Iron Overload, Oxidative Stress and Calcium Mishandling in Cardiomyocytes: Role of the Mitochondrial Permeability Transition Pore

Iron (Fe) plays an essential role in many physiological processes. Hereditary hemochromatosis or frequent blood transfusions often cause iron overload (IO), which can lead to cardiomyopathy and arrhythmias; however, the underlying mechanism is not well defined. In the present study, we assess the hypothesis that IO promotes arrhythmias via reactive oxygen species (ROS) production, mitochondrial membrane potential (∆Ψm) depolarization, and disruption of cytosolic Ca dynamics. In ventricular myocytes isolated from wild type (WT) mice, both cytosolic and mitochondrial Fe levels were elevated following perfusion with the Fe3+/8-hydroxyquinoline (8-HQ) complex. IO promoted mitochondrial superoxide generation (measured using MitoSOX Red) and induced the depolarization of the ΔΨm (measured using tetramethylrhodamine methyl ester, TMRM) in a dose-dependent manner. IO significantly increased the rate of Ca wave (CaW) formation measured in isolated ventricular myocytes using Fluo-4. Furthermore, in ex-vivo Langendorff-perfused hearts, IO increased arrhythmia scores as evaluated by ECG recordings under programmed S1-S2 stimulation protocols. We also carried out similar experiments in cyclophilin D knockout (CypD KO) mice in which the mitochondrial permeability transition pore (mPTP) opening is impaired. While comparable cytosolic and mitochondrial Fe load, mitochondrial ROS production, and depolarization of the ∆Ψm were observed in ventricular myocytes isolated from both WT and CypD KO mice, the rate of CaW formation in isolated cells and the arrhythmia scores in ex-vivo hearts were significantly lower in CypD KO mice compared to those observed in WT mice under conditions of IO. The mPTP inhibitor cyclosporine A (CsA, 1 µM) also exhibited a protective effect. In conclusion, our results suggest that IO induces mitochondrial ROS generation and ∆Ψm depolarization, thus opening the mPTP, thereby promoting CaWs and cardiac arrhythmias. Conversely, the inhibition of mPTP ameliorates the proarrhythmic effects of IO.

Abstract 507: Activation of Transient Receptor Potential Canonical Channel Currents in Iron-Overloaded Cardiac Myocytes

Background: Iron (Fe) overload cardiomyopathy is the leading cause of death in hemochromatotic patients, yet the mechanistic insight is still incomplete and controversial. We investigated alterations of action potentials (APs), ionic currents, and intracellular Ca 2+ (Ca 2+ i ) in Fe-loaded mouse cardiomyocytes, as well as functional impacts of Fe overload on single-cell contraction and whole-heart arrhythmias.

Methods: Cardiomyocytes were isolated from left ventricles of mouse hearts and were superfused with Fe 3+ /8-hydroxyquinoline complex (5-100 μM). APs, L-type Ca 2+ currents (I Ca,L ), total outward K + currents (I K ), and transient receptor potential canonical (TRPC) channel currents were recorded by the patch-clamp technique. Ca 2+ i was evaluated by using Fluo-4. Cell contraction was measured by a video-based edge detection system. Arrhythmias were evaluated in Langendorff-perfused hearts under S 1 -S 2 stimulation protocol.

Results: Persistent Fe (15 μM) treatment prolonged AP duration at 90% repolarization (APD 90 : 46.8 ± 2.8 vs. 203.6 ± 63.4 ms, p<0.05), induced early and delayed afterdepolarizations (EADs: 0 % vs. 45.0 ± 15.0 %; DADs: 4.3 ± 1.4 vs. 27.0 ± 7.0 %, p<0.05, respectively) in mouse cardiomyocytes. Consistently, arrhythmia incidence was increased in Fe 3+ /8-HQ-perfused hearts. Fe treatment decreased peak I Ca,L (16.5 ± 1.7 vs.11.4 ± 1.3 pA/pF, p<0.01) and I K (59.2 ± 3.3 vs. 50.4 ± 3.0 pA/pF, p<0.01), altered Ca 2+ i transient patterns and decreased contractility (4.8 ± 0.5 vs. 3.5 ± 0.4%, p<0.01). During the late phase of Fe treatment, fast Ca 2+ waves and sustained depolarization were induced to generate a secondary (shallow) resting membrane potential (RMP: from -68.8 ± 0.6 to -25.0 ± 3.7 mV) where the myocytes became unexcitable. Gadolinium, a TRPC channel blocker, abolished fast Ca 2+ waves and reversed RMP to the deep level (-62.9 ± 3.5 mV). The involvement of TRPC activation was determined for the first time by recording TRPC current and assessing the effect of functional TRPC channel antibodies.

Conclusions: In mouse cardiomyocytes, Fe overload induced arrhythmogenic APD prolongation and EADs/DADs, aberrant Ca 2+ i dynamics, and impaired contractility. The activation of TRPC channels accounts for an important underlying mechanism.

Toward deciphering the mechanistic role of variations in the Rep1 repeat site in the transcription regulation of SNCA gene

Short structural variants-variants other than single nucleotide polymorphisms-are hypothesized to contribute to many complex diseases, possibly by modulating gene expression. However, the molecular mechanisms by which noncoding short structural variants exert their effects on gene regulation have not been discovered. Here, we study simple sequence repeats (SSRs), a common class of short structural variants. Previously, we showed that repetitive sequences can directly influence the binding of transcription factors to their proximate recognition sites, a mechanism we termed non-consensus binding. In this study, we focus on the SSR termed Rep1, which was associated with Parkinson's disease (PD) and has been implicated in the cis-regulation of the PD-risk SNCA gene. We show that Rep1 acts via the non-consensus binding mechanism to affect the binding of transcription factors from the GATA and ELK families to their specific sites located right next to the Rep1 repeat. Next, we performed an expression analysis to further our understanding regarding the GATA and ELK family members that are potentially relevant for SNCA transcriptional regulation in health and disease. Our analysis indicates a potential role for GATA2, consistent with previous reports. Our study proposes non-consensus transcription factor binding as a potential mechanism through which noncoding repeat variants could exert their pathogenic effects by regulating gene expression.

Effect of densely ionizing radiation on cardiomyocyte differentiation from human-induced pluripotent stem cells

The process of human cardiac development can be faithfully recapitulated in a culture dish with human pluripotent stem cells, where the impact of environmental stressors can be evaluated. The consequences of ionizing radiation exposure on human cardiac differentiation are largely unknown. In this study, human-induced pluripotent stem cell cultures (hiPSCs) were subjected to an external beam of 3.7 MeV α-particles at low mean absorbed doses of 0.5, 3, and 10 cGy. Subsequently, the hiPSCs were differentiated into beating cardiac myocytes (hiPSC-CMs). Pluripotent and cardiac markers and morphology did not reveal differences between the irradiated and nonirradiated groups. While cell number was not affected during CM differentiation, cell number of differentiated CMs was severely reduced by ionizing radiation in a dose-responsive manner. β-adrenergic stimulation causes calcium (Ca²⁺) overload and oxidative stress. Although no significant increase in Ca²⁺ transient amplitude was observed in any group after treatment with 1 μmol/L isoproterenol, the incidence of spontaneous Ca²⁺ waves/releases was more frequent in hiPSC-CMs of the irradiated groups, indicating arrhythmogenic activities at the single cell level. Increased transcript expression of mitochondrial biomarkers (LONP1, TFAM) and mtDNA-encoded genes (MT-CYB, MT-RNR1) was detected upon differentiation of hiPSC-CMs suggesting increased organelle biogenesis. Exposure of hiPSC-CM cultures to 10 cGy significantly upregulated MT-CYB and MT-RNR1 expression, which may reflect an adaptive response to ionizing radiation. Our results indicate that important aspects of differentiation of hiPSCs into cardiac myocytes may be affected by low fluences of densely ionizing radiations such as α-particles.

Newly Identified NO-Sensor Guanylyl Cyclase/Connexin 43 Association Is Involved in Cardiac Electrical Function

BACKGROUND

Guanylyl cyclase, a heme-containing α1β1 heterodimer (GC1), produces cGMP in response to Nitric oxide (NO) stimulation. The NO-GC1-cGMP pathway negatively regulates cardiomyocyte contractility and protects against cardiac hypertrophy-related remodeling. We recently reported that the β1 subunit of GC1 is detected at the intercalated disc with connexin 43 (Cx43). Cx43 forms gap junctions (GJs) at the intercalated disc that are responsible for electrical propagation. We sought to determine whether there is a functional association between GC1 and Cx43 and its role in cardiac homeostasis.

METHODS AND RESULTS

GC1 and Cx43 immunostaining at the intercalated disc and coimmunoprecipitation from membrane fraction indicate that GC1 and Cx43 are associated. Mice lacking the α subunit of GC1 (GCα1 knockout mice) displayed a significant decrease in GJ function (dye-spread assay) and Cx43 membrane lateralization. In a cardiac-hypertrophic model, angiotensin II treatment disrupted the GC1-Cx43 association and induced significant Cx43 membrane lateralization, which was exacerbated in GCα1 knockout mice. Cx43 lateralization correlated with decreased Cx43-containing GJs at the intercalated disc, predictors of electrical dysfunction. Accordingly, an ECG revealed that angiotensin II-treated GCα1 knockout mice had impaired ventricular electrical propagation. The phosphorylation level of Cx43 at serine 365, a protein-kinase A upregulated site involved in trafficking/assembly of GJs, was decreased in these models.

CONCLUSIONS

GC1 modulates ventricular Cx43 location, hence GJ function, and partially protects from electrical dysfunction in an angiotensin II hypertrophy model. Disruption of the NO-cGMP pathway is associated with cardiac electrical disturbance and abnormal Cx43 phosphorylation. This previously unknown NO/Cx43 signaling could be a protective mechanism against stress-induced arrhythmia.

Involvement of mitochondrial permeability transition pore (mPTP) in cardiac arrhythmias: Evidence from cyclophilin D knockout mice

In the present study, we have used a genetic mouse model that lacks cyclophilin D (CypD KO) to assess the cardioprotective effect of mitochondrial permeability transition pore (mPTP) inhibition on Ca2+ waves and Ca2+ alternans at the single cell level, and cardiac arrhythmias in whole-heart preparations. The protonophore carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP) caused mitochondrial membrane potential depolarization to the same extent in cardiomyocytes from both WT and CypD KO mice, however, cardiomyocytes from CypD KO mice exhibited significantly less mPTP opening than cardiomyocytes from WT mice (p<0.05). Consistent with these results, FCCP caused significant increases in CaW rate in WT cardiomyocytes (p<0.05) but not in CypD KO cardiomyocytes. Furthermore, the incidence of Ca2+ alternans after treatment with FCCP and programmed stimulation was significantly higher in WT cardiomyocytes (11 of 13), than in WT cardiomyocytes treated with CsA (2 of 8; p<0.05) or CypD KO cardiomyocytes (2 of 10; p<0.01). (Pseudo-)Lead II ECGs were recorded from ex vivo hearts. We observed ST-T-wave alternans (a precursor of lethal arrhythmias) in 5 of 7 WT hearts. ST-T-wave alternans was not seen in CypD KO hearts (n=5) and in only 1 of 6 WT hearts treated with CsA. Consistent with these results, WT hearts exhibited a significantly higher average arrhythmia score than CypD KO (p<0.01) hearts subjected to FCCP treatment or chemical ischemia-reperfusion (p<0.01). In conclusion, CypD deficiency- induced mPTP inhibition attenuates CaWs and Ca2+ alternans during mitochondrial depolarization, and thereby protects against arrhythmogenesis in the heart.

The effect of PVDF-TrFE scaffolds on stem cell derived cardiovascular cells

Recently, electrospun polyvinylidene fluoride (PVDF) and polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) scaffolds have been developed for tissue engineering applications. These materials have piezoelectric activity, wherein they can generate electric charge with minute mechanical deformations. Since the myocardium is an electroactive tissue, the unique feature of a piezoelectric scaffold is attractive for cardiovascular tissue engineering applications. In this study, we examined the cytocompatibility and function of pluripotent stem cell derived cardiovascular cells including mouse embryonic stem cell-derived cardiomyocytes (mES-CM) and endothelial cells (mES-EC) on PVDF-TrFE scaffolds. MES-CM and mES-EC adhered well to PVDF-TrFE and became highly aligned along the fibers. When cultured on scaffolds, mES-CM spontaneously contracted, exhibited well-registered sarcomeres and expressed classic cardiac specific markers such as myosin heavy chain, cardiac troponin T, and connexin43. Moreover, mES-CM cultured on PVDF-TrFE scaffolds responded to exogenous electrical pacing and exhibited intracellular calcium handling behavior similar to that of mES-CM cultured in 2D. Similar to cardiomyocytes, mES-EC also demonstrated high viability and maintained a mature phenotype through uptake of low-density lipoprotein and expression of classic endothelial cell markers including platelet endothelial cell adhesion molecule, endothelial nitric oxide synthase, and the arterial specific marker, Notch-1. This study demonstrates the feasibility of PVDF-TrFE scaffold as a candidate material for developing engineered cardiovascular tissues utilizing stem cell-derived cells. Biotechnol. Bioeng. 2016;113: 1577-1585. © 2015 Wiley Periodicals, Inc.

Autonomic and endocrine control of cardiovascular function

The function of the heart is to contract and pump oxygenated blood to the body and deoxygenated blood to the lungs. To achieve this goal, a normal human heart must beat regularly and continuously for one’s entire life. Heartbeats originate from the rhythmic pacing discharge from the sinoatrial (SA) node within the heart itself. In the absence of extrinsic neural or hormonal influences, the SA node pacing rate would be about 100 beats per minute. Heart rate and cardiac output, however, must vary in response to the needs of the body’s cells for oxygen and nutrients under varying conditions. In order to respond rapidly to the changing requirements of the body’s tissues, the heart rate and contractility are regulated by the nervous system, hormones, and other factors. Here we review how the cardiovascular system is controlled and influenced by not only a unique intrinsic system, but is also heavily influenced by the autonomic nervous system as well as the endocrine system.

Overexpression of adenylyl cyclase type 5 (AC5) confers a proarrhythmic substrate to the heart

Inhibition of β-adrenergic receptor (β-AR) signaling is one of the most common therapeutic approaches for patients with arrhythmias. Adenylyl cyclase (AC) is the key enzyme responsible for transducing β-AR stimulation to increases in cAMP. The two major AC isoforms in the heart are types 5 and 6. Therefore, it is surprising that prior studies on overexpression of AC5 and AC6 in transgenic (Tg) mice have not examined mediation of arrhythmogenesis. Our goal was to examine the proarrhythmic substrate in AC5Tg hearts. Intracellular calcium ion (Ca(2+) i) was imaged in fluo-4 AM-loaded ventricular myocytes. The sarcoplasmic reticulum (SR) Ca(2+) content, fractional Ca(2+) release, and twitch Ca(2+) transient were significantly higher in the AC5Tg vs. wild-type (WT) myocytes, indicating Ca(2+) overload in AC5Tg myocytes. Action potential (AP) duration was significantly longer in AC5Tg than in WT myocytes. Additionally, AC5Tg myocytes developed spontaneous Ca(2+) waves in a larger fraction compared with WT myocytes, particularly when cells were exposed to isoproterenol. The Ca(2+) waves further induced afterdepolarizations and triggered APs. AC5Tg hearts had increased level of SERCA2a, oxidized Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), and phosphorylation of ryanodine receptors (RyR) at the CaMKII site, especially after isoproterenol treatment. This was consistent with higher reactive oxygen species production in AC5Tg myocytes after isoproterenol treatment. Isoproterenol induced more severe arrhythmias in AC5Tg than in WT mice. We conclude that AC5 overexpression promotes arrhythmogenesis, by inducing SR Ca(2+) overload and hyperactivation of RyR (phosphorylation by CaMKII), which in turn induces spontaneous Ca(2+) waves and afterdepolarizations.

Modulation of intracellular calcium waves and triggered activities by mitochondrial ca flux in mouse cardiomyocytes

Recent studies have suggested that mitochondria may play important roles in the Ca(2+) homeostasis of cardiac myocytes. However, it is still unclear if mitochondrial Ca(2+) flux can regulate the generation of Ca(2+) waves (CaWs) and triggered activities in cardiac myocytes. In the present study, intracellular/cytosolic Ca(2+) (Cai (2+)) was imaged in Fluo-4-AM loaded mouse ventricular myocytes. Spontaneous sarcoplasmic reticulum (SR) Ca(2+) release and CaWs were induced in the presence of high (4 mM) external Ca(2+) (Cao (2+)). The protonophore carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP) reversibly raised basal Cai (2+) levels even after depletion of SR Ca(2+) in the absence of Cao (2+) , suggesting Ca(2+) release from mitochondria. FCCP at 0.01 - 0.1 µM partially depolarized the mitochondrial membrane potential (Δψ m ) and increased the frequency and amplitude of CaWs in a dose-dependent manner. Simultaneous recording of cell membrane potentials showed the augmentation of delayed afterdepolarization amplitudes and frequencies, and induction of triggered action potentials. The effect of FCCP on CaWs was mimicked by antimycin A (an electron transport chain inhibitor disrupting Δψ m ) or Ru360 (a mitochondrial Ca(2+) uniporter inhibitor), but not by oligomycin (an ATP synthase inhibitor) or iodoacetic acid (a glycolytic inhibitor), excluding the contribution of intracellular ATP levels. The effects of FCCP on CaWs were counteracted by the mitochondrial permeability transition pore blocker cyclosporine A, or the mitochondrial Ca(2+) uniporter activator kaempferol. Our results suggest that mitochondrial Ca(2+) release and uptake exquisitely control the local Ca(2+) level in the micro-domain near SR ryanodine receptors and play an important role in regulation of intracellular CaWs and arrhythmogenesis.

Abstract 161: TRPC6 Activator Hyperforin Facilitates Arrhythmias Via Activating Store-operated Calcium Entry

Hyperforin, a major antidepressant constituent of St. John’s wort ( Hypericum perforatum ), exhibits high selectivity in activating the canonical transient potential channel 6 (TRPC6). Our previous studies have shown that TRPC6 may modulate Ca 2+ handling via store-operated Ca 2+ entry (SOCE) in adult ventricular myocytes. In the present study, we aim to test whether hyperforin is arrhythmogenic by facilitating Ca 2+ waves. Intracellular Ca 2+ fluorescence (F/F 0 ) was imaged in Fluo-4-AM loaded ventricular myocytes isolated from adult mice (2-4 month old). SOCE was evaluated by increasing the external Ca 2+ concentration ([Ca 2+ ] o ) from 0 to 1 mM after caffeine (10 mM) and thapsigargin (10 µM) were used to completely deplete SR Ca 2+ . Hyperforin (0.1-10 µM) perfusion increased SOCE up to 3.7 folds (F/F 0 = 3.73 ± 0.42, n = 7, compared to the control F/F 0 = 1.69 ± 0.37, n = 9, p < 0.05), in a concentration dependent fashion. Whole-cell currents were recorded using ramp pulses from -110 mV to +50 mV, while K + , Na + , L-type Ca 2+ , and Na + -Ca 2+ exchange currents were pre-blocked. Hyperforin at a lower concentration (0.1 µM) induced a significant increase of inward current (from -1.44 ± 0.23 pA/pF to -2.56 ± 0.36 pA/pF, n = 8, p < 0.05), which was then inhibited by the SOCE blocker Gd 3+ (1 mM) (-0.43 ± 0.22 pA/pF, n = 8, p < 0.05). Hyperforin promotes spontaneous CaWs ([Ca 2+ ] o 4 mM) by increasing the frequency to 180± 14 % (n = 11. p < 0.05), which can be attenuated by Gd 3+ (1 mM), or SKF-96265 (10 μM). In addition, both the amplitudes of Ca 2+ transients (F/F 0 =1.92 ± 0.09 to 2.09 ± 0.10, n = 12, p < 0.05) and calcium content (control: F/F 0 = 2.03 ± 0.11, n=16 to 2.12 ± 0.13, n = 11, p < 0.05) were enhanced by acute hyperforin perfusion (0.1 µM), when the cells were paced at a pacing cycle length of 2 seconds. The proarrhythmic effect of hyperforin was also confirmed in Langendorff-perfused hearts, from which optical membrane voltage mapping and EKG were simultaneously recorded. In conclusion, the TRPC6 activator hyperforin exhibits an arrhythmogenic effect in the heart. Its action is likely to be mediated by generating an inward current and increasing the calcium load in cardiac myocytes through the SOCE pathway. Caution should be taken when prescribing this drug to patients with heart disease.

Dystrophin-compromised sarcoglycan-δ-knockout diaphragm requires full wild-type embryonic stem cell reconstitution for correction

Limb-girdle muscular dystrophy-2F (LGMD-2F) is an incurable degenerative muscle disorder caused by a mutation in the sarcoglycan-δ (SGδ)-encoding gene (SGCD in humans). The lack of SGδ results in the complete disruption of the sarcoglycan complex (SGC) in the skeletal and cardiac muscle within the larger dystrophin-glycoprotein complex (DGC). The long-term consequences of SG ablation on other members of the DGC are currently unknown. We produced mosaic mice through the injection of wild-type (WT) embryonic stem cells (ESCs) into SGδ-knockout (KO) blastocysts. ESC-derived SGδ was supplied to the sarcolemma of 18-month-old chimeric muscle, which resulted in the restoration of the SGC. Despite SGC rescue, and contrary to previous observations obtained with WT/mdx chimeras (a mouse rescue paradigm for Duchenne muscular dystrophy), low levels of ESC incorporation were insufficient to produce histological corrections in SGδ-KO skeletal muscle or heart. The inefficient process of ESC rescue was more evident in the SGδ-KO diaphragm, which had reduced levels of dystrophin and no compensatory utrophin, and needed almost full WT ESC reconstitution for histological improvement. The results suggest that the SGδ-KO mouse model of LGMD is not amenable to ESC treatment.