Overexpression of inducible 70-kDa heat shock protein in mouse improves structural and functional recovery of skeletal muscles from atrophy

Knockout of Hsp70 genes significantly affects locomotion speed and gene expression in leg skeletal muscles of Drosophila melanogaster

The functions of the heat shock protein 70 (Hsp70) genes were studied using a line of Drosophila melanogaster with a knockout of 6 of these genes out of 13. Namely, the effect of knockout of Hsp70 genes on negative geotaxis climbing (locomotor) speed and the ability to adapt to climbing training (0.5-1.5 h/day, 7 days/wk, 19 days) were examined. Seven- and 23-day-old Hsp70- flies demonstrated a comparable reduction (twofold) in locomotor speed and widespread changes in leg skeletal muscle transcriptome (RNA sequencing) compared with w1118 flies. To identify the functions of genes related to decreased locomotor speed, the overlapped differentially expressed genes at both time points were analyzed: the upregulated genes encoded extracellular proteins, regulators of drug metabolism, and the antioxidant response, whereas downregulated genes encoded regulators of carbohydrate metabolism and transmembrane proteins. In addition, in Hsp70- flies, activation of transcription factors related to disruption of the fibril structure and heat shock response (Hsf) was predicted, using the position weight matrix approach. In control flies, adaptation to chronic exercise training was associated mainly with gene response to a single exercise bout, whereas the predicted transcription factors were related to stress/immune (Hsf, NF-κB, etc.) and early gene response. In contrast, Hsp70- flies demonstrated no adaptation to training as well as a significantly impaired gene response to a single exercise bout. In conclusion, the knockout of Hsp70 genes not only reduced physical performance but also disrupted adaptation to chronic physical training, which is associated with changes in the leg skeletal muscle transcriptome and impaired gene response to a single exercise bout.NEW & NOTEWORTHY Knockout of six heat shock protein 70 (Hsp70) genes in Drosophila melanogaster reduced locomotion (climbing) speed that is associated with genotype-specific differences in leg skeletal muscle gene expression. Disrupted adaptation of Hsp70- flies to chronic exercise training is associated with impaired gene response to a single exercise bout.

Oral glutamine supplementation relieves muscle loss in immobilized rats, altering p38MAPK and FOXO3a signaling pathways

BACKGROUND

Skeletal muscle synthesizes, stores, and releases body L-glutamine (GLN). Muscle atrophy due to disabling diseases triggers the activation of proteolytic and pro-apoptotic cell signaling, thus impairing the body's capacity to manage GLN content. This situation has a poor therapeutic prognosis.

OBJECTIVE

Evaluating if oral GLN supplementation can attenuate muscle wasting mediated by elevated plasma cortisol and activation of caspase-3, p38MAPK, and FOXO3a signaling pathways in soleus and gastrocnemius muscles of rats submitted to 14-day bilateral hindlimbs immobilization.

METHODS

Animals were randomly distributed into six groups: non-immobilized rats (Control), control orally supplemented with GLN (1 g kg-1) in solution with L-alanine (ALA: 0.61 g kg-1; GLN+ALA), control orally supplemented with dipeptide L-alanyl-L-glutamine (DIP; 1.49 g kg-1), hindlimbs immobilized rats (IMOB), IMOB orally GLN+ALA supplemented (GLN+ALA-IMOB), and IMOB orally DIP supplemented (DIP-IMOB). Plasma and muscle GLN concentration, plasma cortisol level, muscle caspase-3 activity, muscle p38MAPK and FOXO3a protein content (total and phosphorylated forms), and muscle cross-sectional area (CSA) were measured.

RESULTS

Compared to controls, IMOB rats presented: a) increased plasma cortisol levels; b) decreased plasma and muscle GLN concentration; c) increased muscle caspase-3 activity; d) increased total and phosphorylated p38MAPK protein content; e) increased FOXO3a and decreased phosphorylated FOXO3a protein content; f) reduced muscle weight and CSA befitting to atrophy. Oral supplementation with GLN+ALA and DIP was able to significantly attenuate these effects.

CONCLUSIONS

These findings attest that oral GLN supplementation in GLN+ALA solution or DIP forms attenuates rats' skeletal muscle mass wasting caused by disuse-mediated muscle atrophy.

The Effect of Heat Shock on Myogenic Differentiation of Human Skeletal-Muscle-Derived Mesenchymal Stem/Stromal Cells

Muscle injuries, degenerative diseases and other lesions negatively affect functioning of human skeletomuscular system and thus quality of life. Therefore, the investigation of molecular mechanisms, stimulating myogenic differentiation of primary skeletal-muscle-derived mesenchymal stem/stromal cells (SM-MSCs), is actual and needed. The aim of the present study was to investigate the myogenic differentiation of CD56 (neural cell adhesion molecule, NCAM)-positive and -negative SM-MSCs and their response to the non-cytotoxic heat stimulus. The SM-MSCs were isolated from the post operation muscle tissue, sorted by flow cytometer according to the CD56 biomarker and morphology, surface profile, proliferation and myogenic differentiation has been investigated. Data show that CD56(+) cells were smaller in size, better proliferated and had significantly higher levels of CD146 (MCAM) and CD318 (CDCP1) compared with the CD56(-) cells. At control level, CD56(+) cells significantly more expressed myogenic differentiation markers MYOD1 and myogenin (MYOG) and better differentiated to the myogenic direction. The non-cytotoxic heat stimulus significantly stronger stimulated expression of myogenic markers in CD56(+) than in CD56(-) cells that correlated with the multinucleated cell formation. Data show that regenerative properties of CD56(+) SM-MSCs can be stimulated by an extracellular stimulus and be used as a promising skeletal muscle regenerating tool in vivo.

Opportunities and Challenges in Stem Cell Aging

Studying aging, as a physiological process that can cause various pathological phenotypes, has attracted lots of attention due to its increasing burden and prevalence. Therefore, understanding its mechanism to find novel therapeutic alternatives for age-related disorders such as neurodegenerative and cardiovascular diseases is essential. Stem cell senescence plays an important role in aging. In the context of the underlying pathways, mitochondrial dysfunction, epigenetic and genetic alterations, and other mechanisms have been studied and as a consequence, several rejuvenation strategies targeting these mechanisms like pharmaceutical interventions, genetic modification, and cellular reprogramming have been proposed. On the other hand, since stem cells have great potential for disease modeling, they have been useful for representing aging and its associated disorders. Accordingly, the main mechanisms of senescence in stem cells and promising ways of rejuvenation, along with some examples of stem cell models for aging are introduced and discussed. This review aims to prepare a comprehensive summary of the findings by focusing on the most recent ones to shine a light on this area of research.

Loss of melusin is a novel, neuronal NO synthase/FoxO3-independent master switch of unloading-induced muscle atrophy

BACKGROUND

Unloading/disuse induces skeletal muscle atrophy in bedridden patients and aged people, who cannot prevent it by means of exercise. Because interventions against known atrophy initiators, such as oxidative stress and neuronal NO synthase (nNOS) redistribution, are only partially effective, we investigated the involvement of melusin, a muscle-specific integrin-associated protein and a recognized regulator of protein kinases and mechanotransduction in cardiomyocytes.

METHODS

Muscle atrophy was induced in the rat soleus by tail suspension and in the human vastus lateralis by bed rest. Melusin expression was investigated at the protein and transcript level and after treatment of tail-suspended rats with atrophy initiator inhibitors. Myofiber size, sarcolemmal nNOS activity, FoxO3 myonuclear localization, and myofiber carbonylation of the unloaded rat soleus were studied after in vivo melusin replacement by cDNA electroporation, and muscle force, myofiber size, and atrogene expression after adeno-associated virus infection. In vivo interference of exogenous melusin with dominant-negative kinases and other atrophy attenuators (Grp94 cDNA; 7-nitroindazole) on size of unloaded rat myofibers was also explored.

RESULTS

Unloading/disuse reduced muscle melusin protein levels to about 50%, already after 6 h in the tail-suspended rat (P < 0.001), and to about 35% after 8 day bed rest in humans (P < 0.05). In the unloaded rat, melusin loss occurred despite of the maintenance of β1D integrin levels and was not abolished by treatments inhibiting mitochondrial oxidative stress, or nNOS activity and redistribution. Expression of exogenous melusin by cDNA transfection attenuated atrophy of 7 day unloaded rat myofibers (-31%), compared with controls (-48%, P = 0.001), without hampering the decrease in sarcolemmal nNOS activity and the increase in myonuclear FoxO3 and carbonylated myofibers. Infection with melusin-expressing adeno-associated virus ameliorated contractile properties of 7 day unloaded muscles (P ≤ 0.05) and relieved myofiber atrophy (-33%) by reducing Atrogin-1 and MurF-1 transcripts (P ≤ 0.002), despite of a two-fold increase in FoxO3 protein levels (P = 0.03). Atrophy attenuation by exogenous melusin did not result from rescue of Akt, ERK, or focal adhesion kinase activity, because it persisted after co-transfection with dominant-negative kinase forms (P < 0.01). Conversely, melusin cDNA transfection, combined with 7-nitroindazole treatment or with cDNA transfection of the nNOS-interacting chaperone Grp94, abolished 7 day unloaded myofiber atrophy.

CONCLUSIONS

Disuse/unloading-induced loss of melusin is an early event in muscle atrophy which occurs independently from mitochondrial oxidative stress, nNOS redistribution, and FoxO3 activation. Only preservation of melusin levels and sarcolemmal nNOS localization fully prevented muscle mass loss, demonstrating that both of them act as independent, but complementary, master switches of muscle disuse atrophy.

Increased Heat Shock Protein Expression Decreases Inflammation in Skeletal Muscle During and after Frostbite Injury

BACKGROUND

Frostbite injury results in serious skeletal muscle damage. The inflammatory response due to frostbite causes local muscle degeneration. Previous studies have shown that heat shock proteins (hsps) can protect against inflammation. In addition, our previous studies showed that increased expression of hsp70 is able to protect skeletal muscle against cryolesion.

METHODS

Therefore, our aim was to determine if the induction of the heat shock proteins are able to minimize inflammation and protect skeletal muscle against frostbite injury.

RESULTS

In the present study, we used the hsp90 inhibitor, 17-dimethylaminoethylamino- 17-demethoxygeldanamycin (17-DMAG), which was administered within 30 minutes following frostbite injury. Rat hind-limb muscles injected with 17-DMAG following frostbite injury exhibited less inflammatory cell infiltration as compared to control rat hind-limb muscles. In agreement with this observation, it has been observed that increased hsp expression resulted in decreased inflammatory cytokine expression. Additionally, we found that the administration of 17-DMAG after frostbite injury can preserve muscle tissue structure as well as function.

CONCLUSION

It has been concluded that compounds such as 17-DMAG that induce the heat shock proteins are able to preserve skeletal muscle function and structure if injected within 30 minutes after frostbite injury. Our studies provide the basis for the development of a potential therapeutic strategy to treat the injury caused by frostbite.

Age-related responses to a bout of mechanotherapy in skeletal muscle of rats

Cyclic compressive loading (CCL) is a massage mimetic that improves muscle regrowth from atrophy in adult rats. Therefore, we tested if a single bout of CCL increases anabolic signaling and protein synthesis in muscle during normal, weight-bearing conditions in gastrocnemius muscle from adult and aged rats. Male Brown Norway/F344 rats at 10 (adult) and 30 (aged) months of age were assigned control or CCL (receiving a single bout of CCL). Twenty-four hours following a single bout of CCL there was no change in protein synthesis, Akt, or GSK3β signaling at either age, despite adult rats having higher abundance and activation of mechanosensitive pathways (integrins and integrin-linked kinase). Murf1 was elevated in response to CCL in both age groups, potentially indicating muscle remodeling. Muscle from aged rats exhibited an increase in heat shock protein (HSP) 25 and HSP70 and in the cold shock protein RNA-binding motif 3 (RBM3), demonstrating a unique stress response to CCL in aged muscle only. Finally, muscle from aged rats exhibited higher basal protein synthesis that was corroborated by elevated eIF2Bε and rpS6 signaling, without an additional effect of CCL. In summary, a single bout of CCL does not have anabolic effects on skeletal muscle during normal, weight-bearing conditions, even though it has previously been shown to improve regrowth from atrophy. These data demonstrate that interventions that may help recover from atrophy do not necessarily induce muscle hypertrophy in unperturbed conditions.NEW & NOTEWORTHY Massage has been demonstrated to be an effective mechanotherapy to improve recovery from atrophy in adult skeletal muscle; however, this study shows that a single bout of massage fails to increase protein synthesis or anabolic signaling in adult or aged skeletal muscle during normal, weight-bearing conditions. Altogether, our data suggest massage is a useful mechanotherapy for preserving skeletal muscle when combined with other interventions but is not an anabolic stimulus on its own.

Heat shock response to exercise in pancreatic islets of obese mice

Chronic obesity imposes an organismal state of low-grade inflammation because the physiological resolution of inflammation is progressively repressed giving rise to cellular senescence and its accompanying Senescence-Associated Secretory Phenotype (SASP), which avoids apoptosis but perpetuates the relay of inflammatory signals from adipose tissue toward the rest of the body. Conversely, resolution of inflammation depends on the integrity of heat shock response (HSR) pathway that leads to the expression of cytoprotective and anti-inflammatory protein chaperones of the 70 kDa family (HSP70). However, chronic exposure to the aforementioned injuring factors leads to SASP, which, in turn, suppresses the HSR. A main metabolic tissue severely jeopardized by obesity-related dysfunctions is the endocrine pancreas, particularly β-cells of the islets of Langerhans. Because exercise is a powerful inducer of HSR and predicted to alleviate negative health outcomes of obesity, we sought whether obesity influence HSP70 expression in pancreatic islets and other metabolic tissues (adipose tissue and skeletal muscle) of adult B6.129SF2/J mice fed on a high-fat diet (HFD) for 13 weeks since the weaning and whether acute exercise as well as moderate-intensity exercise training (8 weeks) could interfere with this scenario. We showed that acute exercise of moderate intensity protects pancreatic islets against cytokine-induced cell death. In addition, acute exercise challenge time-dependently increased islet HSP70 that peaked at 12 h post-exercise in both trained and untrained mice fed on a control diet, suggesting an adequate HSR to exercise training. Unexpectedly, however, neither exercise training nor acute exercise challenges were able to increase islet HSP70 contents in trained mice submitted to HFD, but only in untrained HFD animals. In parallel, HFD disrupted glycemic status which is accompanied by loss of muscular mass resembling sarcopenic obesity that could not be rescued by exercise training. These results suggest that exercise influences HSR in pancreatic islets but obesity undermines islet, muscle and adipose tissue HSR, which is associated with metabolic abnormalities observed in such tissues.

Effects of exercise preconditioning and HSP72 on diaphragm muscle function during mechanical ventilation

BACKGROUND

Mechanical ventilation (MV) is a life-saving measure for patients in respiratory failure. However, prolonged MV results in significant diaphragm atrophy and contractile dysfunction, a condition referred to as ventilator-induced diaphragm dysfunction (VIDD). While there are currently no clinically approved countermeasures to prevent VIDD, increased expression of heat shock protein 72 (HSP72) has been demonstrated to attenuate inactivity-induced muscle wasting. HSP72 elicits cytoprotection via inhibition of NF-κB and FoxO transcriptional activity, which contribute to VIDD. In addition, exercise-induced prevention of VIDD is characterized by an increase in the concentration of HSP72 in the diaphragm. Therefore, we tested the hypothesis that increased HSP72 expression is required for the exercise-induced prevention of VIDD. We also determined whether increasing the abundance of HSP72 in the diaphragm, independent of exercise, is sufficient to prevent VIDD.

METHODS

Cause and effect was determined by inhibiting the endurance exercise-induced increase in HSP72 in the diaphragm of exercise trained animals exposed to prolonged MV via administration of an antisense oligonucleotide targeting HSP72. Additional experiments were performed to determine if increasing HSP72 in the diaphragm via genetic (rAAV-HSP72) or pharmacological (BGP-15) overexpression is sufficient to prevent VIDD.

RESULTS

Our results demonstrate that the exercise-induced increase in HSP72 protein abundance is required for the protective effects of exercise against VIDD. Moreover, both rAAV-HSP72 and BGP-15-induced overexpression of HSP72 were sufficient to prevent VIDD. In addition, modification of HSP72 in the diaphragm is inversely related to the expression of NF-κB and FoxO target genes.

CONCLUSIONS

HSP72 overexpression in the diaphragm is an effective intervention to prevent MV-induced oxidative stress and the transcriptional activity of NF-κB and FoxO. Therefore, overexpression of HSP72 in the diaphragm is a potential therapeutic target to protect against VIDD.

Sex differences in forkhead box O3a signaling response to hindlimb unloading in rat soleus muscle

We tested the hypothesis that there are sex differences in hindlimb unloading-induced activation of the forkhead box subfamily O3a (FoxO3a) signaling pathway in rat soleus muscle. Age-matched male and female Wistar rats were subjected to hindlimb unloading, and the soleus muscle was removed before or 1 or 7 days after unloading. Female rats showed greater percent changes in relative soleus muscle weight than males. FoxO3a phosphorylation was lower in females than in males and was associated with higher levels of protein ubiquitination 7 days after unloading. Heat shock protein 72 (Hsp72) levels were lower in female rats and increased in males during unloading. Female rats showed slightly higher myostatin levels, which showed a non-significant decline in male rats following unloading. Thus, males and females show different responses to the FoxO3a/ubiquitin-proteasome pathway following hindlimb unloading in rat soleus muscle, which may be associated with differences in Hsp72 expression and myostatin signaling.

Stress Effects on Meat Quality: A Mechanistic Perspective

Stress inevitably occurs from the farm to abattoir in modern livestock husbandry. The effects of stress on the behavioral and physiological status and ultimate meat quality have been well documented. However, reports on the mechanism of stress effects on physiological and biochemical changes and their consequent effects on meat quality attributes have been somewhat disjointed and limited. Furthermore, the causes of variability in meat quality traits among different animal species, muscle fibers within an animal, and even positions within a piece of meat in response to stress are still not entirely clear. This review 1st summarizes the primary stress factors, including heat stress, preslaughter handling stress, oxidative stress, and other stress factors affecting animal welfare; carcass quality; and eating quality. This review further delineates potential stress-induced pathways or mediators, including AMP-activated protein kinase-mediated energy metabolism, crosstalk among calcium signaling pathways and reactive oxygen species, protein modification, apoptosis, calpain and cathepsin proteolytic systems, and heat shock proteins that exert effects that cause biochemical changes during the early postmortem period and affect the subsequent meat quality. To obtain meat of high quality, further studies are needed to unravel the intricate mechanisms involving the aforementioned signaling pathways or mediators and their crosstalk.

AMPK Mediates Muscle Mass Change But Not the Transition of Myosin Heavy Chain Isoforms during Unloading and Reloading of Skeletal Muscles in Mice

5′AMP-activated protein kinase (AMPK) plays an important role in the regulation of skeletal muscle mass and fiber-type distribution. However, it is unclear whether AMPK is involved in muscle mass change or transition of myosin heavy chain (MyHC) isoforms in response to unloading or increased loading. Here, we checked whether AMPK controls muscle mass change and transition of MyHC isoforms during unloading and reloading using mice expressing a skeletal-muscle-specific dominant-negative AMPKα1 (AMPK-DN). Fourteen days of hindlimb unloading reduced the soleus muscle weight in wild-type and AMPK-DN mice, but reduction in the muscle mass was partly attenuated in AMPK-DN mice. There was no difference in the regrown muscle weight between the mice after 7 days of reloading, and there was concomitantly reduced AMPKα2 activity, however it was higher in AMPK-DN mice after 14 days reloading. No difference was observed between the mice in relation to the levels of slow-type MyHC I, fast-type MyHC IIa/x, and MyHC IIb isoforms following unloading and reloading. The levels of 72-kDa heat-shock protein, which preserves muscle mass, increased in AMPK-DN-mice. Our results indicate that AMPK mediates the progress of atrophy during unloading and regrowth of atrophied muscles following reloading, but it does not influence the transition of MyHC isoforms.

Temporal regulation of extracellular signal-regulated kinase 1/2 phosphorylation, heat shock protein 70 and activating transcription factor 3 during prostaglandin F-induced luteal regression in pseudopregnant rats following heat stress

The aim of the present study was to investigate the effects of heat stress on heat shock protein (HSP) 70 expression and mitogen-activated protein kinase (MAPK) and protein kinase (PK) B signalling during prostaglandin F (PGF)-induced luteal regression. During pseudopregnancy, rats were exposed to heat stress (HS, 40°C, 2h) for 7 days and treated with PGF or physiological saline on Day 7; serum and ovaries were collected 0, 1, 2, 8 or 24h after PGF treatment. The early inhibitory effect of PGF on progesterone was reduced in HS rats. HSP70 expression in response to PGF was significantly enhanced in HS rats. PGF-induced phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 was significantly greater in the HS group; however, HS rats exhibited elevated basal levels of phosphorylation of p38 MAPK, but not ERK1/2. PGF treatment increased expression of activating transcription factor (ATF) 3 at 2h, which was inhibited by heat stress. Evaluating PKB signalling revealed that phosphorylation of p-Akt (Thr³⁰⁸ and Ser⁴⁷³) was reduced at 8 and 24h after PGF treatment in both non-heat stress (NHS) and HS groups, but there were no significant differences between the HS and NHS groups at any of the time points. In conclusion, the present study provides further evidence that heat stress may enhance HSP70 and affect ERK1/2 and ATF3 expression, but not Akt activation, during PGF-induced luteal regression in pseudopregnant rats.

Therapeutic potential of heat shock protein induction for muscular dystrophy and other muscle wasting conditions

Duchenne muscular dystrophy is the most common and severe of the muscular dystrophies, a group of inherited myopathies caused by different genetic mutations leading to aberrant expression or complete absence of cytoskeletal proteins. Dystrophic muscles are prone to injury, and regenerate poorly after damage. Remorseless cycles of muscle fibre breakdown and incomplete repair lead to progressive and severe muscle wasting, weakness and premature death. Many other conditions are similarly characterized by muscle wasting, including sarcopenia, cancer cachexia, sepsis, denervation, burns, and chronic obstructive pulmonary disease. Muscle trauma and loss of mass and physical capacity can significantly compromise quality of life for patients. Exercise and nutritional interventions are unlikely to halt or reverse the conditions, and strategies promoting muscle anabolism have limited clinical acceptance. Heat shock proteins (HSPs) are molecular chaperones that help proteins fold back to their original conformation and restore function. Since many muscle wasting conditions have pathophysiologies where inflammation, atrophy and weakness are indicated, increasing HSP expression in skeletal muscle may have therapeutic potential. This review will provide evidence supporting HSP induction for muscular dystrophy and other muscle wasting conditions.This article is part of the theme issue 'Heat shock proteins as modulators and therapeutic targets of chronic disease: an integrated perspective'.

BGP-15 improves contractile function of regenerating soleus muscle

This study investigated the effect of the heat shock protein inducer O-[3-piperidino-2-hydroxy-1-propyl]-nicotinic amidoxime (BGP-15) on the morphology and contractile function of regenerating soleus muscles from mice. Cryolesioned soleus muscles from young mice treated daily with BGP-15 (15 mg/Kg) were evaluated on post-cryolesion day 10. At this time point, there was a significant decrease in the cross-sectional area of regenerating myofibers, maximal force, specific tetanic force, and fatigue resistance of regenerating soleus muscles. BGP-15 did not reverse the decrease in myofiber cross-sectional area but effectively prevented the reduction in tetanic force and fatigue resistance of regenerating muscles. In addition, BGP-15 treatment increased the expression of embryonic myosin heavy chain (e-MyHC), MyHC-II and MyHC-I in regenerating muscles. Although BGP-15 did not alter voltage dependent anion-selective channel 2 (VDAC2) expression in cryolesioned muscles, it was able to increase inducible 70-kDa heat shock protein (HSP70) expression. Our results suggest that BGP-15 improves strength recovery in regenerating soleus muscles by accelerating the re-expression of adult MyHC-II and MyHC-I isoforms and HSP70 induction. The beneficial effects of BGP-15 on the contractile function of regenerating muscles reinforce the potential of this molecule to be used as a therapeutic agent.

Mild Aerobic Exercise Training Hardly Affects the Diaphragm of mdx Mice

In the mdx mice model of Duchenne Muscular Dystrophy (DMD), mild endurance exercise training positively affected limb skeletal muscles, whereas few and controversial data exist on the effects of training on the diaphragm. The diaphragm was examined in mdx (C57BL/10ScSn-Dmdmdx) and wild-type (WT, C57BL/10ScSc) mice under sedentary conditions (mdx-SD, WT-SD) and during mild exercise training (mdx-EX, WT-EX). At baseline, and after 30 and 45 days (training: 5 d/wk for 6 weeks), diaphragm muscle morphology and Cx39 protein were assessed. In addition, tissue levels of the chaperonins Hsp60 and Hsp70 and the p65 subunit of nuclear factor-kB (NF-kB) were measured in diaphragm, gastrocnemius, and quadriceps in each experimental group at all time points. Although morphological analysis showed unchanged total area of necrosis/regeneration in the diaphragm after training, there was a trend for larger areas of regeneration than necrosis in the diaphragm of mdx-EX compared to mdx-SD mice. However, the levels of Cx39, a protein associated with active regeneration in damaged muscle, were similar in the diaphragm of mdx-EX and mdx-SD mice. Hsp60 significantly decreased at 45 days in the diaphragm, but not in limb muscles, in both trained and sedentary mdx compared to WT mice. In limb muscles, but not in the diaphragm, Hsp70 and NF-kB p65 levels were increased in mdx mice irrespective of training at 30 and 45 days. Therefore, the diaphragm of mdx mice showed little inflammatory and stress responses over time, and appeared hardly affected by mild endurance training. J. Cell. Physiol. 232: 2044-2052, 2017. © 2016 Wiley Periodicals, Inc.

Growth and repair factors, osteoactivin, matrix metalloproteinase and heat shock protein 72, increase with resolution of inflammation in musculotendinous tissues in a rat model of repetitive grasping

BACKGROUND

Expression of the growth factor osteoactivin (OA) increases during tissue degeneration and regeneration, fracture repair and after denervation-induced disuse atrophy, concomitant with increased matrix metalloproteinases (MMPs). However, OA's expression with repetitive overuse injuries is unknown. The aim of this study was to evaluate: 1) OA expression in an operant rat model of repetitive overuse; 2) expression of MMPs; 3) inflammatory cytokines indicative of injury or inflammation; and 4) the inducible form of heat shock protein 70 (HSPA1A/HSP72) as the latter is known to increase during metabolic stress and to be involved in cellular repair. Young adult female rats performed a high repetition negligible force (HRNF) food retrieval task for up to 6 weeks and were compared to control rats.

METHODS

Flexor digitorum muscles and tendons were collected from 22 young adult female rats performing a HRNF reaching task for 3 to 6 weeks, and 12 food restricted control (FRC) rats. OA mRNA levels were assessed by quantitative polymerase chain reaction (qPCR). OA, MMP-1, -2, -3, and -13 and HSP72 protein expression was assayed using Western blotting. Immunohistochemistry and image analysis was used to evaluate OA and HSP72 expression. ELISA was performed for HSP72 and inflammatory cytokines.

RESULTS

Flexor digitorum muscles and tendons from 6-week HRNF rats showed increased OA mRNA and protein expression compared to FRC rats. MMP-1, -2 and -3 progressively increased in muscles whereas MMP-1 and -3 increased in tendons with HRNF task performance. HSP72 increased in 6-week HRNF muscles and tendons, compared to controls, and co-localized with OA in the myofiber sarcolemma. IL-1alpha and beta increased transiently in tendons or muscles in HRNF week 3 before resolving in week 6.

CONCLUSION

The simultaneous increases of OA with factors involved in tissue repair (MMPs and HSP72) supports a role of OA in tissue regeneration after repetitive overuse.

Deficiency of heat shock transcription factor 1 suppresses heat stress-associated increase in slow soleus muscle mass of mice

AIM

Effects of heat shock transcription factor 1 (HSF1) deficiency on heat stress-associated increase in slow soleus muscle mass of mice were investigated.

METHODS

Both HSF1-null and wild-type mice were randomly assigned to control and heat-stressed groups. Mice in heat-stressed group were exposed to heat stress (41 °C for 60 min) in an incubator without anaesthesia.

RESULTS

Significant increase in wet and dry weights, and protein content of soleus muscle in wild-type mice was observed seven days after the application of the heat stress. However, heat stress had no impact on soleus muscle mass in HSF1-null mice. Neither type of mice exhibited much effect of heat stress on HSF mRNA expression (HSF1, HSF2 and HSF4). On the other hand, heat stress upregulated heat shock proteins (HSPs) at the mRNA (HSP72) and protein (HSP72 and HSP110) levels in wild-type mice, but not in HSF1-null mice. The population of Pax7-positive nuclei relative to total myonuclei of soleus muscle in wild-type mice was significantly increased by heat stress, but not in HSF1-null mice. Furthermore, the absence of HSF1 gene suppressed heat stress-associated phosphorylation of Akt and p70 S6 kinase (p-p70S6K) in soleus muscle.

CONCLUSION

Heat stress-associated increase in skeletal muscle mass may be induced by HSF1 and/or HSF1-mediated stress response that activates muscle satellite cells and Akt/p70S6K signalling pathway.

Repeated exposure to heat stress results in a diaphragm phenotype that resists ventilator-induced diaphragm dysfunction

Controlled mechanical ventilation (CMV) is a life-saving intervention for patients in respiratory failure. Unfortunately, prolonged mechanical ventilation (MV) results in diaphragmatic atrophy and contractile dysfunction, both of which are predicted to contribute to problems in weaning patients from the ventilator. Therefore, developing a strategy to protect the diaphragm against ventilator-induced weakness is important. We tested the hypothesis that repeated bouts of heat stress result in diaphragm resistance against CMV-induced atrophy and contractile dysfunction. Male Wistar rats were randomly divided into six experimental groups: 1) control; 2) single bout of whole body heat stress; 3) repeated bouts of whole body heat stress; 4) 12 h CMV; 5) single bout of whole body heat stress 24 h before CMV; and 6) repeated bouts of whole body heat stress 1, 3, and 5 days before 12 h of CMV. Our results revealed that repeated bouts of heat stress resulted in increased levels of heat shock protein 72 in the diaphragm and protection against both CMV-induced diaphragmatic atrophy and contractile dysfunction at submaximal stimulation frequencies. The specific mechanisms responsible for this protection remain unclear: this heat stress-induced protection against CMV-induced diaphragmatic atrophy and weakness may be partially due to reduced diaphragmatic oxidative stress, diminished activation of signal transducer/transcriptional activator-3, lower caspase-3 activation, and decreased autophagy in the diaphragm.

Coherent correlation networks among protein biomarkers of beef tenderness: What they reveal

The development of proteomic biomarkers for meat tenderness remains an important challenge. The present study used Longissimus thoracis (LT) and Semitendinosus (ST) muscles of young bulls of three continental breeds (Aberdeen Angus, Blond d'Aquitaine and Limousin) to i) identify cellular pathways robustly related with meat tenderness, using potential protein biomarkers and ii) describe biochemical mechanisms underlying muscle to meat conversion. Correlation networks reveal robust correlations, i.e. present for at least two breeds, between potential meat tenderness biomarkers. For the two muscles of the three breeds, DJ-1 and Peroxiredoxin 6 were consistently correlated with Hsp20 and μ-calpain, respectively. For the three breeds, μ-calpain was related to Hsp70-8 in the LT muscle. Various correlations were muscle specific. For the three breeds, DJ-1 was correlated with Hsp27 for the ST, and with ENO3 and LDH-B for the LT muscle. Overall, in the LT, more correlations were found between proteins related to the glycolytic pathway and in the ST, with the small Hsps (Hsp20, 27 and αB-crystallin). Hsp70-Grp75 appeared involved in several relevant biological pathways. At the scientific level, results give insights in biological functions involved in meat tenderness. Further studies are needed to confirm the possible use of these biomarkers in the meat industry to improve assurance of good meat qualities.

Heat shock protein 70 overexpression does not attenuate atrophy in botulinum neurotoxin type A-treated skeletal muscle

Botulinum neurotoxin type A (BoNT/A) is used clinically to induce therapeutic chemical denervation of spastically contracted skeletal muscles. However, BoNT/A administration can also cause atrophy. We sought to determine whether a major proteolytic pathway contributing to atrophy in multiple models of muscle wasting, the ubiquitin proteasome system (UPS), is involved in BoNT/A-induced atrophy. Three and ten days following BoNT/A injection of rat hindlimb, soleus muscle fiber cross-sectional area was reduced 25 and 65%, respectively. The transcriptional activity of NF-κB and Foxo was significantly elevated at 3 days (2- to 4-fold) and 10 days (5- to 6-fold). Muscle RING-finger protein-1 (MuRF1) activity was elevated (2-fold) after 3 days but not 10 days, while atrogin-1 activity was not elevated at any time point. BoNT/A-induced polyubiquitination occurred after 3 days (3-fold increase) but was totally absent after 10 days. Proteasome activity was elevated (1.5- to 2-fold) after 3 and 10 days. We employed the use of heat shock protein 70 (Hsp70) to inhibit NF-κB and Foxo transcriptional activity. Electrotransfer of Hsp70 into rat soleus, before BoNT/A administration, was insufficient to attenuate atrophy. It was also insufficient to decrease BoNT/A-induced Foxo activity at 3 days, although NF-κB activity was abolished. By 10 days both NF-κB and Foxo activation were abolished by Hsp70. Hsp70-overexpression was unable to alter the levels of BoNT/A-induced effects on MuRF1/atrogin-1, polyubiquitination, or proteasome activity. In conclusion, Hsp70 overexpression is insufficient to attenuate BoNT/A-induced atrophy. It remains unclear what proteolytic mechanism/s are contributing to BoNT/A-induced atrophy, although a Foxo-MuRF1-ubiquitin-proteasome contribution may exist, at least in early BoNT/A-induced atrophy. Further clarification of UPS involvement in BoNT/A-induced atrophy is warranted.

Can endurance exercise preconditioning prevention disuse muscle atrophy?

Emerging evidence suggests that exercise training can provide a level of protection against disuse muscle atrophy. Endurance exercise training imposes oxidative, metabolic, and heat stress on skeletal muscle which activates a variety of cellular signaling pathways that ultimately leads to the increased expression of proteins that have been demonstrated to protect muscle from inactivity -induced atrophy. This review will highlight the effect of exercise-induced oxidative stress on endogenous enzymatic antioxidant capacity (i.e., superoxide dismutase, glutathione peroxidase, and catalase), the role of oxidative and metabolic stress on PGC1-α, and finally highlight the effect heat stress and HSP70 induction. Finally, this review will discuss the supporting scientific evidence that these proteins can attenuate muscle atrophy through exercise preconditioning.

Physiological, cellular and biochemical thermal stress response of intertidal shrimps with different vertical distributions: Palaemon elegans and Palaemon serratus

The ability to cope with high temperature variations is a critical factor in intertidal communities. Two species of intertidal rocky shore shrimps (Palaemon sp.) with different vertical distributions were collected from the Portuguese coast in order to test if they were differentially sensitive to thermal stress. Three distinct levels of biological organization (organismal, biochemical, and cellular) were surveyed. The shrimp were exposed to a constant rate of temperature increase of 1°C x h(-1), starting at 20°C until reaching the CTMax (critical thermal maximum). During heat stress, two biomarkers of protein damage were quantified in the muscle via enzyme-linked immunosorbent assays: heat shock proteins HSP70 (hsp70/hsc70) and total ubiquitin. Muscle histopathological alterations caused by temperature were also evaluated. CTMax values were not significantly different between the congeners (P. elegans 33.4 ± 0.5 °C; P. serratus 33.0 ± 0.5 °C). Biomarker levels did not increase along the temperature trial, but P. elegans (higher intertidal) showed higher amounts of HSP70 and total ubiquitin than P. serratus (lower intertidal). HSP70 and total ubiquitin levels showed a positive significant correlation in both species, suggesting that their association is important in thermal tolerance. Histopathological observations of muscle tissue in P. serratus showed no gross alterations due to temperature but did show localized atrophy of muscle fibers at CTMax. In P. elegans, alterations occurred at a larger scale, showing multiple foci of atrophic muscular fascicles caused by necrotic or autolytic processes. In conclusion, Palaemon congeners displayed different responses to stress at a cellular level, with P. elegans having greater biomarker levels and histopathological alterations.

Inverse relationships between biomarkers and beef tenderness according to contractile and metabolic properties of the muscle

Previous proteomic analyses established a list of proteins biomarkers of beef tenderness. The present study quantified the relative abundance of 21 of these proteins by dot-blot technique in the Longissimus thoracis and Semitendinosus muscles of 71 young bulls from three breeds: Aberdeen Angus (AA), Limousin (LI), and Blond d'Aquitaine (BA). For both muscles overall tenderness was estimated by sensory analysis; shear force was measured with a Warner-Bratzler instrument, and an index combining sensory and mechanical measurements was calculated. Multiple regressions based on relative abundances of these proteins were used to propose equations of prediction of the three evaluations of tenderness. Hsp70-1B appeared to be a good biomarker of low tenderness in the three breeds and in the two muscles. Proteins such as lactate dehydrogenase-B, myosin heavy chain IIx, and small heat shock proteins (Hsp27, Hsp20, and αB-crystallin) were related to tenderness but inversely according to the muscle and breed. The results demonstrate that prediction of tenderness must take into account muscle characteristics and animal type.

The Smn-independent beneficial effects of trichostatin A on an intermediate mouse model of spinal muscular atrophy

Spinal muscular atrophy is an autosomal recessive neuromuscular disease characterized by the progressive loss of alpha motor neurons in the spinal cord. Trichostatin A (TSA) is a histone deacetylase inhibitor with beneficial effects in spinal muscular atrophy mouse models that carry the human SMN2 transgene. It is currently unclear whether TSA specifically targets the SMN2 gene or whether other genes respond to TSA and in turn provide neuroprotection in SMA mice. We have taken advantage of the Smn2B/- mouse model that does not harbor the human SMN2 transgene, to test the hypothesis that TSA has its beneficial effects through a non-SMN mediated pathway. TSA increased the median lifespan of Smn2B/- mice from twenty days to eight weeks. As well, there was a significant attenuation of weight loss and improved motor behavior. Pen test and righting reflex both showed significant improvement, and motor neurons in the spinal cord of Smn2B/- mice were protected from degeneration. Both the size and maturity of neuromuscular junctions were significantly improved in TSA treated Smn2B/- mice. Of interest, TSA treatment did not increase the levels of Smn protein in mouse embryonic fibroblasts or myoblasts obtained from the Smn2B/- mice. In addition, no change in the level of Smn transcripts or protein in the brain or spinal cord of TSA-treated SMA model mice was observed. Furthermore, TSA did not increase Smn protein levels in the hind limb muscle, heart, or liver of Smn2B/- mice. We therefore conclude that TSA likely exerts its effects independent of the endogenous mouse Smn gene. As such, identification of the pathways regulated by TSA in the Smn2B/- mice could lead to the development of novel therapeutics for treating SMA.

Neuronal cells but not muscle cells are resistant to oxidative stress mediated protein misfolding and cell death: role of molecular chaperones

Our recent study in a mouse model of familial-Amyotrophic Lateral Sclerosis (f-ALS) revealed that muscle proteins are equally sensitive to misfolding as spinal cord proteins despite the presence of low mutant CuZn-superoxide dismutase, which is considered to be the key toxic element for initiation and progression of f-ALS. More importantly, we observed differential level of heat shock proteins (Hsp's) between skeletal muscle and spinal cord tissues prior to the onset and during disease progression; spinal cord maintains significantly higher level of Hsp's compared to skeletal muscle. In this study, we report two important observations; (i) muscle cells (but not neuronal cells) are extremely vulnerable to protein misfolding and cell death during challenge with oxidative stress and (ii) muscle cells fail to mount Hsp's during challenge unlike neuronal cells. These two findings can possibly explain why muscle atrophy precedes the death of motor neurons in f-ALS mice.

p38 mitogen-activated protein kinase and mitogen-activated protein kinase-activated protein kinase 2 (MK2) signaling in atrophic and hypertrophic denervated mouse skeletal muscle

Background

p38 mitogen-activated protein kinase has been implicated in both skeletal muscle atrophy and hypertrophy. T317 phosphorylation of the p38 substrate mitogen-activated protein kinase-activated protein kinase 2 (MK2) correlates with muscle weight in atrophic and hypertrophic denervated muscle and may influence the nuclear and cytoplasmic distribution of p38 and/or MK2. The present study investigates expression and phosphorylation of p38, MK2 and related proteins in cytosolic and nuclear fractions from atrophic and hypertrophic 6-days denervated skeletal muscles compared to innervated controls.

Methods

Expression and phosphorylation of p38, MK2, Hsp25 (heat shock protein25_rodent/27_human, Hsp25/27) and Hsp70 protein expression were studied semi-quantitatively using Western blots with separated nuclear and cytosolic fractions from innervated and denervated hypertrophic hemidiaphragm and atrophic anterior tibial muscles. Unfractionated innervated and denervated atrophic pooled gastrocnemius and soleus muscles were also studied.

Results

No support was obtained for a differential nuclear/cytosolic localization of p38 or MK2 in denervated hypertrophic and atrophic muscle. The differential effect of denervation on T317 phosphorylation of MK2 in denervated hypertrophic and atrophic muscle was not reflected in p38 phosphorylation nor in the phosphorylation of the MK2 substrate Hsp25. Hsp25 phosphorylation increased 3-30-fold in all denervated muscles studied. The expression of Hsp70 increased 3-5-fold only in denervated hypertrophic muscles.

Conclusions

The study confirms a differential response of MK2 T317 phosphorylation in denervated hypertrophic and atrophic muscles and suggests that Hsp70 may be important for this. Increased Hsp25 phosphorylation in all denervated muscles studied indicates a role for factors other than MK2 in the regulation of this phosphorylation.

AICAR-induced activation of AMPK negatively regulates myotube hypertrophy through the HSP72-mediated pathway in C2C12 skeletal muscle cells

5'-AMP-activated protein kinase (AMPK) plays an important role as a negative regulator of skeletal muscle mass. However, the precise mechanism of AMPK-mediated regulation of muscle mass is not fully clarified. Heat shock proteins (HSPs), stress-induced molecular chaperones, are related with skeletal muscle adaptation, but the association between AMPK and HSPs in skeletal muscle hypertrophy is unknown. Thus, we investigated whether AMPK regulates hypertrophy by mediating HSPs in C2C12 cells. The treatment with AICAR, a potent stimulator of AMPK, decreased 72-kDa HSP (HSP72) expression, whereas there were no changes in the expressions of 25-kDa HSP, 70-kDa heat shock cognate, and heat shock transcription factor 1 in myotubes. Protein content and diameter were less in the AICAR-treated myotubes in those without treatment. AICAR-induced suppression of myotube hypertrophy and HSP72 expression was attenuated in the siRNA-mediated AMPKα knockdown myotubes. AICAR increased microRNA (miR)-1, a modulator of HSP72, and the increase of miR-1 was not induced in AMPKα knockdown condition. Furthermore, siRNA-mediated HSP72 knockdown blocked AICAR-induced inhibition of myotube hypertrophy. AICAR upregulated the gene expression of muscle Ring-finger 1, and this alteration was suppressed in either AMPKα or HSP72 knockdown myotubes. The phosphorylation of p70 S6 kinase Thr(389) was downregulated by AICAR, whereas this was attenuated in AMPKα, but not in HSP72, knockdown myotubes. These results suggest that AMPK inhibits hypertrophy through, in part, an HSP72-associated mechanism via miR-1 and protein degradation pathways in skeletal muscle cells.

Skeletal muscle heat shock protein 70: diverse functions and therapeutic potential for wasting disorders

The stress-inducible 70-kDa heat shock protein (HSP70) is a highly conserved protein with diverse intracellular and extracellular functions. In skeletal muscle, HSP70 is rapidly induced in response to both non-damaging and damaging stress stimuli including exercise and acute muscle injuries. This upregulation of HSP70 contributes to the maintenance of muscle fiber integrity and facilitates muscle regeneration and recovery. Conversely, HSP70 expression is decreased during muscle inactivity and aging, and evidence supports the loss of HSP70 as a key mechanism which may drive muscle atrophy, contractile dysfunction and reduced regenerative capacity associated with these conditions. To date, the therapeutic benefit of HSP70 upregulation in skeletal muscle has been established in rodent models of muscle injury, muscle atrophy, modified muscle use, aging, and muscular dystrophy, which highlights HSP70 as a key therapeutic target for the treatment of various conditions which negatively affect skeletal muscle mass and function. This article will review these important findings and provide perspective on the unanswered questions related to HSP70 and skeletal muscle plasticity which require further investigation.

Inhibition of C2C12 myotube atrophy by a novel HSP70 inducer, celastrol, via activation of Akt1 and ERK1/2 pathways

Celastrol (CEL) is known as a potent inducer of heat shock protein (HSP) in non-muscle cells and exhibits cytoprotective function and inhibitory effects on proteasome and glucocorticoid receptor activities. To investigate an anti-atrophic effect of CEL on skeletal muscle cells, C2C12 myotubes were treated with 150 μM dexamethasone (DEX) for 24h and 1.5 μM CEL was added for the last 6h during the 24h DEX treatment. Compared to the control, the myotube diameter was reduced by a factor of 0.30 by DEX, but CEL treatment almost abrogated the DEX-induced atrophy. CEL treatment also increased expression of HSP72 and phosphorylation of heat shock transcription factor 1 (p-HSF1) 11-fold and 3.4-fold, respectively, as well as accumulation of p-HSF1 in the nucleus. Furthermore, CEL treatment elevated activities of Akt1, p70/S6K and ERK1/2 2.0- to 4.4-fold whereas DEX had no effect on these signaling activities. Inhibition of Akt1 and ERK1/2 pathways by specific inhibitors confirmed CEL-induced anti-atrophic effect. Moreover, DEX-mediated downregulation of FoxO3 phosphorylation and upregulation of MuRF1 expression and proteasome activity were abrogated by CEL treatment. These results demonstrate a novel anti-atrophic function of CEL in muscle cells via both activation of protein anabolic signals and suppression of catabolic signaling activities.

Loss of the inducible Hsp70 delays the inflammatory response to skeletal muscle injury and severely impairs muscle regeneration

Skeletal muscle regeneration following injury is a highly coordinated process that involves transient muscle inflammation, removal of necrotic cellular debris and subsequent replacement of damaged myofibers through secondary myogenesis. However, the molecular mechanisms which coordinate these events are only beginning to be defined. In the current study we demonstrate that Heat shock protein 70 (Hsp70) is increased following muscle injury, and is necessary for the normal sequence of events following severe injury induced by cardiotoxin, and physiological injury induced by modified muscle use. Indeed, Hsp70 ablated mice showed a significantly delayed inflammatory response to muscle injury induced by cardiotoxin, with nearly undetected levels of both neutrophil and macrophage markers 24 hours post-injury. At later time points, Hsp70 ablated mice showed sustained muscle inflammation and necrosis, calcium deposition and impaired fiber regeneration that persisted several weeks post-injury. Through rescue experiments reintroducing Hsp70 intracellular expression plasmids into muscles of Hsp70 ablated mice either prior to injury or post-injury, we confirm that Hsp70 optimally promotes muscle regeneration when expressed during both the inflammatory phase that predominates in the first four days following severe injury and the regenerative phase that predominates thereafter. Additional rescue experiments reintroducing Hsp70 protein into the extracellular microenvironment of injured muscles at the onset of injury provides further evidence that Hsp70 released from damaged muscle may drive the early inflammatory response to injury. Importantly, following induction of physiological injury through muscle reloading following a period of muscle disuse, reduced inflammation in 3-day reloaded muscles of Hsp70 ablated mice was associated with preservation of myofibers, and increased muscle force production at later time points compared to WT. Collectively our findings indicate that depending on the nature and severity of muscle injury, therapeutics which differentially target both intracellular and extracellular localized Hsp70 may optimally preserve muscle tissue and promote muscle functional recovery.

Frogs and estivation: transcriptional insights into metabolism and cell survival in a natural model of extended muscle disuse

Green-striped burrowing frogs (Cyclorana alboguttata) survive in arid environments by burrowing underground and entering into a deep, prolonged metabolic depression known as estivation. Throughout estivation, C. alboguttata is immobilized within a cast-like cocoon of shed skin and ceases feeding and moving. Remarkably, these frogs exhibit very little muscle atrophy despite extended disuse and fasting. Little is known about the transcriptional regulation of estivation or associated mechanisms that may minimize degradative pathways of atrophy. To investigate transcriptional pathways associated with metabolic depression and maintenance of muscle function in estivating burrowing frogs, we assembled a skeletal muscle transcriptome using next-generation short read sequencing and compared gene expression patterns between active and 4 mo estivating C. alboguttata. This identified a complex suite of gene expression changes that occur in muscle during estivation and provides evidence that estivation in burrowing frogs involves transcriptional regulation of genes associated with cytoskeletal remodeling, avoidance of oxidative stress, energy metabolism, the cell stress response, and apoptotic signaling. In particular, the expression levels of genes encoding cell cycle and prosurvival proteins, such as serine/threonine-protein kinase Chk1, cell division protein kinase 2, survivin, and vesicular overexpressed in cancer prosurvival protein 1, were upregulated during estivation. These data suggest that estivating C. alboguttata are able to regulate the expression of genes in several major cellular pathways critical to the survival and viability of cells, thus preserving muscle function while avoiding the deleterious consequences often seen in laboratory models of muscle disuse.

Effects of corticosteroids in the development of limb muscle weakness in a porcine intensive care unit model

Severe muscle wasting is a debilitating condition in critically ill intensive care unit (ICU) patients, characterized by general muscle weakness and dysfunction, resulting in a prolonged mobilization, delayed weaning from the ventilator, and a decreased quality of life post-ICU. The mechanisms underlying limb muscle weakness in ICU patients are complex and involve the impact of primary disease, but also factors common to critically ill ICU patients such as sepsis, mechanical ventilation (MV), immobilization, and systemic administration of corticosteroids (CS). These factors may have additive negative effects on skeletal muscle structure and function, but their respective role alone remain unknown. The primary aim of this study was to examine how CS administration potentiates ventilator and immobilization-related limb muscle dysfunction at the gene level. Comparing biceps femoris gene expression in pigs exposed to MV and CS for 5 days with only MV pigs for the same duration of time showed a distinct deregulation of 186 genes according to microarray. Surprisingly, the decreased force-generation capacity at the single muscle fiber reported in response to the addition of CS administration in mechanically ventilated and immobilized pigs was not associated with an additional upregulation of proteolytic pathways. On the other hand, an altered expression of genes regulating kinase activity, cell cycle, transcription, channel regulation, oxidative stress response, cytoskeletal, sarcomeric, and heat shock protein, as well as protein synthesis at the translational level, appears to play an additive deleterious role for the limb muscle weakness in immobilized ICU patients.