Distinct functional and temporal requirements for zebrafish Hdac1 during neural crest-derived craniofacial and peripheral neuron development

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.

Epigenetics and the evolution of form: Experimental manipulation of a chromatin modification causes species-specific changes to the craniofacial skeleton

A central question in biology is the molecular origins of phenotypic diversity. While genetic changes are key to the genotype-phenotype relationship, alterations to chromatin structure and the physical packaging of histone proteins may also be important drivers of vertebrate divergence. We investigate the impact of such an epigenetic mechanism, histone acetylation, within a textbook example of an adaptive radiation. Cichlids of Lake Malawi have adapted diverse craniofacial structures, and here we investigate how histone acetylation influences morphological variation in these fishes. Specifically, we assessed the effect of inhibiting histone deacetylation using the drug trichostatin A (TSA) on developing facial structures. We examined this during three critical developmental windows in two cichlid species with alternate adult morphologies. Exposure to TSA during neural crest cell (NCC) migration and as postmigratory NCCs proliferate in the pharyngeal arches resulted in significant changes in lateral and ventral shape in Maylandia, but not in Tropheops. This included an overall shortening of the head, widening of the lower jaw, and steeper craniofacial profile, all of which are paedomorphic morphologies. In contrast, treatment with TSA during early chondrogenesis did not result in significant morphological changes in either species. Together, these data suggest a sensitivity to epigenetic alterations that are both time- and species-dependent. We find that morphologies are due to nonautonomous or potentially indirect effects on NCC development, including in part a global developmental delay. Our research bolsters the understanding that proper histone acetylation is essential for early craniofacial development and identifies a species-specific robustness to developmental change. Overall, this study demonstrates how epigenetic regulation may play an important role in both generating and buffering morphological variation.

A. Epigenetic regulation of craniofacial development and disease

BACKGROUND

The formation of the craniofacial complex relies on proper neural crest development. The gene regulatory networks (GRNs) and signaling pathways orchestrating this process have been extensively studied. These GRNs and signaling cascades are tightly regulated as alterations to any stage of neural crest development can lead to common congenital birth defects, including multiple syndromes affecting facial morphology as well as nonsyndromic facial defects, such as cleft lip with or without cleft palate. Epigenetic factors add a hierarchy to the regulation of transcriptional networks and influence the spatiotemporal activation or repression of specific gene regulatory cascades; however less is known about their exact mechanisms in controlling precise gene regulation.

AIMS

In this review, we discuss the role of epigenetic factors during neural crest development, specifically during craniofacial development and how compromised activities of these regulators contribute to congenital defects that affect the craniofacial complex.

T, Mishoe Hernandez L, DeLaurier A. Exposure to valproic acid (VPA) reproduces hdac1 loss of function phenotypes in zebrafish

Histone deacetylases are enzymes that remove acetyl groups from histone tails and are understood to act as repressors of transcriptional activity. Hdac1 has been previously shown to function in eye, pectoral fin, heart, liver, and pharyngeal skeletal development. We show that high doses of Valproic Acid (VPA) reproduce the hdac1 phenotype. We identify tbx5 genes as potential targets of Hdac1 in eye, pectoral fin, and heart development. Using timed exposures, we show that skeletal structures in the pharyngeal arches are impacted by VPA between 24-36 hours post-fertilization, indicating a role for Hdac1 during post-migration patterning, differentiation, or proliferation of cranial neural crest cells.

Sin3a associated protein 130 kDa, sap130, plays an evolutionary conserved role in zebrafish heart development

Hypoplastic left heart syndrome (HLHS) is a congenital heart disease where the left ventricle is reduced in size. A forward genetic screen in mice identified SIN3A associated protein 130 kDa (Sap130), part of the chromatin modifying SIN3A/HDAC complex, as a gene contributing to the etiology of HLHS. Here, we report the role of zebrafish sap130 genes in heart development. Loss of sap130a, one of two Sap130 orthologs, resulted in smaller ventricle size, a phenotype reminiscent to the hypoplastic left ventricle in mice. While cardiac progenitors were normal during somitogenesis, diminution of the ventricle size suggest the Second Heart Field (SHF) was the source of the defect. To explore the role of sap130a in gene regulation, transcriptome profiling was performed after the heart tube formation to identify candidate pathways and genes responsible for the small ventricle phenotype. Genes involved in cardiac differentiation and cardiac function were dysregulated in sap130a, but not in sap130b mutants. Confocal light sheet analysis measured deficits in cardiac output in MZsap130a supporting the notion that cardiomyocyte maturation was disrupted. Lineage tracing experiments revealed a significant reduction of SHF cells in the ventricle that resulted in increased outflow tract size. These data suggest that sap130a is involved in cardiogenesis via regulating the accretion of SHF cells to the growing ventricle and in their subsequent maturation for cardiac function. Further, genetic studies revealed an interaction between hdac1 and sap130a, in the incidence of small ventricles. These studies highlight the conserved role of Sap130a and Hdac1 in zebrafish cardiogenesis.

Bioinformatics Analysis of Hub Genes Involved in Alcohol-Related Hemifacial Microsomia Pathogenesis

OBJECTIVE

Alcohol is a recognized teratogen, and alcohol exposure increases the risk for hemifacial microsomia (HFM) of the fetus during maternal pregnancy. The present study aimed to explore potential mechanisms and verify hub genes of HFM associated with alcohol by bioinformatics methods.

METHODS

First, HFM and alcohol pathogenic genes were obtained. Thereafter, a protein-protein interactional (PPI) network was constructed. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses and molecular complex detection were performed by Metascape. Finally, we used the cytoHubba plugin to screen the hub genes.

RESULTS

A total of 43 HFM genes and 50 optimal alcohol candidate genes were selected. The PPI networks for pathogenic genes contained 93 nodes and 503 edges. Functional enrichment analysis largely focused on tissue formation and development. Two modules were identified from the PPI network, and 10 hub genes were screened out. The genes most relevant to alcohol-induced HFM pathogenesis included CTNNB1, TP53, MYC, HDAC1, and SOX2.

CONCLUSIONS

This study identified some significant hub genes, pathways, and modules of HFM related to alcohol by bioinformatics analyses. Our results suggest that the CTNNB1, TP53, MYC, HDAC1, and SOX B1 gene subfamilies may have played a major role in alcohol-induced HFM.

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.

Epigenetic regulation during melanocyte development and homeostasis

Melanocytes originate in the neural crest as precursor cells which then migrate and proliferate to reach their destination where they differentiate into pigment-producing cells. Melanocytes not only determine the colour of hair, skin and eyes but also protect against the harmful effects of UV irradiation. The establishment of the melanocyte lineage is regulated by a defined set of transcription factors and signalling pathways that direct the specific gene expression programmes underpinning melanoblast specification, survival, migration, proliferation and differentiation. In addition, epigenetic modifiers and replacement histones play key roles in regulating gene expression and its timing during the different steps of this process. Here, we discuss the evidence for the role of epigenetic regulators in melanocyte development and function and how they interact with transcription factors and signalling pathways to establish and maintain this important cell lineage.

A quantitative in vivo assay for craniofacial developmental toxicity of histone deacetylases

Many bony features of the face develop from endochondral ossification of preexisting collagen-rich cartilage structures. The proper development of these cartilage structures is essential to the morphological formation of the face. The developmental programs governing the formation of the pre-bone facial cartilages are sensitive to chemical compounds that disturb histone acetylation patterns and chromatin structure. We have taken advantage of this fact to develop a quantitative morphological assay of craniofacial developmental toxicity based on the distortion and deterioration of facial cartilage structures in zebrafish larvae upon exposure to increasing concentrations of several well-described histone deacetylase inhibitors. In this assay, we measure the angle formed by the developing ceratohyal bone as a precise, sensitive and quantitative proxy for the overall developmental status of facial cartilages. Using the well-established developmental toxicant and histone deacetylase-inhibiting compound valproic acid along with 12 structurally related compounds, we demonstrate the applicability of the ceratohyal angle assay to investigate structure-activity relationships.

Epigenetic Regulation of Cardiac Neural Crest Cells

The cardiac neural crest cells (cNCCs) is a transient, migratory cell population that contribute to the formation of major arteries and the septa and valves of the heart. Abnormal development of cNCCs leads to a spectrum of congenital heart defects that mainly affect the outflow region of the hearts. Signaling molecules and transcription factors are the best studied regulatory events controlling cNCC development. In recent years, however, accumulated evidence supports that epigenetic regulation also plays an important role in cNCC development. Here, we summarize the functions of epigenetic regulators during cNCC development as well as cNCC related cardiovascular defects. These factors include ATP-dependent chromatin remodeling factors, histone modifiers and DNA methylation modulators. In many cases, mutations in the genes encoding these factors are known to cause inborn heart diseases. A better understanding of epigenetic regulators, their activities and their roles during heart development will ultimately contribute to the development of new clinical applications for patients with congenital heart disease.

Histone Modifications and Chondrocyte Fate: Regulation and Therapeutic Implications

The involvement of histone modifications in cartilage development, pathology and regeneration is becoming increasingly evident. Understanding the molecular mechanisms and consequences of histone modification enzymes in cartilage development, homeostasis and pathology provides fundamental and precise perspectives to interpret the biological behavior of chondrocytes during skeletal development and the pathogenesis of various cartilage related diseases. Candidate molecules or drugs that target histone modifying proteins have shown promising therapeutic potential in the treatment of cartilage lesions associated with joint degeneration and other chondropathies. In this review, we summarized the advances in the understanding of histone modifications in the regulation of chondrocyte fate, cartilage development and pathology, particularly the molecular writers, erasers and readers involved. In addition, we have highlighted recent studies on the use of small molecules and drugs to manipulate histone signals to regulate chondrocyte functions or treat cartilage lesions, in particular osteoarthritis (OA), and discussed their potential therapeutic benefits and limitations in preventing articular cartilage degeneration or promoting its repair or regeneration.

DNA methylation and histone modifications are essential for regulation of stem cell formation and differentiation in zebrafish development

The complex processes necessary for embryogenesis require a gene regulatory network that is complex and systematic. Gene expression regulates development and organogenesis, but this process is altered and fine-tuned by epigenetic regulators that facilitate changes in the chromatin landscape. Epigenetic regulation of embryogenesis adjusts the chromatin structure by modifying both DNA through methylation and nucleosomes through posttranslational modifications of histone tails. The zebrafish is a well-characterized model organism that is a quintessential tool for studying developmental biology. With external fertilization, low cost and high fecundity, the zebrafish are an efficient tool for studying early developmental stages. Genetic manipulation can be performed in vivo resulting in quick identification of gene function. Large-scale genome analyses including RNA sequencing, chromatin immunoprecipitation and chromatin structure all are feasible in the zebrafish. In this review, we highlight the key events in zebrafish development where epigenetic regulation plays a critical role from the early stem cell stages through differentiation and organogenesis.

Histones, Their Variants and Post-translational Modifications in Zebrafish Development

Complex multi-cellular organisms are shaped starting from a single-celled zygote, owing to elaborate developmental programs. These programs involve several layers of regulation to orchestrate the establishment of progressively diverging cell type-specific gene expression patterns. In this scenario, epigenetic modifications of chromatin are central in influencing spatiotemporal patterns of gene transcription. In fact, it is generally recognized that epigenetic changes of chromatin states impact on the accessibility of genomic DNA to regulatory proteins. Several lines of evidence highlighted that zebrafish is an excellent vertebrate model for research purposes in the field of developmental epigenetics. In this review, I focus on the dynamic roles recently emerged for histone post-translational modifications (PTMs), histone modifying enzymes, histone variants and histone themselves in the coordination between the precise execution of transcriptional programs and developmental progression in zebrafish. In particular, I first outline a synopsis of the current state of knowledge in this field during early embryogenesis. Then, I present a survey of histone-based epigenetic mechanisms occurring throughout morphogenesis, with a stronger emphasis on cardiac formation. Undoubtedly, the issues addressed in this review take on particular importance in the emerging field of comparative biology of epigenetics, as well as in translational research.

Histone variant dictates fate biasing of neural crest cells to melanocyte lineage

In the neural crest lineage, progressive fate restriction and stem cell assignment are crucial for both development and regeneration. Whereas fate commitment events have distinct transcriptional footprints, fate biasing is often transitory and metastable, and is thought to be moulded by epigenetic programmes. Therefore, the molecular basis of specification is difficult to define. In this study, we established a role for a histone variant, H2a.z.2, in specification of the melanocyte lineage from multipotent neural crest cells. H2a.z.2 silencing reduces the number of melanocyte precursors in developing zebrafish embryos and from mouse embryonic stem cells in vitro We demonstrate that this histone variant occupies nucleosomes in the promoter of the key melanocyte determinant mitf, and enhances its induction. CRISPR/Cas9-based targeted mutagenesis of this gene in zebrafish drastically reduces adult melanocytes, as well as their regeneration. Thereby, our study establishes the role of a histone variant upstream of the core gene regulatory network in the neural crest lineage. This epigenetic mark is a key determinant of cell fate and facilitates gene activation by external instructive signals, thereby establishing melanocyte fate identity.

"Too much guts and not enough brains": (epi)genetic mechanisms and future therapies of Hirschsprung disease - a review

Hirschsprung disease is a neurocristopathy, characterized by aganglionosis in the distal bowel. It is caused by failure of the enteric nervous system progenitors to migrate, proliferate, and differentiate in the gut. Development of an enteric nervous system is a tightly regulated process. Both the neural crest cells and the surrounding environment are regulated by different genes, signaling pathways, and morphogens. For this process to be successful, the timing of gene expression is crucial. Hence, alterations in expression of genes specific for the enteric nervous system may contribute to the pathogenesis of Hirschsprung’s disease. Several epigenetic mechanisms contribute to regulate gene expression, such as modifications of DNA and RNA, histone modifications, and microRNAs. Here, we review the current knowledge of epigenetic and epitranscriptomic regulation in the development of the enteric nervous system and its potential significance for the pathogenesis of Hirschsprung’s disease. We also discuss possible future therapies and how targeting epigenetic and epitranscriptomic mechanisms may open new avenues for novel treatment.

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.

Epigenetic Mechanisms in Hirschsprung Disease

Hirschsprung disease (HSCR, OMIM 142623) is due to a failure of enteric precursor cells derived from neural crest (EPCs) to proliferate, migrate, survive or differentiate during Enteric Nervous System (ENS) formation. This is a complex process which requires a strict regulation that results in an ENS specific gene expression pattern. Alterations at this level lead to the onset of neurocristopathies such as HSCR. Gene expression is regulated by different mechanisms, such as DNA modifications (at the epigenetic level), transcriptional mechanisms (transcription factors, silencers, enhancers and repressors), postranscriptional mechanisms (3′UTR and ncRNA) and regulation of translation. All these mechanisms are finally implicated in cell signaling to determine the migration, proliferation, differentiation and survival processes for correct ENS development. In this review, we have performed an overview on the role of epigenetic mechanisms at transcriptional and posttranscriptional levels on these cellular events in neural crest cells (NCCs), ENS development, as well as in HSCR.

HDAC1-mediated repression of the retinoic acid-responsive gene ripply3 promotes second heart field development

Coordinated transcriptional and epigenetic mechanisms that direct development of the later differentiating second heart field (SHF) progenitors remain largely unknown. Here, we show that a novel zebrafish histone deacetylase 1 (hdac1) mutant allele cardiac really gone (crg) has a deficit of ventricular cardiomyocytes (VCs) and smooth muscle within the outflow tract (OFT) due to both cell and non-cell autonomous loss in SHF progenitor proliferation. Cyp26-deficient embryos, which have increased retinoic acid (RA) levels, have similar defects in SHF-derived OFT development. We found that nkx2.5⁺ progenitors from Hdac1 and Cyp26-deficient embryos have ectopic expression of ripply3, a transcriptional co-repressor of T-box transcription factors that is normally restricted to the posterior pharyngeal endoderm. Furthermore, the ripply3 expression domain is expanded anteriorly into the posterior nkx2.5⁺ progenitor domain in crg mutants. Importantly, excess ripply3 is sufficient to repress VC development, while genetic depletion of Ripply3 and Tbx1 in crg mutants can partially restore VC number. We find that the epigenetic signature at RA response elements (RAREs) that can associate with Hdac1 and RA receptors (RARs) becomes indicative of transcriptional activation in crg mutants. Our study highlights that transcriptional repression via the epigenetic regulator Hdac1 facilitates OFT development through directly preventing expression of the RA-responsive gene ripply3 within SHF progenitors.

Congenital heart defects are the most common malformations found in newborns, with many of these defects disrupting development of the outflow tract, the structure where blood is expelled from the heart. Despite their frequency, we do not have a grasp of the molecular and genetic mechanisms that underlie most congenital heart defects. Here, we show that zebrafish embryos containing a mutation in a gene called histone deacetylase 1 (hdac1) have smaller hearts with a reduction in the size of the ventricle and outflow tract. Hdac1 proteins limit accessibility to DNA and repress gene expression. We find that loss of Hdac1 in zebrafish embryos leads to increased expression of genes that are also induced by excess retinoic acid, a teratogen that induces similar outflow tract defects. Genetic loss-of-function studies support that ectopic expression of ripply3, a common target of both Hdac1 and retinoic acid signaling that is normally restricted to a subset of posterior pharyngeal cells, contributes to the smaller hearts found in zebrafish hdac1 mutants. Our study establishes a mechanism whereby the coordinated repression of genes downstream of Hdac1 and retinoic acid signaling is necessary for normal vertebrate outflow tract development.

Transcriptome analysis of Xenopus orofacial tissues deficient in retinoic acid receptor function

BACKGROUND

Development of the face and mouth is orchestrated by a large number of transcription factors, signaling pathways and epigenetic regulators. While we know many of these regulators, our understanding of how they interact with each other and implement changes in gene expression during orofacial development is still in its infancy. Therefore, this study focuses on uncovering potential cooperation between transcriptional regulators and one important signaling pathway, retinoic acid, during development of the midface.

RESULTS

Transcriptome analyses was performed on facial tissues deficient for retinoic acid receptor function at two time points in development; early (35 hpf) just after the neural crest migrates and facial tissues are specified and later (60 hpf) when the mouth has formed and facial structures begin to differentiate. Functional and network analyses revealed that retinoic acid signaling could cooperate with novel epigenetic factors and calcium-NFAT signaling during early orofacial development. At the later stage, retinoic acid may work with WNT and BMP and regulate homeobox containing transcription factors. Finally, there is an overlap in genes dysregulated in Xenopus embryos with median clefts with human genes associated with similar orofacial defects.

CONCLUSIONS

This study uncovers novel signaling pathways required for orofacial development as well as pathways that could interact with retinoic acid signaling during the formation of the face. We show that frog faces are an important tool for studying orofacial development and birth defects.

Histone deacetylase activity has an essential role in establishing and maintaining the vertebrate neural crest

The neural crest, a progenitor population that drove vertebrate evolution, retains the broad developmental potential of the blastula cells it is derived from, even as neighboring cells undergo lineage restriction. The mechanisms that enable these cells to preserve their developmental potential remain poorly understood. Here, we explore the role of histone deacetylase (HDAC) activity in this process in Xenopus We show that HDAC activity is essential for the formation of neural crest, as well as for proper patterning of the early ectoderm. The requirement for HDAC activity initiates in naïve blastula cells; HDAC inhibition causes loss of pluripotency gene expression and blocks the ability of blastula stem cells to contribute to lineages of the three embryonic germ layers. We find that pluripotent naïve blastula cells and neural crest cells are both characterized by low levels of histone acetylation, and show that increasing HDAC1 levels enhance the ability of blastula cells to be reprogrammed to a neural crest state. Together, these findings elucidate a previously uncharacterized role for HDAC activity in establishing the neural crest stem cell state.

Epigenetic regulators Rbbp4 and Hdac1 are overexpressed in a zebrafish model of RB1 embryonal brain tumor, and are required for neural progenitor survival and proliferation

In this study, we used comparative genomics and developmental genetics to identify epigenetic regulators driving oncogenesis in a zebrafish retinoblastoma 1 (rb1) somatic-targeting model of RB1 mutant embryonal brain tumors. Zebrafish rb1 brain tumors caused by TALEN or CRISPR targeting are histologically similar to human central nervous system primitive neuroectodermal tumors (CNS-PNETs). Like the human oligoneural OLIG2+/SOX10+ CNS-PNET subtype, zebrafish rb1 tumors show elevated expression of neural progenitor transcription factors olig2, sox10, sox8b and the receptor tyrosine kinase erbb3a oncogene. Comparison of rb1 tumor and rb1/rb1 germline mutant larval transcriptomes shows that the altered oligoneural precursor signature is specific to tumor tissue. More than 170 chromatin regulators were differentially expressed in rb1 tumors, including overexpression of chromatin remodeler components histone deacetylase 1 (hdac1) and retinoblastoma binding protein 4 (rbbp4). Germline mutant analysis confirms that zebrafish rb1, rbbp4 and hdac1 are required during brain development. rb1 is necessary for neural precursor cell cycle exit and terminal differentiation, rbbp4 is required for survival of postmitotic precursors, and hdac1 maintains proliferation of the neural stem cell/progenitor pool. We present an in vivo assay using somatic CRISPR targeting plus live imaging of histone-H2A.F/Z-GFP fusion protein in developing larval brain to rapidly test the role of chromatin remodelers in neural stem and progenitor cells. Our somatic assay recapitulates germline mutant phenotypes and reveals a dynamic view of their roles in neural cell populations. Our study provides new insight into the epigenetic processes that might drive pathogenesis in RB1 brain tumors, and identifies Rbbp4 and its associated chromatin remodeling complexes as potential target pathways to induce apoptosis in RB1 mutant brain cancer cells.

This article has an associated First Person interview with the first author of the paper.

Summary: This study shows that chromatin remodelers that are overexpressed in a zebrafish model of RB1 mutant brain cancer are required for neural progenitor proliferation and survival, providing insight into potential mechanisms that drive tumor growth.

Neurocristopathies: New insights 150 years after the neural crest discovery

The neural crest (NC) is a transient, multipotent and migratory cell population that generates an astonishingly diverse array of cell types during vertebrate development. These cells, which originate from the ectoderm in a region lateral to the neural plate in the neural fold, give rise to neurons, glia, melanocytes, chondrocytes, smooth muscle cells, odontoblasts and neuroendocrine cells, among others. Neurocristopathies (NCP) are a class of pathologies occurring in vertebrates, especially in humans that result from the abnormal specification, migration, differentiation or death of neural crest cells during embryonic development. Various pigment, skin, thyroid and hearing disorders, craniofacial and heart abnormalities, malfunctions of the digestive tract and tumors can also be considered as neurocristopathies. In this review we revisit the current classification and propose a new way to classify NCP based on the embryonic origin of the affected tissues, on recent findings regarding the molecular mechanisms that drive NC formation, and on the increased complexity of current molecular embryology techniques.

Histone Deacetylases in Cartilage Homeostasis and Osteoarthritis

The involvement of the epigenome in complex diseases is becoming increasingly clear and more feasible to study due to new genomic and computational technologies. Moreover, therapies altering the activities of proteins that modify and interpret the epigenome are available to treat cancers and neurological disorders. Many additional uses have been proposed for these drugs based on promising preclinical results, including in arthritis models. Understanding the effects of epigenomic drugs on the skeleton is of interest because of its importance in maintaining overall health and fitness. In this review, we summarize ongoing advancements in how one class of epigenetic modifiers, histone deacetylases (Hdacs), controls normal cartilage development and homeostasis, as well as recent work aimed at understanding the alterations in the expression and activities of these enzymes in osteoarthritis (OA). We also review recent studies utilizing Hdac inhibitors and discuss the potential therapeutic benefits and limitations of these drugs for preventing cartilage destruction in OA.

Epigenetics in ENS development and Hirschsprung disease

Hirschsprung disease (HSCR, OMIM 142623) is a neurocristopathy caused by a failure of the enteric nervous system (ENS) progenitors derived from neural crest cells (NCCs), to migrate, proliferate, differentiate or survive to and within the gastrointestinal tract, resulting in aganglionosis in the distal colon. The formation of the ENS is a complex process, which is regulated by a large range of molecules and signalling pathways involving both the NCCs and the intestinal environment. This tightly regulated process needs correct regulation of the expression of ENS specific genes. Alterations in the expression of these genes can have dramatic consequences. Several mechanisms that control the expression of genes have been described, such as DNA modification (epigenetic mechanisms), regulation of transcription (transcription factor, enhancers, repressors and silencers), post-transcriptional regulation (3'UTR and miRNAs) and regulation of translation. In this review, we focus on the epigenetic DNA modifications that have been described so far in the context of the ENS development. Moreover we describe the changes that are found in relation to the onset of HSCR.

Zebrafish as a model for understanding enteric nervous system interactions in the developing intestinal tract

The enteric nervous system (ENS) forms intimate connections with many other intestinal cell types, including immune cells and bacterial consortia resident in the intestinal lumen. In this review, we highlight contributions of the zebrafish model to understanding interactions among these cells. Zebrafish is a powerful model for forward genetic screens, several of which have uncovered genes previously unknown to be important for ENS development. More recently, zebrafish has emerged as a model for testing functions of genes identified in human patients or large-scale human susceptibility screens. In several cases, zebrafish studies have revealed mechanisms connecting intestinal symptoms with other, seemingly unrelated disease phenotypes. Importantly, chemical library screens in zebrafish have provided startling new insights into potential effects of common drugs on ENS development. A key feature of the zebrafish model is the ability to rear large numbers of animals germ free or in association with only specific bacterial species. Studies utilizing these approaches have demonstrated the importance of bacterial signals for normal intestinal development. These types of studies also show how luminal bacteria and the immune system can contribute to inflammatory processes that can feedback to influence ENS development. The excellent optical properties of zebrafish embryos and larvae, coupled with the ease of generating genetically marked cells of both the host and its resident bacteria, allow visualization of multiple intestinal cell types in living larvae and should promote a more in-depth understanding of intestinal cell interactions, especially interactions between other intestinal cell types and the ENS.

Studying the peripheral sympathetic nervous system and neuroblastoma in zebrafish

The zebrafish serves as an excellent model to study vertebrate development and disease. Optically clear embryos, combined with tissue-specific fluorescent reporters, permit direct visualization and measurement of peripheral nervous system formation in real time. Additionally, the model is amenable to rapid cellular, molecular, and genetic approaches to determine how developmental mechanisms contribute to disease states, such as cancer. In this chapter, we describe the development of the peripheral sympathetic nervous system (PSNS) in general, and our current understanding of genetic pathways important in zebrafish PSNS development specifically. We also illustrate how zebrafish genetics is used to identify new mechanisms controlling PSNS development and methods for interrogating the potential role of PSNS developmental pathways in neuroblastoma pathogenesis in vivo using the zebrafish MYCN-driven neuroblastoma model.

Using zebrafish in systems toxicology for developmental toxicity testing

With the high cost and the long-term assessment of developmental toxicity testing in mammals, the vertebrate zebrafish has become a useful alternative model organism for high-throughput developmental toxicity testing. Zebrafish is also very favorable for the 3R perspective in toxicology; however, the methodologies used by research groups vary greatly, posing considerable challenges to integrative analysis. In this review, we discuss zebrafish developmental toxicity testing, focusing on the methods of chemical exposure, the assessment of morphological abnormalities, housing conditions and their effects on the production of healthy embryos, and future directions. Zebrafish as a systems toxicology model has the potential to elucidate developmental toxicity pathways, and to provide a sound basis for human health risk assessments.

Histone Deacetylases in Bone Development and Skeletal Disorders

Histone deacetylases (Hdacs) are conserved enzymes that remove acetyl groups from lysine side chains in histones and other proteins. Eleven of the 18 Hdacs encoded by the human and mouse genomes depend on Zn(2+) for enzymatic activity, while the other 7, the sirtuins (Sirts), require NAD2(+). Collectively, Hdacs and Sirts regulate numerous cellular and mitochondrial processes including gene transcription, DNA repair, protein stability, cytoskeletal dynamics, and signaling pathways to affect both development and aging. Of clinical relevance, Hdacs inhibitors are United States Food and Drug Administration-approved cancer therapeutics and are candidate therapies for other common diseases including arthritis, diabetes, epilepsy, heart disease, HIV infection, neurodegeneration, and numerous aging-related disorders. Hdacs and Sirts influence skeletal development, maintenance of mineral density and bone strength by affecting intramembranous and endochondral ossification, as well as bone resorption. With few exceptions, inhibition of Hdac or Sirt activity though either loss-of-function mutations or prolonged chemical inhibition has negative and/or toxic effects on skeletal development and bone mineral density. Specifically, Hdac/Sirt suppression causes abnormalities in physiological development such as craniofacial dimorphisms, short stature, and bone fragility that are associated with several human syndromes or diseases. In contrast, activation of Sirts may protect the skeleton from aging and immobilization-related bone loss. This knowledge may prolong healthspan and prevent adverse events caused by epigenetic therapies that are entering the clinical realm at an unprecedented rate. In this review, we summarize the general properties of Hdacs/Sirts and the research that has revealed their essential functions in bone forming cells (e.g., osteoblasts and chondrocytes) and bone resorbing osteoclasts. Finally, we offer predictions on future research in this area and the utility of this knowledge for orthopedic applications and bone tissue engineering.

Epigenetic regulation in neural crest development

The neural crest is a migratory and multipotent cell population that plays a crucial role in many aspects of embryonic development. In all vertebrate embryos, these cells emerge from the dorsal neural tube then migrate long distances to different regions of the body, where they contribute to formation of many cell types and structures. These include much of the peripheral nervous system, craniofacial skeleton, smooth muscle, and pigmentation of the skin. The best-studied regulatory events guiding neural crest development are mediated by transcription factors and signaling molecules. In recent years, however, growing evidence supports an important role for epigenetic regulation as an additional mechanism for controlling the timing and level of gene expression at different stages of neural crest development. Here, we summarize the process of neural crest formation, with focus on the role of epigenetic regulation in neural crest specification, migration, and differentiation as well as in neural crest related birth defects and diseases.

Epigenetic regulation of the neural transcriptome and alcohol interference during development

Alcohol intoxicated cells broadly alter their metabolites – among them methyl and acetic acid can alter the DNA and histone epigenetic codes. Together with the promiscuous effect of alcohol on enzyme activities (including DNA methyltransferases) and the downstream effect on microRNA and transposable elements, alcohol is well placed to affect intrinsic transcriptional programs of developing cells. Considering that the developmental consequences of early alcohol exposure so profoundly affect neural systems, it is not unfounded to reason that alcohol exploits transcriptional regulators to challenge canonical gene expression and in effect, intrinsic developmental pathways to achieve widespread damage in the developing nervous system. To fully evaluate the role of epigenetic regulation in alcohol-related developmental disease, it is important to first gather the targets of epigenetic players in neurodevelopmental models. Here, we attempt to review the cellular and genomic windows of opportunity for alcohol to act on intrinsic neurodevelopmental programs. We also discuss some established targets of fetal alcohol exposure and propose pathways for future study. Overall, this review hopes to illustrate the known epigenetic program and its alterations in normal neural stem cell development and further, aims to depict how alcohol, through neuroepigenetics, may lead to neurodevelopmental deficits observed in fetal alcohol spectrum disorders.

HDAC1 and HDAC2 control the specification of neural crest cells into peripheral glia

Schwann cells, the myelinating glia of the peripheral nervous system (PNS), originate from multipotent neural crest cells that also give rise to other cells, including neurons, melanocytes, chondrocytes, and smooth muscle cells. The transcription factor Sox10 is required for peripheral glia specification. However, all neural crest cells express Sox10 and the mechanisms directing neural crest cells into a specific lineage are poorly understood. We show here that histone deacetylases 1 and 2 (HDAC1/2) are essential for the specification of neural crest cells into Schwann cell precursors and satellite glia, which express the early determinants of their lineage myelin protein zero (P0) and/or fatty acid binding protein 7 (Fabp7). In neural crest cells, HDAC1/2 induced expression of the transcription factor Pax3 by binding and activating the Pax3 promoter. In turn, Pax3 was required to maintain high Sox10 levels and to trigger expression of Fabp7. In addition, HDAC1/2 were bound to the P0 promoter and activated P0 transcription. Consistently, in vivo genetic deletion of HDAC1/2 in mouse neural crest cells led to strongly decreased Sox10 expression, no detectable Pax3, virtually no satellite glia, and no Schwann cell precursors in dorsal root ganglia and peripheral nerves. Similarly, in vivo ablation of Pax3 in the mouse neural crest resulted in strongly reduced expression of Sox10 and Fabp7. Therefore, by controlling the expression of Pax3 and the concerted action of Pax3 and Sox10 on their target genes, HDAC1/2 direct the specification of neural crest cells into peripheral glia.

Specific NuRD components are required for fin regeneration in zebrafish

Background

Epimorphic regeneration of a missing appendage in fish and urodele amphibians involves the creation of a blastema, a heterogeneous pool of progenitor cells underneath the wound epidermis. Current evidence indicates that the blastema arises by dedifferentiation of stump tissues in the vicinity of the amputation. In response to tissue loss, silenced developmental programs are reactivated to form a near-perfect copy of the missing body part. However, the importance of chromatin regulation during epimorphic regeneration remains poorly understood.

Results

We found that specific components of the Nucleosome Remodeling and Deacetylase complex (NuRD) are required for fin regeneration in zebrafish. Transcripts of the chromatin remodeler chd4a/Mi-2, the histone deacetylase hdac1/HDAC1/2, the retinoblastoma-binding protein rbb4/RBBP4/7, and the metastasis-associated antigen mta2/MTA were specifically co-induced in the blastema during adult and embryonic fin regeneration, and these transcripts displayed a similar spatial and temporal expression patterns. In addition, chemical inhibition of Hdac1 and morpholino-mediated knockdown of chd4a, mta2, and rbb4 impaired regenerative outgrowth, resulting in reduction in blastema cell proliferation and in differentiation defects.

Conclusion

Altogether, our data suggest that specialized NuRD components are induced in the blastema during fin regeneration and are involved in blastema cell proliferation and redifferentiation of osteoblast precursor cells. These results provide in vivo evidence for the involvement of key epigenetic factors in the cellular reprogramming processes occurring during epimorphic regeneration in zebrafish.