Excitation-contraction coupling gain and cooperativity of the cardiac ryanodine receptor: a modeling approach

Role of ryanodine receptor cooperativity in Ca2+-wave-mediated triggered activity in cardiomyocytes

Ca2+ waves are known to trigger delayed after-depolarizations that can cause malignant cardiac arrhythmias. However, modelling Ca2+ waves using physiologically realistic models has remained a major challenge. Existing models with low Ca2+ sensitivity of ryanodine receptors (RyRs) necessitate large release currents, leading to an unrealistically large Ca2+ transient amplitude incompatible with the experimental observations. Consequently, current physiologically detailed models of delayed after-depolarizations resort to unrealistic cell architectures to produce Ca2+ waves with a normal Ca2+ transient amplitude. Here, we address these challenges by incorporating RyR cooperativity into a physiologically detailed model with a realistic cell architecture. We represent RyR cooperativity phenomenologically through a Hill coefficient within the sigmoid function of RyR open probability. Simulations in permeabilized myocytes with high Ca2+ sensitivity reveal that a sufficiently large Hill coefficient is required for Ca2+ wave propagation via the fire-diffuse-fire mechanism. In intact myocytes, propagating Ca2+ waves can occur only within an intermediate Hill coefficient range. Within this range, the spark rate is neither too low, enabling Ca2+ wave propagation, nor too high, allowing for the maintenance of a high sarcoplasmic reticulum load during diastole of the action potential. Moreover, this model successfully replicates other experimentally observed manifestations of Ca2+-wave-mediated triggered activity, including phase 2 and phase 3 early after-depolarizations and high-frequency voltage-Ca2+ oscillations. These oscillations feature an elevated take-off potential with depolarization mediated by the L-type Ca2+ current. The model also sheds light on the roles of luminal gating of RyRs and the mobile buffer ATP in the genesis of these arrhythmogenic phenomena. KEY POINTS: Existing mathematical models of Ca2+ waves use an excessively large Ca2+-release current or unrealistic diffusive coupling between release units. Our physiologically realistic model, using a Hill coefficient in the ryanodine receptor (RyR) gating function to represent RyR cooperativity, addresses these limitations and generates organized Ca2+ waves at Hill coefficients ranging from ∼5 to 10, as opposed to the traditional value of 2. This range of Hill coefficients gives a spark rate neither too low, thereby enabling Ca2+ wave propagation, nor too high, allowing for the maintenance of a high sarcoplasmic reticulum load during the plateau phase of the action potential. Additionally, the model generates Ca2+-wave-mediated phase 2 and phase 3 early after-depolarizations, and coupled membrane voltage with Ca2+ oscillations mediated by the L-type Ca2+ current. This study suggests that pharmacologically targeting RyR cooperativity could be a promising strategy for treating cardiac arrhythmias linked to Ca2+-wave-mediated triggered activity.

Molecular Mechanism of a FRET Biosensor for the Cardiac Ryanodine Receptor Pathologically Leaky State

Ca2+ leak from cardiomyocyte sarcoplasmic reticulum (SR) via hyperactive resting cardiac ryanodine receptor channels (RyR2) is pro-arrhythmic. An exogenous peptide (DPc10) binding promotes leaky RyR2 in cardiomyocytes and reports on that endogenous state. Conversely, calmodulin (CaM) binding inhibits RyR2 leak and low CaM affinity is diagnostic of leaky RyR2. These observations have led to designing a FRET biosensor for drug discovery targeting RyR2. We used FRET to clarify the molecular mechanism driving the DPc10-CaM interdependence when binding RyR2 in SR vesicles. We used donor-FKBP12.6 (D-FKBP) to resolve RyR2 binding of acceptor-CaM (A-CaM). In low nanomolar Ca2+, DPc10 decreased both FRETmax (under saturating [A-CaM]) and the CaM/RyR2 binding affinity. In micromolar Ca2+, DPc10 decreased FRETmax without affecting CaM/RyR2 binding affinity. This correlates with the analysis of fluorescence-lifetime-detected FRET, indicating that DPc10 lowers occupancy of the RyR2 CaM-binding sites in nanomolar (not micromolar) Ca2+ and lengthens D-FKBP/A-CaM distances independent of [Ca2+]. To observe DPc10/RyR2 binding, we used acceptor-DPc10 (A-DPc10). CaM weakens A-DPc10/RyR2 binding, with this effect being larger in micromolar versus nanomolar Ca2+. Moreover, A-DPc10/RyR2 binding is cooperative in a CaM- and FKBP-dependent manner, suggesting that both endogenous modulators promote concerted structural changes between RyR2 protomers for channel regulation. Aided by the analysis of cryo-EM structures, these insights inform further development of the DPc10-CaM paradigm for therapeutic discovery targeting RyR2.

Molecular Mechanism of a FRET Biosensor for the Cardiac Ryanodine Receptor Pathologically Leaky State

Ca 2+ leak from cardiomyocyte sarcoplasmic reticulum (SR) via hyperactive resting cardiac ryanodine receptor channels (RyR2) is pro-arrhythmic. An exogenous peptide, (DPc10) detects leaky RyR2 in cardiomyocytes. Conversely, calmodulin (CaM) inhibits RyR2 leak. These observations have led to designing a FRET biosensor for drug discovery targeting RyR2. Here we used FRET to understand the molecular mechanism driving the DPc10-CaM interdependence when binding RyR2 in SR vesicles. We used donor-FKBP12.6 (D-FKBP) to resolve RyR2 binding of acceptor-CaM (A-CaM). In low nanomolar Ca 2+ , DPc10 decreased both FRET max (under saturating [A-CaM]) and the CaM/RyR2 binding affinity. In micromolar Ca 2+ , DPc10 decreased FRET max without affecting CaM/RyR2 binding affinity. This correlates with analysis of fluorescence-lifetime-detected FRET indicating that DPc10 lowers occupancy of the RyR2 CaM-binding sites in nanomolar (not micromolar) Ca 2+ and lengthens D-FKBP/A-CaM distances independent of [Ca 2+ ]. To observe DPc10/RyR2 binding, we used acceptor-DPc10 (A-DPc10). CaM weakens A-DPc10/RyR2 binding, this effect being larger in micromolar vs. nanomolar Ca 2+ . Moreover, A-DPc10/RyR2 binding is cooperative in CaM- and FKBP-dependent manner, suggesting that both endogenous modulators promote concerted structural changes between RyR2 protomers for channel regulation. Aided by analysis of cryo-EM structures, these insights inform further development of the DPc10-CaM paradigm for therapeutic discovery targeting RyR2.

Modeling Calcium Cycling in the Heart: Progress, Pitfalls, and Challenges

Intracellular calcium (Ca) cycling in the heart plays key roles in excitation-contraction coupling and arrhythmogenesis. In cardiac myocytes, the Ca release channels, i.e., the ryanodine receptors (RyRs), are clustered in the sarcoplasmic reticulum membrane, forming Ca release units (CRUs). The RyRs in a CRU act collectively to give rise to discrete Ca release events, called Ca sparks. A cell contains hundreds to thousands of CRUs, diffusively coupled via Ca to form a CRU network. A rich spectrum of spatiotemporal Ca dynamics is observed in cardiac myocytes, including Ca sparks, spark clusters, mini-waves, persistent whole-cell waves, and oscillations. Models of different temporal and spatial scales have been developed to investigate these dynamics. Due to the complexities of the CRU network and the spatiotemporal Ca dynamics, it is challenging to model the Ca cycling dynamics in the cardiac system, particularly at the tissue sales. In this article, we review the progress of modeling of Ca cycling in cardiac systems from single RyRs to the tissue scale, the pros and cons of the current models and different modeling approaches, and the challenges to be tackled in the future.

Ultrastructural remodelling of Ca(2+) signalling apparatus in failing heart cells

AIMS

The contraction of a heart cell is controlled by Ca(2+)-induced Ca(2+) release between L-type Ca(2+) channels (LCCs) in the cell membrane/T-tubules (TTs) and ryanodine receptors (RyRs) in the junctional sarcoplasmic reticulum (SR). During heart failure, LCC-RyR signalling becomes defective. The purpose of the present study was to reveal the ultrastructural mechanism underlying the defective LCC-RyR signalling and contractility.

METHODS AND RESULTS

In rat models of heart failure produced by transverse aortic constriction surgery, stereological analysis of transmission electron microscopic images showed that the volume density and the surface area of junctional SRs and those of SR-coupled TTs were both decreased in failing heart cells. The TT-SR junctions were displaced or missing from the Z-line areas. Moreover, the spatial span of individual TT-SR junctions was markedly reduced in failing heart cells. Numerical simulation and junctophilin-2 knockdown experiments demonstrated that the decrease in junction size (and thereby the constitutive LCC and RyR numbers) led to a scattered delay of Ca(2+) release activation.

CONCLUSIONS

The shrinking and eventual absence of TT-SR junctions are important mechanisms underlying the desynchronized and inhomogeneous Ca(2+) release and the decreased contractile strength in heart failure. Maintaining the nanoscopic integrity of TT-SR junctions thus represents a therapeutic strategy against heart failure and related cardiomyopathies.

Computational modeling and numerical methods for spatiotemporal calcium cycling in ventricular myocytes

Intracellular calcium (Ca) cycling dynamics in cardiac myocytes is regulated by a complex network of spatially distributed organelles, such as sarcoplasmic reticulum (SR), mitochondria, and myofibrils. In this study, we present a mathematical model of intracellular Ca cycling and numerical and computational methods for computer simulations. The model consists of a coupled Ca release unit (CRU) network, which includes a SR domain and a myoplasm domain. Each CRU contains 10 L-type Ca channels and 100 ryanodine receptor channels, with individual channels simulated stochastically using a variant of Gillespie’s method, modified here to handle time-dependent transition rates. Both the SR domain and the myoplasm domain in each CRU are modeled by 5 × 5 × 5 voxels to maintain proper Ca diffusion. Advanced numerical algorithms implemented on graphical processing units were used for fast computational simulations. For a myocyte containing 100 × 20 × 10 CRUs, a 1-s heart time simulation takes about 10 min of machine time on a single NVIDIA Tesla C2050. Examples of simulated Ca cycling dynamics, such as Ca sparks, Ca waves, and Ca alternans, are shown.

N-acetylcysteine reverses cardiac myocyte dysfunction in HIV-Tat proteinopathy

HIV cardiomyopathy remains highly prevalent among the estimated 33 million HIV-infected individuals worldwide. This is particularly true in developing countries. Potential mechanisms responsible for myocardial dysfunction following HIV infection include direct effects of HIV proteins. We have previously reported that cardiac myocyte-specific expression of HIV-Tat (Tat) results in a murine cardiomyopathy model. We now report that Tat exhibits decreased myocardial ATP [wild type (WT) vs. Tat transgenic (TG), P < 0.01] and myocyte GSH levels (WT vs. TG, P < 0.01), decreased GSH/GSSG ratio (WT vs. TG, P < 0.01), increased H(2)O(2) levels (WT vs. TG, P < 0.05), and increased catalase (TG vs. WT, P < 0.05) and GPX1 (glutathione peroxidase 1) activities (WT vs. TG, P < 0.05), blunted cardiac myocyte positive inotropy (% peak shortening, WT vs. TG, P < 0.01; +dl/dt, WT vs. TG, P < 0.01) and negative inotropy (-dl/dt, WT vs. TG, P < 0.01), and blunted inotropic responses to Ca(2+) (P < 0.01, for each) and shortened anatomical and functional survival in vitro (P < 0.01). The sulfhydryl donor, N-acetylcysteine (NAC; 10(-4) M), completely reversed both the positive and negative inotropic defects in Tat; increased GSH (P < 0.01) and GSH/GSSG (P < 0.01); reversed H(2)O(2) level (P < 0.05) and GPX1 activity (P < 0.05); and normalized the blunted inotropic response to Ca(2+) (P < 0.01). NAC (10(-7)) M normalized duration of contractile function from <40 min to >120 min (P < 0.01), with no effect on GSH and GSH/GSSG. NAC (10(-4) M) reverses cardiac myocyte dysfunction and markers of oxidative stress. NAC (10(-7) M) enhances myocyte function independent of changes in glutathione. Elucidating the molecular mechanisms involved in the GSH-dependent and GSH-independent salutary effects of NAC should identify novel therapeutic targets for myocardial proteinopathies recently appreciated in human cardiomyopathies.

Mechanisms of abnormal calcium homeostasis in mutations responsible for catecholaminergic polymorphic ventricular tachycardia

Catecholaminergic polymorphic ventricular tachycardia is a heritable arrhythmia unmasked by exertion or stress and is characterized by triggered activity and sudden cardiac death. In this study, we simulated mutations in 2 genes linked to catecholaminergic polymorphic ventricular tachycardia, the first located in calsequestrin (CSQN2) and the second in the ryanodine receptor (RyR2). The aim of the study was to investigate the mechanistic basis for spontaneous Ca2+ release events that lead to delayed afterdepolarizations in affected patients. Sarcoplasmic reticulum (SR) luminal Ca2+ sensing was incorporated into a model of the human ventricular myocyte, and CSQN2 mutations were modeled by simulating disrupted RyR2 luminal Ca2+ sensing. In voltage-clamp mode, the mutant CSQN2 model recapitulated the smaller calcium transients, smaller time to peak calcium transient, and accelerated recovery from inactivation seen in experiments. In current clamp mode, in the presence of beta stimulation, we observed delayed afterdepolarizations, suggesting that accelerated recovery of RyR2 induced by impaired luminal Ca2+ sensing underlies the triggered activity observed in mutant CSQN2-expressing myocytes. In current-clamp mode, in a model of mutant RyR2 that is characterized by reduced FKBP12.6 binding to the RyR2 on beta stimulation, the impaired coupled gating characteristic of these mutations was modeled by reducing cooperativity of RyR2 activation. In current-clamp mode, the mutant RyR2 model exhibited increased diastolic RyR2 open probability that resulted in formation of delayed afterdepolarizations. In conclusion, these minimal order models of mutant CSQN2 and RyR2 provide plausible mechanisms by which defects in RyR2 gating may lead to the cellular triggers for arrhythmia, with implications for the development of targeted therapy.