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

Abnormal development of the enteric nervous system in rat embryos and fetuses with congenital diaphragmatic hernia

BACKGROUND/AIM

Esophageal dilatation, gastroesophageal reflux, and intestinal obstruction have been demonstrated in CDH survivors. Abnormal esophageal and intestinal innervations were recently found in rats and babies with this disease. Our aim was to further characterize these malformations in embryos and fetal rats exposed to nitrofen.

METHODS

Pregnant rats received either 100 mg nitrofen or vehicle on E9.5. Fetuses were recovered at E15, E18, and E21. Sections of esophagus and small bowel were histochemically stained with acetylcholinesterase (AChE) and immunostained for PGP9.5. PGP9.5 gen protein were measured on E21 and PGP9.5 mRNA on E15, E18 and E21. Comparisons between groups were made with non-parametrics tests.

RESULTS

Histochemistry and immunohistochemistry showed deficient innervation in all anatomical areas studied at E15, E18, and E21, and WB confirmed this decrease in E21 fetuses. PGP9.5 messenger was decreased in nitrofen-exposed animals on E18 (esophagus) or E15 (small bowel), and increased on E21 in the esophagus and E18 in small bowel.

CONCLUSIONS

Development of the enteric nervous system of the esophagus, stomach, and small bowel is deficient in rat embryos and fetuses exposed to nitrofen. These anomalies could account in part for the long-term gastrointestinal morbidity observed in CDH survivors.

Characterization of zebrafish intestinal smooth muscle development using a novel sm22α-b promoter

Smooth muscle cells provide structural support for many tissues and control essential physiological processes, such as blood pressure and gastrointestinal motility. Relatively little is known about the early stages of intestinal smooth muscle development and its relationship to the development of the enteric nervous system, which regulates intestinal motility. Here, we report an evolutionarily conserved 523 base pair regulatory element within the promoter of the zebrafish sm22α-b (transgelin1) gene that directs transgene expression in smooth muscle cells of the intestine and other tissues. Comparative genomic analysis identified a conserved motif within this element consisting of two Serum Response Factor binding sites that is also present in the promoters of many mammalian smooth muscle genes. We established a stable line expressing GFP in smooth muscle cell and used this line to describe lineage relationships among cells within different intestinal smooth muscle layers and their co-development with the enteric nervous system (ENS).

CXCR4 controls ventral migration of sympathetic precursor cells

The molecular mechanisms that sort migrating neural crest cells (NCCs) along a shared pathway into two functionally discrete structures, the dorsal root ganglia and sympathetic ganglia (SGs), are unknown. We report here that this patterning is attributable in part to differential expression of the chemokine receptor, CXCR4. We show that (1) a distinct subset of ventrally migrating NCCs express CXCR4 and this subset is destined to form the neural core of the sympathetic ganglia, and (2) the CXCR4 ligand, SDF-1, is a chemoattractant for NCCs in vivo and is expressed adjacent to the future SGs. Reduction of CXCR4 expression in NCCs disrupts their migration toward the future SGs, whereas overexpression of CXCR4 in non-SG-destined NCCs induces them to migrate aberrantly toward the SGs. These data are the first to demonstrate a major role for chemotaxis in the patterning of NCC migration and demonstrate the neural crest is composed of molecularly heterogeneous cell populations.

Distinctive expression patterns of Hedgehog pathway genes in the Ciona intestinalis larva: implications for a role of Hedgehog signaling in postembryonic development and chordate evolution

Members of the Hedgehog (Hh) family are soluble ligands that orchestrate a wide spectrum of developmental processes ranging from left-right axis determination of the embryo to tissue patterning and organogenesis. Tunicates, including ascidians, are the closest relatives of vertebrates, and elucidation of Hh signaling in ascidians should provide an important clue towards better understanding the role of this pathway in development. In previous studies, expression patterns of genes encoding Hh and its downstream factor Gli have been examined up to the tailbud stage in the ascidian embryo, but their expression in the larva has not been reported. Here we show the spatial expression patterns of hedgehog (Ci-hh1, Ci-hh2), patched (Ci-ptc), smoothened (Ci-smo), and Gli (Ci-Gli) orthologs in larvae of the ascidian Ciona intestinalis. The expression patterns of Ci-hh2 and Ci-Gli dramatically change during the period between the late tailbud embryo and the swimming larva. At the larval stage, expression of Ci-Gli was found in a central part of the endoderm and in the visceral ganglion, while Ci-hh2 was expressed in two discrete endodermal regions, anteriorly and posteriorly adjacent to the cells expressing Gli. The expression patterns of these genes suggest that the Hh ligand controls postembryonic development of the endoderm and the central nervous system. Expression of a gene encoding Hh in the anterior and/or pharyngeal endoderm is probably an ancient chordate character; diversification of regulation and targets of the Hh signaling in this region may have played a major role in the evolution of chordate body structures.

Development of the autonomic nervous system: a comparative view

In this review we summarize current understanding of the development of autonomic neurons in vertebrates. The mechanisms controlling the development of sympathetic and enteric neurons have been studied in considerable detail in laboratory mammals, chick and zebrafish, and there are also limited data about the development of sympathetic and enteric neurons in amphibians. Little is known about the development of parasympathetic neurons apart from the ciliary ganglion in chicks. Although there are considerable gaps in our knowledge, some of the mechanisms controlling sympathetic and enteric neuron development appear to be conserved between mammals, avians and zebrafish. For example, some of the transcriptional regulators involved in the development of sympathetic neurons are conserved between mammals, avians and zebrafish, and the requirement for Ret signalling in the development of enteric neurons is conserved between mammals (including humans), avians and zebrafish. However, there are also differences between species in the migratory pathways followed by sympathetic and enteric neuron precursors and in the requirements for some signalling pathways.

Development of enteric neuron diversity

The mature enteric nervous system (ENS) is composed of many different neuron subtypes and enteric glia, which all arise from the neural crest. How this diversity is generated from neural crest-derived cells is a central question in neurogastroenterology, as defects in these processes are likely to underlie some paediatric motility disorders. Here we review the developmental appearance (the earliest age at which expression of specific markers can be localized) and birthdates (the age at which precursors exit the cell cycle) of different enteric neuron subtypes, and their projections to some targets. We then focus on what is known about the mechanisms underlying the generation of enteric neuron diversity and axon pathfinding. Finally, we review the development of the ENS in humans and the etiologies of a number of paediatric motility disorders.

Sonic hedgehog signaling is required for sympathetic nervous system development

Sonic hedgehog (Shh) plays critical roles during nervous system development, yet little is known about its function in the sympathetic nervous system. Using a mouse Shh null line, we examined the roles of Shh during SNS development. Loss of Shh did not prevent formation of the sympathetic trunk, but the ganglia are hypoplastic and misspatterned. Neuronal differentiation was delayed in Shh mutant embryos showing that Shh is required for correct developmental timing in addition to its role in sympathetic nervous system patterning. Immunohistochemical analyses of the ganglia for expression of the pan-neuronal marker beta3-tubulin, the noradrenergic biosynthetic enzyme tyrosine hydroxylase and the glial marker B-FABP showed that Shh is not required for differentiation of sympathetic neurons or glia.

High frequency of cephalic neural crest cells shows coexistence of neurogenic, melanogenic, and osteogenic differentiation capacities

The neural crest (NC) is a vertebrate innovation that distinguishes vertebrates from other chordates and was critical for the development and evolution of a "New Head and Brain." In early vertebrates, the NC was the source of dermal armor of fossil jawless fish. In extant vertebrates, including mammals, the NC forms the peripheral nervous system, melanocytes, and the cartilage and bone of the face. Here, we show that in avian embryos, a large majority of cephalic NC cells (CNCCs) have the ability to differentiate into cell types as diverse as neurons, melanocytes, osteocytes, and chondrocytes. Moreover, we find that the morphogen Sonic hedgehog (Shh) acts on CNCCs to increase endochondral osteogenesis while having no effect on osteoblasts prone to membranous ossification. We have developed culture conditions that demonstrate that "neural-mesenchymal" differentiation abilities are present in more than 90% of CNCCs. A highly multipotent progenitor (able to yield neurons, glia, melanocytes, myofibroblasts, chondrocytes, and osteocytes) comprises 7-13% of the clonogenic cells in the absence and presence of Shh, respectively. This progenitor is a good candidate for a cephalic NC stem cell.

Abnormal nuclear pore formation triggers apoptosis in the intestinal epithelium of elys-deficient zebrafish

BACKGROUND & AIMS

Zebrafish mutants generated by ethylnitrosourea-mutagenesis provide a powerful tool for dissecting the genetic regulation of developmental processes, including organogenesis. One zebrafish mutant, "flotte lotte" (flo), displays striking defects in intestinal, liver, pancreas, and eye formation at 78 hours postfertilization (hpf). In this study, we sought to identify the underlying mutated gene in flo and link the genetic lesion to its phenotype.

METHODS

Positional cloning was employed to map the flo mutation. Subcellular characterization of flo embryos was achieved using histology, immunocytochemistry, bromodeoxyuridine incorporation analysis, and confocal and electron microscopy.

RESULTS

The molecular lesion in flo is a nonsense mutation in the elys (embryonic large molecule derived from yolk sac) gene, which encodes a severely truncated protein lacking the Elys C-terminal AT-hook DNA binding domain. Recently, the human ELYS protein has been shown to play a critical, and hitherto unsuspected, role in nuclear pore assembly. Although elys messenger RNA (mRNA) is expressed broadly during early zebrafish development, widespread early defects in flo are circumvented by the persistence of maternally expressed elys mRNA until 24 hpf. From 72 hpf, elys mRNA expression is restricted to proliferating tissues, including the intestinal epithelium, pancreas, liver, and eye. Cells in these tissues display disrupted nuclear pore formation; ultimately, intestinal epithelial cells undergo apoptosis.

CONCLUSIONS

Our results demonstrate that Elys regulates digestive organ formation.

Architectural malformation of the muscularis propria as a cause for intestinal pseudo-obstruction: two cases and a review of the literature

Two patients are presented with intestinal pseudo-obstruction and malformations of their intestinal muscularis propria. Histopathological findings in each case indicate a distinct alteration of intestinal muscle architecture. The 1st patient, a teenager with lifelong constipation, had an additional circular muscle tunic in 2 dilated segments of his large intestine. The 2nd patient, an infant with multiple malformations, had diffuse abnormal layering of his small intestine. The rare reports of other individuals with similar pathological findings are discussed.

Genetic model system studies of the development of the enteric nervous system, gut motility and Hirschsprung's disease

The enteric nervous system (ENS) is the largest and most complicated subdivision of the peripheral nervous system. Its action is necessary to regulate many of the functions of the gastrointestinal tract including its motility. Whilst the ENS has been studied extensively by developmental biologists, neuroscientists and physiologists for several decades it has only been since the early 1990s that the molecular and genetic basis of ENS development has begun to emerge. Central to this understanding has been the use of genetic model organisms. In this article, we will discuss recent advances that have been achieved using both mouse and zebrafish model genetic systems that have led to new insights into ENS development and the genetic basis of Hirschsprung's disease.

Neuregulin-mediated ErbB3 signaling is required for formation of zebrafish dorsal root ganglion neurons

Dorsal root ganglia (DRGs) arise from trunk neural crest cells that emerge from the dorsal neuroepithelium and coalesce into segmental streams that migrate ventrally along the developing somites. Proper formation of DRGs involves not only normal trunk neural crest migration, but also the ability of DRG progenitors to pause at a particular target location where they can receive DRG-promoting signals. In mammalian embryos, a receptor tyrosine kinase proto-oncogene, ErbB3, is required for proper trunk neural crest migration. Here, we show that in zebrafish mutants lacking ErbB3 function, neural crest cells do not pause at the location where DRGs normally form and DRG neurons are not generated. We also show that these mutants lack trunk neural crest-derived sympathetic neurons, but that cranial neural crest-derived enteric neurons appear normal. We isolated three genes encoding neuregulins, ErbB3 ligands, and show that two neuregulins function together in zebrafish trunk neural crest cell migration and in DRG formation. Together, our results suggest that ErbB3 signaling is required for normal migration of trunk, but not cranial, neural crest cells.