Ca2+ sparks as a plastic signal for skeletal muscle health, aging, and dystrophy

Proteomics-based identification of different training adaptations of aged skeletal muscle following long-term high-intensity interval and moderate-intensity continuous training in aged rats

Aging-associated loss of skeletal muscle mass and force increases the risk of falls, impairs mobility, and leads to a reduced quality of life. High-intensity interval training (HIIT) is superior to moderate-intensity continuous training (MICT) for improving morphological and metabolic adaptations of skeletal muscle in older adults, but the underlying mechanism is unknown. Aged female rats underwent HIIT and MICT for 8 months, and their differential impacts on skeletal muscle proteome were investigated. HIIT resulted in a larger improvement in grip strength and fiber cross-sectional area, with similar increases in inclined plane performance and time to exhaustion. Proteomic analysis showed that common training adaptations of both protocols included changes to muscle contraction, focal adhesion signaling, mitochondrial function, apoptosis and regeneration, and anti-oxidation, whereas protein processing in the endoplasmic reticulum and adipocytokine signaling were specifically altered in the MICT and HIIT groups, respectively. Immunoblotting showed that upregulation of the adiponectin/AMPK signaling pathway may be associated with improvements in autophagy, oxidative stress, mitochondrial function, and apoptosis in aged skeletal muscle following HIIT. Thus, understanding the molecular differences in training adaptations from these two exercise modalities may aid in combatting sarcopenia.

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.

Osmosensation in TRPV2 dominant negative expressing skeletal muscle fibres

Increased plasma osmolarity induces intracellular water depletion and cell shrinkage (CS) followed by activation of a regulatory volume increase (RVI). In skeletal muscle, the hyperosmotic shock-induced CS is accompanied by a small membrane depolarization responsible for a release of Ca(2+) from intracellular pools. Hyperosmotic shock also induces phosphorylation of STE20/SPS1-related proline/alanine-rich kinase (SPAK). TRPV2 dominant negative expressing fibres challenged with hyperosmotic shock present a slower membrane depolarization, a diminished Ca(2+) response, a smaller RVI response, a decrease in SPAK phosphorylation and defective muscle function. We suggest that hyperosmotic shock induces TRPV2 activation, which accelerates muscle cell depolarization and allows the subsequent Ca(2+) release from the sarcoplasmic reticulum, activation of the Na(+) -K(+) -Cl(-) cotransporter by SPAK, and the RVI response. Increased plasma osmolarity induces intracellular water depletion and cell shrinkage followed by activation of a regulatory volume increase (RVI). In skeletal muscle, this is accompanied by transverse tubule (TT) dilatation and by a membrane depolarization responsible for a release of Ca(2+) from intracellular pools. We observed that both hyperosmotic shock-induced Ca(2+) transients and RVI were inhibited by Gd(3+) , ruthenium red and GsMTx4 toxin, three inhibitors of mechanosensitive ion channels. The response was also completely absent in muscle fibres overexpressing a non-permeant, dominant negative (DN) mutant of the transient receptor potential, V2 isoform (TRPV2) ion channel, suggesting the involvement of TRPV2 or of a TRP isoform susceptible to heterotetramerization with TRPV2. The release of Ca(2+) induced by hyperosmotic shock was increased by cannabidiol, an activator of TRPV2, and decreased by tranilast, an inhibitor of TRPV2, suggesting a role for the TRPV2 channel itself. Hyperosmotic shock-induced membrane depolarization was impaired in TRPV2-DN fibres, suggesting that TRPV2 activation triggers the release of Ca(2+) from the sarcoplasmic reticulum by depolarizing TTs. RVI requires the sequential activation of STE20/SPS1-related proline/alanine-rich kinase (SPAK) and NKCC1, a Na(+) -K(+) -Cl(-) cotransporter, allowing ion entry and driving osmotic water flow. In fibres overexpressing TRPV2-DN as well as in fibres in which Ca(2+) transients were abolished by the Ca(2+) chelator BAPTA, the level of P-SPAK(Ser373) in response to hyperosmotic shock was reduced, suggesting a modulation of SPAK phosphorylation by intracellular Ca(2+) . We conclude that TRPV2 is involved in osmosensation in skeletal muscle fibres, acting in concert with P-SPAK-activated NKCC1.

Assessment of calcium sparks in intact skeletal muscle fibers

Maintaining homeostatic Ca(2+) signaling is a fundamental physiological process in living cells. Ca(2+) sparks are the elementary units of Ca(2+) signaling in the striated muscle fibers that appear as highly localized Ca(2+) release events mediated by ryanodine receptor (RyR) Ca(2+) release channels on the sarcoplasmic reticulum (SR) membrane. Proper assessment of muscle Ca(2+) sparks could provide information on the intracellular Ca(2+) handling properties of healthy and diseased striated muscles. Although Ca(2+) sparks events are commonly seen in resting cardiomyocytes, they are rarely observed in resting skeletal muscle fibers; thus there is a need for methods to generate and analyze sparks in skeletal muscle fibers. Detailed here is an experimental protocol for measuring Ca(2+) sparks in isolated flexor digitorm brevis (FDB) muscle fibers using fluorescent Ca(2+) indictors and laser scanning confocal microscopy. In this approach, isolated FDB fibers are exposed to transient hypoosmotic stress followed by a return to isotonic physiological solution. Under these conditions, a robust Ca(2+) sparks response is detected adjacent to the sarcolemmal membrane in young healthy FDB muscle fibers. Altered Ca(2+) sparks response is detected in dystrophic or aged skeletal muscle fibers. This approach has recently demonstrated that membrane-delimited signaling involving cross-talk between inositol (1,4,5)-triphosphate receptor (IP3R) and RyR contributes to Ca(2+) spark activation in skeletal muscle. In summary, our studies using osmotic stress induced Ca(2+) sparks showed that this intracellular response reflects a muscle signaling mechanism in physiology and aging/disease states, including mouse models of muscle dystrophy (mdx mice) or amyotrophic lateral sclerosis (ALS model).

Major aging-associated RNA expressions change at two distinct age-positions

Background

Genome-wide expression profiles are altered during biological aging and can describe molecular regulation of tissue degeneration. Age-regulated mRNA expression trends from cross-sectional studies could describe how aging progresses. We developed a novel statistical methodology to identify age-regulated expression trends in cross-sectional datasets.

Results

We studied six cross-sectional RNA expression profiles from different human tissues. Our methodology, capable of overcoming technical and genetic background differences, identified an age-regulation in four of the tissues. For the identification of expression trends, five regression models were compared and the quadratic model was found as the most suitable for this study. After k-means clustering of the age-associated probes, expression trends were found to change at two major age-positions in brain cortex and in Vastus lateralis muscles. The first age-position was found to occur during the fifth decade and a later one during the eighth decade. In kidney cortex, however, only one age-position was identified correlating with a late age-position. Functional mapping of genes at each age-position suggests that calcium homeostasis and lipid metabolisms are initially affected and subsequently, in elderly mitochondria, apoptosis and hormonal signaling pathways are affected.

Conclusions

Our results suggest that age-associated temporal changes in human tissues progress at distinct age-positions, which differ between tissues and in their molecular composition.

Altered functional differentiation of mesoangioblasts in a genetic myopathy

Mutations underlying genetic cardiomyopathies might affect differentiation commitment of resident progenitor cells. Cardiac mesoangioblasts (cMabs) are multipotent progenitor cells resident in the myocardium. A switch from cardiac to skeletal muscle differentiation has been recently described in cMabs from β-sarcoglycan-null mice (βSG^−/−), a murine model of genetic myopathy with early myocardial involvement. Although complementation with βSG gene was inconsequential, knock-in of miRNA669a (missing in βSG^−/− cMabs) partially rescued the mutation-induced molecular phenotype. Here, we undertook a detailed evaluation of functional differentiation of βSG^−/− cMabs and tested the effects of miRNA669a-induced rescue in vitro. To this end, cMabs were compared with neonatal cardiomyocytes (CMs) and skeletal muscle C2C12 cells, representative of cardiac and skeletal muscle respectively. Consistent with previous data on molecular patterns, electrophysiological and Ca²⁺-handling properties of βSG^−/− cMabs were closer to C2C12 cells than to CM ones. Nevertheless, subtler aspects, including action potential contour, Ca²⁺-spark properties and RyR isoform expression, distinguished βSG^−/− cMabs from C2C12 cells. Contrary to previous reports, wild-type cMabs failed to show functional differentiation towards either cell type. Knock-in of miRNA669a in βSG^−/− cMabs rescued the wild-type functional phenotype, i.e. it completely prevented development of skeletal muscle functional responses. We conclude that miRNA669a expression, ablated by βSG deletion, may prevent functional differentiation of cMabs towards the skeletal muscle phenotype.

What is dynapenia?

Dynapenia (pronounced dahy-nuh-pē-nē-a, Greek translation for poverty of strength, power, or force) is the age-associated loss of muscle strength that is not caused by neurologic or muscular diseases. Dynapenia predisposes older adults to an increased risk for functional limitations and mortality. For the past several decades, the literature has largely focused on muscle size as the primary cause of dynapenia; however, recent findings have clearly demonstrated that muscle size plays a relatively minor role. Conversely, subclinical deficits in the structure and function of the nervous system and/or impairments in the intrinsic force-generating properties of skeletal muscle are potential antecedents to dynapenia. This review highlights in the contributors to dynapenia and the etiology and risk factors that predispose individuals to dynapenia. In addition, we address the role of nutrition in the muscular and neurologic systems for the preservation of muscle strength throughout the life span.

Detection of calcium sparks in intact and permeabilized skeletal muscle fibers

Ca(2+) sparks are the elementary units of Ca(2+) signaling in striated muscle fibers that appear as highly localized Ca(2+) release events through ryanodine receptor (RyR) Ca(2+) release channels in the sarcoplasmic reticulum (SR). While these events are commonly observed in resting cardiac myocytes, they are rarely seen in resting skeletal muscle fibers. Since Ca(2+) spark analysis can provide extensive data on the Ca(2+) handling characteritsics of normal and diseased striated muscle, there has been interest in developing methods for observing Ca(2+) sparks in skeletal muscle. Previously, we discovered that stress generated by osmotic pressure changes induces a robust Ca(2+) spark response confined in close spatial proximity to the sarcolemmal membrane in wild-type intact mammalian muscles. Our studies showed these peripheral Ca(2+) sparks (PCS) were altered in dystrophic or aged skeletal muscles. Other methods to induce Ca(2+) sparks include permeabilization of the sarcolemmal membrane with detergents, such as saponin. In this chapter, we will discuss the methods for isolation of muscle fibers, the techniques for inducing Ca(2+) sparks in these isolated fibers, and provide guidance on the analysis of data from these experiments.

Disrupted membrane structure and intracellular Ca²⁺ signaling in adult skeletal muscle with acute knockdown of Bin1

Efficient intracellular Ca²⁺ ([Ca²⁺]i) homeostasis in skeletal muscle requires intact triad junctional complexes comprised of t-tubule invaginations of plasma membrane and terminal cisternae of sarcoplasmic reticulum. Bin1 consists of a specialized BAR domain that is associated with t-tubule development in skeletal muscle and involved in tethering the dihydropyridine receptors (DHPR) to the t-tubule. Here, we show that Bin1 is important for Ca²⁺ homeostasis in adult skeletal muscle. Since systemic ablation of Bin1 in mice results in postnatal lethality, in vivo electroporation mediated transfection method was used to deliver RFP-tagged plasmid that produced short –hairpin (sh)RNA targeting Bin1 (shRNA-Bin1) to study the effect of Bin1 knockdown in adult mouse FDB skeletal muscle. Upon confirming the reduction of endogenous Bin1 expression, we showed that shRNA-Bin1 muscle displayed swollen t-tubule structures, indicating that Bin1 is required for the maintenance of intact membrane structure in adult skeletal muscle. Reduced Bin1 expression led to disruption of t-tubule structure that was linked with alterations to intracellular Ca²⁺ release. Voltage-induced Ca²⁺ released in isolated single muscle fibers of shRNA-Bin1 showed that both the mean amplitude of Ca²⁺ current and SR Ca²⁺ transient were reduced when compared to the shRNA-control, indicating compromised coupling between DHPR and ryanodine receptor 1. The mean frequency of osmotic stress induced Ca²⁺ sparks was reduced in shRNA-Bin1, indicating compromised DHPR activation. ShRNA-Bin1 fibers also displayed reduced Ca²⁺ sparks' amplitude that was attributed to decreased total Ca²⁺ stores in the shRNA-Bin1 fibers. Human mutation of Bin1 is associated with centronuclear myopathy and SH3 domain of Bin1 is important for sarcomeric protein organization in skeletal muscle. Our study showing the importance of Bin1 in the maintenance of intact t-tubule structure and ([Ca²⁺]i) homeostasis in adult skeletal muscle could provide mechanistic insight on the potential role of Bin1 in skeletal muscle contractility and pathology of myopathy.

DHPR activation underlies SR Ca2+ release induced by osmotic stress in isolated rat skeletal muscle fibers

Changes in skeletal muscle volume induce localized sarcoplasmic reticulum (SR) Ca(2+) release (LCR) events, which are sustained for many minutes, suggesting a possible signaling role in plasticity or pathology. However, the mechanism by which cell volume influences SR Ca(2+) release is uncertain. In the present study, rat flexor digitorum brevis fibers were superfused with isoosmotic Tyrode's solution before exposure to either hyperosmotic (404 mOsm) or hypoosmotic (254 mOsm) solutions, and the effects on cell volume, membrane potential (E(m)), and intracellular Ca(2+) ([Ca(2+)](i)) were determined. To allow comparison with previous studies, solutions were made hyperosmotic by the addition of sugars or divalent cations, or they were made hypoosmotic by reducing [NaCl](o). All hyperosmotic solutions induced a sustained decrease in cell volume, which was accompanied by membrane depolarization (by 14-18 mV; n = 40) and SR Ca(2+) release. However, sugar solutions caused a global increase in [Ca(2+)](i), whereas solutions made hyperosmotic by the addition of divalent cations only induced LCR. Decreasing osmolarity induced an increase in cell volume and a negative shift in E(m) (by 15.04 +/- 1.85 mV; n = 8), whereas [Ca(2+)](i) was unaffected. However, on return to the isoosmotic solution, restoration of cell volume and E(m) was associated with LCR. Both global and localized SR Ca(2+) release were abolished by the dihydropyridine receptor inhibitor nifedipine by sustained depolarization of the sarcolemmal or by the addition of the ryanodine receptor 1 inhibitor tetracaine. Inhibitors of the Na-K-2Cl (NKCC) cotransporter markedly inhibited the depolarization associated with hyperosmotic shrinkage and the associated SR Ca(2+) release. These findings suggest (1) that the depolarization that accompanies a decrease in cell volume is the primary event leading to SR Ca(2+) release, and (2) that volume-dependent regulation of the NKCC cotransporter contributes to the observed changes in E(m). The differing effects of the osmotic agents can be explained by the screening of fixed charges by divalent ions.

Local calcium signals induced by hyper-osmotic stress in mammalian skeletal muscle cells

Strenuous activitiy of skeletal muscle leads to temporary osmotic dysbalance and isolated skeletal muscle fibers exposed to osmotic stress respond with characteristic micro-domain calcium signals. It has been suggested that osmotic stress targets transverse tubular (TT) dihydropyridine receptors (DHPRs) which normally serve as voltage-dependent activators of Ca release via ryanodine receptor (RyR1s) of the sarcoplasmic reticulum (SR). Here, we pursued this hypothesis by imaging the response to hyperosmotic solutions in both mouse skeletal muscle fibers and myotubes. Ca fluctuations in the cell periphery of fibers exposed to osmotic stress were accompanied by a substantial dilation of the peripheral TT. The Ca signals were completely inhibited by a conditioning depolarization that inactivates the DHPR. Dysgenic myotubes, lacking the DHP-receptor-alpha1-subunit, showed strongly reduced, yet not completely inhibited activity when stimulated with solutions of elevated tonicity. The results point to a modulatory, even though not essential, role of the DHP receptor for osmotic stress-induced Ca signals in skeletal muscle.

Calcium sparks

The calcium ion (Ca(2+)) is the simplest and most versatile intracellular messenger known. The discovery of Ca(2+) sparks and a related family of elementary Ca(2+) signaling events has revealed fundamental principles of the Ca(2+) signaling system. A newly appreciated "digital" subsystem consisting of brief, high Ca(2+) concentration over short distances (nanometers to microns) comingles with an "analog" global Ca(2+) signaling subsystem. Over the past 15 years, much has been learned about the theoretical and practical aspects of spark formation and detection. The quest for the spark mechanisms [the activation, coordination, and termination of Ca(2+) release units (CRUs)] has met unexpected challenges, however, and raised vexing questions about CRU operation in situ. Ample evidence shows that Ca(2+) sparks catalyze many high-threshold Ca(2+) processes involved in cardiac and skeletal muscle excitation-contraction coupling, vascular tone regulation, membrane excitability, and neuronal secretion. Investigation of Ca(2+) sparks in diseases has also begun to provide novel insights into hypertension, cardiac arrhythmias, heart failure, and muscular dystrophy. An emerging view is that spatially and temporally patterned activation of the digital subsystem confers on intracellular Ca(2+) signaling an exquisite architecture in space, time, and intensity, which underpins signaling efficiency, stability, specificity, and diversity. These recent advances in "sparkology" thus promise to unify the simplicity and complexity of Ca(2+) signaling in biology.

Immuno-proteomic approach to excitation--contraction coupling in skeletal and cardiac muscle: molecular insights revealed by the mitsugumins

A comprehensive understanding of excitation-contraction (E-C) coupling in skeletal and cardiac muscle requires that all the major components of the Ca(2+) release machinery be resolved. We utilized a unique immuno-proteomic approach to generate a monoclonal antibody library that targets proteins localized to the skeletal muscle triad junction, which provides a structural context to allow efficient E-C coupling. Screening of this library has identified several mitsugumins (MG); proteins that can be localized to the triad junction in mammalian skeletal muscle. Many of these proteins, including MG29 and junctophilin, are important components in maintaining the structural integrity of the triad junction. Other triad proteins, such as calumin, play a more direct role in regulation of muscle Ca(2+) homeostasis. We have recently identified a family of trimeric intracellular cation-selective (TRIC) channels that allow for K(+) movement into the endoplasmic or sarcoplasmic reticulum to counter a portion of the transient negative charge produced by Ca(2+) release into the cytosol. Further study of TRIC channel function and other novel mitsugumins will increase our understanding of E-C coupling and Ca(2+) homoeostasis in muscle physiology and pathophysiology.