Noncoding RNAs as Key Regulators for Cardiac Development and Cardiovascular Diseases

Noncoding RNAs (ncRNAs) play fundamental roles in cardiac development and cardiovascular diseases (CVDs), which are a major cause of morbidity and mortality. With advances in RNA sequencing technology, the focus of recent research has transitioned from studies of specific candidates to whole transcriptome analyses. Thanks to these types of studies, new ncRNAs have been identified for their implication in cardiac development and CVDs. In this review, we briefly describe the classification of ncRNAs into microRNAs, long ncRNAs, and circular RNAs. We then discuss their critical roles in cardiac development and CVDs by citing the most up-to-date research articles. More specifically, we summarize the roles of ncRNAs in the formation of the heart tube and cardiac morphogenesis, cardiac mesoderm specification, and embryonic cardiomyocytes and cardiac progenitor cells. We also highlight ncRNAs that have recently emerged as key regulators in CVDs by focusing on six of them. We believe that this review concisely addresses perhaps not all but certainly the major aspects of current progress in ncRNA research in cardiac development and CVDs. Thus, this review would be beneficial for readers to obtain a recent picture of key ncRNAs and their mechanisms of action in cardiac development and CVDs.

Early Appearance of Epicardial Adipose Tissue through Human Development

Background: Epicardial adipose tissue (EAT) is a visceral fat depot with unique anatomic, biomolecular and genetic features. Due to its proximity to the coronary arteries and myocardium, dysfunctional EAT may contribute to the development and progression of cardiovascular and metabolic-related adiposity-based chronic diseases. The aim of this work was to describe, by morphological techniques, the early origin of EAT. Methods: EAT adipogenesis was studied in 41 embryos from 32 gestational days (GD) to 8 gestational weeks (GW) and in 23 fetuses until full term (from 9 to 36 GW). Results: This process comprises five stages. Stage 1 appears as mesenchyme at 33-35 GD. Stage 2 is characterized by angiogenesis at 42-45 GD. Stage 3 covers up to 34 GW with the appearance of small fibers in the extracellular matrix. Stage 4 is visible around the coronary arteries, as multilocular adipocytes in primitive fat lobules, and Stage 5 is present with unilocular adipocytes in the definitive fat lobules. EAT precursor tissue appears as early as the end of the first gestational month in the atrioventricular grooves. Unilocular adipocytes appear at the eighth gestational month. Conclusions: Due to its early origin, plasticity and clinical implications, factors such as maternal health and nutrition might influence EAT early development in consequence.

Lack of morphometric evidence for ventricular compaction in humans

The remodeling of the compact wall by incorporation of trabecular myocardium, referred to as compaction, receives much attention because it is thought that its failure causes left ventricular non-compaction cardiomyopathy (LVNC). Although the notion of compaction is broadly accepted, the nature and strength of the evidence supporting this process is underexposed. Here, we review the literature that quantitatively investigated the development of the ventricular wall to understand the extent of compaction in humans, mice, and chickens. We queried PubMed using several search terms, screened 1127 records, and selected 56 publications containing quantitative data on ventricular growth. For humans, only 34 studies quantified wall development. The key premise of compaction, namely a reduction of the trabecular layer, was never documented. Instead, the trabecular layer grows slower than the compact wall in later development and this changes wall architecture. There were no reports of a sudden enlargement of the compact layer (from incorporated trabeculae), be it in thickness, area, or volume. Therefore, no evidence for compaction was found. Only in chickens, a sudden increase in compact myocardial thickness layer was reported coinciding with a decrease in trabecular thickness. In mice, morphometric and lineage tracing investigations have yielded conflicting results that allow for limited compaction to occur. In conclusion, compaction in human development is not supported while rapid intrinsic growth of the compact wall is supported in all species. If compaction takes place, it likely plays a much smaller role in determining wall architecture than intrinsic growth of the compact wall.

Quantified growth of the human embryonic heart

The size and growth patterns of the components of the human embryonic heart have remained largely undefined. To provide these data, three-dimensional heart models were generated from immunohistochemically stained sections of ten human embryonic hearts ranging from Carnegie stage 10 to 23. Fifty-eight key structures were annotated and volumetrically assessed. Sizes of the septal foramina and atrioventricular canal opening were also measured. The heart grows exponentially throughout embryonic development. There was consistently less left than right atrial myocardium, and less right than left ventricular myocardium. We observed a later onset of trabeculation in the left atrium compared to the right. Morphometry showed that the rightward expansion of the atrioventricular canal starts in week 5. The septal foramina are less than 0.1 mm² and are, therefore, much smaller than postnatal septal defects. This chronological, graphical atlas of the growth patterns of cardiac components in the human embryo provides quantified references for normal heart development. Thereby, this atlas may support early detection of cardiac malformations in the foetus.This article has an associated First Person interview with the first author of the paper.

Thalidomide remodels developing heart in chick embryo: discovery of a thalidomide mediated hematoma in heart muscle

Despite of medical disaster caused by thalidomide in 1960s, the drug came to clinical use again for the treatment of erythema nodosum leprosum (ENL) and multiple myeloma. Recently, a new generation of children affected by thalidomide intake by their mothers during pregnancy has been identified in Brazil. In the past few years, there is the great enhancement in our understanding of the molecular mechanisms and targets of thalidomide with the help of modern OMICS technologies. However, understanding of cardiac-specific anomalies in fetus due to thalidomide intake by the respective mother has not been explored fully. At organ level, thalidomide causes congenital heart diseases, limb deformities in addition to ocular, and neural and ear abnormalities. The period of morning sickness and cardiogenesis is synchronized in pregnant women. Therefore, thalidomide intake during the first trimester could affect cardiogenesis severely. Thalidomide intake in pregnant women either causes miscarriage or heart abnormalities such as patent ductus arteriosus, ventricular septal defect (VSD), atrial septal defect (ASD), and pulmonary stenosis in survivors. In the present study, we identified a novel morphological defect (lump) in the heart of thalidomide-treated chick embryos. We characterized the lump at morphological, histo-pathological, oxidative stress, electro-physiological, and gene expression level. To our knowledge, here, we report the very first electrophysiological characterization of embryonic heart affected by thalidomide treatment.

The "Dead-End Tract" and Its Role in Arrhythmogenesis

Idiopathic outflow tract ventricular arrhythmias (VAs) represent a significant proportion of all VAs. The mechanism is thought to be catecholamine-mediated delayed after depolarizations and triggered activity, although other etiologies should be considered. In the adult cardiac conduction system it has been demonstrated that sometimes an embryonic branch, the so-called "dead-end tract", persists beyond the bifurcation of the right and left bundle branch (LBB). Several findings suggest an involvement of this tract in idiopathic VAs (IVAs). The aim of this review is to summarize our current knowledge and the possible clinical significance of this tract.