Characterization and temporal development of cores in a mouse model of malignant hyperthermia

A novel, patient-derived RyR1 mutation impairs muscle function and calcium homeostasis in mice

RYR1 is the most commonly mutated gene associated with congenital myopathies, a group of early-onset neuromuscular conditions of variable severity. The functional effects of a number of dominant RYR1 mutations have been established; however, for recessive mutations, these effects may depend on multiple factors, such as the formation of a hypomorphic allele, or on whether they are homozygous or compound heterozygous. Here, we functionally characterize a new transgenic mouse model knocked-in for mutations identified in a severely affected child born preterm and presenting limited limb movement. The child carried the homozygous c.14928C>G RYR1 mutation, resulting in the p.F4976L substitution. In vivo and ex vivo assays revealed that homozygous mice fatigued sooner and their muscles generated significantly less force compared with their WT or heterozygous littermates. Electron microscopy, biochemical, and physiological analyses showed that muscles from RyR1 p.F4976L homozygous mice have the following properties: (1) contain fewer calcium release units and show areas of myofibrillar degeneration, (2) contain less RyR1 protein, (3) fibers show smaller electrically evoked calcium transients, and (4) their SR has smaller calcium stores. In addition, single-channel recordings indicate that RyR1 p.F4976L exhibits higher Po in the presence of 100 μM [Ca2+]. Our mouse model partly recapitulates the clinical picture of the homozygous human patient and provides significant insight into the functional impact of this mutation. These results will help understand the pathology of patients with similar RYR1 mutations.

Characterization of differential distribution patterns between mitofusin isoforms and their interaction in developing skeletal muscles of rat

Skeletal muscle during postnatal development undergoes several structural and biochemical modifications. It is proposed that these changes are closely intertwined with the increase in load-bearing capacity of the muscle (i.e., myofibrils) and molecular machinery to support the energy demand (i.e., mitochondria). Concomitant establishment of the sarcoplasmic reticulum (SR) and mitochondrial network seems to be a major developmental adjustment of skeletal muscle leading to adult phenotype. Here, we have studied oxidativeness, vascularization, and the changes in mitofusins (Mfn) 1-Mfn 2 expression and interaction in the due course of muscle development. Toward this, we used a series of histochemical techniques to compare neonatal and adult limb muscles (Gastrocnemius and Quadriceps) of Wistar rat (Rattus norvegicus). Additionally, we probed the proximity between Mfn 1 and Mfn 2 using a highly sensitive antibody-based proximity ligation assay indicating the change in mitochondrial fusion pattern or mitochondria-SR interaction. The results show that neonatal fibers bear a uniform distribution of mitochondria while a differential pattern of distribution is seen in adults. The distribution of the blood vessels is also quite distinct in adult muscles with a well-formed capillary network but in neonates, only central blood vessels are seen. Interestingly, our Mfn 1-Mfn 2 interaction data show that this interaction is uniformly distributed throughout the neonatal fibers, while it becomes peripherally localized in fibers of adult muscles. This peripheralization of Mfn 1-Mfn 2 interaction must be an important event of muscle development and might be critical to cater to the metabolic needs of adult muscle.

Cytosolic Ca2+ gradients and mitochondrial Ca2+ uptake in resting muscle fibers: A model analysis

Calcium ions (Ca2+) enter mitochondria via the mitochondrial Ca2+ uniporter, driven by electrical and concentration gradients. In this regard, transgenic mouse models, such as calsequestrin knockout (CSQ-KO) mice, with higher mitochondrial Ca2+ concentrations ([Ca2+]mito), should display higher cytosolic Ca2+ concentrations ([Ca2+]cyto). However, repeated measurements of [Ca2+]cyto in quiescent CSQ-KO fibers never showed a difference between WT and CSQ-KO. Starting from the consideration that fluorescent Ca2+ probes (Fura-2 and Indo-1) measure averaged global cytosolic concentrations, in this report we explored the role of local Ca2+ concentrations (i.e., Ca2+ microdomains) in regulating mitochondrial Ca2+ in resting cells, using a multicompartmental diffusional Ca2+ model. Progressively including the inward and outward fluxes of sarcoplasmic reticulum (SR), extracellular space, and mitochondria, we explored their contribution to the local Ca2+ distribution within the cell. The model predicts Ca2+ concentration gradients with hot spots or microdomains even at rest, minor but similar to those of evoked Ca2+ release. Due to their specific localization close to Ca2+ release units (CRU), mitochondria could take up Ca2+ directly from high-concentration microdomains, thus sensibly raising [Ca2+]mito, despite minor, possibly undetectable, modifications of the average [Ca2+]cyto.

Mitochondrial Properties in Skeletal Muscle Fiber

Mitochondria are the primary source of energy production and are implicated in a wide range of biological processes in most eukaryotic cells. Skeletal muscle heavily relies on mitochondria for energy supplements. In addition to being a powerhouse, mitochondria evoke many functions in skeletal muscle, including regulating calcium and reactive oxygen species levels. A healthy mitochondria population is necessary for the preservation of skeletal muscle homeostasis, while mitochondria dysregulation is linked to numerous myopathies. In this review, we summarize the recent studies on mitochondria function and quality control in skeletal muscle, focusing mainly on in vivo studies of rodents and human subjects. With an emphasis on the interplay between mitochondrial functions concerning the muscle fiber type-specific phenotypes, we also discuss the effect of aging and exercise on the remodeling of skeletal muscle and mitochondria properties.

Defective excitation-contraction coupling and mitochondrial respiration precede mitochondrial Ca2+ accumulation in spinobulbar muscular atrophy skeletal muscle

Polyglutamine expansion in the androgen receptor (AR) causes spinobulbar muscular atrophy (SBMA). Skeletal muscle is a primary site of toxicity; however, the current understanding of the early pathological processes that occur and how they unfold during disease progression remains limited. Using transgenic and knock-in mice and patient-derived muscle biopsies, we show that SBMA mice in the presymptomatic stage develop a respiratory defect matching defective expression of genes involved in excitation-contraction coupling (ECC), altered contraction dynamics, and increased fatigue. These processes are followed by stimulus-dependent accumulation of calcium into mitochondria and structural disorganization of the muscle triads. Deregulation of expression of ECC genes is concomitant with sexual maturity and androgen raise in the serum. Consistent with the androgen-dependent nature of these alterations, surgical castration and AR silencing alleviate the early and late pathological processes. These observations show that ECC deregulation and defective mitochondrial respiration are early but reversible events followed by altered muscle force, calcium dyshomeostasis, and dismantling of triad structure.

Neuromuscular symptoms in patients with RYR1-related malignant hyperthermia and rhabdomyolysis

Malignant hyperthermia and exertional rhabdomyolysis have conventionally been considered episodic phenotypes that occur in otherwise healthy individuals in response to an external trigger. However, recent studies have demonstrated a clinical and histopathological continuum between patients with a history of malignant hyperthermia susceptibility and/or exertional rhabdomyolysis and RYR1-related congenital myopathies. We hypothesize that patients with a history of RYR1-related exertional rhabdomyolysis or malignant hyperthermia susceptibility do have permanent neuromuscular symptoms between malignant hyperthermia or exertional rhabdomyolysis episodes. We performed a prospective cross-sectional observational clinical study of neuromuscular features in patients with a history of RYR1-related exertional rhabdomyolysis and/or malignant hyperthermia susceptibility (n = 40) compared with healthy controls (n = 80). Patients with an RYR1-related congenital myopathy, manifesting as muscle weakness preceding other symptoms as well as other (neuromuscular) diseases resulting in muscle weakness were excluded. Study procedures included a standardized history of neuromuscular symptoms, a review of all relevant ancillary diagnostic tests performed up to the point of inclusion and a comprehensive, standardized neuromuscular assessment. Results of the standardized neuromuscular history were compared with healthy controls. Results of the neuromuscular assessment were compared with validated reference values. The proportion of patients suffering from cramps (P < 0.001), myalgia (P < 0.001) and exertional myalgia (P < 0.001) was higher compared with healthy controls. Healthcare professionals were consulted because of apparent neuromuscular symptoms by 17/40 (42.5%) patients and 7/80 (8.8%) healthy controls (P < 0.001). Apart from elevated creatine kinase levels in 19/40 (47.5%) patients and mild abnormalities on muscle biopsies identified in 13/16 (81.3%), ancillary investigations were normal in most patients. The Medical Research Council sum score, spirometry and results of functional measurements were also mostly normal. Three of 40 patients (7.5%) suffered from late-onset muscle weakness, most prominent in the proximal lower extremity muscles. Patients with RYR1 variants resulting in malignant hyperthermia susceptibility and/or exertional rhabdomyolysis frequently report additional neuromuscular symptoms such as myalgia and muscle cramps compared with healthy controls. These symptoms result in frequent consultation of healthcare professionals and sometimes in unnecessary invasive diagnostic procedures. Most patients do have normal strength at a younger age but may develop muscle weakness later in life.

Case report: Unusual episodic myopathy in a patient with novel homozygous deletion of first coding exon of MICU1 gene

We present a patient with unusual episodes of muscular weakness due to homozygous deletion of exon 2 in the MICU1 gene. Forty-three patients from 33 families were previously described with homozygous and compound heterozygous, predominantly loss of function (LoF) variants in the MICU1 gene that lead to autosomal recessive myopathy with extrapyramidal signs. Most described patients developed muscle weakness and elevated CK levels, and half of the patients had progressive extrapyramidal signs and learning disabilities. Our patient had a few severe acute episodes of muscle weakness with minimal myopathy features between episodes and a strongly elevated Creatinine Kinase (CK). Whole exome sequencing (WES) was performed and the homozygous deletion of exon 2 was suspected. To validate the diagnosis, we performed an RNA analysis of all family members. To investigate the possible impact of this deletion on the phenotype, we predicted a new Kozak sequence in exon 4 that could lead to the formation of a truncated MICU1 protein that could partly interact with MCU protein in a mitochondrial Ca2+ complex. We suspect that this unusual phenotype of the proband with MICU1-related myopathy could be explained by the presence of the truncated but partly functional protein. This work helps to define the clinical polymorphism of MICU1 deficiency better.

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.

Ablation of Calsequestrin-1, Ca2+ unbalance, and susceptibility to heat stroke

Introduction: Ca2+ levels in adult skeletal muscle fibers are mainly controlled by excitation-contraction (EC) coupling, a mechanism that translates action potentials in release of Ca2+ from the sarcoplasmic reticulum (SR) release channels, i.e. the ryanodine receptors type-1 (RyR1). Calsequestrin (Casq) is a protein that binds large amounts of Ca2+ in the lumen of the SR terminal cisternae, near sites of Ca2+ release. There is general agreement that Casq is not only important for the SR ability to store Ca2+, but also for modulating the opening probability of the RyR Ca2+ release channels. The initial studies: About 20 years ago we generated a mouse model lacking Casq1 (Casq1-null mice), the isoform predominantly expressed in adult fast twitch skeletal muscle. While the knockout was not lethal as expected, lack of Casq1 caused a striking remodeling of membranes of SR and of transverse tubules (TTs), and mitochondrial damage. Functionally, CASQ1-knockout resulted in reduced SR Ca2+ content, smaller Ca2+ transients, and severe SR depletion during repetitive stimulation. The myopathic phenotype of Casq1-null mice: After the initial studies, we discovered that Casq1-null mice were prone to sudden death when exposed to halogenated anaesthetics, heat and even strenuous exercise. These syndromes are similar to human malignant hyperthermia susceptibility (MHS) and environmental-exertional heat stroke (HS). We learned that mechanisms underlying these syndromes involved excessive SR Ca2+ leak and excessive production of oxidative species: indeed, mortality and mitochondrial damage were significantly prevented by administration of antioxidants and reduction of oxidative stress. Though, how Casq1-null mice could survive without the most important SR Ca2+ binding protein was a puzzling issue that was not solved. Unravelling the mystery: The mystery was finally solved in 2020, when we discovered that in Casq1-null mice the SR undergoes adaptations that result in constitutively active store-operated Ca2+ entry (SOCE). SOCE is a mechanism that allows skeletal fibers to use external Ca2+ when SR stores are depleted. The post-natal compensatory mechanism that allows Casq1-null mice to survive involves the assembly of new SR-TT junctions (named Ca2+ entry units) containing Stim1 and Orai1, the two proteins that mediate SOCE.

Role of mitochondria-associated endoplasmic reticulum membranes in insulin sensitivity, energy metabolism, and contraction of skeletal muscle

Skeletal muscle has a critical role in the regulation of the energy balance of the organism, particularly as the principal tissue responsible for insulin-stimulated glucose disposal and as the major site of peripheral insulin resistance (IR), which has been related to accumulation of lipid intermediates, reduced oxidative capacity of mitochondria and endoplasmic reticulum (ER) stress. These organelles form contact sites, known as mitochondria-associated ER membranes (MAMs). This interconnection seems to be involved in various cellular processes, including Ca²⁺ transport and energy metabolism; therefore, MAMs could play an important role in maintaining cellular homeostasis. Evidence suggests that alterations in MAMs may contribute to IR. However, the evidence does not refer to a specific subcellular location, which is of interest due to the fact that skeletal muscle is constituted by oxidative and glycolytic fibers as well as different mitochondrial populations that appear to respond differently to stimuli and pathological conditions. In this review, we show the available evidence of possible differential responses in the formation of MAMs in skeletal muscle as well as its role in insulin signaling and the beneficial effect it could have in the regulation of energetic metabolism and muscular contraction.

Mutations in proteins involved in E-C coupling and SOCE and congenital myopathies

In skeletal muscle, Ca²⁺ necessary for muscle contraction is stored and released from the sarcoplasmic reticulum (SR), a specialized form of endoplasmic reticulum through the mechanism known as excitation–contraction (E-C) coupling. Following activation of skeletal muscle contraction by the E-C coupling mechanism, replenishment of intracellular stores requires reuptake of cytosolic Ca²⁺ into the SR by the activity of SR Ca²⁺-ATPases, but also Ca²⁺ entry from the extracellular space, through a mechanism called store-operated calcium entry (SOCE). The fine orchestration of these processes requires several proteins, including Ca²⁺ channels, Ca²⁺ sensors, and Ca²⁺ buffers, as well as the active involvement of mitochondria. Mutations in genes coding for proteins participating in E-C coupling and SOCE are causative of several myopathies characterized by a wide spectrum of clinical phenotypes, a variety of histological features, and alterations in intracellular Ca²⁺ balance. This review summarizes current knowledge on these myopathies and discusses available knowledge on the pathogenic mechanisms of disease.

Oxygen Consumption and Basal Metabolic Rate as Markers of Susceptibility to Malignant Hyperthermia and Heat Stroke

Calsequestrin 1 (CASQ1) and Ryanodine receptor 1 (RYR1) are two of the main players in excitation-contraction (EC) coupling. CASQ1-knockout mice and mice carrying a mutation in RYR1 (Y522S) linked to human malignant hyperthermia susceptibility (MHS) both suffer lethal hypermetabolic episodes when exposed to halothane (MHS crises) and to environmental heat (heat stroke, HS). The phenotype of Y522S is more severe than that of CASQ1-null mice. As MHS and HS are hypermetabolic responses, we studied the metabolism of adult CASQ1-null and Y522S mice using wild-type (WT) mice as controls. We found that CASQ1-null and Y522S mice have increased food consumption and higher core temperature at rest. By indirect calorimetry, we then verified that CASQ1-null and Y522S mice show an increased oxygen consumption and a lower respiratory quotient (RQ). The accelerated metabolism of CASQ1-null and Y522S mice was also accompanied with a reduction in body fat. Moreover, both mouse models displayed increased oxygen consumption and a higher core temperature during heat stress. The results collected suggest that metabolic rate, oxygen consumption, and body temperature at rest, all more elevated in Y522S than in CASQ1-null mice, could possibly be used as predictors of the level of susceptibility to hyperthermic crises of mice (and possibly humans).

The Sarcoplasmic Reticulum of Skeletal Muscle Cells: A Labyrinth of Membrane Contact Sites

The sarcoplasmic reticulum of skeletal muscle cells is a highly ordered structure consisting of an intricate network of tubules and cisternae specialized for regulating Ca2+ homeostasis in the context of muscle contraction. The sarcoplasmic reticulum contains several proteins, some of which support Ca2+ storage and release, while others regulate the formation and maintenance of this highly convoluted organelle and mediate the interaction with other components of the muscle fiber. In this review, some of the main issues concerning the biology of the sarcoplasmic reticulum will be described and discussed; particular attention will be addressed to the structure and function of the two domains of the sarcoplasmic reticulum supporting the excitation-contraction coupling and Ca2+-uptake mechanisms.

Ryanodine receptors mediate high intracellular Ca2+ and some myocyte damage following eccentric contractions in rat fast-twitch skeletal muscle

Eccentric contractions (ECC) facilitate cytosolic calcium ion (Ca2+) release from the sarcoplasmic reticulum (SR) and Ca2+ influx from the extracellular space. Ca2+ is a vital signaling messenger that regulates multiple cellular processes via its spatial and temporal concentration ([Ca2+]i) dynamics. We hypothesized that 1) a specific pattern of spatial/temporal intramyocyte Ca2+ dynamics portends muscle damage following ECC and 2) these dynamics would be regulated by the ryanodine receptor (RyR). [Ca2+]i in the tibialis anterior muscles of anesthetized adult Wistar rats was measured by ratiometric (i.e., ratio, R, 340/380 nm excitation) in vivo bioimaging with Fura-2 pre-ECC and at 5 and 24 h post-ECC (5 × 40 contractions). Separate groups of rats received RyR inhibitor dantrolene (DAN; 10 mg/kg ip) immediately post-ECC (+DAN). Muscle damage was evaluated by histological analysis on hematoxylin-eosin stained muscle sections. Compared with control (CONT, no ECC), [Ca2+]i distribution was heterogeneous with increased percent total area of high [Ca2+]i sites (operationally defined as R ≥ 1.39, i.e., ≥1 SD of mean control) 5 h post-ECC (CONT, 14.0 ± 8.0; ECC5h: 52.0 ± 7.4%, P < 0.01). DAN substantially reduced the high [Ca2+]i area 5 h post-ECC (ECC5h + DAN: 6.4 ± 3.1%, P < 0.01) and myocyte damage (ECC24h, 63.2 ± 1.0%; ECC24h + DAN: 29.1 ± 2.2%, P < 0.01). Temporal and spatially amplified [Ca2+]i fluctuations occurred regardless of DAN (ECC vs. ECC + DAN, P > 0.05). These results suggest that the RyR-mediated local high [Ca2+]i itself is related to the magnitude of muscle damage, whereas the [Ca2+]i fluctuation is an RyR-independent phenomenon.

Endothelial cells from umbilical cord of women affected by gestational diabetes: A suitable in vitro model to study mechanisms of early vascular senescence in diabetes

Human umbilical cord endothelial cells (HUVECs) obtained from women affected by gestational diabetes (GD-HUVECs) display durable pro-atherogenic modifications and might be considered a valid in vitro model for studying chronic hyperglycemia effects on early endothelial senescence. Here, we demonstrated that GD- compared to C-HUVECs (controls) exhibited oxidative stress, altered both mitochondrial membrane potential and antioxidant response, significant increase of senescent cells characterized by a reduced NAD-dependent deacetylase sirtuin-1 (SIRT1) activity together with an increase in cyclin-dependent kinase inhibitor-2A (P16), cyclin-dependent kinase inhibitor-1 (P21), and tumor protein p53 (P53) acetylation. This was associated with the p300 activation, and its silencing significantly reduced the GD-HUVECs increased protein levels of P300 and Ac-P53 thus indicating a persistent endothelial senescence via SIRT1/P300/P53/P21 pathway. Overall, our data suggest that GD-HUVECs can represent an "endothelial hyperglycemic memory" model to investigate in vitro the early endothelium senescence in cells chronically exposed to hyperglycemia in vivo.

Altered Ca2+ Handling and Oxidative Stress Underlie Mitochondrial Damage and Skeletal Muscle Dysfunction in Aging and Disease

Skeletal muscle contraction relies on both high-fidelity calcium (Ca²⁺) signals and robust capacity for adenosine triphosphate (ATP) generation. Ca²⁺ release units (CRUs) are highly organized junctions between the terminal cisternae of the sarcoplasmic reticulum (SR) and the transverse tubule (T-tubule). CRUs provide the structural framework for rapid elevations in myoplasmic Ca²⁺ during excitation-contraction (EC) coupling, the process whereby depolarization of the T-tubule membrane triggers SR Ca²⁺ release through ryanodine receptor-1 (RyR1) channels. Under conditions of local or global depletion of SR Ca²⁺ stores, store-operated Ca²⁺ entry (SOCE) provides an additional source of Ca²⁺ that originates from the extracellular space. In addition to Ca²⁺, skeletal muscle also requires ATP to both produce force and to replenish SR Ca²⁺ stores. Mitochondria are the principal intracellular organelles responsible for ATP production via aerobic respiration. This review provides a broad overview of the literature supporting a role for impaired Ca²⁺ handling, dysfunctional Ca²⁺-dependent production of reactive oxygen/nitrogen species (ROS/RNS), and structural/functional alterations in CRUs and mitochondria in the loss of muscle mass, reduction in muscle contractility, and increase in muscle damage in sarcopenia and a wide range of muscle disorders including muscular dystrophy, rhabdomyolysis, central core disease, and disuse atrophy. Understanding the impact of these processes on normal muscle function will provide important insights into potential therapeutic targets designed to prevent or reverse muscle dysfunction during aging and disease.

Untargeted metabolomics profiling of skeletal muscle samples from malignant hyperthermia susceptible patients

PURPOSE

Malignant hyperthermia (MH) is a potentially fatal hypermetabolic condition triggered by certain anesthetics and caused by defective calcium homeostasis in skeletal muscle cells. Recent evidence has revealed impairment of various biochemical pathways in MH-susceptible patients in the absence of anesthetics. We hypothesized that clinical differences between MH-susceptible and control individuals are reflected in measurable differences in myoplasmic metabolites.

METHODS

We performed metabolomic profiling of skeletal muscle samples from MH-negative (control) individuals and MH-susceptible patients undergoing muscle biopsy for diagnosis of MH susceptibility. Cellular metabolites were extracted from 33 fresh and 87 frozen human muscle samples using solid phase microextraction and Metabolon® untargeted biochemical profiling platforms, respectively. Ultra-performance liquid chromatography-high resolution mass spectrometry was used for metabolite identification and validation, followed by analysis of differences in metabolites between the MH-susceptible and MH-negative groups.

RESULTS

Significant fold-change differences between the MH-susceptible and control groups in metabolites from various pathways were found (P value range: 0.009 to < 0.001). These included accumulation of long chain acylcarnitines, diacylglycerols, phosphoenolpyruvate, histidine pathway metabolites, lysophosphatidylcholine, oxidative stress markers, and phosphoinositols, as well as decreased levels of monoacylglycerols. The results from both analytical platforms were in agreement.

CONCLUSION

This metabolomics study indicates a shift from utilization of carbohydrates towards lipids for energy production in MH-susceptible individuals. This shift may result in inefficiency of beta-oxidation, and increased muscle protein turnover, oxidative stress, and/or lysophosphatidylcholine levels.

The mitochondrial calcium homeostasis orchestra plays its symphony: Skeletal muscle is the guest of honor

Skeletal muscle mitochondria are placed in close proximity of the sarcoplasmic reticulum (SR), the main intracellular Ca²⁺ store. During muscle activity, excitation of sarcolemma and of T-tubule triggers the release of Ca²⁺ from the SR initiating myofiber contraction. The rise in cytosolic Ca²⁺ determines the opening of the mitochondrial calcium uniporter (MCU), the highly selective channel of the inner mitochondrial membrane (IMM), causing a robust increase in mitochondrial Ca²⁺ uptake. The Ca²⁺-dependent activation of TCA cycle enzymes increases the synthesis of ATP required for SERCA activity. Thus, Ca²⁺ is transported back into the SR and cytosolic [Ca²⁺] returns to resting levels eventually leading to muscle relaxation. In recent years, thanks to the molecular identification of MCU complex components, the role of mitochondrial Ca²⁺ uptake in the pathophysiology of skeletal muscle has been uncovered. In this chapter, we will introduce the reader to a general overview of mitochondrial Ca²⁺ accumulation. We will tackle the key molecular players and the cellular and pathophysiological consequences of mitochondrial Ca²⁺ dyshomeostasis. In the second part of the chapter, we will discuss novel findings on the physiological role of mitochondrial Ca²⁺ uptake in skeletal muscle. Finally, we will examine the involvement of mitochondrial Ca²⁺ signaling in muscle diseases.

From Mice to Humans: An Overview of the Potentials and Limitations of Current Transgenic Mouse Models of Major Muscular Dystrophies and Congenital Myopathies

Muscular dystrophies are a group of more than 160 different human neuromuscular disorders characterized by a progressive deterioration of muscle mass and strength. The causes, symptoms, age of onset, severity, and progression vary depending on the exact time point of diagnosis and the entity. Congenital myopathies are rare muscle diseases mostly present at birth that result from genetic defects. There are no known cures for congenital myopathies; however, recent advances in gene therapy are promising tools in providing treatment. This review gives an overview of the mouse models used to investigate the most common muscular dystrophies and congenital myopathies with emphasis on their potentials and limitations in respect to human applications.

Dysregulation of Mitochondrial Ca2+ Uptake and Sarcolemma Repair Underlie Muscle Weakness and Wasting in Patients and Mice Lacking MICU1

Muscle function is regulated by Ca²⁺, which mediates excitation-contraction coupling, energy metabolism, adaptation to exercise, and sarcolemmal repair. Several of these actions rely on Ca²⁺ delivery to the mitochondrial matrix via the mitochondrial Ca²⁺ uniporter, the pore of which is formed by mitochondrial calcium uniporter (MCU). MCU's gatekeeping and cooperative activation are controlled by MICU1. Loss-of-protein mutation in MICU1 causes a neuromuscular disease. To determine the mechanisms underlying the muscle impairments, we used MICU1 patient cells and skeletal muscle-specific MICU1 knockout mice. Both these models show a lower threshold for MCU-mediated Ca²⁺ uptake. Lack of MICU1 is associated with impaired mitochondrial Ca²⁺ uptake during excitation-contraction, aerobic metabolism impairment, muscle weakness, fatigue, and myofiber damage during physical activity. MICU1 deficit compromises mitochondrial Ca²⁺ uptake during sarcolemmal injury, which causes ineffective repair of the damaged myofibers. Thus, dysregulation of mitochondrial Ca²⁺ uptake hampers myofiber contractile function, likely through energy metabolism and membrane repair.

Bioenergetic defects in muscle fibers of RYR1 mutant knock-in mice associated with malignant hyperthermia

Mutations in the skeletal muscle ryanodine receptor gene (RYR1) can cause susceptibility to malignant hyperthermia (MH), a potentially lethal genetic condition triggered by volatile anesthetics. MH is associated with hypermetabolism, which has directed research interest into oxidative phosphorylation and muscle bioenergetics. The most common cause of MH in the United Kingdom is the c.7300G>A RYR1 variant, which is present in ∼16% of MH families. Our study focuses on the MH susceptible G2435R-RYR1 knock-in mouse model, which is the murine equivalent of the human c.7300G>A genotype. Using a combination of transcriptomics, protein expression, and functional analysis, we investigated adult muscle fiber bioenergetics in this mouse model. RNA-Seq data showed reduced expression of genes associated with mitochondria and fatty acid oxidation in RYR1 mutants when compared with WT controls. Mitochondrial function was assessed by measuring oxygen consumption rates in permeabilized muscle fibers. Comparisons between WT and homozygous G2435R-RYR1 mitochondria showed a significant increase in complex I-facilitated oxidative phosphorylation in mutant muscle. Furthermore, we observed a gene-dose-specific increase in reactive oxygen species production in G2435R-RYR1 muscle fibers. Collectively, these findings provide evidence of metabolic defects in G2435R-RYR1 knock-in mouse muscle under basal conditions. Differences in metabolic profile could be the result of differential gene expression in metabolic pathways, in conjunction with mitochondrial damage accumulated from chronic exposure to increased oxidative stress.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Skeletal muscle CaV1.1 channelopathies

Ca_V1.1 is specifically expressed in skeletal muscle where it functions as voltage sensor of skeletal muscle excitation-contraction (EC) coupling independently of its functions as L-type calcium channel. Consequently, all known Ca_V1.1-related diseases are muscle diseases and the molecular and cellular disease mechanisms relate to the dual functions of Ca_V1.1 in this tissue. To date, four types of muscle diseases are known that can be linked to mutations in the CACNA1S gene or to splicing defects. These are hypo- and normokalemic periodic paralysis, malignant hyperthermia susceptibility, Ca_V1.1-related myopathies, and myotonic dystrophy type 1. In addition, the Ca_V1.1 function in EC coupling is perturbed in Native American myopathy, arising from mutations in the Ca_V1.1-associated protein STAC3. Here, we first address general considerations concerning the possible roles of Ca_V1.1 in disease and then discuss the state of the art regarding the pathophysiology of the Ca_V1.1-related skeletal muscle diseases with an emphasis on molecular disease mechanisms.

Mouse model of severe recessive RYR1-related myopathy

Ryanodine receptor type I (RYR1)-related myopathies (RYR1 RM) are a clinically and histopathologically heterogeneous group of conditions that represent the most common subtype of childhood onset non-dystrophic muscle disorders. There are no treatments for this severe group of diseases. A major barrier to therapy development is the lack of an animal model that mirrors the clinical severity of pediatric cases of the disease. To address this, we used CRISPR/Cas9 gene editing to generate a novel recessive mouse model of RYR1 RM. This mouse (Ryr1TM/Indel) possesses a patient-relevant point mutation (T4706M) engineered into 1 allele and a 16 base pair frameshift deletion engineered into the second allele. Ryr1TM/Indel mice exhibit an overt phenotype beginning at 14 days of age that consists of reduced body/muscle mass and myofibre hypotrophy. Ryr1TM/Indel mice become progressively inactive from that point onward and die at a median age of 42 days. Histopathological assessment shows myofibre hypotrophy, increased central nuclei and decreased triad number but no clear evidence of metabolic cores. Biochemical analysis reveals a marked decrease in RYR1 protein levels (20% of normal) as compared to only a 50% decrease in transcript. Functional studies at end stage show significantly reduced electrically evoked Ca2+ release and force production. In summary, Ryr1TM/Indel mice exhibit a post-natal lethal recessive form of RYR1 RM that pheno-copies the severe congenital clinical presentation seen in a subgroup of RYR1 RM children. Thus, Ryr1TM/Indel mice represent a powerful model for both establishing the pathomechanisms of recessive RYR1 RM and pre-clinical testing of therapies for efficacy.

Excessive Accumulation of Ca2 + in Mitochondria of Y522S-RYR1 Knock-in Mice: A Link Between Leak From the Sarcoplasmic Reticulum and Altered Redox State

Mice (Y522S or YS), carrying a mutation of the sarcoplasmic reticulum (SR) Ca2+ release channel of skeletal muscle fibers (ryanodine receptor type-1, RyR1) which causes Ca2+ leak, are a widely accepted and intensively studied model for human malignant hyperthermia (MH) susceptibility. Since the involvement of reactive oxygen species (ROS) and of mitochondria in MH crisis has been previously debated, here we sought to determine Ca2+ uptake in mitochondria and its possible link with ROS production in single fibers isolated from flexor digitorum brevis (FDB) of YS mice. We found that Ca2+ concentration in the mitochondrial matrix, as detected with the ratiometric FRET-based 4mtD3cpv probe, was higher in YS than in wild-type (WT) fibers at rest and after Ca2+ release from SR during repetitive electrical stimulation or caffeine administration. Also mitochondrial ROS production associated with contractile activity (detected with Mitosox probe) was much higher in YS fibers than in WT. Importantly, the inhibition of mitochondrial Ca2+ uptake achieved by silencing MCU reduced ROS accumulation in the matrix and Ca2+ release from SR. Finally, inhibition of mitochondrial ROS accumulation using Mitotempo reduced SR Ca2+ release in YS fibers exposed to caffeine. The present results support the view that mitochondria take up larger amounts of Ca2+ in YS than in WT fibers and that mitochondrial ROS production substantially contributes to the increased caffeine-sensitivity and to the enhanced Ca2+ release from SR in YS fibers.

DRP1-mediated mitochondrial shape controls calcium homeostasis and muscle mass

Mitochondrial quality control is essential in highly structured cells such as neurons and muscles. In skeletal muscle the mitochondrial fission proteins are reduced in different physiopathological conditions including ageing sarcopenia, cancer cachexia and chemotherapy-induced muscle wasting. However, whether mitochondrial fission is essential for muscle homeostasis is still unclear. Here we show that muscle-specific loss of the pro-fission dynamin related protein (DRP) 1 induces muscle wasting and weakness. Constitutive Drp1 ablation in muscles reduces growth and causes animal death while inducible deletion results in atrophy and degeneration. Drp1 deficient mitochondria are morphologically bigger and functionally abnormal. The dysfunctional mitochondria signals to the nucleus to induce the ubiquitin-proteasome system and an Unfolded Protein Response while the change of mitochondrial volume results in an increase of mitochondrial Ca²⁺ uptake and myofiber death. Our findings reveal that morphology of mitochondrial network is critical for several biological processes that control nuclear programs and Ca²⁺ handling.

Functional Electrical Stimulation: A Possible Strategy to Improve Muscle Function in Central Core Disease?

Central Core Disease (CCD) is a congenital myopathy characterized by presence of amorphous central areas (or cores) lacking glycolytic/oxidative enzymes and mitochondria in skeletal muscle fibers. Most CCD families are linked to mutations in ryanodine receptor type-1 (RYR1), the gene encoding for the sarcoplasmic reticulum (SR) Ca²⁺ release channel of skeletal muscle. As no treatments are available for CCD, currently management of patients is essentially based on a physiotherapic approaches. Functional electrical stimulation (FES) is a technique used to deliver low energy electrical impulses to artificially stimulate selected skeletal muscle groups. Here we tested the efficacy of FES in counteracting muscle loss and improve function in the lower extremities of a 55-year-old female patient which was diagnosed with CCD at the age of 44. Genetic screening of the RyR1 gene identified a missense mutation (c.7354C>T) in exon 46 resulting in an amino acid substitution (p.R2452W) and a duplication (c.12853_12864dup12) in exon 91. The patient was treated with FES for 26 months and subjected before, during, and after training to a series of functional and structural assessments: measurement of maximum isometric force of leg extensor muscles, magnetic resonance imaging, a complete set of functional tests to assess mobility in activities of daily living, and analysis of muscle biopsies by histology and electron microscopy. All results point to an improvement in muscle structure and function induced by FES suggesting that this approach could be considered as an additional supportive measure to maintain/improve muscle function (and possibly reduce muscle loss) in CCD patients.

Myopathology of Congenital Myopathies: Bridging the Old and the New

Congenital myopathies (CM) are a genetically heterogeneous group of neuromuscular disorders most commonly presenting with neonatal/childhood-onset hypotonia and muscle weakness, a relatively static or slowly progressive disease course, and originally classified into subcategories based on characteristic histopathologic findings in muscle biopsies. This enduring concept of disease definition and classification based on the clinicopathologic phenotype was pioneered in the premolecular era. Advances in molecular genetics have brought into focus the increased blurring of the original seemingly "watertight" categories through broadening of the clinical phenotypes in existing genes, and continuous identification of novel genetic backgrounds. This review summarizes the histopathologic landscape of the 4 "classical" subtypes of CM-nemaline myopathies, core myopathies, centronuclear myopathies, and congenital fiber type disproportion and some of the emerging and novel genetic diseases with a CM presentation.

Inositol trisphosphate receptor-mediated Ca2+ signalling stimulates mitochondrial function and gene expression in core myopathy patients

Core myopathies are a group of childhood muscle disorders caused by mutations of the ryanodine receptor (RyR1), the Ca²⁺ release channel of the sarcoplasmic reticulum. These mutations have previously been associated with elevated inositol trisphosphate receptor (IP3R) levels in skeletal muscle myotubes derived from patients. However, the functional relevance and the relationship of IP3R mediated Ca²⁺ signalling with the pathophysiology of the disease is unclear. It has also been suggested that mitochondrial dysfunction underlies the development of central and diffuse multi-mini-cores, devoid of mitochondrial activity, which is a key pathological consequence of RyR1 mutations. Here we used muscle biopsies of central core and multi-minicore disease patients with RyR1 mutations, as well as cellular and in vivo mouse models of the disease to characterize global cellular and mitochondrial Ca²⁺ signalling, mitochondrial function and gene expression associated with the disease. We show that RyR1 mutations that lead to the depletion of the channel are associated with increased IP₃-mediated nuclear and mitochondrial Ca²⁺ signals and increased mitochondrial activity. Moreover, western blot and microarray analysis indicated enhanced mitochondrial biogenesis at the transcriptional and protein levels and was reflected in increased mitochondrial DNA content. The phenotype was recapitulated by RYR1 silencing in mouse cellular myotube models. Altogether, these data indicate that remodelling of skeletal muscle Ca²⁺ signalling following loss of functional RyR1 mediates bioenergetic adaptation.

Elongated mitochondrial constrictions and fission in muscle fatigue

Mitochondria respond to stress and undergo fusion and fission at variable rates, depending on cell status. To understand mitochondrial behavior during muscle fatigue, we investigated mitochondrial ultrastructure and expression levels of a fission- and stress-related protein in fast-twitch muscle fibers of mice subjected to fatigue testing. Mice were subjected to running at increasing speed until exhaustion at 45 min-1 h. In further experiments, high-intensity muscle stimulation through the sciatic nerve simulated the forced treadmill exercise. We detected a rare phenotype characterized by elongated mitochondrial constrictions (EMCs) connecting two separate segments of the original organelles. EMCs are rare in resting muscles and their frequency increases, albeit still at low levels, in stimulated muscles. The constrictions are accompanied by elevated phosphorylation of Drp1 (Dnm1l) at Ser 616, indicating an increased translocation of Drp1 to the mitochondrial membrane. This is indicative of a mitochondrial stress response, perhaps leading to or facilitating a long-lasting fission event. A close apposition of sarcoplasmic reticulum (SR) to the constricted areas, detected using both transmission and scanning electron microscopy, is highly suggestive of SR involvement in inducing mitochondrial constrictions.

Mitochondrial Calcium Increase Induced by RyR1 and IP3R Channel Activation After Membrane Depolarization Regulates Skeletal Muscle Metabolism

Aim: We hypothesize that both type-1 ryanodine receptor (RyR1) and IP₃-receptor (IP₃R) calcium channels are necessary for the mitochondrial Ca²⁺ increase caused by membrane depolarization induced by potassium (or by electrical stimulation) of single skeletal muscle fibers; this calcium increase would couple muscle fiber excitation to an increase in metabolic output from mitochondria (excitation-metabolism coupling).

Methods: Mitochondria matrix and cytoplasmic Ca²⁺ levels were evaluated in fibers isolated from flexor digitorium brevis muscle using plasmids for the expression of a mitochondrial Ca²⁺ sensor (CEPIA3mt) or a cytoplasmic Ca²⁺ sensor (RCaMP). The role of intracellular Ca²⁺ channels was evaluated using both specific pharmacological inhibitors (xestospongin B for IP₃R and Dantrolene for RyR1) and a genetic approach (shIP₃R1-RFP). O₂ consumption was detected using Seahorse Extracellular Flux Analyzer.

Results: In isolated muscle fibers cell membrane depolarization increased both cytoplasmic and mitochondrial Ca²⁺ levels. Mitochondrial Ca²⁺ uptake required functional inositol IP₃R and RyR1 channels. Inhibition of either channel decreased basal O₂ consumption rate but only RyR1 inhibition decreased ATP-linked O₂ consumption. Cell membrane depolarization-induced Ca²⁺ signals in sub-sarcolemmal mitochondria were accompanied by a reduction in mitochondrial membrane potential; Ca²⁺ signals propagated toward intermyofibrillar mitochondria, which displayed increased membrane potential. These results are compatible with slow, Ca²⁺-dependent propagation of mitochondrial membrane potential from the surface toward the center of the fiber.

Conclusion: Ca²⁺-dependent changes in mitochondrial membrane potential have different kinetics in the surface vs. the center of the fiber; these differences are likely to play a critical role in the control of mitochondrial metabolism, both at rest and after membrane depolarization as part of an “excitation-metabolism” coupling process in skeletal muscle fibers.

Exciting perspectives for Translational Myology in the Abstracts of the 2018Spring PaduaMuscleDays: Giovanni Salviati Memorial - Chapter III - Abstracts of March 16, 2018

Myologists working in Padua (Italy) were able to continue a half-century tradition of studies of skeletal muscles, that started with a research on fever, specifically if and how skeletal muscle contribute to it by burning bacterial toxin. Beside main publications in high-impact-factor journals by Padua myologists, I hope to convince readers (and myself) of the relevance of the editing Basic and Applied Myology (BAM), retitled from 2010 European Journal of Translational Myology (EJTM), of the institution of the Interdepartmental Research Center of Myology of the University of Padova (CIR-Myo), and of a long series of International Conferences organized in Euganei Hills and Padova, that is, the PaduaMuscleDays. The 2018Spring PaduaMuscleDays (2018SpPMD), were held in Euganei Hills and Padua (Italy), in March 14-17, and were dedicated to Giovanni Salviati. The main event of the “Giovanni Salviati Memorial”, was held in the Aula Guariento, Accademia Galileiana di Scienze, Lettere ed Arti of Padua to honor a beloved friend and excellent scientist 20 years after his premature passing. Using the words of Prof. Nicola Rizzuto, we all share his believe that Giovanni “will be remembered not only for his talent and originality as a biochemist, but also for his unassuming and humanistic personality, a rare quality in highly successful people like Giovanni. The best way to remember such a person is to gather pupils and colleagues, who shared with him the same scientific interests and ask them to discuss recent advances in their own fields, just as Giovanni have liked to do”. Since Giovanni’s friends sent many abstracts still influenced by their previous collaboration with him, all the Sessions of the 2018SpPMD reflect both to the research aims of Giovanni Salviati and the traditional topics of the PaduaMuscleDays, that is, basics and applications of physical, molecular and cellular strategies to maintain or recover functions of skeletal muscles. The translational researches summarized in the 2018SpPMD Abstracts are at the appropriate high level to attract approval of Ethical Committees, the interest of International Granting Agencies and approval for publication in top quality, international journals. The abstracts of the March 16, 2018 Padua Muscle Day are listed in this chapter III. All 2018SpPMD Abstracts are indexed at the end of the Chapter IV.

The relationship between form and function throughout the history of excitation-contraction coupling

Franzini-Armstrong reviews the development of the excitation–contraction coupling field over time.

The concept of excitation–contraction coupling is almost as old as Journal of General Physiology. It was understood as early as the 1940s that a series of stereotyped events is responsible for the rapid contraction response of muscle fibers to an initial electrical event at the surface. These early developments, now lost in what seems to be the far past for most young investigators, have provided an endless source of experimental approaches. In this Milestone in Physiology, I describe in detail the experiments and concepts that introduced and established the field of excitation–contraction coupling in skeletal muscle. More recent advances are presented in an abbreviated form, as readers are likely to be familiar with recent work in the field.

Congenital myopathies: clinical phenotypes and new diagnostic tools

Congenital myopathies are a group of genetic muscle disorders characterized clinically by hypotonia and weakness, usually from birth, and a static or slowly progressive clinical course. Historically, congenital myopathies have been classified on the basis of major morphological features seen on muscle biopsy. However, different genes have now been identified as associated with the various phenotypic and histological expressions of these disorders, and in recent years, because of their unexpectedly wide genetic and clinical heterogeneity, next-generation sequencing has increasingly been used for their diagnosis. We reviewed clinical and genetic forms of congenital myopathy and defined possible strategies to improve cost-effectiveness in histological and imaging diagnosis.

How and why are calcium currents curtailed in the skeletal muscle voltage-gated calcium channels?

Voltage‐gated calcium channels represent the sole mechanism converting electrical signals of excitable cells into cellular functions such as contraction, secretion and gene regulation. Specific voltage‐sensing domains detect changes in membrane potential and control channel gating. Calcium ions entering through the channel function as second messengers regulating cell functions, with the exception of skeletal muscle, where Ca_V1.1 essentially does not function as a channel but activates calcium release from intracellular stores. It has long been known that calcium currents are dispensable for skeletal muscle contraction. However, the questions as to how and why the channel function of Ca_V1.1 is curtailed remained obscure until the recent discovery of a developmental Ca_V1.1 splice variant with normal channel functions. This discovery provided new means to study the molecular mechanisms regulating the channel gating and led to the understanding that in skeletal muscle, calcium currents need to be restricted to allow proper regulation of fibre type specification and to prevent mitochondrial damage.

Antioxidant Treatment Reduces Formation of Structural Cores and Improves Muscle Function in RYR1Y522S/WT Mice

Central core disease (CCD) is a congenital myopathy linked to mutations in the ryanodine receptor type 1 (RYR1), the sarcoplasmic reticulum Ca²⁺ release channel of skeletal muscle. CCD is characterized by formation of amorphous cores within muscle fibers, lacking mitochondrial activity. In skeletal muscle of RYR1^Y522S/WT knock-in mice, carrying a human mutation in RYR1 linked to malignant hyperthermia (MH) with cores, oxidative stress is elevated and fibers present severe mitochondrial damage and cores. We treated RYR1^Y522S/WT mice with N-acetylcysteine (NAC), an antioxidant provided ad libitum in drinking water for either 2 or 6 months. Our results show that 2 months of NAC treatment starting at 2 months of age, when mitochondrial and fiber damage was still minimal, (i) reduce formation of unstructured and contracture cores, (ii) improve muscle function, and (iii) decrease mitochondrial damage. The beneficial effect of NAC treatment is also evident following 6 months of treatment starting at 4 months of age, when structural damage was at an advanced stage. NAC exerts its protective effect likely by lowering oxidative stress, as supported by the reduction of 3-NT and SOD2 levels. This work suggests that NAC administration is beneficial to prevent mitochondrial damage and formation of cores and improve muscle function in RYR1^Y522S/WT mice.

Assessing the effects of mitofusin 2 deficiency in the adult heart using 3D electron tomography

The effects of mitofusin 2 (MFN2) deficiency, on mitochondrial morphology and the mitochondria-junctional sarcoplasmic reticulum (jSR) complex in the adult heart, have been previously investigated using 2D electron microscopy, an approach which is unable to provide a 3D spatial assessment of these imaging parameters. Here, we use 3D electron tomography to show that MFN2-deficient mitochondria are larger in volume, more elongated, and less rounded; have fewer mitochondria-jSR contacts, and an increase in the distance between mitochondria and jSR, when compared to WT mitochondria. In comparison to 2D electron microscopy, 3D electron tomography can provide further insights into mitochondrial morphology and the mitochondria-jSR complex in the adult heart.

Allele-Specific Silencing of Mutant mRNA Rescues Ultrastructural and Arrhythmic Phenotype in Mice Carriers of the R4496C Mutation in the Ryanodine Receptor Gene (RYR2)

RATIONALE

Mutations in the cardiac Ryanodine Receptor gene (RYR2) cause dominant catecholaminergic polymorphic ventricular tachycardia (CPVT), a leading cause of sudden death in apparently healthy individuals exposed to emotions or physical exercise.

OBJECTIVE

We investigated the efficacy of allele-specific silencing by RNA interference to prevent CPVT phenotypic manifestations in our dominant CPVT mice model carriers of the heterozygous mutation R4496C in RYR2.

METHODS AND RESULTS

We developed an in vitro mRNA and protein-based assays to screen multiple siRNAs for their ability to selectively silence mutant RYR2-R4496C mRNA over the corresponding wild-type allele. For the most performant of these siRNAs (siRYR2-U10), we evaluated the efficacy of an adeno-associated serotype 9 viral vector (AAV9) expressing miRYR2-U10 in correcting RyR2 (Ryanodine Receptor type 2 protein) function after in vivo delivery by intraperitoneal injection in neonatal and adult RyR2^R4496C/+ (mice heterozygous for the R4496C mutation in the RyR2) heterozygous CPVT mice. Transcriptional analysis showed that after treatment with miRYR2-U10, the ratio between wild-type and mutant RYR2 mRNA was doubled (from 1:1 to 2:1) confirming the ability of miRYR2-U10 to selectively inhibit RYR2-R4496C mRNA, whereas protein quantification showed that total RyR2 was reduced by 15% in the heart of treated mice. Furthermore, AAV9-miRYR2-U10 effectively (1) reduced isoproterenol-induced delayed afterdepolarizations and triggered activity in infected cells, (2) reduced adrenergically mediated ventricular tachycardia in treated mice, (3) reverted ultrastructural abnormalities of junctional sarcoplasmic reticulum and transverse tubules, and (4) attenuated mitochondrial abnormalities.

CONCLUSIONS

The study demonstrates that allele-specific silencing with miRYR2-U10 prevents life-threatening arrhythmias in CPVT mice, suggesting that the reduction of mutant RyR2 may be a novel therapeutic approach for CPVT.

Common and variable clinical, histological, and imaging findings of recessive RYR1-related centronuclear myopathy patients

Mutations in RYR1 give rise to diverse skeletal muscle phenotypes, ranging from classical central core disease to susceptibility to malignant hyperthermia. Next-generation sequencing has recently shown that RYR1 is implicated in a wide variety of additional myopathies, including centronuclear myopathy. In this work, we established an international cohort of 21 patients from 18 families with autosomal recessive RYR1-related centronuclear myopathy, to better define the clinical, imaging, and histological spectrum of this disorder. Early onset of symptoms with hypotonia, motor developmental delay, proximal muscle weakness, and a stable course were common clinical features in the cohort. Ptosis and/or ophthalmoparesis, facial weakness, thoracic deformities, and spinal involvement were also frequent but variable. A common imaging pattern consisted of selective involvement of the vastus lateralis, adductor magnus, and biceps brachii in comparison to adjacent muscles. In addition to a variable prominence of central nuclei, muscle biopsy from 20 patients showed type 1 fiber predominance and a wide range of intermyofibrillary architecture abnormalities. All families harbored compound heterozygous mutations, most commonly a truncating mutation combined with a missense mutation. This work expands the phenotypic characterization of patients with recessive RYR1-related centronuclear myopathy by highlighting common and variable clinical, histological, and imaging findings in these patients.

Aging Effects of Caenorhabditis elegans Ryanodine Receptor Variants Corresponding to Human Myopathic Mutations

Delaying the decline in skeletal muscle function will be critical to better maintenance of an active lifestyle in old age. The skeletal muscle ryanodine receptor, the major intracellular membrane channel through which calcium ions pass to elicit muscle contraction, is central to calcium ion balance and is hypothesized to be a significant factor for age-related decline in muscle function. The nematode Caenorhabditis elegans is a key model system for the study of human aging, and strains were generated with modified C. elegans ryanodine receptors corresponding to human myopathic variants linked with malignant hyperthermia and related conditions. The altered response of these strains to pharmacological agents reflected results of human diagnostic tests for individuals with these pathogenic variants. Involvement of nerve cells in the C. elegans responses may relate to rare medical symptoms concerning the central nervous system that have been associated with ryanodine receptor variants. These single amino acid modifications in C. elegans also conferred a reduction in lifespan and an accelerated decline in muscle integrity with age, supporting the significance of ryanodine receptor function for human aging.

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

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

ALS-Associated Endoplasmic Reticulum Proteins in Denervated Skeletal Muscle: Implications for Motor Neuron Disease Pathology

Alpha-motoneurons and muscle fibres are structurally and functionally interdependent. Both cell types particularly rely on endoplasmic reticulum (ER/SR) functions. Mutations of the ER proteins VAPB, SigR1 and HSP27 lead to hereditary motor neuron diseases (MNDs). Here, we determined the expression profile and localization of these ER proteins/chaperons by immunohistochemistry and immunoblotting in biopsy and autopsy muscle tissue of patients with amyotrophic lateral sclerosis (ALS) and other neurogenic muscular atrophies (NMAs) and compared these patterns to mouse models of neurogenic muscular atrophy. Postsynaptic neuromuscular junction staining for VAPB was intense in normal human and mouse muscle and decreased in denervated Nmd^2J mouse muscle fibres. In contrast, VAPB levels together with other chaperones and autophagy markers were increased in extrasynaptic regions of denervated muscle fibres of patients with MNDs and other NMAs, especially at sites of focal myofibrillar disintegration (targets). These findings did not differ between NMAs due to ALS and other causes. G93A-SOD1 mouse muscle fibres showed a similar pattern of protein level increases in denervated muscle fibres. In addition, they showed globular VAPB-immunoreactive structures together with misfolded SOD1 protein accumulations, suggesting a primary myopathic change. Our findings indicate that altered expression and localization of these ER proteins and autophagy markers are part of the dynamic response of muscle fibres to denervation. The ER is particularly prominent and vulnerable in both muscle fibres and alpha-motoneurons. Thus, ER pathology could contribute to the selective build-up of degenerative changes in the neuromuscular axis in MNDs.

Increased mitochondrial nanotunneling activity, induced by calcium imbalance, affects intermitochondrial matrix exchanges

Exchanges of matrix contents are essential to the maintenance of mitochondria. Cardiac mitochondrial exchange matrix content in two ways: by direct contact with neighboring mitochondria and over longer distances. The latter mode is supported by thin tubular protrusions, called nanotunnels, that contact other mitochondria at relatively long distances. Here, we report that cardiac myocytes of heterozygous mice carrying a catecholaminergic polymorphic ventricular tachycardia-linked RyR2 mutation (A4860G) show a unique and unusual mitochondrial response: a significantly increased frequency of nanotunnel extensions. The mutation induces Ca²⁺ imbalance by depressing RyR2 channel activity during excitation-contraction coupling, resulting in random bursts of Ca²⁺ release probably due to Ca²⁺ overload in the sarcoplasmic reticulum. We took advantage of the increased nanotunnel frequency in RyR2^A4860G+/- cardiomyocytes to investigate and accurately define the ultrastructure of these mitochondrial extensions and to reconstruct the overall 3D distribution of nanotunnels using electron tomography. Additionally, to define the effects of communication via nanotunnels, we evaluated the intermitochondrial exchanges of matrix-targeted soluble fluorescent proteins, mtDsRed and photoactivable mtPA-GFP, in isolated cardiomyocytes by confocal microscopy. A direct comparison between exchanges occurring at short and long distances directly demonstrates that communication via nanotunnels is slower.

Progressive Structural Defects in Canine Centronuclear Myopathy Indicate a Role for HACD1 in Maintaining Skeletal Muscle Membrane Systems

Mutations in HACD1/PTPLA cause recessive congenital myopathies in humans and dogs. Hydroxyacyl-coA dehydratases are required for elongation of very long chain fatty acids, and HACD1 has a role in early myogenesis, but the functions of this striated muscle-specific enzyme in more differentiated skeletal muscle remain unknown. Canine HACD1 deficiency is histopathologically classified as a centronuclear myopathy (CNM). We investigated the hypothesis that muscle from HACD1-deficient dogs has membrane abnormalities in common with CNMs with different genetic causes. We found progressive changes in tubuloreticular and sarcolemmal membranes and mislocalized triads and mitochondria in skeletal muscle from animals deficient in HACD1. Furthermore, comparable membranous abnormalities in cultured HACD1-deficient myotubes provide additional evidence that these defects are a primary consequence of altered HACD1 expression. Our novel findings, including T-tubule dilatation and disorganization, associated with defects in this additional CNM-associated gene provide a definitive pathophysiologic link with these disorders, confirm that dogs deficient in HACD1 are relevant models, and strengthen the evidence for a unifying pathogenesis in CNMs via defective membrane trafficking and excitation-contraction coupling in muscle. These results build on previous work by determining further functional roles of HACD1 in muscle and provide new insight into the pathology and pathogenetic mechanisms of HACD1 CNM. Consequently, alterations in membrane properties associated with HACD1 mutations should be investigated in humans with related phenotypes.

Forty years later: Mitochondria as therapeutic targets in muscle diseases

The hypothesis that mitochondrial dysfunction can be a general mechanism for cell death in muscle diseases is 40 years old. The key elements of the proposed pathogenetic sequence (cytosolic Ca2+ overload followed by excess mitochondrial Ca2+ uptake, functional and then structural damage of mitochondria, energy shortage, worsened elevation of cytosolic Ca2+ levels, hypercontracture of muscle fibers, cell necrosis) have been confirmed in amazing detail by subsequent work in a variety of models. The explicit implication of the hypothesis was that it "may provide the basis for a more rational treatment for some conditions even before their primary causes are known" (Wrogemann and Pena, 1976, Lancet, 1, 672-674). This prediction is being fulfilled, and the potential of mitochondria as pharmacological targets in muscle diseases may soon become a reality, particularly through inhibition of the mitochondrial permeability transition pore and its regulator cyclophilin D.

Age-dependent uncoupling of mitochondria from Ca2⁺ release units in skeletal muscle

Calcium release units (CRUs) and mitochondria control myoplasmic [Ca²⁺] levels and ATP production in muscle, respectively. We recently reported that these two organelles are structurally connected by tethers, which promote proximity and proper Ca²⁺ signaling. Here we show that disposition, ultrastructure, and density of CRUs and mitochondria and their reciprocal association are compromised in muscle from aged mice. Specifically, the density of CRUs and mitochondria is decreased in muscle fibers from aged (>24 months) vs. adult (3-12 months), with an increased percentage of mitochondria being damaged and misplaced from their normal triadic position. A significant reduction in tether (13.8±0.4 vs. 5.5±0.3 tethers/100μm²) and CRU-mitochondrial pair density (37.4±0.8 vs. 27.0±0.7 pairs/100μm²) was also observed in aged mice. In addition, myoplasmic Ca²⁺ transient (1.68±0.08 vs 1.37±0.03) and mitochondrial Ca²⁺ uptake (9.6±0.050 vs 6.58±0.54) during repetitive high frequency tetanic stimulation were significantly decreased. Finally oxidative stress, assessed from levels of 3-nitrotyrosine (3-NT), Cu/Zn superoxide-dismutase (SOD1) and Mn superoxide dismutase (SOD2) expression, were significantly increased in aged mice. The reduced association between CRUs and mitochondria with aging may contribute to impaired cross-talk between the two organelles, possibly resulting in reduced efficiency in activity-dependent ATP production and, thus, to age-dependent decline of skeletal muscle performance.

Skeletal Muscle Microalterations in Patients Carrying Malignant Hyperthermia-Related Mutations of the e-c Coupling Machinery

We have compared the ultrastructure of skeletal muscle biopsies from patients that have survived a [Malignant Hyperthermia, MH] episode and siblings that test positive for MH susceptibility with those from siblings that tested negatives. The aim is to establish whether life long exposure to the MH-related mutation effects may result in subtle abnormalities even in the absence of active episodes and/or clinically detectable deficiencies. Although a specific ultrastructural signature for MH mutants cannot be demonstrated, an MH related pattern of minor alterations does exist. These include the tendency for micro damage to the contractile apparatus and a higher than normal level of mitochondrial abnormalities.

Restricting calcium currents is required for correct fiber type specification in skeletal muscle

Skeletal muscle excitation-contraction (EC) coupling is independent of calcium influx. In fact, alternative splicing of the voltage-gated calcium channel CaV1.1 actively suppresses calcium currents in mature muscle. Whether this is necessary for normal development and function of muscle is not known. However, splicing defects that cause aberrant expression of the calcium-conducting developmental CaV1.1e splice variant correlate with muscle weakness in myotonic dystrophy. Here, we deleted CaV1.1 (Cacna1s) exon 29 in mice. These mice displayed normal overall motor performance, although grip force and voluntary running were reduced. Continued expression of the developmental CaV1.1e splice variant in adult mice caused increased calcium influx during EC coupling, altered calcium homeostasis, and spontaneous calcium sparklets in isolated muscle fibers. Contractile force was reduced and endurance enhanced. Key regulators of fiber type specification were dysregulated and the fiber type composition was shifted toward slower fibers. However, oxidative enzyme activity and mitochondrial content declined. These findings indicate that limiting calcium influx during skeletal muscle EC coupling is important for the secondary function of the calcium signal in the activity-dependent regulation of fiber type composition and to prevent muscle disease.