Calcineurin regulates slow myosin, but not fast myosin or metabolic enzymes, during fast-to-slow transformation in rabbit skeletal muscle cell culture

The Development of Biomimetic Aligned Skeletal Muscles in a Fully 3D Printed Microfluidic Device

Human skeletal muscles are characterized by a unique aligned microstructure of myotubes which is important for their function as well as for their homeostasis. Thus, the recapitulation of the aligned microstructure of skeletal muscles is crucial for the construction of an advanced biomimetic model aimed at drug development applications. Here, we have developed a 3D printed micropatterned microfluid device (3D-PMMD) through the employment of a fused deposition modeling (FDM)-based 3D printer and clear filaments made of biocompatible polyethylene terephthalate glycol (PETG). We could fabricate micropatterns through the adjustment of the printing deposition heights of PETG filaments, leading to the generation of aligned half-cylinder-shaped micropatterns in a dimension range from 100 µm to 400 µm in width and from 60 µm to 150 µm in height, respectively. Moreover, we could grow and expand C2C12 mouse myoblast cells on 3D-PMMD where cells could differentiate into aligned bundles of myotubes with respect to the dimension of each micropattern. Furthermore, our platform was applicable with the electrical pulses stimulus (EPS) modality where we noticed an improvement in myotubes maturation under the EPS conditions, indicating the potential use of the 3D-PMMD for biological experiments as well as for myogenic drug development applications in the future.

Tissue-Engineered Skeletal Muscle Models to Study Muscle Function, Plasticity, and Disease

Skeletal muscle possesses remarkable plasticity that permits functional adaptations to a wide range of signals such as motor input, exercise, and disease. Small animal models have been pivotal in elucidating the molecular mechanisms regulating skeletal muscle adaptation and plasticity. However, these small animal models fail to accurately model human muscle disease resulting in poor clinical success of therapies. Here, we review the potential of in vitro three-dimensional tissue-engineered skeletal muscle models to study muscle function, plasticity, and disease. First, we discuss the generation and function of in vitro skeletal muscle models. We then discuss the genetic, neural, and hormonal factors regulating skeletal muscle fiber-type in vivo and the ability of current in vitro models to study muscle fiber-type regulation. We also evaluate the potential of these systems to be utilized in a patient-specific manner to accurately model and gain novel insights into diseases such as Duchenne muscular dystrophy (DMD) and volumetric muscle loss. We conclude with a discussion on future developments required for tissue-engineered skeletal muscle models to become more mature, biomimetic, and widely utilized for studying muscle physiology, disease, and clinical use.

Mitochondrial T3 receptor and targets

The demonstration that TRα1 mRNA encodes a nuclear thyroid hormone receptor and two proteins imported into mitochondria with molecular masses of 43 and 28 kDa has brought new clues to better understand the pleiotropic influence of iodinated hormones. If p28 activity remains unknown, p43 binds to T3 responsive elements occurring in the organelle genome, and, in the T3 presence, stimulates mitochondrial transcription and the subsequent synthesis of mitochondrial encoded proteins. This influence increases mitochondrial activity and through changes in the mitochondrial/nuclear cross talk affects important nuclear target genes regulating cell proliferation and differentiation, oncogenesis, or apoptosis. In addition, this pathway influences muscle metabolic and contractile phenotype, as well as glycaemia regulation. Interestingly, according to the process considered, p43 exerts opposite or cooperative effects with the well-known T3 pathway, thus allowing a fine tuning of the physiological influence of this hormone.

Primary skeletal muscle cells cultured on gelatin bead microcarriers develop structural and biochemical features characteristic of adult skeletal muscle

A primary skeletal muscle cell culture, in which myoblasts derived from newborn rabbit hindlimb muscles grow on gelatin bead microcarriers in suspension and differentiate into myotubes, has been established previously. In the course of differentiation and beginning spontaneous contractions, these multinucleated myotubes do not detach from their support. Here, we describe the development of the primary myotubes with respect to their ultrastructural differentiation. Scanning electron microscopy reveals that myotubes not only grow around the surface of one carrier bead but also attach themselves to neighboring carriers, forming bridges between carriers. Transmission electron microscopy demonstrates highly ordered myofibrils, T-tubules, and sarcoplasmic reticulum. The functionality of the contractile apparatus is evidenced by contractile activity that occurs spontaneously or can be elicited by electrostimulation. Creatine kinase activity increases steadily until day 20 of culture. Regarding the expression of isoforms of myosin heavy chains (MHC), we could demonstrate that from day 16 on, no non-adult MHC isoform mRNAs are present. Instead, on day 28 the myotubes express predominantly adult fast MHCIId/x mRNA and protein. This MHC pattern resembles that of fast muscles of adult rabbits. In contrast, primary myotubes grown on matrigel-covered culture dishes express substantial amounts of non-adult MHC protein even on day 21. To conclude, primary myotubes grown on microcarriers in their later stages exhibit many features of adult skeletal muscle and characteristics of fast type II fibers. Thus, the culture represents an excellent model of adult fast skeletal muscle, for example, when investigating molecular mechanisms of fast-to-slow fiber-type transformation.

Creatine loading elevates the intracellular phosphorylation potential and alters adaptive responses of rat fast-twitch muscle to chronic low-frequency stimulation

This study tested the hypothesis that elevating the intracellular phosphorylation potential (IPP = [ATP]/[ADP]free) within rat fast-twitch tibialis anterior muscles by creatine (Cr) loading would prevent fast-to-slow fibre transitions induced by chronic low-frequency electrical stimulation (CLFS, 10 Hz, 12 h/day). Creatine-control and creatine-CLFS groups drank a solution of 1% Cr + 5% dextrose, ad libitum, for 10 days before and during 10 days of CLFS; dextrose-control and dextrose-CLFS groups drank 5% dextrose. Cr loading increased total Cr (P < 0.025), phosphocreatine (PCr) (P < 0.003), and the IPP (P < 0.0008) by 34%, 45%, and 64%, respectively. PCr and IPP were 46% (P < 0.002) and 76% (P < 0.02) greater in creatine-CLFS than in dextrose-CLFS. Higher IPP was confirmed by a 58% reduction in phospho-AMP-activated protein kinase α (Thr172) (P < 0.006). In dextrose-CLFS, myosin heavy chain (MyHC) I and IIa transcripts increased 32- and 38-fold (P < 0.006), respectively, whereas MyHC-IIb mRNA decreased by 75% (P < 0.03); the corresponding MyHC-I and MyHC-IIa protein contents increased by 2.0- (P < 0.03) and 2.7-fold (P < 0.05), respectively, and MyHC-IIb decreased by 30% (P < 0.03). In contrast, within creatine-CLFS, MyHC-I and MyHC-IIa mRNA were unchanged and MyHC-IIb mRNA decreased by 75% (P < 0.003); the corresponding MyHC isoform contents were not altered. Oxidative reference enzymes were similarly elevated (P < 0.01) in dextrose-CLFS and creatine-CLFS, but reciprocal reductions in glycolytic reference enzymes occurred only in dextrose-CLFS (P < 0.02). Preservation of the glycolytic potential and greater SERCA2 and parvalbumin contents in creatine-CLFS coincided with prolonged time to peak tension and half-rise time (P < 0.01). These results highlight the IPP as an important physiological regulator of muscle fibre plasticity and demonstrate that training-induced changes typically associated with improvements in muscular endurance or increased power output are not mutually exclusive in Cr-loaded muscles.

Poly-ADP-ribose-polymerase inhibition ameliorates hind limb ischemia reperfusion injury in a murine model of type 2 diabetes

INTRODUCTION

Diabetes is known to increase poly-ADP-ribose-polymerase (PARP) activity and posttranslational poly-ADP-ribosylation of several regulatory proteins involved in inflammation and energy metabolism. These experiments test the hypothesis that PARP inhibition will modulate hind limb ischemia reperfusion (IR) in a mouse model of type-II diabetes and ameliorate the ribosylation and the activity/transnuclear localization of the key glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH).

METHODS

db/db mice underwent 1.5 hours of hind limb ischemia followed by 1, 7, or 24 hours of reperfusion. The treatment group received the PARP inhibitor PJ34 (PJ34) over a 24-hour period; the untreated group received Lactated Ringer (LR) at the same time points. IR muscles were analyzed for indices of PARP activity, fiber injury, metabolic activity, inflammation, GAPDH activity/intracellular localization, and poly-ADP-ribosylation of GAPDH.

RESULTS

PARP activity was significantly lower in the PJ34-treated groups than in the Lactated Ringer group at 7 and 24 hours of reperfusion. There was significantly less muscle fiber injury in the PJ34-treated group than in the Lactated Ringer-treated mice at 24 hours of reperfusion. PJ34 lowered levels of select proinflammatory molecules at 7 hours and 24 hours of IR. There were significant increases in metabolic activity only at 24 hours of IR in the PJ34 group, which temporally correlated with increase in GAPDH activity, decreased GAPDH poly-ADP-ribosylation, and nuclear translocation of GAPDH.

CONCLUSIONS

PJ34 reduced PARP activity, GAPDH ribosylation, and GAPDH translocation; ameliorated muscle fiber injury; and increased metabolic activity after hind limb IR injury in a murine model of type-II diabetes. PARP inhibition might be a therapeutic strategy after IR in diabetic humans.

Regulation of mitochondrial biogenesis and GLUT4 expression by exercise

Endurance exercise training can induce large increases mitochondria and the GLUT4 isoform of the glucose transporter in skeletal muscle. For a long time after the discovery in the 1960s that exercise results in an increase in muscle mitochondria, there was no progress in elucidation of the mechanisms involved. The reason for this lack of progress was that nothing was known regarding how expression of the genes-encoding mitochondrial proteins is coordinately regulated. This situation changed rapidly after discovery of transcription factors that control transcription of genes-encoding mitochondrial proteins and, most importantly, the discovery of peroxisome proliferator-gamma coactivator-1α (PGC-1α). This transcription coactivator binds to and activates transcription factors that regulate transcription of genes-encoding mitochondrial proteins. Thus, PGC-1α activates and coordinates mitochondrial biogenesis. It is now known that exercise rapidly activates and induces increased expression of PGC-1α. The exercise-generated signals that lead to PGC-1α activation and increased expression are the increases in cytosolic Ca(2+) and decreases in ATP and creatine phosphate (∼P). Ca(2+) mediates its effect by activating CAMKII, while the decrease in ∼P mediates its effect via activation of AMPK. Expression of the GLUT4 isoform of the glucose transporter is regulated in parallel with mitochondrial biogenesis via the same signaling pathways. This review describes what is known regarding the regulation of mitochondrial biogenesis and GLUT4 expression by exercise. A major component of this review deals with the physiological and metabolic consequences of the exercise-induced increase in mitochondria and GLUT4.

Extracellular signal-regulated kinase 1/2-mediated phosphorylation of p300 enhances myosin heavy chain I/beta gene expression via acetylation of nuclear factor of activated T cells c1

The nuclear factor of activated T-cells (NFAT) c1 has been shown to be essential for Ca²⁺-dependent upregulation of myosin heavy chain (MyHC) I/β expression during skeletal muscle fiber type transformation. Here, we report activation of extracellular signal-regulated kinase (ERK) 1/2 in Ca²⁺-ionophore-treated C2C12 myotubes and electrostimulated soleus muscle. Activated ERK1/2 enhanced NFATc1-dependent upregulation of a −2.4 kb MyHCI/β promoter construct without affecting subcellular localization of endogenous NFATc1. Instead, ERK1/2-augmented phosphorylation of transcriptional coactivator p300, promoted its recruitment to NFATc1 and increased NFATc1–DNA binding to a NFAT site of the MyHCI/β promoter. In line, inhibition of ERK1/2 signaling abolished the effects of p300. Comparison between wild-type p300 and an acetyltransferase-deficient mutant (p300DY) indicated increased NFATc1–DNA binding as a consequence of p300-mediated acetylation of NFATc1. Activation of the MyHCI/β promoter by p300 depends on two conserved acetylation sites in NFATc1, which affect DNA binding and transcriptional stimulation. NFATc1 acetylation occurred in Ca²⁺-ionophore treated C2C12 myotubes or electrostimulated soleus. Finally, endogenous MyHCI/β gene expression in C2C12 myotubes was strongly inhibited by p300DY and a mutant deficient in ERK phosphorylation sites. In conclusion, ERK1/2-mediated phosphorylation of p300 is crucial for enhancing NFATc1 transactivation function by acetylation, which is essential for Ca²⁺-induced MyHCI/β expression.

Gene regulation mediating fiber-type transformation in skeletal muscle cells is partly glucose- and ChREBP-dependent

Adaptations in the oxidative capacity of skeletal muscle cells can occur under several physiological or pathological conditions. We investigated the effect of increasing extracellular glucose concentration on the expression of markers of energy metabolism in primary skeletal muscle cells and the C2C12 muscle cell line. Growth of myotubes in 25mM glucose (high glucose, HG) compared with 5.55mM led to increases in the expression and activity of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a marker of glycolytic energy metabolism, while oxidative markers peroxisome proliferator-activated receptor γ coactivator 1α and citrate synthase decreased. HG induced metabolic adaptations as are seen during a slow-to-fast fiber transformation. Furthermore, HG increased fast myosin heavy chain (MHC) IId/x but did not change slow MHCI/β expression. Protein phosphatase 2A (PP2A) was shown to mediate the effects of HG on GAPDH and MHCIId/x. Carbohydrate response element-binding protein (ChREBP), a glucose-dependent transcription factor downstream of PP2A, partially mediated the effects of glucose on metabolic markers. The glucose-induced increase in PP2A activity was associated with an increase in p38 mitogen-activated protein kinase activity, which presumably mediates the increase in MHCIId/x promoter activity. Liver X receptor, another possible mediator of glucose effects, induced only an incomplete metabolic shift, mainly increasing the expression of the glycolytic marker. Taken together, HG induces a partial slow-to-fast transformation comprising metabolic enzymes together with an increased expression of MHCIId/x. This work demonstrates a functional role for ChREBP in determining the metabolic type of muscle fibers and highlights the importance of glucose as a signaling molecule in muscle.

Passive mechanical forces upregulate the fast myosin heavy chain IId/x via integrin and p38 MAP kinase activation in a primary muscle cell culture

We have studied the mechanism by which a previously described primary muscle culture growing on microcarriers predominantly expresses fast myosin heavy chain (MHC) IId/x. We have measured MHC IId/x mRNA and protein levels, mRNA of MHC I and markers of muscle metabolism, insulin-like growth factor (IGF)-1 and mechano-growth factor (MGF) transcripts, indicators of the activation of the Akt-mammalian target of rapamycin (mTOR) axis, the p38-, ERK1/2-, and JNK-mitogen-activated protein kinase (MAP) kinase pathways, and of protein phosphatase PP2A, and we have assessed the involvement of integrin. By placing the culture flasks on a rotary shaker, we induce a continuous motion of the culture medium in which the carrier-myotube aggregates are suspended. This motion exerts passive forces on the myotubes that are decisive for the predominance of MHC II expression. These forces act via integrin, which transduces the mechanical signal into activation of PP2A and of p38 MAP-Kinase. The latter presumably is directly responsible for a drastic upregulation of MHC IId/x, whereas MHC I and metabolic markers remain unaffected. At the same time, despite an elevated level of IGF-1 transcription under passive forces, the IGF-1 receptor-Akt-mTOR axis is switched off as evident from the lack of an effect of inhibition of the IGF-1 receptor and from the PP2A-mediated low degree of phosphorylation of Akt and 4E-BP1. Similarly, the ERK1/2- and JNK-MAP kinase pathways are repressed. We conclude that passive stretch exerted on the myotubes by the rotary fluid motion induces a rather selective upregulation of fast MHC II, which goes along with a mild muscle hypertrophy as judged from the amount of protein per cell and is caused by p38 MAP kinase activity elevated via integrin sensing. The direct link between passive stretch and MHC II expression constitutes a novel mechanism, which is expected to become effective physiologically under passive stretch and eccentric contractions of skeletal muscles.

Different roles of H-ras for regulation of myosin heavy chain promoters in satellite cell-derived muscle cell culture during proliferation and differentiation

The effect of constitutively activated proto-oncogene H-ras (H-rasQ61L) on the regulation of myosin heavy chain (MHC) promoter activities was investigated in rabbit satellite cell-derived muscle cell culture during the proliferation stage and early and later stages of differentiation, respectively. During proliferation, overexpression of H-rasQ61L did not affect basal level of activity of the slow MHCI/beta or the fast MHCIId/x promoter luciferase reporter gene construct in transient transfection assays. By contrast, H-rasQ61L affected both MHC promoter activities during differentiation, and this effect changes from inactivation after 2 days to activation after 4 days of differentiation. The activating effect of H-rasQ61L on both MHC promoters after 4 days of differentiation was significantly reduced by LY-294002, a specific inhibitor of the phosphoinositol-3-kinase (PI3K), a downstream target of Ras. Furthermore, the protein kinase Akt (protein kinase B), a downstream target of PI3k, was activated 4 days after initiation of differentiation in myotubes overexpressing H-rasQ61L. By contrast, inhibition of another Ras downstream pathway, mitogen-activated protein kinase kinase 1/2-extracellular signal-regulated protein kinase 1/2 (MKK1/2-ERK1/2-MAPK), increased activities of both MHC promoters, indicating a suppressive role of this pathway. Moreover, the Ras-PI3K-Akt signaling pathway is involved in the activation of MHCI/beta and IId/x promoters in a later stage of differentiation of muscle cells, presumably by a known inhibiting effect of activated Akt on the MKK1/2-ERK1/2-MAPK pathway. The experiments demonstrate that during differentiation of muscle cells activated H-ras is an important regulator of MHC isoform promoter function with opposite effects during early and later stages.

[Exercise and cellular adaptation of muscle]

Resistance training and to a lesser extent endurance training are capable of enhancing protein synthesis in skeletal muscle via various signaling pathways. Additionally, the expression of muscle fiber types responds to different regimes of training stimuli and immobilization as characterized by changes in myosin heavy chain isoforms (I<-->IIA<-->IIX). Eccentric resistance training has been shown to be highly efficient in inducing sarcomeric protein assembly in the longitudinal orientation of muscle cells. However, concentric contractions lead to a hypertrophic response (increased fiber diameter) in muscle which can still be activated in old age. The central signaling pathway to mediate the elevation of protein synthesis in response to training is the mTOR pathway, which is also stimulated by free amino acids. Moreover, adaptation to endurance training is mediated by the calcium-calcineurin-NFATc1 pathway which is strongly activated by the calcium transients involved in the muscle contraction process. High contraction frequency and long duration of training sessions are essential for activation and maintenance of fiber type I expression as well as for induction of transformation of type II into type I fibers. Endurance training sessions should therefore be longer than 30 min and dominated by periods of high frequency contractions. A further factor in the muscular response to training includes the recruitment and integration of satellite cells into muscle fibers. Satellite cells can respond to muscular stretch, activity and injury with increased proliferation and can later be integrated into muscle fibers. Therefore, new myonuclei are available to enhance mRNA synthesis and protein expression in muscle cells. New understanding of the cellular mechanisms of signal transduction in muscle in response to training, bed rest and ageing will help to optimize training and interventions in an ageing population.

Chapter 2. Calcineurin signaling and the slow oxidative skeletal muscle fiber type

Calcineurin, also known as protein phosphatase 2B (PP2B), is a calcium-calmodulin-dependent phosphatase. It couples intracellular calcium to dephosphorylate selected substrates resulting in diverse biological consequences depending on cell type. In mammals, calcineurin's functions include neuronal growth, development of cardiac valves and hypertrophy, activation of lymphocytes, and the regulation of ion channels and enzymes. This chapter focuses on the key roles of calcineurin in skeletal muscle differentiation, regeneration, and fiber type conversion to an oxidative state, all of which are crucial to muscle development, metabolism, and functional adaptations. It seeks to integrate the current knowledge of calcineurin signaling in skeletal muscle and its interactions with other prominent regulatory pathways and their signaling intermediates to form a molecular overview that could provide directions for possible future exploitations in human metabolic health.

Signaling mechanisms in skeletal muscle: acute responses and chronic adaptations to exercise

Physical activity elicits physiological responses in skeletal muscle that result in a number of health benefits, in particular in disease states, such as type 2 diabetes. An acute bout of exercise/muscle contraction improves glucose homeostasis by increasing skeletal muscle glucose uptake, while chronic exercise training induces alterations in the expression of metabolic genes, such as those involved in muscle fiber type, mitochondrial biogenesis, or glucose transporter 4 (GLUT4) protein levels. A primary goal of exercise research is to elucidate the mechanisms that regulate these important metabolic and transcriptional events in skeletal muscle. In this review, we briefly summarize the current literature describing the molecular signals underlying skeletal muscle responses to acute and chronic exercise. The search for possible exercise/contraction-stimulated signaling proteins involved in glucose transport, muscle fiber type, and mitochondrial biogenesis is ongoing. Further research is needed because full elucidation of exercise-mediated signaling pathways would represent a significant step toward the development of new pharmacological targets for the treatment of metabolic diseases such as type 2 diabetes.

Carbonic anhydrases IV and IX: subcellular localization and functional role in mouse skeletal muscle

The subcellular localization of carbonic anhydrase (CA) IV and CA IX in mouse skeletal muscle fibers has been studied immunohistochemically by confocal laser scanning microscopy. CA IV has been found to be located on the plasma membrane as well as on the sarcoplasmic reticulum (SR) membrane. CA IX is not localized in the plasma membrane but in the region of the t-tubular (TT)/terminal SR membrane. CA IV contributes 20% and CA IX 60% to the total CA activity of SR membrane vesicles isolated from mouse skeletal muscles. Our aim was to examine whether SR CA IV and TT/SR CA IX affect muscle contraction. Isolated fiber bundles of fast-twitch extensor digitorum longus and slow-twitch soleus muscle from mouse were investigated for isometric twitch and tetanic contractions and by a fatigue test. The muscle functions of CA IV knockout (KO) fibers and of CA IX KO fibers do not differ from the function of wild-type (WT) fibers. Muscle function of CA IV/XIV double KO mice unexpectedly shows a decrease in rise and relaxation time and in force of single twitches. In contrast, the CA inhibitor dorzolamide, whether applied to WT or to double KO muscle fibers, leads to a significant increase in rise time and force of twitches. It is concluded that the function of mouse skeletal muscle fibers expressing three membrane-associated CAs, IV, IX, and XIV, is not affected by the lack of one isoform but is possibly affected by the lack of all three CAs, as indicated by the inhibition studies.

Metabolic transformation of rabbit skeletal muscle cells in primary culture in response to low glucose

We have investigated the mechanism of the changes in the profile of metabolic enzyme expression that occur in association with fast-to-slow transformation of rabbit skeletal muscle. The hypotheses assessed are: do 1) lowered intracellular ATP concentration or 2) reduction of the muscular glycogen stores act as triggers of metabolic transformation? We find that 3 days of decreased cytosolic ATP content have no impact on the investigated metabolic markers, whereas incubation of the cells with little or no glucose leads to decreases in glycogen in conjunction with decreases in glyceraldehyde-3-phosphate dehydrogenase (GAPDH) promoter activity, GAPDH mRNA and specific GAPDH enzyme activity (indicators of the anaerobic glycolytic pathway), and furthermore to increases in mitochondrial acetoacetyl-CoA thiolase (MAT, also known as ACAT) promoter activity, peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) expression and citrate synthase (CS) specific enzyme activity (all indicators of oxidative metabolic pathways). The AMP-activated protein kinase (AMPK) activity under these conditions is reduced compared to controls. In experiments with two inhibitors of glycogen degradation we show that the observed metabolic transformation caused by low glucose takes place even if intracellular glycogen content is high. These findings for the first time provide evidence that metabolic adaptation of skeletal muscle cells from rabbit in primary culture can be induced not only by elevation of intracellular calcium concentration or by a rise of AMPK activity, but also by reduction of glucose supply. Contrary to expectations, neither an increase in phospho-AMPK nor a reduction of muscular glycogen content are crucial events in the glucose-dependent induction of metabolic transformation in the muscle cell culture system studied.

The Ca(V) 1.2 Ca(2+) channel is expressed in sarcolemma of type I and IIa myofibers of adult skeletal muscle

Although Ca(2+)-dependent signaling pathways are important for skeletal muscle plasticity, the sources of Ca(2+) that activate these signaling pathways are not completely understood. Influx of Ca(2+) through surface membrane Ca(2+) channels may activate these pathways. We examined expression of two L-type Ca(2+) channels in adult skeletal muscle, the Ca(V) 1.1 and Ca(V) 1.2, with isoform-specific antibodies in Western blots and immunocytochemistry assays. Consistent with a large body of work, expression of the Ca(V) 1.1 was restricted to skeletal muscle where it was expressed in T-tubules. Ca(V) 1.2 was also expressed in skeletal muscle, in the sarcolemma of type I and IIa myofibers. Exercise-induced alterations in muscle fiber types cause a concomitant increase in the number of both Ca(V) 1.2 and type IIa-positive fibers. Taken together, these data suggest that the Ca(V) 1.2 Ca(2+) channel is expressed in adult skeletal muscle in a fiber type-specific manner, which may help to maintain oxidative muscle phenotype.

Mechanisms of calcium-induced mitochondrial biogenesis and GLUT4 synthesis

Regularly performed aerobic exercise leads to increases in skeletal muscle mitochondria and glucose transporter 4 (GLUT4) protein content, resulting in an enhanced capacity to oxidize substrates and improvements in insulin- and contraction-mediated glucose uptake. Although the specific mechanisms governing these adaptive responses have not been fully elucidated, accumulating evidence suggests that the increase in cytosolic Ca2+ that occurs with each wave of sacrolemmal depolarization is a key component of these processes. Treating L6 muscle cells with agents that increase Ca2+ without causing reductions in ~P or the activation of 5′-AMP-activated protein kinase leads to increases in GLUT4 and mitochondrial protein contents. This effect is likely controlled through calcium/calmodulin-dependent protein kinase (CaMK), since KN93, a specific CaMK inhibitor, blocks these adaptive responses. Recent findings provide evidence that the activation of p38 mitogen-activated protein kinase (MAPK) is involved in the pathway through which Ca2+/CaMK mediates mitochondrial and GLUT4 biogenesis. p38 MAPK initiates GLUT4 and mitochondrial biogenesis through the activation      of transcription factors and transcriptional coactivators such as myocyte enhancer factor 2 (MEF2) and peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α). Subsequent increases in the content of these proteins further enhance Ca2+-induced GLUT4 and mitochondrial biogenesis. Since decreases in mitochondrial and GLUT4 contents are associated with skeletal muscle insulin resistance, an understanding of the mechanisms by which these processes can be normalized will aid in the prevention and treatment of type 2 diabetes.

Regional specialization of rat quadriceps myosin heavy chain isoforms occurring in distal to proximal parts of middle and deep regions is not mirrored by citrate synthase activity

Myosin heavy chain (MHC) isoform content and citrate synthase (CS) activities were measured in the Quadriceps femoris (QF) muscle of 18 female rats. The muscle group was divided into superficial, middle and deep, distal, central and proximal parts. MHC IIb and IIx were more abundant in superficial regions (P<0.05) with low CS activities compared with deeper parts. The deeper parts expressed all four isoforms (MHC IIb, MHC IIx, MHC IIa and MHC I), with a concomitantly higher CS activity. MHC I, MHC IIa and MHC IIb isoform content varied significantly along the length of the deep regions. Only MHC IIb and CS activity in the proximal middle part correlated (negatively) with each other. This study showed that the QF has regional specialization and that standardization of sampling site is important. Furthermore, CS activity and MHC isoforms are only loosely associated, or not at all.

Myosin heavy chain mRNA isoforms in masseter muscle before and after orthognathic surgery

OBJECTIVES

Orthognathic surgery leads to changed jaw position and force vector of mastication to which the muscles must adapt. The aim of the present study was to determine the relative expression of myosin heavy chain (MyHC) messenger RNA (mRNA) isoforms in different types of human masseter muscle fiber under consideration of change in the number of occlusal contacts before and 6 months after surgery.

STUDY DESIGN

Muscle biopsies were taken from the anterior and posterior parts of both sides in 30 patients with prognathic and retrognathic mandibles. Specific mRNA MyHC analysis was made by real-time polymerase chain reaction to quantify the isoforms I, IIa, and IId/x.

RESULTS

There was a shift in the relative content from type I (46% before, 37% after) to type IIa (29% before, 42% after). This shift correlates with number of teeth in occlusion.

CONCLUSIONS

Correlation between isoform shift and number of teeth in occlusion indicates higher mastication force which stabilizes the treatment result.

Carbonic anhydrase XIV in skeletal muscle: subcellular localization and function from wild-type and knockout mice

The expression of carbonic anhydrase (CA) XIV was investigated in mouse skeletal muscles. Sarcoplasmic reticulum (SR) and sarcolemmal (SL) membrane fractions were isolated from wild-type (WT) and CA XIV knockout (KO) mice. The CA XIV protein of 54 kDa was present in SR and SL membrane fractions as shown by Western blot analysis. CA activity measurements of WT and KO membrane fractions showed that CA XIV accounts for approximately 50% and 66% of the total CA activities determined in the SR and SL fractions, respectively. This indicates the presence of at least one other membrane-associated CA isoform in these membranes, e.g., CA IV, CA IX, or CA XII. Muscle fibers of the extensor digitorum longus (EDL) muscle were immunostained with anti-CA XIV/FITC and anti-sarco(endo)plasmic reticulum Ca(2+)-ATPase 1/TRITC, with anti-CA XIV/FITC and anti-ryanodine receptor/TRITC, or with anti-CA XIV/FITC and anti-monocarboxylate transporter-4/TRITC. CA XIV was expressed in the plasma membrane and in the longitudinal SR but not in the terminal SR. Isometric contraction measurements of single twitches and tetani and a fatigue protocol applied to fiber bundles of the fast-twitch EDL and of the slow-twitch soleus muscle from WT and KO mice showed that the lack of SR membrane-associated CA XIV did not affect maximum force, rise and relaxation times, and fatigue behavior. Thus, it is concluded that a reduction of the total SR CA activity by approximately 50% in CA XIV KO mice does not lead to an impairment of SR function.

Activation of the beta myosin heavy chain promoter by MEF-2D, MyoD, p300, and the calcineurin/NFATc1 pathway

Calcium is a key element in intracellular signaling in skeletal muscle. Changes in intracellular calcium levels are thought to mediate the fast-to-slow transformation of muscle fiber type. One factor implicated in gene regulation in adult muscle is the nuclear factor of activated T-cells (NFAT) isoform c1, whose dephosphorylation by the calcium/calmodulin-dependent phosphatase calcineurin facilitates its nuclear translocation. Here, we report that differentiated C2C12 myotubes predominantly expressing fast-type MyHCII protein undergo fast-to-slow transformation following calcium-ionophore treatment, with several transcription factors and a transcriptional coactivator acting in concert to upregulate the slow myosin heavy chain (MyHC) beta promoter. Transient transfection assays demonstrated that the calcineurin/NFATc1 signaling pathway is essential for MyHCbeta promoter activation during transformation of C2C12 myotubes but is not sufficient for complete fast MyHCIId/x promoter inhibition. Along with NFATc1, myocyte enhancer factor-2D (MEF-2D) and the myogenic transcription factor MyoD transactivated the MyHCbeta promoter in calcium-ionophore-treated myotubes in a calcineurin-dependent manner. To elucidate the mechanism involved in regulating MyHCbeta gene expression, we analyzed the -2.4-kb MyHCbeta promoter construct for cis-regulatory elements. Using electrophoretic mobility shift assays (EMSAs), chromatin immunoprecipitation assays (ChIP), and nuclear complex coimmunoprecipitation (NCcoIP) assays, we demonstrated calcium-ionophore-induced binding of NFATc1 to a NFAT consensus site adjacent to a MyoD-binding E-box. At their respective binding sites, both NFATc1 and MyoD recruited the transcriptional coactivator p300, and in turn, MEF-2D bound to the MyoD complex. The calcium-ionophore-induced effects on the MyHCbeta promoter were shown to be calcineurin-dependent. Together, our findings demonstrate calcium-ionophore-induced activation of the beta MyHC promoter by NFATc1, MyoD, MEF-2D, and p300 in a calcineurin-dependent manner.

Differential gene expression in skeletal muscle cells after membrane depolarization

Skeletal muscle is a highly plastic tissue with a remarkable capacity to adapt itself to challenges imposed by contractile activity. Adaptive response, that include hypertrophy and activation of oxidative mechanisms have been associated with transient changes in transcriptional activity of specific genes. To define the set of genes regulated by a depolarizing stimulus, we used 22 K mouse oligonucleotide microarrays. Total RNA from C2C12 myotubes was obtained at 2, 4, 18, and 24 h after high K+ stimulation. cDNA from control and depolarized samples was labeled with cyanine 3 or 5 dyes prior to microarray hybridization. Loess normalization followed by statistical analysis resulted in 423 differentially expressed genes using an unadjusted P-value < or = 0.01 as cut off. Depolarization affects transcriptional activity of a limited number of genes, mainly associated with metabolism, cell communication and response to stress. A number of genes related to Ca2+ signaling pathways are induced at 4 h, reinforcing the potential role of Ca2+ in early steps of signal transduction that leads to gene expression. Significant changes in the expression of molecules involved in muscle cell structure were observed; K+-depolarization increased Tnni1 and Acta1 mRNA levels in both differentiated C2C12 and rat skeletal muscle cells in primary culture. Of these two, depolarization induced slow Ca2+ transients appear to have a role only in the regulation of Tnni1 transcriptional activity. We suggest that depolarization induced expression of a small set of genes may underlie Ca2+ dependent plasticity of skeletal muscle cells.

Calcineurin and skeletal muscle growth

Recruitment determines the profile of fibre-type-specific genes expressed across the range of muscle fibres associated with slow, fast fatigue-resistant and fast fatiguable motor units. Downstream signalling pathways activated by neural signalling and mechanical load have been the focus of intensive research in past years. It is now known that Ca2+-dependent calcineurin–nuclear factor of activated T cells and insulin-like growth factor 1 pathways and their downstream mediators contribute to these adaptive responses. These pathways regulate gene expression through muscle-specific (myocyte-enhancing factor 2, myoblast determination protein) and non-specific (nuclear factor of activated T cell 2, GATA-2) transcription factors. Transcriptional signals activated with increased contractile activity result in altered expression of fibre-type specific genes, including the myosin heavy chain isoforms and oxidative and glycolytic enzymes and a net change in muscle fibre-type composition. In contrast, transcriptional signals activated by increased load bearing result in hypertrophy or a growth response, a component of which involves satellite cell recruitment and fusion with existing adult myofibres. Calcineurin has been identified as a key mediator in the hypertrophic response, and the current challenge has been to determine the downstream target genes of this pathway. Exciting new data have emerged, showing that myostatin, a negative regulator of muscle growth, and utrophin, a cytoskeletal protein important in maintaining membrane integrity, are downstream targets of calcineurin signalling. Increased understanding of these mediators of muscle growth may provide strategies for the development of effective therapeutics to counter muscle weakness and muscular dystrophy.

The p38α/β Mitogen-activated Protein Kinases Mediate Recruitment of CREB-binding Protein to Preserve Fast Myosin Heavy Chain IId/x Gene Activity in Myotubes

In skeletal muscle, the transformation of fast into slow fiber type is accompanied by shifts in fiber type-specific gene expression that includes down-regulation of the adult fast fiber myosin heavy chain IId/x (MyHCIId/x) gene. Here, we report that the mitogen-activated protein kinases (MAPKs) p38alpha/beta regulate MyHCIId/x gene expression. Electrical stimulation of rabbit skeletal muscle cells with a slow fiber type activity pattern and treatment of C2C12 myotubes with Ca(2+)-ionophore inhibited p38alpha/beta MAPKs and reduced fast fiber type MyHC protein expression and promoter activity. Pharmacological inhibition of p38alpha/beta also down-regulated MyHCII gene expression. In controls, binding of the myocyte enhancer factor-2 (MEF-2) isoforms C and D as a heterodimer to a proximal consensus site within the MyHCIId/x promoter and recruitment of a transcriptional coactivator, the CREB-binding protein CBP, were observed. Overexpression of wild type MEF-2C but not of a MEF-2C mutant that cannot be phosphorylated by p38 induced promoter activity. Mutation of the MEF-2-binding site decreased the inducing effect of overexpressed CBP. Inhibition of p38alpha/beta MAPKs abolished CBP binding, whereas enforced induction of p38 by activated MAPK kinase 6 (MKK6EE) enhanced binding of CBP and increased promoter activity. Furthermore, knockdown of endogenous CBP by RNA interference eliminated promoter activation by MEF-2C or MKK6EE. In electrical stimulated and Ca(2+)-ionophore-treated myotubes, CBP was absent in complex formation at that site. Taken together, the data indicate that p38alpha/beta MAPKs-mediated coactivator recruitment at a proximal MEF-2 site is important for MyHCIId/x gene regulation in skeletal muscle.

Calcineurin A and CaMKIV transactivate PGC-1α promoter, but differentially regulate cytochrome c promoter in rat skeletal muscle

In skeletal muscle, slow-twitch fibers are highly dependent on mitochondrial oxidative metabolism suggesting the existence of common regulatory pathways in the control of slow muscle-specific protein expression and mitochondrial biogenesis. In this study, we determined whether peroxisome proliferator-activated receptor gamma co-activator-1alpha (PGC-1alpha) could transactivate promoters of nuclear-encoded mitochondrial protein (cytochrome c) and muscle-specific proteins (fast troponin I, MyoD). We also investigated if calcineurin A (CnA) and calcium/calmodulin kinase IV (CaMKIV) were involved in the regulation of PGC-1alpha and cytochrome c promoter. For this purpose, we took advantage of the gene electrotransfer technique, which allows acute expression of a gene of interest. Electrotransfer of a PGC-1alpha expression vector into rat Tibialis anterior muscle induced a strong transactivation of cytochrome c promoter (P < 0.001) independent of nuclear respiratory factor 1. PGC-1alpha gene electrotransfer did not transactivate fast troponin I promoter, whereas it did transactivate MyoD promoter (P < 0.05). Finally, whereas electrotransfers of CnA or CaMKIV expression vectors transactivated PGC-1alpha promoter (P < 0.001), gene electrotransfer of CaMKIV was only able to transactivate cytochrome c promoter. Taken together, these data suggest that CnA triggers PGC-1alpha promoter transactivation to drive the expression of non-mitochondrial proteins.

Expression of membrane-bound carbonic anhydrases IV, IX, and XIV in the mouse heart

Expression of membrane-bound carbonic anhydrases (CAs) of CA IV, CA IX, CA XII, and CA XIV has been investigated in the mouse heart. Western blots using microsomal membranes of wild-type hearts demonstrate a 39-, 43-, and 54-kDa band representing CA IV, CA IX, and CA XIV, respectively, but CA XII could not be detected. Expression of CA IX in the CA IV/CA XIV knockout animals was further confirmed using matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Cardiac cells were immunostained using anti-CA/FITC and anti-alpha-actinin/TRITC, as well as anti-CA/FITC and anti-SERCA2/TRITC. Subcellular CA localization was investigated by confocal laser scanning microscopy. CA localization in the sarcolemmal (SL) membrane was examined by double immunostaining using anti-CA/FITC and anti-MCT-1/TRITC. CAs showed a distinct distribution pattern in the sarcoplasmic reticulum (SR) membrane. CA XIV is predominantly localized in the longitudinal SR, whereas CA IX is mainly expressed in the terminal SR/t-tubular region. CA IV is present in both SR regions, whereas CA XII is not found in the SR. In the SL membrane, only CA IV and CA XIV are present. We conclude that CA IV and CA XIV are associated with the SR as well as with the SL membrane, CA IX is located in the terminal SR/t-tubular region, and CA XII is not present in the mouse heart. Therefore, the unique subcellular localization of CA IX and CA XIV in cardiac myocytes suggests different functions of both enzymes in excitation-contraction coupling.

Calcineurin-mediated slow-type fiber expression and growth in reloading condition

PURPOSE

Calcineurin (CaN) signaling pathway has been implicated in the transcriptional regulation of slow muscle fiber genes and in muscle hypertrophy. Our aim was to investigate the functional role of CaN as a regulator of muscle growth and/or muscle fiber type under conditions of recovery from inactivity.

METHODS

Female ICR mice (8 wk of age, 28-32 g) were used. To examine the effects of hindlimb suspension (HS) and reloading on skeletal muscle fiber size and muscle fiber type, animals were designated to 8 wk of HS and subsequent reloading for 4 wk. During reloading, animals were treated with pharmacological inhibitors for CaN (FK506) by intraperitoneal administration (3-5 mg.kg.d). After each experimental period, antigravitational soleus muscle was analyzed.

RESULTS

HS treatment resulted in obvious muscle atrophy and slow-to-fast fiber-type transformation in the soleus muscle. Subsequent reloading for 4 wk following HS induced muscle regrowth and fiber-type reversion toward a slow profile. FK506 administration prevented this kind of reloading-induced transformation of muscle fiber type. Furthermore, it was confirmed that FK506 administration attenuated maintenance of fiber cross-sectional area and reloading-induced fiber regrowth, specifically in slow-type muscle fibers.

CONCLUSION

Reloading-induced fiber-type reversion toward a slow profile is prevented by the pharmacological inhibition of CaN. Additionally, inhibition of CaN prevented maintenance and regrowth of slow-type muscle fibers. These results implicate that the CaN signaling pathway is required in the slow-type muscle fiber program under maintenance and suspension-reloading conditions.

Role of calcineurin in exercise-induced mitochondrial biogenesis

Raising cytosolic Ca2+ induces an increase in mitochondrial biogenesis in myotubes. This phenomenon mimics the adaptive responses of skeletal muscle to exercise. It has been hypothesized that increases in cytosolic Ca2+ during motor nerve activity stimulate mitochondrial biogenesis by activating calcineurin. Overexpression of constitutively active calcineurin increases expression of peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) and induction of genes involved in mitochondrial energy metabolism in muscle cells. The purpose of this study was to determine whether calcineurin plays a role in the stimulation of mitochondrial biogenesis by exercise. Rats were exercised on 5 successive days by means of swimming. Inhibition of calcineurin with cyclosporin did not prevent the exercise-induced increases in PGC-1alpha and a range of mitochondrial proteins. In contrast to the other mitochondrial proteins, the increases in cytochrome oxidase (COX)-I and -IV proteins were blocked by cyclosporin treatment. This inhibitory effect of cyclosporin occurred at the posttranscriptional level, as evidenced by normal increases in COX-I and COX-IV mRNAs in response to exercise in the cyclosporin-treated rats. This toxic effect of cyclosporin may account for the decrease in muscle respiratory capacity reported to occur with cyclosporin treatment. In conclusion, inhibition of calcineurin does not prevent the exercise-induced increase in mitochondrial biogenesis in skeletal muscles, providing evidence that the adaptive response is not mediated by activation of calcineurin.

Effects of Cyclosporine-A on Rat Soleus Muscle Fiber Size and Phenotype

PURPOSE

Organ transplant patients treated with cyclosporine-A (CsA) often exhibit weight loss and muscle weakness. The cellular target of CsA, calcineurin, has been implicated in maintenance of muscle fiber size and in expression of the type I skeletal muscle phenotype. We hypothesized that CsA treatment would cause fiber atrophy, as well as increase type IIa myosin heavy chain (MHC) content and oxidative enzyme activities in the soleus muscle.

METHODS

Rats were treated with CsA for 21 d (20 mg.kg(-1).d(-1); N = 16) and compared with control rats given olive oil vehicle (Veh; N = 16). Soleus muscles were excised bilaterally. MHC content was determined by gel electrophoresis, oxidative enzyme activities by spectrophotometric methods, and fiber type and size by histochemistry.

RESULTS

Lymphocyte count was depressed in CsA rats (P < 0.05), indicating treatment efficacy. Type IIa MHC content was increased in the soleus muscle with CsA (Veh, 10.4 +/- 1.7%; CsA, 15.1 +/- 2.0; P < 0.05) at the expense of type I MHC. Soleus muscle oxidative enzyme activities were also increased with CsA treatment (P < 0.05). Soleus muscle atrophy occurred, reflected by a 22% decrease in fiber cross-sectional area (Veh, 3255 +/- 105 microm(2); CsA, 2533 +/- 125; P < 0.05).

CONCLUSION

These findings indicate that CsA treatment is associated with changes in skeletal muscle fiber size and phenotype. The former may underlie clinical symptoms of transplant patients treated with CsA.

Cultured slow vs. fast skeletal muscle cells differ in physiology and responsiveness to stimulation

In vitro studies have used protein markers to distinguish between myogenic cells isolated from fast and slow skeletal muscles. The protein markers provide some support for the hypothesis that satellite cells from fast and slow muscles are different, but the data are equivocal. To test this hypothesis directly, three-dimensional skeletal muscle constructs were engineered from myogenic cells isolated from fast tibialis anterior (TA) and slow soleus (SOL) muscles of rats and functionality was tested. Time to peak twitch tension (TPT) and half relaxation time (RT(1/2)) were approximately 30% slower in constructs from the SOL. The slower contraction and relaxation times for the SOL constructs resulted in left shift of the force-frequency curve compared with those from the TA. Western blot analysis showed a 60% greater quantity of fast myosin heavy chain in the TA constructs. 14 days of chronic low-frequency electrical stimulation resulted in a 15% slower TPT and a 14% slower RT(1/2), but no change in absolute force production in the TA constructs. In SOL constructs, slow electrical stimulation resulted in an 80% increase in absolute force production with no change in TPT or RT(1/2). The addition of cyclosporine A did not prevent the increase in force in SOL constructs after chronic low-frequency electrical stimulation, suggesting that calcineurin is not responsible for the increase in force. We conclude that myogenic cells associated with a slow muscle are imprinted to produce muscle that contracts and relaxes slowly and that calcineurin activity cannot explain the response to a slow pattern of electrical stimulation.

Depolarization-induced slow Ca2+ transients stimulate transcription of IL-6 gene in skeletal muscle cells

Contracting skeletal muscle produces and releases interleukin-6 (IL-6) in high amounts. Nevertheless, the mechanisms underlying IL-6 expression are not understood. Because inositol-1,4,5-trisphosphate (IP(3))-mediated slow Ca(2+) signals evoked by depolarization of skeletal myotubes appears to play a role in the regulation of gene expression, we examined its involvement on IL-6 transcription. With the use of semiquantitative RT-PCR, we have shown that K(+) depolarization of myotubes induces a transient increase in IL-6 mRNA level, which peaks at 3-4 h and is independent of extracellular Ca(2+). Inhibitors of IP(3)-dependent Ca(2+) signals, like 2-aminoethoxydiphenyl borate (2-APB) and U-73122, decreased activation of IL-6 gene expression as did Ca(2+) signals inhibitor BAPTA-AM, whereas ryanodine, a fast Ca(2+) transient inhibitor, had no effect on IL-6 induction. Depolarization of myotubes transiently transfected with a reporter gene construct, containing 651 bp of IL-6 promoter, induced a twofold increase in promoter activity, which was abolished by either 2-APB or U-73122 and remained unaffected after ryanodine treatment. Site-directed mutagenesis of parental construct allowed us to identify activator protein-1 and NF-kappaB sequences as regulatory elements involved in IL-6 upregulation. Our results provide evidence for involvement of IP(3)-mediated Ca(2+) signals on IL-6 transcription in skeletal muscle cells.

Nerve influence on myosin light chain phosphorylation in slow and fast skeletal muscles

Neural stimulation controls the contractile properties of skeletal muscle fibres through transcriptional regulation of a number of proteins, including myosin isoforms. To study whether neural stimulation is also involved in the control of post-translational modifications of myosin, we analysed the phosphorylation of alkali myosin light chains (MLC1) and regulatory myosin light chains (MLC2) in rat slow (soleus) and fast (extensor digitorum longus EDL) muscles using 2D-gel electrophoresis and mass spectrometry. In control rats, soleus and EDL muscles differed in the proportion of the fast and slow isoforms of MLC1 and MLC2 that they contained, and also in the distribution of the variants with distinct isoelectric points identified on 2D gels. Denervation induced a slow-to-fast transition in myosin isoforms and increased MLC2 phosphorylation in soleus, whereas the opposite changes in myosin isoform expression and MLC2 phosphorylation were observed in EDL. Chronic low-frequency stimulation of EDL, with a pattern mimicking that of soleus, induced a fast-to-slow transition in myosin isoforms, accompanied by a decreased MLC2 phosphorylation. Chronic administration (10 mg x kg(-1) x d(-1) intraperitoneally) of cyclosporin A, a known inhibitor of calcineurin, did not change significantly the distribution of fast and slow MLC2 isoforms or the phosphorylation of MLC2. All changes in MLC2 phosphorylation were paralleled by changes in MLC kinase expression without any variation of the phosphatase subunit, PP1. No variation in MLC1 phosphorylation was detectable after denervation or cyclosporin A administration. These results suggest that the low-frequency neural discharge, typical of soleus, determines low levels of MLC2 phosphorylation together with expression of slow myosin, and that MLC2 phosphorylation is regulated by controlling MLC kinase expression through calcineurin-independent pathways.

Skeletal muscle myofibrillar mRNA expression in heart failure: relationship to local and circulating hormones

Chronic heart failure is characterized by changes in skeletal muscle that contribute to exercise intolerance and muscle weakness. To determine whether changes in the quantity and isoform distribution of key myofibrillar proteins are related to altered gene expression, we measured skeletal muscle myofibrillar mRNA abundance in nine heart failure patients (mean +/- SE; 63 +/- 3 yr) and nine controls (70 +/- 3 yr). In addition, we assessed the relationship of circulating levels of anabolic and catabolic hormones, as well as local expression of insulin-like growth factor (IGF)-I, to myofibrillar mRNA abundance. Heart failure patients were characterized by lower abundance of mRNA encoding the myosin heavy chain (MHC) I isoform (P < 0.01), whereas MHC IIa and MHC IIx mRNA did not differ between groups. Actin mRNA was also lower in heart failure patients compared with controls (P < 0.001). The expression of each MHC isoform transcript correlated with its respective protein product (MHC I: r = 0.656, P < 0.01; MHC IIa: r = 0.489, P < 0.05; MHC IIx: r = 0.505, P < 0.05; n = 18 for all). In addition to changes in myofibrillar transcripts, we found lower (P < 0.01) skeletal muscle IGF-1Ea mRNA content in heart failure patients. Myofibrillar mRNA levels were positively associated with skeletal muscle IGF-1Ea transcript levels (range of r values: 0.663-0.765; P values: <0.01 to <0.001) and modestly associated with circulating markers of immune activation (range of r values: -0.487 to -0.555; P values: <0.05 to <0.03). Our findings suggest that alterations in skeletal muscle MHC content and isoform distribution in heart failure may derive, in part, from changes in MHC gene expression. The relationships of myofibrillar mRNA content to both local and circulating hormones further suggest that alterations in the balance between anabolic and catabolic hormones in heart failure patients may influence skeletal muscle myofibrillar protein phenotype by altering gene expression.

Amplitude modulation of nuclear Ca2+ signals in human skeletal myotubes: A possible role for nuclear Ca2+ buffering

Video-rate confocal microscopy of Indo-1-loaded human skeletal myotubes was used to assess the relationship between the changes in sarcoplasmic ([Ca(2+)](S)) and nuclear ([Ca(2+)](N)) Ca(2+) concentration during low- and high-frequency electrostimulation. A single stimulus of 10 ms duration transiently increased [Ca(2+)] in both compartments with the same time of onset. Rate and amplitude of the [Ca(2+)] rise were significantly lower in the nucleus (4.0- and 2.5-fold, respectively). Similarly, [Ca(2+)](N) decayed more slowly than [Ca(2+)](S) (mono-exponential time constants of 6.1 and 2.5 s, respectively). After return of [Ca(2+)] to the prestimulatory level, a train of 10 stimuli was applied at a frequency of 1 Hz. The amplitude of the first [Ca(2+)](S) transient was 25% lower than that of the preceding single transient. Thereafter, [Ca(2+)](S) increased stepwise to a maximum that equalled that of the single transient. Similarly, the amplitude of the first [Ca(2+)](N) transient was 20% lower than that of the preceding single transient. In contrast to [Ca(2+)](S), [Ca(2+)](N) then increased to a maximum that was 2.3-fold higher than that of the single transient and equalled that of [Ca(2+)](S). In the nucleus, and to a lesser extent in the sarcoplasm, [Ca(2+)] decreased faster at the end of the stimulus train than after the preceding single stimulus (time constants of 3.3 and 2.1 s, respectively). To gain insight into the molecular principles underlying the shaping of the nuclear Ca(2+) signal, a 3-D mathematical model was constructed. Intriguingly, quantitative modelling required the inclusion of a satiable nuclear Ca(2+) buffer. Alterations in the concentration of this putative buffer had dramatic effects on the kinetics of the nuclear Ca(2+) signal. This finding unveils a possible mechanism by which the skeletal muscle can adapt to changes in physiological demand.

FGF6 regulates muscle differentiation through a calcineurin-dependent pathway in regenerating soleus of adult mice

Important functions in myogenesis have been proposed for FGF6, a member of the fibroblast growth factor family accumulating almost exclusively in the myogenic lineage, but its precise role in vivo remains mostly unclear. Here, using FGF6 (-/-) mice and rescue experiments by injection of recombinant FGF6, we dissected the functional role of FGF6 during in vivo myogenesis. We found that the appearance of myotubes was accelerated during regeneration of the soleus of FGF6 (-/-) mice versus wild type mice. This accelerated differentiation was correlated with increased expression of differentiation markers such as CdkIs and calcineurin, as well as structural markers such as MHCI and slow TnI. We showed that an elevated transcript level for calcineurin Aalpha subunit correlated with a positive regulation of calcineurin A activity in regenerating soleus of the FGF6 (-/-) mice. Cyclin D1 and calcineurin were up- and down-regulated, respectively in a dose-dependent manner upon injection of rhFGF6 in regenerating soleus of the mutant mice. We showed an increase of the number of slow oxidative (type I) myofibers, whereas fast oxidative (type IIa) myofibers were decreased in number in regenerating soleus of FGF6 (-/-) mice versus that of wild type mice. In adult soleus, the number of type I myofibers was also higher in FGF6 (-/-) mice than in wild type mice. Taken together these results evidenced a specific phenotype for soleus of the FGF6 (-/-) mice and led us to propose a model accounting for a specific dose-dependent effect of FGF6 in muscle regeneration. At high doses, FGF6 stimulates the proliferation of the myogenic stem cells, whereas at lower doses it regulates both muscle differentiation and muscle phenotype via a calcineurin-signaling pathway.

Time course of physical reconditioning during exercise rehabilitation late after heart transplantation

BACKGROUND

Exercise rehabilitation improves physical capacity in heart transplant recipients. The time course of physical reconditioning and skeletal muscle adaptation late after transplantation are unknown.

METHODS

Twenty-one heart transplant recipients, at 5.2 +/- 2.1 years after transplantation, completed 1 year of an individually tailored home ergometer-training program (2.1 +/- 0.7 sessions weekly with matched heart rates, intensity at 10% below anaerobic threshold). We analyzed time course of physical reconditioning data for each home-training session (n = 2,396). Constant-load tests with consistent blood lactate concentrations were performed quarterly (n = 105) to estimate the time course of skeletal muscle adaptation. Nine heart transplant recipients served as a control group (CG).

RESULTS

After 12 months, exercise capacity for matched heart rates (112 +/- 11 beats/min; CG, 114 +/- 8 beats/min) increased by 35% +/- 19% (from 43 +/- 14 to 58 +/- 18 W; p < 0.001; CG, 53 +/- 18 to 54 +/- 18 W); 24% of the increase was caused by improved skeletal muscle function and 11% by central functioning. Physical reconditioning showed its greatest increase within the first 3 months (+18%; p < 0.001); 50% of the increase consisted of better skeletal muscle or central functioning. Between the 4(th) and 12(th) months, exercise capacity increased continuously (+15%; p < 0.001), mainly because of better skeletal muscle functioning.

CONCLUSIONS

The persistent improvement in exercise capacity along with consistent lactate concentrations during 12 months of training indicates that exercise training could counteract the negative side effects of immunosuppressive treatment on skeletal muscles. Even late after heart transplantation, physical training should be performed regularly to prevent the accelerated decrease in exercise capacity and in skeletal muscle function.

Calcineurin and heat shock protein 72 in functionally overloaded rat plantaris muscle

The involvement of calcineurin (CaN) and heat shock protein (Hsp) 72 in the regulation of fiber size and/or phenotype in response to functional overload (FO) was investigated. In one FO group, the plantaris muscle was overloaded by cutting the distal tendons (5-10 mm length) of the soleus and gastrocnemius of 3-week-old male Wistar rats. Cyclosporin A (CsA), a CaN inhibitor, was injected daily (5 mg/kg body weight, i.p.) in a second group of FO rats (FO+CsA group) for a 2-week period. Compared to age-matched controls (Con), the absolute and relative plantaris weights were increased in both FO groups: the hypertrophic response was attenuated in FO+CsA rats. The mean cross-sectional area of each fiber type was increased (approximately 2.0-fold) in the plantaris of FO rats: CsA treatment attenuated this effect, although the fibers were still larger than in Con rats. The percent composition of myosin heavy chain (MHC) IIb decreased from 54% in Con to 19% in FO rats, whereas types I, IIa, and IIx MHC increased in the FO rats. CsA treatment blunted the shifts in MHC isoforms: the FO+CsA group showed a smaller decrease in type IIb and a smaller increase in type IIx MHC than the FO group. The levels of CaN-A and -B proteins were higher (approximately 2.5-fold) in FO than Con rats, whereas these values were similar in Con and FO+CsA rats. Hsp72 protein levels were higher in FO (3.6-fold) and FO+CsA (5.2-fold) than Con rats, with the values being significantly higher in the FO+CsA than FO rats. CsA treatment in Con rats had no effects on muscle mass, fiber size, MHC composition, and Hsp72 or CaN levels. Combined, these results suggest that CaN levels are related to changes in both fiber size and phenotype, and that Hsp72 levels are more related to the levels of stress added to the muscle rather than to increases in the slow fiber phenotype in functionally overloaded rat plantaris muscles.

Effects of T3 treatment on HSP72 and calcineurin content of functionally overloaded rat plantaris muscle

The content of both heat shock protein 72 (HSP72) and calcineurin (CaN) in skeletal muscle fibers have been reported to be associated with the slow phenotype. The purpose of the present study was to determine the adaptations/contributions of HSP72 and CaN to experimental conditions producing dramatic shifts in fiber phenotype. Two weeks of functional overload (FO) of the rat plantaris by cutting the tendons of its major synergists resulted in a shift towards a slower MHC profile. Two weeks of thyroid hormone (T3) administration (150 microg/kg/day, i.p.) resulted in a shift towards a faster MHC profile in control rats and an attenuation of the shift towards a slower profile in FO rats. HSP72 and CaN-A content were 63% and 47% higher, respectively, in the plantaris of FO than age-matched control rats. These increases were significantly attenuated by T3 treatment in FO rats. CaN-B levels were approximately 50% higher in FO and FO plus T3-treated than control rats. T3 treatment alone had no effect on the levels of HSP72, CaN-A or -B in control rats. Therefore, chronic overload of a muscle results in an increase in the percentage of slow fibers/MHC and enhances the levels of HSP72 and CaN. In turn, these FO-induced adaptations are attenuated by T3 treatment. Combined, these results indicate that muscle HSP72 and CaN protein levels are modulated by mechanical stress and that their levels appear to be related to changes in fiber type/MHC composition, at least in chronically overloaded muscles.

Inhibition of calcineurin increases monocarboxylate transporters 1 and 4 protein and glycolytic enzyme activities in rat soleus muscle

1. The present study was designed to examine the role of calcineurin in muscle metabolic components by the administration of the specific calcineurin inhibitor cyclosporine A (CsA) to rats. 2. Male Wistar rats were divided into either a CsA-treated group (CT) or a vehicle-treated group (VT). Cyclosporine A was administered subcutaneously to rats at a rate of 25 mg/kg bodyweight per day for 10 successive days. Thereafter, changes in muscle enzyme activities and glucose transporter (GLUT)-4 and monocarboxylate transporter (MCT)-1 and MCT-4 proteins in the slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles were examined. 3. There was a significant increase in MCT-1 and MCT-4 proteins in the soleus muscle in the CT group, but not in the EDL muscle. The activities of hexokinase, pyruvate kinase and lactate dehydrogenase in the soleus muscle also increased significantly in the CT group, but a similar increase in enzyme activity was not seen in EDL muscle. The activities of citrate synthase or malate dehydrogenase and the GLUT-4 protein content were not altered by CsA treatment in either the soleus or EDL muscles. 4. These results seem to imply that calcineurin negatively regulates the components of glucose/lactate metabolism, except for GLUT-4, especially in slow-twitch muscle.

Exercise and health: can biotechnology confer similar benefits?

Education and public policies are largely failing to encourage people to exercise. Could our knowledge of exercise biology lead to pharmaceutical treaments that could confer the same benefits as exercise?

Cyclosporin-A inhibits stretch-induced changes in myosin heavy chain expression in C2C12 skeletal muscle cells

Studies in vivo, have shown that passive stretch of skeletal muscle induces changes in contractile protein expression. In the present study the effects of passive stretch upon myosin heavy chain (MyHC) expression were examined in C2C12 cell myotubes. Passive stretch induced an upregulation of adult fast and slow MyHCs, which was prevented by cyclosporin A (CsA), an inhibitor of calcineurin. Calcineurin has been shown to act via the dephosphorylation of NFAT and MEF2 transcriptional factors. In this study no significant change in the phosphorylation state of these factors was observed. In contrast stretch induced an alteration in the levels of the myogenic regulatory factors (MRFs) MyoD, myogenin and myf5. The modulation in the level of these MRFs was also inhibited by CsA. These data indicate that changes in muscle phenotype in C2C12 can be modulated by passive stretch and some of these changes are calcineurin dependent.

The calcineurin activity and MCIP1.4 mRNA levels are increased by innervation in regenerating soleus muscle

The level of active subunit of calcineurin and the calcineurin (Cn) enzyme activity are increased in innervated but not in denervated slow type regenerating skeletal soleus muscle. These nerve-dependent increases were not accompanied by similar increases in the mRNA levels. The changes in the mRNA level of the modulatory calcineurin interacting protein, MCIP1.4, reflected the calcineurin activity and did not increase in denervated regenerating muscles compared to the innervated regenerating controls. The increases in Cn activity and in MCIP1.4 mRNA levels occurred before the switch from fast to slow-type myosin heavy chain isoforms, a phenomenon similarly known to be dependent on innervation. This highlights the role of mediators, acting between the nerve and calcineurin, in the formation of slow fiber identity.

Calcineurin activation influences muscle phenotype in a muscle-specific fashion

Background

The calcium activated protein phosphatase 2B, also known as calcineurin, has been implicated as a cell signaling molecule involved with transduction of physiological signals (free cytosolic Ca²⁺) into molecular signals that influence the expression of phenotype-specific genes in skeletal muscle. In the present study we address the role of calcineurin in mediating adaptations in myosin heavy chain (MHC) isoform expression and muscle mass using 3-month old wild-type (WT) and transgenic mice displaying high-level expression of a constitutively active form of calcineurin (MCK-CN* mice).

Results

Slow muscles, e.g., soleus, were significantly larger (by ~24%), whereas fast muscles, e.g., medial gastrocnemius (MG) and tibialis anterior were significantly smaller (by ~26 and ~16%, respectively) in MCK-CN* mice compared to WT. The masses of mixed phenotype muscles, such as the plantaris and the extensor digitorum longus, were not significantly changed from WT. The soleus, plantaris, MG and diaphragm displayed shifts toward slower MHC isoforms, e.g., soleus from WT mice contained ~52% MHC-I, ~39% MHC-IIa, and ~9% MHC-IIx, whereas MCK-CN* mice had ~67% MHC-I, ~26% MHC-IIa, and ~7% MHC-IIx. The specific isoforms that were either up or down-regulated were muscle-specific. For instance, the proportion of MHC-IIa was decreased in the soleus and diaphragm, but increased in the plantaris and MG of MCK-CN* mice. Also, the proportion of MHC-IIx was unchanged in the soleus, decreased in the diaphragm and increased in the plantaris and MG of MCK-CN* relative to WT mice. Fast to slow shifts in fiber type proportions were evident for the plantaris, but not the soleus. Fast, but not slow, plantaris fibers of MCK-CN* mice had higher oxidative and lower glycolytic properties than WT.

Conclusion

These data suggest that calcineurin activation can influence muscle phenotype and that the specific influence of calcineurin activation on the phenotypic and mass characteristics of a muscle is dependent upon the original phenotypic state of the muscle.

Gene expression in muscle in response to exercise

Muscle has an intrinsic ability to adapt to different types of work by changing fibre type and muscle mass. This process involves quantitative and qualitative changes in gene expression including those of the myosin heavy chain (MyHC) isogenes that encode different types of molecular motors. Increased expression of slow MyHC and of metabolic genes result in increased fatigue resistance. Recently, there has been some insight into how oxidative metabolism, as well as slow myosin expression, is regulated and the role of calcium in initiating switches in gene expression. In relation to muscle mass and power output it has been appreciated that local as well as systemic factors are important. Our group have cloned three types of IGF-I in human muscle which are derived from the IGF-I gene by alternative splicing. The expression of one of these that appears to be an autocrine/paracrine splice variant is only detectable after mechanical stimulation (MGF) and a systemic type (IGF-IEa) that is produced by the liver and other tissue including muscle. As the result of a reading frame shift, the MGF peptide has a different C terminal sequence to IGF-IEa. Interestingly, the MGF C terminal peptide has been found to act as a separate growth factor and to initially activate mononluceated myoblasts (satellite cells). MGF also responds to different signals and has different expression kinetics to IGF-IEa. The mechanotransduction mechanism for this signalling may directly or indirectly involve the dystrophin complex as dystrophic muscle, unlike normal muscle, is unable to express MGF in response to overload. Also the ability to express MGF has been found to decline markedly during ageing. The deficiency in expressing MGF and activating satellite cells in dystrophic and aged muscles may explain why muscle mass is not maintained in these situations. However, in normal muscle MGF appears to initiate local muscle repair with its over expression resulting in hypertrophy.

Transcriptional regulation of acetylcholinesterase-associated collagen ColQ: differential expression in fast and slow twitch muscle fibers is driven by distinct promoters

The presence of a collagenous protein (ColQ) characterizes the collagen-tailed forms of acetylcholinesterase and butyrylcholinesterase at vertebrate neuromuscular junctions which is tethered in the synaptic basal lamina. ColQ subunits, differing mostly by their signal sequences, are encoded by transcripts ColQ-1 and ColQ-1a, which are differentially expressed in slow and fast twitch muscles in mammals. Two distinct promoters, pColQ-1 and pColQ-1a, were isolated from the upstream sequences of human COLQ gene; they showed muscle-specific expression and were activated by myogenic transcriptional elements in cultured myotubes. After in vivo DNA transfection, pColQ-1 showed strong activity in slow twitch muscle (e.g. soleus), whereas pColQ-1a was preferably expressed in fast twitch muscle (e.g. tibialis). Mutation analysis of the ColQ promoters suggested that the muscle fiber type-specific expression pattern of ColQ transcripts were regulated by a slow upsteam regulatory element (SURE) and a fast intronic regulatory element (FIRE). These regulatory elements were responsive to a calcium ionophore and to calcineurin inhibition by cyclosporine A. The slow fiber type-specific expression of ColQ-1 was abolished by the mutation of an NFAT element in pColQ-1. Moreover, both the ColQ promoters contained N-box element that was responsible for the synapse-specific expression of ColQ transcripts. These results explain the specific expression patterns of collagen-tailed acetylcholinesterase in slow and fast muscle fibers.

Genetic loss of calcineurin blocks mechanical overload-induced skeletal muscle fiber type switching but not hypertrophy

The serine/threonine phosphatase calcineurin is an important regulator of calcium-activated intracellular responses in eukaryotic cells. In higher eukaryotes, calcium/calmodulin-mediated activation of calcineurin facilitates direct dephosphorylation and nuclear translocation of the transcription factor nuclear factor of activated T-cells (NFAT). Recently, controversy has surrounded the role of calcineurin in mediating skeletal muscle cell hypertrophy. Here we examined the ability of calcineurin-deficient mice to undergo skeletal muscle hypertrophic growth following mechanical overload (MOV) stimulation or insulin-like growth factor-1 (IGF-1) stimulation. Two distinct models of calcineurin deficiency were employed: calcineurin Abeta gene-targeted mice, which show a approximately 50% reduction in total calcineurin, and calcineurin B1-LoxP-targeted mice crossed with a myosin light chain 1f cre knock-in allele, which show a greater than 80% loss of total calcineurin only in skeletal muscle. Calcineurin Abeta-/- and calcineurin B1-LoxP(fl/fl)-MLC-cre mice show essentially no defects in muscle growth in response to IGF-1 treatment or MOV stimulation, although calcineurin Abeta-/- mice show a basal defect in total fiber number in the plantaris and a mild secondary reduction in growth, consistent with a developmental defect in myogenesis. Both groups of gene-targeted mice show normal increases in Akt activation following MOV or IGF-1 stimulation. However, overload-mediated fiber-type switching was dramatically impaired in calcineurin B1-LoxP(fl/fl)-MLC-cre mice. NFAT-luciferase reporter transgenic mice failed to show a correlation between IGF-1- or MOV-induced hypertrophy and calcineurin-NFAT-dependent signaling in vivo. We conclude that calcineurin expression is important during myogenesis and fiber-type switching, but not for muscle growth in response to hypertrophic stimuli.

Role of calcineurin in striated muscle: development, adaptation, and disease

Striated muscles, cardiac and skeletal muscles, use calcium as a second messenger to respond and adapt to environmental stimuli. Elevations in intracellular calcium activate calcineurin, a serine/threonine phosphatase, resulting in expression of a set of genes involved in remodeling striated muscle. Activation of calcineurin in hearts produces cardiac hypertrophy, and in skeletal muscle promotes cell differentiation and transforms fiber type specificity. In this review we discuss the effects of calcineurin activity on development, adaptation, and disease of striated muscle.

Decrease in resting calcium and calcium entry associated with slow-to-fast transition in unloaded rat soleus muscle

Using fura-2 and the manganese quenching technique, we show here that sarcolemmal permeability to cations (SP-Ca) of slow-twitch muscles is greater than that of fast-twitch ones. This appears to be related to a higher expression and/or activity of stretch-activated channels, whereas leak channel activities are similar. During hindlimb suspension (HU), we found highly correlated decreases in SPCa and resting calcium of soleus muscle toward values of extensor digitorum longus (EDL) muscle. This was significant as soon as 3 days of suspension, contrary to soleus muscle caffeine sensitivity and responsiveness that were not modified after this HU period. After 14 days of HU, SP-Ca, resting calcium, and caffeine response of soleus muscle became similar to that normally observed in EDL muscle. These results demonstrate that the correlated decreases in SP-Ca and resting calcium precede most functional changes due to HU. Given the known shortening of HU soleus muscle, we proposed that this could induce a decrease of SP-Ca and a consequent reduction of resting calcium. According to the crucial role of resting cytosolic free calcium in the maintenance and the adaptation of muscle phenotype, our results suggest that slow-to-fast transition of HU soleus muscle is calcium dependent.