Roles for GFRα1 receptors in zebrafish enteric nervous system development

A targeted CRISPR-Cas9 mediated F0 screen identifies genes involved in establishment of the enteric nervous system

The vertebrate enteric nervous system (ENS) is a crucial network of enteric neurons and glia resident within the entire gastrointestinal tract (GI). Overseeing essential GI functions such as gut motility and water balance, the ENS serves as a pivotal bidirectional link in the gut-brain axis. During early development, the ENS is primarily derived from enteric neural crest cells (ENCCs). Disruptions to ENCC development, as seen in conditions like Hirschsprung disease (HSCR), lead to the absence of ENS in the GI, particularly in the colon. In this study, using zebrafish, we devised an in vivo F0 CRISPR-based screen employing a robust, rapid pipeline integrating single-cell RNA sequencing, CRISPR reverse genetics, and high-content imaging. Our findings unveil various genes, including those encoding opioid receptors, as possible regulators of ENS establishment. In addition, we present evidence that suggests opioid receptor involvement in the neurochemical coding of the larval ENS. In summary, our work presents a novel, efficient CRISPR screen targeting ENS development, facilitating the discovery of previously unknown genes, and increasing knowledge of nervous system construction.

Genetic regulation of enteric nervous system development in zebrafish

The enteric nervous system (ENS) is a complex series of interconnected neurons and glia that reside within and along the entire length of the gastrointestinal tract. ENS functions are vital to gut homeostasis and digestion, including local control of peristalsis, water balance, and intestinal cell barrier function. How the ENS develops during embryological development is a topic of great concern, as defects in ENS development can result in various diseases, the most common being Hirschsprung disease, in which variable regions of the infant gut lack ENS, with the distal colon most affected. Deciphering how the ENS forms from its progenitor cells, enteric neural crest cells, is an active area of research across various animal models. The vertebrate animal model, zebrafish, has been increasingly leveraged to understand early ENS formation, and over the past 20 years has contributed to our knowledge of the genetic regulation that underlies enteric development. In this review, I summarize our knowledge regarding the genetic regulation of zebrafish enteric neuronal development, and based on the most current literature, present a gene regulatory network inferred to underlie its construction. I also provide perspectives on areas for future zebrafish ENS research.

A targeted CRISPR-Cas9 mediated F0 screen identifies genes involved in establishment of the enteric nervous system

The vertebrate enteric nervous system (ENS) is a crucial network of enteric neurons and glia resident within the entire gastrointestinal tract (GI). Overseeing essential GI functions such as gut motility and water balance, the ENS serves as a pivotal bidirectional link in the gut-brain axis. During early development, the ENS is primarily derived from enteric neural crest cells (ENCCs). Disruptions to ENCC development, as seen in conditions like Hirschsprung disease (HSCR), lead to absence of ENS in the GI, particularly in the colon. In this study, using zebrafish, we devised an in vivo F0 CRISPR-based screen employing a robust, rapid pipeline integrating single-cell RNA sequencing, CRISPR reverse genetics, and high-content imaging. Our findings unveil various genes, including those encoding for opioid receptors, as possible regulators of ENS establishment. In addition, we present evidence that suggests opioid receptor involvement in neurochemical coding of the larval ENS. In summary, our work presents a novel, efficient CRISPR screen targeting ENS development, facilitating the discovery of previously unknown genes, and increasing knowledge of nervous system construction.

Who's talking to whom: microbiome-enteric nervous system interactions in early life

The enteric nervous system (ENS) is the intrinsic nervous system of the gastrointestinal tract (GI) and regulates important GI functions, including motility, nutrient uptake, and immune response. The development of the ENS begins during early organogenesis and continues to develop once feeding begins, with ongoing plasticity into adulthood. There has been increasing recognition that the intestinal microbiota and ENS interact during critical periods, with implications for normal development and potential disease pathogenesis. In this review, we focus on insights from mouse and zebrafish model systems to compare and contrast how each model can serve in elucidating the bidirectional communication between the ENS and the microbiome. At the end of this review, we further outline implications for human disease and highlight research innovations that can lead the field forward.

A New Transgenic Tool to Study the Ret Signaling Pathway in the Enteric Nervous System

The receptor tyrosine kinase Ret plays a critical role in regulating enteric nervous system (ENS) development. Ret is important for proliferation, migration, and survival of enteric progenitor cells (EPCs). Ret also promotes neuronal fate, but its role during neuronal differentiation and in the adult ENS is less well understood. Inactivating RET mutations are associated with ENS diseases, e.g., Hirschsprung Disease, in which distal bowel lacks ENS cells. Zebrafish is an established model system for studying ENS development and modeling human ENS diseases. One advantage of the zebrafish model system is that their embryos are transparent, allowing visualization of developmental phenotypes in live animals. However, we lack tools to monitor Ret expression in live zebrafish. Here, we developed a new BAC transgenic line that expresses GFP under the ret promoter. We find that EPCs and the majority of ENS neurons express ret:GFP during ENS development. In the adult ENS, GFP+ neurons are equally present in females and males. In homozygous mutants of ret and sox10-another important ENS developmental regulator gene-GFP+ ENS cells are absent. In summary, we characterize a ret:GFP transgenic line as a new tool to visualize and study the Ret signaling pathway from early development through adulthood.

In toto imaging of early enteric nervous system development reveals that gut colonization is tied to proliferation downstream of Ret

The enteric nervous system is a vast intrinsic network of neurons and glia within the gastrointestinal tract and is largely derived from enteric neural crest cells (ENCCs) that emigrate into the gut during vertebrate embryonic development. Study of ENCC migration dynamics and their genetic regulators provides great insights into fundamentals of collective cell migration and nervous system formation, and these are pertinent subjects for study due to their relevance to the human congenital disease Hirschsprung disease (HSCR). For the first time, we performed in toto gut imaging and single-cell generation tracing of ENCC migration in wild type and a novel ret heterozygous background zebrafish (retwmr1/+) to gain insight into ENCC dynamics in vivo. We observed that retwmr1/+ zebrafish produced fewer ENCCs localized along the gut, and these ENCCs failed to reach the hindgut, resulting in HSCR-like phenotypes. Specifically, we observed a proliferation-dependent migration mechanism, where cell divisions were associated with inter-cell distances and migration speed. Lastly, we detected a premature neuronal differentiation gene expression signature in retwmr1/+ ENCCs. These results suggest that Ret signaling may regulate maintenance of a stem state in ENCCs.

The Polycomb group gene rnf2 is essential for central and enteric neural system development in zebrafish

The development of central nervous system (CNS) and enteric nervous system (ENS) is under precise and strict control in vertebrates. Whether and how the Polycomb repressive complex 1 (PRC1) is involved in it remain unclear. To investigate the role of PRC1 in the nervous system development, using CRISPR/Cas9 technology, we have generated mutant zebrafish lines for the rnf2 gene which encodes Ring1b, the enzymatic component of the PRC1 complex. We show that rnf2 loss of function leads to abnormal migration and differentiation of neural crest and neural precursor cells. rnf2 mutant embryos exhibit aganglionosis, in which the hindgut is devoid of neurons. In particular, the formation of 5-HT serotonin neurons and myelinating glial cells is defective. Furthermore, ectopic expression of ENS marker genes is observed in forebrain of rnf2 mutant embryos. These findings suggest that the rnf2 gene plays an important role in the migration and differentiation of neural precursor cells, and its absence leads to abnormal development of ENS and CNS in zebrafish.

Development, Diversity, and Neurogenic Capacity of Enteric Glia

Enteric glia are a fascinating population of cells. Initially identified in the gut wall as the "support" cells of the enteric nervous system, studies over the past 20 years have unveiled a vast array of functions carried out by enteric glia. They mediate enteric nervous system signalling and play a vital role in the local regulation of gut functions. Enteric glial cells interact with other gastrointestinal cell types such as those of the epithelium and immune system to preserve homeostasis, and are perceptive to luminal content. Their functional versatility and phenotypic heterogeneity are mirrored by an extensive level of plasticity, illustrated by their reactivity in conditions associated with enteric nervous system dysfunction and disease. As one of the hallmarks of their plasticity and extending their operative relationship with enteric neurons, enteric glia also display neurogenic potential. In this review, we focus on the development of enteric glial cells, and the mechanisms behind their heterogeneity in the adult gut. In addition, we discuss what is currently known about the role of enteric glia as neural precursors in the enteric nervous system.

Review: The Role of Wnt/β-Catenin Signalling in Neural Crest Development in Zebrafish

The neural crest (NC) is a multipotent cell population in vertebrate embryos with extraordinary migratory capacity. The NC is crucial for vertebrate development and forms a myriad of cell derivatives throughout the body, including pigment cells, neuronal cells of the peripheral nervous system, cardiomyocytes and skeletogenic cells in craniofacial tissue. NC induction occurs at the end of gastrulation when the multipotent population of NC progenitors emerges in the ectodermal germ layer in the neural plate border region. In the process of NC fate specification, fate-specific markers are expressed in multipotent progenitors, which subsequently adopt a specific fate. Thus, NC cells delaminate from the neural plate border and migrate extensively throughout the embryo until they differentiate into various cell derivatives. Multiple signalling pathways regulate the processes of NC induction and specification. This review explores the ongoing role of the Wnt/β-catenin signalling pathway during NC development, focusing on research undertaken in the Teleost model organism, zebrafish (Danio rerio). We discuss the function of the Wnt/β-catenin signalling pathway in inducing the NC within the neural plate border and the specification of melanocytes from the NC. The current understanding of NC development suggests a continual role of Wnt/β-catenin signalling in activating and maintaining the gene regulatory network during NC induction and pigment cell specification. We relate this to emerging models and hypotheses on NC fate restriction. Finally, we highlight the ongoing challenges facing NC research, current gaps in knowledge, and this field's potential future directions.

Gastrointestinal Neurons Expressing HCN4 Regulate Retrograde Peristalsis

Peristalsis is indispensable for physiological function of the gut. The enteric nervous system (ENS) plays an important role in regulating peristalsis. While the neural network regulating anterograde peristalsis, which migrates from the oral end to the anal end, is characterized to some extent, retrograde peristalsis remains unresolved with regards to its neural regulation. Using forward genetics in zebrafish, we reveal that a population of neurons expressing a hyperpolarization-activated nucleotide-gated channel HCN4 specifically regulates retrograde peristalsis. When HCN4 channels are blocked by an HCN channel inhibitor or morpholinos blocking the protein expression, retrograde peristalsis is specifically attenuated. Conversely, when HCN4(+) neurons expressing channelrhodopsin are activated by illumination, retrograde peristalsis is enhanced while anterograde peristalsis remains unchanged. We propose that HCN4(+) neurons in the ENS forward activating signals toward the oral end and simultaneously stimulate local circuits regulating the circular muscle.

An atlas of neural crest lineages along the posterior developing zebrafish at single-cell resolution

Neural crest cells (NCCs) are vertebrate stem cells that give rise to various cell types throughout the developing body in early life. Here, we utilized single-cell transcriptomic analyses to delineate NCC-derivatives along the posterior developing vertebrate, zebrafish, during the late embryonic to early larval stage, a period when NCCs are actively differentiating into distinct cellular lineages. We identified several major NCC/NCC-derived cell-types including mesenchyme, neural crest, neural, neuronal, glial, and pigment, from which we resolved over three dozen cellular subtypes. We dissected gene expression signatures of pigment progenitors delineating into chromatophore lineages, mesenchyme cells, and enteric NCCs transforming into enteric neurons. Global analysis of NCC derivatives revealed they were demarcated by combinatorial hox gene codes, with distinct profiles within neuronal cells. From these analyses, we present a comprehensive cell-type atlas that can be utilized as a valuable resource for further mechanistic and evolutionary investigations of NCC differentiation.

Zebrafish: A Model Organism for Studying Enteric Nervous System Development and Disease

The Enteric Nervous System (ENS) is a large network of enteric neurons and glia that regulates various processes in the gastrointestinal tract including motility, local blood flow, mucosal transport and secretion. The ENS is derived from stem cells coming from the neural crest that migrate into and along the primitive gut. Defects in ENS establishment cause enteric neuropathies, including Hirschsprung disease (HSCR), which is characterized by an absence of enteric neural crest cells in the distal part of the colon. In this review, we discuss the use of zebrafish as a model organism to study the development of the ENS. The accessibility of the rapidly developing gut in zebrafish embryos and larvae, enables in vivo visualization of ENS development, peristalsis and gut transit. These properties make the zebrafish a highly suitable model to bring new insights into ENS development, as well as in HSCR pathogenesis. Zebrafish have already proven fruitful in studying ENS functionality and in the validation of novel HSCR risk genes. With the rapid advancements in gene editing techniques and their unique properties, research using zebrafish as a disease model, will further increase our understanding on the genetics underlying HSCR, as well as possible treatment options for this disease.

Hypomethylation of GDNF family receptor alpha 1 promotes epithelial-mesenchymal transition and predicts metastasis of colorectal cancer

Tumor metastasis is the major cause of poor prognosis and mortality in colorectal cancer (CRC). However, early diagnosis of highly metastatic CRC is currently difficult. In the present study, we screened for a novel biomarker, GDNF family receptor alpha 1 (GFRA1) based on the expression and methylation data in CRC patients from The Cancer Genome Altlas (TCGA), followed by further analysis of the correlation between the GFRA1 expression, methylation, and prognosis of patients. Our results show DNA hypomethylation-mediated upregulation of GFRA1 in invasive CRC, and it was found to be correlated with poor prognosis of CRC patients. Furthermore, GFRA1 methylation-modified sequences were found to have potential as methylation diagnostic markers of highly metastatic CRC. The targeted demethylation of GFRA1 by dCas9-TET1CD and gRNA promoted CRC metastasis in vivo and in vitro. Mechanistically, demethylation of GFRA1 induces epithelial-mesenchymal transition (EMT) by promoting AKT phosphorylation and increasing c-Jun expression in CRC cells. Collectively, our findings indicate that GFRA1 hypomethylation can promote CRC invasion via inducing EMT, and thus, GFRA1 methylation can be used as a biomarker for the early diagnosis of highly metastasis CRC.

Abnormal DNA methylation, one of important characteristics in tumor cells, is exploited as biomarkers for cancer diagnosis and prognosis prediction. Early diagnosis of highly metastatic CRC will be helpful for the clinical management, thus prolongs patient survival. However, it is currently difficult to make early diagnosis of highly metastatic CRC in clinical practice. Currently, we screened a novel biomarker gene, GFRA1, which associated with the invasion and poor prognosis of CRC. The targeted demethylation of GFRA1 exerted a significant promoting effect on CRC metastasis, and GFRA1 methylation-modified sequences are valuable diagnostic biomarker for CRC metastasis risk assessment. Mechanically, demethylation of GFRA1 induced epithelial-mesenchymal transition (EMT) by upregulating AKT phosphorylation and c-Jun expression in CRC cells. Our results demonstrate the promoting effect of GFRA1 demethylation on CRC invasion and GFRA1 methylation may be a potential prognostic marker for predicting metastasis of CRC.

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

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

Neural crest development: insights from the zebrafish

Our understanding of the neural crest, a key vertebrate innovation, is built upon studies of multiple model organisms. Early research on neural crest cells (NCCs) was dominated by analyses of accessible amphibian and avian embryos, with mouse genetics providing complementary insights in more recent years. The zebrafish model is a relative newcomer to the field, yet it offers unparalleled advantages for the study of NCCs. Specifically, zebrafish provide powerful genetic and transgenic tools, coupled with rapidly developing transparent embryos that are ideal for high-resolution real-time imaging of the dynamic process of neural crest development. While the broad principles of neural crest development are largely conserved across vertebrate species, there are critical differences in anatomy, morphogenesis, and genetics that must be considered before information from one model is extrapolated to another. Here, our goal is to provide the reader with a helpful primer specific to neural crest development in the zebrafish model. We focus largely on the earliest events-specification, delamination, and migration-discussing what is known about zebrafish NCC development and how it differs from NCC development in non-teleost species, as well as highlighting current gaps in knowledge.

Epigenetic factors Dnmt1 and Uhrf1 coordinate intestinal development

Intestinal tract development is a coordinated process involving signaling among the progenitors and developing cells from all three germ layers. Development of endoderm-derived intestinal epithelium has been shown to depend on epigenetic modifications, but whether that is also the case for intestinal tract cell types from other germ layers remains unclear. We found that functional loss of a DNA methylation machinery component, ubiquitin-like protein containing PHD and RING finger domains 1 (uhrf1), leads to reduced numbers of ectoderm-derived enteric neurons and severe disruption of mesoderm-derived intestinal smooth muscle. Genetic chimeras revealed that Uhrf1 functions both cell-autonomously in enteric neuron precursors and cell-non-autonomously in surrounding intestinal cells, consistent with what is known about signaling interactions between these cell types that promote one another's development. Uhrf1 recruits the DNA methyltransferase Dnmt1 to unmethylated DNA during replication. Dnmt1 is also expressed in enteric neurons and smooth muscle progenitors. dnmt1 mutants have fewer enteric neurons and disrupted intestinal smooth muscle compared to wildtypes. Because dnmt1;uhrf1 double mutants have a similar phenotype to dnmt1 and uhrf1 single mutants, Dnmt1 and Uhrf1 must function together during enteric neuron and intestinal muscle development. This work shows that genes controlling epigenetic modifications are important to coordinate intestinal tract development, provides the first demonstration that these genes influence development of the ENS, and advances uhrf1 and dnmt1 as potential new Hirschsprung disease candidates.

Immunohistochemical and ultrastructural analysis of the maturing larval zebrafish enteric nervous system reveals the formation of a neuropil pattern

The gastrointestinal tract is constructed with an intrinsic series of interconnected ganglia that span its entire length, called the enteric nervous system (ENS). The ENS exerts critical local reflex control over many essential gut functions; including peristalsis, water balance, hormone secretions and intestinal barrier homeostasis. ENS ganglia exist as a collection of neurons and glia that are arranged in a series of plexuses throughout the gut: the myenteric plexus and submucosal plexus. While it is known that enteric ganglia are derived from a stem cell population called the neural crest, mechanisms that dictate final neuropil plexus organization remain obscure. Recently, the vertebrate animal, zebrafish, has emerged as a useful model to understand ENS development, however knowledge of its developing myenteric plexus architecture was unknown. Here, we examine myenteric plexus of the maturing zebrafish larval fish histologically over time and find that it consists of a series of tight axon layers and long glial cell processes that wrap the circumference of the gut tube to completely encapsulate it, along all levels of the gut. By late larval stages, complexity of the myenteric plexus increases such that a layer of axons is juxtaposed to concentric layers of glial cells. Ultrastructurally, glial cells contain glial filaments and make intimate contacts with one another in long, thread-like projections. Conserved indicators of vesicular axon profiles are readily abundant throughout the larval plexus neuropil. Together, these data extend our understanding of myenteric plexus architecture in maturing zebrafish, thereby enabling functional studies of its formation in the future.

Local heat-shock mediated multi-color labeling visualizing behaviors of enteric neural crest cells associated with division and neurogenesis in zebrafish gut

BACKGROUND

The enteric nervous system (ENS) is derived from enteric neural crest cells (ENCCs) that migrate into the gut. The zebrafish larva is a good model to study ENCC development due to its simplicity and transparency. However, little is known how individual ENCCs divide and become neurons.

RESULTS

Here, by applying our new method of local heat-shock mediated Cre-recombination around the dorsal vagal area of zebrafish embryos we produced multicolored clones of ENCCs, and performed in vivo time-lapse imaging from ca. 3.5 to 4 days post-fertilization after arrival of ENCCs in the gut. Individual ENCCs migrated in various directions and were highly intermingled. The cell divisions were not restricted to a specific position in the gut. Antibody staining after imaging with anti-HuC/D and anti-Sox10 showed that an ENCC produced two neurons, or formed a neuron and an additional ENCC that further divided. At division, the daughter cells immediately separated. Afterward, some made soma-soma contact with other ENCCs.

CONCLUSIONS

We introduced a new method of visualizing individual ENCCs in the zebrafish gut, describing their behaviors associated with cell division, providing a foundation to study the mechanism of proliferation and neurogenesis in the ENS in vertebrates.

Both Gfrα1a and Gfrα1b Are Involved in the Self-renewal and Maintenance of Spermatogonial Stem Cells in Medaka

Glial cell-derived neurotrophic factor family receptor alpha-1 (GFRα1) plays a crucial role in the self-renewal and maintenance of spermatogonial stem cells (SSCs) from mammals. However, to date, our knowledge about its role in fish SSCs is limited. In the present study, the medaka (Oryzias latipes) gfrα1 duplicate genes, Olgfrα1a and Olgfrα1b, were cloned and characterized. Furthermore, their expression profile and biological activity were investigated. OlGfrα1a and OlGfrα1b predict 524 and 466 amino acid residues, respectively. Both are orthologous to mammalian Gfrα1 by sequence analyses and appear high in spermatogonia by in situ hybridization assay. The knockdown of OlGfrα1a and/or OlGfrα1b via Vivo-Morpholino oligos significantly inhibited the self-renewal and maintenance of SSCs, as evidenced by the decreased proliferation activity of SG3 cells (a spermatogonial stem cell line derived from adult medaka testis) as well as spermatogonia in the testicular organ culture and by the decreased survival rate and expression levels of pluripotency-related genes (klf4, lin28b, bcl6b, and etv5) in SG3 cells. Additionally, our study indicates that OlGfrα1a might function by binding either Gdnfa or Gdnfb (the two medaka Gdnf homologs), whereas OlGfrα1b function by binding Gdnfa not Gdnfb. Taken together, our study indicates that both OlGfrα1a and OlGfrα1b are involved in the self-renewal and maintenance of SSCs by binding Gdnfa and/or Gdnfb, respectively. These findings suggest that the GDNF/GFRα1 signaling pathway might be conserved from mammals to fish species.

Migration and diversification of the vagal neural crest

Arising within the neural tube between the cranial and trunk regions of the body axis, the vagal neural crest shares interesting similarities in its migratory routes and derivatives with other neural crest populations. However, the vagal neural crest is also unique in its ability to contribute to diverse organs including the heart and enteric nervous system. This review highlights the migratory routes of the vagal neural crest and compares them across multiple vertebrates. We also summarize recent advances in understanding vagal neural crest ontogeny and discuss the contribution of this important neural crest population to the cardiovascular system and endoderm-derived organs, including the thymus, lungs and pancreas.

Identification of Variants in RET and IHH Pathway Members in a Large Family With History of Hirschsprung Disease

BACKGROUND & AIMS

Hirschsprung disease (HSCR) is an inherited congenital disorder characterized by absence of enteric ganglia in the distal part of the gut. Variants in ret proto-oncogene (RET) have been associated with up to 50% of familial and 35% of sporadic cases. We searched for variants that affect disease risk in a large, multigenerational family with history of HSCR in a linkage region previously associated with the disease (4q31.3-q32.3) and exome wide.

METHODS

We performed exome sequencing analyses of a family in the Netherlands with 5 members diagnosed with HSCR and 2 members diagnosed with functional constipation. We initially focused on variants in genes located in 4q31.3-q32.3; however, we also performed an exome-wide analysis in which known HSCR or HSCR-associated gene variants predicted to be deleterious were prioritized for further analysis. Candidate genes were expressed in HEK293, COS-7, and Neuro-2a cells and analyzed by luciferase and immunoblot assays. Morpholinos were designed to target exons of candidate genes and injected into 1-cell stage zebrafish embryos. Embryos were allowed to develop and stained for enteric neurons.

RESULTS

Within the linkage region, we identified 1 putative splice variant in the lipopolysaccharide responsive beige-like anchor protein gene (LRBA). Functional assays could not confirm its predicted effect on messenger RNA splicing or on expression of the mab-21 like 2 gene (MAB21L2), which is embedded in LRBA. Zebrafish that developed following injection of the lrba morpholino had a shortened body axis and subtle gut morphological defects, but no significant reduction in number of enteric neurons compared with controls. Outside the linkage region, members of 1 branch of the family carried a previously unidentified RET variant or an in-frame deletion in the glial cell line derived neurotrophic factor gene (GDNF), which encodes a ligand of RET. This deletion was located 6 base pairs before the last codon. We also found variants in the Indian hedgehog gene (IHH) and its mediator, the transcription factor GLI family zinc finger 3 (GLI3). When expressed in cells, the RET-P399L variant disrupted protein glycosylation and had altered phosphorylation following activation by GDNF. The deletion in GDNF prevented secretion of its gene product, reducing RET activation, and the IHH-Q51K variant reduced expression of the transcription factor GLI1. Injection of morpholinos that target ihh reduced the number of enteric neurons to 13% ± 1.4% of control zebrafish.

CONCLUSIONS

In a study of a large family with history of HSCR, we identified variants in LRBA, RET, the gene encoding the RET ligand (GDNF), IHH, and a gene encoding a mediator of IHH signaling (GLI3). These variants altered functions of the gene products when expressed in cells and knockout of ihh reduced the number of enteric neurons in the zebrafish gut.

GDNF family receptor α-1 in the catfish: Possible implication to brain dopaminergic activity

Glial cell line-derived neurotrophic factor (GDNF)is a potent trophic factor that preferentially binds to GDNF family receptor α-1 (GFRα-1)by regulating dopaminergic (DA-ergic) neuronsin brain. Present study aimed to evaluate the significance of GFRα-1 expression during early brain development in catfish. Initially, the full-length cDNA of GFRα-1 was cloned from adult brain which showed high homology with other vertebrate counterparts. Quantitative PCR analysis of tissue distribution revealed ubiquitous expression of GFRα-1 in the tissues analyzed with high levels in female brain and ovary. Significant high expression was evident in brain at 75 and 100 days post hatch females than the respective age-match males. Expression of GFRα-1 was high in brain during the spawning phase when compared to other reproductive phases. Localization of GFRα-1 revealed its presence in preoptic area-hypothalamus which correlated well with the expression profile in discrete areas of brain in adult catfish. Transient silencing of GFRα-1through siRNA lowered expression levels of GFRα-1, which further down regulated the expression of certain brain-specific genes. Expression of GFRα-1 in brain declined significantly upon treatment with the 1-methyl-1,2,3,6-tetrahydropyridinecausing neurodegeneration which further correlated with catecholamines (CA), L-3,4-dihydroxyphenylalanine, DA and norepinephrine levels. Taken together, GFRα-1 plausibly entrains gonadotropin-releasing hormone and gonadotropin axiseither directly or indirectly, at least by partially targeting CA-ergic activity.

Gut feelings: Studying enteric nervous system development, function, and disease in the zebrafish model system

The enteric nervous system (ENS) is the largest part of the peripheral nervous system and is entirely neural crest-derived. It provides the intrinsic innervation of the gut, controlling different aspects of gut function, such as motility. In this review, we will discuss key points of Zebrafish ENS development, genes, and signaling pathways regulating ENS development, as well as contributions of the Zebrafish model system to better understand ENS disorders. During their migration, enteric progenitor cells (EPCs) display a gradient of developmental states based on their proliferative and migratory characteristics, and show spatiotemporal heterogeneity based on gene expression patterns. Many genes and signaling pathways that regulate the migration and proliferation of EPCs have been identified, but later stages of ENS development, especially steps of neuronal and glial differentiation, remain poorly understood. In recent years, Zebrafish have become increasingly important to test candidate genes for ENS disorders (e.g., from genome-wide association studies), to identify environmental influences on ENS development (e.g., through large-scale drug screens), and to investigate the role the gut microbiota play in ENS development and disease. With its unique advantages as a model organism, Zebrafish will continue to contribute to a better understanding of ENS development, function, and disease. Developmental Dynamics 247:268-278, 2018. © 2017 Wiley Periodicals, Inc.

Retinoic acid temporally orchestrates colonization of the gut by vagal neural crest cells

The enteric nervous system arises from neural crest cells that migrate as chains into and along the primitive gut, subsequently differentiating into enteric neurons and glia. Little is known about the mechanisms governing neural crest migration en route to and along the gut in vivo. Here, we report that Retinoic Acid (RA) temporally controls zebrafish enteric neural crest cell chain migration. In vivo imaging reveals that RA loss severely compromises the integrity and migration of the chain of neural crest cells during the window of time window when they are moving along the foregut. After loss of RA, enteric progenitors accumulate in the foregut and differentiate into enteric neurons, but subsequently undergo apoptosis resulting in a striking neuronal deficit. Moreover, ectopic expression of the transcription factor meis3 and/or the receptor ret, partially rescues enteric neuron colonization after RA attenuation. Collectively, our findings suggest that retinoic acid plays a critical temporal role in promoting enteric neural crest chain migration and neuronal survival upstream of Meis3 and RET in vivo.

Enteric nervous system development: A crest cell's journey from neural tube to colon

The enteric nervous system (ENS) is comprised of a network of neurons and glial cells that are responsible for coordinating many aspects of gastrointestinal (GI) function. These cells arise from the neural crest, migrate to the gut, and then continue their journey to colonize the entire length of the GI tract. Our understanding of the molecular and cellular events that regulate these processes has advanced significantly over the past several decades, in large part facilitated by the use of rodents, avians, and zebrafish as model systems to dissect the signals and pathways involved. These studies have highlighted the highly dynamic nature of ENS development and the importance of carefully balancing migration, proliferation, and differentiation of enteric neural crest-derived cells (ENCCs). Proliferation, in particular, is critically important as it drives cell density and speed of migration, both of which are important for ensuring complete colonization of the gut. However, proliferation must be tempered by differentiation among cells that have reached their final destination and are ready to send axonal extensions, connect to effector cells, and begin to produce neurotransmitters or other signals. Abnormalities in the normal processes guiding ENCC development can lead to failure of ENS formation, as occurs in Hirschsprung disease, in which the distal intestine remains aganglionic. This review summarizes our current understanding of the factors involved in early development of the ENS and discusses areas in need of further investigation.

A Novel Zebrafish ret Heterozygous Model of Hirschsprung Disease Identifies a Functional Role for mapk10 as a Modifier of Enteric Nervous System Phenotype Severity

Hirschsprung disease (HSCR) is characterized by absence of enteric neurons from the distal colon and severe intestinal dysmotility. To understand the pathophysiology and genetics of HSCR we developed a unique zebrafish model that allows combined genetic, developmental and in vivo physiological studies. We show that ret mutant zebrafish exhibit cellular, physiological and genetic features of HSCR, including absence of intestinal neurons, reduced peristalsis, and varying phenotype expressivity in the heterozygous state. We perform live imaging experiments using a UAS-GAL4 binary genetic system to drive fluorescent protein expression in ENS progenitors. We demonstrate that ENS progenitors migrate at reduced speed in ret heterozygous embryos, without changes in proliferation or survival, establishing this as a principal pathogenic mechanism for distal aganglionosis. We show, using live imaging of actual intestinal movements, that intestinal motility is severely compromised in ret mutants, and partially impaired in ret heterozygous larvae, and establish a clear correlation between neuron position and organised intestinal motility. We exploited the partially penetrant ret heterozygous phenotype as a sensitised background to test the influence of a candidate modifier gene. We generated mapk10 loss-of-function mutants, which show reduced numbers of enteric neurons. Significantly, we show that introduction of mapk10 mutations into ret heterozygotes enhanced the ENS deficit, supporting MAPK10 as a HSCR susceptibility locus. Our studies demonstrate that ret heterozygous zebrafish is a sensitized model, with many significant advantages over existing murine models, to explore the pathophysiology and complex genetics of HSCR.

Hirschsprung Disease (HSCR) is a common congenital intestinal motility disorder diagnosed at birth by absence of enteric neurons in the distal gut, leading to intestinal obstruction that requires life-saving surgery. HSCR exhibits complex inheritance patterns and its genetic basis is not fully understood. Although well studied by human geneticists, and modelled using mouse, significant questions remain about the cellular and genetic causes of the disease and the relationship between neuron loss and defective intestinal motility. Here we use accessible, transparent zebrafish to address these outstanding questions. We establish that ret mutant zebrafish display key features of HSCR, including absence of intestinal neurons, reduced gut motility and varying phenotype expressivity. Using live imaging, possible in zebrafish but not in mouse, we demonstrate that decreased migration speed of enteric neuron progenitors colonising the gut is the principal defect leading to neuron deficits. By direct examination of gut motility in zebrafish larvae, we establish a clear correlation between neurons and motility patterns. Finally, we show that mapk10 mutations worsen the enteric neuron deficit of ret mutants, indicating that mutations in MAPK10 may increase susceptibility to HSCR. We show many benefits of modelling human genetic diseases in zebrafish and advance our understanding of HSCR.

Molecular fingerprinting delineates progenitor populations in the developing zebrafish enteric nervous system

BACKGROUND

To understand the basis of nervous system development, we must learn how multipotent progenitors generate diverse neuronal and glial lineages. We addressed this issue in the zebrafish enteric nervous system (ENS), a complex neuronal and glial network that regulates essential intestinal functions. Little is currently known about how ENS progenitor subpopulations generate enteric neuronal and glial diversity.

RESULTS

We identified temporally and spatially dependent progenitor subpopulations based on coexpression of three genes essential for normal ENS development: phox2bb, sox10, and ret. Our data suggest that combinatorial expression of these genes delineates three major ENS progenitor subpopulations, (1) phox2bb + /ret- /sox10-, (2) phox2bb + /ret + /sox10-, and (3) phox2bb + /ret + /sox10+, that reflect temporal progression of progenitor maturation during migration. We also found that differentiating zebrafish neurons maintain phox2bb and ret expression, and lose sox10 expression.

CONCLUSIONS

Our data show that zebrafish enteric progenitors constitute a heterogeneous population at both early and late stages of ENS development and suggest that marker gene expression is indicative of a progenitor's fate. We propose that a progenitor's expression profile reveals its developmental state: "younger" wave front progenitors express all three genes, whereas more mature progenitors behind the wave front selectively lose sox10 and/or ret expression, which may indicate developmental restriction. Developmental Dynamics 245:1081-1096, 2016. © 2016 Wiley Periodicals, Inc.

Enteric nervous system development in avian and zebrafish models

Our current understanding of the developmental biology of the enteric nervous system (ENS) and the genesis of ENS diseases is founded almost entirely on studies using model systems. Although genetic studies in the mouse have been at the forefront of this field over the last 20 years or so, historically it was the easy accessibility of the chick embryo for experimental manipulations that allowed the first descriptions of the neural crest origins of the ENS in the 1950s. More recently, studies in the chick and other non-mammalian model systems, notably zebrafish, have continued to advance our understanding of the basic biology of ENS development, with each animal model providing unique experimental advantages. Here we review the basic biology of ENS development in chick and zebrafish, highlighting conserved and unique features, and emphasising novel contributions to our general understanding of ENS development due to technical or biological features.

The zebrafish mutant lessen: an experimental model for congenital enteric neuropathies

BACKGROUND

Congenital enteric neuropathies of the distal intestine (CEN) are characterized by the partial or complete absence of enteric neurons. Over the last decade, zebrafish has emerged as a leading model organism in experimental research. Our aim was to demonstrate that the mutant zebrafish, lessen, expressing CEN characteristics, is an equally valuable animal model alongside mammalian models for CEN, by studying its enteric phenotype.

METHODS

The effect of the lessen mutation on the development of the enteric nervous system (ENS), interstitial cells of Cajal (ICC), and intestinal motility in each intestinal region of mutant and wild-type (wt) zebrafish embryos at 3-6 dpf, was analyzed by immunofluorescent detection of neurochemical markers and motility assays.

KEY RESULTS

Development of intestinal motility in the mutant was delayed and the majority of the observed contractions were disturbed. A significant disturbance in ENS development resulted in a distal intestine that was almost free of neuronal elements, in reduced neuronal density in the proximal and mid-intestine, and in a defect in the expression of neurochemical markers. Furthermore, markedly disturbed development of ICC gave rise to a less dense network of ICC.

CONCLUSIONS & INFERENCES

The observed alterations in intestinal motility, intrinsic innervation and ICC network of the mutant in comparison with the wt zebrafish, are similar to those seen in the oligo- and aganglionic regions of the intestine of CEN patients. It is concluded that the zebrafish mutant lessen is an appropriate animal model to investigate CEN.

In Vitro Conditioned Bone Marrow-Derived Mesenchymal Stem Cells Promote De Novo Functional Enteric Nerve Regeneration, but Not Through Direct-Transdifferentiation

Injury or neurodegenerative disorders of the enteric nervous system (ENS) cause gastrointestinal dysfunctions for which there is no effective therapy. This study, using the benzalkonium chloride-induced rat gastric denervation model, aimed to determine whether transplantation of bone marrow-derived mesenchymal stem cells (BMSC) could promote ENS neuron regeneration and if so, to elucidate the mechanism. Fluorescently labeled BMSC, isolated from either WT (BMSC labeled with bis-benzimide [BBM]) or green fluorescent protein (GFP)-transgenic rats, were preconditioned in vitro using fetal gut culture media containing glial cell-derived neurotrophic factor (GDNF), and transplanted subserosally into the denervated area of rat pylorus. In the nerve-ablated pylorus, grafted BMSC survived and migrated from the subserosa to the submucosa 28 days after transplantation, without apparent dedifferentiation. A massive number of PGP9.5/NSE/HuC/D/Tuj1-positive (but GFP- and BBM-negative) neurons were effectively regenerated in denervated pylorus grafted with preconditioned BMSC, suggesting that they were regenerated de novo, not originating from trans-differentiation of the transplanted BMSC. BMSC transplantation restored both basal pyloric contractility and electric field stimulation-induced relaxation. High levels of GDNF were induced in both in vitro-preconditioned BMSC as well as the previously denervated pylorus after transplantation of preconditioned BMSC. Thus, a BMSC-initiated GDNF-positive feedback mechanism is suggested to promote neuron regeneration and growth. In summary, we have demonstrated that allogeneically transplanted preconditioned BMSC initiate de novo regeneration of gastric neuronal cells/structures that in turn restore gastric contractility in pylorus-denervated rats. These neuronal structures did not originate from the grafted BMSC. Our data suggest that preconditioned allogeneic BMSC may have therapeutic value in treating enteric nerve disorders.

Meis3 is required for neural crest invasion of the gut during zebrafish enteric nervous system development

Loss of Meis3 leads to defects in enteric neural crest cell migration, number, and proliferation during colonization of the gut. This leads to colonic aganglionosis, in which the hindgut is devoid of neurons, identifying it as a novel candidate factor in the etiology of Hirschsprung’s disease during enteric nervous system development.

During development, vagal neural crest cells fated to contribute to the enteric nervous system migrate ventrally away from the neural tube toward and along the primitive gut. The molecular mechanisms that regulate their early migration en route to and entry into the gut remain elusive. Here we show that the transcription factor meis3 is expressed along vagal neural crest pathways. Meis3 loss of function results in a reduction in migration efficiency, cell number, and the mitotic activity of neural crest cells in the vicinity of the gut but has no effect on neural crest or gut specification. Later, during enteric nervous system differentiation, Meis3-depleted embryos exhibit colonic aganglionosis, a disorder in which the hindgut is devoid of neurons. Accordingly, the expression of Shh pathway components, previously shown to have a role in the etiology of Hirschsprung’s disease, was misregulated within the gut after loss of Meis3. Taken together, these findings support a model in which Meis3 is required for neural crest proliferation, migration into, and colonization of the gut such that its loss leads to severe defects in enteric nervous system development.

A Caged Ret Kinase Inhibitor and its Effect on Motoneuron Development in Zebrafish Embryos

Proto-oncogene tyrosine-protein kinase receptor RET is implicated in the development and maintenance of neurons of the central and peripheral nervous systems. Attaching activity-compromising photocleavable groups (caging) to inhibitors could allow for external spatiotemporally controlled inhibition using light, potentially providing novel information on how these kinase receptors are involved in cellular processes. Here, caged RET inhibitors were obtained from 3-substituted pyrazolopyrimidine-based compounds by attaching photolabile groups to the exocyclic amino function. The most promising compound displayed excellent inhibitory effect in cell-free, as well as live-cell assays upon decaging. Furthermore, inhibition could be efficiently activated with light in vivo in zebrafish embryos and was shown to effect motoneuron development.

In vivo visualization of the development of the enteric nervous system using a Tg(-8.3bphox2b:Kaede) transgenic zebrafish

The phox2b gene encodes a transcription factor that is expressed in the developing enteric nervous system (ENS). An enhancer element has been identified in the zebrafish phox2b locus that can drive tissue specific expression of reporter genes in enteric neuron precursor cells. We have generated a transgenic zebrafish line in which the Kaede fluorescent protein is under the control of this phox2b enhancer. This line has stable expression of the Kaede protein in enteric neuron precursor cells over three generations. To demonstrate the utility of this line we compared the migration and division rates of enteric neuron precursor cells in wild type and the zebrafish ENS mutant lessen.

Travelling within the fetal gut: simple rules for an arduous journey

The complex physiology of the gastrointestinal tract is regulated by intricate neural networks embedded within the gut wall. How neural crest cells colonize the intestine to form the enteric nervous system is of great interest to developmental biologists, but also highly relevant for understanding gastrointestinal disorders. A recent paper in BMC Biology addresses this issue with live imaging of gut explants from mouse embryos.

See research article: http://www.biomedcentral.com/1741-7007/12/23.

GDNF and GFRα co-receptor family in the developing feline gut

Glial cell-line derived neurotrophic factor (GDNF) and the GFRα co-receptors play a role in the developing enteric nervous system. The co-receptors elicit their action by binding receptor tyrosine kinase RET. This immunohistochemical study reports the presence of GDNF and its specific co-receptor GFRα1 in the cat gastrointestinal apparatus during development, from stage 9 to 22. At stage 9 and 11, immunoreactivity (IR) to GDNF was observed in the cells of mesenchyme of the anterior gut. From stage 14 to 22, GDNF IR was detected in nervous plexuses; moreover, GDNF and GFRα1 IR appeared localized in gastrointestinal endocrine cells. The presence of GDNF in the enteric nervous system and in the endocrine cells suggests an involvement of this neurotrophic factor in the gastrointestinal development. Moreover, the presence of the co-receptor GFRα1 in endocrine cells and its absence in the enteric nervous system seems to indicate a different mode of transduction of GDNF signal. GFRα2 and GFRα3 co-receptors were not detected.

The developmental etiology and pathogenesis of Hirschsprung disease

The enteric nervous system is the part of the autonomic nervous system that directly controls the gastrointestinal tract. Derived from a multipotent, migratory cell population called the neural crest, a complete enteric nervous system is necessary for proper gut function. Disorders that arise as a consequence of defective neural crest cell development are termed neurocristopathies. One such disorder is Hirschsprung disease (HSCR), also known as congenital megacolon or intestinal aganglionosis. HSCR occurs in 1/5000 live births and typically presents with the inability to pass meconium, along with abdominal distension and discomfort that usually requires surgical resection of the aganglionic bowel. This disorder is characterized by a congenital absence of neurons in a portion of the intestinal tract, usually the distal colon, because of a disruption of normal neural crest cell migration, proliferation, differentiation, survival, and/or apoptosis. The inheritance of HSCR disease is complex, often non-Mendelian, and characterized by variable penetrance. Extensive research has identified a number of key genes that regulate neural crest cell development in the pathogenesis of HSCR including RET, GDNF, GFRα1, NRTN, EDNRB, ET3, ZFHX1B, PHOX2b, SOX10, and SHH. However, mutations in these genes account for only ∼50% of the known cases of HSCR. Thus, other genetic mutations and combinations of genetic mutations and modifiers likely contribute to the etiology and pathogenesis of HSCR. The aims of this review are to summarize the HSCR phenotype, diagnosis, and treatment options; to discuss the major genetic causes and the mechanisms by which they disrupt normal enteric neural crest cell development; and to explore new pathways that may contribute to HSCR pathogenesis.

Loss of ascl1a prevents secretory cell differentiation within the zebrafish intestinal epithelium resulting in a loss of distal intestinal motility

The vertebrate intestinal epithelium is renewed continuously from stem cells at the base of the crypt in mammals or base of the fold in fish over the life of the organism. As stem cells divide, newly formed epithelial cells make an initial choice between a secretory or enterocyte fate. This choice has previously been demonstrated to involve Notch signaling as well as Atonal and Her transcription factors in both embryogenesis and adults. Here, we demonstrate that in contrast to the atoh1 in mammals, ascl1a is responsible for formation of secretory cells in zebrafish. ascl1a-/- embryos lack all intestinal epithelial secretory cells and instead differentiate into enterocytes. ascl1a-/- embryos also fail to induce intestinal epithelial expression of deltaD suggesting that ascl1a plays a role in initiation of Notch signaling. Inhibition of Notch signaling increases the number of ascl1a and deltaD expressing intestinal epithelial cells as well as the number of developing secretory cells during two specific time periods: between 30 and 34hpf and again between 64 and 74hpf. Loss of enteroendocrine products results in loss of anterograde motility in ascl1a-/- embryos. 5HT produced by enterochromaffin cells is critical in motility and secretion within the intestine. We find that addition of exogenous 5HT to ascl1a-/- embryos at near physiological levels (measured by differential pulse voltammetry) induce anterograde motility at similar levels to wild type velocity, distance, and frequency. Removal or doubling the concentration of 5HT in WT embryos does not significantly affect anterograde motility, suggesting that the loss of additional enteroendocrine products in ascl1a-/- embryos also contributes to intestinal motility. Thus, zebrafish intestinal epithelial cells appear to have a common secretory progenitor from which all subtypes form. Loss of enteroendocrine cells reveals the critical need for enteroendocrine products in maintenance of normal intestinal motility.

The emergence of neural activity and its role in the development of the enteric nervous system

The enteric nervous system (ENS) is a vital part of the autonomic nervous system that regulates many gastrointestinal functions, including motility and secretion. All neurons and glia of the ENS arise from neural crest-derived cells that migrate into the gastrointestinal tract during embryonic development. It has been known for many years that a subpopulation of the enteric neural crest-derived cells expresses pan-neuronal markers at early stages of ENS development. Recent studies have demonstrated that some enteric neurons exhibit electrical activity from as early as E11.5 in the mouse, with further maturation of activity during embryonic and postnatal development. This article discusses the maturation of electrophysiological and morphological properties of enteric neurons, the formation of synapses and synaptic activity, and the influence of neural activity on ENS development.

Hirschsprung's disease and variants in genes that regulate enteric neural crest cell proliferation, migration and differentiation

Hirschsprung's disease (HSCR) results from failed colonization of the embryonic gut by enteric neural crest cells (ENCCs); colonization requires RET proto-oncogene (RET) signaling. We sequenced RET to identify coding and splice-site variants in a population-based case group and we tested for associations between HSCR and common variants in RET and candidate genes (ASCL1, homeobox B5 (HOXB5), L1 cell adhesion molecule (L1CAM), paired-like homeobox 2b (PHOX2B), PROK1 and PROKR1) chosen because they are involved in ENCC proliferation, migration and differentiation in animal models. We conducted a nested case-control study of 304 HSCR cases and 1215 controls. Among 38 (12.5%) cases with 34 RET coding and splice-site variants, 18 variants were previously unreported. We confirmed associations with common variants in HOXB5 and PHOX2B but the associations with variants in ASCL1, L1CAM and PROK1 were not significant after multiple comparisons adjustment. RET variants were strongly associated with HSCR (P-values between 10(-3) and 10(-31)) but this differed by race/ethnicity: associations were absent in African-Americans. Our population-based study not only identified novel RET variants in HSCR cases, it showed that common RET variants may not contribute to HSCR in all race/ethnic groups. The findings for HOXB5 and PHOX2B provide supportive evidence that genes regulating ENCC proliferation, migration and differentiation could be risk factors for HSCR.

Genetics of human enteric neuropathies

Knowledge of molecular mechanisms that underlie development of the enteric nervous system has greatly expanded in recent decades. Enteric neuropathies related to aberrant genetic development are thus becoming increasingly recognized. There has been no recent review of these often highly morbid disorders. This review highlights advances in knowledge of the molecular pathogenesis of these disorders from a clinical perspective. It includes diseases characterized by an infantile aganglionic Hirschsprung phenotype and those in which structural abnormalities are less pronounced. The implications for diagnosis, screening and possible reparative approaches are presented.

Development of the zebrafish enteric nervous system

The enteric nervous system (ENS) is composed of neurons and glia that modulate many aspects of intestinal function. The ability to use both forward and reverse genetic approaches and to visualize development in living embryos and larvae has made zebrafish an attractive model in which to study mechanisms underlying ENS development. In this chapter, we review the recent work describing the development and organization of the zebrafish ENS and how this relates to intestinal motility. We also discuss the cellular, molecular, and genetic mechanisms that have been revealed by these studies and how they are providing new insights into human ENS diseases.

Chondrogenic and gliogenic subpopulations of neural crest play distinct roles during the assembly of epibranchial ganglia

In vertebrates, the sensory neurons of the epibranchial (EB) ganglia transmit somatosensory signals from the periphery to the CNS. These ganglia are formed during embryogenesis by the convergence and condensation of two distinct populations of precursors: placode-derived neuroblasts and neural crest- (NC) derived glial precursors. In addition to the gliogenic crest, chondrogenic NC migrates into the pharyngeal arches, which lie in close proximity to the EB placodes and ganglia. Here, we examine the respective roles of these two distinct NC-derived populations during development of the EB ganglia using zebrafish morphant and mutants that lack one or both of these NC populations. Our analyses of mutant and morphant zebrafish that exhibit deficiencies in chondrogenic NC at early stages reveal a distinct requirement for this NC subpopulation during early EB ganglion assembly and segmentation. Furthermore, restoration of wildtype chondrogenic NC in one of these mutants, prdm1a, is sufficient to restore ganglion formation, indicating a specific requirement of the chondrogenic NC for EB ganglia assembly. By contrast, analysis of the sox10 mutant, which lacks gliogenic NC, reveals that the initial assembly of ganglia is not affected. However, during later stages of development, EB ganglia are dispersed in the sox10 mutant, suggesting that glia are required to maintain normal EB ganglion morphology. These results highlight novel roles for two subpopulations of NC cells in the formation and maintenance of EB ganglia: chondrogenic NC promotes the early-stage formation of the developing EB ganglia while glial NC is required for the late-stage maintenance of ganglion morphology.

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).

Neurochemical coding of enteric neurons in adult and embryonic zebrafish (Danio rerio)

Although the morphology and development of the zebrafish enteric nervous system have been extensively studied, the precise neurochemical coding of enteric neurons and their proportional enteric distribution are currently not known. By using immunohistochemistry, we determined the proportional expression and coexpression of neurochemical markers in the embryonic and adult zebrafish intestine. Tyrosine hydroxylase (TH), vasoactive intestinal peptide (VIP), and pituitary adenylate cyclase-activating peptide (PACAP) were observed only in nerve fibers, whereas other markers were also detected in neuronal cell bodies. Calretinin and calbindin had similar distributions. In embryos, all markers, except for choline acetyltransferase (ChAT) and TH, were present from 72 hours postfertilization. Nitrergic neurons, evenly distributed and remaining constant in time, constituted the major neuronal subpopulation. The neuronal proportions of the other markers increased during development and were characterized by regional differences. In the adult, all markers examined were expressed in the enteric nervous system. A large percentage of enteric neurons displayed calbindin and calretinin, and serotonin was the only marker showing significant distribution differences in the three intestinal regions. Colocalization studies showed that serotonin was not coexpressed with any of the other markers. At least five neuronal subpopulations were determined: a serotonergic, a nitrergic noncholinergic, two cholinergic nonnitrergic subpopulations along with one subpopulation expressing both ChAT and neuronal nitric oxide synthase. Analysis of nerve fibers revealed that nitrergic neurons coexpress VIP and PACAP, and that nitrergic neurons innervate the tunica muscularis, whereas serotonergic and cholinergic nonnitrergic neurons innervate the lamina propria and the tunica muscularis.

Zebrafish: an exciting model for investigating the spatio-temporal pattern of enteric nervous system development

AIM

Recently, the zebrafish (Danio rerio) has been shown to be an excellent model for human paediatric research. Advantages over other models include its small size, externally visually accessible development and ease of experimental manipulation. The enteric nervous system (ENS) consists of neurons and enteric glia. Glial cells permit cell bodies and processes of neurons to be arranged and maintained in a proper spatial arrangement, and are essential in the maintenance of basic physiological functions of neurons. Glial fibrillary acidic protein (GFAP) is expressed in astrocytes, but also expressed outside of the central nervous system. The aim of this study was to investigate the spatio-temporal pattern of GFAP expression in developing zebrafish ENS from 24 h post-fertilization (hpf), using transgenic fish that express green fluorescent protein (GFP).

METHODS

Zebrafish embryos were collected from transgenic GFP Tg(GFAP:GFP)(mi2001) adult zebrafish from 24 to 120 hpf, fixed and processed for whole mount immunohistochemistry. Antibodies to Phox2b were used to identify enteric neurons. Specimens were mounted on slides and imaging was performed using a fluorescent laser confocal microscope.

RESULTS

GFAP:GFP labelling outside the spinal cord was identified in embryos from 48 hpf. The patterning was intracellular and consisted of elongated profiles that appeared to migrate away from the spinal cord into the periphery. At 72 and 96 hpf, GFAP:GFP was expressed dorsally and ventrally to the intestinal tract. At 120 hpf, GFAP:GFP was expressed throughout the intestinal wall, and clusters of enteric neurons were identified using Phox2b immunofluorescence along the pathway of GFAP:GFP positive processes, indicative of a migratory pathway of ENS precursors from the spinal cord into the intestine.

CONCLUSION

The pattern of migration of GFAP:GFP expressing cells outside the spinal cord suggests an organized, early developing migratory pathway to the ENS. This shows for the first time that Tg(GFAP:GFP)(mi2001) zebrafish model is an ideal one to study spatio-temporal patterning of early ENS development.

Glial cell line-derived neurotrophic factor defines the path of developing and regenerating axons in the lateral line system of zebrafish

How the peripheral axons of sensory neurons are guided to distant target organs is not well understood. Here we examine this question in the case of the posterior lateral line (PLL) system of zebrafish, where sensory organs are deposited by a migrating primordium. Sensory neurites accompany this primordium during its migration and are thereby guided to their prospective target organs. We show that the inactivation of glial cell line-derived neurotrophic factor (GDNF) signaling leads to defects of innervation and that these defects are due to the inability of sensory axons to track the migrating primordium. GDNF signaling is also used as a guidance cue during axonal regeneration following nerve cut. We conclude that GDNF is a major determinant of directed neuritic growth and of target finding in this system, and we propose that GDNF acts by promoting local neurite outgrowth.

Distribution of glial cell line-derived neurotrophic factor receptor alpha-1 in the brain of adult zebrafish

Glial cell line-derived neurotrophic factor (GDNF) is a potent trophic factor for several types of neurons in the central and peripheral nervous systems. The biological activity of GDNF is mediated by a multicomponent receptor complex that includes a common transmembrane signaling component (the rearranged during transfection (RET) proto-oncogene product, a tyrosine kinase receptor) as well as a GDNF family receptor alpha (GFRalpha) subunit, a high-affinity glycosyl phosphatidylinositol (GPI)-linked binding element. Among the four known GFRalpha subunits, GFRalpha1 preferentially binds to GDNF. In zebrafish (Danio rerio) embryos, the expression of the GFRalpha1a and GFRalpha1b genes has been shown in primary motor neurons, the kidney, and the enteric nervous system. To examine the activity of GFRalpha in the adult brain of a lower vertebrate, we have investigated the localization of GFRalpha1a and GFRalpha1b mRNA and the GFRalpha1 protein in zebrafish. GFRalpha1a and GFRalpha1b transcripts were observed in brain extracts by reverse transcription-polymerase chain reaction. Whole-mount in-situ hybridization experiments revealed a wide distribution of GFRalpha1a and GFRalpha1b mRNAs in various regions of the adult zebrafish brain. These included the olfactory bulbs, dorsal and ventral telencephalic area (telencephalon), preoptic area, dorsal and ventral thalamus, posterior tuberculum and hypothalamus (diencephalon), optic tectum (mesencephalon), cerebellum, and medulla oblongata (rhombencephalon). Finally, expression patterns of the GFRalpha1 protein, detected immunohistochemically, correlated well with the mRNA expression and provided further insights into translational activity at the neuroanatomical level. In conclusion, the current study demonstrated that the presence of GFRalpha1 persists beyond the embryonic development of the zebrafish brain and, together with the GDNF ligand, is probably implicated in the brain physiology of an adult teleost fish.

Glial cells revealed by GFAP immunoreactivity in fish gut

Glial fibrillary acidic protein (GFAP) is a commonly used marker to identify enteric glia in the mammalian gut. Little is however known about enteric glia in other vertebrates. The aim of the present study was to examine the distribution of GFAP immunoreactivity in adult and developing fish. In adult shorthorn sculpin (Myoxocephalus scorpius) and zebrafish (Danio rerio), GFAP immunoreactivity was seen in the myenteric plexus in all regions of the gut. Co-staining for the neuronal markers Hu C/D and acetylated tubulin showed that GFAP immunoreactivity was not associated with nerves. GFAP immunoreactivity was predominantly seen in processes with few glial cell bodies being demonstrated in adult fish. GFAP immunoreactivity was also found in the gut in larval zebrafish from 3 days post-fertilisation, i.e. at approximately the same time that differentiated enteric nerve cells first occur. Immunoreactivity was most prominent in areas with no or a low density of Hu-immunoreactive nerve cell bodies, indicating that the developing glia follows a different pattern from that of enteric neurons. The results suggest that GFAP can be used as a marker for enteric glia in fish, as in birds and mammals. The distribution of GFAP immunoreactivity implies that enteric glia are widespread in the fish gastrointestinal tract. Glia and neurons diverge early during development of the gastrointestinal tract.

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.