Modulation of cardiac ryanodine receptor channels by alkaline earth cations

The role of Zn2+ in shaping intracellular Ca2+ dynamics in the heart

Increasing evidence suggests that Zn2+ acts as a second messenger capable of transducing extracellular stimuli into intracellular signaling events. The importance of Zn2+ as a signaling molecule in cardiovascular functioning is gaining traction. In the heart, Zn2+ plays important roles in excitation-contraction (EC) coupling, excitation-transcription coupling, and cardiac ventricular morphogenesis. Zn2+ homeostasis in cardiac tissue is tightly regulated through the action of a combination of transporters, buffers, and sensors. Zn2+ mishandling is a common feature of various cardiovascular diseases. However, the precise mechanisms controlling the intracellular distribution of Zn2+ and its variations during normal cardiac function and during pathological conditions are not fully understood. In this review, we consider the major pathways by which the concentration of intracellular Zn2+ is regulated in the heart, the role of Zn2+ in EC coupling, and discuss how Zn2+ dyshomeostasis resulting from altered expression levels and efficacy of Zn2+ regulatory proteins are key drivers in the progression of cardiac dysfunction.

The Cardiac Ryanodine Receptor Provides a Suitable Pathway for the Rapid Transport of Zinc (Zn2+)

The sarcoplasmic reticulum (SR) in cardiac muscle is suggested to act as a dynamic storage for Zn²⁺ release and reuptake, albeit it is primarily implicated in the Ca²⁺ signaling required for the cardiac cycle. A large Ca²⁺ release from the SR is mediated by the cardiac ryanodine receptor (RYR2), and while this has a prominent conductance for Ca²⁺ in vivo, it also conducts other divalent cations in vitro. Since Zn²⁺ and permeant Mg²⁺ have similar physical properties, we tested if the RYR2 channel also conducts Zn²⁺. Using the method of planar lipid membranes, we evidenced that the RYR2 channel is permeable to Zn²⁺ with a considerable conductance of 81.1 ± 2.4 pS, which was significantly lower than the values for Ca²⁺ (127.5 ± 1.8 pS) and Mg²⁺ (95.3 ± 1.4 pS), obtained under the same asymmetric conditions. Despite similar physical properties, the intrinsic Zn²⁺ permeability (P_Ca/P_Zn = 2.65 ± 0.19) was found to be ~2.3-fold lower than that of Mg²⁺ (P_Ca/P_Mg = 1.146 ± 0.071). Further, we assessed whether the channel itself could be a direct target of the Zn²⁺ current, having the Zn²⁺ finger extended into the cytosolic vestibular portion of the permeation pathway. We attempted to displace Zn²⁺ from the RYR2 Zn²⁺ finger to induce its structural defects, which are associated with RYR2 dysfunction. Zn²⁺ chelators were added to the channel cytosolic side or strongly competing cadmium cations (Cd²⁺) were allowed to permeate the RYR2 channel. Only the Cd²⁺ current was able to cause the decay of channel activity, presumably as a result of Zn²⁺ to Cd²⁺ replacement. Our findings suggest that the RYR2 channel can provide a suitable pathway for rapid Zn²⁺ escape from the cardiac SR; thus, the channel may play a role in local and/or global Zn²⁺ signaling in cardiomyocytes.

Blocking effect of ferritin on the ryanodine receptor-isoform 2

Iron, an essential element for most living organism, participates in a wide variety of physiological processes. Disturbance in iron homeostasis has been associated with numerous pathologies, particularly in the heart and brain, which are the most susceptible organs. Under iron-overload conditions, the generation of reactive oxygen species leads to impairment in Ca2+ signaling, fundamentally implicated in cardiac and neuronal physiology. Since iron excess is accompanied by increased expression of iron-storage protein, ferritin, we examined whether ferritin has an effect on the ryanodine receptor - isoform 2 (RYR2), which is one of the major components of Ca2+ signaling. Using the method of planar lipid membranes, we show that ferritin induced an abrupt, permanent blockage of the RYR2 channel. The ferritin effect was strongly voltage dependent and competitively antagonized by cytosolic TEA+, an impermeant RYR2 blocker. Our results collectively indicate that monomeric ferritin highly likely blocks the RYR2 channel by a direct electrostatic interaction within the wider region of the channel permeation pathway.

Luminal addition of non-permeant Eu3+ interferes with luminal Ca2+ regulation of the cardiac ryanodine receptor

Dysregulation of the cardiac ryanodine receptor (RYR2) by luminal Ca²⁺ has been implicated in a life-threatening, stress-induced arrhythmogenic disease. The mechanism of luminal Ca²⁺-mediated RYR2 regulation is under debate, and it has been attributed to Ca²⁺ binding on the cytosolic face (the Ca²⁺ feedthrough mechanism) and/or the luminal face of the RYR2 channel (the true luminal mechanism). The molecular nature and location of the luminal Ca²⁺ site is unclear. At the single-channel level, we directly probed the RYR2 luminal face by Eu³⁺, considering the non-permeant nature of trivalent cations and their high binding affinities for Ca²⁺ sites. Without affecting essential determinants of the Ca²⁺ feedthrough mechanism, we found that luminal Eu³⁺ competitively antagonized the activation effect of luminal Ca²⁺ on RYR2 responsiveness to cytosolic caffeine, and no appreciable effect was observed for luminal Ba²⁺ (mimicking the absence of luminal Ca²⁺). Importantly, luminal Eu³⁺ caused no changes in RYR2 gating. Our results indicate that two distinct Ca²⁺ sites (available for luminal Ca²⁺ even when the channel is closed) are likely involved in the true luminal mechanism. One site facing the lumen regulates channel responsiveness to caffeine, while the other site, presumably positioned in the channel pore, governs the gating behavior.

Ultrastructural Localization of Histidine-rich Glycoprotein in Skeletal Muscle Fibers: Colocalization With AMP Deaminase

Histidine-rich glycoprotein (HRG) is a plasma protein synthesized by the liver. We have given the first evidence of a tissue localization of HRG demonstrating its presence in skeletal muscle, associated with the zinc enzyme AMP deaminase (AMPD1). Moreover, we have shown that muscle cells do not synthesize HRG, but they can internalize it from plasma. We have recently demonstrated by confocal laser scanning microscopy that in human skeletal muscle, HRG is mainly localized in the myofibrils, preferentially at the I-band of the sarcomere, in the sarcoplasm, and in the nuclei. Using transmission electron microscopy and immunogold analysis, we carried out this study on human and rat normal skeletal muscles with the purpose to deepen the ultrastructural localization of HRG in skeletal muscle fibers. The immunogold analysis evidenced the presence of HRG in the sarcomeres, mainly in the I-band and to a less extent in the A-band, in the heterochromatin of nuclei, and in the sarcoplasmic reticulum. The colocalization of HRG and skeletal muscle AMPD1 was also analyzed. A colabeling of HRG and AMPD1 was evident at sarcomeric, sarcoplasmic reticulum, and nuclear levels. The significance of these interesting and new results is discussed in this article.

The structural basis of ryanodine receptor ion channel function

Large-conductance Ca²⁺ release channels known as ryanodine receptors (RyRs) mediate the release of Ca²⁺ from an intracellular membrane compartment, the endo/sarcoplasmic reticulum. There are three mammalian RyR isoforms: RyR1 is present in skeletal muscle; RyR2 is in heart muscle; and RyR3 is expressed at low levels in many tissues including brain, smooth muscle, and slow-twitch skeletal muscle. RyRs form large protein complexes comprising four 560-kD RyR subunits, four ∼12-kD FK506-binding proteins, and various accessory proteins including calmodulin, protein kinases, and protein phosphatases. RyRs share ∼70% sequence identity, with the greatest sequence similarity in the C-terminal region that forms the transmembrane, ion-conducting domain comprising ∼500 amino acids. The remaining ∼4,500 amino acids form the large regulatory cytoplasmic "foot" structure. Experimental evidence for Ca²⁺, ATP, phosphorylation, and redox-sensitive sites in the cytoplasmic structure have been described. Exogenous effectors include the two Ca²⁺ releasing agents caffeine and ryanodine. Recent work describing the near atomic structures of mammalian skeletal and cardiac muscle RyRs provides a structural basis for the regulation of the RyRs by their multiple effectors.

K201 (JTV519) is a Ca2+-Dependent Blocker of SERCA and a Partial Agonist of Ryanodine Receptors in Striated Muscle

K201 (JTV-519) may prevent abnormal Ca(2+) leak from the sarcoplasmic reticulum (SR) in the ischemic heart and skeletal muscle (SkM) by stabilizing the ryanodine receptors (RyRs; RyR1 and RyR2, respectively). We tested direct modulation of the SR Ca(2+)-stimulated ATPase (SERCA) and RyRs by K201. In isolated cardiac and SkM SR microsomes, K201 slowed the rate of SR Ca(2+) loading, suggesting potential SERCA block and/or RyR agonism. K201 displayed Ca(2+)-dependent inhibition of SERCA-dependent ATPase activity, which was measured in microsomes incubated with 200, 2, and 0.25 µM Ca(2+) and with the half-maximal K201 inhibitory doses (IC50) estimated at 130, 19, and 9 µM (cardiac muscle) and 104, 13, and 5 µM (SkM SR). K201 (≥5 µM) increased RyR1-mediated Ca(2+) release from SkM microsomes. Maximal K201 doses at 80 µM produced ∼37% of the increase in SkM SR Ca(2+) release observed with the RyR agonist caffeine. K201 (≥5 µM) increased the open probability (Po) of very active ("high-activity") RyR1 of SkM reconstituted into bilayers, but it had no effect on "low-activity" channels. Likewise, K201 activated cardiac RyR2 under systolic Ca(2+) conditions (∼5 µM; channels at Po ∼0.3) but not under diastolic Ca(2+) conditions (∼100 nM; Po < 0.01). Thus, K201-induced the inhibition of SR Ca(2+) leak found in cell-system studies may relate to potentially potent SERCA block under resting Ca(2+) conditions. SERCA block likely produces mild SR depletion in normal conditions but could prevent SR Ca(2+) overload under pathologic conditions, thus precluding abnormal RyR-mediated Ca(2+) release.

Ryanodine receptor gating controls generation of diastolic calcium waves in cardiac myocytes

Calcium waves can form and propagate at low frequencies of spontaneous calcium sparks if the calcium dependence of spark frequency is sufficiently steep, or the number of open RyRs is sufficiently large.

The role of cardiac ryanodine receptor (RyR) gating in the initiation and propagation of calcium waves was investigated using a mathematical model comprising a stochastic description of RyR gating and a deterministic description of calcium diffusion and sequestration. We used a one-dimensional array of equidistantly spaced RyR clusters, representing the confocal scanning line, to simulate the formation of calcium sparks. Our model provided an excellent description of the calcium dependence of the frequency of diastolic calcium sparks and of the increased tendency for the production of calcium waves after a decrease in cytosolic calcium buffering. We developed a hypothesis relating changes in the propensity to form calcium waves to changes of RyR gating and tested it by simulation. With a realistic RyR gating model, increased ability of RyR to be activated by Ca²⁺ strongly increased the propensity for generation of calcium waves at low (0.05–0.1-µM) calcium concentrations but only slightly at high (0.2–0.4-µM) calcium concentrations. Changes in RyR gating altered calcium wave formation by changing the calcium sensitivity of spontaneous calcium spark activation and/or the average number of open RyRs in spontaneous calcium sparks. Gating changes that did not affect RyR activation by Ca²⁺ had only a weak effect on the propensity to form calcium waves, even if they strongly increased calcium spark frequency. Calcium waves induced by modulating the properties of the RyR activation site could be suppressed by inhibiting the spontaneous opening of the RyR. These data can explain the increased tendency for production of calcium waves under conditions when RyR gating is altered in cardiac diseases.

Cardiac ryanodine receptor: Selectivity for alkaline earth metal cations points to the EF-hand nature of luminal binding sites

A growing body of evidence suggests that the regulation of cardiac ryanodine receptor (RYR2) by luminal Ca(2+) is mediated by luminal binding sites located on the RYR2 channel itself and/or its auxiliary protein, calsequestrin. The localization and structure of RYR2-resident binding sites are not known because of the lack of a high-resolution structure of RYR2 luminal regions. To obtain the first structural insight, we probed the RYR2 luminal face stripped of calsequestrin by alkaline earth metal divalents (M(2+): Mg(2+), Ca(2+), Sr(2+) or Ba(2+)). We show that the RYR2 response to caffeine at the single-channel level is significantly modified by the nature of luminal M(2+). Moreover, we performed competition experiments by varying the concentration of luminal M(2+) (Mg(2+), Sr(2+) or Ba(2+)) from 8 mM to 53 mM and investigated its ability to compete with 1mM luminal Ca(2+). We demonstrate that all tested M(2+) bind to exactly the same RYR2 luminal binding sites. Their affinities decrease in the order: Ca(2+)>Sr(2+)>Mg(2+)~Ba(2+), showing a strong correlation with the M(2+) affinity of the EF-hand motif. This indicates that the RYR2 luminal binding regions and the EF-hand motif likely share some structural similarities because the structure ties directly to the function.

Intracellular Zinc Modulates Cardiac Ryanodine Receptor-mediated Calcium Release

Aberrant Zn²⁺ homeostasis is a hallmark of certain cardiomyopathies associated with altered contractile force. In this study, we addressed whether Zn²⁺ modulates cardiac ryanodine receptor gating and Ca²⁺ dynamics in isolated cardiomyocytes. We reveal that Zn²⁺ is a high affinity regulator of RyR2 displaying three modes of operation. Picomolar free Zn²⁺ concentrations potentiate RyR2 responses, but channel activation is still dependent on the presence of cytosolic Ca²⁺. At concentrations of free Zn²⁺ >1 nm, Zn²⁺ is the main activating ligand, and the dependence on Ca²⁺ is removed. Zn²⁺ is therefore a higher affinity activator of RyR2 than Ca²⁺. Millimolar levels of free Zn²⁺ were found to inhibit channel openings. In cardiomyocytes, consistent with our single channel results, we show that Zn²⁺ modulates both the frequency and amplitude of Ca²⁺ waves in a concentration-dependent manner and that physiological levels of Zn²⁺ elicit Ca²⁺ release in the absence of activating levels of cytosolic Ca²⁺. This highlights a new role for intracellular Zn²⁺ in shaping Ca²⁺ dynamics in cardiomyocytes through modulation of RyR2 gating.

Examining a new role for zinc in regulating calcium release in cardiac muscle

It is well established that mammalian cells contain a small but measurable pool of free or labile zinc in the cytosol that is buffered in the high picomolar range. Recent attention has focused on the fact that this pool of free zinc has signalling effects that can be evoked through extracellular stimuli posing the question as to whether zinc should be regarded as a second messenger. Our knowledge of the targets, the biological significance and the molecular mechanisms of zinc signalling is limited but recent evidence suggests that zinc homoeostasis may be intimately linked to intracellular calcium signalling. In this review, we discuss the role of zinc as an intracellular signalling molecule with an emphasis on the potential role of zinc in shaping calcium-dynamics in cardiac muscle. We also consider the evidence that the cardiac ryanodine receptor (RyR2) is a potential zinc signalling target.

Insights into the gating mechanism of the ryanodine-modified human cardiac Ca2+-release channel (ryanodine receptor 2)

Ryanodine receptors (RyRs) are intracellular membrane channels playing key roles in many Ca(2+) signaling pathways and, as such, are emerging novel therapeutic and insecticidal targets. RyRs are so named because they bind the plant alkaloid ryanodine with high affinity and although it is established that ryanodine produces profound changes in all aspects of function, our understanding of the mechanisms underlying altered gating is minimal. We address this issue using detailed single-channel gating analysis, mathematical modeling, and energetic evaluation of state transitions establishing that, with ryanodine bound, the RyR pore adopts an extremely stable open conformation. We demonstrate that stability of this state is influenced by interaction of divalent cations with both activating and inhibitory cytosolic sites and, in the absence of activating Ca(2+), trans-membrane voltage. Comparison of the conformational stability of ryanodine- and Imperatoxin A-modified channels identifies significant differences in the mechanisms of action of these qualitatively similar ligands.

Eudistomin D and penaresin derivatives as modulators of ryanodine receptor channels and sarcoplasmic reticulum Ca2+ ATPase in striated muscle

Eudistomin D (EuD) and penaresin (Pen) derivatives are bioactive alkaloids from marine sponges found to induce Ca(2+) release from striated muscle sarcoplasmic reticulum (SR). Although these alkaloids are believed to affect ryanodine receptor (RyR) gating in a "caffeine-like" manner, no single-channel study confirmed this assumption. Here, EuD and MBED (9-methyl-7-bromoeudistomin D) were contrasted against caffeine on their ability to modulate the SR Ca(2+) loading/leak from cardiac and skeletal muscle SR microsomes as well as the function of RyRs in planar bilayers. The effects of these alkaloids on [(3)H]ryanodine binding and SR Ca(2+) ATPase (SERCA) activity were also tested. MBED (1-5 μM) fully mimicked maximal activating effects of caffeine (20 mM) on SR Ca(2+) leak. At the single-channel level, MBED mimicked the agonistic action of caffeine on cardiac RyR gating (i.e., stabilized long openings characteristic of "high-open-probability" mode). EuD was a partial agonist at the maximal doses tested. The tested Pen derivatives displayed mild to no agonism on RyRs, SR Ca(2+) leak, or [(3)H]ryanodine binding studies. Unlike caffeine, EuD and some Pen derivatives significantly inhibited SERCA at concentrations required to modulate RyRs. Instead, MBED's affinity for RyRs (EC50 ∼ 0.5 μM) was much larger than for SERCA (IC50 > 285 μM). In conclusion, MBED is a potent RyR agonist and, potentially, a better choice than caffeine for microsomal and cell studies due to its reported lack of effects on adenosine receptors and phosphodiesterases. As a high-affinity caffeine-like probe, MBED could also help identify the caffeine-binding site in RyRs.

Luminal Ca2+ controls activation of the cardiac ryanodine receptor by ATP

The synergic effect of luminal Ca²⁺, cytosolic Ca²⁺, and cytosolic adenosine triphosphate (ATP) on activation of cardiac ryanodine receptor (RYR2) channels was examined in planar lipid bilayers. The dose–response of RYR2 gating activity to ATP was characterized at a diastolic cytosolic Ca²⁺ concentration of 100 nM over a range of luminal Ca²⁺ concentrations and, vice versa, at a diastolic luminal Ca²⁺ concentration of 1 mM over a range of cytosolic Ca²⁺ concentrations. Low level of luminal Ca²⁺ (1 mM) significantly increased the affinity of the RYR2 channel for ATP but without substantial activation of the channel. Higher levels of luminal Ca²⁺ (8–53 mM) markedly amplified the effects of ATP on the RYR2 activity by selectively increasing the maximal RYR2 activation by ATP, without affecting the affinity of the channel to ATP. Near-diastolic cytosolic Ca²⁺ levels (<500 nM) greatly amplified the effects of luminal Ca²⁺. Fractional inhibition by cytosolic Mg²⁺ was not affected by luminal Ca²⁺. In models, the effects of luminal and cytosolic Ca²⁺ could be explained by modulation of the allosteric effect of ATP on the RYR2 channel. Our results suggest that luminal Ca²⁺ ions potentiate the RYR2 gating activity in the presence of ATP predominantly by binding to a luminal site with an apparent affinity in the millimolar range, over which local luminal Ca²⁺ likely varies in cardiac myocytes.

Coupled gating of skeletal muscle ryanodine receptors is modulated by Ca2+, Mg2+, and ATP

Coupled gating (synchronous openings and closures) of groups of skeletal muscle ryanodine receptors (RyR1), which mimics RyR1-mediated Ca(2+) release underlying Ca(2+) sparks, was first described by Marx et al. (Marx SO, Ondrias K, Marks AR. Science 281: 818-821, 1998). The nature of the RyR1-RyR1 interactions for coupled gating still needs to be characterized. Consequently, we defined planar lipid bilayer conditions where ∼25% of multichannel reconstitutions contain mixtures of coupled and independently gating RyR1. In ∼10% of the cases, all RyRs (2-10 channels; most frequently 3-4) gated in coupled fashion, allowing for quantification. Our results indicated that coupling required cytosolic solutions containing ATP/Mg(2+) and high (50 mM) luminal Ca(2+) (Ca(lum)) or Sr(2+) solutions. Bursts of coupled activity (events) started and ended abruptly, with all channels activating/deactivating within ∼300 μs. Coupled RyR1 were heterogeneous, where highly active RyR1 ("drivers") seemed open during the entire coupled event (P(o) = 1), while other RyR1s ("followers") displayed abundant flickering and smaller amplitude. Drivers mean open time increased with cytosolic Ca(2+) (Ca(cyt)) or caffeine, whereas followers flicker frequency was Ca(cyt) independent and more sensitive to inhibition by cytosolic Mg(2+). Coupled events were insensitive to varying lumen-to-cytosol Ca(2+) fluxes from ∼1 to 8 pA, which does not corroborate coupling of neighboring RyR1 by local Ca(2+)-induced Ca(2+) release. However, coupling requires specific Ca(lum) sites, as it was lost when Ca(lum) was replaced by luminal Ba(2+) or Mg(2+). In summary, coupled events reveal complex interactions among heterogeneous RyR1, differentially modulated by cytosolic ATP/Mg(2+), Ca(cyt), and Ca(lum,) which under cell-like ionic conditions may parallel synchronous RyR1 gating during Ca(2+) sparks.