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

Small molecules, big effects: the role of microRNAs in regulation of cardiomyocyte death

MicroRNAs (miRNAs) are a class of small non-coding RNAs involved in posttranscriptional regulation of gene expression, and exerting regulatory roles in plethora of biological processes. In recent years, miRNAs have received increased attention for their crucial role in health and disease, including in cardiovascular disease. This review summarizes the role of miRNAs in regulation of cardiac cell death/cell survival pathways, including apoptosis, autophagy and necrosis. It is envisaged that these miRNAs may explain the mechanisms behind the pathogenesis of many cardiac diseases, and, most importantly, may provide new avenues for therapeutic intervention that will limit cardiomyocyte cell death before it irreversibly affects cardiac function. Through an in-depth literature analysis coupled with integrative bioinformatics (pathway and synergy analysis), we dissect here the landscape of complex relationships between the apoptosis-regulating miRNAs in the context of cardiomyocyte cell death (including regulation of autophagy–apoptosis cross talk), and examine the gaps in our current understanding that will guide future investigations.

MicroRNAs in heart failure: Small molecules with major impact

MicroRNAs (miRNAs) have emerged as potent modulators of mammalian gene expression, thereby broadening the spectrum of molecular mechanisms orchestrating human physiological and pathological cellular functions. Growing evidence suggests that these small non-coding RNA molecules are pivotal regulators of cardiovascular development and disease. Importantly, multiple miRNAs have been specifically implicated in the onset and progression of heart failure, thus providing a new platform for battling this multi-faceted disease. This review introduces the basic concepts of miRNA biology, describes representative examples of miRNAs associated with multiple aspects of HF pathogenesis, and explores the prognostic, diagnostic and therapeutic potential of miRNAs in the cardiology clinic.

Critical roles of miRNA-mediated regulation of TGFβ signalling during mouse cardiogenesis

AIMS

MicroRNAs (miRNAs) play critical roles during the development of the cardiovascular system. Blocking miRNA biosynthesis in embryonic hearts through a conditional gene inactivation approach led to differential cardiac defects depending on the Cre drivers used in different studies. The goal of this study is to reveal the cardiogenic pathway that is regulated by the miRNA mechanism at midgestation, a stage that has not been evaluated in previous publications.

METHODS AND RESULTS

We specifically inactivated Dicer1, which is essential for generation of functional mature miRNAs, in the myocardium by crossing cTnt-Cre mice with Dicer1(loxP) mice. cTnt-Cre efficiently inactivates target genes in cardiomyocytes at midgestation. All mutants died between E14.5 and E16.5 with severe myocardial wall defects, including reduced cell proliferation, increased cell death, and spongy myocardial wall. Expression of TGFβ type I receptor (Tgfbr1), which encodes the Type I receptor of TGFβ ligands, was up-regulated in mutant hearts. As expected, TGFβ activity was increased in Dicer1-inactivated hearts. Our further molecular analysis suggested that Tgfbr1 is a direct target of three miRNAs. Reducing TGFβ activities using a pharmacological inhibitor on in vitro cultured hearts, or through an in vivo genetic approach, partially rescued the cardiac defects caused by Dicer1 inactivation.

CONCLUSIONS

We show for the first time that TGFβ signalling is directly regulated by the miRNA mechanism during myocardial wall morphogenesis. Increased TGFβ activity plays a major role in the cardiac defects caused by myocardial deletion of Dicer1. Thus, miRNA-mediated regulation of TGFβ signalling is indispensable for normal cardiogenesis.

microRNA expression profiling of heart tissue during fetal development

microRNAs (miRNAs) are important both in early cardiogenesis and in the process of heart maturation. The aim of this study was to determine the stage-specific expression of miRNAs in human fetal heart in order to identify valuable targets for further study of heart defects. Affymetrix microarrays were used to obtain miRNA expression profiles from human fetal heart tissue at 5, 7, 9 and 23 weeks of gestation. To identify differentially expressed miRNAs at each time-point, linear regression analysis by the R limma algorithm was employed. Hierarchical clustering analysis was conducted with Cluster 3.0 software. Gene Ontology analysis was carried out for miRNAs from different clusters. Commonalities in miRNA families and genomic localization were identified, and the differential expression of selected miRNAs from different clusters was verified by quantitative polymerase chain reaction (qPCR). A total of 703 miRNAs were expressed in human fetal heart. Of these, 288 differentially expressed miRNAs represented 5 clusters with different expression trends. Several clustered miRNAs also shared classification within miRNA families or proximal genomic localization. qPCR confirmed the expression patterns of selected miRNAs. miRNAs within the 5 clusters were predicted to target genes vital for heart development and to be involved in cellular signaling pathways that affect heart structure formation and heart-associated cellular events. In conclusion, to the best of our knowledge, this is the first miRNA expression profiling study of human fetal heart tissue. The stage-specific expression of specific miRNAs suggests potential roles at distinct time-points during fetal heart development.

Atrial fibrillation and microRNAs

Atrial fibrillation (AF) is the most common sustained arrhythmia, especially in the elderly, and has a significant genetic component. Recently, several independent investigators have demonstrated a functional role for small non-coding RNAs (microRNAs) in the pathophysiology of this cardiac arrhythmia. This report represents a systematic and updated appraisal of the main studies that established a mechanistic association between specific microRNAs and AF, focusing both on the regulation of electrical and structural remodeling of cardiac tissue.

Homeobox transcription factor Pitx2: The rise of an asymmetry gene in cardiogenesis and arrhythmogenesis

The homeobox transcription factor Pitx2 displays a highly specific expression pattern during embryogenesis. Gain and loss of function experiments have unraveled its pivotal role in left-right signaling. Conditional deletion in mice has demonstrated a complex and intricate role for Pitx2 in distinct aspects of cardiac development and more recently a link to atrial fibrillation has been proposed based on genome-wide association studies. In this review we will revise the role of Pitx2 in the developing heart, starting from the early events of left-right determination followed by its role in cardiac morphogenesis and ending with its role in cardiac arrhythmogenesis.

Down-regulation of EBAF in the heart with ventricular septal defects and its regulation by histone acetyltransferase p300 and transcription factors smad2 and cited2

As a NODAL pathway inhibitor, EBAF plays a critical role during mammalian cardiac development. As recent tests that have been conducted on gene-targeted mice indicate, its expression is frequently altered where cardiac defects are present. We aimed to explore the EBAF expression pattern and molecular mechanism of EBAF gene for VSD genesis. In this report, we show that the average expression of EBAF in the disease tissues of VSD patients was lower than the expression in normal fetuses without VSD. Further study showed that the expression pattern of EBAF was potentially involved in cardiomyocyte apoptosis by Annexin-V and RT-PCR assays. We also found that abnormal activation of NODAL-PITX2C pathway was associated with down-regulation of EBAF. By luciferase reporter assays, we find that EBAF expression is mediated by transcriptional factors smad2 and cited2. In addition, ChIP assays showed that histone acetyltransferase p300 is involved in the activation of EBAF through inducing hyperacetylation of histone H4 at the EBAF promoter. Co-immunoprecipitation also indicates that the expression of EBAF is regulated by a transcriptional complex including p300, smad2, and cited2. This study revealed a novel regulator mechanism of EBAF, which may be a potential molecular target for halting the onset of VSDs. They also indicate that smad2, cited2, and p300 may play important roles in modulating the confirmation of ventricular septal defects.

MicroRNAs in cardiac regeneration and cardiovascular disease

microRNAs (miRNAs) are a class of small non-coding RNAs, which have been shown important to a wide range of biological process by post-transcriptionally regulating the expression of protein-coding genes. miRNAs have been demonstrated essential to normal cardiac development and function. Recently, numerous studies indicate miRNAs are involved in cardiac regeneration and cardiac disease, including cardiac hypertrophy, myocardial infarction and cardiac arrhythmia. These observations suggest miRNAs play important roles in cardiology. In this review, we summarize the recent progress of studying miRNAs in cardiac regeneration and cardiac disease. We also discuss the diagnostic and therapeutic potential of miRNAs in heart disease.

microRNAs in cardiac development and regeneration

Heart development involves the precise orchestration of gene expression during cardiac differentiation and morphogenesis by evolutionarily conserved regulatory networks. miRNAs (microRNAs) play important roles in the post-transcriptional regulation of gene expression, and recent studies have established critical functions for these tiny RNAs in almost every facet of cardiac development and disease. The realization that miRNAs are amenable to therapeutic manipulation has also generated considerable interest in the potential of miRNA-based drugs for the treatment of a number of human diseases, including cardiovascular disease. In the present review, I discuss well-established and emerging roles of miRNAs in cardiac development, their relevance to congenital heart disease and unresolved questions in the field for future investigation, as well as emerging therapeutic possibilities for cardiac regeneration.

Suppression of Induced microRNA-15b Prevents Rapid Loss of Cardiac Function in a Dicer Depleted Model of Cardiac Dysfunction

Background

Dicer endonuclease, critical for maturation of miRNAs, is depleted in certain forms of cardiomyopathy which results in differential expression of certain microRNAs. We sought to elucidate the mechanisms underlying the rapid loss of cardiac function following cardiac-specific Dicer depletion in adult mice.

Results

Conditional Dicer deletion in the adult murine myocardium demonstrated compromised heart function, mitochondrial dysfunction and oxidant stress. Elevated miR-15b was observed as an early response to Dicer depletion and was found to silence Pim-1 kinase, a protein responsible for maintaining mitochondrial integrity and function. Anti-miRNA based suppression of induced miRNA-15b rescued the function of Dicer-depleted adult heart and attenuated hypertrophy.

Conclusions

Anti-miRNA based suppression of inducible miRNA-15b can prevent rapid loss of cardiac function in a Dicer-depleted adult heart and can be a key approach worthy of therapeutic consideration.

Comparative Analyses of MicroRNA Microarrays during Cardiogenesis: Functional Perspectives

Cardiovascular development is a complex process in which several transcriptional pathways are operative, providing instructions to the developing cardiomyocytes, while coping with contraction and morphogenetic movements to shape the mature heart. The discovery of microRNAs has added a new layer of complexity to the molecular mechanisms governing the formation of the heart. Discrete genetic ablation of the microRNAs processing enzymes, such as Dicer and Drosha, has highlighted the functional roles of microRNAs during heart development. Importantly, selective deletion of a single microRNA, miR-1-2, results in an embryonic lethal phenotype in which both morphogenetic, as well as impaired conduction, phenotypes can be observed. In an effort to grasp the variability of microRNA expression during cardiac morphogenesis, we recently reported the dynamic expression profile during ventricular development, highlighting the importance of miR-27 on the regulation of a key cardiac transcription factor, Mef2c. In this review, we compare the microRNA expression profile in distinct models of cardiogenesis, such as ventricular chamber development, induced pluripotent stem cell (iPS)-derived cardiomyocytes and the aging heart. Importantly, out of 486 microRNAs assessed in the developing heart, 11% (55) displayed increased expression, many of which are also differentially expressed in distinct cardiogenetic experimental models, including iPS-derived cardiomyocytes. A review on the functional analyses of these differentially expressed microRNAs will be provided in the context of cardiac development, highlighting the resolution and power of microarrays analyses on the quest to decipher the most relevant microRNAs in the developing, aging and diseased heart.

Role of microRNAs in Semaphorin function and neural circuit formation

Since the discovery of the first microRNA (miRNA) almost 20 years ago, insight into their functional role has gradually been accumulating. This class of non-coding RNAs has recently been implicated as key molecular regulators in the biology of most eukaryotic cells, contributing to the physiology of various systems including immune, cardiovascular, nervous systems and also to the pathophysiology of cancers. Interestingly, Semaphorins, a class of evolutionarily conserved signalling molecules, are acknowledged to play major roles in these systems also. This, combined with the fact that Semaphorin signalling requires tight spatiotemporal regulation, a hallmark of miRNA expression, suggests that miRNAs could be crucial regulators of Semaphorin function. Here, we review evidence suggesting that Semaphorin signalling is regulated by miRNAs in various systems in health and disease. In particular, we focus on neural circuit formation, including axon guidance, where Semaphorin function was first discovered.

Functions of microRNAs in cardiovascular biology and disease

In 1993, lin-4 was discovered as a critical modulator of temporal development in Caenorhabditis elegans and, most notably, as the first in the class of small, single-stranded noncoding RNAs now defined as microRNAs (miRNAs). Another eight years elapsed before miRNA expression was detected in mammalian cells. Since then, explosive advancements in the field of miRNA biology have elucidated the basic mechanism of miRNA biogenesis, regulation, and gene-regulatory function. The discovery of this new class of small RNAs has augmented the complexity of gene-regulatory programs as well as the understanding of developmental and pathological processes in the cardiovascular system. Indeed, the contributions of miRNAs in cardiovascular development and function have been widely explored, revealing the extensive role of these small regulatory RNAs in cardiovascular physiology.

Partitioning the heart: mechanisms of cardiac septation and valve development

Heart malformations are common congenital defects in humans. Many congenital heart defects involve anomalies in cardiac septation or valve development, and understanding the developmental mechanisms that underlie the formation of cardiac septal and valvular tissues thus has important implications for the diagnosis, prevention and treatment of congenital heart disease. The development of heart septa and valves involves multiple types of progenitor cells that arise either within or outside the heart. Here, we review the morphogenetic events and genetic networks that regulate spatiotemporal interactions between the cells that give rise to septal and valvular tissues and hence partition the heart.

Whole genome expression differences in human left and right atria ascertained by RNA sequencing

BACKGROUND

The left and right atria have different susceptibilities toward developing arrhythmias, with left atrial arrhythmias more commonly observed. To understand the molecular basis for such differences, we catalogued micro (mi)RNA and mRNA expression differences by next generation sequencing.

METHODS AND RESULTS

Four human left-right atrial pairs were subjected to whole-genome expression analyses via next-generation sequencing of small RNAs, including miRNAs, and poly-A-enriched mRNAs. Using a paired sample design, significant differences in the expression of 32 miRNAs were found in between the left and right atria at a probability value of <0.01. Hsa-miR-143 was the most highly expressed miRNA in the atria, as quantified by RNA sequencing. There were 746 and 2292 differentially expressed mRNAs between the left and right atria at false discovery rates of <0.001 and <0.05, respectively. Transcription factor binding elements within 2 kb of RefSeq genes were determined and specific motifs were identified that were enriched in differentially expressed genes. Similarly, specific miRNA target sequences in 3' UTRs were also enriched in differentially expressed genes. In addition, 11 novel noncoding RNAs of unknown function were found to be differentially expressed between the left and right atria.

CONCLUSIONS

There are significant differences in miRNA and mRNA expression profiles between the left and right atria, which may yield insight into increased the arrhythmogenesis of the left atria.

A New Level of Complexity

The discovery of the regulatory role of noncoding RNAs, and micro (mi)RNAs in particular, has added a new layer of complexity to our understanding of cardiovascular development. miRNAs regulate and modulate various steps of cardiovascular morphogenesis, cell proliferation, differentiation, and phenotype modulation. miRNAs simultaneously regulate multiple targets, and many miRNAs can bind to the same target, allowing for a complex pattern of regulation of gene expression. miRNA families are continuously added during evolution paralleling the increased complexity of the cardiovascular system in vertebrates compared with invertebrates. Several lines of evidence suggest that the appearance of miRNAs is at least in part responsible for the formation of complex organ systems and stable regulatory mechanisms in vertebrates. We review the current understanding of miRNAs during cardiovascular development. Further progress in this area will help to decipher quantitative changes in gene expression that provide robustness to cellular phenotypes and regulatory options to diseases processes. miRNAs might also provide clues to better understand congenital heart defects, which are the most common birth defects in human newborns.

MicroRNAs in control of cardiac hypertrophy

MicroRNAs refer to a subfamily of small non-coding RNA species that are designed to influence gene expression in nearly all cell types studied to date. Studies from the past decade have demonstrated that microRNAs are atypically expressed in the cardiovascular system under specific pathological conditions. Gain- and loss-of-function studies using in vitro and in vivo models have revealed distinct roles for specific microRNAs in cardiovascular development, physiological functions, and cardiac pathological conditions. In this review, the current relevant findings on the role of microRNAs in cardiac hypertrophic growth are updated, the target genes of these microRNAs are summarized, and the future of microRNAs as potential therapeutic targets is discussed.

Deletion of the RNaseIII enzyme dicer in thyroid follicular cells causes hypothyroidism with signs of neoplastic alterations

Micro-RNAs (miRNAs) are small non-coding RNAs that regulate gene expression, mainly at mRNA post-transcriptional level. Functional maturation of most miRNAs requires processing of the primary transcript by Dicer, an RNaseIII-type enzyme. To date, the importance of miRNA function for normal organogenesis has been demonstrated in several mouse models of tissue-specific Dicer inactivation. However, the role of miRNAs in thyroid development has not yet been addressed. For the present study, we generated mouse models in which Dicer expression has been inactivated at two different stages of thyroid development in thyroid follicular cells. Regardless of the time of Dicer invalidation, the early stages of thyroid organogenesis, preceding folliculogenesis, were unaffected by the loss of small RNAs, with a bilobate gland in place. Nevertheless, Dicer mutant mice were severely hypothyroid and died soon after weaning unless they were substituted with T4. A conspicuous follicular disorganization was observed in Dicer mutant thyroids together with a strong down regulation of Nis expression. With increasing age, the thyroid tissue showed characteristics of neoplastic alterations as suggested by a marked proliferation of follicular cells and an ongoing de-differentiation in the center of the thyroid gland, with a loss of Pax8, FoxE1, Nis and Tpo expression. Together, our data show that loss of miRNA maturation due to Dicer inactivation severely disturbs functional thyroid differentiation. This suggests that miRNAs are mandatory to fine-tune the expression of thyroid specific genes and to maintain thyroid tissue homeostasis.

MicroRNAs in heart development

MicroRNAs (miRNAs) are a class of small noncoding RNAs of ~22nt in length which are involved in the regulation of gene expression at the posttranscriptional level by degrading their target mRNAs and/or inhibiting their translation. Expressed ubiquitously or in a tissue-specific manner, miRNAs are involved in the regulation of many biological processes such as cell proliferation, differentiation, apoptosis, and the maintenance of normal cellular physiology. Many miRNAs are expressed in embryonic, postnatal, and adult hearts. Aberrant expression or genetic deletion of miRNAs is associated with abnormal cardiac cell differentiation, disruption of heart development, and cardiac dysfunction. This chapter will summarize the history, biogenesis, and processing of miRNAs as well as their function in heart development, remodeling, and disease.

The role of microRNAs in muscle development

MicroRNAs play essential roles during animal development, including in developing muscle. Many microRNAs are expressed during muscle development and some, like miR-1 and miR-133, are muscle specific. Muscle microRNAs are integrated into myogenic regulatory networks: their expression is under the transcriptional and posttranscriptional control of myogenic factors, and they in turn have widespread control of muscle gene expression. This review summarizes recent work characterizing the function of microRNAs in muscle biology and specifically focuses on the genetic analysis of muscle microRNAs in a variety of model organisms including worms, flies, zebrafish, and mice.

The second heart field

Ten years ago, a population of cardiac progenitor cells was identified in pharyngeal mesoderm that gives rise to a major part of the amniote heart. These multipotent progenitor cells, termed the second heart field (SHF), contribute progressively to the poles of the elongating heart tube during looping morphogenesis, giving rise to myocardium, smooth muscle, and endothelial cells. Research into the mechanisms of SHF development has contributed significantly to our understanding of the properties of cardiac progenitor cells and the origins of congenital heart defects. Here recent data concerning the regulation, clinically relevant subpopulations, evolution and lineage relationships of the SHF are reviewed. Proliferation and differentiation of SHF cells are controlled by multiple intercellular signaling pathways and a transcriptional regulatory network that is beginning to be elucidated. Perturbation of SHF development results in common forms of congenital heart defects and particular progenitor cell subpopulations are highly relevant clinically, including cells giving rise to myocardium at the base of the pulmonary trunk and the interatrial septum. A SHF has recently been identified in amphibian, fish, and agnathan embryos, highlighting the important contribution of these cells to the evolution of the vertebrate heart. Finally, SHF-derived parts of the heart share a lineage relationship with craniofacial skeletal muscles revealing that these progenitor cells belong to a broad cardiocraniofacial field of pharyngeal mesoderm. Investigation of the mechanisms underlying the dynamic process of SHF deployment is likely to yield further insights into cardiac development and pathology.

Extending the phenotypes associated with DICER1 mutations

DICER1 is crucial for embryogenesis and early development. Forty different heterozygous germline DICER1 mutations have been reported worldwide in 42 probands that developed as children or young adults, pleuropulmonary blastoma (PPB), cystic nephroma (CN), ovarian sex cord-stromal tumors (especially Sertoli-Leydig cell tumor [SLCT]), and/or multinodular goiter (MNG). We report DICER1 mutations in seven additional families that manifested uterine cervix embryonal rhabdomyosarcoma (cERMS, four cases) and primitive neuroectodermal tumor (cPNET, one case), Wilms tumor (WT, three cases), pulmonary sequestration (PS, one case), and juvenile intestinal polyp (one case). One carrier developed (age 25 years) a pleomorphic sarcoma of the thigh; another carrier had transposition of great arteries (TGA). These observations show that cERMS, cPNET, WT, PS, and juvenile polyps fall within the spectrum of DICER1-related diseases. DICER1 appears to be the first gene implicated in the etiology of cERMS, cPNET, and PS. Young adulthood sarcomas and perhaps congenital malformations such as TGA may also be associated.

MicroRNA-processing enzyme Dicer is required in epicardium for coronary vasculature development

The epicardium is a sheet of epithelial cells covering the heart during early cardiac development. In recent years, the epicardium has been identified as an important contributor to cardiovascular development, and epicardium-derived cells have the potential to differentiate into multiple cardiac cell lineages. Some epicardium-derived cells that undergo epithelial-to-mesenchymal transition and delaminate from the surface of the developing heart subsequently invade the myocardium and differentiate into vascular smooth muscle of the developing coronary vasculature. MicroRNAs (miRNAs) have been implicated broadly in tissue patterning and development, including in the heart, but a role in epicardium is unknown. To examine the role of miRNAs during epicardial development, we conditionally deleted the miRNA-processing enzyme Dicer in the proepicardium using Gata5-Cre mice. Epicardial Dicer mutant mice are born in expected Mendelian ratios but die immediately after birth with profound cardiac defects, including impaired coronary vessel development. We found that loss of Dicer leads to impaired epicardial epithelial-to-mesenchymal transition and a reduction in epicardial cell proliferation and differentiation into coronary smooth muscle cells. These results demonstrate a critical role for Dicer, and by implication miRNAs, in murine epicardial development.

Biogenesis and regulation of cardiovascular microRNAs

MicroRNAs (miRNAs) are important regulators of gene expression and fundamentally impact on cardiovascular function in health and disease. A tight control of miRNA expression is crucial for the maintenance of tissue homeostasis. However, a comprehensive understanding of the various levels of miRNA regulation is in its infancy. We here summarize the current knowledge about regulation of cardiovascular miRNAs at the transcriptional level by transcription factors, during processing by the Drosha and Dicer complexes and the importance of miRNA modification, editing, and decay mechanisms. As an example, miRNA regulation in diabetic and hypoxic cardiovascular disease conditions is discussed. Better knowledge about regulatory mechanisms of miRNAs in cardiovascular disease will probably lead to improved and novel miRNA-based therapeutic therapies.

Tbx1, subpulmonary myocardium and conotruncal congenital heart defects

Conotruncal congenital heart defects, including defects in septation and alignment of the ventricular outlets, account for approximately a third of all congenital heart defects. Failure of the left ventricle to obtain an independent outlet results in incomplete separation of systemic and pulmonary circulation at birth. The embryonic outflow tract, a transient cylinder of myocardium connecting the embryonic ventricles to the aortic sac, plays a critical role in this process during normal development. The outflow tract (OFT) is derived from a population of cardiac progenitor cells called the second heart field that contributes to the arterial pole of the heart tube during cardiac looping. During septation, the OFT is remodeled to form the base of the ascending aorta and pulmonary trunk. Tbx1, the major candidate gene for DiGeorge syndrome, is a critical transcriptional regulator of second heart field development. DiGeorge syndrome patients are haploinsufficient for Tbx1 and present a spectrum of conotruncal anomalies including tetralogy of Fallot, pulmonary atresia, and common arterial trunk. In this review, we focus on the role of Tbx1 in the regulation of second heart field deployment and, in particular, in the development of a specific population of myocardial cells at the base of the pulmonary trunk. Recent data characterizing additional properties and regulators of development of this region of the heart, including the retinoic acid, hedgehog, and semaphorin signaling pathways, are discussed. These findings identify future subpulmonary myocardium as the clinically relevant component of the second heart field and provide new mechanistic insight into a spectrum of common conotruncal congenital heart defects.

MicroRNAs in cardiac disease

MicroRNAs (miRs) are transcriptionally regulated single-strand RNAs that depress protein expression through posttranscriptional mRNA silencing. A host of recent studies have established essential roles for miRs in cardiac development and cardiac health. Regulated myocardial miR expression is observed in a variety of cardiac syndromes, and serum miR levels are being evaluated as disease biomarkers. The manipulation of miR levels in mouse hearts using genetic techniques or engineered miR mimetics and antagonists is elucidating the roles of specific cardiac miRs in cardiac development, and in the cardiac response to injury or stress, and heart disease. The ability to target multiple factors within a single biological response pathway by a given miR has prompted the development of small miR-targeting molecules that can be readily delivered and have sustained in vivo effects. These advances establish a foundation for novel diagnostics and new therapeutic approaches for myocardial infarction, cardiac hypertrophy, and heart failure.

Conditional deletion of Dicer in vascular smooth muscle cells leads to the developmental delay and embryonic mortality

Dicer is a RNAase III enzyme that cleaves double stranded RNA and generates small interfering RNA (siRNA) and microRNA (miRNA). The goal of this study is to examine the role of Dicer and miRNAs in vascular smooth muscle cells (VSMCs). We deleted Dicer in VSMCs of mice, which caused a developmental delay that manifested as early as embryonic day E12.5, leading to embryonic death between E14.5 and E15.5 due to extensive hemorrhage in the liver, brain, and skin. Dicer KO embryos showed dilated blood vessels and a disarray of vascular architecture between E14.5 and E15.5. VSMC proliferation was significantly inhibited in Dicer KOs. The expression of VSMC marker genes were significantly downregulated in Dicer cKO embryos. The vascular structure of the yolk sac and embryo in Dicer KOs was lost to an extent that no blood vessels could be identified after E15.5. Expression of most miRNAs examined was compromised in VSMCs of Dicer KO. Our results indicate that Dicer is required for vascular development and regulates vascular remodeling by modulating VSMC proliferation and differentiation.

Dicer activity in neural crest cells is essential for craniofacial organogenesis and pharyngeal arch artery morphogenesis

MicroRNAs (miRNAs) play important roles in regulating gene expression during numerous biological/pathological processes. Dicer encodes an RNase III endonuclease that is essential for generating most, if not all, functional miRNAs. In this work, we applied a conditional gene inactivation approach to examine the function of Dicer during neural crest cell (NCC) development. Mice with NCC-specific inactivation of Dicer died perinatally. Cranial and cardiac NCC migration into target tissues was not affected by Dicer disruption, but their subsequent development was disturbed. NCC derivatives and their associated mesoderm-derived cells displayed massive apoptosis, leading to severe abnormalities during craniofacial morphogenesis and organogenesis. In addition, the 4th pharyngeal arch artery (PAA) remodeling was affected, resulting in interrupted aortic arch artery type B (IAA-B) in mutant animals. Taken together, our results show that Dicer activity in NCCs is essential for craniofacial development and pharyngeal arch artery morphogenesis.

NKX2-5: an update on this hypermutable homeodomain protein and its role in human congenital heart disease (CHD)

Congenital heart disease (CHD) is among the most prevalent and fatal of all birth defects. Deciphering its causes, however, is complicated, as many patients affected by CHD have no family history of the disease. There is also widespread heterogeneity of cardiac malformations within affected individuals. Nonetheless, there have been tremendous efforts toward a better understanding of the molecular and cellular events leading to CHD. Notably, certain cardiac-specific transcription factors have been implicated in mammalian heart development and disruption of their activity has been demonstrated in CHD. The homeodomain transcription factor NKX2-5 is an important member of this group. Indeed, more than 40 heterozygous NKX2-5 germline mutations have been observed in individuals with CHD, and these are spread along the coding region, with many shown to impact protein function. Thus, NKX2-5 appears to be hypermutable, yet the overall detection frequency in sporadic CHD is about 2% and NKX2-5 mutations are one-time detections with single-positives or private to families. Furthermore, there is lack of genotype-phenotype correlation, in which the same cardiac malformations have been exhibited in different NKX2-5 mutations or the same NKX2-5 mutation associated with diverse malformations. Here, we summarize published NKX2-5 germline mutations and explore different avenues in disease pathogenesis to support the notion of a multifactorial cause of CHD where possibly several genes and associated pathways are involved.