Systolic Ca2 Research Articles

Interplay between mineral bone disorder and cardiac damage in acute kidney injury: from Ca2+ mishandling and preventive role of Klotho in mice to its potential mortality prediction in human

Biomarkers of mineral bone disorders (MBD) including phosphorus, fibroblast growth factor (FGF)-23 and Klotho are strongly altered in patients with acute kidney injury (AKI) who have high cardiac outcomes and mortality rates. However, the crosslink between MBD and cardiac damage after an AKI episode still remains unclear. We tested MBD and cardiac biomarkers in an experimental AKI model after 24 or 72 hours of folic acid injection and we analyzed structural cardiac remodeling, intracellular calcium (Ca2+) dynamics in cardiomyocytes and cardiac rhythm. AKI mice presented high levels of FGF-23, phosphorus and cardiac troponin T and exhibited a cardiac hypertrophy phenotype accompanied by an increase in systolic Ca2+ release 24 hours after AKI. Ca2+ transients and contractile dysfunction were reduced 72 hours after AKI while diastolic sarcoplasmic reticulum Ca2+ leak, pro-arrhythmogenic Ca2+ events and ventricular arrhythmias were increased. These cardiac events were linked to the activation of the calcium/calmodulin-dependent kinase II pathway through the increased phosphorylation of ryanodine receptors and phospholamban specific sites after AKI. Cardiac hypertrophy and the altered intracellular Ca2+ dynamics were prevented in transgenic mice overexpressing Klotho after AKI induction. In a translational retrospective longitudinal clinical study, we determined that combining FGF-23 and phosphorus with cardiac troponin T levels achieved a better prediction of mortality in AKI patients at hospital admission. Thus, monitoring MBD and cardiac damage biomarkers could be crucial to prevent mortality in AKI patients. In this setting, Klotho might be considered as a new cardioprotective therapeutic tool to prevent deleterious cardiac events in AKI conditions.

Antiarrhythmic Effects of Phenytoin, a Novel Heart Failure Diastolic Ryanodine Channel Inhibitor, in Human Failing Ventricle

Heart failure (HF) is a devastating disease and despite advances in medical therapy, mortality remains high due to ventricular arrhythmias. Modifications to the cardiac ryanodine receptor (RyR2) due to stress induced mechanisms ultimately lead to pathological Ca2+ leak during diastole, resulting in delayed after depolarisations and ventricular arrhythmias. Phenytoin, a hydantoin derivative used as an anti-convulsant, is known to exhibit weak Na+ channel blockade in cardiomyocytes. Recent studies revealed that phenytoin inhibits diastolic but not systolic Ca2+ leak from RyR2 channels of human failing hearts and has no effect on Ca2+ release from RyR2 channels from non-failing hearts [1].

Abstract 12001: Atrial Fibrillation Induces Ventricular Dysfunction via Altered Ca 2+ Handling and Oxidative CaMKII Activation in the Human Heart

Introduction: The effects of atrial fibrillation (AF) on left ventricular (LV) function are poorly understood. Clinical data indicate that the arrhythmic component of AF may contribute to LV dysfunction. This study investigated the effects of AF on the human LV. Methods and Results: LV myocardium from patients with preserved LV function (EF>50%) with sinus rhythm (SR, n=31) or rate-controlled AF (n=24) obtained during surgery for aortic stenosis was studied. Clinical data were mostly balanced without changes in LV function and remodeling. LV myocardium from SR and AF patients showed no changes in fibrosis. In functional studies, systolic Ca 2+ transient amplitude of human LV cardiomyocytes (CM) was reduced in AF patients, while diastolic Ca 2+ levels and Ca 2+ transient kinetics were unaltered. These results were confirmed in LV CM from non-failing donors with AF (n=4) vs. SR (n=8). In addition, normofrequent AF was simulated in vitro using arrhythmic or rhythmic electrical culture pacing (both at 60 bpm). After 24h of AF-simulation, human LV CM from non-failing donors (n=9) showed also an impaired Ca 2+ transient amplitude. For a standardized chronic AF-simulation, human iPSC-CM (n=22 differentiations/5 individuals) were tested. 7 days of AF-simulation caused reduced systolic Ca 2+ transient amplitude and sarcoplasmic reticulum Ca 2+ load, which could be explained by an increased diastolic Ca 2+ leak. Mechanistically, oxidative stress markers were higher in the LV of AF patients. Ca 2+ /calmodulin-dependent protein kinase IIδ (CaMKII) was found to be more oxidized at Met281/282 in the LV of AF patients leading to an increased CaMKII activity. Thus, RyR2 phosphorylation was elevated, which likely underlies the depression of LV Ca 2+ handling in AF. In translational contractility experiments of human ventricular trabecula from patients with heart failure (n=8), in vitro AF-simulation for 8h worsened systolic and diastolic contractile function. Conclusions: Here we could firstly show that AF impairs human LV function via increased oxidative stress and CaMKII activity in different patient populations and models. Thus, this translational study demonstrates the detrimental effects and mechanisms of AF in the absence of tachycardia on the human LV.

Abstract 13709: TRPV4 Enhances Ca 2+ Transients and Promotes Arrhythmic Ca 2+ Signaling Following Hypoosmotic Stress and Ischemia-Reperfusion

Introduction: Transient Receptor Potential Vanilloid 4 (TRPV4) is an osmotically activated Ca 2+ channel which enhances cardiomyocyte Ca 2+ cycling and may contribute to arrhythmic Ca 2+ signals in hypoosmotic stress and Ischemia-Reperfusion (I/R). Hypothesis: TRPV4 overexpression enhances Ca 2+ transient (CaT) amplitude and arrhythmic Ca 2+ events in left ventricular (LV) cardiomyocytes exposed to hypoosmotic stress and Langendorff hearts following I/R. Methods: Mouse LV cardiomyocytes from TRPV4 Overexpressors (OEX) and non-transgenic controls (CTL) were exposed to hypoosmotic stress (250 mOsm, 30min). CaT amplitude (paced 0.5 Hz) and arrhythmic Ca 2+ events (prolonged kinetics, spontaneous CaT, Ca 2+ waves) were monitored via confocal imaging (fluo-4, 100Hz). I/R (45min global ischemia, 30min reperfusion) experiments were conducted in OEX and CTL Langendorff hearts expressing the GCaMP6f Ca 2+ sensor (OEX-GCaMP, CTL-GCaMP). CaT and arrhythmic Ca 2+ signals (Ca 2+ waves, alternans, Ca 2+ overload) were monitored in the LV myocardium. Experiments were conducted in the absence and presence of TRPV4 inhibition by HC067047 (HC, 1uM). Results: Incidence of arrhythmic Ca 2+ signals during minutes 0-15 of hypoosmotic stress was increased in OEX cardiomyocytes vs CTL (78 vs 20%, P<0.05, N=9, 5) and prevented in OEX by HC (11%, P<0.05, N=9). Arrhythmia score was increased in OEX vs CTL (1.3±0.7 vs. 0.1±0.1, P<0.05, N=9, 5) and decreased in OEX by HC (0.2±0.2, P<0.05, N=9). Following hypoosmotic stress, CaT amplitude was elevated in OEX vs CTL (F/F 0 = 3.6±0.4 vs 2.1±0.4, P<0.05, N=7, 4) and this effect in OEX was prevented by HC (F/F 0 =2.4±0.4, P<0.05, N=8). Langendorff OEX-GCaMP hearts subjected to I/R demonstrated increased systolic Ca 2+ (F/F 0 =1.8±0.09 vs 1.3±0.1, P<0.05, N=5, 6) during minutes 10-30 of reperfusion, which was attenuated by HC (F/F 0 =1.4±0.08, P<0.05, N=4). OEX-GCaMP hearts were more likely to demonstrate arrhythmic Ca 2+ signals during minutes 15-30 of reperfusion than CTL-GCaMP (27/487 vs 0/415 cells, p<0.05 N=5, 6) and this effect in OEX-GCaMP was prevented by HC (0/633 cells, p<0.05, N=4). Conclusions: TRPV4 activation enhances Ca 2+ transients and promotes arrhythmic Ca 2+ signals, and may provide a novel anti-arrhythmic target in I/R.

A Targeted Treatment Opportunity for HFpEF: Taking Advantage of Diastolic Tone.

A Targeted Treatment Opportunity for HFpEF: Taking Advantage of Diastolic Tone.

RBM20-mutations induce disturbed splicing of calcium relevant genes and guides clinically therapy in different cardiomyopathies

Abstract Background Mutations in the splice factor RBM20 account for ∼3% of genetic cardiomyopathies. Mutations at position R634 in the hotspot RS-domain were found to cause dilative cardiomyopathy (DCM) (R634W) or left ventricular non-compaction cardiomyopathy (LVNC) (R634L), but the pathophysiological mechanisms that govern the heterogeneity in phenotype presentation remain unknown. Purpose We aimed here to identify the molecular events caused by the distinct RBM20 mutations from DCM and LVNC patients using a patient-specific induced stem cell model (iPSC) and test if the currently clinically used β-blockers (Metroprolol) are suitable for different RBM20-dependent cardiomyopathies. Methods We generated iPSC-cardiomyocytes of 2 DCM- and 2 LVNC-patients harboring the RBM20-mutations R634W (DCM) or R634L (LVNC). We investigated alternative splicing, sarcomeric regularity, cAMP-level, kinase-specific phosphorylation of Ca2+ players and Ca2+ handling. To investigate the impact of the genetic background, isogenic rescue lines were generated by CRISPR/Cas9. Different clinical drugs as Metoprolol and Verapamil were used to analyze the pharmacological improvement in vitro. Results We investigated the splicing pattern of the 2 RBM20 mutations in DCM and LVNC iPSC-CMs and observed common isoform changes in titin and a 24bp-insertion in the gene RYR2. The Ca2+ handling gene triadin is misspliced in LVNC-CMs, whereas the structural gene LDB3 is misspliced in DCM-CMs. As a possible consequence of splice defects in sarcomeric genes, both DCM and LVNC-CMs exhibited an irregular sarcomeric structure. The Ca2+ handling gene CAMK2δ was predominantly misspliced in LVNC-CMs leading to CAMK2δ-dependent hyperphosphorylation of its target PLN-Thr17 and subsequently to shortened Ca2+ elimination time and weakened response to β-adrenergic stimulation. By contrast, DCM-CMs exhibited increased Ca2+ sparks and decreased systolic and diastolic Ca2+ levels. RBM20 expression itself was decreased in LVNC-CMs, but not in DCM-CMs. This highlights that 2 distinct RBM20 mutations can lead to different pathological Ca2+ phenotypes. Isogenic CRISPR/Cas9 repair of both RBM20 mutations in LVNC and DCM demonstrated a rescue in gene missplicing, sarcomeric regularity and the Ca2+ handling aberrations and underscored the causative nature of the 2 mutations and their diverging effects. Ca2+ channel blockage with Verapamil showed a significant improvement of some of the LVNC disease characteristics compared to commonly clinically used β-blocker Metoprolol and underpins the potential clinical use of this drug in patients with LVNC. Conclusion We show the first iPSC-model of splice-defect associated RBM20-dependent LVNC and DCM. In summary, our results suggest that the molecular aberrations in alternative splicing differ depending on the distinct mutation in RBM20 and lead to shared and differential pathologies. Verapamil could be a good candidate in the treatment of RBM20-dependent LVNC. Funding Acknowledgement Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Bunderministerium für Bildung und Forschung BMBFGerman Center for Cardiovascular Research DZHK

Specialized pro-resolving mediators prevents cardiac dysfunction by modulating Ca2+ handling and NRF2 axis

Abstract Background and purpose Myocarditis is a severe inflammatory heart disease and a leading cause of sudden death in young adults; but currently no specific treatment is available. Lipoxins and their derivatives promote the resolution of inflammation contributing to recover tissue homeostasis; but their role in cardiac inflammation is poorly understood. Methods and results BML-111, a stable lipoxin A4 receptor agonist, protects against cardiac dysfunction in a murine model of experimental autoimmune myocarditis (EAM) by preventing Ca2+ mishandling. Cardiac proteomic analysis revealed an enhanced cardiac oxidative profile in EAM-induced mice with reduced activation of NRF2, a master antioxidant transcription factor. In vitro analysis showed that 15-epi-lipoxin A4 increased systolic Ca2+ release and sarcoplasmic reticulum (SR)-Ca2+ load in cardiomyocytes isolated from wild-type mice and augmented the rate of SR-Ca2+ uptake by SERCA2a, but failed to induce any functional change in cells from Nrf2−/− mice. BML-111 increased SERCA2a cardiac expression in wild-type mice, and the transcriptional activity of Nrf2 determined SERCA2a expression in human ventricular cells. Human myocarditis-positive myocardium showed a reduced expression of both ATP2A2 (SERCA2a) and NF2L2 (NRF2). Conclusions Our results demonstrated new cardioprotective mechanisms of pro-resolving lipid mediators that may emerge as innovative treatments for myocarditis. Funding Acknowledgement Type of funding sources: Public grant(s) – National budget only. Main funding source(s): ISCIII [PI17/01344]), Sociedad Española de Cardiología: Proyecto Traslacional 2019Fondo Europeo de Desarrollo Regional (FEDER), FSE, and CIBER-CV, a network funded by ISCIII

MCU overexpression evokes disparate dose-dependent effects on mito-ROS and spontaneous Ca2+ release in hypertrophic rat cardiomyocytes.

Cardiac dysfunction in heart failure (HF) and diabetic cardiomyopathy (DCM) is associated with aberrant intracellular Ca2+ handling and impaired mitochondrial function accompanied with reduced mitochondrial calcium concentration (mito-[Ca2+]). Pharmacological or genetic facilitation of mito-Ca2+ uptake was shown to restore Ca2+ transient amplitude in DCM and HF, improving contractility. However, recent reports suggest that pharmacological enhancement of mito-Ca2+ uptake can exacerbate ryanodine receptor-mediated spontaneous sarcoplasmic reticulum (SR) Ca2+ release in ventricular myocytes (VMs) from diseased animals, increasing propensity to stress-induced ventricular tachyarrhythmia. To test whether chronic recovery of mito-[Ca2+] restores systolic Ca2+ release without adverse effects in diastole, we overexpressed mitochondrial Ca2+ uniporter (MCU) in VMs from male rat hearts with hypertrophy induced by thoracic aortic banding (TAB). Measurement of mito-[Ca2+] using genetic probe mtRCamp1h revealed that mito-[Ca2+] in TAB VMs paced at 2 Hz under β-adrenergic stimulation is lower compared with shams. Adenoviral 2.5-fold MCU overexpression in TAB VMs fully restored mito-[Ca2+]. However, it failed to improve cytosolic Ca2+ handling and reduce proarrhythmic spontaneous Ca2+ waves. Furthermore, mitochondrial-targeted genetic probes MLS-HyPer7 and OMM-HyPer revealed a significant increase in emission of reactive oxygen species (ROS) in TAB VMs with 2.5-fold MCU overexpression. Conversely, 1.5-fold MCU overexpression in TABs, that led to partial restoration of mito-[Ca2+], reduced mitochondria-derived reactive oxygen species (mito-ROS) and spontaneous Ca2+ waves. Our findings emphasize the key role of elevated mito-ROS in disease-related proarrhythmic Ca2+ mishandling. These data establish nonlinear mito-[Ca2+]/mito-ROS relationship, whereby partial restoration of mito-[Ca2+] in diseased VMs is protective, whereas further enhancement of MCU-mediated Ca2+ uptake exacerbates damaging mito-ROS emission.NEW & NOTEWORTHY Defective intracellular Ca2+ homeostasis and aberrant mitochondrial function are common features in cardiac disease. Here, we directly compared potential benefits of mito-ROS scavenging and restoration of mito-Ca2+ uptake by overexpressing MCU in ventricular myocytes from hypertrophic rat hearts. Experiments using novel mito-ROS and Ca2+ biosensors demonstrated that mito-ROS scavenging rescued both cytosolic and mito-Ca2+ homeostasis, whereas moderate and high MCU overexpression demonstrated disparate effects on mito-ROS emission, with only a moderate increase in MCU being beneficial.

Myosin Modulation in Hypertrophic Cardiomyopathy and Systolic Heart Failure: Getting Inside the Engine.

Myosin Modulation in Hypertrophic Cardiomyopathy and Systolic Heart Failure: Getting Inside the Engine.

Phosphorylation of RyR2 Ser-2814 by CaMKII mediates β1-adrenergic stress induced Ca2+ -leak from the sarcoplasmic reticulum.

Adrenergic stimulation, while being the central mechanism of cardiac positive inotropy, is a universally acknowledged inductor of undesirable sarcoplasmic reticulum (SR) Ca2+ leak. However, the exact mechanisms for this remained unspecified so far. This study shows that Ca2+/calmodulin‐dependent protein kinase II (CaMKII)‐specific phosphorylation of ryanodine receptor type 2 at Ser‐2814 is the pivotal mechanism by which SR Ca2+ leak develops downstream of β1‐adrenergic stress by increase of the leak/load relationship. Cardiomyocytes with a Ser‐2814 phosphoresistant mutation (S2814A) were protected from isoproterenol‐induced SR Ca2+ leak and consequently displayed improved postrest potentiation of systolic Ca2+ release under adrenergic stress compared to littermate wild‐type cells.

Efficacy of RyR2 inhibitor EL20 in induced pluripotent stem cell-derived cardiomyocytes from a patient with catecholaminergic polymorphic ventricular tachycardia.

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited cardiac arrhythmia syndrome that often leads to sudden cardiac death. The most common form of CPVT is caused by autosomal‐dominant variants in the cardiac ryanodine receptor type‐2 (RYR2) gene. Mutations in RYR2 promote calcium (Ca2+) leak from the sarcoplasmic reticulum (SR), triggering lethal arrhythmias. Recently, it was demonstrated that tetracaine derivative EL20 specifically inhibits mutant RyR2, normalizes Ca2+ handling and suppresses arrhythmias in a CPVT mouse model. The objective of this study was to determine whether EL20 normalizes SR Ca2+ handling and arrhythmic events in induced pluripotent stem cell‐derived cardiomyocytes (iPSC‐CMs) from a CPVT patient. Blood samples from a child carrying RyR2 variant RyR2 variant Arg‐176‐Glu (R176Q) and a mutation‐negative relative were reprogrammed into iPSCs using a Sendai virus system. iPSC‐CMs were derived using the StemdiffTM kit. Confocal Ca2+ imaging was used to quantify RyR2 activity in the absence and presence of EL20. iPSC‐CMs harbouring the R176Q variant demonstrated spontaneous SR Ca2+ release events, whereas administration of EL20 diminished these abnormal events at low nanomolar concentrations (IC50 = 82 nM). Importantly, treatment with EL20 did not have any adverse effects on systolic Ca2+ handling in control iPSC‐CMs. Our results show for the first time that tetracaine derivative EL20 normalized SR Ca2+ handling and suppresses arrhythmogenic activity in iPSC‐CMs derived from a CPVT patient. Hence, this study confirms that this RyR2‐inhibitor represents a promising therapeutic candidate for treatment of CPVT.

Reduction of P62/SQSTM1 on Calcium Handling in Stressed Myocytes

Introduction Cytosolic protein, p62/SQSTM1, regulates the protein turnover rate, and it interacts with various stress signaling proteins in cells. Accumulations of p62/SQSTM1 have been reported in hearts with cardiomyopathy. We recently reported that p62/SQSTM1 protein deletions reduced beta-adrenergic response in myocytes and improved chronic ischemia-induced Calcium (Ca) defect. Here, we studied acute and chronic stress-induced Ca handling in myocytes genetically suppressed p62/SQSTM1 protein. Methods Myocytes (n=3 per group) isolated from Wild-type (WT), and p62/SQSTM1+/- (p62 HET) at Base, 5 days, and 10 days after Isoproterenol (ISO; beta-agonist) injections (150 mg/kg). We measured myocytes contractility (Fraction shortening of sarcomeres), systolic Ca release (CaT), SR Ca contents (CaffT), and Fractional Ca release, (CaT/CaffT) in field-stimulated single myocytes. Myocytes loaded with fluorescent Ca indicators (Fura-2AM; cytosolic Ca loading). Heart tissues in each group were processed for western blot (n=3 per group). Results The baseline myocytes contractility and Ca regulation were not different among the groups. P62 HET myocytes significantly increased contractility and fractional Ca release (the function of SR, F/F0; WT base, 0.58±0.07, WT ISO, 0.84±0.01, p62 HET ISO, 0.79±0.04) after acute ISO stimulation (1 uM, p<0.05), similar to WT myocytes. Interestingly, p62 HET myocytes preserved SR Ca contents, whereas WT myocytes increased SR Ca contents. Chronic ischemia-induced p62 HET myocytes improved fractional Ca release; however, both systolic Ca release (F/F0; WT base, 0.24±0.03, WT ISO 10days, 0.29±0.06, p62 HET ISO 10 days, 0.17±0.02) and SR Ca contents (F/F0; WT base, 0.43±0.05, WT ISO, 0.49±0.10, p62 HET ISO, 0.33±0.04) were proportionally reduced (p<0.05). Conclusion Taken together, suppression of p62/SQSTM1 protein restores stress-induced Ca dysregulation in myocytes. p62/SQSTM1 protein may directly contribute to systolic Ca release via beta-adrenergic signaling.

The structure of the native cardiac thin filament at systolic Ca2+ levels

Every heartbeat relies on cyclical interactions between myosin thick and actin thin filaments orchestrated by rising and falling Ca2+ levels. Thin filaments are comprised of two actin strands, each harboring equally separated troponin complexes, which bind Ca2+ to move tropomyosin cables away from the myosin binding sites and, thus, activate systolic contraction. Recently, structures of thin filaments obtained at low (pCa ∼9) or high (pCa ∼3) Ca2+ levels revealed the transition between the Ca2+-free and Ca2+-bound states. However, in working cardiac muscle, Ca2+ levels fluctuate at intermediate values between pCa ∼6 and pCa ∼7. The structure of the thin filament at physiological Ca2+ levels is unknown. We used cryoelectron microscopy and statistical analysis to reveal the structure of the cardiac thin filament at systolic pCa = 5.8. We show that the two strands of the thin filament consist of a mixture of regulatory units, which are composed of Ca2+-free, Ca2+-bound, or mixed (e.g., Ca2+ free on one side and Ca2+ bound on the other side) troponin complexes. We traced troponin complex conformations along and across individual thin filaments to directly determine the structural composition of the cardiac native thin filament at systolic Ca2+ levels. We demonstrate that the two thin filament strands are activated stochastically with short-range cooperativity evident only on one of the two strands. Our findings suggest a mechanism by which cardiac muscle is regulated by narrow range Ca2+ fluctuations.

Oxidative stress in early metabolic syndrome impairs cardiac RyR2 and SERCA2a activity and modifies the interplay of these proteins during Ca2+ waves

We investigated how oxidative stress (OS) alters Ca2+ handling in ventricular myocytes in early metabolic syndrome (MetS) in sucrose-fed rats. The effects of N-acetyl cysteine (NAC) or dl-Dithiothreitol (DTT) on systolic Ca2+ transients (SCaTs), diastolic Ca2+ sparks (CaS) and Ca2+ waves (CaW), recorded by confocal techniques, and L-type Ca2+ current (ICa), assessed by whole-cell patch clamp, were evaluated in MetS and Control cells. MetS myocytes exhibited decreased SCaTs and CaS frequency but unaffected CaW propagation. In Control cells, NAC/DTT reduced RyR2/SERCA2a activity blunting SCaTs, CaS frequency and CaW propagation, suggesting that basal ROS optimised Ca2+ signalling by maintaining RyR2/SERCA2a function and that these proteins facilitate CaW propagation. Conversely, NAC/DTT in MetS recovered RyR2/SERCA2a function, improving SCaTs and CaS frequency, but unexpectedly decreasing CaW propagation. We hypothesised that OS decreases RyR2/SERCA2a activity at early MetS, and while decreased SERCA2a favours CaW propagation, diminished RyR2 restrains it.

Calcium Dobesilate (CaD) Attenuates High Glucose and High Lipid-Induced Impairment of Sarcoplasmic Reticulum Calcium Handling in Cardiomyocytes.

Calcium dobesilate (CaD) is used effectively in patients with diabetic microvascular disorder, retinopathy, and nephropathy. Here we sought to determine whether it has an effect on cardiomyocytes calcium mishandling that is characteristic of diabetic cardiomyopathy. Cardiomyocytes were sterile isolated and cultured from 1 to 3 days neonatal rats and treated with vehicle (Control), 25 mM glucose+300 μM Palmitic acid (HG+PA), 100 μM CaD (CaD), or HG+PA+CaD to test the effects on calcium signaling (Ca2+ sparks, transients, and SR loads) and reactive oxygen species (ROS) production by confocal imaging. Compared to Control, HG+PA treatment significantly reduced field stimulation-induced calcium transient amplitudes (2.22 ± 0.19 vs. 3.56 ± 0.21, p < 0.01) and the levels of caffeine-induced calcium transients (3.19 ± 0.14 vs. 3.72 ± 0.15, p < 0.01), however significantly increased spontaneous Ca2+ sparks firing levels in single cardiomyocytes (spontaneous frequency 2.65 ± 0.23 vs. 1.72 ± 0.12, p < 0.01) and ROS production (67.12 ± 4.4 vs. 47.65 ± 2.12, p < 0.05), which suggest that HG+PA treatment increases the Spontaneity Ca2+ spark frequency, and then induced partial reduction of SR Ca2+ content and subsequently weaken systolic Ca2+ transient in cardiomyocyte. Remarkably, these impairments in calcium signaling and ROS production were largely prevented by pre-treatment of the cells with CaD. Therefore, CaD may contribute to a good protective effect on patients with calcium mishandling and contractile dysfunction in cardiomyocytes associated with diabetic cardiomyopathy.

Mitochondrial Calcium Uniporter Overexpression Exacerbates Arrhythmogenic Calcium Mishandling in Diseased Rat Ventricular Myocytes

Systolic dysfunction in heart failure (HF) and diabetic cardiomyopathy (DCM) is associated with diminished Ca2+ transient amplitude and impaired mitochondrial function due to reduced mitochondrial [Ca2+]. Pharmacological or genetic facilitation of mito-Ca2+ uptake was shown to restore Ca2+ transient amplitude in DCM and HF, improving contractility. However, recent reports suggest that acute pharmacological enhancement of mito-Ca2+ uptake can exacerbate spontaneous Ca2+ release in myocytes from diseased animals, increasing propensity to stress-induced arrhythmias. To test whether chronic recovery of mito-[Ca2+] restores systolic Ca2+ release without adverse effects in diastole, we overexpressed mitochondrial Ca2+ uniporter (MCU) in ventricular myocytes (VMs) from rat hearts with hypertrophy induced by thoracic aortic banding (TAB). Measurement of mito-[Ca2+] using genetic probe mtRCamp1h revealed that mito-[Ca2+] in TAB VMs paced at 1 Hz under β-adrenergic stimulation is significantly lower than in Shams. Adenoviral MCU overexpression in TAB VMs restored mito-[Ca2+]. Confocal imaging of VMs loaded with fluorescent indicator Rhod-2 revealed that MCU overexpression in TAB VMs reversed the decrease in Ca2+ transient amplitude. Importantly, the decay of caffeine-induced Ca2+ transient in MCU-overexpressing TAB VMs was prolonged, indicative of decreased NCX activity. Moreover, MCU overexpression in TAB VMs was accompanied by a further increase in incidence of proarrhythmic spontaneous Ca2+ waves, suggesting exacerbation of RyR2 dysfunction. Experiments using genetic probe OMM-HyPer showed a 10-fold increase in mito-ROS emission in TAB VMs overexpressing MCU. Importantly, application of mito-ROS scavenger mitoTEMPO (20 µM, 10 min) restored Ca2+ transient amplitude, reduced spontaneous Ca2+ waves and restored mito-[Ca2+] in TAB VMs, rendering them normal. These findings suggest that restoration of mito-[Ca2+] in VMs from diseased hearts by MCU overexpression improves systolic Ca2+ release possibly via reduced NCX-mediated Ca2+ efflux, but exacerbates increased mito-ROS and RyR2 activity, increasing arrhythmic potential.

Exercise Training Stabilizes RyR2-Dependent Ca2+ Release in Post-infarction Heart Failure.

Aim: Dysfunction of the cardiac ryanodine receptor (RyR2) is an almost ubiquitous finding in animal models of heart failure (HF) and results in abnormal Ca2+ release in cardiomyocytes that contributes to contractile impairment and arrhythmias. We tested whether exercise training (ET), as recommended by current guidelines, had the potential to stabilize RyR2-dependent Ca2+ release in rats with post-myocardial infarction HF.Materials and Methods: We subjected male Wistar rats to left coronary artery ligation or sham operations. After 1 week, animals were characterized by echocardiography and randomized to high-intensity interval ET on treadmills or to sedentary behavior (SED). Running speed was adjusted based on a weekly VO2max test. We repeated echocardiography after 5 weeks of ET and harvested left ventricular cardiomyocytes for analysis of RyR2-dependent systolic and spontaneous Ca2+ release. Phosphoproteins were analyzed by Western blotting, and beta-adrenoceptor density was quantified by radioligand binding.Results: ET increased VO2max in HF-ET rats to 127% of HF-SED (P < 0.05). This coincided with attenuated spontaneous SR Ca2+ release in left ventricular cardiomyocytes from HF-ET but also reduced Ca2+ transient amplitude and slowed Ca2+ reuptake during adrenoceptor activation. However, ventricular diameter and fractional shortening were unaffected by ET. Analysis of Ca2+ homeostasis and major proteins involved in the regulation of SR Ca2+ release and reuptake could not explain the attenuated spontaneous SR Ca2+ release or reduced Ca2+ transient amplitude. Importantly, measurements of beta-adrenoceptors showed a normalization of beta1-adrenoceptor density and beta1:beta2-adrenoceptor ratio in HF-ET.Conclusion: ET increased aerobic capacity in post-myocardial infarction HF rats and stabilized RyR2-dependent Ca2+ release. Our data show that these effects of ET can be gained without major alterations in SR Ca2+ regulatory proteins and indicate that future studies should include upstream parts of the sympathetic signaling pathway.

Etiology-Dependent Impairment of Diastolic Cardiomyocyte Calcium Homeostasis in Heart Failure With Preserved Ejection Fraction

BackgroundWhereas heart failure with reduced ejection fraction (HFrEF) is associated with ventricular dilation and markedly reduced systolic function, heart failure with preserved ejection fraction (HFpEF) patients exhibit concentric hypertrophy and diastolic dysfunction. Impaired cardiomyocyte Ca2+ homeostasis in HFrEF has been linked to disruption of membrane invaginations called t-tubules, but it is unknown if such changes occur in HFpEF. ObjectivesThis study examined whether distinct cardiomyocyte phenotypes underlie the heart failure entities of HFrEF and HFpEF. MethodsT-tubule structure was investigated in left ventricular biopsies obtained from HFrEF and HFpEF patients, whereas cardiomyocyte Ca2+ homeostasis was studied in rat models of these conditions. ResultsHFpEF patients exhibited increased t-tubule density in comparison with control subjects. Super-resolution imaging revealed that higher t-tubule density resulted from both tubule dilation and proliferation. In contrast, t-tubule density was reduced in patients with HFrEF. Augmented collagen deposition within t-tubules was observed in HFrEF but not HFpEF hearts. A causative link between mechanical stress and t-tubule disruption was supported by markedly elevated ventricular wall stress in HFrEF patients. In HFrEF rats, t-tubule loss was linked to impaired systolic Ca2+ homeostasis, although diastolic Ca2+ removal was also reduced. In contrast, Ca2+ transient magnitude and release kinetics were largely maintained in HFpEF rats. However, diastolic Ca2+ impairments, including reduced sarco/endoplasmic reticulum Ca2+-ATPase activity, were specifically observed in diabetic HFpEF but not in ischemic or hypertensive models. ConclusionsAlthough t-tubule disruption and impaired cardiomyocyte Ca2+ release are hallmarks of HFrEF, such changes are not prominent in HFpEF. Impaired diastolic Ca2+ homeostasis occurs in both conditions, but in HFpEF, this mechanism for diastolic dysfunction is etiology-dependent.

Electrophysiological Abnormalities in VLCAD Deficient hiPSC-Cardiomyocytes Do not Improve with Carnitine Supplementation.

Patients with a deficiency in very long-chain acyl-CoA dehydrogenase (VLCAD), an enzyme that is involved in the mitochondrial beta-oxidation of long-chain fatty acids, are at risk for developing cardiac arrhythmias. In human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs), VLCAD deficiency (VLCADD) results in a series of abnormalities, including: 1) accumulation of long-chain acylcarnitines, 2) action potential shortening, 3) higher systolic and diastolic intracellular Ca2+ concentrations, and 4) development of delayed afterdepolarizations. In the fatty acid oxidation process, carnitine is required for bidirectional transport of acyl groups across the mitochondrial membrane. Supplementation has been suggested as potential therapeutic approach in VLCADD, but its benefits are debated. Here, we studied the effects of carnitine supplementation on the long-chain acylcarnitine levels and performed electrophysiological analyses in VLCADD patient-derived hiPSC-CMs with a ACADVL gene mutation (p.Val283Ala/p.Glu381del). Under standard culture conditions, VLCADD hiPSC-CMs showed high concentrations of long-chain acylcarnitines, short action potentials, and high delayed afterdepolarizations occurrence. Incubation of the hiPSC-CMs with 400 µM L-carnitine for 48 h led to increased long-chain acylcarnitine levels both in medium and cells. In addition, carnitine supplementation neither restored abnormal action potential parameters nor the increased occurrence of delayed afterdepolarizations in VLCADD hiPSC-CMs. We conclude that long-chain acylcarnitine accumulation and electrophysiological abnormalities in VLCADD hiPSC-CMs are not normalized by carnitine supplementation, indicating that this treatment is unlikely to be beneficial against cardiac arrhythmias in VLCADD patients.

Specialized Pro-Resolving Mediators Protect Against Experimental Autoimmune Myocarditis by Modulating Ca &lt;sup&gt;2+&lt;/sup&gt; Handling and NRF2 Activation

Myocarditis is a severe inflammatory heart disease and a precursor of dilated cardiomyopathy. Despite progress in the understanding of the disease, there is currently no specific treatment available for myocarditis. Lipoxins and their derivatives promote the resolution of inflammation, and help recover tissue homeostasis; however, their role in cardiac inflammation is poorly understood. BML-111, a stable lipoxin A4 receptor agonist, protects against cardiac dysfunction in a murine model of experimental autoimmune myocarditis (EAM) by preventing Ca2+ mishandling. Enhanced cardiac oxidative profile and reduced activation of NRF2, a master antioxidant transcription factor was revealed in EAM-induced mice. Administration of BML-111 to EAM-induced mice prevented all pathological changes, whereas co-administration of the NRF2 inhibitor luteolin to EAM-treated mice blunted the BML-111-induced activation of NRF2 and the accompanying protective effects on cardiac function and intracellular Ca2+ dynamics. In vitro analysis showed that 15-epi-lipoxin A4 increased systolic Ca2+ release and sarcoplasmic reticulum (SR)-Ca2+ load in cardiomyocytes isolated from wild-type mice and augmented the rate of SR-Ca2+ uptake by SERCA2a (SR Ca2+-ATPase), but failed to induce any functional change in cells from Nrf2-/- knockout mice. Mechanistically, BML-111 increased SERCA2a cardiac expression in wild-type mice, and regulation of the expression of NRF2 determined SERCA2a expression in human ventricular cells. Finally, human myocarditis-positive myocardium showed a reduced expression of both ATP2A2 and NFE2L2 coding genes for SERCA2a and NRF2, respectively. Our results highlight new cardioprotective mechanisms of pro-resolving lipid mediators, emerging as innovative treatments for myocarditis.