Cyp26 enzymes are required to balance the cardiac and vascular lineages within the anterior lateral plate mesoderm

Tbx1-dependent and independent pathways promote six gene expression downstream of retinoic acid signaling to determine cardiomyocyte number in zebrafish

Tight regulation of retinoic acid (RA) levels is critical for normal heart development in vertebrates, with early RA signaling restricting the size of the cardiac progenitor field within the anterior lateral plate mesoderm (ALPM). However, the regulatory networks by which RA signaling limits the size of the cardiac progenitor field and consequently cardiomyocyte (CM) number are not fully understood. Here, we identified that the expression of the transcription factors six1b and six2a, whose orthologs regulate outflow tract (OFT) development in mice, are expanded within the ALPM of RA-deficient zebrafish embryos. At 48 h post-fertilization (hpf), RA-deficient six1b;six2a double mutants, but not single six1b or six2a mutants, had a reduction in the number of surplus CMs relative to RA-deficient wild-type embryos. The expansion of six1b, as well as fgf8a, within the ALPM were dependent on tbx1, a factor that is also expanded within the ALPM of RA-deficient zebrafish embryos. However, the restriction of six2a expression by RA was independent of Tbx1. Consistent with a bifurcation of pathways downstream of RA signaling, loss of function experiments demonstrates that tbx1 expansion alone does not contribute to the surplus CMs in RA-deficient embryos. Together, our data indicate that both Tbx1-dependent and independent pathways restrict Six dosage downstream of RA within the ALPM to pattern the CM progenitor field and establish proper heart size.

Retinoic acid signaling and metabolism in heart failure

Nearly 70 years after studies first showed that the offspring of vitamin A (retinol, ROL)-deficient rats exhibit structural cardiac defects and over 20 years since the role of vitamin A's potent bioactive metabolite hormone, all-trans retinoic acid (ATRA), was elucidated in embryonic cardiac development, the role of the vitamin A metabolites, or retinoids, in adult heart physiology and heart and vascular disease, remains poorly understood. Studies have shown that low serum levels of retinoic acid correlate with higher all-cause and cardiovascular mortality, though the relationship between circulating retinol and ATRA levels, cardiac tissue ATRA levels, and intracellular cardiac ATRA signaling in the context of heart and vascular disease has only begun to be addressed. We have recently shown that patients with idiopathic dilated cardiomyopathy show a nearly 40% decline of in situ cardiac ATRA levels, despite adequate local stores of retinol. Moreover, we and others have shown that the administration of ATRA forestalls the development of heart failure (HF) in rodent models. In this review, we summarize key facets of retinoid metabolism and signaling and discuss mechanisms by which impaired ATRA signaling contributes to several HF hallmarks including hypertrophy, contractile dysfunction, poor calcium handling, redox imbalance, and fibrosis. We highlight unresolved issues in cardiac ATRA metabolism whose pursuit will help refine therapeutic strategies aimed at restoring ATRA homeostasis.

Identification and Verification of Potential Markers Related to Myocardial Fibrosis by Bioinformatics Analysis

Mounting evidence indicates that myocardial fibrosis (MF) is frequently intertwined with immune and metabolic disorders. This comprehensive review aims to delve deeply into the crucial role of immune-related signature genes in the pathogenesis and progression of MF. This exploration holds significant importance as understanding the underlying mechanisms of MF is essential for developing effective diagnostic and therapeutic strategies. The dataset GSE9735 about myocardial fibrosis and non-fibrosis was downloaded from GEO database. Differentially expressed genes (DEGs) were identified by 'limma' package in R software. Then, the biological function of DEG was determined by gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis. XCell was used to estimate the composition pattern of matrix and immune cells. Protein-protein interaction (PPI) network was constructed based on STRING analysis software, and Hub genes were screened and functional modules were analyzed. The correlation between hub genes and immune cell subtypes was analyzed. Hub genes with |correlation coefficient|> 0.45 and p-value < 0.05 were used as characteristic biomarkers. Finally, the logistic regression model is used to verify the three markers in the training set and verification set (GSE97358 and GSE225336). A total of 635 DEGs were identified. Functional enrichment analysis shows that inflammation and immune response, extracellular matrix and structural remodeling play an important role in the pathological mechanism of MF. Immune cell infiltration analysis showed that immune cells (Plasma cells, Eosinophils, Chondrocytes and Th2 cells) significantly changed in MF pathological conditions. In PPI network analysis, IL1β, TTN, PTPRC, IGF1, ALDH1A1, CYP26A1, ALDH1A3, MYH11, CSF1R and CD80 were identified as hub genes, among which IL1β, CYP26A1 and GNG2 were regarded as immune-related characteristic markers. The AUC scores of the three biomarkers are all above 0.65, which proves that they have a good discrimination effect in MF. In this study, three immune-related genes were identified as diagnostic biomarkers of MF, which provided a new perspective for exploring the molecular mechanism of MF. This study takes a comprehensive approach to understanding the intricate relationship between myocardial fibrosis and immune metabolism. By identifying key immune-related biomarkers, this study not only reveals the molecular basis of myocardial fibrosis but also paves the way for the development of novel diagnostic tools and therapeutic strategies. These findings are critical for improving patient prognosis and may have broader implications for studying and treating other cardiovascular diseases associated with immune dysregulation.

Cytochrome P450 enzymes in the black-spotted frog (Pelophylax nigromaculatus): molecular characterization and upregulation of expression by sulfamethoxazole

Cytochrome P450 (CYP) enzymes are crucial for the detoxification of xenobiotics, cellular metabolism, and homeostasis. This study investigated the molecular characterization of CYP enzymes in the black-spotted frog, Pelophylax nigromaculatus, and examined the regulation of CYP expression in response to chronic exposure to the antibiotic sulfamethoxazole (SMX) at various environmental concentrations (0, 1, 10, and 100 μg/L). The full-length cDNA of Pn-CYP26B1 was identified. The sequence included open reading frames of 1,536 bp, encoding proteins comprising 511 amino acids. The signature motif, FxxGxxxCxG, was highly conserved when compared with a number of selected animal species. SMX significantly upregulated the expression of the protein CYP26B1 in frog livers at concentrations of 1 and 10 μg/L. SMX showed an affinity for CYP26B1 of -7.6 kcal/mol, indicating a potential mechanism for SMX detoxification or adaptation of the frog. These findings contributed to our understanding of the environmental impact of antibiotics on amphibian species and underscored the importance of CYP enzymes in maintaining biochemical homeostasis under exposure to xenobiotic stress.

Zebrafish as a Model to Study Retinoic Acid Signaling in Development and Disease

Retinoic acid (RA) is a metabolite of vitamin A (retinol) that plays various roles in development to influence differentiation, patterning, and organogenesis. RA also serves as a crucial homeostatic regulator in adult tissues. The role of RA and its associated pathways are well conserved from zebrafish to humans in both development and disease. This makes the zebrafish a natural model for further interrogation into the functions of RA and RA-associated maladies for the sake of basic research, as well as human health. In this review, we explore both foundational and recent studies using zebrafish as a translational model for investigating RA from the molecular to the organismal scale.

Molecular mechanism of ethylparaben on zebrafish embryo cardiotoxicity based on transcriptome analyses

Ethylparaben (EP), one of the parabens, a ubiquitous food and cosmetic preservatives, has caused widespread concern due to its health risks. Recently, studies have found that parabens exposure during pregnancy is negatively correlated with fetal and early childhood development. However, studies about EP on embryo development are few. In this study, the cardiotoxicity effects of EP concentrations ranging from 0 to 20 mg/L on zebrafish embryo development were explored. Results showed that EP exposure induce abnormal cardiac function and morphology, mainly manifested as pericardial effusion and abnormal heart rate in early-stage development of zebrafish embryos. Through transcriptome sequencing followed by Gene Ontology enrichment analysis, and Kyoto Encyclopedia of Genes and Genomes enrichment analysis, we further confirmed that EP exposure ultimately leads to cardiac morphologic abnormalities via the following three mechanisms: 1. Disruption of the retinoic acid signaling pathway related to original cardiac catheter development; 2. Inhibition of gene expression related to myocardial contraction; 3. Orientation development disturbance of heart tube. Moreover, O-Dianisidine staining, whole-mount in situ hybridization at 30 and 48 hours post fertilization (hpf) and hematoxylin-eosin staining results all confirmed the decreased heart's return blood volume, misoriented heart tubes toward either the right or the middle side, and heart loop defects. For the first time, we explored the mechanism by which EP exposure causes abnormal heart development in zebrafish embryos, laying the foundation for further revealing of the EP toxicity on embryonic development.

Pioglitazone Mediates Cardiac Progenitor Formation through Increasing ROS Levels

In order to achieve a sufficient population of cardiac-committed progenitor cells, it is crucial to know the mechanisms of cardiac progenitor formation. Previous studies suggested ROS effect on cardiac commitment events to play a key role in the cell signaling and activate cardiac differentiation of pluripotent stem cells. We previously reported that PPARγ activity is essential for cardiac progenitor cell commitment. Although several studies have conducted the involvement of PPARγ-related signaling pathways in cardiac differentiation, so far, the regulatory mechanisms of these signaling pathways have not been discussed and cleared. In this study, we focus on the role of PPARγ agonist in ROS generation and its further effects on the differentiation of cardiac cells from mESCs. The results of this study show that the presence of ROS is necessary for heart differentiation in the precursor stage of cardiac cells, and the coenzyme Q10 antioxidant precludes proper cardiac differentiation. In addition, this antioxidant prevents the action of pioglitazone in increasing oxygen radicals as well as beating cardiomyocyte differentiation properties. In this case, it can be concluded that PPARγ is required to modulate ROS levels during cardiac differentiation.

GATA4/5/6 family transcription factors are conserved determinants of cardiac versus pharyngeal mesoderm fate

GATA4/5/6 transcription factors play essential, conserved roles in heart development. To understand how GATA4/5/6 modulates the mesoderm-to-cardiac fate transition, we labeled, isolated, and performed single-cell gene expression analysis on cells that express gata5 at precardiac time points spanning zebrafish gastrulation to somitogenesis. We found that most mesendoderm-derived lineages had dynamic gata5/6 expression. In the absence of Gata5/6, the population structure of mesendoderm-derived cells was substantially altered. In addition to the expected absence of cardiac mesoderm, we confirmed a concomitant expansion of cranial-pharyngeal mesoderm. Moreover, Gata5/6 loss led to extensive changes in chromatin accessibility near cardiac and pharyngeal genes. Functional analyses in zebrafish and the tunicate Ciona, which has a single GATA4/5/6 homolog, revealed that GATA4/5/6 acts upstream of tbx1 to exert essential and cell-autonomous roles in promoting cardiac and inhibiting pharyngeal mesoderm identity. Overall, cardiac and pharyngeal mesoderm fate choices are achieved through an evolutionarily conserved GATA4/5/6 regulatory network.

Segmentation and patterning of the vertebrate hindbrain

During early development, the hindbrain is sub-divided into rhombomeres that underlie the organisation of neurons and adjacent craniofacial tissues. A gene regulatory network of signals and transcription factors establish and pattern segments with a distinct anteroposterior identity. Initially, the borders of segmental gene expression are imprecise, but then become sharply defined, and specialised boundary cells form. In this Review, we summarise key aspects of the conserved regulatory cascade that underlies the formation of hindbrain segments. We describe how the pattern is sharpened and stabilised through the dynamic regulation of cell identity, acting in parallel with cell segregation. Finally, we discuss evidence that boundary cells have roles in local patterning, and act as a site of neurogenesis within the hindbrain.

Reorganization of Metabolism during Cardiomyogenesis Implies Time-Specific Signaling Pathway Regulation

Understanding the cell differentiation process involves the characterization of signaling and regulatory pathways. The coordinated action involved in multilevel regulation determines the commitment of stem cells and their differentiation into a specific cell lineage. Cellular metabolism plays a relevant role in modulating the expression of genes, which act as sensors of the extra-and intracellular environment. In this work, we analyzed mRNAs associated with polysomes by focusing on the expression profile of metabolism-related genes during the cardiac differentiation of human embryonic stem cells (hESCs). We compared different time points during cardiac differentiation (pluripotency, embryoid body aggregation, cardiac mesoderm, cardiac progenitor and cardiomyocyte) and showed the immature cell profile of energy metabolism. Highly regulated canonical pathways are thoroughly discussed, such as those involved in metabolic signaling and lipid homeostasis. We reveal the critical relevance of retinoic X receptor (RXR) heterodimers in upstream retinoic acid metabolism and their relationship with thyroid hormone signaling. Additionally, we highlight the importance of lipid homeostasis and extracellular matrix component biosynthesis during cardiomyogenesis, providing new insights into how hESCs reorganize their metabolism during in vitro cardiac differentiation.

Mesoderm patterning by a dynamic gradient of retinoic acid signalling

Retinoic acid (RA), derived from vitamin A, is a major teratogen, clinically recognized in 1983. Identification of its natural presence in the embryo and dissection of its molecular mechanism of action became possible in the animal model with the advent of molecular biology, starting with the cloning of its nuclear receptor. In normal development, the dose of RA is tightly controlled to regulate organ formation. Its production depends on enzymes, which have a dynamic expression profile during embryonic development. As a small molecule, it diffuses rapidly and acts as a morphogen. Here, we review advances in deciphering how endogenously produced RA provides positional information to cells. We compare three mesodermal tissues, the limb, the somites and the heart, and discuss how RA signalling regulates antero-posterior and left-right patterning. A common principle is the establishment of its spatio-temporal dynamics by positive and negative feedback mechanisms and by antagonistic signalling by FGF. However, the response is cell-specific, pointing to the existence of cofactors and effectors, which are as yet incompletely characterized. This article is part of a discussion meeting issue 'Contemporary morphogenesis'.

Vitamin A as a Transcriptional Regulator of Cardiovascular Disease

Vitamin A is a micronutrient and signaling molecule that regulates transcription, cellular differentiation, and organ homeostasis. Additionally, metabolites of Vitamin A are utilized as differentiation agents in the treatment of hematological cancers and skin disorders, necessitating further study into the effects of both nutrient deficiency and the exogenous delivery of Vitamin A and its metabolites on cardiovascular phenotypes. Though vitamin A/retinoids are well-known regulators of cardiac formation, recent evidence has emerged that supports their role as regulators of cardiac regeneration, postnatal cardiac function, and cardiovascular disease progression. We here review findings from genetic and pharmacological studies describing the regulation of both myocyte- and vascular-driven cardiac phenotypes by vitamin A signaling. We identify the relationship between retinoids and maladaptive processes during the pathological hypertrophy of the heart, with a focus on the activation of neurohormonal signaling and fetal transcription factors (Gata4, Tbx5). Finally, we assess how this information might be leveraged to develop novel therapeutic avenues.

The Cdx transcription factors and retinoic acid play parallel roles in antero-posterior position of the pectoral fin field during gastrulation

The molecular regulators that determine the precise position of the vertebrate limb along the anterio-posterior axis have not been identified. One model suggests that a combination of hox genes in the lateral plate mesoderm (LPM) promotes formation of the limb field, however redundancy among duplicated paralogs has made this model difficult to confirm. In this study, we identify an optimal window during mid-gastrulation stages when transient mis-regulation of retinoic acid signaling or the caudal related transcription factor, Cdx4, both known regulators of hox genes, can alter the position of the pectoral fin field. We show that increased levels of either RA or Cdx4 during mid-gastrulation are sufficient to rostrally shift the position of the pectoral fin field at the expense of surrounding gene expression in the anterior lateral plate mesoderm (aLPM). Alternatively, embryos deficient for both Cdx4 and Cdx1a (Cdx-deficient) form pectoral fins that are shifted towards the posterior and reveal an additional effect on size of the pectoral fin buds. Prior to formation of the pectoral fin buds, the fin field in Cdx-deficient embryos is visibly expanded into the posterior LPM (pLPM) region at the expense of surrounding gene expression. The effects on gene expression immediately post-gastrulation and during somitogenesis support a model where RA and Cdx4 act in parallel to regulate the position of the pectoral fin. Our transient method is a potentially useful model for studying the mechanisms of limb positioning along the AP axis.

Bioactive Lipid Signaling in Cardiovascular Disease, Development, and Regeneration

Cardiovascular disease (CVD) remains a leading cause of death globally. Understanding and characterizing the biochemical context of the cardiovascular system in health and disease is a necessary preliminary step for developing novel therapeutic strategies aimed at restoring cardiovascular function. Bioactive lipids are a class of dietary-dependent, chemically heterogeneous lipids with potent biological signaling functions. They have been intensively studied for their roles in immunity, inflammation, and reproduction, among others. Recent advances in liquid chromatography-mass spectrometry techniques have revealed a staggering number of novel bioactive lipids, most of them unknown or very poorly characterized in a biological context. Some of these new bioactive lipids play important roles in cardiovascular biology, including development, inflammation, regeneration, stem cell differentiation, and regulation of cell proliferation. Identifying the lipid signaling pathways underlying these effects and uncovering their novel biological functions could pave the way for new therapeutic strategies aimed at CVD and cardiovascular regeneration.

Retinoic Acid Signaling and Heart Development

As the first organ to form and function in all vertebrates, the heart is crucial to development. Tightly-regulated levels of retinoic acid (RA) are critical for the establishment of the regulatory networks that drive normal cardiac development. Thus, the heart is an ideal organ to investigate RA signaling, with much work remaining to be done in this area. Herein, we highlight the role of RA signaling in vertebrate heart development and provide an overview of the field's inception, its current state, and in what directions it might progress so that it may yield fruitful insight for therapeutic applications within the domain of regenerative medicine.

Regulating Retinoic Acid Availability during Development and Regeneration: The Role of the CYP26 Enzymes

This review focuses on the role of the Cytochrome p450 subfamily 26 (CYP26) retinoic acid (RA) degrading enzymes during development and regeneration. Cyp26 enzymes, along with retinoic acid synthesising enzymes, are absolutely required for RA homeostasis in these processes by regulating availability of RA for receptor binding and signalling. Cyp26 enzymes are necessary to generate RA gradients and to protect specific tissues from RA signalling. Disruption of RA homeostasis leads to a wide variety of embryonic defects affecting many tissues. Here, the function of CYP26 enzymes is discussed in the context of the RA signalling pathway, enzymatic structure and biochemistry, human genetic disease, and function in development and regeneration as elucidated from animal model studies.

Role of carotenoids and retinoids during heart development

The nutritional requirements of the developing embryo are complex. In the case of dietary vitamin A (retinol, retinyl esters and provitamin A carotenoids), maternal derived nutrients serve as precursors to signaling molecules such as retinoic acid, which is required for embryonic patterning and organogenesis. Despite variations in the composition and levels of maternal vitamin A, embryonic tissues need to generate a precise amount of retinoic acid to avoid congenital malformations. Here, we summarize recent findings regarding the role and metabolism of vitamin A during heart development and we survey the association of genes known to affect retinoid metabolism or signaling with various inherited disorders. A better understanding of the roles of vitamin A in the heart and of the factors that affect retinoid metabolism and signaling can help design strategies to meet nutritional needs and to prevent birth defects and disorders associated with altered retinoid metabolism. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

Gene profiling of head mesoderm in early zebrafish development: insights into the evolution of cranial mesoderm

Background

The evolution of the head was one of the key events that marked the transition from invertebrates to vertebrates. With the emergence of structures such as eyes and jaws, vertebrates evolved an active and predatory life style and radiated into diversity of large-bodied animals. These organs are moved by cranial muscles that derive embryologically from head mesoderm. Compared with other embryonic components of the head, such as placodes and cranial neural crest cells, our understanding of cranial mesoderm is limited and is restricted to few species.

Results

Here, we report the expression patterns of key genes in zebrafish head mesoderm at very early developmental stages. Apart from a basic anterior–posterior axis marked by a combination of pitx2 and tbx1 expression, we find that most gene expression patterns are poorly conserved between zebrafish and chick, suggesting fewer developmental constraints imposed than in trunk mesoderm. Interestingly, the gene expression patterns clearly show the early establishment of medial–lateral compartmentalisation in zebrafish head mesoderm, comprising a wide medial zone flanked by two narrower strips.

Conclusions

In zebrafish head mesoderm, there is no clear molecular regionalisation along the anteroposterior axis as previously reported in chick embryos. In contrast, the medial–lateral regionalisation is formed at early developmental stages. These patterns correspond to the distinction between paraxial mesoderm and lateral plate mesoderm in the trunk, suggesting a common groundplan for patterning head and trunk mesoderm. By comparison of these expression patterns to that of amphioxus homologues, we argue for an evolutionary link between zebrafish head mesoderm and amphioxus anteriormost somites.

Electronic supplementary material

The online version of this article (10.1186/s13227-019-0128-3) contains supplementary material, which is available to authorized users.

Recent insights on the role and regulation of retinoic acid signaling during epicardial development

The vitamin A metabolite, retinoic acid, carries out essential and conserved roles in vertebrate heart development. Retinoic acid signals via retinoic acid receptors (RAR)/retinoid X receptors (RXRs) heterodimers to induce the expression of genes that control cell fate specification, proliferation, and differentiation. Alterations in retinoic acid levels are often associated with congenital heart defects. Therefore, embryonic levels of retinoic acid need to be carefully regulated through the activity of enzymes, binding proteins and transporters involved in vitamin A metabolism. Here, we review evidence of the complex mechanisms that control the fetal uptake and synthesis of retinoic acid from vitamin A precursors. Next, we highlight recent evidence of the role of retinoic acid in orchestrating myocardial compact zone growth and coronary vascular development.

Reiterative Mechanisms of Retinoic Acid Signaling during Vertebrate Heart Development

Tightly-regulated levels of retinoic acid (RA) are critical for promoting normal vertebrate development. The extensive history of research on RA has shown that its proper regulation is essential for cardiac progenitor specification and organogenesis. Here, we discuss the roles of RA signaling and its establishment of networks that drive both early and later steps of normal vertebrate heart development. We focus on studies that highlight the drastic effects alternative levels of RA have on early cardiomyocyte (CM) specification and cardiac chamber morphogenesis, consequences of improper RA synthesis and degradation, and known effectors downstream of RA. We conclude with the implications of these findings to our understanding of cardiac regeneration and the etiologies of congenital heart defects.

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.

Retinoic acid signaling in heart development

Retinoic acid (RA) is a vitamin A metabolite that acts as a morphogen and teratogen. Excess or defective RA signaling causes developmental defects including in the heart. The heart develops from the anterior lateral plate mesoderm. Cardiogenesis involves successive steps, including formation of the primitive heart tube, cardiac looping, septation, chamber development, coronary vascularization, and completion of the four-chambered heart. RA is dispensable for primitive heart tube formation. Before looping, RA is required to define the anterior/posterior boundaries of the heart-forming mesoderm as well as to form the atrium and sinus venosus. In outflow tract elongation and septation, RA signaling is required to maintain/differentiate cardiogenic progenitors in the second heart field at the posterior pharyngeal arches level. Epicardium-secreted insulin-like growth factor, the expression of which is regulated by hepatic mesoderm-derived erythropoietin under the control of RA, promotes myocardial proliferation of the ventricular wall. Epicardium-derived RA induces the expression of angiogenic factors in the myocardium to form the coronary vasculature. In cardiogenic events at different stages, properly controlled RA signaling is required to establish the functional heart.

Establishing sharp and homogeneous segments in the hindbrain

Studies of the vertebrate hindbrain have revealed parallel mechanisms that establish sharp segments with a distinct and homogeneous regional identity. Recent work has revealed roles of cell identity regulation and its relationships with cell segregation. At early stages, there is overlapping expression at segment borders of the Egr2 and Hoxb1 transcription factors that specify distinct identities, which is resolved by reciprocal repression. Computer simulations show that this dynamic regulation of cell identity synergises with cell segregation to generate sharp borders. Some intermingling between segments occurs at early stages, and ectopic egr2-expressing cells switch identity to match their new neighbours. This switching is mediated by coupling between egr2 expression and the level of retinoic acid signalling, which acts in a community effect to maintain homogeneous segmental identity. These findings reveal an interplay between cell segregation and the dynamic regulation of cell identity in the formation of sharp patterns in the hindbrain and raise the question of whether similar mechanisms occur in other tissues.

Cell Identity Switching Regulated by Retinoic Acid Signaling Maintains Homogeneous Segments in the Hindbrain

The patterning of tissues to form subdivisions with distinct and homogeneous regional identity is potentially disrupted by cell intermingling. Transplantation studies suggest that homogeneous segmental identity in the hindbrain is maintained by identity switching of cells that intermingle into another segment. We show that switching occurs during normal development and is mediated by feedback between segment identity and the retinoic acid degrading enzymes, cyp26b1 and cyp26c1. egr2, which specifies the segmental identity of rhombomeres r3 and r5, underlies the lower expression level of cyp26b1 and cyp26c1 in r3 and r5 compared with r2, r4, and r6. Consequently, r3 or r5 cells that intermingle into adjacent segments encounter cells with higher cyp26b1/c1 expression, which we find is required for downregulation of egr2b expression. Furthermore, egr2b expression is regulated in r2, r4, and r6 by non-autonomous mechanisms that depend upon the number of neighbors that express egr2b. These findings reveal that a community regulation of retinoid signaling maintains homogeneous segmental identity.

Human Pluripotent Stem Cell-Derived Atrial and Ventricular Cardiomyocytes Develop from Distinct Mesoderm Populations

The ability to direct the differentiation of human pluripotent stem cells (hPSCs) to the different cardiomyocyte subtypes is a prerequisite for modeling specific forms of cardiovascular disease in vitro and for developing novel therapies to treat them. Here we have investigated the development of the human atrial and ventricular lineages from hPSCs, and we show that retinoic acid signaling at the mesoderm stage of development is required for atrial specification. Analyses of early developmental stages revealed that ventricular and atrial cardiomyocytes derive from different mesoderm populations that can be distinguished based on CD235a and RALDH2 expression, respectively. Molecular and electrophysiological characterization of the derivative cardiomyocytes revealed that optimal specification of ventricular and atrial cells is dependent on induction of the appropriate mesoderm. Together these findings provide new insights into the development of the human atrial and ventricular lineages that enable the generation of highly enriched, functional cardiomyocyte populations for therapeutic applications.

The regulation of transcriptome responses in zebrafish embryo exposure to triadimefon

The residue of triadimefon (TDF) (a pesticide) has become the pollutant in water due to its intensive use in agriculture and medicine, and its stability in water leaching from soil and vegetation. In this study, RNA-seq, a high-throughput method was performed, to analyze the global expression of differential expressed genes (DEGs) in zebrafish embryos treated with TDF (10 μg/mL) from fertilization to 72 h post-fertilization (hpf) as compared with that in the control group (without TDF treatment). Two cDNA libraries were generated from treated and non-treated embryos, respectively. With the 79.4% and 78.8% of reads mapped to the reference, it was observed that many differential genes were expressed between the two libraries. The most 20 differentially expressed up-regulated or down-regulated genes were involving in the signaling transduction, the activation of many genes related to cytochrome P450 enzymes, and molecular metabolism. Validation of seven genes expression confirmed RNA-seq results. The transcriptome sequences were further subjected to Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis and showed diverse biological functions and metabolic pathways. The data from this study contributed to a better understanding of the potential consequences of fish exposed to TDF, and to evaluate the potential threat of TDF to fish population in the aquatic environment. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 217-226, 2017.

Mechanisms of retinoic acid signaling during cardiogenesis

Substantial experimental and epidemiological data have highlighted the interplay between nutritional and genetic factors in the development of congenital heart defects. Retinoic acid (RA), a derivative of vitamin A, plays a key role during vertebrate development including the formation of the heart. Retinoids bind to RA and retinoid X receptors (RARs and RXRs) which then regulate tissue-specific genes. Here, we will focus on the roles of RA signaling and receptors in gene regulation during cardiogenesis, and the consequence of deregulated retinoid signaling on heart formation and congenital heart defects.

Retinoic acid catabolizing enzyme CYP26C1 is a genetic modifier in SHOX deficiency

Mutations in the homeobox gene SHOX cause SHOX deficiency, a condition with clinical manifestations ranging from short stature without dysmorphic signs to severe mesomelic skeletal dysplasia. In rare cases, individuals with SHOX deficiency are asymptomatic. To elucidate the factors that modify disease severity/penetrance, we studied a three-generation family with SHOX deficiency. The variant p.Phe508Cys of the retinoic acid catabolizing enzyme CYP26C1 co-segregated with the SHOX variant p.Val161Ala in the affected individuals, while the SHOX mutant alone was present in asymptomatic individuals. Two further cases with SHOX deficiency and damaging CYP26C1 variants were identified in a cohort of 68 individuals with LWD The identified CYP26C1 variants affected its catabolic activity, leading to an increased level of retinoic acid. High levels of retinoic acid significantly decrease SHOX expression in human primary chondrocytes and zebrafish embryos. Individual morpholino knockdown of either gene shortens the pectoral fins, whereas depletion of both genes leads to a more severe phenotype. Together, our findings describe CYP26C1 as the first genetic modifier for SHOX deficiency.

Cyp26 Enzymes Facilitate Second Heart Field Progenitor Addition and Maintenance of Ventricular Integrity

Although retinoic acid (RA) teratogenicity has been investigated for decades, the mechanisms underlying RA-induced outflow tract (OFT) malformations are not understood. Here, we show zebrafish embryos deficient for Cyp26a1 and Cyp26c1 enzymes, which promote RA degradation, have OFT defects resulting from two mechanisms: first, a failure of second heart field (SHF) progenitors to join the OFT, instead contributing to the pharyngeal arch arteries (PAAs), and second, a loss of first heart field (FHF) ventricular cardiomyocytes due to disrupted cell polarity and extrusion from the heart tube. Molecularly, excess RA signaling negatively regulates fibroblast growth factor 8a (fgf8a) expression and positively regulates matrix metalloproteinase 9 (mmp9) expression. Although restoring Fibroblast growth factor (FGF) signaling can partially rescue SHF addition in Cyp26 deficient embryos, attenuating matrix metalloproteinase (MMP) function can rescue both ventricular SHF addition and FHF integrity. These novel findings indicate a primary effect of RA-induced OFT defects is disruption of the extracellular environment, which compromises both SHF recruitment and FHF ventricular integrity.

Retinoic acid (RA) is the most active metabolic product of vitamin A. The embryonic heart is particularly sensitive to inappropriate RA levels, with cardiac outflow tract (OFT) defects among the most common RA-induced malformations. However, the mechanisms underlying these RA-induced defects are not understood. Cyp26 enzymes facilitate degradation of RA and thus are required to limit RA levels in early development. Here, we present evidence that loss of Cyp26 enzymes induces cardiac OFT defects through two mechanisms. First, we find that Cyp26-deficient zebrafish embryos fail to add later-differentiating ventricular cardiac progenitors to the OFT, with some of these progenitors instead contributing to the nearby arch arteries. Second, Cyp26-deficient embryos cannot maintain the integrity of the nascent heart tube, with ventricular cells within the heart tube losing their polarity and being extruded. Our data indicate that excess expression of matrix metalloproteinase 9, an enzyme that degrades the extracellular matrix, underlies both the cardiac progenitor addition and heart tube integrity defects seen in Cyp26-deficient embryos. Our findings highlight perturbation of the extracellular matrix as a major cause of RA-induced cardiac OFT defects that specifically disrupt ventricular development at later stages than previously appreciated.

3D Visualization of Developmental Toxicity of 2,4,6-Trinitrotoluene in Zebrafish Embryogenesis Using Light-Sheet Microscopy

Environmental contamination by trinitrotoluene is of global concern due to its widespread use in military ordnance and commercial explosives. Despite known long-term persistence in groundwater and soil, the toxicological profile of trinitrotoluene and other explosive wastes have not been systematically measured using in vivo biological assays. Zebrafish embryos are ideal model vertebrates for high-throughput toxicity screening and live in vivo imaging due to their small size and transparency during embryogenesis. Here, we used Single Plane Illumination Microscopy (SPIM)/light sheet microscopy to assess the developmental toxicity of explosive-contaminated water in zebrafish embryos and report 2,4,6-trinitrotoluene-associated developmental abnormalities, including defects in heart formation and circulation, in 3D. Levels of apoptotic cell death were higher in the actively developing tissues of trinitrotoluene-treated embryos than controls. Live 3D imaging of heart tube development at cellular resolution by light-sheet microscopy revealed trinitrotoluene-associated cardiac toxicity, including hypoplastic heart chamber formation and cardiac looping defects, while the real time PCR (polymerase chain reaction) quantitatively measured the molecular changes in the heart and blood development supporting the developmental defects at the molecular level. Identification of cellular toxicity in zebrafish using the state-of-the-art 3D imaging system could form the basis of a sensitive biosensor for environmental contaminants and be further valued by combining it with molecular analysis.

Strategies for analyzing cardiac phenotypes in the zebrafish embryo

The molecular mechanisms underlying cardiogenesis are of critical biomedical importance due to the high prevalence of cardiac birth defects. Over the past two decades, the zebrafish has served as a powerful model organism for investigating heart development, facilitated by its powerful combination of optical access to the embryonic heart and plentiful opportunities for genetic analysis. Work in zebrafish has identified numerous factors that are required for various aspects of heart formation, including the specification and differentiation of cardiac progenitor cells, the morphogenesis of the heart tube, cardiac chambers, and atrioventricular canal, and the establishment of proper cardiac function. However, our current roster of regulators of cardiogenesis is by no means complete. It is therefore valuable for ongoing studies to continue pursuit of additional genes and pathways that control the size, shape, and function of the zebrafish heart. An extensive arsenal of techniques is available to distinguish whether particular mutations, morpholinos, or small molecules disrupt specific processes during heart development. In this chapter, we provide a guide to the experimental strategies that are especially effective for the characterization of cardiac phenotypes in the zebrafish embryo.

Excessive feedback of Cyp26a1 promotes cell non-autonomous loss of retinoic acid signaling

Teratogenic levels of retinoic acid (RA) signaling can cause seemingly contradictory phenotypes indicative of both increases and decreases of RA signaling. However, the mechanisms underlying these contradictory phenotypes are not completely understood. Here, we report that using a hyperactive RA receptor to enhance RA signaling in zebrafish embryos leads to defects associated with gain and loss of RA signaling. While the gain-of-function phenotypes arise from an initial increase in RA signaling, using genetic epistasis analysis we found that the loss-of-function phenotypes result from a clearing of embryonic RA that requires a rapid and dramatic increase in cyp26a1 expression. Thus, the sensitivity of cyp26a1 expression to increased RA signaling causes an overcompensation of negative feedback and loss of embryonic RA signaling. Additionally, we used blastula transplantation experiments to test if Cyp26a1, despite its cellular localization, can limit RA exposure to neighboring cells. We find that enhanced Cyp26a1 expression limits RA signaling in the local environment, thus providing the first direct evidence that Cyp26 enzymes can have cell non-autonomous consequences on RA levels within tissues. Therefore, our results provide novel insights into the teratogenic mechanisms of RA signaling and the cellular mechanisms by which Cyp26a1 expression can shape a RA gradient.

Identification of the Full 46 Cytochrome P450 (CYP) Complement and Modulation of CYP Expression in Response to Water-Accommodated Fractions of Crude Oil in the Cyclopoid Copepod Paracyclopina nana

The 46 cytochrome P450 (CYP) gene superfamily was identified in the marine copepod Paracyclopina nana after searching an RNA-seq database and comparing it with other copepod CYP gene families. To annotate the 46 Pn-CYP genes, a phylogenetic analysis of CYP genes was performed using a Bayesian method. Pn-CYP genes were separated into five different clans: CYP2, CYP3, CYP20, CYP26, and mitochondrial. Among these, the principal Pn-CYP genes involved in detoxification were identified by comparing them with those of the copepod Tigriopus japonicus and were examined with respect to their responses to exposure to a water-accommodated fraction (WAF) of crude oil and to the alkylated forms of two polycyclic aromatic hydrocarbons (PAHs; phenanthrene and fluorene). The expression of two Pn-CYP3027 genes (CYP3027F1 and CYP3027F2) was increased in response to WAF exposure and also was upregulated in response to the two alkylated PAHs. In particular, Pn-CYP3027F2 showed the most notable increase in response to 80% WAF exposure. These two responsive CYP genes (Pn-CYP3027F1 and CYP3027F2) were also phylogenetically clustered into the same clade of the WAF- and alkylated PAH-specific CYP genes of the copepod T. japonicus, suggesting that these CYP genes would be those chiefly involved in detoxification in response to WAF exposure in copepods. In this paper, we provide information on the copepod P. nana CYP gene superfamily and also speculate on its potential role in the detoxification of PAHs in marine copepods. Despite the nonlethality of WAF, Pn-CYP3027F2 was rapidly and significantly upregulated in response to WAF that may serve as a useful biomarker of 40% or higher concentration of WAF exposure. This paper will be helpful to better understand the molecular mechanistic events underlying the metabolism of environmental toxicants in copepods.

Input overload: Contributions of retinoic acid signaling feedback mechanisms to heart development and teratogenesis

Appropriate levels of retinoic acid (RA) signaling are critical for normal heart development in vertebrates. A fascinating property of RA signaling is the thoroughness by which positive and negative feedback are employed to promote proper embryonic RA levels. In the present short review, we first cover the advancement of hypotheses regarding the impact of RA signaling on cardiac specification. We then discuss our current understanding of RA signaling feedback mechanisms and the implications of recent studies, which have indicated improperly maintained RA signaling feedback can be a contributing factor to developmental malformations.