Control Cardiomyocytes Research Articles

Chaperone-mediated autophagy protects cardiomyocytes against hypoxic-cell death.

Chaperone-mediated autophagy (CMA) is a chaperone-dependent process of selective cytosolic protein turnover that targets specific proteins to lysosomes for degradation. Enhancing protein degradation mechanisms has been shown to be beneficial in multiple models of cardiac disease, including myocardial infarction (MI) and ischemia-reperfusion (I/R) injury. However, the causal role of CMA in cardiomyocyte injury and death is largely unknown. Hypoxia is an important contributor to both MI and I/R damage, which are major, precedent causes of heart failure. Upregulating CMA was hypothesized to protect against hypoxia-induced cardiomyocyte death. Lysosome-associated membrane protein 2a (Lamp2a) overexpression and knockdown were used to causally study CMA's role in hypoxically stressed cardiomyocytes. LAMP2a protein levels were used as both a primary indicator and driver of CMA function. Hypoxic stress was stimulated by CoCl2 treatment, which increased LAMP2a protein levels (+1.4-fold) and induced cardiomyocyte apoptosis (+3.2-4.0-fold). Lamp2a siRNA knockdown (-3.2-fold) of control cardiomyocytes increased apoptosis (+1.8-fold) suggesting that loss of CMA is detrimental for cardiomyocyte survival. However, there was neither an additive nor a synergistic effect on cell death when Lamp2a-silenced cells were treated with CoCl2. Conversely, Lamp2a overexpression (+3.0-fold) successfully reduced hypoxia-induced apoptosis by ∼50%. LAMP2a was also significantly increased (+1.7-fold) in ischemic heart failure patient samples, similar to hypoxically stressed cardiomyocytes. The failing ischemic hearts may have had insufficient CMA activation. To our knowledge, this study for the first time establishes a protective role for CMA (via Lamp2a overexpression) against hypoxia-induced cardiomyocyte loss and reveals the intriguing possibility that CMA activation may offer a cardioprotective treatment for ischemic heart disease.

MED12 is a Transcriptional Mediator of Cardiac Contractile Function

The Mediator complex plays an essential role in transcription for nearly all genes, and functions by transmitting physiological and developmental signals to RNA polymerase II. The Mediator complex is regulated by a kinase submodule made up of four proteins: MED12, MED13, CDK8, and Cyclin C. MED12 is a pivotal member of this submodule as it is required for CDK8 activity, which regulates Polymerase II‐driven transcription. The objective of this study is to identify the role of MED12 in the heart and determine how it regulates transcription through interactions with transcription factors. MED12 protein levels are increased in human heart failure biopsies, and in the mouse heart after transverse aortic constriction (TAC)‐induced heart failure. Therefore, we hypothesize that increased levels of MED12 contributes to development of heart failure. To test this hypothesis we generated cardiac‐specific MED12 transgenic (cTg) mice by driving increased MED12 expression in cardiomyocytes using the aMHC promoter. We performed serial echocardiography and observed that increased MED12 expression leads to decreased cardiac function, measured as fractional shortening. Decreased cardiac function was accompanied by increased cardiac chamber size and dilation, without evidence of cardiac fibrosis. We performed RNA sequencing on cTg and control cardiomyocytes, to investigate mechanisms of MED12‐induced cardiac dysfunction, and found that increased MED12 levels lead to dysregulated calcium handing gene expression. MED12 is a transcriptional regulator, but does not directly bind DNA. To determine how MED12 regulates expression of calcium handling genes we used cardiomyocytes from our mouse models to screen for transcription factors that interact with MED12. We immunoprecipitated MED12 and identified interacting proteins by mass spectrometry, many of which are regulators of transcription. In follow‐up experiments we demonstrated that MED12 interacts with the transcription factor (TF) MEF2 to regulate calcium handling genes. Preliminary experiments suggest that MED12 also interacts with TFs CREB and SRF. Additional mechanistic studies are ongoing to determine how MED12 coordinates gene expression through multiple transcription factors in cardiomyocytes. Collectively, our data demonstrate that tight regulation of MED12 levels is necessary to maintain normal cardiac function and regulate gene expression in the heart, and increased MED12 expression leads to the development of heart failure.

PO-615-05 ACACETIN, A POTENT TRANSIENT OUTWARD CURRENT BLOCKER, MAY BE A NOVEL THERAPEUTIC FOR KCND3-ENCODED KV4.3 GAIN-OF-FUNCTION-ASSOCIATED J-WAVE SYNDROMES

The transient outward current (Ito) that mediates early (phase 1) repolarization is conducted by the KCND3-encoded Kv4.3 pore-forming α-subunit. KCND3 gain-of-function mutations have been reported previously as a pathogenic substrate for J wave syndromes (JWS), including the Brugada syndrome (BrS) and early repolarization syndrome (ERS), as well as autopsy-negative sudden unexplained death (SUD). Acacetin, a natural flavone, is a potent Ito current blocker. We hypothesize that Acacetin may be a novel therapeutic for KCND3-mediated JWS. KCND3-V392I was identified in an 18-year-old male with JWS/ERS, and a history of cardiac arrest including ventricular tachycardia/fibrillation and atrial fibrillation/flutter. KCND3-V392I was engineered by site directed mutagenesis and expressed in TSA201 cells. Gene-edited/variant-corrected isogenic control and patient-specific pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) from the KCND3-V392I-positive patient were generated. Ito currents and action potentials (APs) were recorded before and after treatment with Acacetin using the whole cell patch-clamp and multielectrode array (MEA) technique. Western blot and immunocytochemistry were performed to investigate KCND3 expression. KCND3-V392I demonstrated a marked gain-of-function phenotype, increasing peak Ito current density by 92% (p<0.05 vs. KCND3-WT) in TSA201 cells and by 61% in Kv4.3-V392I iPSC-CM (p<0.05 vs. isogenic control). KCND3 transcript and translate expression was increased significantly in KCND3-V392I-derived iPSC-CMs (p<0.05 vs. isogenic control). While KCND3-WT revealed an IC50 of 7.2±1.0 μM for Acacetin effect, 30 μM Acacetin dramatically inhibited KCND3-V392I peak Ito current density by 96% (p<0.05 vs. before Acacetin). 10 μM Acacetin, a concentration approaching its IC50 value, inhibited Ito by 54% (P<0.05 vs. before Acacetin) in patient-derived iPSC-CMs and reduced the accentuated AP notch displayed in KCND3-V392I-derived iPSC-CMs. This pre-clinical study provides pharmacological and functional evidence to suggest that Acacetin may be a novel therapeutic for patients with KCND3 gain-of-function-associated JWS by inhibiting Ito and abolishing the accentuated AP notch in patient-derived iPSC-CMs.

Calcium dysregulation increases right ventricular outflow tract arrhythmogenesis in rabbit model of chronic kidney disease.

Chronic kidney disease (CKD) increases the risk of arrhythmia. The right ventricular outflow tract (RVOT) is a crucial site of ventricular tachycardia (VT) origination. We hypothesize that CKD increases RVOT arrhythmogenesis through its effects on calcium dysregulation. We analysed measurements obtained using conventional microelectrodes, patch clamp, confocal microscopy, western blotting, immunohistochemical examination and lipid peroxidation for both control and CKD (induced by 150 mg/kg neomycin and 500 mg/kg cefazolin daily) rabbit RVOT tissues or cardiomyocytes. The RVOT of CKD rabbits exhibited a short action potential duration, high incidence of tachypacing (20 Hz)‐induced sustained VT, and long duration of isoproterenol and tachypacing‐induced sustained and non‐sustained VT. Tachypacing‐induced sustained and non‐sustained VT in isoproterenol‐treated CKD RVOT tissues were attenuated by KB‐R7943 and partially inhibited by KN93 and H89. The CKD RVOT myocytes had high levels of phosphorylated CaMKII and PKA, and an increased expression of tyrosine hydroxylase‐positive neural density. The CKD RVOT myocytes exhibited large levels of I to, I Kr, NCX and L‐type calcium currents, calcium leak and malondialdehyde but low sodium current, SERCA2a activity and SR calcium content. The RVOT in CKD with oxidative stress and autonomic neuron hyperactivity exhibited calcium handling abnormalities, which contributed to the induction of VT.

Cardiac aorta-derived extracellular matrix scaffold enhances critical mediators of angiogenesis in isoproterenol-induced myocardial infarction mice

An incapability to improve lost cardiac muscle caused by acute ischemic injury remains the most important deficiency of current treatments to prevent heart failure. We investigated whether cardiomyocytes culturing on cardiac aorta-derived extracellular matrix scaffold has advantageous effects on cardiomyocytes survival and angiogenesis biomarkers’ expression. Ten male NMRI mice were randomly divided into two groups: (1) control (healthy mice) and (2) myocardial infarction (MI)-induced model group (Isoproterenol/subcutaneously injection/single dose of 85 mg/kg). Two days after isoproterenol injection, all animals were sacrificed to isolate cardiomyocytes from myocardium tissues. The fresh thoracic aorta was obtained from male NMRI mice and decellularized using 4% sodium deoxycholate and 2000 kU DNase-I treatments. Control and MI-derived cardiomyocytes were seeded on decellularized cardiac aorta (DCA) considered three-dimensional (3D) cultures. To compare, the isolated cardiomyocytes from control and MI groups were also cultured as a two-dimensional (2D) culture system for 14 days. The cell viability was examined by MTT assay. The expression levels of Hif-1α and VEGF genes and VEGFR1 protein were tested by real-time PCR and western blotting, respectively. Moreover, the amount of VEGF protein was evaluated in the conditional media of the 2D and 3D systems. The oxidative stress was assessed via MDA assay. Hif-1α and VEGF genes were downregulated in MI groups compared to controls. However, the resulting data showed that decellularized cardiac aorta matrices positively affect the expression of Hif-1α and VEGF genes. The expression level of VEGFR1 protein was significantly (p ≤ 0.01) upregulated in both MI and healthy cell groups cultured on decellularized cardiac aorta matrices as a 3D system compared to the MI cell group cultured in the 2D systems. Furthermore, MDA concentration significantly decreased in 3D-cultured cells (MI and healthy cell groups) rather than the 2D-cultured MI group (p ≤ 0.015). The findings suggest that cardiac aorta-derived extracellular scaffold by preserving VEGF, improving the cell viability, and stimulating angiogenesis via upregulating Hif-1α, VEGF, and VEGFR1 in cardiomyocytes could be considered as a potential approach along with another therapeutic method to reduce the complications of myocardial infarction and control the progressive pathological conditions related to MI.

Cardiomyocyte Na+ and Ca2+ mishandling drives vicious cycle involving CaMKII, ROS, and ryanodine receptors

Cardiomyocyte Na+ and Ca2+ mishandling, upregulated Ca2+/calmodulin-dependent kinase II (CaMKII), and increased reactive oxygen species (ROS) are characteristics of various heart diseases, including heart failure (HF), long QT (LQT) syndrome, and catecholaminergic polymorphic ventricular tachycardia (CPVT). These changes may form a vicious cycle of positive feedback to promote cardiac dysfunction and arrhythmias. In HF rabbit cardiomyocytes investigated in this study, the inhibition of CaMKII, late Na+ current (INaL), and leaky ryanodine receptors (RyRs) all attenuated the prolongation and increased short-term variability (STV) of action potential duration (APD), but in age-matched controls these inhibitors had no or minimal effects. In control cardiomyocytes, we enhanced RyR leak (by low [caffeine] plus isoproterenol mimicking CPVT) which markedly increased STV and delayed afterdepolarizations (DADs). These proarrhythmic changes were significantly attenuated by both CaMKII inhibition and mitochondrial ROS scavenging, with a slight synergy with INaL inhibition. Inducing LQT by elevating INaL (by Anemone toxin II, ATX-II) caused markedly prolonged APD, increased STV, and early afterdepolarizations (EADs). Those proarrhythmic ATX-II effects were largely attenuated by mitochondrial ROS scavenging, and partially reduced by inhibition of CaMKII and pathological leaky RyRs using dantrolene. In human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) bearing LQT3 mutation SCN5A N406K, dantrolene significantly attenuated cell arrhythmias and APD prolongation. Targeting critical components of the Na+–Ca2+–CaMKII–ROS–INaL arrhythmogenic vicious cycle may exhibit important on-target and also trans-target effects (e.g., INaL and RyR inhibition can alter INaL-mediated LQT3 effects). Incorporating this vicious cycle into therapeutic strategies provides novel integrated insight for treating cardiac arrhythmias and diseases.

Insulin signaling alters antioxidant capacity in the diabetic heart

Diabetic cardiomyopathy is associated with an increase in oxidative stress. However, antioxidant therapy has shown a limited capacity to mitigate disease pathology. The molecular mechanisms responsible for the modulation of reactive oxygen species (ROS) production and clearance must be better defined. The objective of this study was to determine how insulin affects superoxide radical (O2•–) levels. O2•– production was evaluated in adult cardiomyocytes isolated from control and Akita (type 1 diabetic) mice by spin-trapping electron paramagnetic resonance spectroscopy. We found that the basal rates of O2•– production were comparable in control and Akita cardiomyocytes. However, culturing cardiomyocytes without insulin resulted in a significant increase in O2•– production only in the Akita group. In contrast, O2•– production was unaffected by high glucose and/or fatty acid supplementation. The increase in O2•– was due in part to a decrease in superoxide dismutase (SOD) activity. The PI3K inhibitor, LY294002, decreased Akita SOD activity when insulin was present, indicating that the modulation of antioxidant activity is through insulin signaling. The effect of insulin on mitochondrial O2•– production was evaluated in Akita mice that underwent a 1-week treatment of insulin. Mitochondria isolated from insulin-treated Akita mice produced less O2•– than vehicle-treated diabetic mice. Quantitative proteomics was performed on whole heart homogenates to determine how insulin affects antioxidant protein expression. Of 29 antioxidant enzymes quantified, thioredoxin 1 was the only one that was significantly enhanced by insulin treatment. In vitro analysis of thioredoxin 1 revealed a previously undescribed capacity of the enzyme to directly scavenge O2•–. These findings demonstrate that insulin has a role in mitigating cardiac oxidative stress in diabetes via regulation of endogenous antioxidant activity.

Cardiac Myoediting Attenuates Cardiac Abnormalities in Human and Mouse Models of Duchenne Muscular Dystrophy.

[Figure: see text].

Generation of microRNA-378a-deficient hiPSC as a novel tool to study its role in human cardiomyocytes

microRNA-378a (miR-378a) is one of the most highly expressed microRNAs in the heart. However, its role in the human cardiac tissue has not been fully understood. It was observed that miR-378a protects cardiomyocytes from hypertrophic growth by regulation of IGF1R and the expression of downstream kinases. Increased levels of miR-378a were reported in the serum of Duchenne muscular dystrophy (DMD) patients and female carriers of DMD gene-associated mutations with developed cardiomyopathy. In order to shed more light on the role of miR-378a in human cardiomyocytes and its potential involvement in DMD-related cardiomyopathy, we generated two human induced pluripotent stem cell (hiPSC) models; one with deletion of miR-378a and the second one with deletion of DMD exon 50 leading to the DMD phenotype. Our results indicate that lack of miR-378a does not influence the pluripotency of hiPSC and their ability to differentiate into cardiomyocytes (hiPSC-CM). miR-378a-deficient hiPSC-CM exhibited, however, significantly bigger size compared to the isogenic control cells, indicating the role of this miRNA in the hypertrophic growth of human cardiomyocytes. In accordance, the level of NFATc3, phosphoAKT, phosphoERK and ERK was higher in these cells compared to the control counterparts. A similar effect was achieved by silencing miR-378a with antagomirs. Of note, the percentage of cells with nuclear localization of NFATc3 was higher in miR-378a-deficient hiPSC-CM. Analysis of electrophysiological properties and Ca2+ oscillations revealed the decrease in the spike slope velocity and lower frequency of calcium spikes in miR-378a-deficient hiPSC-CM. Interestingly, the level of miR-378a increased gradually during cardiac differentiation of hiPSC. Of note, it was low until day 15 in differentiating DMD-deficient hiPSC-CM and then rose to a similar level as in the isogenic control counterparts. In summary, our findings confirmed the utility of hiPSC-based models for deciphering the role of miR-378a in the control and diseased human cardiomyocytes.

L-type channel inactivation balances the increased peak calcium current due to absence of Rad in cardiomyocytes.

The L-type Ca2+ channel (LTCC) provides trigger calcium to initiate cardiac contraction in a graded fashion that is regulated by L-type calcium current (ICa,L) amplitude and kinetics. Inactivation of LTCC is controlled to fine-tune calcium flux and is governed by voltage-dependent inactivation (VDI) and calcium-dependent inactivation (CDI). Rad is a monomeric G protein that regulates ICa,L and has recently been shown to be critical to β-adrenergic receptor (β-AR) modulation of ICa,L. Our previous work showed that cardiomyocyte-specific Rad knockout (cRadKO) resulted in elevated systolic function, underpinned by an increase in peak ICa,L, but without pathological remodeling. Here, we sought to test whether Rad-depleted LTCC contributes to the fight-or-flight response independently of β-AR function, resulting in ICa,L kinetic modifications to homeostatically balance cardiomyocyte function. We recorded whole-cell ICa,L from ventricular cardiomyocytes from inducible cRadKO and control (CTRL) mice. The kinetics of ICa,L stimulated with isoproterenol in CTRL cardiomyocytes were indistinguishable from those of unstimulated cRadKO cardiomyocytes. CDI and VDI are both enhanced in cRadKO cardiomyocytes without differences in action potential duration or QT interval. To confirm that Rad loss modulates LTCC independently of β-AR stimulation, we crossed a β1,β2-AR double-knockout mouse with cRadKO, resulting in a Rad-inducible triple-knockout mouse. Deletion of Rad in cardiomyocytes that do not express β1,β2-AR still yielded modulated ICa,L and elevated basal heart function. Thus, in the absence of Rad, increased Ca2+ influx is homeostatically balanced by accelerated CDI and VDI. Our results indicate that the absence of Rad can modulate the LTCC without contribution of β1,β2-AR signaling and that Rad deletion supersedes β-AR signaling to the LTCC to enhance in vivo heart function.

Molecular and Electrophysiological Role of Diabetes-Associated Circulating Inflammatory Factors in Cardiac Arrhythmia Remodeling in a Metabolic-Induced Model of Type 2 Diabetic Rat.

Background: Diabetic patients have prolonged cardiac repolarization and higher risk of arrhythmia. Besides, diabetes activates the innate immune system, resulting in higher levels of plasmatic cytokines, which are described to prolong ventricular repolarization. Methods: We characterize a metabolic model of type 2 diabetes (T2D) with prolonged cardiac repolarization. Sprague-Dawley rats were fed on a high-fat diet (45% Kcal from fat) for 6 weeks, and a low dose of streptozotozin intraperitoneally injected at week 2. Body weight and fasting blood glucose were measured and electrocardiograms of conscious animals were recorded weekly. Plasmatic lipid profile, insulin, cytokines, and arrhythmia susceptibility were determined at the end of the experimental period. Outward K+ currents and action potentials were recorded in isolated ventricular myocytes by patch-clamp. Results: T2D animals showed insulin resistance, hyperglycemia, and elevated levels of plasma cholesterol, triglycerides, TNFα, and IL-1b. They also developed bradycardia and prolonged QTc-interval duration that resulted in increased susceptibility to severe ventricular tachycardia under cardiac challenge. Action potential duration (APD) was prolonged in control cardiomyocytes incubated 24 h with plasma isolated from diabetic rats. However, adding TNFα and IL-1b receptor blockers to the serum of diabetic animals prevented the increased APD. Conclusions: The elevation of the circulating levels of TNFα and IL-1b are responsible for impaired ventricular repolarization and higher susceptibility to cardiac arrhythmia in our metabolic model of T2D.

LATS2 Deletion Attenuates Myocardial Ischemia-Reperfusion Injury by Promoting Mitochondrial Biogenesis.

Reperfusion therapy is the most effective treatment for acute myocardial infarction, but it can damage cardiomyocytes through a mechanism known as myocardial ischemia/reperfusion injury (MIRI). In this study, we investigated whether the large tumor suppressor kinase 2 (LATS2) contributes to the development of myocardial MIRI by disrupting mitochondrial biogenesis. Our in vitro data demonstrate that cardiomyocyte viability was reduced and apoptosis was increased in response to hypoxia/reoxygenation (H/R) injury. However, suppression of LATS2 by shRNA sustained cardiomyocyte viability by maintaining mitochondrial function. Compared to H/R-treated control cardiomyocytes, cardiomyocytes transfected with LATS2 shRNA exhibited increased mitochondrial respiration, improved mitochondrial ATP generation, and more stable mitochondrial membrane potential. LATS2 suppression increased cardiomyocyte viability and mitochondrial biogenesis in a manner dependent on PGC1α, a key regulator of mitochondrial metabolism. These results identify LATS2 as a new inducer of mitochondrial damage and myocardial MIRI and suggest that approaches targeting LATS2 or mitochondrial biogenesis may be beneficial in the clinical management of cardiac MIRI.

Inositol Trisphosphate Receptors and Nuclear Calcium in Atrial Fibrillation.

The mechanisms underlying atrial fibrillation (AF), the most common clinical arrhythmia, are poorly understood. Nucleoplasmic Ca2+ regulates gene expression, but the nature and significance of nuclear Ca2+-changes in AF are largely unknown. To elucidate mechanisms by which AF alters atrial-cardiomyocyte nuclear Ca2+ ([Ca2+]Nuc) and CaMKII (Ca2+/calmodulin-dependent protein kinase-II)-related signaling. Atrial cardiomyocytes were isolated from control and AF dogs (kept in AF by atrial tachypacing [600 bpm × 1 week]). [Ca2+]Nuc and cytosolic [Ca2+] ([Ca2+]Cyto) were recorded via confocal microscopy. Diastolic [Ca2+]Nuc was greater than [Ca2+]Cyto under control conditions, while resting [Ca2+]Nuc was similar to [Ca2+]Cyto; both diastolic and resting [Ca2+]Nuc increased with AF. IP3R (Inositol-trisphosphate receptor) stimulation produced larger [Ca2+]Nuc increases in AF versus control cardiomyocytes, and IP3R-blockade suppressed the AF-related [Ca2+]Nuc differences. AF upregulated nuclear protein expression of IP3R1 (IP3R-type 1) and of phosphorylated CaMKII (immunohistochemistry and immunoblot) while decreasing the nuclear/cytosolic expression ratio for HDAC4 (histone deacetylase type-4). Isolated atrial cardiomyocytes tachypaced at 3 Hz for 24 hours mimicked AF-type [Ca2+]Nuc changes and L-type calcium current decreases versus 1-Hz-paced cardiomyocytes; these changes were prevented by IP3R knockdown with short-interfering RNA directed against IP3R1. Nuclear/cytosolic HDAC4 expression ratio was decreased by 3-Hz pacing, while nuclear CaMKII phosphorylation was increased. Either CaMKII-inhibition (by autocamtide-2-related peptide) or IP3R-knockdown prevented the CaMKII-hyperphosphorylation and nuclear-to-cytosolic HDAC4 shift caused by 3-Hz pacing. In human atrial cardiomyocytes from AF patients, nuclear IP3R1-expression was significantly increased, with decreased nuclear/nonnuclear HDAC4 ratio. MicroRNA-26a was predicted to target ITPR1 (confirmed by luciferase assay) and was downregulated in AF atrial cardiomyocytes; microRNA-26a silencing reproduced AF-induced IP3R1 upregulation and nuclear diastolic Ca2+-loading. AF increases atrial-cardiomyocyte nucleoplasmic [Ca2+] by IP3R1-upregulation involving miR-26a, leading to enhanced IP3R1-CaMKII-HDAC4 signaling and L-type calcium current downregulation. Graphic Abstract: A graphic abstract is available for this article.

Dioxin Disrupts Dynamic DNA Methylation Patterns in Genes That Govern Cardiomyocyte Maturation

Congenital heart disease (CHD), the leading birth defect worldwide, has a largely unknown etiology, likely to result from complex interactions between genetic and environmental factors during heart development, at a time when the heart adapts to diverse physiological and pathophysiological conditions. Crucial among these is the regulation of cardiomyocyte development and postnatal maturation, governed by dynamic changes in DNA methylation. Previous work from our laboratory has shown that exposure to the environmental toxicant tetrachlorodibenzo-p-dioxin (TCDD) disrupts several molecular networks responsible for heart development and function. To test the hypothesis that the disruption caused by TCDD in the heart results from changes in DNA methylation and gene expression patterns of cardiomyocytes, we established a stable mouse embryonic stem cell line expressing a puromycin resistance selectable marker under control of the cardiomyocyte-specific Nkx2-5 promoter. Differentiation of these cells in the presence of puromycin induces the expression of a large suite of cardiomyocyte-specific markers. To assess the consequences of TCDD treatment on gene expression and DNA methylation in these cardiomyocytes, we subjected them to transcriptome and methylome analyses in the presence of TCDD. Unlike control cardiomyocytes maintained in vehicle, the TCDD-treated cardiomyocytes showed extensive gene expression changes, with a significant correlation between differential RNA expression and DNA methylation in 111 genes, many of which are key elements of pathways that regulate cardiovascular development and function. Our findings provide an important clue toward the elucidation of the complex interactions between genetic and epigenetic mechanisms after developmental TCDD exposure that may contribute to CHD.

Opa1 Reduces Hypoxia-Induced Cardiomyocyte Death by Improving Mitochondrial Quality Control.

Mitochondrial dysfunction contributes to cardiovascular disorders, especially post-infarction cardiac injury, through incompletely characterized mechanisms. Among the latter, increasing evidence points to alterations in mitochondrial quality control, a range of adaptive responses regulating mitochondrial morphology and function. Optic atrophy 1 (Opa1) is a mitochondrial inner membrane GTPase known to promote mitochondrial fusion. In this study, hypoxia-mediated cardiomyocyte damage was induced to mimic post-infarction cardiac injury in vitro. Loss- and gain-of-function assays were then performed to evaluate the impact of Opa1 expression on mitochondrial quality control and cardiomyocyte survival and function. Hypoxic stress reduced cardiomyocyte viability, impaired contractile/relaxation functions, and augmented the synthesis of pro-inflammatory mediators. These effects were exacerbated by Opa1 knockdown, and significantly attenuated by Opa1 overexpression. Mitochondrial quality control was disturbed by hypoxia, as reflected by multiple mitochondrial deficits; i.e., increased fission, defective fusion, impaired mitophagy, decreased biogenesis, increased oxidative stress, and blunted respiration. By contrast, overexpression of Opa1 normalized mitochondrial quality control and sustained cardiomyocyte function. We also found that ERK, AMPK, and YAP signaling can regulate Opa1 expression. These results identify Opa1 as a novel regulator of mitochondrial quality control and highlight a key role for Opa1 in protecting cardiomyocytes against post-infarction cardiac injury.

SUN-556 Cardiac Phenotype in Familial Partial Lipodystrophy

Background Pathogenic variants in Lamin A/C (LMNA) gene are the most common monogenic etiology in Familial Partial Lipodystrophy (FPLD) causing FPLD2. LMNA pathogenic variants have been previously associated with cardiomyopathy, familial arrhythmias or conduction system abnormalities independent of lipodystrophy. We aimed to assess cardiac impacts of FPLD, and to explore the extent of overlap between cardiolaminopathies and FPLD. Methods We conducted a retrospective review of an established cohort of 122 patients (age range: 13-77, M/F 21/101) with FPLD from Michigan (n = 83) and Turkey (n = 39) with an accessible cardiac evaluation. Also, functional syncytia of mature human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) from a FLPD2 patient was studied for assessment of autonomous rhythm and action potential duration with optical mapping using a voltage sensitive dye. Results In the whole study cohort, 95 (78%) patients had cardiac alterations (25% ischemic heart disease, 36% arrhythmia, 16% conduction abnormality, 20% prolonged QT interval, 11% cardiomyopathy, and 15% congestive heart failure). The likelihood of having an arrhythmia (OR; 3.95, 95% CI: 1.49-10.49) and conduction disease (OR: 3.324, 95% CI: 1.33-8.31) was significantly higher in patients with LMNA pathogenic variants. Patients with LMNA pathogenic variants were at high risk for atrial fibrillation/flutter (OR: 6.77, 95% CI: 1.27- 39.18). The time to first arrhythmia was significantly shorter in the LMNA group with a higher hazard rate of 3.04 (95% CI: 1.29-7.17, p = 0.032). Non-482 LMNA pathogenic variants were more likely to be associated with cardiac events (vs. 482 LMNA: OR: 4.74, 95% CI: 1.41- 15.98 for arrhythmia; OR: 17.67, 95% CI: 2.44- 127.68 for atrial fibrillation/flutter; OR: 5.71, 95% CI: 1.37- 23.76 for conduction disease. hiPSC-CMs from a FPLD2 patient had higher frequency of autonomous activity, and shorter Fridericia corrected action potential duration at 80% repolarization compared to control cardiomyocytes. Furthermore, FPLD2 functional syncytia of mature hiPSC-CMs presented several rhythm alterations such as early after-depolarizations, spontaneous quiescence and spontaneous tachyarrhythmia; none of those were observed in the control cell lines. Finally, FPLD2 hiPSC-CMs presented significantly slower recovery in chronotropic changes induced by isoproterenol exposure; which indicates disrupted beta-adrenergic response. Conclusion Our results suggest the need for vigilant cardiac monitoring in FPLD, especially in patients with FPLD2 who have an increased risk to develop cardiac arrhythmias and conduction system diseases. In addition, study of human induced pluripotent stem cell-derived cardiomyocytes may prove useful to understand the mechanism of cardiac disease and arrhythmias and to create precision therapy opportunities in the future.

Increases in [IP3]i aggravates diastolic [Ca2+] and contractile dysfunction in Chagas' human cardiomyocytes.

Chagas cardiomyopathy is the most severe manifestation of human Chagas disease and represents the major cause of morbidity and mortality in Latin America. We previously demonstrated diastolic Ca2+ alterations in cardiomyocytes isolated from Chagas’ patients to different degrees of cardiac dysfunction. In addition, we have found a significant elevation of diastolic [Na+]d in Chagas’ cardiomyocytes (FCII>FCI) that was greater than control. Exposure of cardiomyocytes to agents that enhance inositol 1,4,5 trisphosphate (IP3) generation or concentration like endothelin (ET-1) or bradykinin (BK), or membrane-permeant myoinositol 1,4,5-trisphosphate hexakis(butyryloxy-methyl) esters (IP3BM) caused an elevation in diastolic [Ca2+] ([Ca2+]d) that was always greater in cardiomyocytes from Chagas’ than non- Chagas’ subjects, and the magnitude of the [Ca2+]d elevation in Chagas’ cardiomyocytes was related to the degree of cardiac dysfunction. Incubation with xestospongin-C (Xest-C), a membrane-permeable selective blocker of the IP3 receptors (IP3Rs), significantly reduced [Ca2+]d in Chagas’ cardiomyocytes but did not have a significant effect on non-Chagas’ cells. The effects of ET-1, BK, and IP3BM on [Ca2+]d were not modified by the removal of extracellular [Ca2+]e. Furthermore, cardiomyocytes from Chagas’ patients had a significant decrease in the sarcoplasmic reticulum (SR) Ca2+content compared to control (Control>FCI>FCII), a higher intracellular IP3 concentration ([IP3]i) and markedly depressed contractile properties compared to control cardiomyocytes. These results provide additional and convincing support about the implications of IP3 in the pathogenesis of Chagas cardiomyopathy in patients at different stages of chronic infection. Additionally, these findings open the door for novel therapeutic strategies oriented to improve cardiac function and quality of life of individuals suffering from chronic Chagas cardiomyopathy (CC).

Alcohol Induced Oxidative Stress in neonatal Cardio myocytes

Alcohol Induced Oxidative Stress in neonatal Cardio myocytes

The Role of Membrane Capacitance in Cardiac Impulse Conduction: An Optogenetic Study With Non-excitable Cells Coupled to Cardiomyocytes.

Non-excitable cells (NECs) such as cardiac myofibroblasts that are electrotonically coupled to cardiomyocytes affect conduction velocity (θ) by representing a capacitive load (CL: increased membrane to be charged) and a resistive load (RL: partial depolarization of coupled cardiomyocytes). In this study, we untangled the relative contributions of both loading modalities to NEC-dependent arrhythmogenic conduction slowing. Discrimination between CL and RL was achieved by reversibly removing the RL component by light activation of the halorhodopsin-based hyperpolarizing membrane voltage actuator eNpHR3.0-eYFP (enhanced yellow fluorescent protein) expressed in communication-competent fibroblast-like NIH3T3 cells (3T3HR cells) that served as a model of coupled NECs. Experiments were conducted with strands of neonatal rat ventricular cardiomyocytes coated at increasing densities with 3T3HR cells. Impulse conduction along preparations stimulated at 2.5 Hz was assessed with multielectrode arrays. The relative density of 3T3HR cells was determined by dividing the area showing eYFP fluorescence by the area covered with cardiomyocytes [coverage factor (CF)]. Compared to cardiomyocytes, 3T3HR cells exhibited a depolarized membrane potential (−34 mV) that was shifted to −104 mV during activation of halorhodopsin. Without illumination, 3T3HR cells slowed θ along the preparations from ∼330 mm/s (control cardiomyocyte strands) to ∼100 mm/s (CF = ∼0.6). Illumination of the preparation increased the electrogram amplitudes and induced partial recovery of θ at CF > 0.3. Computer simulations demonstrated that the θ deficit observed during illumination was attributable in full to the CL represented by coupled 3T3HR cells with θ showing a power-law relationship to capacitance with an exponent of −0.78 (simulations) and −0.99 (experiments). The relative contribution of CL and RL to conduction slowing changed as a function of CF with CL dominating at CF ≤ ∼0.3, both mechanisms being equally important at CF = ∼0.5, and RL dominating over CL at CF > 0.5. The finding that RL did not affect θ at CFs ≤ 0.3 is explained by the circumstance that, at the respective moderate levels of cardiomyocyte depolarization, supernormal conduction stabilized propagation. The findings provide experimental estimates for the dependence of θ on membrane capacitance in general and suggest that the myocardium can absorb moderate numbers of electrotonically coupled NECs without showing substantial alterations of θ.

Nogo-C causes post-MI arrhythmia through increasing calcium leakage from sarcoplasmic reticulum

Disturbance of calcium homeostasis in cardiomyocytes plays a pivotal role in the pathogenesis of ventricular arrhythmias after myocardial infarction(MI). Nogo-C is a sarcoplasmic reticulum (SR) protein ubiquitously expressed in tissues including heart. We previously reported that Nogo-C played a critical role in the pathogenesis of MI through inducing cardiomyocyte apoptosis. However, the role and mechanisms of Nogo-C in the regulation of calcium homeostasis and in the pathogenesis of arrhythmia post-MI remain unclear. In the present study, wildtype and Nogo-C knockout mice were subjected with MI operation followed by intraperitoneal injection of caffeine and isoproterenol to induce arrhythmia. We found that Nogo-C knockout mice displayed lower incidence of arrhythmia compared with the high incidence of arrhythmia of wildtype mice. Moreover, Nogo-C overexpression by adenovirus in cardiomyocytes caused decreased SR calcium content, reduced contractile ability, and increased diastolic cytoplasma calcium concentration, as compared with control cells. Oxalate-facilitated 45Ca uptake assay showed no difference of SR calcium reuptake ability between control and Nogo-C overexpressing cardiomyocytes, indicating that the decreased SR calcium content was not through disturbing SR calcium reuptake in Nogo-C overexpressing cells. In contrast, the rate of SR calcium leak, spontaneous calcium wave, and calcium spark markedly increased in Nogo-C overexpressing cardiomyocytes, suggesting that the increased SR calcium leak causes the decreased SR calcium content. Furthermore, We found that Nogo-C overexpression markedly increased, while Nogo-C knockdown decreased Sec61 protein, a newly found calcium leak channel, in cardiomyocytes. Nogo-C bound with Sec61 through its transmembrane domains loop-6 and loop-8, thus inhibited the ubiquitination of Sec61, leading to the increased calcium leak from SR. Together, our data show that Nogo-C is increased in MI hearts, and the increased Nogo-C inhibits the degradation of Sec61 by ubiquitination, leading to increased SR calcium leak, thus promoting arrhythmia after MI.