Regulation of cardiac myocyte cell death and differentiation by myocardin

Heme oxygenase/carbon monoxide system and development of the heart

Progressive differentiation controlled by intercellular signaling between pharyngeal mesoderm, foregut endoderm, and neural crest-derived mesenchyme is required for normal embryonic and fetal development. Gasotransmitters (criteria: 1) a small gas molecule; 2) freely permeable across membranes; 3) endogenously and enzymatically produced and its production regulated; 4) well-defined and specific functions at physiologically relevant concentrations; 5) functions can be mimicked by exogenously applied counterpart; and 6) cellular effects may or may not be second messenger-mediated, but should have specific cellular and molecular targets) are integral to gametogenesis and subsequent embryogenesis, fetal development, and normal heart maturation. Important for in utero development, the heme oxygenase/carbon monoxide system is expressed during gametogenesis, by the placenta, during embryonic development, and by the fetus. Complex sequences of biochemical pathways result in the progressive maturation of the human heart in utero . The resulting myocardial architecture, consisting of working myocardium, coronary arteries and veins, epicardium, valves and cardiac skeleton, endocardial lining, and cardiac conduction system, determines function. Oxygen metabolism in normal and maldeveloping hearts, which develop under reduced and fluctuating oxygen concentrations, is poorly understood. "Normal" hypoxia is critical for heart formation, but "abnormal" hypoxia in utero affects cardiogenesis. The heme oxygenase/carbon monoxide system is important for in utero cardiac development, and other factors also result in alterations of the heme oxygenase/carbon monoxide system during in utero cardiac development. This review will address the role of the heme oxygenase/carbon monoxide system during cardiac development in embryo and fetal development.

Gene editing and cardiac disease modelling for the interpretation of genetic variants of uncertain significance in congenital heart disease

BACKGROUND

Genomic sequencing in congenital heart disease (CHD) patients often discovers novel genetic variants, which are classified as variants of uncertain significance (VUS). Functional analysis of each VUS is required in specialised laboratories, to determine whether the VUS is disease causative or not, leading to lengthy diagnostic delays. We investigated stem cell cardiac disease modelling and transcriptomics for the purpose of genetic variant classification using a GATA4 (p.Arg283Cys) VUS in a patient with CHD.

METHODS

We performed high efficiency CRISPR gene editing with homology directed repair in induced pluripotent stem cells (iPSCs), followed by rapid clonal selection with amplicon sequencing. Genetic variant and healthy matched control cells were compared using cardiomyocyte disease modelling and transcriptomics.

RESULTS

Genetic variant and healthy cardiomyocytes similarly expressed Troponin T (cTNNT), and GATA4. Transcriptomics analysis of cardiomyocyte differentiation identified changes consistent with the patient's clinical human phenotype ontology terms. Further, transcriptomics revealed changes in calcium signalling, and cardiomyocyte adrenergic signalling in the variant cells. Functional testing demonstrated, altered action potentials in GATA4 genetic variant cardiomyocytes were consistent with patient cardiac abnormalities.

CONCLUSIONS

This work provides in vivo functional studies supportive of a damaging effect on the gene or gene product. Furthermore, we demonstrate the utility of iPSCs, CRISPR gene editing and cardiac disease modelling for genetic variant interpretation. The method can readily be applied to other genetic variants in GATA4 or other genes in cardiac disease, providing a centralised assessment pathway for patient genetic variant interpretation.

Supplementation with the Prebiotic High-Esterified Pectin Improves Blood Pressure and Cardiovascular Risk Biomarker Profile, Counteracting Metabolic Malprogramming

Supplementation with the prebiotic pectin is associated with beneficial health effects. We aimed to characterize the cardioprotective actions of chronic high-esterified pectin (HEP) supplementation (10%) in a model of metabolic malprogramming in rats, prone to obesity and associated disorders: the progeny of mild calorie-restricted dams during the first half of pregnancy. Results show that pectin supplementation reverses metabolic malprogramming associated with gestational undernutrition. In this sense, HEP supplementation improved blood pressure, reduced heart lipid content, and regulated cardiac gene expression of atrial natriuretic peptide and lipid metabolism-related genes. Moreover, it caused an elevation in circulating levels of fibroblast growth factor 21 and a higher expression of its co-receptor β-klotho in the heart. Most effects are correlated with the gut levels of beneficial bacteria promoted by HEP. Therefore, chronic HEP supplementation shows cardioprotective actions, and hence, it is worth considering as a strategy to prevent programmed cardiometabolic alterations.

Myocardin suppression increases lipid retention and atherosclerosis via downregulation of ABCA1 in vascular smooth muscle cells

Myocardin (MYOCD) plays an important role in cardiovascular disease. However, its underlying impact on atherosclerosis remains to be elucidated. ATP binding cassette transporter A1 (ABCA1), a key membrane-associated lipid transporter which maintains intracellular lipid homeostasis, has a protective function in atherosclerosis progress. The purpose of this study was to investigate whether and how the effect of MYOCD on atherosclerosis is associated with ABCA1 in vascular smooth muscle cells (VSMCs). We found both MYOCD and ABCA1 expression were dramatically decreased in atherosclerotic patient aortas compared to control. MYOCD knockdown inhibited ABCA1 expression in human aortic vascular smooth muscle cells (HAVSMCs), leading to reduced cholesterol efflux and increased intracellular cholesterol contents. MYOCD overexpression exerted the opposite effect. Mechanistically, MYOCD regulates ABCA1 expression in an SRF-dependent manner. Consistently, apolipoprotein E-deficient mice treated with MYOCD shRNA developed more plaques in the aortic sinus, which is associated with reduced ABCA1 expression, increased cholesterol retention in the aorta, and decreased high-density lipoprotein cholesterol levels in the plasma. Our data suggest that MYOCD deficiency exacerbates atherosclerosis by downregulating ABCA1 dependent cholesterol efflux from VSMCs, thereby providing a novel strategy for the therapeutic treatment of atherosclerotic cardiovascular disease.

Myocardin regulates mitochondrial calcium homeostasis and prevents permeability transition

Myocardin is a transcriptional co-activator required for cardiovascular development, but also promotes cardiomyocyte survival through an unclear molecular mechanism. Mitochondrial permeability transition is implicated in necrosis, while pore closure is required for mitochondrial maturation during cardiac development. We show that loss of myocardin function leads to subendocardial necrosis at E9.5, concurrent with elevated expression of the death gene Nix. Mechanistically, we demonstrate that myocardin knockdown reduces microRNA-133a levels to allow Nix accumulation, leading to mitochondrial permeability transition, reduced mitochondrial respiration, and necrosis. Myocardin knockdown elicits calcium release from the endo/sarcoplasmic reticulum with mitochondrial calcium accumulation, while restoration of microRNA-133a function, or knockdown of Nix rescues calcium perturbations. We observed reduced myocardin and elevated Nix expression within the infarct border-zone following coronary ligation. These findings identify a myocardin-regulated pathway that maintains calcium homeostasis and mitochondrial function during development, and is attenuated during ischemic heart disease. Given the diverse role of Nix and microRNA-133a, these findings may have broader implications to metabolic disease and cancer.

Misoprostol regulates Bnip3 repression and alternative splicing to control cellular calcium homeostasis during hypoxic stress

The cellular response to hypoxia involves the activation of a conserved pathway for gene expression regulated by the transcription factor complex called hypoxia-inducible factor (HIF). This pathway has been implicated in both the adaptive response to hypoxia and in several hypoxic-ischemic-related pathologies. Perinatal hypoxic injury, often associated with prematurity, leads to multi-organ dysfunction resulting in significant morbidity and mortality. Using a rodent model of neonatal hypoxia and several representative cell lines, we observed HIF1α activation and down-stream induction of the cell death gene Bnip3 in brain, large intestine, and heart which was mitigated by administration of the prostaglandin E1 analog misoprostol. Mechanistically, we determined that misoprostol inhibits full-length Bnip3 (Bnip3-FL) expression through PKA-mediated NF-κB (P65) nuclear retention, and the induction of pro-survival splice variants. We observed that the dominant small pro-survival variant of Bnip3 in mouse cells lacks the third exon (Bnip3ΔExon3), whereas human cells produce a pro-survival BNIP3 variant lacking exon 2 (BNIP3ΔExon2). In addition, these small Bnip3 splice variants prevent mitochondrial dysfunction, permeability transition, and necrosis triggered by Bnip3-FL by blocking calcium transfer from the sarco/endoplasmic reticulum to the mitochondria. Furthermore, misoprostol and Bnip3ΔExon3 promote nuclear calcium accumulation, resulting in HDAC5 nuclear export, NFAT activation, and adaptive changes in cell morphology and gene expression. Collectively, our data suggests that misoprostol can mitigate the potential damaging effects of hypoxia on multiple cell types by activating adaptive cell survival pathways through Bnip3 repression and alternative splicing.

Nexilin/NEXN controls actin polymerization in smooth muscle and is regulated by myocardin family coactivators and YAP

Nexilin, encoded by the NEXN gene, is expressed in striated muscle and localizes to Z-discs, influencing mechanical stability. We examined Nexilin/NEXN in smooth muscle cells (SMCs), and addressed if Nexilin localizes to dense bodies and dense bands and whether it is regulated by actin-controlled coactivators from the MRTF (MYOCD, MKL1, MKL2) and YAP/TAZ (YAP1 and WWTR1) families. NEXN expression in SMCs was comparable to that in striated muscles. Immunofluorescence and immunoelectron microscopy suggested that Nexilin localizes to dense bodies and dense bands. Correlations at the mRNA level suggested that NEXN expression might be controlled by actin polymerization. Depolymerization of actin using Latrunculin B repressed the NEXN mRNA and protein in bladder and coronary artery SMCs. Overexpression and knockdown supported involvement of both YAP/TAZ and MRTFs in the transcriptional control of NEXN. YAP/TAZ and MRTFs appeared equally important in bladder SMCs, whereas MRTFs dominated in vascular SMCs. Expression of NEXN was moreover reduced in situations of SMC phenotypic modulation in vivo. The proximal promoter of NEXN conferred control by MRTF-A/MKL1 and MYOCD. NEXN silencing reduced actin polymerization and cell migration, as well as SMC marker expression. NEXN targeting by actin-controlled coactivators thus amplifies SMC differentiation through the actin cytoskeleton, probably via dense bodies and dense bands.

Integrin β3 inhibits hypoxia-induced apoptosis in cardiomyocytes

Hypoxia-induced apoptosis plays an important role in cardiovascular diseases. Integrin β3 is one of the main integrin heterodimer receptors on the surface of cardiac myocytes. However, despite the important role that integrin β3 plays in the cardiovascular disease, its exact role in the hypoxia response remains unclear. Hence, in the present investigation we aimed to study the role of integrin β3 in hypoxia-induced apoptosis in H9C2 cells and primary rat myocardial cells. MTT assay, flow cytometry and TUNEL assay results showed that hypoxia inhibited cardiomyocyte proliferation and induced cardiomyocyte apoptosis. The expression levels of integrin β3 and HIF1α were upregulated in hypoxia-induced cardiomyocytes as revealed by real-time PCR and western blot analysis. Furthermore, knockdown of integrin β3 expression by siRNA increased hypoxia-induced cardiomyocyte apoptosis. In addition, integrin β3 overexpression weakened hypoxia-induced cardiomyocyte apoptosis. The protein expressions of integrin β3 and HIF1α were upregulated in acute myocardial infarction rat cardiac tissues compared with the control rat cardiac tissues. Our data suggest that integrin β3 plays a protective role in cardiomyocytes during hypoxia-induced apoptosis.