Dynamic patterns of retinoic acid synthesis and response in the developing mammalian heart

The molecular mechanisms of cardiac development and related diseases

Cardiac development is a complex and intricate process involving numerous molecular signals and pathways. Researchers have explored cardiac development through a long journey, starting with early studies observing morphological changes and progressing to the exploration of molecular mechanisms using various molecular biology methods. Currently, advancements in stem cell technology and sequencing technology, such as the generation of human pluripotent stem cells and cardiac organoids, multi-omics sequencing, and artificial intelligence (AI) technology, have enabled researchers to understand the molecular mechanisms of cardiac development better. Many molecular signals regulate cardiac development, including various growth and transcription factors and signaling pathways, such as WNT signaling, retinoic acid signaling, and Notch signaling pathways. In addition, cilia, the extracellular matrix, epigenetic modifications, and hypoxia conditions also play important roles in cardiac development. These factors play crucial roles at one or even multiple stages of cardiac development. Recent studies have also identified roles for autophagy, metabolic transition, and macrophages in cardiac development. Deficiencies or abnormal expression of these factors can lead to various types of cardiac development abnormalities. Nowadays, congenital heart disease (CHD) management requires lifelong care, primarily involving surgical and pharmacological treatments. Advances in surgical techniques and the development of clinical genetic testing have enabled earlier diagnosis and treatment of CHD. However, these technologies still have significant limitations. The development of new technologies, such as sequencing and AI technologies, will help us better understand the molecular mechanisms of cardiac development and promote earlier prevention and treatment of CHD in the future.

Epigenetic determinants and non-myocardial signaling pathways contributing to heart growth and regeneration

Congenital heart disease is the most common birth defect worldwide. Defective cardiac myogenesis is either a major presentation or associated with many types of congenital heart disease. Non-myocardial tissues, including endocardium and epicardium, function as a supporting hub for myocardial growth and maturation during heart development. Recent research findings suggest an emerging role of epigenetics in nonmyocytes supporting myocardial development. Understanding how growth signaling pathways in non-myocardial tissues are regulated by epigenetic factors will likely identify new disease mechanisms for congenital heart diseases and shed lights for novel therapeutic strategies for heart regeneration.

Mitral valve transcriptome analysis in thirty-four age-matched Cavalier King Charles Spaniels with or without congestive heart failure caused by myxomatous mitral valve disease

We here report the results of a mitral valve transcriptome study designed to identify genes and molecular pathways involved in development of congestive heart failure (CHF) following myxomatous mitral valve disease (MMVD) in dogs. The study is focused on a cohort of elderly age-matched dogs (n = 34, age ~ 10 years) from a single breed-Cavalier King Charles Spaniels (CKCS)-with a high incidence of MMVD. The cohort comprises 19 dogs (10♀, 9♂) without MMVD-associated CHF, and 15 dogs (6♀, 9♂) with CHF caused by MMVD; i.e., we compare gene expression in breed and age-matched groups of dogs, which only differ with respect to CHF status. We identify 56 genes, which are differentially expressed between the two groups. In this list of genes, we confirm an enrichment of genes related to the TNFβ-signaling pathway, extracellular matrix organization, vascular development, and endothelium damage, which also have been identified in previous studies. However, the genes with the greatest difference in expression between the two groups are CNTN3 and MYH1. Both genes encode proteins, which are predicted to have an effect on the contractile activity of myocardial cells, which in turn may have an effect on valvular performance and hemodynamics across the mitral valve. This may result in shear forces with impact on MMVD progression.

Generation of left ventricle-like cardiomyocytes with improved structural, functional, and metabolic maturity from human pluripotent stem cells

Decreased left ventricle (LV) function caused by genetic mutations or injury often leads to debilitating and fatal cardiovascular disease. LV cardiomyocytes are, therefore, a potentially valuable therapeutical target. Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) are neither homogeneous nor functionally mature, which reduces their utility. Here, we exploit cardiac development knowledge to instruct differentiation of hPSCs specifically toward LV cardiomyocytes. Correct mesoderm patterning and retinoic acid pathway blocking are essential to generate near-homogenous LV-specific hPSC-CMs (hPSC-LV-CMs). These cells transit via first heart field progenitors and display typical ventricular action potentials. Importantly, hPSC-LV-CMs exhibit increased metabolism, reduced proliferation, and improved cytoarchitecture and functional maturity compared with age-matched cardiomyocytes generated using the standard WNT-ON/WNT-OFF protocol. Similarly, engineered heart tissues made from hPSC-LV-CMs are better organized, produce higher force, and beat more slowly but can be paced to physiological levels. Together, we show that functionally matured hPSC-LV-CMs can be obtained rapidly without exposure to current maturation regimes.

Genetic architecture of spatial electrical biomarkers for cardiac arrhythmia and relationship with cardiovascular disease

The 3-dimensional spatial and 2-dimensional frontal QRS-T angles are measures derived from the vectorcardiogram. They are independent risk predictors for arrhythmia, but the underlying biology is unknown. Using multi-ancestry genome-wide association studies we identify 61 (58 previously unreported) loci for the spatial QRS-T angle (N = 118,780) and 11 for the frontal QRS-T angle (N = 159,715). Seven out of the 61 spatial QRS-T angle loci have not been reported for other electrocardiographic measures. Enrichments are observed in pathways related to cardiac and vascular development, muscle contraction, and hypertrophy. Pairwise genome-wide association studies with classical ECG traits identify shared genetic influences with PR interval and QRS duration. Phenome-wide scanning indicate associations with atrial fibrillation, atrioventricular block and arterial embolism and genetically determined QRS-T angle measures are associated with fascicular and bundle branch block (and also atrioventricular block for the frontal QRS-T angle). We identify potential biology involved in the QRS-T angle and their genetic relationships with cardiovascular traits and diseases, may inform future research and risk prediction.

Human multilineage pro-epicardium/foregut organoids support the development of an epicardium/myocardium organoid

The epicardium, the outer epithelial layer that covers the myocardium, derives from a transient organ known as pro-epicardium, crucial during heart organogenesis. The pro-epicardium develops from lateral plate mesoderm progenitors, next to septum transversum mesenchyme, a structure deeply involved in liver embryogenesis. Here we describe a self-organized human multilineage organoid that recreates the co-emergence of pro-epicardium, septum transversum mesenchyme and liver bud. Additionally, we study the impact of WNT, BMP and retinoic acid signaling modulation on multilineage organoid specification. By co-culturing these organoids with cardiomyocyte aggregates, we generated a self-organized heart organoid comprising an epicardium-like layer that fully surrounds  a myocardium-like tissue. These heart organoids recapitulate the impact of epicardial cells on promoting cardiomyocyte proliferation and structural and functional maturation. Therefore, the human heart organoids described herein, open the path to advancing knowledge on how myocardium-epicardium interaction progresses during heart organogenesis in healthy or diseased settings.

Cellular and Engineered Organoids for Cardiovascular Models

An ensemble of in vitro cardiac tissue models has been developed over the past several decades to aid our understanding of complex cardiovascular disorders using a reductionist approach. These approaches often rely on recapitulating single or multiple clinically relevant end points in a dish indicative of the cardiac pathophysiology. The possibility to generate disease-relevant and patient-specific human induced pluripotent stem cells has further leveraged the utility of the cardiac models as screening tools at a large scale. To elucidate biological mechanisms in the cardiac models, it is critical to integrate physiological cues in form of biochemical, biophysical, and electromechanical stimuli to achieve desired tissue-like maturity for a robust phenotyping. Here, we review the latest advances in the directed stem cell differentiation approaches to derive a wide gamut of cardiovascular cell types, to allow customization in cardiac model systems, and to study diseased states in multiple cell types. We also highlight the recent progress in the development of several cardiovascular models, such as cardiac organoids, microtissues, engineered heart tissues, and microphysiological systems. We further expand our discussion on defining the context of use for the selection of currently available cardiac tissue models. Last, we discuss the limitations and challenges with the current state-of-the-art cardiac models and highlight future directions.

Inhibition of RhoA and Cdc42 by miR-133a Modulates Retinoic Acid Signalling during Early Development of Posterior Cardiac Tube Segment

It is well known that multiple microRNAs play crucial roles in cardiovascular development, including miR-133a. Additionally, retinoic acid regulates atrial marker expression. In order to analyse the role of miR-133a as a modulator of retinoic acid signalling during the posterior segment of heart tube formation, we performed functional experiments with miR-133a and retinoic acid by means of microinjections into the posterior cardiac precursors of both primitive endocardial tubes in chick embryos. Subsequently, we subjected embryos to whole mount in situ hybridisation, immunohistochemistry and qPCR analysis. Our results demonstrate that miR-133a represses RhoA and Cdc42, as well as Raldh2/Aldh1a2, and the specific atrial markers Tbx5 and AMHC1, which play a key role during differentiation. Furthermore, we observed that miR-133a upregulates p21 and downregulates cyclin A by repressing RhoA and Cdc42, respectively, thus functioning as a cell proliferation inhibitor. Additionally, retinoic acid represses miR-133a, while it increases Raldh2, Tbx5 and AMHC1. Given that RhoA and Cdc42 are involved in Raldh2 expression and that they are modulated by miR-133a, which is influenced by retinoic acid signalling, our results suggest the presence of a negative feedback mechanism between miR-133a and retinoic acid during early development of the posterior cardiac tube segment. Despite additional unexplored factors being possible contributors to this negative feedback mechanism, miR-133a might also be considered as a potential therapeutic tool for the diagnosis, therapy and prognosis of cardiac diseases.

Enhanced Loss of Retinoic Acid Network Genes in Xenopus laevis Achieves a Tighter Signal Regulation

Retinoic acid (RA) is a major regulatory signal during embryogenesis produced from vitamin A (retinol) by an extensive, autoregulating metabolic and signaling network to prevent fluctuations that result in developmental malformations. Xenopus laevis is an allotetraploid hybrid frog species whose genome includes L (long) and S (short) chromosomes from the originating species. Evolutionarily, the X. laevis subgenomes have been losing either L or S homoeologs in about 43% of genes to generate singletons. In the RA network, out of the 47 genes, about 47% have lost one of the homoeologs, like the genome average. Interestingly, RA metabolism genes from storage (retinyl esters) to retinaldehyde production exhibit enhanced gene loss with 75% singletons out of 28 genes. The effect of this gene loss on RA signaling autoregulation was studied. Employing transient RA manipulations, homoeolog gene pairs were identified in which one homoeolog exhibits enhanced responses or looser regulation than the other, while in other pairs both homoeologs exhibit similar RA responses. CRISPR/Cas9 targeting of individual homoeologs to reduce their activity supports the hypothesis where the RA metabolic network gene loss results in tighter network regulation and more efficient RA robustness responses to overcome complex regulation conditions.

Retinoic acid signaling in heart development: Application in the differentiation of cardiovascular lineages from human pluripotent stem cells

Retinoic acid (RA) signaling plays an important role during heart development in establishing anteroposterior polarity, formation of inflow and outflow tract progenitors, and growth of the ventricular compact wall. RA is also utilized as a key ingredient in protocols designed for generating cardiac cell types from pluripotent stem cells (PSCs). This review discusses the role of RA in cardiogenesis, currently available protocols that employ RA for differentiation of various cardiovascular lineages, and plausible transcriptional mechanisms underlying this fate specification. These insights will inform further development of desired cardiac cell types from human PSCs and their application in preclinical and clinical research.

Patterning of vertebrate cardiac progenitor fields by retinoic acid signaling

The influence of retinoic acid (RA) signaling on vertebrate development has a well-studied history. Cumulatively, we now understand that RA signaling has a conserved requirement early in development restricting cardiac progenitors within the anterior lateral plate mesoderm of vertebrate embryos. Moreover, genetic and pharmacological manipulations of RA signaling in vertebrate models have shown that proper heart development is achieved through the deployment of positive and negative feedback mechanisms, which maintain appropriate RA levels. In this brief review, we present a chronological overview of key work that has led to a current model of the critical role for early RA signaling in limiting the generation of cardiac progenitors within vertebrate embryos. Furthermore, we integrate the previous work in mice and our recent findings using zebrafish, which together show that RA signaling has remarkably conserved influences on the later-differentiating progenitor populations at the arterial and venous poles. We discuss how recognizing the significant conservation of RA signaling on the differentiation of these progenitor populations offers new perspectives and may impact future work dedicated to examining vertebrate heart development.

Retinoic Acid Fluctuation Activates an Uneven, Direction-Dependent Network-Wide Robustness Response in Early Embryogenesis

Robustness is a feature of regulatory pathways to ensure signal consistency in light of environmental changes or genetic polymorphisms. The retinoic acid (RA) pathway, is a central developmental and tissue homeostasis regulatory signal, strongly dependent on nutritional sources of retinoids and affected by environmental chemicals. This pathway is characterized by multiple proteins or enzymes capable of performing each step and their integration into a self-regulating network. We studied RA network robustness by transient physiological RA signaling disturbances followed by kinetic transcriptomic analysis of the recovery during embryogenesis. The RA metabolic network was identified as the main regulated module to achieve signaling robustness using an unbiased pattern analysis. We describe the network-wide responses to RA signal manipulation and found the feedback autoregulation to be sensitive to the direction of the RA perturbation: RA knockdown exhibited an upper response limit, whereas RA addition had a minimal feedback-activation threshold. Surprisingly, our robustness response analysis suggests that the RA metabolic network regulation exhibits a multi-objective optimization, known as Pareto optimization, characterized by trade-offs between competing functionalities. We observe that efficient robustness to increasing RA is accompanied by worsening robustness to reduced RA levels and vice versa. This direction-dependent trade-off in the network-wide feedback response, results in an uneven robustness capacity of the RA network during early embryogenesis, likely a significant contributor to the manifestation of developmental defects.

Retinoic acid signaling is directly activated in cardiomyocytes and protects mouse hearts from apoptosis after myocardial infarction

Retinoic acid (RA) is an essential signaling molecule for cardiac development and plays a protective role in the heart after myocardial infarction (MI). In both cases, the effect of RA signaling on cardiomyocytes, the principle cell type of the heart, has been reported to be indirect. Here we have developed an inducible murine transgenic RA-reporter line using CreER^T2 technology that permits lineage tracing of RA-responsive cells and faithfully recapitulates endogenous RA activity in multiple organs during embryonic development. Strikingly, we have observed a direct RA response in cardiomyocytes during mid-late gestation and after MI. Ablation of RA signaling through deletion of the Aldh1a1/a2/a3 genes encoding RA-synthesizing enzymes leads to increased cardiomyocyte apoptosis in adults subjected to MI. RNA sequencing analysis reveals Tgm2 and Ace1, two genes with well-established links to cardiac repair, as potential targets of RA signaling in primary cardiomyocytes, thereby providing novel links between the RA pathway and heart disease.

High-Throughput Drug Screening System Based on Human Induced Pluripotent Stem Cell-Derived Atrial Myocytes ∼ A Novel Platform to Detect Cardiac Toxicity for Atrial Arrhythmias

Evaluation of proarrhythmic properties is critical for drug discovery. In particular, QT prolongation in electrocardiograms has been utilized as a surrogate marker in many evaluation systems to assess the risk of torsade de pointes and lethal ventricular arrhythmia. Recently, new evaluation systems based on human iPS cell-derived cardiomyocytes have been established. On the other hand, in clinical situations, it has been reported that the incidence of atrial arrhythmias such as atrial fibrillation has been increasing every year, with the prediction of a persistent increase in the near future. As to the increased incidence of atrial arrhythmias, in addition to the increased population of geriatric patients, a wide variety of drug treatments may be related, as an experimental method to detect drug-induced atrial arrhythmia has not been established so far. In the present study, we characterized the atrial-like cardiomyocytes derived from human induced pluripotent stem cells and examined their potential for the evaluation of drug-induced atrial arrhythmia. Atrial-like cardiomyocytes were induced by adding retinoic acid (RA) during the process of myocardial differentiation, and their characteristics were compared to those of RA-free cardiomyocytes. Using gene expression and membrane potential analysis, it was confirmed that the cells with or without RA treatment have atrial or ventricular like cardiomyocytes, respectively. Using the ultra-rapid activating delayed rectifier potassium current (I_Kur) channel inhibitor, which is specific to atrial cardiomyocytes, Pulse width duration (PWD) 30cF prolongation was confirmed only in atrial-like cardiomyocytes. In addition, ventricular like cardiomyocytes exhibited an early after depolarization by treatment with rapidly activating delayed rectifier potassium current (I_Kr) channel inhibitor, which induces ventricular arrhythmia in clinical situations. Here, we have established a high-throughput drug evaluation system using human iPS cell-derived atrial-like cardiomyocytes. Based on the obtained data, the system might be a valuable platform to detect potential risks for drug-induced atrial arrhythmias.

Generation, Characterization, and Application of Inducible Proliferative Adult Human Epicardium-Derived Cells

RATIONALE

In recent decades, the great potential of human epicardium-derived cells (EPDCs) as an endogenous cell source for cardiac regeneration has been recognized. The limited availability and low proliferation capacity of primary human EPDCs and phenotypic differences between EPDCs obtained from different individuals hampers their reproducible use for experimental studies.

AIM

To generate and characterize inducible proliferative adult human EPDCs for use in fundamental and applied research.

METHODS AND RESULTS

Inducible proliferation of human EPDCs was achieved by doxycycline-controlled expression of simian virus 40 large T antigen (LT) with a repressor-based lentiviral Tet-On system. In the presence of doxycycline, these inducible EPDCs (iEPDCs) displayed high and long-term proliferation capacity. After doxycycline removal, LT expression ceased and the iEPDCs regained their cuboidal epithelial morphology. Similar to primary EPDCs, iEPDCs underwent an epithelial-to-mesenchymal transition (EMT) after stimulation with transforming growth factor β3. This was confirmed by reverse transcription-quantitative polymerase chain reaction analysis of epithelial and mesenchymal marker gene expression and (immuno) cytochemical staining. Collagen gel-based cell invasion assays demonstrated that mesenchymal iEPDCs, like primary EPDCs, possess increased invasion and migration capacities as compared to their epithelial counterparts. Mesenchymal iEPDCs co-cultured with sympathetic ganglia stimulated neurite outgrowth similarly to primary EPDCs.

CONCLUSION

Using an inducible LT expression system, inducible proliferative adult human EPDCs were generated displaying high proliferative capacity in the presence of doxycycline. These iEPDCs maintain essential epicardial characteristics with respect to morphology, EMT ability, and paracrine signaling following doxycycline removal. This renders iEPDCs a highly useful new in vitro model for studying human epicardial properties.

Every Beat You Take-The Wilms' Tumor Suppressor WT1 and the Heart

Nearly three decades ago, the Wilms’ tumor suppressor Wt1 was identified as a crucial regulator of heart development. Wt1 is a zinc finger transcription factor with multiple biological functions, implicated in the development of several organ systems, among them cardiovascular structures. This review summarizes the results from many research groups which allowed to establish a relevant function for Wt1 in cardiac development and disease. During development, Wt1 is involved in fundamental processes as the formation of the epicardium, epicardial epithelial-mesenchymal transition, coronary vessel development, valve formation, organization of the cardiac autonomous nervous system, and formation of the cardiac ventricles. Wt1 is further implicated in cardiac disease and repair in adult life. We summarize here the current knowledge about expression and function of Wt1 in heart development and disease and point out controversies to further stimulate additional research in the areas of cardiac development and pathophysiology. As re-activation of developmental programs is considered as paradigm for regeneration in response to injury, understanding of these processes and the molecules involved therein is essential for the development of therapeutic strategies, which we discuss on the example of WT1.

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.

Changes in retinoid metabolism and signaling associated with metabolic remodeling during fasting and in type I diabetes

Liver is the central metabolic hub that coordinates carbohydrate and lipid metabolism. The bioactive derivative of vitamin A, retinoic acid (RA), was shown to regulate major metabolic genes including phosphoenolpyruvate carboxykinase, fatty acid synthase, carnitine palmitoyltransferase 1, and glucokinase among others. Expression levels of these genes undergo profound changes during adaptation to fasting or in metabolic diseases such as type 1 diabetes (T1D). However, it is unknown whether the levels of hepatic RA change during metabolic remodeling. This study investigated the dynamics of hepatic retinoid metabolism and signaling in the fed state, in fasting, and in T1D. Our results show that fed-to-fasted transition is associated with significant decrease in hepatic retinol dehydrogenase (RDH) activity, the rate-limiting step in RA biosynthesis, and downregulation of RA signaling. The decrease in RDH activity correlates with the decreased abundance and altered subcellular distribution of RDH10 while Rdh10 transcript levels remain unchanged. In contrast to fasting, untreated T1D is associated with upregulation of RA signaling and an increase in hepatic RDH activity, which correlates with the increased abundance of RDH10 in microsomal membranes. The dynamic changes in RDH10 protein levels in the absence of changes in its transcript levels imply the existence of posttranscriptional regulation of RDH10 protein. Together, these data suggest that the downregulation of hepatic RA biosynthesis, in part via the decrease in RDH10, is an integral component of adaptation to fasting. In contrast, the upregulation of hepatic RA biosynthesis and signaling in T1D might contribute to metabolic inflexibility associated with this disease.

Specific induction and long-term maintenance of high purity ventricular cardiomyocytes from human induced pluripotent stem cells

Currently, cardiomyocyte (CM) differentiation methods require a purification step after CM induction to ensure the high purity of the cell population. Here we show an improved human CM differentiation protocol with which high-purity ventricular-type CMs can be obtained and maintained without any CM purification process. We induced and collected a mesodermal cell population (platelet-derived growth factor receptor-α (PDGFRα)-positive cells) that can respond to CM differentiation cues, and then stimulated CM differentiation by means of Wnt inhibition. This method reproducibly generated CMs with purities above 95% in several human pluripotent stem cell lines. Furthermore, these CM populations were maintained in culture at such high purity without any further CM purification step for over 200 days. The majority of these CMs (>95%) exhibited a ventricular-like phenotype with a tendency to structural and electrophysiological maturation, including T-tubule-like structure formation and the ability to respond to QT prolongation drugs. This is a simple and valuable method to stably generate CM populations suitable for cardiac toxicology testing, disease modeling and regenerative medicine.

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'.

Human Pluripotent Stem Cell-Derived Cardiovascular Cells: From Developmental Biology to Therapeutic Applications

Advances in our understanding of cardiovascular development have provided a roadmap for the directed differentiation of human pluripotent stem cells (hPSCs) to the major cell types found in the heart. In this Perspective, we review the state of the field in generating and maturing cardiovascular cells from hPSCs based on our fundamental understanding of heart development. We then highlight their applications for studying human heart development, modeling disease-performing drug screening, and cell replacement therapy. With the advancements highlighted here, the promise that hPSCs will deliver new treatments for degenerative and debilitating diseases may soon be fulfilled.

Drug screening platform using human induced pluripotent stem cell-derived atrial cardiomyocytes and optical mapping

Current drug development efforts for the treatment of atrial fibrillation are hampered by the fact that many preclinical models have been unsuccessful in reproducing human cardiac physiology and its response to medications. In this study, we demonstrated an approach using human induced pluripotent stem cell-derived atrial and ventricular cardiomyocytes (hiPSC-aCMs and hiPSC-vCMs, respectively) coupled with a sophisticated optical mapping system for drug screening of atrial-selective compounds in vitro. We optimized differentiation of hiPSC-aCMs by modulating the WNT and retinoid signaling pathways. Characterization of the transcriptome and proteome revealed that retinoic acid pushes the differentiation process into the atrial lineage and generated hiPSC-aCMs. Functional characterization using optical mapping showed that hiPSC-aCMs have shorter action potential durations and faster Ca²⁺ handling dynamics compared with hiPSC-vCMs. Furthermore, pharmacological investigation of hiPSC-aCMs captured atrial-selective effects by displaying greater sensitivity to atrial-selective compounds 4-aminopyridine, AVE0118, UCL1684, and vernakalant when compared with hiPSC-vCMs. These results established that a model system incorporating hiPSC-aCMs combined with optical mapping is well-suited for preclinical drug screening of novel and targeted atrial selective compounds.

A Spatiotemporal Organ-Wide Gene Expression and Cell Atlas of the Developing Human Heart

The process of cardiac morphogenesis in humans is incompletely understood. Its full characterization requires a deep exploration of the organ-wide orchestration of gene expression with a single-cell spatial resolution. Here, we present a molecular approach that reveals the comprehensive transcriptional landscape of cell types populating the embryonic heart at three developmental stages and that maps cell-type-specific gene expression to specific anatomical domains. Spatial transcriptomics identified unique gene profiles that correspond to distinct anatomical regions in each developmental stage. Human embryonic cardiac cell types identified by single-cell RNA sequencing confirmed and enriched the spatial annotation of embryonic cardiac gene expression. In situ sequencing was then used to refine these results and create a spatial subcellular map for the three developmental phases. Finally, we generated a publicly available web resource of the human developing heart to facilitate future studies on human cardiogenesis.

How Dietary Deficiency Studies Have Illuminated the Many Roles of Vitamin A During Development and Postnatal Life

Vitamin A deficiency studies have been carried out since the early 1900s. Initially, these studies led to the identification of fat soluble A as a unique and essential component of the diet of rodents, birds, and humans. Continuing work established that vitamin A deficiency produces biochemical and physiological dysfunction in almost every vertebrate organ system from conception to death. This chapter begins with a review of representative historical and current studies that used the nutritional vitamin A deficiency research model to gain an understanding of the many roles vitamin A plays in prenatal and postnatal development and well-being. This is followed by a discussion of recent studies that show specific effects of vitamin A deficiency on prenatal development and postnatal maintenance of the olfactory epithelium, brain, and heart. Vitamin A deficiency studies have helped define the necessity of vitamin A for the health of all vertebrates, including farm animals, but the breadth of deficient states and their individual effects on health have not been fully determined. Future work is needed to develop tools to assess the complete vitamin A status of an organism and to define the levels of vitamin A that optimally support molecular and systems level processes during all ages and stages of life.

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.

The Role of the Epicardium During Heart Development and Repair

The heart is lined by a single layer of mesothelial cells called the epicardium that provides important cellular contributions for embryonic heart formation. The epicardium harbors a population of progenitor cells that undergo epithelial-to-mesenchymal transition displaying characteristic conversion of planar epithelial cells into multipolar and invasive mesenchymal cells before differentiating into nonmyocyte cardiac lineages, such as vascular smooth muscle cells, pericytes, and fibroblasts. The epicardium is also a source of paracrine cues that are essential for fetal cardiac growth, coronary vessel patterning, and regenerative heart repair. Although the epicardium becomes dormant after birth, cardiac injury reactivates developmental gene programs that stimulate epithelial-to-mesenchymal transition; however, it is not clear how the epicardium contributes to disease progression or repair in the adult. In this review, we will summarize the molecular mechanisms that control epicardium-derived progenitor cell migration, and the functional contributions of the epicardium to heart formation and cardiomyopathy. Future perspectives will be presented to highlight emerging therapeutic strategies aimed at harnessing the regenerative potential of the fetal epicardium for cardiac repair.

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.

Crataegus Extract WS®1442 Stimulates Cardiomyogenesis and Angiogenesis From Stem Cells: A Possible New Pharmacology for Hawthorn?

Extracts from the leaves and flowers of Crataegus spp. (i.e., hawthorn species) have been traditionally used with documented preclinical and clinical activities in cardiovascular medicine. Based on reported positive effects on heart muscle after ischemic injury and the overall cardioprotective profile, the present study addressed potential contributions of Crataegus extracts to cardiopoietic differentiation from stem cells. The quantified Crataegus extract WS^®1442 stimulated cardiomyogenesis from murine and human embryonic stem cells (ESCs). Mechanistically, this effect was found to be induced by promoting differentiation of cardiovascular progenitor cell populations but not by proliferation. Bioassay-guided fractionation, phytochemical and analytical profiling suggested high-molecular weight ingredients as the active principle with at least part of the activity due to oligomeric procyanidines (OPCs) with a degree of polymerization between 3 and 6 (DP3–6). Transcriptome profiling in mESCs suggested two main, plausible mechanisms: These were early, stress-associated cellular events along with the modulation of distinct developmental pathways, including the upregulation of brain-derived neurotrophic factor (BDNF) and retinoic acid as well as the inhibition of transforming growth factor β/bone morphogenetic protein (TGFβ/BMP) and fibroblast growth factor (FGF) signaling. In addition, WS^®1442 stimulated angiogenesis ex vivo in Sca-1⁺ progenitor cells from adult mice hearts. These in vitro data provide evidence for a differentiation promoting activity of WS^®1442 on distinct cardiovascular stem/progenitor cells that could be valuable for therapeutic heart regeneration after myocardial infarction. However, the in vivo relevance of this new pharmacological activity of Crataegus spp. remains to be investigated and active ingredients from bioactive fractions will have to be further characterized.

Modulation of retinoid signaling: therapeutic opportunities in organ fibrosis and repair

The vitamin A metabolite, retinoic acid, is an important signaling molecule during embryonic development serving critical roles in morphogenesis, organ patterning and skeletal and neural development. Retinoic acid is also important in postnatal life in the maintenance of tissue homeostasis, while retinoid-based therapies have long been used in the treatment of a variety of cancers and skin disorders. As the number of people living with chronic disorders continues to increase, there is great interest in extending the use of retinoid therapies in promoting the maintenance and repair of adult tissues. However, there are still many conflicting results as we struggle to understand the role of retinoic acid in the multitude of processes that contribute to tissue injury and repair. This review will assess our current knowledge of the role retinoic acid signaling in the development of fibroblasts, and their transformation to myofibroblasts, and of the potential use of retinoid therapies in the treatment of organ fibrosis.

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.

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.

T-box genes and retinoic acid signaling regulate the segregation of arterial and venous pole progenitor cells in the murine second heart field

The arterial and venous poles of the mammalian heart are hotspots of congenital heart defects (CHD) such as those observed in 22q11.2 deletion (or DiGeorge) and Holt-Oram syndromes. These regions of the heart are derived from late differentiating cardiac progenitor cells of the Second Heart Field (SHF) located in pharyngeal mesoderm contiguous with the elongating heart tube. The T-box transcription factor Tbx1, encoded by the major 22q11.2 deletion syndrome gene, regulates SHF addition to both cardiac poles from a common progenitor population. Despite the significance of this cellular addition the mechanisms regulating the deployment of common progenitor cells to alternate cardiac poles remain poorly understood. Here we demonstrate that Tbx5, mutated in Holt-Oram syndrome and essential for venous pole development, is activated in Tbx1 expressing cells in the posterior region of the SHF at early stages of heart tube elongation. A subset of the SHF transcriptional program, including Tbx1 expression, is subsequently downregulated in Tbx5 expressing cells, generating a transcriptional boundary between Tbx1-positive arterial pole and Tbx5-positive venous pole progenitor cell populations. We show that normal downregulation of the definitive arterial pole progenitor cell program in the posterior SHF is dependent on both Tbx1 and Tbx5. Furthermore, retinoic acid (RA) signaling is required for Tbx5 activation in Tbx1-positive cells and blocking RA signaling at the time of Tbx5 activation results in atrioventricular septal defects at fetal stages. Our results reveal sequential steps of cardiac progenitor cell patterning and provide mechanistic insights into the origin of common forms of CHD.

Hindbrain induction and patterning during early vertebrate development

The hindbrain is a key relay hub of the central nervous system (CNS), linking the bilaterally symmetric half-sides of lower and upper CNS centers via an extensive network of neural pathways. Dedicated neural assemblies within the hindbrain control many physiological processes, including respiration, blood pressure, motor coordination and different sensations. During early development, the hindbrain forms metameric segmented units known as rhombomeres along the antero-posterior (AP) axis of the nervous system. These compartmentalized units are highly conserved during vertebrate evolution and act as the template for adult brainstem structure and function. TALE and HOX homeodomain family transcription factors play a key role in the initial induction of the hindbrain and its specification into rhombomeric cell fate identities along the AP axis. Signaling pathways, such as canonical-Wnt, FGF and retinoic acid, play multiple roles to initially induce the hindbrain and regulate Hox gene-family expression to control rhombomeric identity. Additional transcription factors including Krox20, Kreisler and others act both upstream and downstream to Hox genes, modulating their expression and protein activity. In this review, we will examine the earliest embryonic signaling pathways that induce the hindbrain and subsequent rhombomeric segmentation via Hox and other gene expression. We will examine how these signaling pathways and transcription factors interact to activate downstream targets that organize the segmented AP pattern of the embryonic vertebrate hindbrain.

Alterations in retinoic acid signaling affect the development of the mouse coronary vasculature

BACKGROUND

During the final stages of heart development the myocardium grows and becomes vascularized by means of paracrine factors and cell progenitors derived from the epicardium. There is evidence to suggest that retinoic acid (RA), a metabolite of vitamin A, plays an important role in epicardial-based developmental programming. However, the consequences of altered RA-signaling in coronary development have not been systematically investigated.

RESULTS

We explored the developmental consequences of altered RA-signaling in late cardiogenic events that involve the epicardium. For this, we used a model of embryonic RA excess based on mouse embryos deficient in the retinaldehyde reductase DHRS3, and a complementary model of embryonic RA deficiency based on pharmacological inhibition of RA synthesis. We found that alterations in embryonic RA signaling led to a thin myocardium and aberrant coronary vessel formation and remodeling. Both excess, and deficient RA-signaling are associated with reductions in ventricular coverage and density of coronary vessels, altered vessel morphology, and impaired recruitment of epicardial-derived mural cells. Using a combined transcriptome and proteome profiling approach, we found that RA treatment of epicardial cells influenced key signaling pathways relevant for cardiac development.

CONCLUSIONS

Epicardial RA-signaling plays critical roles in the development of the coronary vasculature needed to support myocardial growth. Developmental Dynamics 247:976-991, 2018. © 2018 Wiley Periodicals, Inc.

Identifying SNPs and candidate genes for three litter traits using single-step GWAS across six parities in Landrace and Large White pigs

Total number born (TNB), number born alive (NBA), and litter weight born alive (LWB) are critically important traits in pig production. The sow's parity is one of the major factors influencing litter traits. Because of monogenic or polygenic contributions and the presence of temporal gene effects in different sows' parities, it is difficult to clarify the biological and genetic background. To systematically explore the genetic mechanism of litter traits, we conducted 18 GWASs using single-step GWAS (ssGWAS) based on two breeds (908 Landrace and 1,130 Large White sow litter records) for each litter trait in different parities. A total of 300 Landrace and 300 Large White sows were genotyped by sequencing (GBS). ssGWAS was performed separately for each breed and each parity due to population stratification and temporal gene effect. In summary, we identified 80 (15 for Landrace and 65 for Large White), 227 (52 for Landrace, 175 for Large White), and 187 (34 for Landrace, 153 for Large White) single nucleotide polymorphisms (SNPs) affecting TNB, NBA, and LWB, respectively. Of them, we suggest that a total of 22 loci (SSC1: 125098202, SSC1: 117560058, SSC14: 147794697, SSC8: 84823302, SSC9: 143554876, and SSC9: 138766097 for Landrace; SSC1: 4023577, SSC1: 3859573, SSC1: 4891063, SSC16: 5197665, SSC10: 32050819, SSC13: 13552924, SSC13: 92819, SSC17: 3579607, SSC13: 196698221, SSC7: 30918403, SSC16: 46221484, SSC16: 46169204, SSC2: 41988642, SSC2: 44475457, SSC2: 42521875, and SSC7: 58411951 for Large White) are shared by TNB, NBA, and LWB. These results indicate the existence of gene temporal effect in each parity. Furthermore, our findings suggest four interesting candidate genes (FBXL7, ALDH1A2, LEPR, and DDX1) associated with litter traits in different parities that have a major effect on embryonic development progression. In conclusion, 22 crucial SNPs and four interesting candidate genes were identified for three litter traits across six parities. These findings advance our understanding of the genetic architecture of litter traits and confirm the presence of temporal gene effects in different parities. Importantly, functional validation studies for findings of particular interest are recommended in litter traits.

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.

Retinoic acid signaling promotes the cytoskeletal rearrangement of embryonic epicardial cells

All- trans-retinoic acid (RA), a vitamin A metabolite, is an important signaling molecule required for the proper development of the heart. The epicardium is the main source of RA in the embryonic heart, yet the cardiogenic functions of epicardial-produced RA are not fully understood. Here, we investigated the roles of RA signaling in the embryonic epicardium using in vivo and in vitro models of excess or deficiency of RA. Our results suggested that RA signaling facilitates the cytoskeletal rearrangement required for the epicardial-to-mesenchymal transition of epicardial cells. In vivo treatment with an inhibitor of RA synthesis delayed the migration of epicardial-derived precursor cells (EPDCs) into the myocardium; the opposite was seen in the case of dehydrogenase/reductase superfamily (DHRS)3-deficient embryos, a mouse model of RA excess. Analysis of the behavior of epicardial cells exposed to RA receptor agonists or inhibitors of RA synthesis in vitro revealed that appropriate levels of RA are important in orchestrating the platelet-derived growth factor-induced loss of epithelial character, cytoskeletal remodeling, and migration, necessary for the infiltration of the myocardium by EPDCs. To understand the molecular mechanisms by which RA regulates epicardial cytoskeletal rearrangement, we used a whole transcriptome profiling approach, which in combination with pull-down and inhibition assays, demonstrated that the Ras homolog gene family, member A (RhoA) pathway is required for the morphologic changes induced by RA in epicardial cells. Collectively, these data demonstrate that RA regulates the cytoskeletal rearrangement of epicardial cells via a signaling cascade that involves the RhoA pathway.-Wang, S., Yu, J., Jones, J. W., Pierzchalski, K., Kane, M. A., Trainor, P. A., Xavier-Neto, J., Moise, A. R. Retinoic acid signaling promotes the cytoskeletal rearrangement of embryonic epicardial cells.

CDC42 is required for epicardial and pro-epicardial development by mediating FGF receptor trafficking to the plasma membrane

The epicardium contributes to multiple cardiac lineages and is essential for cardiac development and regeneration. However, the mechanism of epicardium formation is unclear. This study aimed to establish the cellular and molecular mechanisms underlying the dissociation of pro-epicardial cells (PECs) from the pro-epicardium (PE) and their subsequent translocation to the heart to form the epicardium. We used lineage tracing, conditional deletion, mosaic analysis and ligand stimulation in mice to determine that both villous protrusions and floating cysts contribute to PEC translocation to myocardium in a CDC42-dependent manner. We resolved a controversy by demonstrating that physical contact of the PE with the myocardium constitutes a third mechanism for PEC translocation to myocardium, and observed a fourth mechanism in which PECs migrate along the surface of the inflow tract to reach the ventricles. Epicardial-specific Cdc42 deletion disrupted epicardium formation, and Cdc42 null PECs proliferated less, lost polarity and failed to form villous protrusions and floating cysts. FGF signaling promotes epicardium formation in vivo, and biochemical studies demonstrated that CDC42 is involved in the trafficking of FGF receptors to the cell membrane to regulate epicardium formation.

Highlighted article: During epicardial formation in mice, four different mechanisms of pro-epicardial cell translocation to the myocardium can be identified, with CDC42 playing a key role.

The rs4238326 polymorphism in ALDH1A2 gene potentially associated with non-post traumatic knee osteoarthritis susceptibility: a two-stage population-based study

OBJECTIVE

A recent genome-wide association study reported significant associations of genetic variants within the ALDH1A2 gene with osteoarthritis (OA) of the hand in European populations. However, these findings have not been well generalized to other joints, or to other populations.

METHODS

We performed a two-stage population-based case-control study including 196 non-post traumatic knee OA cases and 442 controls in the first stage and independent 143 non-post traumatic knee OA cases and 238 controls in the second stage in a Chinese population by genotyping eight tagging polymorphisms in ALDH1A2.

RESULTS

In the first stage, the single nucleotide polymorphism (SNP) rs4238326 was found to be potentially associated with knee OA risk (additive model: odds ratio [OR] = 0.70; 95% confidence interval [95% CI] = 0.49-1.01; P = 0.055), which was further confirmed in the second stage with similar effect (additive model: OR = 0.60; 95% CI = 0.38-0.95; P = 0.029). After combining the two stages, we found that the variant C allele of rs4238326 was probably associated with decreased risk of knee OA (additive model: OR = 0.65; 95% CI = 0.49-0.86; P = 0.003). Furthermore, interaction analyses showed that rs4238326 interacted multiplicatively with age to contribute to knee OA risk (interaction P = 0.041).

CONCLUSIONS

These findings indicate that the SNP rs4238326 in ALDH1A2 gene may potentially modify individual susceptibility to knee OA in the Chinese population. Beyond that, further studies are warranted to validate and extend our findings, and future functional studies are required to clarify the possible mechanisms.

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.

Enzymatic Metabolism of Vitamin A in Developing Vertebrate Embryos

Embryonic development is orchestrated by a small number of signaling pathways, one of which is the retinoic acid (RA) signaling pathway. Vitamin A is essential for vertebrate embryonic development because it is the molecular precursor of the essential signaling molecule RA. The level and distribution of RA signaling within a developing embryo must be tightly regulated; too much, or too little, or abnormal distribution, all disrupt embryonic development. Precise regulation of RA signaling during embryogenesis is achieved by proteins involved in vitamin A metabolism, retinoid transport, nuclear signaling, and RA catabolism. The reversible first step in conversion of the precursor vitamin A to the active retinoid RA is mediated by retinol dehydrogenase 10 (RDH10) and dehydrogenase/reductase (SDR family) member 3 (DHRS3), two related membrane-bound proteins that functionally activate each other to mediate the interconversion of retinol and retinal. Alcohol dehydrogenase (ADH) enzymes do not contribute to RA production under normal conditions during embryogenesis. Genes involved in vitamin A metabolism and RA catabolism are expressed in tissue-specific patterns and are subject to feedback regulation. Mutations in genes encoding these proteins disrupt morphogenesis of many systems in a developing embryo. Together these observations demonstrate the importance of vitamin A metabolism in regulating RA signaling during embryonic development in vertebrates.

Probing early heart development to instruct stem cell differentiation strategies

Scientists have studied organs and their development for centuries and, along that path, described models and mechanisms explaining the developmental principles of organogenesis. In particular, with respect to the heart, new fundamental discoveries are reported continuously that keep changing the way we think about early cardiac development. These discoveries are driven by the need to answer long-standing questions regarding the origin of the earliest cells specified to the cardiac lineage, the differentiation potential of distinct cardiac progenitor cells, and, very importantly, the molecular mechanisms underlying these specification events. As evidenced by numerous examples, the wealth of developmental knowledge collected over the years has had an invaluable impact on establishing efficient strategies to generate cardiovascular cell types ex vivo, from either pluripotent stem cells or via direct reprogramming approaches. The ability to generate functional cardiovascular cells in an efficient and reliable manner will contribute to therapeutic strategies aimed at alleviating the increasing burden of cardiovascular disease and morbidity. Here we will discuss the recent discoveries in the field of cardiac progenitor biology and their translation to the pluripotent stem cell model to illustrate how developmental concepts have instructed regenerative model systems in the past and promise to do so in the future. Developmental Dynamics 245:1130-1144, 2016. © 2016 Wiley Periodicals, Inc.

The proepicardium keeps a potential for glomerular marker expression which supports its evolutionary origin from the pronephros

The proepicardium is the embryonic primordium of the epicardium. This transient structure is essential for cardiac development giving rise to the epicardium and supplying the heart with vascular and cardiac connective tissue progenitors. However, their nature and evolutionary origin are poorly-known. We have suggested elsewhere (Pombal et al. Evol. Dev. 10: 210-216, 2008; Cano et al., J. Dev. Biol. 1: 3-19, 2013) that the proepicardium is an evolutionary derivative of the primordium of an ancient external pronephric glomerulus, devoid of its original excretory function. In this study, we describe for the first time expression of two podocyte markers in the chick proepicardium (glepp1 and synaptopodin) and we have shown how these podocyte markers as well as the intermediate mesoderm marker Pax2 are strongly upregulated when the proepicardium is cultured with nephrogenic inducers. Retinoic acid treatment also induced in the proepicardium expression of Hoxb4, a gene which confers to intermediate mesoderm competence to respond to nephrogenic signals. Thus, a latent nephrogenic potential persists in the proepicardium and also that its original glomerular fate can be partially rescued. The transcription factor Wt1, essential for kidney and epicardial development, plays opposite roles in both tissues, inducing epithelial-mesenchymal transition in the proepicardium and promoting epithelialization in the kidneys (Essafi et al., Dev. Cell 21: 559-574, 2011). Consistently with this antithetical function of Wt1, we have observed an upregulation of podocalyxin in the epicardium of mouse embryos with conditional deletion of the Wt1 gene, while this protein is transcriptionally activated by Wt1 in podocytes.

Transcriptional control of cardiac fibroblast plasticity

Cardiac fibroblasts help maintain the normal architecture of the healthy heart and are responsible for scar formation and the healing response to pathological insults. Various genetic, biomechanical, or humoral factors stimulate fibroblasts to become contractile smooth muscle-like cells called myofibroblasts that secrete large amounts of extracellular matrix. Unfortunately, unchecked myofibroblast activation in heart disease leads to pathological fibrosis, which is a major risk factor for the development of cardiac arrhythmias and heart failure. A better understanding of the molecular mechanisms that control fibroblast plasticity and myofibroblast activation is essential to develop novel strategies to specifically target pathological cardiac fibrosis without disrupting the adaptive healing response. This review highlights the major transcriptional mediators of fibroblast origin and function in development and disease. The contribution of the fetal epicardial gene program will be discussed in the context of fibroblast origin in development and following injury, primarily focusing on Tcf21 and C/EBP. We will also highlight the major transcriptional regulatory axes that control fibroblast plasticity in the adult heart, including transforming growth factor β (TGFβ)/Smad signaling, the Rho/myocardin-related transcription factor (MRTF)/serum response factor (SRF) axis, and Calcineurin/transient receptor potential channel (TRP)/nuclear factor of activated T-Cell (NFAT) signaling. Finally, we will discuss recent strategies to divert the fibroblast transcriptional program in an effort to promote cardiomyocyte regeneration. This article is a part of a Special Issue entitled "Fibrosis and Myocardial Remodeling".

Cardiac outflow morphogenesis depends on effects of retinoic acid signaling on multiple cell lineages

BACKGROUND

Retinoic acid (RA), the bioactive derivative of vitamin A, is essential for vertebrate heart development. Both excess and reduced RA signaling lead to cardiovascular malformations affecting the outflow tract (OFT). To address the cellular mechanisms underlying the effects of RA signaling during OFT morphogenesis, we used transient maternal RA supplementation to rescue the early lethality resulting from inactivation of the murine retinaldehyde dehydrogenase 2 (Raldh2) gene.

RESULTS

By embryonic day 13.5, all rescued Raldh2(-/-) hearts exhibit severe, reproducible OFT septation defects, although wild-type and Raldh2(+/-) littermates have normal hearts. Cardiac neural crest cells (cNCC) were present in OFT cushions of Raldh2(-/-) mutant embryos but ectopically located in the periphery of the endocardial cushions, rather than immediately underlying the endocardium. Excess mesenchyme was generated by Raldh2(-/-) mutant endocardium, which displaced cNCC derivatives from their subendocardial, medial position.

CONCLUSIONS

RA signaling affects not only cNCC numbers but also their position relative to endocardial mesenchyme during the septation process. Our study shows that inappropriate coordination between the different cell types of the OFT perturbs its morphogenesis and leads to a severe congenital heart defect, persistent truncus arteriosus.

An in vitro gastrulation model recapitulates the morphogenetic impact of pharmacological inhibitors of developmental signaling pathways

Certain chemical agents act as teratogens, causing birth defects and fetal deaths when pregnant women are exposed to them. The establishment of in vitro models that recapitulate crucial embryonic events is therefore vital to facilitate screening of potential teratogens. Previously, we created a three-dimensional culture method for mouse P19C5 embryonal carcinoma stem cells that, when cultured as embryoid bodies, display elongation morphogenesis resembling gastrulation, which is the critical event resulting in the germ layers and major body axes. Determination of how well this in vitro morphogenesis represents in vivo gastrulation is essential to assess its applicability as well as to identify limitations of the model for detecting teratogenic agents. Here, we investigated the morphological and molecular characteristics of P19C5 morphogenesis using pharmacological agents that are known to cause abnormal patterning in the embryo in vivo by inhibiting major developmental signaling--e.g., involving Wnt, Nodal, Bone morphogenic protein (Bmp), Fibroblast growth factor (Fgf), Retinoic acid, Notch, and Hedgehog pathways. Inhibitors of Wnt, Nodal, Bmp, Fgf, and Retinoic acid signaling caused distinct changes in P19C5 morphogenesis that were quantifiable using morphometric parameters. These five inhibitors, plus the Notch inhibitor, also altered temporal expression profiles of developmental regulator genes in a manner consistent with the in vivo roles of the corresponding signaling pathways. In contrast, the Hedgehog inhibitor did not have any impact on the process, suggesting an absence of active Hedgehog signaling in these embryoid bodies. These results indicate that the P19C5 in vitro gastrulation model is a promising tool to screen for teratogenic agents that interfere with many of the key developmental signals.

Human epicardial cell-conditioned medium contains HGF/IgG complexes that phosphorylate RYK and protect against vascular injury

AIMS

The aim of this study was to evaluate the paracrine activity of human epicardial-derived cells (hEPDCs) to screen for secreted vasoprotective factors and develop therapeutics to treat vascular reperfusion injury.

METHODS AND RESULTS

Epicardial cells support cardiac development, repair, and remodelling after injury in part, through paracrine activity. We hypothesized that secreted ligands from hEPDCs would protect vascular integrity after myocardial infarction (MI) with reperfusion. During simulated ischaemia in culture (24-48 h), concentrated hEPDC-conditioned medium (EPI CdM) increased survival of primary cardiac endothelial cells. In a rat MI model, EPI CdM treatment reduced vascular injury in vivo after reperfusion. By phospho-receptor tyrosine kinase (RTK) arrays, ELISA, and neutralizing antibody screens, we identified hepatocyte growth factor (HGF) as a key vasoprotective factor in EPI CdM. Unexpectedly, we observed that some of the HGF in EPI CdM formed complexes with polyclonal IgG. Following reperfusion, preparations of HGF/IgG complexes provided greater vascular protection than free HGF with IgG. HGF/IgG complexes localized to blood vessels in vivo and increased HGF retention time after administration. In subsequent screens, we found that 'related to tyrosine kinase' (RYK) receptor was phosphorylated after exposure of cardiac endothelial cells to HGF/IgG complexes, but not to free HGF with IgG. The enhanced protection conferred by HGF/IgG complexes was lost after antibody blockade of RYK. Notably, the HGF/IgG complex is the first 'ligand' shown to promote phosphorylation of RYK.

CONCLUSION

Early treatment with HGF/IgG complexes after myocardial ischaemia with reperfusion may rescue tissue through vasoprotection conferred by c-Met and RYK signalling.

Pregestational diabetes induces fetal coronary artery malformation via reactive oxygen species signaling

Hypoplastic coronary artery disease is a congenital coronary artery malformation associated with a high risk of sudden cardiac death. However, the etiology and pathogenesis of hypoplastic coronary artery disease remain undefined. Pregestational diabetes increases reactive oxygen species (ROS) levels and the risk of congenital heart defects. We show that pregestational diabetes in mice induced by streptozotocin significantly increased 4-hydroxynonenal production and decreased coronary artery volume in fetal hearts. Pregestational diabetes also impaired epicardial epithelial-to-mesenchymal transition (EMT) as shown by analyses of the epicardium, epicardial-derived cells, and fate mapping. Additionally, the expression of hypoxia-inducible factor 1α (Hif-1α), Snail1, Slug, basic fibroblast growth factor (bFgf), and retinaldehyde dehydrogenase (Aldh1a2) was decreased and E-cadherin expression was increased in the hearts of fetuses of diabetic mothers. Of note, these abnormalities were all rescued by treatment with N-acetylcysteine (NAC) in diabetic females during gestation. Ex vivo analysis showed that high glucose levels inhibited epicardial EMT, which was reversed by NAC treatment. We conclude that pregestational diabetes in mice can cause coronary artery malformation through ROS signaling. This study may provide a rationale for further clinical studies to investigate whether pregestational diabetes could cause hypoplastic coronary artery disease in humans.