Transient regulation of c-fos, alpha B-crystallin, and hsp70 in muscle during recovery from contractile activity

Alpha B-Crystallin in Muscle Disease Prevention: The Role of Physical Activity

HSPB5 or alpha B-crystallin (CRYAB), originally identified as lens protein, is one of the most widespread and represented of the human small heat shock proteins (sHSPs). It is greatly expressed in tissue with high rates of oxidative metabolism, such as skeletal and cardiac muscles, where HSPB5 dysfunction is associated with a plethora of human diseases. Since HSPB5 has a major role in protecting muscle tissues from the alterations of protein stability (i.e., microfilaments, microtubules, and intermediate filament components), it is not surprising that this sHSP is specifically modulated by exercise. Considering the robust content and the protective function of HSPB5 in striated muscle tissues, as well as its specific response to muscle contraction, it is then realistic to predict a specific role for exercise-induced modulation of HSPB5 in the prevention of muscle diseases caused by protein misfolding. After offering an overview of the current knowledge on HSPB5 structure and function in muscle, this review aims to introduce the reader to the capacity that different exercise modalities have to induce and/or activate HSPB5 to levels sufficient to confer protection, with the potential to prevent or delay skeletal and cardiac muscle disorders.

A novel homozygous initiation codon variant associated with infantile alpha-Bcrystallinopathy in a Chinese family

BACKGROUND

Due to inconsistencies with reported myofibrillar myopathy (MFM), including autosomal dominant inheritance, late onset and a slowly progressive course, the severe, recessively inherited form of CRYAB (alpha-B crystallin) gene-related infantile MFM has been suggested. Here, we report an infant in a Chinese family with fatal neonatal-onset hypertonic MFM with a novel CRYAB homozygous variant (c.3G > A (p.Met1?)).

METHODS

Muscle biopsy indicated that muscle fibers showed a uniformly small diameter, cell atrophy, and visible focal muscle fiber degeneration and necrosis consistent with myogenic myopathy. We performed the whole exome sequencing of pathogenic genes and identified it as MFM.

RESULTS

The proband presented with profound muscle stiffness, progressive respiratory distress and a concurrent abnormal increase in myocardial enzymogram, and the patient died in the 17th month of life. Muscle biopsy and electron microscopy results were consistent with ultramicroscopic myogenic damage and pathological changes. Mutation analysis of the proband identified a novel rare homozygous mutation in the initiation codon of the CRYAB gene, which was inherited from currently asymptomatic, heterozygous carrier parents, and his heterozygous biological brother is unaffected.

CONCLUSIONS

This article reports one infant with CRYAB-related neonatal onset MFM with a novel homozygous variant in CRYAB. To our knowledge, this is the first reported case of infantile alpha-Bcrystallinopathy in the Chinese population.

The early response of αB-crystallin to a single bout of aerobic exercise in mouse skeletal muscles depends upon fiber oxidative features

Besides its substantial role in eye lens, αB-crystallin (HSPB5) retains fundamental function in striated muscle during physiological or pathological modifications. In this study, we aimed to analyse the cellular and molecular factors driving the functional response of HSPB5 protein in different muscles from mice subjected to an acute bout of non-damaging endurance exercise or in C2C12 myocytes upon exposure to pro-oxidant environment, chosen as “in vivo” and “in vitro” models of a physiological stressing conditions, respectively.

To this end, red (GR) and white gastrocnemius (GW), as sources of slow-oxidative and fast-glycolytic/oxidative fibers, as well as the soleus (SOL), mainly composed of slow-oxidative type fibers, were obtained from BALB/c mice, before (CTRL) and at different times (0′, 15′, 30′ 120′) following 1-h of running. Although the total level of HSPB5 protein was not affected by exercise, we found a significantly increase of phosphorylated HSPB5 (p-HSPB5) only in GR and SOL skeletal muscle with a higher amount of type I and IIA/X myofibers. The fiber-specific activation of HSPB5 was correlated to its interaction with the actin filaments, as well as to an increased level of lipid peroxidation and carbonylated proteins. The role of the pro-oxidant environment in HSPB5 response was investigated in terminally differentiated C2C12 myotubes, where most of HSPB5/pHSPB5 pool was present in the cytosolic compartment in standard culture conditions. As a result of exposure to pro-oxidizing, but not cytotoxic, H₂O₂ concentration, the p-38MAPK-mediated phosphorylation of HSPB5 resulted functional to promote its interaction with the myofibrillar components, such as β-actin, desmin and filamin 1.

This study provides novel information on the molecular pathway underlying the HSPB5 physiological function in skeletal muscle, confirming the contribution of the pro-oxidant environment in HSPB5 activation and interaction with substrate/client myofibrillar proteins, offering new insights for the study of myofibrillar myopathies and cardiomyopathies.

Activation of AMP-Activated Protein Kinase and Stimulation of Energy Metabolism by Acetic Acid in L6 Myotube Cells

Previously, we found that orally administered acetic acid decreased lipogenesis in the liver and suppressed lipid accumulation in adipose tissue of Otsuka Long-Evans Tokushima Fatty rats, which exhibit hyperglycemic obesity with hyperinsulinemia and insulin resistance. Administered acetic acid led to increased phosphorylation of AMP-activated protein kinase (AMPK) in both liver and skeletal muscle cells, and increased transcripts of myoglobin and glucose transporter 4 (GLUT4) genes in skeletal muscle of the rats. It was suggested that acetic acid improved the lipid metabolism in skeletal muscles. In this study, we examined the activation of AMPK and the stimulation of GLUT4 and myoglobin expression by acetic acid in skeletal muscle cells to clarify the physiological function of acetic acid in skeletal muscle cells. Acetic acid added to culture medium was taken up rapidly by L6 cells, and AMPK was phosphorylated upon treatment with acetic acid. We observed increased gene and protein expression of GLUT4 and myoglobin. Uptake of glucose and fatty acids by L6 cells were increased, while triglyceride accumulation was lower in treated cells compared to untreated cells. Furthermore, treated cells also showed increased gene and protein expression of myocyte enhancer factor 2A (MEF2A), which is a well-known transcription factor involved in the expression of myoglobin and GLUT4 genes. These results indicate that acetic acid enhances glucose uptake and fatty acid metabolism through the activation of AMPK, and increases expression of GLUT4 and myoglobin.

Alpha B-crystallin induction in skeletal muscle cells under redox imbalance is mediated by a JNK-dependent regulatory mechanism

The small heat shock protein α-B-crystallin (CRYAB) is critically involved in stress-related cellular processes such as differentiation, apoptosis, and redox homeostasis. The up-regulation of CRYAB plays a key role in the cytoprotective and antioxidant response, but the molecular pathway driving its expression in muscle cells during oxidative stress still remains unknown. Here we show that noncytotoxic exposure to sodium meta-arsenite (NaAsO2) inducing redox imbalance is able to increase the CRYAB content of C2C12 myoblasts in a transcription-dependent manner. Our in silico analysis revealed a genomic region upstream of the Cryab promoter containing two putative antioxidant-responsive elements motifs and one AP-1-like binding site. The redox-sensitive transcription factors Nrf2 and the AP-1 component c-Jun were found to be up-regulated in NaAsO2-treated cells, and we demonstrated a specific NaAsO2-mediated increase of c-Jun and Nrf2 binding activity to the genomic region identified, supporting their putative involvement in CRYAB regulation following a shift in redox balance. These changes also correlated with a specific phosphorylation of JNK and p38 MAPK kinases, the well-known molecular mediators of signaling pathways leading to the activation of these transcription factors. Pretreatment of C2C12 cells with the JNK inhibitor SP600125 induced a decrease in c-Jun and Nrf2 content and was able to counteract the NaAsO2-mediated increase in CRYAB expression. Thus these data show a direct role of JNK in CRYAB regulation under redox imbalance and also point to a previously unrecognized link between c-Jun and Nrf2 transcription factors and redox-induced CRYAB expression in muscle cells.

2-D DIGE analysis of the mitochondrial proteome from human skeletal muscle reveals time course-dependent remodelling in response to 14 consecutive days of endurance exercise training

Adaptation of skeletal muscle to repeated bouts of endurance exercise increases aerobic capacity and improves mitochondrial function. However, the adaptation of human skeletal muscle mitochondrial proteome to short-term endurance exercise training has not been investigated. Eight sedentary males cycled for 60 min at 80% of peak oxygen consumption (VO(2peak) ) each day for 14 consecutive days, resulting in an increase in VO(2peak) of 17.5±3.8% (p<0.01). Mitochondria-enriched protein fractions from skeletal muscle biopsies taken from m. vastus lateralis at baseline, and on the morning following the 7th and 14th training sessions were subjected to 2-D DIGE analysis with subsequent MS followed by database interrogation to identify the proteins of interest. Thirty-one protein spots were differentially expressed after either 7 or 14 days of training (ANOVA, p<0.05). These proteins included subunits of the electron transport chain, enzymes of the tricarboxylic acid cycle, phosphotransfer enzymes, and regulatory factors in mitochondrial protein synthesis, oxygen transport, and antioxidant capacity. Several proteins demonstrated a time course-dependent induction during training. Our results illustrate the phenomenon of skeletal muscle plasticity with the extensive remodelling of the mitochondrial proteome occurring after just 7 days of exercise training suggestive of enhanced capacity for adenosine triphosphate generation at a cellular level.

Myofibrillar myopathies

Myofibrillar myopathies represent a group of muscular dystrophies with a similar morphologic phenotype. They are characterized by a distinct pathologic pattern of myofibrillar dissolution associated with disintegration of the Z-disk, accumulation of myofibrillar degradation products, and ectopic expression of multiple proteins and sometimes congophilic material. The clinical features of myofibrillar myopathies are more variable. These include progressive muscle weakness, that often involves or begins in distal muscles but limb-girdle or scapuloperoneal distributions can also occur. Cardiomyopathy and peripheral neuropathy are frequent associated features. EMG of the affected muscles reveals myopathic motor unit potentials and abnormal irritability often with myotonic discharges. Rarely, neurogenic motor unit potentials or slow nerve conductions are present. The generic diagnosis of myofibrillar myopathies is based on muscle biopsy findings in frozen sections. To date, all myofibrillar myopathy mutations have been traced to Z-disk-associated proteins, namely, desmin, αB-crystallin, myotilin, ZASP, filamin C and Bag3. However, in the majority of the myofibrillar myopathy patients the disease gene awaits discovery.

Myofibrillar myopathies

Myofibrillar myopathies (MFMs) represent a group of muscular dystrophies with a similar morphological phenotype. The diagnosis is established by muscle biopsy. The MFMs are characterized by a distinct pathological pattern of myofibrillar dissolution associated with disintegration of the Z-disk, accumulation of myofibrillar degradation products, and ectopic expression of multiple proteins that include desmin, αB-crystallin, dystrophin, and sometimes congophilic material. The clinical features of MFMs are more variable. These include progressive muscle weakness that often involves or begins in distal muscles, but limb-girdle or scapuloperoneal distributions can also occur. Cardiomyopathy and peripheral neuropathy are frequent associated features. Electromyography of the affected muscles reveals myopathic motor unit potentials and abnormal irritability, often with myotonic discharges. Rarely, neurogenic motor unit potentials or slowing of nerve conduction velocities are present. To date, all MFM mutations have appeared in Z-disk-associated proteins: namely, desmin, αB-crystallin, myotilin, ZASP, filamin C, and Bag3. However, in the majority of patients with MFM, the disease gene awaits discovery.

Specific attenuation of protein kinase phosphorylation in muscle with a high mitochondrial content

Acute contractile activity increases the activation of protein kinases involved in signal transduction. We hypothesized that the contractile activity-induced kinase phosphorylation would occur to a lesser degree in muscle with elevated mitochondrial content. We compared red and white sections of tibialis anterior (TA) muscle with two- to threefold differences in mitochondrial volume, and we increased the mitochondrial content in the TA muscle by 40% with unilateral chronic stimulation-induced contractile activity (10 Hz, 7 days, 3 h/day). Both the chronically stimulated and the contralateral control muscles were then acutely stimulated in situ for 15 min (10 Hz). We investigated 1) the total protein content and 2) the phosphorylation of kinases important for mitochondrial biogenesis in skeletal muscle, including AMPKalpha and p44, p42, and p38 MAPKs, as well as Akt by immunoblotting. In response to chronic stimulation, a selective upregulation of kinase protein content was observed, suggesting unique transcriptional/translational processing for these enzymes. Inverse relationships were observed between mitochondrial volume and 1) kinase protein content and 2) basal levels of kinase phosphorylation. In general, the kinase phosphorylation response to acute exercise depended, in part, on the oxidative capacity of the fiber type, evidenced by a greater absolute level of acute contractile activity-induced kinase signaling in muscle with a lower mitochondrial volume. The attenuation of contraction-evoked kinase phosphorylation in muscle with high mitochondrial content suggests that these proteins may become less sensitive to upstream signaling and require greater stimulation for activation to propagate these adaptive cues downstream toward transcription or translation events.

Effects of acetate on lipid metabolism in muscles and adipose tissues of type 2 diabetic Otsuka Long-Evans Tokushima Fatty (OLETF) rats

We have reported that orally administrated acetate contributed to suppression of lipogenesis in the liver and to reduction of lipid accumulation in the adipose tissue of Otsuka Long-Evans Tokushima Fatty (OLETF) rats. The aim of this study was to investigate the effect of acetate on skeletal muscle and adipose tissues. Treatment with acetate showed a higher rate of oxygen consumption and a smaller size of lipid droplets in white adipose and brown adipose tissues. An analysis by Northern blotting revealed that the transcripts of myoglobin and Glut4 genes in the abdominal muscle of the OLETF rats were increased by acetate treatment, while the transcripts of lipolytic genes increased in the white adipose and brown adipose tissues. It is possible that acetate has effects on lipid metabolism in the skeletal muscles and the adipose tissues, and has functions that work against obesity and obesity-linked type 2 diabetes.

Kinase-specific responsiveness to incremental contractile activity in skeletal muscle with low and high mitochondrial content

Muscle contractions activate protein kinases, leading to signal transduction. We hypothesized that kinase activation would be influenced by mitochondrial content, as well as by contractile activity-induced increases in muscle O(2) consumption (Vo(2)). Kinase phosphorylation in high-oxidative red and low-oxidative white tibialis anterior (TA) muscle (RTA and WTA, respectively) with 2.5-fold differences in mitochondrial content were compared. Stimulation of the TA muscle elicited large increases in Vo(2) (3- to 6-fold and 4- to 60-fold above resting levels in WTA and RTA, respectively). At rest, AMP-activated protein kinase (AMPK), p38, p42, and p44 activation were nearly twofold greater in WTA than in RTA, suggesting an inverse relationship between mitochondrial content and kinase activation in resting muscle. During contractions, similar degrees of phosphorylation in RTA and WTA were evident as a function of Vo(2) for p38 and p42. During increases in Vo(2) up to sixfold above rest, greater responses were observed in RTA than in WTA for AMPK and p44, whereas Akt activation was greater in WTA. In RTA, elevations in Vo(2) elicited increases in AMPK and p44 activation, whereas Akt, p38, and p42 were less sensitive to increments in Vo(2). Reactive oxygen species (ROS) production was greater in mitochondria from white muscle, but when it was calculated in the context of the whole muscle, ROS production was twofold greater in red than in white myofibers. Thus mitochondrial content influences ROS production and is inversely related to kinase activation in resting muscle. During contractions, kinases are differentially sensitive to contraction-induced increments in Vo(2), suggesting that muscle mitochondrial content is important, but it is not the sole determinant of kinase activation during exercise of different intensities.

Opposite pathobiochemical fate of pyruvate kinase and adenylate kinase in aged rat skeletal muscle as revealed by proteomic DIGE analysis

Sarcopenia is the drastic loss of skeletal muscle mass and strength during ageing. In order to better understand the molecular pathogenesis of age-related muscle wasting, we have performed a DIGE analysis of young adult versus old rat skeletal muscle. Proteomic profiling revealed that out of 2493 separated 2-D spots, 69 proteins exhibited a drastically changed expression. Age-dependent alterations in protein abundance indicated dramatic changes in metabolism, contractile activity, myofibrillar remodelling and stress response. In contrast to decreased levels of pyruvate kinase (PK), enolase and phosphofructokinase, the mitochondrial ATP synthase, succinate dehydrogenase, malate dehydrogenase, isocitrate dehydrogenase and adenylate kinase (AK) were increased in senescent fibres. Higher expression levels of myoglobin and fatty acid binding-protein indicated a shift to more aerobic-oxidative metabolism in a slower-twitching aged fibre population. The drastic increase in alphaB-crystallin and myotilin demonstrated substantial filament remodelling during ageing. An immunoblotting survey of selected muscle proteins confirmed the pathobiochemical transition process in aged muscle metabolism. The proteomic analysis of aged muscle has identified a large cohort of new biomarkers of sarcopenia including opposite changes in PK and AK, which might be useful for the design of improved diagnostic procedures and/or therapeutic strategies to counteract ageing-induced muscle degeneration.

Aging skeletal muscle shows a drastic increase in the small heat shock proteins αB-crystallin/HspB5 and cvHsp/HspB7

Most heat shock proteins operate as molecular chaperones and play a central role in the maintenance of normal cellular function. In skeletal muscle, members of the alpha-crystallin domain-containing family of small heat shock proteins are believed to form a cohort of essential stress proteins. Since alphaB-crystallin (alphaBC/HspB5) and the cardiovascular heat shock protein (cvHsp/HspB7) are both implicated in the molecular response to fibre transformation and muscle wasting, it was of interest to investigate the fate of these stress proteins in young adult versus aged muscle. The age-related loss of skeletal muscle mass and strength, now generally referred to as sarcopenia, is one of the most striking features of the senescent organism. In order to better understand the molecular pathogenesis of age-related muscle wasting, we have performed a two-dimensional gel electrophoretic analysis, immunoblotting and confocal microscopy study of aged rat gastrocnemius muscle. Fluorescent labelling of the electrophoretically separated soluble muscle proteome revealed an overall relatively comparable protein expression pattern of young adult versus aged fibres, but clearly an up-regulation of alphaBC and cvHsp. This was confirmed by immunofluorescence microscopy and immunoblot analysis, which showed a dramatic age-induced increase in these small heat shock proteins. Immunodecoration of other major stress proteins showed that they were not affected or less drastically changed in their expression in aged muscle. These findings indicate that the increase in muscle-specific small heat shock proteins constitutes an essential cellular response to fibre aging and might therefore be a novel therapeutic option to treat sarcopenia of old age.

Maximal eccentric exercise induces a rapid accumulation of small heat shock proteins on myofibrils and a delayed HSP70 response in humans

In this study the stress protein response to unaccustomed maximal eccentric exercise in humans was investigated. Eleven healthy males performed 300 maximal eccentric actions with the quadriceps muscle. Biopsies from vastus lateralis were collected at 30 min and 4, 8, 24, 96, and 168 h after exercise. Cellular regulation and localization of heat shock protein (HSP) 27, alpha B-crystallin, and HSP70 were analyzed by immunohistochemistry, ELISA technique, and Western blotting. Additionally, mRNA levels of HSP27, alpha B-crystallin, and HSP70 were quantified by Northern blotting. After exercise (30 min), 81 +/- 8% of the myofibers showed strong HSP27 staining (P < 0.01) that gradually decreased during the following week. alpha B-Crystallin mimicked the changes observed in HSP27. After exercise (30 min), the ELISA analysis showed a 49 +/- 13% reduction of the HSP27 level in the cytosolic fraction (P < 0.01), whereas Western blotting revealed a 15-fold increase of the HSP27 level in the myofibrillar fraction (P < 0.01). The cytosolic HSP70 level increased to 203 +/- 37% of the control level 24 h after exercise (P < 0.05). After 4 days, myofibrillar-bound HSP70 had increased approximately 10-fold (P < 0.01) and was accompanied by strong staining on cross sections. mRNA levels of HSP27, alpha B-crystallin, and HSP70 were all elevated the first day after exercise (P < 0.01); HSP70 mRNA showed the largest increase (20-fold at 8 h). HSP27 and alpha B-crystallin seemed to respond immediately to maximal eccentric exercise by binding to cytoskeletal/myofibrillar proteins, probably to function as stabilizers of disrupted myofibrillar structures. Later, mRNA and total HSP protein levels, especially HSP70, increased, indicating that HSPs play a role in skeletal muscle recovery and remodeling/adaptation processes to high-force exercise.

Proteolytic mRNA expression in response to acute resistance exercise in human single skeletal muscle fibers

The purpose of this study was to characterize changes in mRNA expression of select proteolytic markers in human slow-twitch [myosin heavy chain (MHC) I] and fast-twitch (MHC IIa) single skeletal muscle fibers following a bout of resistance exercise (RE). Muscle biopsies were obtained from the vastus lateralis of eight young healthy sedentary men [23 +/- 2 yr (mean +/- SD), 93 +/- 17 kg, 183 +/- 6 cm] before and 4 and 24 h after 3 x 10 repetitions of bilateral knee extensions at 65% of one repetition maximum. The mRNA levels of TNF-alpha, calpains 1 and 2, muscle RING (really interesting novel gene) finger-1 (MuRF-1), atrogin-1, caspase-3, B-cell leukemia/lymphoma (Bcl)-2, and Bcl-2-associated X protein (Bax) were quantified using real-time RT-PCR. Generally, MHC I fibers had higher (1.6- to 5.0-fold, P < 0.05) mRNA expression pre- and post-RE. One exception was a higher (1.6- to 3.9-fold, P < 0.05) Bax-to-Bcl-2 mRNA ratio in MHC IIa fibers pre- and post-RE. RE increased (1.4- to 4.8-fold, P < 0.05) MuRF-1 and caspase-3 mRNA levels 4-24 h post-RE in both fiber types, whereas Bax-to-Bcl-2 mRNA ratio increased 2.2-fold (P < 0.05) at 4 h post-RE only in MHC I fibers. These results suggest that MHC I fibers have a greater proteolytic mRNA expression pre- and post-RE compared with MHC IIa fibers. The greatest mRNA induction following RE was in MuRF-1 and caspase-3 in both fiber types. This altered and specific proteolytic mRNA expression among slow- and fast-twitch muscle fibers indicates that the ubiquitin/proteasomal and caspase pathways may play an important role in muscle remodeling with RE.

Differential expression of the fast skeletal muscle proteome following chronic low-frequency stimulation

Physiological and biochemical responses of skeletal muscle fibres to enhanced neuromuscular activity under conditions of maximum activation can be studied experimentally by chronic low-frequency stimulation of fast muscles. Stimulation-induced changes in the expression pattern of the rabbit fast skeletal muscle proteome were evaluated by two-dimensional gel electrophoresis and compared to the altered isoform expression profile of established transformation markers such as the Ca2+-ATPase, calsequestrin and the myosin heavy chain. Sixteen muscle proteins exhibited a marked change in their expression level. This included albumin with a 4-fold increase in abundance. In contrast, glycolytic enzymes, such as enolase and aldolase, showed a decreased expression. Concomitant changes were observed with marker elements of the contractile apparatus. While the fast isoforms of troponin T and myosin light chain 2 were drastically down-regulated, their slow counterparts exhibited increased expression. Interestingly, mitochondrial creatine kinase expression increased while the cytosolic isoform of this key muscle enzyme decreased. The expression of the small heat shock protein HSP-B5/alphaB-crystallin and the oxygen carrier protein myoglobin were both increased 2-fold following stimulation. The observed changes indicate that the conversion into fatigue-resistant red fibres depends on: (i) the optimum utilization of free fatty acids via albumin transportation, (ii) a rearrangement of the creatine kinase isozyme pattern for enhanced mitochondrial activity, (iii) an increased availability of oxygen for aerobic metabolism via myoglobin transport, (iv) the conversion of the contractile apparatus to isoforms with slower twitch characteristics and (v) the up-regulation of chaperone-like proteins for stabilising myofibrillar components during the fast-to-slow transition process.

Skeletal muscle HSP72 and norepinephrine response to static magnetic field in rat

The present work was undertaken in order to investigate the noradrenergic system and skeletal muscle heat shock protein 72 (HSP72) response to static magnetic field (MF) in male rats. At thermoneutrality (25 degrees C), the exposition of rats 1 hour/day for 5 consecutive days to MF of 128 mT (m tesla) induced an increase in norepinephrine content in gastrocnemius muscle (+25%, p < 0.05) but had no effect at 67 mT (+1%, p > 0.05), indicating a stimulatory effect of sub-acute MF exposure on the noradrenergic system activity. Moreover, exposed rats to MF displayed a non-significant increase of HSP72 levels in gastrocnemius muscles (+29%, p > 0.05). The results indicate that noradrenergic systems in rat's gastrocnemius muscles are affected by MF exposure. Interestingly, sub-acute exposure insufficiency increased HSP72 levels in gastrocnemius muscles.

Myoglobin: an essential hemoprotein in striated muscle

Myoglobin is a cytoplasmic hemoprotein, expressed solely in cardiac myocytes and oxidative skeletal muscle fibers, that reversibly binds O2 by its heme residue, a porphyrin ring:iron ion complex. Since the initial discovery of its structure over 40 years ago, wide-ranging work by many investigators has added importantly to our understanding of its function and regulation. Functionally, myoglobin is well accepted as an O2-storage protein in muscle, capable of releasing O2 during periods of hypoxia or anoxia. Myoglobin is also thought to buffer intracellular O2 concentration when muscle activity increases and to facilitate intracellular O2 diffusion by providing a parallel path that augments simple diffusion of dissolved O2. The use of gene targeting and other molecular biological techniques has revealed important new insights into the developmental and environmental regulation of myoglobin and provided additional functions for this hemoprotein such as scavenging nitric oxide and reactive O2 species. These recent findings, coupled with additional emerging technologies and the discovery of other tissue globins, provide a framework for addressing new questions about myoglobin and readdressing old ones.

Heat shock proteins and neuromuscular disease

The heat shock proteins are families of proteins with known activities that include chaperoning nascent peptides within the cell and cytoprotection. Most work on the nervous system has related to the role of heat shock proteins in neuroprotection from either hypoxic-ischemic or traumatic injury. The role of these proteins during normal physiological activity and injury is still under investigation. Heat shock proteins in neuromuscular disease have been investigated to some extent but were largely neglected until recently. The goal of this review is to summarize the evidence linking heat shock proteins with neuromuscular disease and to provide some insight into the roles or functions of these proteins in disease states.

Regulation of Egr-1, SRF, and Sp1 mRNA expression in contracting skeletal muscle cells

The early cellular signals associated with contractile activity initiate the activation and induction of transcription factors that regulate changes in skeletal muscle phenotype. The transcription factors Egr-1, Sp1, and serum response factor (SRF) are potentially important mediators of mitochondrial biogenesis based on the prevalence of binding sites for them in the promoter regions of genes encoding mitochondrial proteins, including PGC-1α, the important regulator of mitochondrial biogenesis. Thus, to further define a role for transcription factors at the onset of contractile activity, we examined the time-dependent alterations in Egr-1, Sp1, and SRF mRNA and the levels in electrically stimulated mouse C2C12skeletal muscle cells. Early transient increases in Egr-1 mRNA levels within 30 min ( P < 0.05) of contractile activity led to threefold increases ( P < 0.05) in Egr-1 protein by 60 min. The increase in Egr-1 mRNA was not because of increased stability, as Egr-1 mRNA half-life after 30 min of stimulation showed only a 58% decline. Stimulation of muscle cells had no effect on Sp1 mRNA but led to progressive increases ( P < 0.05) in SRF mRNA by 30 and 60 min. This was not matched by increases in SRF protein but occurred coincident with increases ( P < 0.05) in SRF-serum response element DNA binding at 30 and 60 min as a result of SRF phosphorylation on serine-103. To assess the importance of the recovery period, 12 h of continuous contractile activity was compared with four successive 3-h bouts, with an intervening 21-h recovery period after each bout. Continuous contractile activity led to a twofold increase ( P < 0.05) in Egr-1 mRNA, no change in SRF mRNA, and a 43% decrease in Sp1 mRNA expression. The recovery period prevented the decline in Sp1 mRNA, produced a decrease in Egr-1 mRNA, and had no effect on SRF mRNA. Thus continuous and intermittent contractile activity evoked different specific transcription factor expression patterns, which may ultimately contribute to divergent qualitative, or temporal patterns of, phenotypic adaptation in muscle.

Are exercise‐induced genes induced by exercise?

Numerous human in vivo studies on skeletal muscle gene expression have investigated the effects of given interventions. These have been founded on the assumption that presampling can be regarded as a representative control for postintervention sampling. However, many genes are responsive to the metabolic status, which varies during the day, so that observed differences in gene expression between the pre- and post-sample may therefore be a result of the daily variations rather than an intervention. Furthermore, the sampling itself can cause a local stress response, which may also influence the expression of some genes in later samples from the same localized area. To test this, we performed a short-term human endurance exercise study in which muscle biopsies were obtained from healthy untrained individuals (n=14) before and in the hours after exercise to measure the expression of mRNA for previously reported exercise-related genes (e.g., PPARgamma coactivator-1alpha (PGC-1alpha), pyruvate dehydrogenase kinase 4 (PDK4), MyoD, p21, (heat shock protein 72 (HSP72), lipoprotein lipase (LPL), citrate synthase (CS), and glucose transporter 4 (GLUT4)). To test for changes unrelated to exercise, one half of the subjects did not exercise. As suspected, several presumed exercise-induced genes were induced even without the exercise. Our data demonstrate that presampling is not always a representative control for postintervention sampling, illustrating that use of presampling can cause erroneous interpretations of the underlying induction signals.

Effects of aerobic training on pyruvate dehydrogenase and pyruvate dehydrogenase kinase in human skeletal muscle

This study examined the effects of short- and long-term aerobic training on the stable up-regulation of pyruvate dehydrogenase (PDH) and PDH kinase (PDK) in human skeletal muscle. We hypothesized that 8 weeks, but not 1 week, of aerobic training would increase total PDH (PDHt) and PDK activities compared to pretraining, and this would be detectable at the level of gene transcription (mRNA) and/or gene translation (protein). Resting muscle biopsies were taken before and after 1 and 8 weeks of aerobic cycle exercise training. PDHt and PDK activities, and their respective protein and mRNA expression, did not differ after 1 week of aerobic training. PDHt activity increased 31% after 8 weeks and this may be partially due to a 1.3-fold increase in PDH-E(1)alpha protein expression. PDK activity approximately doubled after 8 weeks of aerobic training and this was attributed to a 1.3-fold increase in PDK2 isoform protein expression. Similar to 1 week, no changes were observed at the mRNA level after 8 weeks of training. These findings suggest that aerobically trained human skeletal muscle has an increased maximal capacity to utilize carbohydrates, evident by increased PDHt, but increased metabolic control sensitivity to pyruvate through increased contribution of PDK2 to total PDK activity.

Exercise-induced gene expression in soleus muscle is dependent on time after spinal cord injury in rats

Cycling exercise attenuates atrophy in hindlimb muscles and causes changes in spinal cord properties after spinal cord injury in rats. We hypothesized that exercising soleus muscle expresses genes that are potentially beneficial to the injured spinal cord. Rats underwent spinal cord injury at T10 and were exercised on a motor-driven bicycle. Soleus muscle and lumbar spinal cord tissue were used for messenger RNA (mRNA) analysis. Gene expression of brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) was elevated 11- and 14-fold, respectively, in soleus muscle after one bout of exercise performed 5 days after spinal cord transection. Also, c-fos and heat shock protein-27 (HSP27) mRNA abundance were increased 11- and 7-fold, respectively. When exercise was started 2 days after the injury, the changes in gene expression were not observed. By contrast, at 2 but not at 5 days after transection, expression of the HSP27 gene was elevated sixfold in the lumbar spinal cord, independent of exercise. Electromyographic activity in soleus muscles was also decreased at 2 days, indicating that the spinal cord was less permissive to exercise at this early time. Long-term exercise for 4 weeks attenuated muscle atrophy equally well in rats started at 2 days or 5 days after injury. We conclude that BDNF and GDNF released from exercising muscle may be involved in exercise-induced plasticity of the spinal cord. Furthermore, the data suggest that the lumbar spinal cord undergoes time-dependent changes that temporarily impede the ability of the muscle to respond to exercise.

Different skeletal muscle HSP70 responses to high-intensity strength training and low-intensity endurance training

Heat shock protein, e.g. HSP70, can be induced in human skeletal muscle undergoing exercise training, and plays important role in adaptation to stress. This study was designed to investigate the effects of high-intensity strength training and low-intensity endurance training on the HSP70 response to exercise, bearing in mind whether HSP70 is induced in the well-trained muscle during low-intensity endurance training. Six well-trained rowers (male, aged 18 years) underwent a training program which consisted of 3 weeks high-intensity training (HIT) and 3 weeks low-intensity endurance training (ET), followed by 1 week of recovery each (R1 and R2, respectively). HSP70 (2.5 microg total protein loaded) was determined by Western blot with reference to a series of known amount of standard HSP70. HSP70 mRNA was analyzed by RT-PCR, and the relative percentage change was referred to the baseline level (before training). HSP70 increased significantly at the end of HIT (from 51 to 73 ng), decreased at the end of R1(66 ng), and remained unchanged throughout ET and R2. HSP70 mRNA increased significantly after HIT (257%) and decreased gradually afterwards (194%, 166%, and 119% for R1, ET, and R2, respectively). It can be concluded that: (1) HSP70 was induced by high-intensity training, but not by endurance training at low intensity, and (2) there was a discrepancy in terms of HSP70 regulation between the protein and mRNA levels, suggesting that posttranscriptional regulation may play a role in HSP70 expression in human skeletal muscle in response to exercise.

Influence of pre-exercise muscle glycogen content on exercise-induced transcriptional regulation of metabolic genes

Transcription of metabolic genes is transiently induced during recovery from exercise in skeletal muscle of humans. To determine whether pre-exercise muscle glycogen content influences the magnitude and/or duration of this adaptive response, six male subjects performed one-legged cycling exercise to lower muscle glycogen content in one leg and then, the following day, completed 2.5 h low intensity two-legged cycling exercise. Nuclei and mRNA were isolated from biopsies obtained from the vastus lateralis muscle of the control and reduced glycogen (pre-exercise glycogen = 609 +/- 47 and 337 +/- 33 mmol kg(-1) dry weight, respectively) legs before and after 0, 2 and 5 h of recovery. Exercise induced a significant (P < 0.05) increase (2- to 3-fold) in transcription of the pyruvate dehydrogenase kinase 4 (PDK4) and uncoupling protein 3 (UCP3) genes in the reduced glycogen leg only. Although PDK4, lipoprotein lipase (LPL) and hexokinase II (HKII) mRNA were elevated in the reduced glycogen leg before exercise, no consistent difference was found between the two legs in response to exercise. In a second study, six subjects completed two trials (separated by 2 weeks) consisting of 3 h of two-legged knee extensor exercise with either control (398 +/- 52 mmol kg(-1) dry weight) or low (240 +/- 38 mmol kg(-1) dry weight) pre-exercise muscle glycogen. Exercise induced a significantly greater increase in PDK4 transcription in the low glycogen (> 6-fold) than in the control (< 3-fold) trial. Induction of PDK4 and UCP3 mRNA in response to exercise was also significantly higher in the low glycogen (11.4- and 3.5-fold, respectively) than in the control (5.0- and 1.7-fold, respectively) trial. These data indicate that low muscle glycogen content enhances the transcriptional activation of some metabolic genes in response to exercise, raising the possibility that signalling mechanisms sensitive to glycogen content and/or FFA availability may be linked to the transcriptional control of exercise-responsive genes.

The scientific basis for high-intensity interval training: optimising training programmes and maximising performance in highly trained endurance athletes

While the physiological adaptations that occur following endurance training in previously sedentary and recreationally active individuals are relatively well understood, the adaptations to training in already highly trained endurance athletes remain unclear. While significant improvements in endurance performance and corresponding physiological markers are evident following submaximal endurance training in sedentary and recreationally active groups, an additional increase in submaximal training (i.e. volume) in highly trained individuals does not appear to further enhance either endurance performance or associated physiological variables [e.g. peak oxygen uptake (VO2peak), oxidative enzyme activity]. It seems that, for athletes who are already trained, improvements in endurance performance can be achieved only through high-intensity interval training (HIT). The limited research which has examined changes in muscle enzyme activity in highly trained athletes, following HIT, has revealed no change in oxidative or glycolytic enzyme activity, despite significant improvements in endurance performance (p < 0.05). Instead, an increase in skeletal muscle buffering capacity may be one mechanism responsible for an improvement in endurance performance. Changes in plasma volume, stroke volume, as well as muscle cation pumps, myoglobin, capillary density and fibre type characteristics have yet to be investigated in response to HIT with the highly trained athlete. Information relating to HIT programme optimisation in endurance athletes is also very sparse. Preliminary work using the velocity at which VO2max is achieved (V(max)) as the interval intensity, and fractions (50 to 75%) of the time to exhaustion at V(max) (T(max)) as the interval duration has been successful in eliciting improvements in performance in long-distance runners. However, V(max) and T(max) have not been used with cyclists. Instead, HIT programme optimisation research in cyclists has revealed that repeated supramaximal sprinting may be equally effective as more traditional HIT programmes for eliciting improvements in endurance performance. Further examination of the biochemical and physiological adaptations which accompany different HIT programmes, as well as investigation into the optimal HIT programme for eliciting performance enhancements in highly trained athletes is required.

Exercise and skeletal muscle gene expression

1. Skeletal muscle is a complex and heterogenous tissue capable of remarkable adaptation in response to exercise training. The role of gene transcription, as an initial target to control protein synthesis, is poorly understood. 2. Mature myofibres contain several hundred nuclei, all of which maintain transcriptional competency, although the localized responsiveness of nuclei is not well known. Myofibres are capable of hypertrophy. These processes require the activation and myogenic differentiation of mononuclear satellite cells that fuse with the enlarging or repairing myofibre. 3. A single bout of exercise in human subjects is capable of activating the expression of many diverse groups of genes. 4. The impact of repeated exercise bouts, typical of exercise training, on gene expression has yet to receive systematic investigation. 5. The molecular programme elicited by resistance exercise and endurance exercise differs markedly. Muscular hypertrophy following resistance exercise is dependent on the activation of satellite cells and their subsequent myogenic maturation. Endurance exercise requires the simultaneous activation of mitochondrial and nuclear genes to enable mitochondrial biogenesis. 6. Future analysis of the regulation of genes by exercise may combine high-throughput technologies, such as gene-chips, enabling the rapid detection and analysis of changes in the expression of many thousands of genes.

Activation of MEF2 by muscle activity is mediated through a calcineurin-dependent pathway

Gene expression in skeletal muscles of adult vertebrates is altered profoundly by changing patterns of contractile work. Here we observed that the functional activity of MEF2 transcription factors is stimulated by sustained periods of endurance exercise or motor nerve pacing, as assessed by expression in trans genic mice of a MEF2-dependent reporter gene (desMEF2-lacZ). This response is accompanied by transformation of specialized myofiber subtypes, and is blocked either by cyclosporin A, a specific chemical inhibitor of calcineurin, or by forced expression of the endogenous calcineurin inhibitory protein, myocyte-enriched calcineurin interacting protein 1. Calcineurin removes phosphate groups from MEF2, and augments the potency of the transcriptional activation domain of MEF2 fused to a heterologous DNA binding domain. Across a broad range, the enzymatic activity of calcineurin correlates directly with expression of endogenous genes that are transcriptionally activated by muscle contractions. These results delineate a molecular pathway in which calcineurin and MEF2 participate in the adaptive mechanisms by which skeletal myofibers acquire specialized contractile and metabolic properties as a function of changing patterns of muscle contraction.

Molecular adaptations in human skeletal muscle to endurance training under simulated hypoxic conditions

This study was performed to explore changes in gene expression as a consequence of exercise training at two levels of intensity under normoxic and normobaric hypoxic conditions (corresponding to an altitude of 3,850 m). Four groups of human subjects trained five times a week for a total of 6 wk on a bicycle ergometer. Muscle biopsies were taken, and performance tests were carried out before and after the training period. Similar increases in maximal O(2) uptake (8.3-13.1%) and maximal power output (11.4-20.8%) were found in all groups. RT-PCR revealed elevated mRNA concentrations of the alpha-subunit of hypoxia-inducible factor 1 (HIF-1) after both high- (+82.4%) and low (+78.4%)-intensity training under hypoxic conditions. The mRNA of HIF-1alpha(736), a splice variant of HIF-1alpha newly detected in human skeletal muscle, was shown to be changed in a similar pattern as HIF-1alpha. Increased mRNA contents of myoglobin (+72.2%) and vascular endothelial growth factor (+52.4%) were evoked only after high-intensity training in hypoxia. Augmented mRNA levels of oxidative enzymes, phosphofructokinase, and heat shock protein 70 were found after high-intensity training under both hypoxic and normoxic conditions. Our findings suggest that HIF-1 is specifically involved in the regulation of muscle adaptations after hypoxia training. Fine-tuning of the training response is recognized at the molecular level, and with less sensitivity also at the structural level, but not at global functional responses like maximal O(2) uptake or maximal power output.

Regulatory elements governing transcription in specialized myofiber subtypes

Skeletal myofibers of vertebrates acquire specialized metabolic and physiological properties as a consequence of developmental cues in the embryo and different patterns of contractile activity in the adult. The myoglobin gene is regulated stringently in muscle fibers, such that high myoglobin expression is observed in mitochondria-rich, oxidative myofibers (Types I and IIa) compared with glycolytic fibers (Type IIb). Using germ-line transgenesis and somatic cell gene transfer methods, we defined discrete regions of the murine and human genes encoding myoglobin that are sufficient to confer muscle- and fiber type-specific expression to reporter genes. Mutational analysis confirms the importance of A/T-rich, MEF2-binding motifs in myoglobin gene regulation, as suggested by previous studies using different experimental approaches. In addition, we demonstrated a previously unsuspected role for an intragenic E-box motif as a negative regulatory element contributing to the tightly regulated variation in myoglobin gene expression among particular myofiber subtypes.

Effects of body temperature during exercise training on myocardial adaptations

This study determined the role of body temperature during chronic exercise on myocardial stress proteins and antioxidant enzymes as well as functional recovery after an ischemic insult. Male Sprague-Dawley rats were exercised for 3, 6, or 9 wk in a 23 degrees C room (3WK, 6WK, and 9WK, respectively) or in a 4-8 degrees C environment with wetted fur (3WKC, 6WKC, and 9WKC, respectively). The colder room prevented elevations in core temperature. During weeks 3-9 the animals ran 5 days/wk up a 6% grade at 20 m/min for 60 min. Myocardial heat shock protein 70 (HSP 70) increased 12.3-fold (P < 0.05) in 9WK versus sedentary (SED) rats but was unchanged in the cold-room runners. Compared with SED rats, alphaB-crystallin was 90% higher in 9WKC animals, HSP 90 was 50% higher in 3WKC and 6WKC animals, and catalase was 23% higher in 3WK animals (P < 0.05 for all). Cytosolic superoxide dismutase increased and mitochondrial SOD decreased (P < 0.05) in 3WK and 6WK rats compared with 3WKC and 6WKC rats. Antioxidant enzymes returned to SED values in all runners by 9 wk. No differences were observed among any of the groups for glucose-regulated protein 75, heme oxygenase-1, or glutathione peroxidase. Mechanical recovery of isolated working hearts after 22.5 min of global ischemia was enhanced in 9WK (P < 0.05) but not in 9WKC rats. We conclude that exercise training results in dynamic changes in cardioprotective proteins over time which are influenced by core temperature. In addition, cardioprotection resulting from chronic exercise appears to be due to increased HSP 70.

Invited Review: contractile activity-induced mitochondrial biogenesis in skeletal muscle

Chronic contractile activity produces mitochondrial biogenesis in muscle. This adaptation results in a significant shift in adenine nucleotide metabolism, with attendant improvements in fatigue resistance. The vast majority of mitochondrial proteins are derived from the nuclear genome, necessitating the transcription of genes, the translation of mRNA into protein, the targeting of the protein to a mitochondrial compartment via the import machinery, and the assembly of multisubunit enzyme complexes in the respiratory chain or matrix. Putative signals involved in initiating this pathway of gene expression in response to contractile activity likely arise from combinations of accelerations in ATP turnover or imbalances between mitochondrial ATP synthesis and cellular ATP demand, and Ca(2+) fluxes. These rapid events are followed by the activation of exercise-responsive kinases, which phosphorylate proteins such as transcription factors, which subsequently bind to upstream regulatory regions in DNA, to alter transcription rates. Contractile activity increases the mRNA levels of nuclear-encoded proteins such as cytochrome c and mitochondrial transcription factor A (Tfam) and mRNA levels of upstream transcription factors like c-jun and nuclear respiratory factor-1 (NRF-1). mRNA level changes are often most evident during the postexercise recovery period, and they can occur as a result of contractile activity-induced increases in transcription or mRNA stability. Tfam is imported into mitochondria and controls the expression of mitochondrial DNA (mtDNA). mtDNA contributes only 13 protein products to the respiratory chain, but they are vital for electron transport and ATP synthesis. Contractile activity increases Tfam expression and accelerates its import into mitochondria, resulting in increased mtDNA transcription and replication. The result of this coordinated expression of the nuclear and the mitochondrial genomes, along with poorly understood changes in phospholipid synthesis, is an expansion of the muscle mitochondrial reticulum. Further understanding of 1) regulation of mtDNA expression, 2) upstream activators of NRF-1 and other transcription factors, 3) the identity of mRNA stabilizing proteins, and 4) potential of contractile activity-induced changes in apoptotic signals are warranted.

Differential effects of exercise on insulin-signaling gene expression in human skeletal muscle

Skeletal muscle insulin sensitivity is enhanced after acute exercise and short-term endurance training. We investigated the impact of exercise on the gene expression of key insulin-signaling proteins in humans. Seven untrained subjects (4 women and 3 men) completed 9 days of cycling at 63 +/- 2% of peak O(2) uptake for 60 min/day. Muscle biopsies were taken before, immediately after, and 3 h after the exercise bouts (on days 1 and 9). The gene expression of insulin receptor substrate-2 and the p85 alpha subunit of phosphatidylinositol 3-kinase was significantly higher 3 h after a single exercise bout, although short-term training ameliorated this effect. Gene expression of insulin receptor and insulin receptor substrate-1 was not significantly altered at any time point. These results suggest that exercise may have a transitory impact on the expression of insulin receptor substrate-2 and phosphatidylinositol 3-kinase; however, the predominant actions of exercise on insulin sensitivity appear not to reside in the transcriptional activation of the genes encoding major insulin-signaling proteins.

Transgene overexpression of alphaB crystallin confers simultaneous protection against cardiomyocyte apoptosis and necrosis during myocardial ischemia and reperfusion

We investigated whether enhanced expression of alphaB crystallin, a stress-inducible molecular chaperone of the small heat shock family, can protect myocardial contractile apparatus against ischemia reperfusion (I/R) injury. Transgenic mice overexpressing alphaB crystallin were generated using the 0.76 kb rat alphaB crystallin cDNA cloned into a pCAGGS plasmid driven by a human cytomegalovirus expression system. Southern analysis confirmed transgene integration and Northern and Western blotting characterized expression (3.1-fold and 6.9-fold elevations in myocardial mRNA and protein levels, respectively). Extent of functional recovery over a 3 h reperfusion period following a 20 min ischemic period in transgenic and wild-type mouse hearts was assessed using an ex vivo work-performing heart preparation. The transgenic group displayed significantly higher values of DP at R45 min (29.14+/-1.9 mm Hg vs. 17.6+/-0.7 mm Hg), R60 min (31.56+/-1.7 mm Hg vs. 17.8+/-0.8 mm Hg), and R75 min (32.5+/-2.2 mm Hg vs. 16.9+/-0.9 mm Hg), and of dLVP/dt at R45 min (1740.2+/-111.5 mm Hg.s-1 vs. 548.7+/-82.2 mm Hg.s-1) and R60 min (1199.8+/-104.6 mm Hg.s-1 vs. 466.9+/-61.1 mm Hg.s-1). The transgenic group also displayed development of less oxidative stress, decreased extent of infarction, and attenuated cardiomyocyte apoptotic cell death. Transgene overexpression of alphaB crystallin was therefore successful in diminishing the independent contributory effects of both necrosis and apoptosis on I/R-induced cell death.

Effects of contractile activity on mitochondrial transcription factor A expression in skeletal muscle

Mitochondrial transcription factor A (Tfam) is a nuclear-encoded gene product that is imported into mitochondria and is required for the transcription of mitochondrial DNA (mtDNA). We hypothesized that conditions known to produce mitochondrial biogenesis in skeletal muscle would be preceded by an increase in Tfam expression. Therefore, rat muscle was stimulated (10 Hz, 3 h/day). Tfam mRNA levels were significantly elevated (by 55%) at 4 days and returned to control levels at 14 days. Tfam import into intermyofibrillar (IMF) mitochondria was increased by 52 and 61% (P < 0.05) at 5 and 7 days, respectively. This corresponded to an increase in the level of import machinery components. Immunoblotting data indicated that IMF Tfam protein content was increased by 63% (P < 0.05) at 7 days of stimulation. This was associated with a 49% (P < 0.05) increase in complex formation at the mtDNA promoter and a 65% (P < 0.05) increase in the levels of a mitochondrial transcript, cytochrome-c oxidase (COX) subunit III. Similarly, COX enzyme activity was elevated by 71% (P < 0.05) after 7 days of contractile activity. These results indicate that early events in mitochondrial biogenesis include increases in Tfam mRNA, followed by accelerations in mitochondrial import and increased Tfam content, which correspond with increased binding to the mtDNA promoter region. This was accompanied by increased mitochondrial transcript levels and elevated COX activity. These data support the role of Tfam as a regulatory protein involved in contractile activity-induced mitochondrial biogenesis.

Transcriptional regulation of gene expression in human skeletal muscle during recovery from exercise

Exercise training elicits a number of adaptive changes in skeletal muscle that result in an improved metabolic efficiency. The molecular mechanisms mediating the cellular adaptations to exercise training in human skeletal muscle are unknown. To test the hypothesis that recovery from exercise is associated with transcriptional activation of specific genes, six untrained male subjects completed 60-90 min of exhaustive one-legged knee extensor exercise for five consecutive days. On day 5, nuclei were isolated from biopsies of the vastus lateralis muscle of the untrained and the trained leg before exercise and from the trained leg immediately after exercise and after 15 min, 1 h, 2 h, and 4 h of recovery. Transcriptional activity of the uncoupling protein 3 (UCP3), pyruvate dehydrogenase kinase 4 (PDK4), and heme oxygenase-1 (HO-1) genes (relative to beta-actin) increased by three- to sevenfold in response to exercise, peaking after 1-2 h of recovery. Increases in mRNA levels followed changes in transcription, peaking between 2 and 4 h after exercise. Lipoprotein lipase and carnitine pamitoyltransferase I gene transcription and mRNA levels showed similar but less dramatic induction patterns, with increases ranging from two- to threefold. In a separate study, a single 4-h bout of cycling exercise (n = 4) elicited from 5 to >20-fold increases in UCP3, PDK4, and HO-1 transcription, suggesting that activation of these genes may be related to the duration or intensity of exercise. These data demonstrate that exercise induces transient increases in transcription of metabolic genes in human skeletal muscle. Moreover, the findings suggest that the cumulative effects of transient increases in transcription during recovery from consecutive bouts of exercise may represent the underlying kinetic basis for the cellular adaptations associated with exercise training.

Effect of contractile activity on protein turnover in skeletal muscle mitochondrial subfractions

To determine the role of intramitochondrial protein synthesis (PS) and degradation (PD) in contractile activity-induced mitochondrial biogenesis, we evaluated rates of [(35)S]methionine incorporation into protein in isolated rat muscle subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria. Rates of PS ranged from 47 to 125% greater (P < 0.05) in IMF compared with SS mitochondria. Intense, acute in situ contractile activity (10 Hz, 5 min) of fast-twitch gastrocnemius muscle resulted in a 50% decrease in PS (P < 0.05) in SS but not IMF mitochondria. Recovery, or continued contractile activity (55 min), reestablished PS in SS mitochondria. In contrast, PS was not affected in either SS or IMF mitochondria after prolonged (60-min) contractile activity in the presence or absence of a recovery period. PD was not influenced by 5 min of contractile activity in the presence or absence of recovery but was reduced after 60 min of contractions followed by recovery. Chronic stimulation (10 Hz, 3 h/day, 14 days) increased muscle cytochrome-c oxidase activity by 2.2-fold but reduced PS in IMF mitochondria by 29% (P < 0.05; n = 4). PS in SS mitochondria and PD in both subfractions were not changed by chronic stimulation. Thus acute contractile activity exerts differential effects on protein turnover in IMF and SS mitochondria, and it appears that intramitochondrial PS does not limit the extent of chronic contractile activity-induced mitochondrial biogenesis.

Altered ischemic induction of immediate early gene and heat shock protein 70 mRNAs after preconditioning in rat hearts

Immediate early genes and heat shock protein (HSP) 70s, which may play a role in adaptation and cellular protection, respectively, are induced by ischemia in hearts. We examined if the induction of immediate early gene (c-fos, c-myc, c-jun, and junB) and HSP70 mRNAs by ischemia is affected by ischemic preconditioning. Transient ischemia (5 or 10 minute) was applied to Wistar rat (n=75) hearts, by tightening a snare placed around left coronary arterial branches 7 days before applying ischemia. Rats without earlier ischemia (control group, C) and rats with 5-minute ischemia 12 or 24 hours earlier (EI12 or 24 group) were given 10-minute ischemia and sacrificed at 0, 0.5, 1, 2, and 4 hour. RNA was extracted from the ischemic region and Northern blot analysis was performed. The induction of c-fos and c-myc mRNAs was significantly increased in EI12 but not in EI24 compared with that in C. The induction of c-jun and junB mRNAs showed no change in both EI12 and EI24 compared with that in C. The induction of HSP72 and 73 mRNAs was decreased in EI12 and decreased further in EI24. Thus, ischemic preconditioning altered the induction of immediate early gene and HSP70 mRNAs by ischemia. The effect of preconditioning differed among genes studied and changed with time after preconditioning. Ischemic preconditioning alters protective and adaptive responses to ischemia at the gene level.

HSP70 expression is increased during the day in a diurnal animal, the golden-mantled ground squirrel Spermophilus lateralis

Heat shock protein 70 (HSP70) gene expression was studied in a seasonal hibernator, the diurnal ground squirrel, Spermophilus lateralis. RNA transcripts of 2.7 and 2.9 kb hybridizing to an HSP70 cDNA were expressed in both brain and peripheral tissues of pre-hibernation euthermic animals; higher levels of expression were observed during the day than during nighttime samples. A decline in the expression of both transcripts occurred in all tissues examined during hibernation that remained low throughout the hibernation season, including the interbout euthermic periods and regardless of time of day. Quantitative comparisons showed pre-hibernation nighttime HSP70 expression to be as low as that observed during hibernation, despite the drastic increase in metabolic state and nearly 30 degrees C difference in body temperature. In contrast to HSP70, some mRNAs, such as beta-actin and HSP60, remained relatively constant, while others, such as glyceraldehyde 3-phosphate dehydrogenase, increased in specific tissues during the hibernation season. These results indicate that the expression of a highly conserved gene involved in protection from cellular stress, HSP70, can vary with an animal's arousal state.

Cytochrome c transcriptional activation and mRNA stability during contractile activity in skeletal muscle

We evaluated contractile activity-induced alterations in cytochrome c transcriptional activation and mRNA stability with unilateral chronic stimulation (10 Hz, 3 h/day) of the rat tibialis anterior (TA) muscle for 1, 2, 3, 4, 5, and 7 days (n = 3-11/group). Transcriptional activation was assessed by direct plasmid DNA injection into the TA with a chloramphenicol acetyltransferase (CAT) reporter gene linked to 326 bp of the cytochrome c promoter. Cytochrome c mRNA in stimulated muscles increased by 1.3- to 1. 7-fold above control between 1 and 7 days. Cytochrome c protein was increased after 5 days of stimulation to reach levels that were 1. 9-fold higher than control by 7 days. Cytochrome c mRNA stability, determined with an in vitro decay assay, was greater in stimulated TA than in control between 2 and 4 days, likely mediated by the induction of a cytosolic factor. In contrast, cytochrome c transcriptional activation was elevated only after 5 days of stimulation when mRNA stability had returned to control levels. Thus the contractile activity-induced increase in cytochrome c mRNA was due to an early increase in mRNA stability, followed by an elevation in transcriptional activation, leading to an eventual increase in cytochrome c protein levels.

Acute exercise can improve cardioprotection without increasing heat shock protein content

The aim of this study was to determine the effects of acute bouts of exercise on myocardial recovery after ischemia and heat shock protein expression. Adult female Sprague-Dawley rats were divided into five groups: 1) 1-day run (1DR; n = 6) and 2) 3-day run (3DR; n = 7), in which rats ran for 100 min at a speed of 20 m/min up a 6 degrees grade for 1 or 3 consecutive days; 3) 1-day cold run (1CR), in which rats ran the same as 1DR but with wet fur at 8 degrees C, which prevented an elevation of core temperature (n = 8); 4) heat shock sedentary (HS), in which rats had their core temperatures raised to 42 degrees C one time for 15 min (n = 5); and 5) sedentary control (n=15). Cardiac function was analyzed 24 h after the last treatment using an isolated, working heart model. Nonpaced hearts were initially perfused under normoxic conditions, then underwent 17 min of global, normothermic (37 degrees C) ischemia, and, finally, were allowed to recover for 30 min under normoxic conditions. The concentration of the 72-kDa heat shock protein (HSP 72) was measured in each left ventricle. Compared with that in the sedentary group, recovery of cardiac output x systolic pressure (CO x SP) was enhanced (P < 0.05) in all treatment groups when the postischemic value was covaried with the preischemic value. No differences in CO x SP were found (P > 0.05) between the following groups: 1DR vs. 3DR, 1DR vs. HS, and 1DR vs. 1CR. Heat shock protein concentration was significantly greater (P < 0.05) than that in the sedentary controls in HS, 1DR, and 3DR groups, but not for 1CR. The concentration of HSP 72 was not significantly correlated with postischemic CO x SP (R2 = 0.197, P > 0.05). We conclude that acute bouts of exercise can produce cardioprotective effects without an elevation of HSP 72.

A missense mutation in the alphaB-crystallin chaperone gene causes a desmin-related myopathy

Desmin-related myopathies (DRM) are inherited neuromuscular disorders characterized by adult onset and delayed accumulation of aggregates of desmin, a protein belonging to the type III intermediate filament family, in the sarcoplasma of skeletal and cardiac muscles. In this paper, we have mapped the locus for DRM in a large French pedigree to a 26-cM interval in chromosome 11q21-23. This region contains the alphaB-crystallin gene (CRYAB), a candidate gene encoding a 20-kD protein that is abundant in lens and is also present in a number of non-ocular tissues, including cardiac and skeletal muscle. AlphaB-crystallin is a member of the small heat shock protein (shsp) family and possesses molecular chaperone activity. We identified an R120G missense mutation in CRYAB that co-segregates with the disease phenotype in this family. Muscle cell lines transfected with the mutant CRYAB cDNA showed intracellular aggregates that contain both desmin and alphaB-crystallin as observed in muscle fibers from DRM patients. These results are the first to identify a defect in a molecular chaperone as a cause for an inherited human muscle disorder.