miRNA-processing enzyme Dicer is necessary for cardiac outflow tract alignment and chamber septation

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.

Posttranscriptional Regulation by Proteins and Noncoding RNAs

Posttranscriptional regulation comprises those mechanisms occurring after the initial copy of the DNA sequence is transcribed into an intermediate RNA molecule (i.e., messenger RNA) until such a molecule is used as a template to generate a protein. A subset of these posttranscriptional regulatory mechanisms essentially are destined to process the immature mRNA toward its mature form, conferring the adequate mRNA stability, providing the means for pertinent introns excision, and controlling mRNA turnover rate and quality control check. An additional layer of complexity is added in certain cases, since discrete nucleotide modifications in the mature RNA molecule are added by RNA editing, a process that provides large mature mRNA diversity. Moreover, a number of posttranscriptional regulatory mechanisms occur in a cell- and tissue-specific manner, such as alternative splicing and noncoding RNA-mediated regulation. In this chapter, we will briefly summarize current state-of-the-art knowledge of general posttranscriptional mechanisms, while major emphases will be devoted to those tissue-specific posttranscriptional modifications that impact on cardiac development and congenital heart disease.

Integrated small RNA, mRNA and protein omics reveal a miRNA network orchestrating metabolic maturation of the developing human heart

BACKGROUND

As the fetal heart develops, cardiomyocyte proliferation potential decreases while fatty acid oxidative capacity increases in a highly regulated transition known as cardiac maturation. Small noncoding RNAs, such as microRNAs (miRNAs), contribute to the establishment and control of tissue-specific transcriptional programs. However, small RNA expression dynamics and genome-wide miRNA regulatory networks controlling maturation of the human fetal heart remain poorly understood.

RESULTS

Transcriptome profiling of small RNAs revealed the temporal expression patterns of miRNA, piRNA, circRNA, snoRNA, snRNA and tRNA in the developing human heart between 8 and 19 weeks of gestation. Our analysis demonstrated that miRNAs were the most dynamically expressed small RNA species throughout mid-gestation. Cross-referencing differentially expressed miRNAs and mRNAs predicted 6200 mRNA targets, 2134 of which were upregulated and 4066 downregulated as gestation progressed. Moreover, we found that downregulated targets of upregulated miRNAs, including hsa-let-7b, miR-1-3p, miR-133a-3p, miR-143-3p, miR-499a-5p, and miR-30a-5p predominantly control cell cycle progression. In contrast, upregulated targets of downregulated miRNAs, including hsa-miR-1276, miR-183-5p, miR-1229-3p, miR-615-3p, miR-421, miR-200b-3p and miR-18a-3p, are linked to energy sensing and oxidative metabolism. Furthermore, integrating miRNA and mRNA profiles with proteomes and reporter metabolites revealed that proteins encoded in mRNA targets and their associated metabolites mediate fatty acid oxidation and are enriched as the heart develops.

CONCLUSIONS

This study presents the first comprehensive analysis of the small RNAome of the maturing human fetal heart. Our findings suggest that coordinated activation and repression of miRNA expression throughout mid-gestation is essential to establish a dynamic miRNA-mRNA-protein network that decreases cardiomyocyte proliferation potential while increasing the oxidative capacity of the maturing human fetal heart. Our results provide novel insights into the molecular control of metabolic maturation of the human fetal heart.

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.

Circulating microRNA: Myocardium-derived prenatal biomarker of ventricular septal defects

Background: Recently, circulating microRNAs (miRNAs) from maternal blood and amniotic fluid have been used as biomarkers for ventricular septal defect (VSD) diagnosis. However, whether circulating miRNAs are associated with fetal myocardium remains unknown.

Methods: Dimethadione (DMO) induced a VSD rat model. The miRNA expression profiles of the myocardium, amniotic fluid and maternal serum were analyzed. Differentially expressed microRNAs (DE-microRNAs) were verified by qRT–PCR. The target gene of miR-1-3p was confirmed by dual luciferase reporter assays. Expression of amniotic fluid-derived DE-microRNAs was verified in clinical samples.

Results: MiRNAs were differentially expressed in VSD fetal rats and might be involved in cardiomyocyte differentiation and apoptosis. MiR-1-3p, miR-1b and miR-293-5p were downregulated in the myocardium and upregulated in amniotic fluid/maternal serum. The expression of amniotic fluid-derived DE-microRNAs (miR-1-3p, miR-206 and miR-184) was verified in clinical samples. Dual luciferase reporter assays confirmed that miR-1-3p directly targeted SLC8A1/NCX1.

Conclusion: MiR-1-3p, miR-1b and miR-293-5p are downregulated in VSD myocardium and upregulated in circulation and may be released into circulation by cardiomyocytes. MiR-1-3p targets SLC8A1/NCX1 and participates in myocardial apoptosis. MiR-1-3p upregulation in circulation is a direct and powerful indicator of fetal VSD and is expected to serve as a prenatal VSD diagnostic marker.

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.

Individual and joint effects of genetic polymorphisms in microRNA-machinery genes on congenital heart disease susceptibility

Single-nucleotide polymorphisms in miRNA-machinery genes may alter the biogenesis of miRNAs affecting disease susceptibility. In this case-control study, we aimed to evaluate the impact of three single-nucleotide polymorphisms (DICER rs1057035, DROSHA rs10719, and XPO5 rs11077) and their combined effect in a genetic risk score model on congenital heart disease (CHD) risk. A total of 639 participants was recruited, including 125 patients with CHD (65 males; age 9.2 ± 10 years) and 514 healthy controls (289 males; age 15.8 ± 18 years). Genotyping of polymorphisms in miRNA-machinery genes was performed using a TaqMan®SNP genotyping assay. A genetic risk score was calculated by summing the number of risk alleles of selected single-nucleotide polymorphisms. There was a significantly increased risk of CHD in patients with XPO5 rs11077 CC genotype as compared to AC heterozygote and AA homozygote patients (ORadjusted = 1.7; 95% CI: 1.1-2.8; p = 0.018). A clear tendency to significance was also found for DROSHA rs10719 AA genotype and CHD risk for both codominant and recessive models (ORadjusted = 1.8; 95% CI: 0.91-3.8; p = 0.09 and ORadjusted = 1.9; 95% CI: 0.92-4; p = 0.08, respectively). The resulting genetic risk score predicted a 1.73 risk for CHD per risk allele (95% CI: 1.2-2.5; p = 0.002). Subjects in the top tertile of genetic risk score were estimated to have more than three-fold increased risk of CHD compared with those in the bottom tertile (ORadjusted = 3.52; 95% CI: 1.4-9; p = 0.009). Our findings show that the genetic variants in miRNA-machinery genes might participate in the development of CHD.

miRNA in cardiac development and regeneration

Ischemic heart disease is one of the main causes of morbidity and mortality in the world. In adult mammalian hearts, most cardiomyocytes are terminally differentiated and have extremely limited capacity of proliferation, making it impossible to regenerate the heart after injuries such as myocardial infarction. MicroRNAs (miRNAs), a class of non-coding single-stranded RNA, which are involved in mRNA silencing and the regulation of post-transcriptional gene expression, have been shown to play a crucial role in cardiac development and cardiomyocyte proliferation. Muscle specific miRNAs such as miR-1 are key regulators of cardiomyocyte maturation and growth, while miR-199-3p and other miRNAs display potent activity to induce proliferation of cardiomyocytes. Given their small size and relative pleiotropic effects, miRNAs have gained significant attraction as promising therapeutic targets or tools in cardiac regeneration. Increasing number of studies demonstrated that overexpression or inhibition of specific miRNAs could induce cardiomyocyte proliferation and cardiac regeneration. Some common targets of pro-proliferation miRNAs, such as the Hippo-Yap signaling pathway, were identified in multiple species, highlighting the power of miRNAs as probes to dissect core regulators of biological processes. A number of miRNAs have been shown to improve heart function after myocardial infarction in mice, and one trial in swine also demonstrated promising outcomes. However, technical difficulties, especially in delivery methods, and adverse effects, such as uncontrolled proliferation, remain. In this review, we summarize the recent progress in miRNA research in cardiac development and regeneration, examine the mechanisms of miRNA regulating cardiomyocyte proliferation, and discuss its potential as a new strategy for cardiac regeneration therapy.

A comparative analysis of heart microRNAs in vertebrates brings novel insights into the evolution of genetic regulatory networks

BACKGROUND

During vertebrate evolution, the heart has undergone remarkable changes that lead to morphophysiological differences in the fully formed heart of these species, such as chamber septation, heart rate frequency, blood pressure, and cardiac output volume. Despite these differences, the heart developmental process is guided by a core gene set conserved across vertebrates. Nonetheless, the regulatory mechanisms controlling the expression of genes involved in heart development and maintenance are largely uncharted. MicroRNAs (miRNAs) have been described as important regulatory elements in several biological processes, including heart biology. These small RNA molecules are broadly conserved in sequence and genomic context in metazoans. Mutations may occur in miRNAs and/or genes that contribute to the establishment of distinct repertoires of miRNA-target interactions, thereby favoring the differential control of gene expression and, consequently, the origin of novel phenotypes. In fact, several studies showed that miRNAs are integrated into genetic regulatory networks (GRNs) governing specific developmental programs and diseases. However, studies integrating miRNAs in vertebrate heart GRNs under an evolutionary perspective are still scarce.

RESULTS

We comprehensively examined and compared the heart miRNome of 20 species representatives of the five major vertebrate groups. We found 54 miRNA families with conserved expression and a variable number of miRNA families with group-specific expression in fishes, amphibians, reptiles, birds, and mammals. We also detected that conserved miRNAs present higher expression levels and a higher number of targets, whereas the group-specific miRNAs present lower expression levels and few targets.

CONCLUSIONS

Both the conserved and group-specific miRNAs can be considered modulators orchestrating the core and peripheral genes of heart GRNs of vertebrates, which can be related to the morphophysiological differences and similarities existing in the heart of distinct vertebrate groups. We propose a hypothesis to explain evolutionary differences in the putative functional roles of miRNAs in the heart GRNs analyzed. Furthermore, we present new insights into the molecular mechanisms that could be helping modulate the diversity of morphophysiology in the heart organ of vertebrate species.

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.

Rutin treats myocardial damage caused by pirarubicin via regulating miR-22-5p-regulated RAP1/ERK signaling pathway

Our experiments have previously demonstrated that rutin (RUT) can improve myocardial damage caused by pirarubicin (THP). However, the underlying molecular mechanisms remain uncertain. In this study, we developed an microRNA (miRNA) chip by replicating the rat model of THP-induced myocardial injury and identified miR-22-5p and the RAP1-member of RAS oncogene family/extracellular regulated protein kinases (RAP1/ERK) signaling pathway as an object of study. Also, in vivo experiments demonstrated that THP caused abnormal changes in the electrocardiogram, cardiac function, and histomorphology in rats (P < .01). THP also reduces the expression of miR-22-5p (P < .01) and increases the levels of RAP1/ERK signaling pathway-related proteins (P < .01, P < .05). RUT significantly improved THP-induced myocardial damage (P < .01), increased the expression of miR-22-5p (P < .01), and decreased the levels of RAP1/ERK signaling pathway-related proteins (P < .01, P < .05). In vitro studies confirmed that Rap1a is one of the target genes of miR-22-5p. miR-22-5p overexpression in cardiomyocytes can affect the RAP1/ERK pathway and reduce reactive oxygen species production and cardiomyocyte apoptosis caused by THP (P < .01), which is consistent with the effect of RUT. Our results indicate that RUT treats THP-induced myocardial damage, which may be achieved by upregulating miR-22-5p, causing changes in its target gene Rap1a and the RAP1/ERK pathway.

Comprehensive Overview of Non-coding RNAs in Cardiac Development

Cardiac development in the human embryo is characterized by the interactions of several transcription and growth factors leading the heart from a primordial linear tube into a synchronous contractile four-chamber organ. Studies on cardiogenesis showed that cell proliferation, differentiation, fate specification and morphogenesis are spatiotemporally coordinated by cell-cell interactions and intracellular signalling cross-talks. In recent years, research has focused on a class of inter- and intra-cellular modulators called non-coding RNAs (ncRNAs), transcribed from the noncoding portion of the DNA and involved in the proper formation of the heart. In this chapter, we will summarize the current state of the art on the roles of three major forms of ncRNAs [microRNAs (miRNAs), long ncRNAs (lncRNAs) and circular RNAs (circRNAs)] in orchestrating the four sequential phases of cardiac organogenesis.

Aberrant expression of miR-29b-3p influences heart development and cardiomyocyte proliferation by targeting NOTCH2

Objectives

microRNA‐29 (miR‐29) family have shown different expression patterns in cardiovascular diseases. Our study aims to explore the effect and mechanism of miR‐29 family on cardiac development.

Materials and methods

A total of 13 patients with congenital heart disease (CHD) and 7 controls were included in our study. Tissues were obtained from the right ventricular outflow tract (RVOT) after surgical resection or autopsy. The next‐generation sequencing was applied to screen the microRNA expression profiles of CHD. Quantitative RT‐PCR and Western blot were employed to measure genes expression. Tg Cmlc2: GFP reporter zebrafish embryos were injected with microRNA (miRNA) to explore its role in cardiac development in vivo. Dual‐luciferase reporter assay was designed to validate the target gene of miRNAs. CCK‐8 and EdU incorporation assays were performed to evaluate cardiomyocyte proliferation.

Results

Our study showed miR‐29b‐3p expression was significantly increased in the RVOT of the CHD patients. Injection of miR‐29b‐3p into zebrafish embryos induced higher mortality and malformation rates, developmental delay, cardiac malformation and dysfunction. miR‐29b‐3p inhibited cardiomyocyte proliferation, and its inhibitor promoted cardiomyocyte proliferation in vitro and in vivo. Furthermore, we identified that miR‐29b‐3p influenced cardiomyocyte proliferation by targeting NOTCH2, which was down‐regulated in the RVOT of the CHD patients.

Conclusion

This study reveals that miR‐29b‐3p functions as a novel regulator of cardiac development and inhibits cardiomyocyte proliferation via NOTCH2, which provides novel insights into the aetiology and potential treatment of CHD.

Mature miR-99a Upregulation in the Amniotic Fluid Samples from Female Fetus Down Syndrome Pregnancies: A Pilot Study

Background and Objective: Although Down syndrome is the most frequent aneuploidy, its pathogenic molecular mechanisms are not yet fully understood. The aim of our study is to quantify-by qRT-PCR-the expression levels of both the mature forms and the pri-miRNAs of the microRNAs resident on chromosome 21 (miR(21)) in the amniotic fluid samples from Down syndrome singleton pregnancies and to estimate the impact of the differentially expressed microRNAs on Down syndrome fetal heart and amniocytes transcriptomes. Materials and methods: We collected amniotic fluid samples harvested by trained obstetricians as part of the second trimester screening/diagnostic procedure for aneuploidies to assess the trisomy 21 status by QF-PCR and karyotyping. Next, we evaluated-by Taqman qRT-PCR-the expression levels of both the mature forms and the pri-miRNA precursors of the microRNAs resident on chromosome 21 in amniotic fluid samples from singleton Down syndrome and euploid pregnancies. Further, we combined miRWalk 3.0 microRNA target prediction with GEO DataSets analysis to estimate the impact of hsa-miR-99a abnormal expression on Down syndrome heart and amniocytes transcriptome. Results: We found a statistically significant up-regulation of the mature form of miR-99a, but not pri-miR-99a, in the amniotic fluid samples from Down syndrome pregnancies with female fetuses. GATHER functional enrichment analysis of miRWalk3.0-predicted targets from Down syndrome amniocytes and fetal hearts transcriptome GEODataSets outlined both focal adhesion and cytokine-cytokine receptor interaction signaling as novel signaling pathways impacted by miR-99a and associated with cardiac defects in female Down syndrome patients. Conclusions: The significant overexpression of miR-99a, but not pri-miR-99a, points towards an alteration of the post-transcriptional mechanisms of hsa-miR-99a maturation and/or stability in the female trisomic milieu, with a potential impact on signaling pathways important for proper development of the heart.

The Role of Non-Coding RNA in Congenital Heart Diseases

Cardiovascular development is a complex developmental process starting with the formation of an early straight heart tube, followed by a rightward looping and the configuration of atrial and ventricular chambers. The subsequent step allows the separation of these cardiac chambers leading to the formation of a four-chambered organ. Impairment in any of these developmental processes invariably leads to cardiac defects. Importantly, our understanding of the developmental defects causing cardiac congenital heart diseases has largely increased over the last decades. The advent of the molecular era allowed to bridge morphogenetic with genetic defects and therefore our current understanding of the transcriptional regulation of cardiac morphogenesis has enormously increased. Moreover, the impact of environmental agents to genetic cascades has been demonstrated as well as of novel genomic mechanisms modulating gene regulation such as post-transcriptional regulatory mechanisms. Among post-transcriptional regulatory mechanisms, non-coding RNAs, including therein microRNAs and lncRNAs, are emerging to play pivotal roles. In this review, we summarize current knowledge on the functional role of non-coding RNAs in distinct congenital heart diseases, with particular emphasis on microRNAs and long non-coding RNAs.

Epigenetics, cardiovascular disease, and cellular reprogramming

Under the seeming disorder of "junk" sequences the last decade has seen developments in our understanding of non-coding RNA's (ncRNAs). It's a complex revised order and nowhere is this more relevant than in the developing heart whereby old rules have been set aside to make room for new ones. The development of the mammalian heart has been studied at the genetic and cellular level for several decades because these areas were considered ideal control points. As such, detailed mechanisms governing cell lineages are well described. Emerging evidence suggests a complex new order regulated by epigenetic mechanisms mark cardiac cell lineage. Indeed, molecular cardiologists are in the process of shedding light on the roles played by ncRNAs, nucleic acid methylation and histone/chromatin modifications in specific pathologies of the heart. The aim of this article is to discuss some of the recent advances in the field of cardiovascular epigenetics that are related to direct cell reprogramming and repair. As such, we explore ncRNAs as nodes regulating signaling networks and attempt to make sense of regulatory disorder by reinforcing the importance of epigenetic components in the developmental program.

Shifts in ovine cardiopulmonary microRNA expression in late gestation and the perinatal period

Background

MicroRNAs (miRNAs) have been identified as important contributors to the regulation of early fetal cardiopulmonary development. However, miRNA expression profiles during late gestation and the early neonatal period are not fully elaborated in large mammals such as sheep (ovis aries). The aim of this study was to sequence miRNA from cardiopulmonary tissues in late gestation and neonate sheep to identify changes in miRNA expression.

Methods

Illumina HiSeq next-generation deep sequencing (NGS) was performed on ovine tissues from the left (LV) and right ventricles (RV), lungs and pulmonary artery (PA) of preterm fetuses (128 days), near-term fetuses (140 days) (term = 148 days) and neonatal lambs (5 days). NGS reads were mapped to the sheep genome (OviAri) and published miRNA sequences.

Results

Of 1345 cardiopulmonary miRNAs that were sequenced, relatively few major shifts in miRNA expression were detected with increased age from near term to neonates, and were confirmed by quantitative real-time PCR: bta-miR-146a (lung), bta-miR-22-3p (lung, LV), hsa-miR-335* (lung, PA), and miR-210 (lung, PA, LV).

Conclusions

Sequencing of miRNA led to identification of four predominant miRNA in ovine cardiopulmonary tissues which alter expression during late gestation and the early neonatal period, concurrent with important functional changes in heart and lungs.

At the heart of programming: the role of micro-RNAs

Epidemiological and experimental observations tend to prove that environment, lifestyle or nutritional challenges influence heart functions together with genetic factors. Furthermore, when occurring during sensitive windows of heart development, these environmental challenges can induce an 'altered programming' of heart development and shape the future heart disease risk. In the etiology of heart diseases driven by environmental challenges, epigenetics has been highlighted as an underlying mechanism, constituting a bridge between environment and heart health. In particular, micro-RNAs which are involved in each step of heart development and functions seem to play a crucial role in the unfavorable programming of heart diseases. This review describes the latest advances in micro-RNA research in heart diseases driven by early exposure to challenges and discusses the use of micro-RNAs as potential targets in the reversal of the pathophysiology.

Association of MicroRNA Biogenesis Genes Polymorphisms with Ischemic Stroke Susceptibility and Post-Stroke Mortality

BACKGROUND AND PURPOSE

MicroRNA (miRNA) expression has been examined in multiple conditions, including various cancers, neurological diseases, and cerebrovascular diseases, particularly stroke. Existing evidence indicates that miRNA biosynthesis and function play crucial roles in ischemic stroke physiology and pathology. In this study, we selected six known polymorphisms in miRNA-biogenesis genes; DICER rs13078A>T, rs3742330A>G; DROSHA rs10719T>C, rs6877842G>C; Ran GTPase (RAN) rs14035C>T; exportin 5 (XPO5) rs11077A>C.

METHODS

We analyzed the associations between these polymorphisms and disease status and clinical factors in 585 ischemic stroke patients and 403 controls. Genotyping was performed with the polymerase chain reaction-restriction fragment length polymorphism method.

RESULTS

The DICER rs3742330A>G (AA vs. AG+GG: adjusted odds ratio [AOR], 1.360; 95% confidence interval [CI], 1.024 to 1.807; P=0.034) and DROSHA rs10719T>C polymorphisms (TT vs. CC: AOR, 2.038; 95% CI, 1.113 to 3.730; P=0.021) were associated with ischemic stroke prevalence. During a mean follow-up of 4.80±2.11 years, 99 (5.91%) of the stroke patients died. In multivariate Cox proportional hazard regression models, a significant association was found between RAN rs14035 and survival of large artery disease patients with ischemic stroke (CC vs. TT: adjusted hazard ratio, 5.978; P=0.015).

CONCLUSIONS

An association was identified between the DICER and DROSHA polymorphisms and ischemic stroke. Specifically, polymorphisms (rs3742330 and rs10719) were more common in stroke patients, suggesting that they may be associated with an increased risk of ischemic stroke.

Adverse Intrauterine Environment and Cardiac miRNA Expression

Placental insufficiency, high altitude pregnancies, maternal obesity/diabetes, maternal undernutrition and stress can result in a poor setting for growth of the developing fetus. These adverse intrauterine environments result in physiological changes to the developing heart that impact how the heart will function in postnatal life. The intrauterine environment plays a key role in the complex interplay between genes and the epigenetic mechanisms that regulate their expression. In this review we describe how an adverse intrauterine environment can influence the expression of miRNAs (a sub-set of non-coding RNAs) and how these changes may impact heart development. Potential consequences of altered miRNA expression in the fetal heart include; Hypoxia inducible factor (HIF) activation, dysregulation of angiogenesis, mitochondrial abnormalities and altered glucose and fatty acid transport/metabolism. It is important to understand how miRNAs are altered in these adverse environments to identify key pathways that can be targeted using miRNA mimics or inhibitors to condition an improved developmental response.

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.

MicroRNAs in injury and repair

Organ damage and resulting pathologies often involve multiple deregulated pathways. MicroRNAs (miRNAs) are short, non-coding RNAs that regulate a multitude of genes at the post-transcriptional level. Since their discovery over two decades ago, miRNAs have been established as key players in the molecular mechanisms of mammalian biology including the maintenance of normal homeostasis and the regulation of disease pathogenesis. In recent years, there has been substantial progress in innovative techniques to measure miRNAs along with advances in targeted delivery of agents modulating their expression. This has expanded the scope of miRNAs from being important mediators of cell signaling to becoming viable quantitative biomarkers and therapeutic targets. Currently, miRNA therapeutics are in clinical trials for multiple disease areas and vast numbers of patents have been filed for miRNAs involved in various pathological states. In this review, we summarize miRNAs involved in organ injury and repair, specifically with regard to organs that are the most susceptible to injury: the liver, heart and kidney. In addition, we review the current state of knowledge on miRNA biology, miRNA biomarkers and nucleotide-based therapeutics designed to target miRNAs to prevent organ injury and promote repair.

Epigenetic Biomarkers and Cardiovascular Disease: Circulating MicroRNAs

MicroRNAs (miRNAs) are a class of small noncoding RNA (20-25 nucleotides) involved in gene regulation. In recent years, miRNAs have emerged as a key epigenetic mechanism in the development and physiology of the cardiovascular system. These molecular species regulate basic functions in virtually all cell types, and are therefore directly associated with the pathophysiology of a large number of cardiovascular diseases. Since their relatively recent discovery in extracellular fluids, miRNAs have been studied as potential biomarkers of disease. A wide array of studies have proposed miRNAs as circulating biomarkers of different cardiovascular pathologies (eg, myocardial infarction, coronary heart disease, and heart failure, among others), which may have superior physicochemical and biochemical properties than the conventional protein indicators currently used in clinical practice. In the present review, we provide a brief introduction to the field of miRNAs, paying special attention to their potential clinical application. This includes their possible role as new diagnostic or prognostic biomarkers in cardiovascular disease.

Non-coding RNAs in Development and Disease: Background, Mechanisms, and Therapeutic Approaches

Advances in RNA-sequencing techniques have led to the discovery of thousands of non-coding transcripts with unknown function. There are several types of non-coding linear RNAs such as microRNAs (miRNA) and long non-coding RNAs (lncRNA), as well as circular RNAs (circRNA) consisting of a closed continuous loop. This review guides the reader through important aspects of non-coding RNA biology. This includes their biogenesis, mode of actions, physiological function, as well as their role in the disease context (such as in cancer or the cardiovascular system). We specifically focus on non-coding RNAs as potential therapeutic targets and diagnostic biomarkers.

Aneurysm-Specific miR-221 and miR-146a Participates in Human Thoracic and Abdominal Aortic Aneurysms

Altered microRNA expression is implicated in cardiovascular diseases. Our objective was to determine microRNA signatures in thoracic aortic aneurysms (TAAs) and abdominal aortic aneurysms (AAAs) compared with control non-aneurysmal aortic specimens. We evaluated the expression of fifteen selected microRNA in human TAA and AAA operative specimens compared to controls. We observed significant upregulation of miR-221 and downregulation of miR-1 and -133 in TAA specimens. In contrast, upregulation of miR-146a and downregulation of miR-145 and -331-3p were found only for AAA specimens. Upregulation of miR-126 and -486-5p and downregulation of miR-30c-2*, -155, and -204 were observed in specimens of TAAs and AAAs. The data reveal microRNA expression signatures unique to aneurysm location and common to both thoracic and abdominal pathologies. Thus, changes in miR-1, -29a, -133a, and -221 are involved in TAAs and miR-145, -146, and -331-3p impact AAAs. This work validates prior studies on microRNA expression in aneurysmal diseases.

Current Perspectives in Cardiac Laterality

The heart is the first organ to break symmetry in the developing embryo and onset of dextral looping is the first indication of this event. Looping is a complex process that progresses concomitantly to cardiac chamber differentiation and ultimately leads to the alignment of the cardiac regions in their final topology. Generation of cardiac asymmetry is crucial to ensuring proper form and consequent functionality of the heart, and therefore it is a highly regulated process. It has long been known that molecular left/right signals originate far before morphological asymmetry and therefore can direct it. The use of several animal models has led to the characterization of a complex regulatory network, which invariably converges on the Tgf-β signaling molecule Nodal and its downstream target, the homeobox transcription factor Pitx2. Here, we review current data on the cellular and molecular bases of cardiac looping and laterality, and discuss the contribution of Nodal and Pitx2 to these processes. A special emphasis will be given to the morphogenetic role of Pitx2 and to its modulation of transcriptional and functional properties, which have also linked laterality to atrial fibrillation.

HIRA Gene is Lower Expressed in the Myocardium of Patients with Tetralogy of Fallot

Background:

The most typical cardiac abnormality is conotruncal defects (CTDs) in patients with 22q11 deletion syndrome (22q11DS). HIRA (histone cell cycle regulator) gene, as one of the candidate genes located at the critical region of 22q11DS, was reported as possibly relevant to CTD in animal models. This study aimed to analyze the level of expression of the HIRA gene in tetralogy of Fallot (TOF) patients and the potential DNA sequence variations in the promoter region.

Methods:

The messenger RNA (mRNA) expression was examined with quantitative real-time polymerase chain reaction in 39 myocardial tissues of the right ventricular outflow tract (RVOT) from TOF patients and 4 myocardial tissues of RVOT from noncardiac death children. The protein expression was detected using immunohistochemistry in 12 TOF patients and 4 controls. A total of 100 TOF cases and 200 healthy controls were recruited for DNA sequencing.

Results:

The mRNA and protein expressions of the HIRA gene in the myocardium of the TOF patients were both significantly lower as compared to the controls (P < 0.05). Five single nucleotide polymorphisms (SNPs), including g.4111A>G (rs1128399), g.4265C>A (rs4585115), g.4369T>G (rs2277837), g.4371C>A (rs148516780), and g.4543T>C (rs111802956), were found in the promoter region of the HIRA gene. There were no significant differences of frequencies in these SNPs between the TOF patients and the controls (P > 0.05).

Conclusion:

The abnormal lower expression of the HIRA gene in the myocardium may participate in the pathogenesis of TOF.

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.

Overview of MicroRNAs in Cardiac Hypertrophy, Fibrosis, and Apoptosis

MicroRNAs (miRNAs) are non-coding RNAs that play essential roles in modulating the gene expression in almost all biological events. In the past decade, the involvement of miRNAs in various cardiovascular disorders has been explored in numerous in vitro and in vivo studies. In this paper, studies focused upon the discovery of miRNAs, their target genes, and functionality are reviewed. The selected miRNAs discussed herein have regulatory effects on target gene expression as demonstrated by miRNA/3′ end untranslated region (3′UTR) interaction assay and/or gain/loss-of-function approaches. The listed miRNA entities are categorized according to the biological relevance of their target genes in relation to three cardiovascular pathologies, namely cardiac hypertrophy, fibrosis, and apoptosis. Furthermore, comparison across 86 studies identified several candidate miRNAs that might be of particular importance in the ontogenesis of cardiovascular diseases as they modulate the expression of clusters of target genes involved in the progression of multiple adverse cardiovascular events. This review illustrates the involvement of miRNAs in diverse biological signaling pathways and provides an overview of current understanding of, and progress of research into, of the roles of miRNAs in cardiovascular health and disease.

MicroRNA and Heart Failure

Heart failure (HF) imposes significant economic and public health burdens upon modern society. It is known that disturbances in neurohormonal status play an important role in the pathogenesis of HF. Therapeutics that antagonize selected neurohormonal pathways, specifically the renin-angiotensin-aldosterone and sympathetic nervous systems, have significantly improved patient outcomes in HF. Nevertheless, mortality remains high with about 50% of HF patients dying within five years of diagnosis thus mandating ongoing efforts to improve HF management. The discovery of short noncoding microRNAs (miRNAs) and our increasing understanding of their functions, has presented potential therapeutic applications in complex diseases, including HF. Results from several genome-wide miRNA studies have identified miRNAs differentially expressed in HF cohorts suggesting their possible involvement in the pathogenesis of HF and their potential as both biomarkers and as therapeutic targets. Unravelling the functional relevance of miRNAs within pathogenic pathways is a major challenge in cardiovascular research. In this article, we provide an overview of the role of miRNAs in the cardiovascular system. We highlight several HF-related miRNAs reported from selected cohorts and review their putative roles in neurohormonal signaling.

MicroRNA Signature and Cardiovascular Dysfunction

The worldwide increase in the prevalence of obesity and type 2 diabetes and the associated elevated risk of cardiovascular disease (CVD) has emphasized the need to seek new therapeutic targets to offset the negative impact on human health outcomes. In this regards, microRNAs (miRNAs), a class of small noncoding RNAs that mediate posttranscriptional gene silencing, have received considerable interest. miRNAs repress gene expression by their ability to pair with target sequences in the 3' untranslated region of the messenger RNA. miRNAs play a crucial role in the biogenesis and function of the cardiovascular system and are implicated as dynamic regulators of cardiac and vascular signaling and pathophysiology. Numerous miRNAs have been identified as novel biomarkers and potential therapeutic targets for CVD. In this review, we discuss the contribution of miRNAs to the regulation of CVD, their role in macrovascular/microvascular (dys)function, their potential as important biomarkers for the early detection of CVD, and, finally, as therapeutic targets.

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.

Noncoding RNAs and pancreatic cancer

Noncoding RNAs (ncRNAs) represent a class of RNA molecules that typically do not code for proteins. Emerging data suggest that ncRNAs play an important role in several physiological and pathological conditions such as cancer. The best-characterized ncRNAs are the microRNAs (miRNAs), which are short, approximately 22-nucleotide sequences of RNA of approximately 22-nucleotide in length that regulate gene expression at the posttranscriptional level, through transcript degradation or translational repression. MiRNAs can function as master gene regulators, impacting a variety of cellular pathways important to normal cellular functions as well as cancer development and progression. In addition to miRNAs, long ncRNAs, which are transcripts longer than 200 nucleotides, have recently emerged as novel drivers of tumorigenesis. However, the molecular mechanisms of their regulation and function, and the significance of other ncRNAs such as piwi-interacting RNAs in pancreas carcinogenesis are largely unknown. This review summarizes the growing body of evidence supporting the vital roles of ncRNAs in pancreatic cancer, focusing on their dysregulation through both genetic and epigenetic mechanisms, and highlighting the promise of ncRNAs in diagnostic and therapeutic applications of pancreatic cancer.

MicroRNAs in congenital heart disease

Congenital heart disease (CHD) is a broad term which encompasses a spectrum of pathology, the most common phenotypes include atrial septal defects (ASDs), ventricular septal defects (VSDs), patent ductus arteriosus (PAD) and tetralogy of Fallot (TOF). The impact of CHD is profound and it is estimated to be responsible for over 40% of prenatal deaths. MicroRNAs (miRs) are small, highly conserved, non-coding RNAs which have complex roles in a variety of pathophysiological states. miRs are post-transcriptional negative regulators of gene expression. Individual miRs are known to exert effects in multiple target genes, therefore the altered expression of a single miR could influence an entire gene network resulting in complex pathological states. Recent evidences suggest a role in the dysregulation of miRs in CHD. Mouse knock out models have contributed to our knowledge base revealing specific patterns of miR expression in cardiovascular physiology and pathological states. Specific miRs necessary for embryonic cardiac development have been revealed. Dysregulation of these miRs has been shown to cause structural abnormalities in the heart and vasculature, thus furthering our understanding of the processes which result in CHD. These advances have provided new insight into the signalling pathways responsible for CHD. Furthermore, this new appreciation for miRs in the development of CHD has uncovered their potential for new therapeutic targets where modulated miR activity may reduce the burden of disease. Here, we summarize current knowledge of the cause-effect relationships of miRs in CHD and consider their potential as a therapeutic targets and biomarkers in this clinical setting.

MicroRNAs and Cardiac Regeneration

The human heart has a limited capacity to regenerate lost or damaged cardiomyocytes after cardiac insult. Instead, myocardial injury is characterized by extensive cardiac remodeling by fibroblasts, resulting in the eventual deterioration of cardiac structure and function. Cardiac function would be improved if these fibroblasts could be converted into cardiomyocytes. MicroRNAs (miRNAs), small noncoding RNAs that promote mRNA degradation and inhibit mRNA translation, have been shown to be important in cardiac development. Using this information, various researchers have used miRNAs to promote the formation of cardiomyocytes through several approaches. Several miRNAs acting in combination promote the direct conversion of cardiac fibroblasts into cardiomyocytes. Moreover, several miRNAs have been identified that aid the formation of inducible pluripotent stem cells and miRNAs also induce these cells to adopt a cardiac fate. MiRNAs have also been implicated in resident cardiac progenitor cell differentiation. In this review, we discuss the current literature as it pertains to these processes, as well as discussing the therapeutic implications of these findings.

miR-27 and miR-125 Distinctly Regulate Muscle-Enriched Transcription Factors in Cardiac and Skeletal Myocytes

MicroRNAs are noncoding RNAs of approximately 22–24 nucleotides which are capable of interacting with the 3′ untranslated region of coding RNAs (mRNAs), leading to mRNA degradation and/or protein translation blockage. In recent years, differential microRNA expression in distinct cardiac development and disease contexts has been widely reported, yet the role of individual microRNAs in these settings remains largely unknown. We provide herein evidence of the role of miR-27 and miR-125 regulating distinct muscle-enriched transcription factors. Overexpression of miR-27 leads to impair expression of Mstn and Myocd in HL1 atrial cardiomyocytes but not in Sol8 skeletal muscle myoblasts, while overexpression of miR-125 resulted in selective upregulation of Mef2d in HL1 atrial cardiomyocytes and downregulation in Sol8 cells. Taken together our data demonstrate that a single microRNA, that is, miR-27 or miR-125, can selectively upregulate and downregulate discrete number of target mRNAs in a cell-type specific manner.

Targeting microRNAs in heart failure

MicroRNAs play pivotal roles in cardiac disease, and their therapeutic modulation raises exciting and unique opportunities, as well as challenges in the path toward clinical development and implementation. In this review, we provide a detailed overview of recent studies highlighting the important role of microRNAs in heart failure (HF) and the potential use of microRNA-based technology for diagnosis, prevention, and treatment of HF. We will focus on the strategies presently used for microRNA-based therapy by discussing their use and drawbacks, as well as the challenges and future directions for their development in the context of human HF.

Mechanisms and therapeutic potential of microRNAs in hypertension

Hypertension is the major risk factor for the development of stroke, coronary artery disease, heart failure and renal disease. The underlying cellular and molecular mechanisms of hypertension are complex and remain largely elusive. MicroRNAs (miRNAs) are short, noncoding RNA fragments of 22-26 nucleotides and regulate protein expression post-transcriptionally by targeting the 3'-untranslated region of mRNA. A growing body of recent research indicates that miRNAs are important in the pathogenesis of arterial hypertension. Herein, we summarize the current knowledge regarding the mechanisms of miRNAs in cardiovascular remodeling, focusing specifically on hypertension. We also review recent progress of the miRNA-based therapeutics including pharmacological and nonpharmacological therapies (such as exercise training) and their potential applications in the management of hypertension.

Decoding the complex genetic causes of heart diseases using systems biology

The pace of disease gene discovery is still much slower than expected, even with the use of cost-effective DNA sequencing and genotyping technologies. It is increasingly clear that many inherited heart diseases have a more complex polygenic aetiology than previously thought. Understanding the role of gene-gene interactions, epigenetics, and non-coding regulatory regions is becoming increasingly critical in predicting the functional consequences of genetic mutations identified by genome-wide association studies and whole-genome or exome sequencing. A systems biology approach is now being widely employed to systematically discover genes that are involved in heart diseases in humans or relevant animal models through bioinformatics. The overarching premise is that the integration of high-quality causal gene regulatory networks (GRNs), genomics, epigenomics, transcriptomics and other genome-wide data will greatly accelerate the discovery of the complex genetic causes of congenital and complex heart diseases. This review summarises state-of-the-art genomic and bioinformatics techniques that are used in accelerating the pace of disease gene discovery in heart diseases. Accompanying this review, we provide an interactive web-resource for systems biology analysis of mammalian heart development and diseases, CardiacCode ( http://CardiacCode.victorchang.edu.au/ ). CardiacCode features a dataset of over 700 pieces of manually curated genetic or molecular perturbation data, which enables the inference of a cardiac-specific GRN of 280 regulatory relationships between 33 regulator genes and 129 target genes. We believe this growing resource will fill an urgent unmet need to fully realise the true potential of predictive and personalised genomic medicine in tackling human heart disease.

A Fleeting Glimpse Inside microRNA, Epigenetics, and Micropeptidomics

MicroRNAs (miRs) are important regulators of gene expression in numerous biological processes. Their maturation process is herein described, including the most updated insights from the current literature. Circa 2000 miR sequences have been identified in the human genome, with over 50,000 miR-target interactions, including enzymes involved in epigenetic modulation of gene expression. Moreover, some "pieces of RNA" previously annotated as noncoding have been recently found to encode micropeptides that carry out critical mechanistic functions in the cell. Advanced techniques now available will certainly allow a precise scanning of the genome looking for micropeptides hidden within the "noncoding" RNA.

microRNA and Cardiac Regeneration

Heart diseases are a very common health problem in developed as well as developing countries. In particular, ischemic heart disease and heart failure represent a plague for the patients and for the society. Loss of cardiac tissue after myocardial infarction or dysfunctioning tissue in nonischemic cardiomyopathies may result in cardiac failure. Despite great advancements in the treatment of these diseases, there is a substantial unmet need for novel therapies, ideally addressing repair and regeneration of the damaged or lost myocardium. Along this line, cardiac cell based therapies have gained substantial attention. Three main approaches are currently under investigation: stem cell therapy with either embryonic or adult stem cells; generation of patient-specific induced pluripotent stem cells; stimulation of endogenous regeneration trough direct reprogramming of fibroblasts into cardiomyocytes, activation of resident cardiac stem cells or induction of native resident cardiomyocytes to reenter the cell cycle. All these strategies need to be optimized since their efficiency is low.It has recently become clear that cardiac signaling and transcriptional pathways are intimately intertwined with microRNA molecules which act as modulators of cardiac development, function, and disease. Moreover, miRNA also regulates stem cell differentiation. Here we describe how miRNA may circumvent hurdles that hamper the field of cardiac regeneration and stem cell therapy, and how miRNA may result as the most suitable solution for the damaged heart.

Regulation of Cardiac Cell Fate by microRNAs: Implications for Heart Regeneration

microRNAs are post-transcriptional regulators of gene expression that have been shown to be central players in the establishment of cellular programs, often acting as switches that control the choice between proliferation and differentiation during development and in adult tissues. The heart develops from two small patches of cells in the mesoderm, the heart fields, which originate the different cardiac cell types, including cardiomyocytes, vascular smooth muscle and endothelial cells. These progenitors proliferate and differentiate to establish a highly connected three-dimensional structure, involving a robust succession of gene expression programs strongly influenced by microRNAs. Although the mammalian heart has conventionally been viewed as a post-mitotic organ, cardiac cells have recently been shown to display some regenerative potential, which is nonetheless insufficient to regenerate heart lesions, in contrast with other vertebrates like the zebrafish. Both the proliferation of adult cardiac stem cells and the ability of cardiomyocytes to re-enter the cell cycle have been proposed to sustain these regenerative processes. Here we review the role of microRNAs in the control of stem cell and cardiomyocyte dependent cardiac regeneration processes, and discuss potential applications for the treatment of cardiac injury.

Genetic networks governing heart development

Animal genomes contain a code for construction of the body plan from a fertilized egg. Understanding how genome information is deciphered to create the complex multilayered regulatory systems that drive organismal development, and which become altered in disease, is one of the greatest challenges in the biological sciences. The development of methods that effectively represent and communicate the complexity inherent in gene regulatory networks remains a major barrier. This review introduces the philosophy of systems biology and discusses recent progress in understanding the development of the heart at a systems biology level.

How the proteome packages the genome for cardiovascular development

The devastating impact of congenital heart defects has made mechanisms of vertebrate heart and vascular development an active area of study. Because myocyte death is a common feature of acquired cardiovascular diseases and the adult heart does not regenerate, the need exists to understand whether features of the developing heart and vasculature-which are more plastic-can be exploited therapeutically in the disease setting. We know that a core network of transcription factors governs commitment to the cardiovascular lineage, and recent studies using genetic loss-of-function approaches and unbiased genomic studies have revealed the role for various chromatin modulatory events. We reason that chromatin structure itself is a causal feature that influences transcriptome complexity along a developmental continuum, and the purpose of this article is to highlight the areas in which 'omics technologies have the potential to reveal new principles of phenotypic plasticity in development. We review the major mechanisms of chromatin structural regulation, highlighting what is known about their actions to control cardiovascular differentiation. We discuss emergent mechanisms of regulation that have been identified on the basis of genomic and proteomic studies of cardiac nuclei and identify current challenges to an integrated understanding of chromatin structure and cardiovascular phenotype.

MiRiad Roles for MicroRNAs in Cardiac Development and Regeneration

Cardiac development is an exquisitely regulated process that is sensitive to perturbations in transcriptional activity and gene dosage. Accordingly, congenital heart abnormalities are prevalent worldwide, and are estimated to occur in approximately 1% of live births. Recently, small non-coding RNAs, known as microRNAs, have emerged as critical components of the cardiogenic regulatory network, and have been shown to play numerous roles in the growth, differentiation, and morphogenesis of the developing heart. Moreover, the importance of miRNA function in cardiac development has facilitated the identification of prospective therapeutic targets for patients with congenital and acquired cardiac diseases. Here, we discuss findings attesting to the critical role of miRNAs in cardiogenesis and cardiac regeneration, and present evidence regarding the therapeutic potential of miRNAs for cardiovascular diseases.