Dantrolene stabilizes domain interactions within the ryanodine receptor

1,4-Benzothiazepines with Cyclopropanol Groups and Their Structural Analogues Exhibit Both RyR2-Stabilizing and SERCA2a-Stimulating Activities

To discover new multifunctional agents for the treatment of cardiovascular diseases, we designed and synthesized a series of compounds with a cyclopropyl alcohol moiety and evaluated them in biochemical assays. Biological screening identified derivatives with dual activity: preventing Ca2+ leak through ryanodine receptor 2 (RyR2) and enhancing cardiac sarco-endoplasmic reticulum (SR) Ca2+ load by activation of Ca2+-dependent ATPase 2a (SERCA2a). The compounds that stabilize RyR2 at micro- and nanomolar concentrations are either structurally related to RyR-stabilizing drugs or Rycals or have structures similar to them. The novel compounds also demonstrate a good ability to increase ATP hydrolysis mediated by SERCA2a activity in cardiac microsomes, e.g., the half-maximal effective concentration (EC50) was as low as 383 nM for compound 12a, which is 1,4-benzothiazepine with two cyclopropanol groups. Our findings indicate that these derivatives can be considered as new lead compounds to improve cardiac function in heart failure.

Antidotes in Clinical Toxicology-Critical Review

Poisoning and overdose are very important aspects in medicine and toxicology. Chemical weapons pose a threat to civilians, and emergency medicine principles must be followed when dealing with patients who have been poisoned or overdosed. Antidotes have been used for centuries and modern research has led to the development of new antidotes that can accelerate the elimination of toxins from the body. Although some antidotes have become less relevant due to modern intensive care techniques, they can still save lives or reduce the severity of toxicity. The availability of antidotes is crucial, especially in developing countries where intensive care facilities may be limited. This article aims to provide information on specific antidotes, their recommended uses, and potential risks and new uses. In the case of poisoning, supportive therapies are most often used; however, in many cases, the administration of an appropriate antidote saves the patient's life. In this review, we reviewed the literature on selected antidotes used in the treatment of poisonings. We also characterised the antidotes (bio)chemically. We described the cases in which they are used together with the dosage recommendations. We also analysed the mechanisms of action. In addition, we described alternative methods of using a given substance as a drug, an example of which is N-acetylcysteine, which can be used in the treatment of COVID-19. This article was written as part of the implementation of the project of the Polish Ministry of Education and Science, "Toxicovigilance, poisoning prevention, and first aid in poisoning with xenobiotics of current clinical importance in Poland", grant number SKN/SP/570184/2023.

Dantrolene inhibition of ryanodine channels (RyR2) in artificial lipid bilayers depends on FKBP12.6

Dantrolene is a neutral hydantoin that is clinically used as a skeletal muscle relaxant to prevent overactivation of the skeletal muscle calcium release channel (RyR1) in response to volatile anesthetics. Dantrolene has aroused considerable recent interest as a lead compound for stabilizing calcium release due to overactive cardiac calcium release channels (RyR2) in heart failure. Previously, we found that dantrolene produces up to a 45% inhibition RyR2 with an IC50 of 160 nM, and that this inhibition requires the physiological association between RyR2 and CaM. In this study, we tested the hypothesis that dantrolene inhibition of RyR2 in the presence of CaM is modulated by RyR2 phosphorylation at S2808 and S2814. Phosphorylation was altered by incubations with either exogenous phosphatase (PP1) or kinases; PKA to phosphorylate S2808 or endogenous CaMKII to phosphorylate S2814. We found that PKA caused selective dissociation of FKBP12.6 from the RyR2 complex and a loss of dantrolene inhibition. Rapamycin-induced FKBP12.6 dissociation from RyR2 also resulted in the loss of dantrolene inhibition. Subsequent incubations of RyR2 with exogenous FKBP12.6 reinstated dantrolene inhibition. These findings indicate that the inhibitory action of dantrolene on RyR2 depends on RyR2 association with FKBP12.6 in addition to CaM as previously found.

RyR2-targeting therapy prevents left ventricular remodeling and ventricular tachycardia in post-infarction heart failure

BACKGROUND

Dantrolene binds to the Leu⁶⁰¹-Cys⁶²⁰ region of the N-terminal domain of cardiac ryanodine receptor (RyR2), which corresponds to the Leu⁵⁹⁰-Cys⁶⁰⁹ region of the skeletal ryanodine receptor, and suppresses diastolic Ca²⁺ leakage through RyR2.

OBJECTIVE

We investigated whether the chronic administration of dantrolene prevented left ventricular (LV) remodeling and ventricular tachycardia (VT) after myocardial infarction (MI) by the same mechanism with the mutation V3599K of RyR2, which indicated that the inhibition of diastolic Ca²⁺ leakage occurred by enhancing the binding affinity of calmodulin (CaM) to RyR2.

METHODS AND RESULTS

A left anterior descending coronary artery ligation MI model was developed in mice. Wild-type (WT) were divided into four groups: sham-operated mice (WT-Sham), sham-operated mice treated with dantrolene (WT-Sham-DAN), MI mice (WT-MI), and MI mice treated with dantrolene (WT-MI-DAN). Homozygous V3599K RyR2 knock-in (KI) mice were divided into two groups: sham-operated mice (KI-Sham) and MI mice (KI-MI). The mice were followed for 12 weeks. Survival was significantly higher in the WT-MI-DAN (73%) and KI-MI groups (70%) than the WT-MI group (40%). Echocardiography, pathological tissue, and epinephrine-induced VT studies showed that LV remodeling and VT were prevented in the WT-MI-DAN and KI-MI groups compared to the WT-MI group. An increase in diastolic Ca²⁺ spark frequency and a decrease in the binding affinity of CaM to the RyR2 were observed at 12 weeks after MI in the WT-MI group, although significant improvements in these values were observed in the WT-MI-DAN and KI-MI groups.

CONCLUSIONS

Pharmacological or genetic stabilization of RyR2 tetrameric structure improves survival after MI by suppressing LV remodeling and proarrhythmia.

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.

Stabilizing cardiac ryanodine receptor with dantrolene treatment prevents left ventricular remodeling in pressure-overloaded heart failure mice

Dantrolene (DAN) directly binds to cardiac ryanodine receptor 2 (RyR2) through Leu⁶⁰¹-Cys⁶²⁰ in the N-terminal domain and subsequently inhibits diastolic Ca²⁺ leakage through RyR2. We previously reported that therapy using RyR2 V3599K mutation, which inhibits diastolic Ca²⁺ leakage by enhancing calmodulin (CaM) binding ability to RyR2, prevents left ventricular (LV) remodeling in transverse aortic constriction (TAC) heart failure. Here, we examined whether chronic administration of DAN prevents LV remodeling in TAC heart failure via the same mechanism as genetic therapy. A pressure-overloaded hypertrophy mouse model was developed using TAC. Wild-type (WT) mice were divided into three groups: sham-operated mice (Sham group), TAC mice (TAC group), and TAC mice treated with DAN (TAC-DAN group, 20 mg/kg/day, i.p.). They were then followed up for 8 weeks. The survival rate was higher in the TAC-DAN group (83%) than in the TAC group (49%), and serial echocardiography studies and pathological tissue analysis showed that LV remodeling was significantly prevented in the TAC-DAN group compared to the TAC group. An increase in the diastolic Ca²⁺ spark frequency and a decrease in the binding affinity of CaM to RyR2 were observed at 8 weeks in the TAC group but not in the TAC-DAN group. Stabilization of RyR2 with DAN prevented LV remodeling and improved survival after TAC by enhancing CaM binding to RyR2 and inhibiting RyR2-mediated diastolic Ca²⁺ leakage.

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.

Therapeutic Approaches of Ryanodine Receptor-Associated Heart Diseases

Cardiac diseases are the leading causes of death, with a growing number of cases worldwide, posing a challenge for both healthcare and research. Therefore, the most relevant aim of cardiac research is to unravel the molecular pathomechanisms and identify new therapeutic targets. Cardiac ryanodine receptor (RyR2), the Ca²⁺ release channel of the sarcoplasmic reticulum, is believed to be a good therapeutic target in a group of certain heart diseases, collectively called cardiac ryanopathies. Ryanopathies are associated with the impaired function of the RyR, leading to heart diseases such as congestive heart failure (CHF), catecholaminergic polymorphic ventricular tachycardia (CPVT), arrhythmogenic right ventricular dysplasia type 2 (ARVD2), and calcium release deficiency syndrome (CRDS). The aim of the current review is to provide a short insight into the pathological mechanisms of ryanopathies and discuss the pharmacological approaches targeting RyR2.

Emerging Antiarrhythmic Drugs for Atrial Fibrillation

Atrial fibrillation (AF), the most common cardiac arrhythmia worldwide, is driven by complex mechanisms that differ between subgroups of patients. This complexity is apparent from the different forms in which AF presents itself (post-operative, paroxysmal and persistent), each with heterogeneous patterns and variable progression. Our current understanding of the mechanisms responsible for initiation, maintenance and progression of the different forms of AF has increased significantly in recent years. Nevertheless, antiarrhythmic drugs for the management of AF have not been developed based on the underlying arrhythmia mechanisms and none of the currently used drugs were specifically developed to target AF. With the increased knowledge on the mechanisms underlying different forms of AF, new opportunities for developing more effective and safer AF therapies are emerging. In this review, we provide an overview of potential novel antiarrhythmic approaches based on the underlying mechanisms of AF, focusing both on the development of novel antiarrhythmic agents and on the possibility of repurposing already marketed drugs. In addition, we discuss the opportunity of targeting some of the key players involved in the underlying AF mechanisms, such as ryanodine receptor type-2 (RyR2) channels and atrial-selective K⁺-currents (I_K2P and I_SK) for antiarrhythmic therapy. In addition, we highlight the opportunities for targeting components of inflammatory signaling (e.g., the NLRP3-inflammasome) and upstream mechanisms targeting fibroblast function to prevent structural remodeling and progression of AF. Finally, we critically appraise emerging antiarrhythmic drug principles and future directions for antiarrhythmic drug development, as well as their potential for improving AF management.

Calcium dysregulation in heart diseases: Targeting calcium channels to achieve a correct calcium homeostasis

Intracellular calcium signaling is a universal language source shared by the most part of biological entities inside cells that, all together, give rise to physiological and functional anatomical units, the organ. Although preferentially recognized as signaling between cell life and death processes, in the heart it assumes additional relevance considered the importance of calcium cycling coupled to ATP consumption in excitation-contraction coupling. The concerted action of a plethora of exchangers, channels and pumps inward and outward calcium fluxes where needed, to convert energy and electric impulses in muscle contraction. All this without realizing it, thousands of times, every day. An improper function of those proteins (i.e., variation in expression, mutations onset, dysregulated channeling, differential protein-protein interactions) being part of this signaling network triggers a short circuit with severe acute and chronic pathological consequences reported as arrhythmias, cardiac remodeling, heart failure, reperfusion injury and cardiomyopathies. By acting with chemical, peptide-based and pharmacological modulators of these players, a correction of calcium homeostasis can be achieved accompanied by an amelioration of clinical symptoms. This review will focus on all those defects in calcium homeostasis which occur in the most common cardiac diseases, including myocardial infarction, arrhythmia, hypertrophy, heart failure and cardiomyopathies. This part will be introduced by the state of the art on the proteins involved in calcium homeostasis in cardiomyocytes and followed by the therapeutic treatments that to date, are able to target them and to revert the pathological phenotype.

Pediatric Catecholaminergic Polymorphic Ventricular Tachycardia: A Translational Perspective for the Clinician-Scientist

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a rare and potentially lethal inherited arrhythmia disease characterized by exercise or emotion-induced bidirectional or polymorphic ventricular tachyarrhythmias. The median age of disease onset is reported to be approximately 10 years of age. The majority of CPVT patients have pathogenic variants in the gene encoding the cardiac ryanodine receptor, or calsequestrin 2. These lead to mishandling of calcium in cardiomyocytes resulting in after-depolarizations, and ventricular arrhythmias. Disease severity is particularly pronounced in younger individuals who usually present with cardiac arrest and arrhythmic syncope. Risk stratification is imprecise and long-term prognosis on therapy is unknown despite decades of research focused on pediatric CPVT populations. The purpose of this review is to summarize contemporary data on pediatric CPVT, highlight knowledge gaps and present future research directions for the clinician-scientist to address.

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.

The Medicinal Chemistry of Caffeine

The purine alkaloid caffeine is the most widely consumed psychostimulant drug in the world and has multiple beneficial pharmacological activities, for example, in neurodegenerative diseases. However, despite being an extensively studied bioactive natural product, the mechanistic understanding of caffeine's pharmacological effects is incomplete. While several molecular targets of caffeine such as adenosine receptors and phosphodiesterases have been known for decades and inspired numerous medicinal chemistry programs, new protein interactions of the xanthine are continuously discovered providing potentially improved pharmacological understanding and a molecular basis for future medicinal chemistry. In this Perspective, we gather knowledge on the confirmed protein interactions, structure activity relationship, and chemical biology of caffeine on well-known and upcoming targets. The diversity of caffeine's molecular activities on receptors and enzymes, many of which are abundant in the CNS, indicates a complex interplay of several mechanisms contributing to neuroprotective effects and highlights new targets as attractive subjects for drug discovery.

Patient-specific induced pluripotent stem cells as “disease-in-a-dish” models for inherited cardiomyopathies and channelopathies – 15 years of research

Among inherited cardiac conditions, a special place is kept by cardiomyopathies (CMPs) and channelopathies (CNPs), which pose a substantial healthcare burden due to the complexity of the therapeutic management and cause early mortality. Like other inherited cardiac conditions, genetic CMPs and CNPs exhibit incomplete penetrance and variable expressivity even within carriers of the same pathogenic deoxyribonucleic acid variant, challenging our understanding of the underlying pathogenic mechanisms. Until recently, the lack of accurate physiological preclinical models hindered the investigation of fundamental cellular and molecular mechanisms. The advent of induced pluripotent stem cell (iPSC) technology, along with advances in gene editing, offered unprecedented opportunities to explore hereditary CMPs and CNPs. Hallmark features of iPSCs include the ability to differentiate into unlimited numbers of cells from any of the three germ layers, genetic identity with the subject from whom they were derived, and ease of gene editing, all of which were used to generate “disease-in-a-dish” models of monogenic cardiac conditions. Functionally, iPSC-derived cardiomyocytes that faithfully recapitulate the patient-specific phenotype, allowed the study of disease mechanisms in an individual-/allele-specific manner, as well as the customization of therapeutic regimen. This review provides a synopsis of the most important iPSC-based models of CMPs and CNPs and the potential use for modeling disease mechanisms, personalized therapy and deoxyribonucleic acid variant functional annotation.

Pathological conformations of disease mutant Ryanodine Receptors revealed by cryo-EM

Ryanodine Receptors (RyRs) are massive channels that release Ca²⁺ from the endoplasmic and sarcoplasmic reticulum. Hundreds of mutations are linked to malignant hyperthermia (MH), myopathies, and arrhythmias. Here, we explore the first MH mutation identified in humans by providing cryo-EM snapshots of the pig homolog, R615C, showing that it affects an interface between three solenoid regions. We also show the impact of apo-calmodulin (apoCaM) and how it can induce opening by bending of the bridging solenoid, mediated by its N-terminal lobe. For R615C RyR1, apoCaM binding abolishes a pathological ‘intermediate’ conformation, distributing the population to a mixture of open and closed channels, both different from the structure without apoCaM. Comparisons show that the mutation primarily affects the closed state, inducing partial movements linked to channel activation. This shows that disease mutations can cause distinct pathological conformations of the RyR and facilitate channel opening by disrupting interactions between different solenoid regions.

Ryanodine Receptors (RyRs) release Ca2+ from the endoplasmic and sarcoplasmic reticulum. Mutations in RyR are linked to malignant hyperthermia (MH), myopathies, and arrhythmias. Here, a collection of cryoEM structures provides insights into the molecular consequences of MHrelated RyR mutation R615C, and how apoCaM opens RyR1.

In vivo Ca2+ dynamics during cooling after eccentric contractions in rat skeletal muscle

The effect of cooling on in vivo intracellular calcium ion concentration [Ca²⁺]_i after eccentric contractions (ECs) remains to be determined. We tested the hypothesis that cryotherapy following ECs promotes an increased [Ca²⁺]_i and induces greater muscle damage in two muscles with substantial IIb and IIx fiber populations. The thin spinotrapezius (SPINO) muscles of Wistar rats were used for in vivo [Ca²⁺]_i imaging, and tibialis anterior (TA) muscles provided greater fidelity and repeatability of contractile function measurements. SPINO [Ca²⁺]_i was estimated using fura 2-AM and the magnitude, location, and temporal profile of [Ca²⁺]_i determined as the temperature near the muscle surface post-ECs was decreased from 30°C (control) to 20°C or 10°C. Subsequently, in the TA, the effect of post-ECs cooling to 10°C on muscle contractile performance was determined at 1 and 2 days after ECs. TA muscle samples were examined by hematoxylin and eosin staining to assess damage. In SPINO, reducing the muscle temperature from 30°C to 10°C post-ECs resulted in a 3.7-fold increase in the spread of high [Ca²⁺]_i sites generated by ECs (P < 0.05). These high [Ca²⁺]_i sites demonstrated partial reversibility when rewarmed to 30°C. Dantrolene, a ryanodine receptor Ca²⁺ release inhibitor, reduced the presence of high [Ca²⁺] sites at 10°C. In the TA, cooling exacerbated ECs-induced muscle strength deficits via enhanced muscle fiber damage (P < 0.05). By demonstrating that cooling post-ECs potentiates [Ca²⁺]_i derangements, this in vivo approach supports a putative mechanistic basis for how postexercise cryotherapy might augment muscle fiber damage and decrease subsequent exercise performance.

Mitochondrial and Sarcoplasmic Reticulum Interconnection in Cardiac Arrhythmia

Ca²⁺ plays a pivotal role in mitochondrial energy production, contraction, and apoptosis. Mitochondrial Ca²⁺-targeted fluorescent probes have demonstrated that mitochondria Ca²⁺ transients are synchronized with Ca²⁺ fluxes occurring in the sarcoplasmic reticulum (SR). The presence of specialized proteins tethering SR to mitochondria ensures the local Ca²⁺ flux between these organelles. Furthermore, communication between SR and mitochondria impacts their functionality in a bidirectional manner. Mitochondrial Ca²⁺ uptake through the mitochondrial Ca²⁺ uniplex is essential for ATP production and controlled reactive oxygen species levels for proper cellular signaling. Conversely, mitochondrial ATP ensures the proper functioning of SR Ca²⁺-handling proteins, which ensures that mitochondria receive an adequate supply of Ca²⁺. Recent evidence suggests that altered SR Ca²⁺ proteins, such as ryanodine receptors and the sarco/endoplasmic reticulum Ca²⁺ ATPase pump, play an important role in maintaining proper cardiac membrane excitability, which may be initiated and potentiated when mitochondria are dysfunctional. This recognized mitochondrial role offers the opportunity to develop new therapeutic approaches aimed at preventing cardiac arrhythmias in cardiac disease.

CaMKII-dependent ryanodine receptor phosphorylation mediates sepsis-induced cardiomyocyte apoptosis

Sepsis is associated with cardiac dysfunction, which is at least in part due to cardiomyocyte apoptosis. However, the underlying mechanisms are far from being understood. Using the colon ascendens stent peritonitis mouse model of sepsis (CASP), we examined the subcellular mechanisms that mediate sepsis‐induced apoptosis. Wild‐type (WT) CASP mice hearts showed an increase in apoptosis respect to WT‐Sham. CASP transgenic mice expressing a CaMKII inhibitory peptide (AC3‐I) were protected against sepsis‐induced apoptosis. Dantrolene, used to reduce ryanodine receptor (RyR) diastolic sarcoplasmic reticulum (SR) Ca²⁺ release, prevented apoptosis in WT‐CASP. To examine whether CaMKII‐dependent RyR2 phosphorylation mediates diastolic Ca²⁺ release and apoptosis in sepsis, we evaluated apoptosis in mutant mice hearts that have the CaMKII phosphorylation site of RyR2 (Serine 2814) mutated to Alanine (S2814A). S2814A CASP mice did not show increased apoptosis. Consistent with RyR2 phosphorylation‐dependent enhancement in diastolic SR Ca²⁺ release leading to mitochondrial Ca²⁺ overload, mitochondrial Ca²⁺ retention capacity was reduced in mitochondria isolated from WT‐CASP compared to Sham and this reduction was absent in mitochondria from CASP S2814A or dantrolene‐treated mice. We conclude that in sepsis, CaMKII‐dependent RyR2 phosphorylation results in diastolic Ca²⁺ release from SR which leads to mitochondrial Ca²⁺ overload and apoptosis.

Targeting Ca2 + Handling Proteins for the Treatment of Heart Failure and Arrhythmias

Diseases of the heart, such as heart failure and cardiac arrhythmias, are a growing socio-economic burden. Calcium (Ca²⁺) dysregulation is key hallmark of the failing myocardium and has long been touted as a potential therapeutic target in the treatment of a variety of cardiovascular diseases (CVD). In the heart, Ca²⁺ is essential for maintaining normal cardiac function through the generation of the cardiac action potential and its involvement in excitation contraction coupling. As such, the proteins which regulate Ca²⁺ cycling and signaling play a vital role in maintaining Ca²⁺ homeostasis. Changes to the expression levels and function of Ca²⁺-channels, pumps and associated intracellular handling proteins contribute to altered Ca²⁺ homeostasis in CVD. The remodeling of Ca²⁺-handling proteins therefore results in impaired Ca²⁺ cycling, Ca²⁺ leak from the sarcoplasmic reticulum and reduced Ca²⁺ clearance, all of which contributes to increased intracellular Ca²⁺. Currently, approved treatments for targeting Ca²⁺ handling dysfunction in CVD are focused on Ca²⁺ channel blockers. However, whilst Ca²⁺ channel blockers have been successful in the treatment of some arrhythmic disorders, they are not universally prescribed to heart failure patients owing to their ability to depress cardiac function. Despite the progress in CVD treatments, there remains a clear need for novel therapeutic approaches which are able to reverse pathophysiology associated with heart failure and arrhythmias. Given that heart failure and cardiac arrhythmias are closely associated with altered Ca²⁺ homeostasis, this review will address the molecular changes to proteins associated with both Ca²⁺-handling and -signaling; their potential as novel therapeutic targets will be discussed in the context of pre-clinical and, where available, clinical data.

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

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

Dantrolene prevents ventricular tachycardia by stabilizing the ryanodine receptor in pressure- overload induced failing hearts

Aberrant Ca²⁺ release from cardiac ryanodine receptors (RyR2) has been shown to be one of the most important causes of lethal arrhythmia in various types of failing hearts. We previously showed that dantrolene, a specific agent for the treatment of malignant hyperthermia, inhibits Ca²⁺ leakage from the RyR2 by correcting the defective inter-domain interaction between the N-terminal (1-619 amino acids) and central (2000-2500 amino acids) domains of the RyR2 and allosterically enhancing the binding affinity of calmodulin to the RyR2 in diseased hearts. In this study, we examined whether dantrolene inhibits this Ca²⁺ leakage, thereby preventing the pharmacologically inducible ventricular tachycardia in ventricular pressure-overloaded failing hearts. Ventricular tachycardia (VT) was easily induced after an injection of epinephrine in mice after 8 weeks of transverse aortic constriction-induced pressure-overload. Pretreatment with dantrolene almost completely inhibited the pharmacologically inducible VT. In the presence of dantrolene, the occurrence of both Ca²⁺ sparks and spontaneous Ca²⁺ transients was inhibited, which was associated with enhanced calmodulin binding affinity to the RyR2. These results suggest that dantrolene could be a new potent agent in the treatment of lethal arrhythmia in cases of acquired heart failure.

Dantrolene Requires Mg2+ and ATP To Inhibit the Ryanodine Receptor

Dantrolene is a ryanodine receptor (RyR) inhibitor, which is used to relax muscles in malignant hyperthermia syndrome. Although dantrolene binds to the RyR protein, its mechanism of action is unknown, mainly because of the controversial results showing that dantrolene inhibited Ca²⁺ release from intact fibers and sarcoplasmic reticulum (SR) vesicles, but failed to inhibit single RyR channel currents in bilayers. Accordingly, it was concluded that an important factor for dantrolene's action was lost during the purification procedure of RyR. Recently, Mg²⁺ was demonstrated to be the essential factor for dantrolene to inhibit Ca²⁺ release in skinned muscle fibers. The aim of the present study was to confirm these results in Ca²⁺ release and bilayer experiments, using SR vesicles and solubilized channels, respectively. Our Ca²⁺ release experiments demonstrated that the effect of dantrolene and Mg²⁺ was cooperative and that ATP enhanced the inhibiting effect of dantrolene. Namely, 10 µM dantrolene reduced RyR channel open probability by ∼50% in the presence of 3 mM free Mg²⁺ and 1 mM ATP, whereas channel activity further decreased to ∼20% of control when [ATP] was increased to 2 mM. Our data provide important complementary information that supports the direct, Mg²⁺-dependent mechanism of dantrolene's action and suggests that dantrolene also requires ATP to inhibit RyR.

Dantrolene sodium increases calcium binding by human recombinant cardiac calsequestrin and calcium loading by sheep cardiac sarcoplasmic reticulum

AIM

Dantrolene interacts with ryanodine receptors in skeletal and cardiac muscle affecting sarcoplasmic reticulum calcium release. Since dantrolene is lipophilic it could also affect intra-sarcoplasmic reticulum calcium handling proteins such as calsequestrin. This study investigated whether dantrolene (1-30 µmol/L) alters the polymerization state and the calcium binding capacity of recombinant cardiac calsequestrin and cardiac sarcoplasmic reticulum calcium handling.

METHODS

Human recombinant cardiac calsequestrin was used to make simultaneous measurements of turbidity (to indicate calsequestrin polymerization) and calcium binding to calsequestrin in the presence and absence of dantrolene. Caffeine-induced Ca²⁺ transients were used to investigate the effects of dantrolene on sarcoplasmic reticulum calcium loading and release in saponin-permeabilized cardiomyocytes laid down in monolayers in 96-well array plates.

RESULTS

Dantrolene (1-30 µmol/L) increased the polymerization state of calsequestrin and its calcium binding capacity. In the presence of dantrolene, calsequestrin-dependent turbidity increased 2.5-11.5-fold at 1.0 mmol/L calcium added to unbuffered Ca²⁺ solutions and 3-10-fold when calcium was raised from 0.06 to 30 µmol/L. The dantrolene-dependent increase in turbidity at 30 µmol/L dantrolene was associated with a 3-fold increase in the number of calcium ions bound per calsequestrin molecule at 30 and 100 µmol/L calcium. The caffeine-induced releasable calcium loaded by the sarcoplasmic reticulum at 0.63 µmol/L free calcium in permeabilized cardiomyocytes was also increased in the presence of 30 µmol/L dantrolene.

CONCLUSION

Dantrolene alters the polymerization state and the calcium binding properties of cardiac calsequestrin and increases sarcoplasmic reticulum calcium loading in permeabilized cardiomyocytes.

Atrial Infarction-Induced Spontaneous Focal Discharges and Atrial Fibrillation in Sheep: Role of Dantrolene-Sensitive Aberrant Ryanodine Receptor Calcium Release

BACKGROUND

The mechanisms underlying spontaneous atrial fibrillation (AF) associated with atrial ischemia/infarction are incompletely elucidated. Here, we investigate the mechanisms underlying spontaneous AF in an ovine model of left atrial myocardial infarction (LAMI).

METHODS AND RESULTS

LAMI was created by ligating the atrial branch of the left anterior descending coronary artery. ECG loop recorders were implanted to monitor AF episodes. In 7 sheep, dantrolene-a ryanodine receptor blocker-was administered in vivo during the 8-day observation period (LAMI-D, 2.5 mg/kg, IV, BID). LAMI animals experienced numerous spontaneous AF episodes during the 8-day monitoring period that were suppressed by dantrolene (LAMI, 26.1±5.1; sham, 4.3±1.1; LAMI-D, 2.8±0.8; mean±SEM episodes per sheep, P<0.01). Optical mapping showed spontaneous focal discharges (SFDs) originating from the ischemic/normal-zone border. SFDs were calcium driven, rate dependent, and enhanced by isoproterenol (0.03 µmol/L, from 210±87 to 3816±1450, SFDs per sheep) but suppressed by dantrolene (to 55.8±32.8, SFDs per sheep, mean±SEM). SFDs initiated AF-maintaining reentrant rotors anchored by marked conduction delays at the ischemic/normal-zone border. NOS1 (NO synthase-1) protein expression decreased in ischemic zone myocytes, whereas NADPH (nicotinamide adenine dinucleotide phosphate, reduced form) oxidase and xanthine oxidase enzyme activities and reactive oxygen species (DCF [6-carboxy-2',7'-dichlorodihydrofluorescein diacetate]-fluorescence) increased. CaM (calmodulin) aberrantly increased [³H]ryanodine binding to cardiac RyR2 (ryanodine receptors) in the ischemic zone. Dantrolene restored the physiological binding of CaM to RyR2.

CONCLUSIONS

Atrial ischemia causes spontaneous AF episodes in sheep, caused by SFDs that initiate reentry. Nitroso-redox imbalance in the ischemic zone is associated with intense reactive oxygen species production and altered RyR2 responses to CaM. Dantrolene administration normalizes the CaM response, prevents LAMI-related SFDs, and AF initiation. These findings provide novel insights into the mechanisms underlying ischemia-related atrial arrhythmias.

Functional recovery after dantrolene-supplementation of cold stored hearts using an ex vivo isolated working rat heart model

The ryanodine receptor antagonist dantrolene inhibits calcium release from the sarcoplasmic reticulum and reduces cardiac ischaemia-reperfusion injury (IRI) in global warm ischaemia models however the cardioprotective potential of dantrolene under hypothermic conditions is unknown. This study addresses whether the addition of dantrolene during cardioplegia and hypothermic storage of the donor heart can improve functional recovery and reduce IRI. Using an ex vivo isolated working heart model, Wistar rat (3 month and 12 month) hearts were perfused to acquire baseline haemodynamic measurements of aortic flow, coronary flow, cardiac output, pulse pressure and heart rate. Hearts were arrested and stored in Celsior preservation solution supplemented with 0.2–40 μM dantrolene for 6 hours at 4°C, then reperfused (15 min Langendorff, 30 min working mode). In 3-month hearts, supplementation with 1 μM dantrolene significantly improved aortic flow and cardiac output compared to unsupplemented controls however lactate dehydrogenase (LDH) release and contraction bands were comparable. In contrast, 40 μM dantrolene-supplementation yielded poor cardiac recovery, increased post-reperfusion LDH but reduced contraction bands. All 3-month hearts stored in dantrolene displayed significantly reduced cleaved-caspase 3 intensities compared to controls. Analysis of cardioprotective signalling pathways showed no changes in AMPKα however dantrolene increased STAT3 and ERK1/2 signaling in a manner unrelated to functional recovery and AKT activity was reduced in 1 μM dantrolene-stored hearts. In contrast to 3-month hearts, no significant improvements were observed in the functional recovery of 12-month hearts following prolonged storage in 1 μM dantrolene. Conclusions: Dantrolene supplementation at 1 μM during hypothermic heart preservation improved functional recovery of young, but not older (12 month) hearts. Although the molecular mechanisms responsible for dantrolene-mediated cardioprotection are unclear, our studies show no correlation between improved functional recovery and SAFE and RISK pathway activation.

Association of riluzole and dantrolene improves significant recovery after acute spinal cord injury in rats

BACKGROUND CONTEXT

Damage to the spinal cord can result in irreversible impairment or complete loss of motor, sensory, and autonomic functions. Riluzole and dantrolene have been shown to provide neuroprotection by reducing neuronal apoptosis after brain and spinal cord injury (SCI) in several animal models of neurologic disorders. As these drugs protect the injured spinal cord through different mechanisms, we investigated the cumulative effects of riluzole and dantrolene.

PURPOSE

This study aimed to investigate the neuroprotective efficacy of the combined administration of riluzole and dantrolene in experimental thoracic SCI.

STUDY DESIGN

Twenty-nine Wistar rats were laminectomized at T12 and divided in five groups. Rats in GI (n=6) underwent laminectomy alone and were treated with placebo. Rats in GII (n=6) underwent laminectomy followed by SCI and were treated with placebo. Rats in GIII (n=5) underwent laminectomy followed by SCI and were treated with riluzole and placebo 15 minutes and 1 hour after laminectomy, respectively. Rats in GIV (n=6) underwent laminectomy followed by SCI and were treated with placebo and dantrolene 15 minutes and 1 hour after laminectomy, respectively. Rats in GV (n=6) underwent laminectomy followed by SCI and were treated with riluzole and dantrolene 15 minutes and 1 hour after laminectomy, respectively. A compressive trauma was performed to induce SCI.

METHODS

Behavioral testing of hind limb function was performed using the Basso Beattie Bresnahan locomotor rating scale, which revealed significant recovery in the group treated with the association of riluzole and dantrolene compared with other groups. After euthanasia, the spinal cord was evaluated using light microscopy and immunochemistry with anti-NeuN and transferase dUTP nick-end-labeling (TUNEL) staining.

RESULTS

Animals treated with the association of riluzole and dantrolene showed a larger number of NeuN-positive neurons adjacent to the epicenter of injury (p≤.05). Furthermore, the TUNEL staining was similar between animals treated with riluzole and dantrolene and those that did not receive spinal cord trauma (p>.05).

CONCLUSIONS

These results showed that riluzole and dantrolene have a synergistic effect in neuroprotection after traumatic SCI by decreasing apoptotic cell death.

Repurposing Dantrolene for Long-Term Combination Therapy to Potentiate Antisense-Mediated DMD Exon Skipping in the mdx Mouse

Duchenne muscular dystrophy (DMD) is caused by mutations in DMD, resulting in loss of dystrophin, which is essential to muscle health. DMD “exon skipping” uses anti-sense oligo-nucleotides (AONs) to force specific exon exclusion during mRNA processing to restore reading frame and rescue of partially functional dystrophin protein. Although exon-skipping drugs in humans show promise, levels of rescued dystrophin protein remain suboptimal. We previously identified dantrolene as a skip booster when combined with AON in human DMD cultures and short-term mdx dystrophic mouse studies. Here, we assess the effect of dantrolene/AON combination on DMD exon-23 skipping over long-term mdx treatment under conditions that better approximate potential human dosing. To evaluate the dantrolene/AON combination treatment effect on dystrophin induction, we assayed three AON doses, with and without oral dantrolene, to assess multiple outcomes across different muscles. Meta-analyses of the results of statistical tests from both the quadriceps and diaphragm assessing contributions of dantrolene beyond AON, across all AON treatment groups, provide strong evidence that dantrolene modestly boosts exon skipping and dystrophin rescue while reducing muscle pathology in mdx mice (p < 0.0087). These findings support a trial of combination dantrolene/AON to increase exon-skipping efficacy and highlight the value of combinatorial approaches and Food and Drug Administration (FDA) drug re-purposing for discovery of unsuspected therapeutic application and rapid translation.

The emerging role of calmodulin regulation of RyR2 in controlling heart rhythm, the progression of heart failure and the antiarrhythmic action of dantrolene

Cardiac output and rhythm depend on the release and the take-up of calcium from the sarcoplasmic reticulum (SR). Excessive diastolic calcium leak from the SR due to dysfunctional calcium release channels (RyR2) contributes to the formation of delayed after-depolarizations, which underlie the fatal arrhythmias that occur in heart failure and inherited syndromes. Calmodulin (CaM) is a calcium-binding protein that regulates target proteins and acts as a calcium sensor. CaM is comprised of two calcium-binding EF-hand domains and a flexible linker. CaM is an accessory protein that partially inhibits RyR2 channel activity. CaM is critical for normal cardiac function, and altered CaM binding and efficacy may contribute to defects in SR calcium release. The present paper reviews CaM binding to RyR2 and how it regulates RyR2 channel activity. It then goes on to review how mutations in the CaM amino acid sequence give rise to inherited syndromes such as Catecholaminergic Polymorphic Ventricular Tachychardia (CPVT) and long QT syndrome (LQTS). In addition, the role of reduced CaM binding to RyR2 that results from RyR2 phosphorylation or from oxidation of either RyR2 or CaM contributes to the progression of heart failure is reviewed. Finally, this manuscript reviews recent evidence that CaM binding to RyR2 is required for the inhibitory action of a pharmaceutical agent (dantrolene) on RyR2. Dantrolene is a clinically used muscle relaxant that has recently been found to exert antiarrhythmic effects against SR Ca²⁺ overload arrhythmias.

Strenuous exercise triggers a life-threatening response in mice susceptible to malignant hyperthermia

In humans, hyperthermic episodes can be triggered by halogenated anesthetics [malignant hyperthermia (MH) susceptibility] and by high temperature [environmental heat stroke (HS)]. Correlation between MH susceptibility and HS is supported by extensive work in mouse models that carry a mutation in ryanodine receptor type-1 (RYR1^Y522S/WT) and calsequestrin-1 knockout (CASQ1-null), 2 proteins that control Ca²⁺ release in skeletal muscle. As overheating episodes in humans have also been described during exertion, here we subjected RYR1^Y522S/WT and CASQ1-null mice to an exertional-stress protocol (incremental running on a treadmill at 34°C and 40% humidity). The mortality rate was 80 and 78.6% in RYR1^Y522S/WT and CASQ1-null mice, respectively, vs. 0% in wild-type mice. Lethal crises were characterized by hyperthermia and rhabdomyolysis, classic features of MH episodes. Of importance, pretreatment with azumolene, an analog of the drug used in humans to treat MH crises, reduced mortality to 0 and 12.5% in RYR1^Y522S/WT and CASQ1-null mice, respectively, thanks to a striking reduction of hyperthermia and rhabdomyolysis. At the molecular level, azumolene strongly prevented Ca²⁺-dependent activation of calpains and NF-κB by lowering myoplasmic Ca²⁺ concentration and nitro-oxidative stress, parameters that were elevated in RYR1^Y522S/WT and CASQ1-null mice. These results suggest that common molecular mechanisms underlie MH crises and exertional HS in mice.-Michelucci, A., Paolini, C., Boncompagni, S., Canato, M., Reggiani, C., Protasi, F. Strenuous exercise triggers a life-threatening response in mice susceptible to malignant hyperthermia.

Dantrolene requires Mg2+ to arrest malignant hyperthermia

Malignant hyperthermia (MH) is a clinical syndrome of skeletal muscle that presents as a hypermetabolic response to volatile anesthetic gases, where susceptible persons may develop lethally high body temperatures. Genetic predisposition mainly arises from mutations on the skeletal muscle ryanodine receptor (RyR). Dantrolene is administered to alleviate MH symptoms, but its mechanism of action and its influence on the Ca²⁺ transients elicited by MH triggers are unknown. Here, we show that Ca²⁺ release in the absence of Mg²⁺ is unaffected by the presence of dantrolene but that dantrolene becomes increasingly effective as cytoplasmic-free [Mg²⁺] (free [Mg²⁺]_cyto) passes mM levels. Furthermore, we found in human muscle susceptible to MH that dantrolene was ineffective at reducing halothane-induced repetitive Ca²⁺ waves in the presence of resting levels of free [Mg²⁺]_cyto (1 mM). However, an increase of free [Mg²⁺]_cyto to 1.5 mM could increase the period between Ca²⁺ waves. These results reconcile previous contradictory reports in muscle fibers and isolated RyRs, where Mg²⁺ is present or absent, respectively, and define the mechanism of action of dantrolene is to increase the Mg²⁺ affinity of the RyR (or "stabilize" the resting state of the channel) and suggest that the accumulation of the metabolite Mg²⁺ from MgATP hydrolysis is required to make dantrolene administration effective in arresting an MH episode.

In vivo Ca2+ dynamics induced by Ca2+ injection in individual rat skeletal muscle fibers

In contrast to cardiomyocytes, store overload‐induced calcium ion (Ca²⁺) release (SOICR) is not considered to constitute a primary Ca²⁺ releasing system from the sarcoplasmic reticulum (SR) in skeletal muscle myocytes. In the latter, voltage‐induced Ca²⁺ release (VICR) is regarded as the dominant mechanism facilitating contractions. Any role of the SOICR in the regulation of cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) and its dynamics in skeletal muscle in vivo remains poorly understood. By means of in vivo single fiber Ca²⁺ microinjections combined with bioimaging techniques, we tested the hypothesis that the [Ca²⁺]_i dynamics following Ca²⁺ injection would be amplified and fiber contraction facilitated by SOICR. The circulation‐intact spinotrapezius muscle of adult male Wistar rats (n = 34) was exteriorized and loaded with Fura‐2 AM to monitor [Ca²⁺]_i dynamics. Groups of rats underwent the following treatments: (1) 0.02, 0.2, and 2.0 mmol/L Ca²⁺ injections, (2) 2.0 mmol/L Ca²⁺ with inhibition of ryanodine receptors (RyR) by dantrolene sodium (DAN), and (3) 2.0 mmol/L Ca²⁺ with inhibition of SR Ca²⁺ ATPase (SERCA) by cyclopiazonic acid (CPA). A quantity of 0.02 mmol/L Ca²⁺ injection yielded no detectable response, whereas peak evoked [Ca²⁺]_i increased 9.9 ± 1.8% above baseline for 0.2 mmol/L and 23.8 ± 4.3% (P < 0.05) for 2.0 mmol/L Ca²⁺ injections. The peak [Ca²⁺]_i in response to 2.0 mmol/L Ca²⁺ injection was largely abolished by DAN and CPA (−85.8%, −71.0%, respectively, both P < 0.05 vs. unblocked) supporting dependence of the [Ca²⁺]_i dynamics on Ca²⁺ released by SOICR rather than injected Ca²⁺ itself. Thus, this investigation demonstrates the presence of a robust SR‐evoked SOICR operant in skeletal muscle in vivo.

Baclofen in the Therapeutic of Sequele of Traumatic Brain Injury: Spasticity

Traumatic brain injury (TBI) is an alteration in brain function, caused by an external force, which may be a hit on the skull, rapid acceleration or deceleration, penetration of an object, or shock waves from an explosion. Traumatic brain injury is a major cause of morbidity and mortality worldwide, with a high prevalence rate in pediatric patients, in which treatment options are still limited, not available at present neuroprotective drugs. Although the therapeutic management of these patients is varied and dependent on the severity of the injury, general techniques of drug types are handled, as well as physical and surgical. Baclofen is a muscle relaxant used to treat spasticity and improve mobility in patients with spinal cord injuries, relieving pain and muscle stiffness. Pharmacological support with baclofen is contradictory, because disruption of its oral administration may cause increased muscle tone syndrome and muscle spasm, prolonged seizures, hyperthermia, dysesthesia, hallucinations, or even multisystem organ failure. Combined treatments must consider the pathophysiology of broader alterations than only excitation/inhibition context, allowing the patient's reintegration with the greatest functionality.

Antiarrhythmic effects of dantrolene in human diseased cardiomyocytes

BACKGROUND

Cardiac type 2 ryanodine receptors (RyR₂s) play a pivotal role in cellular electrophysiology and contractility. Increased RyR₂-mediated diastolic sarcoplasmic reticulum (SR) Ca²⁺ release is linked to heart failure (HF) and arrhythmias. Dantrolene, a drug used for the treatment of malignant hyperthermia, is known to stabilize RyRs in skeletal muscle.

OBJECTIVE

The purpose of this study was to investigate the effects of dantrolene on arrhythmogenic triggers and contractile function in human atrial fibrillation (AF) and HF cardiomyocytes (CM).

METHODS

Human CM were isolated from either patients with HF (ventricular) or patients with AF (atrial), and Ca²⁺ imaging, patch-clamp, or muscle strip experiments were performed.

RESULTS

After exposure to dantrolene, human atrial AF and left ventricular HF CM showed significant reductions in proarrhythmic SR Ca²⁺ spark frequency and diastolic SR Ca²⁺ leak. Moreover, dantrolene decreased the frequency of Ca²⁺ waves and spontaneous Ca²⁺ transients in HF CM. Patch-clamp experiments revealed that dantrolene significantly suppressed delayed afterdepolarizations in HF and AF CM. Importantly, dantrolene had no effect on action potential duration in AF or in HF CM. In addition, dantrolene had neutral effects on contractile force of human isometrically twitching ventricular HF trabeculae.

CONCLUSION

Our study showed that dantrolene beneficially influenced disrupted SR Ca²⁺ homeostasis in human HF and AF CM. Cellular arrhythmogenic triggers were potently suppressed by dantrolene, whereas action potential duration and contractility were not affected. As a clinically approved drug for the treatment of malignant hyperthermia, dantrolene may be a potential antiarrhythmic drug for patients with rhythm disorders and merits further clinical investigation.

Mediation of Autophagic Cell Death by Type 3 Ryanodine Receptor (RyR3) in Adult Hippocampal Neural Stem Cells

Cytoplasmic Ca²⁺ actively engages in diverse intracellular processes from protein synthesis, folding and trafficking to cell survival and death. Dysregulation of intracellular Ca²⁺ levels is observed in various neuropathological states including Alzheimer’s and Parkinson’s diseases. Ryanodine receptors (RyRs) and inositol 1,4,5-triphosphate receptors (IP₃Rs), the main Ca²⁺ release channels located in endoplasmic reticulum (ER) membranes, are known to direct various cellular events such as autophagy and apoptosis. Here we investigated the intracellular Ca²⁺-mediated regulation of survival and death of adult hippocampal neural stem (HCN) cells utilizing an insulin withdrawal model of autophagic cell death (ACD). Despite comparable expression levels of RyR and IP₃R transcripts in HCN cells at normal state, the expression levels of RyRs—especially RyR3—were markedly upregulated upon insulin withdrawal. While treatment with the RyR agonist caffeine significantly promoted the autophagic death of insulin-deficient HCN cells, treatment with its inhibitor dantrolene prevented the induction of autophagy following insulin withdrawal. Furthermore, CRISPR/Cas9-mediated knockout of the RyR3 gene abolished ACD of HCN cells. This study delineates a distinct, RyR3-mediated ER Ca²⁺ regulation of autophagy and programmed cell death in neural stem cells. Our findings provide novel insights into the critical, yet understudied mechanisms underlying the regulatory function of ER Ca²⁺ in neural stem cell biology.

Structural and functional interactions within ryanodine receptor

The ryanodine receptor/Ca2+ release channel plays a pivotal role in skeletal and cardiac muscle excitation-contraction coupling. Defective regulation leads to neuromuscular disorders and arrhythmogenic cardiac disease. This mini-review focuses on channel regulation through structural intra- and inter-subunit interactions and their implications in ryanodine receptor pathophysiology.

Critical Role of Intracellular RyR1 Calcium Release Channels in Skeletal Muscle Function and Disease

The skeletal muscle Ca²⁺ release channel, also known as ryanodine receptor type 1 (RyR1), is the largest ion channel protein known and is crucial for effective skeletal muscle contractile activation. RyR1 function is controlled by Ca_v1.1, a voltage gated Ca²⁺ channel that works mainly as a voltage sensor for RyR1 activity during skeletal muscle contraction and is also fine-tuned by Ca²⁺, several intracellular compounds (e.g., ATP), and modulatory proteins (e.g., calmodulin). Dominant and recessive mutations in RyR1, as well as acquired channel alterations, are the underlying cause of various skeletal muscle diseases. The aim of this mini review is to summarize several current aspects of RyR1 function, structure, regulation, and to describe the most common diseases caused by hereditary or acquired RyR1 malfunction.

FRET-based trilateration of probes bound within functional ryanodine receptors

To locate the biosensor peptide DPc10 bound to ryanodine receptor (RyR) Ca(2+) channels, we developed an approach that combines fluorescence resonance energy transfer (FRET), simulated-annealing, cryo-electron microscopy, and crystallographic data. DPc10 is identical to the 2460-2495 segment within the cardiac muscle RyR isoform (RyR2) central domain. DPc10 binding to RyR2 results in a pathologically elevated Ca(2+) leak by destabilizing key interactions between the RyR2 N-terminal and central domains (unzipping). To localize the DPc10 binding site within RyR2, we measured FRET between five single-cysteine variants of the FK506-binding protein (FKBP) labeled with a donor probe, and DPc10 labeled with an acceptor probe (A-DPc10). Effective donor positions were calculated from simulated-annealing constrained by both the RyR cryo-EM map and the FKBP atomic structure docked to the RyR. FRET to A-DPc10 was measured in permeabilized cardiomyocytes via confocal microscopy, converted to distances, and used to trilaterate the acceptor locus within RyR. Additional FRET measurements between donor-labeled calmodulin and A-DPc10 were used to constrain the trilaterations. Results locate the DPc10 probe within RyR domain 3, ?35 Å from the previously docked N-terminal domain crystal structure. This multiscale approach may be useful in mapping other RyR sites of mechanistic interest within FRET range of FKBP.

Antiarrhythmic Effects of Dantrolene in Patients with Catecholaminergic Polymorphic Ventricular Tachycardia and Replication of the Responses Using iPSC Models

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a highly malignant inherited arrhythmogenic disorder. Type 1 CPVT (CPVT1) is caused by cardiac ryanodine receptor (RyR2) gene mutations resulting in abnormal calcium release from sarcoplasmic reticulum. Dantrolene, an inhibitor of sarcoplasmic Ca²⁺ release, has been shown to rescue this abnormal Ca²⁺ release in vitro. We assessed the antiarrhythmic efficacy of dantrolene in six patients carrying various RyR2 mutations causing CPVT. The patients underwent exercise stress test before and after dantrolene infusion. Dantrolene reduced the number of premature ventricular complexes (PVCs) on average by 74% (range 33-97) in four patients with N-terminal or central mutations in the cytosolic region of the RyR2 protein, while dantrolene had no effect in two patients with mutations in or near the transmembrane domain. Induced pluripotent stem cells (iPSCs) were generated from all the patients and differentiated into spontaneously beating cardiomyocytes (CMs). The antiarrhythmic effect of dantrolene was studied in CMs after adrenaline stimulation by Ca²⁺ imaging. In iPSC derived CMs with RyR2 mutations in the N-terminal or central region, dantrolene suppressed the Ca²⁺ cycling abnormalities in 80% (range 65-97) of cells while with mutations in or near the transmembrane domain only in 23 or 32% of cells. In conclusion, we demonstrate that dantrolene given intravenously shows antiarrhythmic effects in a portion of CPVT1 patients and that iPSC derived CM models replicate these individual drug responses. These findings illustrate the potential of iPSC models to individualize drug therapy of inherited diseases.

Essential Role of Calmodulin in RyR Inhibition by Dantrolene

Dantrolene is the first line therapy of malignant hyperthermia. Animal studies suggest that dantrolene also protects against heart failure and arrhythmias caused by spontaneous Ca(2+) release. Although dantrolene inhibits Ca(2+) release from the sarcoplasmic reticulum of skeletal and cardiac muscle preparations, its mechanism of action has remained controversial, because dantrolene does not inhibit single ryanodine receptor (RyR) Ca(2+) release channels in lipid bilayers. Here we test the hypothesis that calmodulin (CaM), a physiologic RyR binding partner that is lost during incorporation into lipid bilayers, is required for dantrolene inhibition of RyR channels. In single channel recordings (100 nM cytoplasmic [Ca(2+)] + 2 mM ATP), dantrolene caused inhibition of RyR1 (rabbit skeletal muscle) and RyR2 (sheep) with a maximal inhibition of Po (Emax) to 52 ± 4% of control only after adding physiologic [CaM] = 100 nM. Dantrolene inhibited RyR2 with an IC50 of 0.16 ± 0.03 µM. Mutant N98S-CaM facilitated dantrolene inhibition with an IC50 = 5.9 ± 0.3 nM. In mouse cardiomyocytes, dantrolene had no effect on cardiac Ca(2+) release in the absence of CaM, but reduced Ca(2+) wave frequency (IC50 = 0.42 ± 0.18 µM, Emax = 47 ± 4%) and amplitude (IC50 = 0.19 ± 0.04 µM, Emax = 66 ± 4%) in the presence of 100 nM CaM. We conclude that CaM is essential for dantrolene inhibition of RyR1 and RyR2. Its absence explains why dantrolene inhibition of single RyR channels has not been previously observed.

Effects of shakuyakukanzoto and its absorbed components on twitch contractions induced by physiological Ca2+ release in rat skeletal muscle

Shakuyakukanzoto (SKT) is a kampo medicine composed of equal proportions of Glycyrrhizae radix (G. radix) and Paeoniae radix (P. radix). A double-blind study reported that SKT significantly ameliorated painful muscle cramp in cirrhosis patients without the typical severe side effects of muscle weakness and central nervous system (CNS) depression. Previous basic studies reported that SKT and its active components induced relaxation by a direct action on skeletal muscle and that SKT did not depress CNS functions; however, why SKT has a lower incidence of muscle weakness remains unknown. In the present study, we investigated which components are absorbed into the blood of rats after a single oral administration of SKT to identify the active components of SKT. We also investigated the effects of SKT and its components on the twitch contraction induced by physiological Ca²⁺ release. Our study demonstrated that SKT and five G. radix isolates, which are responsible for the antispasmodic effect of SKT, did not inhibit the twitch contraction in contrast to dantrolene sodium, a direct-acting peripheral muscle relaxant, indicating that the mechanisms of muscle contraction of SKT and dantrolene in skeletal muscle differ. These findings suggest that SKT does not reduce the contractile force in skeletal muscle under physiological conditions, i.e., SKT may have a low risk of causing muscle weakness in clinical use. Considering that most muscle relaxants and anticonvulsants cause various harmful side effects such as weakness and CNS depression, SKT appears to have a benign safety profile.

Dantrolene rescues aberrant N-terminus intersubunit interactions in mutant pro-arrhythmic cardiac ryanodine receptors

AIMS

The ryanodine receptor (RyR2) is an intracellular Ca(2+) release channel essential for cardiac excitation-contraction coupling. Abnormal RyR2 channel function results in the generation of arrhythmias and sudden cardiac death. The present study was undertaken to investigate the mechanistic basis of RyR2 dysfunction in inherited arrhythmogenic cardiac disease.

METHODS AND RESULTS

We present several lines of complementary evidence, indicating that the arrhythmia-associated L433P mutation disrupts RyR2 N-terminus self-association. A combination of yeast two-hybrid, co-immunoprecipitation, and chemical cross-linking assays collectively demonstrate that a RyR2 N-terminal fragment carrying the L433P mutation displays substantially reduced self-interaction compared with wild type. Moreover, sucrose density gradient centrifugation reveals that the L433P mutation impairs tetramerization of the full-length channel. [(3)H]Ryanodine-binding assays demonstrate that disrupted N-terminal intersubunit interactions within RyR2(L433P) confer an altered sensitivity to Ca(2+) activation. Calcium imaging of RyR2(L433P)-expressing cells reveals substantially prolonged Ca(2+) transients and reduced Ca(2+) store content indicating defective channel closure. Importantly, dantrolene treatment reverses the L433P mutation-induced impairment and restores channel function.

CONCLUSION

The N-terminus domain constitutes an important structural determinant for the functional oligomerization of RyR2. Our findings are consistent with defective N-terminus self-association as a molecular mechanism underlying RyR2 channel deregulation in inherited arrhythmogenic cardiac disease. Significantly, the therapeutic action of dantrolene may occur via the restoration of normal RyR2 N-terminal intersubunit interactions.

Hypothermia and hyperthermia in the ambulatory surgical patient

Homeotherms, including humans, are able to maintain a relatively constant temperature despite variations in their thermal environment. We normally maintain a narrow thermoregulatory threshold range of approximately 0.2°C, and little change in core temperature is required to trigger compensatory mechanisms to either cool or warm our core temperature back to normothermia. This article focuses on the mechanisms and consequences of hypothermia and hyperthermia in the surgical patient and reviews techniques to prevent and treat these conditions.

Accelerated activation of SOCE current in myotubes from two mouse models of anesthetic- and heat-induced sudden death

Store-operated calcium entry (SOCE) channels play an important role in Ca²⁺ signaling. Recently, excessive SOCE was proposed to play a central role in the pathogenesis of malignant hyperthermia (MH), a pharmacogenic disorder of skeletal muscle. We tested this hypothesis by characterizing SOCE current (I_SkCRAC) magnitude, voltage dependence, and rate of activation in myotubes derived from two mouse models of anesthetic- and heat-induced sudden death: 1) type 1 ryanodine receptor (RyR1) knock-in mice (Y524S/+) and 2) calsequestrin 1 and 2 double knock-out (dCasq-null) mice. I_SkCRAC voltage dependence and magnitude at -80 mV were not significantly different in myotubes derived from wild type (WT), Y524S/+ and dCasq-null mice. However, the rate of I_SkCRAC activation upon repetitive depolarization was significantly faster at room temperature in myotubes from Y524S/+ and dCasq-null mice. In addition, the maximum rate of I_SkCRAC activation in dCasq-null myotubes was also faster than WT at more physiological temperatures (35-37°C). Azumolene (50 µM), a more water-soluble analog of dantrolene that is used to reverse MH crises, failed to alter I_SkCRAC density or rate of activation. Together, these results indicate that while an increased rate of I_SkCRAC activation is a common characteristic of myotubes derived from Y524S/+ and dCasq-null mice and that the protective effects of azumolene are not due to a direct inhibition of SOCE channels.

Dantrolene enhances antisense-mediated exon skipping in human and mouse models of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) causes profound and progressive muscle weakness and loss, resulting in early death. DMD is usually caused by frameshifting deletions in the gene DMD, which leads to absence of dystrophin protein. Dystrophin binds to F-actin and components of the dystrophin-associated glycoprotein complex and protects the sarcolemma from contraction-induced injury. Antisense oligonucleotide-mediated exon skipping is a promising therapeutic approach aimed at restoring the DMD reading frame and allowing expression of an intact dystrophin glycoprotein complex. To date, low levels of dystrophin protein have been produced in humans by this method. We performed a small-molecule screen to identify existing drugs that enhance antisense-directed exon skipping. We found that dantrolene, currently used to treat malignant hyperthermia, potentiates antisense oligomer-guided exon skipping to increase exon skipping to restore the mRNA reading frame, the sarcolemmal dystrophin protein, and the dystrophin glycoprotein complex in skeletal muscles of mdx mice when delivered intramuscularly or intravenously. Further, dantrolene synergized with multiple weekly injections of antisense to increase muscle strength and reduce serum creatine kinase in mdx mice. Dantrolene similarly promoted antisense-mediated exon skipping in reprogrammed myotubes from DMD patients. Ryanodine and Rycal S107, which, like dantrolene, targets the ryanodine receptor, also promoted antisense-driven exon skipping, implicating the ryanodine receptor as the critical molecular target.

The calcium signaling pathway regulates leydig cell steroidogenesis through a transcriptional cascade involving the nuclear receptor NR4A1 and the steroidogenic acute regulatory protein

In the gonads and adrenal glands, the transient increase in steroidogenesis after hormonal stimulation requires modulation of steroidogenic acute regulatory protein (Star) expression and activity in a tightly regulated process involving cAMP and Ca(2+). In Leydig cells, the cAMP and Ca(2+) pathways account for most if not all of LH-induced steroidogenesis. Although the cAMP-activated molecular network has been well characterized in Leydig cells, little is known about the molecular cascade triggered by the Ca(2+) signaling pathway and the transcription factors responsible for mediating the genomic response. It is established that LH induces an increase in cytoplasmic Ca(2+) from the endoplasmic reticulum primarily through the ryanodine receptors. Previous reports also suggested a role of the Ca(2+) signaling pathway in Star expression based on the fact that inhibition of the Ca(2+)/calmodulin (CaM) protein kinase pathway greatly impaired Star expression in Leydig and adrenal cells. In this study, we used ryanodine receptors and CaM antagonists to show that the increase in intracellular Ca(2+) level is an essential modulator of progesterone synthesis through the regulation of Star gene expression in MA-10 Leydig cells. Furthermore, we mapped a Ca(2+)/CaM-sensitive element in the Star promoter, which led to the identification of the nuclear receptor 4A1 (NR4A1) as a key mediator of the Ca(2+)/CaM signaling pathway in these cells. These data provide new insights into the Ca(2+) molecular pathway essential for steroidogenesis in Leydig cells.