MicroRNA-processing enzyme Dicer is required in epicardium for coronary vasculature development

miR-194-3p regulates epithelial-mesenchymal transition in embryonic epicardial cells via p120/β-catenin signaling

The epicardium is integral to cardiac development and facilitates endogenous heart regeneration and repair. While miR-194-3p is associated with cellular migration and invasion, its impact on epicardial cells remains uncharted. In this work we use gain-of-function and loss-of-function methodologies to investigate the function of miR-194-3p in cardiac development. We culture embryonic epicardial cells in vitro and subject them to transforming growth factor β (TGF-β) treatment to induce epithelial-mesenchymal transition (EMT) and monitor miR-194-3p expression. In addition, the effects of miR-194-3p mimics and inhibitors on epicardial cell development and changes in EMT are investigated. To validate the binding targets of miR-194-3p and its ability to recover the target gene-phenotype, we produce a mutant vector p120-catenin-3'UTR-MUT. In epicardial cells, TGF-β-induced EMT results in a notable overexpression of miR-194-3p. The administration of miR-194-3p mimics promotes EMT, which is correlated with elevated levels of mesenchymal markers. Conversely, miR-194-3p inhibitor attenuates EMT. Further investigations reveal a negative correlation between miR-194-3p and p120-catenin, which influences β-catenin level in the cell adhesion pathway. The suppression of EMT caused by the miR-194-3p inhibitor is balanced by silencing of p120-catenin. In conclusion, miR-194-3p directly targets p120-catenin and modulates its expression, which in turn alters β-catenin expression, critically influencing the EMT process in the embryonic epicardial cells via the cell adhesion mechanism.

miRNAs in Heart Development and Disease

Cardiovascular diseases (CVD) are a group of disorders that affect the heart and blood vessels. They include conditions such as myocardial infarction, coronary artery disease, heart failure, arrhythmia, and congenital heart defects. CVDs are the leading cause of death worldwide. Therefore, new medical interventions that aim to prevent, treat, or manage CVDs are of prime importance. MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression at the posttranscriptional level and play important roles in various biological processes, including cardiac development, function, and disease. Moreover, miRNAs can also act as biomarkers and therapeutic targets. In order to identify and characterize miRNAs and their target genes, scientists take advantage of computational tools such as bioinformatic algorithms, which can also assist in analyzing miRNA expression profiles, functions, and interactions in different cardiac conditions. Indeed, the combination of miRNA research and bioinformatic algorithms has opened new avenues for understanding and treating CVDs. In this review, we summarize the current knowledge on the roles of miRNAs in cardiac development and CVDs, discuss the challenges and opportunities, and provide some examples of recent bioinformatics for miRNA research in cardiovascular biology and medicine.

Novel Insights into the Molecular Mechanisms Governing Embryonic Epicardium Formation

The embryonic epicardium originates from the proepicardium, an extracardiac primordium constituted by a cluster of mesothelial cells. In early embryos, the embryonic epicardium is characterized by a squamous cell epithelium resting on the myocardium surface. Subsequently, it invades the subepicardial space and thereafter the embryonic myocardium by means of an epithelial-mesenchymal transition. Within the myocardium, epicardial-derived cells present multilineage potential, later differentiating into smooth muscle cells and contributing both to coronary vasculature and cardiac fibroblasts in the mature heart. Over the last decades, we have progressively increased our understanding of those cellular and molecular mechanisms driving proepicardial/embryonic epicardium formation. This study provides a state-of-the-art review of the transcriptional and emerging post-transcriptional mechanisms involved in the formation and differentiation of the embryonic epicardium.

Post-Transcriptional and Epigenetic Regulation of Estrogen Signaling

Post-translational and epigenetic regulation are important mechanisms controlling functions of genes and proteins. Although the "classic" estrogen receptors (ERs) have been acknowledged to function in mediating estrogen effects via transcriptional mechanisms, estrogenic agents modulate the turnover of several proteins via post-transcriptional and post-translational pathways including epigenetics. For instance, the metabolic and angiogenic action of G-protein coupled estrogen receptor (GPER) in vascular endothelial cells has been recently elucidated. By interacting with GPER, 17β-estradiol and the GPER agonist G1 enhance endothelial stability of 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 (PFKFB3) and capillary tube formation by increasing ubiquitin-specific peptidase 19 levels, thereby reducing PFKFB3 ubiquitination and proteasomal degradation. In addition to ligands, the functional expression and trafficking of ERs can be modulated by post-translational modification, including palmitoylation. MicroRNAs (miRNAs), the most abundant form of endogenous small RNAs in humans, regulate multiple target genes and are at the center of the multi-target regulatory network. This review also discusses the emerging evidence of how miRNAs affect glycolytic metabolism in cancer, as well as their regulation by estrogens. Restoring dysregulated miRNA expression represents a promising strategy to counteract the progression of cancer and other disease conditions. Accordingly, estrogen post-transcriptional regulatory and epigenetic mechanisms represent novel targets for pharmacological and nonpharmacological intervention for the treatment and prevention of hormone-sensitive noncommunicable diseases, including estrogen-sensitive cancers of the reproductive system in women. SIGNIFICANCE STATEMENT: The effects of estrogen are mediated by several mechanisms that are not limited to the transcriptional regulation of target genes. Slowing down the turnover of master regulators of metabolism by estrogens allows cells to rapidly adapt to environmental cues. Identification of estrogen-targeted microRNAs may lead to the development of novel RNA therapeutics that disrupt pathological angiogenesis in estrogen-dependent cancers.

Differential requirement for DICER1 activity during the development of mitral and tricuspid valves

Mitral and tricuspid valves are essential for unidirectional blood flow in the heart. They are derived from similar cell sources, and yet congenital dysplasia affecting both valves is clinically rare, suggesting the presence of differential regulatory mechanisms underlying their development. Here, we specifically inactivated Dicer1 in the endocardium during cardiogenesis and found that Dicer1 deletion caused congenital mitral valve stenosis and regurgitation, whereas it had no impact on other valves. We showed that hyperplastic mitral valves were caused by abnormal condensation and extracellular matrix (ECM) remodeling. Our single-cell RNA sequencing analysis revealed impaired maturation of mesenchymal cells and abnormal expression of ECM genes in mutant mitral valves. Furthermore, expression of a set of miRNAs that target ECM genes was significantly lower in tricuspid valves compared to mitral valves, consistent with the idea that the miRNAs are differentially required for mitral and tricuspid valve development. We thus reveal miRNA-mediated gene regulation as a novel molecular mechanism that differentially regulates mitral and tricuspid valve development, thereby enhancing our understanding of the non-association of inborn mitral and tricuspid dysplasia observed clinically.

The Role of Bmp- and Fgf Signaling Modulating Mouse Proepicardium Cell Fate

Bmp and Fgf signaling are widely involved in multiple aspects of embryonic development. More recently non coding RNAs, such as microRNAs have also been reported to play essential roles during embryonic development. We have previously demonstrated that microRNAs, i.e., miR-130, play an essential role modulating Bmp and Fgf signaling during early stages of cardiomyogenesis. More recently, we have also demonstrated that microRNAs are capable of modulating cell fate decision during proepicardial/septum transversum (PE/ST) development, since over-expression of miR-23 blocked while miR-125, miR-146, miR-223 and miR-195 enhanced PE/ST-derived cardiomyogenesis, respectively. Importantly, regulation of these microRNAs is distinct modulated by Bmp2 and Fgf2 administration in chicken. In this study, we aim to dissect the functional role of Bmp and Fgf signaling during mouse PE/ST development, their implication regulating post-transcriptional modulators such as microRNAs and their impact on lineage determination. Mouse PE/ST explants and epicardial/endocardial cell cultures were distinctly administrated Bmp and Fgf family members. qPCR analyses of distinct microRNAs, cardiomyogenic, fibrogenic differentiation markers as well as key elements directly epithelial to mesenchymal transition were evaluated. Our data demonstrate that neither Bmp2/Bmp4 nor Fgf2/Fgf8 signaling is capable of inducing cardiomyogenesis, fibrogenesis or inducing EMT in mouse PE/ST explants, yet deregulation of several microRNAs is observed, in contrast to previous findings in chicken PE/ST. RNAseq analyses in mouse PE/ST and embryonic epicardium identified novel Bmp and Fgf family members that might be involved in such cell fate differences, however, their implication on EMT induction and cardiomyogenic and/or fibrogenic differentiation is limited. Thus our data support the notion of species-specific differences regulating PE/ST cardiomyogenic lineage commitment.

Critical role of the BAF chromatin remodeling complex during murine neural crest development

The BAF complex plays an important role in the development of a wide range of tissues by modulating gene expression programs at the chromatin level. However, its role in neural crest development has remained unclear. To determine the role of the BAF complex, we deleted BAF155/BAF170, the core subunits required for the assembly, stability, and functions of the BAF complex in neural crest cells (NCCs). Neural crest-specific deletion of BAF155/BAF170 leads to embryonic lethality due to a wide range of developmental defects including craniofacial, pharyngeal arch artery, and OFT defects. RNAseq and transcription factor enrichment analysis revealed that the BAF complex modulates the expression of multiple signaling pathway genes including Hippo and Notch, essential for the migration, proliferation, and differentiation of the NCCs. Furthermore, we demonstrated that the BAF complex is essential for the Brg1-Yap-Tead-dependent transcription of target genes in NCCs. Together, our results demonstrate an important role of the BAF complex in modulating the gene regulatory network essential for neural crest development.

Neural crest cells (NCCs) are a multipotent and migratory cell population that is induced at the neural plate border during neurulation and contributes to the formation of a wide range of tissues. Defects in the development, differentiation, or migration of NCCs lead to various birth defects including craniofacial and heart anomalies. Here, by genetically deleting BAF155/BAF170, the core subunits required for the assembly, stability, and functions of the BAF chromatin remodeling complex, we demonstrate that the BAF complex is essential for the proliferation, survival, and differentiation of the NCCs. Neural crest-specific deletion of BAF155/BAF170 leads to embryonic lethality due to a wide range of developmental defects including craniofacial and cardiovascular defects. By performing RNAseq and transcription factor enrichment analysis we show that the BAF complex modulates the expression of multiple signaling pathway genes including Hippo and Notch, essential for the development of the NCCs. Furthermore, the BAF complex component physically interacts with the Hippo signaling components in NCCs to regulate gene expression. We demonstrated that the BAF complex is essential for the Brg1-Yap-Tead-dependent transcription of target genes in NCCs. Together, our results demonstrate a critical role of the BAF complex in modulating the gene regulatory network essential for the proper development of neural crest and neural crest-derived tissues.

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.

MiR-195 enhances cardiomyogenic differentiation of the proepicardium/septum transversum by Smurf1 and Foxp1 modulation

Cardiovascular development is a complex developmental process in which multiple cell lineages are involved, namely the deployment of first and second heart fields. Beside the contribution of these cardiogenic fields, extracardiac inputs to the developing heart are provided by the migrating cardiac neural crest cells and the proepicardial derived cells. The proepicardium (PE) is a transitory cauliflower-like structure located between the cardiac and hepatic primordia. The PE is constituted by an internal mesenchymal component surrounded by an external epithelial lining. With development, cells derived from the proepicardium migrate to the neighboring embryonic heart and progressive cover the most external surface, leading to the formation of the embryonic epicardium. Experimental evidence in chicken have nicely demonstrated that epicardial derived cells can distinctly contribute to fibroblasts, endothelial and smooth muscle cells. Surprisingly, isolation of the developing PE anlage and ex vivo culturing spontaneously lead to differentiation into beating cardiomyocytes, a process that is enhanced by Bmp but halted by Fgf administration. In this study we provide a comprehensive characterization of the developmental expression profile of multiple microRNAs during epicardial development in chicken. Subsequently, we identified that miR-125, miR-146, miR-195 and miR-223 selectively enhance cardiomyogenesis both in the PE/ST explants as well as in the embryonic epicardium, a Smurf1- and Foxp1-driven process. In addition we identified three novel long non-coding RNAs with enhanced expression in the PE/ST, that are complementary regulated by Bmp and Fgf administration and well as by microRNAs that selectively promote cardiomyogenesis, supporting a pivotal role of these long non coding RNAs in microRNA-mediated cardiomyogenesis of the PE/ST cells.

Applications of miRNAs in cardiac development, disease progression and regeneration

Development of the complex human heart is tightly regulated at multiple levels, maintaining multipotency and proliferative state in the embryonic cardiovascular progenitors and thereafter suppressing progenitor characteristics to allow for terminal differentiation and maturation. Small regulatory microRNAs (miRNAs) are at the level of post-transcriptional gene suppressors, which enhance the degradation or decay of their target protein-coding mRNAs. These miRNAs are known to play roles in a large number of biological events, cardiovascular development being no exception. A number of critical cardiac-specific miRNAs have been identified, of which structural developmental defects have been linked to dysregulation of miRNAs in the proliferating cardiac stem cells. These miRNAs present in the stem cell niche are lost when the cardiac progenitors terminally differentiate, resulting in the postnatal mitotic arrest of the heart. Therapeutic applications of these miRNAs extend to the realm of heart failure, whereby the death of heart cells in the ageing heart cannot be replaced due to the arrest of cell division. By utilizing miRNA therapy to control cell cycling, the regenerative potential of matured myocardium can be restored. This review will address the various cardiac progenitor-related miRNAs that control the development and proliferative potential of the heart.

RNAase III-Type Enzyme Dicer Regulates Mitochondrial Fatty Acid Oxidative Metabolism in Cardiac Mesenchymal Stem Cells

Cardiac mesenchymal stem cells (C-MSC) play a key role in maintaining normal cardiac function under physiological and pathological conditions. Glycolysis and mitochondrial oxidative phosphorylation predominately account for energy production in C-MSC. Dicer, a ribonuclease III endoribonuclease, plays a critical role in the control of microRNA maturation in C-MSC, but its role in regulating C-MSC energy metabolism is largely unknown. In this study, we found that Dicer knockout led to concurrent increase in both cell proliferation and apoptosis in C-MSC compared to Dicer floxed C-MSC. We analyzed mitochondrial oxidative phosphorylation by quantifying cellular oxygen consumption rate (OCR), and glycolysis by quantifying the extracellular acidification rate (ECAR), in C-MSC with/without Dicer gene deletion. Dicer gene deletion significantly reduced mitochondrial oxidative phosphorylation while increasing glycolysis in C-MSC. Additionally, Dicer gene deletion selectively reduced the expression of β-oxidation genes without affecting the expression of genes involved in the tricarboxylic acid (TCA) cycle or electron transport chain (ETC). Finally, Dicer gene deletion reduced the copy number of mitochondrially encoded 1,4-Dihydronicotinamide adenine dinucleotide (NADH): ubiquinone oxidoreductase core subunit 6 (MT-ND6), a mitochondrial-encoded gene, in C-MSC. In conclusion, Dicer gene deletion induced a metabolic shift from oxidative metabolism to aerobic glycolysis in C-MSC, suggesting that Dicer functions as a metabolic switch in C-MSC, which in turn may regulate proliferation and environmental adaptation.

The Hippo Signaling Pathway in Cardiac Development and Diseases

Heart disease continues to be the leading cause of morbidity and mortality worldwide. Cardiac malformation during development could lead to embryonic or postnatal death. However, matured heart tissue has a very limited regenerative capacity. Thus, loss of cardiomyocytes from injury or diseases in adults could lead to heart failure. The Hippo signaling pathway is a newly identified signaling cascade that modulates regenerative response by regulating cardiomyocyte proliferation in the embryonic heart, as well as in postnatal hearts after injury. In this review, we summarize recent findings highlighting the function and regulation of the Hippo signaling pathway in cardiac development and diseases.

Semaphorin 3E/PlexinD1 signaling is required for cardiac ventricular compaction

Left ventricular noncompaction (LVNC) is one of the most common forms of genetic cardiomyopathy characterized by excessive trabeculation and impaired myocardial compaction during fetal development. Patients with LVNC are at higher risk of developing left/right ventricular failure or both. Although the key regulators for cardiac chamber development are well studied, the role of semaphorin (Sema)/plexin signaling in this process remains poorly understood. In this article, we demonstrate that genetic deletion of Plxnd1, a class-3 Sema receptor in endothelial cells, leads to severe cardiac chamber defects. They were characterized by excessive trabeculation and noncompaction similar to patients with LVNC. Loss of Plxnd1 results in decreased expression of extracellular matrix proteolytic genes, leading to excessive deposition of cardiac jelly. We demonstrate that Plxnd1 deficiency is associated with an increase in Notch1 expression and its downstream target genes. In addition, inhibition of the Notch signaling pathway partially rescues the excessive trabeculation and noncompaction phenotype present in Plxnd1 mutants. Furthermore, we demonstrate that Semaphorin 3E (Sema3E), one of PlexinD1's known ligands, is expressed in the developing heart and is required for myocardial compaction. Collectively, our study uncovers what we believe to be a previously undescribed role of the Sema3E/PlexinD1 signaling pathway in myocardial trabeculation and the compaction process.

Neural crest-specific deletion of Rbfox2 in mice leads to craniofacial abnormalities including cleft palate

Alternative splicing (AS) creates proteomic diversity from a limited size genome by generating numerous transcripts from a single protein-coding gene. Tissue-specific regulators of AS are essential components of the gene regulatory network, required for normal cellular function, tissue patterning, and embryonic development. However, their cell-autonomous function in neural crest development has not been explored. Here, we demonstrate that splicing factor Rbfox2 is expressed in the neural crest cells (NCCs), and deletion of Rbfox2 in NCCs leads to cleft palate and defects in craniofacial bone development. RNA-Seq analysis revealed that Rbfox2 regulates splicing and expression of numerous genes essential for neural crest/craniofacial development. We demonstrate that Rbfox2-TGF-β-Tak1 signaling axis is deregulated by Rbfox2 deletion. Furthermore, restoration of TGF-β signaling by Tak1 overexpression can rescue the proliferation defect seen in Rbfox2 mutants. We also identified a positive feedback loop in which TGF-β signaling promotes expression of Rbfox2 in NCCs.

Abnormalities affecting the head and face – such as a cleft lip or palate – are among the most common of all birth defects. These tissues normally develop from cells in the embryo known as the neural crest cells, and specifically a subset of these cells called the cranial neural crest cells. Most cases of cleft lip or palate are linked back to genes that affect the biology of this group of cells.

The list of genes implicated in the impaired development of cranial neural crest cells code for proteins with a wide range of different activities. Some encode transcription factors – proteins that switch genes on or off. Others code for chromatin remodeling factors, which control how the DNA is packed inside cells. However, the role of another group of proteins – the splicing factors – remains unclear and warrants further investigation.

When a gene is switched on its genetic code is first copied into a short-lived molecule called a transcript. These transcripts are then edited to form templates to build proteins. Splicing is one way that a transcript can be edited, which involves different pieces of the transcript being cut out and the remaining pieces being pasted together to form alternative versions of the final template. Splicing factors control this process.

Cibi et al. now show that neural crest cells from mice make a splicing factor called Rbfox2 and that deleting this gene for this protein from only these cells leads to mice with a cleft palate and defects in the bones of their head and face.

Further analysis helped to identify the transcripts that are spliced by Rbfox2, and the effects that these splicing events have on gene activity in mouse tissues that develop from cranial neural crest cells. Cibi et al. went on to find a signaling pathway that was impaired in the mutant cells that lacked Rbfox2. Forcing the mutant cells to over-produce one of the proteins involved in this signaling pathway (a protein named Tak1) was enough to compensate for the some of the defects caused by a lack of Rbfox2, suggesting it acts downstream of the splicing regulator.

Lastly, Cibi et al. showed that another protein in this signaling pathway, called TGF-β, acted to increase how much Rbfox2 was made by neural crest cells. Together these findings may be relevant in human disease studies, given that altered TGF-β signaling is a common feature in many birth defects seen in humans.

Deficiency in the secreted protein Semaphorin3d causes abnormal parathyroid development in mice

Primary hyperparathyroidism (PHPT) is a common endocrinopathy characterized by hypercalcemia and elevated levels of parathyroid hormone. The primary cause of PHPT is a benign overgrowth of parathyroid tissue causing excessive secretion of parathyroid hormone. However, the molecular etiology of PHPT is incompletely defined. Here, we demonstrate that semaphorin3d (Sema3d), a secreted glycoprotein, is expressed in the developing parathyroid gland in mice. We also observed that genetic deletion of Sema3d leads to parathyroid hyperplasia, causing PHPT. In vivo and in vitro experiments using histology, immunohistochemistry, biochemical, RT-qPCR, and immunoblotting assays revealed that Sema3d inhibits parathyroid cell proliferation by decreasing the epidermal growth factor receptor (EGFR)/Erb-B2 receptor tyrosine kinase (ERBB) signaling pathway. We further demonstrate that EGFR signaling is elevated in Sema3d ^-/- parathyroid glands and that pharmacological inhibition of EGFR signaling can partially rescue the parathyroid hyperplasia phenotype. We propose that because Sema3d is a secreted protein, it may be possible to use recombinant Sema3d or derived peptides to inhibit parathyroid cell proliferation causing hyperplasia and hyperparathyroidism. Collectively, these findings identify Sema3d as a negative regulator of parathyroid growth.

Functional Role of Non-Coding RNAs during Epithelial-To-Mesenchymal Transition

Epithelial-to-mesenchymal transition (EMT) is a key biological process involved in a multitude of developmental and pathological events. It is characterized by the progressive loss of cell-to-cell contacts and actin cytoskeletal rearrangements, leading to filopodia formation and the progressive up-regulation of a mesenchymal gene expression pattern enabling cell migration. Epithelial-to-mesenchymal transition is already observed in early embryonic stages such as gastrulation, when the epiblast undergoes an EMT process and therefore leads to the formation of the third embryonic layer, the mesoderm. Epithelial-to-mesenchymal transition is pivotal in multiple embryonic processes, such as for example during cardiovascular system development, as valve primordia are formed and the cardiac jelly is progressively invaded by endocardium-derived mesenchyme or as the external cardiac cell layer is established, i.e., the epicardium and cells detached migrate into the embryonic myocardial to form the cardiac fibrous skeleton and the coronary vasculature. Strikingly, the most important biological event in which EMT is pivotal is cancer development and metastasis. Over the last years, understanding of the transcriptional regulatory networks involved in EMT has greatly advanced. Several transcriptional factors such as Snail, Slug, Twist, Zeb1 and Zeb2 have been reported to play fundamental roles in EMT, leading in most cases to transcriptional repression of cell⁻cell interacting proteins such as ZO-1 and cadherins and activation of cytoskeletal markers such as vimentin. In recent years, a fundamental role for non-coding RNAs, particularly microRNAs and more recently long non-coding RNAs, has been identified in normal tissue development and homeostasis as well as in several oncogenic processes. In this study, we will provide a state-of-the-art review of the functional roles of non-coding RNAs, particularly microRNAs, in epithelial-to-mesenchymal transition in both developmental and pathological EMT.

Epithelialization of mouse ovarian tumor cells originating in the fallopian tube stroma

Epithelial ovarian carcinoma accounts for 90% of all ovarian cancer and is the most deadly gynecologic malignancy. Recent studies have suggested that fallopian tube fimbriae can be the origin of cells for high-grade serous subtype of epithelial ovarian carcinoma (HGSOC). A mouse HGSOC model with conditional Dicer-Pten double knockout (Dicer-Pten DKO) developed primary tumors, intriguingly, from the fallopian tube stroma. We examined the growth and epithelial phenotypes of the Dicer-Pten DKO mouse tumor cells contributable by each gene knockout. Unlike human ovarian epithelial cancer cells that expressed full-length E-cadherin, the Dicer-Pten DKO stromal tumor cells expressed cleaved E-cadherin fragments and metalloproteinase 2, a mixture of epithelial and mesenchymal markers. Although the Dicer-Pten DKO tumor cells lost the expression of mature microRNAs as expected, they showed high levels of tRNA fragment expression and enhanced AKT activation due to the loss of PTEN function. Introduction of a Dicer1-expressing construct into the DKO mouse tumor cells significantly reduced DNA synthesis and the cell growth rate, with concurrent diminished adhesion and ZO1 epithelial staining. Hence, it is likely that the loss of Dicer promoted mesenchymal-epithelial transition in fallopian tube stromal cells, and in conjunction with Pten loss, further promoted cell proliferation and epithelial-like tumorigenesis.

Cardiac repair by epicardial EMT: Current targets and a potential role for the primary cilium

Despite therapeutic advances that have prolonged life, myocardial infarction (MI) remains a leading cause of death worldwide and imparts a significant economic burden. The advancement of treatments to improve cardiac repair post-MI requires the discovery of new targeted treatment strategies. Recent studies have highlighted the importance of the epicardial covering of the heart in both cardiac development and lower vertebrate cardiac regeneration. The epicardium serves as a source of cardiac cells including smooth muscle cells, endothelial cells and cardiac fibroblasts. Mammalian adult epicardial cells are typically quiescent. However, the fetal genetic program is reactivated post-MI, and epicardial epithelial-to-mesenchymal transition (EMT) occurs as an inherent mechanism to support neovascularization and cardiac healing. Unfortunately, endogenous EMT is not enough to encourage sufficient repair. Recent developments in our understanding of the mechanisms supporting the EMT process has led to a number of studies directed at augmenting epicardial EMT post-MI. With a focus on the role of the primary cilium, this review outlines the newly demonstrated mechanisms supporting EMT, the role of epicardial EMT in cardiac development, and promising advances in augmenting epicardial EMT as potential therapeutics to support cardiac repair post-MI.

Regulating microRNA expression: at the heart of diabetes mellitus and the mitochondrion

Type 2 diabetes mellitus is a major risk factor for cardiovascular disease and mortality. Uncontrolled type 2 diabetes mellitus results in a systemic milieu of increased circulating glucose and fatty acids. The development of insulin resistance in cardiac tissue decreases cellular glucose import and enhances mitochondrial fatty acid uptake. While triacylglycerol and cytotoxic lipid species begin to accumulate in the cardiomyocyte, the energy substrate utilization ratio of free fatty acids to glucose changes to almost entirely free fatty acids. Accumulating evidence suggests a role of miRNA in mediating this metabolic transition. Energy substrate metabolism, apoptosis, and the production and response to excess reactive oxygen species are regulated by miRNA expression. The current momentum for understanding the dynamics of miRNA expression is limited by a lack of understanding of how miRNA expression is controlled. While miRNAs are important regulators in both normal and pathological states, an additional layer of complexity is added when regulation of miRNA regulators is considered. miRNA expression is known to be regulated through a number of mechanisms, which include, but are not limited to, epigenetics, exosomal transport, processing, and posttranscriptional sequestration. The purpose of this review is to outline how mitochondrial processes are regulated by miRNAs in the diabetic heart. Furthermore, we will highlight the regulatory mechanisms, such as epigenetics, exosomal transport, miRNA processing, and posttranslational sequestration, that participate as regulators of miRNA expression. Additionally, current and future treatment strategies targeting dysfunctional mitochondrial processes in the diseased myocardium, as well as emerging miRNA-based therapies, will be summarized.

More than Just a Simple Cardiac Envelope; Cellular Contributions of the Epicardium

The adult pumping heart is formed by distinct tissue layers. From inside to outside, the heart is composed by an internal endothelial layer, dubbed the endocardium, a thick myocardial component which supports the pumping capacity of the heart and exteriorly covered by a thin mesothelial layer named the epicardium. Cardiac insults such as coronary artery obstruction lead to ischemia and thus to an irreversible damage of the myocardial layer, provoking in many cases heart failure and death. Thus, searching for new pathways to regenerate the myocardium is an urgent biomedical need. Interestingly, the capacity of heart regeneration is present in other species, ranging from fishes to neonatal mammals. In this context, several lines of evidences demonstrated a key regulatory role for the epicardial layer. In this manuscript, we provide a state-of-the-art review on the developmental process leading to the formation of the epicardium, the distinct pathways controlling epicardial precursor cell specification and determination and current evidences on the regenerative potential of the epicardium to heal the injured heart.

Functions of miRNAs during Mammalian Heart Development

MicroRNAs (miRNAs) play essential roles during mammalian heart development and have emerged as attractive therapeutic targets for cardiovascular diseases. The mammalian embryonic heart is mainly derived from four major cell types during development. These include cardiomyocytes, endocardial cells, epicardial cells, and neural crest cells. Recent data have identified various miRNAs as critical regulators of the proper differentiation, proliferation, and survival of these cell types. In this review, we briefly introduce the contemporary understanding of mammalian cardiac development. We also focus on recent developments in the field of cardiac miRNAs and their functions during the development of different cell types.

Hippo Signaling Mediators Yap and Taz Are Required in the Epicardium for Coronary Vasculature Development

Formation of the coronary vasculature is a complex and precisely coordinated morphogenetic process that begins with the formation of epicardium. The epicardium gives rise to many components of the coronary vasculature, including fibroblasts, smooth muscle cells, and endothelium. Hippo signaling components have been implicated in cardiac development and regeneration. However, a role of Hippo signaling in the epicardium has not been explored. Employing a combination of genetic and pharmacological approaches, we demonstrate that inhibition of Hippo signaling mediators Yap and Taz leads to impaired epicardial epithelial-to-mesenchymal transition (EMT) and a reduction in epicardial cell proliferation and differentiation into coronary endothelial cells. We provide evidence that Yap and Taz control epicardial cell behavior, in part by regulating Tbx18 and Wt1 expression. Our findings show a role for Hippo signaling in epicardial cell proliferation, EMT, and cell fate specification during cardiac organogenesis.

In Vitro Culture of Epicardial Cells From Mouse Embryonic Heart

During embryogenesis, the epicardial contribution to coronary vasculature development has been very well established. Cells derived from the epicardium differentiate into smooth muscle cells, fibroblasts and endothelial cells that contribute to the formation of coronary vessels. Here we have established an in vitro culture method for embryonic epicardial cells. Using genetic labelling, we have demonstrated that the majority of the migrating cells in our explant culture are of epicardial origin. Epicardial explant cells also retain the expression of epicardial markers (Wt1 and Tbx18). Furthermore, we provide evidence that epicardial explant cells undergo epithelial to mesenchymal transition (EMT), migrate and differentiate into smooth muscle cells after Transforming growth factor beta 1 (TGF-β1) treatment in a manner indistinguishable from that of epicardial cells in vivo. In conclusion, we provide a novel method for the culture of embryonic epicardial cells, which will help to explore the role of specific genes in epicardial cell biology.

New Insights Into the Role of RNA-Binding Proteins in the Regulation of Heart Development

The regulation of gene expression during development takes place both at the transcriptional and posttranscriptional levels. RNA-binding proteins (RBPs) regulate pre-mRNA processing, mRNA localization, stability, and translation. Many RBPs are expressed in the heart and have been implicated in heart development, function, or disease. This chapter will review the current knowledge about RBPs in the developing heart, focusing on those that regulate posttranscriptional gene expression. The involvement of RBPs at each stage of heart development will be considered in turn, including the establishment of specific cardiac cell types and formation of the primitive heart tube, cardiac morphogenesis, and postnatal maturation and aging. The contributions of RBPs to cardiac birth defects and heart disease will also be considered in these contexts. Finally, the interplay between RBPs and other regulatory factors in the developing heart, such as transcription factors and miRNAs, will be discussed.

The role of microRNAs in cardiac development and regenerative capacity

The mammalian heart has long been considered to be a postmitotic organ. It was thought that, in the postnatal period, the heart underwent a transition from hyperplasic growth (more cells) to hypertrophic growth (larger cells) due to the conversion of cardiomyocytes from a proliferative state to one of terminal differentiation. This hypothesis was gradually disproven, as data were published showing that the myocardium is a more dynamic tissue in which cardiomyocyte karyokinesis and cytokinesis produce new cells, leading to the hyperplasic regeneration of some of the muscle mass lost in various pathological processes. microRNAs have been shown to be critical regulators of cardiomyocyte differentiation and proliferation and may offer the novel opportunity of regenerative hyperplasic therapy. Here we summarize the relevant processes and recent progress regarding the functions of specific microRNAs in cardiac development and regeneration.

Semaphorin signaling in cardiovascular development

Semaphorins were originally identified as neuronal guidance molecules mediating their attractive or repulsive signals by forming complexes with plexin and neuropilin receptors. Subsequent research has identified functions for semaphorin signaling in many organs and tissues outside of the nervous system. Vital roles for semaphorin signaling in vascular patterning and cardiac morphogenesis have been demonstrated, and impaired semaphorin signaling has been associated with various human cardiovascular disorders, including persistent truncus arteriosus, sinus bradycardia and anomalous pulmonary venous connections. Here, we review the functions of semaphorins and their receptors in cardiovascular development and disease and highlight important recent discoveries in the field.

Tbx5 is required for avian and Mammalian epicardial formation and coronary vasculogenesis

RATIONALE

Holt-Oram syndrome is an autosomal dominant heart-hand syndrome caused by mutations in the TBX5 gene. Overexpression of Tbx5 in the chick proepicardial organ impaired coronary blood vessel formation. However, the potential activity of Tbx5 in the epicardium itself, and the role of Tbx5 in mammalian coronary vasculogenesis, remains largely unknown.

OBJECTIVE

To evaluate the consequences of altered Tbx5 gene dosage during proepicardial organ and epicardial development in the embryonic chick and mouse.

METHODS AND RESULTS

Retroviral-mediated knockdown or upregulation of Tbx5 expression in the embryonic chick proepicardial organ and proepicardial-specific deletion of Tbx5 in the embryonic mouse (Tbx5(epi-/)) impaired normal proepicardial organ cell development, inhibited epicardial and coronary blood vessel formation, and altered developmental gene expression. The generation of epicardial-derived cells and their migration into the myocardium were impaired between embryonic day (E) 13.5 to 15.5 in mutant hearts because of delayed epicardial attachment to the myocardium and subepicardial accumulation of epicardial-derived cells. This caused defective coronary vasculogenesis associated with impaired vascular smooth muscle cell recruitment and reduced invasion of cardiac fibroblasts and endothelial cells into myocardium. In contrast to wild-type hearts that exhibited an elaborate ventricular vascular network, Tbx5(epi-/-) hearts displayed a marked decrease in vascular density that was associated with myocardial hypoxia as exemplified by hypoxia inducible factor-1α upregulation and increased binding of hypoxyprobe-1. Tbx5(epi-/-) mice with such myocardial hypoxia exhibited reduced exercise capacity when compared with wild-type mice.

CONCLUSIONS

Our findings support a conserved Tbx5 dose-dependent requirement for both proepicardial and epicardial progenitor cell development in chick and in mouse coronary vascular formation.

MicroRNAs in heart failure: Small molecules with major impact

MicroRNAs (miRNAs) have emerged as potent modulators of mammalian gene expression, thereby broadening the spectrum of molecular mechanisms orchestrating human physiological and pathological cellular functions. Growing evidence suggests that these small non-coding RNA molecules are pivotal regulators of cardiovascular development and disease. Importantly, multiple miRNAs have been specifically implicated in the onset and progression of heart failure, thus providing a new platform for battling this multi-faceted disease. This review introduces the basic concepts of miRNA biology, describes representative examples of miRNAs associated with multiple aspects of HF pathogenesis, and explores the prognostic, diagnostic and therapeutic potential of miRNAs in the cardiology clinic.

microRNAs in cardiac development and regeneration

Heart development involves the precise orchestration of gene expression during cardiac differentiation and morphogenesis by evolutionarily conserved regulatory networks. miRNAs (microRNAs) play important roles in the post-transcriptional regulation of gene expression, and recent studies have established critical functions for these tiny RNAs in almost every facet of cardiac development and disease. The realization that miRNAs are amenable to therapeutic manipulation has also generated considerable interest in the potential of miRNA-based drugs for the treatment of a number of human diseases, including cardiovascular disease. In the present review, I discuss well-established and emerging roles of miRNAs in cardiac development, their relevance to congenital heart disease and unresolved questions in the field for future investigation, as well as emerging therapeutic possibilities for cardiac regeneration.

Suppression of Induced microRNA-15b Prevents Rapid Loss of Cardiac Function in a Dicer Depleted Model of Cardiac Dysfunction

Background

Dicer endonuclease, critical for maturation of miRNAs, is depleted in certain forms of cardiomyopathy which results in differential expression of certain microRNAs. We sought to elucidate the mechanisms underlying the rapid loss of cardiac function following cardiac-specific Dicer depletion in adult mice.

Results

Conditional Dicer deletion in the adult murine myocardium demonstrated compromised heart function, mitochondrial dysfunction and oxidant stress. Elevated miR-15b was observed as an early response to Dicer depletion and was found to silence Pim-1 kinase, a protein responsible for maintaining mitochondrial integrity and function. Anti-miRNA based suppression of induced miRNA-15b rescued the function of Dicer-depleted adult heart and attenuated hypertrophy.

Conclusions

Anti-miRNA based suppression of inducible miRNA-15b can prevent rapid loss of cardiac function in a Dicer-depleted adult heart and can be a key approach worthy of therapeutic consideration.

Semaphorin 3d signaling defects are associated with anomalous pulmonary venous connections

Total anomalous pulmonary venous connection (TAPVC) is a potentially lethal congenital disorder that occurs when the pulmonary veins do not connect normally to the left atrium, allowing mixing of pulmonary and systemic blood. In contrast to the extensive knowledge of arterial vascular patterning, little is known about the patterning of veins. Here we show that the secreted guidance molecule semaphorin 3d (Sema3d) is crucial for the normal patterning of pulmonary veins. Prevailing models suggest that TAPVC occurs when the midpharyngeal endothelial strand (MES), the precursor of the common pulmonary vein, does not form at the proper location on the dorsal surface of the embryonic common atrium. However, we found that TAPVC occurs in Sema3d mutant mice despite normal formation of the MES. In these embryos, the maturing pulmonary venous plexus does not anastomose uniquely with the properly formed MES. In the absence of Sema3d, endothelial tubes form in a region that is normally avascular, resulting in aberrant connections. Normally, Sema3d provides a repulsive cue to endothelial cells in this area, establishing a boundary. Sequencing of SEMA3D in individuals with anomalous pulmonary veins identified a phenylalanine-to-leucine substitution that adversely affects SEMA3D function. These results identify Sema3d as a crucial pulmonary venous patterning cue and provide experimental evidence for an alternate developmental model to explain abnormal pulmonary venous connections.

Epithelial-mesenchymal transitions: insights from development

Epithelial-mesenchymal transition (EMT) is a crucial, evolutionarily conserved process that occurs during development and is essential for shaping embryos. Also implicated in cancer, this morphological transition is executed through multiple mechanisms in different contexts, and studies suggest that the molecular programs governing EMT, albeit still enigmatic, are embedded within developmental programs that regulate specification and differentiation. As we review here, knowledge garnered from studies of EMT during gastrulation, neural crest delamination and heart formation have furthered our understanding of tumor progression and metastasis.

Synthetic lethality between Rb, p53 and Dicer or miR-17-92 in retinal progenitors suppresses retinoblastoma formation

Synthetic lethality is a promising strategy for specific targeting of cancer cells that carry mutations that are absent in normal cells. This approach may help overcome the challenge associated with targeting dysfunctional tumour suppressors, such as p53 and Rb (refs 1, 2). Here we show that Dicer1 targeting prevents retinoblastoma formation in mice by synthetic lethality with combined inactivation of p53 and Rb. Although Dicer1 functions as a haploinsufficient tumour suppressor, its complete loss of function is selected against during tumorigenesis(3-5). We show that Dicer1 deficiency is tolerated in Rb-deficient retinal progenitor cells harbouring an intact p53 pathway, but not in the absence of p53. This synthetic lethality is mediated by the oncogenic miR-17-92 cluster because its deletion phenocopies Dicer1 loss in this context. miR-17-92 inactivation suppresses retinoblastoma formation in mice and co-silencing of miR-17/20a and p53 cooperatively decreases the viability of human retinoblastoma cells. These data provide an explanation for the selective pressure against loss of Dicer1 during tumorigenesis and a proof-of-concept that targeting miRNAs may potentially represent a general approach for synthetic lethal targeting of cancer cells that harbour specific cancer-inducing genotypes.

Functional importance of Dicer protein in the adaptive cellular response to hypoxia

The processes by which cells sense and respond to ambient oxygen concentration are fundamental to cell survival and function, and they commonly target gene regulatory events. To date, however, little is known about the link between the microRNA pathway and hypoxia signaling. Here, we show in vitro and in vivo that chronic hypoxia impairs Dicer (DICER1) expression and activity, resulting in global consequences on microRNA biogenesis. We show that von Hippel-Lindau-dependent down-regulation of Dicer is key to the expression and function of hypoxia-inducible factor α (HIF-α) subunits. Specifically, we show that EPAS1/HIF-2α is regulated by the Dicer-dependent microRNA miR-185, which is down-regulated by hypoxia. Full expression of hypoxia-responsive/HIF target genes in chronic hypoxia (e.g. VEGFA, FLT1/VEGFR1, KDR/VEGFR2, BNIP3L, and SLC2A1/GLUT1), the function of which is to regulate various adaptive responses to compromised oxygen availability, is also dependent on hypoxia-mediated down-regulation of Dicer function and changes in post-transcriptional gene regulation. Therefore, functional deficiency of Dicer in chronic hypoxia is relevant to both HIF-α isoforms and hypoxia-responsive/HIF target genes, especially in the vascular endothelium. These findings have relevance to emerging therapies given that we show that the efficacy of RNA interference under chronic hypoxia, but not normal oxygen availability, is Dicer-dependent. Collectively, these findings show that the down-regulation of Dicer under chronic hypoxia is an adaptive mechanism that serves to maintain the cellular hypoxic response through HIF-α- and microRNA-dependent mechanisms, thereby providing an essential mechanistic insight into the oxygen-dependent microRNA regulatory pathway.

MicroRNAs and stem cells: control of pluripotency, reprogramming, and lineage commitment

Stem cells hold great promise for regenerative medicine and the treatment of cardiovascular diseases. The mechanisms regulating self-renewal, pluripotency, and differentiation are not fully understood. MicroRNAs (miRs) are small noncoding RNAs controlling gene expression, either by inducing mRNA degradation or by blocking mRNA translation. The expression of miRs was shown to regulate various aspects of stem cell functions, including the maintenance and induction of pluripotency for reprogramming. In addition, some miRs control cell fate decisions. This review summarizes the role of miRs in reprogramming and embryonic stem cell self-renewal, and specifically addresses the regulation of cardiovascular cell fate decisions by miRs.

Distinct compartments of the proepicardial organ give rise to coronary vascular endothelial cells

The proepicardial organ is an important transient structure that contributes cells to various cardiac lineages. However, its contribution to the coronary endothelium has been disputed, with conflicting data arising in chick and mouse. Here we resolve this conflict by identifying two proepicardial markers, Scleraxis (Scx) and Semaphorin3D (Sema3D), that genetically delineate heretofore uncharacterized proepicardial subcompartments. In contrast to previously fate-mapped Tbx18/WT-1-expressing cells that give rise to vascular smooth muscle, Scx- and Sema3D-expressing proepicardial cells give rise to coronary vascular endothelium both in vivo and in vitro. Furthermore, Sema3D(+) and Scx(+) proepicardial cells contribute to the early sinus venosus and cardiac endocardium, respectively, two tissues linked to vascular endothelial formation at later stages. Taken together, our studies demonstrate that the proepicardial organ is a molecularly compartmentalized structure, reconciling prior chick and mouse data and providing a more complete understanding of the progenitor populations that establish the coronary vascular endothelium.