MicroRNA-208a is a regulator of cardiac hypertrophy and conduction in mice

microRNAs in hypertrophy and heart failure

microRNAs (miRNAs) are highly conserved, short non-coding RNAs that have become established as having an important role in regulatory networks. We review here the progress made in our understanding of heart-related microRNAs over the last two years, focusing mainly on hypertrophic growth and heart failure.

Pervasive roles of microRNAs in cardiovascular biology

First recognized as regulators of development in worms and fruitflies, microRNAs are emerging as pivotal modulators of mammalian cardiovascular development and disease. Individual microRNAs modulate the expression of collections of messenger RNA targets that often have related functions, thereby governing complex biological processes. The wideranging functions of microRNAs in the cardiovascular system have provided new perspectives on disease mechanisms and have revealed intriguing therapeutic targets, as well as diagnostics, for a variety of cardiovascular disorders.

What causes a broken heart--molecular insights into heart failure

Our understanding of the molecular processes which regulate cardiac function has grown immeasurably in recent years. Even with the advent of β-blockers, angiotensin inhibitors and calcium modulating agents, heart failure (HF) still remains a seriously debilitating and life-threatening condition. Here, we review the molecular changes which occur in the heart in response to increased load and the pathways which control cardiac hypertrophy, calcium homeostasis, and immune activation during HF. These can occur as a result of genetic mutation in the case of hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) or as a result of ischemic or hypertensive heart disease. In the majority of cases, calcineurin and CaMK respond to dysregulated calcium signaling and adrenergic drive is increased, each of which has a role to play in controlling blood pressure, heart rate, and left ventricular function. Many major pathways for pathological remodeling converge on a set of transcriptional regulators such as myocyte enhancer factor 2 (MEF2), nuclear factors of activated T cells (NFAT), and GATA4 and these are opposed by the action of the natriuretic peptides ANP and BNP. Epigenetic modification has emerged in recent years as a major influence cardiac physiology and histone acetyl transferases (HATs) and histone deacetylases (HDACs) are now known to both induce and antagonize hypertrophic growth. The newly emerging roles of microRNAs in regulating left ventricular dysfunction and fibrosis also has great potential for novel therapeutic intervention. Finally, we discuss the role of the immune system in mediating left ventricular dysfunction and fibrosis and ways this can be targeted in the setting of viral myocarditis.

MicroRNA-27a regulates beta cardiac myosin heavy chain gene expression by targeting thyroid hormone receptor beta1 in neonatal rat ventricular myocytes

MicroRNAs (miRNAs), small noncoding RNAs, are negative regulators of gene expression and play important roles in gene regulation in the heart. To examine the role of miRNAs in the expression of the two isoforms of the cardiac myosin heavy chain (MHC) gene, α- and β-MHC, which regulate cardiac contractility, endogenous miRNAs were downregulated in neonatal rat ventricular myocytes (NRVMs) using lentivirus-mediated small interfering RNA (siRNA) against Dicer, an essential enzyme for miRNA biosynthesis, and MHC expression levels were examined. As a result, Dicer siRNA could downregulate endogenous miRNAs simultaneously and the β-MHC gene but not α-MHC, which implied that specific miRNAs could upregulate the β-MHC gene. Among 19 selected miRNAs, miR-27a was found to most strongly upregulate the β-MHC gene but not α-MHC. Moreover, β-MHC protein was downregulated by silencing of endogenous miR-27a. Through a bioinformatics screening using TargetScan, we identified thyroid hormone receptor β1 (TRβ1), which negatively regulates β-MHC transcription, as a target of miR-27a. Moreover, miR-27a was demonstrated to modulate β-MHC gene regulation via thyroid hormone signaling and to be upregulated during the differentiation of mouse embryonic stem (ES) cells or in hypertrophic hearts in association with β-MHC gene upregulation. These findings suggested that miR-27a regulates β-MHC gene expression by targeting TRβ1 in cardiomyocytes.

Common microRNA signatures in cardiac hypertrophic and atrophic remodeling induced by changes in hemodynamic load

Background

Mechanical overload leads to cardiac hypertrophy and mechanical unloading to cardiac atrophy. Both conditions produce similar transcriptional changes including a re-expression of fetal genes, despite obvious differences in phenotype. MicroRNAs (miRNAs) are discussed as superordinate regulators of global gene networks acting mainly at the translational level. Here, we hypothesized that defined sets of miRNAs may determine the direction of cardiomyocyte plasticity responses.

Methodology/Principal Findings

We employed ascending aortic stenosis (AS) and heterotopic heart transplantation (HTX) in syngenic Lewis rats to induce mechanical overloading and unloading, respectively. Heart weight was 26±3% higher in AS (n = 7) and 33±2% lower in HTX (n = 7) as compared to sham-operated (n = 6) and healthy controls (n = 7). Small RNAs were enriched from the left ventricles and subjected to quantitative stem-loop specific RT-PCR targeting a panel of 351 miRNAs. In total, 153 miRNAs could be unambiguously detected. Out of 72 miRNAs previously implicated in the cardiovascular system, 40 miRNAs were regulated in AS and/or HTX. Overall, HTX displayed a slightly broader activation pattern for moderately regulated miRNAs. Surprisingly, however, the regulation of individual miRNA expression was strikingly similar in direction and amplitude in AS and HTX with no miRNA being regulated in opposite direction. In contrast, fetal hearts from Lewis rats at embryonic day 18 exhibited an entirely different miRNA expression pattern.

Conclusions

Taken together, our findings demonstrate that opposite changes in cardiac workload induce a common miRNA expression pattern which is markedly different from the fetal miRNA expression pattern. The direction of postnatal adaptive cardiac growth does, therefore, not appear to be determined at the level of single miRNAs or a specific set of miRNAs. Moreover, miRNAs themselves are not reprogrammed to a fetal program in response to changes in hemodynamic load.

miRNAs got rhythm

Despite significant advances in treatments, cardiovascular disease (CVD) remains the leading cause of human morbidity and mortality in developed countries. The development of novel and efficient treatment strategies requires an understanding of the basic molecular mechanisms underlying cardiac function. MicroRNAs (miRNAs) are a family of small nonprotein-coding RNAs that have emerged as important regulators in cardiac and vascular developmental and pathological processes, including cardiac arrhythmia, fibrosis, hypertrophy and ischemia, heart failure and vascular atherosclerosis. The miRNA acts as an adaptor for the miRNA-induced silencing complex (miRISC) to specifically recognize and regulate particular mRNAs. Mature miRNAs recognize their target mRNAs by base-pairing interactions between nucleotides 2 and 8 of the miRNA (the seed region) and complementary nucleotides in the 3'-untranslated region (3'-UTR) of mRNAs and miRISCs subsequently inhibit gene expression by targeting mRNAs for translational repression or cleavage. In this review we summarize the basic mechanisms of action of miRNAs as they are related to cardiac arrhythmia and address the potential for miRNAs to be therapeutically manipulated in the treatment of arrhythmias.

MicroRNA-218 regulates vascular patterning by modulation of Slit-Robo signaling

RATIONALE

Establishment of a functional vasculature requires the interconnection and remodeling of nascent blood vessels. Precise regulation of factors that influence endothelial cell migration and function is essential for these stereotypical vascular patterning events. The secreted Slit ligands and their Robo receptors constitute a critical signaling pathway controlling the directed migration of both neurons and vascular endothelial cells during embryonic development, but the mechanisms of their regulation are incompletely understood.

OBJECTIVE

To identify microRNAs regulating aspects of the Slit-Robo pathway and vascular patterning.

METHODS AND RESULTS

Here, we provide evidence that microRNA (miR)-218, which is encoded by an intron of the Slit genes, inhibits the expression of Robo1 and Robo2 and multiple components of the heparan sulfate biosynthetic pathway. Using in vitro and in vivo approaches, we demonstrate that miR-218 directly represses the expression of Robo1, Robo2, and glucuronyl C5-epimerase (GLCE), and that an intact miR-218-Slit-Robo regulatory network is essential for normal vascularization of the retina. Knockdown of miR-218 results in aberrant regulation of this signaling axis, abnormal endothelial cell migration, and reduced complexity of the retinal vasculature.

CONCLUSIONS

Our findings link Slit gene expression to the posttranscriptional regulation of Robo receptors and heparan sulfate biosynthetic enzymes, allowing for precise control over vascular guidance cues influencing the organization of blood vessels during development.

MicroRNAs in cardiac hypertrophy: angels or devils

MicroRNAs (miRNAs) are short noncoding RNA molecules that can regulate gene expression via affecting mRNA stability or translation efficiency. miRNAs mediate many important cellular processes and emerge as a newly discovered regulator of gene expression. In cardiac hypertrophy, miRNAs expression is aberrantly altered. Some of these miRNAs can promote cardiac hypertrophy, whereas others can inhibit the process. In this review, we summarize the up- and downregulated miRNAs during cardiac hypertrophy and discuss about their roles in cardiac hypertrophy. The studies on miRNAs shed new light on the mechanism of cardiac hypertrophy and suggest that they may be promising therapeutic targets in tackling cardiac hypertrophy.

Posttranscriptional mechanisms involving microRNA-27a and b contribute to fast-specific and glucocorticoid-mediated myostatin expression in skeletal muscle

Expression of the antigrowth factor myostatin (MSTN) differs between fast and slow skeletal muscles and is increased in nearly every form of muscle atrophy, but the contribution of transcriptional vs. posttranscriptional mechanisms to its differing expression in these states has not been defined. We show here that levels of mature MSTN mRNA were sixfold greater in fast vs. slow muscle and were increased twofold in fast muscle in response to dexamethasone (Dex) injection in vivo and in C₂C₁₂ myotubes following Dex treatment in vitro, but that levels of MSTN pre-mRNA, a readout of transcription, only minimally and nonsignificantly differed in these states. Moreover, Dex treatment with or without cotransfection with a glucocorticoid receptor expression construct had only modest effects on mouse MSTN promoter activity in C₂C₁₂ myotubes. We therefore explored the potential contribution of posttranscriptional mechanisms, and the role of the microRNAs miR-27a and b in particular, on MSTN expression. The MSTN 3'-untranslated region (UTR) contains a putative recognition sequence for miR-27a and b that is conserved across a wide range of vertebrate species. Cotransfection of a MSTN 3'-UTR-luciferase construct with a miR-27b expression construct significantly attenuated by approximately half while mutation of the miR-27 recognition sequence significantly increased by approximately twofold the activity of a MSTN 3'-UTR construct and decreased mRNA degradation of a luciferase reporter construct in C₂C₁₂ myotubes. Expression of miR-27a and b was almost sixfold greater in slow-twitch than in fast-twitch muscle in vivo, and miR-27a expression was significantly decreased by nearly half by glucocorticoid treatment in vitro. Finally, the miR-27a and b promoters were activated by cotransfection with the slow-specific signaling molecules calcineurin and peroxisome proliferator-activated receptor-γ coactivator-1α. The present data represent the first demonstration that posttranscriptional mechanisms involving miR-27a and b may contribute to fast-specific and glucocorticoid-dependent myostatin expression in muscle.

Analysis of microRNA knockouts in mice

MicroRNAs (miRNAs) are small noncoding RNAs that act as potent regulators of gene expression. The discovery of miRNAs with specific temporal and spatial expression patterns revealed a hidden layer of post-transcriptional gene regulation. Furthermore, differential expression of miRNAs during disease progression identified miRNAs as relevant candidate genes in human pathologies. Currently the exact roles of miRNAs in human development and disease progression remain largely unknown. There have been recent efforts to study the loss of these genes in vivo and this review will discuss published miRNA knockout mouse models, highlighting their potential mechanisms of action in vivo.

MicroRNA-based therapeutic approaches in the cardiovascular system

MicroRNAs (miRNAs) are endogenous small ribonucleotides that participate in the orchestration of the genome by regulating target messenger RNA translation. MiRNAs control physiological processes and misexpression of miRNAs is pathogenically involved in many diseases including cardiovascular diseases. Normalization of miRNA expression and thus downstream target networks may have enormous therapeutic chances but also risks. We here highlight and discuss recent advances in the development and use of miRNA therapeutics to target miRNAs in vivo that may translate into novel therapeutic strategies for cardiovascular diseases in the future.

Genome-wide association analysis identifies multiple loci related to resting heart rate

Higher resting heart rate is associated with increased cardiovascular disease and mortality risk. Though heritable factors play a substantial role in population variation, little is known about specific genetic determinants. This knowledge can impact clinical care by identifying novel factors that influence pathologic heart rate states, modulate heart rate through cardiac structure and function or by improving our understanding of the physiology of heart rate regulation. To identify common genetic variants associated with heart rate, we performed a meta-analysis of 15 genome-wide association studies (GWAS), including 38,991 subjects of European ancestry, estimating the association between age-, sex- and body mass-adjusted RR interval (inverse heart rate) and approximately 2.5 million markers. Results with P < 5 × 10(-8) were considered genome-wide significant. We constructed regression models with multiple markers to assess whether results at less stringent thresholds were likely to be truly associated with RR interval. We identified six novel associations with resting heart rate at six loci: 6q22 near GJA1; 14q12 near MYH7; 12p12 near SOX5, c12orf67, BCAT1, LRMP and CASC1; 6q22 near SLC35F1, PLN and c6orf204; 7q22 near SLC12A9 and UfSp1; and 11q12 near FADS1. Associations at 6q22 400 kb away from GJA1, at 14q12 MYH6 and at 1q32 near CD34 identified in previously published GWAS were confirmed. In aggregate, these variants explain approximately 0.7% of RR interval variance. A multivariant regression model including 20 variants with P < 10(-5) increased the explained variance to 1.6%, suggesting that some loci falling short of genome-wide significance are likely truly associated. Future research is warranted to elucidate underlying mechanisms that may impact clinical care.

Mechanical stretch up-regulates microRNA-26a and induces human airway smooth muscle hypertrophy by suppressing glycogen synthase kinase-3β

Airway smooth muscle hypertrophy is one of the hallmarks of airway remodeling in severe asthma. Several human diseases have been now associated with dysregulated microRNA (miRNA) expression. miRNAs are a class of small non-coding RNAs, which negatively regulate gene expression at the post-transcriptional level. Here, we identify miR-26a as a hypertrophic miRNA of human airway smooth muscle cells (HASMCs). We show that stretch selectively induces the transcription of miR-26a located in the locus 3p21.3 of human chromosome 3. The transcription factor CCAAT enhancer-binding protein α (C/EBPα) directly activates miR-26a expression through the transcriptional machinery upon stretch. Furthermore, stretch or enforced expression of miR-26a induces HASMC hypertrophy, and miR-26 knockdown reverses this effect, suggesting that miR-26a is a hypertrophic gene. We identify glycogen synthase kinase-3β (GSK-3β), an anti-hypertrophic protein, as a target gene of miR-26a. Luciferase reporter assays demonstrate that miR-26a directly interact with the 3'-untranslated repeat of the GSK-3β mRNA. Stretch or enforced expression of miR-26a attenuates the endogenous GSK-3β protein levels followed by the induction of HASMC hypertrophy. miR-26 knockdown reverses this effect, suggesting that miR-26a-induced hypertrophy occurs via its target gene GSK-3β. Overall, as a first time, our study unveils that miR-26a is a mechanosensitive gene, and it plays an important role in the regulation of HASMC hypertrophy.

MicroRNAs in mouse development and disease

MicroRNAs, small non-coding RNAs which act as repressors of target genes, were discovered in 1993, and since then have been shown to play important roles in the development of numerous systems. Consistent with this role, they are also implicated in the pathogenesis of multiple diseases. Here we review the involvement of microRNAs in mouse development and disease, with particular reference to deafness as an example.

Clinical applications of miRNAs in cardiac remodeling and heart failure

MicroRNAs (miRNAs or miRs) are short, highly conserved noncoding RNAs that regulate gene expression at the post-transcriptional level by inhibiting translation or promoting the degradation of target mRNA. Even though the field of miRNA biology is relatively young, growing lines of evidence suggest that miRNAs play a key role pathogenesis of heart failure through their ability to regulate genes that govern the process of adaptive and maladaptive cardiac remodeling. Herein, we review the biology of miRNAs in relation to their role in modulating various aspects of the cardiac remodeling process, as well as discuss the potential applications of miRNA biology to the field of heart failure.

Connexins: sensors and regulators of cell cycling

It is nowadays well established that gap junctions are critical gatekeepers of cell proliferation, by controlling the intercellular exchange of essential growth regulators. In recent years, however, it has become clear that the picture is not as simple as originally anticipated, as structural precursors of gap junctions can affect cell cycling by performing actions not related to gap junctional intercellular communication. Indeed, connexin hemichannels also foresee a pathway for cell growth communication, albeit between the intracellular compartment and the extracellular environment, while connexin proteins as such can directly or indirectly influence the production of cell cycle regulators independently of their channel activities. Furthermore, a novel set of connexin-like proteins, the pannexins, have lately joined in as regulators of the cell proliferation process, which they can affect as either single units or as channel entities. In the current paper, these multifaceted aspects of connexin-related signalling in cell cycling are reviewed.

MicroRNA profiling during mouse ventricular maturation: a role for miR-27 modulating Mef2c expression

AIMS

non-coding RNA has been recently demonstrated to be a novel mechanism for modulation of gene expression at the post-transcriptional level. The importance of microRNAs in the cardiovascular system is now apparent. Mutations of distinct microRNAs have provided evidence for fundamental roles of microRNAs during cardiovascular development. However, there is limited information about global microRNA profiles during mouse heart development. In this study, we have gained insight from the expression profiles of microRNAs during mouse ventricular development by microarray and qRT-PCR analysis.

METHODS AND RESULTS

our microarray analysis reveals that relatively few microRNAs display either increasing or decreasing expression profiles during ventricular chamber formation. Interestingly, most of the differentially expressed microRNAs display a rather discrete peak of expression at particular developmental stages. Furthermore, we demonstrate that microRNA-27b (miR-27b) displays an overt myocardial expression during heart development and that the transcription factor-encoding gene Mef2c is an miR-27b target.

CONCLUSION

our data present a comprehensive profile of microRNA expression during ventricular maturation, providing an entry point for investigation of the functional roles of the most abundantly and differentially expressed microRNAs during cardiogenesis.

Dynamic microRNA expression programs during cardiac differentiation of human embryonic stem cells: role for miR-499

BACKGROUND

MicroRNAs (miRNAs) are a newly discovered endogenous class of small, noncoding RNAs that play important posttranscriptional regulatory roles by targeting messenger RNAs for cleavage or translational repression. Human embryonic stem cells are known to express miRNAs that are often undetectable in adult organs, and a growing body of evidence has implicated miRNAs as important arbiters of heart development and disease.

METHODS AND RESULTS

To better understand the transition between the human embryonic and cardiac "miRNA-omes," we report here the first miRNA profiling study of cardiomyocytes derived from human embryonic stem cells. Analyzing 711 unique miRNAs, we have identified several interesting miRNAs, including miR-1, -133, and -208, that have been previously reported to be involved in cardiac development and disease and that show surprising patterns of expression across our samples. We also identified novel miRNAs, such as miR-499, that are strongly associated with cardiac differentiation and that share many predicted targets with miR-208. Overexpression of miR-499 and -1 resulted in upregulation of important cardiac myosin heavy-chain genes in embryoid bodies; miR-499 overexpression also caused upregulation of the cardiac transcription factor MEF2C.

CONCLUSIONS

Taken together, our data give significant insight into the regulatory networks that govern human embryonic stem cell differentiation and highlight the ability of miRNAs to perturb, and even control, the genes that are involved in cardiac specification of human embryonic stem cells.

The emerging role of microRNAs as a therapeutic target for cardiovascular disease

MicroRNAs (miRNAs) are a class of small non-coding RNAs that are endogenously transcribed and processed into approximately 21- to approximately 23-nucleotide products. They are believed to function predominantly as sequence-targeted modifiers of gene expression through inhibition of post-transcriptional processes, including messenger RNA degradation and translational repression. Rapid expansion of functional studies of miRNAs in recent years has established a new paradigm in which miRNAs 'fine-tune' gene expression in complex biologic networks. In this review, we summarize the role of miRNA in heart development and cardiac pathogenesis, and discuss the implication of miRNAs as innovative therapeutic approaches for cardiovascular diseases.

Molecular distinction between physiological and pathological cardiac hypertrophy: experimental findings and therapeutic strategies

Cardiac hypertrophy can be defined as an increase in heart mass. Pathological cardiac hypertrophy (heart growth that occurs in settings of disease, e.g. hypertension) is a key risk factor for heart failure. Pathological hypertrophy is associated with increased interstitial fibrosis, cell death and cardiac dysfunction. In contrast, physiological cardiac hypertrophy (heart growth that occurs in response to chronic exercise training, i.e. the 'athlete's heart') is reversible and is characterized by normal cardiac morphology (i.e. no fibrosis or apoptosis) and normal or enhanced cardiac function. Given that there are clear functional, structural, metabolic and molecular differences between pathological and physiological hypertrophy, a key question in cardiovascular medicine is whether mechanisms responsible for enhancing function of the athlete's heart can be exploited to benefit patients with pathological hypertrophy and heart failure. This review summarizes key experimental findings that have contributed to our understanding of pathological and physiological heart growth. In particular, we focus on signaling pathways that play a causal role in the development of pathological and physiological hypertrophy. We discuss molecular mechanisms associated with features of cardiac hypertrophy, including protein synthesis, sarcomeric organization, fibrosis, cell death and energy metabolism and provide a summary of profiling studies that have examined genes, microRNAs and proteins that are differentially expressed in models of pathological and physiological hypertrophy. How gender and sex hormones affect cardiac hypertrophy is also discussed. Finally, we explore how knowledge of molecular mechanisms underlying pathological and physiological hypertrophy may influence therapeutic strategies for the treatment of cardiovascular disease and heart failure.

MicroRNA regulatory networks in cardiovascular development

The heart, more than any other organ, requires precise functionality on a second-to-second basis throughout the lifespan of the organism. Even subtle perturbations in cardiac structure or function have catastrophic consequences, resulting in lethal forms of congenital and adult heart disease. Such intolerance of the heart to variability necessitates especially robust regulatory mechanisms to govern cardiac gene expression. Recent studies have revealed central roles for microRNAs (miRNAs) as governors of gene expression during cardiovascular development and disease. The integration of miRNAs into the genetic circuitry of the heart provides a rich and robust array of regulatory interactions to control cardiac gene expression. miRNA regulatory networks also offer opportunities for therapeutically modulating cardiac function through the manipulation of pathogenic and protective miRNAs. We discuss the roles of miRNAs as regulators of cardiac form and function, unresolved questions in the field, and issues for the future.

Intronic microRNAs support their host genes by mediating synergistic and antagonistic regulatory effects

BACKGROUND

MicroRNA-mediated control of gene expression via translational inhibition has substantial impact on cellular regulatory mechanisms. About 37% of mammalian microRNAs appear to be located within introns of protein coding genes, linking their expression to the promoter-driven regulation of the host gene. In our study we investigate this linkage towards a relationship beyond transcriptional co-regulation.

RESULTS

Using measures based on both annotation and experimental data, we show that intronic microRNAs tend to support their host genes by regulation of target gene expression with significantly correlated expression patterns. We used expression data of three differentiating cell types and compared gene expression profiles of host and target genes. Many microRNA target genes show expression patterns significantly correlated with the expressions of the microRNA host genes. By calculating functional similarities between host and predicted microRNA target genes based on GO annotations, we confirm that many microRNAs link host and target gene activity in an either synergistic or antagonistic manner.

CONCLUSIONS

These two regulatory effects may result from fine tuning of target gene expression functionally related to the host or knock-down of remaining opponent target gene expression. This finding allows to extend the common practice of mapping large scale gene expression data to protein associated genes with functionality of co-expressed intronic microRNAs.

MicroRNAs in cardiac development and remodeling

MicroRNAs (miRNAs) are a class of highly conserved, small, noncoding RNAs that play roles in a wide range of biologic processes. Dysregulated miRNA expression has been associated with human cardiovascular disease, and studies using animal models have shown that miRNAs are essential for cardiac development and remodeling. These previously unrecognized small molecules shed new light on the regulatory mechanisms underlying cardiac development and pathology, suggesting the potential importance of miRNAs as diagnostic markers and therapeutic targets for cardiovascular disease.

Identification of candidate genes potentially relevant to chamber-specific remodeling in postnatal ventricular myocardium

Molecular predisposition of postnatal ventricular myocardium to chamber-dependent (concentric or eccentric) remodeling remains largely elusive. To this end, we compared gene expression in the left (LV) versus right ventricle (RV) in newborn piglets, using a differential display reverse transcription-PCR (DDRT-PCR) technique. Out of more than 5600 DDRT-PCR bands, a total of 153 bands were identified as being differentially displayed. Of these, 96 bands were enriched in the LV, whereas the remaining 57 bands were predominant in the RV. The transcripts, displaying over twofold LV-RV expression differences, were sequenced and identified by BLAST comparison to known mRNA sequences. Among the genes, whose expression was not previously recognized as being chamber-dependent, we identified a small cohort of key regulators of muscle cell growth/proliferation (MAP3K7IP2, MSTN, PHB2, APOBEC3F) and gene expression (PTPLAD1, JMJD1C, CEP290), which may be relevant to the chamber-dependent predisposition of ventricular myocardium to respond differentially to pressure (LV) and volume (RV) overloads after birth. In addition, our data demonstrate chamber-dependent alterations in expression of as yet uncharacterized novel genes, which may also be suitable candidates for association studies in animal models of LV/RV hypertrophy.

Expression of microRNA-208 is associated with adverse clinical outcomes in human dilated cardiomyopathy

BACKGROUND

Recently, microRNA-208 (miR-208) encoded by the alpha-myosin heavy chain (MHC) gene, has been shown to be involved in pathological cardiac growth, fibrosis, and up-regulation of beta-MHC expression. A recent study has also reported 2 additional myosin-expressed miRNAs (miR-208b and miR-499). The aim of this study was to determine whether miR-208, miR-208b, and miR-499 are expressed with MHC mRNA in human dilated cardiomyopathy (DCM), and whether these levels are related to left ventricular (LV) function and to clinical outcomes.

METHODS AND RESULTS

Endomyocardial biopsy tissues were obtained from 82 patients with DCM and 21 subjects without LV dysfunction as controls. Levels of miR-208, miR-208b, and miR-499 were higher in DCM patients than in controls. Levels of alpha-MHC mRNA were lower in patients with DCM than in controls, whereas beta-MHC mRNA levels were higher in patients with DCM compared with controls. Levels of miR-208 were correlated with beta-MHC mRNA levels and myocardial collagen volume, whereas levels of miR-208b and miR-499 showed no correlation. After a mean follow-up of 517 days, an increase in miR-208 levels was shown to be a strong predictor of clinical outcomes (RR 3.4, 95% CI 1.1-11.2).

CONCLUSIONS

This study suggests that myocardial expression of miR-208 is associated with MHC mRNA expression and with poor clinical outcomes in patients with DCM. We conclude that miR-208 may therefore be involved in the progression of human DCM.

MicroRNAs in cardiovascular diseases: biology and potential clinical applications

Cardiovascular diseases represent one of the major causes for increasing rates of human morbidity and mortality across the world. This reinforces the necessity for the development of novel diagnostics and therapies for the early identification and cure of heart diseases. MicroRNAs are evolutionarily conserved small regulatory non-coding RNAs that regulate the expression of large number of genes. They are involved in several cellular pathophysiological pathways and have been shown to play a significant role in the pathogenesis of many disease states. Recent studies have correlated dysregulated miRNA expressions to diseased hearts and also shown the relevance of miRNA in growth, development, function, and stress responsiveness of the heart. The possibility of exploiting miRNAs to develop diagnostic markers or manipulating them to obtain therapeutic effects is very attractive since they have very specific targets in a particular cellular pathway. In this review we will summarize the role played by miRNAs in the heart and discuss the scope of utilizing miRNA-based strategies in the clinics for the benefit of mankind.

MicroRNAs in cardiac remodeling and disease

MicroRNAs (miRNAs) are a large sub-group of small non-coding RNAs, which have been demonstrated to post-transcriptionally regulate the expression of protein-coding genes in a wide-range biological process. miRNAs have been shown to be essential for normal heart development and cardiac function. Recent data suggest that miRNAs are involved in the etiology of cardiac disease and the remodeling of hearts, including cardiac hypertrophy, myocardial infarction, and cardiac arrhythmias. In this review, we focus on the recent progress in the understanding of the function of miRNAs in cardiac remodeling and disease. We will also discuss the diagnostic and therapeutic potential of miRNAs in heart disease.

Uncoupling of expression of an intronic microRNA and its myosin host gene by exon skipping

The ancient MYH7b gene, expressed in striated muscle and brain, encodes a sarcomeric myosin and the intronic microRNA miR-499. We find that skipping of an exon introduces a premature termination codon in the transcript that downregulates MYH7b protein production without affecting microRNA expression. Among other genes, endogenous miR-499 targets the 3' untranslated region of the transcription factor Sox6, which in turn acts as a repressor of MYH7b transcriptional activity. Thus, concerted transcription and alternative splicing uncouple the level of expression of MYH7b and miR-499 when their coexpression is not required.

microRNAs in heart disease: putative novel therapeutic targets?

microRNAs (miRs) are short, approximately 22-nucleotide-long non-coding RNAs involved in the control of gene expression. They guide ribonucleoprotein complexes that effect translational repression or messenger RNA degradation to targeted messenger RNAs. miRs were initially thought to be peculiar to the developmental regulation of the nematode worm, in which they were first described in 1993. Since then, hundreds of different miRs have been reported in diverse organisms, and many have been implicated in the regulation of physiological processes of adult animals. Of importance, misexpression of miRs has been uncovered as a pathogenic mechanism in several diseases. Here, we first outline the biogenesis and mechanism of action of miRs, and then discuss their relevance to heart biology, pathology, and medicine.

MicroRNAs: a novel class of potential therapeutic targets for cardiovascular diseases

Currently, cardiovascular diseases remain one of the leading causes of morbidity and mortality in the world, indicating the need for innovative therapies and diagnosis for heart disease. MicroRNAs (miRNAs) have recently emerged as one of the central players in regulating gene expression. Numerous studies have documented the implications of miRNAs in nearly every pathological process of the cardiovascular system, including cardiac arrhythmia, cardiac hypertrophy, heart failure, cardiac fibrosis, cardiac ischemia and vascular atherosclerosis. More surprisingly, forced expression or suppression of a single miRNA is enough to cause or alleviate the pathological alteration, underscoring the therapeutic potential of miRNAs in cardiovascular diseases. In this review we summarize the key miRNAs that can solely modulate the cardiovascular pathological process and discuss the mechanisms by which they exert their function and the perspective of these miRNAs as novel therapeutic targets and/or diagnostic markers. In addition, current approaches for manipulating the action of miRNAs will be introduced.

MicroRNA group disorganization in aging

Among non-coding RNAs, microRNAs may be one of the best known subgroups, due to their unique function of negatively controlling gene expression, by either degrading target messages or binding to their 3'-untranslated region to inhibit translation. Thus gene expression can be repressed through post-transcriptional regulation, implemented as a 'dimmer switch', in contrast to the all-or-none mode of suppression. Work from our laboratory and others shows that during aging, dysregulated expression of microRNAs generally occurs in groups, suggesting that their actions may be functionally coordinated as a 'pack' by common transcriptional regulators; the accumulation of these 'pack' disorganizations may be the underlying culprit contributing to the pathoetiology of many age-dependent disease states. The fact that many microRNAs are coordinated in their expression, due to either the close proximity of their genomic locations or sharing the same transcriptional regulation, suggests that future strategies for correcting age-dependent microRNA disorganization may need to involve a system biology, rather than a reductionist, approach. Therefore, understanding age-dependent changes of microRNA expression in 'packs' may open an entirely new frontier, i.e. how particular groups of non-coding RNAs, functioning together, contribute to mechanisms regulating aging and longevity.

A family of microRNAs encoded by myosin genes governs myosin expression and muscle performance

Myosin is the primary regulator of muscle strength and contractility. Here we show that three myosin genes, Myh6, Myh7, and Myh7b, encode related intronic microRNAs (miRNAs), which, in turn, control muscle myosin content, myofiber identity, and muscle performance. Within the adult heart, the Myh6 gene, encoding a fast myosin, coexpresses miR-208a, which regulates the expression of two slow myosins and their intronic miRNAs, Myh7/miR-208b and Myh7b/miR-499, respectively. miR-208b and miR-499 play redundant roles in the specification of muscle fiber identity by activating slow and repressing fast myofiber gene programs. The actions of these miRNAs are mediated in part by a collection of transcriptional repressors of slow myofiber genes. These findings reveal that myosin genes not only encode the major contractile proteins of muscle, but act more broadly to influence muscle function by encoding a network of intronic miRNAs that control muscle gene expression and performance.

Essential amino acids increase microRNA-499, -208b, and -23a and downregulate myostatin and myocyte enhancer factor 2C mRNA expression in human skeletal muscle

Essential amino acids (EAA) stimulate muscle protein synthesis in humans. However, little is known about whether microRNAs (miRNA) and genes associated with muscle growth are expressed differently following EAA ingestion. Our purpose in this experiment was to determine whether miRNA and growth-related mRNA expressed in skeletal muscle are up- or downregulated in humans following the ingestion of EAA. We hypothesized that EAA would alter miRNA expression in skeletal muscle as well as select growth-related genes. Muscle biopsies were obtained from the vastus lateralis of 7 young adult participants (3 male, 4 female) before and 3 h after ingesting 10 g of EAA. Muscle samples were analyzed for muscle miRNA (miR-499, -208b, -23a, -1, -133a, and -206) and muscle-growth related genes [MyoD1, myogenin, myostatin, myocyte enhancer factor C (MEF2C), follistatin-like-1 (FSTL1), histone deacytylase 4, and serum response factor mRNA] before and after EAA ingestion using real-time PCR. Following EAA ingestion, miR-499, -208b, -23a, -1, and pri-miR-206 expression increased (P < 0.05). The muscle-growth genes MyoD1 and FSTL1 mRNA expression increased (P < 0.05), and myostatin and MEF2C mRNA were downregulated following EAA ingestion (P < 0.05). We conclude that miRNA and growth-related genes expressed in skeletal muscle are rapidly altered within hours following EAA ingestion. Further work is needed to determine whether these miRNA are post-transcriptional regulators of growth-related genes following an anabolic stimulus.