Ryanodine receptor activity and store-operated Ca2+ entry: Critical regulators of Ca2+ content and function in skeletal muscle

Store-operated Ca2+ entry (SOCE) is critical to cell function. In skeletal muscle, SOCE has evolved alongside excitation-contraction coupling (EC coupling); as a result, it displays unique properties compared to SOCE in other cells. The plasma membrane of skeletal muscle is mostly internalized as the tubular system, with the tubules meeting the sarcoplasmic reticulum (SR) terminal cisternae, forming junctions where the proteins that regulate EC coupling and SOCE are positioned. In this review, we describe the properties and roles of SOCE based on direct measurements of Ca2+ influx during SR Ca2+ release and leak. SOCE is activated immediately and locally as the [Ca2+ ] of the junctional SR terminal cisternae ([Ca2+ ]jSR ) depletes. [Ca2+ ]jSR changes rapidly and steeply with increasing activity of the SR ryanodine receptor isoform 1 (RyR1). The high fidelity of [Ca2+ ]jSR with RyR1 activity probably depends on the SR Ca2+ -buffer calsequestrin that is located immediately behind RyR1 inside the SR. This arrangement provides in-phase activation and deactivation of SOCE with a large dynamic range, allowing precise grading of SOCE flux. The in-phase activation of SOCE as the SR partially depletes traps Ca2+ in the cytoplasm, preventing net Ca2+ loss. Mild presentation of RyR1 leak can occur under physiological conditions, providing fibre Ca2+ redistribution without changing fibre Ca2+ content. This condition preserves normal contractile function at the same time as increasing basal metabolic rate. However, higher RyR1 leak drives excess cytoplasmic and mitochondrial Ca2+ load, setting a deleterious intracellular environment that compromises the function of the skeletal muscle.

Ryanodine receptor leak triggers fiber Ca2+ redistribution to preserve force and elevate basal metabolism in skeletal muscle

Muscle contraction depends on tightly regulated Ca²⁺ release. Aberrant Ca²⁺ leak through ryanodine receptor 1 (RyR1) on the sarcoplasmic reticulum (SR) membrane can lead to heatstroke and malignant hyperthermia (MH) susceptibility, as well as severe myopathy. However, the mechanism by which Ca²⁺ leak drives these pathologies is unknown. Here, we investigate the effects of four mouse genotypes with increasingly severe RyR1 leak in skeletal muscle fibers. We find that RyR1 Ca²⁺ leak initiates a cascade of events that cause precise redistribution of Ca²⁺ among the SR, cytoplasm, and mitochondria through altering the Ca²⁺ permeability of the transverse tubular system membrane. This redistribution of Ca²⁺ allows mice with moderate RyR1 leak to maintain normal function; however, severe RyR1 leak with RYR1 mutations reduces the capacity to generate force. Our results reveal the mechanism underlying force preservation, increased ATP metabolism, and susceptibility to MH in individuals with gain-of-function RYR1 mutations.

Effects of high-intensity intermittent exercise on the contractile properties of human type I and type II skeletal muscle fibers

In vitro studies have shown that alterations in redox state can cause a range of opposing effects on the properties of the contractile apparatus in skeletal muscle fibers. To test whether and how redox changes occurring in vivo affect the contractile properties, vastus lateralis muscle fibers from seven healthy young adults were examined at rest (PRE) and following (POST) high-intensity intermittent cycling exercise. Individual mechanically skinned muscle fibers were exposed to heavily buffered solutions at progressively higher free [Ca²⁺] to determine their force-Ca²⁺ relationship. Following acute exercise, Ca²⁺ sensitivity was significantly decreased in type I fibers (by 0.06 pCa unit) but not in type II fibers (0.01 pCa unit). Specific force decreased after the exercise in type II fibers (-18%) but was unchanged in type I fibers. Treatment with the reducing agent dithiothreitol (DTT) caused a small decrease in Ca²⁺-sensitivity in type II fibers at PRE (by ∼0.014 pCa units) and a significantly larger decrease at POST (∼0.035 pCa units), indicating that the exercise had increased S-glutathionylation of fast troponin I. DTT treatment also increased specific force (by ∼4%), but only at POST. In contrast, DTT treatment had no effect on either parameter in type I fibers at either PRE or POST. In type I fibers, the decreased Ca²⁺ sensitivity was not due to reversible oxidative changes and may have contributed to a decrease in power production during vigorous exercises. In type II fibers, exercise-induced redox changes help counter the decline in Ca²⁺-sensitivity while causing a small decline in maximum force.NEW & NOTEWORTHY This study identified important cellular changes occurring in human skeletal muscle fibers following high-intensity intermittent exercise: 1) a decrease in contractile apparatus Ca²⁺ sensitivity in type I but not type II fibers, 2) a decrease in specific force only in type II muscle fibers, and 3) a redox-dependent increase in Ca²⁺ sensitivity occurring only in type II fibers, which would help maintain muscle performance by countering the normal metabolite-induced decline in Ca²⁺ sensitivity.

Effects of S-glutathionylation on the passive force-length relationship in skeletal muscle fibres of rats and humans

This study investigated the effect of S-glutathionylation on passive force in skeletal muscle fibres, to determine whether activity-related redox reactions could modulate the passive force properties of muscle. Mechanically-skinned fibres were freshly obtained from human and rat muscle, setting sarcomere length (SL) by laser diffraction. Larger stretches were required to produce passive force in human fibres compared to rat fibres, but there were no fibre-type differences in either species. When fibres were exposed to glutathione disulfide (GSSG; 20 mM, 15 min) whilst stretched (at a SL where passive force reached ~ 20% of maximal Ca²⁺-activated force, denoted as SL_{20 % max}), passive force was subsequently decreased at all SLs in both type I and type II fibres of rat and human (e.g., passive force at SL_{20 % max} decreased by 12 to 25%). This decrease was fully reversed by subsequent reducing treatment with dithiothreitol (DTT; 10 mM for 10 min). If freshly skinned fibres were initially treated with DTT, there was an increase in passive force in type II fibres (by 10 ± 3% and 9 ± 2% in rat and human fibres, respectively), but not in type I fibres. These results indicate that (i) S-glutathionylation, presumably in titin, causes a decrease in passive force in skeletal muscle fibres, but the reduction is relatively smaller than that reported in cardiac muscle, (ii) in rested muscle in vivo, there appears to be some level of reversible oxidative modification, probably involving S-glutathionylation of titin, in type II fibres, but not in type I fibres.

Cell specific differences in the protein abundances of GAPDH and Na(+),K(+)-ATPase in skeletal muscle from aged individuals

Na(+), K(+)-ATPase (NKA) isoforms (α1,α2,α3,β1,β2,β3) are involved in the maintenance of membrane potential and hence are important regulators of cellular homeostasis. Given the age-related decline in skeletal muscle function, we investigated whether the natural physiological process of aging is associated with altered abundance of NKA isoforms (α1,α2,α3,β1,β2,β3) or of the commonly used control protein, glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Importantly, measurements were made in both whole muscle or specific fiber types obtained from skeletal muscle biopsies. Seventeen healthy older (AGED, 69.4 ± 3.5 years, mean ± SD) and 14 younger (YOUNG, 25.5 ± 2.8 years) adults underwent a muscle biopsy for biochemical analyses. Comparing homogenates from AGED and YOUNG individuals revealed higher β3 isoform (p<0.05) and lower GAPDH (p<0.05). Analysis of individual fibers in muscle from YOUNG individuals, showed greater α3 and β2 isoforms, and more GAPDH in Type II compared with Type I fibers (p<0.05). In the AGED, GAPDH was higher in Type II compared with Type I fibers (p<0.05), there were no fiber type differences in the NKA isoforms (p>0.05). Compared with the same fiber type in YOUNG, α1 was greater (Type I) and α3 lower (Type II), while in both fiber types, β2 was lower, β3 greater and GAPDH lower, in muscle from AGED individuals (all p<0.05). Overall, we demonstrate that (i) GAPDH is an inappropriate choice of protein for normalization in all skeletal muscle research and (ii) full understanding of the role of NKA isoforms in human skeletal muscle requires consideration of age and muscle fiber type.

Ca(2+) leakage out of the sarcoplasmic reticulum is increased in type I skeletal muscle fibres in aged humans

KEY POINTS

The amount of Ca(2+) stored in the sarcoplasmic reticulum (SR) of muscle fibres is decreased in aged individuals, and an important question is whether this results from increased Ca(2+) leakage out through the Ca(2+) release channels (ryanodine receptors; RyRs). The present study examined the effects of blocking the RyRs with Mg(2+), or applying a strong reducing treatment, on net Ca(2+) accumulation by the SR in skinned muscle fibres from Old (∼70 years) and Young (∼24 years) adults. Raising cytoplasmic [Mg(2+)] and reducing treatment increased net SR Ca(2+) accumulation in type I fibres of Old subjects relative to that in Young. The densities of RyRs and dihydropyridine receptors were not significantly changed in the muscle of Old subjects. These findings indicate that oxidative modification of the RyRs causes increased Ca(2+) leakage from the SR in muscle fibres in Old subjects, which probably deleteriously affects normal muscle function both directly and indirectly.

ABSTRACT

The present study examined whether the lower Ca(2+) storage levels in the sarcoplasmic reticulum (SR) in vastus lateralis muscle fibres in Old (70 ± 4 years) relative to Young (24 ± 4 years) human subjects is the result of increased leakage of Ca(2+) out of the SR through the Ca(2+) release channels/ryanodine receptors (RyRs) and due to oxidative modification of the RyRs. SR Ca(2+) accumulation in mechanically skinned muscle fibres was examined in the presence of 1, 3 or 10 mm cytoplasmic Mg(2+) because raising [Mg(2+)] strongly inhibits Ca(2+) efflux through the RyRs. In type I fibres of Old subjects, SR Ca(2+) accumulation in the presence of 1 mm Mg(2+) approached saturation at shorter loading times than in Young subjects, consistent with Ca(2+) leakage limiting net uptake, and raising [Mg(2+)] to 10 mm in such fibres increased maximal SR Ca(2+) accumulation. No significant differences were seen in type II fibres. Treatment with dithiothreitol (10 mm for 5 min), a strong reducing agent, also increased maximal SR Ca(2+) accumulation at 1 mm Mg(2+) in type I fibres of Old subjects but not in other fibres. The densities of dihydropyridine receptors and RyRs were not significantly different in muscles of Old relative to Young subjects. These findings indicate that Ca(2+) leakage from the SR is increased in type I fibres in Old subjects by reversible oxidative modification of the RyRs; this increased SR Ca(2+) leak is expected to have both direct and indirect deleterious effects on Ca(2+) movements and muscle function.

Contractile properties and sarcoplasmic reticulum calcium content in type I and type II skeletal muscle fibres in active aged humans

KEY POINTS

Muscle weakness in old age is due in large part to an overall loss of skeletal muscle tissue, but it remains uncertain how much also stems from alterations in the properties of the individual muscle fibres. This study examined the contractile properties and amount of stored intracellular calcium in single muscle fibres of Old (70 ± 4 years) and Young (22 ± 3 years) adults. The maximum level of force production (per unit cross-sectional area) in fast twitch fibres in Old subjects was lower than in Young subjects, and the fibres were also less sensitive to activation by calcium. The amount of calcium stored inside muscle fibres and available to trigger contraction was also lower in both fast- and slow-twitch muscle fibres in the Old subjects. These findings indicate that muscle weakness in old age stems in part from an impaired capacity for force production in the individual muscle fibres.

ABSTRACT

This study examined the contractile properties and sarcoplasmic reticulum (SR) Ca(2+) content in mechanically skinned vastus lateralis muscle fibres of Old (70 ± 4 years) and Young (22 ± 3 years) humans to investigate whether changes in muscle fibre properties contribute to muscle weakness in old age. In type II fibres of Old subjects, specific force was reduced by ∼17% and Ca(2+) sensitivity was also reduced (pCa50 decreased ∼0.05 pCa units) relative to that in Young. S-Glutathionylation of fast troponin I (TnIf ) markedly increased Ca(2+) sensitivity in type II fibres, but the increase was significantly smaller in Old versus Young (+0.136 and +0.164 pCa unit increases, respectively). Endogenous and maximal SR Ca(2+) content were significantly smaller in both type I and type II fibres in Old subjects. In fibres of Young, the SR could be nearly fully depleted of Ca(2+) by a combined caffeine and low Mg(2+) stimulus, whereas in fibres of Old the amount of non-releasable Ca(2+) was significantly increased (by > 12% of endogenous Ca(2+) content). Western blotting showed an increased proportion of type I fibres in Old subjects, and increased amounts of calsequestrin-2 and calsequestrin-like protein. The findings suggest that muscle weakness in old age is probably attributable in part to (i) an increased proportion of type I fibres, (ii) a reduction in both maximum specific force and Ca(2+) sensitivity in type II fibres, and also a decreased ability of S-glutathionylation of TnIf to counter the fatiguing effects of metabolites on Ca(2+) sensitivity, and (iii) a reduction in the amount of releasable SR Ca(2+) in both fibre types.

Single-fiber expression and fiber-specific adaptability to short-term intense exercise training of Na+-K+-ATPase α- and β-isoforms in human skeletal muscle

The Na(+)-K(+)-ATPase (NKA) plays a key role in muscle excitability, but little is known in human skeletal muscle about fiber-type-specific differences in NKA isoform expression or adaptability. A vastus lateralis muscle biopsy was taken in 17 healthy young adults to contrast NKA isoform protein relative abundance between type I and IIa fibers. We further investigated muscle fiber-type-specific NKA adaptability in eight of these adults following 4-wk repeated-sprint exercise (RSE) training, comprising three sets of 5 × 4-s sprints, 3 days/wk. Single fibers were separated, and myosin heavy chain (I and IIa) and NKA (α1-3 and β1-3) isoform abundance were determined via Western blotting. All six NKA isoforms were expressed in both type I and IIa fibers. No differences between fiber types were found for α1-, α2-, α3-, β1-, or β3-isoform abundances. The NKA β2-isoform was 27% more abundant in type IIa than type I fibers (P < 0.05), with no other fiber-type-specific trends evident. RSE training increased β1 in type IIa fibers (pretraining 0.70 ± 0.25, posttraining 0.84 ± 0.24 arbitrary units, 42%, P < 0.05). No training effects were found for other NKA isoforms. Thus human skeletal muscle expresses all six NKA isoforms and not in a fiber-type-specific manner; this points to their different functional roles in skeletal muscle cells. Detection of elevated NKA β1 after RSE training demonstrates the sensitivity of the single-fiber Western blotting technique for fiber-type-specific intervention effects.

Sarcoplasmic reticulum Ca2+ uptake and leak properties, and SERCA isoform expression, in type I and type II fibres of human skeletal muscle

The Ca(2+) uptake properties of the sarcoplasmic reticulum (SR) were compared between type I and type II fibres of vastus lateralis muscle of young healthy adults. Individual mechanically skinned muscle fibres were exposed to solutions with the free [Ca(2+)] heavily buffered in the pCa range (-log10[Ca(2+)]) 7.3-6.0 for set times and the amount of net SR Ca(2+) accumulation determined from the force response elicited upon emptying the SR of all Ca(2+). Western blotting was used to determine fibre type and the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) isoform present in every fibre examined. Type I fibres contained only SERCA2 and displayed half-maximal Ca(2+) uptake rate at ∼pCa 6.8, whereas type II fibres contained only SERCA1 and displayed half-maximal Ca(2+) uptake rate at ∼pCa 6.6. Maximal Ca(2+) uptake rate was ∼0.18 and ∼0.21 mmol Ca(2+) (l fibre)(-1) s(-1) in type I and type II fibres, respectively, in good accord with previously measured SR ATPase activity. Increasing free [Mg(2+)] from 1 to 3 mM had no significant effect on the net Ca(2+) uptake rate at pCa 6.0, indicating that there was little or no calcium-induced calcium release occurring through the Ca(2+) release channels during uptake in either fibre type. Ca(2+) leakage from the SR at pCa 8.5, which is thought to occur at least in part through the SERCA, was ∼2-fold lower in type II fibres than in type I fibres, and was little affected by the presence of ADP, in marked contrast to the larger SR Ca(2+) leak observed in rat muscle fibres under the same conditions. The higher affinity of Ca(2+) uptake in the type I human fibres can account for the higher relative level of SR Ca(2+) loading observed in type I compared to type II fibres, and the SR Ca(2+) leakage characteristics of the human fibres suggest that the SERCAs are regulated differently from those in rat and contribute comparatively less to resting metabolic rate.

Endogenous and maximal sarcoplasmic reticulum calcium content and calsequestrin expression in type I and type II human skeletal muscle fibres

The relationship between sarcoplasmic reticulum (SR) Ca(2+) content and calsequestrin (CSQ) isoforms was investigated in human skeletal muscle. A fibre-lysing assay was used to quantify the endogenous Ca(2+) content and maximal Ca(2+) capacity of the SR in skinned segments of type I and type II fibres from vastus lateralis muscles of young healthy adults. Western blotting of individual fibres showed the great majority contained either all fast or all slow isoforms of myosin heavy chain (MHC), troponins C and I, tropomyosin and SERCA, and that the strontium sensitivity of the force response was closely indicative of the troponin C isoform present. The endogenous SR Ca(2+) content was slightly lower in type I compared to type II fibres (0.76 ± 0.03 and 0.85 ± 0.02 mmol Ca(2+) per litre of fibre, respectively), with virtually all of this Ca(2+) evidently being in the SR, as it could be rapidly released with a caffeine-low [Mg(2+)] solution (only 0.08 ± 0.01 and <0.07 mmol l(-1), respectively, remaining). The maximal Ca(2+) content that could be reached with SR Ca(2+) loading was 1.45 ± 0.04 and 1.79 ± 0.03 mmol l(-1) in type I and type II fibres, respectively (P < 0.05). In non-lysed skinned fibres, where the SR remained functional, repeated cycles of caffeine-induced Ca(2+) release and subsequent Ca(2+) reloading similarly indicated that (i) maximal SR Ca(2+) content was lower in type I fibres than in type II fibres (P < 0.05), and (ii) the endogenous Ca(2+) content represented a greater percentage of maximal content in type I fibres compared to type II fibres (∼59% and 41%, respectively, P < 0.05). Type II fibres were found on average to contain ∼3-fold more CSQ1 and ∼5-fold less CSQ2 than type I fibres (P < 0.001). The findings are consistent with the SR Ca(2+) content characteristics in human type II fibres being primarily determined by the CSQ1 abundance, and in type I fibres by the combined amounts of both CSQ1 and CSQ2.

S-glutathionylation of troponin I (fast) increases contractile apparatus Ca2+ sensitivity in fast-twitch muscle fibres of rats and humans

Oxidation can decrease or increase the Ca2+ sensitivity of the contractile apparatus in rodent fast-twitch (type II) skeletal muscle fibres, but the reactions and molecular targets involved are unknown. This study examined whether increased Ca2+ sensitivity is due to S-glutathionylation of particular cysteine residues. Skinned muscle fibres were directly activated in heavily buffered Ca2+ solutions to assess contractile apparatus Ca2+ sensitivity. Rat type II fibres were subjected to S-glutathionylation by successive treatments with 2,2′-dithiodipyridine (DTDP) and glutathione (GSH), and displayed a maximal increase in pCa50 (−log10 [Ca2+] at half-maximal force) of ∼0.24 pCa units, with little or no effect on maximum force or Hill coefficient. Partial similar effect was produced by exposure to oxidized gluthathione (GSSG, 10 mM) for 10 min at pH 7.1, and near-maximal effect by GSSG treatment at pH 8.5. None of these treatments significantly altered Ca2+ sensitivity in rat type I fibres. Western blotting showed that both the DTDP–GSH and GSSG–pH 8.5 treatments caused marked S-glutathionylation of the fast troponin I isoform (TnI(f)) present in type II fibres, but not of troponin C (TnC) or myosin light chain 2. Both the increased Ca2+ sensitivity and glutathionylation of TnI(f) were blocked by N-ethylmaleimide (NEM). S-nitrosoglutathione (GSNO) also increased Ca2+ sensitivity, but only in conditions where it caused S-glutathionylation of TnI(f). In human type II fibres from vastus lateralis muscle, DTDP–GSH treatment also caused similar increased Ca2+ sensitivity and S-glutathionylation of TnI(f). When the slow isoform of TnI in type I fibres of rat was partially substituted (∼30%) with TnI(f), DTDP–GSH treatment caused a significant increase in Ca2+ sensitivity (∼0.08 pCa units). TnIf in type II fibres from toad and chicken muscle lack Cys133 present in mammalian TnIf, and such fibres showed no change in Ca2+ sensitivity with DTDP–GSH nor any S-glutathionylation of TnI(f) (latter examined only in toad). Following 40 min of cycling exercise in human subjects (at ∼60% peak oxygen consumption), TnI(f) in vastus lateralis muscle displayed a marked increase in S-glutathionylation (∼4-fold). These findings show that S-glutathionylation of TnI(f), most probably at Cys133, increases the Ca2+ sensitivity of the contractile apparatus, and that this occurs in exercising humans, with likely beneficial effects on performance.