Protein geometry and placement in the cardiac dyad influence macroscopic properties of calcium-induced calcium release

In cardiac ventricular myocytes, events crucial to excitation-contraction coupling take place in spatially restricted microdomains known as dyads. The movement and dynamics of calcium (Ca2+) ions in the dyad have often been described by assigning continuously valued Ca2+ concentrations to one or more dyadic compartments. However, even at its peak, the estimated number of free Ca2+ ions present in a single dyad is small (approximately 10-100 ions). This in turn suggests that modeling dyadic calcium dynamics using laws of mass action may be inappropriate. In this study, we develop a model of stochastic molecular signaling between L-type Ca2+ channels (LCCs) and ryanodine receptors (RyR2s) that describes: a), known features of dyad geometry, including the space-filling properties of key dyadic proteins; and b), movement of individual Ca2+ ions within the dyad, as driven by electrodiffusion. The model enables investigation of how local Ca2+ signaling is influenced by dyad structure, including the configuration of key proteins within the dyad, the location of Ca2+ binding sites, and membrane surface charges. Using this model, we demonstrate that LCC-RyR2 signaling is influenced by both the stochastic dynamics of Ca2+ ions in the dyad as well as the shape and relative positioning of dyad proteins. Results suggest the hypothesis that the relative placement and shape of the RyR2 proteins helps to "funnel" Ca2+ ions to RyR2 binding sites, thus increasing excitation-contraction coupling gain.     

心肌兴奋收缩耦联过程中的钙调控

联系以膜电位变化为特征的细胞兴奋和以肌丝滑行为基础的肌肉收缩的中介过程通常称为兴奋收缩耦联。在所有参与调控心肌收缩功能的离子中,钙离子被认为是最重要的介导因子,因此验明钙离子参与介导心肌兴奋收缩耦联的方式和途径等特征无疑有益于更好地理解心脏的生理功能。     

心肌兴奋收缩耦联过程中的钙调控

联系以膜电位变化为特征的细胞兴奋和以肌丝滑行为基础的肌肉收缩的中介过程通常称为兴奋收缩耦联。在所有参与调控心肌收缩功能的离子中,钙离子被认为是最重要的介导因子,因此验明钙离子参与介导心肌兴奋收缩耦联的方式和途径等特征无疑有益于更好地理解心脏的生理功能。     

Ryanodine受体相关的肌肉疾病及其研究进展

Ryanodine受体(ryanodine receptor,RyR)是位于细胞内肌质网(sarcoplasmic reticulum,SR)膜上的钙释放通道,是骨骼肌和心肌细胞兴奋-收缩偶联过程中的关键蛋白。RyR结构和功能的改变,往往导致肌细胞兴奋-收缩的解偶联,从而引发一些相关的肌肉疾病。目前的研究肯定这些疾病的发病机制都与RyR密切相关,本文就此方面的最新研究进展进行综述,为预防和治疗这些疾病提供理论依据。     

Calmodulin kinase II inhibition protects against structural heart disease

Beta-adrenergic receptor (betaAR) stimulation increases cytosolic Ca(2+) to physiologically augment cardiac contraction, whereas excessive betaAR activation causes adverse cardiac remodeling, including myocardial hypertrophy, dilation and dysfunction, in individuals with myocardial infarction. The Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) is a recently identified downstream element of the betaAR-initiated signaling cascade that is linked to pathological myocardial remodeling and to regulation of key proteins involved in cardiac excitation-contraction coupling. We developed a genetic mouse model of cardiac CaMKII inhibition to test the role of CaMKII in betaAR signaling in vivo. Here we show CaMKII inhibition substantially prevented maladaptive remodeling from excessive betaAR stimulation and myocardial infarction, and induced balanced changes in excitation-contraction coupling that preserved baseline and betaAR-stimulated physiological increases in cardiac function. These findings mark CaMKII as a determinant of clinically important heart disease phenotypes, and suggest CaMKII inhibition can be a highly selective approach for targeting adverse myocardial remodeling linked to betaAR signaling.     

Maintained contractile reserve in a transgenic mouse model of myocardial stunning

Cardiac excitation-contraction (E-C) coupling is impaired at the myofilament level in the reversible postischemic dysfunction known as "stunned" myocardium. We characterized tension development and calcium cycling in intact isolated trabeculae from transgenic (TG) mice expressing the major proteolytic degradation fragment of troponin I (TnI) found in stunned myocardium (TnI(1-193)) and determined the ATPase activity of myofibrils extracted from TG and non-TG mouse hearts. The phenotype of these mice at baseline recapitulates that of stunning. Here, we address the question of whether contractile reserve is preserved in these mice, as it is in genuine stunned myocardium. During twitch contractions, calcium cycling was normal, whereas tension was greatly reduced, compared with non-TG controls. A decrease in maximum Ca2+-activated tension and Ca2+ desensitization of the myofilaments accounted for this contractile dysfunction. The decrease in maximum tension was paralleled by an equivalent decrease in maximum Ca2+-activated myofibrillar ATPase activity. Exposure to high calcium or isoproterenol recruited a sizable contractile reserve in TG muscles, which was proportionately similar to that in control muscles but scaled downward in amplitude. These results suggest that calcium regulatory pathways and beta-adrenergic signal transduction remain intact in isolated trabeculae from stunned TG mice, further recapitulating key features of genuine stunned myocardium.     

Myocardial calcium signalling and arrhythmia pathogenesis

Myocardial calcium signalling is a vital component of the normal physiological function of the heart. Key amongst the many roles calcium plays is its use as the primary signalling component of excitation-contraction coupling, the intracellular process that links cardiomyocyte depolarisation to contraction. Defective cellular calcium handling, due to abnormalities of the various components which mediate and control excitation-contraction coupling, is widely recognised as a significant patho-physiological event in the contractile dysfunction of the failing heart. In addition, similar defects also appear to be increasingly recognised as mediators of certain forms of cardiac arrhythmias. Such defects include single gene defects in excitation-contraction coupling components that lead to inherited sudden death arrhythmia syndromes. Alternatively, arrhythmogenesis occurring within the context of acquired cardiac disease, in particular heart failure, also appears to be highly dependent on abnormal calcium homeostasis. In this article we review the defects in cardiomyocyte calcium homeostasis that lead to particular pro-arrhythmogenic phenomena and discuss recent insights gained into a variety of inherited and acquired arrhythmia syndromes that appear to involve defective calcium signalling as a central component of their patho-physiology. Potential opportunities for new anti arrhythmic therapeutic strategies based on these recent insights are also discussed.     

Regulation of sodium and calcium channels by signaling complexes

Membrane depolarization and intracellular calcium transients generated by activation of voltage-gated sodium and calcium channels are local signals, which initiate physiological processes such as action potential conduction, synaptic transmission, and excitation-contraction coupling. Targeting of effector proteins and regulatory proteins to ion channels is an important mechanism to ensure speed, specificity, and precise regulation of signaling events in response to local stimuli. In this article, we review recent experimental results showing that sodium and calcium channels form local signaling complexes, in which effector proteins, anchoring proteins, and regulatory proteins interact directly with ion channels. The intracellular domains of these channels serve as signaling platforms, mediating their participation in intracellular signaling processes. These protein-protein interactions are important for efficient synaptic transmission and for regulation of ion channels by neurotransmitters and intracellular second messengers. These localized signaling complexes are essential for normal function and regulation of electrical excitability, synaptic transmission, and excitation-contraction coupling.     

Regulation of Sodium and Calcium Channels by Signaling Complexes

Membrane depolarization and intracellular calcium transients generated by activation of voltage-gated sodium and calcium channels are local signals, which initiate physiological processes such as action potential conduction, synaptic transmission, and excitation-contraction coupling. Targeting of effector proteins and regulatory proteins to ion channels is an important mechanism to ensure speed, specificity, and precise regulation of signaling events in response to local stimuli. In this article, we review recent experimental results showing that sodium and calcium channels form local signaling complexes, in which effector proteins, anchoring proteins, and regulatory proteins interact directly with ion channels. The intracellular domains of these channels serve as signaling platforms, mediating their participation in intracellular signaling processes. These protein-protein interactions are important for efficient synaptic transmission and for regulation of ion channels by neurotransmitters and intracellular second messengers. These localized signaling complexes are essential for normal function and regulation of electrical excitability, synaptic transmission, and excitation-contraction coupling.     

Calcium handling in myocardium from amphibian,avian, and mammalian species: the search for two components

Summary Steps involved in excitation-contraction coupling in mammalian myocardium have been derived using a relatively limited number of animal species. However, the use of animal models for investigations into excitation-contraction coupling in normal and disease states has encompassed a wide range of animal species. We addressed the question as to whether excitation-contraction coupling as currently understood applies to intracellular calcium handling in myocardium from multiple mammalian species, amphibian, and avian myocardium. The bioluminescent calcium indicator aequorin was used to record intracellular calcium transients in both ventricular and atrial tissue. We report that in all mammalian and avian species studied the calcium transient recorded in both ventricular and atrial myocardium is monophasic and reflects calcium release and re-uptake by the sarcoplasmic reticulum. In contrast, the Ca²⁺ transient recorded from salamander myocardium is prolonged relative to mammalian and avian myocardium, and appears to reflect in part trans-sarcolemmal calcium entry. Only in diseased myocardium derived from human and swine myocardium was a second component detected in the calcium transient. These data indicate that sarcoplasmic reticulum calcium handling is pivotal in excitation-contraction coupling for multiple species with differing physiologies. Also, in disease states, intracellular calcium handling is often affected with resultant alterations in the time-course and/or configuration of the calcium transient.     

Specific absence of the alpha 1 subunit of the dihydropyridine receptor in mice with muscular dysgenesis

Muscular dysgenesis is a lethal mutation in mice that results in a complete absence of skeletal muscle contraction due to the failure of depolarization of the transverse tubular membrane to trigger calcium release from the sarcoplasmic reticulum. In order to determine whether the defect in muscular dysgenesis leads to a specific loss of one of the components of excitation-contraction coupling or to a generalized loss of all components of excitation-contraction coupling, we have analyzed skeletal muscle from control and dysgenic mice for the sarcoplasmic reticulum and transverse tubular proteins which are believe to function in excitation-contraction coupling. We report that the proteins involved in sarcoplasmic reticulum calcium transport, storage, and release [Ca2+ + Mg2+)-ATPase, calsequestrin, and calcium release channel) are present in dysgenic muscle. Also present in dysgenic muscle is the 175/150-kDa glycoprotein subunit (alpha 2) of the dihydropyridine receptor. However, the 170-kDa dihydropyridine binding subunit (alpha 1) of the dihydropyridine receptor is absent in dysgenic muscle. These results suggest that the specific absence of the alpha 1 subunit of the dihydropyridine receptor is responsible for the defects in muscular dysgenesis and that the alpha 1 subunit of the dihydropyridine receptor is essential for excitation-contraction coupling in skeletal muscle.