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

Longevity mechanisms in cardiac aging: exploring calcium dysregulation and senescence

Cardiac aging is a multistep process that results in a loss of various structural and functional heart abilities, increasing the risk of heart disease. Since its remarkable discovery in the early 1800s, when limestone is heated, calcium's importance has been defined in numerous ways. It can help stiffen shells and bones, function as a reducing agent in chemical reactions, and play a central role in cellular signalling. The movement of calcium ions in and out of cells and between those is referred to as calcium signalling. It influences the binding of the ligand, enzyme activity, electrochemical gradients, and other cellular processes. Calcium signalling is critical for both contraction and relaxation under the sliding filament model of heart muscle. However, with age, the heart undergoes changes that lead to increases in cardiac dysfunction, such as myocardial fibrosis, decreased cardiomyocyte function, and noxious disturbances in calcium homeostasis. Additionally, when cardiac tissues age, cellular senescence, a state of irreversible cell cycle arrest, accumulates and begins to exacerbate tissue inflammation and fibrosis. This review explores the most recent discoveries regarding the role of senescent cell accumulation and calcium signalling perturbances in cardiac aging. Additionally, new treatment strategies are used to reduce aged-related heart dysfunction by targeting senescent cells and modulating calcium homeostasis.

Genetic Interactions of Phase II Xenobiotic-Metabolizing Enzymes GSTO1 and GCLC in Relation to Alcohol Abuse and Psoriasis Risk

The present pilot study aimed to investigate whether common single nucleotide polymorphisms (SNPs) in the gene encoding glutathione S-transferase omega 1 (GSTO1), both individually and in combination with variants of the catalytic subunit of the glutamate cysteine ligase (GCLC) gene and environmental risk factors, are associated with the risk of psoriasis. The research included a total of 944 participants, comprising 474 individuals diagnosed with psoriasis and 470 healthy control subjects. Five common SNPs in the GSTO1 gene-specifically, rs11191736, rs34040810, rs2289964, rs11191979, and rs187304410-were genotyped in the study groups using the MassARRAY-4 system. The allele rs187304410-A (OR = 0.19, 95% CI 0.04-0.86, Pperm = 0.02) and the genotype rs187304410-G/A (OR = 0.19, 95% CI 0.04-0.85, Pperm = 0.01) were found to be associated with psoriasis in females. The model-based multifactor dimensionality reduction approach facilitated the identification of higher-order epistatic interactions between the variants of the GSTO1 and GCLC genes (Pperm < 0.0001). These interactions, along with the risk factor of alcohol abuse, collectively contribute to the pathogenesis of psoriasis. This study is the first to demonstrate that polymorphisms in the GSTO1 gene, both individually and in combination with variants of the GCLC gene and alcohol abuse, are associated with an increased risk of psoriasis.

RyR2-mediated calcium signaling regulates T-cell activation and Th1 differentiation

T helper cell differentiation is one of the key developmental events in the peripheral immune regulations, resulting in better adaptation to the nature of infection and inflammation. While it is known that several factors are involved in this differentiation, including subsets of antigen-presenting cells, cytokine environment, and metabolic activation, how calcium signaling plays a role in this event has remained elusive and sometimes controversial. In this report, we show that ER membrane Ca2+ ion channel ryanodine receptor 2 (RyR2) may be an important regulator in this event. RyR2-deficient T cells show greater retention of Ca2+ in the ER and have reduced SOCE activation, leading a delayed entry of NFAT2 into the nuclei. This delay causes a significant bias toward Th1 both in cytokine profiles and in T-bet expression, likely as a result of increased Il12rb2 and Stat4 expression. Interestingly, such a bias permits better host protection against intracellular Listeria Monocytogenes infection. Our work suggests the possibility that RyR2 may be regulated in T-cell activation for biased Th polarization, which may provide a target for fine-tuning T-cell differentiation in future clinical settings.

Development of selective RyR2 inhibitors with a pharmacophore containing a parabanic acid skeleton

Gene mutations resulting in dysfunction of the ryanodine receptor type 2 (RyR2), a huge Ca2+ release channel that controls the concentration of Ca2+ in the cytosol of cardiac muscle cells, can cause fatal heart arrhythmias. However, no RyR2 inhibitors have yet been developed for clinical usage. In this work, we discovered an isoform-selective RyR2 inhibitor 1 with a parabanic acid skeleton by screening a large chemical library. A detailed structure-activity relationship study of compound 1 showed that the parabanic acid skeleton was essential for inhibitory activity, and led to the development of the 15.5-fold more active inhibitor 18 through modifications at both side chains. Compound 18 selectively inhibited RyR2 among wild-type RyRs, and also inhibited RyR2 containing established pathogenic mutations, RyR2(R4495C) and RyR2(R2474S). These findings highlight the potential of the parabanic acid skeleton as a part of a pharmacophore for medicinal chemistry.

Glutathione S-transferase: A keystone in Parkinson's disease pathogenesis and therapy

Parkinson's disease is a progressive neurodegenerative disorder that predominantly affects motor function due to the loss of dopaminergic neurons in the substantia nigra. It presents significant challenges, impacting millions worldwide with symptoms such as tremors, rigidity, bradykinesia, and postural instability, leading to decreased quality of life and increased morbidity. The pathogenesis of Parkinson's disease is multifaceted, involving complex interactions between genetic susceptibility, environmental factors, and aging, with oxidative stress playing a central role in neuronal degeneration. Glutathione S-Transferase enzymes are critical in the cellular defense mechanism against oxidative stress, catalysing the conjugation of the antioxidant glutathione to various toxic compounds, thereby facilitating their detoxification. Recent research underscores the importance of Glutathione S-Transferase in the pathophysiology of Parkinson's disease, revealing that genetic polymorphisms in Glutathione S-Transferase genes influence the risk and progression of the disease. These genetic variations can affect the enzymatic activity of Glutathione S-Transferase, thereby modulating an individual's capacity to detoxify reactive oxygen species and xenobiotics, which are implicated in Parkinson's disease neuropathological processes. Moreover, biochemical studies have elucidated the role of Glutathione S-Transferase in not only maintaining cellular redox balance but also in modulating various cellular signalling pathways, highlighting its neuroprotective potential. From a therapeutic perspective, targeting Glutathione S-Transferase pathways offers promising avenues for the development of novel treatments aimed at enhancing neuroprotection and mitigating disease progression. This review explores the evident and hypothesized roles of Glutathione S-Transferase in Parkinson's disease, providing a comprehensive overview of its importance and potential as a target for therapeutic intervention.

Structural insights into transmembrane helix S0 facilitated RyR1 channel gating by Ca2+/ATP

The type-1 ryanodine receptor (RyR1) is an intracellular calcium release channel for skeletal muscle excitation-contraction coupling. Previous structural studies showed that the RyR1 activity is modulated by the exogenous regulators including caffeine, ryanodine, PCB-95 and diamide. An additional transmembrane helix, located adjacent to S1 and S4, has been observed in some structures, although its function remains unclear. Here, we report that using a mild purification procedure, this helix is co-purified with RyR1 and is designated as S0. When RyR1 is coupled with S0, it can be activated by Ca2+ to an open state; however when decoupled from S0, it remains in primed state. S0 regulates the channel conformation by directly affecting the TM domain via the pVSD-S0-S4/S5 linker coupling, which facilitates the dilation of S6. Our results demonstrate that S0 is an essential component of RyR1 and plays a key role in the physiological regulation of RyR1 channel gating.

Application of scRNA-seq in Dental Research: Seeking Regenerative Clues From the Structure of Tooth and Periodontium in Physical or Pathological States

This review presents a comprehensive overview of single-cell RNA sequencing (scRNA-seq) analyses used to study tooth and periodontal tissues. The intricate cellular composition of both teeth and periodontium are revealed, leading to the identification of new cell types and tracing lineage profiles for each cell type. Herein, we summarize the progression of dental and periodontal tissue formation, tooth homeostasis, and regenerative mechanisms. scRNA-seq analyses have demonstrated that the cellular constituent ratio of dental and periodontal tissues transforms homeostasis or injury repair. Importantly, single-cell data in the diseased tissue demonstrated a change in both cell types and intercellular communication patterns compared to the normal state. These findings provide valuable insights into the underlying disease mechanisms at the cellular level in the context of single-cell vision, thereby facilitating the investigation of potential therapeutic interventions.

Structural instability of ryanodine receptor 2 causes endoplasmic reticulum (ER) dysfunction as well as sarcoplasmic reticulum (SR) dysfunction

The type 2 ryanodine receptor (RyR2) is a giant Ca2+ (Ca)-releasing channel on the sarcoplasmic reticulum (SR) membrane, with subunits composed of 5000 amino acids constituting a homotetrameric channel. The N-terminal (1-220) and central (2300-2500) domain interactions (inter-subunit zipping interfaces) within RyR2 are located in close proximity to each other between different neighboring subunits and play an important "cornerstone" role in maintaining the tetrameric structure of RyR2. External stress such as oxidative stress causes Ca leak by destabilizing RyR2 (instability of the tetrameric structure) due to domain unzipping between N-terminal (1-220) and central (2300-2500) domains, followed by dissociation of calmodulin (CaM: binds to the RyR2 and stabilize RyR2) from RyR2. Ca leak from SR causes arrhythmias and myocardial dysfunction. RyR2 is also present in the endoplasmic reticulum (ER), thus it is not surprising that undesired Ca release from RyR2 on the ER is closely associated with various diseases involving ER dysfunction such as neurodegenerative diseases, diabetes, metabolic dysfunction-associated steatotic liver disease, chronic kidney disease, and autoimmune diseases. Pharmacological or genetic (point mutations within RyR2 that increase CaM-RyR2 affinity: knock-in RyR2-V3599K) RyR2 structural stabilization has shown potential therapeutic effects not only for SR failure-related diseases (malignant hyperthermia, arrhythmia, and heart failure) but also for ER failure-related disease. RyR2-stabilizers may function as a panacea for aging-related diseases.

The C. elegans uv1 Neuroendocrine Cells Provide Mechanosensory Feedback of Vulval Opening

Neuroendocrine cells react to physical, chemical, and synaptic signals originating from tissues and the nervous system, releasing hormones that regulate various body functions beyond the synapse. Neuroendocrine cells are often embedded in complex tissues making direct tests of their activation mechanisms and signaling effects difficult to study. In the nematode worm Caenorhabditis elegans, four uterine-vulval (uv1) neuroendocrine cells sit above the vulval canal next to the egg-laying circuit, releasing tyramine and neuropeptides that feedback to inhibit egg laying. We have previously shown uv1 cells are mechanically deformed during egg laying, driving uv1 Ca2+ transients. However, whether egg-laying circuit activity, vulval opening, and/or egg release triggered uv1 Ca2+ activity was unclear. Here, we show uv1 responds directly to mechanical activation. Optogenetic vulval muscle stimulation triggers uv1 Ca2+ activity following muscle contraction even in sterile animals. Direct mechanical prodding with a glass probe placed against the worm cuticle triggers robust uv1 Ca2+ activity similar to that seen during egg laying. Direct mechanical activation of uv1 cells does not require other cells in the egg-laying circuit, synaptic or peptidergic neurotransmission, or transient receptor potential vanilloid and Piezo channels. EGL-19 L-type Ca2+ channels, but not P/Q/N-type or ryanodine receptor Ca2+ channels, promote uv1 Ca2+ activity following mechanical activation. L-type channels also facilitate the coordinated activation of uv1 cells across the vulva, suggesting mechanical stimulation of one uv1 cell cross-activates the other. Our findings show how neuroendocrine cells like uv1 report on the mechanics of tissue deformation and muscle contraction, facilitating feedback to local circuits to coordinate behavior.

Structure-Function Diversity of Calcium-Binding Proteins (CaBPs): Key Roles in Cell Signalling and Disease

Calcium (Ca2+) signalling is a fundamental cellular process, essential for a wide range of physiological functions. It is regulated by various mechanisms, including a diverse family of Ca2+-binding proteins (CaBPs), which are structurally and functionally similar to calmodulin (CaM). The CaBP family consists of six members (CaBP1, CaBP2, CaBP4, CaBP5, CaBP7, and CaBP8), each exhibiting unique localisation, structural features, and functional roles. In this review, we provide a structure-function analysis of the CaBP family, highlighting the key similarities and differences both within the family and in comparison to CaM. It has been shown that CaBP1-5 share similar structural and interaction characteristics, while CaBP7 and CaBP8 form a distinct subfamily with unique properties. This review of current CaBP knowledge highlights the critical gaps in our understanding, as some CaBP members are less well characterised than others. We also examine pathogenic mutations within CaBPs and their functional impact, showing the need for further research to improve treatment options for associated disorders.

Regulation of calcium homeostasis in endoplasmic reticulum-mitochondria crosstalk: implications for skeletal muscle atrophy

This review comprehensively explores the critical role of calcium as an essential small-molecule biomessenger in skeletal muscle function. Calcium is vital for both regulating muscle excitation-contraction coupling and for the development, maintenance, and regeneration of muscle cells. The orchestrated release of calcium from the endoplasmic reticulum (ER) is mediated by receptors such as the ryanodine receptor (RYR) and inositol 1,4,5-trisphosphate receptor (IP3R), which is crucial for skeletal muscle contraction. The sarcoendoplasmic reticulum calcium ATPase (SERCA) pump plays a key role in recapturing calcium, enabling the muscle to return to a relaxed state. A pivotal aspect of calcium homeostasis involves the dynamic interaction between mitochondria and the ER. This interaction includes local calcium signaling facilitated by RYRs and a "quasi-synaptic" mechanism formed by the IP3R-Grp75-VDAC/MCU axis, allowing rapid calcium uptake by mitochondria with minimal interference at the cytoplasmic level. Disruption of calcium transport can lead to mitochondrial calcium overload, triggering the opening of the mitochondrial permeability transition pore and subsequent release of reactive oxygen species and cytochrome C, ultimately resulting in muscle damage and atrophy. This review explores the complex relationship between the ER and mitochondria and how these organelles regulate calcium levels in skeletal muscle, aiming to provide valuable perspectives for future research on the pathogenesis of muscle diseases and the development of prevention strategies.

Mechanisms and Countermeasures for Muscle Atrophy in Microgravity

Previous studies have revealed that muscle atrophy emerges as a significant challenge faced by astronauts during prolonged missions in space. A loss in muscle mass results in a weakening of skeletal muscle strength and function, which will not only contribute to a decline in overall physical performance but also elevate the risk of various age-related diseases. Skeletal muscle atrophy in the microgravity environment is thought to be associated with changes in energy metabolism, protein metabolism, calcium ion homeostasis, myostatin levels, and apoptosis. Modulating some pathways could be a promising approach to mitigating muscle atrophy in the microgravity environment. This review serves as a comprehensive summary of research on the impact of microgravity on skeletal muscle, with the aim of providing insights into its pathogenesis and the development of effective treatments.

Structural dynamics of human ribosomes in situ reconstructed by exhaustive high-resolution template matching

Protein synthesis is central to life and requires the ribosome, which catalyzes the stepwise addition of amino acids to a polypeptide chain by undergoing a sequence of structural transformations. Here, we employed high-resolution template matching (HRTM) on cryoelectron microscopy (cryo-EM) images of directly cryofixed living cells to obtain a set of ribosomal configurations covering the entire elongation cycle, with each configuration occurring at its native abundance. HRTM's position and orientation precision and ability to detect small targets (∼300 kDa) made it possible to order these configurations along the reaction coordinate and to reconstruct molecular features of any configuration along the elongation cycle. Visualizing the cycle's structural dynamics by combining a sequence of >40 reconstructions into a 3D movie readily revealed component and ligand movements, some of them surprising, such as spring-like intramolecular motion, providing clues about the molecular mechanisms involved in some still mysterious steps during chain elongation.

Nernst-Planck-Gaussian finite element modelling of Ca2+ electrodiffusion in amphibian striated muscle transverse tubule-sarcoplasmic reticular triadic junctional domains

Introduction

Intracellular Ca2+ signalling regulates membrane permeabilities, enzyme activity, and gene transcription amongst other functions. Large transmembrane Ca2+ electrochemical gradients and low diffusibility between cell compartments potentially generate short-lived, localised, high-[Ca2+] microdomains. The highest concentration domains likely form between closely apposed membranes, as at amphibian skeletal muscle transverse tubule-sarcoplasmic reticular (T-SR, triad) junctions.

Materials and methods

Finite element computational analysis characterised the formation and steady state and kinetic properties of the Ca2+ microdomains using established empirical physiological and anatomical values. It progressively incorporated Fick diffusion and Nernst-Planck electrodiffusion gradients, K+, Cl-, and Donnan protein, and calmodulin (CaM)-mediated Ca2+ buffering. It solved for temporal-spatial patterns of free and buffered Ca2+, Gaussian charge differences, and membrane potential changes, following Ca2+ release into the T-SR junction.

Results

Computational runs using established low and high Ca2+ diffusibility (D Ca2+) limits both showed that voltages arising from intracytosolic total [Ca2+] gradients and the counterions little affected microdomain formation, although elevated D Ca2+ reduced attained [Ca2+] and facilitated its kinetics. Contrastingly, adopting known cytosolic CaM concentrations and CaM-Ca2+ affinities markedly increased steady-state free ([Ca2+]free) and total ([Ca2+]), albeit slowing microdomain formation, all to extents reduced by high D Ca2+. However, both low and high D Ca2+ yielded predictions of similar, physiologically effective, [Ca2+-CaM]. This Ca2+ trapping by the relatively immobile CaM particularly increased [Ca2+] at the junction centre. [Ca2+]free, [Ca2+-CaM], [Ca2+], and microdomain kinetics all depended on both CaM-Ca2+ affinity and D Ca2+. These changes accompanied only small Gaussian (∼6 mV) and surface charge (∼1 mV) effects on tubular transmembrane potential at either D Ca2+.

Conclusion

These physical predictions of T-SR Ca2+ microdomain formation and properties are compatible with the microdomain roles in Ca2+ and Ca2+-CaM-mediated signalling but limited the effects on tubular transmembrane potentials. CaM emerges as a potential major regulator of both the kinetics and the extent of microdomain formation. These possible cellular Ca2+ signalling roles are discussed in relation to possible feedback modulation processes sensitive to the μM domain but not nM bulk cytosolic, [Ca2+]free, and [Ca2+-CaM], including ryanodine receptor-mediated SR Ca2+ release; Na+, K+, and Cl- channel-mediated membrane excitation and stabilisation; and Na+/Ca2+ exchange transport.

A new stress test for ryanodine receptors

JGP study (Steinz et al. https://doi.org/10.1085/jgp.202313515) reveals that oxidative stress can induce stable posttranslational modifications of RyR1 that increase the channel's open probability and could therefore disrupt muscle contractility.

Stable oxidative posttranslational modifications alter the gating properties of RyR1

The ryanodine receptor type 1 (RyR1) is a Ca2+ release channel that regulates skeletal muscle contraction by controlling Ca2+ release from the sarcoplasmic reticulum (SR). Posttranslational modifications (PTMs) of RyR1, such as phosphorylation, S-nitrosylation, and carbonylation are known to increase RyR1 open probability (Po), contributing to SR Ca2+ leak and skeletal muscle dysfunction. PTMs on RyR1 have been linked to muscle dysfunction in diseases like breast cancer, rheumatoid arthritis, Duchenne muscle dystrophy, and aging. While reactive oxygen species (ROS) and oxidative stress induce PTMs, the impact of stable oxidative modifications like 3-nitrotyrosine (3-NT) and malondialdehyde adducts (MDA) on RyR1 gating remains unclear. Mass spectrometry and single-channel recordings were used to study how 3-NT and MDA modify RyR1 and affect Po. Both modifications increased Po in a dose-dependent manner, with mass spectrometry identifying 30 modified residues out of 5035 amino acids per RyR1 monomer. Key modifications were found in domains critical for protein interaction and channel activation, including Y808/3NT in SPRY1, Y1081/3NT and H1254/MDA in SPRY2&3, and Q2107/MDA and Y2128/3NT in JSol, near the binding site of FKBP12. Though these modifications did not directly overlap with FKBP12 binding residues, they promoted FKBP12 dissociation from RyR1. These findings provide detailed insights into how stable oxidative PTMs on RyR1 residues alter channel gating, advancing our understanding of RyR1-mediated Ca2+ release in conditions associated with oxidative stress and muscle weakness.

Role of ryanodine receptor cooperativity in Ca2+-wave-mediated triggered activity in cardiomyocytes

Ca2+ waves are known to trigger delayed after-depolarizations that can cause malignant cardiac arrhythmias. However, modelling Ca2+ waves using physiologically realistic models has remained a major challenge. Existing models with low Ca2+ sensitivity of ryanodine receptors (RyRs) necessitate large release currents, leading to an unrealistically large Ca2+ transient amplitude incompatible with the experimental observations. Consequently, current physiologically detailed models of delayed after-depolarizations resort to unrealistic cell architectures to produce Ca2+ waves with a normal Ca2+ transient amplitude. Here, we address these challenges by incorporating RyR cooperativity into a physiologically detailed model with a realistic cell architecture. We represent RyR cooperativity phenomenologically through a Hill coefficient within the sigmoid function of RyR open probability. Simulations in permeabilized myocytes with high Ca2+ sensitivity reveal that a sufficiently large Hill coefficient is required for Ca2+ wave propagation via the fire-diffuse-fire mechanism. In intact myocytes, propagating Ca2+ waves can occur only within an intermediate Hill coefficient range. Within this range, the spark rate is neither too low, enabling Ca2+ wave propagation, nor too high, allowing for the maintenance of a high sarcoplasmic reticulum load during diastole of the action potential. Moreover, this model successfully replicates other experimentally observed manifestations of Ca2+-wave-mediated triggered activity, including phase 2 and phase 3 early after-depolarizations and high-frequency voltage-Ca2+ oscillations. These oscillations feature an elevated take-off potential with depolarization mediated by the L-type Ca2+ current. The model also sheds light on the roles of luminal gating of RyRs and the mobile buffer ATP in the genesis of these arrhythmogenic phenomena. KEY POINTS: Existing mathematical models of Ca2+ waves use an excessively large Ca2+-release current or unrealistic diffusive coupling between release units. Our physiologically realistic model, using a Hill coefficient in the ryanodine receptor (RyR) gating function to represent RyR cooperativity, addresses these limitations and generates organized Ca2+ waves at Hill coefficients ranging from ∼5 to 10, as opposed to the traditional value of 2. This range of Hill coefficients gives a spark rate neither too low, thereby enabling Ca2+ wave propagation, nor too high, allowing for the maintenance of a high sarcoplasmic reticulum load during the plateau phase of the action potential. Additionally, the model generates Ca2+-wave-mediated phase 2 and phase 3 early after-depolarizations, and coupled membrane voltage with Ca2+ oscillations mediated by the L-type Ca2+ current. This study suggests that pharmacologically targeting RyR cooperativity could be a promising strategy for treating cardiac arrhythmias linked to Ca2+-wave-mediated triggered activity.

Gain-of-Function and Loss-of-Function Mutations in the RyR2-Expressing Gene Are Responsible for the CPVT1-Related Arrhythmogenic Activities in the Heart

Mutations in the ryanodine receptor (RyR2) gene have been linked to arrhythmia and possibly sudden cardiac death (SCD) during acute emotional stress, physical activities, or catecholamine perfusion. The most prevalent disorder is catecholaminergic polymorphic ventricular tachycardia (CPVT1). Four primary mechanisms have been proposed to describe CPVT1 with a RyR2 mutation: (a) gain-of-function, (b) destabilization of binding proteins, (c) store-overload-induced Ca2+ release (SOICR), and (d) loss of function. The goal of this study was to use computational models to understand these four mechanisms and how they might contribute to arrhythmia. To this end, we have developed a local control stochastic model of a ventricular cardiac myocyte and used it to investigate how the Ca2+ dynamics in the mutant RyR2 are responsible for the development of an arrhythmogenic episode under the condition of β-adrenergic (β-AR) stimulation or pauses afterward. Into the model, we have incorporated 20,000 distinct cardiac dyads consisting of stochastically gated L-type Ca2+ channels (LCCs) and ryanodine receptors (RyR2s) and the intervening dyadic cleft to analyze the alterations in Ca2+ dynamics. Recent experimental findings were incorporated into the model parameters to test these proposed mechanisms and their role in triggering arrhythmias. The model could not find any connection between SOICR and the destabilization of binding proteins as the arrhythmic mechanisms in the mutant myocyte. On the other hand, the model was able to observe loss-of-function and gain-of-function mutations resulting in EADs (Early Afterdepolarizations) and variations in action potential amplitudes and durations as the precursors to generate arrhythmia, respectively. These computational studies demonstrate how GOF and LOF mutations can lead to arrhythmia and cast doubt on the feasibility of SOICR as a mechanism of arrhythmia.

Electrophysical cardiac remodeling at the molecular level: Insights into ryanodine receptor activation and calcium-induced calcium release from a stochastic explicit-particle model

We present the first-ever, fully discrete, stochastic model of triggered cardiac Ca2+ dynamics. Using anatomically accurate subcellular cardiac myocyte geometries, we simulate the molecular players involved in Ca2+ handling using high-resolution stochastic and explicit-particle methods at the level of an individual cardiac dyadic junction. Integrating data from multiple experimental sources, the model not only replicates the findings of traditional in silico studies and complements in vitro experimental data but also reveals new insights into the molecular mechanisms driving cardiac dysfunction under stress and disease conditions. We improve upon older, nondiscrete models using the same realistic geometry by incorporating molecular mechanisms for spontaneous, as well as triggered calcium-induced calcium release (CICR). Action potentials are used to activate L-type calcium channels (LTCC), triggering CICR through ryanodine receptors (RyRs) on the surface of the sarcoplasmic reticulum. These improvements allow for the specific focus on the couplon: the structure-function relationship between LTCC and RyR. We investigate the electrophysical effects of normal and diseased action potentials on CICR and interrogate the effects of dyadic junction deformation through detubulation and orphaning of RyR. Our work demonstrates the importance of the electrophysical integrity of the calcium release unit on CICR fidelity, giving insights into the molecular basis of heart disease. Finally, we provide a unique, detailed, molecular view of the CICR process using advanced rendering techniques. This easy-to-use model comes complete with tutorials and the necessary software for use and analysis to maximize usability and reproducibility. Our work focuses on quantifying, qualifying, and visualizing the behavior of the molecular species that underlie the function and dysfunction of subcellular cardiomyocyte systems.

Exploring paraptosis as a therapeutic approach in cancer treatment

A variety of cell death pathways play critical roles in the onset and progression of multiple diseases. Paraptosis, a unique form of programmed cell death, has gained significant attention in recent years. Unlike apoptosis and necrosis, paraptosis is characterized by cytoplasmic vacuolization, swelling of the endoplasmic reticulum and mitochondria, and the absence of caspase activation. Numerous natural products, synthetic compounds, and newly launched nanomedicines have been demonstrated to prime cell death through the paraptotic program and may offer novel therapeutic strategies for cancer treatment. This review summarizes recent findings, delineates the intricate network of signaling pathways underlying paraptosis, and discusses the potential therapeutic implications of targeting paraptosis in cancer treatment. The aim of this review is to expand our understanding of this unique cell death process and explore the potential therapeutic implications of targeting paraptosis in cancer treatment.

[From gene to cell: Functional validation of RYR1 variants]

Genetic screening of rare diseases allows identification of the responsible gene(s) in about 50% of patients. The remaining cases are in a diagnostic deadlock as current knowledge fails to identify the correct gene or determine if the detected variant on the gene is pathogenic. These are named "variants of unknown significance" (VUS). In the case of neuromuscular diseases, the RYR1 gene is often implicated, with the majority of variants classified as VUS, requiring reliable classification to help patient diagnosis. Our project aims to create an efficient classification pipeline, integrating artificial intelligence, structural biology data, and functional analyses to enhance genetic diagnosis of RYR1-related diseases.

Ca2+ Signaling in Cardiovascular Fibroblasts

Fibrogenesis is a physiological process required for wound healing and tissue repair. It is induced by activation of quiescent fibroblasts, which first proliferate and then change their phenotype into migratory, contractile myofibroblasts. Myofibroblasts secrete extracellular matrix proteins, such as collagen, to form a scar. Once the healing process is terminated, most myofibroblasts undergo apoptosis. However, in some tissues, such as the heart, myofibroblasts remain active and sensitive to neurohumoral factors and inflammatory mediators, which lead eventually to excessive organ fibrosis. Many cellular processes involved in fibroblast activation, including cell proliferation, protein secretion and cell contraction, are highly regulated by intracellular Ca2+ signals. This review summarizes current research on Ca2+ signaling pathways underlying fibroblast activation. We present receptor- and ion channel-mediated Ca2+ signaling pathways, discuss how localized Ca2+ signals of the cell nucleus may be involved in fibroblast activation and present Ca2+-sensitive transcription pathways relevant for fibroblast biology. When investigated, we highlight how the function of Ca2+-handling proteins changes during cardiac and pulmonary fibrosis. Many aspects of Ca2+ signaling remain unexplored in different types of cardiovascular fibroblasts in relation to pathologies, and a better understanding of Ca2+ signaling in fibroblasts will help to design targeted therapies against fibrosis.

HuR-dependent expression of RyR2 contributes to calcium-mediated thermogenesis in brown adipocytes

Several uncoupling protein 1 (UCP1)-independent thermogenic pathways have been described in thermogenic adipose tissue, including calcium-mediated thermogenesis in beige adipocytes via sarco/endoplasmic reticulum ATPase (SERCA). We have previously shown that adipocyte-specific deletion of the RNA binding protein human antigen R (HuR) results in thermogenic dysfunction independent of UCP1 expression. RNA sequencing revealed the downregulation of several genes involved in calcium ion transport upon HuR deletion. The goal of this work was to define the HuR-dependent mechanisms of calcium driven thermogenesis in brown adipocytes. We generated (BAT)-specific HuR-deletion (BAT-HuR -/- ) mice and show that their body weight, glucose tolerance, brown and white adipose tissue weights, and total lipid droplet size were not significantly different compared to wild-type. Similar to our initial findings in Adipo-HuR -/- mice, mice with BAT-specific HuR deletion are cold intolerant following acute thermal challenge at 4°C, demonstrating specificity of acute HuR-dependent thermogenesis to BAT. We also found decreased expression of ryanodine receptor 2 (RyR2), but no changes in RyR2, SERCA1, SERCA2, or UCP1 expression, in BAT from BAT-HuR -/- mice. Next, we used Fluo-4 calcium indicator dye to show that genetic deletion or pharmacological inhibition of HuR blunts the increase in cytosolic calcium concentration in SVF-derived primary brown adipocytes. Moreover, we saw a similar blunting in β-adrenergic-mediated heat generation, as assessed by ERtherm AC fluorescence, in SVF-derived brown adipocytes following HuR inhibition or deletion. Mechanistically, we show that HuR directly binds and reduces the decay rate of RyR2 mRNA in brown adipocytes, and stabilization of RyR2 via S107 rescues β-adrenergic-mediated cytosolic calcium increase and heat generation in HuR deficient brown adipocytes. In conclusion, our results suggest that HuR-dependent control of RyR2 expression plays a significant role in the thermogenic function of brown adipose tissue through modulation of SR calcium cycling.

Modulation of heteromeric glycine receptor function through high concentration clustering

Ion channels are targeted by many drugs for treating neurological, musculoskeletal, renal and other diseases. These drugs bind to and alter the function of individual channels to achieve desired therapeutic effects. However, many ion channels function in high concentration clusters in their native environment. It is unclear if and how clustering modulates ion channel function. Human heteromeric glycine receptors (GlyRs) are the major inhibitory neurotransmitter receptors in the spinal cord and are active targets for developing chronic pain medications. We show that the α2β heteromeric GlyR assembles with the master postsynaptic scaffolding gephyrin (GPHN) into micron-sized clustered at the plasma membrane after heterologous expression. The inhibitory trans- synaptic adhesion protein neuroligin-2 (NL2) further increases both the cluster sizes and GlyR concentration. The apparent glycine affinity increases monotonically as a function of GlyR concentration but not with cluster size. We also show that ligand re-binding to adjacent GlyRs alters kinetics but not chemical equilibrium. A positively charged N- terminus sequence of the GlyR β subunit was further identified essential for glycine affinity modulation through clustering. Taken together, we propose a mechanism where clustering enhances local electrostatic potential, which in turn concentrates ions and ligands, modulating the function of GlyR. This mechanism is likely universal across ion channel clusters found ubiquitously in biology and provides new perspectives in possible pharmaceutical development.

PKD2: An Important Membrane Protein in Organ Development

PKD2 was first identified as the pathogenic protein for autosomal dominant polycystic kidney disease (ADPKD) and is widely recognized as an ion channel. Subsequent studies have shown that PKD2 is widely expressed in various animal tissues and plays a crucial role in tissue and organ development. Additionally, PKD2 is conserved from single-celled organisms to vertebrates. Here, we provide an overview of recent advances in the function of PKD2 in key model animals, focusing on the establishment of left-right organ asymmetry, renal homeostasis, cardiovascular development, and signal transduction in reproduction and mating. We specifically focus on the roles of PKD2 in development and highlight future prospects for PKD2 research.

Quantitative Single-Molecule Analysis of Ryanodine Receptor 2 Subunit Assembly in Cardiac and Neuronal Tissues

We developed a method for ex vivo receptor encapsulation and single-molecule imaging techniques from neuronal and cardiac tissues, illustrating the method's broad applicability for measuring membrane receptor assembly. Ryanodine receptor 2 (RyR2) is a tetrameric Ca2+ channel governing intracellular Ca2+ dynamics, which is critical for muscle contraction. Employing GFP-RyR2 knock-in mice, we isolated individual receptor proteins in tissue-specific nanovesicles and performed subunit counting analyses to yield quantitative assessment of stoichiometric distributions across different organs. With this method, we explored the potential heterogeneity of brain-derived RyR2, which has been reported to form heteromeric assemblies with other ryanodine receptor isoforms.

Modeling the mechanism of Ca2+ release in skeletal muscle by DHPRs easing inhibition at RyR I1-sites

Ca2+ release from the sarcoplasmic reticulum (SR) plays a central role in excitation-contraction coupling (ECC) in skeletal muscles. However, the mechanism by which activation of the voltage-sensors/dihydropyridine receptors (DHPRs) in the membrane of the transverse tubular system leads to activation of the Ca2+-release channels/ryanodine receptors (RyRs) in the SR is not fully understood. Recent observations showing that a very small Ca2+ leak through RyR1s in mammalian skeletal muscle can markedly raise the background [Ca2+] in the junctional space (JS) above the Ca2+ level in the bulk of the cytosol indicate that there is a diffusional barrier between the JS and the cytosol at large. Here, I use a mathematical model to explore the hypothesis that a sudden rise in Ca2+ leak through DHPR-coupled RyR1s, caused by reduced inhibition at the RyR1 Ca2+/Mg2+ inhibitory I1-sites when the associated DHPRs are activated, is sufficient to enable synchronized responses that trigger a regenerative rise of Ca2+ release that remains under voltage control. In this way, the characteristic response to Ca2+ of RyR channels is key not only for the Ca2+ release mechanism in cardiac muscle and other tissues, but also for the DHPR-dependent Ca2+ release in skeletal muscle.

Cryo-EM investigation of ryanodine receptor type 3

Ryanodine Receptor isoform 3 (RyR3) is a large ion channel found in the endoplasmic reticulum membrane of many different cell types. Within the hippocampal region of the brain, it is found in dendritic spines and regulates synaptic plasticity. It controls myogenic tone in arteries and is upregulated in skeletal muscle in early development. RyR3 has a unique functional profile with a very high sensitivity to activating ligands, enabling high gain in Ca2+-induced Ca2+ release. Here we solve high-resolution cryo-EM structures of RyR3 in non-activating and activating conditions, revealing structural transitions that occur during channel opening. Addition of activating ligands yields only open channels, indicating an intrinsically high open probability under these conditions. RyR3 has reduced binding affinity to the auxiliary protein FKBP12.6 due to several sequence variations in the binding interface. We map disease-associated sequence variants and binding sites for known pharmacological agents. The N-terminal region contains ligand binding sites for a putative chloride anion and ATP, both of which are targeted by sequence variants linked to epileptic encephalopathy.

Rapid small-scale nanobody-assisted purification of ryanodine receptors for cryo-EM

Ryanodine receptors (RyRs) are large Ca2+ release channels residing in the endoplasmic or sarcoplasmic reticulum membrane. Three isoforms of RyRs have been identified in mammals, the disfunction of which has been associated with a series of life-threatening diseases. The need for large amounts of native tissue or eukaryotic cell cultures limits advances in structural studies of RyRs. Here, we report a method that utilizes nanobodies to purify RyRs from only 5 mg of total protein. The purification process, from isolated membranes to cryo-EM grade protein, is achieved within 4 h on the bench, yielding protein usable for cryo-EM analysis. This is demonstrated by solving the structures of rabbit RyR1, solubilized in detergent, reconstituted into lipid nanodiscs or liposomes, and bovine RyR2 reconstituted in nanodisc, and mouse RyR2 in detergent. The reported method facilitates structural studies of RyRs directed toward drug development and is useful in cases where the amount of starting material is limited.

Structural changes and contractility in muscle assessed by magnetic resonance imaging in individuals with ryanodine receptor 1-related rhabdomyolysis or myalgia

INTRODUCTION/AIMS

Ryanodine receptor 1 (RYR1)-related myopathies associated with variants in the RYR1 gene present with a wide range of symptoms and severity. Two of the milder phenotypes associated with dominant pathogenic variants in RYR1 are rhabdomyolysis and myalgia. Only a few studies have investigated the muscle function and structure of individuals with RYR1-related rhabdomyolysis/myalgia objectively, showing inconsistent results. This study aimed to describe structural changes and contractility of muscles in individuals with RYR1-related rhabdomyolysis/myalgia.

METHODS

We investigated 15 individuals with dominant variants in the RYR1-gene and compared them with 15 age-, sex-, and body mass index (BMI)-matched controls using MRI, stationary isokinetic dynamometry, and comprehensive clinical evaluation.

RESULTS

No significant differences were found between individuals with RYR1-related rhabdomyolysis/myalgia and healthy controls in peak torque, fat fraction, cross-sectional area, contractile cross-sectional area, or contractility (p > .05) in muscles of the lower back (MRI data only), thigh, or calf. On clinical examination, three individuals exhibited weakness in hip or back extension on the Medical Research Council (MRC) test and eight had muscle hypertrophy. Individuals with weakness were not hypertrophic.

DISCUSSION

Most individuals with RYR1-related rhabdomyolysis/myalgia have close to normal strength, and normal fat fraction and contractility of muscles, and therefore constitute a mild phenotype of RYR1-related myopathies.

Conductive Microgel Annealed Scaffolds Enhance Myogenic Potential of Myoblastic Cells

Conductive biomaterials may capture native or exogenous bioelectric signaling, but incorporation of conductive moieties is limited by cytotoxicity, poor injectability, or insufficient stimulation. Microgel annealed scaffolds are promising as hydrogel-based materials due to their inherent void space that facilitates cell migration and proliferation better than nanoporous bulk hydrogels. Conductive microgels are generated from poly(ethylene) glycol (PEG and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) to explore the interplay of void volume and conductivity on myogenic differentiation. PEDOT: PSS increases microgel conductivity two-fold while maintaining stiffness, annealing strength, and viability of associated myoblastic cells. C2C12 myoblasts exhibit increases in the late-stage differentiation marker myosin heavy chain as a function of both porosity and conductivity. Myogenin, an earlier marker, is influenced only by porosity. Human skeletal muscle-derived cells exhibit increased Myod1, insulin like growth factor-1, and insulin-like growth factor binding protein 2 at earlier time points on conductive microgel scaffolds compared to non-conductive scaffolds. They also secrete more vascular endothelial growth factor at early time points and express factors that led to macrophage polarization patterns observe during muscle repair. These data indicate that conductivity aids myogenic differentiation of myogenic cell lines and primary cells, motivating the need for future translational studies to promote muscle repair.

Physiological role for S-nitrosylation of RyR1 in skeletal muscle function and development

Excitation-contraction coupling in skeletal muscle myofibers depends upon Ca2+ release from the sarcoplasmic reticulum through the ryanodine receptor/Ca2+-release channel RyR1. The RyR1 contains ∼100 Cys thiols of which ∼30 comprise an allosteric network subject to posttranslational modification by S-nitrosylation, S-palmitoylation and S-oxidation. However, the role and function of these modifications is not understood. Although aberrant S-nitrosylation of multiple unidentified sites has been associated with dystrophic diseases, malignant hyperthermia and other myopathic syndromes, S-nitrosylation in physiological situations is reportedly specific to a single (1 of ∼100) Cys in RyR1, Cys3636 in a manner gated by pO2. Using mice expressing a form of RyR1 with a Cys3636→Ala point mutation to prevent S-nitrosylation at this site, we showed that Cys3636 was the principal target of endogenous S-nitrosylation during normal muscle function. The absence of Cys3636 S-nitrosylation suppressed stimulus-evoked Ca2+ release at physiological pO2 (at least in part by altering the regulation of RyR1 by Ca2+/calmodulin), eliminated pO2 coupling, and diminished skeletal myocyte contractility in vitro and measures of muscle strength in vivo. Furthermore, we found that abrogation of Cys3636 S-nitrosylation resulted in a developmental defect reflected in diminished myofiber diameter, altered fiber subtypes, and altered expression of genes implicated in muscle development and atrophy. Thus, our findings establish a physiological role for pO2-coupled S-nitrosylation of RyR1 in skeletal muscle contractility and development and provide foundation for future studies of RyR1 modifications in physiology and disease.

Purinergic agonists increase [Ca2+]i in rat conjunctival goblet cells through ryanodine receptor type 3

ATP and benzoylbenzoyl-ATP (BzATP) increase free cytosolic Ca2+ concentration ([Ca2+]i) in conjunctival goblet cells (CGCs) resulting in mucin secretion. The purpose of this study was to investigate the source of the Ca2+i mobilized by ATP and BzATP. First-passage cultured rat CGCs were incubated with Fura-2/AM, and [Ca2+]i was measured under several conditions with ATP and BzATP stimulation. The following conditions were used: 1) preincubation with the Ca2+ chelator EGTA, 2) preincubation with the SERCA inhibitor thapsigargin (10-6 M), which depletes ER Ca2+ stores, 3) preincubation with phospholipase C (PLC) or protein kinase A (PKA) inhibitor, or 4) preincubation with the voltage-gated calcium channel antagonist nifedipine (10-5 M) and the ryanodine receptor (RyR) antagonist dantrolene (10-5 M). Immunofluorescence microscopy (IF) and quantitative reverse transcription polymerase chain reaction (RT-qPCR) were used to investigate RyR presence in rat and human CGCs. ATP-stimulated peak [Ca2+]i was significantly lower after chelating Ca2+i with 2 mM EGTA in Ca2+-free buffer. The peak [Ca2+]i increase in CGCs preincubated with thapsigargin, the PKA inhibitor H89, nifedipine, and dantrolene, but not the PLC inhibitor, was reduced for ATP at 10-5 M and BzATP at 10-4 M. Incubating CGCs with dantrolene alone decreased [Ca2+]i and induced CGC cell death at a high concentration. RyR3 was detected in rat and human CGCs with IF and RT-qPCR. We conclude that ATP- and BzATP-induced Ca2+i increases originate from the ER and that RyR3 may be an essential regulator of CGC [Ca2+]i. This study contributes to the understanding of diseases arising from defective Ca2+ signaling in nonexcitable cells.NEW & NOTEWORTHY ATP and benzoylbenzoyl-ATP (BzATP) induce mucin secretion through an increase in free cytosolic calcium concentration ([Ca2+]i) in conjunctival goblet cells (CGCs). The mechanisms through which ATP and BzATP increase [Ca2+]i in CGCs are unclear. Ryanodine receptors (RyRs) are fundamental in [Ca2+]i regulation in excitable cells. Herein, we find that ATP and BzATP increase [Ca2+]i through the activation of protein kinase A, voltage-gated calcium channels, and RyRs, and that RyRs are crucial for nonexcitable CGCs' Ca2+i homeostasis.

Gene structure, expression and function analysis of the MyoD gene in the Pacific white shrimp Litopenaeus vannamei

The Pacific white shrimp Litopenaeus vannamei is a representative species of decapod crustacean and an economically important marine aquaculture species worldwide. However, research on the genes involved in muscle growth and development in shrimp is still lacking. MyoD is recognized as a crucial regulator of myogenesis and plays an essential role in muscle growth and differentiation in various animals. Nonetheless, little information is available concerning the function of this gene among crustaceans. In this study, we identified a sequence of the MyoD gene (LvMyoD) with a conserved bHLH domain in the L. vannamei genome. Phylogenetic analysis revealed that both the overall protein sequence and specific functional sites of LvMyoD are highly conserved with those of other crustacean species and that they are evolutionarily closely related to vertebrate MyoD and Myf5. LvMyoD expression is initially high during early muscle development in shrimp and gradually decreases after 40 days post-larval development. In adults, the muscle-specific expression of LvMyoD was confirmed through RT-qPCR analysis. Knockdown of LvMyoD inhibited the growth of the shrimp in body length and weight. Histological observation and transcriptome sequencing of muscle samples after RNA interference (RNAi) revealed nuclear agglutination and looseness in muscle fibers. Additionally, we observed significant effects on the expression of genes involved in heat shock proteins, myosins, actins, protein synthesis, and glucose metabolism. These findings suggest that LvMyoD plays a critical role in regulating muscle protein synthesis and muscle cell differentiation. Overall, this study highlights the involvement of LvMyoD in myogenesis and muscle growth, suggesting that it is a potentially important regulatory target for shrimp breeding efforts.

The ion channels of endomembranes

The endomembrane system consists of organellar membranes in the biosynthetic pathway [endoplasmic reticulum (ER), Golgi apparatus, and secretory vesicles] as well as those in the degradative pathway (early endosomes, macropinosomes, phagosomes, autophagosomes, late endosomes, and lysosomes). These endomembrane organelles/vesicles work together to synthesize, modify, package, transport, and degrade proteins, carbohydrates, and lipids, regulating the balance between cellular anabolism and catabolism. Large ion concentration gradients exist across endomembranes: Ca2+ gradients for most endomembrane organelles and H+ gradients for the acidic compartments. Ion (Na+, K+, H+, Ca2+, and Cl-) channels on the organellar membranes control ion flux in response to cellular cues, allowing rapid informational exchange between the cytosol and organelle lumen. Recent advances in organelle proteomics, organellar electrophysiology, and luminal and juxtaorganellar ion imaging have led to molecular identification and functional characterization of about two dozen endomembrane ion channels. For example, whereas IP3R1-3 channels mediate Ca2+ release from the ER in response to neurotransmitter and hormone stimulation, TRPML1-3 and TMEM175 channels mediate lysosomal Ca2+ and H+ release, respectively, in response to nutritional and trafficking cues. This review aims to summarize the current understanding of these endomembrane channels, with a focus on their subcellular localizations, ion permeation properties, gating mechanisms, cell biological functions, and disease relevance.

Cancer-associated muscle weakness - From triggers to molecular mechanisms

Skeletal muscle weakness is a debilitating consequence of many malignancies. Muscle weakness has a negative impact on both patient wellbeing and outcome in a range of cancer types and can be the result of loss of muscle mass (i.e. muscle atrophy, cachexia) and occur independently of muscle atrophy or cachexia. There are multiple cancer specific triggers that can initiate the progression of muscle weakness, including the malignancy itself and the tumour environment, as well as chemotherapy, radiotherapy and malnutrition. This can induce weakness via different routes: 1) impaired intrinsic capacity (i.e., contractile dysfunction and intramuscular impairments in excitation-contraction coupling or crossbridge cycling), 2) neuromuscular disconnection and/or 3) muscle atrophy. The mechanisms that underlie these pathways are a complex interplay of inflammation, autophagy, disrupted protein synthesis/degradation, and mitochondrial dysfunction. The current lack of therapies to treat cancer-associated muscle weakness highlight the critical need for novel interventions (both pharmacological and non-pharmacological) and mechanistic insight. Moreover, most research in the field has placed emphasis on directly improving muscle mass to improve muscle strength. However, accumulating evidence suggests that loss of muscle function precedes atrophy. This review primarily focuses on cancer-associated muscle weakness, independent of cachexia, and provides a solid background on the underlying mechanisms, methodology, current interventions, gaps in knowledge, and limitations of research in the field. Moreover, we have performed a mini-systematic review of recent research into the mechanisms behind muscle weakness in specific cancer types, along with the main pathways implicated.

Disrupted Endoplasmic Reticulum Ca2+ Handling: A Harβinger of β-Cell Failure

The β-cell workload increases in the setting of insulin resistance and reduced β-cell mass, which occurs in type 2 and type 1 diabetes, respectively. The prolonged elevation of insulin production and secretion during the pathogenesis of diabetes results in β-cell ER stress. The depletion of β-cell Ca2+ER during ER stress activates the unfolded protein response, leading to β-cell dysfunction. Ca2+ER is involved in many pathways that are critical to β-cell function, such as protein processing, tuning organelle and cytosolic Ca2+ handling, and modulating lipid homeostasis. Mutations that promote β-cell ER stress and deplete Ca2+ER stores are associated with or cause diabetes (e.g., mutations in ryanodine receptors and insulin). Thus, improving β-cell Ca2+ER handling and reducing ER stress under diabetogenic conditions could preserve β-cell function and delay or prevent the onset of diabetes. This review focuses on how mechanisms that control β-cell Ca2+ER are perturbed during the pathogenesis of diabetes and contribute to β-cell failure.

Interplay between Mg2+ and Ca2+ at multiple sites of the ryanodine receptor

RyR1 is an intracellular Ca2+ channel important in excitable cells such as neurons and muscle fibers. Ca2+ activates it at low concentrations and inhibits it at high concentrations. Mg2+ is the main physiological RyR1 inhibitor, an effect that is overridden upon activation. Despite the significance of Mg2+-mediated inhibition, the molecular-level mechanisms remain unclear. In this work we determined two cryo-EM structures of RyR1 with Mg2+ up to 2.8 Å resolution, identifying multiple Mg2+ binding sites. Mg2+ inhibits at the known Ca2+ activating site and we propose that the EF hand domain is an inhibitory divalent cation sensor. Both divalent cations bind to ATP within a crevice, contributing to the precise transmission of allosteric changes within the enormous channel protein. Notably, Mg2+ inhibits RyR1 by interacting with the gating helices as validated by molecular dynamics. This structural insight enhances our understanding of how Mg2+ inhibition is overcome during excitation.

The Pentameric Ligand-Gated Ion Channel Family: A New Member of the Voltage Gated Ion Channel Superfamily?

In this report we present seven lines of bioinformatic evidence supporting the conclusion that the Pentameric Ligand-gated Ion Channel (pLIC) Family is a member of the Voltage-gated Ion Channel (VIC) Superfamily. In our approach, we used the Transporter Classification Database (TCDB) as a reference and applied a series of bioinformatic methods to search for similarities between the pLIC family and members of the VIC superfamily. These include: (1) sequence similarity, (2) compatibility of topology and hydropathy profiles, (3) shared domains, (4) conserved motifs, (5) similarity of Hidden Markov Model profiles between families, (6) common 3D structural folds, and (7) clustering analysis of all families. Furthermore, sequence and structural comparisons as well as the identification of a 3-TMS repeat unit in the VIC superfamily suggests that the sixth transmembrane segment evolved into a re-entrant loop. This evidence suggests that the voltage-sensor domain and the channel domain have a common origin. The classification of the pLIC family within the VIC superfamily sheds light onto the topological origins of this family and its evolution, which will facilitate experimental verification and further research into this superfamily by the scientific community.

Skeletal muscle differentiation induces wide-ranging nucleosome repositioning in muscle gene promoters

In a previous report, we demonstrated that Cbx1, PurB and Sp3 inhibited cardiac muscle differentiation by increasing nucleosome density around cardiac muscle gene promoters. Since cardiac and skeletal muscle express many of the same proteins, we asked if Cbx1, PurB and Sp3 similarly regulated skeletal muscle differentiation. In a C2C12 model of skeletal muscle differentiation, Cbx1 and PurB knockdown increased myotube formation. In contrast, Sp3 knockdown inhibited myotube formation, suggesting that Sp3 played opposing roles in cardiac muscle and skeletal muscle differentiation. Consistent with this finding, Sp3 knockdown also inhibited various muscle-specific genes. The Cbx1, PurB and Sp3 proteins are believed to influence gene-expression in part by altering nucleosome position. Importantly, we developed a statistical approach to determine if changes in nucleosome positioning were significant and applied it to understanding the architecture of muscle-specific genes. Through this novel statistical approach, we found that during myogenic differentiation, skeletal muscle-specific genes undergo a set of unique nucleosome changes which differ significantly from those shown in commonly expressed muscle genes. While Sp3 binding was associated with nucleosome loss, there appeared no correlation with the aforementioned nucleosome changes. In summary, we have identified a novel role for Sp3 in skeletal muscle differentiation and through the application of quantifiable MNase-seq have discovered unique fingerprints of nucleosome changes for various classes of muscle genes during myogenic differentiation.

Phosphorylation of RyR2 simultaneously expands the dyad and rearranges the tetramers

We have previously demonstrated that type II ryanodine receptors (RyR2) tetramers can be rapidly rearranged in response to a phosphorylation cocktail. The cocktail modified downstream targets indiscriminately, making it impossible to determine whether phosphorylation of RyR2 was an essential element of the response. Here, we used the β-agonist isoproterenol and mice homozygous for one of the following clinically relevant mutations: S2030A, S2808A, S2814A, or S2814D. We measured the length of the dyad using transmission electron microscopy (TEM) and directly visualized RyR2 distribution using dual-tilt electron tomography. We found that the S2814D mutation, by itself, significantly expanded the dyad and reorganized the tetramers, suggesting a direct link between the phosphorylation state of the tetramer and its microarchitecture. S2808A and S2814A mutant mice, as well as wild types, had significant expansions of their dyads in response to isoproterenol, while S2030A mutants did not. In agreement with functional data from these mutants, S2030 and S2808 were necessary for a complete β-adrenergic response, unlike S2814 mutants. Additionally, all mutants had unique effects on the organization of their tetramer arrays. Lastly, the correlation of structural with functional changes suggests that tetramer-tetramer contacts play an important functional role. We thus conclude that both the size of the dyad and the arrangement of the tetramers are linked to the state of the channel tetramer and can be dynamically altered by a β-adrenergic receptor agonist.

RyR2-dependent modulation of neuronal hyperactivity: A potential therapeutic target for treating Alzheimer's disease

Increasing evidence suggests that simply reducing β-amyloid (Aβ) plaques may not significantly affect the progression of Alzheimer's disease (AD). There is also increasing evidence indicating that AD progression is driven by a vicious cycle of soluble Aβ-induced neuronal hyperactivity. In support of this, it has recently been shown that genetically and pharmacologically limiting ryanodine receptor 2 (RyR2) open time prevents neuronal hyperactivity, memory impairment, dendritic spine loss and neuronal cell death in AD mouse models. By contrast, increased RyR2 open probability (Po) exacerbates the onset of familial AD-associated neuronal dysfunction and induces AD-like defects in the absence of AD-causing gene mutations. Thus, RyR2-dependent modulation of neuronal hyperactivity represents a promising new target for combating AD.

Targeting mitochondrial Ca2+ uptake for the treatment of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a rare adult-onset neurodegenerative disease characterized by progressive motor neuron (MN) loss, muscle denervation and paralysis. Over the past several decades, researchers have made tremendous efforts to understand the pathogenic mechanisms underpinning ALS, with much yet to be resolved. ALS is described as a non-cell autonomous condition with pathology detected in both MNs and non-neuronal cells, such as glial cells and skeletal muscle. Studies in ALS patient and animal models reveal ubiquitous abnormalities in mitochondrial structure and function, and disturbance of intracellular calcium homeostasis in various tissue types, suggesting a pivotal role of aberrant mitochondrial calcium uptake and dysfunctional calcium signalling cascades in ALS pathogenesis. Calcium signalling and mitochondrial dysfunction are intricately related to the manifestation of cell death contributing to MN loss and skeletal muscle dysfunction. In this review, we discuss the potential contribution of intracellular calcium signalling, particularly mitochondrial calcium uptake, in ALS pathogenesis. Functional consequences of excessive mitochondrial calcium uptake and possible therapeutic strategies targeting mitochondrial calcium uptake or the mitochondrial calcium uniporter, the main channel mediating mitochondrial calcium influx, are also discussed.

S-acylation of Ca2+ transport proteins: molecular basis and functional consequences

Calcium (Ca2+) regulates a multitude of cellular processes during fertilization and throughout adult life by acting as an intracellular messenger to control effector functions in excitable and non-excitable cells. Changes in intracellular Ca2+ levels are driven by the co-ordinated action of Ca2+ channels, pumps, and exchangers, and the resulting signals are shaped and decoded by Ca2+-binding proteins to drive rapid and long-term cellular processes ranging from neurotransmission and cardiac contraction to gene transcription and cell death. S-acylation, a lipid post-translational modification, is emerging as a critical regulator of several important Ca2+-handling proteins. S-acylation is a reversible and dynamic process involving the attachment of long-chain fatty acids (most commonly palmitate) to cysteine residues of target proteins by a family of 23 proteins acyltransferases (zDHHC, or PATs). S-acylation modifies the conformation of proteins and their interactions with membrane lipids, thereby impacting intra- and intermolecular interactions, protein stability, and subcellular localization. Disruptions of S-acylation can alter Ca2+ signalling and have been implicated in the development of pathologies such as heart disease, neurodegenerative disorders, and cancer. Here, we review the recent literature on the S-acylation of Ca2+ transport proteins of organelles and of the plasma membrane and highlight the molecular basis and functional consequence of their S-acylation as well as the therapeutic potential of targeting this regulation for diseases caused by alterations in cellular Ca2+ fluxes.

Contractile and Genetic Characterization of Cardiac Constructs Engineered from Human Induced Pluripotent Stem Cells: Modeling of Tuberous Sclerosis Complex and the Effects of Rapamycin

The implementation of three-dimensional tissue engineering concurrently with stem cell technology holds great promise for in vitro research in pharmacology and toxicology and modeling cardiac diseases, particularly for rare genetic and pediatric diseases for which animal models, immortal cell lines, and biopsy samples are unavailable. It also allows for a rapid assessment of phenotype-genotype relationships and tissue response to pharmacological manipulation. Mutations in the TSC1 and TSC2 genes lead to dysfunctional mTOR signaling and cause tuberous sclerosis complex (TSC), a genetic disorder that affects multiple organ systems, principally the brain, heart, skin, and kidneys. Here we differentiated healthy (CC3) and tuberous sclerosis (TSP8-15) human induced pluripotent stem cells (hiPSCs) into cardiomyocytes to create engineered cardiac tissue constructs (ECTCs). We investigated and compared their mechano-elastic properties and gene expression and assessed the effects of rapamycin, a potent inhibitor of the mechanistic target of rapamycin (mTOR). The TSP8-15 ECTCs had increased chronotropy compared to healthy ECTCs. Rapamycin induced positive inotropic and chronotropic effects (i.e., increased contractility and beating frequency, respectively) in the CC3 ECTCs but did not cause significant changes in the TSP8-15 ECTCs. A differential gene expression analysis revealed 926 up- and 439 down-regulated genes in the TSP8-15 ECTCs compared to their healthy counterparts. The application of rapamycin initiated the differential expression of 101 and 31 genes in the CC3 and TSP8-15 ECTCs, respectively. A gene ontology analysis showed that in the CC3 ECTCs, the positive inotropic and chronotropic effects of rapamycin correlated with positively regulated biological processes, which were primarily related to the metabolism of lipids and fatty and amino acids, and with negatively regulated processes, which were predominantly associated with cell proliferation and muscle and tissue development. In conclusion, this study describes for the first time an in vitro TSC cardiac tissue model, illustrates the response of normal and TSC ECTCs to rapamycin, and provides new insights into the mechanisms of TSC.

Rycal S48168 (ARM210) for RYR1-related myopathies: a phase one, open-label, dose-escalation trial

Background

RYR1-related myopathies (RYR1-RM) are caused by pathogenic variants in the RYR1 gene which encodes the type 1 ryanodine receptor (RyR1). RyR1 is the sarcoplasmic reticulum (SR) calcium release channel that mediates excitation-contraction coupling in skeletal muscle. RyR1 sub-conductance, SR calcium leak, reduced RyR1 expression, and oxidative stress often contribute to RYR1-RM pathogenesis. Loss of RyR1-calstabin1 association, SR calcium leak, and increased RyR1 open probability were observed in 17 RYR1-RM patient skeletal muscle biopsies and improved following ex vivo treatment with Rycal compounds. Thus, we initiated a first-in-patient trial of Rycal S48168 (ARM210) in ambulatory adults with genetically confirmed RYR1-RM.

Methods

Participants received 120 mg (n = 3) or 200 mg (n = 4) S48168 (ARM210) daily for 29 days. The primary endpoint was safety and tolerability. Exploratory endpoints included S48168 (ARM210) pharmacokinetics (PK), target engagement, motor function measure (MFM)-32, hand grip and pinch strength, timed functional tests, PROMIS fatigue scale, semi-quantitative physical exam strength measurements, and oxidative stress biomarkers. The trial was registered with clinicaltrials.gov (NCT04141670) and was conducted at the National Institutes of Health Clinical Center between October 28, 2019 and December 12, 2021.

Findings

S48168 (ARM210) was well-tolerated, did not cause any serious adverse events, and exhibited a dose-dependent PK profile. Three of four participants who received the 200 mg/day dose reported improvements in PROMIS-fatigue at 28 days post-dosing, and also demonstrated improved proximal muscle strength on physical examination.

Interpretation

S48168 (ARM210) demonstrated favorable safety, tolerability, and PK, in RYR1-RM affected individuals. Most participants who received 200 mg/day S48168 (ARM210) reported decreased fatigue, a key symptom of RYR1-RM. These results set the foundation for a randomized, double-blind, placebo-controlled proof of concept trial to determine efficacy of S48168 (ARM210) in RYR1-RM.

Funding

NINDS and NINR Intramural Research Programs, NIH Clinical Center Bench to Bedside Award (2017-551673), ARMGO Pharma Inc., and its development partner Les Laboratoires Servier.

Optimization of CHARMM force field parameters for ryanodine receptor inhibitory drug dantrolene using FFTK and FFParam

CONTEXT

Ryanodine receptors (RyRs) are large intracellular ligand-gated calcium release ion channels. Mutations in human RyR1 in combination with a volatile anesthetic or muscle relaxant are known to cause leaky RyRs resulting in malignant hyperthermia (MH). This has long been primarily treated with the RyR inhibitory drug dantrolene. Alternatives to dantrolene as a RyR inhibitor may be found through computer-aided drug design. Additionally, molecular dynamics (MD) studies of dantrolene interacting with RyRs may reveal its full mechanism of action. The availability of accurate force field parameters is important for the success of both.

METHODS

In this study, force field parameters for dantrolene were obtained from the CHARMM General Force Field (CGenFF) program and optimized using the force field toolkit (FFTK) and FFParam programs. The obtained parameters were then validated by a comparison between calculated and experimental IR spectra and normal mode analysis, among other techniques.

Basic science of cardiac contractility modulation therapy: Molecular and electrophysiological mechanisms

In heart failure with reduced ejection fraction and heart failure with preserved ejection fraction, profound cellular and molecular changes have recently been documented in the failing myocardium. These changes include altered calcium handling and metabolic efficiency of the cardiac myocyte, reactivation of the fetal gene program, changes in the electrophysiological properties of the heart, and accumulation of collagen (fibrosis) at the interstitial level. Cardiac contractility modulation therapy is an innovative device-based therapy currently approved for heart failure with reduced ejection fraction in patients with narrow QRS complex and under investigation for the treatment of heart failure with preserved ejection fraction. This therapy is based on the delivery of high-voltage biphasic electrical signals to the septal wall of the right ventricle during the absolute refractory period of the myocardium. At the cellular level, in patients with heart failure with reduced ejection fraction, cardiac contractility modulation therapy has been shown to restore calcium handling and improve the metabolic status of cardiac myocytes, reverse the heart failure-associated fetal gene program, and reduce the extent of interstitial fibrosis. This review summarizes the preclinical literature on the use of cardiac contractility modulation therapy in heart failure with reduced and preserved ejection fraction, correlating the molecular and electrophysiological effects with the clinical benefits demonstrated by this therapy.

RyR-mediated calcium release in hippocampal health and disease

Hippocampal synaptic plasticity is widely considered the cellular basis of learning and spatial memory processes. This article highlights the central role of Ca2+ release from the endoplasmic reticulum (ER) in hippocampal synaptic plasticity and hippocampus-dependent memory in health and disease. The key participation of ryanodine receptor (RyR) channels, which are the principal Ca2+ release channels expressed in the hippocampus, in these processes is emphasized. It is proposed that the increased neuronal oxidative tone displayed by hippocampal neurons during aging or Alzheimer's disease (AD) leads to excessive activation of RyR-mediated Ca2+ release, a process that is highly redox-sensitive, and that this abnormal response contributes to and aggravates these deleterious conditions.