Forkhead transcription factor foxe1 regulates chondrogenesis in zebrafish

Clinical and Genetic Correlation in Neurocristopathies: Bridging a Precision Medicine Gap

Neurocristopathies (NCPs) encompass a spectrum of disorders arising from issues during the formation and migration of neural crest cells (NCCs). NCCs undergo epithelial-mesenchymal transition (EMT) and upon key developmental gene deregulation, fetuses and neonates are prone to exhibit diverse manifestations depending on the affected area. These conditions are generally rare and often have a genetic basis, with many following Mendelian inheritance patterns, thus making them perfect candidates for precision medicine. Examples include cranial NCPs, like Goldenhar syndrome and Axenfeld-Rieger syndrome; cardiac-vagal NCPs, such as DiGeorge syndrome; truncal NCPs, like congenital central hypoventilation syndrome and Waardenburg syndrome; and enteric NCPs, such as Hirschsprung disease. Additionally, NCCs' migratory and differentiating nature makes their derivatives prone to tumors, with various cancer types categorized based on their NCC origin. Representative examples include schwannomas and pheochromocytomas. This review summarizes current knowledge of diseases arising from defects in NCCs' specification and highlights the potential of precision medicine to remedy a clinical phenotype by targeting the genotype, particularly important given that those affected are primarily infants and young children.

The evolutionary loss of the Eh1 motif in FoxE1 in the lineage of placental mammals

Forkhead box E1 (FoxE1) protein is a transcriptional regulator known to play a major role in the development of the thyroid gland. By performing sequence alignments, we detected a deletion in FoxE1, which occurred in the evolution of mammals, near the point of divergence of placental mammals. This deletion led to the loss of the majority of the Eh1 motif, which was important for interactions with transcriptional corepressors. To investigate a potential mechanism for this deletion, we analyzed replication through the deletion area in mammalian cells with two-dimensional gel electrophoresis, and in vitro, using a primer extension reaction. We demonstrated that the area of the deletion presented an obstacle for replication in both assays. The exact position of polymerization arrest in primer extension indicated that it was most likely caused by a quadruplex DNA structure. The quadruplex structure hypothesis is also consistent with the exact borders of the deletion. The exact roles of these evolutionary changes in FoxE1 family proteins are still to be determined.

Disruption of the foxe1 gene in zebrafish reveals conserved functions in development of the craniofacial skeleton and the thyroid

Introduction: Mutations in the FOXE1 gene are implicated in cleft palate and thyroid dysgenesis in humans.

Methods: To investigate whether zebrafish could provide meaningful insights into the etiology of developmental defects in humans related to FOXE1, we generated a zebrafish mutant that has a disruption in the nuclear localization signal in the foxe1 gene, thereby restraining nuclear access of the transcription factor. We characterized skeletal development and thyroidogenesis in these mutants, focusing on embryonic and larval stages.

Results: Mutant larvae showed aberrant skeletal phenotypes in the ceratohyal cartilage and had reduced whole body levels of Ca, Mg and P, indicating a critical role for foxe1 in early skeletal development. Markers of bone and cartilage (precursor) cells were differentially expressed in mutants in post-migratory cranial neural crest cells in the pharyngeal arch at 1 dpf, at induction of chondrogenesis at 3 dpf and at the start of endochondral bone formation at 6 dpf. Foxe1 protein was detected in differentiated thyroid follicles, suggesting a role for the transcription factor in thyroidogenesis, but thyroid follicle morphology or differentiation were unaffected in mutants.

Discussion: Taken together, our findings highlight the conserved role of Foxe1 in skeletal development and thyroidogenesis, and show differential signaling of osteogenic and chondrogenic genes related to foxe1 mutation.

Zebrafish as an emerging tool for drug discovery and development for thyroid diseases

Zebrafish is a useful model for understanding human genetics and diseases and has evolved into a prominent scientific research model. The genetic structure of zebrafish is 70% identical to that of humans. Its small size, low cost, and transparent embryo make it a valuable tool in experimentation. Zebrafish and mammals possess the same molecular mechanism of thyroid organogenesis and development. Thus, thyroid hormone signaling, embryonic development, thyroid-related disorders, and novel genes involved in early thyroid development can all be studied using zebrafish as a model. Here in this review, we emphasize the evolving role of zebrafish as a possible tool for studying the thyroid gland in the context of physiology and pathology. The transcription factors nkx2.1a, pax2a, and hhex which contribute a pivotal role in the differentiation of thyroid primordium are discussed. Further, we have described the role of zebrafish as a model for thyroid cancer, evaluation of defects in thyroid hormone transport, thyroid hormone (TH) metabolism, and as a screening tool to study thyrotoxins. Hence, the present review highlights the role of zebrafish as a novel approach to understand thyroid development and organogenesis.

Thyroid and endostyle development in cyclostomes provides new insights into the evolutionary history of vertebrates

Background

The endostyle is an epithelial exocrine gland found in non-vertebrate chordates (amphioxi and tunicates) and the larvae of modern lampreys. It is generally considered to be an evolutionary precursor of the thyroid gland of vertebrates. Transformation of the endostyle into the thyroid gland during the metamorphosis of lampreys is thus deemed to be a recapitulation of a past event in vertebrate evolution. In 1906, Stockard reported that the thyroid gland in hagfish, the sister cyclostome group of lampreys, develops through an endostyle-like primordium, strongly supporting the plesiomorphy of the lamprey endostyle. However, the findings in hagfish thyroid development were solely based on this single study, and these have not been confirmed by modern molecular, genetic, and morphological data pertaining to hagfish thyroid development over the last century.

Results

Here, we showed that the thyroid gland of hagfish undergoes direct development from the ventrorostral pharyngeal endoderm, where the previously described endostyle-like primordium was not found. The developmental pattern of the hagfish thyroid, including histological features and regulatory gene expression profiles, closely resembles that found in modern jawed vertebrates (gnathostomes). Meanwhile, as opposed to gnathostomes but similar to non-vertebrate chordates, lamprey and hagfish share a broad expression domain of Nkx2-1/2-4, a key regulatory gene, in the pharyngeal epithelium during early developmental stages.

Conclusions

Based on the direct development of the thyroid gland both in hagfish and gnathostomes, and the shared expression profile of thyroid-related transcription factors in the cyclostomes, we challenge the plesiomorphic status of the lamprey endostyle and propose an alternative hypothesis where the lamprey endostyle could be obtained secondarily in crown lampreys.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01282-7.

The crucial role of model systems in understanding the complexity of cell signaling in human neurocristopathies

Animal models are useful to study the molecular, cellular, and morphogenetic mechanisms underlying normal and pathological development. Cell-based study models have emerged as an alternative approach to study many aspects of human embryonic development and disease. The neural crest (NC) is a transient, multipotent, and migratory embryonic cell population that generates a diverse group of cell types that arises during vertebrate development. The abnormal formation or development of the NC results in neurocristopathies (NCPs), which are characterized by a broad spectrum of functional and morphological alterations. The impaired molecular mechanisms that give rise to these multiphenotypic diseases are not entirely clear yet. This fact, added to the high incidence of these disorders in the newborn population, has led to the development of systematic approaches for their understanding. In this article, we have systematically reviewed the ways in which experimentation with different animal and cell model systems has improved our knowledge of NCPs, and how these advances might contribute to the development of better diagnostic and therapeutic tools for the treatment of these pathologies. This article is categorized under: Congenital Diseases > Genetics/Genomics/Epigenetics Congenital Diseases > Stem Cells and Development Congenital Diseases > Molecular and Cellular Physiology Neurological Diseases > Genetics/Genomics/Epigenetics.

The Mandibular and Hyoid Arches-From Molecular Patterning to Shaping Bone and Cartilage

The mandibular and hyoid arches collectively make up the facial skeleton, also known as the viscerocranium. Although all three germ layers come together to assemble the pharyngeal arches, the majority of tissue within viscerocranial skeletal components differentiates from the neural crest. Since nearly one third of all birth defects in humans affect the craniofacial region, it is important to understand how signalling pathways and transcription factors govern the embryogenesis and skeletogenesis of the viscerocranium. This review focuses on mouse and zebrafish models of craniofacial development. We highlight gene regulatory networks directing the patterning and osteochondrogenesis of the mandibular and hyoid arches that are actually conserved among all gnathostomes. The first part of this review describes the anatomy and development of mandibular and hyoid arches in both species. The second part analyses cell signalling and transcription factors that ensure the specificity of individual structures along the anatomical axes. The third part discusses the genes and molecules that control the formation of bone and cartilage within mandibular and hyoid arches and how dysregulation of molecular signalling influences the development of skeletal components of the viscerocranium. In conclusion, we notice that mandibular malformations in humans and mice often co-occur with hyoid malformations and pinpoint the similar molecular machinery controlling the development of mandibular and hyoid arches.

Unraveling the Complex Interplay Between Transcription Factors and Signaling Molecules in Thyroid Differentiation and Function, From Embryos to Adults

Thyroid differentiation of progenitor cells occurs during embryonic development and in the adult thyroid gland, and the molecular bases of these complex and finely regulated processes are becoming ever more clear. In this Review, we describe the most recent advances in the study of transcription factors, signaling molecules and regulatory pathways controlling thyroid differentiation and development in the mammalian embryo. We also discuss the maintenance of the adult differentiated phenotype to ensure the biosynthesis of thyroid hormones. We will focus on endoderm-derived thyroid epithelial cells, which are responsible for the formation of the thyroid follicle, the functional unit of the thyroid gland. The use of animal models and pluripotent stem cells has greatly aided in providing clues to the complicated puzzle of thyroid development and function in adults. The so-called thyroid transcription factors - Nkx2-1, Foxe1, Pax8 and Hhex - were the first pieces of the puzzle identified in mice. Other transcription factors, either acting upstream of or directly with the thyroid transcription factors, were subsequently identified to, almost, complete the puzzle. Among them, the transcription factors Glis3, Sox9 and the cofactor of the Hippo pathway Taz, have emerged as important players in thyroid differentiation and development. The involvement of signaling molecules increases the complexity of the puzzle. In this context, the importance of Bmps, Fgfs and Shh signaling at the onset of development, and of TSH, IGF1 and TGFβ both at the end of terminal differentiation in embryos and in the adult thyroid, are well recognized. All of these aspects are covered herein. Thus, readers will be able to visualize the puzzle of thyroid differentiation with most - if not all - of the pieces in place.

Systematic review of hormonal and genetic factors involved in the nonsyndromic disorders of the lower jaw

Mandibular disorders are among the most common birth defects in humans, yet the etiological factors are largely unknown. Most of the neonates affected by mandibular abnormalities have a sequence of secondary anomalies, including airway obstruction and feeding problems, that reduce the quality of life. In the event of lacking corrective surgeries, patients with mandibular congenital disorders suffer from additional lifelong problems such as sleep apnea and temporomandibular disorders, among others. The goal of this systematic review is to gather evidence on hormonal and genetic factors that are involved in signaling pathways and interactions that are potentially associated with the nonsyndromic mandibular disorders. We found that members of FGF and BMP pathways, including FGF8/10, FGFR2/3, BMP2/4/7, BMPR1A, ACVR1, and ACVR2A/B, have a prominent number of gene-gene interactions among all identified genes in this review. Gene ontology of the 154 genes showed that the functional gene sets are involved in all aspects of cellular processes and organogenesis. Some of the genes identified by the genome-wide association studies of common mandibular disorders are involved in skeletal formation and growth retardation based on animal models, suggesting a potential direct role as genetic risk factors in the common complex jaw disorders. Developmental Dynamics 248:162-172, 2019. © 2018 Wiley Periodicals, Inc.

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.

Exploration of the association between FOXE1 gene polymorphism and differentiated thyroid cancer: a meta-analysis

BACKGROUND

Several association analyses and linkage researches indicated that inherited genetic variations effectively influence differentiated thyroid carcinogenesis.

METHODS

The results from 15 published studies on differentiated thyroid carcinoma (DTC) were combined. The genetic model included rs965513, rs944289 and rs1867277. Meta-analyses were performed and cochran's χ² based Q-statistic and I² test were performed to assess heterogeneity using STATA software.

RESULTS

Significant results were noticed for rs965513(Odds Ratio(OR) = 1.162(1.117, 1.208)), rs944289(OR = 1.082(1.035, 1.131)) and rs1867277(OR = 1.415(1.324, 1.512)). In the subgroup analysis by ethnicity, rs965513 polymorphism conferred that risk of Caucasians (OR = 1.168(1.122, 1.215)) was more than that of East Asians of 1.35 (OR = 0.897(0.680, 1.193)).

CONCLUSION

This meta-analysis revealed that common variations of FOXE1 (rs965513, rs944289 and rs1867277) were risk factors associated with increased DTC susceptibility.

How zebrafish research has helped in understanding thyroid diseases

Next-generation sequencing technologies have revolutionized the identification of disease-causing genes, accelerating the discovery of new mutations and new candidate genes for thyroid diseases. To face this flow of novel genetic information, it is important to have suitable animal models to study the mechanisms regulating thyroid development and thyroid hormone availability and activity. Zebrafish ( Danio rerio), with its rapid external embryonic development, has been extensively used in developmental biology. To date, almost all of the components of the zebrafish thyroid axis have been characterized and are structurally and functionally comparable with those of higher vertebrates. The availability of transgenic fluorescent zebrafish lines allows the real-time analysis of thyroid organogenesis and its alterations. Transient morpholino-knockdown is a solution to silence the expression of a gene of interest and promptly obtain insights on its contribution during the development of the zebrafish thyroid axis. The recently available tools for targeted stable gene knockout have further increased the value of zebrafish to the study of thyroid disease. All of the reported zebrafish models can also be used to screen small compounds and to test new drugs and may allow the establishment of experimental proof of concept to plan subsequent clinical trials.

Zebrafish models of orofacial clefts

Zebrafish is a model organism that affords experimental advantages toward investigating the normal function of genes associated with congenital birth defects. Here we summarize zebrafish studies of genes implicated in orofacial cleft (OFC). The most common use of zebrafish in this context has been to explore the normal function an OFC-associated gene product in craniofacial morphogenesis by inhibiting expression of its zebrafish ortholog. The most frequently deployed method has been to inject embryos with antisense morpholino oligonucleotides targeting the desired transcript. However, improvements in targeted mutagenesis strategies have led to widespread adoption of CRISPR/Cas9 technology. A second application of zebrafish has been for functional assays of gene variants found in OFC patients; such in vivo assays are valuable because the success of in silico methods for testing allele severity has been mixed. Finally, zebrafish have been used to test the tissue specificity of enhancers that harbor single nucleotide polymorphisms associated with risk for OFC. We review examples of each of these approaches in the context of genes that are implicated in syndromic and non-syndromic OFC. Developmental Dynamics 246:897-914, 2017. © 2017 Wiley Periodicals, Inc.

Development of the thyroid gland

Thyroid hormones are crucial for organismal development and homeostasis. In humans, untreated congenital hypothyroidism due to thyroid agenesis inevitably leads to cretinism, which comprises irreversible brain dysfunction and dwarfism. Elucidating how the thyroid gland - the only source of thyroid hormones in the body - develops is thus key for understanding and treating thyroid dysgenesis, and for generating thyroid cells in vitro that might be used for cell-based therapies. Here, we review the principal mechanisms involved in thyroid organogenesis and functional differentiation, highlighting how the thyroid forerunner evolved from the endostyle in protochordates to the endocrine gland found in vertebrates. New findings on the specification and fate decisions of thyroid progenitors, and the morphogenesis of precursor cells into hormone-producing follicular units, are also discussed.

Disorders of thyroid morphogenesis

Developmental anomalies of the thyroid gland, defined as thyroid dysgenesis, underlie the majority of cases of congenital hypothyroidism. Thyroid dysgenesis is predominantly a sporadic disorder although a reported familial enrichment, variation of incidence by ethnicity and the monogenic defects associated mainly with athyreosis or orthotopic thyroid hypoplasia, suggest a genetic contribution. Of note, the most common developmental anomaly, thyroid ectopy, remains unexplained. Ectopy may result from multiple genetic or epigenetic variants in the germline and/or at the somatic level. This review provides a brief overview of the monogenic defects in candidate genes that have been identified so far and of the syndromes which are known to be associated with thyroid dysgenesis.

Determinants of orofacial clefting I: Effects of 5-Aza-2'-deoxycytidine on cellular processes and gene expression during development of the first branchial arch

In this study, we identify gene targets and cellular events mediating the teratogenic action(s) of 5-Aza-2'-deoxycytidine (AzaD), an inhibitor of DNA methylation, on secondary palate development. Exposure of pregnant mice (on gestation day (GD) 9.5) to AzaD for 12h resulted in the complete penetrance of cleft palate (CP) in fetuses. Analysis of cells of the embryonic first branchial arch (1-BA), in fetuses exposed to AzaD, revealed: 1) significant alteration in expression of genes encoding several morphogenetic factors, cell cycle inhibitors and regulators of apoptosis; 2) a decrease in cell proliferation; and, 3) an increase in apoptosis. Pyrosequencing of selected genes, displaying pronounced differential expression in AzaD-exposed 1-BAs, failed to reveal significant alterations in CpG methylation levels in their putative promoters or gene bodies. CpG methylation analysis suggested that the effects of AzaD on gene expression were likely indirect.

ATX-LPA1 axis contributes to proliferation of chondrocytes by regulating fibronectin assembly leading to proper cartilage formation

The lipid mediator lysophosphatidic acid (LPA) signals via six distinct G protein-coupled receptors to mediate both unique and overlapping biological effects, including cell migration, proliferation and survival. LPA is produced extracellularly by autotaxin (ATX), a secreted lysophospholipase D, from lysophosphatidylcholine. ATX-LPA receptor signaling is essential for normal development and implicated in various (patho)physiological processes, but underlying mechanisms remain incompletely understood. Through gene targeting approaches in zebrafish and mice, we show here that loss of ATX-LPA₁ signaling leads to disorganization of chondrocytes, causing severe defects in cartilage formation. Mechanistically, ATX-LPA₁ signaling acts by promoting S-phase entry and cell proliferation of chondrocytes both in vitro and in vivo, at least in part through β1-integrin translocation leading to fibronectin assembly and further extracellular matrix deposition; this in turn promotes chondrocyte-matrix adhesion and cell proliferation. Thus, the ATX-LPA₁ axis is a key regulator of cartilage formation.

Two Forkhead transcription factors regulate cardiac progenitor specification by controlling the expression of receptors of the fibroblast growth factor and Wnt signaling pathways

Cardiogenesis involves the coordinated regulation of multiple biological processes by a finite set of transcription factors (TFs). Here, we show that the Forkhead TFs Checkpoint suppressor homologue (CHES-1-like) and Jumeau (Jumu), which govern cardiac progenitor cell divisions by regulating Polo kinase activity, play an additional, mutually redundant role in specifying the cardiac mesoderm (CM) as eliminating the functions of both Forkhead genes in the same Drosophila embryo results in defective hearts with missing hemisegments. This process is mediated by the Forkhead TFs regulating the fibroblast growth factor receptor Heartless (Htl) and the Wnt receptor Frizzled (Fz): CHES-1-like and jumu exhibit synergistic genetic interactions with htl and fz in CM specification, thereby implying that they function through the same genetic pathways, and transcriptionally activate the expression of both receptor-encoding genes. Furthermore, ectopic overexpression of either htl or fz in the mesoderm partially rescues the defective CM specification phenotype in embryos lacking both Forkhead genes. Together, these data emphasize the functional redundancy that leads to robustness in the cardiac progenitor specification process, and illustrate the pleiotropic functions of Forkhead TFs in different aspects of cardiogenesis.

Summary: Checkpoint suppressor homologue and Jumeau, which are known to govern cardiac progenitor cell divisions, play additional, mutually redundant roles in specifying cardiac mesoderm in Drosophila.

Zebrafish Craniofacial Development: A Window into Early Patterning

The formation of the face and skull involves a complex series of developmental events mediated by cells derived from the neural crest, endoderm, mesoderm, and ectoderm. Although vertebrates boast an enormous diversity of adult facial morphologies, the fundamental signaling pathways and cellular events that sculpt the nascent craniofacial skeleton in the embryo have proven to be highly conserved from fish to man. The zebrafish Danio rerio, a small freshwater cyprinid fish from eastern India, has served as a popular model of craniofacial development since the 1990s. Unique strengths of the zebrafish model include a simplified skeleton during larval stages, access to rapidly developing embryos for live imaging, and amenability to transgenesis and complex genetics. In this chapter, we describe the anatomy of the zebrafish craniofacial skeleton; its applications as models for the mammalian jaw, middle ear, palate, and cranial sutures; the superior imaging technology available in fish that has provided unprecedented insights into the dynamics of facial morphogenesis; the use of the zebrafish to decipher the genetic underpinnings of craniofacial biology; and finally a glimpse into the most promising future applications of zebrafish craniofacial research.

A single nucleotide polymorphism associated with isolated cleft lip and palate, thyroid cancer and hypothyroidism alters the activity of an oral epithelium and thyroid enhancer near FOXE1

Three common diseases, isolated cleft lip and cleft palate (CLP), hypothyroidism and thyroid cancer all map to the FOXE1 locus, but causative variants have yet to be identified. In patients with CLP, the frequency of coding mutations in FOXE1 fails to account for the risk attributable to this locus, suggesting that the common risk alleles reside in nearby regulatory elements. Using a combination of zebrafish and mouse transgenesis, we screened 15 conserved non-coding sequences for enhancer activity, identifying three that regulate expression in a tissue specific pattern consistent with endogenous foxe1 expression. These three, located -82.4, -67.7 and +22.6 kb from the FOXE1 start codon, are all active in the oral epithelium or branchial arches. The -67.7 and +22.6 kb elements are also active in the developing heart, and the -67.7 kb element uniquely directs expression in the developing thyroid. Within the -67.7 kb element is the SNP rs7850258 that is associated with all three diseases. Quantitative reporter assays in oral epithelial and thyroid cell lines show that the rs7850258 allele (G) associated with CLP and hypothyroidism has significantly greater enhancer activity than the allele associated with thyroid cancer (A). Moreover, consistent with predicted transcription factor binding differences, the -67.7 kb element containing rs7850258 allele G is significantly more responsive to both MYC and ARNT than allele A. By demonstrating that this common non-coding variant alters FOXE1 expression, we have identified at least in part the functional basis for the genetic risk of these seemingly disparate disorders.

Molecular cloning and functional characterization of two forms of Pax8 in the rainbow trout, Oncorhynchus mykiss

We have identified two distinct Pax8 (a and b) mRNAs from the thyroid gland of the rainbow trout (Oncorhynchus mykiss), which seemed to be generated by alternative splicing. Both Pax8a and Pax8b proteins were predicted to possess the paired domain, octapeptide, and partial homeodomain, while Pax8b lacked the carboxy-terminal portion due to an insertion in the coding region of the mRNA. RT-PCR analysis showed each of Pax8a and Pax8b mRNAs to be abundantly expressed in the thyroid and kidney. In situ hybridization histochemistry further detected the expression of Pax8 mRNA in the epithelial cells of the thyroid follicles of the adult trout and in the thyroid primordial cells of the embryo. The functional properties of Pax8a and Pax8b were investigated by dual luciferase assay. The transcriptional regulation by the rat thyroid peroxidase (TPO) promoter was found to be increased by Pax8a, but not by Pax8b. Pax8a further showed synergistic transcriptional activity with rat Nkx2-1 for the human TPO upstream region including the enhancer and promoter. On the other hand, Pax8b decreased the synergistic activity of Pax8a and Nkx2-1. Electrophoretic mobility shift assay additionally indicated that not only Pax8a but also Pax8b can bind to the TPO promoter and enhancer, implying that the inhibitory effect of Pax8b might result from the lack of the functional carboxy-terminal portion. Collectively, the results suggest that for the trout thyroid gland, Pax8a may directly increase TPO gene expression in cooperation with Nkx2-1 while Pax8b may work as a non-activating competitor for the TPO transcription.

Genome-wide map of quantified epigenetic changes during in vitro chondrogenic differentiation of primary human mesenchymal stem cells

Background

For safe clinical application of engineered cartilage made from mesenchymal stem cells (MSCs), molecular mechanisms for chondrogenic differentiation must be known in detail. Changes in gene expression and extracellular matrix synthesis have been extensively studied, but the epigenomic modifications underlying these changes have not been described. To this end we performed whole-genome chromatin immunoprecipitation and deep sequencing to quantify six histone modifications, reduced representation bisulphite sequencing to quantify DNA methylation and mRNA microarrays to quantify gene expression before and after 7 days of chondrogenic differentiation of MSCs in an alginate scaffold. To add to the clinical relevance of our observations, the study is based on primary bone marrow-derived MSCs from four donors, allowing us to investigate inter-individual variations.

Results

We see two levels of relationship between epigenetic marking and gene expression. First, a large number of genes ontogenetically linked to MSC properties and the musculoskeletal system are epigenetically prepatterned by moderate changes in H3K4me3 and H3K9ac near transcription start sites. Most of these genes remain transcriptionally unaltered. Second, transcriptionally upregulated genes, more closely associated with chondrogenesis, are marked by H3K36me3 in gene bodies, highly increased H3K4me3 and H3K9ac on promoters and 5' end of genes, and increased H3K27ac and H3K4me1 marking in at least one enhancer region per upregulated gene. Within the 7-day time frame, changes in promoter DNA methylation do not correlate significantly with changes in gene expression. Inter-donor variability analysis shows high level of similarity between the donors for this data set.

Conclusions

Histone modifications, rather than DNA methylation, provide the primary epigenetic control of early differentiation of MSCs towards the chondrogenic lineage.

Transgenic zebrafish illuminate the dynamics of thyroid morphogenesis and its relationship to cardiovascular development

Among the various organs derived from foregut endoderm, the thyroid gland is unique in that major morphogenic events such as budding from foregut endoderm, descent into subpharyngeal mesenchyme and growth expansion occur in close proximity to cardiovascular tissues. To date, research on thyroid organogenesis was missing one vital tool-a transgenic model that allows to track the dynamic changes in thyroid size, shape and location relative to adjacent cardiovascular tissues in live embryos. In this study, we generated a novel transgenic zebrafish line, tg(tg:mCherry), in which robust and thyroid-specific expression of a membrane version of mCherry enables live imaging of thyroid development in embryos from budding stage throughout formation of functional thyroid follicles. By using various double transgenic models in which EGFP expression additionally labels cardiovascular structures, a high coordination was revealed between thyroid organogenesis and cardiovascular development. Early thyroid development was found to proceed in intimate contact with the distal ventricular myocardium and live imaging confirmed that thyroid budding from the pharyngeal floor is tightly coordinated with the descent of the heart. Four-dimensional imaging of live embryos by selective plane illumination microscopy and 3D-reconstruction of confocal images of stained embryos yielded novel insights into the role of specific pharyngeal vessels, such as the hypobranchial artery (HA), in guiding late thyroid expansion along the pharyngeal midline. An important role of the HA was corroborated by the detailed examination of thyroid development in various zebrafish models showing defective cardiovascular development. In combination, our results from live imaging as well es from 3D-reconstruction of thyroid development in tg(tg:mCherry) embryos provided a first dynamic view of late thyroid organogenesis in zebrafish-a critical resource for the design of future studies addressing the molecular mechanisms of these thyroid-vasculature interactions.

Accelerated evolution of 3'avian FOXE1 genes, and thyroid and feather specific expression of chicken FoxE1

Background

The forkhead transcription factor gene E1 (FOXE1) plays an important role in regulation of thyroid development, palate formation and hair morphogenesis in mammals. However, avian FOXE1 genes have not been characterized and as such, codon evolution of FOXE1 orthologs in a broader evolutionary context of mammals and birds is not known.

Results

In this study we identified the avian FOXE1 gene in chicken, turkey and zebra finch, all of which consist of a single exon. Chicken and zebra finch FOXE1 are uniquely located on the sex-determining Z chromosome. In situ hybridization shows that chicken FOXE1 is specifically expressed in the developing thyroid. Its expression is initiated at the placode stage and is maintained during the stages of vesicle formation and follicle primordia. Based on this expression pattern, we propose that avian FOXE1 may be involved in regulating the evagination and morphogenesis of thyroid. Chicken FOXE1 is also expressed in growing feathers. Sequence analysis identified two microdeletions in the avian FOXE1 genes, corresponding to the loss of a transferable repression domain and an engrailed homology motif 1 (Eh1) C-terminal to the forkhead domain. The avian FOXE1 proteins exhibit a significant sequence divergence of the C-terminus compared to those of amphibian and mammalian FOXE1. The codon evolution analysis (dN/dS) of FOXE1 shows a significantly increased dN/dS ratio in the avian lineages, consistent with either a relaxed purifying selection or positive selection on a few residues in avian FOXE1 evolution. Further site specific analysis indicates that while relaxed purifying selection is likely to be a predominant cause of accelerated evolution at the 3'-region of avian FOXE1, a few residues might have evolved under positive selection.

Conclusions

We have identified three avian FOXE1 genes based on synteny and sequence similarity as well as characterized the expression pattern of the chicken FOXE1 gene during development. Our evolutionary analyses suggest that while a relaxed purifying selection is likely to be the dominant force driving accelerated evolution of avian FOXE1 genes, a few residues may have evolved adaptively. This study provides a basis for future genetic and comparative biochemical studies of FOXE1.

The eye as an organizer of craniofacial development

The formation and invagination of the optic stalk coincides with the migration of cranial neural crest (CNC) cells, and a growing body of data reveals that the optic stalk and CNC cells communicate to lay the foundations for periocular and craniofacial development. Following migration, the interaction between the developing eye and surrounding periocular mesenchyme (POM) continues, leading to induction of transcriptional regulatory cascades that regulate craniofacial morphogenesis. Studies in chick, mice, and zebrafish have revealed a remarkable level of genetic and mechanistic conservation, affirming the power of each animal model to shed light on the broader morphogenic process. This review will focus on the role of the developing eye in orchestrating craniofacial morphogenesis, utilizing morphogenic gradients, paracrine signaling, and transcriptional regulatory cascades to establish an evolutionarily-conserved facial architecture. We propose that in addition to the forebrain, the eye functions during early craniofacial morphogenesis as a key organizer of facial development, independent of its role in vision.

Morphogenesis of the thyroid gland

Congenital hypothyroidism is mainly due to structural defects of the thyroid gland, collectively known as thyroid dysgenesis. The two most prevalent forms of this condition are abnormal localization of differentiated thyroid tissue (thyroid ectopia) and total absence of the gland (athyreosis). The clinical picture of thyroid dysgenesis suggests that impaired specification, proliferation and survival of thyroid precursor cells and loss of concerted movement of these cells in a distinct spatiotemporal pattern are major causes of malformation. In normal development the thyroid primordium is first distinguished as a thickening of the anterior foregut endoderm at the base of the prospective tongue. Subsequently, this group of progenitors detaches from the endoderm, moves caudally and ultimately differentiates into hormone-producing units, the thyroid follicles, at a distant location from the site of specification. In higher vertebrates later stages of thyroid morphogenesis are characterized by shape remodeling into a bilobed organ and the integration of a second type of progenitors derived from the caudal-most pharyngeal pouches that will differentiate into C-cells. The present knowledge of thyroid developmental dynamics has emerged from embryonic studies mainly in chicken, mouse and more recently also in zebrafish. This review will highlight the key morphogenetic steps of thyroid organogenesis and pinpoint which crucial regulatory mechanisms are yet to be uncovered. Considering the co-incidence of thyroid dysgenesis and congenital heart malformations the possible interactions between thyroid and cardiovascular development will also be discussed.