Calmodulin kinase modulates Ca2+ release in mouse skeletal muscle

Transcriptomic regulatory analysis of skeletal muscle development in landrace pigs

The development of pig skeletal muscle is a complex dynamic regulation process, which mainly includes the formation of primary and secondary muscle fibers, the remodeling of muscle fibers, and the maturation of skeletal muscle; However, the regulatory mechanism of the entire developmental process remains unclear. This study analyzed the whole-transcriptome data of skeletal muscles at 27 developmental nodes (E33-D180) in Landrace pigs, and their key regulatory factors in the development process were identified using the bioinformatics method. Firstly, we constructed a transcriptome expression map of skeletal muscle development from embryo to adulthood in Landrace pig. Subsequently, due to drastic change in gene expression, the perinatal periods including E105, D0 and D9, were focused, and the genes related to the process of muscle fiber remodeling and volume expansion were revealed. Then, though conjoint analysis with miRNA and lncRNA transcripts, a ceRNA network were identified, which consist of 11 key regulatory genes (such as CHAC1, RTN4IP1 and SESN1), 7 miRNAs and 43 lncRNAs, and they potentially play an important role in the process of muscle fiber differentiation, muscle fiber remodeling and volume expansion, intramuscular fat deposition, and other skeletal muscle developmental events. In summary, we reveal candidate genes and underlying molecular regulatory networks associated with perinatal skeletal muscle fiber type remodeling and expansion. These data provide new insights into the molecular regulation of mammalian skeletal muscle development and diversity.

Affinity Purification-Mass Spectrometry and Single Fiber Physiology/Proteomics Reveals Mechanistic Insights of C18ORF25

C18ORF25 was recently shown to be phosphorylated at S67 by AMP-activated protein kinase (AMPK) in the skeletal muscle, following acute exercise in humans. Phosphorylation was shown to improve the ex vivo skeletal muscle contractile function in mice, but our understanding of the molecular mechanisms is incomplete. Here, we profiled the interactome of C18ORF25 in mouse myotubes using affinity purification coupled to mass spectrometry. This analysis included an investigation of AMPK-dependent and S67-dependent protein/protein interactions. Several nucleocytoplasmic and contractile-associated proteins were identified, which revealed a subset of GTPases that associate with C18ORF25 in an AMPK- and S67 phosphorylation-dependent manner. We confirmed that C18ORF25 is localized to the nucleus and the contractile apparatus in the skeletal muscle. Mice lacking C18Orf25 display defects in calcium handling specifically in fast-twitch muscle fibers. To investigate these mechanisms, we developed an integrated single fiber physiology and single fiber proteomic platform. The approach enabled a detailed assessment of various steps in the excitation-contraction pathway including SR calcium handling and force generation, followed by paired single fiber proteomic analysis. This enabled us to identify >700 protein/phenotype associations and 36 fiber-type specific differences, following loss of C18Orf25. Taken together, our data provide unique insights into the function of C18ORF25 and its role in skeletal muscle physiology.

Enhanced capacity for CaMKII signaling mitigates calcium release related contractile fatigue with high intensity exercise

BACKGROUND

We tested whether enhancing the capacity for calcium/calmodulin-dependent protein kinase type II (CaMKII) signaling would delay fatigue of excitation-induced calcium release and improve contractile characteristics of skeletal muscle during fatiguing exercise.

METHODS

Fast and slow type muscle, gastrocnemius medialis (GM) and soleus (SOL), of rats and mouse interosseus (IO) muscle fibers, were transfected with pcDNA3-based plasmids for rat α and β CaMKII or empty controls. Levels of CaMKII, its T287-phosphorylation (pT287-CaMKII), and phosphorylation of components of calcium release and re-uptake, ryanodine receptor 1 (pS2843-RyR1) and phospholamban (pT17-PLN), were quantified biochemically. Sarcoplasmic calcium in transfected muscle fibers was monitored microscopically during trains of electrical excitation based on Fluo-4 FF fluorescence (n = 5-7). Effects of low- (n = 6) and high- (n = 8) intensity exercise on pT287-CaMKII and contractile characteristics were studied in situ.

RESULTS

Co-transfection with αCaMKII-pcDNA3/βCaMKII-pcDNA3 increased α and βCaMKII levels in SOL (+45.8 %, +250.5 %) and GM (+40.4 %, +89.9 %) muscle fibers compared to control transfection. High-intensity exercise increased pT287-βCaMKII and pS2843-RyR1 levels in SOL (+269 %, +151 %) and GM (+354 %, +119 %), but decreased pT287-αCaMKII and p17-PLN levels in GM compared to SOL (-76 % vs. +166 %; 0 % vs. +128 %). α/β CaMKII overexpression attenuated the decline of calcium release in muscle fibers with repeated excitation, and mitigated exercise-induced deterioration of rates in force production, and passive force, in a muscle-dependent manner, in correlation with pS2843-RyR1 and pT17-PLN levels (|r| > 0.7).

CONCLUSION

Enhanced capacity for α/β CaMKII signaling improves fatigue-resistance of active and passive contractile muscle properties in association with RyR1- and PLN-related improvements in sarcoplasmic calcium release.

CaMKII oxidation is a critical performance/disease trade-off acquired at the dawn of vertebrate evolution

Antagonistic pleiotropy is a foundational theory that predicts aging-related diseases are the result of evolved genetic traits conferring advantages early in life. Here we examine CaMKII, a pluripotent signaling molecule that contributes to common aging-related diseases, and find that its activation by reactive oxygen species (ROS) was acquired more than half-a-billion years ago along the vertebrate stem lineage. Functional experiments using genetically engineered mice and flies reveal ancestral vertebrates were poised to benefit from the union of ROS and CaMKII, which conferred physiological advantage by allowing ROS to increase intracellular Ca²⁺ and activate transcriptional programs important for exercise and immunity. Enhanced sensitivity to the adverse effects of ROS in diseases and aging is thus a trade-off for positive traits that facilitated the early and continued evolutionary success of vertebrates.

Natural selection may favor traits underlying aging-related diseases if they benefit the young. Wang et al. find that oxidative activation of CaMKII provides physiological benefits critical to the initial and continued success of vertebrates but at the cost of disease, frailty, and shortened lifespan.

Increased nuclear but not cytoplasmic activities of CELF1 protein leads to muscle wasting

mRNA processing is highly regulated during development through changes in RNA-binding protein (RBP) activities. CUG-BP, Elav-like family member 1 (CELF1, also called CUGBP1) is an RBP, the expression of which decreases in skeletal muscle soon after birth. CELF1 regulates multiple nuclear and cytoplasmic RNA processing events. In the nucleus, CELF1 regulates networks of postnatal alternative splicing (AS) transitions, while in the cytoplasm, CELF1 regulates mRNA stability and translation. Stabilization and misregulation of CELF1 has been implicated in human diseases including myotonic dystrophy type 1, Alzheimer's disease and multiple cancers. To understand the contribution of nuclear and cytoplasmic CELF1 activity to normal and pathogenic skeletal muscle biology, we generated transgenic mice for doxycycline-inducible and skeletal muscle-specific expression of active CELF1 mutants engineered to be localized predominantly to either the nucleus or the cytoplasm. Adult mice expressing nuclear, but not cytoplasmic, CELF1 are characterized by strong histopathological defects, muscle loss within 10 days and changes in AS. In contrast, mice expressing cytoplasmic CELF1 display changes in protein levels of targets known to be regulated at the level of translation by CELF1, with minimal changes in AS. These changes are in the absence of overt histopathological changes or muscle loss. RNA-sequencing revealed extensive gene expression and AS changes in mice overexpressing nuclear and naturally localized CELF1 protein, with affected genes involved in cytoskeleton dynamics, membrane dynamics, RNA processing and zinc ion binding. These results support a stronger role for nuclear CELF1 functions as compared to cytoplasmic CELF1 functions in skeletal muscle wasting.

Effect of formoterol, a long-acting β2-adrenergic agonist, on muscle strength and power output, metabolism, and fatigue during maximal sprinting in men

The aim was to investigate the effect of the long-acting β2-adrenergic agonist formoterol on muscle strength and power output, muscle metabolism, and phosphorylation of CaMKII Thr287 and FXYD1 during maximal sprinting. In a double-blind crossover study, 13 males [V̇o2 max: 45.0 ± 0.2 (means ± SE) ml·min−1·kg−1] performed a 30-s cycle ergometer sprint after inhalation of either 54 μg of formoterol (FOR) or placebo (PLA). Before and after the sprint, muscle biopsies were collected from vastus lateralis and maximal voluntary contraction (MVC), and contractile properties of quadriceps were measured. Oxygen uptake was measured during the sprint. During the sprint, peak power, mean power, and end power were 4.6 ± 0.8, 3.9 ± 1.1, and 9.5 ± 3.2% higher ( P < 0.05) in FOR than in PLA, respectively. Net rates of glycogenolysis and glycolysis were 45.7 ± 21.0 and 28.5 ± 13.4% higher ( P < 0.05) in FOR than in PLA, respectively, and the decrease in ATP content was lower ( P < 0.05) in FOR than in PLA (3.7 ± 1.5 vs. 8.0 ± 1.6 mmol/kg dry weight). There was no difference in breakdown of phosphocreatine and oxygen uptake between treatments. Before and after the sprint, MVC and peak twitch force were higher ( P < 0.05) in FOR than in PLA. No differences were observed in phosphorylation of CaMKII Thr287 and FXYD1 between treatments before the sprint, whereas phosphorylation of CaMKII Thr287 and FXYD1 was greater ( P < 0.05) in FOR than in PLA after the sprint. In conclusion, formoterol-induced enhancement in power output during maximal sprinting is associated with increased rates of glycogenolysis and glycolysis that may counteract development of fatigue.

The RNA-binding protein Rbfox1 regulates splicing required for skeletal muscle structure and function

The Rbfox family of RNA-binding proteins is highly conserved with established roles in alternative splicing (AS) regulation. High-throughput studies aimed at understanding transcriptome remodeling have revealed skeletal muscle as displaying one of the largest number of AS events. This finding is consistent with requirements for tissue-specific protein isoforms needed to sustain muscle-specific functions. Rbfox1 is abundant in vertebrate brain, heart and skeletal muscle. Genome-wide genetic approaches have linked the Rbfox1 gene to autism, and a brain-specific knockout mouse revealed a critical role for this splicing regulator in neuronal function. Moreover, a Caenorhabditis elegans Rbfox1 homolog regulates muscle-specific splicing. To determine the role of Rbfox1 in muscle function, we developed a conditional knockout mouse model to specifically delete Rbfox1 in adult tissue. We show that Rbfox1 is required for muscle function but a >70% loss of Rbfox1 in satellite cells does not disrupt muscle regeneration. Deep sequencing identified aberrant splicing of multiple genes including those encoding myofibrillar and cytoskeletal proteins, and proteins that regulate calcium handling. Ultrastructure analysis of Rbfox1(-/-) muscle by electron microscopy revealed abundant tubular aggregates. Immunostaining showed mislocalization of the sarcoplasmic reticulum proteins Serca1 and Ryr1 in a pattern indicative of colocalization with the tubular aggregates. Consistent with mislocalization of Serca1 and Ryr1, calcium handling was drastically altered in Rbfox1(-/-) muscle. Moreover, muscle function was significantly impaired in Rbfox1(-/-) muscle as indicated by decreased force generation. These results demonstrate that Rbfox1 regulates a network of AS events required to maintain multiple aspects of muscle physiology.

CaMKII content affects contractile, but not mitochondrial, characteristics in regenerating skeletal muscle

Background

The multi-meric calcium/calmodulin-dependent protein kinase II (CaMKII) is the main CaMK in skeletal muscle and its expression increases with endurance training. CaMK family members are implicated in contraction-induced regulation of calcium handling, fast myosin type IIA expression and mitochondrial biogenesis. The objective of this study was to investigate the role of an increased CaMKII content for the expression of the contractile and mitochondrial phenotype in vivo. Towards this end we attempted to co-express alpha- and beta-CaMKII isoforms in skeletal muscle and characterised the effect on the contractile and mitochondrial phenotype.

Results

Fast-twitch muscle m. gastrocnemius (GM) and slow-twitch muscle m. soleus (SOL) of the right leg of 3-month old rats were transfected via electro-transfer of injected expression plasmids for native α/β CaMKII. Effects were identified from the comparison to control-transfected muscles of the contralateral leg and non-transfected muscles. α/β CaMKII content in muscle fibres was 4-5-fold increased 7 days after transfection. The transfection rate was more pronounced in SOL than GM muscle (i.e. 12.6 vs. 3.5%). The overexpressed α/β CaMKII was functional as shown through increased threonine 287 phosphorylation of β-CaMKII after isometric exercise and down-regulated transcripts COXI, COXIV, SDHB after high-intensity exercise in situ. α/β CaMKII overexpression under normal cage activity accelerated excitation-contraction coupling and relaxation in SOL muscle in association with increased SERCA2, ANXV and fast myosin type IIA/X content but did not affect mitochondrial protein content. These effects were observed on a background of regenerating muscle fibres.

Conclusion

Elevated CaMKII content promotes a slow-to-fast type fibre shift in regenerating muscle but is not sufficient to stimulate mitochondrial biogenesis in the absence of an endurance stimulus.

The role of in vivo Ca²⁺ signals acting on Ca²⁺-calmodulin-dependent proteins for skeletal muscle plasticity

Skeletal muscle fibres are highly heterogeneous regarding size, metabolism and contractile function. They also show a large capacity for adaptations in response to alterations in the activation pattern. A major part of this activity-dependent plasticity relies on transcriptional alterations controlled by intracellular Ca(2+) signals. In this review we discuss how intracellular Ca(2+) fluctuations induced by activation patterns likely to occur in vivo control muscle properties via effects on Ca(2+)-calmodulin-dependent proteins. We focus on two such Ca(2+) decoders: calcineurin and Ca(2+)-calmodulin-dependent protein kinase II. Inherent Ca(2+) transients during contractions differ rather little between slow- and fast-twitch muscle fibres and this difference is unlikely to have any significant impact on the activity of Ca(2+) decoders. The major exception to this is fatigue-induced changes in Ca(2+) transients that occur in fast-twitch fibres exposed to high-intensity activation typical of slow-twitch motor units. In conclusion, the cascade from neural stimulation pattern to Ca(2+)-dependent transcription is likely to be central in maintaining the fibre phenotypes in both fast- and slow-twitch fibres. Moreover, changes in Ca(2+) signalling (e.g. induced by endurance training) can result in altered muscle properties (e.g. increased mitochondrial biogenesis) and this plasticity involves other signalling pathways.

Interaction between Calcineurin and Ca/Calmodulin Kinase-II in Modulating Cellular Functions

Roles of calcineurin (CaN), a Ca²⁺/calmodulin- (CaM-) dependent protein phosphatase, and Ca²⁺/CaM-dependent protein kinase-II (CaMKII) in modulating K⁺ channel activity and the intracellular Ca²⁺ concentration ([Ca²⁺]_i) have been investigated in renal tubule epithelial cells. The channel current through the cell membrane was recorded with the patch-clamp technique, and [Ca²⁺]_i was monitored using fura-2 imaging. We found that a CaN-inhibitor, cyclosporin A (CyA), lowered the K⁺ channel activity and elevated [Ca²⁺]_i, suggesting that CyA closes K⁺ channels and opens Ca²⁺-release channels of the cytosolic Ca²⁺-store. Moreover, both of these responses were blocked by KN-62, an inhibitor of CaMKII. It is suggested that the CyA-mediated response results from the activation of CaMKII. Indeed, Western blot analysis revealed that CyA increased phospho-CaMKII, an active form of CaMKII. These findings suggest that CaN-dependent dephosphorylation inhibits CaMKII-mediated phosphorylation, and the inhibition of CaN increases phospho-CaMKII, which results in the stimulation of CaMKII-dependent cellular actions.

Ca2+-calmodulin-dependent protein kinase II represses cardiac transcription of the L-type calcium channel alpha(1C)-subunit gene (Cacna1c) by DREAM translocation

Recent studies have demonstrated that changes in the activity of calcium-calmodulin-dependent protein kinase II (CaMKII) induce a unique cardiomyocyte phenotype through the regulation of specific genes involved in excitation-contraction (E-C)-coupling. To explain the transcriptional effects of CaMKII we identified a novel CaMKII-dependent pathway for controlling the expression of the pore-forming α-subunit (Cav1.2) of the L-type calcium channel (LTCC) in cardiac myocytes. We show that overexpression of either cytosolic (δC) or nuclear (δB) CaMKII isoforms selectively downregulate the expression of the Cav1.2. Pharmacological inhibition of CaMKII activity induced measurable changes in LTCC current density and subsequent changes in cardiomyocyte calcium signalling in less than 24 h. The effect of CaMKII on the α1C-subunit gene (Cacna1c) promoter was abolished by deletion of the downstream regulatory element (DRE), which binds transcriptional repressor DREAM/calsenilin/KChIP3. Imaging DREAM-GFP (green fluorescent protein)-expressing cardiomyocytes showed that CaMKII potentiates the calcium-induced nuclear translocation of DREAM. Thereby CaMKII increases DREAM binding to the DRE consensus sequence of the endogenous Cacna1c gene. By mathematical modelling we demonstrate that the LTCC downregulation through the Ca2+-CaMKII-DREAM cascade constitutes a physiological feedback mechanism enabling cardiomyocytes to adjust the calcium intrusion through LTCCs to the amount of intracellular calcium detected by CaMKII.

Calcium/calmodulin kinase II-dependent acetylcholine receptor cycling at the mammalian neuromuscular junction in vivo

At the mammalian skeletal neuromuscular junction, cycling of nicotinic ACh receptors (nAChRs) is critical for the maintenance of a high postsynaptic receptor density. However, the mechanisms that regulate nAChRs recycling in living animals remain unknown. Using in vivo time-lapse imaging, fluorescence recovery after photobleaching, and biochemical pull down assays, we demonstrated that recycling of internalized nAChRs into fully functional and denervated synapses was promoted by both direct muscle stimulation and pharmacologically induced intracellular calcium elevations. Most of internalized nAChRs are recycled directly into synaptic sites. Chelating of intracellular calcium below resting level drastically decreased cycling of nAChRs. Furthermore we found that calcium-dependent AChR recycling is mediated by Ca(2+)/calmodulin-dependent kinase II (CaMKII). Inhibition of CaMKII selectively blocked recycling and caused intracellular accumulation of internalized nAChRs, whereas internalization of surface receptors remained unaffected. Electroporation of CaMKII-GFP isoforms into the sternomastoid muscle showed that muscle-specific CaMKIIβm isoform is highly expressed at the neuromuscular junction (NMJ) and precisely colocalized with nAChRs at crests of synaptic folds while the CaMKIIγ and δ isoforms are poorly expressed in synaptic sites. These results indicate that Ca(2+) along with CaMKII activity are critical for receptor recycling and may provide a mechanism by which the postsynaptic AChR density is maintained at the NMJ in vivo.

Role of calcineurin-mediated dephosphorylation in modulation of an inwardly rectifying K+ channel in human proximal tubule cells

Activity of an inwardly rectifying K(+) channel with inward conductance of about 40 pS in cultured human renal proximal tubule epithelial cells (RPTECs) is regulated at least in part by protein phosphorylation and dephosphorylation. In this study, we examined involvement of calcineurin (CaN), a Ca(2+)/calmodulin (CaM)-dependent phosphatase, in modulating K(+) channel activity. In cell-attached mode of the patch-clamp technique, application of a CaN inhibitor, cyclosporin A (CsA, 5 microM) or FK520 (5 microM), significantly suppressed channel activity. Intracellular Ca(2+) concentration ([Ca(2+)]( i )) estimated by fura-2 imaging was elevated by these inhibitors. Since inhibition of CaN attenuates some dephosphorylation with increase in [Ca(2+)]( i ), we speculated that inhibiting CaN enhances Ca(2+)-dependent phosphorylation, which might result in channel suppression. To verify this hypothesis, we examined effects of inhibitors of PKC and Ca(2+)/CaM-dependent protein kinase-II (CaMKII) on CsA-induced channel suppression. Although the PKC inhibitor GF109203X (500 nM) did not influence the CsA-induced channel suppression, the CaMKII inhibitor KN62 (20 microM) prevented channel suppression, suggesting that the channel suppression resulted from CaMKII-dependent processes. Indeed, Western blot analysis showed that CsA increased phospho-CaMKII (Thr286), an activated CaMKII in inside-out patches, application of CaM (0.6 microM) and CaMKII (0.15 U/ml) to the bath at 10(-6) M Ca(2+) significantly suppressed channel activity, which was reactivated by subsequent application of CaN (800 U/ml). These results suggest that CaN plays an important role in supporting K(+) channel activity in RPTECs by preventing CaMKII-dependent phosphorylation.

Regulation of excitation-contraction coupling in mouse cardiac myocytes: integrative analysis with mathematical modelling

Background

The cardiomyocyte is a prime example of inherently complex biological system with inter- and cross-connected feedback loops in signalling, forming the basic properties of intracellular homeostasis. Functional properties of cells and tissues have been studied e.g. with powerful tools of genetic engineering, combined with extensive experimentation. While this approach provides accurate information about the physiology at the endpoint, complementary methods, such as mathematical modelling, can provide more detailed information about the processes that have lead to the endpoint phenotype.

Results

In order to gain novel mechanistic information of the excitation-contraction coupling in normal myocytes and to analyze sophisticated genetically engineered heart models, we have built a mathematical model of a mouse ventricular myocyte. In addition to the fundamental components of membrane excitation, calcium signalling and contraction, our integrated model includes the calcium-calmodulin-dependent enzyme cascade and the regulation it imposes on the proteins involved in excitation-contraction coupling. With the model, we investigate the effects of three genetic modifications that interfere with calcium signalling: 1) ablation of phospholamban, 2) disruption of the regulation of L-type calcium channels by calcium-calmodulin-dependent kinase II (CaMK) and 3) overexpression of CaMK. We show that the key features of the experimental phenotypes involve physiological compensatory and autoregulatory mechanisms that bring the system to a state closer to the original wild-type phenotype in all transgenic models. A drastic phenotype was found when the genetic modification disrupts the regulatory signalling system itself, i.e. the CaMK overexpression model.

Conclusion

The novel features of the presented cardiomyocyte model enable accurate description of excitation-contraction coupling. The model is thus an applicable tool for further studies of both normal and defective cellular physiology. We propose that integrative modelling as in the present work is a valuable complement to experiments in understanding the causality within complex biological systems such as cardiac myocytes.

Multiple signalling pathways redundantly control glucose transporter GLUT4 gene transcription in skeletal muscle

Increased glucose transporter GLUT4 expression in skeletal muscle is an important benefit of regular exercise, resulting in improved insulin sensitivity and glucose tolerance. The Ca(2+)-calmodulin-dependent kinase II (CaMKII), calcineurin and AMPK pathways have been implicated in GLUT4 gene regulation based on pharmacological evidence. Here, we have used a more specific genetic approach to establish the relative role of the three pathways in fast and slow muscles. Plasmids coding for protein inhibitors of CaMKII or calcineurin were co-transfected in vivo with a GLUT4 enhancer-reporter construct either in normal mice or in mice expressing a kinase dead (KD) AMPK mutant. GLUT4 reporter activity was not inhibited in the slow soleus muscle by blocking either CaMKII or calcineurin alone, but was inhibited by blocking both pathways. GLUT4 reporter activity was likewise unchanged in the soleus of KD-AMPK mice, but was significantly reduced by incapacitation of either CaMKII or calcineurin in these mice. On the other hand, in the fast tibialis anterior (TA) muscle, calcineurin appears to exert a prominent role in the control of GLUT4 reporter activity, independent of CaMKII and AMPK. The results point to a muscle type-specific and redundant regulation of GLUT4 enhancer based on the interplay of multiple signalling pathways, all of which are known to affect myocyte enhancing factor 2 (MEF2) transcriptional activity, a point of convergence of different pathways on muscle gene regulation.

Modelling sarcoplasmic reticulum calcium ATPase and its regulation in cardiac myocytes

When developing large-scale mathematical models of physiology, some reduction in complexity is necessarily required to maintain computational efficiency. A prime example of such an intricate cell is the cardiac myocyte. For the predictive power of the cardiomyocyte models, it is vital to accurately describe the calcium transport mechanisms, since they essentially link the electrical activation to contractility. The removal of calcium from the cytoplasm takes place mainly by the Na(+)/Ca(2+) exchanger, and the sarcoplasmic reticulum Ca(2+) ATPase (SERCA). In the present study, we review the properties of SERCA, its frequency-dependent and beta-adrenergic regulation, and the approaches of mathematical modelling that have been used to investigate its function. Furthermore, we present novel theoretical considerations that might prove useful for the elucidation of the role of SERCA in cardiac function, achieving a reduction in model complexity, but at the same time retaining the central aspects of its function. Our results indicate that to faithfully predict the physiological properties of SERCA, we should take into account the calcium-buffering effect and reversible function of the pump. This 'uncomplicated' modelling approach could be useful to other similar transport mechanisms as well.

Endogenous calpain-3 activation is primarily governed by small increases in resting cytoplasmic [Ca2+] and is not dependent on stretch

Proteolytically active calpain-3/p94 is clearly vital for normal muscle function, since its absence leads to limb girdle muscular dystrophy 2A, but its function and regulatory control are poorly understood. Here we use single muscle fibers, individually skinned by microdissection, to investigate the diffusibility and autolytic activation of calpain-3 in situ. Virtually all calpain-3 present in mature muscle fibers is tightly bound in the vicinity of the titin N2A line and triad junctions and remains so irrespective of fiber stretching or raised [Ca(2+)]. Most calpain-3 is evidently bound within the contractile filament lattice, because (i) its slow diffusional loss is slowed further by locking myosin and actin into rigor and (ii) detergent dispersion of membranes causes rapid washout of most ryanodine receptors and sarcoplasmic reticulum Ca(2+) pumps with little accompanying washout of calpain-3. Calpain-3 autolyzes (becoming proteolytically active) in a tightly calcium-dependent manner. It remains in its nonactivated full-length form if [Ca(2+)] is maintained at < or = 50 nm, the normal resting level, even with brief increases to 2-20 mum during repeated tetanic contractions, but it becomes active (though still bound) if [Ca(2+)] is kept slightly elevated at 200 nm ( approximately 20% autolysis in 1 h). Calpain-3 did not spontaneously autolyze even when free in solution with 200 nm Ca(2+) for up to 60 min. These findings explain why calpain-3 remains quiescent with normal exercise but is activated following eccentric (stretching) contractions, when resting [Ca(2+)] is elevated, and how a protease such as calpain-3 can be very Ca(2+)-sensitive yet highly specific in its actions.

Model of excitation-contraction coupling of rat neonatal ventricular myocytes

The neonatal rat ventricular myocyte culture is one of the most popular experimental cardiac cell models. To our knowledge, the excitation-contraction coupling (ECC) of these cells, i.e., the process linking the electrical activity to the cytosolic Ca2+ transient and contraction, has not been previously analyzed, nor has it been presented as a complete system in detail. Neonatal cardiomyocytes are in the postnatal developmental stage, and therefore, the features of their ECC differ vastly from those of adult ventricular myocytes. We present the first complete analysis of ECC in these cells by characterizing experimentally the action potential and calcium signaling and developing the first mathematical model of ECC in neonatal cardiomyocytes that we know of. We show that in comparison to adult cardiomyocytes, neonatal cardiomyocytes have long action potentials, heterogeneous cytosolic Ca2+ signals, weaker sarcoplasmic reticulum Ca2+ handling, and stronger sarcolemmal Ca2+ handling, with a significant contribution by the Na+/Ca2+ exchanger. The developed model reproduces faithfully the ECC of rat neonatal cardiomyocytes with a novel description of spatial cytosolic [Ca2+] signals. Simulations also demonstrate how an increase in the cell size (hypertrophy) affects the ECC in neonatal cardiomyocytes. This model of ECC in developing cardiomyocytes provides a platform for developing future models of cardiomyocytes at different developmental stages.

Evidence for the involvement of CaMKII and AMPK in Ca2+-dependent signaling pathways regulating FA uptake and oxidation in contracting rodent muscle

Calcium-calmodulin/dependent protein kinase II (CaMKII), AMP-activated protein kinase (AMPK), and extracellular signal-regulated kinase (ERK1/2) have each been implicated in the regulation of substrate metabolism during exercise. The purpose of this study was to determine whether CaMKII is involved in the regulation of FA uptake and oxidation and, if it is involved, whether it does so independently of AMPK and ERK1/2. Rat hindquarters were perfused at rest with (n = 16) or without (n = 10) 3 mM caffeine, or during electrical stimulation (n = 14). For each condition, rats were subdivided and treated with 10 muM of either KN92 or KN93, inactive and active CaMKII inhibitors, respectively. Both caffeine treatment and electrical stimulation significantly increased FA uptake and oxidation. KN93 abolished caffeine-induced FA uptake, decreased contraction-induced FA uptake by 33%, and abolished both caffeine- and contraction-induced FA oxidation (P < 0.05). Caffeine had no effect on ERK1/2 phosphorylation (P > 0.05) and increased alpha(2)-AMPK activity by 68% (P < 0.05). Electrical stimulation increased ERK1/2 phosphorylation and alpha(2)-AMPK activity by 51% and 3.4-fold, respectively (P < 0.05). KN93 had no effect on caffeine-induced alpha(2)-AMPK activity, ERK1/2 phosphorylation, or contraction-induced ERK1/2 phosphorylation (P > 0.05). Alternatively, it decreased contraction-induced alpha(2)-AMPK activity by 51% (P < 0.05), suggesting that CaMKII lies upstream of AMPK. These results demonstrate that regulation of contraction-induced FA uptake and oxidation occurs in part via Ca(2+)-independent activation of ERK1/2 as well as Ca(2+)-dependent activation of CaMKII and AMPK.

Impaired calcium release during fatigue

Impaired calcium release from the sarcoplasmic reticulum (SR) has been identified as a contributor to fatigue in isolated skeletal muscle fibers. The functional importance of this phenomenon can be quantified by the use of agents, such as caffeine, which can increase SR Ca2+release during fatigue. A number of possible mechanisms for impaired calcium release have been proposed. These include reduction in the amplitude of the action potential, potentially caused by extracellular K+accumulation, which may reduce voltage sensor activation but is counteracted by a number of mechanisms in intact animals. Reduced effectiveness of SR Ca2+channel opening is caused by the fall in intracellular ATP and the rise in Mg2+concentrations that occur during fatigue. Reduced Ca2+available for release within the SR can occur if inorganic phosphate enters the SR and precipitates with Ca2+. Further progress requires the development of methods that can identify impaired SR Ca2+release in intact, blood-perfused muscles and that can distinguish between the various mechanisms proposed.

Skeletal Muscle Fatigue: Cellular Mechanisms

Repeated, intense use of muscles leads to a decline in performance known as muscle fatigue. Many muscle properties change during fatigue including the action potential, extracellular and intracellular ions, and many intracellular metabolites. A range of mechanisms have been identified that contribute to the decline of performance. The traditional explanation, accumulation of intracellular lactate and hydrogen ions causing impaired function of the contractile proteins, is probably of limited importance in mammals. Alternative explanations that will be considered are the effects of ionic changes on the action potential, failure of SR Ca2+release by various mechanisms, and the effects of reactive oxygen species. Many different activities lead to fatigue, and an important challenge is to identify the various mechanisms that contribute under different circumstances. Most of the mechanistic studies of fatigue are on isolated animal tissues, and another major challenge is to use the knowledge generated in these studies to identify the mechanisms of fatigue in intact animals and particularly in human diseases.

Activation of Ca(2+)-dependent protein kinase II during repeated contractions in single muscle fibres from mouse is dependent on the frequency of sarcoplasmic reticulum Ca(2+) release

AIM

To investigate the importance and contribution of calmodulin-dependent protein kinase II (CaMKII) activity on sarcoplasmic reticulum (SR) Ca(2+)-release in response to different work intensities in single, intact muscle fibres.

METHODS

CaMKII activity was blocked in single muscle fibres using either the inhibitory peptide AC3-I or the pharmacological inhibitor KN-93. The effect on tetanic force production and [Ca(2+)](i) was determined during work of different intensities. The activity of CaMKII was assessed by mathematical modelling.

RESULTS

Using a standard protocol to induce fatigue (50x 70 Hz, 350 ms duration, every 2 s) the number of stimuli needed to induce fatigue was decreased from 47 +/- 3 contractions in control to 33 +/- 3 with AC3-I. KN-93 was a more potent inhibitor, decreasing the number of contractions needed to induce fatigue to 15 +/- 3. Tetanic [Ca(2+)](i) was 100 +/- 11%, 97 +/- 11% and 67 +/- 11% at the end of stimulation in control, AC3-I and KN-93 respectively. A similar inhibition was obtained using a high intensity protocol (20x 70 Hz, 200 ms duration, every 300 ms). However, using a long interval protocol (25x 70 Hz, 350 ms duration, every 5 s) no change was observed in either tetanic [Ca(2+)](i) or force when inhibiting CaMKII. A mathematical model used to investigate the activation pattern of CaMKII suggests that there is a threshold of active CaMKII that has to be surpassed in order for CaMKII to affect SR Ca(2+) release.

CONCLUSION

Our results show that CaMKII is crucial for maintaining proper SR Ca(2+) release and that this is regulated in a work intensity manner.

Roles of the calcineurin and CaMK signaling pathways in fast-to-slow fiber type transformation of cultured adult mouse skeletal muscle fibers

Two Ca2+-dependent signaling pathways, mediated by the Ca2+-activated phosphatase calcineurin and by the Ca2+-activated kinase Ca2+/calmodulin-dependent kinase (CaMK), are both believed to function in fast-to-slow skeletal muscle fiber type transformation, but questions about the relative importance of the two pathways still remain. Here, the differential gene expression during fast-to-slow fiber type transformation was studied using cultured adult flexor digitorum brevis (FDB) fibers and a custom minimicroarray system containing 21 fiber type-specific marker genes. After 3 days of culture, unstimulated fibers showed a generally slower gene expression profile; 3 days of electric field stimulation of cultured FDB fibers with a slow fiber-type pattern transformed the fibers to an even slower gene expression profile. Unstimulated FDB fibers overexpressing constitutively active calcineurin featured a slower gene expression profile, except four genes, indicating that transformation occurred, but was incomplete with activation of the calcineurin pathway alone. In both unstimulated FDB fibers and slow-type electrically stimulated FDB fibers, blocking of CaMK pathway with KN93 generated a faster gene expression profile compared with the negative control KN92, indicating that CaMK pathway functions during the transformation induced by both unstimulated culturing and slow fiber-type electrical stimulation. Moreover, neither the calcineurin nor the CaMK pathway alone could maximally activate the transformation, and coordination of the two pathways is required to accomplish a complete fast-to-slow fiber type transformation.

Regulation and function of Ca2+-calmodulin-dependent protein kinase II of fast-twitch rat skeletal muscle

The activation and function of Ca(2+)-calmodulin-dependent kinase II (CaMKII) in contracting rat skeletal muscle was examined. The increase in autonomous activity and phosphorylation at Thr(287) of CaMKII of gastrocnemius muscle in response to contractions in situ was rapid and transient, peaking at 1-3 min, but reversed after 30 min of contractions. There was a positive correlation between CaMKII phosphorylation at Thr(287) and autonomous CaMKII activity. In contrast to the rapid and transient increase in autonomous CaMKII activity, the phosphorylation of the putative CaMKII substrate trisk95/triadin was rapid and sustained during contractions. There were no changes in CaMKII activity and phosphorylation or trisk95 phosphorylation in the resting contralateral muscles during stimulation. When fast-twitch muscles were contracted ex vivo, CaMKII inhibition resulted in a greater magnitude of fatigue as well as blunted CaMKII and trisk95 phosphorylation, identifying trisk95 as a physiological CaMKII substrate. In summary, skeletal muscle CaMKII activation was rapid and sustained during exercise/contraction and is mediated by factors within the contracting muscle, probably through allosteric activation via Ca(2+)-CaM. CaMKII may signal through trisk95 to modulate Ca(2+) release in fast-twitch rat skeletal muscle during exercise/contraction.

Ca2+-calmodulin-dependent protein kinase expression and signalling in skeletal muscle during exercise

Ca2+ signalling is proposed to play an important role in skeletal muscle function during exercise. Here, we examined the expression of multifunctional Ca2+-calmodulin-dependent protein kinases (CaMK) in human skeletal muscle and show that CaMKII and CaMKK, but not CaMKI or CaMKIV, are expressed. Furthermore, the effect of exercise duration and intensity on skeletal muscle CaMKII activity and phosphorylation of downstream targets was examined. Eight healthy men exercised at approximately 67% of peak pulmonary O2 uptake(VO2peak) with muscle samples taken at rest and after 1, 10, 30, 60 and 90 min of exercise. Ten other men exercised for three consecutive 10 min bouts at 35%, 60% and 85% VO2peak with muscle samples taken at rest, at the end of each interval and 30 min post-exercise. There was a rapid and transient increase in autonomous CaMKII activity and CaMKII phosphorylation at Thr287 in skeletal muscle during exercise. Furthermore, the phosphorylation of phospholamban (PLN) at Thr17, which was identified as a CaMKII substrate in skeletal muscle, was rapidly (< 1 min) increased by exercise, and remained phosphorylated 5-fold above basal level during 90 min of exercise. The phosphorylation of serum response factor at Ser103, a putative CaMKII substrate, was higher after 30 min of exercise. PLN phosphorylation at Thr17 was higher with increasing exercise intensities. These data indicate that CaMKII is the major multifunctional CaMK in skeletal muscle and its activation occurs rapidly and is sustained during continuous exercise, with the activation being greater during intense exercise.

Differential effects of repetitive activity on sarcoplasmic reticulum responses in rat muscles of different oxidative potential

We investigated the hypothesis that muscles of different oxidative potential would display differences in sarcoplasmic reticulum (SR) Ca2+ handling responses to repetitive contractile activity and recovery. Repetitive activity was induced in two muscles of high oxidative potential, namely, soleus (SOL) and red gastrocnemius (RG), and in white gastrocnemius (WG), a muscle of low oxidative potential, by stimulation in adult male rats. Measurements of SR properties, performed in crude homogenates, were made on control and stimulated muscles at the start of recovery (R0) and at 25 min of recovery (R25). Maximal Ca2+-ATPase activity (Vmax, micromol x g protein(-1) x min(-1)) at R0 was lower in stimulated SOL (105 +/- 9 vs. 135 +/- 7) and RG (269 +/- 22 vs. 317 +/- 26) and higher (P < 0.05) in WG (795 +/- 32 vs. 708 +/- 34). At R25, Vmax remained lower (P < 0.05) in SOL and RG but recovered in WG. Ca2+ uptake, measured at 2,000 nM, was depressed (P < 0.05) in SOL and RG by 34 and 13%, respectively, in stimulated muscles at R0 and remained depressed (P < 0.05) at R25. In contrast, Ca2+ uptake was elevated (P < 0.05) in stimulated WG at R0 by 9% and remained elevated (P < 0.05) at R25. Ca2+ release, unaltered in SOL and RG at both R0 and R25, was increased (P < 0.05) in stimulated WG at both R0 and R25. We conclude that SR Ca2+-handling responses to repetitive contractile activity and recovery are related to the oxidative potential of muscle.

Exercise increases Ca2+-calmodulin-dependent protein kinase II activity in human skeletal muscle

There is evidence in rodents that Ca2+-calmodulin-dependent protein kinase II (CaMKII) activity is higher in contracting skeletal muscle, and this kinase may regulate skeletal muscle function and metabolism during exercise. To investigate the effect of exercise on CaMKII in human skeletal muscle, healthy men (n = 8) performed cycle ergometer exercise for 40 min at 76 +/- 1% peak pulmonary O2 uptake (VO2peak), with skeletal muscle samples taken at rest and after 5 and 40 min of exercise. CaMKII expression and activities were examined by immunoblotting and in vitro kinase assays, respectively. There were no differences in maximal (+ Ca2+, CaM) CaMKII activity during exercise compared with rest. Autonomous (- Ca2+, CaM) CaMKII activity was 9 +/- 1% of maximal at rest, remained unchanged at 5 min, and increased to 17 +/- 1% (P < 0.01) at 40 min. CaMKII autophosphorylation at Thr287 was 50-70% higher during exercise, with no differences in CaMKII expression. The effect of maximal aerobic exercise on CaMKII was also examined (n = 9), with 0.7- to 1.5-fold increases in autonomous CaMKII activity, but no change in maximal CaMKII activity. CaMKIV was not detected in human skeletal muscle. In summary, exercise increases the activity of CaMKII in skeletal muscle, suggesting that it may have a role in regulating skeletal muscle function and metabolism during exercise in humans.