Deficiency of inducible nitric oxide synthase attenuates immobilization-induced skeletal muscle atrophy in mice

Effects of resistance training on alleviating hypoxia-induced muscle atrophy: Focus on acetylation of FoxO1

This study aims to explore the role of FoxO1 and its acetylation in the alleviation of hypoxia-induced muscle atrophy by resistance training. Forty male Sprague-Dawley rats were randomly divided into four groups: normoxic control group (C), normoxic resistance training group (R), hypoxic control group (H) and hypoxic resistance training group (HR). Rats in R and HR groups were trained on an incremental weight-bearing ladder every other day, while those in H and HR groups were kept in an environment containing 12.4% O2 . After 4 weeks, muscles were collected for analysis. Differentiated L6 myoblasts were analysed in vitro after hypoxia exposure and plasmids transfection (alteration in FoxO1 acetylation). The lean body mass loss, wet weight and fibre cross-sectional area of extensor digitorum longus of rats were decreased after 4 weeks hypoxia, and the adverse reactions above was reversed by resistance training. At the same time, the increase in hypoxia-induced autophagy was suppressed, which was accompanied by a decrease in the expression of nuclear FoxO1 and cytoplasmic Ac-FoxO1 by resistance training. The L6 myotube diameter increased and the expression of autophagic proteins were inhibited under hypoxia via intervening by FoxO1 deacetylation. Overall, resistance training alleviates hypoxia-induced muscle atrophy by inhibiting nuclear FoxO1 and cytoplasmic Ac-FoxO1-mediated autophagy.

α-Hydroxyisocaproic Acid Decreases Protein Synthesis but Attenuates TNFα/IFNγ Co-Exposure-Induced Protein Degradation and Myotube Atrophy via Suppression of iNOS and IL-6 in Murine C2C12 Myotube

There is ongoing debate as to whether or not α-hydroxyisocaproic acid (HICA) positively regulates skeletal muscle protein synthesis resulting in the gain or maintenance of skeletal muscle. We investigated the effects of HICA on mouse C2C12 myotubes under normal conditions and during cachexia induced by co-exposure to TNFα and IFNγ. The phosphorylation of AMPK or ERK1/2 was significantly altered 30 min after HICA treatment under normal conditions. The basal protein synthesis rates measured by a deuterium-labeling method were significantly lowered by the HICA treatment under normal and cachexic conditions. Conversely, myotube atrophy induced by TNFα/IFNγ co-exposure was significantly improved by the HICA pretreatment, and this improvement was accompanied by the inhibition of iNOS expression and IL-6 production. Moreover, HICA also suppressed the TNFα/IFNγ co-exposure-induced secretion of 3-methylhistidine. These results demonstrated that HICA decreases basal protein synthesis under normal or cachexic conditions; however, HICA might attenuate skeletal muscle atrophy via maintaining a low level of protein degradation under cachexic conditions.

Peroxiredoxin2 Deficiency Aggravates Aging-Induced Insulin Resistance and Declines Muscle Strength

This study examined the role of peroxiredoxin2 (Prx2) in aging-induced insulin resistance and reduction in skeletal muscle function in young (2-month-old) and old (24-month-old) Prx2 knockout (KO) and wild-type mice. Plasma insulin levels increased with aging in Prx2 KO mice but not in wild-type mice. Insulin sensitivity in the whole-body and skeletal muscle as assessed with the hyperinsulinemic-euglycemic clamp was lower in Prx2 KO mice than in wild-type mice in the old group but was not significantly different between the two genotypes in the young group. Insulin-induced activation of intracellular signaling molecules was also suppressed in old Prx2 KO mice compared to their wild-type littermates. Oxidative stress, inflammation, and p53 expression levels in skeletal muscle were higher in Prx2 KO mice than in wild-type mice in the old group but were not different between the two genotypes in the young group. p53 expression was negatively correlated with skeletal muscle insulin sensitivity in old mice. Skeletal muscle mass was similar between the two genotypes but grip strength was reduced in old Prx2 KO mice compared to old wild-type mice. These results suggest that Prx2 plays a protective role in aging-induced insulin resistance and declines in muscle strength by suppressing oxidative stress.

Beta-lapachone attenuates immobilization-induced skeletal muscle atrophy in mice

Skeletal muscle atrophy reduces quality of life and increases morbidity and mortality in patients with chronic conditions. Oxidative stress is a key factor contributing to skeletal muscle atrophy by altering both protein synthesis and protein degradation pathways. Beta-lapachone (Beta-L) is known to act as a pro-oxidant in cancer cells but suppresses oxidative stress in normal cells and tissues. In the present study, we examined whether Beta-L (100 mg/kg body weight) prevents immobilization-induced skeletal muscle atrophy in male C57BL/6N mice. Skeletal muscle atrophy was induced by immobilization of left hindlimbs for two weeks, and right hindlimbs were used as controls. The muscle weights of gastrocnemius (0.132 ± 0.003 g vs. 0.115 ± 0.003 g in Beta-L and SLS, respectively, p < 0.01) and tibialis anterior (0.043 ± 0.001 vs. 0.027 ± 0.002 in Beta-L and SLS, respectively, p < 0.001) were significantly heavier in Beta-L-treated mice than that in SLS-treated mice in immobilization group, which was accompanied by improved skeletal muscle function as tested by treadmill exhaustion and grip strength test. Immobilization increased H₂O₂ levels, while Beta-L treatment normalized such levels (1.6 ± 0.16 μM vs. 2.7 ± 0.44 μM in Beta-L and vehicle, respectively, p < 0.05). Oxidative stress makers were also normalized by Beta-L treatment. Protein synthesis signaling pathways were unaltered in the case of both immobilization and Beta-L treatment. However, protein catabolic, ubiquitin-proteasomal, and autophagy-lysosomal pathways were stimulated by immobilization and were normalized by Beta-L treatment. Upregulation of transforming growth factor β and Smad 2/3 after immobilization was significantly diminished by Beta-L treatment. These results suggest that Beta-L attenuates the loss of muscle weight and function induced by immobilization through suppression of oxidative stress.

Glutathione and Nitric Oxide: Key Team Players in Use and Disuse of Skeletal Muscle

Glutathione (GSH) is the main non-enzymatic antioxidant playing an important role in detoxification, signal transduction by modulation of protein thiols redox status and direct scavenging of radicals. The latter function is not only performed against reactive oxygen species (ROS) but GSH also has a fundamental role in buffering nitric oxide (NO), a physiologically-produced molecule having-multifaceted functions. The efficient rate of GSH synthesis and high levels of GSH-dependent enzymes are characteristic features of healthy skeletal muscle where, besides the canonical functions, it is also involved in muscle contraction regulation. Moreover, NO production in skeletal muscle is a direct consequence of contractile activity and influences several metabolic myocyte pathways under both physiological and pathological conditions. In this review, we will consider the homeostasis and intersection of GSH with NO and then we will restrict the discussion on their role in processes related to skeletal muscle function and degeneration.

Muscle Changes During Atrophy

Muscle atrophy typically is a direct effect of protein degradation induced by a diversity of pathophysiologic states such as disuse, immobilization, denervation, aging, sepsis, cachexia, glucocorticoid treatment, hereditary muscular disorders, cancer, diabetes and obesity, kidney and heart failure, and others. Muscle atrophy is defined by changes in the muscles, consisting in shrinkage of myofibers, changes in the types of fiber and myosin isoforms, and a net loss of cytoplasm, organelles and overall a protein loss. Although in the literature there are extensive studies in a range of animal models, the paucity of human data is a reality. This chapter is focused on various aspects of muscle wasting and describes the transitions of myofiber types during the progression of muscle atrophy in several pathological states. Clinical conditions associated with muscle atrophy have been grouped based on the fast-to-slow or slow-to-fast fiber-type shifts. We have also summarized the ultrastructural and histochemical features characteristic for muscle atrophy in clinical and experimental models for aging, cancer, diabetes and obesity, and heart failure and arrhythmia.

TLR4-defective (C3H/HeJ) mice are not protected from cast immobilization-induced muscle atrophy

Recent studies have shown that activation of Toll‐like receptor (TLR)4 signaling may be an important factor in muscle atrophy and excessive inflammatory response associated with immobilization. To examine the role of TLR4 signaling on cast immobilization‐induced skeletal muscle atrophy, we tested the hypothesis that muscle atrophy and inflammation after cast immobilization is reduced in TLR4‐defective mice. TLR4‐defective (C3H/HeJ) and wild type (C3H/HeN) mice were divided into control and cast‐immobilization groups. Cast immobilization was imposed for 14 days. Cast immobilization increased TLR4 mRNA expression in the gastrocnemius and decreased muscle mass and cross‐sectional area (CSA) of the gastrocnemius fibers. However, there was no difference in the gastrocnemius muscle mass and CSA between TLR4‐defective and wild type mice. Cast immobilization‐induced increase in ubiquitin E3 ligases (MAFbx/Atrogin‐1 and MuRF1), inflammatory cytokines, and macrophage/monocyte marker mRNAs were unaffected by defective TLR4. Our findings in C3H/HeJ mice suggested that TLR4 signaling might not play an essential role in immobilization‐induced muscle atrophy.

IL-1β-Induced Accumulation of Amyloid: Macroautophagy in Skeletal Muscle Depends on ERK

The pathology of inclusion body myositis (IBM) involves an inflammatory response and β-amyloid deposits in muscle fibres. It is believed that MAP kinases such as the ERK signalling pathway mediate the inflammatory signalling in cells. Further, there is evidence that autophagic activity plays a crucial role in the pathogenesis of IBM. Using a well established in vitro model of IBM, the autophagic pathway, MAP kinases, and accumulation of β-amyloid were examined. We demonstrate that stimulation of muscle cells with IL-1β and IFN-γ led to an increased phosphorylation of ERK. The ERK inhibitor PD98059 diminished the expression of proinflammatory markers as well as the accumulation of β-amyloid. In addition, IL-1β and IFN-γ led to an increase of autophagic activity, upregulation of APP, and subsequent accumulation of β-sheet aggregates. Taken together, the data demonstrate that the ERK pathway contributes to formation of β-amyloid and regulation of autophagic activity in muscle cells exposed to proinflammatory cell stress. This suggests that ERK serves as an important mediator between inflammatory mechanisms and protein deposition in skeletal muscle and is a crucial element of the pathology of IBM.

L-Citrulline Protects Skeletal Muscle Cells from Cachectic Stimuli through an iNOS-Dependent Mechanism

Dietary L-citrulline is thought to modulate muscle protein turnover by increasing L-arginine availability. To date, the direct effects of increased L-citrulline concentrations in muscle have been completely neglected. Therefore, we determined the role of L-citrulline in regulating cell size during catabolic conditions by depriving mature C2C12 myotubes of growth factors (serum free; SF) or growth factors and nutrients (HEPES buffered saline; HBS). Cells were treated with L-citrulline or equimolar concentrations of L-arginine (positive control) or L-alanine (negative control) and changes in cell size and protein turnover were assessed. In myotubes incubated in HBS or SF media, L-citrulline improved rates of protein synthesis (HBS: +63%, SF: +37%) and myotube diameter (HBS: +18%, SF: +29%). L-citrulline treatment substantially increased iNOS mRNA expression (SF: 350%, HBS: 750%). The general NOS inhibitor L-NAME and the iNOS specific inhibitor aminoguanidine prevented these effects in both models. Depriving myotubes in SF media of L-arginine or L-leucine, exacerbated wasting which was not attenuated by L-citrulline. The increased iNOS mRNA expression was temporally associated with increases in mRNA of the endogenous antioxidants SOD1, SOD3 and catalase. Furthermore, L-citrulline prevented inflammation (LPS) and oxidative stress (H₂O₂) induced muscle cell wasting. In conclusion, we demonstrate a novel direct protective effect of L-citrulline on skeletal muscle cell size independent of L-arginine that is mediated through induction of the inducible NOS (iNOS) isoform. This discovery of a nutritional modulator of iNOS mRNA expression in skeletal muscle cells could have substantial implications for the treatment of muscle wasting conditions.

Multifidus Muscle Changes After Back Injury Are Characterized by Structural Remodeling of Muscle, Adipose and Connective Tissue, but Not Muscle Atrophy: Molecular and Morphological Evidence

STUDY DESIGN

Longitudinal case-controlled animal study.

OBJECTIVE

To investigate putative cellular mechanisms to explain structural changes in muscle and adipose and connective tissues of the back muscles after intervertebral disc (IVD) injury.

SUMMARY OF BACKGROUND DATA

Structural back muscle changes are ubiquitous with back pain/injury and considered relevant for outcome, but their exact nature, time course, and cellular mechanisms remain elusive. We used an animal model that produces phenotypic back muscle changes after IVD injury to study these issues at the cellular/molecular level.

METHODS

Multifidus muscle was harvested from both sides of the spine at L1-L2 and L3-L4 IVDs in 27 castrated male sheep at 3 (n = 10) or 6 (n = 17) months after a surgical anterolateral IVD injury at both levels. Ten control sheep underwent no surgery (3 mo, n = 4; 6 mo, n = 6). Tissue was harvested at L4 for histological analysis of cross-sectional area of muscle and adipose and connective tissue (whole muscle), plus immunohistochemistry to identify proportion and cross-sectional area of individual muscle fiber types in the deepest fascicle. Quantitative polymerase chain reaction measured gene expression of typical cytokines/signaling molecules at L2.

RESULTS

Contrary to predictions, there was no multifidus muscle atrophy (whole muscle or individual fiber). There was increased adipose and connective tissue (fibrotic proliferation) cross-sectional area and slow-to-fast muscle fiber transition at 6 but not 3 months. Within the multifidus muscle, increases in the expression of several cytokines (tumor necrosis factor α and interleukin-1β) and molecules that signal trophic/atrophic processes for the 3 tissue types (e.g., growth factor pathway [IGF-1, PI3k, Akt1, mTOR], potent tissue modifiers [calcineurin, PCG-1α, and myostatin]) were present.

CONCLUSION

This study provides cellular evidence that refutes the presence of multifidus muscle atrophy accompanying IVD degeneration at this intermediate time point. Instead, adipose/connective tissue increased in parallel with the expression of the genes that provide putative mechanisms for multifidus structural remodeling. This provides novel targets for pharmacological and physical interventions.

LEVEL OF EVIDENCE

N/A.

TLR2 deficiency attenuates skeletal muscle atrophy in mice

Oxidative stress and inflammation are associated with skeletal muscle atrophy. Because the activation of toll-like receptor (TLR) 2 induces oxidative stress and inflammation, TLR2 may be directly linked to skeletal muscle atrophy. This study examined the role of TLR2 in skeletal muscle atrophy in wild-type (WT) and TLR2 knockout (KO) mice. Immobilization for 2 weeks increased the expression of cytokine genes and the levels of carbonylated proteins and nitrotyrosine in the skeletal muscle, but these increases were lower in the TLR2 KO mice. Muscle weight loss and a reduction in treadmill running times induced by immobilization were also attenuated in TLR2 KO mice. Furthermore, immobilization increased the protein levels of forkhead box O 1/3, atrogin-1 and muscle ring finger 1 in the WT mice, which was attenuated in TLR2 KO mice. In addition, immobilization-associated increases in ubiquitinated protein levels were lower in the TLR2 KO mice. Immobilization increased the phosphorylation of Akt and p70S6K similarly in WT and KO mice. Furthermore, cardiotoxin injection into the skeletal muscle increased the protein levels of atrogin-1, interleukin-6, and nitrotyrosine and increased the levels of ubiquitinated proteins, although these levels were increased to a lesser extent in TLR2 KO mice. These results suggest that TLR2 is involved in skeletal muscle atrophy, and the inhibition of TLR2 offers a potential target for preventing skeletal muscle atrophy.

Deficiency of clusterin exacerbates high-fat diet-induced insulin resistance in male mice

The present study examined the role of clusterin in insulin resistance in high fat-fed wild-type and clusterin knockout (KO) mice. The plasma levels of glucose and C-peptide and islet size were increased in clusterin KO mice after an 8-week high-fat diet. In an ip glucose tolerance test, the area under the curve for glucose was not different, whereas the area under the curve for insulin was higher in clusterin KO mice. In a hyperinsulinemic-euglycemic clamp, the clamp insulin levels were higher in clusterin KO mice after the high-fat diet. After adjusting for the clamp insulin levels, the glucose infusion rate, suppression of hepatic glucose production, and glucose uptake were lower in clusterin KO mice in the high fat-fed group. The plasma levels of clusterin and clusterin mRNA levels in the skeletal muscle and liver were increased by the high-fat diet. The mRNA levels of the antioxidant enzymes were lower, and the mRNA levels of nicotinamide adenine dinucleotide phosphate oxidase (NOX) 1 and cytokines and protein carbonylation were higher in the skeletal muscle and liver in clusterin KO mice after the high-fat diet. Palmitate-induced gene expressions of NOX1 and cytokines were higher in the primary cultured hepatocytes of clusterin KO mice compared with the wild-type mice. Clusterin inhibited the gene expression and reactive oxygen species generation by palmitate in the hepatocytes and C2C12. AKT phosphorylation by insulin was reduced in the hepatocytes of clusterin KO mice. These results suggest that clusterin plays a protective role against high-fat diet-induced insulin resistance through the suppression of oxidative stress and inflammation.

Nutritional strategies to counteract muscle atrophy caused by disuse and to improve recovery

Periods of immobilisation are often associated with pathologies and/or ageing. These periods of muscle disuse induce muscle atrophy which could worsen the pathology or elderly frailty. If muscle mass loss has positive effects in the short term, a sustained/uncontrolled muscle mass loss is deleterious for health. Muscle mass recovery following immobilisation-induced atrophy could be critical, particularly when it is uncompleted as observed during ageing. Exercise, the best way to recover muscle mass, is not always applicable. So, other approaches such as nutritional strategies are needed to limit muscle wasting and to improve muscle mass recovery in such situations. The present review discusses mechanisms involved in muscle atrophy following disuse and during recovery and emphasises the effect of age in these mechanisms. In addition, the efficiency of nutritional strategies proposed to limit muscle mass loss during disuse and to improve protein gain during recovery (leucine supplementation, whey proteins, antioxidants and anti-inflammatory compounds, energy intake) is also discussed.

Paracrine effects of IGF-1 overexpression on the functional decline due to skeletal muscle disuse: molecular and functional evaluation in hindlimb unloaded MLC/mIgf-1 transgenic mice

Slow-twitch muscles, devoted to postural maintenance, experience atrophy and weakness during muscle disuse due to bed-rest, aging or spaceflight. These conditions impair motion activities and can have survival implications. Human and animal studies demonstrate the anabolic role of IGF-1 on skeletal muscle suggesting its interest as a muscle disuse countermeasure. Thus, we tested the role of IGF-1 overexpression on skeletal muscle alteration due to hindlimb unloading (HU) by using MLC/mIgf-1 transgenic mice expressing IGF-1 under the transcriptional control of MLC promoter, selectively activated in skeletal muscle. HU produced atrophy in soleus muscle, in terms of muscle weight and fiber cross-sectional area (CSA) reduction, and up-regulation of atrophy gene MuRF1. In parallel, the disuse-induced slow-to-fast fiber transition was confirmed by an increase of the fast-type of the Myosin Heavy Chain (MHC), a decrease of PGC-1α expression and an increase of histone deacetylase-5 (HDAC5). Consistently, functional parameters such as the resting chloride conductance (gCl) together with ClC-1 chloride channel expression were increased and the contractile parameters were modified in soleus muscle of HU mice. Surprisingly, IGF-1 overexpression in HU mice was unable to counteract the loss of muscle weight and the decrease of fiber CSA. However, the expression of MuRF1 was recovered, suggesting early effects on muscle atrophy. Although the expression of PGC-1α and MHC were not improved in IGF-1-HU mice, the expression of HDAC5 was recovered. Importantly, the HU-induced increase of gCl was fully contrasted in IGF-1 transgenic mice, as well as the changes in contractile parameters. These results indicate that, even if local expression does not seem to attenuate HU-induced atrophy and slow-to-fast phenotype transition, it exerts early molecular effects on gene expression which can counteract the HU-induced modification of electrical and contractile properties. MuRF1 and HDAC5 can be attractive therapeutic targets for pharmacological countermeasures and then deserve further investigations.

Mechanisms for fiber-type specificity of skeletal muscle atrophy

PURPOSE OF REVIEW

There are a variety of pathophysiologic conditions that are known to induce skeletal muscle atrophy. However, muscle wasting can occur through multiple distinct signaling pathways with differential sensitivity between selective skeletal muscle fiber subtypes. This review summarizes some of the underlying molecular mechanisms responsible for fiber-specific muscle mass regulation.

RECENT FINDINGS

Peroxisome proliferator-activated receptor gamma coactivator 1-alpha protects slow-twitch oxidative fibers from denervation/immobilization (disuse)-induced muscle atrophies. Nutrient-related muscle atrophies, such as those induced by cancer cachexia, sepsis, chronic heart failure, or diabetes, are largely restricted to fast-twitch glycolytic fibers, of which the underlying mechanism is usually related to abnormality of protein degradation, including proteasomal and lysosomal pathways. In contrast, nuclear factor kappaB activation apparently serves a dual function by inducing both fast-twitch fiber atrophy and slow-twitch fiber degeneration.

SUMMARY

Fast-twitch glycolytic fibers are more vulnerable than slow-twitch oxidative fibers under a variety of atrophic conditions related to signaling transduction of Forkhead box O family, autophagy inhibition, transforming growth factor beta family, and nuclear factor-kappaB. The resistance of oxidative fibers may result from the protection of peroxisome proliferator-activated receptor gamma coactivator 1-alpha.

Hemin, heme oxygenase-1 inducer, attenuates immobilization-induced skeletal muscle atrophy in mice

AIMS

The present study examined the effect of the heme oxygenase (HO)-1 inducer hemin on skeletal muscle atrophy induced by single limb immobilization in mice.

MAIN METHODS

Immobilization was conducted in the left hindlimb of C57BL/6 mice for 1 week and the right hindlimb was used as a control. Hemin (30 mg/kg) was administered intraperitoneally once a day during the immobilization period. Gastrocnemius muscles were used for analysis. Muscle weight was measured to quantify degree of atrophy, and exhaustion treadmill test was performed to assess muscle function.

KEY FINDINGS

Immobilization increased HO-1 protein levels in skeletal muscle, which was further increased by hemin treatment. Immobilization induced weight loss and a functional reduction in skeletal muscle, which were attenuated by hemin treatment. Gene expression and protein levels of MuRF1 and atrogin-1 were increased by immobilization and hemin treatment attenuated the increment. The phosphorylation of mTOR and p70S6k was decreased by immobilization in skeletal muscle and hemin had no effect on mTOR and p70S6k phosphorylation. Gene expression of the antioxidants superoxide dismutase and glutathione peroxidase 1 in skeletal muscle was reduced by immobilization and hemin treatment recovered the reduction. Immobilization increased levels of carbonylated protein and nitrotyrosine in skeletal muscle, which was reversed by hemin treatment. Gene expression of inflammatory cytokines was increased by immobilization and was normalized as a result of hemin treatment.

SIGNIFICANCE

These results suggest that hemin attenuates immobilization-induced skeletal muscle atrophy through the suppression of protein degradation via its anti-oxidant and anti-inflammatory properties.