Molecular cloning and characterization of the ryanodine receptor/junctional channel complex cDNA from skeletal muscle sarcoplasmic reticulum

Ryanodine receptor type 1 content decrease-induced endoplasmic reticulum stress is a hallmark of myopathies

BACKGROUND

Decreased ryanodine receptor type 1 (RyR1) protein levels are a well-described feature of recessive RYR1-related myopathies. The aim of the present study was twofold: (1) to determine whether RyR1 content is also decreased in other myopathies and (2) to investigate the mechanisms by which decreased RyR1 protein triggers muscular disorders.

METHODS

We used publicly available datasets, muscles from human inflammatory and mitochondrial myopathies, an inducible muscle-specific RYR1 recessive mouse model and RyR1 knockdown in C2C12 muscle cells to measure RyR1 content and endoplasmic reticulum (ER) stress markers. Proteomics, lipidomics, molecular biology and transmission electron microscopy approaches were used to decipher the alterations associated with the reduction of RyR1 protein levels.

RESULTS

RYR1 transcripts were reduced in muscle samples of patients suffering from necrotizing myopathy (P = 0.026), inclusion body myopathy (P = 0.003), polymyositis (P < 0.001) and juvenile dermatomyositis (P < 0.001) and in muscle samples of myotonic dystrophy type 2 (P < 0.001), presymptomatic (P < 0.001) and symptomatic (P < 0.001) Duchenne muscular dystrophy, Becker muscular dystrophy (P = 0.004) and limb-girdle muscular dystrophy type 2A (P = 0.004). RyR1 protein content was also significantly decreased in inflammatory myopathy (-75%, P < 0.001) and mitochondrial myopathy (-71%, P < 0.001) muscles. Proteomics data showed that depletion of RyR1 protein in C2C12 myoblasts leads to myotubes recapitulating the common molecular alterations observed in myopathies. Mechanistically, RyR1 protein depletion reduces ER-mitochondria contact length (-26%, P < 0.001), Ca2+ transfer to mitochondria (-48%, P = 0.002) and the mitophagy gene Parkinson protein 2 transcripts (P = 0.037) and induces mitochondrial accumulation (+99%, P = 0.005) and dysfunction (P < 0.001). This was associated to the accumulation of deleterious sphingolipid species. Our data showed increased levels of the ER stress marker chaperone-binding protein/glucose regulated protein 78, GRP78-Bip, in RyR1 knockdown myotubes (+45%, P = 0.046), in mouse RyR1 recessive muscles (+58%, P = 0.001) and in human inflammatory (+96%, P = 0.006) and mitochondrial (+64%, P = 0.049) myopathy muscles. This was accompanied by increased protein levels of the pro-apoptotic protein CCAAT-enhancer-binding protein homologous protein, CHOP-DDIT3, in RyR1 knockdown myotubes (+27%, P < 0.001), mouse RyR1 recessive muscles (+63%, P = 0.009), human inflammatory (+50%, P = 0.038) and mitochondrial (+51%, P = 0.035) myopathy muscles. In publicly available datasets, the decrease in RYR1 content in myopathies was also associated to increased ER stress markers and RYR1 transcript levels are inversely correlated with ER stress markers in the control population.

CONCLUSIONS

Decreased RyR1 is commonly observed in myopathies and associated to ER stress in vitro, in mouse muscle and in human myopathy muscles, suggesting a potent role of RyR1 depletion-induced ER stress in the pathogenesis of myopathies.

Aberrant mitochondrial dynamics contributes to diaphragmatic weakness induced by mechanical ventilation

In critical care patients, the ""temporary inactivity of the diaphragm caused by mechanical ventilation (MV) triggers a series of events leading to diaphragmatic dysfunction and atrophy, commonly known as ventilator-induced diaphragm dysfunction (VIDD). While mitochondrial dysfunction related to oxidative stress is recognized as a crucial factor in VIDD, the exact molecular mechanism remains poorly understood. In this study, we observe that 6 h of MV triggers aberrant mitochondrial dynamics, resulting in a reduction in mitochondrial size and interaction, associated with increased expression of dynamin-related protein 1 (DRP1). This effect can be prevented by P110, a molecule that inhibits the recruitment of DRP1 to the mitochondrial membrane. Furthermore, isolated mitochondria from the diaphragms of ventilated patients exhibited increased production of reactive oxygen species (ROS). These mitochondrial changes were associated with the rapid oxidation of type 1 ryanodine receptor (RyR1) and a decrease in the stabilizing subunit calstabin 1. Subsequently, we observed that the sarcoplasmic reticulum (SR) in the ventilated diaphragms showed increased calcium leakage and reduced contractile function. Importantly, the mitochondrial fission inhibitor P110 effectively prevented all of these alterations. Taken together, the results of our study illustrate that MV leads, in the diaphragm, to both mitochondrial fragmentation and dysfunction, linked to the up-/down-regulation of 320 proteins, as assessed through global comprehensive quantitative proteomics analysis, primarily associated with mitochondrial function. These outcomes underscore the significance of developing compounds aimed at modulating the balance between mitochondrial fission and fusion as potential interventions to mitigate VIDD in human patients.

Molecular Aspects Implicated in Dantrolene Selectivity with Respect to Ryanodine Receptor Isoforms

Dantrolene is an intra-cellularly acting skeletal muscle relaxant used for the treatment of the rare genetic disorder, malignant hyperthermia (MH). In most cases, MH susceptibility is caused by dysfunction of the skeletal ryanodine receptor (RyR1) harboring one of nearly 230 single-point MH mutations. The therapeutic effect of dantrolene is the result of a direct inhibitory action on the RyR1 channel, thus suppressing aberrant Ca2+ release from the sarcoplasmic reticulum. Despite the almost identical dantrolene-binding sequence exits in all three mammalian RyR isoforms, dantrolene appears to be an isoform-selective inhibitor. Whereas RyR1 and RyR3 channels are competent to bind dantrolene, the RyR2 channel, predominantly expressed in the heart, is unresponsive. However, a large body of evidence suggests that the RyR2 channel becomes sensitive to dantrolene-mediated inhibition under certain pathological conditions. Although a consistent picture of the dantrolene effect emerges from in vivo studies, in vitro results are often contradictory. Hence, our goal in this perspective is to provide the best possible clues to the molecular mechanism of dantrolene’s action on RyR isoforms by identifying and discussing potential sources of conflicting results, mainly coming from cell-free experiments. Moreover, we propose that, specifically in the case of the RyR2 channel, its phosphorylation could be implicated in acquiring the channel responsiveness to dantrolene inhibition, interpreting functional findings in the structural context.

Three-dimensional perspective on ryanodine receptor mutations causing skeletal and cardiac muscle-related diseases

Mutations in RyR alter the cell's Ca2+ homeostasis and can cause serious health problems for which few effective therapies are available. Until recently, there was little structural context for the hundreds of mutations linked to muscular disorders reported for this large channel. Growing knowledge of the three-dimensional structure of RyR starts to illustrate the fine control of Ca2+ release. Current efforts directed towards understanding how disease mutations impinge in such processes will be crucial for future design of novel therapies. In this review article we discuss the up-to-date information about mutations according to their role in the 3D structure, and classified them to provide context from a structural perspective.

Targeting ryanodine receptors to treat human diseases

This Review provides an update on ryanodine receptors (RyRs) and their role in human diseases of heart, muscle, and brain. Calcium (Ca2+) is a requisite second messenger in all living organisms. From C. elegans to mammals, Ca2+ is necessary for locomotion, bodily functions, and neural activity. However, too much of a good thing can be bad. Intracellular Ca2+ overload can result in loss of function and death. Intracellular Ca2+ release channels evolved to safely provide large, rapid Ca2+ signals without exposure to toxic extracellular Ca2+. RyRs are intracellular Ca2+ release channels present throughout the zoosphere. Over the past 35 years, our knowledge of RyRs has advanced to the level of atomic-resolution structures revealing their role in the mechanisms underlying the pathogenesis of human disorders of heart, muscle, and brain. Stress-induced RyR-mediated intracellular Ca2+ leak in the heart can promote heart failure and cardiac arrhythmias. In skeletal muscle, RyR1 leak contributes to muscle weakness in inherited myopathies, to age-related loss of muscle function and cancer-associated muscle weakness, and to impaired muscle function in muscular dystrophies, including Duchenne. In the brain, leaky RyR channels contribute to cognitive dysfunction in Alzheimer's disease, posttraumatic stress disorder, and Huntington's disease. Novel therapeutics targeting dysfunctional RyRs are showing promise.

Disrupted T-tubular network accounts for asynchronous calcium release in MTM1-deficient skeletal muscle

In mammalian skeletal muscle, the propagation of surface membrane depolarization into the interior of the muscle fibre along the transverse (T) tubular network is essential for the synchronized release of calcium from the sarcoplasmic reticulum (SR) via ryanodine receptors (RyRs) in response to the conformational change in the voltage-sensor dihydropyridine receptors. Deficiency in 3-phosphoinositide phosphatase myotubularin (MTM1) has been reported to disrupt T-tubules, resulting in impaired SR calcium release. Here confocal calcium transients recorded in muscle fibres of MTM1-deficient mice were compared with the results from a model where propagation of the depolarization along the T-tubules was modelled mathematically with disruptions in the network assumed to modify the access and transmembrane resistance as well as the capacitance. If, in simulations, T-tubules were assumed to be partially or completely inaccessible to the depolarization and RyRs at these points to be prime for calcium-induced calcium release, all the features of measured SR calcium release could be reproduced. We conclude that the inappropriate propagation of the depolarization into the fibre interior is the initial critical cause of severely impaired SR calcium release in MTM1 deficiency, while the Ca²⁺ -triggered opening of RyRs provides an alleviating support to the diseased process. KEY POINTS: Myotubular myopathy is a fatal disease due to genetic deficiency in the phosphoinositide phosphatase MTM1. Although the causes are known and corresponding gene therapy strategies are being developed, there is no mechanistic understanding of the disease-associated muscle function failure. Resolving this issue is of primary interest not only for a fundamental understanding of how MTM1 is critical for healthy muscle function, but also for establishing the related cellular mechanisms most primarily or stringently affected by the disease, which are thus of potential interest as therapy targets. The mathematical modelling approach used in the present work proves that the disease-associated alteration of the plasma membrane invagination network is sufficient to explain the dysfunctions of excitation-contraction coupling, providing the first integrated quantitative framework that explains the associated contraction failure.

Neuronal microRNAs safeguard ER Ca2+ homeostasis and attenuate the unfolded protein response upon stress

Ca2+ is a critical mediator of neurotransmitter release, synaptic plasticity, and gene expression, but also excitotoxicity. Ca2+ signaling and homeostasis are coordinated by an intricate network of channels, pumps, and calcium-binding proteins, which must be rapidly regulated at all expression levels. Τhe role of neuronal miRNAs in regulating ryanodine receptors (RyRs) and inositol 1,4,5-triphosphate receptors (IP3Rs) was investigated to understand the underlying mechanisms that modulate ER Ca2+ release. RyRs and IP3Rs are critical in mounting and propagating cytosolic Ca2+ signals by functionally linking the ER Ca2+ content, while excessive ER Ca2+ release via these receptors is central to the pathophysiology of a wide range of neurological diseases. Herein, two brain-restricted microRNAs, miR-124-3p and miR-153-3p, were found to bind to RyR1-3 and IP3R3 3'UTRs, and suppress their expression at both the mRNA and protein level. Ca2+ imaging studies revealed that overexpression of these miRNAs reduced ER Ca2+ release upon RyR/IP3R activation, but had no effect on [Ca2+]i under resting conditions. Interestingly, treatments that cause excessive ER Ca2+ release decreased expression of these miRNAs and increased expression of their target ER Ca2+ channels, indicating interdependence of miRNAs, RyRs, and IP3Rs in Ca2+ homeostasis. Furthermore, by maintaining the ER Ca2+ content, miR-124 and miR-153 reduced cytosolic Ca2+ overload and preserved protein-folding capacity by attenuating PERK signaling. Overall, this study shows that miR-124-3p and miR-153-3p fine-tune ER Ca2+ homeostasis and alleviate ER stress responses.

Acute RyR1 Ca2+ leak enhances NADH-linked mitochondrial respiratory capacity

Sustained ryanodine receptor (RyR) Ca²⁺ leak is associated with pathological conditions such as heart failure or skeletal muscle weakness. We report that a single session of sprint interval training (SIT), but not of moderate intensity continuous training (MICT), triggers RyR1 protein oxidation and nitrosylation leading to calstabin1 dissociation in healthy human muscle and in in vitro SIT models (simulated SIT or S-SIT). This is accompanied by decreased sarcoplasmic reticulum Ca²⁺ content, increased levels of mitochondrial oxidative phosphorylation proteins, supercomplex formation and enhanced NADH-linked mitochondrial respiratory capacity. Mechanistically, (S-)SIT increases mitochondrial Ca²⁺ uptake in mouse myotubes and muscle fibres, and decreases pyruvate dehydrogenase phosphorylation in human muscle and mouse myotubes. Countering Ca²⁺ leak or preventing mitochondrial Ca²⁺ uptake blunts S-SIT-induced adaptations, a result supported by proteomic analyses. Here we show that triggering acute transient Ca²⁺ leak through RyR1 in healthy muscle may contribute to the multiple health promoting benefits of exercise.

High-resolution structure of the membrane-embedded skeletal muscle ryanodine receptor

The type 1 ryanodine receptor (RyR)/calcium release channel on the sarcoplasmic reticulum (SR) is required for skeletal muscle excitation-contraction coupling and is the largest known ion channel, composed of four 565-kDa protomers. Cryogenic electron microscopy (cryo-EM) studies of the RyR have primarily used detergent to solubilize the channel; in the present study, we have used cryo-EM to solve high-resolution structures of the channel in liposomes using a gel-filtration approach with on-column detergent removal to form liposomes and incorporate the channel simultaneously. This allowed us to resolve the structure of the channel in the primed and open states at 3.4 and 4.0 Å, respectively, with a single dataset. This method offers validation for detergent-based structures of the RyR and offers a starting point for utilizing a chemical gradient mimicking the SR, where Ca²⁺ concentrations are millimolar in the lumen and nanomolar in the cytosol.

Ryanodine receptor 1-related disorders: an historical perspective and proposal for a unified nomenclature

The RYR1 gene, which encodes the sarcoplasmic reticulum calcium release channel or type 1 ryanodine receptor (RyR1) of skeletal muscle, was sequenced in 1988 and RYR1 variations that impair calcium homeostasis and increase susceptibility to malignant hyperthermia were first identified in 1991. Since then, RYR1-related myopathies (RYR1-RM) have been described as rare, histopathologically and clinically heterogeneous, and slowly progressive neuromuscular disorders. RYR1 variants can lead to dysfunctional RyR1-mediated calcium release, malignant hyperthermia susceptibility, elevated oxidative stress, deleterious post-translational modifications, and decreased RyR1 expression. RYR1-RM-affected individuals can present with delayed motor milestones, contractures, scoliosis, ophthalmoplegia, and respiratory insufficiency.

Historically, RYR1-RM-affected individuals were diagnosed based on morphologic features observed in muscle biopsies including central cores, cores and rods, central nuclei, fiber type disproportion, and multi-minicores. However, these histopathologic features are not always specific to RYR1-RM and often change over time. As additional phenotypes were associated with RYR1 variations (including King-Denborough syndrome, exercise-induced rhabdomyolysis, lethal multiple pterygium syndrome, adult-onset distal myopathy, atypical periodic paralysis with or without myalgia, mild calf-predominant myopathy, and dusty core disease) the overlap among diagnostic categories is ever increasing. With the continuing emergence of new clinical subtypes along the RYR1 disease spectrum and reports of adult-onset phenotypes, nuanced nomenclatures have been reported (RYR1- [related, related congenital, congenital] myopathies). In this narrative review, we provide historical highlights of RYR1 research, accounts of the main diagnostic disease subtypes and propose RYR1-related disorders (RYR1-RD) as a unified nomenclature to describe this complex and evolving disease spectrum.

Role of defective calcium regulation in cardiorespiratory dysfunction in Huntington's disease

Huntington’s disease (HD) is a progressive, autosomal dominant neurodegenerative disorder affecting striatal neurons beginning in young adults with loss of muscle coordination and cognitive decline. Less appreciated is the fact that patients with HD also exhibit cardiac and respiratory dysfunction, including pulmonary insufficiency and cardiac arrhythmias. The underlying mechanism for these symptoms is poorly understood. In the present study we provide insight into the cause of cardiorespiratory dysfunction in HD and identify a potentially novel therapeutic target. We now show that intracellular calcium (Ca²⁺) leak via posttranslationally modified ryanodine receptor/intracellular calcium release (RyR) channels plays an important role in HD pathology. RyR channels were oxidized, PKA phosphorylated, and leaky in brain, heart, and diaphragm both in patients with HD and in a murine model of HD (Q175). HD mice (Q175) with endoplasmic reticulum Ca²⁺ leak exhibited cognitive dysfunction, decreased parasympathetic tone associated with cardiac arrhythmias, and reduced diaphragmatic contractile function resulting in impaired respiratory function. Defects in cognitive, motor, and respiratory functions were ameliorated by treatment with a novel Rycal small-molecule drug (S107) that fixes leaky RyR. Thus, leaky RyRs likely play a role in neuronal, cardiac, and diaphragmatic pathophysiology in HD, and RyRs are a potential novel therapeutic target.

This study explores the role of ryanodine receptor calcium channels in the brain, the heart, and the diaphragm and central versus peripheral pathophysiological mechanisms in Huntington’s disease.

Ryanodine Receptors: A Potential Treatment Target in Various Neurodegenerative Disease

Progressive neuronal demise is a key contributor to the key pathogenic event implicated in many different neurodegenerative disorders (NDDs). There are several therapeutic strategies available; however, none of them are particularly effective. Targeted neuroprotective therapy is one such therapy, which seems a compelling option, yet remains challenging due to the internal heterogeneity of the mechanisms underlying various NDDs. An alternative method to treat NDDs is to exploit common modalities involving molecularly distinct subtypes and thus develop specialized drugs with broad-spectrum characteristics. There is mounting evidence which supports for the theory that dysfunctional ryanodine receptors (RyRs) disrupt intracellular Ca²⁺ homeostasis, contributing to NDDs significantly. This review aims to provide direct and indirect evidence on the intersection of NDDs and RyRs malfunction, and to shed light on novel strategies to treat RyRs-mediated disease, modifying pharmacological therapies such as the potential therapeutic role of dantrolene, a RyRs antagonist.

Single Amino Acid Changes in the Ryanodine Receptor in the Human Population Have Effects In Vivo on Caenorhabditis elegans Neuro-Muscular Function

The ryanodine receptor mediates intracellular calcium ion release with excitation of nerve and muscle cells. Ryanodine receptor missense variants cause a number of myopathologies, such as malignant hyperthermia, and have been linked with various neuropathologies, including Alzheimer's disease. We characterized the consequences of ryanodine receptor variants in vivo. Eight Caenorhabditis elegans strains, with ryanodine receptor modifications equivalent to human myopathic RYR1 variants, were generated by genome editing. In humans, these variants are rare and confer sensitivity to the inhalational anaesthetic halothane when heterozygous. Increased sensitivity to halothane was found in both homozygous and heterozygous C. elegans. Close analysis revealed distinct subtle locomotion defects, due to the different single amino acid residue changes, even in the absence of the external triggering agent. Distinct pre- and postsynaptic consequences of the variants were characterized through the responses to cholinergic pharmacological agents. The range of phenotypes reflects the complexity of the regulatory inputs to the ryanodine receptor and the criticality of the calcium ion channel opening properties, in different cell types and with age. Ryanodine receptors with these single amino acid residue changes still function as calcium ion channels, but with altered properties which are likely to have subtle consequences for human carriers of such variants. The long-term consequences of subtly altered calcium ion signalling could be cumulative and may be focussed in the smaller nerve cells rather than the more robust muscle cells. It was important to assess phenotypes in vivo to properly appreciate consequences for a whole organism.

Ryanodine Receptor Calcium Leak in Circulating B-Lymphocytes as a Biomarker in Heart Failure

BACKGROUND

Advances in congestive heart failure (CHF) management depend on biomarkers for monitoring disease progression and therapeutic response. During systole, intracellular Ca²⁺ is released from the sarcoplasmic reticulum into the cytoplasm through type-2 ryanodine receptor/Ca²⁺ release channels. In CHF, chronically elevated circulating catecholamine levels cause pathological remodeling of type-2 ryanodine receptor/Ca²⁺ release channels resulting in diastolic sarcoplasmic reticulum Ca²⁺ leak and decreased myocardial contractility. Similarly, skeletal muscle contraction requires sarcoplasmic reticulum Ca²⁺ release through type-1 ryanodine receptors (RyR1), and chronically elevated catecholamine levels in CHF cause RyR1-mediated sarcoplasmic reticulum Ca²⁺ leak, contributing to myopathy and weakness. Circulating B-lymphocytes express RyR1 and catecholamine-responsive signaling cascades, making them a potential surrogate for defects in intracellular Ca²⁺ handling because of leaky RyR channels in CHF.

METHODS

Whole blood was collected from patients with CHF, CHF following left-ventricular assist device implant, and controls. Blood was also collected from mice with ischemic CHF, ischemic CHF+S107 (a drug that specifically reduces RyR channel Ca²⁺ leak), and wild-type controls. Channel macromolecular complex was assessed by immunostaining RyR1 immunoprecipitated from lymphocyte-enriched preparations. RyR1 Ca²⁺ leak was assessed using flow cytometry to measure Ca²⁺ fluorescence in B-lymphocytes in the absence and presence of RyR1 agonists that empty RyR1 Ca²⁺ stores within the endoplasmic reticulum.

RESULTS

Circulating B-lymphocytes from humans and mice with CHF exhibited remodeled RyR1 and decreased endoplasmic reticulum Ca²⁺ stores, consistent with chronic intracellular Ca²⁺ leak. This Ca²⁺ leak correlated with circulating catecholamine levels. The intracellular Ca²⁺ leak was significantly reduced in mice treated with the Rycal S107. Patients with CHF treated with left-ventricular assist devices exhibited a heterogeneous response.

CONCLUSIONS

In CHF, B-lymphocytes exhibit remodeled leaky RyR1 channels and decreased endoplasmic reticulum Ca²⁺ stores consistent with chronic intracellular Ca²⁺ leak. RyR1-mediated Ca²⁺ leak in B-lymphocytes assessed using flow cytometry provides a surrogate measure of intracellular Ca²⁺ handling and systemic sympathetic burden, presenting a novel biomarker for monitoring response to pharmacological and mechanical CHF therapy.

Interactions among ryanodine receptor isotypes contribute to muscle fiber type development and function

Mutations affecting ryanodine receptor (RyR) calcium release channels commonly underlie congenital myopathies. Although these channels are known principally for their essential roles in muscle contractility, mutations in the human RYR1 gene result in a broad spectrum of phenotypes, including muscle weakness, altered proportions of fiber types, anomalous muscle fibers with cores or centrally placed nuclei, and dysmorphic craniofacial features. Currently, it is unknown which phenotypes directly reflect requirements for RyRs and which result secondarily to aberrant muscle function. To identify biological processes requiring RyR function, skeletal muscle development was analyzed in zebrafish embryos harboring protein-null mutations. RyR channels contribute to both muscle fiber development and function. Loss of some RyRs had modest effects, altering muscle fiber-type specification in the embryo without compromising viability. In addition, each RyR-encoding gene contributed to normal swimming behavior and muscle function. The RyR channels do not function in a simple additive manner. For example, although isoform RyR1a is sufficient for muscle contraction in the absence of RyR1b, RyR1a normally attenuates the activity of the co-expressed RyR1b channel in slow muscle. RyR3 also acts to modify the functions of other RyR channels. Furthermore, diminished RyR-dependent contractility affects both muscle fiber maturation and craniofacial development. These findings help to explain some of the heterogeneity of phenotypes that accompany RyR1 mutations in humans.

Role of Ryanodine Type 2 Receptors in Elementary Ca2+ Signaling in Arteries and Vascular Adaptive Responses

Background Hypertension is the major risk factor for cardiovascular disease, the most common cause of death worldwide. Resistance arteries are capable of adapting their diameter independently in response to pressure and flow-associated shear stress. Ryanodine receptors (RyRs) are major Ca2+-release channels in the sarcoplasmic reticulum membrane of myocytes that contribute to the regulation of contractility. Vascular smooth muscle cells exhibit 3 different RyR isoforms (RyR1, RyR2, and RyR3), but the impact of individual RyR isoforms on adaptive vascular responses is largely unknown. Herein, we generated tamoxifen-inducible smooth muscle cell-specific RyR2-deficient mice and tested the hypothesis that vascular smooth muscle cell RyR2s play a specific role in elementary Ca2+ signaling and adaptive vascular responses to vascular pressure and/or flow. Methods and Results Targeted deletion of the Ryr2 gene resulted in a complete loss of sarcoplasmic reticulum-mediated Ca2+-release events and associated Ca2+-activated, large-conductance K+ channel currents in peripheral arteries, leading to increased myogenic tone and systemic blood pressure. In the absence of RyR2, the pulmonary artery pressure response to sustained hypoxia was enhanced, but flow-dependent effects, including blood flow recovery in ischemic hind limbs, were unaffected. Conclusions Our results establish that RyR2-mediated Ca2+-release events in VSCM s specifically regulate myogenic tone (systemic circulation) and arterial adaptation in response to changes in pressure (hypoxic lung model), but not flow. They further suggest that vascular smooth muscle cell-expressed RyR2 deserves scrutiny as a therapeutic target for the treatment of vascular responses in hypertension and chronic vascular diseases.

The binding interactions that maintain excitation-contraction coupling junctions in skeletal muscle

Calcium for contraction of skeletal muscles is released via tetrameric ryanodine receptor (RYR1) channels of the sarcoplasmic reticulum (SR), which are assembled in ordered arrays called couplons at junctions where the SR abuts T tubules or plasmalemma. Voltage-gated Ca2+ (CaV1.1) channels, found in tubules or plasmalemma, form symmetric complexes called CaV tetrads that associate with and activate underlying RYR tetramers during membrane depolarization by conveying a conformational change. Intriguingly, CaV tetrads regularly skip every other RYR tetramer within the array; therefore, the RYRs underlying tetrads (named V), but not the voltage sensor-lacking (C) RYRs, should be activated by depolarization. Here we hypothesize that the checkerboard association is maintained solely by reversible binary interactions between CaVs and RYRs and test this hypothesis using a quantitative model of the energies that govern CaV1.1-RYR1 binding, which are assumed to depend on number and location of bound CaVs. A Monte Carlo simulation generates large statistical samples and distributions of state variables that can be compared with quantitative features in freeze-fracture images of couplons from various sources. This analysis reveals two necessary model features: (1) the energy of a tetramer must have wells at low and high occupation by CaVs, so that CaVs positively cooperate in binding RYR (an allosteric effect), and (2) a large energy penalty results when two CaVs bind simultaneously to adjacent RYR protomers in adjacent tetramers (a steric clash). Under the hypothesis, V and C channels will eventually reverse roles. Role reversal justifies the presence of sensor-lacking C channels, as a structural and functional reserve for control of muscle contraction.

Ryanodine receptor dysfunction in human disorders

Regulation of intracellular calcium (Ca²⁺) is critical in all cell types. The ryanodine receptor (RyR), an intracellular Ca²⁺ release channel located on the sarco/endoplasmic reticulum (SR/ER), releases Ca²⁺ from intracellular stores to activate critical functions including muscle contraction and neurotransmitter release. Dysfunctional RyR-mediated Ca²⁺ handling has been implicated in the pathogenesis of inherited and non-inherited conditions including heart failure, cardiac arrhythmias, skeletal myopathies, diabetes, and neurodegenerative diseases. Here we have reviewed the evidence linking human disorders to RyR dysfunction and describe novel approaches to RyR-targeted therapeutics.

Role of defective Ca2+ signaling in skeletal muscle weakness: Pharmacological implications

The misbehaving attitude of Ca²⁺ signaling pathways could be the probable reason in many muscular disorders such as myopathies, systemic disorders like hypoxia, sepsis, cachexia, sarcopenia, heart failure, and dystrophy. The present review throws light upon the calcium flux regulating signaling channels like ryanodine receptor complex (RyR1), SERCA (Sarco-endoplasmic Reticulum Calcium ATPase), DHPR (Dihydropyridine Receptor) or Cav1.1 and Na+/Ca²⁺ exchange pump in detail and how remodelling of these channels contribute towards disturbed calcium homeostasis. Understanding these pathways will further provide an insight for establishing new therapeutic approaches for the prevention and treatment of muscle atrophy under stress conditions, targeting calcium ion channels and associated regulatory proteins.

Ryanodine Receptor Structure and Function in Health and Disease

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

IP3 receptor signaling and endothelial barrier function

The endothelium, a monolayer of endothelial cells lining vessel walls, maintains tissue-fluid homeostasis by restricting the passage of the plasma proteins and blood cells into the interstitium. The ion Ca²⁺, a ubiquitous secondary messenger, initiates signal transduction events in endothelial cells that is critical to control of vascular tone and endothelial permeability. The ion Ca²⁺ is stored inside the intracellular organelles and released into the cytosol in response to environmental cues. The inositol 1,4,5-trisphosphate (IP₃) messenger facilitates Ca²⁺ release through IP₃ receptors which are Ca²⁺-selective intracellular channels located within the membrane of the endoplasmic reticulum. Binding of IP₃ to the IP₃Rs initiates assembly of IP₃R clusters, a key event responsible for amplification of Ca²⁺ signals in endothelial cells. This review discusses emerging concepts related to architecture and dynamics of IP₃R clusters, and their specific role in propagation of Ca²⁺ signals in endothelial cells.

Ca2+ transport and signalling in enamel cells

Dental enamel is one of the most remarkable examples of matrix-mediated biomineralization. Enamel crystals form de novo in a rich extracellular environment in a stage-dependent manner producing complex microstructural patterns that are visually stunning. This process is orchestrated by specialized epithelial cells known as ameloblasts which themselves undergo striking morphological changes, switching function from a secretory role to a cell primarily engaged in ionic transport. Ameloblasts are supported by a host of cell types which combined represent the enamel organ. Fully mineralized enamel is the hardest tissue found in vertebrates owing its properties partly to the unique mixture of ionic species represented and their highly organized assembly in the crystal lattice. Among the main elements found in enamel, Ca2+ is the most abundant ion, yet how ameloblasts modulate Ca2+ dynamics remains poorly known. This review describes previously proposed models for passive and active Ca2+ transport, the intracellular Ca2+ buffering systems expressed in ameloblasts and provides an up-dated view of current models concerning Ca2+ influx and extrusion mechanisms, where most of the recent advances have been made. We also advance a new model for Ca2+ transport by the enamel organ.

Intracellular calcium release channels: an update

Ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (IP3 Rs) are calcium (Ca2+ ) release channels on the endo/sarcoplasmic reticulum (ER/SR). Here we summarize the latest advances in the field, describing the recently discovered mechanistic roles of intracellular Ca2+ release channels in the regulation of mitochondrial fitness and endothelial function, providing novel therapeutic options for the treatment of heart failure, hypertension, and diabetes mellitus.

Physiology and pathophysiology of excitation-contraction coupling: the functional role of ryanodine receptor

Calcium (Ca2+) release from intracellular stores plays a key role in the regulation of skeletal muscle contraction. The type 1 ryanodine receptors (RyR1) is the major Ca2+ release channel on the sarcoplasmic reticulum (SR) of myocytes in skeletal muscle and is required for excitation-contraction (E-C) coupling. This article explores the role of RyR1 in skeletal muscle physiology and pathophysiology.

The Mitochondrial Ca2+ Uniporter: Structure, Function, and Pharmacology

Mitochondrial Ca²⁺ uptake is crucial for an array of cellular functions while an imbalance can elicit cell death. In this chapter, we briefly reviewed the various modes of mitochondrial Ca²⁺ uptake and our current understanding of mitochondrial Ca²⁺ homeostasis in regards to cell physiology and pathophysiology. Further, this chapter focuses on the molecular identities, intracellular regulators as well as the pharmacology of mitochondrial Ca²⁺ uniporter complex.

Structures of the colossal RyR1 calcium release channel

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

Review of novel therapeutic targets for improving heart failure treatment based on experimental and clinical studies

Heart failure (HF) is a major public health priority due to its epidemiological transition and the world's aging population. HF is typified by continuous loss of contractile function with reduced, normal, or preserved ejection fraction, elevated vascular resistance, fluid and autonomic imbalance, and ventricular dilatation. Despite considerable advances in the treatment of HF over the past few decades, mortality remains substantial. Pharmacological treatments including β-blockers, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, and aldosterone antagonists have been proven to prolong the survival of patients with HF. However, there are still instances where patients remain symptomatic, despite optimal use of existing therapeutic agents. This understanding that patients with chronic HF progress into advanced stages despite receiving optimal treatment has increased the quest for alternatives, exploring the roles of additional pathways that contribute to the development and progression of HF. Several pharmacological targets associated with pathogenesis of HF have been identified and novel therapies have emerged. In this work, we review recent evidence from proposed mechanisms to the outcomes of experimental and clinical studies of the novel pharmacological agents that have emerged for the treatment of HF.

Molecular characterization of a ryanodine receptor gene from Spodoptera exigua and its upregulation by chlorantraniliprole

Chlorantraniliprole is a novel diamide insecticide that targets the insect ryanodine receptor, a Ca(2+) release channel. Spodoptera exigua is a significant insect pest, and chlorantraniliprole is the most popular diamide insecticide used against this pest. To better understand the effects of diamides on RyR expression and [Ca(2+)], we isolated the SeRyR cDNA and investigated changes in SeRyR expression as a result of the application of chlorantraniliprole. The full-length cDNAs of SeRyR contain an open reading frame (ORF) of 15,357 bp with a predicted protein consisting of 5118 amino acids. SeRyR shares 77-92% identity with other insect RyR isoforms and 45-47% identity with vertebrate RyR isoforms. Furthermore, the relative expression abundances of RyR mRNA extracted from S. exigua fat body cells after 24 h of culture in 0.1, 1, 10, 100 nM, 1 µM and 100 µM of chlorantraniliprole changed 1.04-, 0.89-, 1.83-, 2.58-, 4.03- and 3.12-fold compared to blank control, respectively. The regression equation for the relative expression levels of SeRyR after 24 h as a function of the chlorantraniliprole concentration was Y = 0.6455 + 0.8188LgX, R(2) = 0.97093 for the cell line IOZCAS-Spex-II. These results outline the effects of chlorantraniliprole on the expression of SeRyR and provide a basis for the discovery of a compound that may exhibit selective insect activity.

Genome-Wide Association and Trans-ethnic Meta-Analysis for Advanced Diabetic Kidney Disease: Family Investigation of Nephropathy and Diabetes (FIND)

Diabetic kidney disease (DKD) is the most common etiology of chronic kidney disease (CKD) in the industrialized world and accounts for much of the excess mortality in patients with diabetes mellitus. Approximately 45% of U.S. patients with incident end-stage kidney disease (ESKD) have DKD. Independent of glycemic control, DKD aggregates in families and has higher incidence rates in African, Mexican, and American Indian ancestral groups relative to European populations. The Family Investigation of Nephropathy and Diabetes (FIND) performed a genome-wide association study (GWAS) contrasting 6,197 unrelated individuals with advanced DKD with healthy and diabetic individuals lacking nephropathy of European American, African American, Mexican American, or American Indian ancestry. A large-scale replication and trans-ethnic meta-analysis included 7,539 additional European American, African American and American Indian DKD cases and non-nephropathy controls. Within ethnic group meta-analysis of discovery GWAS and replication set results identified genome-wide significant evidence for association between DKD and rs12523822 on chromosome 6q25.2 in American Indians (P = 5.74x10^-9). The strongest signal of association in the trans-ethnic meta-analysis was with a SNP in strong linkage disequilibrium with rs12523822 (rs955333; P = 1.31x10^-8), with directionally consistent results across ethnic groups. These 6q25.2 SNPs are located between the SCAF8 and CNKSR3 genes, a region with DKD relevant changes in gene expression and an eQTL with IPCEF1, a gene co-translated with CNKSR3. Several other SNPs demonstrated suggestive evidence of association with DKD, within and across populations. These data identify a novel DKD susceptibility locus with consistent directions of effect across diverse ancestral groups and provide insight into the genetic architecture of DKD.

Type 2 diabetes is the most common cause of severe kidney disease worldwide and diabetic kidney disease (DKD) associates with premature death. Individuals of non-European ancestry have the highest burden of type 2 DKD; hence understanding the causes of DKD remains critical to reducing health disparities. Family studies demonstrate that genes regulate the onset and progression of DKD; however, identifying these genes has proven to be challenging. The Family Investigation of Diabetes and Nephropathy consortium (FIND) recruited a large multi-ethnic collection of individuals with type 2 diabetes with and without kidney disease in order to detect genes associated with DKD. FIND discovered and replicated a DKD-associated genetic locus on human chromosome 6q25.2 (rs955333) between the SCAF8 and CNKSR genes. Findings were supported by significantly different expression of genes in this region from kidney tissue of subjects with, versus without DKD. The present findings identify a novel kidney disease susceptibility locus in individuals with type 2 diabetes which is consistent across subjects of differing ancestries. In addition, FIND results provide a rich catalogue of genetic variation in DKD patients for future research on the genetic architecture regulating this common and devastating disease.

Cholesterol removal from adult skeletal muscle impairs excitation-contraction coupling and aging reduces caveolin-3 and alters the expression of other triadic proteins

Cholesterol and caveolin are integral membrane components that modulate the function/location of many cellular proteins. Skeletal muscle fibers, which have unusually high cholesterol levels in transverse tubules, express the caveolin-3 isoform but its association with transverse tubules remains contentious. Cholesterol removal impairs excitation–contraction (E–C) coupling in amphibian and mammalian fetal skeletal muscle fibers. Here, we show that treating single muscle fibers from adult mice with the cholesterol removing agent methyl-β-cyclodextrin decreased fiber cholesterol by 26%, altered the location pattern of caveolin-3 and of the voltage dependent calcium channel Cav1.1, and suppressed or reduced electrically evoked Ca²⁺ transients without affecting membrane integrity or causing sarcoplasmic reticulum (SR) calcium depletion. We found that transverse tubules from adult muscle and triad fractions that contain ~10% attached transverse tubules, but not SR membranes, contained caveolin-3 and Cav1.1; both proteins partitioned into detergent-resistant membrane fractions highly enriched in cholesterol. Aging entails significant deterioration of skeletal muscle function. We found that triad fractions from aged rats had similar cholesterol and RyR1 protein levels compared to triads from young rats, but had lower caveolin-3 and glyceraldehyde 3-phosphate dehydrogenase and increased Na⁺/K⁺-ATPase protein levels. Both triad fractions had comparable NADPH oxidase (NOX) activity and protein content of NOX2 subunits (p47^phox and gp91^phox), implying that NOX activity does not increase during aging. These findings show that partial cholesterol removal impairs E–C coupling and alters caveolin-3 and Cav1.1 location pattern, and that aging reduces caveolin-3 protein content and modifies the expression of other triadic proteins. We discuss the possible implications of these findings for skeletal muscle function in young and aged animals.

Potassium dependent rescue of a myopathy with core-like structures in mouse

Myopathies decrease muscle functionality. Mutations in ryanodine receptor 1 (RyR1) are often associated with myopathies with microscopic core-like structures in the muscle fiber. In this study, we identify a mouse RyR1 model in which heterozygous animals display clinical and pathological hallmarks of myopathy with core-like structures. The RyR1 mutation decreases sensitivity to activated calcium release and myoplasmic calcium levels, subsequently affecting mitochondrial calcium and ATP production. Mutant muscle shows a persistent potassium leak and disrupted expression of regulators of potassium homeostasis. Inhibition of KATP channels or increasing interstitial potassium by diet or FDA-approved drugs can reverse the muscle weakness, fatigue-like physiology and pathology. We identify regulators of potassium homeostasis as biomarkers of disease that may reveal therapeutic targets in human patients with myopathy of central core disease (CCD). Altogether, our results suggest that amelioration of potassium leaks through potassium homeostasis mechanisms may minimize muscle damage of myopathies due to certain RyR1 mutations.

Themes and variations in ER/SR calcium release channels: structure and function

Calcium (Ca(2+)) release from reticular stores is a vital regulatory signal in eukaryotes. Recent structural data on large NH(2)-terminal regions of IP(3)Rs and RyRs and their tetrameric arrangement in the full-length context reveal striking mechanistic similarities in Ca(2+) release channel function. A common ancestor found in unicellular genomes underscores the fundamentality of these elements to Ca(2+) release channels.

Amplifying long transcripts of ryanodine receptors of five agricultural pests by transcriptome analysis and gap filling

Ryanodine receptor (RyR) is an intracellular calcium release channel that plays a key role in excitation contraction coupling. Insect RyR is the target of diamide insecticides. Better understanding of insect RyR is necessary for studying the molecular mode of action and potential resistance mechanism of diamide insecticides. However, molecular manipulation of the full RyR gene is difficult because of its length (approximately 15 kb). At present, RyR genes have been reported only in a limited number of insects. Here, we developed an efficient strategy to amplify full-length transcripts of insect RyR genes. First, we searched the transcriptomes of five insects, Bemisia tabaci, Cnaphalocrocis medinalis, Chilo suppressalis, Laodelphgax striatellus, and Plutella xylostella, yielding 85 RyR contigs in total. Second, the relative positions of these contigs in RyR transcripts were determined by aligning them with 12 well-annotated RyRs. Third, we designed primers to fill gaps between contigs and used rapid amplification of cDNA ends (RACE) to amplify both 5'- and 3'-ends. Last, we assembled all fragments into long transcripts. As a result, full-length transcripts of three insects, C. suppressalis, L. striatellus, and P. xylostella, were obtained. The RyR transcript of B. tabaci was near full length, containing an intact ORF. Northern blot analysis indicated that RyR genes were expressed in all five insects. Sequence analyses showed that the amplified insect segments contained typical RyRs characteristics, such as EF-hand, motif GVRAGGGIGD, and six transmembrane domains. Seven lepidopteran-specific amino acid residues were found to be located in the C-terminal region of RyR proteins, which might be associated with the specificity of RyRs to diamide insecticides.

Alterations in ryanodine receptors and related proteins in heart failure

Sarcoplasmic reticulum (SR) Ca(2+) release plays an essential role in mediating cardiac myocyte contraction. Depolarization of the plasma membrane results in influx of Ca(2+) through l-type Ca(2+) channels (LTCCs) that in turn triggers efflux of Ca(2+) from the SR through ryanodine receptor type-2 channels (RyR2). This process known as Ca(2+)-induced Ca(2+)release (CICR) occurs within the dyadic region, where the adjacent transverse (T)-tubules and SR membranes allow RyR2 clusters to release SR Ca(2+) following Ca(2+) influx through adjacent LTCCs. SR Ca(2+) released during systole binds to troponin-C and initiates actin-myosin cross-bridging, leading to muscle contraction. During diastole, the cytosolic Ca(2+) concentration is restored by the resequestration of Ca(2+) into the SR by SR/ER Ca(2+)-ATPase (SERCA2a) and by the extrusion of Ca(2+) via the Na(+)/Ca(2+)-exchanger (NCX1). This whole process, entitled excitation-contraction (EC) coupling, is highly coordinated and determines the force of contraction, providing a link between the electrical and mechanical activities of cardiac muscle. In response to heart failure (HF), the heart undergoes maladaptive changes that result in depressed intracellular Ca(2+) cycling and decreased SR Ca(2+) concentrations. As a result, the amplitude of CICR is reduced resulting in less force production during EC coupling. In this review, we discuss the specific proteins that alter the regulation of Ca(2+) during HF. In particular, we will focus on defects in RyR2-mediated SR Ca(2+) release. This article is part of a Special Issue entitled: Heart failure pathogenesis and emerging diagnostic and therapeutic interventions.

Cloning, characterisation and expression profiling of the cDNA encoding the ryanodine receptor in diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae)

BACKGROUND

The rynodine receptors (RyRs) are the main targets of diamide insecticides such as chlorantraniliprole. To provide the basis for a good understanding of the molecular mechanisms of diamide insecticide resistance, an RyR gene from Plutella xylostella was cloned and characterised in the present paper.

RESULTS

A full-length cDNA sequence of RyR was cloned from P. xylostella through RT-PCR and rapid amplification of cDNA ends (RACE). The gene (named PxRyR1) is 15 753 bp long, with an open reading frame of 15 354 bp, encoding a predicted RyR of 5117 amino acids. An alternative splicing of the PxRyR1 was also cloned and named PxRyR2. The PxRyR1 shares 77-93% identity with other insect RyRs. Quantitative real-time PCR analysis showed that the PxRyR was expressed at a high level in second-instar larvae and adults, at a low level in prepupae and pupae and abundantly in the body wall muscle and head (respectively 6.00 and 3.12 times the expression in the gut). Western blot analysis with anti-RyR antibodies showed that the RyR was mainly present in the body wall muscle and head, but barely present in the haemocyte and gut.

CONCLUSIONS

There are at least two alternative splices of PxRyR expressed in all developmental stages and tissues in P. xylostella at various levels. The results provided the basis for further understanding of the mechanisms of resistance to diamide insecticides in P. xylostella.

Ryanodine receptor patents

Research over the past two decades has implicated dysfunction of the ryanodine receptor (RyR), a Ca(2+) release channel on the sarcoplasmic reticulum (SR) required for excitation-contraction (EC) coupling, in the pathogenesis of cardiac and skeletal myopathies. These discoveries have led to the development of novel drugs, screening tools, and research methods. The patents associated with these advances tell the story of the initial discovery of RyRs as a target for plant alkaloids, to their central role in cardiac and skeletal muscle excitation-contraction coupling, and ongoing clinical trials with a novel class of drugs called RycalsTM that inhibit pathological intracellular Ca(2+) leak. Additionally, these patents highlight questions, controversies, and future directions of the RyR field.

Modulation of the expression of ryanodine receptor mRNA from Plutella xylostella as a result of diamide insecticide application

Ryanodine receptors (RyRs), members of the largest family of calcium channel proteins, have been studied because of their key roles in calcium signalling within cells. With the development of diamide insecticides that exhibit a novel mode of action on the RyRs from Lepidoptera, research on insect RyRs has become more attractive in the field of plant protection. To enhance our understanding of the effects of diamides on RyRs, we cloned the Plutella xylostella RyR gene (Px-RyR), which is the most serious pest of Brassicaceae plants throughout the world. Furthermore, we investigated the modulation of the expression of Px-RyR as a result of the application of diamide insecticides. The full-length cDNAs of Px-RyR contain an open reading frame (ORF) of 15,372bp with a predicted protein consisting of 5123 amino acids. Px-RyR possesses a high level of overall amino acid homology with other isoforms (77-92% identity with insect isoforms and 45-47% identity with vertebrate isoforms). The weight of Px. gradually decreased as the concentration of the diamides increased. However, the relative expression levels of the RyRs from larvae were dependent on the insecticide concentration and gradually increased with increasing insecticide concentrations.

Ryanodine receptor: a new therapeutic target to control diabetic cardiomyopathy

Diabetes mellitus is a major risk factor for cardiovascular complications. Intracellular Ca(2+) release plays an important role in the regulation of muscle contraction. Sarcoplasmic reticulum Ca(2+) release is controlled by dedicated molecular machinery, composed of a complex of cardiac ryanodine receptors (RyR2s). Acquired and genetic defects in this complex result in a spectrum of abnormal Ca(2+) release phenotypes in heart. Cardiovascular dysfunction is a leading cause for mortality of diabetic individuals due, in part, to a specific cardiomyopathy, and to altered vascular reactivity. Cardiovascular complications result from multiple parameters, including glucotoxicity, lipotoxicity, fibrosis, and mitochondrial uncoupling. In diabetic subjects, oxidative stress arises from an imbalance between production of reactive oxygen and nitrogen species and capability of the system to readily detoxify reactive intermediates. To date, the etiology underlying diabetes-induced reductions in myocyte and cardiac contractility remains incompletely understood. However, numerous studies, including work from our laboratory, suggest that these defects stem in part from perturbation in intracellular Ca(2+) cycling. Since the RyR2s are one of the well-characterized redox-sensitive ion channels in heart, this article summarizes recent findings on redox regulation of cardiac Ca(2+) transport systems and discusses contributions of redox regulation to pathological cardiac function in diabetes.

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.

Ryanodine receptor studies using genetically engineered mice

Ryanodine receptors (RyR) regulate intracellular Ca(2+) release in many cell types and have been implicated in a number of inherited human diseases. Over the past 15 years genetically engineered mouse models have been developed to elucidate the role that RyRs play in physiology and pathophysiology. To date these models have implicated RyRs in fundamental biological processes including excitation-contraction coupling and long term plasticity as well as diseases including malignant hyperthermia, cardiac arrhythmias, heart failure, and seizures. In this review we summarize the RyR mouse models and how they have enhanced our understanding of the RyR channels and their roles in cellular physiology and disease.

Quantitative structure-activity relationship (QSAR) studies for predicting activation of the ryanodine receptor type 1 channel complex (RyR1) by polychlorinated biphenyl (PCB) congeners

A quantitative structure-activity relationship (QSAR) was developed to predict the congener specific ryanodine receptor type RyR1 activity of all 209 polychlorinated biphenyl (PCB) congeners. A three-variable QSAR equation was obtained via stepwise forward linear regression on an unsupervised forward selection reduced data set from an initial database. Application of the QSAR towards predicting EC(2x) values for all 209 PCB congeners indicated good agreement in substitution pattern trends between the experimental and estimated data sets. The QSAR model predicts a less than two-fold increase in maximal potency among all congeners outside the experimental database, and it appears that no high-potency PCB congeners with EC(2x) values much less than 0.2 microM exist. Increasing RyR1-neuro toxicity equivalents with increasing homologue number and Aroclor chlorination likely reflect indirect molecular controls on toxicity, since congeners with multiple ortho substituents-the primary structural feature controlling a lack of coplanarity and resulting neurotoxicity-are more likely to be found in higher homologues.

New and selective ryanodine receptor activators for insect control

Diamide insecticides have emerged as one of the most promising new classes of insecticide chemistry owing to their excellent insecticidal efficacy and high margins of mammalian safety. Chlorantraniliprole and flubendiamide, the first two insecticides from this class, demonstrate exceptional activity across a broad range of pests in the order Lepidoptera. This chemistry has been confirmed to control insects via activation of ryanodine receptors which leads to uncontrolled calcium release in muscle. The high levels of mammalian safety are attributed to a strong selectivity for insect over mammalian receptors.

Cell Lines as In Vitro Models for Drug Screening and Toxicity Studies

Cell culture is highly desirable, as it provides systems for ready, direct access and evaluation of tissues. The use of tissue culture is a valuable tool to study problems of clinical relevance, especially those related to diseases, screening, and studies of cell toxicity mechanisms. Ready access to the cells provides the possibility for easy studies of cellular mechanisms that may suggest new potential drug targets and, in the case of pathological-derived tissue, it has an interesting application in the evaluation of therapeutic agents that potentially may treat the dysfunction. However, special considerations must be addressed to establish stable in vitro function. In primary culture, these factors are primarily linked to greater demands of tissue to adequately survive and develop differentiated conditions in vitro. Additional requirements include the use of special substrates (collagen, laminin, extracellular matrix preparations, etc.), growth factors and soluble media supplements, some of which can be quite complex in their composition. These demands, along with difficulties in obtaining adequate tissue amounts, have prompted interest in developing immortalized cell lines which can provide unlimited tissue amounts. However, cell lines tend to exhibit problems in stability and/or viability, though they serve as a feasible alternative, especially regarding new potential applications in cell transplant therapy. In this regard, stem cells may also be a source for the generation of various cell types in vitro. This review will address aspects of cell culture system application, with focus on immortalized cell lines, in studying cell function and dysfunction with the primary aim being to identify cell targets for drug screening.

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.

Dantrolene: mechanisms of neuroprotection and possible clinical applications in the neurointensive care unit

Calcium plays a central role in neuronal function and injury. Dantrolene, an inhibitor of the ryanodine receptor, inhibits intracellular calcium release from the sarco-endoplasmic reticulum. We review the available data of dantrolene as a potential neuroprotective agent and briefly summarize its other pharmacologic effects that may have potential applications for patients in the neurointensive care unit (NICU). Areas with the need for continued research are identified.

Stressed out: the skeletal muscle ryanodine receptor as a target of stress

Over the past century, understanding the mechanisms underlying muscle fatigue and weakness has been the focus of much investigation. However, the dominant theory in the field, that lactic acidosis causes muscle fatigue, is unlikely to tell the whole story. Recently, dysregulation of sarcoplasmic reticulum (SR) Ca(2+) release has been associated with impaired muscle function induced by a wide range of stressors, from dystrophy to heart failure to muscle fatigue. Here, we address current understandings of the altered regulation of SR Ca(2+) release during chronic stress, focusing on the role of the SR Ca(2+) release channel known as the type 1 ryanodine receptor.

Personal recollections on the discovery of the ryanodine receptors of muscle

The intracellular Ca(2+) release channels are indispensable molecular machinery in practically all eukaryotic cells of multicellular animals. They serve a key role in cell signaling by way of Ca(2+) as a second messenger. In response to a signaling event, the channels release Ca(2+) from intracellular stores. The resulting rise in cytoplasmic Ca(2+) concentration triggers the cell to carry out its specialized role, after which the intracellular Ca(2+) concentration must be reduced so that the signaling event can again be repeated. There are two types of intracellular Ca(2+) release channels, i.e., the ryanodine receptors and the inositol triphosphate receptors. My focus in this minireview is to present a personal account, from the vantage point our laboratory, of the discovery, isolation, and characterization of the ryanodine receptors from mammalian muscle. There are three isoforms: ryanodine receptor 1 (RyR1), first isolated from rabbit fast twitch skeletal muscle; ryanodine receptor 2 (RyR2), first isolated from dog heart; and ryanodine receptor 3 (RyR3), first isolated from bovine diaphragm muscle. The ryanodine receptors are the largest channel structures known. The RyR isoforms are very similar albeit with important differences. Natural mutations in humans in these receptors have already been associated with a number of muscle diseases.

Ryanodine receptor type 1 (RyR1) possessing malignant hyperthermia mutation R615C exhibits heightened sensitivity to dysregulation by non-coplanar 2,2',3,5',6-pentachlorobiphenyl (PCB 95)

Malignant hyperthermia (MH) susceptibility is conferred by inheriting one of >60 missense mutations within the highly regulated microsomal Ca(2+) channel known as ryanodine receptor type 1 (RyR1). Although MH susceptible patients lack overt clinical signs, a potentially lethal MH syndrome can be triggered by exposure to halogenated alkane anesthetics. This study compares how non-coplanar 2,2',3,5',6-pentachlorobiphenyl (PCB 95), a congener identified in environmental and human samples, alters the binding properties of [(3)H]ryanodine to RyR1 in vitro. Junctional sarcoplasmic reticulum (SR) was isolated from skeletal muscle dissected from wild type pigs ((Wt)RyR1) and pigs homozygous for MH mutation R615C ((MH)RyR1), a mutation also found in humans. Although the level of (Wt)RyR1 and (MH)RyR1 expression is the same, (MH)RyR1 shows heightened sensitivity to activation and altered regulation by physiological cations. We report here that (MH)RyR1 shows more pronounced activation by Ca(2+), and is less sensitive to channel inhibition by Ca(2+) and Mg(2+), compared to (Wt)RyR1. In a buffer containing 100nM free Ca(2+), conditions typically found in resting cells, PCB 95 (50-1000nM) enhances the activity of (MH)RyR1 whereas it has no detectable effect on (Wt)RyR1. PCB 95 (2microM) decreases channel inhibition by Mg(2+) to a greater extent in (MH)RyR1 (IC(50) increased nine-fold) compared to (Wt)RyR1 (IC(50) increased by 2.5-fold). PCB95 reduces inhibition by Ca(2+) two-fold more with (MH)RyR1 than (Wt)RyR1. Our data suggest that non-coplanar PCBs are more potent and efficacious toward (MH)RyR1 than (Wt)RyR1, and have more profound effects on its cation regulation. Considering the important roles of Ca(2+) and Mg(2+) in regulating Ca(2+) signals involving RyR channels, these data provide the first mechanistic evidence that a genetic mutation known to confer susceptibility to pharmacological agents also enhances sensitivity to an environmental contaminant.