Expression levels of RyR1 and RyR3 control resting free Ca2+ in skeletal muscle

Chronic repetitive cooling and caffeine-induced intracellular Ca2+ elevation differentially impact adaptations in slow- and fast-twitch rat skeletal muscles

Intracellular Ca2+ concentration ([Ca2+]i) is considered important in the regulation of skeletal muscle mass. This study tested the hypothesis that chronic repeated cooling and/or caffeine ingestion would acutely increase [Ca2+]i and hypertrophy muscles potentially in a fiber-type-dependent manner. Control rats and those fed caffeine were subjected to repeated bidiurnal treatments of percutaneous icing, under anesthesia, to reduce the muscle temperature below ∼5°C. The predominantly fast-twitch tibialis anterior (TA) and slow-twitch soleus (SOL) muscles were evaluated after 28 days of intervention. The [Ca2+]i elevating response to icing was enhanced by caffeine loading only in the SOL muscle, with the response present across a significantly higher temperature range than in the TA muscle under caffeine-loading conditions. In both the TA and SOL muscles, myofiber cross-sectional area (CSA) was decreased by chronic caffeine treatment (mean reductions of 10.5% and 20.4%, respectively). However, in the TA, but not the SOL, CSA was restored by icing (+15.4 ± 4.3% vs. noniced, P < 0.01). In the SOL, but not TA, icing + caffeine increased myofiber number (20.5 ± 6.7%, P < 0.05) and satellite cell density (2.5 ± 0.3-fold) in cross sections. These contrasting muscle responses to cooling and caffeine may reflect fiber-type-specific [Ca2+]i responses and/or differential responses to elevated [Ca2+]i.

Intestinal enteroendocrine cells rely on ryanodine and IP3 calcium store receptors for mechanotransduction

Enteroendocrine cells (EECs) are specialized sensors of luminal forces and chemicals in the gastrointestinal (GI) epithelium that respond to stimulation with a release of signalling molecules such as serotonin (5-HT). For mechanosensitive EECs, force activates Piezo2 channels, which generate a very rapidly activating and inactivating (∼10 ms) cationic (Na+ , K+ , Ca2+ ) receptor current. Piezo2 receptor currents lead to a large and persistent increase in intracellular calcium (Ca2+ ) that lasts many seconds to sometimes minutes, suggesting signal amplification. However, intracellular calcium dynamics in EEC mechanotransduction remain poorly understood. The aim of this study was to determine the role of Ca2+ stores in EEC mechanotransduction. Mechanical stimulation of a human EEC cell model (QGP-1) resulted in a rapid increase in cytoplasmic Ca2+ and a slower decrease in ER stores Ca2+ , suggesting the involvement of intracellular Ca2+ stores. Comparing murine primary colonic EECs with colonocytes showed expression of intercellular Ca2+ store receptors, a similar expression of IP3 receptors, but a >30-fold enriched expression of Ryr3 in EECs. In mechanically stimulated primary EECs, Ca2+ responses decreased dramatically by emptying stores and pharmacologically blocking IP3 and RyR1/3 receptors. RyR3 genetic knockdown by siRNA led to a significant decrease in mechanosensitive Ca2+ responses and 5-HT release. In tissue, pressure-induced increase in the Ussing short circuit current was significantly decreased by ryanodine receptor blockade. Our data show that mechanosensitive EECs use intracellular Ca2+ stores to amplify mechanically induced Ca2+ entry, with RyR3 receptors selectively expressed in EECs and involved in Ca2+ signalling, 5-HT release and epithelial secretion. KEY POINTS: A population of enteroendocrine cells (EECs) are specialized mechanosensors of the gastrointestinal (GI) epithelium that respond to mechanical stimulation with the release of important signalling molecules such as serotonin. Mechanical activation of these EECs leads to an increase in intracellular calcium (Ca2+ ) with a longer duration than the stimulus, suggesting intracellular Ca2+ signal amplification. In this study, we profiled the expression of intracellular Ca2+ store receptors and found an enriched expression of the intracellular Ca2+ receptor Ryr3, which contributed to the mechanically evoked increases in intracellular calcium, 5-HT release and epithelial secretion. Our data suggest that mechanosensitive EECs rely on intracellular Ca2+ stores and are selective in their use of Ryr3 for amplification of intracellular Ca2+ . This work advances our understanding of EEC mechanotransduction and may provide novel diagnostic and therapeutic targets for GI motility disorders.

Transcriptome response of proliferating muscle satellite cells to thermal challenge in commercial turkey

Thermal stress poses a threat to agricultural systems through increased risk to animal growth, health, and production. Exposure of poultry, especially hatchlings, to extreme temperatures can seriously affect muscle development and thus compromise subsequent meat quality. This study was designed to characterize transcriptional changes induced in turkey muscle satellite cells (SCs) cultured from commercial birds under thermal challenge to determine the applicability of previous results obtained for select research lines. Satellite cells isolated from the pectoralis major muscle of 1-week old commercial fast-growing birds (Nicholas turkey, NCT) and from a slower-growing research line (RBC2) were proliferated in culture at 38°C or 43°C for 72 h. RNAseq analysis found statistically significant differences in gene expression among treatments and between turkey lines with a greater number of genes altered in the NCT SCs suggesting early myogenesis. Pathway analysis identified cell signaling and regulation of Ca2+ as important responses. Expression of the intercellular signaling Wnt genes, particularly Wnt5a and 7a was significantly altered by temperature with differential response between lines. The peripheral calcium channel RYR3 gene was among the genes most highly upregulated by heat stress. Increased expression of RYR3 would likely result in higher resting cytosolic calcium levels and increased overall gene transcription. Although responses in the calcium signaling pathway were similar among the RBC2 and NCT lines, the magnitude of expression changes was greater in the commercially selected birds. These results provide evidence into how SC activity, cellular fate, and ultimately muscle development are altered by heat stress and commercial selection.

Transcriptional profiles of genes related to electrophysiological function in Scn5a+/- murine hearts

The Scn5a gene encodes the major pore-forming Nav 1.5 (α) subunit, of the voltage-gated Na+ channel in cardiomyocytes. The key role of Nav 1.5 in action potential initiation and propagation in both atria and ventricles predisposes organisms lacking Scn5a or carrying Scn5a mutations to cardiac arrhythmogenesis. Loss-of-function Nav 1.5 genetic abnormalities account for many cases of the human arrhythmic disorder Brugada syndrome (BrS) and related conduction disorders. A murine model with a heterozygous Scn5a deletion recapitulates many electrophysiological phenotypes of BrS. This study examines the relationships between its Scn5a+/- genotype, resulting transcriptional changes, and the consequent phenotypic presentations of BrS. Of 62 selected protein-coding genes related to cardiomyocyte electrophysiological or homeostatic function, concentrations of mRNA transcribed from 15 differed significantly from wild type (WT). Despite halving apparent ventricular Scn5a transcription heterozygous deletion did not significantly downregulate its atrial expression, raising possibilities of atria-specific feedback mechanisms. Most of the remaining 14 genes whose expression differed significantly between WT and Scn5a+/- animals involved Ca2+ homeostasis specifically in atrial tissue, with no overlap with any ventricular changes. All statistically significant changes in expression were upregulations in the atria and downregulations in the ventricles. This investigation demonstrates the value of future experiments exploring for and clarifying links between transcriptional control of Scn5a and of genes whose protein products coordinate Ca2+ regulation and examining their possible roles in BrS.

Ion Channel Gene Mutations Causing Skeletal Muscle Disorders: Pathomechanisms and Opportunities for Therapy

Skeletal muscle ion channelopathies (SMICs) are a large heterogeneous group of rare genetic disorders caused by mutations in genes encoding ion channel subunits in the skeletal muscle mainly characterized by myotonia or periodic paralysis, potentially resulting in long-term disabilities. However, with the development of new molecular technologies, new genes and new phenotypes, including progressive myopathies, have been recently discovered, markedly increasing the complexity in the field. In this regard, new advances in SMICs show a less conventional role of ion channels in muscle cell division, proliferation, differentiation, and survival. Hence, SMICs represent an expanding and exciting field. Here, we review current knowledge of SMICs, with a description of their clinical phenotypes, cellular and molecular pathomechanisms, and available treatments.

Gain-of-Function STIM1 L96V Mutation Causes Myogenesis Alteration in Muscle Cells From a Patient Affected by Tubular Aggregate Myopathy

Tubular Aggregate Myopathy (TAM) is a hereditary ultra-rare muscle disorder characterized by muscle weakness and cramps or myasthenic features. Biopsies from TAM patients show the presence of tubular aggregates originated from sarcoplasmic reticulum due to altered Ca²⁺ homeostasis. TAM is caused by gain-of-function mutations in STIM1 or ORAI1, proteins responsible for Store-Operated-Calcium-Entry (SOCE), a pivotal mechanism in Ca²⁺ signaling. So far there is no cure for TAM and the mechanisms through which STIM1 or ORAI1 gene mutation lead to muscle dysfunction remain to be clarified. It has been established that post-natal myogenesis critically relies on Ca²⁺ influx through SOCE. To explore how Ca²⁺ homeostasis dysregulation associated with TAM impacts on muscle differentiation cascade, we here performed a functional characterization of myoblasts and myotubes deriving from patients carrying STIM1 L96V mutation by using fura-2 cytofluorimetry, high content imaging and real-time PCR. We demonstrated a higher resting Ca²⁺ concentration and an increased SOCE in STIM1 mutant compared with control, together with a compensatory down-regulation of genes involved in Ca²⁺ handling (RyR1, Atp2a1, Trpc1). Differentiating STIM1 L96V myoblasts persisted in a mononuclear state and the fewer multinucleated myotubes had distinct morphology and geometry of mitochondrial network compared to controls, indicating a defect in the late differentiation phase. The alteration in myogenic pathway was confirmed by gene expression analysis regarding early (Myf5, Mef2D) and late (DMD, Tnnt3) differentiation markers together with mitochondrial markers (IDH3A, OGDH). We provided evidences of mechanisms responsible for a defective myogenesis associated to TAM mutant and validated a reliable cellular model usefull for TAM preclinical studies.

Elementary calcium signaling in arterial smooth muscle

Vascular smooth muscle cells (VSMCs) of small peripheral arteries contribute to blood pressure control by adapting their contractile state. These adaptations depend on the VSMC cytosolic Ca²⁺ concentration, regulated by complex local elementary Ca²⁺ signaling pathways. Ca²⁺ sparks represent local, transient, rapid calcium release events from a cluster of ryanodine receptors (RyRs) in the sarcoplasmic reticulum. In arterial SMCs, Ca²⁺ sparks activate nearby calcium-dependent potassium channels, cause membrane hyperpolarization and thus decrease the global intracellular [Ca²⁺] to oppose vasoconstriction. Arterial SMC Ca_v1.2 L-type channels regulate intracellular calcium stores content, which in turn modulates calcium efflux through RyRs. Ca_v3.2 T-type channels contribute to a minor extend to Ca²⁺ spark generation in certain types of arteries. Their localization within cell membrane caveolae is essential. We summarize present data on local elementary calcium signaling (Ca²⁺ sparks) in arterial SMCs with focus on RyR isoforms, large-conductance calcium-dependent potassium (BK_Ca) channels, and cell membrane-bound calcium channels (Ca_v1.2 and Ca_v3.2), particularly in caveolar microdomains.

Identification of Differentially Expressed Genes in Different Types of Broiler Skeletal Muscle Fibers Using the RNA-seq Technique

The difference in muscle fiber types is very important to the muscle development and meat quality of broilers. At present, the molecular regulation mechanisms of skeletal muscle fiber-type transformation in broilers are still unclear. In this study, differentially expressed genes between breast and leg muscles in broilers were analyzed using RNA-seq. A total of 767 DEGs were identified. Compared with leg muscle, there were 429 upregulated genes and 338 downregulated genes in breast muscle. Gene Ontology (GO) enrichment indicated that these DEGs were mainly involved in cellular processes, single organism processes, cells, and cellular components, as well as binding and catalytic activity. KEGG analysis shows that a total of 230 DEGs were mapped to 126 KEGG pathways and significantly enriched in the four pathways of glycolysis/gluconeogenesis, starch and sucrose metabolism, insulin signalling pathways, and the biosynthesis of amino acids. Quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) was used to verify the differential expression of 7 selected DEGs, and the results were consistent with RNA-seq data. In addition, the expression profile of MyHC isoforms in chicken skeletal muscle cells showed that with the extension of differentiation time, the expression of fast fiber subunits (types IIA and IIB) gradually increased, while slow muscle fiber subunits (type I) showed a downward trend after 4 days of differentiation. The differential genes screened in this study will provide some new ideas for further understanding the molecular mechanism of skeletal muscle fiber transformation in broilers.

Whole-genome sequencing association analysis reveals the genetic architecture of meat quality traits in Chinese Qingyu pigs

The Chinese Qingyu pig breed is an invaluable indigenous genetic resource. However, few studies have investigated the genetic architecture of meat quality traits in Qingyu pigs. Here, 30 purebred Qingyu pigs were subjected to whole-genome sequencing. After quality control, 18 436 759 SNPs were retained. Genome-wide association studies (GWAS) were then performed for meat pH and color at three postmortem time points (45 min, 24 h, and 48 h) using single-marker regression analysis. In total, 11 and 69 SNPs were associated with meat pH and color of the longissimus thoracis muscle (LTM), respectively, while 54 and 29 SNPs were associated with meat pH and color of the semimembranosus muscle (SM), respectively. Seven SNPs associated with pork pH were shared by all three postmortem time points. Several candidate genes for meat traits were identified, including four genes (CXXC5, RYR3, BNIP3, and MYCT1) related to skeletal muscle development, regulation of Ca²⁺ release in the muscle, and anaerobic respiration, which are promising candidates for selecting superior meat quality traits in Qingyu pigs. To our knowledge, this is the first study investigating the postmortem genetic architecture of pork pH and color in Qingyu pigs. Our findings further the current understanding of the genetic factors influencing meat quality.

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.

Intracellular calcium leak lowers glucose storage in human muscle, promoting hyperglycemia and diabetes

Most glucose is processed in muscle, for energy or glycogen stores. Malignant Hyperthermia Susceptibility (MHS) exemplifies muscle conditions that increase [Ca2+]cytosol. 42% of MHS patients have hyperglycemia. We show that phosphorylated glycogen phosphorylase (GPa), glycogen synthase (GSa) - respectively activated and inactivated by phosphorylation - and their Ca2+-dependent kinase (PhK), are elevated in microsomal extracts from MHS patients' muscle. Glycogen and glucose transporter GLUT4 are decreased. [Ca2+]cytosol, increased to MHS levels, promoted GP phosphorylation. Imaging at ~100 nm resolution located GPa at sarcoplasmic reticulum (SR) junctional cisternae, and apo-GP at Z disk. MHS muscle therefore has a wide-ranging alteration in glucose metabolism: high [Ca2+]cytosol activates PhK, which inhibits GS, activates GP and moves it toward the SR, favoring glycogenolysis. The alterations probably cause these patients' hyperglycemia. For basic studies, MHS emerges as a variable stressor, which forces glucose pathways from the normal to the diseased range, thereby exposing novel metabolic links.

Caffeine as a tool to investigate sarcoplasmic reticulum and intracellular calcium dynamics in human skeletal muscles

Caffeine is worldwide used for its power to increase cognitive and physical performance. The ergogenic effects of caffeine, however, do not depend on a direct action on muscles. Actually, the actions of caffeine on skeletal muscles, take place at millimolar concentrations which are far above the micromolar level reached after a regular consumption of coffee or similar drinks, and close to a lethal concentration. At millimolar concentrations caffeine exerts a powerful effect on sarcoplasmic reticulum (SR) activating the release of calcium via ryanodine receptors and, possibly, inhibiting calcium reuptake. For this reason caffeine has become a valuable tool for studying SR function and for diagnostics of SR related muscle disorders. This review aims to briefly describe the effects and the mechanism of action of caffeine on sarcoplasmic reticulum and to focus on its use to study intracellular calcium dynamics in human muscle fibers in physiological and pathological conditions.

Co-expression Network Approach Reveals Functional Similarities among Diseases Affecting Human Skeletal Muscle

Diseases affecting skeletal muscle exhibit considerable heterogeneity in intensity, etiology, phenotypic manifestation and gene expression. Systems biology approaches using network theory, allows for a holistic understanding of functional similarities amongst diseases. Here we propose a co-expression based, network theoretic approach to extract functional similarities from 20 heterogeneous diseases comprising of dystrophinopathies, inflammatory myopathies, neuromuscular, and muscle metabolic diseases. Utilizing this framework we identified seven closely associated disease clusters with 20 disease pairs exhibiting significant correlation (p < 0.05). Mapping the diseases onto a human protein-protein interaction network enabled the inference of a common program of regulation underlying more than half the muscle diseases considered here and referred to as the “protein signature.” Enrichment analysis of 17 protein modules identified as part of this signature revealed a statistically non-random dysregulation of muscle bioenergetic pathways and calcium homeostasis. Further, analysis of mechanistic similarities of less explored significant disease associations [such as between amyotrophic lateral sclerosis (ALS) and cerebral palsy (CP)] using a proposed “functional module” framework revealed adaptation of the calcium signaling machinery. Integrating drug-gene information into the quantitative framework highlighted the presence of therapeutic opportunities through drug repurposing for diseases affecting the skeletal muscle.

Smooth Muscle Cell Genome Browser: Enabling the Identification of Novel Serum Response Factor Target Genes

Genome-scale expression data on the absolute numbers of gene isoforms offers essential clues in cellular functions and biological processes. Smooth muscle cells (SMCs) perform a unique contractile function through expression of specific genes controlled by serum response factor (SRF), a transcription factor that binds to DNA sites known as the CArG boxes. To identify SRF-regulated genes specifically expressed in SMCs, we isolated SMC populations from mouse small intestine and colon, obtained their transcriptomes, and constructed an interactive SMC genome and CArGome browser. To our knowledge, this is the first online resource that provides a comprehensive library of all genetic transcripts expressed in primary SMCs. The browser also serves as the first genome-wide map of SRF binding sites. The browser analysis revealed novel SMC-specific transcriptional variants and SRF target genes, which provided new and unique insights into the cellular and biological functions of the cells in gastrointestinal (GI) physiology. The SRF target genes in SMCs, which were discovered in silico, were confirmed by proteomic analysis of SMC-specific Srf knockout mice. Our genome browser offers a new perspective into the alternative expression of genes in the context of SRF binding sites in SMCs and provides a valuable reference for future functional studies.

Human adaptation to arsenic-rich environments

Adaptation drives genomic changes; however, evidence of specific adaptations in humans remains limited. We found that inhabitants of the northern Argentinean Andes, an arid region where elevated arsenic concentrations in available drinking water is common, have unique arsenic metabolism, with efficient methylation and excretion of the major metabolite dimethylated arsenic and a less excretion of the highly toxic monomethylated metabolite. We genotyped women from this population for 4,301,332 single nucleotide polymorphisms (SNPs) and found a strong association between the AS3MT (arsenic [+3 oxidation state] methyltransferase) gene and mono- and dimethylated arsenic in urine, suggesting that AS3MT functions as the major gene for arsenic metabolism in humans. We found strong genetic differentiation around AS3MT in the Argentinean Andes population, compared with a highly related Peruvian population (FST = 0.014) from a region with much less environmental arsenic. Also, 13 of the 100 SNPs with the highest genome-wide Locus-Specific Branch Length occurred near AS3MT. In addition, our examination of extended haplotype homozygosity indicated a selective sweep of the Argentinean Andes population, in contrast to Peruvian and Colombian populations. Our data show that adaptation to tolerate the environmental stressor arsenic has likely driven an increase in the frequencies of protective variants of AS3MT, providing the first evidence of human adaptation to a toxic chemical.

Myoplasmic resting Ca2+ regulation by ryanodine receptors is under the control of a novel Ca2+-binding region of the receptor

Passive SR (sarcoplasmic reticulum) Ca²⁺ leak through the RyR (ryanodine receptor) plays a critical role in the mechanisms that regulate [Ca²⁺]_rest (intracellular resting myoplasmic free Ca²⁺ concentration) in muscle. This process appears to be isoform-specific as expression of either RyR1 or RyR3 confers on myotubes different [Ca²⁺]_rest. Using chimaeric RyR3–RyR1 receptors expressed in dyspedic myotubes, we show that isoform-dependent regulation of [Ca²⁺]_rest is primarily defined by a small region of the receptor encompassing amino acids 3770–4007 of RyR1 (amino acids 3620–3859 of RyR3) named as the CLR (Ca²⁺ leak regulatory) region. [Ca²⁺]_rest regulation by the CLR region was associated with alteration of RyRs’ Ca²⁺-activation profile and changes in SR Ca²⁺-leak rates. Biochemical analysis using Tb³⁺-binding assays and intrinsic tryptophan fluorescence spectroscopy of purified CLR domains revealed that this determinant of RyRs holds a novel Ca²⁺-binding domain with conformational properties that are distinctive to each isoform. Our data suggest that the CLR region provides channels with unique functional properties that modulate the rate of passive SR Ca²⁺ leak and confer on RyR1 and RyR3 distinctive [Ca²⁺]_rest regulatory properties. The identification of a new Ca²⁺-binding domain of RyRs with a key modulatory role in [Ca²⁺]_rest regulation provides new insights into Ca²⁺-mediated regulation of RyRs.

This paper reports the finding of a new class of Ca²⁺-binding domain of intracellular Ca²⁺ channels from muscle cells. This domain provides channels with distinctive properties that result in channel-specific modulation of the intracellular resting Ca²⁺ concentration.

Amyloid beta a4 precursor protein-binding family B member 1 (FE65) interactomics revealed synaptic vesicle glycoprotein 2A (SV2A) and sarcoplasmic/endoplasmic reticulum calcium ATPase 2 (SERCA2) as new binding proteins in the human brain

FE65 is a cytosolic adapter protein and an important binding partner of amyloid precursor protein. Dependent on Thr668 phosphorylation in amyloid precursor protein, which influences amyloidogenic amyloid precursor protein processing, FE65 undergoes nuclear translocation, thereby transmitting a signal from the cell membrane to the nucleus. As this translocation may be relevant in Alzheimer disease, and as FE65 consists of three protein-protein interaction domains able to bind and affect a variety of other proteins and downstream signaling pathways, the identification of the FE65 interactome is of central interest in Alzheimer disease research. In this study, we identified 121 proteins as new potential FE65 interacting proteins in a pulldown/mass spectrometry approach using human post-mortem brain samples as protein pools for recombinantly expressed FE65. Co-immunoprecipitation assays further validated the interaction of FE65 with the candidates SV2A and SERCA2. In parallel, we investigated the whole cell proteome of primary hippocampal neurons from FE65/FE65L1 double knockout mice. Notably, the validated FE65 binding proteins were also found to be differentially abundant in neurons derived from the FE65 knockout mice relative to wild-type control neurons. SERCA2 is an important player in cellular calcium homeostasis, which was found to be up-regulated in double knockout neurons. Indeed, knock-down of FE65 in HEK293T cells also evoked an elevated sensitivity to thapsigargin, a stressor specifically targeting the activity of SERCA2. Thus, our results suggest that FE65 is involved in the regulation of intracellular calcium homeostasis. Whereas transfection of FE65 alone caused a typical dot-like phenotype in the nucleus, co-transfection of SV2A significantly reduced the percentage of FE65 dot-positive cells, pointing to a possible role for SV2A in the modulation of FE65 intracellular targeting. Given that SV2A has a signaling function at the presynapse, its effect on FE65 intracellular localization suggests that the SV2A/FE65 interaction might play a role in synaptic signal transduction.

Morphogenesis of T-tubules in heart cells: the role of junctophilin-2

The T-tubule (TT) system forms the structural basis for excitation-contraction coupling in heart and muscle cells. The morphogenesis of the TT system is a key step in the maturation of heart cells because it does not exist in neonatal cardiomyocytes. In the present study, we quantified the morphological changes in TTs during heart cell maturation and investigated the role of junctophilin-2 (JP2), a protein known to anchor the sarcoplasmic reticulum (SR) to TT, in changes to TT morphological parameters. Analysis of confocal images showed that the transverse elements of TTs increased, while longitudinal elements decreased during the maturation of TTs. Fourier transform analysis showed that the power of ∼2 μm spatial components increased with cardiomyocytes maturation. These changes were preceded by increased expression of JP2, and were reversed by JP2 knockdown. These findings indicate that JP2 is required for the morphogenesis of TTs during heart development.

Functional classification of skeletal muscle networks. II. Applications to pathophysiology

In our preceding companion paper (Wang Y, Winters J, Subramaniam S. J Appl Physiol. doi: 10.1152/japplphysiol.01514.2011), we used extensive expression profile data on normal human subjects, in combination with legacy knowledge to classify skeletal muscle function into four models, namely excitation-activation, mechanical, metabolic, and signaling-production model families. In this paper, we demonstrate how this classification can be applied to study two well-characterized myopathies: amyotrophic lateral sclerosis (ALS) and Duchenne muscular dystrophy (DMD). Using skeletal muscle profile data from ALS and DMD patients compared with that from normal subjects, normal young in the case of DMD, we delineate molecular mechanisms that are causative and consequential to skeletal muscle dysfunction. In ALS, our analysis establishes the metabolic role and specifically identifies the mechanisms of calcium dysregulation and defects in mitochondrial transport of materials as important for muscle dysfunction. In DMD, we illustrate how impaired mechanical function is strongly coordinated with other three functional networks, resulting in transformation of the skeletal muscle into hybrid forms as a compensatory mechanism. Our functional models also provide, in exquisite detail, the mechanistic role of myriad proteins in these four families in normal and disease function.

Triadin/Junctin double null mouse reveals a differential role for Triadin and Junctin in anchoring CASQ to the jSR and regulating Ca(2+) homeostasis

Triadin (Tdn) and Junctin (Jct) are structurally related transmembrane proteins thought to be key mediators of structural and functional interactions between calsequestrin (CASQ) and ryanodine receptor (RyRs) at the junctional sarcoplasmic reticulum (jSR). However, the specific contribution of each protein to the jSR architecture and to excitation-contraction (e-c) coupling has not been fully established. Here, using mouse models lacking either Tdn (Tdn-null), Jct (Jct-null) or both (Tdn/Jct-null), we identify Tdn as the main component of periodically located anchors connecting CASQ to the RyR-bearing jSR membrane. Both proteins proved to be important for the structural organization of jSR cisternae and retention of CASQ within them, but with different degrees of impact. Our results also suggest that the presence of CASQ is responsible for the wide lumen of the jSR cisternae. Using Ca(2+) imaging and Ca(2+) selective microelectrodes we found that changes in e-c coupling, SR Ca(2+)content and resting [Ca(2+)] in Jct, Tdn and Tdn/Jct-null muscles are directly correlated to the effect of each deletion on CASQ content and its organization within the jSR. These data suggest that in skeletal muscle the disruption of Tdn/CASQ link has a more profound effect on jSR architecture and myoplasmic Ca(2+) regulation than Jct/CASQ association.

Genomic profiling reveals Pitx2 controls expression of mature extraocular muscle contraction-related genes

PURPOSE

To assess the influence of the Pitx2 transcription factor on the global gene expression profile of extraocular muscle (EOM) of mice.

METHODS

Mice with a conditional knockout of Pitx2, designated Pitx2(Δflox/Δflox) and their control littermates Pitx2(flox/flox), were used. RNA was isolated from EOM obtained at 3, 6, and 12 weeks of age and processed for microarray-based profiling. Pairwise comparisons were performed between mice of the same age and differentially expressed gene lists were generated. Select genes from the profile were validated using real-time quantitative polymerase chain reaction and protein immunoblot. Ultrastructural analysis was performed to evaluate EOM sarcomeric structure.

RESULTS

The number of differentially expressed genes was relatively small. Eleven upregulated and 23 downregulated transcripts were identified common to all three age groups in the Pitx2-deficient extraocular muscle compared with littermate controls. These fell into a range of categories including muscle-specific structural genes, transcription factors, and ion channels. The differentially expressed genes were primarily related to muscle contraction. We verified by protein and ultrastructural analysis that myomesin 2 was expressed in the Pitx2-deficient mice, and this was associated with development of M lines evident in their orbital region.

CONCLUSIONS

The global transcript expression analysis uncovered that Pitx2 primarily regulates a relatively select number of genes associated with muscle contraction. Pitx2 loss led to the development of M line structures, a feature more typical of other skeletal muscle.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Orthograde dihydropyridine receptor signal regulates ryanodine receptor passive leak

The skeletal muscle dihydropyridine receptor (DHPR) and ryanodine receptor (RyR1) are known to engage a form of conformation coupling essential for muscle contraction in response to depolarization, referred to as excitation-contraction coupling. Here we use WT and Ca(V)1.1 null (dysgenic) myotubes to provide evidence for an unexplored RyR1-DHPR interaction that regulates the transition of the RyR1 between gating and leak states. Using double-barreled Ca(2+)-selective microelectrodes, we demonstrate that the lack of Ca(V)1.1 expression was associated with an increased myoplasmic resting [Ca(2+)] ([Ca(2+)](rest)), increased resting sarcolemmal Ca(2+) entry, and decreased sarcoplasmic reticulum (SR) Ca(2+) loading. Pharmacological control of the RyR1 leak state, using bastadin 5, reverted the three parameters to WT levels. The fact that Ca(2+) sparks are not more frequent in dysgenic than in WT myotubes adds support to the hypothesis that the leak state is a conformation distinct from gating RyR1s. We conclude from these data that this orthograde DHPR-to-RyR1 signal inhibits the transition of gated RyR1s into the leak state. Further, it suggests that the DHPR-uncoupled RyR1 population in WT muscle has a higher propensity to be in the leak conformation. RyR1 leak functions are to keep [Ca(2+)](rest) and the SR Ca(2+) content in the physiological range and thus maintain normal intracellular Ca(2+) homeostasis.

Functional expression of transgenic 1sDHPR channels in adult mammalian skeletal muscle fibres

We investigated the effects of the overexpression of two enhanced green fluorescent protein (EGFP)-tagged α1sDHPR variants on Ca2+ currents (ICa), charge movements (Q) and SR Ca2+ release of muscle fibres isolated from adult mice. Flexor digitorum brevis (FDB)muscles were transfected by in vivo electroporation with plasmids encoding for EGFP-α1sDHPR-wt and EGFP-α1sDHPR-T935Y (an isradipine-insensitive mutant). Two-photon laser scanning microscopy (TPLSM) was used to study the subcellular localization of transgenic proteins, while ICa, Q and Ca2+ release were studied electrophysiologically and optically under voltage-clamp conditions. TPLSM images demonstrated that most of the transgenic α1sDHPR was correctly targeted to the transverse tubular system (TTS). Immunoblotting analysis of crude extracts of transfected fibres demonstrated the synthesis of bona fide transgenic EGFP-α1sDHPR-wt in quantities comparable to that of native α1sDHPR. Though expression of both transgenic variants of the alpha subunit of the dihydropyridine receptor (α1sDHPR) resulted in ∼50% increase in Q, they surprisingly had no effect on the maximal Ca2+ conductance (gCa) nor the SR Ca2+ release. Nonetheless, fibres expressing EGFP-α1sDHPR-T935Y exhibited up to 70% isradipine-insensitive ICa (ICa-ins) with a right-shifted voltage dependence compared to that in control fibres. Interestingly, Qand SRCa2+ release also displayed right-shifted voltage dependence in fibres expressing EGFP-α1sDHPR-T935Y. In contrast, the midpoints of the voltage dependence of gCa, Q and Ca2+ release were not different from those in control fibres and in fibres expressing EGFP-α1sDHPR-wt. Overall, our results suggest that transgenic α1sDHPRs are correctly trafficked and inserted in the TTS membrane, and that a substantial fraction of the mworks as conductive Ca2+ channels capable of physiologically controlling the release of Ca2+ from the SR. A plausible corollary of this work is that the expression of transgenic variants of the α1sDHPR leads to the replacement of native channels interacting with the ryanodine receptor 1 (RyR1), thus demonstrating the feasibility of molecular remodelling of the triads in adult skeletal muscle fibres.

Impaired Orai1-mediated resting Ca2+ entry reduces the cytosolic [Ca2+] and sarcoplasmic reticulum Ca2+ loading in quiescent junctophilin 1 knock-out myotubes

In the absence of store depletion, plasmalemmal Ca(2+) permeability in resting muscle is very low, and its contribution in the maintenance of Ca(2+) homeostasis at rest has not been studied in detail. Junctophilin 1 knock-out myotubes (JP1 KO) have a severe reduction in store-operated Ca(2+) entry, presumably caused by physical alteration of the sarcoplasmic reticulum (SR) and T-tubule junction, leading to disruption of the SR signal sent by Stim1 to activate Orai1. Using JP1 KO myotubes as a model, we assessed the contribution of the Orai1-mediated Ca(2+) entry pathway on overall Ca(2+) homeostasis at rest with no store depletion. JP1 KO myotubes have decreased Ca(2+) entry, [Ca(2+)](rest), and intracellular Ca(2+) content compared with WT myotubes and unlike WT myotubes, are refractory to BTP2, a Ca(2+) entry blocker. JP1 KO myotubes show down-regulation of Orai1 and Stim1 proteins, suggesting that this pathway may be important in the control of resting Ca(2+) homeostasis. WT myotubes stably transduced with Orai1(E190Q) had similar alterations in their resting Ca(2+) homeostasis as JP1 KO myotubes and were also unresponsive to BTP2. JP1 KO cells show decreased expression of TRPC1 and -3 but overexpress TRPC4 and -6; on the other hand, the TRPC expression profile in Orai1(E190Q) myotubes was comparable with WT. These data suggest that an important fraction of resting plasmalemmal Ca(2+) permeability is mediated by the Orai1 pathway, which contributes to the control of [Ca(2+)](rest) and resting Ca(2+) stores and that this pathway is defective in JP1 KO myotubes.

Ablation of skeletal muscle triadin impairs FKBP12/RyR1 channel interactions essential for maintaining resting cytoplasmic Ca2+

Previously, we have shown that lack of expression of triadins in skeletal muscle cells results in significant increase of myoplasmic resting free Ca(2+) ([Ca(2+)](rest)), suggesting a role for triadins in modulating global intracellular Ca(2+) homeostasis. To understand this mechanism, we study here how triadin alters [Ca(2+)](rest), Ca(2+) release, and Ca(2+) entry pathways using a combination of Ca(2+) microelectrodes, channels reconstituted in bilayer lipid membranes (BLM), Ca(2+), and Mn(2+) imaging analyses of myotubes and RyR1 channels obtained from triadin-null mice. Unlike WT cells, triadin-null myotubes had chronically elevated [Ca(2+)](rest) that was sensitive to inhibition with ryanodine, suggesting that triadin-null cells have increased basal RyR1 activity. Consistently, BLM studies indicate that, unlike WT-RyR1, triadin-null channels more frequently display atypical gating behavior with multiple and stable subconductance states. Accordingly, pulldown analysis and fluorescent FKBP12 binding studies in triadin-null muscles revealed a significant impairment of the FKBP12/RyR1 interaction. Mn(2+) quench rates under resting conditions indicate that triadin-null cells also have higher Ca(2+) entry rates and lower sarcoplasmic reticulum Ca(2+) load than WT cells. Overexpression of FKBP12.6 reverted the null phenotype, reducing resting Ca(2+) entry, recovering sarcoplasmic reticulum Ca(2+) content levels, and restoring near normal [Ca(2+)](rest). Exogenous FKBP12.6 also reduced the RyR1 channel P(o) but did not rescue subconductance behavior. In contrast, FKBP12 neither reduced P(o) nor recovered multiple subconductance gating. These data suggest that elevated [Ca(2+)](rest) in triadin-null myotubes is primarily driven by dysregulated RyR1 channel activity that results in part from impaired FKBP12/RyR1 functional interactions and a secondary increased Ca(2+) entry at rest.

RyR1-mediated Ca2+ leak and Ca2+ entry determine resting intracellular Ca2+ in skeletal myotubes

The control of resting free Ca(2+) in skeletal muscle is thought to be a balance of channels, pumps, and exchangers in both the sarcolemma and sarcoplasmic reticulum. We explored these mechanisms using pharmacologic and molecular perturbations of genetically engineered (dyspedic) muscle cells that constitutively lack expression of the skeletal muscle sarcoplasmic reticulum Ca(2+) release channels, RyR1 and RyR3. We demonstrate here that expression of RyR1 is responsible for more than half of total resting Ca(2+) concentration ([Ca(2+)](rest)) measured in wild type cells. The elevated [Ca(2+)](rest) in RyR1-expressing cells is not a result of active gating of the RyR1 channel but instead is accounted for by the RyR1 ryanodine-insensitive Ca(2+) leak conformation. In addition, we demonstrate that basal sarcolemmal Ca(2+) influx is also governed by RyR1 expression and contributes in the regulation of [Ca(2+)](rest) in skeletal myotubes.

Antioxidant treatment of hindlimb-unloaded mouse counteracts fiber type transition but not atrophy of disused muscles

Oxidative stress was proposed as a trigger of muscle impairment in various muscle diseases. The hindlimb-unloaded (HU) rodent is a model of disuse inducing atrophy and slow-to-fast transition of postural muscles. Here, mice unloaded for 14 days were chronically treated with the selective antioxidant trolox. After HU, atrophy was more pronounced in the slow-twitch soleus muscle (Sol) than in the fast-twitch gastrocnemius and tibialis anterior muscles, and was absent in extensor digitorum longus muscle. In accord with the phenotype transition, HU Sol showed a reduced expression of myosin heavy chain type 2A (MHC-2A) and increase in MHC-2X and MHC-2B isoforms. In parallel, HU Sol displayed an increased sarcolemma chloride conductance related to an increased expression of ClC-1 channels, changes in excitability parameters, a positive shift of the mechanical threshold, and a decrease of the resting cytosolic calcium concentration. Moreover, the level of lipoperoxidation increased proportionally to the degree of atrophy of each muscle type. As expected, trolox treatment fully prevented oxidative stress in HU mice. Atrophy was not prevented but the drug significantly attenuated Sol phenotypic transition and excitability changes. Trolox treatment had no effect on control mice. These results suggest possible benefits of antioxidants in protecting muscle against disuse.

The cell boundary theorem: a simple law of the control of cytosolic calcium concentration

Many molecular biological interventions in current use, as well as inheritable disease conditions, modify the intracellular endowment of molecules that bind Ca(2+) or channels and pumps that transfer it to and from intracellular storage organelles. A simple law, named the "cell boundary theorem," states that intracellular alterations cannot directly result in changes in the cytosolic concentration, [Ca(2+)](i), in a true resting state. A demonstration of the validity of this theorem is provided. Several examples are then discussed of interventions or diseases that increase leak of Ca(2+) from storage organelles and result in greater resting [Ca(2+)](i). According to the theorem, the increase in [Ca(2+)](i) cannot be a direct consequence of the greater leak. Its primary cause must be a change of the fluxes at the level of the plasmalemma, caused in turn by the increase in leak through some sort of "store-operated Ca(2+) entry." While the law is discussed in terms of Ca(2+) homeostasis, it applies to any solute that may be transported by the plasma membrane.

The extract of Ginkgo biloba EGb 761 reactivates a juvenile profile in the skeletal muscle of sarcopenic rats by transcriptional reprogramming

Background

Sarcopenia is a major public health problem in industrialized nations, placing an increasing burden on public healthcare systems because the loss of skeletal muscle mass and strength that characterizes this affection increases the dependence and the risk of injury caused by sudden falls in elderly people. Albeit exercise and caloric restriction improve sarcopenia-associated decline of the muscular performances, a more suitable and focused pharmacological treatment is still lacking.

Methodology/Principal Findings

In order to evaluate such a possible treatment, we investigated the effects of EGb 761, a Ginkgo biloba extract used in chronic age-dependent neurological disorders, on the function of the soleus muscle in aged rats. EGb 761 induced a gain in muscular mass that was associated with an improvement of the muscular performances as assessed by biochemical and electrophysiological tests. DNA microarray analysis shows that these modifications are accompanied by the transcriptional reprogramming of genes related to myogenesis through the TGFβ signaling pathway and to energy production via fatty acids and glucose oxidation. EGb 761 restored a more juvenile gene expression pattern by regenerating the aged muscle and reversing the age-related metabolic shift from lipids to glucose utilization.

Conclusions/Significance

Thus, EGb 761 may represent a novel treatment for sarcopenia both more manageable and less cumbersome than exercise and caloric restriction.

Altered expression of triadin 95 causes parallel changes in localized Ca2+ release events and global Ca2+ signals in skeletal muscle cells in culture

The 95 kDa triadin (Trisk 95), an integral protein of the sarcoplasmic reticular membrane in skeletal muscle, interacts with both the ryanodine receptor (RyR) and calsequestrin. While its role in the regulation of calcium homeostasis has been extensively studied, data are not available on whether the overexpression or the interference with the expression of Trisk 95 would affect calcium sparks the localized events of calcium release (LCRE). In the present study LCRE and calcium transients were studied using laser scanning confocal microscopy on C2C12 cells and on primary cultures of skeletal muscle. Liposome- or adenovirus-mediated Trisk 95 overexpression and shRNA interference with triadin translation were used to modify the level of the protein. Stable overexpression in C2C12 cells significantly decreased the amplitude and frequency of calcium sparks, and the frequency of embers. In line with these observations, depolarization-evoked calcium transients were also suppressed. Similarly, adenoviral transfection of Trisk 95 into cultured mouse skeletal muscle cells significantly decreased both the frequency and amplitude of spontaneous global calcium transients. Inhibition of endogenous triadin expression by RNA interference caused opposite effects. Primary cultures of rat skeletal muscle cells expressing endogenous Trisk 95 readily generated spontaneous calcium transients but rarely produced calcium sparks. Their transfection with specific shRNA sequence significantly reduced the triadin-specific immunoreactivity. Functional experiments on these cells revealed that while caffeine-evoked calcium transients were reduced, LCRE appeared with higher frequency. These results suggest that Trisk 95 negatively regulates RyR function by suppressing localized calcium release events and global calcium signals in cultured muscle cells.

Full length ryanodine receptor subtype 3 encodes spontaneous calcium oscillations in native duodenal smooth muscle cells

Two isoforms of the ryanodine receptor subtype 3 (RYR3) have been described in smooth muscle. The RYR3 short isoform (RYR3S) negatively regulates the calcium-induced calcium release mechanism encoded by the RYR2, whereas the role of the full length isoform of RYR3 (RYR3L) was still unclear. Here, we describe RYR-dependent spontaneous Ca(2+) oscillations measured in 10% of native duodenum myocytes. We investigated the role of RYR3 isoforms in these spontaneous Ca(2+) signals. Inhibition of RYR3S expression by antisense oligonucleotides revealed that both RYR2 and RYR3L were able to propagate spontaneous Ca(2+) waves that were distinguishable by frequency analysis. When RYR3L expression was inhibited, the spontaneous Ca(2+) oscillations were never observed, indicating that RYR3S inhibited the function of RYR2. RYR2 expression inhibition led to Ca(2+) oscillations identical to those observed in control cells suggesting that RYR3S did not functionally interact with RYR3L. The presence and frequency of RYR3L-dependent Ca(2+) oscillations were dependent on sarcoplasmic reticulum Ca(2+) content as revealed by long-term changes of the extracellular Ca(2+) concentration. Our study shows that, in native duodenal myocytes, the spontaneous Ca(2+) waves are encoded by the RYR3L alone, which activity is regulated by sarcoplasmic reticulum Ca(2+) loading.

Triadins modulate intracellular Ca(2+) homeostasis but are not essential for excitation-contraction coupling in skeletal muscle

To unmask the role of triadin in skeletal muscle we engineered pan-triadin-null mice by removing the first exon of the triadin gene. This resulted in a total lack of triadin expression in both skeletal and cardiac muscle. Triadin knockout was not embryonic or birth-lethal, and null mice presented no obvious functional phenotype. Western blot analysis of sarcoplasmic reticulum (SR) proteins in skeletal muscle showed that the absence of triadin expression was associated with down-regulation of Junctophilin-1, junctin, and calsequestrin but resulted in no obvious contractile dysfunction. Ca(2+) imaging studies in null lumbricalis muscles and myotubes showed that the lack of triadin did not prevent skeletal excitation-contraction coupling but reduced the amplitude of their Ca(2+) transients. Additionally, null myotubes and adult fibers had significantly increased myoplasmic resting free Ca(2+).[(3)H]Ryanodine binding studies of skeletal muscle SR vesicles detected no differences in Ca(2+) activation or Ca(2+) and Mg(2+) inhibition between wild-type and triadin-null animals. Subtle ultrastructural changes, evidenced by the appearance of longitudinally oriented triads and the presence of calsequestrin in the sacs of the longitudinal SR, were present in fast but not slow twitch-null muscles. Overall, our data support an indirect role for triadin in regulating myoplasmic Ca(2+) homeostasis and organizing the molecular complex of the triad but not in regulating skeletal-type excitation-contraction coupling.

T-tubule formation in cardiacmyocytes: two possible mechanisms?

We have followed the differentiation of transverse (T) tubules and of the associations between sarcoplasmic reticulum (SR) and either the plasmalemma (peripheral couplings) or the T tubules (dyads) in postnatal rat ventricular myocytes using electron microscopy. Dyads and peripheral couplings are collectively called Ca(2+) Release Units (CRUs) because they are the sites at which Ca(2+) is released from the SR. Profiles of T tubules, caveolae and dyads are mostly at the cell edge in early postnatal days and are found with increased frequency in the cell interior during the first two postnatal weeks. Using ferritin to trace continuity of T tubules lumen with the extracellular space, we find that some of T tubules (between approximately 6 and 25%), either singly or within dyads, lack ferritin in their lumen. The percentage of tubules that do not contain ferritin decreases slightly during postnatal differentiation and is not very different at the cells' edges and interior. We propose that T tubules form as invaginations of the plasmalemma that penetrate inward driven by accrual of membrane lipids and specific proteins. This occurs by a dual mechanism: either by the independent flow of SR and T tubule proteins into the two separate membranes or by the fusion of preformed vesicle tandems into the dyads. Most of the CRUs (approximately 86%) are constituted by peripheral couplings and ferritin containing dyads, thus constituting CRUs in which Ca(2+ )release from the SR is initiated by a membrane depolarization. In the remaining CRUs, activation of Ca(2+) release must be dependent on some other mechanisms.

Regulation of ryanodine receptors from skeletal and cardiac muscle during rest and excitation

1. In muscle, intracellular calcium concentration, hence skeletal muscle force and cardiac output, is regulated by uptake and release of calcium from the sarcoplasmic reticulum (SR). The ryanodine receptor (RyR) forms the calcium release channel in the SR. 2. Calcium release through RyRs is modulated by a wide variety of endogenous molecules, including small diffusible ligands such as ATP, Ca2+ and Mg2+. The regulation of RyR channels by ATP, Ca2+ and Mg2+ is a complex interplay of several regulatory mechanisms, which are still being unravelled. Consequently, it is not clearly known how RyRs are regulated in resting muscle and during contraction. 3. The present paper reviews factors controlling the activity of RyRs in skeletal and cardiac muscle with an emphasis on mechanistic insights derived from single channel recording methods. 4. In addition, the nature of dihydropyridine receptor (DHPR) control of RyRs in skeletal muscle derived from experiments with peptide fragments of the DHPR II-III loop is reviewed. 5. Finally, recent experiments on coupled RyRs in lipid bilayers and their potential for resolving the elusive mechanisms controlling calcium release during cardiac contraction are discussed.

RyR isoforms and fibre type-specific expression of proteins controlling intracellular calcium concentration in skeletal muscles

Muscle fibres which shorten with high maximum shortening velocity also exhibit fast kinetics of contraction, i.e. short values of time to peak tension and time to half relaxation. This short review aims to discuss the molecular basis of such correlation, to reach, based on the available literature, an answer to the question whether there is a correlation in expression of proteins determining shortening velocity, myosin isoforms in the first place, and proteins controlling cytosolic calcium concentration and its variations at rest or during contraction. Although the isoforms of RyR, the sarcoplasmic calcium release channels, do not show a tightly coordinated expression with myosin isoforms, other proteins involved in controlling intracellular calcium do. This is likely sufficient to guarantee the correlation between maximum shortening velocity and speed of isometric contraction.

Modulation of calcium signalling by dominant negative splice variant of ryanodine receptor subtype 3 in native smooth muscle cells

The ryanodine receptor subtype 3 (RYR3) is expressed ubiquitously but its physiological function varies from cell to cell. Here, we investigated the role of a dominant negative RYR3 isoform in Ca2+ signalling in native smooth muscle cells. We used intranuclear injection of antisense oligonucleotides to specifically inhibit endogenous RYR3 isoform expression. In mouse duodenum myocytes expressing RYR2 subtype and both spliced and non-spliced RYR3 isoforms, RYR2 and non-spliced RYR3 were activated by caffeine whereas the spliced RYR3 was not. Only RYR2 was responsible for the Ca2+-induced Ca2+ release mechanism that amplified Ca2+ influx- or inositol 1,4,5-trisphosphate-induced Ca2+ signals. However, the spliced RYR3 negatively regulated RYR2 leading to the decrease of amplitude and upstroke velocity of Ca2+ signals. Immunostaining in injected cells showed that the spliced RYR3 was principally expressed near the plasma membrane whilst the non-spliced isoform was revealed around the nucleus. This study shows for the first time that the short isoform of RYR3 controls Ca2+ release through RYR2 in native smooth muscle cells.

Concerted vs. sequential. Two activation patterns of vast arrays of intracellular Ca2+ channels in muscle

To signal cell responses, Ca²⁺ is released from storage through intracellular Ca²⁺ channels. Unlike most plasmalemmal channels, these are clustered in quasi-crystalline arrays, which should endow them with unique properties. Two distinct patterns of local activation of Ca²⁺ release were revealed in images of Ca²⁺ sparks in permeabilized cells of amphibian muscle. In the presence of sulfate, an anion that enters the SR and precipitates Ca²⁺, sparks became wider than in the conventional, glutamate-based solution. Some of these were “protoplatykurtic” (had a flat top from early on), suggesting an extensive array of channels that activate simultaneously. Under these conditions the rate of production of signal mass was roughly constant during the rise time of the spark and could be as high as 5 μm³ ms⁻¹, consistent with a release current >50 pA since the beginning of the event. This pattern, called “concerted activation,” was observed also in rat muscle fibers. When sulfate was combined with a reduced cytosolic [Ca²⁺] (50 nM) these sparks coexisted (and interfered) with a sequential progression of channel opening, probably mediated by Ca²⁺-induced Ca²⁺ release (CICR). Sequential propagation, observed only in frogs, may require parajunctional channels, of RyR isoform β, which are absent in the rat. Concerted opening instead appears to be a property of RyR α in the amphibian and the homologous isoform 1 in the mammal.

A probable role of dihydropyridine receptors in repression of Ca2+ sparks demonstrated in cultured mammalian muscle

To activate skeletal muscle contraction, action potentials must be sensed by dihydropyridine receptors (DHPRs) in the T tubule, which signal the Ca(2+) release channels or ryanodine receptors (RyRs) in the sarcoplasmic reticulum (SR) to open. We demonstrate here an inhibitory effect of the T tubule on the production of sparks of Ca(2+) release. Murine primary cultures were confocally imaged for Ca(2+) detection and T tubule visualization. After 72 h of differentiation, T tubules extended from the periphery for less than one-third of the myotube radius. Spontaneous Ca(2+) sparks were found away from the region of cells where tubules were found. Immunostaining showed RyR1 and RyR3 isoforms in all areas, implying inhibition of both isoforms by a T tubule component. To test for a role of DHPRs in this inhibition, we imaged myotubes from dysgenic mice (mdg) that lack DHPRs. These exhibited T tubule development similar to that of normal myotubes, but produced few sparks, even in regions where tubules were absent. To increase spark frequency, a high-Ca(2+) saline with 1 mM caffeine was used. Wild-type cells in this saline plus 50 microM nifedipine retained the topographic suppression pattern of sparks, but dysgenic cells in high-Ca(2+) saline did not. Shifted excitation and emission ratios of indo-1 in the cytosol or mag-indo-1 in the SR were used to image [Ca(2+)] in these compartments. Under the conditions of interest, wild-type and mdg cells had similar levels of free [Ca(2+)] in cytosol and SR. These data suggest that DHPRs play a critical role in reducing the rate of spontaneous opening of Ca(2+) release channels and/or their susceptibility to Ca(2+)-induced activation, thereby suppressing the production of Ca(2+) sparks.