Amino acids 1-1,680 of ryanodine receptor type 1 hold critical determinants of skeletal type for excitation-contraction coupling. Role of divergence domain D2

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.

Calcium Release Channels: Structure and Function of IP3 Receptors and Ryanodine Receptors

Ca²⁺-release channels are giant membrane proteins that control the release of Ca²⁺ from the endoplasmic and sarcoplasmic reticulum. The two members, ryanodine receptors (RyRs) and inositol-1,4,5-trisphosphate Receptors (IP₃Rs), are evolutionarily related and are both activated by cytosolic Ca²⁺. They share a common architecture, but RyRs have evolved additional modules in the cytosolic region. Their massive size allows for the regulation by tens of proteins and small molecules, which can affect the opening and closing of the channels. In addition to Ca²⁺, other major triggers include IP₃ for the IP₃Rs, and depolarization of the plasma membrane for a particular RyR subtype. Their size has made them popular targets for study via electron microscopic methods, with current structures culminating near 3Å. The available structures have provided many new mechanistic insights int the binding of auxiliary proteins and small molecules, how these can regulate channel opening, and the mechanisms of disease-associated mutations. They also help scrutinize previously proposed binding sites, as some of these are now incompatible with the structures. Many questions remain around the structural effects of post-translational modifications, additional binding partners, and the higher-order complexes these channels can make in situ. This review summarizes our current knowledge about the structures of Ca²⁺-release channels and how this informs on their function.

Tissue-specific isoforms of the single C. elegans Ryanodine receptor gene unc-68 control specific functions

Ryanodine receptors (RyR) are essential regulators of cellular calcium homeostasis and signaling. Vertebrate genomes contain multiple RyR gene isoforms, expressed in different tissues and executing different functions. In contrast, invertebrate genomes contain a single RyR-encoding gene and it has long been proposed that different transcripts generated by alternative splicing may diversify their functions. Here, we analyze the expression and function of alternative exons in the C. elegans RyR gene unc-68. We show that specific isoform subsets are created via alternative promoters and via alternative splicing in unc-68 Divergent Region 2 (DR2), which actually corresponds to a region of high sequence variability across vertebrate isoforms. The expression of specific unc-68 alternative exons is enriched in different tissues, such as in body wall muscle, neurons and pharyngeal muscle. In order to infer the function of specific alternative promoters and alternative exons of unc-68, we selectively deleted them by CRISPR/Cas9 genome editing. We evaluated pharyngeal function, as well as locomotor function in swimming and crawling with high-content computer-assisted postural and behavioral analysis. Our data provide a comprehensive map of the pleiotropic impact of isoform-specific mutations and highlight that tissue-specific unc-68 isoforms fulfill distinct functions. As a whole, our work clarifies how the C. elegans single RyR gene unc-68 can fulfill multiple tasks through tissue-specific isoforms, and provide a solid foundation to further develop C. elegans as a model to study RyR channel functions and malfunctions.

Preclinical model systems of ryanodine receptor 1-related myopathies and malignant hyperthermia: a comprehensive scoping review of works published 1990-2019

BACKGROUND

Pathogenic variations in the gene encoding the skeletal muscle ryanodine receptor (RyR1) are associated with malignant hyperthermia (MH) susceptibility, a life-threatening hypermetabolic condition and RYR1-related myopathies (RYR1-RM), a spectrum of rare neuromuscular disorders. In RYR1-RM, intracellular calcium dysregulation, post-translational modifications, and decreased protein expression lead to a heterogenous clinical presentation including proximal muscle weakness, contractures, scoliosis, respiratory insufficiency, and ophthalmoplegia. Preclinical model systems of RYR1-RM and MH have been developed to better understand underlying pathomechanisms and test potential therapeutics.

METHODS

We conducted a comprehensive scoping review of scientific literature pertaining to RYR1-RM and MH preclinical model systems in accordance with the PRISMA Scoping Reviews Checklist and the framework proposed by Arksey and O'Malley. Two major electronic databases (PubMed and EMBASE) were searched without language restriction for articles and abstracts published between January 1, 1990 and July 3, 2019.

RESULTS

Our search yielded 5049 publications from which 262 were included in this review. A majority of variants tested in RYR1 preclinical models were localized to established MH/central core disease (MH/CCD) hot spots. A total of 250 unique RYR1 variations were reported in human/rodent/porcine models with 95% being missense substitutions. The most frequently reported RYR1 variant was R614C/R615C (human/porcine total n = 39), followed by Y523S/Y524S (rabbit/mouse total n = 30), I4898T/I4897T/I4895T (human/rabbit/mouse total n = 20), and R163C/R165C (human/mouse total n = 18). The dyspedic mouse was utilized by 47% of publications in the rodent category and its RyR1-null (1B5) myotubes were transfected in 23% of publications in the cellular model category. In studies of transfected HEK-293 cells, 57% of RYR1 variations affected the RyR1 channel and activation core domain. A total of 15 RYR1 mutant mouse strains were identified of which ten were heterozygous, three were compound heterozygous, and a further two were knockout. Porcine, avian, zebrafish, C. elegans, canine, equine, and drosophila model systems were also reported.

CONCLUSIONS

Over the past 30 years, there were 262 publications on MH and RYR1-RM preclinical model systems featuring more than 200 unique RYR1 variations tested in a broad range of species. Findings from these studies have set the foundation for therapeutic development for MH and RYR1-RM.

Regulatory mechanisms of ryanodine receptor/Ca2+ release channel revealed by recent advancements in structural studies

Ryanodine receptors (RyRs) are huge homotetrameric Ca²⁺ release channels localized to the sarcoplasmic reticulum. RyRs are responsible for the release of Ca²⁺ from the SR during excitation–contraction coupling in striated muscle cells. Recent revolutionary advancements in cryo-electron microscopy have provided a number of near-atomic structures of RyRs, which have enabled us to better understand the architecture of RyRs. Thus, we are now in a new era understanding the gating, regulatory and disease-causing mechanisms of RyRs. Here we review recent advances in the elucidation of the structures of RyRs, especially RyR1 in skeletal muscle, and their mechanisms of regulation by small molecules, associated proteins and disease-causing mutations.

Calcium Dynamics Mediated by the Endoplasmic/Sarcoplasmic Reticulum and Related Diseases

The flow of intracellular calcium (Ca2+) is critical for the activation and regulation of important biological events that are required in living organisms. As the major Ca2+ repositories inside the cell, the endoplasmic reticulum (ER) and the sarcoplasmic reticulum (SR) of muscle cells are central in maintaining and amplifying the intracellular Ca2+ signal. The morphology of these organelles, along with the distribution of key calcium-binding proteins (CaBPs), regulatory proteins, pumps, and receptors fundamentally impact the local and global differences in Ca2+ release kinetics. In this review, we will discuss the structural and morphological differences between the ER and SR and how they influence localized Ca2+ release, related diseases, and the need for targeted genetically encoded calcium indicators (GECIs) to study these events.

Distinct Components of Retrograde Ca(V)1.1-RyR1 Coupling Revealed by a Lethal Mutation in RyR1

The molecular basis for excitation-contraction coupling in skeletal muscle is generally thought to involve conformational coupling between the L-type voltage-gated Ca(2+) channel (CaV1.1) and the type 1 ryanodine receptor (RyR1). This coupling is bidirectional; in addition to the orthograde signal from CaV1.1 to RyR1 that triggers Ca(2+) release from the sarcoplasmic reticulum, retrograde signaling from RyR1 to CaV1.1 results in increased amplitude and slowed activation kinetics of macroscopic L-type Ca(2+) current. Orthograde coupling was previously shown to be ablated by a glycine for glutamate substitution at RyR1 position 4242. In this study, we investigated whether the RyR1-E4242G mutation affects retrograde coupling. L-type current in myotubes homozygous for RyR1-E4242G was substantially reduced in amplitude (∼80%) relative to that observed in myotubes from normal control (wild-type and/or heterozygous) myotubes. Analysis of intramembrane gating charge movements and ionic tail current amplitudes indicated that the reduction in current amplitude during step depolarizations was a consequence of both decreased CaV1.1 membrane expression (∼50%) and reduced channel Po (∼55%). In contrast, activation kinetics of the L-type current in RyR1-E4242G myotubes resembled those of normal myotubes, unlike dyspedic (RyR1 null) myotubes in which the L-type currents have markedly accelerated activation kinetics. Exogenous expression of wild-type RyR1 partially restored L-type current density. From these observations, we conclude that mutating residue E4242 affects RyR1 structures critical for retrograde communication with CaV1.1. Moreover, we propose that retrograde coupling has two distinct and separable components that are dependent on different structural elements of RyR1.

The Central domain of RyR1 is the transducer for long-range allosteric gating of channel opening

The ryanodine receptors (RyRs) are intracellular calcium channels responsible for rapid release of Ca²⁺ from the sarcoplasmic/endoplasmic reticulum (SR/ER) to the cytoplasm, which is essential for the excitation-contraction (E-C) coupling of cardiac and skeletal muscles. The near-atomic resolution structure of closed RyR1 revealed the molecular details of this colossal channel, while the long-range allosteric gating mechanism awaits elucidation. Here, we report the cryo-EM structures of rabbit RyR1 in three closed conformations at about 4 Å resolution and an open state at 5.7 Å. Comparison of the closed RyR1 structures shows a breathing motion of the cytoplasmic platform, while the channel domain and its contiguous Central domain remain nearly unchanged. Comparison of the open and closed structures shows a dilation of the S6 tetrahelical bundle at the cytoplasmic gate that leads to channel opening. During the pore opening, the cytoplasmic “O-ring” motif of the channel domain and the U-motif of the Central domain exhibit coupled motion, while the Central domain undergoes domain-wise displacement. These structural analyses provide important insight into the E-C coupling in skeletal muscles and identify the Central domain as the transducer that couples the conformational changes of the cytoplasmic platform to the gating of the central pore.

Structural mapping of divergent regions in the type 1 ryanodine receptor using fluorescence resonance energy transfer

Ryanodine receptors (RyRs) release Ca(2+) to initiate striated muscle contraction. Three highly divergent regions (DRs) in the RyR protein sequence (DR1, DR2, and DR3) may confer isoform-specific functional properties to the RyRs. We used cell-based fluorescence resonance energy transfer (FRET) measurements to localize these DRs to the cryoelectron microscopic (cryo-EM) map of the skeletal muscle RyR isoform (RyR1). FRET donors were targeted to RyR1 using five different FKBP12.6 variants labeled with Alexa Fluor 488. FRET was then measured to the FRET acceptors, Cy3NTA or Cy5NTA, targeted to decahistidine tags introduced within the DRs. DR2 and DR3 were localized to separate positions within the "clamp" region of the RyR1 cryo-EM map, which is presumed to interface with Cav1.1. DR1 was localized to the "handle" region, near the regulatory calmodulin-binding site on the RyR. These localizations provide insights into the roles of DRs in RyR allosteric regulation during excitation contraction coupling.

Myoplasmic resting Ca2+ regulation by ryanodine receptors is under the control of a novel Ca2+-binding region of the receptor

Passive SR (sarcoplasmic reticulum) Ca²⁺ leak through the RyR (ryanodine receptor) plays a critical role in the mechanisms that regulate [Ca²⁺]_rest (intracellular resting myoplasmic free Ca²⁺ concentration) in muscle. This process appears to be isoform-specific as expression of either RyR1 or RyR3 confers on myotubes different [Ca²⁺]_rest. Using chimaeric RyR3–RyR1 receptors expressed in dyspedic myotubes, we show that isoform-dependent regulation of [Ca²⁺]_rest is primarily defined by a small region of the receptor encompassing amino acids 3770–4007 of RyR1 (amino acids 3620–3859 of RyR3) named as the CLR (Ca²⁺ leak regulatory) region. [Ca²⁺]_rest regulation by the CLR region was associated with alteration of RyRs’ Ca²⁺-activation profile and changes in SR Ca²⁺-leak rates. Biochemical analysis using Tb³⁺-binding assays and intrinsic tryptophan fluorescence spectroscopy of purified CLR domains revealed that this determinant of RyRs holds a novel Ca²⁺-binding domain with conformational properties that are distinctive to each isoform. Our data suggest that the CLR region provides channels with unique functional properties that modulate the rate of passive SR Ca²⁺ leak and confer on RyR1 and RyR3 distinctive [Ca²⁺]_rest regulatory properties. The identification of a new Ca²⁺-binding domain of RyRs with a key modulatory role in [Ca²⁺]_rest regulation provides new insights into Ca²⁺-mediated regulation of RyRs.

This paper reports the finding of a new class of Ca²⁺-binding domain of intracellular Ca²⁺ channels from muscle cells. This domain provides channels with distinctive properties that result in channel-specific modulation of the intracellular resting Ca²⁺ concentration.

Molecular cloning and characterization of a ryanodine receptor gene in brown planthopper (BPH), Nilaparvata lugens (Stål)

BACKGROUND

Ryanodine receptors (RyRs) are a distinct class of intracellular calcium (Ca(2+)) release channel. The recent discovery of diamide insecticides has prompted studies on insect RyRs. However, information about the structure and function of insect RyRs is still limited. In this study, we isolated and characterized a full-length RyR cDNA (named NlRyR) from the brown planthopper, Nilaparvata lugens (Stål) (Homoptera: Delphacidae), a serious rice pest throughout Asia.

RESULTS

The composite NlRyR gene contains an open reading frame of 15 423 bp encoding a protein of 5140 amino acid residues, which shares high sequence identity (78-81%) with other insect homologues, except for two regions (IDR1: 4379-4732; IDR2: 1307-1529) with markedly low identity (44-48 and 38-41%, respectively). All hallmarks of the RyR proteins are conserved in the NlRyR protein, including the RyR domain as well as mannosyltransferase, IP3 R and RyR (pfam02815) (MIR) and RyR and IP3 R homology (pfam01365) (RIH) domains. Expression analysis of NlRyR revealed significant differences in mRNA expression levels among N. lugens developmental stages. Furthermore, three alternative splicing sites were identified in NlRyR, one of which forms the mutually exclusive exons A/B and is conserved in various insect species. Diagnostic PCR assays showed that the splice variant containing exon A was predominantly detected in all developmental stages.

CONCLUSION

NlRyR may play an important role in the control of developmental processes of N. lugens. Alternative splicing may generate the functional diversity of NlRyR. The results provided the basis for further structural and functional characterization of NlRyR.

RNAi suppression of the ryanodine receptor gene results in decreased susceptibility to chlorantraniliprole in Colorado potato beetle Leptinotarsa decemlineata

Leptinotarsadecemlineata is the most important pest in potato and causes serious yield loss each year. Chlorantraniliprole acts on insect ryanodine receptors (RyRs) and is among the most active compounds against L. decemlineata. Here we cloned and characterized a 15,792-bp full-length LdRyR cDNA that encoded a 5128-amino acid protein. LdRyR shares 85-92% amino acid similarities with other insect RyR homologues, and 59-61% similarities with those from Caenorhabditis elegans and Homo sapiens. All hallmarks of the RyR proteins are conserved in LdRyR. LdRyR has a MIR domain, two RIH domains, three SPRY domains, four copies of RyR domain and a RIH-associated domain in the N-terminus, and it possesses two consensus calcium ion-binding EF-hand motifs and six predicted transmembrane helices in the C-terminus. Temporal, spatial and tissue-specific expression patterns of LdRyR were evaluated. LdRyR expression level was increased constantly from egg to wandering stages, dropped in pupal stage and was increased again in the adult stage. It was widely expressed in the head, thorax and abdomen of day 3 fourth-instar larvae. Moreover, it was ubiquitously expressed in all inspected tissues including epidermis, foregut, midgut, ileum, rectum, fat body, ventral ganglia and Malpighian tubules in day 3 fourth-instar larvae. Dietary introduction of double-stranded RNA of LdRyR significantly reduced the mRNA levels of the target gene in the larvae and adults, respectively, and significantly decreased chlorantraniliprole-induced mortalities. Thus, our results suggested that LdRyR encoded a functional ryanodine receptor in L. decemlineata.

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

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

Dominant and recessive RYR1 mutations in adults with core lesions and mild muscle symptoms

INTRODUCTION

Ryanodine receptor gene (RYR1) mutations have been associated with central core disease (CCD), multiminicore/minicore/multicore disease (MmD), and susceptibility to malignant hyperthermia (MH).

METHODS

Patients with muscle symptoms in adulthood, who had features compatible with CCD/MmD, underwent clinical, histological, and genetic (RYR1 and SEPN1 genes) evaluations. Published cases of CCD and MmD with adult onset were also reviewed.

RESULTS

Eight patients fulfilled the criteria for further analysis. Five RYR1 mutations, 4 of them unreported, were detected in 3 patients. Compound heterozygosity was proven in 1 case.

CONCLUSIONS

To our knowledge, this is the only report of adult onset associated with recessive RYR1 mutations and central core/multiminicores on muscle biopsy. Although adult patients with CCD, MmD, and minimally symptomatic MH with abnormal muscle biopsy findings usually have a mild clinical course, differential diagnosis and carrier screening is crucial for prevention of potentially life-threatening reactions to general anesthesia.

Reduced gain of excitation-contraction coupling in triadin-null myotubes is mediated by the disruption of FKBP12/RyR1 interaction

Several studies have suggested that triadin (Tdn) may be a critical component of skeletal EC-coupling. However, using Tdn-null mice we have shown that triadin ablation results in no significant disruption of skeletal EC-coupling. To analyze the role of triadin in EC-coupling signaling here we used whole-cell voltage clamp and simultaneous recording of intracellular Ca²+ release to characterize the retrograde and orthograde signaling between RyR1 and DHPR in cultured myotubes. DHPR Ca²+ currents elicited by depolarization of Wt and Tdn-null myotubes displayed similar current densities and voltage dependence. However, kinetic analysis of the Ca²+ current shows that activation time constant of the slow component was slightly decreased in Tdn-null cells. Voltage-evoked Ca²+ transient of Tdn-null myotubes showed small but significant reduction in peak fluorescence amplitude but no differences in voltage dependence. This difference in Ca²+ amplitude was averted by over-expression of FKBP12.6. Our results show that bi-directional signaling between DHPR and RyR1 is preserved nearly intact in Tdn-null myotubes and that the effect of triadin ablation on Ca²+ transients appears to be secondary to the reduced FKBP12 binding capacity of RyR1 in Tdn-null myotubes. These data suggest that skeletal triadins do not play a direct role in skeletal EC-coupling.

Ryanodine receptors

Excitation-contraction coupling involves the faithful conversion of electrical stimuli to mechanical shortening in striated muscle cells, enabled by the ubiquitous second messenger, calcium. Crucial to this process are ryanodine receptors (RyRs), the sentinels of massive intracellular calcium stores contained within the sarcoplasmic reticulum. In response to sarcolemmal depolarization, RyRs release calcium into the cytosol, facilitating mobilization of the myofilaments and enabling cell contraction. In order for the cells to relax, calcium must be rapidly resequestered or extruded from the cytosol. The sustainability of this cycle is crucially dependent upon precise regulation of RyRs by numerous cytosolic metabolites and by proteins within the lumen of the sarcoplasmic reticulum and those directly associated with the receptors in a macromolecular complex. In addition to providing the majority of the calcium necessary for contraction of cardiac and skeletal muscle, RyRs act as molecular switchboards that integrate a multitude of cytosolic signals such as dynamic and steady calcium fluctuations, β-adrenergic stimulation (phosphorylation), nitrosylation and metabolic states, and transduce these signals to the channel pore to release appropriate amounts of calcium. Indeed, dysregulation of calcium release via RyRs is associated with life-threatening diseases in both skeletal and cardiac muscle. In this paper, we briefly review some of the most outstanding structural and functional attributes of RyRs and their mechanism of regulation. Further, we address pathogenic RyR dysfunction implicated in cardiovascular disease and skeletal myopathies.

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.

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.

Insect ryanodine receptors: molecular targets for novel pest control chemicals

Ryanodine receptors (RyRs) are a distinct class of ligand-gated calcium channels controlling the release of calcium from intracellular stores. They are located on the sarcoplasmic reticulum of muscle and the endoplasmic reticulum of neurons and many other cell types. Ryanodine, a plant alkaloid and an important ligand used to characterize and purify the receptor, has served as a natural botanical insecticide, but attempts to generate synthetic commercial analogues of ryanodine have proved unsuccessful. Recently two classes of synthetic chemicals have emerged resulting in commercial insecticides that target insect RyRs. The phthalic acid diamide class has yielded flubendiamide, the first synthetic ryanodine receptor insecticide to be commercialized. Shortly after the discovery of the phthalic diamides, the anthranilic diamides were discovered. This class has produced the insecticides Rynaxypyr and Cyazypyr. Here we review the structure and functions of insect RyRs and address the modes of action of phthalic acid diamides and anthranilic diamides on insect ryanodine receptors. Particularly intersting is the inherent selectivity both chemical classes exhibit for insect RyRs over their mammalian counterparts. The future prospects for RyRs as a commercially-validated target site for insect control chemicals are also considered.

A dihydropyridine receptor alpha1s loop region critical for skeletal muscle contraction is intrinsically unstructured and binds to a SPRY domain of the type 1 ryanodine receptor

The II-III loop of the dihydropyridine receptor (DHPR) alpha(1s) subunit is a modulator of the ryanodine receptor (RyR1) Ca(2+) release channel in vitro and is essential for skeletal muscle contraction in vivo. Despite its importance, the structure of this loop has not been reported. We have investigated its structure using a suite of NMR techniques which revealed that the DHPR II-III loop is an intrinsically unstructured protein (IUP) and as such belongs to a burgeoning structural class of functionally important proteins. The loop does not possess a stable tertiary fold: it is highly flexible, with a strong N-terminal helix followed by nascent helical/turn elements and unstructured segments. Its residual structure is loosely globular with the N and C termini in close proximity. The unstructured nature of the II-III loop may allow it to easily modify its interaction with RyR1 following a surface action potential and thus initiate rapid Ca(2+) release and contraction. The in vitro binding partner for the II-III was investigated. The II-III loop interacts with the second of three structurally distinct SPRY domains in RyR1, whose function is unknown. This interaction occurs through two preformed N-terminal alpha-helical regions and a C-terminal hydrophobic element. The A peptide corresponding to the helical N-terminal region is a common probe of RyR function and binds to the same SPRY domain as the full II-III loop. Thus the second SPRY domain is an in vitro binding site for the II-III loop. The possible in vivo role of this region is discussed.

Allosterically coupled calcium and magnesium binding sites are unmasked by ryanodine receptor chimeras

We studied cation regulation of wild-type ryanodine receptor type 1 ((WT)RyR1), type 3 ((WT)RyR3), and RyR3/RyR1 chimeras (Ch) expressed in 1B5 dyspedic myotubes. Using [(3)H]ryanodine binding to sarcoplasmic reticulum (SR) membranes, Ca(2+) titrations with (WT)RyR3 and three chimeras show biphasic activation that is allosterically coupled to an attenuated inhibition relative to (WT)RyR1. Chimeras show biphasic Mg(2+) inhibition profiles at 3 and 10 microM Ca(2+), no observable inhibition at 20 microM Ca(2+) and monophasic inhibition at 100 microM Ca(2+). Ca(2+) imaging of intact myotubes expressing Ch-4 exhibit caffeine-induced Ca(2+) transients with inhibition kinetics that are significantly slower than those expressing (WT)RyR1 or (WT)RyR3. Four new aspects of RyR regulation are evident: (1) high affinity (H) activation and low affinity (L) inhibition sites are allosterically coupled, (2) Ca(2+) facilitates removal of the inherent Mg(2+) block, (3) (WT)RyR3 exhibits reduced cooperativity between H activation sites when compared to (WT)RyR1, and (4) uncoupling of these sites in Ch-4 results in decreased rates of inactivation of caffeine-induced Ca(2+) transients.

Bridging the myoplasmic gap: recent developments in skeletal muscle excitation–contraction coupling

Conformational coupling between the L-type voltage-gated Ca(2+) channel (or 1,4-dihydropyridine receptor; DHPR) and the ryanodine-sensitive Ca(2+) release channel of the sarcoplasmic reticulum (RyR1) is the mechanistic basis for excitation-contraction (EC) coupling in skeletal muscle. In this article, recent findings regarding the roles of the individual cytoplasmic domains (the amino- and carboxyl-termini, cytoplasmic loops I-II, II-III, and III-IV) of the DHPR alpha(1S) subunit in bi-directional communication with RyR1 will be discussed.

Conformation-dependent stability of junctophilin 1 (JP1) and ryanodine receptor type 1 (RyR1) channel complex is mediated by their hyper-reactive thiols

Junctophilin 1 (JP1), a 72-kDa protein localized at the skeletal muscle triad, is essential for stabilizing the close apposition of T-tubule and sarcoplasmic reticulum membranes to form junctions. In this study we report that rapid and selective labeling of hyper-reactive thiols found in both JP1 and ryanodine receptor type 1 (RyR1) with 7-diethylamino-3-(4'-maleimidylphenyl)-4-methylcoumarin, a fluorescent thiol-reactive probe, proceeded 12-fold faster under conditions that minimize RyR1 gating (e.g. 10 mM Mg2+) compared with conditions that promote high channel activity (e.g. 100 microM Ca2+, 10 mM caffeine, 5 mM ATP). The reactivity of these thiol groups was very sensitive to oxidation by naphthoquinone, H2O2, NO, or O2, all known modulators of the RyR1 channel complex. Using preparative SDS-PAGE, in-gel tryptic digestion, high pressure liquid chromatography, and mass spectrometry-based peptide sequencing, we identified 7-diethylamino-3-(4'-maleimidylphenyl)-4-methylcoumarin-thioether adducts on three cysteine residues of JP1 (101, 402, and 627); the remaining five cysteines of JP1 were unlabeled. Co-immunoprecipitation experiments demonstrated a physical interaction between JP1 and RyR1 that, like thiol reactivity, was sensitive to RyR1 conformation and chemical status of the hyper-reactive cysteines of JP1 and RyR1. These findings support a model in which JP1 interacts with the RyR1 channel complex in a conformationally sensitive manner and may contribute integral redox-sensing properties through reactive sulfhydryl chemistry.

Bidirectional signaling between calcium channels of skeletal muscle requires multiple direct and indirect interactions

We have defined regions of the skeletal muscle ryanodine receptor (RyR1) essential for bidirectional signaling with dihydropyridine receptors (DHPRs) and for the organization of DHPR into tetrad arrays by expressing RyR1-RyR3 chimerae in dyspedic myotubes. RyR1-RyR3 constructs bearing RyR1 residues 1-1681 restored wild-type DHPR tetrad arrays and, in part, skeletal-type excitation-contraction (EC) coupling (orthograde signaling) but failed to enhance DHPR Ca(2+) currents (retrograde signaling) to WT RyR1 levels. Within this region, the D2 domain (amino acids 1272-1455), although ineffective on its own, dramatically enhanced the formation of tetrads and EC coupling rescue by constructs that otherwise are only partially effective. These findings suggest that the orthograde signal and DHPR tetrad formation require the contributions of numerous RyR regions. Surprisingly, we found that RyR3, although incapable of supporting EC coupling or tetrad formation, restored a significant level of Ca(2+) current, revealing a functional interaction with the skeletal muscle DHPR. Thus, our data support the hypotheses that (i) the structural/functional link between RyR1 and the skeletal muscle DHPR requires multiple interacting regions, (ii) the D2 domain of RyR1 plays a key role in stabilizing this interaction, and (iii) a form of retrograde signaling from RyR3 to the DHPR occurs in the absence of direct protein-protein interactions.

Ryanodine receptors and ventricular arrhythmias: emerging trends in mutations, mechanisms and therapies

It has been six years since the first reported link between mutations in the cardiac ryanodine receptor Ca(2+) release channel (RyR2) and catecholaminergic polymorphic ventricular tachycardia (CPVT), a malignant stress-induced arrhythmia. In this time, rapid advances have been made in identifying new mutations, and in understanding how these mutations disrupt normal channel function to cause VT that frequently degenerates into ventricular fibrillation (VF) and sudden death. Functional characterisation of these RyR2 Ca(2+) channelopathies suggests that mutations alter the ability of RyR2 to sense its intracellular environment, and that channel modulation via covalent modification, Ca(2+)- and Mg(2+)-dependent regulation and structural feedback mechanisms are catastrophically disturbed. This review reconciles the current status of RyR2 mutation-linked etiopathology, the significance of mutational clustering within the RyR2 polypeptide and the mechanisms underlying channel dysfunction. We will also review new data that explores the link between abnormal Ca(2+) release and the resultant cardiac electrical instability in VT and VF, and how these recent developments impact on novel anti-arrhythmic therapies. Finally, we evaluate the concept that mechanistic differences between CPVT and other arrhythmogenic disorders may preclude a common therapeutic strategy to normalise RyR2 function in cardiac disease.

Effects of peptide C corresponding to the Glu724-Pro760 region of the II-III loop of the DHP (dihydropyridine) receptor alpha1 subunit on the domain- switch-mediated activation of RyR1 (ryanodine receptor 1) Ca2+ channels

The Leu720-Leu764 region of the II-III loop of the dihydropyridine receptor is believed to be important for both orthograde and retrograde communications with the RyR (ryanodine receptor), but its actual role has not yet been resolved. Our recent studies suggest that voltage-dependent activation of the RyR channel is mediated by a pair of interacting N-terminal and central domains, designated as the 'domain switch'. To investigate the effect of peptide C (a peptide corresponding to residues Glu724-Pro760) on domain- switch-mediated activation of the RyR, we measured Ca2+ release induced by DP (domain peptide) 1 or DP4 (which activates the RyR by mediation of the domain switch) and followed the Ca2+ release time course using a luminal Ca2+ probe (chlortetracycline) under Ca2+-clamped conditions. Peptide C produced a significant potentiation of the domain-switch-mediated Ca2+ release, provided that the Ca2+ concentration was sufficiently low (e.g. 0.1 microM) and the Ca2+ channel was only partially activated by the domain peptide. However, at micromolar Ca2+ concentrations, peptide C inhibits activation. Covalent cross-linking of fluorescently labelled peptide C to the RyR and screening of the fluorescently labelled tryptic fragments permitted us to localize the peptide-C-binding site to residues 450-1400, which may represent the primary region involved in physical coupling. Based on the above findings, we propose that the physiological role of residues Glu724-Pro760 is to facilitate depolarization-induced and domain-switch-mediated RyR activation at sub- or near-threshold concentrations of cytoplasmic Ca2+ and to suppress activation upon an increase of cytoplasmic Ca2+.

Ryanodine Receptor Type 1 (RyR1) Mutations C4958S and C4961S Reveal Excitation-coupled Calcium Entry (ECCE) Is Independent of Sarcoplasmic Reticulum Store Depletion

Bi-directional signaling between ryanodine receptor type 1 (RyR1) and dihydropyridine receptor (DHPR) in skeletal muscle serves as a prominent example of conformational coupling. Evidence for a physiological mechanism that upon depolarization of myotubes tightly couples three calcium channels, DHPR, RyR1, and a Ca(2+) entry channel with SOCC-like properties, has recently been presented. This form of conformational coupling, termed excitation-coupled calcium entry (ECCE) is triggered by the alpha(1s)-DHPR voltage sensor and is highly dependent on RyR1 conformation. In this report, we substitute RyR1 cysteines 4958 or 4961 within the TXCFICG motif, common to all ER/SR Ca(2+) channels, with serine. When expressed in skeletal myotubes, C4958S- and C4961S-RyR1 properly target and restore L-type current via the DHPR. However, these mutants do not respond to RyR activators and do not support skeletal type EC coupling. Nonetheless, depolarization of cells expressing C4958S- or C4961S-RyR1 triggers calcium entry via ECCE that resembles that for wild-type RyR1, except for substantially slowed inactivation and deactivation kinetics. ECCE in these cells is completely independent of store depletion, displays a cation selectivity of Ca(2+)>Sr(2+) approximately Ba(2+), and is fully inhibited by SKF-96365 or 2-APB. Mutation of other non-CXXC motif cysteines within the RyR1 transmembrane assembly (C3635S, C4876S, and C4882S) did not replicate the phenotype observed with C4958S- and C4961S-RyR1. This study demonstrates the essential role of Cys(4958) and Cys(4961) within an invariant CXXC motif for stabilizing conformations of RyR1 that influence both its function as a release channel and its interaction with ECCE channels.

Localization of a disease-associated mutation site in the three-dimensional structure of the cardiac muscle ryanodine receptor

The cardiac muscle ryanodine receptor (RyR2) functions as a calcium release channel in the heart. Up to 40 mutations in RyR2 have been linked to genetic forms of sudden cardiac death. These mutations are largely clustered in three regions of the sequence of the polypeptide: one near the N terminus, one in the central region, and the third in the C-terminal region. The central region includes 11 mutations, and an isoleucine-proline motif (positions 2427 and 2428) in the same region is predicted to contribute to the binding of FKBP12.6 protein. We have mapped the central mutation site in the three-dimensional structure of RyR2 by green fluorescent protein insertion, cryoelectron microscopy, and single-particle image processing. The central mutation site was mapped to a "bridge" of density that connects cytoplasmic domains 5 and 6, which have been suggested to undergo conformational changes during channel gating. Moreover, the location of this central mutation site is not close to that of the FKBP12.6-binding site mapped previously by cryoelectron microscopy.

Ryanodine receptors

RyRs are large homotetrameric proteins that are approximately 4/5 cytoplasmic and approximately 1/5 transmembrane and luminal in mass. Mutations in RyRs produce human disease and many of these disease-causing mutations are in the cytoplasmic domains. To elucidate the mechanisms of a disease and to develop interventions, it is crucial to determine how the alterations in the cytoplasmic domains communicate with the transmembrane pore of this channel. One of the major activators of all three RyR isoforms is Ca2+ and some of the disease-causing mutations are thought to alter the sensitivity of the channels to Ca2+ activation. This review examines the current state of structural understanding of the RyR channel activation.

Maurocalcine and domain A of the II-III loop of the dihydropyridine receptor Cav 1.1 subunit share common binding sites on the skeletal ryanodine receptor

Maurocalcine is a scorpion venom toxin of 33 residues that bears a striking resemblance to the domain A of the dihydropyridine voltage-dependent calcium channel type 1.1 (Cav1.1) subunit. This domain belongs to the II-III loop of Cav1.1, which is implicated in excitation-contraction coupling. Besides the structural homology, maurocalcine also modulates RyR1 channel activity in a manner akin to a synthetic peptide of domain A. Because of these similarities, we hypothesized that maurocalcine and domain A may bind onto an identical region(s) of RyR1. Using a set of RyR1 fragments, we demonstrate that peptide A and maurocalcine bind onto two discrete RyR1 regions: fragments 3 and 7 encompassing residues 1021-1631 and 3201-3661, respectively. The binding onto fragment 7 is of greater importance and was thus further investigated. We found that the amino acid region 3351-3507 of RyR1 (fragment 7.2) is sufficient for these interactions. Proof that peptide A and maurocalcine bind onto the same site is provided by competition experiments in which binding of fragment 7.2 to peptide A is inhibited by preincubation with maurocalcine. Moreover, when expressed in COS-7 cells, RyR1 carrying a deletion of fragment 7 shows a loss of interaction with both peptide A and maurocalcine. At the functional level, this deletion abolishes the maurocalcine induced stimulation of [3H]ryanodine binding onto microsomes of transfected COS-7 cells without affecting the caffeine and ATP responses.

Expression levels of RyR1 and RyR3 control resting free Ca2+ in skeletal muscle

To better understand the role of the transient expression of ryanodine receptor (RyR) type 3 (RyR3) on Ca(2+) homeostasis during the development of skeletal muscle, we have analyzed the effect of expression levels of RyR3 and RyR1 on the overall physiology of cultured myotubes and muscle fibers. Dyspedic myotubes were infected with RyR1 or RyR3 containing virions at 0.2, 0.4, 1.0, and 4.0 moieties of infection (MOI), and analysis of their pattern of expression, caffeine sensitivity, and resting free Ca(2+) concentration ([Ca(2+)](r)) was performed. Although increased MOI resulted in increased expression of each receptor isoform, it did not significantly affect the immunopattern of RyRs or the expression levels of calsequestrin, triadin, or FKBP-12. Interestingly, myotubes expressing RyR3 always had significantly higher [Ca(2+)](r) and lower caffeine EC(50) than did cells expressing RyR1. Although some of the increased sensitivity of RyR3 to caffeine could be attributed to the higher [Ca(2+)](r) in RyR3-expressing cells, studies of [(3)H]ryanodine binding demonstrated intrinsic differences in caffeine sensitivity between RyR1 and RyR3. Tibialis anterior (TA) muscle fibers at different stages of postnatal development exhibited a transient increase in [Ca(2+)](r) coordinately with their level of RyR3 expression. Similarly, adult soleus fibers, which also express RyR3, had higher [Ca(2+)](r) than did adult TA fibers, which exclusively express RyR1. These data show that in skeletal muscle, RyR3 increases [Ca(2+)](r) more than RyR1 does at any expression level. These data suggest that the coexpression of RyR1 and RyR3 at different levels may constitute a novel mechanism by which to regulate [Ca(2+)](r) in skeletal muscle.

Evidence for conformational coupling between two calcium channels

Ryanodine receptor 1 (RyR1, the sarcoplasmic reticulum Ca(2+) release channel) and alpha(1S)dihydropyridine receptor (DHPR, the surface membrane voltage sensor) of skeletal muscle belong to separate membrane systems but are functionally and structurally linked. Four alpha(1S)DHPRs associated with the four identical subunits of a RyR form a tetrad. We treated skeletal muscle cell lines with ryanodine, at concentrations that block RyRs, and determined whether this treatment affects the distance between DHPRs in the tetrad. We find a substantial ( approximately 2-nm) shift in the alpha(1S)DHPR positions, indicating that ryanodine induces large conformational changes in the RyR1 cytoplasmic domain and that the alpha(1S)DHPR-RyR complex acts as a unit.

Location of divergent region 2 on the three-dimensional structure of cardiac muscle ryanodine receptor/calcium release channel

Ryanodine receptors (RyRs) are a family of calcium release channels found on intracellular calcium-handing organelles. Molecular cloning studies have identified three different RyR isoforms, which are 66-70% identical in amino acid sequence. In mammals, the three isoforms are encoded by three separate genes located on different chromosomes. The major variations among the isoforms occur in three regions, known as divergent regions 1, 2, and 3 (DR1, DR2, and DR3). In the present study, a modified RyR2 (cardiac isoform) cDNA was constructed, into which was inserted a green fluorescent protein (GFP)-encoding cDNA within DR2, specifically after amino acid residue Thr1366 (RyR2(T1366-GFP)). HEK293 cells expressing RyR2(T1366-GFP) cDNAs showed caffeine-sensitive and ryanodine-sensitive calcium release, demonstrating that RyR2(T1366-GFP) forms functional calcium release channels. Cells expressing RyR2(T1366-GFP) were identified readily by the characteristic fluorescence of GFP, indicating that the overall structure of the inserted GFP was retained. Cryo-electron microscopy (cryo-EM) of purified RyR2(T1366-GFP) showed structurally intact receptors, and a three-dimensional reconstruction was obtained by single-particle image processing. The location of the inserted GFP was obtained by comparing this three-dimensional reconstruction to one obtained for wild-type RyR2. The inserted GFP and, consequently Thr1366 within DR2, was mapped on the three-dimensional structure of RyR2 to domain 6, one of the characteristic cytoplasmic domains that form part of the multi-domain "clamp" regions of RyR2. The three-dimensional location of DR2 suggests that it plays roles in the RyR conformational changes that occur during channel gating, and possibly in RyR's interaction with the dihydropyridine receptor in excitation-contraction coupling. This study further demonstrates the feasibility and reliability of the GFP insertion/cryo-EM approach for correlating RyR's amino acid sequence with its three-dimensional structure, thereby enhancing our understanding of the structural basis of RyR function.