Zebrafish Mef2ca and Mef2cb are essential for both first and second heart field cardiomyocyte differentiation

Gata6 functions in zebrafish endoderm to regulate late differentiating arterial pole cardiogenesis

Mutations in GATA6 are associated with congenital heart disease, most notably conotruncal structural defects. However, how GATA6 regulates cardiac morphology during embryogenesis is undefined. We used knockout and conditional mutant zebrafish alleles to investigate the spatiotemporal role of gata6 during cardiogenesis. Loss of gata6 specifically impacts atrioventricular valve formation and recruitment of epicardium, with a prominent loss of arterial pole cardiac cells, including those of the ventricle and outflow tract. However, there are no obvious defects in cardiac progenitor cell specification, proliferation or death. Conditional loss of gata6 starting at 24 h is sufficient to disrupt the addition of late differentiating cardiomyocytes at the arterial pole, with decreased expression levels of anterior secondary heart field (SHF) markers spry4 and mef2cb. Conditional loss of gata6 in the endoderm is sufficient to phenocopy the straight knockout, resulting in a significant loss of ventricular and outflow tract tissue. Exposure to a Dusp6 inhibitor largely rescues the loss of ventricular cells in gata6-/- larvae. Thus, gata6 functions in endoderm are mediated by FGF signaling to regulate the addition of anterior SHF progenitor derivatives during heart formation.

Transcriptome analysis reveals EBF1 ablation-induced injuries in cardiac system

Rationale: Regulatory processes of transcription factors (TFs) shape heart development and influence the adult heart's response to stress, contributing to cardiac disorders. Despite their significance, the precise mechanisms underpinning TF-mediated regulation remain elusive. Here, we identify that EBF1, as a TF, is highly expressed in human heart tissues. EBF1 is reported to be associated with human cardiovascular disease, but its roles are unclear in heart. In this study, we investigated EBF1 function in cardiac system. Methods: RNA-seq was utilized to profile EBF1 expression patterns. CRISPR/Cas9 was utilized to knock out EBF1 to investigate its effects. Human pluripotent stem cells (hPSCs) differentiated into cardiac lineages were used to mimic cardiac development. Cardiac function was evaluated on mouse model with Ebf1 knockout by using techniques such as echocardiography. RNA-seq was conducted to analyze transcriptional perturbations. ChIP-seq was employed to elucidate EBF1-bound genes and the underlying regulatory mechanisms. Results: EBF1 was expressed in some human and mouse cardiomyocyte. Knockout of EBF1 inhibited cardiac development. ChIP-seq indicated EBF1's binding on promoters of cardiogenic TFs pivotal to cardiac development, facilitating their transcriptional expression and promoting cardiac development. In mouse, Ebf1 depletion triggered transcriptional perturbations of genes, resulting in cardiac remodeling. Mechanistically, we found that EBF1 directly bound to upstream chromatin regions of cardiac hypertrophy-inducing genes, contributing to cardiac hypertrophy. Conclusions: We uncover the mechanisms underlying EBF1-mediated regulatory processes, shedding light on cardiac development, and the pathogenesis of cardiac remodeling. These findings emphasize EBF1's critical role in orchestrating diverse aspects of cardiac processes and provide a promising therapeutic intervention for cardiomyopathy.

Dihalogenated nitrophenols in drinking water: Prevalence, resistance to household treatment, and cardiotoxic impact on zebrafish embryo

Dihalogenated nitrophenols (2,6-DHNPs), an emerging group of aromatic disinfection byproducts (DBPs) detected in drinking water, have limited available information regarding their persistence and toxicological risks. The present study found that 2,6-DHNPs are resistant to major drinking water treatment processes (sedimentation and filtration) and households methods (boiling, filtration, microwave irradiation, and ultrasonic cleaning). To further assess their health risks, we conducted a series of toxicology studies using zebrafish embryos as the model organism. Our findings reveal that these emerging 2,6-DHNPs showed lethal toxicity 248 times greater than that of the regulated DBP, dichloroacetic acid. Specifically, at sublethal concentrations, exposure to 2,6-DHNPs generated reactive oxygen species (ROS), caused apoptosis, inhibited cardiac looping, and induced cardiac failure in zebrafish. Remarkably, the use of a ROS scavenger, N-acetyl-l-cysteine, considerably mitigated these adverse effects, emphasizing the essential role of ROS in 2,6-DHNP-induced cardiotoxicity. Our findings highlight the cardiotoxic potential of 2,6-DHNPs in drinking water even at low concentrations of 19 μg/L and the beneficial effect of N-acetyl-l-cysteine in alleviating the 2,6-DHNP-induced cardiotoxicity. This study underscores the urgent need for increased scrutiny of these emerging compounds in public health discussions.

Identification of two MEF2s and their role in inhibiting the transcription of the mstn2a gene in the yellowfin seabream, Acanthopagrus latus (Hottuyn, 1782)

Myocyte-specific enhancer binding factor 2 (MEF2), which belongs to the MADS superfamily, is a pivotal and conserved transcription factor that combines with the E-box motif to control the expression of muscle genes. Myostatin (mstn), a muscle growth inhibitor, is a vital member of the TGF-β superfamily. Currently, an understanding of the mechanisms of A. latus mstn (Almstn) transcriptional regulation mediated by MEF2 in fish muscle development is lacking. In the present study, two AlMEF2s (AlMEF2A and AlMEF2B) and Almstn2a were characterized from Acanthopagrus latus. AlMEF2A and AlMEF2B had 456 and 315 amino acid (aa) residues, respectively. Two typical regions, a MADS-box, MEF2, and transcriptionally activated (TAD) domains, are present in both AlMEF2s. The expression profiles of the two AlMEF2 genes were similar. The AlMEF2 genes were mainly expressed in the brain, white muscle, and liver, while Almstn2a expression was higher in the brain than in other tissues. Moreover, the expression trends of AlMEF2s and Almstn2a were significantly changed after starvation and refeeding in the five groups. Additionally, truncation experiments showed that -987 to +168 and -105 to +168 were core promoters of Almstn2a that responded to AlMEF2A and AlMEF2B, respectively. The point mutation experiment confirmed that Almstn2a transcription relies on the mutation binding sites 1 or 5 (M1/5) and mutation binding sites 4 or 5 (M4/5) for AlMEF2A and AlMEF2B regulation, respectively. The electrophoretic mobile shift assay (EMSA) further verified that M1 (-527 to -512) was a pivotal site where AlMEF2A acted on the Almstn2a gene. Furthermore, a siRNA interference gene expression experiment showed that reduced levels of AlMEF2A or AlMEF2B could prominently increase Almstn2a transcription. These results provide new information about the regulation of Almstn2a transcriptional activity by AlMEF2s and a theoretical basis for the regulatory mechanisms involved in muscle development in fish.

Medicine designed to combat diseases of affluence affects the early development of fish. How do plastic microparticles contribute?

The incidence of diseases of affluence, such as diabetes mellitus, cardiovascular diseases, high blood pressure, and high cholesterol has been reported to rise. Consequently, the concentrations of residues of drugs designed to treat these diseases have been rising in water bodies. Moreover, the toxicity of these pharmaceuticals towards fish and other non-target organisms can be even enhanced by microplastic particles that are reportedly present in surface water. Therefore, the aim of this study was to describe the effects of three highly prescribed drugs, in particular metoprolol, enalapril, and metformin on fish early-life stages. Also, it was hypothesized that polystyrene microparticles will increase the toxicity of metoprolol to fish early-life stages. Embryonal acute toxicity tests on Danio rerio and Cyprinus carpio were carried out in order to describe the possible toxic effects of metoprolol, enalapril, and metformin. Also, the acute toxicity of polystyrene microparticles and the combination of metoprolol with polystyrene microparticles were tested on D. rerio embryos. Additionally, a 31-day long embryo-larval subchronic toxicity test was carried out with C. carpio in order to describe the long-term effects of low concentrations of metoprolol. The results of the study show that both metoprolol and enalapril have the potential to disrupt the early development of the heart in the embryonal stages of fish. Also, enalapril and metformin together with polystyrene microparticles seem to possibly disrupt the reproduction cycle and act as endocrine disruptors. Both pure polystyrene microparticles and the combination of them with metoprolol affect inflammatory processes in organisms. Additionally, metformin alters several metabolism pathways in fish early-life stages. The results of the study bring new evidence that even low, environmentally-relevant concentrations of pharmaceuticals have the potential to disrupt the early development of fish, particularly on a molecular level.

mef2ca and mef2cb Double Mutant Zebrafish Show Altered Craniofacial Phenotype and Motor Behaviour

The transcription factor MEF2C is crucial in neuronal, cardiac, bone and cartilage molecular processes, as well as for craniofacial development. MEF2C was associated with the human disease MRD20, whose patients show abnormal neuronal and craniofacial development. Zebrafish mef2ca;mef2cb double mutants were analysed for abnormalities in craniofacial and behaviour development through phenotypic analysis. Quantitative PCR was performed to investigate the expression levels of neuronal marker genes in mutant larvae. The motor behaviour was analysed by the swimming activity of 6 dpf larvae. We found that mef2ca;mef2cb double mutants display several abnormal phenotypes during early development, including those already described in zebrafish carrying mutations in each paralog, but also (i) a severe craniofacial phenotype (comprising both cartilaginous and dermal bone structures), (ii) developmental arrest due to the disruption of cardiac oedema and (iii) clear alterations in behaviour. We demonstrate that the defects observed in zebrafish mef2ca;mef2cb double mutants are similar to those previously described in MEF2C-null mice and MRD20 patients, confirming the usefulness of these mutant lines as a model for studies concerning MRD20 disease, the identification of new therapeutic targets and screening for possible rescue strategies.

Chicken embryo as a model in second heart field development

Previously, a single source of progenitor cells was thought to be responsible for the formation of the cardiac muscle. However, the second heart field has recently been identified as an additional source of myocardial progenitor cells. The chicken embryo, which develops in the egg, outside the mother can easily be manipulated in vivo and in vitro. Hence, it was an excellent model for establishing the concept of the second heart field. Here, our review will focus on the chicken model, specifically its role in understanding the second heart field. In addition to discussing historical aspects, we provide an overview of recent findings that have helped to define the chicken second heart field progenitor cells. A better understanding of the second heart field development will provide important insights into the congenital malformations affecting cardiac muscle formation and function.

Variable paralog expression underlies phenotype variation

Human faces are variable; we look different from one another. Craniofacial disorders further increase facial variation. To understand craniofacial variation and how it can be buffered, we analyzed the zebrafish mef2ca mutant. When this transcription factor encoding gene is mutated, zebrafish develop dramatically variable craniofacial phenotypes. Years of selective breeding for low and high penetrance of mutant phenotypes produced strains that are either resilient or sensitive to the mef2ca mutation. Here, we compared gene expression between these strains, which revealed that selective breeding enriched for high and low mef2ca paralog expression in the low- and high-penetrance strains, respectively. We found that mef2ca paralog expression is variable in unselected wild-type zebrafish, motivating the hypothesis that heritable variation in paralog expression underlies mutant phenotype severity and variation. In support, mutagenizing the mef2ca paralogs, mef2aa, mef2b, mef2cb, and mef2d demonstrated modular buffering by paralogs. Specifically, some paralogs buffer severity while others buffer variability. We present a novel, mechanistic model for phenotypic variation where variable, vestigial paralog expression buffers development. These studies are a major step forward in understanding the mechanisms of facial variation, including how some genetically resilient individuals can overcome a deleterious mutation.

Design and Microinjection of Morpholino Antisense Oligonucleotides and mRNA into Zebrafish Embryos to Elucidate Specific Gene Function in Heart Development

The morpholino oligomer-based knockdown system has been used to identify the function of various gene products through loss or reduced expression. Morpholinos (MOs) have the advantage in biological stability over DNA oligos because they are not susceptible to enzymatic degradation. For optimal effectiveness, MOs are injected into 1-4 cell stage embryos. The temporal efficacy of knockdown is variable, but MOs are believed to lose their effects due to dilution eventually. Morpholino dilution and injection amount should be closely controlled to minimize the occurrence of off-target effects while maintaining on-target efficacy. Additional complementary tools, such as CRISPR/Cas9 should be performed against the target gene of interest to generate mutant lines and to confirm the morphant phenotype with these lines. This article will demonstrate how to design, prepare, and microinject a translation-blocking morpholino against hand2 into the yolk of 1-4 cell stage zebrafish embryos to knockdown hand2 function and rescue these "morphants" by co-injection of mRNA encoding the corresponding cDNA. Subsequently, the efficacy of the morpholino microinjections is assessed by first verifying the presence of morpholino in the yolk (co-injected with phenol red) and then by phenotypic analysis. Moreover, cardiac functional analysis to test for knockdown efficacy will be discussed. Finally, assessing the effect of morpholino-induced blockage of gene translation via western blotting will be explained.

Perfluorooctane Sulfonamide (PFOSA) Induces Cardiotoxicity via Aryl Hydrocarbon Receptor Activation in Zebrafish

Perfluorooctane sulfonamide (PFOSA), a precursor of perfluorooctanesulfonate (PFOS), is widely used during industrial processes, though little is known about its toxicity, particularly to early life stage organisms that are generally sensitive to xenobiotic exposure. Here, following exposure to concentrations of 0.01, 0.1, 1, 10, and 100 μg/L PFOSA, transcriptional, morphological, physiological, and biochemical assays were used to evaluate the potential effects on aquatic organisms. The top Tox functions in exposed zebrafish were related to cardiac diseases predicted by Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway, and Ingenuity Pathway Analysis (IPA) analysis. Consistent with impacts predicted by transcriptional changes, abnormal cardiac morphology, disordered heartbeat signals, as well as reduced heart rate and cardiac output were observed following the exposure of 0.1, 1, 10, or 100 μg/L PFOSA. Furthermore, these PFOSA-induced cardiac effects were either prevented or alleviated by supplementation with an aryl hydrocarbon receptor (AHR) antagonist or ahr2-morpholino knock-down, uncovering a seminal role of AHR in PFOSA-induced cardiotoxicity. Our results provide the first evidence in fish that PFOSA can impair proper heart development and function and raises concern for PFOSA analogues in the natural environment.

Hand2 delineates mesothelium progenitors and is reactivated in mesothelioma

The mesothelium lines body cavities and surrounds internal organs, widely contributing to homeostasis and regeneration. Mesothelium disruptions cause visceral anomalies and mesothelioma tumors. Nonetheless, the embryonic emergence of mesothelia remains incompletely understood. Here, we track mesothelial origins in the lateral plate mesoderm (LPM) using zebrafish. Single-cell transcriptomics uncovers a post-gastrulation gene expression signature centered on hand2 in distinct LPM progenitor cells. We map mesothelial progenitors to lateral-most, hand2-expressing LPM and confirm conservation in mouse. Time-lapse imaging of zebrafish hand2 reporter embryos captures mesothelium formation including pericardium, visceral, and parietal peritoneum. We find primordial germ cells migrate with the forming mesothelium as ventral migration boundary. Functionally, hand2 loss disrupts mesothelium formation with reduced progenitor cells and perturbed migration. In mouse and human mesothelioma, we document expression of LPM-associated transcription factors including Hand2, suggesting re-initiation of a developmental program. Our data connects mesothelium development to Hand2, expanding our understanding of mesothelial pathologies.

Shutdown corner, a large deletion mutant isolated from a haploid mutagenesis screen in zebrafish

Morphogenesis, the formation of three-dimensional organ structures, requires precise coupling of genetic regulation and complex cell behaviors. The genetic networks governing many morphogenetic systems, including that of the embryonic eye, are poorly understood. In zebrafish, several forward genetic screens have sought to identify factors regulating eye development. These screens often look for eye defects at stages after the optic cup is formed and when retinal neurogenesis is under way. This approach can make it difficult to identify mutants specific for morphogenesis, as opposed to neurogenesis. To this end, we carried out a forward genetic, small-scale haploid mutagenesis screen in zebrafish (Danio rerio) to identify factors that govern optic cup morphogenesis. We screened ∼100 genomes and isolated shutdown corner (sco), a mutant that exhibits multiple tissue defects and harbors a ∼10-Mb deletion that encompasses 89 annotated genes. Using a combination of live imaging and antibody staining, we found cell proliferation, cell death, and tissue patterning defects in the sco optic cup. We also observed other phenotypes, including paralysis, neuromuscular defects, and ocular vasculature defects. To date, the largest deletion mutants reported in zebrafish are engineered using CRISPR-Cas9 and are less than 300 kb. Because of the number of genes within the deletion interval, shutdown corner [Df(Chr05:sco)z207] could be a useful resource to the zebrafish community, as it may be helpful for gene mapping, understanding genetic interactions, or studying many genes lost in the mutant.

Nxhl Controls Angiogenesis by Targeting VE-PTP Through Interaction With Nucleolin

Precise regulation of angiogenesis is required for organ development, wound repair, and tumor progression. Here, we identified a novel gene, nxhl (New XingHuo light), that is conserved in vertebrates and that plays a crucial role in vascular integrity and angiogenesis. Bioinformatic analysis uncovered its essential roles in development based on co-expression with several key developmental genes. Knockdown of nxhl in zebrafish causes global and pericardial edema, loss of blood circulation, and vascular defects characterized by both reduced vascularization in intersegmental vessels and decreased sprouting in the caudal vein plexus. The nxhl gene also affects human endothelial cell behavior in vitro. We found that nxhl functions in part by targeting VE-PTP through interaction with NCL (nucleolin). Loss of ptprb (a VE-PTP ortholo) in zebrafish resulted in defects similar to nxhl knockdown. Moreover, nxhl deficiency attenuates tumor invasion and proteins (including VE-PTP and NCL) associated with angiogenesis and EMT. These findings illustrate that nxhl can regulate angiogenesis via a novel nxhl-NCL-VE-PTP axis, providing a new therapeutic target for modulating vascular formation and function, especially for cancer treatment.

Short exposure to glyphosate induces locomotor, craniofacial, and bone disorders in zebrafish (Danio rerio) embryos

Glyphosate [N-(phosphonomethyl)glycine] is the active ingredient in widely used broad-spectrum herbicides. Even though the toxicity mechanism of this herbicide in vertebrates is poorly understood, evidence suggests that glyphosate is an endocrine disruptor capable of producing morphological anomalies as well as cardiotoxic and neurotoxic effects. We used the zebrafish model to assess the effects of early life glyphosate exposure on the development of cartilage and bone tissues and organismal responses. We found functional alterations, including a reduction in the cardiac rate, significant changes in the spontaneous tail movement pattern, and defects in craniofacial development. These effects were concomitant with alterations in the level of the estrogen receptor alpha osteopontin and bone sialoprotein. We also found that embryos exposed to glyphosate presented spine deformities as adults. These developmental alterations are likely induced by changes in protein levels related to bone and cartilage formation.

Transcriptional Regulation of Postnatal Cardiomyocyte Maturation and Regeneration

During the postnatal period, mammalian cardiomyocytes undergo numerous maturational changes associated with increased cardiac function and output, including hypertrophic growth, cell cycle exit, sarcomeric protein isoform switching, and mitochondrial maturation. These changes come at the expense of loss of regenerative capacity of the heart, contributing to heart failure after cardiac injury in adults. While most studies focus on the transcriptional regulation of embryonic or adult cardiomyocytes, the transcriptional changes that occur during the postnatal period are relatively unknown. In this review, we focus on the transcriptional regulators responsible for these aspects of cardiomyocyte maturation during the postnatal period in mammals. By specifically highlighting this transitional period, we draw attention to critical processes in cardiomyocyte maturation with potential therapeutic implications in cardiovascular disease.

Enhanced Canonical Wnt Signaling During Early Zebrafish Development Perturbs the Interaction of Cardiac Mesoderm and Pharyngeal Endoderm and Causes Thyroid Specification Defects

Background: Congenital hypothyroidism due to thyroid dysgenesis is a frequent congenital endocrine disorder for which the molecular mechanisms remain unresolved in the majority of cases. This situation reflects, in part, our still limited knowledge about the mechanisms involved in the early steps of thyroid specification from the endoderm, in particular the extrinsic signaling cues that regulate foregut endoderm patterning. In this study, we used small molecules and genetic zebrafish models to characterize the role of various signaling pathways in thyroid specification. Methods: We treated zebrafish embryos during different developmental periods with small-molecule compounds known to manipulate the activity of Wnt signaling pathway and observed effects in thyroid, endoderm, and cardiovascular development using whole-mount in situ hybridization and transgenic fluorescent reporter models. We used the antisense morpholino (MO) technique to create a zebrafish acardiac model. For thyroid rescue experiments, bone morphogenetic protein (BMP) pathway induction in zebrafish embryos was obtained by manipulation of heat-shock inducible transgenic lines. Results: Combined analyses of thyroid and cardiovascular development revealed that overactivation of Wnt signaling during early development leads to impaired thyroid specification concurrent with severe defects in the cardiac specification. When using a model of MO-induced blockage of cardiomyocyte differentiation, a similar correlation was observed, suggesting that defective signaling between cardiac mesoderm and endodermal thyroid precursors contributes to thyroid specification impairment. Rescue experiments through transient overactivation of BMP signaling could partially restore thyroid specification in models with defective cardiac development. Conclusion: Collectively, our results indicate that BMP signaling is critically required for thyroid cell specification and identify cardiac mesoderm as a likely source of BMP signals.

From Stripes to a Beating Heart: Early Cardiac Development in Zebrafish

The heart is the first functional organ to form during vertebrate development. Congenital heart defects are the most common type of human birth defect, many originating as anomalies in early heart development. The zebrafish model provides an accessible vertebrate system to study early heart morphogenesis and to gain new insights into the mechanisms of congenital disease. Although composed of only two chambers compared with the four-chambered mammalian heart, the zebrafish heart integrates the core processes and cellular lineages central to cardiac development across vertebrates. The rapid, translucent development of zebrafish is amenable to in vivo imaging and genetic lineage tracing techniques, providing versatile tools to study heart field migration and myocardial progenitor addition and differentiation. Combining transgenic reporters with rapid genome engineering via CRISPR-Cas9 allows for functional testing of candidate genes associated with congenital heart defects and the discovery of molecular causes leading to observed phenotypes. Here, we summarize key insights gained through zebrafish studies into the early patterning of uncommitted lateral plate mesoderm into cardiac progenitors and their regulation. We review the central genetic mechanisms, available tools, and approaches for modeling congenital heart anomalies in the zebrafish as a representative vertebrate model.

Cardiac Development: A Glimpse on Its Translational Contributions

Cardiac development is a complex developmental process that is initiated soon after gastrulation, as two sets of precardiac mesodermal precursors are symmetrically located and subsequently fused at the embryonic midline forming the cardiac straight tube. Thereafter, the cardiac straight tube invariably bends to the right, configuring the first sign of morphological left–right asymmetry and soon thereafter the atrial and ventricular chambers are formed, expanded and progressively septated. As a consequence of all these morphogenetic processes, the fetal heart acquired a four-chambered structure having distinct inlet and outlet connections and a specialized conduction system capable of directing the electrical impulse within the fully formed heart. Over the last decades, our understanding of the morphogenetic, cellular, and molecular pathways involved in cardiac development has exponentially grown. Multiples aspects of the initial discoveries during heart formation has served as guiding tools to understand the etiology of cardiac congenital anomalies and adult cardiac pathology, as well as to enlighten novels approaches to heal the damaged heart. In this review we provide an overview of the complex cellular and molecular pathways driving heart morphogenesis and how those discoveries have provided new roads into the genetic, clinical and therapeutic management of the diseased hearts.

Mef2c factors are required for early but not late addition of cardiomyocytes to the ventricle

During heart formation, the heart grows and undergoes dramatic morphogenesis to achieve efficient embryonic function. Both in fish and amniotes, much of the growth occurring after initial heart tube formation arises from second heart field (SHF)-derived progenitor cell addition to the arterial pole, allowing chamber formation. In zebrafish, this process has been extensively studied during embryonic life, but it is unclear how larval cardiac growth occurs beyond 3 days post-fertilisation (dpf). By quantifying zebrafish myocardial growth using live imaging of GFP-labelled myocardium we show that the heart grows extensively between 3 and 5 dpf. Using methods to assess cell division, cellular development timing assay and Kaede photoconversion, we demonstrate that proliferation, CM addition, and hypertrophy contribute to ventricle growth. Mechanistically, we show that reduction in Mef2c activity (mef2ca^+/-;mef2cb^-/-), downstream or in parallel with Nkx2.5 and upstream of Ltbp3, prevents some CM addition and differentiation, resulting in a significantly smaller ventricle by 3 dpf. After 3 dpf, however, CM addition in mef2ca^+/-;mef2cb^-/- mutants recovers to a normal pace, and the heart size gap between mutants and their siblings diminishes into adulthood. Thus, as in mice, there is an early time window when SHF contribution to the myocardium is particularly sensitive to loss of Mef2c activity.

Dynamic transcriptome profiling toward understanding the development of the human embryonic heart during different Carnegie stages

Transcriptional regulation participates in heart development. However, the transcriptomes of human embryonic hearts during Carnegie stage (CS)10-CS16 have not been elucidated. Here, we found marked changes in the morphology and transcriptome of the human embryonic heart from CS10 to CS11. At CS12-CS14, the embryonic heart undergoes hypoxia-to-aerobic transformation. At CS14-CS16, transcriptome functions were related to energy metabolism, regulation of cholesterol, and processes related to inorganic substances. Moreover, the transcriptomes of cardiac progenitor cells derived from human embryonic stem cells (hESCs) most overlapped with those of human embryonic hearts at CS10. Cardiomyocytes derived from hESCs considerably overlapped with embryonic hearts at CS14-CS16. Overall, these results provide a new perspective into the characteristics of human embryonic heart development.

Exploring the Expression of Cardiac Regulators in a Vertebrate Extremophile: The Cichlid Fish Oreochromis (Alcolapia) alcalica

Although it is widely accepted that the cellular and molecular mechanisms of vertebrate cardiac development are evolutionarily conserved, this is on the basis of data from only a few model organisms suited to laboratory studies. Here, we investigate gene expression during cardiac development in the extremophile, non-model fish species, Oreochromis (Alcolapia) alcalica. We first characterise the early development of O. alcalica and observe extensive vascularisation across the yolk prior to hatching. We further investigate heart development by identifying and cloning O. alcalica orthologues of conserved cardiac transcription factors gata4, tbx5, and mef2c for analysis by in situ hybridisation. Expression of these three key cardiac developmental regulators also reveals other aspects of O. alcalica development, as these genes are expressed in developing blood, limb, eyes, and muscle, as well as the heart. Our data support the notion that O. alcalica is a direct-developing vertebrate that shares the highly conserved molecular regulation of the vertebrate body plan. However, the expression of gata4 in O. alcalica reveals interesting differences in the development of the circulatory system distinct from that of the well-studied zebrafish. Understanding the development of O. alcalica embryos is an important step towards providing a model for future research into the adaptation to extreme conditions; this is particularly relevant given that anthropogenic-driven climate change will likely result in more freshwater organisms being exposed to less favourable conditions.

Systems genetics analysis identifies calcium-signaling defects as novel cause of congenital heart disease

BACKGROUND

Congenital heart disease (CHD) occurs in almost 1% of newborn children and is considered a multifactorial disorder. CHD may segregate in families due to significant contribution of genetic factors in the disease etiology. The aim of the study was to identify pathophysiological mechanisms in families segregating CHD.

METHODS

We used whole exome sequencing to identify rare genetic variants in ninety consenting participants from 32 Danish families with recurrent CHD. We applied a systems biology approach to identify developmental mechanisms influenced by accumulation of rare variants. We used an independent cohort of 714 CHD cases and 4922 controls for replication and performed functional investigations using zebrafish as in vivo model.

RESULTS

We identified 1785 genes, in which rare alleles were shared between affected individuals within a family. These genes were enriched for known cardiac developmental genes, and 218 of these genes were mutated in more than one family. Our analysis revealed a functional cluster, enriched for proteins with a known participation in calcium signaling. Replication in an independent cohort confirmed increased mutation burden of calcium-signaling genes in CHD patients. Functional investigation of zebrafish orthologues of ITPR1, PLCB2, and ADCY2 verified a role in cardiac development and suggests a combinatorial effect of inactivation of these genes.

CONCLUSIONS

The study identifies abnormal calcium signaling as a novel pathophysiological mechanism in human CHD and confirms the complex genetic architecture underlying CHD.

Cardiac Na+-Ca2+ exchanger 1 (ncx1h) is critical for the ventricular cardiomyocyte formation via regulating the expression levels of gata4 and hand2 in zebrafish

Ca²⁺ signaling is critical for heart development; however, the precise roles and regulatory pathways of Ca²⁺ transport proteins in cardiogenesis remain largely unknown. Sodium-calcium exchanger 1 (Ncx1) is responsible for Ca²⁺ efflux in cardiomyocytes. It is involved in cardiogenesis, while the mechanism is unclear. Here, using the forward genetic screening in zebrafish, we identified a novel mutation at a highly-conserved leucine residue in ncx1 gene (mutant^LDD353/ncx1h^L154P) that led to smaller hearts with reduced heart rate and weak contraction. Mechanistically, the number of ventricular but not atrial cardiomyocytes was reduced in ncx1h^L154P zebrafish. These defects were mimicked by knockdown or knockout of ncx1h. Moreover, ncx1h^L154P had cytosolic and mitochondrial Ca²⁺ overloading and Ca²⁺ transient suppression in cardiomyocytes. Furthermore, ncx1h^L154P and ncx1h morphants downregulated cardiac transcription factors hand2 and gata4 in the cardiac regions, while overexpression of hand2 and gata4 partially rescued cardiac defects including the number of ventricular myocytes. These findings demonstrate an essential role of the novel 154th leucine residue in the maintenance of Ncx1 function in zebrafish, and reveal previous unrecognized critical roles of the 154th leucine residue and Ncx1 in the formation of ventricular cardiomyocytes by at least partially regulating the expression levels of gata4 and hand2.

Toxic effects of dechlorane plus on the common carp (Cyprinus carpio) embryonic development

Dechlorane Plus (DP) is a widely used chlorinated flame retardant, which has been extensively detected in the environment. Although DP content in the surface water is low, it can pose a continuous exposure risk to aquatic organisms due to its strong bioaccumulation. Considering that the related studies on the toxicity mechanism of DP exposure are limited, the effect of DP on carp embryo development was evaluated. In the present work, carp embryos were exposed to different concentrations (0, 30, 60, and 120 μg/L) of DP at 3 h post-fertilization (hpf). The expression levels of neural and skeletal development-associated genes, such as sox2, sox19a, Mef2c and BMP4, were detected with quantitative PCR, and the changes in different developmental toxicity endpoints were observed. Our results demonstrated that the expression levels of sox2, sox19a, Mef2c and BMP4 were significantly altered and several developmental abnormalities were found in DP-exposed carp embryos, such as DNA damage, increased mortality rate, delayed hatching time, reduced hatching rate, decreased body length, and increased morphological deformities. In addition, the activities of reactive oxygen species and malondialdehyde were remarkably higher in 60 and 120 μg/L DP exposure groups than in control group. These results suggest that DP can exhibit a unique modes of action, which lead to aberration occurrence in the early development stage of common carps, which may be related to some gene damage and oxidative stress. Besides, the parameters evaluated here can be used as tools to access the environmental risk for biota and humans exposed to DP.

Pbx4 limits heart size and fosters arch artery formation by partitioning second heart field progenitors and restricting proliferation

Vertebrate heart development requires the integration of temporally distinct differentiating progenitors. However, few signals are understood that restrict the size of the later-differentiating outflow tract (OFT). We show that improper specification and proliferation of second heart field (SHF) progenitors in zebrafish lazarus (lzr) mutants, which lack the transcription factor Pbx4, produces enlarged hearts owing to an increase in ventricular and smooth muscle cells. Specifically, Pbx4 initially promotes the partitioning of the SHF into anterior progenitors, which contribute to the OFT, and adjacent endothelial cell progenitors, which contribute to posterior pharyngeal arches. Subsequently, Pbx4 limits SHF progenitor (SHFP) proliferation. Single cell RNA sequencing of nkx2.5+ cells revealed previously unappreciated distinct differentiation states and progenitor subpopulations that normally reside within the SHF and arterial pole of the heart. Specifically, the transcriptional profiles of Pbx4-deficient nkx2.5+ SHFPs are less distinct and display characteristics of normally discrete proliferative progenitor and anterior, differentiated cardiomyocyte populations. Therefore, our data indicate that the generation of proper OFT size and arch arteries requires Pbx-dependent stratification of unique differentiation states to facilitate both homeotic-like transformations and limit progenitor production within the SHF.

Maternal Larp6 controls oocyte development, chorion formation and elevation

La-related protein 6 (Larp6) is a conserved RNA-binding protein found across eukaryotes that has been suggested to regulate collagen biogenesis, muscle development, ciliogenesis, and various aspects of cell proliferation and migration. Zebrafish have two Larp6 family genes: larp6a and larp6b Viable and fertile single and double homozygous larp6a and larp6b zygotic mutants revealed no defects in muscle structure, and were indistinguishable from heterozygous or wild-type siblings. However, larp6a mutant females produced eggs with chorions that failed to elevate fully and were fragile. Eggs from larp6b single mutant females showed minor chorion defects, but chorions from eggs laid by larp6a;larp6b double mutant females were more defective than those from larp6a single mutants. Electron microscopy revealed defective chorionogenesis during oocyte development. Despite this, maternal zygotic single and double mutants were viable and fertile. Mass spectrometry analysis provided a description of chorion protein composition and revealed significant reductions in a subset of zona pellucida and lectin-type proteins between wild-type and mutant chorions that paralleled the severity of the phenotype. We conclude that Larp6 proteins are required for normal oocyte development, chorion formation and egg activation.

Zebrafish Vestigial Like Family Member 4b Is Required for Valvulogenesis Through Sequestration of Transcription Factor Myocyte Enhancer Factor 2c

A variety of cardiac transcription factors/cofactors, signaling pathways, and downstream structural genes integrate to form the regulatory hierarchies to ensure proper cardiogenesis in vertebrate. Major interaction proteins of the transcription cofactor vestigial like family member 4 (VGLL4) include myocyte enhancer factor 2 (MEF2) and TEA domain transcription factors (TEAD), both of which play important roles in embryonic cardiac development and in adulthood. In this study, we identified that the deficiency of zebrafish vgll4b paralog, a unique family member expressed in developing heart, led to an impaired valve development. Mechanistically, in vgll4b mutant embryos the disruption of Vgll4b-Mef2c complex, rather than that of Vgll4b-Tead complex, resulted in an aberrant expression of krüppel-like factor 2a (klf2a) in endocardium. Such misexpression of klf2a eventually evoked the valvulogenesis defects. Our findings suggest that zebrafish Vgll4b plays an important role in modulating the transcription activity of Mef2c on klf2a during valve development in a blood-flow-independent manner.

miR-190-5p in human diseases

miRNAs, a major class of small noncoding RNAs approximately 18-25 nucleotides in length, function by repressing the expression of target genes through binding to complementary sequences in the 3'-UTRs of target genes. Emerging evidence has highlighted their important roles in numerous diseases, including human cancers. Recently, miR-190 has been shown to be dysregulated in various types of human cancers that participates in cancer-related biological processes, including proliferation, apoptosis, metastasis, drug resistance, by regulating associated target genes, and to predict cancer diagnosis and prognosis. In this review, we summarized the roles of miR-190-5p in human diseases, especially in human cancers. Then we classified its target genes in tumorigenesis and progression, which might provide evidence for cancer diagnosis and prognosis, promising tools for cancer treatment, or leads for further investigation.

Common Carp mef2 Genes: Evolution and Expression

The MEF2 (myocyte enhancer factor 2) family belongs to the MADS-box superfamily of eukaryotic transcription factors. The vertebrate genes compose four distinct subfamilies designated MEF2A, -B, -C, and -D. There are multiple mef2 genes in the common carp (Cyprinus carpio). So far, the embryonic expression patterns of these genes and the evolution of fish mef2 genes have been barely investigated. In this study, we completed the coding information of C. carpio mef2ca2 and mef2d1 genes via gene cloning and presented two mosaic mef2 sequences as evidence for recombination. We also analyzed the phylogenetic relationship and conserved synteny of mef2 genes and proposed a new evolutionary scenario. In our version, MEF2B and the other three vertebrate subfamilies were generated in parallel from the single last ancestor via two rounds of whole genome duplication events that occurred at the dawn of vertebrates. Moreover, we examined the expression patterns of C. carpio mef2 genes during embryogenesis, by using whole-mount in situ hybridization, and found the notochord to be a new expression site for these genes except for mef2ca1&2. Our results thus provide new insights into the evolution and expression of mef2 genes.

Differential expression of myosin heavy chain isoforms in cardiac segments of gnathostome vertebrates and its evolutionary implications

Background

Immunohistochemical studies of hearts from the lesser spotted dogfish, Scyliorhinus canicula (Chondrichthyes) revealed that the pan-myosin heavy chain (pan-MyHC) antibody MF20 homogeneously labels all the myocardium, while the pan-MyHC antibody A4.1025 labels the myocardium of the inflow (sinus venosus and atrium) but not the outflow (ventricle and conus arteriosus) cardiac segments, as opposed to other vertebrates. We hypothesized that the conventional pattern of cardiac MyHC isoform distribution present in most vertebrates, i.e. MYH6 in the inflow and MYH7 in the outflow segments, has evolved from a primitive pattern that persists in Chondrichthyes. In order to test this hypothesis, we conducted protein detection techniques to identify the MyHC isoforms expressed in adult dogfish cardiac segments and to assess the pan-MyHC antibodies reactivity against the cardiac segments of representative species from different vertebrate groups.

Results

Western and slot blot results confirmed the specificity of MF20 and A4.1025 for MyHC in dogfish and their differential reactivity against distinct myocardial segments. HPLC-ESI-MS/MS and ESI-Quadrupole-Orbitrap revealed abundance of MYH6 and MYH2 in the inflow and of MYH7 and MYH7B in the outflow segments. Immunoprecipitation showed higher affinity of A4.1025 for MYH2 and MYH6 than for MYH7 and almost no affinity for MYH7B. Immunohistochemistry showed that A4.1025 signals are restricted to the inflow myocardial segments of elasmobranchs, homogeneous in all myocardial segments of teleosts and acipenseriforms, and low in the ventricle of polypteriforms.

Conclusions

The cardiac inflow and outflow segments of the dogfish show predominance of fast- and slow-twitch MyHC isoforms respectively, what can be considered a synapomorphy of gnathostomes. The myocardium of the dogfish contains two isomyosins (MYH2 and MYH7B) not expressed in the adult heart of other vertebrates. We propose that these isomyosins lost their function in cardiac contraction during the evolution of gnathostomes, the later acquiring a regulatory role in myogenesis through its intronic miRNA. Loss of MYH2 and MYH7B expression in the heart possibly occurred before the origin of Osteichthyes, being the latter reacquired in polypteriforms. We raise the hypothesis that the slow tonic MYH7B facilitates the peristaltic contraction of the conus arteriosus of fish with a primitive cardiac anatomical design and of the vertebrate embryo.

Physiological Responses to Swimming-Induced Exercise in the Adult Zebrafish Regenerating Heart

Exercise promotes a set of physiological responses known to provide long-term health benefits and it can play an important role in cardioprotection. In the present study, we examined cardiac responses to exercise training in the adult zebrafish and in the context of cardiac regeneration. We found that swimming-induced exercise increased cardiomyocyte proliferation and that this response was also found under regenerating conditions, when exercise was performed either prior to and after ventricular cryoinjury (CI). Exercise prior to CI resulted in a mild improvement in cardiac function and lesion recovery over the non-exercise condition. Transcriptomic profiling of regenerating ventricles in cryoinjured fish subjected to exercise identified genes possibly involved in the cardioprotective effects of exercise and that could represent potential targets for heart regeneration strategies. Taken together, our results suggest that exercise constitutes a physiological stimulus that may help promote cardiomyogenic mechanisms of the vertebrate heart through the induction of cardiomyocyte proliferation. The zebrafish exercise model may be useful for investigating the potential cardioprotective effects of exercise in teleost fish and to contribute to further identify and develop novel avenues in basic research to promote heart regeneration.

Toxic effects of cyhalofop-butyl on embryos of the Yellow River carp (Cyprinus carpio var.): alters embryos hatching, development failure, mortality of embryos, and apoptosis

As a universal environmental contaminant, the herbicide cyhalofop-butyl is considered to have infested effects on the embryonic development of aquatic species. The present study focused on an assessment of the impacts of cyhalofop-butyl on Yellow River carp embryos. It was found that cyhalofop-butyl inhibited the hatching of the embryos, and the hatching rate decreased with higher concentrations of the herbicide. The mortality rate was increased on exposure to cyhalofop-butyl and was significantly higher in the 1.6 and 2 mg/L treatment groups over 48 h. All of the embryos of the 2 mg/L treatment group died within the 48 h post-hatching stage. And the transcription of several embryos related to apoptosis was also influenced by cyhalofop-butyl exposure. Further, cyhalofop-butyl exposure leads to a series of morphological changes (pericardial edema, tail deformation, and spine deformation) in embryos, which were consistent with significant modifications in the associated genes. These results provided a scientific basis for further studies into the effects of cyhalofop-butyl on aquatic organisms.

Hippo signaling determines the number of venous pole cells that originate from the anterior lateral plate mesoderm in zebrafish

The differentiation of the lateral plate mesoderm cells into heart field cells constitutes a critical step in the development of cardiac tissue and the genesis of functional cardiomyocytes. Hippo signaling controls cardiomyocyte proliferation, but the role of Hippo signaling during early cardiogenesis remains unclear. Here, we show that Hippo signaling regulates atrial cell number by specifying the developmental potential of cells within the anterior lateral plate mesoderm (ALPM), which are incorporated into the venous pole of the heart tube and ultimately into the atrium of the heart. We demonstrate that Hippo signaling acts through large tumor suppressor kinase 1/2 to modulate BMP signaling and the expression of hand2, a key transcription factor that is involved in the differentiation of atrial cardiomyocytes. Collectively, these results demonstrate that Hippo signaling defines venous pole cardiomyocyte number by modulating both the number and the identity of the ALPM cells that will populate the atrium of the heart.

Mef2 and the skeletal muscle differentiation program

Mef2 is a conserved and significant transcription factor in the control of muscle gene expression. In cell culture Mef2 synergises with MyoD-family members in the activation of gene expression and in the conversion of fibroblasts into myoblasts. Amongst its in vivo roles, Mef2 is required for both Drosophila muscle development and mammalian muscle regeneration. Mef2 has functions in other cell-types too, but this review focuses on skeletal muscle and surveys key findings on Mef2 from its discovery, shortly after that of MyoD, up to the present day. In particular, in vivo functions, underpinning mechanisms and areas of uncertainty are highlighted. We describe how Mef2 sits at a nexus in the gene expression network that controls the muscle differentiation program, and how Mef2 activity must be regulated in time and space to orchestrate specific outputs within the different aspects of muscle development. A theme that emerges is that there is much to be learnt about the different Mef2 proteins (from different paralogous genes, spliced transcripts and species) and how the activity of these proteins is controlled.

Promoter architecture and transcriptional regulation of musculoskeletal embryonic nuclear protein 1b (mustn1b) gene in zebrafish

BACKGROUND

Mustn1 is a specific musculoskeletal protein that plays a critical role in myogenesis and chondrogenesis in vertebrates. Whole-mount in situ hybridization revealed that mustn1b mRNAs are specifically expressed in skeletal and cardiac muscles in Zebrafish embryos. However, the precise function and the regulatory elements required for its muscle-specific expression are largely unknown.

RESULTS

The purpose of this study was to explore and uncover the target genomic regions that regulate mustn1b gene expression by in vivo functional characterization of the mustn1b promoter. We report here stable expression analyses of eGFP from fluorescent transgenic reporter Zebrafish line containing a 0.8kb_mustn1b-Tol2-eGFP construct. eGFP expression was specifically found in the skeletal and cardiac muscle tissues. We show that reporter Zebrafish lines generated replicate the endogenous mustn1b expression pattern in early Zebrafish embryos. Specific site directed-mutagenesis analysis revealed that promoter activity resides in two annotated genomic regulatory regions, each one corresponding to a specific functional transcription factor binding site.

CONCLUSIONS

Our data indicate that mustn1b is specifically expressed in skeletal and cardiac muscle tissues and its muscle specificity is controlled by the 0.2-kb promoter and flanking sequences and in vivo regulated by the action of two sequence-specific families of transcription factors. Developmental Dynamics 246:992-1000, 2017. © 2017 Wiley Periodicals, Inc.

MEF2C loss-of-function mutation associated with familial dilated cardiomyopathy

Background:

The MADS-box transcription factor myocyte enhancer factor 2C (MEF2C) is required for the cardiac development and postnatal adaptation and in mice-targeted disruption of the MEF2C gene results in dilated cardiomyopathy (DCM). However, in humans, the association of MEF2C variation with DCM remains to be investigated.

Methods:

The coding regions and splicing boundaries of the MEF2C gene were sequenced in 172 unrelated patients with idiopathic DCM. The available close relatives of the index patient harboring an identified MEF2C mutation and 300 unrelated, ethnically matched healthy individuals used as controls were genotyped for MEF2C. The functional effect of the mutant MEF2C protein was characterized in contrast to its wild-type counterpart by using a dual-luciferase reporter assay system.

Results:

A novel heterozygous MEF2C mutation, p.Y157X, was detected in an index patient with adult-onset DCM. Genetic screen of the mutation carrier’s family members revealed that the mutation co-segregated with DCM, which was transmitted as an autosomal dominant trait with complete penetrance. The non-sense mutation was absent in 300 control individuals. Functional analyses unveiled that the mutant MEF2C protein had no transcriptional activity. Furthermore, the mutation abolished the synergistic transactivation between MEF2C and GATA4 as well as HAND1, two other transcription factors that have been associated with DCM.

Conclusions:

This study indicates MEF2C as a new gene responsible for human DCM, which provides novel insight into the mechanism underpinning DCM, suggesting potential implications for development of innovative prophylactic and therapeutic strategies for DCM, the most prevalent form of primary myocardial disease.

Nuclear Pores Regulate Muscle Development and Maintenance by Assembling a Localized Mef2C Complex

Nuclear pore complexes (NPCs) are multiprotein channels connecting the nucleus with the cytoplasm. NPCs have been shown to have tissue-specific composition, suggesting that their function can be specialized. However, the physiological roles of NPC composition changes and their impacts on cellular processes remain unclear. Here we show that the addition of the Nup210 nucleoporin to NPCs during myoblast differentiation results in assembly of an Mef2C transcriptional complex required for efficient expression of muscle structural genes and microRNAs. We show that this NPC-localized complex is essential for muscle growth, myofiber maturation, and muscle cell survival and that alterations in its activity result in muscle degeneration. Our findings suggest that NPCs regulate the activity of functional gene groups by acting as scaffolds that promote the local assembly of tissue-specific transcription complexes and show how nuclear pore composition changes can be exploited to regulate gene expression at the nuclear periphery.

MEF2C loss-of-function mutation contributes to congenital heart defects

Congenital heart disease (CHD) is the most common type of developmental abnormality in humans, and is a leading cause for substantially increased morbidity and mortality in affected individuals. Increasing studies demonstrates a pivotal role of genetic defects in the pathogenesis of CHD, and presently mutations in more than 60 genes have been associated with CHD. Nevertheless, CHD is of pronounced genetic heterogeneity, and the genetic basis underpinning CHD in a large proportion of patients remains unclear. In the present study, the whole coding exons and splicing donors/acceptors of the MEF2C gene, which codes for a transcription factor essential for normal cardiovascular development, were sequenced in 200 unrelated patients affected with CHD, and a novel heterozygous missense mutation, p.L38P, was identified in an index patient with patent ductus arteriosus (PDA) and ventricular septal defect (VSD). Genetic scan of the mutation carrier's family members available showed that the mutation was present in all affected family members but absent in unaffected family members. Analysis of the proband's pedigree revealed that the mutation co-segregated with PDA, which was transmitted as an autosomal dominant trait with complete penetrance. The mutation changed the amino acid that was completely conserved evolutionarily, and did not exist in 300 unrelated, ethnically-matched healthy individuals used as controls. Functional deciphers by using a dual-luciferase reporter assay system unveiled that the mutant MEF2C protein had a significantly reduced transcriptional activity. Furthermore, the mutation significantly diminished the synergistic activation between MEF2C and GATA4, another cardiac core transcription factor that has been causally linked to CHD. In conclusion, this is the first report on the association of a MEF2C loss-of-function mutation with an increased vulnerability to CHD in humans, which provides novel insight into the molecular mechanisms underlying CHD, implying potential implications for early diagnosis and timely prophylaxis of CHD.

Antagonistic regulation of cell-cycle and differentiation gene programs in neonatal cardiomyocytes by homologous MEF2 transcription factors

Cardiomyocytes acquire their primary specialized function (contraction) before exiting the cell cycle. In this regard, proliferation and differentiation must be precisely coordinated for proper cardiac morphogenesis. Here, we have investigated the complex transcriptional mechanisms employed by cardiomyocytes to coordinate antagonistic cell-cycle and differentiation gene programs through the molecular dissection of the core cardiac transcription factor, MEF2. Knockdown of individual MEF2 proteins, MEF2A, -C, and -D, in primary neonatal cardiomyocytes resulted in radically distinct and opposite effects on cellular homeostasis and gene regulation. MEF2A and MEF2D were absolutely required for cardiomyocyte survival, whereas MEF2C, despite its major role in cardiac morphogenesis and direct reprogramming, was dispensable for this process. Inhibition of MEF2A or -D also resulted in the activation of cell-cycle genes and down-regulation of markers of terminal differentiation. In striking contrast, the regulation of cell-cycle and differentiation gene programs by MEF2C was antagonistic to that of MEF2A and -D. Computational analysis of regulatory regions from MEF2 isoform-dependent gene sets identified the Notch and Hedgehog signaling pathways as key determinants in coordinating MEF2 isoform-specific control of antagonistic gene programs. These results reveal that mammalian MEF2 family members have distinct transcriptional functions in cardiomyocytes and suggest that these differences are critical for proper development and maturation of the heart. Analysis of MEF2 isoform-specific function in neonatal cardiomyocytes has yielded insight into an unexpected transcriptional regulatory mechanism by which these specialized cells utilize homologous members of a core cardiac transcription factor to coordinate cell-cycle and differentiation gene programs.

Micro- and Macrostructured PLGA/Gelatin Scaffolds Promote Early Cardiogenic Commitment of Human Mesenchymal Stem Cells In Vitro

The biomaterial scaffold plays a key role in most tissue engineering strategies. Its surface properties, micropatterning, degradation, and mechanical features affect not only the generation of the tissue construct in vitro, but also its in vivo functionality. The area of myocardial tissue engineering still faces significant difficulties and challenges in the design of bioactive scaffolds, which allow composition variation to accommodate divergence in the evolving myocardial structure. Here we aimed at verifying if a microstructured bioartificial scaffold alone can provoke an effect on stem cell behavior. To this purpose, we fabricated microstructured bioartificial polymeric constructs made of PLGA/gelatin mimicking anisotropic structure and mechanical properties of the myocardium. We found that PLGA/gelatin scaffolds promoted adhesion, elongation, ordered disposition, and early myocardial commitment of human mesenchymal stem cells suggesting that these constructs are able to crosstalk with stem cells in a precise and controlled manner. At the same time, the biomaterial degradation kinetics renders the PLGA/gelatin constructs very attractive for myocardial regeneration approaches.

Glyphosate induces cardiovascular toxicity in Danio rerio

Glyphosate is a broad spectrum herbicide used aggressively in agricultural practices as well as home garden care. Although labeled "safe" by the chemical industry, doses tested by industry do not mimic chronic exposures to sublethal doses that organisms in the environment are exposed to over long periods of time. Given the widespread uses of and exposure to glyphosate, studies on developmental toxicity are needed. Here we utilize the zebrafish vertebrate model system to study early effects of glyphosate on the developing heart. Treatment by embryo soaking with 50μg/ml glyphosate starting at gastrulation results in structural abnormalities in the atrium and ventricle, irregular heart looping, situs inversus as well as decreased heartbeats by 48h as determined by live imaging and immunohistochemistry. Vasculature in the body was also affected as determined using fli-1 transgenic embryos. To determine if the effects noted at 48h post fertilization are due to early stage alterations in myocardial precursors, we also investigate cardiomyocyte development with a Mef2 antibody and by mef2ca in situ hybridization and find alterations in the Mef2/mef2ca staining patterns during early cardiac patterning stages. We conclude that glyphosate is developmentally toxic to the zebrafish heart.

The Function of the MEF2 Family of Transcription Factors in Cardiac Development, Cardiogenomics, and Direct Reprogramming

Proper formation of the mammalian heart requires precise spatiotemporal transcriptional regulation of gene programs in cardiomyocytes. Sophisticated regulatory networks have evolved to not only integrate the activities of distinct transcription factors to control tissue-specific gene programs but also, in many instances, to incorporate multiple members within these transcription factor families to ensure accuracy and specificity in the system. Unsurprisingly, perturbations in this elaborate transcriptional circuitry can lead to severe cardiac abnormalities. Myocyte enhancer factor–2 (MEF2) transcription factor belongs to the evolutionarily conserved cardiac gene regulatory network. Given its central role in muscle gene regulation and its evolutionary conservation, MEF2 is considered one of only a few core cardiac transcription factors. In addition to its firmly established role as a differentiation factor, MEF2 regulates wide variety of, sometimes antagonistic, cellular processes such as cell survival and death. Vertebrate genomes encode multiple MEF2 family members thereby expanding the transcriptional potential of this core transcription factor in the heart. This review highlights the requirement of the MEF2 family and their orthologs in cardiac development in diverse animal model systems. Furthermore, we describe the recently characterized role of MEF2 in direct reprogramming and genome-wide cardiomyocyte gene regulation. A thorough understanding of the regulatory functions of the MEF2 family in cardiac development and cardiogenomics is required in order to develop effective therapeutic strategies to repair the diseased heart.

The AP-1 transcription factor component Fosl2 potentiates the rate of myocardial differentiation from the zebrafish second heart field

The vertebrate heart forms through successive phases of cardiomyocyte differentiation. Initially, cardiomyocytes derived from first heart field (FHF) progenitors assemble the linear heart tube. Thereafter, second heart field (SHF) progenitors differentiate into cardiomyocytes that are accreted to the poles of the heart tube over a well-defined developmental window. Although heart tube elongation deficiencies lead to life-threatening congenital heart defects, the variables controlling the initiation, rate and duration of myocardial accretion remain obscure. Here, we demonstrate that the AP-1 transcription factor, Fos-like antigen 2 (Fosl2), potentiates the rate of myocardial accretion from the zebrafish SHF. fosl2 mutants initiate accretion appropriately, but cardiomyocyte production is sluggish, resulting in a ventricular deficit coupled with an accumulation of SHF progenitors. Surprisingly, mutant embryos eventually correct the myocardial deficit by extending the accretion window. Overexpression of Fosl2 also compromises production of SHF-derived ventricular cardiomyocytes, a phenotype that is consistent with precocious depletion of the progenitor pool. Our data implicate Fosl2 in promoting the progenitor to cardiomyocyte transition and uncover the existence of regulatory mechanisms to ensure appropriate SHF-mediated cardiomyocyte contribution irrespective of embryonic stage.

Function Over Form: Modeling Groups of Inherited Neurological Conditions in Zebrafish

Zebrafish are a unique cell to behavior model for studying the basic biology of human inherited neurological conditions. Conserved vertebrate genetics and optical transparency provide in vivo access to the developing nervous system as well as high-throughput approaches for drug screens. Here we review zebrafish modeling for two broad groups of inherited conditions that each share genetic and molecular pathways and overlap phenotypically: neurodevelopmental disorders such as Autism Spectrum Disorders (ASD), Intellectual Disability (ID) and Schizophrenia (SCZ), and neurodegenerative diseases, such as Cerebellar Ataxia (CATX), Hereditary Spastic Paraplegia (HSP) and Charcot-Marie Tooth Disease (CMT). We also conduct a small meta-analysis of zebrafish orthologs of high confidence neurodevelopmental disorder and neurodegenerative disease genes by looking at duplication rates and relative protein sizes. In the past zebrafish genetic models of these neurodevelopmental disorders and neurodegenerative diseases have provided insight into cellular, circuit and behavioral level mechanisms contributing to these conditions. Moving forward, advances in genetic manipulation, live imaging of neuronal activity and automated high-throughput molecular screening promise to help delineate the mechanistic relationships between different types of neurological conditions and accelerate discovery of therapeutic strategies.

Phosphorylation-dependent degradation of MEF2C contributes to regulate G2/M transition

The Myocyte Enhancer Factor 2C (MEF2C) transcription factor plays a critical role in skeletal muscle differentiation, promoting muscle-specific gene transcription. Here we report that in proliferating cells MEF2C is degraded in mitosis by the Anaphase Promoting Complex/Cyclosome (APC/C) and that this downregulation is necessary for an efficient progression of the cell cycle. We show that this mechanism of degradation requires the presence on MEF2C of a D-box (R-X-X-L) and 2 phospho-motifs, pSer98 and pSer110. Both the D-box and pSer110 motifs are encoded by the ubiquitous alternate α1 exon. These two domains mediate the interaction between MEF2C and CDC20, a co-activator of APC/C. We further report that in myoblasts, MEF2C regulates the expression of G2/M checkpoint genes (14–3–3γ, Gadd45b and p21) and the sub-cellular localization of CYCLIN B1. The importance of controlling MEF2C levels during the cell cycle is reinforced by the observation that modulation of its expression affects the proliferation rate of colon cancer cells. Our findings show that beside the well-established role as pro-myogenic transcription factor, MEF2C can also function as a regulator of cell proliferation.