The epicardium is an embryonic tissue layer essential for heart morphogenesis, providing progenitor cells and regulatory signals that support myocardial growth and coronary vessel formation. Epicardial cells arise from the proepicardium (PE) and spread over the myocardium to form the embryonic epicardium (EE), a transition that requires tight coordination between proliferation, migration, and lineage priming. However, the molecular mechanisms controlling this developmental timing remain incompletely understood. Here, we identify Trim71 as a key regulator of epicardial cell behaviour during the PE-to-EE transition. Trim71 is enriched in the PE and subsequently downregulated as cells acquire migratory competence. Functional analyses show that loss of Trim71 function decreases proliferation while promoting migration, as well as inducing the expression of epicardial commitment markers, suggesting that Trim71 is a controller of a progenitor-like state. We further demonstrate that Trim71 is necessary for these processes through a reciprocal feedback loop with the microRNAs let-7c and miR-30c. Our findings establish Trim71 as a temporal gatekeeper that coordinates the balance between progenitor maintenance and migration during early epicardial development. This Trim71-miRNAs axis constitutes a novel post-transcriptional layer of regulation that ensures the correct timing of epicardium development during cardiogenesis.

Cardiogenesis is initiated soon during gastrulation as bilateral precardiac mesoderm, progressive and symmetrically determined at both sides of the developing embryo. The precardiac mesoderm subsequently fuse at the embryonic midline constituting an embryonic linear heart tube. As development progress, the embryonic tube heart suffers an invariably rightward looping generating a prospective embryonic ventricular and atrial chambers. As cardiac development progresses, the primitive atrial and ventricular chambers enlarged and distinct left and right compartments emerge by formation of the interatrial and interventricular septa, respectively. The last steps of cardiac morphogenesis are represented by the completion of atrial and ventricular septation, resulting in the configuration of a double circuitry with distinct systemic and pulmonary chambers, each of them with distinct inlets and outlets connections. The heart development is resulted of the activation and repression of several cardiac specific gene programs in different cell types. These cardiac specific gene pathways are modulated at transcriptional and post-transcriptional levels by key cardiac transcription factors and different paracrine and autocrine signals. Over the last decade, a novel layer of complexity has emerged with the discovery of non-coding RNAs, particularly microRNAs and lncRNAs. MicroRNAs are endogenous ≈22-nt non-coding RNAs that participate in the regulation of gene expression at post-transcriptional level on most cellular processes. In this line, miR-1/miR-133 cluster is one of the muscle-specific cluster microRNAs. Proper expression of members of this cluster is essential for cardiogenesis. Furthermore, dysregulation expression of miR-1 and miR-133 expression exerts an important role in heart failure and related pathological processes. Herein, we provide a state-of-the-art review of the contribution of miR-1/miR-133 cluster in cardiogenesis and heart failure development.

The heart is the first organ to develop during embryogenesis and its development relies on coordinated interactions between the myocardium and the embryonic epicardium (EE), the third layer of the heart derived from the proepicardium (PE). A subset of epicardial cells undergoes an epithelial-to-mesenchymal transition (EMT), invades the subepicardial space and leads to epicardial-derived cells (EPDCs) colonizing the embryonic myocardium and differentiating into multiple cardiac lineages. However, the regulatory landscape governing these processes remains incomplete. In recent years, long non-coding RNAs (lncRNAs) have emerged as key regulators of cardiac development. Previous data from our laboratory identified two murine lncRNAs, Elektra and Alien, with differential expression during the transition from the PE to the EE. In this study, we performed a multi-omic characterization of both lncRNAs across embryonic and adult tissues. Their transcriptional regulation was evaluated through cardiogenic transcription factors analysis, while their molecular interactomes were identified via RNA pull-down (PD) and mass spectrometry (MS) assays and subsequently validated by RNA immunoprecipitation (RIP). Functional impact was analyzed through loss-of-function experiments and RT-qPCR, focusing on cell migration and EMT dynamics, evidencing distinct roles for each lncRNA. Elektra regulated the expression of ion channel genes in the myocardium through interaction with Qki protein, while Alien modulates the epicardial EMT process by interacting with Wt1 and controlling EMT-related genes, including Snai1, Snai2, Cdh1 and Cdh2. Altogether, our findings reveal that Elektra and Alien exert important roles in cardiac development by regulating myocardial ion channel expression and epicardial EMT, respectively, supporting new insights into lncRNA-mediated regulation of heart morphogenesis in mice.

Cardiogenesis begins during gastrulation with the bilateral specification of precardiac mesoderm on either side of the embryo. These fields fuse at the embryonic midline to form the linear heart tube, which subsequently undergoes rightward looping and chamber formation. Expansion and septation of the primitive atrial and ventricular chambers establish separate systemic and pulmonary circuits with correctly aligned inflow and outflow tracts. These morphogenetic events depend on the coordinated activation and repression of cell type-specific cardiac gene programmes controlled at transcriptional and post-transcriptional levels by cardiac transcription factors and paracrine/autocrine signals. Non-coding RNAs, particularly microRNAs and long non-coding RNAs, add an essential regulatory layer. MicroRNAs are endogenous ≈22-nt RNAs that modulate gene expression predominantly at the post-transcriptional level. The muscle-enriched miR-1/miR-133 cluster is essential for normal cardiogenesis, whereas dysregulation of miR-1 and miR-133 contributes to heart failure and its associated pathological processes. Here, we provide a state-of-the-art review of the genomic regulation, developmental functions, context-dependent interactions, and translational potential of the miR-1/miR-133 cluster in cardiogenesis and heart failure.

IntroductionCardiovascular diseases (CVDs) are the leading cause of death globally, taking an estimated 17.9 million lives each year. Most heart cardiomyopathies result in an increased need for protein production that translates into an increased endoplasmic reticulum stress and therefore in the activation of the unfolded protein response pathway (UPR). The sustained activation of this pathway produces cell death and worsens the course of the disease. The role of lncRNAs in UPR signalling and their impact in several cardiomyopathies is beginning to be addressed.MethodsTo conduct our study we have performed real time PCR (qPCR), immunochemistry (IMQ), SeaHorse mithocondrial activity, Western blot (WB), Mass spectrometry (MS) and cell viability analysis.ResultsOur results demonstrate a sex-dependent regulation of Walar, Walaa, Wallrd, Walrad and Walras lncRNAs in different dilated cardiomyopathy (DCM) and hypertrophic cardiomyopathy (HCM) murine experimental models. Functional assays demonstrated that Walras overexpression leads to unfolded protein response (UPR) pathway activation and increased apoptosis, and additionally it also impairs mitochondrial function. Mechanistically, Walras physically interacts with calumenin (CALU), repressing its protein levels by promoting proteosomal degradation. Finally, we proved that APO02340.1, a Walras human homologue exerts a similar role.DiscussionOur data demonstrate that Walras and APO02340.1 modulate UPR associated apoptosis by regulating CALU protein turnover and thus acting as deletereous factors in several cardiomyophaties.

[This corrects the article DOI: 10.3389/fphys.2026.1740128.].

The epicardium provides essential cellular and molecular cues required for proper cardiogenesis and cardiac repair. Epicardial-derived cells (EPDCs) play a pivotal role in establishing cardiac structure, contributing to coronary vasculature formation, connective tissue organization, and post-ischemic cardiac remodeling. During EPDC emergence, the epicardium must preserve a precise balance between cellular motility and epithelial integrity. However, the mechanisms determining why some epicardial cells undergo epithelial-to-mesenchymal transition to become EPDCs while others retain an epithelial state remain unclear. We show that miR-200b is expressed in a subset of epicardial cells during embryonic EPDC formation. Gain-and loss-of-function experiments reveal that miR-200b regulates the overall number of EPDCs by modulating the proportion of symmetric and asymmetric cell divisions. RNA pull-down coupled with RNA-seq, together with in vitro and ex vivo functional assays, identified filamin A (FLNA)-a key regulator of spindle positioning during asymmetric division-as a direct miR-200b target in epicardial cells. FLNA loss reduced asymmetric divisions, supporting its role in promoting this division mode. Overall, our study defines a miR-200b-FLNA axis that governs symmetric versus asymmetric division to control epicardial tissue dynamics during cardiogenesis. Additionally, altered miR-200b expression after myocardial infarction in mice and humans suggests a potential role post-MI.

Background: Methylene blue (MB), a versatile redox agent, is emerging as a promising therapeutic in diseases associated with mitochondrial dysfunction. Its ability to optimize the electron transport chain increases ATP synthesis (30–40%) and reduces oxidative stress, protecting cellular components such as mitochondrial DNA. The protective role of this compound has been described in several neurodegenerative disease such as Alzheimer’s and Parkinson’s diseases. However, its role in cardiovascular disease has been poorly explored. Methods: In this study, we explored the impact of MB on murine (HL1) and human (AC16) cardiomyocyte redox signaling and cellular survival using RT-Qpcr analysis and immunochemistry assays. Results: Our results revealed that MB increased functional mitochondria, reversed H2O2-induced oxidative damage, and modulated antioxidant gene expression. Furthermore, it regulated the microRNA16–UPR signaling axis, reducing CHOP expression and promoting cell survival. Conclusions: These findings underscore its potential in cardioprotective therapy; however, its putative use as a drug requires in vivo validation in preclinical animal models.

Cardiovascular diseases are the leading cause of death worldwide in the human population. Currently, both noninvasive and invasive conventional diagnostic methods are used in cardiovascular diseases. Noninvasive techniques such as electrocardiogram are widely accepted; yet, it is hindered by poor sensitivity and specificity. On the other hand, invasive examinations such as coronary arteriography and intravascular ultrasound are not resolute enough during the early stages of cardiovascular pathological changes. Consequently, there is an urgent need to identify noninvasive biomarkers that are highly sensitive, specific, and effective for the early-stage diagnosis of cardiovascular diseases. Noncoding RNAs have recently emerged as a novel layer of gene regulation, and their expression is widely impaired in pathological conditions, including cardiovascular diseases. In this context, circRNAs are highly stable molecules that are easy detectable in liquid biopsies, thus providing as good candidate for clinical diagnosis. Within this section, we provide a state-of-the art review of the current identification of circRNAs as biomarkers in distinct cardiovascular pathological conditions.

BACKGROUND:The heart is the first functional organ to develop in the vertebrate embryos. In mice, the primitive tubular heart begins beating at embryonic day (E) 8.0-E.8.5 and undergoes rightward looping to form the atrial and ventricular chambers. The proepicardium, a transient cell cluster at the sinus venous-lateral plate mesenchyme junction migrates onto the heart and gives rise to the embryonic epicardium, a squamous epithelium that plays a key role in cardiac development. Despite advances in understanding epicardial lineage contributions, the molecular mechanisms governing these processes remain poorly understood. METHODS:To characterize the transcriptional and post-transcriptional regulation of epicardial development, we performed RNA sequencing at two critical timepoints, proepicardium formation and embryonic epicardium establishment. We analysed differentially expressed coding and non-coding RNAs, focusing on microRNAs and their potential regulatory interactions. RESULTS:We identified a complex network involving differentially expressed mRNAs, microRNAs and lncRNAs between proepicardium and embryonic epicardium. Notably, with miR-495 and let-7c emerged as key regulators of epicardial cell migration, an essential process for proper epicardium formation and epicardial-derived cell migration. Our findings also reveal that these microRNAs not only regulate target gene expression but also modulate other microRNAs, suggesting a novel regulatory mechanism in epicardial development. Additionally, Foxf1 inhibition modulates let-7c, promoting the expression of key cardiogenic lineage markers in epicardial cells. CONCLUSION:Our study highlights the role of Foxf1 in regulating miR-495 and let-7c, which in turn modulate epicardial cell migration and myocardial specification. These finding provide new insights into the intricate interplay between transcription factors and microRNAs in governing cardiogenesis.

EDITORIAL article Front. Cell Dev. Biol., 12 April 2024Sec. Molecular and Cellular Pathology Volume 12 - 2024 | https://doi.org/10.3389/fcell.2024.1392713

Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Ministry of Science and Innovation LMNA is a gene encoding lamin A/C proteins of the cell nuclear lamina. Pathogenic variants in LMNA cause a group of diseases known as laminopathies, a common cause of dilated cardiomyopathy (DCM). LMNA is the second most frequently mutated gene in DCM. Although molecular mechanisms underlying the cardiac involvement are unclear, previous studies have shown a reduction of the lamin expression in LMNA heterozygous mutation carriers. We identified a missense variant in LMNA, c.1871G>A (p.Arg624His), in a family affected with DCM. In this study, we analysed the impact of the p.Arg624His variant on the mRNA expression through quantitative PCR (qPCR) to demonstrate its pathogenicity. RNA was isolated from patient’s peripheral blood lymphocytes. Quantitative reverse transcription PCR (RT-qPCR) with LMNA specific primers was performed to measure lamin RNA levels. Whole-exome sequencing (WES) through next-generation sequencing (NGS) yielded an heterozygous missense variant in the LMNA gene (NM_170707.4(LMNA): c.1871G>A; p.Arg624His; ClinVar ID 66870) classified as variant of uncertain significance (VUS) in five members of a family affected with DCM. Computational (In-Silico) programs predict a damaging effect and variant is absent from controls in population databases. RT-qPCR demonstrated a significant reduction in the LMNA mRNA expression between the control (II.1) and affected family members (II.3, III.1) (Figure 2). According to evidence of co-segregation, variant frequency, results of in-silico predictors and mRNA reduction, LMNA p.Arg624His variant has been reclassified as pathogenic. In this study we reclassified to pathogenic a variant of uncertain significance in LMNA which reduces mRNA expression and leads to cardiac laminopathy. Further investigations are needed to determine the effect on the protein.

A large diversity of epigenetic factors, such as microRNAs and histones modifications, are known to be capable of regulating gene expression without altering DNA sequence itself. In particular, miR-1 is considered the first essential microRNA in cardiac development. In this study, miR-1 potential role in early cardiac chamber differentiation was analyzed through specific signaling pathways. For this, we performed in chick embryos functional experiments by means of miR-1 microinjections into the posterior cardiac precursors—of both primitive endocardial tubes—committed to sinoatrial region fates. Subsequently, embryos were subjected to whole mount in situ hybridization, immunohistochemistry and RT-qPCR analysis. As a relevant novelty, our results revealed that miR-1 increased Amhc1, Tbx5 and Gata4, while this microRNA diminished Mef2c and Cripto expressions during early differentiation of the cardiac sinoatrial region. Furthermore, we observed in this developmental context that miR-1 upregulated CrabpII and Rarß and downregulated CrabpI, which are three crucial factors in the retinoic acid signaling pathway. Interestingly, we also noticed that miR-1 directly interacted with Hdac4 and Calm1/Calmodulin, as well as with Erk2/Mapk1, which are three key factors actively involved in Mef2c regulation. Our study shows, for the first time, a key role of miR-1 as an epigenetic regulator in the early differentiation of the cardiac sinoatrial region through orchestrating opposite actions between retinoic acid and Mef2c, fundamental to properly assign cardiac cells to their respective heart chambers. A better understanding of those molecular mechanisms modulated by miR-1 will definitely help in fields applied to therapy and cardiac regeneration and repair.

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.

Background: Cardiac development is a complex developmental process. The early cardiac straight tube is composed of an external myocardial layer and an internal endocardial lining. Soon after rightward looping, the embryonic heart becomes externally covered by a new epithelial lining, the embryonic epicardium. A subset of these embryonic epicardial cells migrate and colonize the embryonic myocardium, contributing to distinct cell types. In recent years, our understanding of the molecular mechanisms that govern proepicardium and embryonic epicardium formation has greatly increased. We have recently witnessed a novel layer of complexity governing gene regulation with the discovery of non-coding RNAs. Our laboratory recently identified three distinct lncRNAs, adjacent to the Wt1, Bmp4 and Fgf8 chicken gene loci, with enhanced expression in the proepicardium that are distinctly regulated by Bmp, Fgf and thymosin 4, providing support of their plausible implication in epicardial formation. Methods: Expression of lncRNAs was analyzed in different chicken and mouse tissues as well as their subcellular distribution in chicken proepicardial, epicardial, ventricle explants and in different murine cardiac cell types. lncRNA transcriptional regulation was analyzed by using siRNAs and expression vectors of different transcription factors in chicken and mouse. Antisense oligonucleotides were used to inhibit Gm14014 expression. Cardiac injury was induced ex vivo in mouse ventricle explants by cryoinjury. Furthermore, RT-qPCR, immunocytochemistry, RNA pulldown, Western blot, viability and cell migration assays were conducted to investigate the biological function of Wt1_76127 and Gm14014. Results: We demonstrated that Wt1_76127 in chicken and its evolutionarily conserved homologue Gm14014 in mice are widely distributed in different embryonic and adult tissues and distinctly regulated by cardiac enriched transcription factors, particularly Mef2c and Nkx2.5. Gm14014 is distinctly regulated in mouse ventricular cryoinjury ex vivo models, displaying a negative correlation with epicardial and epithelial to mesenchymal transition markers. Furthermore, silencing assays demonstrated that mouse Gm14014, but not chicken Wt1_76127, is essential for epicardial, but not endocardial or myocardial cell migration. Such process is governed by partnering with Myl9, promoting cytoskeletal remodeling. Conclusion: Our data evidence that Gm14014 plays a pivotal role in epicardial cell migration essential for heart regeneration.

Background: Lipoprotein(a) [Lp(a)] is a proatherogenic particle associated with increased cardiovascular risk. It is mainly genetically determined; so, the aim of our study is to evaluate the levels of Lp(a) in the relatives of a prospective cohort of patients who have suffered from an acute coronary syndrome (ACS) with Lp(a) ≥ 50 mg/dL. Methods: We conducted a multicenter prospective study, in which consecutive patients who had suffered from an ACS and presented Lp(a) ≥ 50 mg/dL and their first-degree relatives were included. Results: We included 413 subjects, of which 56.4% were relatives of the patients. Family history of early ischemic heart disease was present in 57.5%, and only 20.6% were receiving statin treatment. The family cohort was younger (37.5 vs. 59.1 years; p < 0.001), and 4% had ischemic heart disease and fewer cardiovascular risk factors. Mean Lp(a) levels were 64.9 mg/dL, 59.4% had levels ≥ 50 mg/dL, and 16.1% had levels ≥ 100 mg/dL. When comparing the patients with respect to their relatives, the mean level of Lp(a) was lower but without significant differences regarding the levels of LDLc, ApoB, and non-HDL. However, relatives with Lp(a) ≥ 50 mg/dL, had values similar to the group of patients with ACS (96.8 vs. 103.8 mg/dL; p = 0.18). No differences were found in Lp(a) levels in relatives based on the other lipid parameters. Conclusions: Overall, 59.4% of the first-degree relatives of patients who suffered from an ACS with Lp(a) ≥ 50 mg/dL also had elevated levels. Relatives with elevated Lp(a) had similar levels as patients.

Cardiac development is a complex developmental process that results in the formation of the four-chambered organ from a single linear heart tube. In the early stages of development, the linear heart tube is composed of only two tissue layers, an external myocardium that is internally lined by the endocardium. Subsequently, the proepicardium emerges at the septum transversum and soon thereafter proepicardial cells will migrate into the naked myocardium, leading to the formation of the embryonic epicardium. As cardiac development proceeds, epicardial-derived cells migrate into the subepicardial space and subsequently invade the developing ventricular chambers leading to the contribution of distinct cardiovascular cell types such as the cardiac fibroskeleton and different components of coronary vasculature. At present, the molecular mechanisms that regulate the transitional process from the proepicardium to the embryonic myocardium are largely unexplored. In this study we have implemented an ex vivo proepicardium/septum transversum (PE/ST)-embryonic myocardium explant model and we demonstrated that miR-223, but not miR-195, is capable of modulating PE/ST migration, a process that seems to be mediated by Slug expression. Thus, our study demonstrates for the first time the implication of distinct microRNAs in the PE/ST to embryonic myocardium transition in chicken embryonic hearts.

Excess lipid accumulation in the heart is associated with lipotoxicity and cardiac dysfunction due to excessive fatty acid oxidation. Peroxisome proliferator-activated receptor gamma (PPARγ) modulates the expression of key molecules involved in the FA metabolic pathway. Cardiomyocyte-specific overexpression of PPARγ causes dilated cardiomyopathy associated with lipotoxicity in mice. miR-130b-3p has been shown to be downregulated in the plasma of idiopathic dilated cardiomyopathy patients, but its role in modulating cardiomyocyte lipotoxicity via PPARγ remains unclear. Our objective was to investigate the protective role of miR-130b-3p against palmitate-induced lipotoxicity in cardiomyocytes through the modulation of the PPARγ signaling pathway. Human cardiomyoblasts were treated with palmitate. Intracellular lipid accumulation and expression of PPARγ and its downstream targets (CD36, FABP3, CAV1, VLDLR) were analyzed. Mitochondrial oxidative stress was assessed via MitoTracker Green and Redox Sensor Red staining and expression of CPT1B and SOD2. Endoplasmic reticulum stress and apoptosis were determined by examining GRP78, ATF6, XBP1s, CHOP, and caspase-3 expression. miR-130b-3p overexpression was achieved using transfection methods, and its effect on these parameters was evaluated. Luciferase assays were used to confirm PPARγ as a direct target of miR-130b-3p. Palmitate treatment led to increased lipid accumulation and upregulation of PPARγ and its downstream targets in human cardiomyoblasts. Palmitate also increased mitochondrial oxidative stress, endoplasmic reticulum stress and apoptosis. miR-130b-3p overexpression reduced PPARγ expression and its downstream signaling, alleviated mitochondrial oxidative stress and decreased endoplasmic reticulum stress and apoptosis in palmitate-stimulated cardiomyoblasts. Luciferase assays confirmed PPARγ as a direct target of miR-130b-3p. Our findings suggest that miR-130b-3p plays a protective role against palmitate-induced lipotoxicity in cardiomyocytes by modulating the PPARγ signaling pathway.

Myocardial cell fate specification takes place during the early stages of heart development as the precardiac mesoderm is configured into two symmetrical sets of bilateral precursor cells. Molecular cues of the surrounding tissues specify and subsequently determine the early cardiomyocytes, that finally matured as the heart is completed at early postnatal stages. Over the last decade, we have greatly enhanced our understanding of the transcriptional regulation of cardiac development and thus of myocardial cell fate. The recent discovery of a novel layer of gene regulation by non-coding RNAs has flourished their implication in epigenetic, transcriptional and post-transcriptional regulation of cardiac development. In this review, we revised the current state-of-the-art knowledge on the functional role of non-coding RNAs during myocardial cell fate.

Abstract Funding Acknowledgements None. Introduction A large diversity of epigenetic factors, such as microRNAs and histones modifications, are known to be capable of regulating gene expression without altering DNA sequence itself. In particular, miR-1 is considered the first essential microRNA in cardiac development. Purpose The aim of this work is to analyze the miR-1 modulation role in cardiac chamber differentiation through specific signaling pathways. Methods By means of microinjections in both primitive endocardial tubes of chick embryo culture, gain- and loss-of-function experiments were performed with premiR-1 and anti-miR-1, respectively. Subsequently, embryos were subjected to whole mount in situ hybridization with Tbx5, Gata4 and AMHC1 probes, as well as immunohistochemistry with Mef2c, HDAC4, Calmodulin/Calm1 and MAPK1/Erk2 antibodies. Also, we carried out RT-qPCR analysis of control and experimental embryos, including CRABPI, CRABPII and retinoic acid receptors (RAR and RXR). Results Our results reveal that miR-1 increases specific atrial gene expression such as Tbx5, Gata4 and AMHC1, while this microRNA diminishes Mef2c expression. Furthermore, we observed that miR-1 upregulates CRABPII and RARß, and downregulates CRABPI, which are three crucial factors in retinoic acid signaling pathway. Interestingly, we also observed that miR-1 actively interacts with HDAC4, Calmodulin and Erk2/MAPK1, key factors involved in Mef2c regulation. Conclusion All these data suggest that miR-1 functions as an epigenetic factor modulating complementary actions performed by retinoic acid and Mef2c, which are required to properly assign cells as sinoatrial precursors.