Comparison of the structure-function of five newly members of the calcin family

Calcins are a group of scorpion toxin peptides specifically binding to ryanodine receptors (RyRs) with high affinity, and have the ability to activate and stabilize RyR in a long-lasting subconductance state. Five newly calcins synthesized compounds exhibit typical structural characteristics of a specific family through chemical synthesis and virtual analysis. As the calcins from the same species, Petersiicalcin1 and Petersiicalcin2, Jendekicalcin2 and Jendekicalcin3, have only one residue difference. Both Petersiicalcin1 and Petersiicalcin2 exhibited different affinities in stimulating [3H]ryanodine binding, but the residue mutation resulted in a 2.7 folds difference. Other calcins also exhibited a stimulatory effect on [3H]ryanodine binding to RyR1, however, their affinities were significantly lower than that of Petersiiicalcin1 and Petersiiicalcin2. The channel domain of RyR1 was found to be capable of binding with the basic residues of these calcins, which also exhibited interactions with the S6 helices on RyR1. Dynamic simulations were conducted for Petersiicalcin1 and Petersiicalcin2, which demonstrated their ability to form a highly stable conformation and resulting in an asymmetric tetramer structure of RyR1. The discovery of five newly calcins further enriches the diversity of the natural calcin family, which provides more native peptides for the structure-function analysis between calcin and RyRs.

Propofol directly binds and inhibits skeletal muscle ryanodine receptor 1 (RyR1)

As the primary Ca 2+ release channel in skeletal muscle sarcoplasmic reticulum (SR), mutations in the type 1 ryanodine receptor (RyR1) or its binding partners underlie a constellation of muscle disorders, including malignant hyperthermia (MH). In patients with MH mutations, exposure to triggering drugs such as the halogenated volatile anesthetics biases RyR1 to an open state, resulting in uncontrolled Ca 2+ release, sarcomere tension and heat production. Restoration of Ca 2+ into the SR also consumes ATP, generating a further untenable metabolic load. When anesthetizing patients with known MH mutations, the non-triggering intravenous general anesthetic propofol is commonly substituted for triggering anesthetics. Evidence of direct binding of anesthetic agents to RyR1 or its binding partners is scant, and the atomic-level interactions of propofol with RyR1 are entirely unknown. Here, we show that propofol decreases RyR1 opening in heavy SR vesicles and planar lipid bilayers, and that it inhibits activator-induced Ca 2+ release from SR in human skeletal muscle. In addition to confirming direct binding, photoaffinity labeling using m- azipropofol (AziP m ) revealed several putative propofol binding sites on RyR1. Prediction of binding affinity by molecular dynamics simulation suggests that propofol binds at least one of these sites at clinical concentrations. These findings invite the hypothesis that in addition to propofol not triggering MH, it may also be protective against MH by inhibiting induced Ca 2+ flux through RyR1.

Trans-scale thermal signaling in biological systems

Biochemical reactions in cells serve as the endogenous source of heat, maintaining a constant body temperature. This process requires proper control; otherwise, serious consequences can arise due to the unwanted but unavoidable responses of biological systems to heat. This review aims to present a range of responses to heat in biological systems across various spatial scales. We begin by examining the impaired thermogenesis of malignant hyperthermia in model mice and skeletal muscle cells, demonstrating that the progression of this disease is caused by a positive feedback loop between thermally driven Ca2+ signaling and thermogenesis at the subcellular scale. After we explore thermally driven force generation in both muscle and non-muscle cells, we illustrate how in vitro assays using purified proteins can reveal the heat-responsive properties of proteins and protein assemblies. Building on these experimental findings, we propose the concept of 'trans-scale thermal signaling'.

Allosteric modulation of ryanodine receptor RyR1 by nucleotide derivatives

The coordinated release of Ca2+ from the sarcoplasmic reticulum (SR) is critical for excitation-contraction coupling. This release is facilitated by ryanodine receptors (RyRs) that are embedded in the SR membrane. In skeletal muscle, activity of RyR1 is regulated by metabolites such as ATP, which upon binding increase channel open probability (Po). To obtain structural insights into the mechanism of RyR1 priming by ATP, we determined several cryo-EM structures of RyR1 bound individually to ATP-γ-S, ADP, AMP, adenosine, adenine, and cAMP. We demonstrate that adenine and adenosine bind RyR1, but AMP is the smallest ATP derivative capable of inducing long-range (>170 Å) structural rearrangements associated with channel activation, establishing a structural basis for key binding site interactions that are the threshold for triggering quaternary structural changes. Our finding that cAMP also induces these structural changes and results in increased channel opening suggests its potential role as an endogenous modulator of RyR1 conductance.

Cryo-EM analysis of scorpion toxin binding to Ryanodine Receptors reveals subconductance that is abolished by PKA phosphorylation

Calcins are peptides from scorpion venom with the unique ability to cross cell membranes, gaining access to intracellular targets. Ryanodine Receptors (RyR) are intracellular ion channels that control release of Ca2+ from the endoplasmic and sarcoplasmic reticulum. Calcins target RyRs and induce long-lived subconductance states, whereby single-channel currents are decreased. We used cryo-electron microscopy to reveal the binding and structural effects of imperacalcin, showing that it opens the channel pore and causes large asymmetry throughout the cytosolic assembly of the tetrameric RyR. This also creates multiple extended ion conduction pathways beyond the transmembrane region, resulting in subconductance. Phosphorylation of imperacalcin by protein kinase A prevents its binding to RyR through direct steric hindrance, showing how posttranslational modifications made by the host organism can determine the fate of a natural toxin. The structure provides a direct template for developing calcin analogs that result in full channel block, with potential to treat RyR-related disorders.

CaV1.1 Calcium Channel Signaling Complexes in Excitation-Contraction Coupling: Insights from Channelopathies

In skeletal muscle, excitation-contraction (EC) coupling relies on the mechanical coupling between two ion channels: the L-type voltage-gated calcium channel (CaV1.1), located in the sarcolemma and functioning as the voltage sensor of EC coupling, and the ryanodine receptor 1 (RyR1), located on the sarcoplasmic reticulum serving as the calcium release channel. To this day, the molecular mechanism by which these two ion channels are linked remains elusive. However, recently, skeletal muscle EC coupling could be reconstituted in heterologous cells, revealing that only four proteins are essential for this process: CaV1.1, RyR1, and the cytosolic proteins CaVβ1a and STAC3. Due to the crucial role of these proteins in skeletal muscle EC coupling, any mutation that affects any one of these proteins can have devastating consequences, resulting in congenital myopathies and other pathologies.Here, we summarize the current knowledge concerning these four essential proteins and discuss the pathophysiology of the CaV1.1, RyR1, and STAC3-related skeletal muscle diseases with an emphasis on the molecular mechanisms. Being part of the same signalosome, mutations in different proteins often result in congenital myopathies with similar symptoms or even in the same disease.

Mice with R2509C-RYR1 mutation exhibit dysfunctional Ca2+ dynamics in primary skeletal myocytes

Type 1 ryanodine receptor (RYR1) is a Ca2+ release channel in the sarcoplasmic reticulum (SR) of the skeletal muscle and plays a critical role in excitation-contraction coupling. Mutations in RYR1 cause severe muscle diseases, such as malignant hyperthermia, a disorder of Ca2+-induced Ca2+ release (CICR) through RYR1 from the SR. We recently reported that volatile anesthetics induce malignant hyperthermia (MH)-like episodes through enhanced CICR in heterozygous R2509C-RYR1 mice. However, the characterization of Ca2+ dynamics has yet to be investigated in skeletal muscle cells from homozygous mice because these animals die in utero. In the present study, we generated primary cultured skeletal myocytes from R2509C-RYR1 mice. No differences in cellular morphology were detected between wild type (WT) and mutant myocytes. Spontaneous Ca2+ transients and cellular contractions occurred in WT and heterozygous myocytes, but not in homozygous myocytes. Electron microscopic observation revealed that the sarcomere length was shortened to ∼1.7 µm in homozygous myocytes, as compared to ∼2.2 and ∼2.3 µm in WT and heterozygous myocytes, respectively. Consistently, the resting intracellular Ca2+ concentration was higher in homozygous myocytes than in WT or heterozygous myocytes, which may be coupled with a reduced Ca2+ concentration in the SR. Finally, using infrared laser-based microheating, we found that heterozygous myocytes showed larger heat-induced Ca2+ transients than WT myocytes. Our findings suggest that the R2509C mutation in RYR1 causes dysfunctional Ca2+ dynamics in a mutant-gene dose-dependent manner in the skeletal muscles, in turn provoking MH-like episodes and embryonic lethality in heterozygous and homozygous mice, respectively.

It takes two to tango: Rycals and ATP snuggle up to bind ryanodine receptors

In this issue of Structure, Melville and colleagues used cryo-EM to study the binding of ryanodine receptors to Rycals, compounds with the potential to treat skeletal and cardiac muscle disorders. Unexpectedly, they found that Rycal packs against an ATP in a peripheral pocket, which stabilizes the closed channel state.

[Therapeutic effects of novel type1 ryanodine receptor inhibitor on skeletal muscle diseases]

Type 1 ryanodine receptor (RyR1) plays a key role in Ca²⁺ release from the sarcoplasmic reticulum (SR) during excitation-contraction coupling of skeletal muscle. Mutations in RyR1 hyperactivate the channel to cause malignant hyperthermia (MH). MH is a serious complication characterized by skeletal muscle rigidity and elevated body temperature in response to commonly used inhalational anesthetics. Thus far, more than 300 mutations in RyR1 gene have been reported in patients with MH. Some heat stroke triggered by exercise or environmental heat stress is also related to MH mutations in the RyR1 gene. The only drug approved for ameliorating the symptoms of MH is dantrolene, which has been first developed in 1960s as a muscle relaxant. However, dantrolene has several disadvantages for clinical use: poor water solubility which makes rapid preparation difficult in emergency situations and long plasma half-life, which causes long-lasting side effects such as muscle weakness. Here we show that a novel RyR1-selective inhibitor, 6,7-(methylenedioxy)-1-octyl-4-quinolone-3-carboxylic acid (Compound 1, Cpd1), effectively rescues MH and heat stroke in new mouse model relevant to MH. Cpd1 has great advantages of higher water solubility and shorter plasma half-life compared to dantrolene. Our data suggest that Cpd1 has the potential to be a promising new candidate for effective treatment of patients carrying RyR1 mutations.

Using hiPSC-CMs to Examine Mechanisms of Catecholaminergic Polymorphic Ventricular Tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a potentially lethal inherited cardiac arrhythmia condition, triggered by physical or acute emotional stress, that predominantly expresses early in life. Gain-of-function mutations in the cardiac ryanodine receptor gene (RYR2) account for the majority of CPVT cases, causing substantial disruption of intracellular calcium (Ca²⁺ ) homeostasis particularly during the periods of β-adrenergic receptor stimulation. However, the highly variable penetrance, patient outcomes, and drug responses observed in clinical practice remain unexplained, even for patients with well-established founder RyR2 mutations. Therefore, investigation of the electrophysiological consequences of CPVT-causing RyR2 mutations is crucial to better understand the pathophysiology of the disease. The development of strategies for reprogramming human somatic cells to human induced pluripotent stem cells (hiPSCs) has provided a unique opportunity to study inherited arrhythmias, due to the ability of hiPSCs to differentiate down a cardiac lineage. Employment of genome editing enables generation of disease-specific cell lines from healthy and diseased patient-derived hiPSCs, which subsequently can be differentiated into cardiomyocytes. This paper describes the means for establishing an hiPSC-based model of CPVT in order to recapitulate the disease phenotype in vitro and investigate underlying pathophysiological mechanisms. The framework of this approach has the potential to contribute to disease modeling and personalized medicine using hiPSC-derived cardiomyocytes. © 2021 Wiley Periodicals LLC.

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.

Structure and function of STAC proteins: Calcium channel modulators and critical components of muscle excitation-contraction coupling

In skeletal muscle tissue, an intriguing mechanical coupling exists between two ion channels from different membranes: the L-type voltage-gated calcium channel (Ca_V1.1), located in the plasma membrane, and ryanodine receptor 1 (RyR1) located in the sarcoplasmic reticulum membrane. Excitable cells rely on Ca_vs to initiate Ca²⁺ entry in response to action potentials. RyRs can amplify this signal by releasing Ca²⁺ from internal stores. Although this process can be mediated through Ca²⁺ as a messenger, an overwhelming amount of evidence suggests that RyR1 has recruited Ca_V1.1 directly as its voltage sensor. The exact mechanisms that underlie this coupling have been enigmatic, but a recent wave of reports have illuminated the coupling protein STAC3 as a critical player. Without STAC3, the mechanical coupling between Ca_v1.1 and RyR1 is lost, and muscles fail to contract. Various sequence variants of this protein have been linked to congenital myopathy. Other STAC isoforms are expressed in the brain and may serve as regulators of L-type Ca_Vs. Despite the short length of STACs, several points of contacts have been proposed between them and Ca_Vs. However, it is currently unclear whether STAC3 also forms direct interactions with RyR1, and whether this modulates RyR1 function. In this review, we discuss the 3D architecture of STAC proteins, the biochemical evidence for their interactions, the relevance of these connections for functional modulation, and their involvement in myopathy.

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.

Insights into channel modulation mechanism of RYR1 mutants using Ca2+ imaging and molecular dynamics

Molecular bases of pathogenic enhancement of Ca²⁺ release channel activities in RYR1 carrying disease-associated mutations at the N-terminal region were studied. Functional studies and MD simulation revealed that the interactions between domains have a strong correlation with channel activity.

Type 1 ryanodine receptor (RYR1) is a Ca²⁺ release channel in the sarcoplasmic reticulum in skeletal muscle and plays an important role in excitation–contraction coupling. Mutations in the RYR1 gene cause severe muscle diseases such as malignant hyperthermia (MH), which is a disorder of CICR via RYR1. Thus far, >300 mutations in RYR1 have been reported in patients with MH. However, owing to a lack of comprehensive analysis of the structure–function relationship of mutant RYR1, the mechanism remains largely unknown. Here, we combined functional studies and molecular dynamics (MD) simulations of RYR1 bearing disease-associated mutations at the N-terminal region. When expressed in HEK293 cells, the mutant RYR1 caused abnormalities in Ca²⁺ homeostasis. MD simulations of WT and mutant RYR1s were performed using crystal structure of the N-terminal domain (NTD) monomer, consisting of A, B, and C domains. We found that the mutations located around the interdomain region differentially affected hydrogen bonds/salt bridges. Particularly, mutations at R402, which increase the open probability of the channel, cause clockwise rotation of BC domains with respect to the A domain by alteration of the interdomain interactions. Similar results were also obtained with artificial mutations that mimic alteration of the interactions. Our results reveal the importance of interdomain interactions within the NTD in the regulation of the RYR1 channel and provide insights into the mechanism of MH caused by the mutations at the NTD.

A multi-dimensional analysis of genotype-phenotype discordance in malignant hyperthermia susceptibility

BACKGROUND

Malignant hyperthermia (MH) susceptibility is an inherited condition, diagnosed either by the presence of a pathogenic genetic variant or by in vitro caffeine-halothane contracture testing. Through a multi-dimensional approach, we describe the implications of discordance between genetic and in vitro test results in a patient with a family history of possible MH.

METHODS

The patient, whose brother had a possible MH reaction, underwent the caffeine-halothane contracture test (CHCT) according to the North American MH Group protocol. Screening of the complete RYR1 and CACNA1S transcripts was done using Sanger sequencing. Additional functional analyses included skinned myofibre calcium-induced calcium release sensitivity, calcium signalling assays in cultured myotubes, and in silico evaluation of the effect of any genetic variants on their chemical environment.

RESULTS

The patient's CHCT result was negative but she carried an RYR1 variant c.1209C>G, p.Ile403Met, that is listed as pathogenic by the European Malignant Hyperthermia Group. Functional tests indicated a gain-of-function effect with a weak impact, and the variant was predicted to affect the folding stability of the 3D structure of the RyR1 protein. Based on American College of Medical Genetics and Genomics/Association of Molecular Pathology guidelines, this variant would be characterised as a variant of uncertain significance.

CONCLUSIONS

Available data do not confirm or exclude an increased risk of MH for this patient. Further research is needed to correlate RyR1 functional assays, including the current gold standard testing for MH susceptibility, with clinical phenotypes. The pathogenicity of genetic variants associated with MH susceptibility should be re-evaluated.

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.

Biophysical Investigation of Sodium Channel Interaction with β-Subunit Variants Associated with Arrhythmias

Background: Voltage-gated sodium (Na_V) channels help regulate electrical activity of the plasma membrane. Mutations in associated subunits can result in pathological outcomes. Here we examined the interaction of Na_V channels with cardiac arrhythmia-linked mutations in SCN2B and SCN4B, two genes that encode auxiliary β-subunits. Materials and Methods: To investigate changes in SCN2B ^R137H and SCN4B ^I80T function, we combined three-dimensional X-ray crystallography with electrophysiological measurements on Na_V1.5, the dominant subtype in the heart. Results: SCN4B ^I80T alters channel activity, whereas SCN2B ^R137H does not have an apparent effect. Structurally, the SCN4B ^I80T perturbation alters hydrophobic packing of the subunit with major structural changes and causes a thermal destabilization of the folding. In contrast, SCN2B ^R137H leads to structural changes but overall protein stability is unaffected. Conclusion: SCN4B ^I80T data suggest a functionally important region in the interaction between Na_V1.5 and β4 that, when disrupted, could lead to channel dysfunction. A lack of apparent functional effects of SCN2B ^R137H on Na_V1.5 suggests an alternative working mechanism, possibly through other Na_V channel subtypes present in heart tissue. Indeed, mapping the structural variations of SCN2B ^R137H onto neuronal Na_V channel structures suggests altered interaction patterns.

Trimethylamine-N-oxide acutely increases cardiac muscle contractility

Cardiovascular disease is a major cause of morbidity and mortality among patients with chronic kidney disease (CKD). Trimethylamine-N-oxide (TMAO), a uremic metabolite that is elevated in the setting of CKD, has been implicated as a nontraditional risk factor for cardiovascular disease. While association studies have linked elevated plasma levels of TMAO to adverse cardiovascular outcomes, its direct effect on cardiac and smooth muscle function remains to be fully elucidated. We hypothesized that pathological concentrations of TMAO would acutely increase cardiac and smooth muscle contractility. These effects may ultimately contribute to cardiac dysfunction during CKD. High levels of TMAO significantly increased paced, ex vivo human cardiac muscle biopsy contractility (P < 0.05). Similarly, TMAO augmented contractility in isolated mouse hearts (P < 0.05). Reverse perfusion of TMAO through the coronary arteries via a Langendorff apparatus also enhanced cardiac contractility (P < 0.05). In contrast, the precursor molecule, trimethylamine (TMA), did not alter contractility (P > 0.05). Multiphoton microscopy, used to capture changes in intracellular calcium in paced, adult mouse hearts ex vivo, showed that TMAO significantly increased intracellular calcium fluorescence (P < 0.05). Interestingly, acute administration of TMAO did not have a statistically significant influence on isolated aortic ring contractility (P > 0.05). We conclude that TMAO directly increases the force of cardiac contractility, which corresponds with TMAO-induced increases in intracellular calcium but does not acutely affect vascular smooth muscle or endothelial function of the aorta. It remains to be determined if this acute inotropic action on cardiac muscle is ultimately beneficial or harmful in the setting of CKD.NEW & NOTEWORTHY We demonstrate for the first time that elevated concentrations of TMAO acutely augment myocardial contractile force ex vivo in both murine and human cardiac tissue. To gain mechanistic insight into the processes that led to this potentiation in cardiac contraction, we used two-photon microscopy to evaluate intracellular calcium in ex vivo whole hearts loaded with the calcium indicator dye Fluo-4. Acute treatment with TMAO resulted in increased Fluo-4 fluorescence, indicating that augmented cytosolic calcium plays a role in the effects of TMAO on force production. Lastly, TMAO did not show an effect on aortic smooth muscle contraction or relaxation properties. Our results demonstrate novel, acute, and direct actions of TMAO on cardiac function and help lay the groundwork for future translational studies investigating the complex multiorgan interplay involved in cardiovascular pathogenesis during CKD.

Cardiac arrest in a mother and daughter and the identification of a novel RYR2 variant, predisposing to low penetrant catecholaminergic polymorphic ventricular tachycardia in a four‐generation Canadian family

Background

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a rare inherited arrhythmia syndrome characterized by adrenergically driven ventricular arrhythmia predominantly caused by pathogenic variants in the cardiac ryanodine receptor (RyR2). We describe a novel variant associated with cardiac arrest in a mother and daughter.

Methods

Initial sequencing of the RYR2 gene identified a novel variant (c.527G > T, p.R176L) in the index case (the mother), and her daughter. Structural analysis demonstrated the variant was located within the N‐terminal domain of RyR2, likely leading to a gain‐of‐function effect facilitating enhanced calcium ion release. Four generation cascade genetic and clinical screening was carried out.

Results

Thirty‐eight p.R176L variant carriers were identified of 94 family members with genetic testing, and 108 family members had clinical evaluations. Twelve carriers were symptomatic with previous syncope and 2 additional survivors of cardiac arrest were identified. Thirty‐two had clinical features suggestive of CPVT. Of 52 noncarriers, 11 had experienced previous syncope with none exhibiting any clinical features of CPVT. A documented arrhythmic event rate of 2.89/1000 person‐years across all carriers was calculated.

Conclusion

The substantial variability in phenotype and the lower than previously reported penetrance is illustrative of the importance of exploring family variants beyond first‐degree relatives.

Arrhythmia mutations in calmodulin can disrupt cooperativity of Ca2+ binding and cause misfolding

KEY POINTS

Mutations in the calmodulin protein (CaM) are associated with arrhythmia syndromes. This study focuses on understanding the structural characteristics of CaM disease mutants and their interactions with the voltage-gated calcium channel Ca_V 1.2. Arrhythmia mutations in CaM can lead to loss of Ca²⁺ binding, uncoupling of Ca²⁺ binding cooperativity, misfolding of the EF-hands and altered affinity for the calcium channel. These results help us to understand how different CaM mutants have distinct effects on structure and interactions with protein targets to cause disease.

ABSTRACT

Calmodulinopathies are life-threatening arrhythmia syndromes that arise from mutations in calmodulin (CaM), a calcium sensing protein whose sequence is completely conserved across all vertebrates. These mutations have been shown to interfere with the function of cardiac ion channels, including the voltage-gated Ca²⁺ channel Ca_V 1.2 and the ryanodine receptor (RyR2), in a mutation-specific manner. The ability of different CaM disease mutations to discriminate between these channels has been enigmatic. We present crystal structures of several C-terminal lobe mutants and an N-terminal lobe mutant in complex with the Ca_V 1.2 IQ domain, in conjunction with binding assays and complementary structural biology techniques. One mutation (D130G) causes a pathological conformation, with complete separation of EF-hands within the C-lobe and loss of Ca²⁺ binding in EF-hand 4. Another variant (Q136P) has severely reduced affinity for the IQ domain, and shows changes in the CD spectra under Ca²⁺ -saturating conditions when unbound to the IQ domain. Ca²⁺ binding to a pair of EF-hands normally proceeds with very high cooperativity, but we found that N98S CaM can adopt different conformations with either one or two Ca²⁺ ions bound to the C-lobe, possibly disrupting the cooperativity. An N-lobe variant (N54I), which causes severe stress-induced arrhythmia, does not show any major changes in complex with the IQ domain, providing a structural basis for why this mutant does not affect function of Ca_V 1.2. These findings show that different CaM mutants have distinct effects on both the CaM structure and interactions with protein targets, and act via distinct pathological mechanisms to cause disease.

Excitation-contraction coupling in skeletal muscle: recent progress and unanswered questions

Excitation-contraction coupling (ECC) is a physiological process that links excitation of muscles by the nervous system to their mechanical contraction. In skeletal muscle, ECC is initiated with an action potential, generated by the somatic nervous system, which causes a depolarisation of the muscle fibre membrane (sarcolemma). This leads to a rapid change in the transmembrane potential, which is detected by the voltage-gated Ca²⁺ channel dihydropyridine receptor (DHPR) embedded in the sarcolemma. DHPR transmits the contractile signal to another Ca²⁺ channel, ryanodine receptor (RyR1), embedded in the membrane of the sarcoplasmic reticulum (SR), which releases a large amount of Ca²⁺ ions from the SR that initiate muscle contraction. Despite the fundamental role of ECC in skeletal muscle function of all vertebrate species, the molecular mechanism underpinning the communication between the two key proteins involved in the process (DHPR and RyR1) is still largely unknown. The goal of this work is to review the recent progress in our understanding of ECC in skeletal muscle from the point of view of the structure and interactions of proteins involved in the process, and to highlight the unanswered questions in the field.

Slaying a giant: Structures of calmodulin and protein kinase a bound to the cardiac ryanodine receptor

Ryanodine Receptors are Ca²⁺ release channels expressed in the Endoplasmic and Sarcoplasmic Reticulum membranes. Gong et al [1] reported cryo-EM structures of the cardiac RyR2 complexed to Calmodulin, which can downregulate channel opening. Haji-Ghassemi et al [2] report crystal structures of an RyR2 domain with PKA, a kinase promoting channel opening.

The Cardiac Ryanodine Receptor Phosphorylation Hotspot Embraces PKA in a Phosphorylation-Dependent Manner

Ryanodine receptors (RyRs) are intracellular Ca²⁺ release channels controlling essential cellular functions. RyRs are targeted by cyclic AMP (cAMP)-dependent protein kinase A (PKA), a controversial regulation implicated in disorders ranging from heart failure to Alzheimer's. Using crystal structures, we show that the phosphorylation hotspot domain of RyR2 embraces the PKA catalytic subunit, with an extensive interface not seen in PKA complexes with peptides. We trapped an intermediary open-form PKA bound to the RyR2 domain and an ATP analog, showing that PKA can engage substrates in an open form. Phosphomimetics or prior phosphorylation at nearby sites in RyR2 either enhance or reduce the activity of PKA. Finally, we show that a phosphomimetic at S2813, a well-known target site for calmodulin-dependent kinase II, induces the formation of an alpha helix in the phosphorylation domain, resulting in increased interactions and PKA activity. This shows that the different phosphorylation sites in RyR2 are not independent.