of sympathetic axons into target tissues involves neurotrophins and GDNF. Relatively little is known about the development of parasympathetic ganglia, but GDNF appears to play a role in the migration of some cranial ganglion precursors to their correct location, and both GDNF and neurturin are involved in the growth of parasympathetic axons into particular targets.