Continuous contractile activity induces fiber type specific expression of HSP70 in skeletal muscle

Role of Heat Shock Proteins in Atrial Fibrillation: From Molecular Mechanisms to Diagnostic and Therapeutic Opportunities

Heat shock proteins (HSPs) are endogenous protective proteins and biomarkers of cell stress response, of which examples are HSP70, HSP60, HSP90, and small HSPs (HSPB). HSPs protect cells and organs, especially the cardiovascular system, against harmful and cytotoxic conditions. More recent attention has focused on the roles of HSPs in the irreversible remodeling of atrial fibrillation (AF), which is the most common arrhythmia in clinical practice and a significant contributor to mortality. In this review, we investigated the relationship between HSPs and atrial remodeling mechanisms in AF. PubMed was searched for studies using the terms "Heat Shock Proteins" and "Atrial Fibrillation" and their relevant abbreviations up to 10 July 2022. The results showed that HSPs have cytoprotective roles in atrial cardiomyocytes during AF by promoting reverse electrical and structural remodeling. Heat shock response (HSR) exhaustion, followed by low levels of HSPs, causes proteostasis derailment in cardiomyocytes, which is the basis of AF. Furthermore, potential implications of HSPs in the management of AF are discussed in detail. HSPs represent reliable biomarkers for predicting and staging AF. HSP inducers may serve as novel therapeutic modalities in postoperative AF. HSP induction, either by geranylgeranylacetone (GGA) or by other compounds presently in development, may therefore be an interesting new approach for upstream therapy for AF, a strategy that aims to prevent AF whilst minimizing the ventricular proarrhythmic risks of traditional anti-arrhythmic agents.

Overexpression of Heat Shock Protein 70 Ameliorates Meat Quality of Broilers Subjected to Pre-Slaughter Transport at High Ambient Temperatures by Improving Energy Status of Pectoralis Major Muscle and Antioxidant Capacity

The induction of heat shock protein 70 (HSP70) potentially mediates meat-quality development under stress conditions. To investigate the effects and mechanism of HSP70 on the meat quality of the pectoralis major (PM) muscles of broilers exposed to pre-slaughter transport, a total of 168 broilers were intraperitoneally injected with L-glutamine (Gln) or saline. Twenty-four hours later, broilers were subjected to transport or held under normal living conditions. The results indicated that acute Gln supplementation significantly increased HSP70 expression in the PM of transported broilers (p < 0.05). The overexpression of HSP70 significantly alleviated the decreases in muscle pH and water-holding capacity and improved the shrinking of muscle fibers induced by transport (p < 0.05). HSP70 induction increased ATP content, decreased the activities of glycolytic enzymes, and lowered the phosphorylation level of AMP-activated protein kinase in transported broilers (p < 0.05). In addition, the overexpression of HSP70 greatly increased total superoxide dismutase and the total antioxidant capability and decreased the levels of reactive oxygen species, malonaldehyde, and carbonyls in the PM of transported broilers (p < 0.05). Overall, this work indicated that HSP70 could effectively improve the meat quality of transported broilers by improving the energy status, inhibiting glycolytic influx, and restoring redox homeostasis.

Impact of adrenaline and metabolic stress on exercise-induced intracellular signaling and PGC-1α mRNA response in human skeletal muscle

This study tested the hypothesis that elevated plasma adrenaline or metabolic stress enhances exercise‐induced PGC‐1α mRNA and intracellular signaling in human muscle. Trained (VO₂‐max: 53.8 ± 1.8 mL min⁻¹ kg⁻¹) male subjects completed four different exercise protocols (work load of the legs was matched): C – cycling at 171 ± 6 W for 60 min (control); A – cycling at 171 ± 6 W for 60 min, with addition of intermittent arm exercise (98 ± 4 W). DS – cycling at 171 ± 6 W interspersed by 30 sec sprints (513 ± 19 W) every 10 min (distributed sprints); and CS – cycling at 171 ± 6 W for 40 min followed by 20 min of six 30 sec sprints (clustered sprints). Sprints were followed by 3:24 min:sec at 111 ± 4 W. A biopsy was obtained from m. vastus lateralis at rest and immediately, and 2 and 5 h after exercise. Muscle PGC‐1α mRNA content was elevated (P < 0.05) three‐ to sixfold 2 h after exercise relative to rest in C, A, and DS, with no differences between protocols. AMPK and p38 phosphorylation was higher (P < 0.05) immediately after exercise than at rest in all protocols, and 1.3‐ to 2‐fold higher (P < 0.05) in CS than in the other protocols. CREB phosphorylation was higher (P < 0.05) 2 and 5 h after exercise than at rest in all protocols, and higher (P < 0.05) in DS than CS 2 h after exercise. This suggests that neither plasma adrenaline nor muscle metabolic stress determines the magnitude of PGC‐1α mRNA response in human muscle. Furthermore, higher exercise‐induced changes in AMPK, p38, and CREB phosphorylation are not associated with differences in the PGC‐1α mRNA response.

Skeletal muscle Heat shock protein 60 increases after endurance training and induces peroxisome proliferator-activated receptor gamma coactivator 1 α1 expression

Heat shock protein 60 (Hsp60) is a chaperone localizing in skeletal muscle mitochondria, whose role is poorly understood. In the present study, the levels of Hsp60 in fibres of the entire posterior group of hindlimb muscles (gastrocnemius, soleus, and plantaris) were evaluated in mice after completing a 6-week endurance training program. The correlation between Hsp60 levels and the expression of four isoforms of peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC1α) were investigated only in soleus. Short-term overexpression of hsp60, achieved by in vitro plasmid transfection, was then performed to determine whether this chaperone could have a role in the activation of the expression levels of PGC1α isoforms. The levels of Hsp60 protein were fibre-type specific in the posterior muscles and endurance training increased its content in type I muscle fibers. Concomitantly with the increased levels of Hsp60 released in the blood stream of trained mice, mitochondrial copy number and the expression of three isoforms of PGC1α increased. Overexpressing hsp60 in cultured myoblasts induced only the expression of PGC1 1α, suggesting a correlation between Hsp60 overexpression and PGC1 1 α activation.

Distinct muscle apoptotic pathways are activated in muscles with different fiber types in a rat model of critical illness myopathy

Critical illness myopathy (CIM) is associated with severe muscle atrophy and fatigue in affected patients. Apoptotic signaling is involved in atrophy and is elevated in muscles from patients with CIM. In this study we investigated underlying mechanisms of apoptosis-related pathways in muscles with different fiber type composition in a rat model of CIM using denervation and glucocorticoid administration (denervation and steroid-induced myopathy, DSIM). Soleus and tibialis anterior (TA) muscles showed severe muscle atrophy (40-60% of control muscle weight) and significant apoptosis in interstitial as well as myofiber nuclei that was similar between the two muscles with DSIM. Caspase-3 and -8 activities, but not caspase-9 and -12, were elevated in TA and not in soleus muscle, while the caspase-independent proteins endonuclease G (EndoG) and apoptosis inducing factor (AIF) were not changed in abundance nor differentially localized in either muscle. Anti-apoptotic proteins HSP70, -27, and apoptosis repressor with a caspase recruitment domain (ARC) were elevated in soleus compared to TA muscle and ARC was significantly decreased with induction of DSIM in soleus. Results indicate that apoptosis is a significant process associated with DSIM in both soleus and TA muscles, and that apoptosis-associated processes are differentially regulated in muscles of different function and fiber type undergoing atrophy due to DSIM. We conclude that interventions combating apoptosis with CIM may need to be directed towards inhibiting caspase-dependent as well as -independent mechanisms to be able to affect muscles of all fiber types.

The role of estrogen receptor-α in estrogen-mediated regulation of basal and exercise-induced Hsp70 and Hsp27 expression in rat soleus

We examined the influence of estrogen receptor-alpha (ERα) activation on estrogen-mediated regulation of heat shock proteins 70 (Hsp70) and 27 (Hsp27) in soleus. Ovariectomized rats received estrogen (EST), an ERα agonist (propyl pyrazole triol, PPT), both (EST+PPT), or a sham, and they served as either unexercised controls or were subjected to exercise by having to run downhill (17 m/min, -13.5° grade) for 90 min. At 72 h postexercise, soleus muscles were removed and either immunohistochemically stained for Hsp70 and myosin heavy chain or homogenized for Western blotting for Hsp70 and Hsp27. Elevated (p < 0.05) basal Hsp70 in both type I and II fibres in the unexercised EST, PPT, and EST+PPT groups relative to unexercised sham animals was noted. Compared with Hsp70 levels in the unexercised animals, that in exercised animals was elevated (p < 0.05) in both sham and PPT groups but not in EST and EST+PPT groups. Western blot determined that Hsp27 levels were not significantly different between groups. Hence, the ability of estrogen to augment resting type I and type II muscle fibre Hsp70 content is primarily mediated via muscle ERα. However, the blunted Hsp70 response following damaging exercise in estrogen-supplemented animals does not appear to be fully accounted for by ERα-mediated effects.

Subcellular fractionation reveals HSP72 does not associate with SERCA in human skeletal muscle following damaging eccentric and concentric exercise

Through its upregulation and/or translocation, heat shock protein 72 (HSP72) is involved in protection and repair of key proteins after physiological stress. In human skeletal muscle we investigated HSP72 protein after eccentric (ECC1) and concentric (CONC) exercise and repeated eccentric exercise (ECC2; 8 wk later) and whether it translocated from its normal cytosolic location to membranes/myofibrils. HSP72 protein increased ∼2-fold 24 h after ECC1, with no apparent change after CONC or ECC2. In resting (nonstressed) human skeletal muscle the total pool of HSP72 protein was present almost exclusively in the cytosolic fraction, and after each exercise protocol the distribution of HSP72 protein remained unaltered. Overall, the amount of HSP72 protein in the cytosol increased 24 h after ECC1, matching the fold increase that was measured in total HSP72 protein. To better ascertain the capabilities and limitations of HSP72, using quantitative Western blotting we determined the HSP72 protein content to be 11.4 μmol/kg wet weight in resting human vastus lateralis muscle, which is comprised of Type I (slow-twitch) and Type II (fast-twitch) fibers. HSP72 protein content was similar in individual Type I or II fiber segments. After physiological stress, HSP72 content can increase and, although the functional consequences of increased amounts of HSP72 protein are poorly understood, it has been shown to bind to and protect protein pumps like SERCA and Na+-K+-ATPase. Given no translocation of cytosolic HSP72, these findings suggest eccentric contractions, unlike other forms of stress such as heat, do not trigger tight binding of HSP72 to its primary membrane-bound target proteins, in particular SERCA.

Mechanisms of exercise-induced mitochondrial biogenesis in skeletal muscle: implications for health and disease

Mitochondria have paradoxical functions within cells. Essential providers of energy for cellular survival, they are also harbingers of cell death (apoptosis). Mitochondria exhibit remarkable dynamics, undergoing fission, fusion, and reticular expansion. Both nuclear and mitochondrial DNA (mtDNA) encode vital sets of proteins which, when incorporated into the inner mitochondrial membrane, provide electron transport capacity for ATP production, and when mutated lead to a broad spectrum of diseases. Acute exercise can activate a set of signaling cascades in skeletal muscle, leading to the activation of the gene expression pathway, from transcription, to post-translational modifications. Research has begun to unravel the important signals and their protein targets that trigger the onset of mitochondrial adaptations to exercise. Exercise training leads to an accumulation of nuclear- and mtDNA-encoded proteins that assemble into functional complexes devoted to mitochondrial respiration, reactive oxygen species (ROS) production, the import of proteins and metabolites, or apoptosis. This process of biogenesis has important consequences for metabolic health, the oxidative capacity of muscle, and whole body fitness. In contrast, the chronic muscle disuse that accompanies aging or muscle wasting diseases provokes a decline in mitochondrial content and function, which elicits excessive ROS formation and apoptotic signaling. Research continues to seek the molecular underpinnings of how regular exercise can be used to attenuate these decrements in organelle function, maintain skeletal muscle health, and improve quality of life.

The expression of heat shock protein in human skeletal muscle: effects of muscle fibre phenotype and training background

AIM

Exercise-induced adaptations of skeletal muscle are related to training mode and can be muscle fibre type specific. This study aimed to investigate heat shock protein expression in type I and type II muscle fibres in resting skeletal muscle of subjects with different training backgrounds.

METHODS

Three groups of subjects were included: healthy active not engaged in any training programme (ACT, n = 12), resistance trained (RES, n = 6) and endurance trained (END, n = 8). Biopsies were obtained from vastus lateralis, and immunohistochemistry was performed using monoclonal antibodies against myosin heavy chain I and IIA, αB-crystallin, HSP27, HSP60 and HSP70.

RESULTS

In ACT and RES, but not in END, a fibre type-specific expression with higher staining intensity in type I than type II fibres was seen for αB-crystallin. The opposite (II > I) was found for HSP27 in subjects from ACT (6 of 12 subjects) and RES (3 of 6), whereas all subjects from END displayed uniform staining. HSP60 showed no fibre-specific expression. HSP70 displayed a fibre-specific expression pattern (I > II) in ACT (4 of 12), but not in END or RES.

CONCLUSION

This study shows that the level of expression of the different HSPs in human skeletal muscle is influenced by muscle fibre phenotype. The fibre type-specific expression of HSP70 is influenced by resistance and endurance training, whereas those of αB-crystallin and HSP27 is influenced only by endurance training, suggesting the existence of a training-modality-specific action on the adaptive processes including heat shock proteins in human skeletal muscle.

Isolated hearts treated with skeletal muscle homogenates exhibit altered function

Skeletal muscle fiber damage and necrosis can result in the release of intracellular molecules into the extracellular environment. These molecules, termed damage-associated molecular patterns (DAMPs), can act as signals capable of initiating immune and/or inflammatory responses through interactions with pattern recognition receptors. To investigate whether skeletal muscle DAMPs interact with the heart and alter cardiac function, isolated rat hearts were perfused for 75 min with buffer containing 1 μg/ml of either soleus (slow), white gastrocnemius (WG, fast), or heat-stressed white gastrocnemius (HSWG) skeletal muscle homogenates. Left ventricular developed pressure (LVDP) and rates of pressure increase/decrease (± dP/dt) were measured using the Langendorff technique. Compared to controls, no changes in LVDP or +dP/dt were observed over the 75-min perfusion when homogenates from the WG muscles were added. In contrast, at 30 min and thereafter, a decreased LVDP and +dP/dt was observed in the hearts treated with soleus muscle homogenates. The hearts treated with HSWG homogenates also showed a decrease in LVDP from 45 min until the end of perfusion. These results suggest that molecules present in slow muscle and heat-stressed muscle are capable of altering cardiac function. Thus, muscle fiber type and/or heat shock protein content of skeletal muscles may be factors that influence cardiac function following skeletal muscle damage.

Heat shock protein 70 (Hsp70) inhibits oxidative phosphorylation and compensates ATP balance through enhanced glycolytic activity

To address possible effects of heat shock protein 70 (Hsp70) on energy metabolism, we established a cell line expressing different levels of Hsp70 and evaluated changes in glucose and lactate metabolites, as well as ATP levels accordingly. In addition, activities of enzymes involved in glycolysis [phosphofructokinase (PFK) and lactate dehydrogenase (LDH)], Krebs cycle [citric synthase (CS)], and oxidative phosphorylation {NADH dehydrogenase [complex I (CI)] and ubiquinol:cytochrome-c reductase [complex III (CIII)]} were analyzed. The results show that both glucose consumption and lactate excretion were elevated significantly in cells expressing increased levels of Hsp70. Simultaneously, the activities of glycolytic enzymes PFK and LDH were increased markedly in cells overexpressing Hsp70. Activities of enzymes CI and CIII, both involved in oxidative phosphorylation, decreased upon increased expression of Hsp70. These findings were supported by nonsignificant reductions of CS activities in cells that overexpressed Hsp70, whereas intracellular ATP levels remained constant over a wide range of Hsp70 expression. In conclusion, overexpression of Hsp70 in HeLa cells results in downregulation of oxidative phosphorylation, in particular, multiprotein CIII, the main source of reactive oxygen species. In exchange, upregulation of the glycolytic pathway compensates for the homeostasis of cellular ATP supply.

Training alters the skeletal muscle antioxidative capacity in non-insulin-dependent type 2 diabetic men

The present study analyzes the oxidative stress situation in the skeletal muscle of overweight/obese men suffering from non-insulin-dependent type 2 diabetes mellitus [T2DM, n=16, years=61±7, body mass index (BMI)=31±4 kg/m(2) ] and BMI-matched non-diabetic male control subjects (CON, n=7, years=53±6, BMI=30±4 kg/m(2) ). Furthermore, it investigates whether physical training can alter the skeletal muscle antioxidative capacity of T2DM patients at rest. Molecule content analyses (immunohistochemical stainings) of 8-iso-prostaglandin-F2α (8-Iso-PGF), superoxide dismutase-2 (SOD2), glutathione peroxidase-1 (GPX1), peroxiredoxin isoforms (PRDX 1-6) and heat-shock-protein-70 (HSP70) were performed in biopsies taken from the vastus lateralis muscle. Under basal conditions, 8-Iso-PGF was significantly decreased in T2DM patients (-35.7%), whereas PRDX2 and PRDX6 were significantly increased relative to CON (+82.6%; +82.3%). Differences were neither observed in SOD2 nor in GPX1 or PRDX1, 3, 4, 5 density. Regular physical activity (moderate endurance or resistance training twice a week for 3 months) did not alter PRDX1, 2, 3, 4, 6 in the skeletal muscle of T2DM patients, but significantly increased SOD2 (+65.9%), GPX1 (+62.4%), PRDX5 (+37.5%), and HSP70 (+48.5%). Overweight/obese men with non-insulin-dependent T2DM exhibit up-regulated cytosolic peroxiredoxin contents relative to BMI-matched controls. Regular training further up-regulates cytosolic and mitochondrial antioxidative enzymes in T2DM patients and improves their cellular protection systems. This may contribute to a retardation of the disease's progression.

Regulation of survival gene hsp70

Rapid expression of the survival gene, inducible heat shock protein 70 (hsp70), is critical for mounting cytoprotection against severe cellular stress, like elevated temperature. Hsp70 protein chaperones the refolding of heat-denatured peptides to minimize proteolytic degradation as a part of an eukaryotically conserved phenomenon referred to as the heat shock response. The physiologic stress associated with exercise, which can include elevated temperature, mechanical damage, hypoxia, lowered pH, and reactive oxygen species generation, may promote protein unfolding, leading to hsp70 gene expression in skeletal myofibers. Although the pre-transcriptional activation of hsp70 gene expression has been thoroughly reviewed, discussion of downstream hsp70 gene regulation is less extensive. The purpose of this brief review was to examine all levels of hsp70 gene regulation in response to heat stress and exercise with a special focus on skeletal myofibers where data are available. In general, while heat stress represses bulk gene expression, hsp70 mRNA expression is enhanced. Post-transcriptionally, intronless hsp70 mRNA circumvents a host of decay pathways, as well as heat stress-repressed pre-mRNA splicing and nuclear export. Pre-translationally, hsp70 mRNA is excluded from stress granules and preferentially translated during heat stress-repressed global cap-dependent translation. Post-translationally, nascent Hsp70 protein is thermodynamically stable at elevated temperatures, allowing for the commencement of chaperoning activity early after synthesis to attenuate the heat shock response and protect against subsequent injury. This review demonstrates that hsp70 mRNA expression is closely coupled with functional protein translation.

hsp70 mRNA temporal localization in rat skeletal myofibers and blood vessels post-exercise

Rapid transcription of the survival transcript, inducible heat shock protein 70 (hsp70), is critical for mounting cytoprotection against severe cellular stress, like elevated temperature. Previous investigations have demonstrated that exercise-induced expression of Hsp70 protein occurs in a fiber-specific pattern; however, the activation pattern of hsp70 mRNA expression remains unclear in skeletal muscle. Consequentially, the temporal localization of hsp70 mRNA was characterized via in situ hybridization (ISH) experiments examining fast-muscle, white vastus: 1, 3, 10, and 24 h after a single bout of intense treadmill running (1 h, 30 m/min, 6% grade) in rats. The role that the physiologic temperature stress associated with exercise (raising core body temperature to 40.0°C for 15 min (HS-40.0°C)) might play in inducing hsp70 mRNA expression was also explored. In skeletal muscle myofibers (SkM), hsp70 mRNA ISH signal was observed to be concentrated in a punctate manner that was associated with nuclei post-exercise. HS-40°C treatment produced minimal detectable hsp70 mRNA ISH signal in SkM. In large intermyofibrillar blood vessels (BV), peak hsp70 mRNA signal, distributed throughout the vessel wall, was observed 1 h post-exercise. In BV, no differences in hsp70 mRNA signal were observed between HS-40°C and EX-1 h. Results indicate that the majority of hsp70 mRNA is retained in a perinuclear localization in SkM post-exercise. They further suggest a muscle-type specific time course for peak hsp70 mRNA expression. This investigation suggests that the physiologic rise in core temperature associated with exercise per se is not the key stimulus responsible for inducing hsp70 mRNA transcription in SkM.

Moderate and high intensity sprint exercise induce differential responses in COX4I2 and PDK4 gene expression in Thoroughbred horse skeletal muscle

REASONS FOR PERFORMING STUDY

The role of molecular signalling pathways in the phenotypic adaptation of skeletal muscle to different exercise stimuli in the Thoroughbred horse has not been reported previously.

OBJECTIVE

To examine CKM, COX4I1, COX4I2 and PDK4 gene expression following high intensity sprint and moderate intensity treadmill exercise stimuli in skeletal muscle of Thoroughbred horses.

MATERIALS AND METHODS

Two groups of trained 3-year-old Thoroughbred horses participated. Group A (n = 6 females, n = 3 males) participated in an incremental step test (moderate intensity) to fatigue or HR(max) on a Sato high speed treadmill (distance = 5418.67 m ± 343.21). Group B (n = 8 females) participated in routine 'work' (sprint) on an all-weather gallop (distance = 812.83 m ± 12.53). Biopsy samples were obtained from the gluteus medius pre-exercise (T(0)), immediately post exercise (T(1)) and 4 h post exercise (T(2)). For physiological relevance venous blood samples were collected to measure plasma lactate and creatine kinase concentrations. Changes in mRNA expression were determined by quantitative real-time RT-PCR for creatine kinase muscle (CKM), cytochrome c oxidase subunit IV isoform 1 (COX4I1), cytochrome c oxidase subunit IV isoform 2 (COX4I2) and pyruvate dehydrogenase kinase, isozyme 4 (PDK4) genes. Statistical significance (α < 0.05) was determined using Student's t tests.

RESULTS

COX4I2 mRNA expression decreased significantly in Group A and remained unchanged in Group B between T(0) vs. T(2) (-1.7-fold, P = 0.017; -1.0-fold, P = 0.859). PDK4 mRNA expression increased significantly in Group B but not in Group A between T(0) vs. T(1) (3.8-fold, P = 0.039; 1.4-fold, P = 0.591). There were no significant changes in the expression in CKM or COX4I1 mRNA abundance in either group.

CONCLUSIONS

Different exercise protocols elicit variable transcriptional responses in key exercise relevant genes in equine skeletal muscle due to variation in metabolic demand.

Transcriptional adaptations following exercise in thoroughbred horse skeletal muscle highlights molecular mechanisms that lead to muscle hypertrophy

Background

Selection for exercise-adapted phenotypes in the Thoroughbred racehorse has provided a valuable model system to understand molecular responses to exercise in skeletal muscle. Exercise stimulates immediate early molecular responses as well as delayed responses during recovery, resulting in a return to homeostasis and enabling long term adaptation. Global mRNA expression during the immediate-response period has not previously been reported in skeletal muscle following exercise in any species. Also, global gene expression changes in equine skeletal muscle following exercise have not been reported. Therefore, to identify novel genes and key regulatory pathways responsible for exercise adaptation we have used equine-specific cDNA microarrays to examine global mRNA expression in skeletal muscle from a cohort of Thoroughbred horses (n = 8) at three time points (before exercise, immediately post-exercise, and four hours post-exercise) following a single bout of treadmill exercise.

Results

Skeletal muscle biopsies were taken from the gluteus medius before (T₀), immediately after (T₁) and four hours after (T₂) exercise. Statistically significant differences in mRNA abundance between time points (T₀ vs T₁ and T₀ vs T₂) were determined using the empirical Bayes moderated t-test in the Bioconductor package Linear Models for Microarray Data (LIMMA) and the expression of a select panel of genes was validated using real time quantitative reverse transcription PCR (qRT-PCR). While only two genes had increased expression at T₁ (P < 0.05), by T₂ 932 genes had increased (P < 0.05) and 562 genes had decreased expression (P < 0.05). Functional analysis of genes differentially expressed during the recovery phase (T₂) revealed an over-representation of genes localized to the actin cytoskeleton and with functions in the MAPK signalling, focal adhesion, insulin signalling, mTOR signaling, p53 signaling and Type II diabetes mellitus pathways. At T₁, using a less stringent statistical approach, we observed an over-representation of genes involved in the stress response, metabolism and intracellular signaling. These findings suggest that protein synthesis, mechanosensation and muscle remodeling contribute to skeletal muscle adaptation towards improved integrity and hypertrophy.

Conclusions

This is the first study to characterize global mRNA expression profiles in equine skeletal muscle using an equine-specific microarray platform. Here we reveal novel genes and mechanisms that are temporally expressed following exercise providing new knowledge about the early and late molecular responses to exercise in the equine skeletal muscle transcriptome.

Protective effects of exercise preconditioning on hindlimb unloading-induced atrophy of rat soleus muscle

AIM

A chronic decrease in the activation and loading levels of skeletal muscles as occurs with hindlimb unloading (HU) results in a number of detrimental changes. Several proteolytic pathways are involved with an increase in myofibrillar protein degradation associated with HU. Exercise can be used to counter this increase in proteolytic activity and, thus, may be able to protect against some of the detrimental changes associated with chronic decreased use. The purpose of the present study was to determine the potential of a single bout of preconditioning endurance exercise in attenuating the effects of 2 weeks of HU on the mass, phenotype and force-related properties of the soleus muscle in adult rats.

METHODS

Male Wistar rats were subjected to HU for 2 weeks. One half of the rats performed a single bout of treadmill exercise for 25 min immediately prior to the 2 weeks of HU.

RESULTS

Soleus mass, maximum tetanic tension, myofibrillar protein content, fatigue resistance and percentage of type I (slow) myosin heavy chain were decreased in HU rats. In addition, markers for the cathepsin, calpain, caspase and ATP-ubiquitin-proteasome proteolytic pathways were increased. The preconditioning endurance exercise bout attenuated all of the detrimental changes associated with HU, and increased HSP72 mRNA expression and protein levels.

CONCLUSION

These findings indicate that exercise preconditioning may be an effective countermeasure to the detrimental effects of chronic decreases in activation and loading levels on skeletal muscles and that an elevation in HSP72 may be one of the mechanisms associated with these responses.

Transcriptional and post-transcriptional regulation of mitochondrial biogenesis in skeletal muscle: effects of exercise and aging

Acute contractile activity of skeletal muscle initiates the activation of signaling kinases. This promotes the phosphorylation of transcription factors, leading to enhanced DNA binding and transcriptional activation and/or repression. The mRNA products of nuclear genes encoding mitochondrial proteins are translated in the cytosol and imported into pre-existing mitochondria. When contractile activity is repeated, the recapitulation of these cellular events progressively leads to an expansion of the mitochondrial reticulum within muscle. This has physiologically relevant health benefit, including enhanced lipid metabolism and reduced muscle fatigability. In aging skeletal muscle, the response to contractile activity appears to be attenuated, suggesting that a greater contractile stimulus is required to attain a similar phenotype adaptation. This review summarizes our current understanding of the effects of exercise on the gene expression pathway leading to organelle biogenesis in muscle.

Thermal preconditioning prevents fiber type transformation of the unloading induced-atrophied muscle in rats

Muscle atrophy is accompanied by a slow-to-fast transformation of the slow muscle, e.g., the soleus muscle, which is characterized by a decrease in the expression of the slow myosin heavy chain (MyHC) isoform. Heat stress before hindlimb unloading, i.e., thermal preconditioning, has been shown to reduce the rate of disuse-induced muscle atrophy. The present study examined whether thermal preconditioning could prevent a slow-to-fast transformation of the MyHC isoform through the induction of heat-shock protein (HSP) 72. Thermally preconditioned rats (Heat + HU) were individually placed in an environmentally controlled heat chamber for 1 h before hindlimb unloading for 2 weeks (HU). Although the mean fiber cross-sectional areas of the soleus muscle decreased in the HU and Heat + HU group, the loss of myofibrillar protein was attenuated in the Heat + HU group. Furthermore, a slow-to-fast transformation of MyHC isoform was inhibited in the Heat + HU group with the overexpression of HSP72. These results indicate that thermal preconditioning before hindlimb unloading attenuates the decrease of the slow MyHC isoform in the soleus muscle. Therefore, thermal preconditioning provides a new approach to prevent disuse-induced fiber type transformation of skeletal muscle.

Effects of ovarian sex hormones and downhill running on fiber-type-specific HSP70 expression in rat soleus

This study examined the influence of the ovarian sex hormones, estrogen and progesterone, on the fiber-type-specific response of the inducible 70-kDa heat shock protein (HSP70) to damaging exercise in rat soleus. Ovariectomized female rats were divided into three treatment groups (n = 16 per group): sham (S), progesterone (P; 25 mg pellet), and estrogen (E; 0.25 mg pellet). Each treatment group was divided into control and exercised groups. After 8 days of sham or hormone treatment, animals ran downhill intermittently for 90 min (17 m/min, -13.5 degrees grade) on a treadmill, and soleus muscles were removed 24 h postexercise. HSP70 expression was assessed in whole muscle homogenates by Western blotting and in individual muscle fiber types by immunohistochemical analysis of serial cross sections of soleus samples. Comparisons between control groups showed that HSP70 expression in soleus was increased (P < 0.05) in E compared with both S and P. No difference (P > 0.05) was observed between S and P. Following downhill running, HSP70 content in soleus was increased (P < 0.05) compared with control in S and P, but not (P > 0.05) in E. As a result, soleus HSP70 content following downhill running was not different (P > 0.05) between any of the treatment groups. Under all conditions, HSP70 content was higher in type I vs. type II fibers, and the effects of both estrogen and exercise on HSP70 expression in soleus were also more pronounced in type I vs. type II fibers. These results demonstrate that 1) estrogen regulates HSP70 expression in skeletal muscle, increasing basal HSP70 expression and preventing further increases in HSP70 in response to exercise; 2) progesterone is not involved in the regulation of HSP70 expression in skeletal muscle; and 3) the effects of estrogen and exercise on HSP70 expression in skeletal muscle are fiber type specific.

The exercise-induced stress response in skeletal muscle: failure during aging

Mammalian adult skeletal muscle adapts to the stress of contractile activity with increased gene expression by yielding a family of highly conserved cytoprotective proteins known as heat shock proteins (HSPs). Although the exercise-induced stress response of both animal and human skeletal muscle is now well documented, the precise mechanisms underlying this adaptation remain unclear. The induction of HSPs after exercise is severely blunted in the muscle of older individuals. This review focuses on the effects of different forms of exercise and training on the induction of HSPs in the muscles of adult individuals, and examines the proposed mechanisms underlying this adaptation. Furthermore, the functional effect of the inability of the muscles of older individuals to adapt in this way is discussed, together with the proposed mechanisms underlying this maladaptation.

Cellular distribution of Hsp70 expression in rat skeletal muscles. Effects of moderate exercise training and chronic hypoxia

Rat hindlimb muscles constitutively express the inducible heat shock protein 72 (Hsp70), apparently in proportion to the slow myosin content. Since it remains controversial whether chronic Hsp70 expression reflects the overimposed stress, we investigated Hsp70 cellular distribution in fast muscles of the posterior rat hindlimb after (1) mild exercise training (up to 30 m/min treadmill run for 1 h/day), which induces a remodeling in fast fiber composition, or (2) prolonged exposure to normobaric hypoxia (10%O(2)), which does not affect fiber-type composition. Both conditions increased significantly protein Hsp70 levels in the skeletal muscle. Immunohistochemistry showed the labeling for Hsp70 in subsets of both slow/type 1 and fast/type 2A myofibers of control, sedentary, and normoxic rats. Endurance training increased about threefold the percentage of Hsp70-positive myofibers (P < 0.001), and changed the distribution of Hsp70 immunoreactivity, which involved a larger subset of both type 2A and intermediate type 2A/2X myofibers (P < 0.001) and vascular smooth muscle cells. Hypoxia induced Hsp70 immunoreactivity in smooth muscle cells of veins and did not increase the percentage of Hsp70-positive myofibers; however, sustained exposure to hypoxia affected the distribution of Hsp70 immunoreactivity, which appeared detectable in a very small subset of type 2A fibers, whereas it concentrated in type 1 myofibers (P < 0.05) together with the labeling for heme-oxygenase isoform 1, a marker of oxidative stress. Therefore, the chronic induction of Hsp70 expression in rat skeletal muscles is not obligatory related to the slow fiber phenotype but reveals the occurrence of a stress response.

Elevation in heat shock protein 72 mRNA following contractions in isolated single skeletal muscle fibers

The purpose of the present study was 1) to develop a stable model for measuring contraction-induced elevations in mRNA in single skeletal muscle fibers and 2) to utilize this model to investigate the response of heat shock protein 72 (HSP72) mRNA following an acute bout of fatiguing contractions. Living, intact skeletal muscle fibers were microdissected from lumbrical muscle of Xenopus laevis and either electrically stimulated for 15 min of tetanic contractions (EX; n=26) or not stimulated to contract (REST; n=14). The relative mean developed tension of EX fibers decreased to 29+/-7% of initial peak tension at the stimulation end point. Following treatment, individual fibers were allowed to recover for 1 (n=9), 2 (n=8), or 4 h (n=9) prior to isolation of total cellular mRNA. HSP72, HSP60, and cardiac alpha-actin mRNA content were then assessed in individual fibers using quantitative PCR detection. Relative HSP72 mRNA content was significantly (P<0.05) elevated at the 2-h postcontraction time point relative to REST fibers when normalized to either HSP60 (18.5+/-7.5-fold) or cardiac alpha-actin (14.7+/-4.3-fold), although not at the 1- or 4-h time points. These data indicate that 1) extraction of RNA followed by relative quantification of mRNA of select genes in isolated single skeletal muscle fibers can be reliably performed, 2) HSP60 and cardiac alpha-actin are suitable endogenous normalizing genes in skeletal muscle following contractions, and 3) a significantly elevated content of HSP72 mRNA is detectable in skeletal muscle 2 h after a single bout of fatiguing contractions, despite minimal temperature changes and without influence from extracellular sources.

Heat shock protein accumulation and heat shock transcription factor activation in rat skeletal muscle during compensatory hypertrophy

AIM

To assess the stress/heat shock protein (HSP) and heat shock factor activation response in overloaded (hypertrophied) plantaris muscles.

METHODS

Male Sprague-Dawley rats (n = 5 per time point) underwent unilateral removal of the left gastrocnemius muscle. After 1, 2, 3, 5, 7, 14 and 28 days, plantaris muscles were removed, weighted rapidly frozen in liquid nitrogen. Total protein content was determined and HSP 25 and HSP 72 contents were assessed by Western blotting. Heat shock transcription factor (HSF) activation was assessed by electrophoretic mobility shift assay (EMSA).

RESULTS

While plantaris muscle mass was significantly increased 3 days after the imposition of overload and remained elevated thereafter confirming muscle hypertrophy, muscle protein content was not increased until 7 days after the imposition of overload. HSP 72 content was significantly increased at 3 days, while HSP 25 content was not significantly increased until 7 days after synergistic muscle removal. HSF activation was detected at 1, 2 and 3 days of overload but undetectable thereafter. The addition of HSF1- and HSF2-specific antibodies to extracts prior to EMSA failed to supershift the HSF-heat shock element complex.

CONCLUSION

The temporal pattern of both HSF activation and HSP expression in skeletal muscle undergoing hypertrophy suggests the increased level of the observed HSPs may be both a consequence of both the immediate stress of overload and the hypertrophic process. The inability of HSF1- and HSF2-specific antibodies to cause supershifts suggests the HSF detected during overload may not be HSF1 or HSF2.

Differentially transcribed genes in skeletal muscle of Duroc and Taoyuan pigs

The objective of this study was to compare gene transcription profiles of LM between two pig breeds, Duroc and Taoyuan, which display dramatically different postnatal muscle growth. We isolated LM from neonatal pigs, and the Duroc muscle length and mass were greater (P < 0.01) than for Taoyuan pigs; however, insignificant differences in the muscle fiber area and the percentage of fiber types were found. A human high-density complementary DNA (cDNA) microarray consisting of 9,182 probes was used to compare gene transcription profiles of LM between the two breeds. The results showed that the transcription level of 73 genes and 44 genes in Duroc LM were upregulated and down-regulated by at least 1.75-fold (P < 0.05) compared with Taoyuan, respectively. The strongly upregulated genes in Duroc pigs included those encoding the complex of myofibrillar proteins (e.g., myosin light and heavy chains, and troponin), ribosomal proteins, transcription regulatory proteins (e.g., skeletal muscle LIM protein 1 [SLIM1] and high-mobility group proteins), and energy metabolic enzymes (e.g., electron-transferring flavo-protein dehydrogenase, NADH dehydrogenase, malate dehydrogenase, and ATP synthases). The highly transcribed genes that encode energy metabolic enzymes indicate a more glycolytic metabolism in Duroc LM, thereby favoring carbohydrates rather than lipids for use as energy substrates in this tissue. The over-transcribed genes that encode skeletal muscle-predominant proteins or transcription regulators that control myogenesis and/or muscle growth suggest a general mechanism for the observed higher rate of postnatal muscle growth in Duroc pigs. The transcription of one such gene, SLIM1, was more highly transcribed (P < 0.01) in Duroc LM at birth and at postnatal d 7 than in Taoyuan. The transcription of SLIM1 increased (P < 0.05) in Duroc LM from neonate through 7 d of age, whereas its transcription remained essentially constant in Taoyuan during this period. These results suggest that SLIM1 may be useful for the development of markers associated with the postnatal muscle growth of pigs.

Muscle fiber type-specific response of Hsp70 expression in human quadriceps following acute isometric exercise

To investigate the time course of fiber type-specific heat shock protein 70 (Hsp70) expression in human skeletal muscle after acute exercise, 10 untrained male volunteers performed single-legged isometric knee extensor exercise at 60% of their maximal voluntary contraction (MVC) with a 50% duty cycle (5-s contraction and 5-s relaxation) for 30 min. Muscle biopsies were collected from the vastus lateralis before (Pre) exercise in the rested control leg (C) and immediately after exercise (Post) in the exercised leg (E) only and on recovery days 1 (R1), 2 (R2), 3 (R3), and 6 (R6) from both legs. As demonstrated by Western blot analysis, whole muscle Hsp70 content was unchanged (P > 0.05) immediately after exercise (Pre vs. Post), was increased (P < 0.05) by approximately 43% at R1, and remained elevated throughout the entire recovery period in E only. Hsp70 expression was also assessed in individual muscle fiber types I, IIA, and IIAX/IIX by immunohistochemistry. There were no fiber type differences (P > 0.05) in basal Hsp70 expression. Immediately after exercise, Hsp70 expression was increased (P < 0.05) in type I fibers by approximately 87% but was unchanged (P > 0.05) in type II fibers (Pre vs. Post). At R1 and throughout recovery, Hsp70 content in E was increased above basal levels (P < 0.05) in all fiber types, but Hsp70 expression was always highest (P < 0.05) in type I fibers. Hsp70 content in C was not different from Pre at any time throughout recovery. Glycogen depletion was observed at Post in all type II, but not type I, fibers, suggesting that the fiber type differences in exercise-induced Hsp70 expression were not related to glycogen availability. These results demonstrate that the time course of exercise-induced Hsp70 expression in human skeletal muscle is fiber type specific.

Elevated core and muscle temperature to levels comparable to exercise do not increase heat shock protein content of skeletal muscle of physically active men

AIM

Exercise-associated hyperthermia is routinely cited as the signal responsible for inducing an increased production of heat shock proteins (HSPs) following exercise. This hypothesis, however, has not been tested in human skeletal muscle. The aim of the present study was to therefore investigate the role of increased muscle and core temperature in contributing to the exercise-induced production of the major HSP families in human skeletal muscle.

METHODS

Seven physically active males underwent a passive heating protocol of 1 h duration during which the temperature of the core and vastus lateralis muscle were increased to similar levels to those typically occurring during moderately demanding aerobic exercise protocols. One limb was immersed in a tank containing water maintained at approximately 45 degrees C whilst the contra-lateral limb remained outside the tank and was not exposed to heat stress. Muscle biopsies were obtained from the vastus lateralis of both legs immediately prior to and at 48 h and 7 days post-heating.

RESULTS

The heating protocol induced significant increases (P < 0.05) in rectal (1.5 +/- 0.2 degrees C) and muscle temperature of the heated leg (3.6 +/- 0.5 degrees C). Muscle temperature of the non-heated limb showed no significant change (P > 0.05) following heating (pre: 36.1 +/- 0.5, post: 35.7 +/- 0.2 degrees C). Heating failed to induce a significant increase (P > 0.05) in muscle content of HSP70, HSC70, HSP60, HSP27, alphaB-crystallin, MnSOD protein content or in the activity of superoxide dismutase and catalase.

CONCLUSIONS

These data demonstrate that increases in both systemic and local muscle temperature per se do not appear to mediate the exercise-induced production of HSPs in human skeletal muscle and suggest that non-heat stress factors associated with contractile activity are of more importance in mediating this response.

Endurance training modulates the muscular transcriptome response to acute exercise

We hypothesized that in untrained individuals (n=6) a single bout of ergometer endurance exercise provokes a concerted response of muscle transcripts towards a slow-oxidative muscle phenotype over a 24-h period. We further hypothesized this response during recovery to be attenuated after six weeks of endurance training. We monitored the expression profile of 220 selected transcripts in muscle biopsies before as well as 1, 8, and 24 h after a 30-min near-maximal bout of exercise. The generalized gene response of untrained vastus lateralis muscle peaked after 8 h of recovery (P=0.001). It involved multiple transcripts of oxidative metabolism and glycolysis. Angiogenic and cell regulatory transcripts were transiently reduced after 1 h independent of the training state. In the trained state, the induction of most transcripts 8 h after exercise was less pronounced despite a moderately higher relative exercise intensity, partially because of increased steady-state mRNA concentration, and the level of metabolic and extracellular RNAs was reduced during recovery from exercise. Our data suggest that the general response of the transcriptome for regulatory and metabolic processes is different in the trained state. Thus, the response is specifically modified with repeated bouts of endurance exercise during which muscle adjustments are established.

Slower skeletal muscle phenotypes are critical for constitutive expression of Hsp70 in overloaded rat plantaris muscle

Heat shock protein 72 (Hsp70) is constitutively expressed in rat hindlimb muscles, reportedly in proportion to their content of type I myosin heavy chain. This distribution pattern has been suggested to result from the higher recruitment and activity of such muscles and/or a specific relationship between myosin phenotype and Hsp70 content. To differentiate between these possibilities, the fiber-specific distribution of Hsp70 was examined in male Sprague-Dawley rat plantaris under control conditions, following a fast-to-slow phenotypic shift in response to surgically induced overload (O) and in response to O when the phenotypic shift was prevented by 3,5,3'-triiodo-dl-thyronine administration. Constitutive expression of Hsp70 was restricted to type I and IIa fibers in plantaris from control rats, and this fiber-specific pattern of expression was maintained following O of up to 28 days, although Hsp70 content in the O muscle doubled. When O (for 40 days) of the plantaris was combined with 3,5,3'-triiodo-dl-thyronine administration, despite typical hypertrophy in the overloaded plantaris, prevention of the normal phenotypic transformation also blocked the increased expression of Hsp70 observed in euthyroid controls. Collectively, these data suggest that chronic changes in constitutive expression of Hsp70 with altered contractile activity appear critically dependent on fast-to-slow phenotypic remodeling.

Hindlimb unloading increases muscle content of cytosolic but not nuclear Id2 and p53 proteins in young adult and aged rats

This study tested the hypothesis that inhibitor of differentiation-2 (Id2), p53, and heat shock proteins (HSP) are responsive to suspension-induced muscle atrophy. Fourteen days of hindlimb suspension were used to unload the hindlimbs and induce atrophy in gastrocnemius muscles of young adult and aged rats. Following suspension, medial gastrocnemius muscle wet weight was reduced by approximately 30%, and the muscle wet weight normalized to the animal body weight decreased by 11 and 15% in young adult and aged animals, respectively. mRNA abundances of Id2, p53, HSP70-2, and HSP27 did not change with suspension, whereas HSP70-1 mRNA content was lower in the suspended muscle compared with the control muscle in both young adult and aged animals. Our immunoblot analyses indicated that protein expressions of HSP70 and HSP60 were not different between suspended and control muscles in both ages, whereas HSP27 protein content was increased in suspended muscle relative to control muscle only in young adult animals. Id2 and p53 protein contents were elevated in the cytosolic fraction of suspended muscle compared with the control muscle in both young and aged animals, but these changes were not found in the nuclear protein fraction. Furthermore, compared with young adult, aged muscles had a lower HSP70-1 mRNA content but higher HSP70-2 mRNA content and protein contents of Id2, p53, HSP70, and HSP27. These findings are consistent with the hypothesis that Id2 and p53 are responsive to unloading-induced muscle atrophy. Moreover, our data indicate that aging is accompanied with altered abundances of HSP70-1 and HSP70-2 mRNA, in addition to Id2, p53, HSP70, and HSP27 protein in rat gastrocnemius muscle.

Analysis of global mRNA expression in human skeletal muscle during recovery from endurance exercise

To search for novel transcriptional pathways that are activated in skeletal muscle after endurance exercise, we used cDNA microarrays to measure global mRNA expression after an exhaustive bout of high-intensity cycling (approximately 75 min). Healthy, young, sedentary males performed the cycling bout, and skeletal muscle biopsies were taken from the vastus lateralis before, and at 3 and 48 h after exercise. We examined mRNA expression in individual muscle samples from four subjects using cDNA microarrays, used repeated-measures significance analysis of microarray (SAM) to determine statistically significant expression changes, and confirmed selected results using real-time RT-PCR. In total, the expression of 118 genes significantly increased 3 h postcycling and 8 decreased. At 48 h, the expression of 29 genes significantly increased and 5 decreased. Many of these are potentially important novel genes involved in exercise recovery and adaptation, including several involved in 1) metabolism and mitochondrial biogenesis (FOXO1, PPARdelta, PPARgamma, nuclear receptor binding protein 2, IL-6 receptor, ribosomal protein L2, aminolevulinate delta-synthase 2); 2) the oxidant stress response (metalothioneins 1B, 1F, 1G, 1H, 1L, 2A, 3, interferon regulatory factor 1); and 3) electrolyte transport across membranes [Na+-K+-ATPase (beta3), SERCA3, chloride channel 4]. Others include genes involved in cell stress, proteolysis, apoptosis, growth, differentiation, and transcriptional activation, as well as all three nuclear receptor subfamily 4A family members (Nur77, Nurr1, and Nor1). This study is the first to characterize global mRNA expression during recovery from endurance exercise, and the results provide potential insight into 1) the transcriptional contributions to homeostatic recovery in human skeletal muscle after endurance exercise, and 2) the transcriptional contributions from a single bout of endurance exercise to the adaptive processes that occur after a period of endurance exercise training.

Effects of alpha-AMPK knockout on exercise-induced gene activation in mouse skeletal muscle

We tested the hypothesis that 5'AMP-activated protein kinase (AMPK) plays an important role in regulating the acute, exercise-induced activation of metabolic genes in skeletal muscle, which were dissected from whole-body alpha2- and alpha1-AMPK knockout (KO) and wild-type (WT) mice at rest, after treadmill running (90 min), and in recovery. Running increased alpha1-AMPK kinase activity, phosphorylation (P) of AMPK, and acetyl-CoA carboxylase (ACC)beta in alpha2-WT and alpha2-KO muscles and increased alpha2-AMPK kinase activity in alpha2-WT. In alpha2-KO muscles, AMPK-P and ACCbeta-P were markedly lower compared with alpha2-WT. However, in alpha1-WT and alpha1-KO muscles, AMPK-P and ACCbeta-P levels were identical at rest and increased similarly during exercise in the two genotypes. The alpha2-KO decreased peroxisome-proliferator-activated receptor gamma coactivator (PGC)-1alpha, uncoupling protein-3 (UCP3), and hexokinase II (HKII) transcription at rest but did not affect exercise-induced transcription. Exercise increased the mRNA content of PGC-1alpha, Forkhead box class O (FOXO)1, HKII, and pyruvate dehydrogenase kinase 4 (PDK4) similarly in alpha2-WT and alpha2-KO mice, whereas glucose transporter GLUT 4, carnitine palmitoyltransferase 1 (CPTI), lipoprotein lipase, and UCP3 mRNA were unchanged by exercise in both genotypes. CPTI mRNA was lower in alpha2-KO muscles than in alpha2-WT muscles at all time-points. In alpha1-WT and alpha1-KO muscles, running increased the mRNA content of PGC-1alpha and FOXO1 similarly. The alpha2-KO was associated with lower muscle adenosine 5'-triphosphate content, and the inosine monophosphate content increased substantially at the end of exercise only in alpha2-KO muscles. In addition, subcutaneous injection of 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside (AICAR) increased the mRNA content of PGC-1alpha, HKII, FOXO1, PDK4, and UCP3, and alpha2-KO abolished the AICAR-induced increases in PGC-1alpha and HKII mRNA. In conclusion, KO of the alpha2- but not the alpha1-AMPK isoform markedly diminished AMPK activation during running. Nevertheless, exercise-induced activation of the investigated genes in mouse skeletal muscle was not impaired in alpha1- or alpha2-AMPK KO muscles. Although it cannot be ruled out that activation of the remaining alpha-isoform is sufficient to increase gene activation during exercise, the present data do not support an essential role of AMPK in regulating exercise-induced gene activation in skeletal muscle.

Mitochondrial assembly: protein import

The protein import process of mitochondria is vital for the assembly of the hundreds of nuclear-derived proteins into an expanding organelle reticulum. Most of our knowledge of this complex multisubunit network comes from studies of yeast and fungal systems, with little information known about the protein import process in mammalian cells, particularly skeletal muscle. However, growing evidence indicates that the protein import machinery can respond to changes in the energy status of the cell. In particular, contractile activity, a powerful inducer of mitochondrial biogenesis, has been shown to alter the stoichiometry of the protein import apparatus via changes in several protein import machinery components. These adaptations include the induction of cytosolic molecular chaperones that transport precursors to the matrix, the up-regulation of outer membrane import receptors, and the increase in matrix chaperonins that facilitate the import and proper folding of the protein for subsequent compartmentation in the matrix or inner membrane. The physiological importance of these changes is an increased capacity for import into the organelle at any given precursor concentration. Defects in the protein import machinery components have been associated with mitochondrial disorders. Thus, contractile activity may serve as a possible mechanism for up-regulation of mitochondrial protein import and compensation for mitochondrial phenotype alterations observed in diseased muscle.

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.

Constitutive expression of inducible Hsp70 is linked to natural shifts in skeletal muscle phenotype

AIM

Constitutive expression of heat shock protein 70 (Hsp70) is elevated in frequently recruited, metabolically efficient rodent striated muscle. We aimed to assess the relative importance of muscle phenotype vs. increased contractile activity on this pattern of expression using the rat diaphragm, which undergoes a dramatic and sustained increase in recruitment with parturition and development.

METHODS

Diaphragms were collected from rats of various ages (20 day fetus, 1 and 3 days, and 1, 3, 6 and 12 weeks postpartum; PP), and assessed for changes in oxidative capacity, Hsp70 and Type I myosin heavy chain (MHCI) (used as a marker of muscle phenotype changes).

RESULTS

Oxidative capacity of the diaphragm (as indicated by citrate synthase activity) and whole body growth rate (% increase in body weight per week), factors thought to require chaperone activity, increased rapidly, peaked at 3-6 weeks PP and declined late in development. In contrast, at 1 week PP, increased contractile activity in the diaphragm had not altered the expression of Hsp70 protein or mRNA from fetal levels. Significant increases in Hsp70 were not observed until between 1 and 3 weeks, achieving their highest levels at 12 weeks PP. Both MHC I protein (r = 0.69, P = 0.001) and mRNA (r = 0.76, P = 0.001) were significantly correlated with their Hsp70 counterparts.

CONCLUSIONS

Expression of Hsp70 in the developing diaphragm represents an adaptation associated with a shift towards a slower, more metabolically efficient adult phenotype rather than simply a response to contractile stress.

Skeletal muscle HSP72 response to mechanical unloading: influence of endurance training

AIMS

It has been shown that increased contractile activity results in heat shock protein 72 (HSP72) accumulation in various skeletal muscles. By contrast, there is no consensus for muscle HSP72 response to muscle disuse for short duration (5-8 days). On the basis of a greater constitutive HSP72 expression in slow-twitch muscles we tested the hypothesis that mechanical unloading for a longer period (2 weeks) would affect this phenotype to a greater extent. Secondly, we evaluated the effects of a physiological muscle heat shock protein (HSP) enhancer (endurance training) on HSP response to unloading and muscle remodelling.

METHODS

Adult male Wistar rats were assigned randomly to four groups: (1) sedentary weight-bearing; (2) hindlimb-unloaded (HU) via tail suspension for 2 week; (3) trained on a treadmill (6 week) and (4) trained 6 week and then HU for 2 week.

RESULTS

Unloading resulted in a preferential atrophy of slow muscles [soleus (SOL), adductor longus (AL)] and a slow-to-fast fibre transition with no change in HSP72 level. HSP72 levels were significantly lower in fast muscles [extensor digitorum longus (EDL) and plantaris (PLA)], and did not change with mechanical unloading. Endurance training was accompanied by a small (SOL) or a large (EDL, PLA) increase in HSP72 level with no change in AL. Training-induced accumulation of HSP72 disappeared with subsequent unloading in the SOL and PLA whereas HSP72 content remained elevated in EDL.

CONCLUSION

The results of this study indicate that (1) after 2 weeks of unloading no change occurred in HSP72 protein levels of slow-twitch muscles despite a slow-to-fast fibre transition; and (2) the training-induced increase of HSP72 content in skeletal muscles did not attenuate fibre transition.

Effects of voluntary exercise and genetic selection for high activity levels on HSP72 expression in house mice

We studied expression of heat shock protein 72 (HSP72) in female mice from four replicate lines that had been selectively bred for high voluntary wheel running (S) and from four random-bred control lines (C). Mice from generation 23 were sampled after 6 days of wheel access, and those from generation 14 were sampled after 8 wk of access to wheels either free to rotate or locked. Mice from S lines ran ∼2.6 times as many revolutions per day as did those from C lines. Western blotting of tissues from generation 23 mice indicated that S mice had elevated HSP72 expression in triceps surae muscle, but levels in spleen, kidney, heart, and lung were similar in S and C mice. HSP72 expression in triceps surae from generation 14 mice was measured by ELISA and analyzed with a two-way analysis of covariance. The interaction between wheel type and line type (S vs. C) was statistically significant, and subsequent analyses indicated that S mice had significantly elevated HSP72 expression only when housed with free wheels. Mice with the previously described mini-muscle phenotype (Houle-Leroy P, Guderley H, Swallow JG, and Garland T Jr. Am J Physiol Regul Integr Comp Physiol 284: R433-R443, 2003) occurred in both generations and had elevated HSP72 expression in triceps surae. For the generation 23 sample, wheel running as a covariate had a significant negative association with HSP72 expression, and the effect of line type was still statistically significant. Therefore, the increased HSP72 expression of S mice is not a simple proximate effect of their increased wheel running.

Differential responses of HSPs to heat stress in slow and fast regions of rat gastrocnemius muscle

In a recent study, we showed that the rat slow soleus and fast plantaris muscles exhibited different time courses for the response of specific heat shock proteins (HSPs) after 1 h of heat stress. We hypothesized that these differential responses were related, in part, to the varying fiber type composition of these muscles. To further test this hypothesis, we now have determined the responses of Hsp60, Hsp72, and Hsc73 during the 60 h following exposure to a single bout of heat stress in the deep (relatively high percentage of slow fibers) and superficial regions (only fast fibers) of the adult rat gastrocnemius muscle. The temperature of the musculature in the left hindlimb was elevated to approximately 42 degrees C for 1 h, while the right hindlimb served as a control. Two hours after the heat stress, the Hsp60 levels were increased by 1.3- and 2.0-fold in the deep and superficial regions, respectively. The Hsp72 levels were increased (1.8-fold) in the deep region at 8 h after heat stress, whereas in the superficial region these levels were increased between 4 and 48 h (peak at 36 h by 10-fold) after the heat stress. No changes were observed for Hsc73 in either region of the muscle. Combined with our previous data, the results indicate that the responses of HSPs in the rat hindlimb muscles after a single exposure to heat stress are related to fiber type composition of the muscle or muscle region or to the inherent properties of each HSP. From a clinical viewpoint, these data indicate that specific regions (most likely based on fiber type composition) within a muscle may be affected differentially by any intervention inducing HSPs.

Endogenous expression and developmental changes of HSP72 in rat skeletal muscles

The purpose of the present study was to determine whether endogenous factor(s) contributes to the expression of heat shock proteins (HSPs) during the early developmental stages of rat skeletal muscles. HSP72 was expressed in both the soleus and plantaris muscles at embryonic day 22 (E22). On the basis of myosin heavy chain (MHC) immunohistochemistry, HSP72 was specifically expressed in slow type I fibers in both muscles. These slow fibers were observed throughout the entire cross section of the soleus muscle and only in the deep region (close to the bone) of the plantaris muscle. These results indicate that the expression of HSP72 is related to endogenous factors associated with type I fibers, because E22 rats have minimal exogenous influences and the soleus and plantaris muscles of E22 rats have similar metabolic and contractile profiles at this stage of development. We then examined the changes in HSP72 and heat shock cognate (HSC) 73 in the same two muscles from E22 to postnatal day 56 via Western blotting. The level of HSP72 in the soleus muscle gradually increased in parallel with the increment in the type I MHC isoform. Compared with the soleus, only a small amount of HSP72 could be detected in the plantaris muscle throughout the developmental period. For both muscles, HSC73 reached levels observed in adult muscles at postnatal day 3, and these levels were unchanged thereafter. These results indicate that the expression of HSP72, but not HSC73, is influenced by both endogenous and exogenous factors during the embryonic and early developmental periods.

Effects of training status on fibers of the musculus vastus lateralis in professional road cyclists

OBJECTIVE

To evaluate possible changes occurring in muscle fibers related to the training status of professional road cyclists.

DESIGN

A comparative study was performed on two groups of male road cyclists: ten 21-yr-old cyclists with a history of 3 yr of sport competition (RC21) and ten 25-yr-old cyclists with a history of 7 yr of competition (RC25). The control group was formed by two subgroups of five nontrained, sedentary volunteers who were matched for age with the study subjects (NT21 and NT25). Biopsies of the vastus lateralis muscle were obtained to determine the fiber variables: percentage, cross-sectional area, mitochondrial volume, and capillary density.

RESULTS

Control group variables were within the normal range. According to their training status, cyclists showed an increased percentage of type I and IIC fibers (RC25 > RC21 > NT) and decreased percentage of type IIA (RC25 < RC21 < NT) and IIB fibers (RC25 = RC21 < NT), an increased cross-sectional area of all fiber types after 3 yr of training (RC25 = RC21 > NT) except IIB fibers (RC25 > RC21), an increased mitochondrial volume in all fiber types (RC25 > RC21 > NT) except type IIA fibers (RC25 > RC21 = NT21), and an increased capillary density (RC25 > RC21 > NT).

CONCLUSIONS

Findings indicate a progressive increase in the muscle fiber types that are mostly implicated in endurance sports, accompanied by phenotypic changes in the fiber population of lesser participation.

Overexpression of heat shock protein 60/10 in myocardium of patients with chronic atrial fibrillation

BACKGROUND

Cardiomyocytes respond to chronic atrial fibrillation with increased expression of heat shock protein 60 (HSP60). The aim of this study was to investigate whether expression of the coprotein HSP10 is also increased.

METHODS

Right atrial samples from 16 patients undergoing elective cardiac operation were excised and immediately frozen in liquid nitrogen. Eight patients had chronic atrial fibrillation and 8 patients were in sinus rhythm. The HSP60 and HSP10 protein levels were determined by SDS-PAGE, Western blot, and quantified by optical densitometry according to the immunoreactive bands of actin.

RESULTS

In myocardial samples from patients with chronic atrial fibrillation we found simultaneous upregulation of both stress proteins. HSP60 expression was more than 2.3-fold and HSP10 expression was more than 2.4-fold increased in atrial myocardium of patients with chronic atrial fibrillation.

CONCLUSIONS

These results indicate functional upregulation of mitochondrial HSP60 and HSP10 in response to chronic atrial fibrillation.

Glucocorticoid treatment induces expression of small heat shock proteins in human satellite cell populations: consequences for a desmin-related myopathy involving the R120G alpha B-crystallin mutation

A missense mutation (R120G) of the molecular chaperone alpha B-crystallin has recently been linked to a familial form of desmin-related myopathy characterized by intrasarcoplasmic aggregates of desmin. It was previously demonstrated that the mutant R120G had a defective chaperone-like function. However, the cellular and physiopathological consequences of R120G mutant expression in human muscle cells are as yet unclear. Thus, we developed a cellular model for the study of this R120G alpha B-crystallin-related desmin-related myopathy. We demonstrate that dexamethasone enhances alpha B-crystallin and HSP27 expression in normal and desmin-related myopathy-derived muscle cells. In the undifferentiated desmin-related myopathy satellite cell population no intracytoplasmic aggregates were observed. However, in differentiated satellite cells derived from a related myopathy patient, we observed an enhanced plasma membrane localization of alpha B-crystallin following glucocorticoid. We also observed that the protective effect against stress of alpha B-crystallin is altered upon glucocorticoid-induced small heat shock protein expression for the desmin-related myopathy-derived cells.

Muscle type-specific response of HSP60, HSP72, and HSC73 during recovery after elevation of muscle temperature

An original method to induce heat stress was used to clarify the time course of changes in heat shock proteins (HSPs) in rat skeletal muscles during recovery after a single bout of heat stress. One hindlimb was inserted into a stainless steel can and directly heated by raising the air temperature inside the can via a flexible heater twisted around the steel can. Muscle temperature was increased gradually and maintained at 42 degrees C for 60 min. Core rectal and contralateral muscle temperatures were increased <1.5 degrees C during the heat stress. HSP60, HSP72, and heat shock cognate (HSC) 73 content in the slow soleus and fast plantaris in both limbs were determined immediately (0 h) and 2, 4, 8, 12, 24, 36, 48, or 60 h after heat stress. Within 0-4 h, all HSPs were approximately 1.5- to 2.2-fold higher in heat-stressed than contralateral soleus. Compared with the contralateral plantaris, the heat-stressed plantaris had a higher (1.5-fold) HSP60 content immediately and 2 h after heat stress and a higher (2.5- to 6.8-fold) HSP72 content between 24 and 48 h after heat stress. Plantaris HSC73 content was not affected by heat stress. This unique heat-stress method provides advantages over existing systems; muscle temperature can be controlled precisely during heating and the HSP response can be compared between muscles in heat-stressed and contralateral limbs of individual rats. Results show a differential response of HSPs in the soleus and plantaris during recovery after heat stress; soleus demonstrated a more rapid and broader HSP response to heat stress than plantaris.

Dynamics of stimulation-induced muscle adaptation: insights from varying the duty cycle

We sought to gain insight into the dynamics of the signalling process that initiates adaptive change in mammalian skeletal muscles in response to chronic neuromuscular stimulation. Programmable miniature stimulators were implanted into rabbits and used to impose one of the following patterns on the dorsiflexors of one ankle: 10 Hz delivered in equal on/off periods of 30 s, 30 min, or 12 h (all equivalent in terms of aggregate impulse activity to continuous 5 Hz). Two further groups received continuous stimulation at 5 Hz or 10 Hz. In every case the stimulation pattern was maintained continuously for 6 weeks. Tibialis anterior muscles stimulated intermittently with equal on/off periods of 30 s, 30 min and 12 h had contractile characteristics that were significantly slower than the contralateral, unstimulated muscles but did not differ from those of muscles stimulated continuously at 5 Hz. Muscles stimulated continuously at 10 Hz were significantly slower than either contralateral muscles or muscles stimulated with any of the other patterns. Corresponding changes were seen in myosin heavy chain isoform composition. The fatigue index, defined as the fraction of tension remaining after 5 min of a standard fatigue test, was 0.4 for muscles in the contralateral group but equal to or greater than 0.85 for muscles of all the stimulated groups. These results were interpreted with the help of a simple model of the growth and decay of a putative signalling substance based on first order kinetics. The model suggests a rate constant for the accumulation of the signalling substance that is greater than 30 h(-1), and a rate constant for its removal that is greater than 50 h(-1).

Heat shock protects L6 myotubes from catabolic effects of dexamethasone and prevents downregulation of NF-kappaB

Glucocorticoids are the most important mediator of muscle cachexia in various catabolic conditions. Recent studies suggest that the transcription factor NF-kappaB acts as a suppressor of genes in the ubiquitin-proteasome proteolytic pathway and that glucocorticoids increase muscle proteolysis by downregulating NF-kappaB activity. The heat shock (stress) response, characterized by the induction of heat shock proteins, confers a protective effect against a variety of harmful stimuli. In the present study, we tested the hypothesis that the heat shock response protects muscle cells from the catabolic effects of dexamethasone and prevents downregulation of NF-kappaB. Cultured L6 myotubes were subjected to heat shock (43 degrees C for 1 h) followed by recovery at 37 degrees C for 1 h. Thereafter, cells were treated for 6 h with 1 microM dexamethasone, during which period protein degradation was measured as release of TCA-soluble radioactivity from proteins that had been prelabeled with [(3)H]tyrosine. Heat shock resulted in increased protein and mRNA levels for heat shock protein 70. The increase in protein degradation induced by dexamethasone was prevented in cells expressing the heat shock response. In the same cells, dexamethasone-induced downregulation of NF-kappaB DNA binding activity was blocked. The present results suggest that the heat shock response may protect muscle cells from the catabolic effects of dexamethasone and that this effect of heat shock may be related to inhibited downregulation of NF-kappaB activity.