Ca2+ mishandling and mitochondrial dysfunction: a converging road to prediabetic and diabetic cardiomyopathy.

Diabetic cardiomyopathy is defined as the myocardial dysfunction that suffers patients with diabetes mellitus (DM) in the absence of hypertension and structural heart diseases such as valvular or coronary artery dysfunctions. Since the impact of DM on cardiac function is rather silent and slow, early stages of diabetic cardiomyopathy, known as prediabetes, are poorly recognized, and, on many occasions, cardiac illness is diagnosed only after a severe degree of dysfunction was reached. Therefore, exploration and recognition of the initial pathophysiological mechanisms that lead to cardiac dysfunction in diabetic cardiomyopathy are of vital importance for an on-time diagnosis and treatment of the malady. Among the complex and intricate mechanisms involved in diabetic cardiomyopathy, Ca2+ mishandling and mitochondrial dysfunction have been described as pivotal early processes. In the present review, we will focus on these two processes and the molecular pathway that relates these two alterations to the earlier stages and the development of diabetic cardiomyopathy.

Non-ß-Blocking Carvedilol Analog, VK-II-86, Prevents Ouabain-Induced Cardiotoxicity.

BACKGROUND: It has been shown that carvedilol and its non ß-blocking analog, VK-II-86, inhibit spontaneous Ca2+ release from the sarcoplasmic reticulum (SR). The aim of this study is to determine whether carvedilol and VK-II-86 suppress ouabain-induced arrhythmogenic Ca2+ waves and apoptosis in cardiac myocytes. Methods and Results: Rat cardiac myocytes were exposed to toxic doses of ouabain (50 µmol/L). Cell length (contraction) was monitored in electrically stimulated and non-stimulated conditions. Ouabain treatment increased contractility, frequency of spontaneous contractions and apoptosis compared to control cells. Carvedilol (1 µmol/L) or VK-II-86 (1 µmol/L) did not affect ouabain-induced inotropy, but significantly reduced the frequency of Ca2+ waves, spontaneous contractions and cell death evoked by ouabain treatment. This antiarrhythmic effect was not associated with a reduction in Ca2+ calmodulin-dependent protein kinase II (CaMKII) activity, phospholamban and ryanodine receptor phosphorylation or SR Ca2+ load. Similar results could be replicated in human cardiomyocytes derived from stem cells and in a mathematical model of human myocytes. CONCLUSIONS: Carvedilol and VK-II-86 are effective to prevent ouabain-induced apoptosis and spontaneous contractions indicative of arrhythmogenic activity without affecting inotropy and demonstrated to be effective in human models, thus emerging as a therapeutic tool for the prevention of digitalis-induced arrhythmias and cardiac toxicity.

Calcium-calmodulin-dependent protein kinase mediates the intracellular signalling pathways of cardiac apoptosis in mice with impaired glucose tolerance.

KEY POINTS: Spontaneous sarcoplasmic reticulum (SR) Ca2+ release events increased in fructose-rich diet mouse (FRD) myocytes vs. control diet (CD) mice, in the absence of significant changes in SR Ca2+ load. In HEK293 cells, hyperglycaemia significantly enhanced [3 H]ryanodine binding and Ca2+ /calmodulin-dependent protein kinase II (CaMKII) phosphorylation of RyR2-S2814 residue vs. normoglycaemia. These increases were prevented by CaMKII inhibition. FRD significantly augmented cardiac apoptosis in WT vs. CD-WT mice, which was prevented by co-treatment with the reactive oxygen species scavenger Tempol. Oxidative stress was also increased in FRD-SR-autocamide inhibitory peptide (AIP) mice, expressing the SR-targeted CaMKII inhibitor AIP, without any significant enhancement of apoptosis vs. CD-SR-AIP mice. FRD produced mitochondrial swelling and membrane depolarization in FRD-WT mice but not in FRD-S2814A mice, in which the CaMKII site on ryanodine receptor 2 was ablated. FRD decreased mitochondrial area, mean Feret diameter and the mean distance between SR and the outer mitochondrial membrane vs. CD hearts. This remodelling was prevented in AC3I mice, with cardiac-targeted CaMKII inhibition. ABSTRACT: The impact of cardiac apoptosis in pre-diabetic stages of diabetic cardiomyopathy is unknown. We show that myocytes from fructose-rich diet (FRD) animals exhibit arrhythmias produced by exacerbated Ca2+ /calmodulin-protein kinase (CaMKII) activity, ryanodine receptor 2 (RyR2) phosphorylation and sarcoplasmic reticulum (SR) Ca2+ leak. We tested the hypothesis that this mechanism also underlies cardiac apoptosis in pre-diabetes. We generated a pre-diabetic model in FRD mice. FRD mice showed an increase in oxidative stress, hypertrophy and systolic dysfunction. FRD myocytes exhibited enhanced SR Ca2+ spontaneous events in the absence of SR Ca2+ load alterations vs. control-diet (CD) myocytes. In HEK293 cells, hyperglycaemia significantly enhanced [3 H]ryanodine binding and CaMKII phosphorylation of RyR2-S2814 residue vs. normoglycaemia. CaMKII inhibition prevented hyperglycaemia-induced alterations. FRD also evoked cardiac apoptosis in WT mice vs. CD-WT mice. Co-treatment with the reactive oxygen species scavenger Tempol prevented FRD-induced apoptosis in WT mice. In contrast, FRD enhanced oxidative stress but not apoptosis in FRD-SR-AIP mice, in which a CaMKII inhibitor is targeted to the SR. FRD produced mitochondrial membrane depolarization in WT mice but not in S2814A mice, in which the CaMKII phosphorylation site on RyR2 was ablated. Furthermore, FRD decreased mitochondrial area, mean Feret diameter and mean SR-mitochondrial distance vs. CD-WT hearts. This remodelling was prevented in AC3I mice, with cardiac-targeted CaMKII inhibition. CaMKII phosphorylation of RyR2, SR Ca2+ leak and mitochondrial membrane depolarization are critically involved in the apoptotic pathway of the pre-diabetic heart. The FRD-induced decrease in SR-mitochondrial distance is likely to additionally favour Ca2+ transit between the two organelles.

Ca2+ Sparks and Ca2+ waves are the subcellular events underlying Ca2+ overload during ischemia and reperfusion in perfused intact hearts.

Abnormal intracellular Ca(2+) cycling plays a key role in cardiac dysfunction, particularly during the setting of ischemia/reperfusion (I/R). During ischemia, there is an increase in cytosolic and sarcoplasmic reticulum (SR) Ca(2+). At the onset of reperfusion, there is a transient and abrupt increase in cytosolic Ca(2++), which occurs timely associated with reperfusion arrhythmias. However, little is known about the subcellular dynamics of Ca(2+) increase during I/R, and a possible role of the SR as a mechanism underlying this increase has been previously overlooked. The aim of the present work is to test two main hypotheses: (1) An increase diastolic Ca(2+) sparks frequency (cspf) constitutes a mayor substrate for the ischemia-induced diastolic Ca(2+) increase; (2) an increase in cytosolic Ca(2+) pro-arrhythmogenic events (Ca(2+) waves), mediates the abrupt diastolic Ca(2+) rise at the onset of reperfusion. We used confocal microscopy on mouse intact hearts loaded with Fluo-4. Hearts were submitted to global I/R (12/30 min) to assess epicardial Ca(2+) sparks in the whole heart. Intact heart sparks were faster than in isolated myocytes whereas cspf was not different. During ischemia, cspf significantly increased relative to preischemia (2.07±0.33 vs. 1.13±0.20 sp/s/100 µm, n=29/34, 7 hearts). Reperfusion significantly changed Ca(2+) sparks kinetics, by prolonging Ca(2+) sparks rise time and decreased cspf. However, it significantly increased Ca(2+) wave frequency relative to ischemia (0.71±0.14 vs. 0.38±0.06 w/s/100 µm, n=32/33, 7 hearts). The results show for the first time the assessment of intact perfused heart Ca(2+) sparks and provides direct evidence of increased Ca(2+) sparks in ischemia that transform into Ca(2+) waves during reperfusion. These waves may constitute a main trigger for reperfusion arrhythmias.

Chasing cardiac physiology and pathology down the CaMKII cascade.

Calcium dynamics is central in cardiac physiology, as the key event leading to the excitation-contraction coupling (ECC) and relaxation processes. The primary function of Ca(2+) in the heart is the control of mechanical activity developed by the myofibril contractile apparatus. This key role of Ca(2+) signaling explains the subtle and critical control of important events of ECC and relaxation, such as Ca(2+) influx and SR Ca(2+) release and uptake. The multifunctional Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) is a signaling molecule that regulates a diverse array of proteins involved not only in ECC and relaxation but also in cell death, transcriptional activation of hypertrophy, inflammation, and arrhythmias. CaMKII activity is triggered by an increase in intracellular Ca(2+) levels. This activity can be sustained, creating molecular memory after the decline in Ca(2+) concentration, by autophosphorylation of the enzyme, as well as by oxidation, glycosylation, and nitrosylation at different sites of the regulatory domain of the kinase. CaMKII activity is enhanced in several cardiac diseases, altering the signaling pathways by which CaMKII regulates the different fundamental proteins involved in functional and transcriptional cardiac processes. Dysregulation of these pathways constitutes a central mechanism of various cardiac disease phenomena, like apoptosis and necrosis during ischemia/reperfusion injury, digitalis exposure, post-acidosis and heart failure arrhythmias, or cardiac hypertrophy. Here we summarize significant aspects of the molecular physiology of CaMKII and provide a conceptual framework for understanding the role of the CaMKII cascade on Ca(2+) regulation and dysregulation in cardiac health and disease.

CaMKII-dependent phosphorylation of cardiac ryanodine receptors regulates cell death in cardiac ischemia/reperfusion injury.

Ca(2+)-calmodulin kinase II (CaMKII) activation is deleterious in cardiac ischemia/reperfusion (I/R). Moreover, inhibition of CaMKII-dependent phosphorylations at the sarcoplasmic reticulum (SR) prevents CaMKII-induced I/R damage. However, the downstream targets of CaMKII at the SR level, responsible for this detrimental effect, remain unclear. In the present study we aimed to dissect the role of the two main substrates of CaMKII at the SR level, phospholamban (PLN) and ryanodine receptors (RyR2), in CaMKII-dependent I/R injury. In mouse hearts subjected to global I/R (45/120min), phosphorylation of the primary CaMKII sites, S2814 on cardiac RyR2 and of T17 on PLN, significantly increased at the onset of reperfusion whereas PKA-dependent phosphorylation of RyR2 and PLN did not change. Similar results were obtained in vivo, in mice subjected to regional myocardial I/R (1/24h). Knock-in mice with an inactivated serine 2814 phosphorylation site on RyR2 (S2814A) significantly improved post-ischemic mechanical recovery, reduced infarct size and decreased apoptosis. Conversely, knock-in mice, in which CaMKII site of RyR2 is constitutively activated (S2814D), significantly increased infarct size and exacerbated apoptosis. In S2814A and S2814D mice subjected to regional myocardial ischemia, infarct size was also decreased and increased respectively. Transgenic mice with double-mutant non-phosphorylatable PLN (S16A/T17A) in the PLN knockout background (PLNDM) also showed significantly increased post-ischemic cardiac damage. This effect cannot be attributed to PKA-dependent PLN phosphorylation and was not due to the enhanced L-type Ca(2+) current, present in these mice. Our results reveal a major role for the phosphorylation of S2814 site on RyR2 in CaMKII-dependent I/R cardiac damage. In contrast, they showed that CaMKII-dependent increase in PLN phosphorylation during reperfusion opposes rather than contributes to I/R damage.

Role of CaMKII in post acidosis arrhythmias: a simulation study using a human myocyte model.

Postacidotic arrhythmias have been associated to increased sarcoplasmic reticulum (SR) Ca(2+) load and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) activation. However, the molecular mechanisms underlying these arrhythmias are still unclear. To better understand this process, acidosis produced by CO2 increase from 5% to 30%, resulting in intracellular pH (pHi) change from 7.15 to 6.7, was incorporated into a myocyte model of excitation-contraction coupling and contractility, including acidotic inhibition of L-type Ca(2+) channel (I(CaL)), Na(+)-Ca(2+) exchanger, Ca(2+) release through the SR ryanodine receptor (RyR2) (I(rel)), Ca(2+) reuptake by the SR Ca(2+) ATPase2a (I(up)), Na(+)-K(+) pump, K(+) efflux through the inward rectifier K(+) channel and the transient outward K(+) flow (I(to)) together with increased activity of the Na(+)-H(+) exchanger (I(NHE)). Simulated CaMKII regulation affecting I(rel), I(up), I(CaL), I(NHE) and I(to) was introduced in the model to partially compensate the acidosis outcome. Late Na(+) current increase by CaMKII was also incorporated. Using this scheme and assuming that diastolic Ca(2+) leak through the RyR2 was modulated by the resting state of this channel and the difference between SR and dyadic cleft [Ca(2+)], postacidotic delayed after depolarizations (DADs) were triggered upon returning to normal pHi after 6 min acidosis. The model showed that DADs depend on SR Ca(2+) load and on increased Ca(2+) leak through RyR2. This postacidotic arrhythmogenic pattern relies mainly on CaMKII effect on I(CaL) and I(up), since its individual elimination produced the highest DAD reduction. The model further revealed that during the return to normal pHi, DADs are fully determined by SR Ca(2+) load at the end of acidosis. Thereafter, DADs are maintained by SR Ca(2+) reloading by Ca(2+) influx through the reverse NCX mode during the time period in which [Na(+)]i is elevated.

The multifunctional Ca(2+)/calmodulin-dependent protein kinase II delta (CaMKIIδ) phosphorylates cardiac titin's spring elements.

Titin-based passive stiffness is post-translationally regulated by several kinases that phosphorylate specific spring elements located within titin's elastic I-band region. Whether titin is phosphorylated by calcium/calmodulin dependent protein kinase II (CaMKII), an important regulator of cardiac function and disease, has not been addressed. The aim of this work was to determine whether CaMKIIδ, the predominant CaMKII isoform in the heart, phosphorylates titin, and to use phosphorylation assays and mass spectrometry to study which of titin's spring elements might be targeted by CaMKIIδ. It was found that CaMKIIδ phosphorylates titin in mouse LV skinned fibers, that the CaMKIIδ sites can be dephosphorylated by protein phosphatase 1 (PP1), and that under baseline conditions, in both intact isolated hearts and skinned myocardium, about half of the CaMKIIδ sites are phosphorylated. Mass spectrometry revealed that both the N2B and PEVK segments are targeted by CaMKIIδ at several conserved serine residues. Whether phosphorylation of titin by CaMKIIδ occurs in vivo, was tested in several conditions using back phosphorylation assays and phospho-specific antibodies to CaMKIIδ sites. Reperfusion following global ischemia increased the phosphorylation level of CaMKIIδ sites on titin and this effect was abolished by the CaMKII inhibitor KN-93. No changes in the phosphorylation level of the PEVK element were found suggesting that the increased phosphorylation level of titin in IR (ischemia reperfusion) might be due to phosphorylation of the N2B element. The findings of these studies show for the first time that titin can be phosphoryalated by CaMKIIδ, both in vitro and in vivo, and that titin's molecular spring region that determines diastolic stiffness is a target of CaMKIIδ.

Interplay between SERCA, 4E-BP, and eIF4E in the Drosophila heart.

Appropriate cardiac performance depends on a tightly controlled handling of Ca2+ in a broad range of species, from invertebrates to mammals. The role of the Ca2+ ATPase, SERCA, in Ca2+ handling is pivotal, and its activity is regulated, inter alia, by interacting with distinct proteins. Herein, we give evidence that 4E binding protein (4E-BP) is a novel regulator of SERCA activity in Drosophila melanogaster during cardiac function. Flies over-expressing 4E-BP showed improved cardiac performance in young individuals associated with incremented SERCA activity. Moreover, we demonstrate that SERCA interacts with translation initiation factors eIF4E-1, eIF4E-2 and eIF4E-4 in a yeast two-hybrid assay. The specific identification of eIF4E-4 in cardiac tissue leads us to propose that the interaction of elF4E-4 with SERCA may be the basis of the cardiac effects observed in 4E-BP over-expressing flies associated with incremented SERCA activity.

Angiotensin II-induced oxidative stress resets the Ca2+ dependence of Ca2+-calmodulin protein kinase II and promotes a death pathway conserved across different species.

RATIONALE: Angiotensin (Ang) II-induced apoptosis was reported to be mediated by different signaling molecules. Whether these molecules are either interconnected in a single pathway or constitute different and alternative cascades by which Ang II exerts its apoptotic action, is not known. OBJECTIVE: To investigate in cultured myocytes from adult cat and rat, 2 species in which Ang II has opposite inotropic effects, the signaling cascade involved in Ang II-induced apoptosis. METHODS AND RESULTS: Ang II (1 micromol/L) reduced cat/rat myocytes viability by approximately 40%, in part, because of apoptosis (TUNEL/caspase-3 activity). In both species, apoptosis was associated with reactive oxygen species (ROS) production, Ca(2+)/calmodulin-dependent protein kinase (CaMK)II, and p38 mitogen-activated protein kinase (p38MAPK) activation and was prevented by the ROS scavenger MPG (2-mercaptopropionylglycine) or the NADPH oxidase inhibitor DPI (diphenyleneiodonium) by CaMKII inhibitors (KN-93 and AIP [autocamtide 2-related inhibitory peptide]) or in transgenic mice expressing a CaMKII inhibitory peptide and by the p38MAPK inhibitor, SB202190. Furthermore, p38MAPK overexpression exacerbated Ang II-induced cell mortality. Moreover, although KN-93 did not affect Ang II-induced ROS production, it prevented p38MAPK activation. Results further show that CaMKII can be activated by Ang II or H(2)O(2), even in the presence of the Ca(2+) chelator BAPTA-AM, in myocytes and in EGTA-Ca(2+)-free solutions in the presence of the calmodulin inhibitor W-7 in in vitro experiments. CONCLUSIONS: (1) The Ang II-induced apoptotic cascade converges in both species, in a common pathway mediated by ROS-dependent CaMKII activation which results in p38MAPK activation and apoptosis. (2) In the presence of Ang II or ROS, CaMKII may be activated at subdiastolic Ca(2+) concentrations, suggesting a new mechanism by which ROS reset the Ca(2+) dependence of CaMKII to extremely low Ca(2+) levels.

Is Bauman's "liquid modernity" influencing the way we are doing science?

This commentary analyzes the possible effects of lightness-a typical attribute of modern (liquid) society, according to Bauman-on the way we are doing science. We share our opinion in an attempt to discern whether some unwanted practices that may affect our scientific results (such as technology misuse, bonus rewards, publishing under pressure, or indolence for getting accurate results) can be attributed, at least partially, to the liquid characteristic of modern society. We also examine whether the different systems that support science favor these actions, conspiring against what should be the primary goal of science: the search for truth. We finally consider several aspects that should be taken into account to rescue science from the intrusion of weightless actions.

Na+/K+-ATPase inhibition by ouabain induces CaMKII-dependent apoptosis in adult rat cardiac myocytes.

The positive inotropic effect produced by Na(+)/K(+)-ATPase inhibition has been used for the treatment of heart failure for over 200 years. Recently, administration of toxic doses of ouabain has been shown to induce cardiac myocyte apoptosis. However, whether prolonged administration of non-toxic doses of ouabain can also promote cardiac myocyte cell death has never been explored. The aim of this study was to assess whether non-toxic doses of ouabain can induce myocyte apoptosis and if so, to examine the underlying mechanisms. For this purpose, cardiac myocytes from rat and cat, two species with different sensitivity to digitalis, were cultured for 24h in the presence or absence of 2 microM (rat) and 25 nm-2 microM ouabain (cat). Cell viability and apoptosis assays showed that ouabain produced, in the rat, a 43+/-5% decrease in cell viability due to apoptosis (enhanced caspase-3 activity, increased Bax/Bcl-2 and TUNEL-positive nuclei) and necrosis (LDH release and trypan blue staining). Similar results were obtained with 25 nM ouabain in the cat. Ouabain-induced reduction in cell viability was prevented by the NCX inhibitor KB-R7943 and by the CaMKII inhibitors, KN93 and AIP. Furthermore, CaMKII overexpression exacerbated ouabain-induced cell mortality which in contrast was reduced in transgenic mice with chronic CaMKII inhibition. However, KN93 failed to affect ouabain-induced inotropy. In addition, whereas ERK(1/2) inhibition with PD-98059 had no effect on cell mortality, PI3K inhibition with wortmannin, exacerbated myocyte death. We conclude that ouabain triggers an apoptotic cascade that involves NCX and CaMKII as a downstream effector. Ouabain simultaneously activates an antiapoptotic cascade involving PI3K/AKT which is however, insufficient to completely repress apoptosis. The finding that KN93 prevents ouabain-induced apoptosis without affecting inotropy suggests the potential use of CaMKII inhibitors as an adjunct to digitalis treatment for cardiovascular disease.

Ca(2+)/calmodulin-dependent protein kinase II contributes to intracellular pH recovery from acidosis via Na(+)/H(+) exchanger activation.

The Na(+)/H(+) exchanger (NHE-1) plays a key role in pH(i) recovery from acidosis and is regulated by pH(i) and the ERK1/2-dependent phosphorylation pathway. Since acidosis increases the activity of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in cardiac muscle, we examined whether CaMKII activates the exchanger by using pharmacological tools and highly specific genetic approaches. Adult rat cardiomyocytes, loaded with the pH(i) indicator SNARF-1/AM were subjected to different protocols of intracellular acidosis. The rate of pH(i) recovery from the acid load (dpH(i)/dt)-an index of NHE-1 activity in HEPES buffer or in NaHCO(3) buffer in the presence of inhibition of anion transporters-was significantly decreased by the CaMKII inhibitors KN-93 or AIP. pH(i) recovery from acidosis was faster in CaMKII-overexpressing myocytes than in overexpressing beta-galactosidase myocytes (dpH(i)/dt: 0.195+/-0.04 vs. 0.045+/-0.010 min(-)(1), respectively, n=8) and slower in myocytes from transgenic mice with chronic cardiac CaMKII inhibition (AC3-I) than in controls (AC3-C). Inhibition of CaMKII and/or ERK1/2 indicated that stimulation of NHE-1 by CaMKII was independent of and additive to the ERK1/2 cascade. In vitro studies with fusion proteins containing wild-type or mutated (Ser/Ala) versions of the C-terminal domain of NHE-1 indicate that CaMKII phosphorylates NHE-1 at residues other than the canonical phosphorylation sites for the kinase (Ser648, Ser703, and Ser796). These results provide new mechanistic insights and unequivocally demonstrate a role of the already multifunctional CaMKII on the regulation of the NHE-1 activity. They also prove clinically important in multiple disorders which, like ischemia/reperfusion injury or hypertrophy, are associated with increased NHE-1 and CaMKII.

The signalling pathway of CaMKII-mediated apoptosis and necrosis in the ischemia/reperfusion injury.

Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) plays an important role mediating apoptosis/necrosis during ischemia-reperfusion (IR). We explored the mechanisms of this deleterious effect. Langendorff perfused rat and transgenic mice hearts with CaMKII inhibition targeted to sarcoplasmic reticulum (SR-AIP) were subjected to global IR. The onset of reperfusion increased the phosphorylation of Thr(17) site of phospholamban, without changes in total protein, consistent with an increase in CaMKII activity. Instead, there was a proportional decrease in the phosphorylation of Ser2815 site of ryanodine receptors (RyR2) and the amount of RyR2 at the onset of reperfusion, i.e. the ratio Ser2815/RyR2 did not change. Inhibition of the reverse Na(+)/Ca(2+)exchanger (NCX) mode (KBR7943) diminished phospholamban phosphorylation, reduced apoptosis/necrosis and enhanced mechanical recovery. CaMKII-inhibition (KN-93), significantly decreased phospholamban phosphorylation, infarct area, lactate dehydrogenase release (LDH) (necrosis), TUNEL positive nuclei, caspase-3 activity, Bax/Bcl-2 ratio and Ca(2+)-induced mitochondrial swelling (apoptosis), and increased contractile recovery when compared with non-treated IR hearts or IR hearts pretreated with the inactive analog, KN-92. Blocking SR Ca(2+) loading and release (thapsigargin/dantrolene), mitochondrial Ca(2+) uniporter (ruthenium red/RU360), or mitochondrial permeability transition pore (cyclosporine A), significantly decreased infarct size, LDH release and apoptosis. SR-AIP hearts failed to show an increase in the phosphorylation of Thr(17) of phospholamban at the onset of reflow and exhibited a significant decrease in infarct size, apoptosis and necrosis respect to controls. The results reveal an apoptotic-necrotic pathway mediated by CaMKII-dependent phosphorylations at the SR, which involves the reverse NCX mode and the mitochondria as trigger and end effectors, respectively, of the cascade.

Determinants of Ca2+ release restitution: Insights from genetically altered animals and mathematical modeling.

Each heartbeat is followed by a refractory period. Recovery from refractoriness is known as Ca2+ release restitution (CRR), and its alterations are potential triggers of Ca2+ arrhythmias. Although the control of CRR has been associated with SR Ca2+ load and RYR2 Ca2+ sensitivity, the relative role of some of the determinants of CRR remains largely undefined. An intriguing point, difficult to dissect and previously neglected, is the possible independent effect of SR Ca2+ content versus the velocity of SR Ca2+ refilling on CRR. To assess these interrogations, we used isolated myocytes with phospholamban (PLN) ablation (PLNKO), knock-in mice with pseudoconstitutive CaMKII phosphorylation of RYR2 S2814 (S2814D), S2814D crossed with PLNKO mice (SDKO), and a previously validated human cardiac myocyte model. Restitution of cytosolic Ca2+ (Fura-2 AM) and L-type calcium current (ICaL; patch-clamp) was evaluated with a two-pulse (S1/S2) protocol. CRR and ICaL restitution increased as a function of the (S2-S1) coupling interval, following an exponential curve. When SR Ca2+ load was increased by increasing extracellular [Ca2+] from 2.0 to 4.0 mM, CRR and ICaL restitution were enhanced, suggesting that ICaL restitution may contribute to the faster CRR observed at 4.0 mM [Ca2+]. In contrast, ICaL restitution did not differ among the different mouse models. For a given SR Ca2+ load, CRR was accelerated in S2814D myocytes versus WT, but not in PLNKO and SDKO myocytes versus WT and S2814D, respectively. The model mimics all experimental data. Moreover, when the PLN ablation-induced decrease in RYR2 expression was corrected, the model revealed that CRR was accelerated in PLNKO and SDKO versus WT and S2814D myocytes, consistent with the enhanced velocity of refilling, SR [Ca2+] recovery, and CRR. We speculate that refilling rate might enhance CRR independently of SR Ca2+ load.

Unbalance Between Sarcoplasmic Reticulum Ca2 + Uptake and Release: A First Step Toward Ca2 + Triggered Arrhythmias and Cardiac Damage.

The present review focusses on the regulation and interplay of cardiac SR Ca2+ handling proteins involved in SR Ca2+ uptake and release, i.e., SERCa2/PLN and RyR2. Both RyR2 and SERCA2a/PLN are highly regulated by post-translational modifications and/or different partners' proteins. These control mechanisms guarantee a precise equilibrium between SR Ca2+ reuptake and release. The review then discusses how disruption of this balance alters SR Ca2+ handling and may constitute a first step toward cardiac damage and malignant arrhythmias. In the last part of the review, this concept is exemplified in different cardiac diseases, like prediabetic and diabetic cardiomyopathy, digitalis intoxication and ischemia-reperfusion injury.

Ablation of phospholamban rescues reperfusion arrhythmias but exacerbates myocardium infarction in hearts with Ca2+/calmodulin kinase II constitutive phosphorylation of ryanodine receptors.

AIMS: Abnormal Ca2+ release from the sarcoplasmic reticulum (SR), associated with Ca2+-calmodulin kinase II (CaMKII)-dependent phosphorylation of RyR2 at Ser2814, has consistently been linked to arrhythmogenesis and ischaemia/reperfusion (I/R)-induced cell death. In contrast, the role played by SR Ca2+ uptake under these stress conditions remains controversial. We tested the hypothesis that an increase in SR Ca2+ uptake is able to attenuate reperfusion arrhythmias and cardiac injury elicited by increased RyR2-Ser2814 phosphorylation. METHODS AND RESULTS: We used WT mice, which have been previously shown to exhibit a transient increase in RyR2-Ser2814 phosphorylation at the onset of reperfusion; mice with constitutive pseudo-phosphorylation of RyR2 at Ser2814 (S2814D) to exacerbate CaMKII-dependent reperfusion arrhythmias and cardiac damage, and phospholamban (PLN)-deficient-S2814D knock-in (SDKO) mice resulting from crossbreeding S2814D with phospholamban knockout deficient (PLNKO) mice. At baseline, S2814D and SDKO mice had structurally normal hearts. Moreover none of the strains were arrhythmic before ischaemia. Upon cardiac I/R, WT, and S2814D hearts exhibited abundant arrhythmias that were prevented by PLN ablation. In contrast, PLN ablation increased infarct size compared with WT and S2814D hearts. Mechanistically, the enhanced SR Ca2+ sequestration evoked by PLN ablation in SDKO hearts prevented arrhythmogenic events upon reperfusion by fragmenting SR Ca2+ waves into non-propagated and non-arrhythmogenic events (mini-waves). Conversely, the increase in SR Ca2+ sequestration did not reduce but rather exacerbated I/R-induced SR Ca2+ leak, as well as mitochondrial alterations, which were greatly avoided by inhibition of RyR2. These results indicate that the increase in SR Ca2+ uptake is ineffective in preventing the enhanced SR Ca2+ leak of PLN ablated myocytes from either entering into nearby mitochondria and/or activating additional CaMKII pathways, contributing to cardiac damage. CONCLUSION: Our results demonstrate that increasing SR Ca2+ uptake by PLN ablation can prevent the arrhythmic events triggered by CaMKII-dependent phosphorylation of RyR2-induced SR Ca2+ leak. These findings underscore the benefits of increasing SERCA2a activity in the face of SR Ca2+ triggered arrhythmias. However, enhanced SERCA2a cannot prevent but rather exacerbates I/R cardiac injury.