Global analysis of gene expression patterns during disuse atrophy in rat skeletal muscle

Combination of Parenteral Amino Acid Infusion and Intermittent Loading Exercise Ameliorates Progression of Postoperative Sarcopenia in Rat Model

Postoperative sarcopenia is associated with poor outcomes in hospitalized patients. However, few studies have focused on short-term postoperative sarcopenia. Furthermore, the influence of nutritional management using amino acids (AAs) comprising a peripheral parenteral nutrition (PPN) solution and its combination with exercise (Exc) is unclear. Hence, we established a postoperative sarcopenic rat model to evaluate the effects of parenteral AA infusion combined with Exc on skeletal muscles and investigate the underlying mechanisms involved in the amelioration of muscle atrophy. Male F344 rats underwent surgery followed by hindlimb suspension (HS) for 5 days. The rats were divided into AA (-), AA (+), AA (-)-Exc, and AA (+)-Exc groups. They were continuously administered a PPN solution with or without AA at 98 kcal/kg/day. The Exc groups were subjected to intermittent loading for 1 h per day. Postoperative sarcopenic rats exhibited decreased muscle strength and mass and an upregulated ubiquitin-proteasome system, autophagy-lysosome system, and fast-twitch fiber-related genes, especially in the AA (-) group. The AA (+)-Exc group exhibited attenuated decreased muscle strength, increased gastrocnemius mass, and a suppressed upregulation of muscle atrophy- and fast-twitch fiber-related genes. Therefore, parenteral AA infusion combined with Exc may be effective in preventing postoperative sarcopenia in hospitalized patients.

Metformin Pre-Treatment as a Means of Mitigating Disuse-Induced Rat Soleus Muscle Wasting

Currently, no ideal treatment exists to combat skeletal muscle disuse-induced atrophy and loss of strength. Because the activity of AMP-activated protein kinase (AMPK) in rat soleus muscle is suppressed at the early stages of disuse, we hypothesized that pre-treatment of rats with metformin (an AMPK activator) would exert beneficial effects on skeletal muscle during disuse. Muscle disuse was performed via hindlimb suspension (HS). Wistar rats were divided into four groups: (1) control (C), (2) control + metformin for 10 days (C+Met), (3) HS for 7 days (HS), (4) metformin treatment for 7 days before HS and during the first 3 days of 1-week HS (HS+Met). Anabolic and catabolic markers were assessed using WB and RT-PCR. Treatment with metformin partly prevented an HS-induced decrease in rat soleus weight and size of slow-twitch fibers. Metformin prevented HS-related slow-to-fast fiber transformation. Absolute soleus muscle force in the HS+Met group was increased vs. the HS group. GSK-3β (Ser9) phosphorylation was significantly increased in the HS+Met group vs. the HS group. Metformin pre-treatment partly prevented HS-induced decrease in 18S+28S rRNA content and attenuated upregulation of calpain-1 and ubiquitin. Thus, pre-treatment of rats with metformin can ameliorate disuse-induced reductions in soleus muscle weight, the diameter of slow-type fibers, and absolute muscle strength.

The Contribution of Genetics to Muscle Disuse, Retraining, and Aging

Genetic background may partly explain differences in muscle responses to internal or external stimuli. Muscle disuse involves various degrees of skeletal muscle atrophy due to inactivity and mechanical unloading. Whether and to which extent genetic background impacts disuse atrophy and retraining in individuals of different ages are currently unclear. Here, we provide a brief overview of relevant literature on the contribution of genetics to muscle disuse, retraining, and aging, and offer a perspective on unanswered questions on the subject that may open new venues for research.

Translational therapy from preclinical animal models for muscle degeneration after rotator cuff injury

Chronic rotator cuff tears are debilitating diseases which significantly affect patients’ quality of life and pose substantial financial burden to the society. The intraoperative reparability of injured tendon and postoperative probability of tendon retear are highly associated with the quality of torn muscles, specifically, the severity of muscle atrophy and fatty infiltration. Animal models that reproduce the characteristic muscle pathology after rotator cuff injury have been developed and used to provide insight into the underlying biology and pathophysiology. In this review, we briefly summarize the current information obtained from preclinical animal studies regarding the degenerative change of cuff muscle subsequent to tendon release and/or suprascapular nerve denervation. Importantly, we focus on the potential translational therapeutic targets or agents for the prevention or reversal of muscle atrophy and fatty infiltration. While further studies are warranted to assess the safety and efficacy of novel therapies derived from these preclinical animal research, we believe that their clinical translation for the treatment of rotator cuff disorders is on the horizon.

The Translational potential of this article

Novel therapeutic strategies described in this review from preclinical animal studies hold a great translational potential for preventing or reversing rotator cuff muscle pathology, while further assessments on their safety and efficacy are warranted.

Biology of Activating Transcription Factor 4 (ATF4) and Its Role in Skeletal Muscle Atrophy

Activating transcription factor 4 (ATF4) is a multifunctional transcription regulatory protein in the basic leucine zipper superfamily. ATF4 can be expressed in most if not all mammalian cell types, and it can participate in a variety of cellular responses to specific environmental stresses, intracellular derangements, or growth factors. Because ATF4 is involved in a wide range of biological processes, its roles in human health and disease are not yet fully understood. Much of our current knowledge about ATF4 comes from investigations in cultured cell models, where ATF4 was originally characterized and where further investigations continue to provide new insights. ATF4 is also an increasingly prominent topic of in vivo investigations in fully differentiated mammalian cell types, where our current understanding of ATF4 is less complete. Here, we review some important high-level concepts and questions concerning the basic biology of ATF4. We then discuss current knowledge and emerging questions about the in vivo role of ATF4 in one fully differentiated cell type, mammalian skeletal muscle fibers.

Mitochondrial aberrations during the progression of disuse atrophy differentially affect male and female mice

BACKGROUND

Disuse decreases muscle size and is predictive of mortality across multiple pathologies. Detriments to mitochondrial function are hypothesized to underlie disuse-induced muscle atrophy. Little data exist on early mechanisms contributing to onset of these pathologies, nor is it known how they differ between sexes. The purpose of this study was to examine differential and conserved responses to mitochondrial quality control in male and female mice during the development and progression of disuse-induced atrophy.

METHODS

One hundred C57BL/6J mice (50 male and 50 female) were hindlimb unloaded to induce disuse atrophy for 0 (con), 24, 48, 72, or 168 h. At designated time-points, extensor digitorum longus, gastrocnemius, and soleus muscles were collected for analysis of mitochondrial quality control markers.

RESULTS

One hundred sixty-eight hours of disuse resulted in ~25% lower oxidative muscle fibre CSA in both male (P = 0.003) and female (P = 0.02) mice without any differences due to disuse in glycolytic fibres. In male mice, 48 h of unloading was sufficient to result in ~67% greater mitochondrial oxidative stress as assessed by the reporter gene pMitoTimer compared with 0 h (P = 0.002), this mitochondrial stress preceded detectable muscle loss. However in female mice, mitochondrial oxidative stress did not occur until 168 h of disuse (~40% greater mitochondrial oxidative stress in 168 h compared with 0 h of disuse, P < 0.0001). Blunted oxidative stress in female mice appeared to coincide with greater inductions of autophagy and mitophagy in female mice (~3-fold greater BNIP3 and ~6-fold greater LC3II/I ratio P < 0.0001 and P = 0.038 respectively). Male mice overall had greater reactive oxygen species (ROS) production compared with female mice. Female mice had a greater induction of ROS within 24 h of disuse (~4-fold greater compared with 0 h, P < 0.0001); whereas male mice did not have greater ROS production until 168 h of disuse (~2-fold greater, P < 0.0001). Although all muscle types exhibited some alterations to mitochondrial quality control, such as increased markers of mitophagy and fission, the soleus muscle in both male and female mice exhibited consistent alterations to various markers of mitochondrial quality. Markers of mitochondrial translation were approximately 30-50% lower within 24 h of unloading in both male and female soleus muscle (P value ranges: <0.0001-0.03).

CONCLUSIONS

Disuse negatively affects mitochondria differentially between sexes during development of muscle wasting. Acutely, female mice may forgo muscle mass to maintain mitochondrial quality compared with male mice. These differences may contribute to divergent clinical manifestations of atrophy.

Out of Control: The Role of the Ubiquitin Proteasome System in Skeletal Muscle during Inflammation

The majority of critically ill intensive care unit (ICU) patients with severe sepsis develop ICU-acquired weakness (ICUAW) characterized by loss of muscle mass, reduction in myofiber size and decreased muscle strength leading to persisting physical impairment. This phenotype results from a dysregulated protein homeostasis with increased protein degradation and decreased protein synthesis, eventually causing a decrease in muscle structural proteins. The ubiquitin proteasome system (UPS) is the predominant protein-degrading system in muscle that is activated during diverse muscle atrophy conditions, e.g., inflammation. The specificity of UPS-mediated protein degradation is assured by E3 ubiquitin ligases, such as atrogin-1 and MuRF1, which target structural and contractile proteins, proteins involved in energy metabolism and transcription factors for UPS-dependent degradation. Although the regulation of activity and function of E3 ubiquitin ligases in inflammation-induced muscle atrophy is well perceived, the contribution of the proteasome to muscle atrophy during inflammation is still elusive. During inflammation, a shift from standard- to immunoproteasome was described; however, to which extent this contributes to muscle wasting and whether this changes targeting of specific muscular proteins is not well described. This review summarizes the function of the main proinflammatory cytokines and acute phase response proteins and their signaling pathways in inflammation-induced muscle atrophy with a focus on UPS-mediated protein degradation in muscle during sepsis. The regulation and target-specificity of the main E3 ubiquitin ligases in muscle atrophy and their mode of action on myofibrillar proteins will be reported. The function of the standard- and immunoproteasome in inflammation-induced muscle atrophy will be described and the effects of proteasome-inhibitors as treatment strategies will be discussed.

Maslinic Acid Attenuates Denervation-Induced Loss of Skeletal Muscle Mass and Strength

Maslinic acid (MA) is a pentacyclic triterpene abundant in olive peels. MA reportedly increases skeletal muscle mass and strength in older adults; however, the underlying mechanism is unknown. This study aimed to investigate the effects of MA on denervated muscle atrophy and strength and to explore the underlying molecular mechanism. Mice were fed either a control diet or a 0.27% MA diet. One week after intervention, the sciatic nerves of both legs were cut to induce muscle atrophy. Mice were examined 14 days after denervation. MA prevented the denervation-induced reduction in gastrocnemius muscle mass and skeletal muscle strength. Microarray gene expression profiling in gastrocnemius muscle demonstrated several potential mechanisms for muscle maintenance. Gene set enrichment analysis (GSEA) revealed different enriched biological processes, such as myogenesis, PI3/AKT/mTOR signaling, TNFα signaling via NF-κB, and TGF-β signaling in MA-treated mice. In addition, qPCR data showed that MA induced Igf1 expression and suppressed the expressions of Atrogin-1, Murf1 and Tgfb. Altogether, our results suggest the potential of MA as a new therapeutic and preventive dietary ingredient for muscular atrophy and strength.

Transcriptomic links to muscle mass loss and declines in cumulative muscle protein synthesis during short-term disuse in healthy younger humans

Muscle disuse leads to a rapid decline in muscle mass, with reduced muscle protein synthesis (MPS) considered the primary physiological mechanism. Here, we employed a systems biology approach to uncover molecular networks and key molecular candidates that quantitatively link to the degree of muscle atrophy and/or extent of decline in MPS during short-term disuse in humans. After consuming a bolus dose of deuterium oxide (D₂ O; 3 mL.kg^-1 ), eight healthy males (22 ± 2 years) underwent 4 days of unilateral lower-limb immobilization. Bilateral muscle biopsies were obtained post-intervention for RNA sequencing and D₂ O-derived measurement of MPS, with thigh lean mass quantified using dual-energy X-ray absorptiometry. Application of weighted gene co-expression network analysis identified 15 distinct gene clusters ("modules") with an expression profile regulated by disuse and/or quantitatively connected to disuse-induced muscle mass or MPS changes. Module scans for candidate targets established an experimentally tractable set of candidate regulatory molecules (242 hub genes, 31 transcriptional regulators) associated with disuse-induced maladaptation, many themselves potently tied to disuse-induced reductions in muscle mass and/or MPS and, therefore, strong physiologically relevant candidates. Notably, we implicate a putative role for muscle protein breakdown-related molecular networks in impairing MPS during short-term disuse, and further establish DEPTOR (a potent mTOR inhibitor) as a critical mechanistic candidate of disuse driven MPS suppression in humans. Overall, these findings offer a strong benchmark for accelerating mechanistic understanding of short-term muscle disuse atrophy that may help expedite development of therapeutic interventions.

MERG1A Protein Abundance Increases in the Atrophied Skeletal Muscle of Denervated Mice, But Does Not Affect NFκB Activity

Skeletal muscle atrophy may occur with disease, injury, decreased muscle use, starvation, and normal aging. No reliably effective treatments for atrophy are available, thus research into the mechanisms contributing to muscle loss is essential. The ERG1A K+ channel contributes to muscle loss by increasing ubiquitin proteasome proteolysis (UPP) in the skeletal muscle of both unweighted and cachectic mice. Because the mechanisms which produce atrophy vary based upon the initiating factor, here we investigate atrophy produced by denervation. Using immunohistochemistry and immunoblots, we demonstrate that ERG1A protein abundance increases significantly in the Gastrocnemius muscle of rodents 7 days after both sciatic nerve transection and hind limb unweighting. Further, we reveal that ectopic expression of a Merg1a encoded plasmid in normal mouse Gastrocnemius muscle has no effect on activity of the NFκB transcription factor family, a group of proteins which contribute to muscle atrophy by modulation of the UPP. Further, although NFκB activity increases significantly after denervation, we show that expression of a plasmid encoding a dominant negative Merg1a mutant in Gastrocnemius muscle prior to denervation, has no effect on NFκB activity. Thus, although the ERG1A K+ channel increases UPP, it does not do so through modulation of NFκB transcription factors.

Morphological Evidence of Telocytes in Skeletal Muscle Interstitium of Exercised and Sedentary Rodents

Skeletal muscle atrophy, resulting from states of hypokinesis or immobilization, leads to morphological, metabolic, and functional changes within the muscle tissue, a large variety of which are supported by the stromal cells populating the interstitium. Telocytes represent a recently discovered population of stromal cells, which has been increasingly identified in several human organs and appears to participate in sustaining cross-talk, promoting regenerative mechanisms and supporting differentiation of local stem cell niche. The aim of this morphologic study was to investigate the presence of Telocytes in the tibialis anterior muscle of healthy rats undergoing an endurance training protocol for either 4 weeks or 16 weeks compared to sedentary rats. Histomorphometric analysis of muscle fibers diameter revealed muscle atrophy in sedentary rats. Telocytes were identified by double-positive immunofluorescence staining for CD34/CD117 and CD34/vimentin. The results showed that Telocytes were significantly reduced in sedentary rats at 16 weeks, while rats subjected to regular exercise maintained a stable Telocytes population after 16 weeks. Understanding of the relationship between Telocytes and exercise offers new chances in the field of regenerative medicine, suggesting possible triggers for Telocytes in sarcopenia and other musculoskeletal disorders, promoting adapted physical activity and rehabilitation programmes in clinical practice.

Alternative splicing transitions associate with emerging atrophy phenotype during denervation-induced skeletal muscle atrophy

Alternative splicing (AS) presents a key posttranscriptional regulatory mechanism associated with numerous physiological processes. However, little is known about its role in skeletal muscle atrophy. In this study, we used a rat model of denervated skeletal muscle atrophy and performed RNA-sequencing to analyze transcriptome profiling of tibialis anterior muscle at multiple time points following denervation. We found that AS is a novel mechanism involving muscle atrophy, which is independent changes at the transcript level. Bioinformatics analysis further revealed that AS transitions are associated with the appearance of the atrophic phenotype. Moreover, we found that the inclusion of multiple highly conserved exons of Obscn markedly increased at 3 days after denervation. In addition, we confirmed that this newly transcript inhibited C2C12 cell proliferation and exacerbated myotube atrophy. Finally, our study revealed that a large number of RNA-binding proteins were upregulated when the atrophy phenotype appeared. Our data emphasize the importance of AS in this process.

RNA sequencing (RNA-seq) analysis of gene expression provides new insights into hindlimb unloading-induced skeletal muscle atrophy

Background

Weightlessness-induced skeletal muscle atrophy, accompanied by complex biochemical and physiological changes, has potentially damaged consequences. However, there is still an insufficient effective strategy to treat skeletal muscle atrophy. Therefore, exploring the molecular mechanisms regulating skeletal muscle atrophy and effective protection is necessary.

Methods

RNA sequencing (RNA-seq) analysis was used to detect differentially expressed genes (DEGs) in the soleus muscle at 12, 24, 36 hours, three days, and seven days after hindlimb unloading in rats. Pearson correlation heatmaps and principal component analysis (PCA) were applied to analyze DEGs’ expression profiles. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) were used for cluster analysis of DEGs. Ingenuity pathway analysis (IPA) was used to analyze specific biological processes further.

Results

At different time points (12, 24, 36 hours, three days, seven days) after hindlimb unloading, the expression levels of 712, 1,109, 1,433, 1,162, and 1,182 genes in rat soleus muscle were upregulated, respectively, whereas the expression levels of 1,186, 1,324, 1,632, 1,446, and 1,596 genes were downregulated, respectively. PCA revealed that rat soleus muscle showed three different transcriptional phases within seven days after hindlimb unloading. KEGG and GO annotation indicated that the first transcriptional phase primarily involved the activation of stress responses, including oxidative stress, and the inhibition of cell proliferation and angiogenesis; the second transcriptional phase primarily involved the activation of proteolytic systems and, to a certain degree, inflammatory responses; and the third transcriptional phase primarily involved extensive activation of the proteolytic system, significant inhibition of energy metabolism, and activation of the aging process and slow-to-fast muscle conversion.

Conclusions

Different physiological processes in rat skeletal muscles were activated sequentially after unloading. From these activated biological processes, the three transcriptional phases after skeletal muscle unloading can be successively defined as the stress response phase, the atrophic initiation phase, and the atrophic phase. Our study not only helps in the understanding of the molecular mechanisms underlying weightlessness-induced muscle atrophy but may also provide an important time window for the treatment and prevention of weightlessness-induced muscle atrophy.

Master Regulators of Muscle Atrophy: Role of Costamere Components

The loss of muscle mass and force characterizes muscle atrophy in several different conditions, which share the expression of atrogenes and the activation of their transcriptional regulators. However, attempts to antagonize muscle atrophy development in different experimental contexts by targeting contributors to the atrogene pathway showed partial effects in most cases. Other master regulators might independently contribute to muscle atrophy, as suggested by our recent evidence about the co-requirement of the muscle-specific chaperone protein melusin to inhibit unloading muscle atrophy development. Furthermore, melusin and other muscle mass regulators, such as nNOS, belong to costameres, the macromolecular complexes that connect sarcolemma to myofibrils and to the extracellular matrix, in correspondence with specific sarcomeric sites. Costameres sense a mechanical load and transduce it both as lateral force and biochemical signals. Recent evidence further broadens this classic view, by revealing the crucial participation of costameres in a sarcolemmal “signaling hub” integrating mechanical and humoral stimuli, where mechanical signals are coupled with insulin and/or insulin-like growth factor stimulation to regulate muscle mass. Therefore, this review aims to enucleate available evidence concerning the early involvement of costamere components and additional putative master regulators in the development of major types of muscle atrophy.

Gene-Metabolite Network Linked to Inhibited Bioenergetics in Association With Spaceflight-Induced Loss of Male Mouse Quadriceps Muscle

Prolonged residence of mice in spaceflight is a scientifically robust and ethically ratified model of muscle atrophy caused by continued unloading. Under the Rodent Research Program of the National Aeronautics and Space Administration (NASA), we assayed the large-scale mRNA and metabolomic perturbations in the quadriceps of C57BL/6j male mice that lived in spaceflight (FLT) or on the ground (control or CTR) for approximately 4 weeks. The wet weights of the quadriceps were significantly reduced in FLT mice. Next-generation sequencing and untargeted mass spectroscopic assays interrogated the gene-metabolite landscape of the quadriceps. A majority of top-ranked differentially suppressed genes in FLT encoded proteins from the myosin or troponin families, suggesting sarcomere alterations in space. Significantly enriched gene-metabolite networks were found linked to sarcomeric integrity, immune fitness, and oxidative stress response; all inhibited in space as per in silico prediction. A significant loss of mitochondrial DNA copy numbers in FLT mice underlined the energy deprivation associated with spaceflight-induced stress. This hypothesis was reinforced by the transcriptomic sequencing-metabolomics integrative analysis that showed inhibited networks related to protein, lipid, and carbohydrate metabolism, and adenosine triphosphate (ATP) synthesis and hydrolysis. Finally, we discovered important upstream regulators, which could be targeted for next-generation therapeutic intervention for chronic disuse of the musculoskeletal system. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

Altering aspects of mitochondrial quality to improve musculoskeletal outcomes in disuse atrophy

Muscle atrophy is a significant moderator for disease prognosis; as such, interventions to mitigate disuse-induced muscle loss are imperative to improve clinical interventions. Mitochondrial deteriorations may underlie disuse-induced myopathies; therefore, improving mitochondrial quality may be an enticing therapeutic intervention. However, different mitochondria-based treatments may have divergent impacts on the prognosis of disuse atrophy. Therefore, the purpose of this study was to investigate different mitochondria-centered interventions during disuse atrophy in hindlimb unloaded male and female mice. Male and female mice overexpressing peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) or mitochondrially targeted catalase (MCAT) and their respective wild-type (WT) littermate controls were hindlimb unloaded for 7 days to induce disuse atrophy or allowed normal ambulatory activity (cage control; CON). After designated interventions, animals were euthanized, and tissues were collected for measures of mitochondrial quality control and protein turnover. Although PGC-1α overexpression mitigated ubiquitin-proteasome activation (MuRF1 and Atrogin mRNA content), this did not correspond to phenotypic protections from disuse-induced atrophy. Rather, PGC-1α mice appeared to have a greater reliance on autophagic protein breakdown compared with WT mice. In MCAT mice, females exhibited a mitigated response to disuse atrophy; however, this effect was not noted in males. Despite these phenotypic differences, there were no clear cellular signaling differences between MCAT hindlimb unloaded females and MCAT fully loaded females. PGC-1α overexpression does not protect against phenotypic alterations during disuse atrophy but appears to shift catabolic pathways moderating atrophy. However, increased mitochondrially targeted catalase activity appears to blunt disuse atrophy within highly oxidative muscles specifically in female mice.NEW & NOTEWORTHY We present data suggesting that mitochondria-based interventions may mitigate disuse atrophy. However, the efficacy of mitochondria-based interventions may vary depending on the specific target of the intervention and the sex of the organism. Females appear to be more responsive to increased mitochondrial catalase as a potential therapeutic for mitigating disuse atrophy.

Impact of sedentarism due to the COVID-19 home confinement on neuromuscular, cardiovascular and metabolic health: Physiological and pathophysiological implications and recommendations for physical and nutritional countermeasures

The COVID-19 pandemic is an unprecedented health crisis as entire populations have been asked to self-isolate and live in home-confinement for several weeks to months, which in itself represents a physiological challenge with significant health risks. This paper describes the impact of sedentarism on the human body at the level of the muscular, cardiovascular, metabolic, endocrine and nervous systems and is based on evidence from several models of inactivity, including bed rest, unilateral limb suspension, and step-reduction. Data form these studies show that muscle wasting occurs rapidly, being detectable within two days of inactivity. This loss of muscle mass is associated with fibre denervation, neuromuscular junction damage and upregulation of protein breakdown, but is mostly explained by the suppression of muscle protein synthesis. Inactivity also affects glucose homeostasis as just few days of step reduction or bed rest, reduce insulin sensitivity, principally in muscle. Additionally, aerobic capacity is impaired at all levels of the O₂ cascade, from the cardiovascular system, including peripheral circulation, to skeletal muscle oxidative function. Positive energy balance during physical inactivity is associated with fat deposition, associated with systemic inflammation and activation of antioxidant defences, exacerbating muscle loss. Importantly, these deleterious effects of inactivity can be diminished by routine exercise practice, but the exercise dose-response relationship is currently unknown. Nevertheless, low to medium-intensity high volume resistive exercise, easily implementable in home-settings, will have positive effects, particularly if combined with a 15-25% reduction in daily energy intake. This combined regimen seems ideal for preserving neuromuscular, metabolic and cardiovascular health.

Down-Regulation of Mitochondrial Metabolism after Tendon Release Primes Lipid Accumulation in Rotator Cuff Muscle

Atrophy and fat accumulation are debilitating aspects of muscle diseases and are rarely prevented. Using a vertical approach combining anatomic techniques with omics methodology in a tenotomy-induced sheep model of rotator cuff disease, we tested whether mitochondrial dysfunction is implicated in muscle wasting and perturbed lipid metabolism, speculating that both can be prevented by the stimulation of β-oxidation with l-carnitine. The infraspinatus muscle lost 22% of its volume over the first 6 weeks after tenotomy before the area-percentage of lipid increased from 8% to 18% at week 16. Atrophy was associated with the down-regulation of mitochondrial transcripts and protein and a slow-to-fast shift in muscle composition. Correspondingly, amino acid levels were increased 2 weeks after tendon release, when the levels of high-energy phosphates and glycerophospholipids were lowered. l-Carnitine administration (0.9 g/kg per day) prevented atrophy over the first 2 weeks, and mitigated alterations of glutamate, glycerophospholipids, and carnitine levels in released muscle, but did not prevent the level decrease in high-energy phosphates or protein constituents of mitochondrial respiration, promoting the accumulation of longer lipids with an increasing saturation. We conclude that the early phase of infraspinatus muscle degeneration after tendon release involves the elimination of oxidative characteristics associated with an aberrant accumulation of lipid species but is largely unrelated to the prevention of atrophy with oral l-carnitine administration.

MT-102 prevents tissue wasting and improves survival in a rat model of severe cancer cachexia

BACKGROUND

Cachexia, a common manifestation of malignant cancer, is associated with wasting of skeletal muscle and fat tissue. In this study, we investigated the effects of a new first in class anabolic catabolic transforming agent on skeletal muscle in a rat model of cancer cachexia.

METHODS

Young male Wistar Han rats were intraperitoneally inoculated with 10⁸ Yoshida hepatoma AH-130 cells and once daily treated with 0.3 mg kg^-1 , 3 mg kg^-1 MT-102, or placebo by gavage.

RESULTS

Three mg kg^-1 d^-1 MT-102 not only prevented progressive loss of fat mass (-6 ± 2 g vs -12 ± 1 g; P < 0.001); lean mass (+1 ± 10 g vs. -37 ± 2 g; P < 0.001) and body weight (+1 ± 13 g vs. -60 ± 2 g; P < 0.001) were remained. Quality of life was also improved as indicated by a higher food intake 12.9 ± 3.1 g and 4.3 ± 0.5 g, 3 mg kg^-1 d^-1 MT-102 vs. placebo, respectively, P < 0.001) and a higher spontaneous activity (52 369 ± 6521 counts/24 h and 29 509 ± 1775 counts/24 h, 3 mg·kg^-1 d^-1 MT-102 vs. placebo, respectively, P < 0.01) on Day 11. Most importantly, survival was improved (HR = 0.29; 95% CI: 0.16-0.51, P < 0.001). The molecular mechanisms behind these effects involve reduction of overall protein degradation and activation of protein synthesis, assessed by measurement of proteasome and caspase-6 activity or Western blot analysis, respectively.

CONCLUSIONS

The present study shows that 3 mg kg^-1 MT-102 reduces catabolism, while inducing anabolism in skeletal muscle leading to an improved survival.

Mechanisms of exercise-induced preconditioning in skeletal muscles

Endurance exercise training promotes numerous biochemical adaptations within skeletal muscle fibers culminating into a phenotype that is safeguarded against numerous perils including doxorubicin-induced myopathy and inactivity-induced muscle atrophy. This exercise-induced protection of skeletal muscle fibers is commonly termed "exercise preconditioning". This review will discuss the biochemical mechanisms responsible for exercise-induced protection of skeletal muscle fibers against these harmful events. The first segment of this report highlights the evidence that endurance exercise training provides cytoprotection to skeletal muscle fibers against several potentially damaging insults. The second and third sections of the review will discuss the cellular adaptations responsible for exercise-induced protection of skeletal muscle fibers against doxorubicin-provoked damage and inactivity-induced fiber atrophy, respectively. Importantly, we also identify gaps in our understanding of exercise preconditioning in hopes of stimulating future research.

Skeletal Muscle Atrophy: Discovery of Mechanisms and Potential Therapies

Skeletal muscle atrophy proceeds through a complex molecular signaling network that is just beginning to be understood. Here, we discuss examples of recently identified molecular mechanisms of muscle atrophy and how they highlight an immense need and opportunity for focused biochemical investigations and further unbiased discovery work.

Exercise and Muscle Atrophy

The incidence of muscle atrophy is increasing with each passing year, which imposes a huge burden on the quality of life of patients. It is a public health issue that causes a growing concern around the world. Exercise is one of the key strategies to prevent and treat various diseases. Appropriate exercise is conducive to compensatory muscle hypertrophy, to improve muscle strength and elasticity, and to train muscle coordination, which is also beneficial to the recovery of skeletal muscle function and the regeneration of muscle cells. Sequelae of paralysis of patients with limb dyskinesia caused by muscle atrophy will be significantly alleviated after regular exercise therapy. Furthermore, exercise therapy can slow down or even reverse muscle atrophy. This article aims to introduce the characteristics of muscle atrophy and summarize the role and mechanism of exercise in the treatment of muscle atrophy in the existing studies, in order to further explore the mechanism of exercise to protect muscle atrophy and provide protection for patients with muscular atrophy.

Short-term creatine supplementation changes protein metabolism signaling in hindlimb suspension

The effect of a short-term creatine supplementation on hindlimb suspension (HS)-induced muscle atrophy was investigated. Creatine monohydrate (5 g/kg b.w. per day) or placebo, divided in 2 daily doses, was given by oral gavage for 5 days. Rats were maintained in HS with dietary supplementation concomitantly for 5 days. Body weight, soleus and EDL muscle masses, and cross-sectional areas (CSA) of the muscle fibers were measured. Signaling pathways associated with skeletal muscle mass regulation (FST, MSTN, FAK, IGF-1, MGF, Akt, mTOR, atrogin-1, and MuRF1 expressions, and Akt, S6, GSK3B, and 4EBP1 proteins) were evaluated in the muscles. Soleus muscle exhibited more atrophy than the EDL muscle due to HS. Creatine supplementation attenuated the decrease of wet weight and increased p-4EBP1 protein in the EDL muscle of HS rats. Also, creatine increased mTOR and atrogin-1 expressions in the same muscle and condition. In the absence of HS, creatine supplementation increased FAK and decreased MGF expressions in the EDL muscle. Creatine attenuated the increase in FST expression due to HS in the soleus muscle. MuRF1 expression increased in the soleus muscle due to creatine supplementation in HS animals whereas atrogin-1 expression increased still further in this group compared with untreated HS rats. In conclusion, short-term creatine supplementation changed protein metabolism signaling in soleus and EDL muscles. However, creatine supplementation only slightly attenuated the mass loss of both muscles and did not prevent the CSA reduction and muscle strength decrease induced by HS for 5 days.

Muscle metabolism and atrophy: let's talk about sex

Skeletal muscle health is a strong predictor of overall health and longevity. Pathologies affecting skeletal muscle such as cancer cachexia, intensive care unit treatment, muscular dystrophies, and others are associated with decreased quality of life and increased mortality. Recent research has begun to determine that these muscular pathologies appear to present and develop differently between males and females. However, to our knowledge, there has yet to be a comprehensive review on musculoskeletal differences between males and females and how these differences may contribute to sex differences in muscle pathologies. Herein, we present a review of the current literature on muscle phenotype and physiology between males and females and how these differences may contribute to differential responses to atrophic stimuli. In general, females appear to be more susceptible to disuse induced muscle wasting, yet protected from inflammation induced (such as cancer cachexia) muscle wasting compared to males. These differences may be due in part to differences in muscle protein turnover, satellite cell content and proliferation, hormonal interactions, and mitochondrial differences between males and females. However, more works specifically examining muscle pathologies in females are necessary to more fully understand the inherent sex-based differences in muscle pathologies between the sexes and how they may correspond to different clinical treatments.

Skeletal muscle atrogenes: From rodent models to human pathologies

Skeletal muscle atrophy is a common side effect of most human diseases. Muscle loss is not only detrimental for the quality of life but it also dramatically impairs physiological processes of the organism and decreases the efficiency of medical treatments. While hypothesized for years, the existence of an atrophying programme common to all pathologies is still incompletely solved despite the discovery of several actors and key regulators of muscle atrophy. More than a decade ago, the discovery of a set of genes, whose expression at the mRNA levels were similarly altered in different catabolic situations, opened the way of a new concept: the presence of atrogenes, i.e. atrophy-related genes. Importantly, the atrogenes are referred as such on the basis of their mRNA content in atrophying muscles, the regulation at the protein level being sometimes more complicate to elucidate. It should be noticed that the atrogenes are markers of atrophy and that their implication as active inducers of atrophy is still an open question for most of them. While the atrogene family has grown over the years, it has mostly been incremented based on data coming from rodent models. Whether the rodent atrogenes are valid for humans still remain to be established. An "atrogene" was originally defined as a gene systematically up- or down-regulated in several catabolic situations. Even if recent works often restrict this notion to the up-regulation of a limited number of proteolytic enzymes, it is important to keep in mind the big picture view. In this review, we provide an update of the validated and potential rodent atrogenes and the metabolic pathways they belong, and based on recent work, their relevance in human physio-pathological situations. We also propose a more precise definition of the atrogenes that integrates rapid recovery when catabolic stimuli are stopped or replaced by anabolic ones.

Acute estradiol treatment reduces skeletal muscle protein breakdown markers in early- but not late-postmenopausal women

OBJECTIVES

Menopause and decline in estradiol (E₂) may contribute to sarcopenia (i.e., age-related decline in muscle mass and strength) in women. E₂ may directly impact skeletal muscle protein breakdown via estrogen receptor (ER) signaling, primarily ERα. It is not yet known whether: 1) E₂ regulates pathways of skeletal muscle protein breakdown; 2) E₂-mediated changes in protein breakdown markers are associated with ERα activation and insulin sensitivity; and 3) the effects of E₂ on protein breakdown markers differ by increasing time since menopause.

STUDY DESIGN

We studied 27 women who were ≤6 years past menopause (early postmenopausal, EPM; n = 13) or ≥10 years past menopause (late postmenopausal, LPM; n = 14). Fasted skeletal muscle samples were collected following 1 week of transdermal E₂ or placebo treatment in a randomized cross-over design.

MAIN OUTCOME MEASURES

We analyzed for cytosolic protein content of the: 1) structural proteins myosin heavy chain (MHC) and tropomyosin; and 2) protein regulatory markers: protein kinase B (Akt), muscle-specific ring finger protein1 (MuRF1), atrogin1, and forkhead box O3 (FOXO3) using Western blot.

RESULTS

In response to acute E₂, FOXO3 activation (dephosphorylation) and MuRF1 protein expression decreased in EPM but increased in LPM women (p < 0.05). ERα activation was not associated with these protein breakdown markers, but FOXO3 activation tended to be inversely correlated (r = -0.318, p = 0.065) to insulin sensitivity.

CONCLUSIONS

These preliminary studies suggest the effects of E₂ on skeletal muscle protein breakdown markers were dependent on time since menopause, which is consistent with our previous study on insulin sensitivity.

The Pathway to Cancer Cachexia: MicroRNA-Regulated Networks in Muscle Wasting Based on Integrative Meta-Analysis

Cancer cachexia is a multifactorial syndrome that leads to significant weight loss. Cachexia affects 50%-80% of cancer patients, depending on the tumor type, and is associated with 20%-40% of cancer patient deaths. Besides the efforts to identify molecular mechanisms of skeletal muscle atrophy-a key feature in cancer cachexia-no effective therapy for the syndrome is currently available. MicroRNAs are regulators of gene expression, with therapeutic potential in several muscle wasting disorders. We performed a meta-analysis of previously published gene expression data to reveal new potential microRNA-mRNA networks associated with muscle atrophy in cancer cachexia. We retrieved 52 differentially expressed genes in nine studies of muscle tissue from patients and rodent models of cancer cachexia. Next, we predicted microRNAs targeting these differentially expressed genes. We also include global microRNA expression data surveyed in atrophying skeletal muscles from previous studies as background information. We identified deregulated genes involved in the regulation of apoptosis, muscle hypertrophy, catabolism, and acute phase response. We further predicted new microRNA-mRNA interactions, such as miR-27a/Foxo1, miR-27a/Mef2c, miR-27b/Cxcl12, miR-27b/Mef2c, miR-140/Cxcl12, miR-199a/Cav1, and miR-199a/Junb, which may contribute to muscle wasting in cancer cachexia. Finally, we found drugs targeting MSTN, CXCL12, and CAMK2B, which may be considered for the development of novel therapeutic strategies for cancer cachexia. Our study has broadened the knowledge of microRNA-regulated networks that are likely associated with muscle atrophy in cancer cachexia, pointing to their involvement as potential targets for novel therapeutic strategies.

Doxorubicin Exposure Causes Subacute Cardiac Atrophy Dependent on the Striated Muscle-Specific Ubiquitin Ligase MuRF1

Background Anthracycline chemotherapeutics, such as doxorubicin, are used widely in the treatment of numerous malignancies. The primary dose-limiting adverse effect of anthracyclines is cardiotoxicity that often presents as heart failure due to dilated cardiomyopathy years after anthracycline exposure. Recent data from animal studies indicate that anthracyclines cause cardiac atrophy. The timing of onset and underlying mechanisms are not well defined, and the relevance of these findings to human disease is unclear. Methods and Results Wild-type mice were sacrificed 1 week after intraperitoneal administration of doxorubicin (1-25 mg/kg), revealing a dose-dependent decrease in cardiac mass ( R2=0.64; P<0.0001) and a significant decrease in cardiomyocyte cross-sectional area (336±29 versus 188±14 µm2; P<0.0001). Myocardial tissue analysis identified a dose-dependent upregulation of the ubiquitin ligase, MuRF1 (muscle ring finger-1; R2=0.91; P=0.003) and a molecular profile of muscle atrophy. To investigate the determinants of doxorubicin-induced cardiac atrophy, we administered doxorubicin 20 mg/kg to mice lacking MuRF1 (MuRF1-/-) and wild-type littermates. MuRF1-/- mice were protected from cardiac atrophy and exhibited no reduction in contractile function. To explore the clinical relevance of these findings, we analyzed cardiac magnetic resonance imaging data from 70 patients in the DETECT-1 cohort and found that anthracycline exposure was associated with decreased cardiac mass evident within 1 month and persisting to 6 months after initiation. Conclusions Doxorubicin causes a subacute decrease in cardiac mass in both mice and humans. In mice, doxorubicin-induced cardiac atrophy is dependent on MuRF1. These findings suggest that therapies directed at preventing or reversing cardiac atrophy might preserve the cardiac function of cancer patients receiving anthracyclines.

Muscle Atrophy Marker Expression Differs between Rotary Cell Culture System and Animal Studies

Muscular atrophy, defined as the loss of muscle tissue, is a serious issue for immobilized patients on Earth and for humans during spaceflight, where microgravity prevents normal muscle loading. In vitro modeling is an important step in understanding atrophy mechanisms and testing countermeasures before animal trials. The most ideal environment for modeling must be empirically determined to best mimic known responses in vivo. To simulate microgravity conditions, murine C2C12 myoblasts were cultured in a rotary cell culture system (RCCS). Alginate encapsulation was compared against polystyrene microcarrier beads as a substrate for culturing these adherent muscle cells. Changes after culture under simulated microgravity were characterized by assessing mRNA expression of MuRF1, MAFbx, Caspase 3, Akt2, mTOR, Ankrd1, and Foxo3. Protein concentration of myosin heavy chain 4 (Myh4) was used as a differentiation marker. Cell morphology and substrate structure were evaluated with brightfield and fluorescent imaging. Differentiated C2C12 cells encapsulated in alginate had a significant increase in MuRF1 only following simulated microgravity culture and were morphologically dissimilar to normal cultured muscle tissue. On the other hand, C2C12 cells cultured on polystyrene microcarriers had significantly increased expression of MuRF1, Caspase 3, and Foxo3 and easily identifiable multinucleated myotubes. The extent of differentiation was higher in simulated microgravity and protein synthesis more active with increased Myh4, Akt2, and mTOR. The in vitro microcarrier model described herein significantly increases expression of several of the same atrophy markers as in vivo models. However, unlike animal models, MAFbx and Ankrd1 were not significantly increased and the fold change in MuRF1 and Foxo3 was lower than expected. Using a standard commercially available RCCS, the substrates and culture methods described only partially model changes in mRNAs associated with atrophy in vivo.

Sarcolemmal loss of active nNOS (Nos1) is an oxidative stress-dependent, early event driving disuse atrophy

Skeletal muscle atrophy following unloading or immobilization represents a major invalidating event in bedridden patients. Among mechanisms involved in atrophy development, a controversial role is played by neuronal NOS (nNOS; NOS1), whose dysregulation at the protein level and/or subcellular distribution also characterizes other neuromuscular disorders. This study aimed to investigate unloading-induced changes in nNOS before any evidence of myofiber atrophy, using vastus lateralis biopsies obtained from young healthy subjects after a short bed-rest and rat soleus muscles after exposure to short unloading periods. Our results showed that (1) changes in nNOS subcellular distribution using NADPH-diaphorase histochemistry to detect enzyme activity were observed earlier than using immunofluorescence to visualize the protein; (2) loss of active nNOS from the physiological subsarcolemmal localization occurred before myofiber atrophy, i.e. in 8-day bed-rest biopsies and in 6 h-unloaded rat soleus, and was accompanied by increased nNOS activity in the sarcoplasm; (3) nNOS (Nos1) transcript and protein levels decreased significantly in the rat soleus after 6 h and 1 day unloading, respectively, to return to ambulatory levels after 4 and 7 days of unloading, respectively; (4) unloading-induced nNOS redistribution appeared dependent on mitochondrial-derived oxidant species, indirectly measured by tropomyosin disulfide bonds which had increased significantly in the rat soleus already after a 6 h-unloading bout; (5) activity of displaced nNOS molecules is required for translocation of the FoxO3 transcription factor to myofiber nuclei. FoxO3 nuclear localization in rat soleus increased after 6 h unloading (about four-fold the ambulatory level), whereas it did not when nNOS expression and activity were inhibited in vivo before and during 6 h unloading. In conclusion, this study demonstrates that the redistribution of active nNOS molecules from sarcolemma to sarcoplasm not only is ahead of the atrophy of unloaded myofibers, and is induced by increased production of mitochondrial superoxide anion, but also drives FoxO3 activation to initiate muscle atrophy. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Effects of different types of exercise on skeletal muscle atrophy, antioxidant capacity and growth factors expression following myocardial infarction

AIMS

Myocardial infarction (MI) is accompanied with skeletal muscle abnormalities. The aims are to explore an optimal exercise mode to improve cardiac function and prevent skeletal muscle atrophy, and detect the possible mechanisms of exercise-induced inhibition of muscle atrophy.

MAIN METHODS

Rats were subjected to four weeks of different types of exercise after MI surgery (resistance training, RT; moderated-intensity continuous aerobic exercise, MCE and high-intensity intermittent aerobic exercise, HIA). Cardiac function, histological changes of heart and skeletal muscle, oxidative stress, antioxidant capacity and the expression of muscle atrophy-related factors were detected in skeletal muscle.

KEY FINDINGS

The three types of exercise improved heart function, reduced cardiac fibrosis and increased muscle weight and cross-section area (CSA) of muscle fibers in different degrees. The survival rates of MI rats intervened by RT and MCE were higher than HIA. Exercise down-regulated the mRNA levels of murf1 and atrogin-1, decreased reactive oxygen species level, increased antioxidant capacity, regulated the expression of insulin-like growth factor 1 (IGF1), mechano growth factor (MGF), Neuregulin1 (NRG1) and Myostatin (MSTN), and activated Akt and Erk1/2 signalings in soleus muscle. Furthermore, CSA of muscle fibers and the expression of IGF1, MGF, NRG1 in skeletal muscle had correlations with cardiac function.

SIGNIFICANCE

RT and MCE are the first two choices for the early exercise rehabilitation following MI. All types of exercise can effectively inhibit skeletal muscle atrophy through increasing the antioxidant capacity, reducing oxidative stress and protein degradation, and regulating the growth factors expression in skeletal muscle.

Proteomic identification of potential markers of myosteatosis in human urine

Myosteatosis, the infiltration of fat in skeletal muscle, is associated with lower skeletal muscle density (SMD) as detected by computed tomography (CT). It increases with aging and obesity and is thought to play a role in the aetiology of insulin resistance and type II diabetes. The clinical significance of myosteatosis in cancer cachexia, however, remains to be determined. Along with demonstrable subcutaneous and visceral lipolysis, myosteatosis may also be a key component of the syndrome. We aimed to investigate the use of human urine as a non-invasive way to screen for molecular biomarkers of myosteatosis/reduced SMD using SELDI-TOF mass spectrometry. Pre-operative CT scans of patients undergoing surgery for upper gastrointestinal or hepatopancreaticobiliary cancer were analysed at the level of the third lumbar vertebrae. Myosteatosis was inferred as the presence of reduced SMD, which was defined as Hounsfield units for skeletal muscle <39.5 (two standard deviations below a normal healthy cohort). Urine was analysed by mass spectrometry using CM10 and IMAC30 SELDI-chips. Peaks observed in the CM10 and IMAC30 chip types, showed marked expressional differences between control and myosteatosis, were further investigated by mascot SELDI matrix matching. A total of 55 patients was recruited; 31 patients were found to be myosteatotic on CT scan. Application of the IMAC30-derived model to the entire cohort showed a sensitivity of 97%, specificity of 71% and an overall correctness of 85%. Application of the CM10 chipset-based model to the entire cohort, showed a 77% sensitivity, 67% specificity and 73% overall correctness. Analysis of the peaks of interest resulted in the identification of significant fragments of cathepsin C, argin, arylsulfatase A and glial fibrillary acidic protein. We identified several potential urinary molecular biomarkers associated with reduced SMD in cancer. Such markers are potentially useful in deriving a clinical screening test for myosteatosis.

Muscle Atrophy Induced by Mechanical Unloading: Mechanisms and Potential Countermeasures

Prolonged periods of skeletal muscle inactivity or mechanical unloading (bed rest, hindlimb unloading, immobilization, spaceflight and reduced step) can result in a significant loss of musculoskeletal mass, size and strength which ultimately lead to muscle atrophy. With advancement in understanding of the molecular and cellular mechanisms involved in disuse skeletal muscle atrophy, several different signaling pathways have been studied to understand their regulatory role in this process. However, substantial gaps exist in our understanding of the regulatory mechanisms involved, as well as their functional significance. This review aims to update the current state of knowledge and the underlying cellular mechanisms related to skeletal muscle loss during a variety of unloading conditions, both in humans and animals. Recent advancements in understanding of cellular and molecular mechanisms, including IGF1-Akt-mTOR, MuRF1/MAFbx, FOXO, and potential triggers of disuse atrophy, such as calcium overload and ROS overproduction, as well as their role in skeletal muscle protein adaptation to disuse is emphasized. We have also elaborated potential therapeutic countermeasures that have shown promising results in preventing and restoring disuse-induced muscle loss. Finally, identified are the key challenges in this field as well as some future prospectives.

Integration of miRNA and mRNA expression profiles reveals microRNA-regulated networks during muscle wasting in cardiac cachexia

Cardiac cachexia (CC) is a common complication of heart failure (HF) associated with muscle wasting and poor patient prognosis. Although different mechanisms have been proposed to explain muscle wasting during CC, its pathogenesis is still not understood. Here, we described an integrative analysis between miRNA and mRNA expression profiles of muscle wasting during CC. Global gene expression profiling identified 1,281 genes and 19 miRNAs differentially expressed in muscle wasting during CC. Several of these deregulated genes are known or putative targets of the altered miRNAs, including miR-29a-3p, miR-29b-3p, miR-210-5p, miR-214, and miR-489. Gene ontology analysis on integrative mRNA/miRNA expression profiling data revealed miRNA interactions affecting genes that regulate extra-cellular matrix (ECM) organization, proteasome protein degradation, citric acid cycle and respiratory electron transport. We further identified 11 miRNAs, including miR-29a-3p and miR-29b-3p, which target 21 transcripts encoding the collagen proteins related to ECM organization. Integrative miRNA and mRNA global expression data allowed us to identify miRNA target genes involved in skeletal muscle wasting in CC. Our functional experiments in C2C12 cells confirmed that miR-29b down-regulates collagen genes and contributes to muscle cell atrophy. Collectively, our results suggest that key ECM-associated miRNAs and their target genes may contribute to CC in HF.

Tetramethylpyrazine ameliorated disuse-induced gastrocnemius muscle atrophy in hindlimb unloading rats through suppression of Ca2+/ROS-mediated apoptosis

The purpose of this study was to examine the possible mechanism underlying the protective effect of tetramethylpyrazine (TMP) against disuse-induced muscle atrophy. Sprague−Dawley rats were randomly assigned to receive 14 days of hindlimb unloading (HLU, a model of disuse atrophy) or cage controls. The rats were given TMP (60 mg/kg body mass) or vehicle (water) by gavage. Compared with vehicle treatment, TMP significantly attenuated the loss of gastrocnemius muscle mass (−33.56%, P < 0.01), the decrease of cross-sectional area of slow fiber (−10.99%, P < 0.05) and fast fiber (−15.78%, P < 0.01) during HLU. Although TMP failed to further improve recovery of muscle function or fatigability compared with vehicle treatment, it can suppress the higher level of lactate (−22.71%, P < 0.01) induced by HLU. Besides, TMP could effectually reduce the increased protein expression of muscle RING-finger protein 1 induced by HLU (−14.52%, P < 0.01). Furthermore, TMP can ameliorate the calcium overload (−54.39%, P < 0.05), the increase of malondialdehyde content (−19.82%, P < 0.05), the decrease of superoxide dismutase activity (21.34%, P < 0.05), and myonuclear apoptosis (−78.22%, P < 0.01) induced by HLU. Moreover, TMP significantly reduced HLU-induced increase of Bax to B-cell lymphoma 2 (−36.36%, P < 0.01) and cytochrome c release (−36.16%, P < 0.05). In conclusion, TMP attenuated HLU-induced gastrocnemius muscle atrophy through suppression of Ca2+/reactive oxygen species increase and consequent proteolysis and apoptosis. Therefore, TMP might exhibit therapeutic effect against oxidative stress, cytosolic calcium overload, and mitochondrial damage in disuse-induced muscle atrophy.

Prevention of muscle wasting and osteoporosis: the value of examining novel animal models

Bone mass and skeletal muscle mass are controlled by factors such as genetics, diet and nutrition, growth factors and mechanical stimuli. Whereas increased mechanical loading of the musculoskeletal system stimulates an increase in the mass and strength of skeletal muscle and bone, reduced mechanical loading and disuse rapidly promote a decrease in musculoskeletal mass, strength and ultimately performance (i.e. muscle atrophy and osteoporosis). In stark contrast to artificially immobilised laboratory mammals, animals that experience natural, prolonged bouts of disuse and reduced mechanical loading, such as hibernating mammals and aestivating frogs, consistently exhibit limited or no change in musculoskeletal performance. What factors modulate skeletal muscle and bone mass, and what physiological and molecular mechanisms protect against losses of muscle and bone during dormancy and following arousal? Understanding the events that occur in different organisms that undergo natural periods of prolonged disuse and suffer negligible musculoskeletal deterioration could not only reveal novel regulatory factors but also might lead to new therapeutic options. Here, we review recent work from a diverse array of species that has revealed novel information regarding physiological and molecular mechanisms that dormant animals may use to conserve musculoskeletal mass despite prolonged inactivity. By highlighting some of the differences and similarities in musculoskeletal biology between vertebrates that experience disparate modes of dormancy, it is hoped that this Review will stimulate new insights and ideas for future studies regarding the regulation of atrophy and osteoporosis in both natural and clinical models of muscle and bone disuse.

Neo-epitope Peptides as Biomarkers of Disease Progression for Muscular Dystrophies and Other Myopathies

For several decades, serological biomarkers of neuromuscular diseases as dystrophies, myopathies and myositis have been limited to routine clinical biochemistry panels. Gauging the pathological progression is a prerequisite for proper treatment and therefore identifying accessible, easy to monitor biomarkers that can predict the disease progression would be an important advancement. Most muscle diseases involve accelerated muscle fiber degradation, inflammation, fatty tissue substitution and/or fibrosis. All these pathological traits have been shown to give rise to serological peptide biomarkers in other tissues, underlining the potential application of existing biomarkers of such traits in muscle disorders. A significant quantity of tissue is involved in these pathological mechanisms alongside with qualitative changes in protein turnover in myofibrillar, extra-cellular matrix and immunological cell protein fractions accompanied by alterations in body fluids. We propose that protein and peptides can leak out of the afflicted muscles and can be of use in diagnosis, prediction of pathology trajectory and treatment efficacy. Proteolytic cleavage systems are especially modulated during a range of muscle pathologies, thereby giving rise to peptides that are differentially released during disease manifestation. Therefore, we believe that pathology-specific post-translational modifications like cleavages can give rise to neoepitope peptides that may represent a promising class of peptides for discovery of biomarkers pertaining to neuromuscular diseases.

The Molecular Mechanisms of Calpains Action on Skeletal Muscle Atrophy

Skeletal muscle atrophy is associated with a loss of muscle protein which may result from both increased proteolysis and decreased protein synthesis. Investigations on cell signaling pathways that regulate muscle atrophy have promoted our understanding of this complicated process. Emerging evidence implicates that calpains play key roles in dysregulation of proteolysis seen in muscle atrophy. Moreover, studies have also shown that abnormally activated calpain results muscle atrophy via its downstream effects on ubiquitin-proteasome pathway (UPP) and Akt phosphorylation. This review will discuss the role of calpains in regulation of skeletal muscle atrophy mainly focusing on its collaboration with either UPP or Akt in atrophy conditions in hope to stimulate the interest in development of novel therapeutic interventions for skeletal muscle atrophy.

Gadd45a Protein Promotes Skeletal Muscle Atrophy by Forming a Complex with the Protein Kinase MEKK4

Skeletal muscle atrophy is a serious and highly prevalent condition that remains poorly understood at the molecular level. Previous work found that skeletal muscle atrophy involves an increase in skeletal muscle Gadd45a expression, which is necessary and sufficient for skeletal muscle fiber atrophy. However, the direct mechanism by which Gadd45a promotes skeletal muscle atrophy was unknown. To address this question, we biochemically isolated skeletal muscle proteins that associate with Gadd45a as it induces atrophy in mouse skeletal muscle fibers in vivo. We found that Gadd45a interacts with multiple proteins in skeletal muscle fibers, including, most prominently, MEKK4, a mitogen-activated protein kinase kinase kinase that was not previously known to play a role in skeletal muscle atrophy. Furthermore, we found that, by forming a complex with MEKK4 in skeletal muscle fibers, Gadd45a increases MEKK4 protein kinase activity, which is both sufficient to induce skeletal muscle fiber atrophy and required for Gadd45a-mediated skeletal muscle fiber atrophy. Together, these results identify a direct biochemical mechanism by which Gadd45a induces skeletal muscle atrophy and provide new insight into the way that skeletal muscle atrophy occurs at the molecular level.

Tenotomy immobilization as a model to investigate skeletal muscle fibrosis (with emphasis on Secreted frizzled-related protein 2)

The pathological endpoint of congenital and senile myopathies is chronic muscle degeneration characterized by the atrophy of contractile elements, accompanied by fibrosis and fatty infiltration of the interstitium. Tenotomy is the release of preload that causes abrupt shortening of the muscle and models atrophy and fibrosis without prominent inflammatory response. Fibrosis in the skeletal muscle is known to be triggered by transforming growth factor (TGF)-β, which is activated by inflammatory events. As these were lacking, tenotomy provided an opportunity to investigate transcriptional events on a background without inflammation. An unbiased look at the transcriptome of tenotomy-immobilized soleus muscle revealed that the majority of the transcriptional changes took place in the first 4 wk. Regarding atrophy, proteasomal and lysosomal pathways were actively involved in accompanying cathepsins and calpains in the breakdown of the macromolecular contractile machinery. The transcriptome provided clear-cut evidence for the upregulation of collagens and several extracellular matrix components that define fibrotic remodeling of the skeletal muscle architecture as well as activation of the fibro-adipogenic precursors. Concomitantly, Sfrp2, a Wnt antagonist as well as a procollagen processor, accompanied fibrosis in skeletal muscle with an expression that was stringently confined to the slow-twitch fibers. An interpreted mechanistic scenario construed the kinetic events initiated through the abnormal shortening of the muscle fibers as enough to trigger the resident latent TGF-β in the extracellular matrix, leading to the activation of fibroadipogenic precursors. As in the heart, Sfrp2 shows itself to be a therapeutic target for the prevention of irreversible fibrosis in degenerative skeletal muscle conditions.

Spatial Geometries of Self-Assembled Chitohexaose Monolayers Regulate Myoblast Fusion

Myoblast fusion into functionally-distinct myotubes to form in vitro skeletal muscle constructs under differentiation serum-free conditions still remains a challenge. Herein, we report that our microtopographical carbohydrate substrates composed of bioactive hexa-N-acetyl-d-glucosamine (GlcNAc6) modulated the efficiency of myoblast fusion without requiring horse serum or any differentiation medium during cell culture. Promotion of the differentiation of dissociated mononucleated skeletal myoblasts (C2C12; a mouse myoblast cell line) into robust myotubes was found only on GlcNAc6 micropatterns, whereas the myoblasts on control, non-patterned GlcNAc6 substrates or GlcNAc6-free patterns exhibited an undifferentiated form. We also examined the possible role of GlcNAc6 micropatterns with various widths in the behavior of C2C12 cells in early and late stages of myogenesis through mRNA expression of myosin heavy chain (MyHC) isoforms. The spontaneous contraction of myotubes was investigated via the regulation of glucose transporter type 4 (GLUT4), which is involved in stimulating glucose uptake during cellular contraction. Narrow patterns demonstrated enhanced glucose uptake rate and generated a fast-twitch muscle fiber type, whereas the slow-twitch muscle fiber type was dominant on wider patterns. Our findings indicated that GlcNAc6-mediated integrin interactions are responsible for guiding myoblast fusion forward along with myotube formation.

Muscle wasting and aging: Experimental models, fatty infiltrations, and prevention

Identification of cost-effective interventions to maintain muscle mass, muscle strength, and physical performance during muscle wasting and aging is an important public health challenge. It requires understanding of the cellular and molecular mechanisms involved. Muscle-deconditioning processes have been deciphered by means of several experimental models, bringing together the opportunities to devise comprehensive analysis of muscle wasting. Studies have increasingly recognized the importance of fatty infiltrations or intermuscular adipose tissue for the age-mediated loss of skeletal-muscle function and emphasized that this new important factor is closely linked to inactivity. The present review aims to address three main points. We first mainly focus on available experimental models involving cell, animal, or human experiments on muscle wasting. We next point out the role of intermuscular adipose tissue in muscle wasting and aging and try to highlight new findings concerning aging and muscle-resident mesenchymal stem cells called fibro/adipogenic progenitors by linking some cellular players implicated in both FAP fate modulation and advancing age. In the last part, we review the main data on the efficiency and molecular and cellular mechanisms by which exercise, replacement hormone therapies, and β-hydroxy-β-methylbutyrate prevent muscle wasting and sarcopenia. Finally, we will discuss a potential therapeutic target of sarcopenia: glucose 6-phosphate dehydrogenase.

Maternal conjugated linoleic acid supplementation reverses high-fat diet-induced skeletal muscle atrophy and inflammation in adult male rat offspring

A high-saturated-fat diet (HFD) during pregnancy and lactation leads to metabolic disorders in offspring concomitant with increased adiposity and a proinflammatory phenotype in later life. During the fetal period, the impact of maternal diet on skeletal muscle development is poorly described, despite this tissue exerting a major influence on life-long metabolic health. This study investigated the effect of a maternal HFD on skeletal muscle anabolic, catabolic, and inflammatory signaling in adult rat offspring. Furthermore, the actions of maternal-supplemented conjugated linoleic acid (CLA) on these measures of muscle phenotype were investigated. A purified control diet (CD; 10% kcal fat), a CD supplemented with CLA (CLA; 10% kcal fat, 1% total fat as CLA), a high-fat (HFD; 45% kcal fat from lard), or a HFD supplemented with CLA (HFCLA; 45% kcal fat from lard, 1% total fat as CLA) was fed ad libitum to female Sprague-Dawley rats for 10 days before mating and throughout gestation and lactation. Male offspring received a standard chow diet from weaning, and the gastrocnemius was collected for analysis at day 150. Offspring from HF and HFCLA mothers displayed lower muscular protein content accompanied by elevated monocyte chemotactic protein-1, IL-6, and IL-1β concentrations. Phosphorylation of NF-κBp65 (Ser536) and expression of the catabolic E3 ligase muscle ring finger 1 (MuRF1) were increased in HF offspring, an effect reversed by maternal CLA supplementation. The present study demonstrates the importance of early life interventions to ameliorate the negative effects of poor maternal diet on offspring skeletal muscle development.

In Vivo Rodent Models of Skeletal Muscle Adaptation to Decreased Use

Skeletal muscle possesses plasticity and adaptability to external and internal physiological changes. Due to these characteristics, skeletal muscle shows dramatic changes depending on its response to stimuli such as physical activity, nutritional changes, disease status, and environmental changes. Modulation of the rate of protein synthesis/degradation plays an important role in atrophic responses. The purpose of this review is to describe different features of skeletal muscle adaptation with various models of deceased use. In this review, four models were addressed: immobilization, spinal cord transection, hindlimb unloading, and aging. Immobilization is a form of decreased use in which skeletal muscle shows electrical activity, tension development, and motion. These results differ by muscle group. Spinal cord transection was selected to simulate spinal cord injury. Similar to the immobilization model, dramatic atrophy occurs in addition to fiber type conversion in this model. Despite the fact that electromyography shows unremarkable changes in muscle after hindlimb unloading, decreased muscle mass and contractile force are observed. Lastly, aging significantly decreases the numbers of muscle fibers and motor units. Skeletal muscle responses to decreased use include decreased strength, decreased fiber numbers, and fiber type transformation. These four models demonstrated different changes in the skeletal muscle. This review elucidates the different skeletal muscle adaptations in these four decreased use animal models and encourages further studies.

Comparative Analysis of Muscle Transcriptome between Pig Genotypes Identifies Genes and Regulatory Mechanisms Associated to Growth, Fatness and Metabolism

Iberian ham production includes both purebred (IB) and Duroc-crossbred (IBxDU) Iberian pigs, which show important differences in meat quality and production traits, such as muscle growth and fatness. This experiment was conducted to investigate gene expression differences, transcriptional regulation and genetic polymorphisms that could be associated with the observed phenotypic differences between IB and IBxDU pigs. Nine IB and 10 IBxDU pigs were slaughtered at birth. Morphometric measures and blood samples were obtained and samples from Biceps femoris muscle were employed for compositional and transcriptome analysis by RNA-Seq technology. Phenotypic differences were evident at this early age, including greater body size and weight in IBxDU and greater Biceps femoris intramuscular fat and plasma cholesterol content in IB newborns. We detected 149 differentially expressed genes between IB and IBxDU neonates (p < 0.01 and Fold-Change > 1. 5). Several were related to adipose and muscle tissues development (DLK1, FGF21 or UBC). The functional interpretation of the transcriptomic differences revealed enrichment of functions and pathways related to lipid metabolism in IB and to cellular and muscle growth in IBxDU pigs. Protein catabolism, cholesterol biosynthesis and immune system were functions enriched in both genotypes. We identified transcription factors potentially affecting the observed gene expression differences. Some of them have known functions on adipogenesis (CEBPA, EGRs), lipid metabolism (PPARGC1B) and myogenesis (FOXOs, MEF2D, MYOD1), which suggest a key role in the meat quality differences existing between IB and IBxDU hams. We also identified several polymorphisms showing differential segregation between IB and IBxDU pigs. Among them, non-synonymous variants were detected in several transcription factors as PPARGC1B and TRIM63 genes, which could be associated to altered gene function. Taken together, these results provide information about candidate genes, metabolic pathways and genetic polymorphisms potentially involved in phenotypic differences between IB and IBxDU pigs associated to meat quality and production traits.

Microarray Analysis Gene Expression Profiles in Laryngeal Muscle After Recurrent Laryngeal Nerve Injury

OBJECTIVES

The pathophysiology of recurrent laryngeal nerve (RLN) transection injury is rare in that it is characteristically followed by a high degree of spontaneous reinnervation, with reinnervation of the laryngeal adductor complex (AC) preceding that of the abducting posterior cricoarytenoid (PCA) muscle. Here, we aim to elucidate the differentially expressed myogenic factors following RLN injury that may be at least partially responsible for the spontaneous reinnervation.

METHODS

F344 male rats underwent RLN injury (n = 12) or sham surgery (n = 12). One week after RLN injury, larynges were harvested following euthanasia. The mRNA was extracted from PCA and AC muscles bilaterally, and microarray analysis was performed using a full rat genome array.

RESULTS

Microarray analysis of denervated AC and PCA muscles demonstrated dramatic differences in gene expression profiles, with 205 individual probes that were differentially expressed between the denervated AC and PCA muscles and only 14 genes with similar expression patterns.

CONCLUSIONS

The differential expression patterns of the AC and PCA suggest different mechanisms of reinnervation. The PCA showed the gene patterns of Wallerian degeneration, while the AC expressed the gene patterns of reinnervation by adjacent axonal sprouting. This finding may reveal important therapeutic targets applicable to RLN and other peripheral nerve injuries.

Effects of age and hindlimb immobilization and remobilization on fast troponin T precursor mRNA alternative splicing in rat gastrocnemius muscle

Fast skeletal muscle troponin T (TNNT3) is an important component of the skeletal muscle contractile machinery. The precursor mRNA (pre-mRNA) encoding TNNT3 is alternatively spliced, and changes in the pattern of TNNT3 splice form expression are associated with alterations in thin-filament calcium sensitivity and force production during muscle contraction and thereby regulate muscle function. Interestingly, during aging, the muscle force/cross-sectional area is reduced, suggesting that loss of mass does not completely account for the impaired muscle function that develops during the aging process. Therefore, in this study, we tested the hypothesis that age and changes in muscle loading are associated with alterations in Tnnt3 alternative splicing in the rat gastrocnemius muscle. We found that the relative abundance of several Tnnt3 splice forms varied significantly with age among 2-, 9-, and 18-month-old rats and that the pattern correlated with changes in body mass rather than muscle mass. Hindlimb immobilization for 7 days resulted in dramatic alterations in splice form relative abundance such that the pattern was similar to that observed in lighter animals. Remobilization for 7 days restored the splicing pattern toward that observed in the nonimmobilized limb, even though muscle mass had not yet begun to recover. In conclusion, the results suggest that Tnnt3 pre-mRNA alternative splicing is modulated rapidly (i.e., within days) in response to changes in the load placed on the muscle. Moreover, the results show that restoration of Tnnt3 alternative splicing to control patterns is initiated prior to an increase in muscle mass.

Vitamin D treatment protects against and reverses oxidative stress induced muscle proteolysis

Vitamin D is known to have a biological role in many extra skeletal tissues in the body including muscle. Vitamin D deficiency has been associated with preferential atrophy of type II fibres in human muscle. Vitamin D at physiological concentrations is known to protect cells against oxidative damage. In this study we examined whether vitamin D deficiency induces muscle oxidative stress in a rat model and further if pre or post treatment of C2C12 muscle cells with vitamin D offers protection against oxidative stress induced muscle proteolysis. Protein carbonylation as a marker of protein oxidation was increased in both the deficient muscle and vehicle-treated C2C12 cells. Vitamin D deficiency led to an increase in activities of the glutathione-dependent enzymes and decrease in SOD and catalase enzymes in the rat muscle. Higher nitrate levels indicative of nitrosative stress were observed in the deficient muscle compared to control muscle. Rehabilitation with vitamin D could reverse the alterations in oxidative and nitrosative stress parameters. Increase in total protein degradation, 20S proteasomal enzyme activity, muscle atrophy gene markers and expression of proteasome subunit genes induced by oxidative stress were corrected both by pre/post treatment of C2C12 muscle cells with vitamin D. Increase in SOD activity in the presence of vitamin D indicates antioxidant potential of vitamin D in the muscle. The data presented indicates that vitamin D deficiency leads to mild oxidative stress in the muscle which may act as a trigger for increased proteolysis in the vitamin D deficient muscle.