The ryanodine receptor: a pivotal Ca2+ regulatory protein and potential therapeutic drug target

The ryanodine receptor (RyR) calcium release channel is an essential intracellular ion channel that is central to Ca(2+) signaling and contraction in the heart and skeletal muscle. The rapid release of Ca(2+) from the internal sarcoplasmic reticulum Ca(2+) stores through the RyR during excitation-contraction coupling is facilitated by the unique arrangement of the surface and sarcoplasmic reticulum membrane systems. Debilitating and sometimes fatal skeletal and cardiomyopathies result from changes in RyR activity that disrupt normal Ca(2+) signaling. Such changes can be caused by point mutations in many different regions of the RyR protein or acquired as a result of stress associated with exercise, heart failure, age or drugs. In general, both inherited and acquired changes include an increase in RyR channel activity. Because of its central function, the RyR is a potential therapeutic target for the inherited disorders and many of the acquired disorders. The RyR is currently used as a therapeutic target in malignant hyperthermia where dantrolene is effective and to relieve ventricular arrhythmia, with the use of JTV519 and flecainide. These drugs show that the RyR is a valid therapeutic target, but have side effects that prevent their chronic use. Thus there is an urgent need for the development of skeletal and cardiac specific drugs to treat these diverse muscle disorders. In this review, we discuss the mutations that cause skeletal myopathies and cardiac arrhythmias and how these mutations pinpoint residues within the RyR protein that are functionally significant and might be developed as targets for therapeutic drugs.

3D Mapping of the SPRY2 domain of ryanodine receptor 1 by single-particle cryo-EM

The type 1 skeletal muscle ryanodine receptor (RyR1) is principally responsible for Ca(2+) release from the sarcoplasmic reticulum and for the subsequent muscle contraction. The RyR1 contains three SPRY domains. SPRY domains are generally known to mediate protein-protein interactions, however the location of the three SPRY domains in the 3D structure of the RyR1 is not known. Combining immunolabeling and single-particle cryo-electron microscopy we have mapped the SPRY2 domain (S1085-V1208) in the 3D structure of RyR1 using three different antibodies against the SPRY2 domain. Two obstacles for the image processing procedure; limited amount of data and signal dilution introduced by the multiple orientations of the antibody bound in the tetrameric RyR1, were overcome by modifying the 3D reconstruction scheme. This approach enabled us to ascertain that the three antibodies bind to the same region, to obtain a 3D reconstruction of RyR1 with the antibody bound, and to map SPRY2 to the periphery of the cytoplasmic domain of RyR1. We report here the first 3D localization of a SPRY2 domain in any known RyR isoform.

Cyclization of the intrinsically disordered α1S dihydropyridine receptor II-III loop enhances secondary structure and in vitro function

A key component of excitation contraction (EC) coupling in skeletal muscle is the cytoplasmic linker (II-III loop) between the second and third transmembrane repeats of the α(1S) subunit of the dihydropyridine receptor (DHPR). The II-III loop has been previously examined in vitro using a linear II-III loop with unrestrained N- and C-terminal ends. To better reproduce the loop structure in its native environment (tethered to the DHPR transmembrane domains), we have joined the N and C termini using intein-mediated technology. Circular dichroism and NMR spectroscopy revealed a structural shift in the cyclized loop toward a protein with increased α-helical and β-strand structure in a region of the loop implicated in its in vitro function and also in a critical region for EC coupling. The affinity of binding of the II-III loop binding to the SPRY2 domain of the skeletal ryanodine receptor (RyR1) increased 4-fold, and its ability to activate RyR1 channels in lipid bilayers was enhanced 3-fold by cyclization. These functional changes were predicted consequences of the structural enhancement. We suggest that tethering the N and C termini stabilized secondary structural elements in the DHPR II-III loop and may reflect structural and dynamic characteristics of the loop that are inherent in EC coupling.

The β(1a) subunit of the skeletal DHPR binds to skeletal RyR1 and activates the channel via its 35-residue C-terminal tail

Although it has been suggested that the C-terminal tail of the β(1a) subunit of the skeletal dihyropyridine receptor (DHPR) may contribute to voltage-activated Ca(2+) release in skeletal muscle by interacting with the skeletal ryanodine receptor (RyR1), a direct functional interaction between the two proteins has not been demonstrated previously. Such an interaction is reported here. A peptide with the sequence of the C-terminal 35 residues of β(1a) bound to RyR1 in affinity chromatography. The full-length β(1a) subunit and the C-terminal peptide increased [(3)H]ryanodine binding and RyR1 channel activity with an AC(50) of 450-600 pM under optimal conditions. The effect of the peptide was dependent on cytoplasmic Ca(2+), ATP, and Mg(2+) concentrations. There was no effect of the peptide when channel activity was very low as a result of Mg(2+) inhibition or addition of 100 nM Ca(2+) (without ATP). Maximum increases were seen with 1-10 μM Ca(2+), in the absence of Mg(2+) inhibition. A control peptide with the C-terminal 35 residues in a scrambled sequence did not bind to RyR1 or alter [(3)H]ryanodine binding or channel activity. This high-affinity in vitro functional interaction between the C-terminal 35 residues of the DHPR β(1a) subunit and RyR1 may support an in vivo function of β(1a) during voltage-activated Ca(2+) release.

The voltage-gated calcium-channel beta subunit: more than just an accessory

Voltage-gated Ca(2+) channels (VGCCs) are involved in a number of excitatory processes in the cell that regulate muscle contraction, neurotransmitter release, gene regulation, and neuronal migration. They consist of a central pore-forming alpha(1) subunit together with a number of associated auxiliary subunits including a cytoplasmic beta subunit. With the aid of X-ray crystallography, it has been found that the beta subunits of VGCCs (beta(2a), beta(3), and beta(4)) interact strongly with the I-II loop of the pore-forming alpha(1) subunit. Here we discuss the potential interaction sites of beta(1a) with its alpha(1) subunit as well as the skeletal ryanodine receptor. We suggest that not only can beta(1a) interact with the alpha(1) subunit I-II loop, but more subtle interactions may be possible through the II-III loop via the beta(1a) SH3 domain. Such findings could have important implications with respect to EC coupling.

Unique isoform-specific properties of calsequestrin in the heart and skeletal muscle

Calcium signaling in myocytes is dependent on the cardiac ryanodine receptor (RyR2) calcium release channel and the calcium buffering protein in the sarcoplasmic reticulum, cardiac calsequestrin (CSQ2). The overall properties of CSQ2 and its regulation of RyR2 have not been explored in detail or directly compared with skeletal CSQ1 and its regulation of the skeletal RyR1, with physiological ionic strength and Ca(2+) concentrations. We find that there are major differences between the two isoforms under these physiological conditions. Ca(2+) binding to CSQ2 is 50% lower than to CSQ1. Only approximately 30% of CSQ2 is bound to cardiac junctional face membrane (JFM), compared with approximately 70% of CSQ1 and the ratio of CSQ2 to RyR2 is only 50% of the CSQ1/RyR1 ratio. Chemical crosslinking shows that CSQ2 is mostly monomer/dimer, while CSQ1 is mostly polymerized. In single channel lipid bilayer experiments, CSQ2 monomers and/or dimers increase the open probability of both RyR1 and RyR2 channels, while CSQ1 polymers decrease the activity of RyR1. We speculate that CSQ2 facilitates high rates of Ca(2+) release through RyR2 during systole, while CSQ1 curtails RyR1 opening in response to a single action potential to maintain Ca(2+) and allow repeated Ca(2+) release and graded activation with increased stimulation frequency.

In vitro modulation of the cardiac ryanodine receptor activity by Homer1

The Homer protein family allows clustering and/or functional modulation of many proteins from different calcium signalling complexes including those formed by the ryanodine receptor (RyR) Ca(2+) release channel in skeletal muscle and the heart. Homer1b/c and the cardiac RyR (RyR2) are strongly expressed in the heart and neurons where their interaction with each other may modulate Ca(2+) signalling. However, functional interactions between Homer1b and RyR2 have been poorly defined. Our preliminary data and similar consensus binding sites for Homer in RyR2 and skeletal RyR (RyR1) proteins, led to the hypothesis that Homer may similarly regulate both RyR isoforms. Single-channel and [(3)H]ryanodine binding data showed that RyR2 and RyR1 activity increased to a maximum with ~50-100 nM Homer1b and fell with Homer1b > 200 nM. Homer1b (50 nM) activated RyR2 and RyR1 at all cytosolic [Ca(2+)]; estimated EC(50) value of RyR2 diminished from ~2.8 microM Ca(2+) (control) to ~1.9 microM Ca(2+) in the presence of 50 nM Homer1b. Short Homer1 (lacking the coiled-coil multimerisation domain) and Homer1b similarly modulated RyR2, indicating an action through ligand binding, not mutimerisation. These actions of Homer were generally similar in RyR2 and RyR1. The strong functional interactions suggest that Homer1 is likely to be an endogenous modulator of RyR channels in the heart and neurons as well as in skeletal muscle.

Phosphorylation of skeletal muscle calsequestrin enhances its Ca2+ binding capacity and promotes its association with junctin

Calcium signaling, intrinsic to skeletal and cardiac muscle function, is critically dependent on the amount of calcium stored within the sarcoplasmic reticulum. Calsequestrin, the main calcium buffer in the sarcoplasmic reticulum, provides a pool of calcium for release through the ryanodine receptor and acts as a luminal calcium sensor for the channel via its interactions with triadin and junctin. We examined the influence of phosphorylation of calsequestrin on its ability to store calcium, to polymerise and to regulate ryanodine receptors by binding to triadin and junctin. Our hypothesis was that these parameters might be altered by phosphorylation of threonine 353, which is located near the calcium and triadin/junctin binding sites. Although phosphorylation increased the calcium binding capacity of calsequestrin nearly 2-fold, it did not alter calsequestrin polymerisation, its binding to triadin or junctin or inhibition of ryanodine receptor activity at 1 mM luminal calcium. Phosphorylation was required for calsequestrin binding to junctin when calcium concentration was low (100 nM), and ryanodine receptors were activated by dephosphorylated calsequestrin when it bound to triadin alone. These novel data shows that phosphorylated calsequestrin is required for high capacity calcium buffering and suggest that ryanodine receptor inhibition by calsequestrin is mediated by junctin.

Alternative splicing of RyR1 alters the efficacy of skeletal EC coupling

Alternative splicing of ASI residues (Ala(3481)-Gln(3485)) in the skeletal muscle ryanodine receptor (RyR1) is developmentally regulated: the residues are present in adult ASI(+)RyR1, but absent in the juvenile ASI(-)RyR1 which is over-expressed in adult myotonic dystrophy type 1 (DM1). Although this splicing switch may influence RyR1 function in developing muscle and DM1, little is known about the properties of the splice variants. We examined excitation-contraction (EC) coupling and the structure and interactions of the ASI domain (Thr(3471)-Gly(3500)) in the splice variants. Depolarisation-dependent Ca(2+) release was enhanced by >50% in myotubes expressing ASI(-)RyR1 compared with ASI(+)RyR1, although DHPR L-type currents and SR Ca(2+) content were unaltered, while ASI(-)RyR1 channel function was actually depressed. The effect on EC coupling did not depend on changes in ASI domain secondary structure. Probing RyR1 function with peptides possessing the ASI domain sequence indicated that the domain contributes to an inhibitory module in RyR1. The action of the peptide depended on a sequence of basic residues and their alignment in an alpha-helix adjacent to the ASI splice site. This is the first evidence that the ASI residues contribute to an inhibitory module in RyR1 that influences EC coupling. Implications for development and DM1 are discussed.

Redox potential and the response of cardiac ryanodine receptors to CLIC-2, a member of the glutathione S-transferase structural family

The type 2 chloride intracellular channel, CLIC-2, is a member of the glutathione S-transferase structural family and a suppressor of cardiac ryanodine receptor (RyR2) Ca2+ channels located in the membrane of the sarcoplasmic reticulum (SR). Modulators of RyR2 activity can alter cardiac contraction. Since both CLIC-2 and RyR2 are modified by redox reactions, we speculated that the action of CLIC-2 on RyR2 may depend on redox potential. We used a GSH:GSSG buffer system to produce mild changes in redox potential to influence redox sensors in RyR2 and CLIC-2. RyR2 activity was modified only when both luminal and cytoplasmic solutions contained the GSH:GSSG buffer and the effects were reversed by removing the buffer from one of the solutions. Channel activity increased with an oxidizing redox potential and decreased when the potential was more reducing. Addition of cytoplasmic CLIC-2 inhibited RyR2 with oxidizing redox potentials, but activated RyR2 under reducing conditions. The results suggested that both RyR2 and CLIC-2 contain redox sensors. Since cardiac ischemia involves a destructive Ca2+ overload that is partly due to oxidation-induced increase in RyR2 activity, we speculate that the properties of CLIC-2 place it in an ideal position to limit ischemia-induced cellular damage in cardiac muscle.

Muscle-specific GSTM2-2 on the luminal side of the sarcoplasmic reticulum modifies RyR ion channel activity

We show that a glutathione transferase (GST) protein, which is recognised by an antibody against the muscle-specific human GSTM2-2 (hGSTM2-2), is associated with the lumen of the sarcoplasmic reticulum (SR) of cardiac muscle, but not skeletal muscle. We further show that hGSTM2-2 modifies both cardiac and skeletal ryanodine receptor (RyR) activity when it binds to the luminal domain of the RyR channel complex. The properties of hGSTM2-2 were compared with those of the calsequestrin (CSQ), a Ca(2+) binding protein also present in the lumen of the SR which, like GSTM2-2, contains a thioredoxin-fold structure and modifies RyR activity (Wei, L., Varsanyi, M., Dulhunty, A. F., Beard, N. A. (2006). The Biophysical Journal, 91, 1288-1301). The glutathione transferase activity of hGSTM2-2 is strong, while CSQ is essentially inactive. Conversely CSQ is a strong Ca(2+) binder, but hGSTM2-2 is not. The effects of luminal hGSTM2-2 on RyR activity differ from those of CSQ in that hGSTM2-2 activates RyRs by increasing their open probability and conductance and the effects are independent of luminal Ca(2+) concentration. The results suggest that GSTM2-2 can interact with specific luminal sites on the RyR complex and that the interaction is likely to be within the pore of the RyR channel. The differences between the effects of CSQ and hGSTM2-2 suggest that the thioredoxin fold is not a major determinant of the luminal actions of either protein. The results indicate that GSTM2-2 is a novel luminal regulator of the RyR channels in the heart.

Triadin binding to the C-terminal luminal loop of the ryanodine receptor is important for skeletal muscle excitation contraction coupling

Ca²⁺ release from intracellular stores is controlled by complex interactions between multiple proteins. Triadin is a transmembrane glycoprotein of the junctional sarcoplasmic reticulum of striated muscle that interacts with both calsequestrin and the type 1 ryanodine receptor (RyR1) to communicate changes in luminal Ca²⁺ to the release machinery. However, the potential impact of the triadin association with RyR1 in skeletal muscle excitation–contraction coupling remains elusive. Here we show that triadin binding to RyR1 is critically important for rapid Ca²⁺ release during excitation–contraction coupling. To assess the functional impact of the triadin-RyR1 interaction, we expressed RyR1 mutants in which one or more of three negatively charged residues (D4878, D4907, and E4908) in the terminal RyR1 intraluminal loop were mutated to alanines in RyR1-null (dyspedic) myotubes. Coimmunoprecipitation revealed that triadin, but not junctin, binding to RyR1 was abolished in the triple (D4878A/D4907A/E4908A) mutant and one of the double (D4907A/E4908A) mutants, partially reduced in the D4878A/D4907A double mutant, but not affected by either individual (D4878A, D4907A, E4908A) mutations or the D4878A/E4908A double mutation. Functional studies revealed that the rate of voltage- and ligand-gated SR Ca²⁺ release were reduced in proportion to the degree of interruption in triadin binding. Ryanodine binding, single channel recording, and calcium release experiments conducted on WT and triple mutant channels in the absence of triadin demonstrated that the luminal loop mutations do not directly alter RyR1 function. These findings demonstrate that junctin and triadin bind to different sites on RyR1 and that triadin plays an important role in ensuring rapid Ca²⁺ release during excitation–contraction coupling in skeletal muscle.