Eric A. Sobie

Calcium Biology of the Transverse Tubules in Heart

Abstract: Ca2+ sparks in heart muscle are activated on depolarization by the influx of Ca2+ through dihydropyridine receptors in the sarcolemmal (SL) and transverse tubule (TT) membranes. The cardiac action potential is thus able to synchronize the [Ca2+]i transient as Ca2+ release is activated throughout the cell. Increases in the amount of Ca2+ within the sarcoplasmic reticulum (SR) underlie augmented Ca2+ release globally and an increase in the sensitivity of the ryanodine receptors (RyRs) to be triggered by the local [Ca2+]i. In a similar manner, phosphorylation of the RyRs by protein kinase A (PKA) increases the sensitivity of the RyRs to be activated by local [Ca2+]i. Heart failure and other cardiac diseases are associated with changes in SR Ca2+ content, phosphorylation state of the RyRs, [Ca2+]i signaling defects and arrhythmias. Additional changes in transverse tubules and nearby junctional SR may contribute to alterations in local Ca2+ signaling. Here we briefly discuss how TT organization can influence Ca2+ signaling and how changes in SR Ca2+ release triggering can influence excitation‐contraction (EC) coupling. High speed imaging methods are used in combination with single cell patch clamp experiments to investigate how abnormal Ca2+ signaling may be regulated in health and disease. Three issues are examined in this presentation: (1) normal Ca2+‐induced Ca2+ release and Ca2+ sparks, (2) abnormal SR Ca2+ release in disease, and (3) the triggering and propagation of waves of elevated [Ca2+]i.

Models of cardiac excitation–contraction coupling in ventricular myocytes

Mathematical and computational modeling of cardiac excitation-contraction coupling has produced considerable insights into how the heart muscle contracts. With the increase in biophysical and physiological data available, the modeling has become more sophisticated with investigations spanning in scale from molecular components to whole cells. These modeling efforts have provided insight into cardiac excitation-contraction coupling that advanced and complemented experimental studies. One goal is to extend these detailed cellular models to model the whole heart. While this has been done with mechanical and electrophysiological models, the complexity and fast time course of calcium dynamics have made inclusion of detailed calcium dynamics in whole heart models impractical. Novel methods such as the probability density approach and moment closure technique which increase computational efficiency might make this tractable.

Moment Closure for Local Control Models of Calcium-Induced Calcium Release in Cardiac Myocytes

In prior work, we introduced a probability density approach to modeling local control of Ca2+-induced Ca2+ release in cardiac myocytes, where we derived coupled advection-reaction equations for the time-dependent bivariate probability density of subsarcolemmal subspace and junctional sarcoplasmic reticulum (SR) [Ca2+] conditioned on Ca2+ release unit (CaRU) state. When coupled to ordinary differential equations (ODEs) for the bulk myoplasmic and network SR [Ca2+], a realistic but minimal model of cardiac excitation-contraction coupling was produced that avoids the computationally demanding task of resolving spatial aspects of global Ca2+ signaling, while accurately representing heterogeneous local Ca2+ signals in a population of diadic subspaces and junctional SR depletion domains. Here we introduce a computationally efficient method for simulating such whole cell models when the dynamics of subspace [Ca2+] are much faster than those of junctional SR [Ca2+]. The method begins with the derivation of a system of ODEs describing the time-evolution of the moments of the univariate probability density functions for junctional SR [Ca2+] jointly distributed with CaRU state. This open system of ODEs is then closed using an algebraic relationship that expresses the third moment of junctional SR [Ca2+] in terms of the first and second moments. In simulated voltage-clamp protocols using 12-state CaRUs that respond to the dynamics of both subspace and junctional SR [Ca2+], this moment-closure approach to simulating local control of excitation-contraction coupling produces high-gain Ca2+ release that is graded with changes in membrane potential, a phenomenon not exhibited by common pool models. Benchmark simulations indicate that the moment-closure approach is nearly 10,000-times more computationally efficient than corresponding Monte Carlo simulations while leading to nearly identical results. We conclude by applying the moment-closure approach to study the restitution of Ca2+-induced Ca2+ release during simulated two-pulse voltage-clamp protocols.

Predicting Local SR Ca2+ Dynamics during Ca2+ Wave Propagation in Ventricular Myocytes

Of the many ongoing controversies regarding the workings of the sarcoplasmic reticulum (SR) in cardiac myocytes, two unresolved and interconnected topics are 1), mechanisms of calcium (Ca(2+)) wave propagation, and 2), speed of Ca(2+) diffusion within the SR. Ca(2+) waves are initiated when a spontaneous local SR Ca(2+) release event triggers additional release from neighboring clusters of SR release channels (ryanodine receptors (RyRs)). A lack of consensus regarding the effective Ca(2+) diffusion constant in the SR (D(Ca,SR)) severely complicates our understanding of whether dynamic local changes in SR [Ca(2+)] can influence wave propagation. To address this problem, we have implemented a computational model of cytosolic and SR [Ca(2+)] during Ca(2+) waves. Simulations have investigated how dynamic local changes in SR [Ca(2+)] are influenced by 1), D(Ca,SR); 2), the distance between RyR clusters; 3), partial inhibition or stimulation of SR Ca(2+) pumps; 4), SR Ca(2+) pump dependence on cytosolic [Ca(2+)]; and 5), the rate of transfer between network and junctional SR. Of these factors, D(Ca,SR) is the primary determinant of how release from one RyR cluster alters SR [Ca(2+)] in nearby regions. Specifically, our results show that local increases in SR [Ca(2+)] ahead of the wave can potentially facilitate Ca(2+) wave propagation, but only if SR diffusion is relatively slow. These simulations help to delineate what changes in [Ca(2+)] are possible during SR Ca(2+)release, and they broaden our understanding of the regulatory role played by dynamic changes in [Ca(2+)](SR).

Expression of a sorcin missense mutation in the heart modulates excitation-contraction coupling

Sorcin is a Ca2+ binding protein implicated in the regulation of intracellular Ca2+ cycling and cardiac excitation‐contraction coupling. Structural and human genetic studies suggest that a naturally occurring sequence variant encoding L112‐sorcin disrupts an E‐F hand Ca2+ binding domain and may be responsible for a heritable form of hypertension and hypertrophic heart disease. We generated transgenic mice overexpressing L112‐sorcin in the heart and characterized the effects on Ca2+ regulation and cardiac function both in vivo and in dissociated cardiomyocytes. Hearts of sorcinF112L transgenic mice were mildly dilated but ventricular function was preserved and systemic blood pressure was normal. SorcinF112L myocytes were smaller than control cells and displayed complex alterations in Ca2+ regulation and contractility, including a slowed inactivation of L‐type Ca2+ current, enhanced Ca2+ spark width, duration, and frequency, and increased Na+‐Ca2+ exchange activity. In contrast, mice with cardiac‐specific overexpression of wild‐type sorcin displayed directionally opposite effects on L‐type Ca2+ channel function and Ca2+ spark behavior. These data further define the role of sorcin in cardiac excitation‐contraction coupling and highlight its negative regulation of SR calcium release. Our results also suggest that additional factors may be responsible for the development of cardiac hypertrophy and hypertension in humans expressing the L112‐sorcin sequence variant.—Collis, L. P., Meyers, M. B., Zhang, J., Phoon, C. K. L., Sobie, E. A., Coetzee, W. A., Fishman, G. I. Expression of a sorcin missense mutation in the heart modulates excitation‐contraction coupling. FASEB J. 21, 475–487 (2007)

The challenge of molecular medicine: complexity versus Occam’s razor

Everything should be made as simple as possible, but not simpler. —Albert Einstein’s comment on Occam’s Razor The goal of molecular medicine is to find treatments for human diseases by the clever and effective application of the tools of molecular and cell biology. To do this, an animal model (or a set of animal models) of the disease is devised, investigated, and characterized. Novel therapies are conceived and tested on the animal model(s) until a rescue from the pathology is achieved. The rescue strategy is then developed for human trial.