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.

The outermost layer of the heart, the epicardium, is an essential cell population that contributes, through epithelial-to-mesenchymal transition (EMT), to form different cell types and provide paracrine signals to the developing heart. Despite its quiescent state during adulthood, in response to cardiac injury, adult epicardium reactivates and recapitulates many aspects of embryonic cardiogenesis supporting cardiac tissue remodeling. Thus, the epicardium has been considered as a crucial source of cell progenitors with important contribution to developing and injured heart. Although several studies have provided evidence about cell fate determination in the epicardium, to date it is unclear whether epicardial-derived cells (EPDCs) come from specific, and previously predetermined, epicardial cell subpopulations or if they are derived from a common progenitor. In recent years, different approaches have been used to study cell heterogeneity within the epicardial layer by using different experimental models. However, generated data are still insufficient to understand the complexity of this epithelial layer. In this review, we summarize the previous works supporting the cellular composition, molecular signatures and diversity within the developing and adult epicardium.

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

Transcriptional regulation constitutes a key step in gene expression regulation. Myocyte enhancer factor 2C (MEF2C) is a transcription factor of MADS box family involved in the early development of several cell types including muscle cells. Over the last decade a novel layer of complexity modulating gene regulation is emerging as non-coding RNAs have been identified, impacting in both transcriptional and post-transcriptional regulation. microRNAs represent the most studied and abundantly expressed subtype of small non-coding RNAs and their functional role have been widely documented. On the other hand, our knowledge on the transcriptional and post-transcriptional regulatory mechanisms that drive microRNA expression is still incipient. We recently demonstrated that MEF2C is able to transactive the long, but not short, regulatory element upstream of miR-23a-miR-27a-miR-24-2 transcriptional start site. However, MEF2C over-expression and silencing, respectively, displayed distinct effects on each of the miR-23a-miR-27a-miR-24-2 mature cluster members, without affecting pri-miRNA expression levels, supporting thus additional MEF2C-driven regulatory mechanisms. Within this study we demonstrated a complex post-transcriptional regulatory mechanisms directed by MEF2C in the regulation of miR-23a-miR-27a-miR-24-2 cluster members, distinctly involving different domains of the MEF2C transcription factor and the physical interaction with pre-miRNAs and RNA interacting proteins such as Ksrp, HnRNPa3 and Ddx17.

Atrial fibrillation (AF) is a common arrhythmia. Better prevention and treatment of AF are needed to reduce AF-associated morbidity and mortality. Several major mechanisms cause AF in patients, including a genetic predisposition to develop AF. Genome-wide association studies have identified genetic variants associated with AF populations, with the strongest hits clustering on chromosome 4q25, close to the gene coding for the homeobox transcription PITX2. Because of the inherent complexity of the human heart, experimental and basic research on PITX2-dependent AF is not sufficient for understanding atrial functional proprieties. Linking PITX2 to ion channels, cells, tissues, atria and the whole heart, computational models are necessary for achieving a quantitative understanding of atrial structure and function in PITX2-dependent AF. Computational approaches are used to capture all that we know about PITX2-dependent AF and to develop improved therapies. In the present review, we discuss advances in atrial modelling and focus on the mechanistic links between PITX2 and AF. Challenges in applying models for improving patient health are described, as well as a summary of future perspectives.

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.

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 is 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 nov-elty, our results revealed that miR-1 increases Tbx5, Gata4 and AMHC1, while this microRNA diminishes Mef2c expression, during early differentiation of cardiac sinoatrial region. Further-more, we observed in this developmental context that miR-1 upregulates CRABPII and RARß, and downregulates CRABPI, which are three crucial factors in retinoic acid signaling pathway. Interestingly, we also noticed that miR-1 directly interacts with HDAC4 and 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 early differenti-ation of 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.