Localization of an NH(2)-terminal disease-causing mutation hot spot to the "clamp" region in the three-dimensional structure of the cardiac ryanodine receptor

Structural Insight Into Ryanodine Receptor Channelopathies

The ryanodine receptors (RyRs) are large cation-selective ligand-gated channels that are expressed in the sarcoplasmic reticulum (SR) membrane. They mediate the controlled release of Ca²⁺ from SR and play an important role in many cellular processes. The mutations in RyRs are associated with several skeletal muscle and cardiac conditions, including malignant hyperthermia (MH), central core disease (CCD), catecholaminergic polymorphic ventricular tachycardia (CPVT), and arrhythmogenic right ventricular dysplasia (ARVD). Recent breakthroughs in structural biology including cryo-electron microscopy (EM) and X-ray crystallography allowed the determination of a number of near-atomic structures of RyRs, including wildtype and mutant structures as well as the structures in complex with different modulating molecules. This allows us to comprehend the physiological gating and regulatory mechanisms of RyRs and the underlying pathological mechanisms of the disease-causing mutations. In this review, based on the insights gained from the available high-resolution structures of RyRs, we address several questions: 1) what are the gating mechanisms of different RyR isoforms; 2) how RyRs are regulated by multiple channel modulators, including ions, small molecules, and regulatory proteins; 3) how do disease-causing mutations affect the structure and function of RyRs; 4) how can these structural information aid in the diagnosis of the related diseases and the development of pharmacological therapies.

Phenytoin Reduces Activity of Cardiac Ryanodine Receptor 2; A Potential Mechanism for Its Cardioprotective Action

Phenytoin is a hydantoin derivative that is used clinically for the treatment of epilepsy and has been reported to have antiarrhythmic actions on the heart. In a failing heart, the elevated diastolic Ca2+ leak from the sarcoplasmic reticulum can be normalized by the cardiac ryanodine receptor 2 (RyR2) inhibitor, dantrolene, without inhibiting Ca2+ release during systole or affecting Ca2+ release in normal healthy hearts. Unfortunately, dantrolene is hepatotoxic and unsuitable for chronic long-term administration. Because phenytoin and dantrolene belong to the hydantoin class of compounds, we test the hypothesis that dantrolene and phenytoin have similar inhibitory effects on RyR2 using a single-channel recording of RyR2 activity in artificial lipid bilayers. Phenytoin produced a reversible inhibition of RyR2 channels from sheep and human failing hearts. It followed a hyperbolic dose response with maximal inhibition of ∼50%, Hill coefficient ∼1, and IC50 ranging from 10 to 20 µM. It caused inhibition at diastolic cytoplasmic [Ca2+] but not at Ca2+ levels in the dyadic cleft during systole. Notably, phenytoin inhibits RyR2 from failing human heart but not from healthy heart, indicating that phenytoin may selectively target defective RyR2 channels in humans. We conclude that phenytoin could effectively inhibit RyR2-mediated release of Ca2+ in a manner paralleling that of dantrolene. Moreover, the IC50 of phenytoin in RyR2 is at least threefold lower than for other ion channels and clinically used serum levels, pointing to phenytoin as a more human-safe alternative to dantrolene for therapies against heart failure and cardiac arrythmias. SIGNIFICANCE STATEMENT: We show that phenytoin, a Na channel blocker used clinically for treatment of epilepsy, is a diastolic inhibitor of cardiac calcium release channels [cardiac ryanodine receptor 2 (RyR2)] at doses threefold lower than its current therapeutic levels. Phenytoin inhibits RyR2 from failing human heart and not from healthy heart, indicating that phenytoin may selectively target defective RyR2 channels in humans and pointing to phenytoin as a more human-safe alternative to dantrolene for therapies against heart failure and cardiac arrhythmias.

Kv2.1 mediates spatial and functional coupling of L-type calcium channels and ryanodine receptors in mammalian neurons

The voltage-gated K⁺ channel Kv2.1 serves a major structural role in the soma and proximal dendrites of mammalian brain neurons, tethering the plasma membrane (PM) to endoplasmic reticulum (ER). Although Kv2.1 clustering at neuronal ER-PM junctions (EPJs) is tightly regulated and highly conserved, its function remains unclear. By identifying and evaluating proteins in close spatial proximity to Kv2.1-containing EPJs, we discovered that a significant role of Kv2.1 at EPJs is to promote the clustering and functional coupling of PM L-type Ca²⁺ channels (LTCCs) to ryanodine receptor (RyR) ER Ca²⁺ release channels. Kv2.1 clustering also unexpectedly enhanced LTCC opening at polarized membrane potentials. This enabled Kv2.1-LTCC-RyR triads to generate localized Ca²⁺ release events (i.e., Ca²⁺ sparks) independently of action potentials. Together, these findings uncover a novel mode of LTCC regulation and establish a unique mechanism whereby Kv2.1-associated EPJs provide a molecular platform for localized somatodendritic Ca²⁺ signals in mammalian brain neurons.

Ryanodine receptor-bound calmodulin is essential to protect against catecholaminergic polymorphic ventricular tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is caused by a single point mutation in the cardiac type 2 ryanodine receptor (RyR2). Using a knockin (KI) mouse model (R2474S/+), we previously reported that a single point mutation within the RyR2 sensitizes the channel to agonists, primarily mediated by defective interdomain interaction within the RyR2 and subsequent dissociation of calmodulin (CaM) from the RyR2. Here, we examined whether CPVT can be genetically rescued by enhancing the binding affinity of CaM to the RyR2. We first determined whether there is a possible amino acid substitution within the CaM-binding domain in the RyR2 (3584-3603 residues) that can enhance its binding affinity to CaM and found that V3599K substitution showed the highest binding affinity of CaM to the CaM-binding domain. Hence, we generated a heterozygous KI mouse model (V3599K/+) with a single amino acid substitution in the CaM-binding domain of the RyR2 and crossbred it with the heterozygous CPVT-associated R2474S/+-KI mouse to obtain a double-heterozygous R2474S/V3599K-KI mouse model. The CPVT phenotypes - bidirectional or polymorphic ventricular tachycardia, spontaneous Ca2+ transients, and Ca2+ sparks - were all inhibited in the R2474S/V3599K mice. Thus, enhancement of the CaM-binding affinity of the RyR2 is essential to prevent CPVT-associated arrhythmogenesis.

Site-specific labeling of proteins for electron microscopy

Electron microscopy is commonly employed to determine the subunit organization of large macromolecular assemblies. However, the field lacks a robust molecular labeling methodology for unambiguous identification of constituent subunits. We present a strategy that exploits the unique properties of an unnatural amino acid in order to enable site-specific attachment of a single, readily identifiable protein label at any solvent-exposed position on the macromolecular surface. Using this method, we show clear labeling of a subunit within the 26S proteasome lid subcomplex that has not been amenable to labeling by traditional approaches.

Single-particle cryo-EM of the ryanodine receptor channel in an aqueous environment

Ryanodine receptors (RyRs) are tetrameric ligand-gated Ca2+ release channels that are responsible for the increase of cytosolic Ca2+ concentration leading to muscle contraction. Our current understanding of RyR channel gating and regulation is greatly limited due to the lack of a high-resolution structure of the channel protein. The enormous size and unwieldy shape of Ca2+ release channels make X-ray or NMR methods difficult to apply for high-resolution structural analysis of the full-length functional channel. Single-particle electron cryo-microscopy (cryo-EM) is one of the only effective techniques for the study of such a large integral membrane protein and its molecular interactions. Despite recent developments in cryo-EM technologies and break-through single-particle cryo-EM studies of ion channels, cryospecimen preparation, particularly the presence of detergent in the buffer, remains the main impediment to obtaining atomic-resolution structures of ion channels and a multitude of other integral membrane protein complexes. In this review we will discuss properties of several detergents that have been successfully utilized in cryo-EM studies of ion channels and the emergence of the detergent alternative amphipol to stabilize ion channels for structure-function characterization. Future structural studies of challenging specimen like ion channels are likely to be facilitated by cryo-EM amenable detergents or alternative surfactants.

Conformational dynamics inside amino-terminal disease hotspot of ryanodine receptor

The N-terminal region of both skeletal and cardiac ryanodine receptor is a disease mutation hotspot. Recently, a crystal structure of the RyR1 fragment (residues 1-559) was solved. This N-terminal structure contains three separate domains, A, B, and C, and was docked into a central vestibule in a full-length RyR1 cryo-EM map. Here, we reconstructed three-dimensional cryo-EM structures of two GFP-tagged RyR2s with GFP inserted after residue Glu-310 and Ser-437, respectively. The structures of RyR2E310-GFP and RyR2S437-GFP displayed an extra mass on domain B and C, directly validating the predicted docking model. Next, we revealed domain movements in molecular dynamics flexible fitting models in both the closed and open state cryo-EM maps. To further probe the conformational changes, we generated FRET pairs by inserting CFP or YFP in two selected domains, FRET studies of three dual-insertion pairs and three co-expressed single-insertion pairs showed the dynamic structural changes within the N-terminal domains.

N-terminus oligomerization regulates the function of cardiac ryanodine receptors

The ryanodine receptor (RyR) is an ion channel composed of four identical subunits mediating calcium efflux from the endo/sarcoplasmic reticulum of excitable and non-excitable cells. We present several lines of evidence indicating that the RyR2 N-terminus is capable of self-association. A combination of yeast two-hybrid screens, co-immunoprecipitation analysis, chemical crosslinking and gel filtration assays collectively demonstrate that a RyR2 N-terminal fragment possesses the intrinsic ability to oligomerize, enabling apparent tetramer formation. Interestingly, N-terminus tetramerization mediated by endogenous disulfide bond formation occurs in native RyR2, but notably not in RyR1. Disruption of N-terminal inter-subunit interactions within RyR2 results in dysregulation of channel activation at diastolic Ca(2+) concentrations from ryanodine binding and single channel measurements. Our findings suggest that the N-terminus interactions mediating tetramer assembly are involved in RyR channel closure, identifying a crucial role for this structural association in the dynamic regulation of intracellular Ca(2+) release.

Two potential calmodulin-binding sequences in the ryanodine receptor contribute to a mobile, intra-subunit calmodulin-binding domain

Calmodulin (CaM), a 16 kDa ubiquitous calcium-sensing protein, is known to bind tightly to the calcium release channel/ryanodine receptor (RyR), and modulate RyR function. CaM binding studies using RyR fragments or synthetic peptides have revealed the presence of multiple, potential CaM-binding regions in the primary sequence of RyR. In the present study, we inserted GFP into two of these proposed CaM-binding sequences and mapped them onto the three-dimensional structure of intact cardiac RyR2 by cryo-electron microscopy. Interestingly, we found that the two potential CaM-binding regions encompassing, Arg3595 and Lys4269, respectively, are in close proximity and are adjacent to the previously mapped CaM-binding sites. To monitor the conformational dynamics of these CaM-binding regions, we generated a fluorescence resonance energy transfer (FRET) pair, a dual CFP- and YFP-labeled RyR2 (RyR2R3595-CFP/K4269-YFP) with CFP inserted after Arg3595 and YFP inserted after Lys4269. We transfected HEK293 cells with the RyR2R3595-CFP/K4269-YFP cDNA, and examined their FRET signal in live cells. We detected significant FRET signals in transfected cells that are sensitive to the channel activator caffeine, suggesting that caffeine is able to induce conformational changes in these CaM-binding regions. Importantly, no significant FRET signals were detected in cells co-transfected with cDNAs encoding the single CFP (RyR2R3595-CFP) and single YFP (RyR2K4269-YFP) insertions, indicating that the FRET signal stemmed from the interaction between R3595-CFP and K4269-YFP that are in the same RyR subunit. These observations suggest that multiple regions in the RyR2 sequence may contribute to an intra-subunit CaM-binding pocket that undergoes conformational changes during channel gating.

Ligand-dependent conformational changes in the clamp region of the cardiac ryanodine receptor

Global conformational changes in the three-dimensional structure of the Ca(2+) release channel/ryanodine receptor (RyR) occur upon ligand activation. A number of ligands are able to activate the RyR channel, but whether these structurally diverse ligands induce the same or different conformational changes in the channel is largely unknown. Here we constructed a fluorescence resonance energy transfer (FRET)-based probe by inserting a CFP after residue Ser-2367 and a YFP after residue Tyr-2801 in the cardiac RyR (RyR2) to yield a CFP- and YFP-dual labeled RyR2 (RyR2(Ser-2367-CFP/Tyr-2801-YFP)). Both of these insertion sites have previously been mapped to the "clamp" region in the four corners of the square-shaped cytoplasmic assembly of the three-dimensional structure of RyR2. Using this novel FRET probe, we monitored the extent of conformational changes in the clamp region of RyR2(Ser-2367-CFP/Tyr-2801-YFP) induced by various ligands. We also monitored the extent of Ca(2+) release induced by the same ligands in HEK293 cells expressing RyR2(Ser-2367-CFP/Tyr-2801-YFP). We detected conformational changes in the clamp region for the ligands caffeine, aminophylline, theophylline, ATP, and ryanodine but not for Ca(2+) or 4-chloro-m-cresol, although they all induced Ca(2+) release. Interestingly, caffeine is able to induce further conformational changes in the clamp region of the ryanodine-modified channel, suggesting that ryanodine does not lock RyR in a fixed conformation. Our data demonstrate that conformational changes in the clamp region of RyR are ligand-dependent and suggest the existence of multiple ligand dependent RyR activation mechanisms associated with distinct conformational changes.

In cardiomyocytes, binding of unzipping peptide activates ryanodine receptor 2 and reciprocally inhibits calmodulin binding

RATIONALE

One hypothesis for elevated Ca(2+) leak through cardiac ryanodine receptors (ryanodine receptor 2 [RyR2]) in heart failure is interdomain unzipping that can enhance aberrant channel activation. A peptide (domain peptide corresponding to RyR2 residues 2460-2495 [DPc10]) corresponding to RyR2 central domain residues 2460-2495 recapitulates this arrhythmogenic RyR2 leakiness by unzipping N-terminal and central domains. Calmodulin (CaM) and FK506-binding protein (FKBP12.6) bind to RyR2 and stabilize the closed channel. Little is known about DPc10 binding to the RyR2 and how that may interact with binding (and effects) of CaM and FKBP12.6 to RyR2.

OBJECTIVE

To measure, directly in cardiac myocytes, the kinetics and binding affinity of DPc10 to RyR2 and how that affects RyR2 interaction with FKBP12.6 and CaM.

METHODS AND RESULTS

We used permeabilized rat ventricular myocytes and fluorescently labeled DPc10, FKBP12.6, and CaM. DPc10 access to its binding site is extremely slow in resting RyR2 but is accelerated by promoting RyR opening or unzipping (by unlabeled DPc10). RyR2-bound CaM (but not FKBP12.6) drastically slowed DPc10 binding. Conversely, DPc10 binding significantly reduced CaM (but not FKBP12.6) binding to the RyR2. Fluorescence resonance energy transfer measurements indicate that DPc10-binding and CaM-binding sites are separate and allow triangulation of the structural DPc10 binding locus on RyR2 vs FKBP12.6-binding and CaM-binding sites.

CONCLUSIONS

DPc10-RyR2 binding is sterically limited by the resting zipped RyR2 state. CaM binding to RyR2 stabilizes this zipped state, whereas RyR2 activation or prebound DPc10 enhances DPc10 access. DPc10-binding and CaM-binding sites are distinct but are allosterically interacting RyR2 sites. Neither DPc10 nor FKBP12.6 influences RyR2 binding of the other.

Super-resolution imaging of EC coupling protein distribution in the heart

The cardiac ryanodine receptor (RyR) plays a central role in the control of contractile function of the heart. In cardiac ventricular myocytes RyRs and associated Ca(2+) handling proteins, including membrane Ca(2+) channels, Ca(2+) pumps and other sarcolemmal and sarcoplasmic reticulum proteins interact to set the time course and amplitude of the electrically triggered cytosolic Ca(2+) transient. It has become increasingly clear that protein distribution and clustering on the nanometer scale is critical in determining the interaction of these proteins and the resulting properties of cardiac Ca(2+) handling. Such intricate near-molecular scale detail cannot be visualized with conventional fluorescence microscopy techniques (e.g. confocal microscopy) but it has recently become accessible with optical super-resolution techniques. These techniques retain the advantages of fluorescent marker technology, i.e. high specificity and excellent contrast, but have a spatial resolution approaching 10nm, i.e. objects not much further apart than 10nm can be distinguished, previously only attainable with electron microscopy. We review the use of these novel imaging techniques for the study of protein distribution in cardiac ventricular myocytes and discuss technical considerations as well as recent findings using super-resolution imaging. An emphasis is on single molecule localization based super-resolution approaches and their use to reveal the complexity of RyR cluster morphology, placement and relationship to other excitation-contraction coupling proteins. Super-resolution imaging approaches have already demonstrated their utility for the study of cardiac structure-function relationships and we anticipate that their use will rapidly increase and help improve our understanding of cardiac Ca(2+) regulation.

Identification of ATP-binding regions in the RyR1 Ca²⁺ release channel

ATP is an important modulator of gating in type 1 ryanodine receptor (RyR1), also known as a Ca²⁺ release channel in skeletal muscle cells. The activating effect of ATP on this channel is achieved by directly binding to one or more sites on the RyR1 protein. However, the number and location of these sites have yet to be determined. To identify the ATP-binding regions within RyR1 we used 2N₃ATP-2′,3′-Biotin-LC-Hydrazone (BioATP-HDZ), a photo-reactive ATP analog to covalently label the channel. We found that BioATP-HDZ binds RyR1 specifically with an IC₅₀ = 0.6±0.2 mM, comparable with the reported EC50 for activation of RyR1 with ATP. Controlled proteolysis of labeled RyR1 followed by sequence analysis revealed three fragments with apparent molecular masses of 95, 45 and 70 kDa that were crosslinked by BioATP-HDZ and identified as RyR1 sequences. Our analysis identified four glycine-rich consensus motifs that can potentially constitute ATP-binding sites and are located within the N-terminal 95-kDa fragment. These putative nucleotide-binding sequences include amino acids 699–704, 701–706, 1081–1084 and 1195–1200, which are conserved among the three RyR isoforms. Located next to the N-terminal disease hotspot region in RyR1, these sequences may communicate the effects of ATP-binding to channel function by tuning conformational motions within the neighboring cytoplasmic regulatory domains. Two other labeled fragments lack ATP-binding consensus motifs and may form non-canonical ATP-binding sites. Based on domain topology in the 3D structure of RyR1 it is also conceivable that the identified ATP-binding regions, despite their wide separation in the primary sequence, may actually constitute the same non-contiguous ATP-binding pocket within the channel tetramer.

Bidirectional coupling between ryanodine receptors and Ca2+ release-activated Ca2+ (CRAC) channel machinery sustains store-operated Ca2+ entry in human T lymphocytes

The expression and functional significance of ryanodine receptors (RyR) were investigated in resting and activated primary human T cells. RyR1, RyR2, and RyR3 transcripts were detected in human T cells. RyR1/2 transcript levels increased, whereas those of RyR3 decreased after T cell activation. RyR1/2 protein immunoreactivity was detected in activated but not in resting T cells. The RyR agonist caffeine evoked Ca(2+) release from the intracellular store in activated T cells but not in resting T cells, indicating that RyR are functionally up-regulated in activated T cells compared with resting T cells. In the presence of store-operated Ca(2+) entry (SOCE) via plasmalemmal Ca(2+) release-activated Ca(2+) (CRAC) channels, RyR blockers reduced the Ca(2+) leak from the endoplasmic reticulum (ER) and the magnitude of SOCE, suggesting that a positive feedback relationship exists between RyR and CRAC channels. Overexpression of fluorescently tagged RyR2 and stromal interaction molecule 1 (STIM1), an ER Ca(2+) sensor gating CRAC channels, in HEK293 cells revealed that RyR are co-localized with STIM1 in the puncta formed after store depletion. These data indicate that in primary human T cells, the RyR are coupled to CRAC channel machinery such that SOCE activates RyR via a Ca(2+)-induced Ca(2+) release mechanism, which in turn reduces the Ca(2+) concentration within the ER lumen in the vicinity of STIM1, thus facilitating SOCE by reducing store-dependent CRAC channel inactivation. Treatment with RyR blockers suppressed activated T cell expansion, demonstrating the functional importance of RyR in T cells.

Ryanodine receptors: structure and function

Ryanodine receptors (RyRs) are huge ion channels that are responsible for the release of Ca(2+) from the sarco/endoplasmic reticulum. RyRs form homotetramers with a mushroom-like shape, consisting of a large cytoplasmic head and transmembrane stalk. Ca(2+) is a major physiological ligand that triggers opening of RyRs, but a plethora of modulatory proteins and small molecules in the cytoplasm and sarco/endoplasmic reticulum lumen have been recognized. Over 300 mutations in RyRs are associated with severe skeletal muscle disorders or triggered cardiac arrhythmias. With the advent of high-resolution structures of individual domains, many of these can be mapped onto the three-dimensional structure.

FRET-based localization of fluorescent protein insertions within the ryanodine receptor type 1

Fluorescent protein (FP) insertions have often been used to localize primary structure elements in mid-resolution 3D cryo electron microscopic (EM) maps of large protein complexes. However, little is known as to the precise spatial relationship between the location of the fused FP and its insertion site within a larger protein. To gain insights into these structural considerations, Förster resonance energy transfer (FRET) measurements were used to localize green fluorescent protein (GFP) insertions within the ryanodine receptor type 1 (RyR1), a large intracellular Ca(2+) release channel that plays a key role in skeletal muscle excitation contraction coupling. A series of full-length His-tagged GFP-RyR1 fusion constructs were created, expressed in human embryonic kidney (HEK)-293T cells and then complexed with Cy3NTA, a His-tag specific FRET acceptor. FRET efficiency values measured from each GFP donor to Cy3NTA bound to each His tag acceptor site were converted into intermolecular distances and the positions of each inserted GFP were then triangulated relative to a previously published X-ray crystal structure of a 559 amino acid RyR1 fragment. We observed that the chromophoric centers of fluorescent proteins inserted into RyR1 can be located as far as 45 Å from their insertion sites and that the fused proteins can also be located in internal cavities within RyR1. These findings should prove useful in interpreting structural results obtained in cryo EM maps using fusions of small fluorescent proteins. More accurate point-to-point distance information may be obtained using complementary orthogonal labeling systems that rely on fluorescent probes that bind directly to amino acid side chains.

Ca(2+) signalling by IP(3) receptors

The Ca(2) (+) signals evoked by inositol 1,4,5-trisphosphate (IP(3)) are built from elementary Ca(2) (+) release events involving progressive recruitment of IP(3) receptors (IP(3)R), intracellular Ca(2) (+) channels that are expressed in almost all animal cells. The smallest events ('blips') result from opening of single IP(3)R. Larger events ('puffs') reflect the near-synchronous opening of a small cluster of IP(3)R. These puffs become more frequent as the stimulus intensity increases and they eventually trigger regenerative Ca(2) (+) waves that propagate across the cell. This hierarchical recruitment of IP(3)R is important in allowing Ca(2) (+) signals to be delivered locally to specific target proteins or more globally to the entire cell. Co-regulation of IP(3)R by Ca(2) (+) and IP(3), the ability of a single IP(3)R rapidly to mediate a large efflux of Ca(2) (+) from the endoplasmic reticulum, and the assembly of IP(3)R into clusters are key features that allow IP(3)R to propagate Ca(2) (+) signals regeneratively. We review these properties of IP(3)R and the structural basis of IP(3)R behavior.

The deletion of exon 3 in the cardiac ryanodine receptor is rescued by β strand switching

Mutations in the cardiac Ryanodine Receptor (RYR2) are linked to triggered arrhythmias. Removal of exon 3 results in a severe form of catecholaminergic polymorphic ventricular tachycardia (CPVT). This exon encodes secondary structure elements that are crucial for folding of the N-terminal domain (NTD), raising the question of why the deletion is neither lethal nor confers a loss of function. We determined the 2.3 Å crystal structure of the NTD lacking exon 3. The removal causes a structural rescue whereby a flexible loop inserts itself into the β trefoil domain and increases thermal stability. The exon 3 deletion is not tolerated in the corresponding RYR1 domain. The rescue shows a novel mechanism by which RYR2 channels can adjust their Ca²⁺ release properties through altering the structure of the NTD. Despite the rescue, the deletion affects interfaces with other RYR2 domains. We propose that relative movement of the NTD is allosterically coupled to the pore region.

Mutation-linked defective interdomain interactions within ryanodine receptor cause aberrant Ca²⁺release leading to catecholaminergic polymorphic ventricular tachycardia

BACKGROUND

The molecular mechanism by which catecholaminergic polymorphic ventricular tachycardia is induced by single amino acid mutations within the cardiac ryanodine receptor (RyR2) remains elusive. In the present study, we investigated mutation-induced conformational defects of RyR2 using a knockin mouse model expressing the human catecholaminergic polymorphic ventricular tachycardia-associated RyR2 mutant (S2246L; serine to leucine mutation at the residue 2246).

METHODS AND RESULTS

All knockin mice we examined produced ventricular tachycardia after exercise on a treadmill. cAMP-dependent increase in the frequency of Ca²⁺ sparks was more pronounced in saponin-permeabilized knockin cardiomyocytes than in wild-type cardiomyocytes. Site-directed fluorescent labeling and quartz microbalance assays of the specific binding of DP2246 (a peptide corresponding to the 2232 to 2266 region: the 2246 domain) showed that DP2246 binds with the K201-binding sequence of RyR2 (1741 to 2270). Introduction of S2246L mutation into the DP2246 increased the affinity of peptide binding. Fluorescence quench assays of interdomain interactions within RyR2 showed that tight interaction of the 2246 domain/K201-binding domain is coupled with domain unzipping of the N-terminal (1 to 600)/central (2000 to 2500) domain pair in an allosteric manner. Dantrolene corrected the mutation-caused domain unzipping of the domain switch and stopped the exercise-induced ventricular tachycardia.

CONCLUSIONS

The catecholaminergic polymorphic ventricular tachycardia-linked mutation of RyR2, S2246L, causes an abnormally tight local subdomain-subdomain interaction within the central domain involving the mutation site, which induces defective interaction between the N-terminal and central domains. This results in an erroneous activation of Ca²⁺ channel in a diastolic state reflecting on the increased Ca²⁺ spark frequency, which then leads to lethal arrhythmia.

Localization of the dantrolene-binding sequence near the FK506-binding protein-binding site in the three-dimensional structure of the ryanodine receptor

Dantrolene is believed to stabilize interdomain interactions between the NH2-terminal and central regions of ryanodine receptors by binding to the NH2-terminal residues 590-609 in skeletal ryanodine receptor (RyR1) and residues 601-620 in cardiac ryanodine receptor (RyR2). To gain further insight into the structural basis of dantrolene action, we have attempted to localize the dantrolene-binding sequence in RyR1/RyR2 by using GFP as a structural marker and three-dimensional cryo-EM. We inserted GFP into RyR2 after residues Arg-626 and Tyr-846 to generate GFP-RyR2 fusion proteins, RyR2Arg-626-GFP and RyR2Tyr-846-GFP. Insertion of GFP after residue Arg-626 abolished the binding of a bulky GST- or cyan fluorescent protein-tagged FKBP12.6 but not the binding of a smaller, nontagged FKBP12.6, suggesting that residue Arg-626 and the dantrolene-binding sequence are located near the FKBP12.6-binding site. Using cryo-EM, we have mapped the three-dimensional location of Tyr-846-GFP to domain 9, which is also adjacent to the FKBP12.6-binding site. To further map the three-dimensional location of the dantrolene-binding sequence, we generated 10 FRET pairs based on four known three-dimensional locations (FKBP12.6, Ser-437-GFP, Tyr-846-GFP, and Ser-2367-GFP). Based on the FRET efficiencies of these FRET pairs and the corresponding distance relationships, we mapped the three-dimensional location of Arg-626-GFP or -cyan fluorescent protein, hence the dantrolene-binding sequence, to domain 9 near the FKBP12.6-binding site but distant to the central region around residue Ser-2367. An allosteric mechanism by which dantrolene stabilizes interdomain interactions between the NH2-terminal and central regions is proposed.

IP(3) receptors: toward understanding their activation

Inositol 1,4,5-trisphosphate receptors (IP(3)R) and their relatives, ryanodine receptors, are the channels that most often mediate Ca(2+) release from intracellular stores. Their regulation by Ca(2+) allows them also to propagate cytosolic Ca(2+) signals regeneratively. This brief review addresses the structural basis of IP(3)R activation by IP(3) and Ca(2+). IP(3) initiates IP(3)R activation by promoting Ca(2+) binding to a stimulatory Ca(2+)-binding site, the identity of which is unresolved. We suggest that interactions of critical phosphate groups in IP(3) with opposite sides of the clam-like IP(3)-binding core cause it to close and propagate a conformational change toward the pore via the adjacent N-terminal suppressor domain. The pore, assembled from the last pair of transmembrane domains and the intervening pore loop from each of the four IP(3)R subunits, forms a structure in which a luminal selectivity filter and a gate at the cytosolic end of the pore control cation fluxes through the IP(3)R.

Ryanodine receptors: structure, expression, molecular details, and function in calcium release

Ryanodine receptors (RyRs) are located in the sarcoplasmic/endoplasmic reticulum membrane and are responsible for the release of Ca(2+) from intracellular stores during excitation-contraction coupling in both cardiac and skeletal muscle. RyRs are the largest known ion channels (> 2MDa) and exist as three mammalian isoforms (RyR 1-3), all of which are homotetrameric proteins that interact with and are regulated by phosphorylation, redox modifications, and a variety of small proteins and ions. Most RyR channel modulators interact with the large cytoplasmic domain whereas the carboxy-terminal portion of the protein forms the ion-conducting pore. Mutations in RyR2 are associated with human disorders such as catecholaminergic polymorphic ventricular tachycardia whereas mutations in RyR1 underlie diseases such as central core disease and malignant hyperthermia. This chapter examines the current concepts of the structure, function and regulation of RyRs and assesses the current state of understanding of their roles in associated disorders.

Three-dimensional structure of recombinant type 1 inositol 1,4,5-trisphosphate receptor

IP3Rs (inositol 1,4,5-trisphosphate receptors) are the intracellular channels that mediate release of Ca2+ from the endoplasmic reticulum in response to the many stimuli that evoke Ins(1,4,5)P3 formation. We characterized and purified type 1 IP3R heterologously expressed in Sf9 insect cells, and used the purified IP3R1 to determine its three-dimensional structure by electron microscopy and single-particle analysis. Recombinant IP3R1 has 4-fold symmetry with overall dimensions of approx. 19.5 nm x 19.5 nm x 17.5 nm. It comprises a small domain, which is likely to include the pore, linked by slender bridges to a large cytoplasmic domain with four petal-like regions. Our structures of recombinant IP3R1 and native cerebellar IP3R have similar appearances and dimensions. The only notable difference is the absence of a central stigma-like domain from the cytoplasmic region of recombinant IP3R1. The first structure of a recombinant IP3R is an important step towards developing three-dimensional structures of IP3R that better contribute to our understanding of the structural basis of IP3R activation.

Dynamic, inter-subunit interactions between the N-terminal and central mutation regions of cardiac ryanodine receptor

Naturally occurring mutations in the cardiac ryanodine receptor (RyR2) have been linked to certain types of cardiac arrhythmias and sudden death. Two mutation hotspots that lie in the N-terminal and central regions of RyR2 are predicted to interact with one another and to form an important channel regulator switch. To monitor the conformational dynamics involving these regions, we generated a fluorescence resonance energy transfer (FRET) pair. A yellow fluorescent protein (YFP) was inserted into RyR2 after residue Ser437 in the N-terminal region, and a cyan fluorescent protein (CFP) was inserted after residue Ser2367 in the central region, to form a dual YFP- and CFP-labeled RyR2 (RyR2(S437-YFP/S2367-CFP)). We transfected HEK293 cells with RyR2(S437-YFP/S2367-CFP) cDNAs, and then examined them by using confocal microscopy and by measuring the FRET signal in live cells. The FRET signals are influenced by modulators of RyR2, by domain peptides that mimic the effects of disease causing RyR2 mutations, and by various drugs. Importantly, FRET signals were also readily detected in cells co-transfected with single CFP (RyR2(S437-YFP)) and single YFP (RyR2(S2367-CFP)) labeled RyR2, indicating that the interaction between the N-terminal and central mutation regions is an inter-subunit interaction. Our studies demonstrate that FRET analyses of this CFP- and YFP-labeled RyR2 can be used not only for investigating the conformational dynamics associated with RyR2 channel gating, but potentially, also for identifying drugs that are capable of stabilizing the conformations of RyR2.

Catecholaminergic polymorphic ventricular tachycardia is caused by mutation-linked defective conformational regulation of the ryanodine receptor

RATIONALE

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is caused by a single point mutation in a well-defined region of the cardiac type 2 ryanodine receptor (RyR)2. However, the underlying mechanism by which a single mutation in such a large molecule produces drastic effects on channel function remains unresolved.

OBJECTIVE

Using a knock-in (KI) mouse model with a human CPVT-associated RyR2 mutation (R2474S), we investigated the molecular mechanism by which CPVT is induced by a single point mutation within the RyR2.

METHODS AND RESULTS

The R2474S/+ KI mice showed no apparent structural or histological abnormalities in the heart, but they showed clear indications of other abnormalities. Bidirectional or polymorphic ventricular tachycardia was induced after exercise on a treadmill. The interaction between the N-terminal (amino acids 1 to 600) and central (amino acids 2000 to 2500) domains of the RyR2 (an intrinsic mechanism to close Ca(2+) channels) was weakened (domain unzipping). On protein kinase A-mediated phosphorylation of the RyR2, this domain unzipping further increased, resulting in a significant increase in the frequency of spontaneous Ca(2+) transients. cAMP-induced aberrant Ca(2+) release events (Ca(2+) sparks/waves) occurred at much lower sarcoplasmic reticulum Ca(2+) content as compared to the wild type. Addition of a domain-unzipping peptide, DPc10 (amino acids 2460 to 2495), to the wild type reproduced the aforementioned abnormalities that are characteristic of the R2474S/+ KI mice. Addition of DPc10 to the (cAMP-treated) KI cardiomyocytes produced no further effect.

CONCLUSIONS

A single point mutation within the RyR2 sensitizes the channel to agonists and reduces the threshold of luminal [Ca(2+)] for activation, primarily mediated by defective interdomain interaction within the RyR2.

Dissociation of calmodulin from cardiac ryanodine receptor causes aberrant Ca(2+) release in heart failure

AIMS

Calmodulin (CaM) is well known to modulate the channel function of the cardiac ryanodine receptor (RyR2). However, the possible role of CaM on the aberrant Ca(2+) release in diseased hearts remains unclear. In this study, we investigated the state of RyR2-bound CaM and channel dysfunctions in pacing-induced failing hearts.

METHODS AND RESULTS

The characteristics of CaM binding to RyR2 and the role of CaM on the aberrant Ca(2+) release were assessed in normal and failing canine hearts. The affinity of CaM binding to RyR2 was lower in failing sarcoplasmic reticulum (SR) than in normal SR. Addition of FK506, which dissociates FKBP12.6 from RyR2, to normal SR reduced the CaM-binding affinity. Dantrolene restored a normal level of the CaM-binding affinity in either FK506-treated (normal) SR or failing SR, suggesting that the defective inter-domain interaction between the N-terminal domain and the central domain of RyR2 (the therapeutic target of dantrolene) is involved in the reduction of the CaM-binding affinity in failing hearts. In saponin-permeabilized cardiomyocytes, the frequency of spontaneous Ca(2+) sparks was much more increased in failing cardiomyocytes than in normal cardiomyocytes, whereas the addition of a high concentration of CaM attenuated the aberrant increase of Ca(2+) sparks.

CONCLUSION

The defective inter-domain interaction between N-terminal and central domains within RyR2 reduces the binding affinity of CaM to RyR2, thereby causing the spontaneous Ca(2+) release events in failing hearts. Correction of the defective CaM binding may be a new strategy to protect against the aberrant Ca(2+) release in heart failure.

Defective calmodulin binding to the cardiac ryanodine receptor plays a key role in CPVT-associated channel dysfunction

Calmodulin (CaM), one of the accessory proteins of the cardiac ryanodine receptor (RyR2), is known to play a significant role in the channel regulation of the RyR2. However, the possible involvement of calmodulin in the pathogenic process of catecholaminergic polymorphic ventricular tachycardia (CPVT) has not been investigated. In this study, we investigated the state of RyR2-bound CaM and channel dysfunctions using a knock-in (KI) mouse model with CPVT-linked RyR2 mutation (R2474S). Without added effectors, the affinity of CaM binding to the RyR2 was indistinguishable between KI and WT hearts. In response to cAMP (1 micromol/L), the RyR2 phosphorylation at Ser2808 increased in both WT and KI hearts to the same extent. However, cAMP caused a significant decrease of the CaM-binding affinity in KI hearts, but the affinity was unchanged in WT. Dantrolene restored a normal level of CaM-binding affinity in the cAMP-treated KI hearts, suggesting that defective inter-domain interaction between the N-terminal domain and the central domain of the RyR2 (the target of therapeutic effect of dantrolene) is involved in the cAMP-induced reduction of the CaM-binding affinity. In saponin-permeabilized cardiomyocytes, the addition of cAMP increased the frequency of spontaneous Ca(2+) sparks to a significantly larger extent in KI cardiomyocytes than in WT cardiomyocytes, whereas the addition of a high concentration of CaM attenuated the aberrant increase of Ca(2+) sparks. In conclusion, CPVT mutation causes defective inter-domain interaction, significant reduction in the ability of CaM binding to the RyR2, spontaneous Ca(2+) leak, and then lethal arrhythmia.

Crystal structures of the N-terminal domains of cardiac and skeletal muscle ryanodine receptors: insights into disease mutations

Ryanodine receptors (RyRs) are channels governing the release of Ca(2+) from the sarcoplasmic or endoplasmic reticulum. They are required for the contraction of both skeletal (RyR1) and cardiac (RyR2) muscles. Mutations in both RyR1 and RyR2 have been associated with severe genetic disorders, but high-resolution data describing the disease variants in detail have been lacking. Here we present the crystal structures of the N-terminal domains of both RyR2 (1-217) and RyR1 (9-205) at 2.55 A and 2.9 A, respectively. The domains map in a hot spot region for disease mutations. Both structures consist of a core beta trefoil domain flanked by an alpha helix. Crystal structures of two RyR2 disease mutants, A77V (2.2 A) and V186M (1.7 A), show that the mutations cause distinct local changes in the surface of the protein. A RyR2 deletion mutant causes significant changes in the thermal stability. The disease positions highlight two putative binding interfaces required for normal RyR function.

Ryanodine receptor mutations in arrhythmia: The continuing mystery of channel dysfunction

Mutations in RyR2 are causative of an inherited disorder which often results in sudden cardiac death. Dysfunctional channel behaviour has been the subject of many investigations varying from single channel analysis through to complex animal models. This review discusses recent advances in the field, describes the controversy surrounding the exact consequences of RyR2 mutation and how the disparate data may be reconciled. This heterogeneity of function with respect to the effects of polymorphisms, phosphorylation, cytosolic and luminal Ca(2+) as well as inter-domain interactions may have important implications for the recent pharmaceutical therapies which have been put forward. We surmise that a comprehensive characterisation of mutations on a case-by-case basis may be beneficial for the development of specifically targeted therapies.

The RYR2-encoded ryanodine receptor/calcium release channel in patients diagnosed previously with either catecholaminergic polymorphic ventricular tachycardia or genotype negative, exercise-induced long QT syndrome: a comprehensive open reading frame mutational analysis

OBJECTIVES

This study was undertaken to determine the spectrum and prevalence of mutations in the RYR2-encoded cardiac ryanodine receptor in cases with exertional syncope and normal corrected QT interval (QTc).

BACKGROUND

Mutations in RYR2 cause type 1 catecholaminergic polymorphic ventricular tachycardia (CPVT1), a cardiac channelopathy with increased propensity for lethal ventricular dysrhythmias. Most RYR2 mutational analyses target 3 canonical domains encoded by <40% of the translated exons. The extent of CPVT1-associated mutations localizing outside of these domains remains unknown as RYR2 has not been examined comprehensively in most patient cohorts.

METHODS

Mutational analysis of all RYR2 exons was performed using polymerase chain reaction, high-performance liquid chromatography, and deoxyribonucleic acid sequencing on 155 unrelated patients (49% females, 96% Caucasian, age at diagnosis 20 +/- 15 years, mean QTc 428 +/- 29 ms), with either clinical diagnosis of CPVT (n = 110) or an initial diagnosis of exercise-induced long QT syndrome but with QTc <480 ms and a subsequent negative long QT syndrome genetic test (n = 45).

RESULTS

Sixty-three (34 novel) possible CPVT1-associated mutations, absent in 400 reference alleles, were detected in 73 unrelated patients (47%). Thirteen new mutation-containing exons were identified. Two-thirds of the CPVT1-positive patients had mutations that localized to 1 of 16 exons.

CONCLUSIONS

Possible CPVT1 mutations in RYR2 were identified in nearly one-half of this cohort; 45 of the 105 translated exons are now known to host possible mutations. Considering that approximately 65% of CPVT1-positive cases would be discovered by selective analysis of 16 exons, a tiered targeting strategy for CPVT genetic testing should be considered.

Förster resonance energy transfer measurements of ryanodine receptor type 1 structure using a novel site-specific labeling method

BACKGROUND

While the static structure of the intracellular Ca(2+) release channel, the ryanodine receptor type 1 (RyR1) has been determined using cryo electron microscopy, relatively little is known concerning changes in RyR1 structure that accompany channel gating. Förster resonance energy transfer (FRET) methods can resolve small changes in protein structure although FRET measurements of RyR1 are hampered by an inability to site-specifically label the protein with fluorescent probes.

METHODOLOGY/PRINCIPAL FINDINGS

A novel site-specific labeling method is presented that targets a FRET acceptor, Cy3NTA to 10-residue histidine (His) tags engineered into RyR1. Cy3NTA, comprised of the fluorescent dye Cy3, coupled to two Ni(2+)/nitrilotriacetic acid moieties, was synthesized and functionally tested for binding to His-tagged green fluorescent protein (GFP). GFP fluorescence emission and Cy3NTA absorbance spectra overlapped significantly, indicating that FRET could occur (Förster distance = 6.3 nm). Cy3NTA bound to His(10)-tagged GFP, quenching its fluorescence by 88%. GFP was then fused to the N-terminus of RyR1 and His(10) tags were placed either at the N-terminus of the fused GFP or between GFP and RyR1. Cy3NTA reduced fluorescence of these fusion proteins by 75% and this quenching could be reversed by photobleaching Cy3, thus confirming GFP-RyR1 quenching via FRET. A His(10) tag was then placed at amino acid position 1861 and FRET was measured from GFP located at either the N-terminus or at position 618 to Cy3NTA bound to this His tag. While minimal FRET was detected between GFP at position 1 and Cy3NTA at position 1861, 53% energy transfer was detected from GFP at position 618 to Cy3NTA at position 1861, thus indicating that these sites are in close proximity to each other.

CONCLUSIONS/SIGNIFICANCE

These findings illustrate the potential of this site-specific labeling system for use in future FRET-based experiments to elucidate novel aspects of RyR1 structure.

Ryanodine receptor-mediated arrhythmias and sudden cardiac death

The cardiac ryanodine receptor-Ca²⁺ release channel (RyR2) is an essential sarcoplasmic reticulum (SR) transmembrane protein that plays a central role in excitation–contraction coupling (ECC) in cardiomyocytes. Aberrant spontaneous, diastolic Ca²⁺ leak from the SR due to dysfunctional RyR2 contributes to the formation of delayed after-depolarisations, which are thought to underlie the fatal arrhythmia that occurs in both heart failure (HF) and in catecholaminergic polymorphic ventricular tachycardia (CPVT). CPVT is an inherited disorder associated with mutations in either the RyR2 or a SR luminal protein, calsequestrin. RyR2 shows normal function at rest in CPVT but the RyR2 dysfunction is unmasked by physical exercise or emotional stress, suggesting abnormal RyR2 activation as an underlying mechanism. Several potential mechanisms have been advanced to explain the dysfunctional RyR2 observed in HF and CPVT, including enhanced RyR2 phosphorylation status, altered RyR2 regulation at luminal/cytoplasmic sites and perturbed RyR2 intra/inter-molecular interactions. This review considers RyR2 dysfunction in the context of the structural and functional modulation of the channel, and potential therapeutic strategies to stabilise RyR2 function in cardiac pathology.

CLIC2-RyR1 interaction and structural characterization by cryo-electron microscopy

Chloride intracellular channel 2 (CLIC2), a newly discovered small protein distantly related to the glutathione transferase (GST) structural family, is highly expressed in cardiac and skeletal muscle, although its physiological function in these tissues has not been established. In the present study, [3H]ryanodine binding, Ca2+ efflux from skeletal sarcoplasmic reticulum (SR) vesicles, single channel recording, and cryo-electron microscopy were employed to investigate whether CLIC2 can interact with skeletal ryanodine receptor (RyR1) and modulate its channel activity. We found that: (1) CLIC2 facilitated [3H]ryanodine binding to skeletal SR and purified RyR1, by increasing the binding affinity of ryanodine for its receptor without significantly changing the apparent maximal binding capacity; (2) CLIC2 reduced the maximal Ca2+ efflux rate from skeletal SR vesicles; (3) CLIC2 decreased the open probability of RyR1 channel, through increasing the mean closed time of the channel; (4) CLIC2 bound to a region between domains 5 and 6 in the clamp-shaped region of RyR1; (5) and in the same clamp region, domains 9 and 10 became separated after CLIC2 binding, indicating CLIC2 induced a conformational change of RyR1. These data suggest that CLIC2 can interact with RyR1 and modulate its channel activity. We propose that CLIC2 functions as an intrinsic stabilizer of the closed state of RyR channels.

Luminal Ca(2+) activation of cardiac ryanodine receptors by luminal and cytoplasmic domains

The ryanodine receptors form the calcium release channel in the membrane of the sarcoplasmic reticulum (SR, the main intracellular Ca(2+) store). The importance of ryanodine receptors (RyRs) to cardiac pacemaking and rhythmicity is highlighted by more than 69 mutations, RyR mutations, which underlie arrhythmias and sudden cardiac death. Although most of these mutations lie in cytoplasmic domains, they all cause increased RyR activation by Ca(2+) in the SR lumen. Presented here is a review of the mechanisms by which cytoplasmic domains of the RyR can determine luminal activation.

Ryanodine receptor as a new therapeutic target of heart failure and lethal arrhythmia

Abnormal intracellular Ca(2+) handling by the sarcoplasmic reticulum (SR) is a critical factor in the development of heart failure (HF). Not only decreased Ca(2+) uptake, but also uncoordinated Ca(2+) release plays a significant role in contractile and relaxation dysfunction. Spontaneous Ca(2+) release through ryanodine receptor (RyR) 2, a huge tetrameric protein, during diastole leads to a decrease in the SR Ca(2+) content, and also triggers delayed after depolarization that is a substrate for lethal arrhythmia. Several disease-linked mutations of RyR have been reported in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT) or arrhythmogenic right ventricular cardiomyopathy type 2 (ARVC2). The unique distribution of these mutation sites has lead to the concept that an interaction among the putative regulatory domains within RyR may play a key role in regulating channel opening, and that there seems to be a common abnormality in the channel disorder of HF and CPVT/ARVC2. Recent knowledge gained from pathological conditions may lead to the development of a new therapeutic strategy for the treatment of HF or cardiac arrhythmia.

Structural and functional characterization of ryanodine receptor-natrin toxin interaction

Cysteine-rich secretory proteins (CRISPs) are widely distributed, and notably occur in the mammalian reproductive tract and in the salivary glands of venomous reptiles. Most CRISPs can inhibit ion channels, such as the cyclic nucleotide-gated ion channel, potassium channel, and calcium channel. Natrin is a CRISP that has been purified from snake venom. Its targets include the calcium-activated potassium channel, the voltage-gated potassium channel, and the calcium release channel/ryanodine receptor (RyR). Immunoprecipitation experiments showed that natrin binds specifically to type 1 RyR (RyR1) from skeletal muscle. Natrin was found to inhibit both the binding of ryanodine to RyR1, and the calcium-channel activity of RyR1. Cryo-electron microscopy and single-particle image reconstruction analysis revealed that natrin binds to the clamp domains of RyR1. Docking of the crystal structure of natrin into our cryo-electron microscopy density map of the RyR1 + natrin complex suggests that natrin inhibits RyR1 by stabilizing a domain-domain interaction, and that the cysteine-rich domain of natrin is crucial for binding. These findings help reveal how natrin toxin inhibits the RyR calcium release channel, and they allow us to posit a generalized mechanism that governs the interaction between CRISPs and ion channels.

Subnanometer-resolution electron cryomicroscopy-based domain models for the cytoplasmic region of skeletal muscle RyR channel

The skeletal muscle Ca(2+) release channel (RyR1), a homotetramer, regulates the release of Ca(2+) from the sarcoplasmic reticulum to initiate muscle contraction. In this work, we have delineated the RyR1 monomer boundaries in a subnanometer-resolution electron cryomicroscopy (cryo-EM) density map. In the cytoplasmic region of each RyR1 monomer, 36 alpha-helices and 7 beta-sheets can be resolved. A beta-sheet was also identified close to the membrane-spanning region that resembles the cytoplasmic pore structures of inward rectifier K(+) channels. Three structural folds, generated for amino acids 12-565 using comparative modeling and cryo-EM density fitting, localize close to regions implicated in communication with the voltage sensor in the transverse tubules. Eleven of the 15 disease-related residues for these domains are mapped to the surface of these models. Four disease-related residues are found in a basin at the interfaces of these regions, creating a pocket in which the immunophilin FKBP12 can fit. Taken together, these results provide a structural context for both channel gating and the consequences of certain malignant hyperthermia and central core disease-associated mutations in RyR1.

Localization of PKA phosphorylation site, Ser(2030), in the three-dimensional structure of cardiac ryanodine receptor

PKA (protein kinase A)-dependent phosphorylation of the cardiac Ca2+-release channel/RyR2 (type 2 ryanodine receptor)is believed to directly dissociate FKBP12.6 (12.6 kDa FK506-binding protein) from the channel, causing abnormal channel activation and Ca2+ release. To gain insight into the structural basis of the regulation of RyR2 by PKA, we determined the three-dimensional location of the PKA site Ser2030. GFP (green fluorescent protein) was inserted into RyR2-wt (wild-type RyR2)and RyR2 mutant, A4860G, after Thr2023. The resultant GFP-RyR2 fusion proteins, RyR2T2023-GFP and RyR2(A4860G)T2023-GFP, were expressed in HEK-293 (human embryonic kidney) cells and functionally characterized. Ca2+-release assays revealed that both GFP-RyR2 fusion proteins formed caffeine- and ryanodine-sensitive Ca2+-release channels. Further analyses using[3H]ryanodine binding demonstrated that the insertion of GFPinto RyR2-wt after Thr2023 reduced the sensitivity of the channelto activation by Ca2+ or caffeine. RyR2(A4860G)T2023-GFP was found to be structurally more stable than RyR2T2023-GFP and was subsequently used as a basis for three-dimensional reconstruction. Cryo-electronmicroscopy and single particle image processing of the purified RyR2(A4860G)T2023-GFP protein revealed the location of the inserted GFP, and hence the Ser2030 PKA site in domain 4,a region that may be involved in signal transduction between the transmembrane and cytoplasmic domains. Like the Ser2808 PKA site reported previously, the Ser2030 site is not located close to the FKBP12.6-binding site mapped previously, indicating that neither of these PKA sites is directly involved in FKBP12.6 binding. On the basis of the three-dimensional localizations of a number of residues or regions, a model for the subunit organization in the structure of RyR2 is proposed.

Sarcoplasmic reticulum calcium leak and cardiac arrhythmias

Ventricular arrhythmias deteriorating into sudden cardiac death are a major cause of mortality worldwide. The recent linkage of a genetic form of cardiac arrhythmia to mutations in the gene encoding RyR2 (ryanodine receptor 2) has uncovered an important role of this SR (sarcoplasmic reticulum) calcium release channel in triggering arrhythmias. Mutant RyR2 channels give rise to spontaneous release of calcium (Ca(2+)) from the SR during diastole, which enhances the probability of ventricular arrhythmias. Several molecular mechanisms have been proposed to explain the gain-of-function phenotype observed in mutant RyR2 channels. Despite considerable differences between the models discussed in the present review, each predicts spontaneous diastolic Ca(2+) leak from the SR due to incomplete closure of the RyR2 channel. Enhanced SR Ca(2+) leak is also observed in common structural diseases of the heart, such as heart failure. In heart failure, defective channel regulation in the absence of inherited mutations may also increase SR Ca(2+) leak and initiate cardiac arrhythmias. Therefore inhibition of diastolic Ca(2+) leak through SR Ca(2+) release channels has emerged as a new and promising therapeutic target for cardiac arrhythmias.

Identification of target domains of the cardiac ryanodine receptor to correct channel disorder in failing hearts

BACKGROUND

We previously demonstrated that defective interdomain interaction between N-terminal (0 to 600) and central regions (2000 to 2500) of ryanodine receptor 2 (RyR2) induces Ca2+ leak in failing hearts and that K201 (JTV519) inhibits the Ca2+ leak by correcting the defective interdomain interaction. In the present report, we identified the K201-binding domain and characterized the role of this novel domain in the regulation of the RyR2 channel.

METHODS AND RESULTS

An assay using a quartz-crystal microbalance technique (a very sensitive mass-measuring technique) revealed that K201 specifically bound to recombinant RyR2 fragments 1741 to 2270 and 1981 to 2520 but not to other RyR2 fragments from the 1 to 2750 region (1 to 610, 494 to 1000, 741 to 1260, 985 to 1503, 1245 to 1768, 2234 to 2750). By further analysis of the fragment(1741-2270), K201 was found to specifically bind to its subfragment(2114-2149). With the use of the peptide matching this subfragment (DP(2114-2149)) as a carrier, the RyR2 was fluorescently labeled with methylcoumarin acetate (MCA) in a site-directed manner. After tryptic digestion, the major MCA-labeled fragment of RyR2 (155 kDa) was detected by an antibody raised against the central region (Ab(2132)). Moreover, of several recombinant RyR2 fragments, only fragment(2234-2750) was specifically MCA labeled; this suggests that the K201-binding domain(2114-2149) binds with domain(2234-2750). Addition of DP(2114-2149) to the MCA-labeled sarcoplasmic reticulum interfered with the interaction between domain(2114-2149) and domain(2234-2750), causing domain unzipping, as evidenced by an increased accessibility of the bound MCA to a large-size fluorescence quencher. In failing cardiomyocytes, the frequency of spontaneous Ca2+ spark was markedly increased compared with normal cardiomyocytes, whereas incorporation of DP(2114-2149) markedly decreased the frequency of spontaneous Ca2+ spark.

CONCLUSIONS

We first identified the K201-binding site as domain(2114-2149) of RyR2. Interruption of the interdomain interaction between the domain(2114-2149) and central domain(2234-2750) seems to mediate stabilization of RyR2 in failing hearts, which may lead to a novel therapeutic strategy against heart failure and perhaps lethal arrhythmia.

Distinct mechanisms for dysfunctions of mutated ryanodine receptor isoforms

Ryanodine receptor (RyR) is the Ca(2+)-induced Ca(2+) release channel in cells. RyR1 and RyR2 are its isoforms expressed in the skeletal and cardiac muscles, respectively. Their missense mutations, which are clustered in three regions that correspond to each other, cause hereditary disorders such as malignant hyperthermia and central core disease in skeletal muscle and catecholaminergic polymorphic ventricular tachycardia in cardiac muscle. Their pathogeneses, however, are not well understood. The following hypotheses are favorably discussed in this article: phenotypes with RyR1 and RyR2 mutations are mainly caused by dysregulations of their functions through the interdomain interaction and luminal Ca(2+), respectively.

A domain peptide of the cardiac ryanodine receptor regulates channel sensitivity to luminal Ca2+ via cytoplasmic Ca2+ sites

The clustering of cardiac RyR mutations, linked to sudden cardiac death (SCD), into several regions in the amino acid sequence underlies the hypothesis that these mutations interfere with stabilising interactions between different domains of the RyR2. SCD mutations cause increased channel sensitivity to cytoplasmic and luminal Ca(2+). A synthetic peptide corresponding to part of the central domain (DPc10:(2460)G-P(2495)) was designed to destabilise the interaction of the N-terminal and central domains of wild-type RyR2 and mimic the effects of SCD mutations. With Ca(2+) as the sole regulating ion, DPc10 caused increased channel activity which could be reversed by removal of the peptide whereas in the presence of ATP DPc10 caused no activation. In support of the domain destablising hypothesis, the corresponding peptide (DPc10-mut) containing the CPVT mutation R2474S did not affect channel activity under any circumstances. DPc10-induced activation was due to a small increase in RyR2 sensitivity to cytoplasmic Ca(2+) and a large increase in the magnitude of luminal Ca(2+) activation. The increase in the luminal Ca(2+) response appeared reliant on the luminal-to-cytoplasmic Ca(2+) flux in the channel, indicating that luminal Ca(2+) was activating the RyR2 via its cytoplasmic Ca(2+) sites. DPc10 had no significant effect on the RyR2 gating associated with luminal Ca(2+) sensing sites. The results were fitted by the luminal-triggered Ca(2+) feed-through model and the effects of DPc10 were explained entirely by perturbations in cytoplasmic Ca(2+)-activation mechanism.

Ryanodine receptor mutations in arrhythmias: advances in understanding the mechanisms of channel dysfunction

The cardiac ryanodine receptor (RyR2) mediates rapid Ca2+ efflux from intracellular stores to effect myocyte contraction during the process of EC (excitation–contraction) coupling. It is now known that mutations in this channel perturb Ca2+ release function, leading to triggered arrhythmias that may cause SCD (sudden cardiac death). Resolving the precise molecular mechanisms by which SCD-linked RyR2 dysfunction occurs currently constitutes a burgeoning area of cardiac research. So far, defective channel phosphorylation, accessory protein binding, luminal/cytosolic Ca2+ sensing, and the disruption of interdomain interactions represent the main candidate mechanisms for explaining aberrant SR (sarcoplasmic reticulum) Ca2+ release via mutants of RyR2. It appears increasingly unlikely that a single exclusive common mechanism underlies every case of mutant channel dysfunction, and that each of these potential mechanisms may contribute to the resultant phenotype. The present review will consider very recent mechanistic developments in this field, including new observations from mutant RyR2 transgenic mouse models, peptide-probe studies, and the implications of functional and phenotypic heterogeneity of RyR2 mutations and polymorphisms.