Changes in muscle proteins and spermidine content in response to unloading and clenbuterol treatment

Proteomic analysis revealed different responses to hypergravity of soleus and extensor digitorum longus muscles in mice

Investigating protein abundance profiles is important to understand the differences in the slow and fast skeletal muscle characteristics. The profiles in soleus (Sol) and extensor digitorum longus (EDL) muscles in mice exposed to 1 g or 3 g for 28 d were compared. The biological implications of the profiles revealed that hypergravity exposure activated a larger number of pathways involved in protein synthesis in Sol. In contrast, the inactivation of signalling pathways involved in oxidative phosphorylation were conspicuous in EDL. These results suggested that the reactivity of molecular pathways in Sol and EDL differed. Additionally, the levels of spermidine synthase and spermidine, an important polyamine for cell growth, increased in both muscles following hypergravity exposure, whereas the level of spermine oxidase (SMOX) increased in EDL alone. The SMOX level was negatively correlated with spermine content, which is involved in muscle atrophy, and was higher in EDL than Sol, even in the 1 g group. These results indicated that the contribution of SMOX to the regulation of spermidine and spermine contents in Sol and EDL differed. However, contrary to expectations, the difference in the SMOX level did not have a significant impact on the growth of these muscles following hypergravity exposure. SIGNIFICANCE: The skeletal muscle-specific protein abundance profiles result in differences in the characteristics of slow and fast skeletal muscles. We investigated differences in the profiles in mouse slow-twitch Sol and fast-twitch EDL muscles following 28-d of 1 g and 3 g exposure by LC-MS/MS analysis and label-free quantitation. A two-step solubilisation of the skeletal muscle proteins increased the coverage of proteins identified by LC-MS/MS analysis. Additionally, this method reduced the complexity of samples more easily than protein or peptide fractionation by SDS-PAGE and offline HPLC while maintaining the high operability of samples and was reproducible. A larger number of hypergravity-responsive proteins as well as a prominent increase in the wet weights was observed in Sol than EDL muscles. The biological implications of the difference in the protein abundance profiles in 1 g and 3 g groups revealed that the reactivity of each molecular pathway in Sol and EDL muscles to hypergravity exposure differed significantly. In addition, we found that the biosynthetic and interconversion pathway of polyamines, essential factors for cell growth and survival in mammals, was responsive to hypergravity exposure; spermidine and spermine contents in Sol and EDL muscles were regulated by different mechanisms even in the 1 g group. However, our results indicated that the difference in the mechanism regulating polyamine contents is unlikely to have a significant effect on the differences in Sol and EDL muscle growth following hypergravity exposure.

A Systems Biology Approach Using Transcriptomic Data Reveals Genes and Pathways in Porcine Skeletal Muscle Affected by Dietary Lysine

Nine crossbred finishing barrows (body weight 94.4 ± 6.7 kg) randomly assigned to three dietary treatments were used to investigate the effects of dietary lysine on muscle growth related metabolic and signaling pathways. Muscle samples were collected from the longissimus dorsi of individual pigs after feeding the lysine-deficient (4.30 g/kg), lysine-adequate (7.10 g/kg), or lysine-excess (9.80 g/kg) diet for five weeks, and the total RNA was extracted afterwards. Affymetrix Porcine Gene 1.0 ST Array was used to quantify the expression levels of 19,211 genes. Statistical ANOVA analysis of the microarray data showed that 674 transcripts were differentially expressed (at p ≤ 0.05 level); 60 out of 131 transcripts (at p ≤ 0.01 level) were annotated in the NetAffx database. Ingenuity pathway analysis showed that dietary lysine deficiency may lead to: (1) increased muscle protein degradation via the ubiquitination pathway as indicated by the up-regulated DNAJA1, HSP90AB1 and UBE2B mRNA; (2) reduced muscle protein synthesis via the up-regulated RND3 and ZIC1 mRNA; (3) increased serine and glycine synthesis via the up-regulated PHGDH and PSPH mRNA; and (4) increased lipid accumulation via the up-regulated ME1, SCD, and CIDEC mRNA. Dietary lysine excess may lead to: (1) decreased muscle protein degradation via the down-regulated DNAJA1, HSP90AA1, HSPH1, and UBE2D3 mRNA; and (2) reduced lipid biosynthesis via the down-regulated CFD and ME1 mRNA. Collectively, dietary lysine may function as a signaling molecule to regulate protein turnover and lipid metabolism in the skeletal muscle of finishing pigs.

Polyamines, androgens, and skeletal muscle hypertrophy

The naturally occurring polyamines, spermidine, spermine, and their precursor putrescine, play indispensible roles in both prokaryotic and eukaryotic cells, from basic DNA synthesis to regulation of cell proliferation and differentiation. The rate-limiting polyamine biosynthetic enzymes, ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase, are essential for mammalian development, with knockout of the genes encoding these enzymes, Odc1 and Amd1, causing early embryonic lethality in mice. In muscle, the involvement of polyamines in muscle hypertrophy is suggested by the concomitant increase in cardiac and skeletal muscle mass and polyamine levels in response to anabolic agents including β-agonists. In addition to β-agonists, androgens, which increase skeletal mass and strength, have also been shown to stimulate polyamine accumulation in a number of tissues. In muscle, androgens act via the androgen receptor to regulate expression of polyamine biosynthetic enzyme genes, including Odc1 and Amd1, which may be one mechanism via which androgens promote muscle growth. This review outlines the role of polyamines in proliferation and hypertrophy, and explores their possible actions in mediating the anabolic actions of androgens in muscle.

The effect of a low dose of clenbuterol on rat soleus muscle submitted to joint immobilization

The aim of the present study was to evaluate the effect of joint immobilization on morphometric parameters and glycogen content of soleus muscle treated with clenbuterol. Male Wistar (3-4 months old) rats were divided into 4 groups (N = 6 for each group): control, clenbuterol, immobilized, and immobilized treated with clenbuterol. Immobilization was performed with acrylic resin orthoses and 10 microg/kg body weight clenbuterol was administered subcutaneously for 7 days. The following parameters were measured the next day on soleus muscle: weight, glycogen content, cross-sectional area, and connective tissue content. The clenbuterol group showed an increase in glycogen (81.6%, 0.38 +/- 0.09 vs 0.69 +/- 0.06 mg/100 g; P < 0.05) without alteration in weight, cross-sectional area or connective tissue compared with the control group. The immobilized group showed a reduction in muscle weight (34.2%, 123.5 +/- 5.3 vs 81.3 +/- 4.6 mg; P < 0.05), glycogen content (31.6%, 0.38 +/- 0.09 vs 0.26 +/- 0.05 mg/100 mg; P < 0.05) and cross-sectional area (44.1%, 2574.9 +/- 560.2 vs 1438.1 +/- 352.2 microm(2); P < 0.05) and an increase in connective tissue (216.5%, 8.82 +/- 3.55 vs 27.92 +/- 5.36%; P < 0.05). However, the immobilized + clenbuterol group showed an increase in weight (15.9%; 81.3 +/- 4.6 vs 94.2 +/- 4.3 mg; P < 0.05), glycogen content (92.3%, 0.26 +/- 0.05 vs 0.50 +/- 0.17 mg/100 mg; P < 0.05), and cross-sectional area (19.9%, 1438.1 +/- 352.2 vs 1724.8 +/- 365.5 microm(2); P < 0.05) and a reduction in connective tissue (52.2%, 27.92 +/- 5.36 vs 13.34 +/- 6.86%; P < 0.05). Statistical analysis was performed using Kolmogorov-Smirnov and homoscedasticity tests. For the muscle weight and muscle glycogen content, two-way ANOVA and the Tukey test were used. For the cross-sectional area and connective tissue content, Kruskal-Wallis and Tukey tests were used. This study emphasizes the importance of anabolic pharmacological protection during immobilization to minimize skeletal muscle alterations resulting from disuse.

Role of beta-adrenoceptor signaling in skeletal muscle: implications for muscle wasting and disease

The importance of beta-adrenergic signaling in the heart has been well documented, but it is only more recently that we have begun to understand the importance of this signaling pathway in skeletal muscle. There is considerable evidence regarding the stimulation of the beta-adrenergic system with beta-adrenoceptor agonists (beta-agonists). Although traditionally used for treating bronchospasm, it became apparent that some beta-agonists could increase skeletal muscle mass and decrease body fat. These so-called "repartitioning effects" proved desirable for the livestock industry trying to improve feed efficiency and meat quality. Studying beta-agonist effects on skeletal muscle has identified potential therapeutic applications for muscle wasting conditions such as sarcopenia, cancer cachexia, denervation, and neuromuscular diseases, aiming to attenuate (or potentially reverse) the muscle wasting and associated muscle weakness, and to enhance muscle growth and repair after injury. Some undesirable cardiovascular side effects of beta-agonists have so far limited their therapeutic potential. This review describes the physiological significance of beta-adrenergic signaling in skeletal muscle and examines the effects of beta-agonists on skeletal muscle structure and function. In addition, we examine the proposed beneficial effects of beta-agonist administration on skeletal muscle along with some of the less desirable cardiovascular effects. Understanding beta-adrenergic signaling in skeletal muscle is important for identifying new therapeutic targets and identifying novel approaches to attenuate the muscle wasting concomitant with many diseases.