Structural factors that determine the ability of adenosine and related compounds to activate the cardiac ryanodine receptor

Significance of Zn2+ in RyR1 for Structural Integrity and Ligand Binding: Insight from Molecular Dynamics

Ryanodine receptor type 1 (RyR1) is a Ca2+-release channel central to skeletal muscle excitation-contraction (EC) coupling. RyR1's cryo-EM structures reveal a zinc-finger motif positioned within the cytoplasmic C-terminal domain (CTD). Yet, owing to limitations in cryo-EM resolution, RyR1 structures lack precision in detailing the metal coordination structure, prompting the need for an accurate model. In this study, we employed molecular dynamics (MD) simulations and the density functional theory (DFT) method to refine the binding characteristics of Zn2+ in the zinc-finger site of the RyR1 channel. Our findings also highlight substantial conformational changes in simulations conducted in the absence of Zn2+. Notably, we observed a loss of contact at the interface between protein domains proximal to the zinc-finger site, indicating a crucial role of Zn2+ in maintaining structural integrity and interdomain interactions within RyR1. Furthermore, this study provides valuable insights into the modulation of ATP, Ca2+, and caffeine binding, shedding light on the intricate relationship between Zn2+ coordination and the dynamic behavior of RyR1. Our integrative approach combining MD simulations and DFT calculations enhances our understanding of the molecular mechanisms governing ligand binding in RyR1.

Mutations in proteins involved in E-C coupling and SOCE and congenital myopathies

In skeletal muscle, Ca²⁺ necessary for muscle contraction is stored and released from the sarcoplasmic reticulum (SR), a specialized form of endoplasmic reticulum through the mechanism known as excitation–contraction (E-C) coupling. Following activation of skeletal muscle contraction by the E-C coupling mechanism, replenishment of intracellular stores requires reuptake of cytosolic Ca²⁺ into the SR by the activity of SR Ca²⁺-ATPases, but also Ca²⁺ entry from the extracellular space, through a mechanism called store-operated calcium entry (SOCE). The fine orchestration of these processes requires several proteins, including Ca²⁺ channels, Ca²⁺ sensors, and Ca²⁺ buffers, as well as the active involvement of mitochondria. Mutations in genes coding for proteins participating in E-C coupling and SOCE are causative of several myopathies characterized by a wide spectrum of clinical phenotypes, a variety of histological features, and alterations in intracellular Ca²⁺ balance. This review summarizes current knowledge on these myopathies and discusses available knowledge on the pathogenic mechanisms of disease.

RyR1-related myopathy mutations in ATP and calcium binding sites impair channel regulation

The type 1 ryanodine receptor (RyR1) is an intracellular calcium (Ca²⁺) release channel on the sarcoplasmic/endoplasmic reticulum that is required for skeletal muscle contraction. RyR1 channel activity is modulated by ligands, including the activators Ca²⁺ and ATP. Patients with inherited mutations in RyR1 may exhibit muscle weakness as part of a heterogeneous, complex disorder known as RYR1-related myopathy (RYR1-RM) or more recently termed RYR1-related disorders (RYR1-RD). Guided by high-resolution structures of skeletal muscle RyR1, obtained using cryogenic electron microscopy, we introduced mutations into putative Ca²⁺ and ATP binding sites and studied the function of the resulting mutant channels. These mutations confirmed the functional significance of the Ca²⁺ and ATP binding sites identified by structural studies based on the effects on channel regulation. Under normal conditions, Ca²⁺ activates RyR1 at low concentrations (µM) and inhibits it at high concentrations (mM). Mutations in the Ca²⁺-binding site impaired both activating and inhibitory regulation of the channel, suggesting a single site for both high and low affinity Ca²⁺-dependent regulation of RyR1 function. Mutation of residues that interact with the adenine ring of ATP abrogated ATP binding to the channel, whereas mutating residues that interact with the triphosphate tail only affected the degree of activation. In addition, patients with mutations at the Ca²⁺ or ATP binding sites suffer from muscle weakness, therefore impaired RyR1 channel regulation by either Ca²⁺ or ATP may contribute to the pathophysiology of RYR1-RM in some patients.

The V2475F CPVT1 mutation yields distinct RyR2 channel populations that differ in their responses to cytosolic Ca2+ and Mg2

Catecholaminergic polymorphic ventricular tachycardia type 1 (CPVT1) is a lethal genetic disease causing arrhythmias and sudden cardiac death in children and young adults and is linked to mutations in the cardiac ryanodine receptor (RyR2). The effects of CPVT1 mutations on RyR2 ion-channel function are often investigated using purified recombinant RyR2 channels homozygous for the mutation. However, CPVT1 patients are heterozygous for the disease, so this approach does not reveal the true changes to RyR2 function across the entire RyR2 population of channels in the heart. We therefore investigated the native cardiac RyR2 single-channel abnormalities in mice heterozygous for the CPVT1 mutation, V2475F(+/-)-RyR2, and applied molecular modelling techniques to investigate the possible structural changes that could initiate any altered function. We observed that increased sensitivity of cardiac V2475F(+/-)-RyR2 channels to both activating and inactivating levels of cytosolic Ca²⁺ , plus attenuation of Mg²⁺ inhibition, were the most marked changes. Severity of abnormality was not uniform across all channels, giving rise to multiple sub-populations with differing functional characteristics. For example, 46% of V2475F(+/-)-RyR2 channels exhibited reduced Mg²⁺ inhibition and 23% were actually activated by Mg²⁺ . Using homology modelling, we discovered that V2475 is situated at a hinge between two regions of the RyR2 helical domain 1 (HD1). Our model proposes that detrimental functional changes to RyR2 arise because mutation at this critical site reduces the angle between these regions. Our results demonstrate the necessity of characterising the total heterozygous population of CPVT1-mutated channels in order to understand CPVT1 phenotypes in patients. KEY POINTS: RyR2 mutations can cause type-1 catecholaminergic polymorphic ventricular tachycardia (CPVT1), a lethal, autosomal-dominant arrhythmic disease. However, the changes in RyR2 ion-channel function that result from the many different patient mutations are rarely investigated in detail and often only recombinant RyR2, homozygous for the mutation, is studied. As CPVT1 is a heterozygous disease and the tetrameric RyR2 channels expressed in the heart will contain varying numbers of mutated monomers, we have investigated the range of RyR2 single-channel abnormalities found in the hearts of mice heterozygous for the CPVT1 mutation, V2475F(+/-)-RyR2. Specific alterations to ligand regulation of V2475F(+/-)-RyR2 were observed. Multiple sub-populations of channels exhibited varying degrees of abnormality. In particular, an increased sensitivity to activating and inactivating cytosolic [Ca²⁺ ], and reduced sensitivity to Mg²⁺ inhibition were evident. Our results provide mechanistic insight into the changes to RyR2 gating that destabilise sarcoplasmic reticulum Ca²⁺ -release causing life-threatening arrhythmias in V2475F(+/-)-CPVT1 patients.

The Ryanodine Receptor as a Sensor for Intracellular Environments in Muscles

The ryanodine receptor (RyR) is a Ca²⁺ release channel in the sarcoplasmic reticulum of skeletal and cardiac muscles and plays a key role in excitation-contraction coupling. The activity of the RyR is regulated by the changes in the level of many intracellular factors, such as divalent cations (Ca²⁺ and Mg²⁺), nucleotides, associated proteins, and reactive oxygen species. Since these intracellular factors change depending on the condition of the muscle, e.g., exercise, fatigue, or disease states, the RyR channel activity will be altered accordingly. In this review, we describe how the RyR channel is regulated under various conditions and discuss the possibility that the RyR acts as a sensor for changes in the intracellular environments in muscles.

Electrical polarity-dependent gating and a unique subconductance of RyR2 induced by S-adenosyl methionine via the ATP binding site

S-Adenosyl-l-methionine (SAM) was used to probe the functional effects exerted via the RyR2 adenine nucleotide binding site. Single channel experiments revealed that SAM applied to the cytoplasmic face of RyR2 had complex voltage dependent effects on channel gating and conductance. At positive transmembrane holding potentials, SAM caused a striking reduction in channel openings and a reduced channel conductance. In contrast, at negative potentials SAM promoted a clearly resolved subconductance state. At membrane potentials between -75 and -25 mV the open probability of the subconductance state was independent of voltage. ATP, but not the non-adenosine based RyR activator 4-chloro-m-cresol interfered with the effects of SAM at both negative and positive potentials. This suggests that ATP and SAM interact with a common binding site. Molecular docking showed SAM bound to the adenine nucleotide-binding site and formed a hydrogen bond to Glu4886 in the C-terminal end of the S6 alpha helix. In this configuration SAM may alter the conformation of the RyR2 ion conduction pathway. This work provides novel insight into potential functional outcomes of ligand binding to the RyR adenine nucleotide binding site.

Structural Basis for the Modulation of Ryanodine Receptors

Historically, ryanodine receptors (RyRs) have presented unique challenges for high-resolution structural determination despite long-standing interest in their role in excitation-contraction coupling. Owing to their large size (nearly 2.2 MDa), high-resolution structures remained elusive until the advent of cryogenic electron microscopy (cryo-EM) techniques. In recent years, structures for both RyR1 and RyR2 have been solved at near-atomic resolution. Furthermore, recent reports have delved into their more complex structural associations with key modulators - proteins such as the dihydropyridine receptor (DHPR), FKBP12/12.6, and calmodulin (CaM), as well as ions and small molecules including Ca²⁺, ATP, caffeine, and PCB95. This review addresses the modulation of RyR1 and RyR2, in addition to the impact of such discoveries on intracellular Ca²⁺ dynamics and biophysical properties.

Molecular basis for allosteric regulation of the type 2 ryanodine receptor channel gating by key modulators

As a switch for the release of Ca²⁺ from the sarco(endo)plasmic reticulum of cardiomyocytes, the type 2 ryanodine receptor (RyR2) is subject to sophisticated regulation by a broad spectrum of modulators. Dysregulation of RyR2-mediated Ca²⁺ release is linked to life-threatening cardiac arrhythmias. The regulatory mechanism of RyR2 by key modulators, such as Ca²⁺, FKBP12.6, ATP, and caffeine, remains unclear. This study provides important insights into the long-range allosteric regulation of RyR2 channel gating by these modulators and serves as an important framework for mechanistic understanding of the regulation of this key player in the excitation–contraction coupling of cardiac muscles.

The type 2 ryanodine receptor (RyR2) is responsible for releasing Ca²⁺ from the sarcoplasmic reticulum of cardiomyocytes, subsequently leading to muscle contraction. Here, we report 4 cryo-electron microscopy (cryo-EM) structures of porcine RyR2 bound to distinct modulators that, together with our published structures, provide mechanistic insight into RyR2 regulation. Ca²⁺ alone induces a contraction of the central domain that facilitates the dilation of the S6 bundle but is insufficient to open the pore. The small-molecule agonist PCB95 helps Ca²⁺ to overcome the barrier for opening. FKBP12.6 induces a relaxation of the central domain that decouples it from the S6 bundle, stabilizing RyR2 in a closed state even in the presence of Ca²⁺ and PCB95. Although the channel is open when PCB95 is replaced by caffeine and adenosine 5′-triphosphate (ATP), neither of the modulators alone can sufficiently counter the antagonistic effect to open the channel. Our study marks an important step toward mechanistic understanding of the sophisticated regulation of this key channel whose aberrant activity engenders life-threatening cardiac disorders.

Promiscuous attraction of ligands within the ATP binding site of RyR2 promotes diverse gating behaviour

ATP is an essential constitutive regulator of cardiac ryanodine receptors (RyR2), enabling small changes in cytosolic Ca²⁺ to trigger large changes in channel activity. With recent landmark determinations of the full structures of RyR1 (skeletal isoform) and RyR2 using cryo-EM, and identification of the RyR1 ATP binding site, we have taken the opportunity to model the binding of fragments of ATP into RyR2 in order to investigate how the structure of the ATP site dictates the functional responses of ligands attracted there. RyR2 channel gating was assessed under voltage-clamp conditions and by [³H]ryanodine binding studies. We show that even the triphosphate (PPPi) moiety alone was capable of activating RyR2 but produced two distinct effects (activation or irreversible inactivation) that we suggest correspond to two preferred binding locations within the ATP site. Combinations of complementary fragments of ATP (Pi + ADP or PPi + AMP) could not reproduce the effects of ATP, however, the presence of adenosine prevented the inactivating PPPi effects, allowing activation similar to that of ATP. RyR2 appears to accommodate diverse types of molecules, including PPPi, deep within the ATP binding site. The most effective ligands, however, have at least three phosphate groups that are guided into place by a nucleoside.

Ryanodine Receptor Structure and Function in Health and Disease

Ryanodine receptors (RyRs) are ubiquitous intracellular calcium (Ca2+) release channels required for the function of many organs including heart and skeletal muscle, synaptic transmission in the brain, pancreatic beta cell function, and vascular tone. In disease, defective function of RyRs due either to stress (hyperadrenergic and/or oxidative overload) or genetic mutations can render the channels leaky to Ca2+ and promote defective disease-causing signals as observed in heat failure, muscular dystrophy, diabetes mellitus, and neurodegerative disease. RyRs are massive structures comprising the largest known ion channel-bearing macromolecular complex and exceeding 3 million Daltons in molecular weight. RyRs mediate the rapid release of Ca2+ from the endoplasmic/sarcoplasmic reticulum (ER/SR) to stimulate cellular functions through Ca2+-dependent processes. Recent advances in single-particle cryogenic electron microscopy (cryo-EM) have enabled the determination of atomic-level structures for RyR for the first time. These structures have illuminated the mechanisms by which these critical ion channels function and interact with regulatory ligands. In the present chapter we discuss the structure, functional elements, gating and activation mechanisms of RyRs in normal and disease states.

Dampened activity of ryanodine receptor channels in mutant skeletal muscle lacking TRIC-A

KEY POINTS

The role of trimeric intracellular cation (TRIC) channels is not known, although evidence suggests they may regulate ryanodine receptors (RyR) via multiple mechanisms. We therefore investigated whether Tric-a gene knockout (KO) alters the single-channel function of skeletal RyR (RyR1). We find that RyR1 from Tric-a KO mice are more sensitive to inhibition by divalent cations, although they respond normally to cytosolic Ca²⁺ , ATP, caffeine and luminal Ca²⁺ . In the presence of Mg²⁺ , ATP cannot effectively activate RyR1 from Tric-a KO mice. Additionally, RyR1 from Tric-a KO mice are not activated by protein kinase A phosphorylation, demonstrating a defect in the ability of β-adrenergic stimulation to regulate sarcoplasmic reticulum (SR) Ca²⁺ -release. The defective RyR1 gating that we describe probably contributes significantly to the impaired SR Ca²⁺ -release observed in skeletal muscle from Tric-a KO mice, further highlighting the importance of TRIC-A for normal physiological regulation of SR Ca²⁺ -release in skeletal muscle.

ABSTRACT

The type A trimeric intracellular cation channel (TRIC-A) is a major component of the nuclear and sarcoplasmic reticulum (SR) membranes of cardiac and skeletal muscle, and is localized closely with ryanodine receptor (RyR) channels in the SR terminal cisternae. The skeletal muscle of Tric-a knockout (KO) mice is characterized by Ca²⁺ overloaded and swollen SR and by changes in the properties of SR Ca²⁺ release. We therefore investigated whether RyR1 gating behaviour is modified in the SR from Tric-a KO mice by incorporating native RyR1 into planar phospholipid bilayers under voltage-clamp conditions. We find that RyR1 channels from Tric-a KO mice respond normally to cytosolic Ca²⁺ , ATP, adenine, caffeine and to luminal Ca²⁺ . However, the channels are more sensitive to the inactivating effects of divalent cations, thus, in the presence of Mg²⁺ , ATP is inadequate as an activator. Additionally, channels are not characteristically activated by protein kinase A even though the phosphorylation levels of Ser2844 are similar to controls. The results of the present study suggest that TRIC-A functions as an excitatory modulator of RyR1 channels within the SR terminal cisternae. Importantly, this regulatory action of TRIC-A appears to be independent of (although additive to) any indirect consequences to RyR1 activity that arise as a result of K⁺ fluxes across the SR via TRIC-A.

Structural Basis for Gating and Activation of RyR1

The type-1 ryanodine receptor (RyR1) is an intracellular calcium (Ca(2+)) release channel required for skeletal muscle contraction. Here, we present cryo-EM reconstructions of RyR1 in multiple functional states revealing the structural basis of channel gating and ligand-dependent activation. Binding sites for the channel activators Ca(2+), ATP, and caffeine were identified at interdomain interfaces of the C-terminal domain. Either ATP or Ca(2+) alone induces conformational changes in the cytoplasmic assembly ("priming"), without pore dilation. In contrast, in the presence of all three activating ligands, high-resolution reconstructions of open and closed states of RyR1 were obtained from the same sample, enabling analyses of conformational changes associated with gating. Gating involves global conformational changes in the cytosolic assembly accompanied by local changes in the transmembrane domain, which include bending of the S6 transmembrane segment and consequent pore dilation, displacement, and deformation of the S4-S5 linker and conformational changes in the pseudo-voltage-sensor domain.

Structures of the colossal RyR1 calcium release channel

Ryanodine receptors (RyRs) are intracellular cation channels that mediate the rapid and voluminous release of Ca2+ from the sarcoplasmic reticulum (SR) as required for excitation-contraction coupling in cardiac and skeletal muscle. Understanding of the architecture and gating of RyRs has advanced dramatically over the past two years, due to the publication of high resolution cryo-electron microscopy (cryoEM) reconstructions and associated atomic models of multiple functional states of the skeletal muscle receptor, RyR1. Here we review recent advances in our understanding of RyR architecture and gating, and highlight remaining gaps in understanding which we anticipate will soon be filled.

ATP interacts with the CPVT mutation-associated central domain of the cardiac ryanodine receptor

BACKGROUND

This study was designed to determine whether the cardiac ryanodine receptor (RyR2) central domain, a region associated with catecholamine polymorphic ventricular tachycardia (CPVT) mutations, interacts with the RyR2 regulators, ATP and the FK506-binding protein 12.6 (FKBP12.6).

METHODS

Wild-type (WT) RyR2 central domain constructs (G(2236)to G(2491)) and those containing the CPVT mutations P2328S and N2386I, were expressed as recombinant proteins. Folding and stability of the proteins were examined by circular dichroism (CD) spectroscopy and guanidine hydrochloride chemical denaturation.

RESULTS

The far-UV CD spectra showed a soluble stably-folded protein with WT and mutant proteins exhibiting a similar secondary structure. Chemical denaturation analysis also confirmed a stable protein for both WT and mutant constructs with similar two-state unfolding. ATP and caffeine binding was measured by fluorescence spectroscopy. Both ATP and caffeine bound with an EC50 of ~200-400μM, and the affinity was the same for WT and mutant constructs. Sequence alignment with other ATP binding proteins indicated the RyR2 central domain contains the signature of an ATP binding pocket. Interaction of the central domain with FKBP12.6 was tested by glutaraldehyde cross-linking and no association was found.

CONCLUSIONS

The RyR2 central domain, expressed as a 'correctly' folded recombinant protein, bound ATP in accord with bioinformatics evidence of conserved ATP binding sequence motifs. An interaction with FKBP12.6 was not evident. CPVT mutations did not disrupt the secondary structure nor binding to ATP.

GENERAL SIGNIFICANCE

Part of the RyR2 central domain CPVT mutation cluster, can be expressed independently with retention of ATP binding.

Diadenosine pentaphosphate is a potent activator of cardiac ryanodine receptors revealing a novel high-affinity binding site for adenine nucleotides

BACKGROUND AND PURPOSE

Diadenosine polyphosphates are normally present in cells at low levels, but significant increases in concentrations can occur during cellular stress. The aim of this study was to investigate the effects of diadenosine pentaphosphate (Ap5A) and an oxidized analogue, oAp5A on the gating of sheep cardiac ryanodine receptors (RyR2).

EXPERIMENTAL APPROACH

RyR2 channel function was monitored after incorporation into planar bilayers under voltage-clamp conditions.

KEY RESULTS

With10 micromol.L(-1) cytosolic Ca2+, a significant 'hump' or plateau at the base of the dose-response relationship to Ap5A was revealed. Open probability (Po) was significantly increased to a plateau of approximately 0.2 in the concentration range 100 pmol x L(-1)-10 micromol x L(-1). High Po values were observed at >10 micromol x L(-1) Ap5A, and Po values close to 1 could be achieved. Nanomolar levels of ATP and adenosine also revealed a hump at the base of the dose-response relationships, although GTP did not activate at any concentration, indicating a common, high-affinity binding site on RyR2 for adenine-based compounds. The oxidized analogue, oAp5A, did not significantly activate RyR2 via the high-affinity binding site; however, it could fully open the channel with an EC(50) of 16 micromol.L(-1) (Ap5A EC(50) = 140 micromol x L(-1)). Perfusion experiments suggest that oAp5A and Ap5A dissociate slowly from their binding sites on RyR2.

CONCLUSIONS AND IMPLICATIONS

The ability of Ap5A compounds to increase Po even in the presence of ATP and their slow dissociation from the channel may enable these compounds to act as physiological regulators of RyR2, particularly under conditions of cellular stress.

Single-channel characterization of the rabbit recombinant RyR2 reveals a novel inactivation property of physiological concentrations of ATP

Ryanodine receptor 2 (RyR2) cDNA has been available for more than 15 years; however, due to the complex nature of ligand gating in this channel, many aspects of recombinant RyR2 function have been unresearched. We established a stable, inducible HEK 293 cell line expressing full-length rabbit RyR2 cDNA and assessed the single-channel properties of the recombinant RyR2, with particular reference to ligand regulation with Ca2+ as the permeant ion. We found that the single-channel conductances of recombinant RyR2 and RyR2 isolated from cardiac muscle are essentially identical, as is irreversible modification by ryanodine. Although it is known that RyR2 expressed in HEK 293 cells is not associated with FKBP12.6, we demonstrate that these channels do not exhibit any discernable disorganized gating characteristics or subconductance states. We also show that the gating of recombinant RyR2 is indistinguishable from that of channels isolated from cardiac muscle when activated by cytosolic Ca2+, caffeine or suramin. The mechanisms underlying ATP activation are also similar; however, the experiments highlighted a novel effect of ATP at physiologically relevant concentrations of 5-10 mM. With Ca2+ as permeant ion, 5-10 mM ATP consistently inactivated recombinant channels (15/16 experiments). Such inactivation was rarely observed with native RyR2 isolated from cardiac muscle (1 in 16 experiments). However, if the channels were purified, inactivation by ATP was then revealed in all experiments. This action of ATP may be relevant for inactivation of sarcoplasmic reticulum Ca2+ release during cardiac excitation-contraction coupling or may represent unnatural behavior that is revealed when RyR2 is purified or expressed in noncardiac systems.

S-Adenosyl-l-methionine activates the cardiac ryanodine receptor

S-Adenosyl-l-methionine (SAM) is the biological methyl-group donor for the enzymatic methylation of numerous substrates including proteins. SAM has been reported to activate smooth muscle derived ryanodine receptor calcium release channels. Therefore, we examined the effects of SAM on the cardiac isoform of the ryanodine receptor (RyR2). SAM increased cardiac sarcoplasmic reticulum [(3)H]ryanodine binding in a concentration-dependent manner by increasing the affinity of RyR2 for ryanodine. Activation occurred at physiologically relevant concentrations. SAM, which contains an adenosine moiety, enhanced ryanodine binding in the absence but not in the presence of an ATP analogue. S-Adenosyl-l-homocysteine (SAH) is the product of the loss of the methyl-group from SAM and inhibits methylation reactions. SAH did not activate RyR2 but did inhibit SAM-induced RyR2 activation. SAH did not alter adenine nucleotide activation of RyR2. These data suggest SAM activates RyR2 via a site that interacts with, but is distinct from, the adenine nucleotide binding site.

Maximum Phosphorylation of the Cardiac Ryanodine Receptor at Serine-2809 by Protein Kinase A Produces Unique Modifications to Channel Gating and Conductance Not Observed at Lower Levels of Phosphorylation

It is suggested that protein kinase A (PKA)-dependent phosphorylation of cardiac ryanodine receptors (RyR2) is linked to the development of heart failure and the generation of fatal cardiac arrhythmias. It is also suggested that RyR2 is phosphorylated to 75% of maximum levels in heart failure resulting in leaky, unregulated channels gating in subconductance states. We now demonstrate that this is unlikely, as RyR2 isolated from nonfailing cardiac muscle is phosphorylated to 75% of maximum at serine-2809, and in this situation, RyR2 displays low open probability (P(o)) (0.059+/-0.010 [SEM]; n=30) and normal regulation of gating by Ca(2+) and other ligands. However, when serine-2809 is PKA phosphorylated to maximum levels on RyR2, unique changes in channel behavior are observed. The channel displays enhanced single-channel conductance, very long open states causing large increases in P(o), and no evidence of subconductance states. Dephosphorylation of channels by protein phosphatase 1 (from 75% to near 0% at serine-2809) also enhances RyR2 channel activity through abbreviation of closed lifetimes. We propose that channels phosphorylated to 75% of maximum at serine-2809 occupy a natural low point in the RyR2 activity landscape. This optimizes channel control, which can be accomplished either by enhanced or decreased phosphorylation, making the channel particularly sensitive to the kinase:phosphatase balance. Pathological situations such as heart failure might upset this balance and thereby permit prolonged stoichiometric phosphorylation of serine-2809, which would be required for dysregulation of SR Ca(2+) release.

Modulation of the skeletal muscle Ca2+ release channel/ryanodine receptor by adenosine and its metabolites: a structure-activity approach

Activation of ryanodine receptor (RyR) from skeletal muscle sarcoplasmic reticulum by adenosine and adenosine's metabolites was studied. The purines tested increased the [3H]-ryanodine binding as follows: xanthine>adenosine>adenine >inosine>/=uric acid>hypoxanthine. The enhanced [3H]-ryanodine binding did not involve change in the RyR-Ca(2+) sensitivity and was due mainly to lower values in the affinity constant (K(d)) that corresponded with an increase in the association rate constant (K(+1)). [3H]-ryanodine maximum binding (B(max)) was much less affected. Adenosine and inosine effects were dependent on the presence beta-glycosidic bond within the ribose ring, since the combination of adenine or hypoxanthine with ribose was not able to emulate the nucleosides' original activation. Competition experiments with AMP-PCP, a non-hydrolyzable analogue of ATP, evidenced a nucleotide's inhibitory influence on the adenosine and xanthine activation of the RyR. As a result of a Quantitative Structure-Activity Relationship (QSAR) study, we found a significant correlation between the modulation by adenosine and its metabolites on RyR activity and the components of their calculated dipole moment vector. Our results show that the ribose moiety and the dipole moment vector could be factors that make possible the modulation of the RyR activity by adenosine and its metabolites.

Structural characteristics that govern binding to, and modulation through, the cardiac ryanodine receptor nucleotide binding site

Comparative molecular field analysis (CoMFA) predicts that the large electrostatic field around the phosphate groups of ATP plays a crucial role in stabilizing the open state of the cardiac ryanodine receptor (RyR) channel. We therefore investigated the effects of adenosine-5'-(beta,gamma-methylenetriphosphate) (AMP-PCP), an ATP analog with lower negative charge in this region, on the gating of the cardiac RyR channel. In the presence of 10 microM cytosolic Ca2+, AMP-PCP exhibited approximately 50% of the efficacy of ATP and optimal doses increased open probability (Po) to only 0.441 +/- 0.156 (n = 4), thus confirming the predictive ability of our preliminary CoMFA model. We also reveal that AMP-PCP has a higher affinity than ATP for the cardiac RyR, demonstrating that the structural properties required for tight binding to RyR differ from those necessary for recruiting long open states and high Po values. CoMFA identified very strong correlations between the structures of adenine-based ligands and their affinity for RyR and different (but also highly significant) correlations between structure and the ability to activate the channel. Analysis indicates that ATP may be more effective than other adenine nucleotides because it can convert the greatest amount of binding energy into conformational changes that stabilize the open channel state.

Regulation of the calcium release channel from rabbit skeletal muscle by the nucleotides ATP, AMP, IMP and adenosine

1. Nucleotide activation of skeletal muscle ryanodine receptors (RyRs) was studied in planar lipid bilayers in order to understand RyR regulation in vivo under normal and fatigued conditions. With 'resting' calcium (100 nM cytoplasmic and 1 mM luminal), RyRs had an open probability (P(o)) of approximately 0.01 in the absence of nucleotides and magnesium. ATP reversibly activated RyRs with P(o) at saturation (P(max)) approximately 0.33 and K(a) (concentration for half-maximal activation) approximately 0.36 mM and with a Hill coefficient (n(H)) of approximately 1.8 in RyRs when P(max) < 0.5 and approximately 4 when P(max) > 0.5. 2. AMP was a much weaker agonist (P(max) approximately 0.09) and adenosine was weaker still (P(max) approximately 0.01-0.02), whereas inosine monophosphate (IMP), the normal metabolic end product of ATP hydrolysis, produced no activation at all. 3. Adenosine acted as a competitive antagonist that reversibly inhibited ATP- and AMP-activated RyRs with n(H) approximately 1 and K(i) approximately 0.06 mM at [ATP] < 0.5 mM, increasing 4-fold for each 2-fold increase in [ATP] above 0.5 mM. This is explained by the binding of a single adenosine preventing the cooperative binding of two ATP or AMP molecules, with dissociation constants of 0.4, 0.45 and 0.06 mM for ATP, AMP and adenosine, respectively. Importantly, IMP (< or = 8 mM) had no inhibitory effect whatsoever on ATP-activated RyRs. 4. Mean open (tau(o)) and closed (tau(c)) dwell-times were more closely related to P(o) than to the nucleotide species or individual RyRs. At P(o) < 0.2, RyR regulation occurred via changes in tau(c), whereas at higher P(o) this also occurred via changes in tau(o). The detailed properties of activation and competitive inhibition indicated complex channel behaviour that could be explained in terms of a model involving interactions between different subunits of the RyR homotetramer. 5. The results also show how deleterious adenosine accumulation is to the function of RyRs in skeletal muscle and, by comparison with voltage sensor-controlled Ca(2+) release, indicate that voltage sensor activation requires ATP binding to the RyR to be effective.

Characteristics of irreversible ATP activation suggest that native skeletal ryanodine receptors can be phosphorylated via an endogenous CaMKII

Phosphorylation of skeletal muscle ryanodine receptor (RyR) calcium release channels by endogenous kinases incorporated into lipid bilayers with native sarcoplasmic reticulum vesicles was investigated during exposure to 2 mM cytoplasmic ATP. Activation of RyRs after 1-min exposure to ATP was reversible upon ATP washout. In contrast, activation after 5 to 8 min was largely irreversible: the small fall in activity with washout was significantly less than that after brief ATP exposure. The irreversible activation was reduced by acid phosphatase and was not seen after exposure to nonhydrolyzable ATP analogs. The data suggested that the channel complex was phosphorylated after addition of ATP and that phosphorylation reduced the RyR's sensitivity to ATP, adenosine, and Ca(2+). The endogenous kinase was likely to be a calcium calmodulin kinase II (CaMKII) because the CaMKII inhibitor KN-93 and an inhibitory peptide for CaMKII prevented the phosphorylation-induced irreversible activation. In contrast, phosphorylation effects remained unchanged with inhibitory peptides for protein kinase C and A. The presence of CaMKIIbeta in the SR vesicles was confirmed by immunoblotting. The results suggest that CaMKII is anchored to skeletal muscle RyRs and that phosphorylation by this kinase alters the enhancement of channel activity by ATP and Ca(2+).