Modulation of cardiac ryanodine receptors of swine and rabbit by a phosphorylation-dephosphorylation mechanism

Ablation of three major phospho-sites in RyR2 preserves the global adrenergic response but creates an arrhythmogenic substrate

Background

Ryanodine receptor 2 (RyR2) is one of the first substrates undergoing phosphorylation upon catecholaminergic stimulation. Yet, the role of RyR2 phosphorylation in the adrenergic response remains debated. To date, three residues in RyR2 are known to undergo phosphorylation upon adrenergic stimulation. We generated a model of RyR2 phospho-ablation of all three canonical phospho-sites (RyR2-S2031A/S2808A/S2814A, triple phospho-mutant, TPM) to elucidate the role of phosphorylation at these residues in the adrenergic response.

Methods

Cardiac structure and function, cellular Ca 2+ dynamics and electrophysiology, and RyR2 channel activity both under basal conditions and under isoproterenol (Iso) stimulation were systematically evaluated. We used echocardiography and electrocardiography in anesthetized mice, single-cell Ca 2+ imaging and whole-cell patch clamp in isolated adult cardiomyocytes, and biochemical assays.

Results

Iso stimulation produced normal chronotropic and inotropic responses in TPM mice as well as an increase in the global Ca 2+ transients in isolated cardiomyocytes. Functional studies revealed fewer Ca 2+ sparks in permeabilized TPM myocytes, and reduced RyR2-mediated Ca 2+ leak in intact myocytes under Iso stimulation, suggesting that the canonical sites may regulate RyR2-mediated Ca 2+ leak. TPM mice also displayed increased propensity for arrhythmia. TPM myocytes were prone to develop early afterdepolarizations (EADs), which were abolished by chelating intracellular Ca 2+ with EGTA, indicating that EADs require SR Ca 2+ release. EADs were also blocked by a low concentration of tetrodotoxin, further suggesting reactivation of the sodium current ( I Na ) as the underlying cause.

Conclusion

Phosphorylation of the three canonical residues on RyR2 may not be essential for the global adrenergic responses. However, these sites play a vital role in maintaining electrical stability during catecholamine stimulation by fine-tuning RyR2-mediated Ca 2+ leak. These findings underscore the importance of RyR2 phosphorylation and a finite diastolic Ca 2+ leak in maintaining electrical stability during catecholamine stimulation.

Clustering properties of the cardiac ryanodine receptor in health and heart failure

The cardiac ryanodine receptor (RyR2) is an intracellular Ca2+ release channel vital for the function of the heart. Physiologically, RyR2 is triggered to release Ca2+ from the sarcoplasmic reticulum (SR) which enables cardiac contraction; however, spontaneous Ca2+ leak from RyR2 has been implicated in the pathophysiology of heart failure (HF). RyR2 channels have been well documented to assemble into clusters within the SR membrane, with the organisation of RyR2 clusters recently gaining interest as a mechanism by which the occurrence of pathological Ca2+ leak is regulated, including in HF. In this review, we explain the terminology relating to key nanoscale RyR2 clustering properties as both single clusters and functionally grouped Ca2+ release units, with a focus on the advancements in super-resolution imaging approaches which have enabled the detailed study of cluster organisation. Further, we discuss proposed mechanisms for modulating RyR2 channel organisation and the debate regarding the potential impact of cluster organisation on Ca2+ leak activity. Finally, recent experimental evidence investigating the nanoscale remodelling and functional alterations of RyR2 clusters in HF is discussed with consideration of the clinical implications.

Allosteric modulation of ryanodine receptor RyR1 by nucleotide derivatives

The coordinated release of Ca2+ from the sarcoplasmic reticulum (SR) is critical for excitation-contraction coupling. This release is facilitated by ryanodine receptors (RyRs) that are embedded in the SR membrane. In skeletal muscle, activity of RyR1 is regulated by metabolites such as ATP, which upon binding increase channel open probability (Po). To obtain structural insights into the mechanism of RyR1 priming by ATP, we determined several cryo-EM structures of RyR1 bound individually to ATP-γ-S, ADP, AMP, adenosine, adenine, and cAMP. We demonstrate that adenine and adenosine bind RyR1, but AMP is the smallest ATP derivative capable of inducing long-range (>170 Å) structural rearrangements associated with channel activation, establishing a structural basis for key binding site interactions that are the threshold for triggering quaternary structural changes. Our finding that cAMP also induces these structural changes and results in increased channel opening suggests its potential role as an endogenous modulator of RyR1 conductance.

Timing mechanisms to control heart rhythm and initiate arrhythmias: roles for intracellular organelles, signalling pathways and subsarcolemmal Ca2

Rhythms of electrical activity in all regions of the heart can be influenced by a variety of intracellular membrane bound organelles. This is true both for normal pacemaker activity and for abnormal rhythms including those caused by early and delayed afterdepolarizations under pathological conditions. The influence of the sarcoplasmic reticulum (SR) on cardiac electrical activity is widely recognized, but other intracellular organelles including lysosomes and mitochondria also contribute. Intracellular organelles can provide a timing mechanism (such as an SR clock driven by cyclic uptake and release of Ca²⁺, with an important influence of intraluminal Ca²⁺), and/or can act as a Ca²⁺ store involved in signalling mechanisms. Ca²⁺ plays many diverse roles including carrying electric current, driving electrogenic sodium-calcium exchange (NCX) particularly when Ca²⁺ is extruded across the surface membrane causing depolarization, and activation of enzymes which target organelles and surface membrane proteins. Heart function is also influenced by Ca²⁺ mobilizing agents (cADP-ribose, nicotinic acid adenine dinucleotide phosphate and inositol trisphosphate) acting on intracellular organelles. Lysosomal Ca²⁺ release exerts its effects via calcium/calmodulin-dependent protein kinase II to promote SR Ca²⁺ uptake, and contributes to arrhythmias resulting from excessive beta-adrenoceptor stimulation. A separate arrhythmogenic mechanism involves lysosomes, mitochondria and SR. Interacting intracellular organelles, therefore, have profound effects on heart rhythms and NCX plays a central role. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Preserved cardiac performance and adrenergic response in a rabbit model with decreased ryanodine receptor 2 expression

Ryanodine receptor 2 (RyR2) is an ion channel in the heart responsible for releasing into the cytosol most of the Ca2+ required for contraction. Proper regulation of RyR2 is critical, as highlighted by the association between channel dysfunction and cardiac arrhythmia. Lower RyR2 expression is also observed in some forms of heart disease; however, there is limited information on the impact of this change on excitation-contraction (e-c) coupling, Ca2+-dependent arrhythmias, and cardiac performance. We used a constitutive knock-out of RyR2 in rabbits (RyR2-KO) to assess the extent to which a stable decrease in RyR2 expression modulates Ca2+ handling in the heart. We found that homozygous knock-out of RyR2 in rabbits is embryonic lethal. Remarkably, heterozygotes (KO+/-) show ~50% loss of RyR2 protein without developing an overt phenotype at the intact animal and whole heart levels. Instead, we found that KO+/- myocytes show (1) remodeling of RyR2 clusters, favoring smaller groups in which channels are more densely arranged; (2) lower Ca2+ spark frequency and amplitude; (3) slower rate of Ca2+ release and mild but significant desynchronization of the Ca2+ transient; and (4) a significant decrease in the basal phosphorylation of S2031, likely due to increased association between RyR2 and PP2A. Our data show that RyR2 deficiency, although remarkable at the molecular and subcellular level, has only a modest impact on global Ca2+ release and is fully compensated at the whole-heart level. This highlights the redundancy of RyR2 protein expression and the plasticity of the e-c coupling apparatus.

Activation of endogenous protein phosphatase 1 enhances the calcium sensitivity of the ryanodine receptor type 2 in murine ventricular cardiomyocytes

KEY POINTS

Increased protein phosphatase 1 (PP-1) activity has been found in end stage human heart failure. Although PP-1 has been extensively studied, a detailed understanding of its role in the excitation-contraction coupling mechanism, in normal and diseased hearts, remains elusive. The present study investigates the functional effect of the PP-1 activity on local Ca²⁺ release events in ventricular cardiomyocytes, by using an activating peptide (PDP3) for the stimulation of the endogenous PP-1 protein. We report that acute de-phosphorylation may increase the sensitivity of RyR2 channels to Ca²⁺ in situ, and that the RyR2-serine2808 phosphorylation site may mediate such a process. Our approach unmasks the functional importance of PP-1 in the regulation of RyR2 activity, suggesting a potential role in the generation of a pathophysiological sarcoplasmic reticulum Ca²⁺ leak in the diseased heart.

ABSTRACT

Changes in cardiac ryanodine receptor (RyR2) phosphorylation are considered to be important regulatory and disease related post-translational protein modifications. The extent of RyR2 phosphorylation is mainly determined by the balance of the activities of protein kinases and phosphatases, respectively. Increased protein phosphatase-1 (PP-1) activity has been observed in heart failure, although the regulatory role of this enzyme on intracellular Ca²⁺ handling remains poorly understood. To determine the physiological and pathophysiological significance of increased PP-1 activity, we investigated how the PP-1 catalytic subunit (PP-1c) alters Ca²⁺ sparks in permeabilized cardiomyocytes and we also applied a PP-1-disrupting peptide (PDP3) to specifically activate endogenous PP-1, including the one anchored on the RyR2 macromolecular complex. We compared wild-type and transgenic mice in which the usually highly phosphorylated site RyR2-S2808 has been ablated to investigate its involvement in RyR2 modulation (S2808A^+/+ ). In wild-type myocytes, PP-1 increased Ca²⁺ spark frequency by two-fold, followed by depletion of the sarcoplasmic reticulum Ca²⁺ store. Similarly, PDP3 transiently increased spark frequency and decreased sarcoplasmic reticulum Ca²⁺ load. RyR2 Ca²⁺ sensitivity, which was assessed by Ca²⁺ spark recovery analysis, was increased in the presence of PDP3 compared to a negative control peptide. S2808A^+/+ cardiomyocytes did not respond to both PP-1c and PDP3 treatment. Our results suggest an increased Ca²⁺ sensitivity of RyR2 upon de-phosphorylation by PP-1. Furthermore, we have confirmed the S2808 site as a target for PP-1 and as a potential link between RyR2s modulation and the cellular response.

Challenges and Opportunities for Therapeutic Targeting of Calmodulin Kinase II in Heart

Heart failure remains a major health burden around the world. Despite great progress in delineation of molecular mechanisms underlying development of disease, standard therapy has not advanced at the same pace. The multifunctional signaling molecule Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) has received considerable attention over recent years for its central role in maladaptive remodeling and arrhythmias in the setting of chronic disease. However, these basic science discoveries have yet to translate into new therapies for human patients. This review addresses both the promise and barriers to developing translational therapies that target CaMKII signaling to abrogate pathologic remodeling in the setting of chronic disease. Efforts in small molecule design are discussed, as well as alternative targeting approaches that exploit novel avenues for compound delivery and/or genetic approaches to affect cardiac CaMKII signaling. These alternative strategies provide hope for overcoming some of the challenges that have limited the development of new therapies.

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.

Regulation of sarcoplasmic reticulum Ca2+ release by serine-threonine phosphatases in the heart

The amount and timing of Ca²⁺ release from the sarcoplasmic reticulum (SR) during cardiac cycle are the main determinants of cardiac contractility. Reversible phosphorylation of the SR Ca²⁺ release channel, ryanodine receptor type 2 (RyR2) is the central mechanism of regulation of Ca²⁺ release in cardiomyocytes. Three major serine-threonine phosphatases including PP1, PP2A and PP2B (calcineurin) have been implicated in modulation of RyR2 function. Changes in expression levels of these phosphatases, their activity and targeting to the RyR2 macromolecular complex were demonstrated in many animal models of cardiac disease and humans and are implicated in cardiac arrhythmia and heart failure. Here we review evidence in support of regulation of RyR2-mediated SR Ca²⁺ release by serine-threonine phosphatases and the role and mechanisms of dysregulation of phosphatases in various disease states.

The calcium-frequency response in the rat ventricular myocyte: an experimental and modelling study

KEY POINTS

In the majority of species, including humans, increased heart rate increases cardiac contractility. This change is known as the force-frequency response (FFR). The majority of mammals have a positive force-frequency relationship (FFR). In rat the FFR is controversial. We derive a species- and temperature-specific data-driven model of the rat ventricular myocyte. As a measure of the FFR, we test the effects of changes in frequency and extracellular calcium on the calcium-frequency response (CFR) in our model and three altered models. The results show a biphasic peak calcium-frequency response, due to biphasic behaviour of the ryanodine receptor and the combined effect of the rapid calmodulin buffer and the frequency-dependent increase in diastolic calcium. Alterations to the model reveal that inclusion of Ca(2+) /calmodulin-dependent protein kinase II (CAMKII)-mediated L-type channel and transient outward K(+) current activity enhances the positive magnitude calcium-frequency response, and the absence of CAMKII-mediated increase in activity of the sarco/endoplasmic reticulum Ca(2+) -ATPase induces a negative magnitude calcium-frequency response.

ABSTRACT

An increase in heart rate affects the strength of cardiac contraction by altering the Ca(2+) transient as a response to physiological demands. This is described by the force-frequency response (FFR), a change in developed force with pacing frequency. The majority of mammals, including humans, have a positive FFR, and cardiac contraction strength increases with heart rate. However, the rat and mouse are exceptions, with the majority of studies reporting a negative FFR, while others report either a biphasic or a positive FFR. Understanding the differences in the FFR between humans and rats is fundamental to interpreting rat-based experimental findings in the context of human physiology. We have developed a novel model of rat ventricular electrophysiology and calcium dynamics, derived predominantly from experimental data recorded under physiological conditions. As a measure of FFR, we tested the effects of changes in stimulation frequency and extracellular calcium concentration on the simulated Ca(2+) transient characteristics and showed a biphasic peak calcium-frequency relationship, consistent with recent observations of a shift from negative to positive FFR when approaching the rat physiological frequency range. We tested the hypotheses that (1) inhibition of Ca(2+) /calmodulin-dependent protein kinase II (CAMKII)-mediated increase in sarco/endoplasmic reticulum Ca(2+) -ATPase (SERCA) activity, (2) CAMKII modulation of SERCA, L-type channel and transient outward K(+) current activity and (3) Na(+) /K(+) pump dynamics play a significant role in the rat FFR. The results reveal a major role for CAMKII modulation of SERCA in the peak Ca(2+) -frequency response, driven most significantly by the cytosolic calcium buffering system and changes in diastolic Ca(2+) .

Maximal acceleration of Ca2+ release refractoriness by β-adrenergic stimulation requires dual activation of kinases PKA and CaMKII in mouse ventricular myocytes

KEY POINTS

Refractoriness of calcium release in heart cells is altered in several disease states, but the physiological mechanisms that regulate this process are incompletely understood. We examined refractoriness of calcium release in mouse ventricular myocytes and investigated how activation of different intracellular signalling pathways influenced this process. We found that refractoriness of calcium release is abbreviated by stimulation of the 'fight-or-flight' response, and that simultaneous activation of multiple intracellular signalling pathways contributes to this response. Data obtained under several conditions at the subcellular, microscopic level were consistent with results obtained at the cellular level. The results provide insight into regulation of cardiac calcium release and how alterations to this process may increase arrhythmia risk under different conditions.

ABSTRACT

Time-dependent refractoriness of calcium (Ca(2+)) release in cardiac myocytes is an important factor in determining whether pro-arrhythmic release patterns develop. At the subcellular level of the Ca(2+) spark, recent studies have suggested that recovery of spark amplitude is controlled by local sarcoplasmic reticulum (SR) refilling whereas refractoriness of spark triggering depends on both refilling and the sensitivity of the ryanodine receptor (RyR) release channels that produce sparks. Here we studied regulation of Ca(2+) spark refractoriness in mouse ventricular myocytes by examining how β-adrenergic stimulation influenced sequences of Ca(2+) sparks originating from individual RyR clusters. Our protocol allowed us to separately measure recovery of spark amplitude and delays between successive sparks, and data were interpreted quantitatively through simulations with a stochastic mathematical model. We found that, compared with spark sequences measured under control conditions: (1) β-adrenergic stimulation with isoproterenol (isoprenaline) accelerated spark amplitude recovery and decreased spark-to-spark delays; (2) activating protein kinase A (PKA) with forskolin accelerated amplitude recovery but did not affect spark-to-spark delays; (3) inhibiting PKA with H89 retarded amplitude recovery and increased spark-to-spark delays; (4) preventing phosphorylation of the RyR at serine 2808 with a knock-in mouse prevented the decrease in spark-to-spark delays seen with β-adrenergic stimulation; (5) inhibiting either PKA or Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) during β-adrenergic stimulation prevented the decrease in spark-to-spark delays seen without inhibition. The results suggest that activation of either PKA or CaMKII is sufficient to speed SR refilling, but activation of both kinases appears necessary to observe increased RyR sensitivity. The data provide novel insight into β-adrenergic regulation of Ca(2+) release refractoriness in mouse myocytes.

Direct measurements of SR free Ca reveal the mechanism underlying the transient effects of RyR potentiation under physiological conditions

AIMS

Most of the calcium that activates contraction is released from the sarcoplasmic reticulum (SR) through the ryanodine receptor (RyR). It is controversial whether activators of the RyR produce a maintained increase in the amplitude of the systolic Ca transient. We therefore aimed to examine the effects of activation of the RyR in large animals under conditions designed to be as physiological as possible while simultaneously measuring SR and cytoplasmic Ca.

METHODS AND RESULTS

Experiments were performed on ventricular myocytes from canine and ovine hearts. Cytoplasmic Ca was measured with fluo-3 and SR Ca with mag-fura-2. Application of caffeine resulted in a brief increase in the amplitude of the systolic Ca transient accompanied by an increase of action potential duration. These effects disappeared with a rate constant of ∼3 s(-1). Similar effects were seen in cells taken from sheep in which heart failure had been induced by rapid pacing. The decrease of Ca transient amplitude was accompanied by a decrease of SR Ca content. During this phase, the maximum (end-diastolic) SR Ca content fell while the minimum systolic increased.

CONCLUSIONS

This study shows that, under conditions designed to be as physiological as possible, potentiation of RyR opening has no maintained effect on the systolic Ca transient. This result makes it unlikely that potentiation of the RyR has a maintained role in positive inotropy.

CaMKII regulation of cardiac ryanodine receptors and inositol triphosphate receptors

Ryanodine receptors (RyRs) and inositol triphosphate receptors (InsP3Rs) are structurally related intracellular calcium release channels that participate in multiple primary or secondary amplified Ca(2+) signals, triggering muscle contraction and oscillatory Ca(2+) waves, or activating transcription factors. In the heart, RyRs play an indisputable role in the process of excitation-contraction coupling as the main pathway for Ca(2+) release from sarcoplasmic reticulum (SR), and a less prominent role in the process of excitation-transcription coupling. Conversely, InsP3Rs are believed to contribute in subtle ways, only, to contraction of the heart, and in more important ways to regulation of transcription factors. Because uncontrolled activity of either RyRs or InsP3Rs may elicit life-threatening arrhythmogenic and/or remodeling Ca(2+) signals, regulation of their activity is of paramount importance for normal cardiac function. Due to their structural similarity, many regulatory factors, accessory proteins, and post-translational processes are equivalent for RyRs and InsP3Rs. Here we discuss regulation of RyRs and InsP3Rs by CaMKII phosphorylation, but touch on other kinases whenever appropriate. CaMKII is emerging as a powerful modulator of RyR and InsP3R activity but interestingly, some of the complexities and controversies surrounding phosphorylation of RyRs also apply to InsP3Rs, and a clear-cut effect of CaMKII on either channel eludes investigators for now. Nevertheless, some effects of CaMKII on global cellular activity, such as SR Ca(2+) leak or force-frequency potentiation, appear clear now, and this constrains the limits of the controversies and permits a more tractable approach to elucidate the effects of phosphorylation at the single channel level.

Modeling CaMKII-mediated regulation of L-type Ca(2+) channels and ryanodine receptors in the heart

Excitation-contraction coupling (ECC) in the cardiac myocyte is mediated by a number of highly integrated mechanisms of intracellular Ca²⁺ transport. Voltage- and Ca²⁺-dependent L-type Ca²⁺ channels (LCCs) allow for Ca²⁺ entry into the myocyte, which then binds to nearby ryanodine receptors (RyRs) and triggers Ca²⁺ release from the sarcoplasmic reticulum in a process known as Ca²⁺-induced Ca²⁺ release. The highly coordinated Ca²⁺-mediated interaction between LCCs and RyRs is further regulated by the cardiac isoform of the Ca²⁺/calmodulin-dependent protein kinase (CaMKII). Because CaMKII targets and modulates the function of many ECC proteins, elucidation of its role in ECC and integrative cellular function is challenging and much insight has been gained through the use of detailed computational models. Multiscale models that can both reconstruct the detailed nature of local signaling events within the cardiac dyad and predict their functional consequences at the level of the whole cell have played an important role in advancing our understanding of CaMKII function in ECC. Here, we review experimentally based models of CaMKII function with a focus on LCC and RyR regulation, and the mechanistic insights that have been gained through their application.

Role of inhibitory autophosphorylation of calcium/calmodulin-dependent kinase II (αCAMKII) in persistent (>24 h) hippocampal LTP and in LTD facilitated by novel object-place learning and recognition in mice

Experience-dependent synaptic plasticity is widely expressed in the mammalian brain and is believed to underlie memory formation. Persistent forms of synaptic plasticity in the hippocampus, such as long-term potentiation (LTP) and long-term depression (LTD) are particularly of interest, as evidence is accumulating that they are expressed as a consequence of, or at the very least in association with, hippocampus-dependent novel learning events. Learning-facilitated plasticity describes the property of hippocampal synapses to express persistent synaptic plasticity when novel spatial learning is combined with afferent stimulation that is subthreshold for induction of changes in synaptic strength. In mice it occurs following novel object recognition and novel object-place recognition. Calmodulin-dependent kinase II (CAMKII) is strongly expressed in synapses and has been shown to be required for hippocampal LTP in vitro and for spatial learning in the water maze. Here, we show that in mice that undergo persistent inhibitory autophosphorylation of αCAMKII, object-place learning is intact. Furthermore, these animals demonstrate a higher threshold for induction of persistent (>24 h) hippocampal LTP in the hippocampal CA1 region during unrestrained behaviour. The transgenic mice also express short-term depression in response to afferent stimulation frequencies that are ineffective in controls. Furthermore, they express stronger LTD in response to novel learning of spatial configurations compared to controls. These findings support that modulation of αCAMKII activity via autophosphorylation at the Thr305/306 site comprises a key mechanism for the maintenance of synaptic plasticity within a dynamic range. They also indicate that a functional differentiation occurs in the way spatial information is encoded: whereas LTP is likely to be critically involved in the encoding of space per se, LTD appears to play a special role in the encoding of the content or features of space.

Genetic ablation of ryanodine receptor 2 phosphorylation at Ser-2808 aggravates Ca(2+)-dependent cardiomyopathy by exacerbating diastolic Ca2+ release

Phosphorylation of the cardiac ryanodine receptor (RyR2) by protein kinase A (PKA) at Ser-2808 is suggested to mediate the physiological 'fight or flight' response and contribute to heart failure by rendering the sarcoplasmic reticulum (SR) leaky for Ca(2+). In the present study, we examined the potential role of RyR2 phosphorylation at Ser-2808 in the progression of Ca(2+)-dependent cardiomyopathy (CCM) by using mice genetically modified to feature elevated SR Ca(2+) leak while expressing RyR2s that cannot be phosphorylated at this site (S2808A). Surprisingly, rather than alleviating the disease phenotype, constitutive dephosphorylation of Ser-2808 aggravated CCM as manifested by shortened survival, deteriorated in vivo cardiac function, exacerbated SR Ca(2+) leak and mitochondrial injury. Notably, the deteriorations of cardiac function, myocyte Ca(2+) handling, and mitochondria integrity were consistently worse in mice with heterozygous ablation of Ser-2808 than in mice with complete ablation. Wild-type (WT) and CCM myocytes expressing unmutated RyR2s exhibited a high level of baseline phosphorylation at Ser-2808. Exposure of these CCM cells to protein phosphatase 1 caused a transitory increase in Ca(2+) leak attributable to partial dephosphorylation of RyR2 tetramers at Ser-2808 from more fully phosphorylated state. Thus, exacerbated Ca(2+) leak through partially dephosphorylated RyR2s accounts for the prevalence of the disease phenotype in the heterozygous S2808A CCM mice. These results do not support the importance of RyR2 hyperphosphorylation in Ca(2+)-dependent heart disease, and rather suggest roles for the opposite process, the RyR2 dephosphorylation at this residue in physiological and pathophysiological Ca(2+) signalling.

Cardiac sarcoplasmic reticulum calcium leak: basis and roles in cardiac dysfunction

Synchronized SR calcium (Ca) release is critical to normal cardiac myocyte excitation-contraction coupling, and ideally this release shuts off completely between heartbeats. However, other SR Ca release events are referred to collectively as SR Ca leak (which includes Ca sparks and waves as well as smaller events not detectable as Ca sparks). Much, but not all, of the SR Ca leak occurs via ryanodine receptors and can be exacerbated in pathological states such as heart failure. The extent of SR Ca leak is important because it can (a) reduce SR Ca available for release, causing systolic dysfunction; (b) elevate diastolic [Ca]i, contributing to diastolic dysfunction; (c) cause triggered arrhythmias; and (d) be energetically costly because of extra ATP used to repump Ca. This review addresses quantitative aspects and manifestations of SR Ca leak and its measurement, and how leak is modulated by Ca, associated proteins, and posttranslational modifications in health and disease.

Integrins protect cardiomyocytes from ischemia/reperfusion injury

Ischemic damage is recognized to cause cardiomyocyte (CM) death and myocardial dysfunction, but the role of cell-matrix interactions and integrins in this process has not been extensively studied. Expression of α7β1D integrin, the dominant integrin in normal adult CMs, increases during ischemia/reperfusion (I/R), while deficiency of β1 integrins increases ischemic damage. We hypothesized that the forced overexpression of integrins on the CM would offer protection from I/R injury. Tg mice with CM-specific overexpression of integrin α7β1D exposed to I/R had a substantial reduction in infarct size compared with that of α5β1D-overexpressing mice and WT littermate controls. Using isolated CMs, we found that α7β1D preserved mitochondrial membrane potential during hypoxia/reoxygenation (H/R) injury via inhibition of mitochondrial Ca2+ overload but did not alter H/R effects on oxidative stress. Therefore, we assessed Ca2+ handling proteins in the CM and found that β1D integrin colocalized with ryanodine receptor 2 (RyR2) in CM T-tubules, complexed with RyR2 in human and rat heart, and specifically bound to RyR2 amino acids 165-175. Integrins stabilized the RyR2 interdomain interaction, and this stabilization required integrin receptor binding to its ECM ligand. These data suggest that α7β1D integrin modifies Ca2+ regulatory pathways and offers a means to protect the myocardium from ischemic injury.

Rate-dependent Ca2+ signalling underlying the force-frequency response in rat ventricular myocytes: a coupled electromechanical modeling study

BACKGROUND

Rate-dependent effects on the Ca2+ sub-system in a rat ventricular myocyte are investigated. Here, we employ a deterministic mathematical model describing various Ca2+ signalling pathways under voltage clamp (VC) conditions, to better understand the important role of calmodulin (CaM) in modulating the key control variables Ca2+/calmodulin-dependent protein kinase-II (CaMKII), calcineurin (CaN), and cyclic adenosine monophosphate (cAMP) as they affect various intracellular targets. In particular, we study the frequency dependence of the peak force generated by the myofilaments, the force-frequency response (FFR).

METHODS

Our cell model incorporates frequency-dependent CaM-mediated spatially heterogenous interaction of CaMKII and CaN with their principal targets (dihydropyridine (DHPR) and ryanodine (RyR) receptors and the SERCA pump). It also accounts for the rate-dependent effects of phospholamban (PLB) on the SERCA pump; the rate-dependent role of cAMP in up-regulation of the L-type Ca2+ channel (ICa,L); and the enhancement in SERCA pump activity via phosphorylation of PLB.

RESULTS

Our model reproduces positive peak FFR observed in rat ventricular myocytes during voltage-clamp studies both in the presence/absence of cAMP mediated β-adrenergic stimulation. This study provides quantitative insight into the rate-dependence of Ca2+-induced Ca2+-release (CICR) by investigating the frequency-dependence of the trigger current (ICa,L) and RyR-release. It also highlights the relative role of the sodium-calcium exchanger (NCX) and the SERCA pump at higher frequencies, as well as the rate-dependence of sarcoplasmic reticulum (SR) Ca2+ content. A rigorous Ca2+ balance imposed on our investigation of these Ca2+ signalling pathways clarifies their individual roles. Here, we present a coupled electromechanical study emphasizing the rate-dependence of isometric force developed and also investigate the temperature-dependence of FFR.

CONCLUSIONS

Our model provides mechanistic biophysically based explanations for the rate-dependence of CICR, generating useful and testable hypotheses. Although rat ventricular myocytes exhibit a positive peak FFR in the presence/absence of beta-adrenergic stimulation, they show a characteristic increase in the positive slope in FFR due to the presence of Norepinephrine or Isoproterenol. Our study identifies cAMP-mediated stimulation, and rate-dependent CaMKII-mediated up-regulation of ICa,L as the key mechanisms underlying the aforementioned positive FFR.

The promise of CaMKII inhibition for heart disease: preventing heart failure and arrhythmias

INTRODUCTION

Calcium-calmodulin-dependent protein kinase II (CaMKII) has emerged as a central mediator of cardiac stress responses which may serve several critical roles in the regulation of cardiac rhythm, cardiac contractility and growth. Sustained and excessive activation of CaMKII during cardiac disease has, however, been linked to arrhythmias, and maladaptive cardiac remodeling, eventually leading to heart failure (HF) and sudden cardiac death.

AREAS COVERED

In the current review, the authors describe the unique structural and biochemical properties of CaMKII and focus on its physiological effects in cardiomyocytes. Furthermore, they provide evidence for a role of CaMKII in cardiac pathologies, including arrhythmogenesis, myocardial ischemia and HF development. The authors conclude by discussing the potential for CaMKII as a target for inhibition in heart disease.

EXPERT OPINION

CaMKII provides a promising nodal point for intervention that may allow simultaneous prevention of HF progression and development of arrhythmias. For future studies and drug development there is a strong rationale for the development of more specific CaMKII inhibitors. In addition, an improved understanding of the differential roles of CaMKII subtypes is required.

ß-Adrenergic stimulation increases RyR2 activity via intracellular Ca2+ and Mg2+ regulation

Here we investigate how ß-adrenergic stimulation of the heart alters regulation of ryanodine receptors (RyRs) by intracellular Ca²⁺ and Mg²⁺ and the role of these changes in SR Ca²⁺ release. RyRs were isolated from rat hearts, perfused in a Langendorff apparatus for 5 min and subject to 1 min perfusion with 1 µM isoproterenol or without (control) and snap frozen in liquid N₂ to capture their phosphorylation state. Western Blots show that RyR2 phosphorylation was increased by isoproterenol, confirming that RyR2 were subject to normal ß-adrenergic signaling. Under basal conditions, S2808 and S2814 had phosphorylation levels of 69% and 15%, respectively. These levels were increased to 83% and 60%, respectively, after 60 s of ß-adrenergic stimulation consistent with other reports that ß-adrenergic stimulation of the heart can phosphorylate RyRs at specific residues including S2808 and S2814 causing an increase in RyR activity. At cytoplasmic [Ca²⁺] <1 µM, ß-adrenergic stimulation increased luminal Ca²⁺ activation of single RyR channels, decreased luminal Mg²⁺ inhibition and decreased inhibition of RyRs by mM cytoplasmic Mg²⁺. At cytoplasmic [Ca²⁺] >1 µM, ß-adrenergic stimulation only decreased cytoplasmic Mg²⁺ and Ca²⁺ inhibition of RyRs. The K_a and maximum levels of cytoplasmic Ca²⁺ activation site were not affected by ß-adrenergic stimulation.

Our RyR2 gating model was fitted to the single channel data. It predicted that in diastole, ß-adrenergic stimulation is mediated by 1) increasing the activating potency of Ca²⁺ binding to the luminal Ca²⁺ site and decreasing its affinity for luminal Mg²⁺ and 2) decreasing affinity of the low-affinity Ca²⁺/Mg²⁺ cytoplasmic inhibition site. However in systole, ß-adrenergic stimulation is mediated mainly by the latter.

Lamotrigine increases intracellular Ca(2+) levels and Ca(2+)/calmodulin-dependent kinase II activation in mouse dorsal root ganglion neurones

AIM

Lamotrigine is a neuroprotective agent that is used clinically for the treatment of seizures and neuropathic pain. A significant volume of literature has reported that lamotrigine exerts analgesic effect by blocking Ca(2+) channels. However, little is known regarding the effect of lamotrigine on the intracellular Ca(2+) concentration ([Ca(2+)](i)). The aim of this study was to determine whether lamotrigine modulates [Ca(2+)](i) in sensory neurones.

METHODS

Lamotrigine-induced changes in [Ca(2+)](i) were measured in mouse dorsal root ganglion (DRG) neurones using the Ca(2+)-sensitive fluorescent indicator Fluo 3-AM and a confocal laser scanning microscope. Ca(2+)/calmodulin-dependent kinase II (CaMKII) activation was assessed by the fluorescence intensity using immunocytochemical procedures.

RESULTS

Treatment with 1, 10, 30 or 100 μM lamotrigine transiently increased [Ca(2+)](i) in DRG neurones in a dose-dependent manner. Treatment with 100 μM lamotrigine induced a significant (threefold) increase in the Ca(2+) peak in the presence or absence of extracellular Ca(2+). The lamotrigine-induced Ca(2+) increase was abolished or decreased by the treatment with a specific PLC inhibitor (U73122), IP3R antagonist (xestospongin C) or RyR antagonist (dantrolene). In some cells, treatment with 100 μM lamotrigine caused a transient Ca(2+) increase, and the Ca(2+) levels quickly fell to below the basal Ca(2+) level observed prior to lamotrigine application. The decrease in basal Ca(2+) levels was blocked by the treatment with a CaMKII inhibitor (KN93). Immunocytochemical analysis indicated that lamotrigine treatment increased the expression of phosphorylated CaMKII in DRG neurones.

CONCLUSION

Treatment with lamotrigine increased [Ca(2+)](i) apparently as a result of Ca(2+) release from intracellular stores and CaMKII activity.

Calcium flux balance in the heart

This article reviews the consequences of the need for the cardiac cell to be in calcium flux balance in the steady state. We first discuss how this steady state condition affects the control of resting [Ca(2+)]i. The next section considers how sarcoplasmic reticulum (SR) Ca content is controlled by a feedback mechanism whereby changes of SR Ca affect the amplitude of the Ca transient and this, in turn, controls sarcolemmal Ca fluxes. Subsequent sections review the effects of altering the activity of individual Ca handling proteins. Increasing the activity of the SR Ca-ATPase (SERCA) increases both the amplitude and rate constant of decay of the systolic Ca transient. The Ca flux balance condition requires that this must be achieved with no change of Ca efflux placing constraints on the magnitude of change of amplitude and decay rate. We analyze the quantitative dependence of Ca transient amplitude and SR content on SERCA activity. Increasing the open probability of the RyR during systole is predicted to have no steady state effect on the amplitude of the systolic Ca transient. We discuss the effects of changing the amplitude of the L-type Ca current in the context of both triggering Ca release from the SR and loading the cell with calcium. These manoeuvres are considered in the context of the effects of β-adrenergic stimulation. Finally, we review calcium flux balance in the presence of Ca waves.

Posttranslational modifications of cardiac ryanodine receptors: Ca(2+) signaling and EC-coupling

In cardiac muscle, a number of posttranslational protein modifications can alter the function of the Ca(2+) release channel of the sarcoplasmic reticulum (SR), also known as the ryanodine receptor (RyR). During every heartbeat RyRs are activated by the Ca(2+)-induced Ca(2+) release mechanism and contribute a large fraction of the Ca(2+) required for contraction. Some of the posttranslational modifications of the RyR are known to affect its gating and Ca(2+) sensitivity. Presently, research in a number of laboratories is focused on RyR phosphorylation, both by PKA and CaMKII, or on RyR modifications caused by reactive oxygen and nitrogen species (ROS/RNS). Both classes of posttranslational modifications are thought to play important roles in the physiological regulation of channel activity, but are also known to provoke abnormal alterations during various diseases. Only recently it was realized that several types of posttranslational modifications are tightly connected and form synergistic (or antagonistic) feed-back loops resulting in additive and potentially detrimental downstream effects. This review summarizes recent findings on such posttranslational modifications, attempts to bridge molecular with cellular findings, and opens a perspective for future work trying to understand the ramifications of crosstalk in these multiple signaling pathways. Clarifying these complex interactions will be important in the development of novel therapeutic approaches, since this may form the foundation for the implementation of multi-pronged treatment regimes in the future. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

CaMKIIδC slows [Ca]i decline in cardiac myocytes by promoting Ca sparks

Acute activation of calcium/calmodulin-dependent protein kinase (CaMKII) in permeabilized phospholamban knockout (PLN-KO) mouse myocytes phosphorylates ryanodine receptors (RyRs) and activates spontaneous local sarcoplasmic reticulum (SR) Ca release events (Ca sparks) even at constant SR Ca load. To assess how CaMKII regulates SR Ca release in intact myocytes (independent of SR Ca content changes or PLN effects), we compared Ca sparks in PLN-KO versus mice, which also have transgenic cardiac overexpression of CaMKIIδC in the PLN-KO background (KO/TG). Compared with PLN-KO mice, these KO/TG cardiomyocytes exhibited 1), increased twitch Ca transient and fractional release (both by ∼35%), but unaltered SR Ca load; 2), increased resting Ca spark frequency (300%) despite a lower diastolic [Ca]i, which also slowed twitch [Ca]i decline (suggesting CaMKII-dependent RyR Ca sensitization); 3), elevated Ca spark amplitude and rate of Ca release (which might indicate that more RyR channels participate in a single spark); 4), prolonged Ca spark rise time (which implies that CaMKII either delays RyR closure or prolongs the time when openings can occur); 5), more frequent repetitive sparks at single release sites. Analysis of repetitive sparks from individual Ca release sites indicates that CaMKII enhanced RyR Ca sensitivity, but did not change the time course of SR Ca refilling. These results demonstrate that there are dramatic CaMKII-mediated effects on RyR Ca release that occur via regulation of both RyR activation and termination processes.

Calcium signaling in cardiac myocytes

Calcium (Ca(2+)) is a critical regulator of cardiac myocyte function. Principally, Ca(2+) is the link between the electrical signals that pervade the heart and contraction of the myocytes to propel blood. In addition, Ca(2+) controls numerous other myocyte activities, including gene transcription. Cardiac Ca(2+) signaling essentially relies on a few critical molecular players--ryanodine receptors, voltage-operated Ca(2+) channels, and Ca(2+) pumps/transporters. These moieties are responsible for generating Ca(2+) signals upon cellular depolarization, recovery of Ca(2+) signals following cellular contraction, and setting basal conditions. Whereas these are the central players underlying cardiac Ca(2+) fluxes, networks of signaling mechanisms and accessory proteins impart complex regulation on cardiac Ca(2+) signals. Subtle changes in components of the cardiac Ca(2+) signaling machinery, albeit through mutation, disease, or chronic alteration of hemodynamic demand, can have profound consequences for the function and phenotype of myocytes. Here, we discuss mechanisms underlying Ca(2+) signaling in ventricular and atrial myocytes. In particular, we describe the roles and regulation of key participants involved in Ca(2+) signal generation and reversal.

Ca²+-dependent phosphorylation of RyR2 can uncouple channel gating from direct cytosolic Ca²+ regulation

Phosphorylation of the cardiac ryanodine receptor (RyR2) is thought to be important not only for normal cardiac excitation-contraction coupling but also in exacerbating abnormalities in Ca²+ homeostasis in heart failure. Linking phosphorylation to specific changes in the single-channel function of RyR2 has proved very difficult, yielding much controversy within the field. We therefore investigated the mechanistic changes that take place at the single-channel level after phosphorylating RyR2 and, in particular, the idea that PKA-dependent phosphorylation increases RyR2 sensitivity to cytosolic Ca²+. We show that hyperphosphorylation by exogenous PKA increases open probability (P(o)) but, crucially, RyR2 becomes uncoupled from the influence of cytosolic Ca²+; lowering [Ca²+] to subactivating levels no longer closes the channels. Phosphatase (PP1) treatment reverses these gating changes, returning the channels to a Ca²+-sensitive mode of gating. We additionally found that cytosolic incubation with Mg²+/ATP in the absence of exogenously added kinase could phosphorylate RyR2 in approximately 50% of channels, thereby indicating that an endogenous kinase incorporates into the bilayer together with RyR2. Channels activated by the endogenous kinase exhibited identical changes in gating behavior to those activated by exogenous PKA, including uncoupling from the influence of cytosolic Ca²+. We show that the endogenous kinase is both Ca²+-dependent and sensitive to inhibitors of PKC. Moreover, the Ca²+-dependent, endogenous kinase-induced changes in RyR2 gating do not appear to be related to phosphorylation of serine-2809. Further work is required to investigate the identity and physiological role of this Ca²+-dependent endogenous kinase that can uncouple RyR2 gating from direct cytosolic Ca²+ regulation.

In the RyR2(R4496C) mouse model of CPVT, β-adrenergic stimulation induces Ca waves by increasing SR Ca content and not by decreasing the threshold for Ca waves

RATIONALE

mutations of the ryanodine receptor (RyR) cause catecholaminergic polymorphic ventricular tachycardia (CPVT). These mutations predispose to the generation of Ca waves and delayed afterdepolarizations during adrenergic stimulation. Ca waves occur when either sarcoplasmic reticulum (SR) Ca content is elevated above a threshold or the threshold is decreased. Which of these occurs in cardiac myocytes expressing CPVT mutations is unknown.

OBJECTIVE

we tested whether the threshold SR Ca content is different between control and CPVT and how it relates to SR Ca content during β-adrenergic stimulation.

METHODS AND RESULTS

ventricular myocytes from the RyR2 R4496C(+/-) mouse model of CPVT and wild-type (WT) controls were voltage-clamped; diastolic SR Ca content was measured and compared with the Ca wave threshold. The results showed the following. (1) In 1 mmol/L [Ca(2+)](o), β-adrenergic stimulation with isoproterenol (1μmol/L) caused Ca waves only in R4496C. (2) SR Ca content and Ca wave threshold in R4496C were lower than those in WT. (3) β-Adrenergic stimulation increased SR Ca content by a similar amount in both R4496C and WT. (4) β-Adrenergic stimulation increased the threshold for Ca waves. (5) During β-adrenergic stimulation in R4496C, but not WT, the increase of SR Ca was sufficient to reach threshold and produce Ca waves.

CONCLUSIONS

in the R4496C CPVT model, the RyR is leaky, and this lowers both SR Ca content and the threshold for waves. β-Adrenergic stimulation produces Ca waves by increasing SR Ca content and not by lowering threshold.

Calcium/calmodulin-dependent kinase II regulation of cardiac ion channels

Calcium/calmodulin-dependent kinase II (CaMKII) is a multifunctional serine/threonine kinase expressed abundantly in the heart. CaMKII targets numerous proteins involved in excitation-contraction coupling and excitability, and its activation may simultaneously contribute to heart failure and cardiac arrhythmias. In this review, we summarize the modulatory effects of CaMKII on cardiac ion channel function and expression and illustrate potential implications in the onset of arrhythmias via a computer model.

Role of CaMKII in RyR leak, EC coupling and action potential duration: a computational model

During heart failure, the ability of the sarcoplasmic reticulum (SR) to store Ca(2+) is severely impaired resulting in abnormal Ca(2+) cycling and excitation-contraction (EC) coupling. Recently, it has been proposed that "leaky" ryanodine receptors (RyRs) contribute to diminished Ca(2+) levels in the SR. Various groups have experimentally investigated the effects of RyR phosphorylation mediated by Ca(2+)/calmodulin-dependent kinase II (CaMKII) on RyR behavior. Some of these results are difficult to interpret since RyR gating is modulated by many external proteins and ions, including Ca(2+). Here, we present a mathematical model representing CaMKII-RyR interaction in the canine ventricular myocyte. This is an extension of our previous model which characterized CaMKII phosphorylation of L-type Ca(2+) channels (LCCs) in the cardiac dyad. In this model, it is assumed that upon phosphorylation, RyR Ca(2+)-sensitivity is increased. Individual RyR phosphorylation is modeled as a function of dyadic CaMKII activity, which is modulated by local Ca(2+) levels. The model is constrained by experimental measurements of Ca(2+) spark frequency and steady state RyR phosphorylation. It replicates steady state RyR (leak) fluxes in the range measured in experiments without the addition of a separate passive leak pathway. Simulation results suggest that under physiological conditions, CaMKII phosphorylation of LCCs ultimately has a greater effect on RyR flux as compared with RyR phosphorylation. We also show that phosphorylation of LCCs decreases EC coupling gain significantly and increases action potential duration. These results suggest that LCC phosphorylation sites may be a more effective target than RyR sites in modulating diastolic RyR flux.

Increased Ca(2+) leak and spatiotemporal coherence of Ca(2+) release in cardiomyocytes during beta-adrenergic stimulation

beta-Adrenergic receptor (beta-AR) stimulation of cardiac muscle has been proposed to enhance Ca(2+) release from the sarcoplasmic reticulum (SR) through ryanodine receptors (RyRs). However, the anticipated increase in RyR Ca(2+) sensitivity has proven difficult to study in intact cardiomyocytes, due to accompanying alterations in SR Ca(2+) content, inward Ca(2+) current (I(Ca)) and diastolic cytosolic Ca(2+) concentration ([Ca(2+)](i)). Here, we studied whole-cell Ca(2+) release and spontaneous Ca(2+) leak (Ca(2+) sparks) in guinea-pig ventricular myocytes with confocal Ca(2+) imaging before and during beta-AR stimulation by isoproterenol (Iso), but under otherwise nearly identical experimental conditions. The extent of SR Ca(2+) loading was controlled under whole-cell voltage-clamp conditions. UV flash-induced uncaging of Ca(2+) from DM-nitrophen was employed as an invariant trigger for whole-cell Ca(2+) release. At matched SR Ca(2+) content, we found that Iso enhanced the spatiotemporal coherence of whole-cell Ca(2+) release, evident from spatially intercorrelated release and accelerated release kinetics that resulted in moderately (20%) increased release amplitude. This may arise from higher RyR Ca(2+) sensitivity, and was also reflected in spontaneous SR Ca(2+) leak. At comparable SR Ca(2+) content and cytosolic [Ca(2+)](i), we observed an approximately 4-fold increase in Ca(2+) spark frequency in Iso that also appeared in quiescent cells within 2 min without increased SR Ca(2+) content. This was likely to have been mediated by Ca(2+)/calmodulin-dependent protein kinase (CaMKII), rather than cAMP dependent protein kinase (PKA). We conclude that Iso increases the propensity of RyRs to open, both in response to rapid elevations of [Ca(2+)](i) and at diastolic [Ca(2+)](i). While this could be beneficial in enhancing and synchronizing systolic whole-cell SR Ca(2+) release, the same behaviour could also be proarrhythmogenic during diastole.

Beta-adrenergic signaling accelerates and synchronizes cardiac ryanodine receptor response to a single L-type Ca2+ channel

As the most prototypical G protein-coupled receptor, beta-adrenergic receptor (betaAR) regulates the pace and strength of heart beating by enhancing and synchronizing L-type channel (LCC) Ca(2+) influx, which in turn elicits greater sarcoplasmic reticulum (SR) Ca(2+) release flux via ryanodine receptors (RyRs). However, whether and how betaAR-protein kinase A (PKA) signaling directly modulates RyR function remains elusive and highly controversial. By using unique single-channel Ca(2+) imaging technology, we measured the response of a single RyR Ca(2+) release unit, in the form of a Ca(2+) spark, to its native trigger, the Ca(2+) sparklet from a single LCC. We found that acute application of the selective betaAR agonist isoproterenol (1 microM, < or = 20 min) increased triggered spark amplitude in an LCC unitary current-independent manner. The increased ratio of Ca(2+) release flux underlying a Ca(2+) spark to SR Ca(2+) content indicated that betaAR stimulation helps to recruit additional RyRs in synchrony. Quantification of sparklet-spark kinetics showed that betaAR stimulation synchronized the stochastic latency and increased the fidelity (i.e., chance of hit) of LCC-RyR intermolecular signaling. The RyR modulation was independent of the increased SR Ca(2+) content. The PKA antagonists Rp-8-CPT-cAMP (100 microM) and H89 (10 microM) both eliminated these effects, indicating that betaAR acutely modulates RyR activation via the PKA pathway. These results demonstrate unequivocally that RyR activation by a single LCC is accelerated and synchronized during betaAR stimulation. This molecular mechanism of sympathetic regulation will permit more fundamental studies of altered betaAR effects in cardiovascular diseases.

Calcium-Induced Calcium Release in Skeletal Muscle

Calcium-induced calcium release (CICR) was first discovered in skeletal muscle. CICR is defined as Ca2+ release by the action of Ca2+ alone without the simultaneous action of other activating processes. CICR is biphasically dependent on Ca2+ concentration; is inhibited by Mg2+, procaine, and tetracaine; and is potentiated by ATP, other adenine compounds, and caffeine. With depolarization of the sarcoplasmic reticulum (SR), a potential change of the SR membrane in which the luminal side becomes more negative, CICR is activated for several seconds and is then inactivated. All three types of ryanodine receptors (RyRs) show CICR activity. At least one RyR, RyR1, also shows non-CICR Ca2+ release, such as that triggered by the t-tubule voltage sensor, by clofibric acid, and by SR depolarization. Maximum rates of CICR, at the optimal Ca2+ concentration in the presence of physiological levels of ATP and Mg2+ determined in skinned fibers and fragmented SR, are much lower than the rate of physiological Ca2+ release. The primary event of physiological Ca2+ release, the Ca2+ spark, is the simultaneous opening of multiple channels, the coordinating mechanism of which does not appear to be CICR because of the low probability of CICR opening under physiological conditions. The coordination may require Ca2+, but in that case, some other stimulus or stimuli must be provided simultaneously, which is not CICR by definition. Thus CICR does not appear to contribute significantly to physiological Ca2+ release. On the other hand, CICR appears to play a key role in caffeine contracture and malignant hyperthermia. The potentiation of voltage-activated Ca2+ release by caffeine, however, does not seem to occur through secondary CICR, although the site where caffeine potentiates voltage-activated Ca2+ release might be the same site where caffeine potentiates CICR.

Ryanodine receptor-mediated arrhythmias and sudden cardiac death

The cardiac ryanodine receptor-Ca²⁺ release channel (RyR2) is an essential sarcoplasmic reticulum (SR) transmembrane protein that plays a central role in excitation–contraction coupling (ECC) in cardiomyocytes. Aberrant spontaneous, diastolic Ca²⁺ leak from the SR due to dysfunctional RyR2 contributes to the formation of delayed after-depolarisations, which are thought to underlie the fatal arrhythmia that occurs in both heart failure (HF) and in catecholaminergic polymorphic ventricular tachycardia (CPVT). CPVT is an inherited disorder associated with mutations in either the RyR2 or a SR luminal protein, calsequestrin. RyR2 shows normal function at rest in CPVT but the RyR2 dysfunction is unmasked by physical exercise or emotional stress, suggesting abnormal RyR2 activation as an underlying mechanism. Several potential mechanisms have been advanced to explain the dysfunctional RyR2 observed in HF and CPVT, including enhanced RyR2 phosphorylation status, altered RyR2 regulation at luminal/cytoplasmic sites and perturbed RyR2 intra/inter-molecular interactions. This review considers RyR2 dysfunction in the context of the structural and functional modulation of the channel, and potential therapeutic strategies to stabilise RyR2 function in cardiac pathology.

CaMKII-induced shift in modal gating explains L-type Ca(2+) current facilitation: a modeling study

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) plays an important role in L-type Ca(2+) channel (LCC) facilitation: the Ca(2+)-dependent augmentation of Ca(2+) current (I(CaL)) exhibited during rapid repeated depolarization. Multiple mechanisms may underlie facilitation, including an increased rate of recovery from Ca(2+)-dependent inactivation and a shift in modal gating distribution from mode 1, the dominant mode of LCC gating, to mode 2, a mode in which openings are prolonged. We hypothesized that the primary mechanism underlying facilitation is the shift in modal gating distribution resulting from CaMKII-mediated LCC phosphorylation. We developed a stochastic model describing the dynamic interactions among CaMKII, LCCs, and phosphatases as a function of dyadic Ca(2+) and calmodulin levels, and we incorporated it into an integrative model of the canine ventricular myocyte. The model reproduces behaviors at physiologic protein levels and allows for dynamic transition between modes, depending on the LCC phosphorylation state. Simulations showed that a CaMKII-dependent shift in LCC distribution toward mode 2 accounted for the I(CaL) positive staircase. Moreover, simulations demonstrated that experimentally observed changes in LCC inactivation and recovery kinetics may arise from modal gating shifts, rather than from changes in intrinsic inactivation properties. The model therefore serves as a powerful tool for interpreting I(CaL) experiments.

Arrhythmia mechanisms in the failing heart

BACKGROUND

Heart failure (HF) claims over 200,000 lives annually in the United States alone. Approximately 50% of these deaths are sudden and unexpected, and presumably the consequence of lethal ventricular tachyarrhythmias. Electrical remodeling that occurs at the cellular and tissue network levels predisposes patients with HF to malignant arrhythmias. Our limited understanding of fundamental arrhythmia mechanisms has hampered the development of effective treatment strategies for these patients.

METHODS AND CONCLUSIONS

In this review, we outline recent advances in our understanding of arrhythmia mechanisms in the failing heart, highlighting various aspects of remodeling of ion channels, calcium handling proteins, and gap junction-related molecules. As will be discussed, these changes promote the prolongation of the action potential, the enhancement of spatio-temporal gradients of repolarization, the formation of calcium-mediated triggers and conduction abnormalities, all of which combine to form an electrophysiological substrate that is ripe for the genesis of lethal arrhythmias and sudden cardiac death.

Expression of active p21-activated kinase-1 induces Ca2+ flux modification with altered regulatory protein phosphorylation in cardiac myocytes

p21-Activated kinase-1 (Pak1) is a serine-threonine kinase that associates with and activates protein phosphatase 2A in adult ventricular myocytes and, thereby, induces increased Ca2+ sensitivity of skinned-fiber tension development mediated by dephosphorylation of myofilament proteins (Ke Y, Wang L, Pyle WG, de Tombe PP, Solaro RJ. Circ Res 94: 194-200, 2004). We test the hypothesis that activation of Pak1 also moderates cardiac contractility through regulation of intracellular Ca2+ fluxes. We found no difference in field-stimulated intracellular Ca2+ concentration ([Ca2+]i) transient amplitude and extent of cell shortening between myocytes expressing constitutively active Pak1 (CA-Pak1) and controls expressing LacZ; however, time to peak shortening was significantly faster and rate of [Ca2+]i decay and time of relengthening were slower. Neither caffeine-releasable sarcoplasmic reticulum (SR) Ca2+ content nor fractional release was different in CA-Pak1 myocytes compared with controls. Isoproterenol application revealed a significantly blunted increase in [Ca2+]i transient amplitude, as well as a slowed rate of [Ca2+]i decay, increased SR Ca2+ content, and increased cell shortening, in CA-Pak1 myocytes. We found no significant change in phospholamban phosphorylation at Ser16 or Thr17 in CA-Pak1 myocytes. Analysis of cardiac troponin I revealed a significant reduction in phosphorylated species that are primarily attributable to Ser(23/24) in CA-Pak1 myocytes. Nonstimulated, spontaneous SR Ca2+ release sparks were significantly smaller in amplitude in CA-Pak1 than LacZ myocytes. Propagation of spontaneous Ca2+ waves resulting from SR Ca2+ overload was significantly slower in CA-Pak1 myocytes. Our data indicate that CA-Pak1 expression has significant effects on ventricular myocyte contractility through altered myofilament Ca2+ sensitivity and modification of the [Ca2+]i transient.

CaMKII and its role in cardiac arrhythmia

Calcium/calmodulin-dependent kinase II (CaMKII) is a multifunctional serine/threonine kinase widely distributed in a number of tissue types. Activation of CaMKII has been linked to important downstream physiological processes, including apoptosis, hypertrophy, and arrhythmia in the heart. Pharmacological or genetic inhibition of CaMKII has been shown to improve health outcomes in a number of animal models. In this review, we summarize the structural and functional properties of CaMKII and detail its role in cardiac arrhythmia, structural heart disease, and sudden death.

Dysregulated sarcoplasmic reticulum calcium release: potential pharmacological target in cardiac disease

In the heart, Ca(2+) released from the intracellular Ca(2+) storage site, the sarcoplasmic reticulum (SR), is the principal determinant of cardiac contractility. SR Ca(2+) release is controlled by dedicated molecular machinery, composed of the cardiac ryanodine receptor (RyR2) and a number of accessory proteins, including FKBP12.6, calsequestrin (CASQ2), triadin (TRD) and junctin (JN). Acquired and genetic defects in the components of the release channel complex result in a spectrum of abnormal Ca(2+) release phenotypes ranging from arrhythmogenic spontaneous Ca(2+) releases and Ca(2+) alternans to the uniformly diminished systolic Ca(2+) release characteristic of heart failure. In this article, we will present an overview of the structure and molecular components of the SR and Ca(2+) release machinery and its modulation by different intracellular factors, such as Ca(2+) levels inside the SR as well as phosphorylation and redox modification of RyR2s. We will also discuss the relationships between abnormal SR Ca(2+) release and various cardiac disease phenotypes, including, arrhythmias and heart failure, and consider SR Ca(2+) release as a potential therapeutic target.

The mitochondrial ryanodine receptor in rat heart: A pharmaco-kinetic profile

A protein discovered within inner mitochondrial membranes (IMM), designated as the mitochondrial ryanodine receptor (mRyR), has been recognized recently as a modulator of Ca(2+) fluxes in mitochondria. The present study provides fundamental pharmacological and electrophysiological properties of this mRyR. Rat cardiac IMM fused to lipid bilayers revealed the presence of a mitochondrial channel with gating characteristics similar to those of classical sarcoplasmic reticulum RyR (SR-RyR), but a variety of other mitochondrial channels obstructed clean recordings. Mitochondrial vesicles were thus solubilized and subjected to sucrose sedimentation to obtain mRyR-enriched fractions. Reconstitution of sucrose-purified fractions into lipid bilayers yielded Cs(+)-conducting, Ca(2+)-sensitive, large conductance (500-800 pS) channels with signature properties of SR-RyRs. Cytosolic Ca(2+) increased the bursting frequency and mean open time of the channel. Micromolar concentrations of ryanodine induced the appearance of subconductance states or inhibited channel activity altogether, while Imperatoxin A (IpTx(a)), a specific activator of RyRs, reversibly induced the appearance of distinct subconductance states. Remarkably, the cardiac mRyR displayed a Ca(2+) dependence of [(3)H]ryanodine binding curve similar to skeletal RyR (RyR1), not cardiac RyR (RyR2). Overall, the mRyR displayed elemental attributes that are present in single channel lipid bilayer recordings of SR-RyRs, although some exquisite differences were also noted. These results therefore provide the first direct evidence that a unique RyR occurs in mitochondrial membranes.

Calmodulin kinase II inhibition prevents arrhythmic activity induced by alpha and beta adrenergic agonists in rabbit pulmonary veins

The autonomic nervous system and calcium regulation play important roles in the pathophysiology of atrial fibrillation. Calmodulin regulates the calcium homeostasis and may mediate the proarrhythmic effects of autonomic nervous agents. The purpose of this study was to compare the effects of beta- and alpha-adrenoceptor agonists on the pulmonary vein electrical activity and evaluate whether calmodulin kinase II inhibitors may change the effects of the adrenoceptor agonists on the pulmonary vein arrhythmogenesis. Conventional microelectrodes were used to record the action potentials in isolated rabbit pulmonary vein tissue specimens before and after the administration of isoproterenol, phenylephrine and KN-93 (a calmodulin kinase II inhibitor). In the tissue preparation, isoproterenol (0, 0.1, 3 microM) increased the beating rates (1.5+/-0.2, 1.6+/-0.2, 2.3+/-0.3 Hz, n=10, P<0.001) with the genesis of early afterdepolarizations (EADs, 0%, 40%, 50%, P<0.05) and increased the amplitude of the delayed afterdepolarizations (DADs, 0.6+/-0.3, 1.7+/-0.4, 3.9+/-1.0 mV, P<0.05). Phenylephrine (0, 1, 10 microM) also increased the beating rates (1.4+/-0.2, 1.6+/-0.2, 1.9+/-0.2 Hz, n=12, P<0.001), incidence of EADs (0%, 8%, 50%, P<0.05) and amplitude of the DADs (0.4+/-0.2, 1.2+/-0.4, 2.6+/-0.8 mV, P<0.05). KN-93 did not change the pulmonary vein beating rates or action potential duration. However, in the presence of KN-93 (1 microM), isoproterenol (3 microM) and phenylephrine (10 microM) did not induce any EADs or DADs in the pulmonary veins. In conclusion, calmodulin kinase II inhibition may prevent adrenergic induced pulmonary vein arrhythmogenesis.

Ca2+/calmodulin kinase II increases ryanodine binding and Ca2+-induced sarcoplasmic reticulum Ca2+ release kinetics during beta-adrenergic stimulation

We aimed to define the relative contribution of both PKA and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) cascades to the phosphorylation of RyR2 and the activity of the channel during beta-adrenergic receptor (betaAR) stimulation. Rat hearts were perfused with increasing concentrations of the beta-agonist isoproterenol in the absence and the presence of CaMKII inhibition. CaMKII was inhibited either by preventing the Ca(2+) influx to the cell by low [Ca](o) plus nifedipine or by the specific inhibitor KN-93. We immunodetected RyR2 phosphorylated at Ser2809 (PKA and putative CaMKII site) and at Ser2815 (CaMKII site) and measured [(3)H]-ryanodine binding and fast Ca(2+) release kinetics in sarcoplasmic reticulum (SR) vesicles. SR vesicles were isolated in conditions that preserved the phosphorylation levels achieved in the intact heart and were actively and equally loaded with Ca(2+). Our results demonstrated that Ser2809 and Ser2815 of RyR2 were dose-dependently phosphorylated under betaAR stimulation by PKA and CaMKII, respectively. The isoproterenol-induced increase in the phosphorylation of Ser2815 site was prevented by the PKA inhibitor H-89 and mimicked by forskolin. CaMKII-dependent phosphorylation of RyR2 (but not PKA-dependent phosphorylation) was responsible for the beta-induced increase in the channel activity as indicated by the enhancement of the [(3)H]-ryanodine binding and the velocity of fast SR Ca(2+) release. The present results show for the first time a dose-dependent increase in the phosphorylation of Ser2815 of RyR2 through the PKA-dependent activation of CaMKII and a predominant role of CaMKII-dependent phosphorylation of RyR2, over that of PKA-dependent phosphorylation, on SR-Ca(2+) release during betaAR stimulation.