Plasma beta-hydroxy-beta-methylbutyrate availability after enteral administration during critical illness after trauma: An exploratory study

BACKGROUND

During critical illness skeletal muscle wasting occurs rapidly. Although beta-hydroxy-beta-methylbutyrate (HMB) is a potential treatment to attenuate this process, the plasma appearance and muscle concentration is uncertain.

METHODS

This was an exploratory study nested within a blinded, parallel group, randomized clinical trial in which critically ill patients after trauma received enteral HMB (3 g daily) or placebo. Plasma samples were collected at 0, 60, and 180 min after study supplement administration on day 1. Needle biopsies of the vastus lateralis muscle were collected (baseline and day 7 of the HMB treatment intervention period). An external standard curve was used to calculate HMB concentrations in plasma and muscle.

RESULTS

Data were available for 16 participants (male n = 12 (75%), median [interquartile range] age 50 [29-58] years) who received placebo and 18 participants (male n = 14 (78%), age 49 [34-55] years) who received HMB. Plasma HMB concentrations were similar at baseline but increased after HMB (T = 60 min: placebo 0.60 [0.44-1.31]  µM; intervention 51.65 [22.76-64.72]  µM). Paired muscle biopsies were collected from 11 participants (placebo n = 7, HMB n = 4). Muscle HMB concentrations were similar at baseline between groups (2.35 [2.17-2.95]; 2.07 [1.78-2.31] µM). For participants in the intervention group who had the repeat biopsy within 4 h of HMB administration, concentrations were greater (7.2 and 12.3 µM) than those who had the repeat biopsy >4 h after HMB (2.7 and 2.1 µM).

CONCLUSION

In this exploratory study, enteral HMB administration increased plasma HMB availability. The small sample size limits interpretation of the muscle HMB findings.

Inflammation and altered metabolism impede efficacy of functional electrical stimulation in critically ill patients

BACKGROUND

Critically ill patients suffer from acute muscle wasting, which is associated with significant physical functional impairment. We describe data from nested muscle biopsy studies from two trials of functional electrical stimulation (FES) that did not shown improvements in physical function.

METHODS

Primary cohort: single-centre randomized controlled trial. Additional healthy volunteer data from patients undergoing elective hip arthroplasty. Validation cohort: Four-centre randomized controlled trial.

INTERVENTION

FES cycling for 60-90min/day.

ANALYSES

Skeletal muscle mRNA expression of 223 genes underwent hierarchal clustering for targeted analysis and validation.

RESULTS

Positively enriched pathways between healthy volunteers and ICU participants were "stress response", "response to stimuli" and "protein metabolism", in keeping with published data. Positively enriched pathways between admission and day 7 ICU participants were "FOXO-mediated transcription" (admission = 0.48 ± 0.94, day 7 = - 0.47 ± 1.04 mean log2 fold change; P = 0.042), "Fatty acid metabolism" (admission = 0.50 ± 0.67, day 7 = 0.07 ± 1.65 mean log2 fold change; P = 0.042) and "Interleukin-1 processing" (admission = 0.88 ± 0.50, day 7 = 0.97 ± 0.76 mean log2 fold change; P = 0.054). Muscle mRNA expression of UCP3 (P = 0.030) and DGKD (P = 0.040) decreased in both cohorts with no between group differences. Changes in IL-18 were not observed in the validation cohort (P = 0.268). Targeted analyses related to intramuscular mitochondrial substrate oxidation, fatty acid oxidation and intramuscular inflammation showed PPARγ-C1α; (P < 0.001), SLC25A20 (P = 0.017) and UCP3 (P < 0.001) decreased between admission and day 7 in both arms. LPIN-1 (P < 0.001) and SPT1 (P = 0.044) decreased between admission and day 7. IL-18 (P = 0.011) and TNFRSF12A (P = 0.009) increased in both arms between admission and day 7. IL-1β (P = 0.007), its receptor IL-1R1 (P = 0.005) and IL-6R (P = 0.001) decreased in both arms between admission and day 7. No between group differences were seen in any of these (all p > 0.05).

CONCLUSIONS

Intramuscular inflammation and altered substrate utilization are persistent in skeletal muscle during first week of critical illness and are not improved by the application of Functional Electrical Stimulation-assisted exercise. Future trials of exercise to prevent muscle wasting and physical impairment are unlikely to be successful unless these processes are addressed by other means than exercise alone.

Iron overload and impaired iron handling contribute to the dystrophic pathology in models of Duchenne muscular dystrophy

BACKGROUND

Oxidative stress is implicated in the pathophysiology of Duchenne muscular dystrophy (DMD, caused by mutations in the dystrophin gene), which is the most common and severe of the muscular dystrophies. To our knowledge, the distribution of iron, an important modulator of oxidative stress, has not been assessed in DMD. We tested the hypotheses that iron accumulation occurs in mouse models of DMD and that modulation of iron through the diet or chelation could modify disease severity.

METHODS

We assessed iron distribution and total elemental iron using LA-ICP-MS on skeletal muscle cross-sections of 8-week-old Bl10 control mice and dystrophic mdx mice (with moderate dystrophy) and dystrophin/utrophin-null mice (dko, with severe dystrophy). In addition, mdx mice (4 weeks) were treated with either an iron chelator (deferiprone 150 mg/kg/day) or iron-enriched feed (containing 1% added iron as carbonyl iron). Immunoblotting was used to determine the abundance of iron- and mitochondria-related proteins. (Immuno)histochemical and mRNA assessments of fibrosis and inflammation were also performed.

RESULTS

We observed a significant increase in total elemental iron in hindlimb muscles of dko mice (+50%, P < 0.05) and in the diaphragm of mdx mice (+80%, P < 0.05), with both tissues exhibiting severe pathology. Iron dyshomeostasis was further evidenced by an increase in the storage protein ferritin (dko: +39%, P < 0.05) and ferroportin compared with Bl10 control mice (mdx: +152% and dko: +175%, P < 0.05). Despite having features of iron overload, dystrophic muscles had lower protein expression of ALAS-1, the rate-limiting enzyme for haem synthesis (dko -44%, P < 0.05), and the haem-containing protein myoglobin (dko -54%, P < 0.05). Deferiprone treatment tended to decrease muscle iron levels in mdx mice (-30%, P < 0.1), which was associated with lower oxidative stress and fibrosis, but suppressed haem-containing proteins and mitochondrial content. Increasing iron via dietary intervention elevated total muscle iron (+25%, P < 0.05) but did not aggravate the pathology.

CONCLUSIONS

Muscles from dystrophic mice have increased iron levels and dysregulated iron-related proteins that are associated with dystrophic pathology. Muscle iron levels were manipulated by iron chelation and iron enriched feed. Iron chelation reduced fibrosis and reactive oxygen species (ROS) but also suppressed haem-containing proteins and mitochondrial activity. Conversely, iron supplementation increased ferritin and haem-containing proteins but did not alter ROS, fibrosis, or mitochondrial activity. Further studies are required to investigate the contribution of impaired ferritin breakdown in the dysregulation of iron homeostasis in DMD.

Dystrophin deficiency disrupts muscle clock expression and mitochondrial quality control in mdx mice

Impaired oxidative capacity and mitochondrial function contribute to the dystrophic pathology in muscles of patients with Duchenne muscular dystrophy (DMD) and in relevant mouse models of the disease. Emerging evidence suggests an association between disrupted core clock expression and mitochondrial quality control, but this has not been established in muscles lacking dystrophin. We examined the diurnal regulation of muscle core clock and mitochondrial quality control expression in dystrophin-deficient C57BL/10ScSn-Dmdmdx (mdx) mice, an established model of DMD. Male C57BL/10 (BL/10; n = 18) and mdx mice (n = 18) were examined every 4 h beginning at the dark cycle. Throughout the entire light-dark cycle, extensor digitorum longus (EDL) muscles from mdx mice had decreased core clock mRNA expression (Arntl, Cry1, Cry2, Nr1d2; P < 0.05) and disrupted mitochondrial quality control mRNA expression related to biogenesis (decreased; Ppargc1a, Esrra; P < 0.05), fission (increased; Dnm1l; P < 0.01), fusion (decreased; Opa1, Mfn1; P < 0.05), and autophagy/mitophagy (decreased: Bnip3; P < 0.05; increased: Becn1; P < 0.05). Cosinor analysis revealed a decrease in the rhythmicity parameters mesor and amplitude for Arntl, Cry1, Cry2, Per2, and Nr1d1 (P < 0.001) in mdx mice. Diurnal oscillations in Esrra, Sirt1, Map1lc3b, and Sqstm1 were absent in mdx mice, along with decreased mesor and amplitude of Ppargc1a mRNA expression (P < 0.01). The expression of proteins involved in mitochondrial biogenesis (decreased: PPARGC1A, P < 0.05) and autophagy/mitophagy (increased: MAP1LC3BII, SQSTM1, BNIP3; P < 0.05) were also dysregulated in tibialis anterior muscles of mdx mice. These findings suggest that dystrophin deficiency in mdx mice impairs the regulation of the core clock and mitochondrial quality control, with relevance to DMD and related disorders.

Iron accumulation in skeletal muscles of old mice is associated with impaired regeneration after ischaemia-reperfusion damage

BACKGROUND

Oxidative stress is implicated in the insidious loss of muscle mass and strength that occurs with age. However, few studies have investigated the role of iron, which is elevated during ageing, in age-related muscle wasting and blunted repair after injury. We hypothesized that iron accumulation leads to membrane lipid peroxidation, muscle wasting, increased susceptibility to injury, and impaired muscle regeneration.

METHODS

To examine the role of iron in age-related muscle atrophy, we compared the skeletal muscles of 3-month-old with 22- to 24-month-old 129SvEv FVBM mice. We assessed iron distribution and total elemental iron using laser ablation inductively coupled plasma mass spectrometry and Perls' stain on skeletal muscle cross-sections. In addition, old mice underwent ischaemia-reperfusion (IR) injury (90 min ischaemia), and muscle regeneration was assessed 14 days after injury. Immunoblotting was used to determine lipid peroxidation (4HNE) and iron-related proteins. To determine whether muscle iron content can be altered, old mice were treated with deferiprone (DFP) in the drinking water, and we assessed its effects on muscle regeneration after injury.

RESULTS

We observed a significant increase in total elemental iron (+43%, P < 0.05) and lipid peroxidation (4HNE: +76%, P < 0.05) in tibialis anterior muscles of old mice. Iron was further increased after injury (adult: +81%, old: +135%, P < 0.05) and associated with increased lipid peroxidation (+41%, P < 0.05). Administration of DFP did not impact iron or measures of lipid peroxidation in skeletal muscle or modulate muscle mass. Increased muscle iron concentration and lipid peroxidation were associated with less efficient regeneration, evident from the smaller fibres in cross-sections of tibialis anterior muscles (-24%, P < 0.05) and an increased percentage of fibres with centralized nuclei (+4124%, P < 0.05) in muscles of old compared with adult mice. Administration of DFP lowered iron after IR injury (PRE: -32%, P < 0.05 and POST: -41%, P < 0.05), but did not translate to structural improvements.

CONCLUSIONS

Muscles from old mice have increased iron levels, which are associated with increased lipid peroxidation, increased susceptibility to IR injury, and impaired muscle regeneration. Our results suggest that iron is involved in effective muscle regeneration, highlighting the importance of iron homeostasis in muscle atrophy and muscle repair.

Glycine Protects Muscle Cells From Wasting in vitro via mTORC1 Signaling

Glycine supplementation can protect skeletal muscles of mice from cancer-induced wasting, but the mechanisms underlying this protection are not well-understood. The aim of this study was to determine whether exogenous glycine directly protects skeletal muscle cells from wasting. C2C12 muscle cells were exposed to non-inflammatory catabolic stimuli via two models: serum withdrawal (SF) for 48 h; or incubation in HEPES buffered saline (HBS) for up to 5 h. Cells were supplemented with glycine or equimolar concentrations of L-alanine. SF- and HBS-treated myotubes (with or without L-alanine) were ~20% and ~30% smaller than control myotubes. Glycine-treated myotubes were up to 20% larger (P < 0.01) compared to cells treated with L-alanine in both models of muscle cell atrophy. The mTORC1 inhibitor rapamycin prevented the glycine-stimulated protection of myotube diameter, and glycine-stimulated S6 phosphorylation, suggesting that mTORC1 signaling may be necessary for glycine's protective effects in vitro. Increasing glycine availability may be beneficial for muscle wasting conditions associated with inadequate nutrient intake.

Deletion of suppressor of cytokine signaling 3 (SOCS3) in muscle stem cells does not alter muscle regeneration in mice after injury

Muscles of older animals are more susceptible to injury and regenerate poorly, in part due to a persistent inflammatory response. The janus kinase (Jak)/signal transducer and activator of transcription (Stat) pathway mediates inflammatory signaling and is tightly regulated by the suppressor of cytokine signaling (SOCS) proteins, especially SOCS3. SOCS3 expression is altered in the muscle of aged animals and may contribute to the persistent inflammation and impaired regeneration. To test this hypothesis, we performed myotoxic injuries on mice with a tamoxifen-inducible deletion of SOCS3 specifically within the muscle stem cell compartment. Muscle stem cell-specific SOCS3 deletion reduced muscle mass at 14 days post-injury (-14%, P < 0.01), altered the myogenic transcriptional program, and reduced myogenic fusion based on the number of centrally-located nuclei per muscle fiber. Despite the delay in myogenesis, muscles with a muscle stem cell-specific deletion of SOCS3 were still able to regenerate after a single bout or multiple bouts of myotoxic injury. A reduction in SOCS3 expression in muscle stem cells is unlikely to be responsible for the incomplete muscle repair in aged animals.

Glycine metabolism in skeletal muscle: implications for metabolic homeostasis

PURPOSE OF REVIEW

The review summarizes the recent literature on the role of glycine in skeletal muscle during times of stress.

RECENT FINDINGS

Supplemental glycine protects muscle mass and function under pathological conditions. In addition, mitochondrial dysfunction in skeletal muscle leads to increased cellular serine and glycine production and activation of NADPH-generating pathways and glutathione metabolism. These studies highlight how glycine availability modulates cellular homeostasis and redox status.

SUMMARY

Recent studies demonstrate that supplemental glycine effectively protects muscles in a variety of wasting models, including cancer cachexia, sepsis, and reduced caloric intake. The underlying mechanisms responsible for the effects of glycine remain unclear but likely involve receptor-mediated responses and modulation of intracellular metabolism. Future research to understand these mechanisms will provide insight into glycine's therapeutic potential. Our view is that glycine holds considerable promise for improving health by protecting muscles during different wasting conditions.

Muscle-specific deletion of SOCS3 does not reduce the anabolic response to leucine in a mouse model of acute inflammation

Excessive inflammation reduces skeletal muscle protein synthesis leading to wasting and weakness. The janus kinase/signal transducers and activators of transcription-3 (JAK/STAT3) pathway is important for the regulation of inflammatory signaling. As such, suppressor of cytokine signaling-3 (SOCS3), the negative regulator of JAK/STAT signaling, is thought to be important in the control of muscle homeostasis. We hypothesized that muscle-specific deletion of SOCS3 would impair the anabolic response to leucine during an inflammatory insult. Twelve week old (n=8 per group) SOCS3 muscle-specific knockout mice (SOCS3-MKO) and littermate controls (WT) were injected with lipopolysaccharide (LPS, 1mg/kg) or saline and were studied during fasted conditions or after receiving 0.5g/kg leucine 3h after the injection of LPS. Markers of inflammation, anabolic signaling, and protein synthesis were measured 4h after LPS injection. LPS injection robustly increased mRNA expression of inflammatory molecules (Socs3, Socs1, Il-6, Ccl2, Tnfα and Cd68). In muscles from SOCS3-MKO mice, the Socs3 mRNA response to LPS was significantly blunted (∼6-fold) while STAT3 Tyr705 phosphorylation was exacerbated (18-fold). Leucine administration increased protein synthesis in both WT (∼1.6-fold) and SOCS3-MKO mice (∼1.5-fold) compared to basal levels. LPS administration blunted this effect, but there were no differences between WT and SOCS3-MKO mice. Muscle-specific SOCS3 deletion did not alter the response of AKT, mTOR, S6 or 4EBP1 under any treatment conditions. Therefore, SOCS3 does not appear to mediate the early inflammatory or leucine-induced changes in protein synthesis in skeletal muscle.

Glycine restores the anabolic response to leucine in a mouse model of acute inflammation

Amino acids, especially leucine, potently stimulate protein synthesis and reduce protein breakdown in healthy skeletal muscle and as a result have received considerable attention as potential treatments for muscle wasting. However, the normal anabolic response to amino acids is impaired during muscle-wasting conditions. Although the exact mechanisms of this anabolic resistance are unclear, inflammation and ROS are believed to play a central role. The nonessential amino acid glycine has anti-inflammatory and antioxidant properties and preserves muscle mass in calorie-restricted and tumor-bearing mice. We hypothesized that glycine would restore the normal muscle anabolic response to amino acids under inflammatory conditions. Relative rates of basal and leucine-stimulated protein synthesis were measured using SUnSET methodology 4 h after an injection of 1 mg/kg lipopolysaccharide (LPS). Whereas leucine failed to stimulate muscle protein synthesis in LPS-treated mice pretreated with l-alanine (isonitrogenous control), leucine robustly stimulated protein synthesis (+51%) in mice pretreated with 1 g/kg glycine. The improvement in leucine-stimulated protein synthesis was accompanied by a higher phosphorylation status of mTOR, S6, and 4E-BP1 compared with l-alanine-treated controls. Despite its known anti-inflammatory action in inflammatory cells, glycine did not alter the skeletal muscle inflammatory response to LPS in vivo or in vitro but markedly reduced DHE staining intensity, a marker of oxidative stress, in muscle cross-sections and attenuated LPS-induced wasting in C2C12 myotubes. Our observations in male C57BL/6 mice suggest that glycine may represent a promising nutritional intervention for the attenuation of skeletal muscle wasting.

The level of FoxO1 and IL-15 in skeletal muscle, serum and synovial fluid in people with knee osteoarthritis: a case control study

The molecular regulation of muscle function in knee osteoarthritis is unclear. Elevated muscle atrophy regulation marker expression was associated with reduced muscle strength in knee osteoarthritis. The level of protein expression appears to be different between muscle, knee joint and serum, suggesting that inflammation is regulated differently within these tissues.

INTRODUCTION

Impaired muscle function is common in knee osteoarthritis (OA). Numerous biochemical molecules have been implicated in the development of OA; however, these have only been identified in the joint and serum. We compared the expression of interleukin-15 (IL-15) and Forkhead box protein-O1 (FoxO1) in muscle of patients with knee OA and asymptomatic individuals and examined whether IL-15 was also present in the joint and serum.

METHODS

Muscle and blood samples were collected from 19 patients with knee OA and 10 age-matched asymptomatic individuals. Synovial fluid and muscle biopsies were collected from the OA group during knee replacement surgery. IL-15 and FoxO1 were measured in the skeletal muscle. IL-15 abundance was also analysed in the serum of both groups and synovial fluid from the OA group. Knee extensor strength was measured and correlated with IL-15 and FoxO1 in the muscle.

RESULTS

FoxO1 protein expression was higher (p = 0.04), whereas IL-15 expression was lower (p = 0.02) in the muscle of the OA group. Strength was also lower in the OA group and was inversely correlated with FoxO1 expression. No correlation was found between IL-15 in the joint, muscle or serum.

CONCLUSION

Skeletal muscle, particularly the quadriceps, is affected in people with knee OA where elevated FoxO1 protein expression was associated with reduced muscle strength. While IL-15 protein expression in the muscle was lower in the knee OA group, no correlation was found between the expression of IL-15 protein in the muscle, joint and serum, which suggests that inflammation is regulated differently within these tissues. Australian Clinical Trials Registry (ACTR) number: ACTRN12613000467730 ( http://www.anzctr.org.au/TrialSearch.aspx?searchTxt=ACTRN12613000467730&isBasic=True ).

Muscle atrophy in patients with Type 2 Diabetes Mellitus: roles of inflammatory pathways, physical activity and exercise

Muscle atrophy is caused by an imbalance in contractile protein synthesis and degradation which can be triggered by various conditions including Type 2 Diabetes Mellitus (T2DM). Reduced muscle quality in patients with T2DM adversely affects muscle function, the capacity to perform activities of daily living, quality of life and ultimately may increase the risk of premature mortality. Systemic inflammation initiated by obesity and prolonged overnutrition not only contributes to insulin resistance typical of T2DM, but also promotes muscle atrophy via decreased muscle protein synthesis and increased ubiquitin-proteasome, lysosomal-proteasome and caspase 3- mediated protein degradation. Emerging evidence suggests that the inflammation-sensitive Nuclear Factor κ B (NF-κB) and Signal Transducer and Activator of Transcription 3 (STAT3) pathways may contribute to muscle atrophy in T2DM. In contrast, exercise appears to be an effective tool in promoting muscle hypertrophy, in part due to its effect on systemic and local (skeletal muscle) inflammation. The current review discusses the role inflammation plays in muscle atrophy in T2DM and the role of exercise training in minimising the effect of inflammatory markers on skeletal muscle. We also report original data from a cohort of obese patients with T2DM compared to age-matched controls and demonstrate that patients with T2DM have 60% higher skeletal muscle expression of the atrophy transcription factor FoxO1. This review concludes that inflammatory pathways in muscle, in particular, NF-κB, potentially contribute to T2DM-mediated muscle atrophy. Further in-vivo and longitudinal human research is required to better understand the role of inflammation in T2DM-mediated atrophy and the anti-inflammatory effect of exercise training under these conditions.

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.

Early myogenic responses to acute exercise before and after resistance training in young men

To enable dynamic regulation of muscle mass and myofiber repair following injury, a satellite cell precursor population exists to supply additional nuclei. Activated satellite cells express many genes and associated proteins necessary for maturation and incorporation into the damaged fiber. There is little knowledge about the response of these markers following whole-body resistance exercise training. We investigated the impact of 12 weeks of progressive whole-body resistance training on the expression of MRFs, PAX7, NCAM, and FA1, incorporating both acute and chronic resistance exercise components. Ten young recreationally active males (21.2 ± 3.5 years) performed 12 weeks of whole-body resistance training at 70-85% of their predetermined one-repetition maximum (1RM). At the initiation and completion of the training period, muscular strength was assessed by RM and dynamometer testing, and vastus lateralis samples were obtained prior to and 3 h following an acute resistance exercise test (both whole-body and isometric exercises). Increased mRNA expression of PAX7 (threefold), NCAM (threefold), MYF5 (threefold), MYOD (threefold) and MYOGENIN (twofold) was observed 3 h after the acute resistance exercise test, both pre and posttraining. Similarly, PAX7 (11-fold) and FA1 (twofold) protein abundance increased after acute exercise, while resting NCAM (eightfold) and FA1 (threefold) protein abundance increased following 12 weeks of resistance training. It is possible that these molecular changes are primarily due to the preceding exercise bout, and are not modified by long-term or whole-body exercise training.

Ribosome biogenesis adaptation in resistance training-induced human skeletal muscle hypertrophy

Resistance training (RT) has the capacity to increase skeletal muscle mass, which is due in part to transient increases in the rate of muscle protein synthesis during postexercise recovery. The role of ribosome biogenesis in supporting the increased muscle protein synthetic demands is not known. This study examined the effect of both a single acute bout of resistance exercise (RE) and a chronic RT program on the muscle ribosome biogenesis response. Fourteen healthy young men performed a single bout of RE both before and after 8 wk of chronic RT. Muscle cross-sectional area was increased by 6 ± 4.5% in response to 8 wk of RT. Acute RE-induced activation of the ERK and mTOR pathways were similar before and after RT, as assessed by phosphorylation of ERK, MNK1, p70S6K, and S6 ribosomal protein 1 h postexercise. Phosphorylation of TIF-IA was also similarly elevated following both RE sessions. Cyclin D1 protein levels, which appeared to be regulated at the translational rather than transcriptional level, were acutely increased after RE. UBF was the only protein found to be highly phosphorylated at rest after 8 wk of training. Also, muscle levels of the rRNAs, including the precursor 45S and the mature transcripts (28S, 18S, and 5.8S), were increased in response to RT. We propose that ribosome biogenesis is an important yet overlooked event in RE-induced muscle hypertrophy that warrants further investigation.

Citrulline does not prevent skeletal muscle wasting or weakness in limb-casted mice

BACKGROUND

Increasing arginine (Arg) availability reduces atrophy in cultured skeletal muscle cells. Supplementation with its metabolic precursor citrulline (Cit) is more effective at improving skeletal muscle Arg concentrations.

OBJECTIVE

We tested the hypothesis that Cit supplementation would attenuate skeletal muscle atrophy and loss of function during hindlimb immobilization in mice.

METHODS

Male C57BL/6JArc mice underwent 14 d of unilateral hindlimb immobilization/plaster casting and were supplemented with ~0.81 g Cit · kg⁻¹ · d⁻¹ (CIT group) or Ala (ALA group) mixed into their food. The uncasted contralateral limb (internal control) and an uncasted group (CON) served as controls. Muscle atrophy was evaluated with mass, fiber area, and in situ muscle function.

RESULTS

Tibialis anterior (TA) muscle mass [ALA: 37.6 ± 0.92 mg; CIT: 38.3 ± 1.25 mg] and peak tetanic force (ALA: 1150 ± 38.5 mN; CIT: 1150 ± 52.0 mN) were lower (P < 0.001) in the ALA (53.9 ± 0.42 mg) and CIT (1760 ± 28.5 mN) groups than in the CON group. No difference was found between ALA and CIT groups for TA mass, fiber area, or peak force. The mRNA expression of the nitric oxide synthase 2, inducible (Nos2; ~15-fold) and B-cell chronic lymphoid leukemia/lymphoma 2/adenovirus E1B 19 kDa interacting protein 3 (Bnip3; ~17-fold) genes and the ratio of microtubule-associated protein light chain 3BII to 3BI (LC3BII:LC3BI) (50.5% ± 17.7%) were higher (P < 0.05) in the ALA group than in the CON group, suggesting increased autophagy. In the CIT group, Bnip3 mRNA was lower (-70%; P < 0.05) and Nos2 mRNA tended to be lower (-45%; P = 0.05) than in the ALA group, whereas LC3BII:LC3BI was not different from the CON group.

CONCLUSIONS

Cit treatment of male mice did not affect therapeutically relevant outcome measures such as skeletal muscle mass and peak muscle force after 14 d of hindlimb immobilization.

Short communication: Bovine-derived proteins activate STAT3 in human skeletal muscle in vitro

Bovine milk contains biologically active peptides that may modulate growth and development within humans. In this study, targeted bovine-derived proteins were evaluated for their effects on signal transducer and activator of transcription-3 (STAT3) phosphorylation in human skeletal muscle cells. Following an acute exposure, bovine-derived acidic fibroblast growth factor-1 (FGF) and leukemia inhibitory factor (LIF) activated STAT3 in differentiating myotubes. Chronic exposure to FGF and LIF during the proliferative phase reduced myoblast proliferation and elevated MyoD and creatine kinase (CKM) mRNA expression without altering apoptotic genes. In mature myotubes, neither FGF nor LIF elicited any action. Together, these data indicate that a reduction in proliferation in the presence of bovine-derived FGF or LIF may stimulate early maturation of myoblasts.

Arginine protects muscle cells from wasting in vitro in an mTORC1-dependent and NO-independent manner

Amino acids are potent regulators of muscle protein synthesis and breakdown and have received considerable attention for the treatment of muscle wasting conditions. Arginine is critically involved in numerous physiological functions including providing substrate for the production of creatine, urea and nitric oxide (NO) and in the synthesis of new proteins. However, little is known about the direct effects of arginine on skeletal muscle protein synthesis during catabolic conditions. The aims of this study were to determine whether exogenous arginine could protect skeletal muscle cells from wasting directly and whether this effect was dependent on production of NO and/or activation of the rapamycin-sensitive mechanistic target of rapamycin complex 1 (mTORC1) signalling pathway. To explore these aims, we deprived mature C2C12 myotubes from nutrients and growth factors by incubating them in HEPES buffered saline with arginine or equimolar concentrations of alanine (control). Our results show that arginine: increased the ratio of phosphorylated to total mTOR (146 %), S6 (40 %) and 4EBP1 (69 %); increased protein synthesis (69 %) during the first hour of treatment; and increased myotube diameter by ~15 %. Experiments using the NO synthase inhibitor L-NG-Nitroarginine Methyl Ester showed a NO-independent protection from muscle wasting. On the other hand, the mTORC1 inhibitor rapamycin prevented increases in phosphorylated S6, protein synthesis and myotube diameter. The activation of mTORC1 and protein synthesis by arginine was not associated with changes in the phosphorylation status of Akt, but rather increased the expression of the amino acid-sensitive type III PI3-kinase Vps34 signalling protein. These data support a direct role for arginine in the regulation of mTORC1 in skeletal muscle.

The effects of osteoarthritis and age on skeletal muscle strength, Na+-K+-ATPase content, gene and isoform expression

Knee osteoarthritis (OA) is a debilitating disorder prevalent in older populations that is accompanied by declines in muscle mass, strength, and physical activity. In skeletal muscle, the Na(+)-K(+) pump (NKA) is pivotal in ion homeostasis and excitability and is modulated by disuse and exercise training. This study examined the effects of OA and aging on muscle NKA in 36 older adults (range 55-81 yr), including 19 with OA (69.9 ± 6.5 yr, mean ± SD) and 17 asymptomatic controls (CON, 66.8 ± 6.4 yr). Participants completed knee extensor strength testing and a physical activity questionnaire. A vastus lateralis muscle biopsy was analyzed for NKA content ([(3)H]ouabain binding sites), α1-3- and β1-3-isoform protein abundance (immunoblotting), and mRNA (real-time RT-PCR). The association between age and NKA content was investigated within the OA and CON groups and in pooled data. The NKA content was also contrasted between subgroups below and above the median age of 68.5 yr. OA had lower strength (-40.8%, P = 0.005), but higher NKA α2- (∼34%, P = 0.006) and α3-protein (100%, P = 0.016) abundance than CON and performed more incidental physical activity (P = 0.035). No differences were found between groups for NKA content, abundance of other NKA isoforms, or gene expression. There was a negative correlation between age and NKA content within OA (r = -0.63, P = 0.03) and with both groups combined (r = -0.47, P = 0.038). The NKA content was 25.5% lower in the older (69-81 yr) than in the younger (55-68 yr) subgroup. Hence older age, but not knee OA, was related to lowered muscle NKA content in older adults.

Unchanged [3H]ouabain binding site content but reduced Na+-K+ pump α2-protein abundance in skeletal muscle in older adults

Aging is associated with reduced muscle mass, weakness, and increased fatigability. In skeletal muscle, the Na+-K+ pump (NKA) is important in regulating Na+-K+ gradients, membrane excitability, and thus contractility, but the effects of aging on muscle NKA are unclear. We investigated whether aging is linked with reduced muscle NKA by contrasting muscle NKA isoform gene expression and protein abundance, and NKA total content in 17 Elderly (66.8 ± 6.4 yr, mean ± SD) and 16 Young adults (23.9 ± 2.2 yr). Participants underwent peak oxygen consumption assessment and a vastus lateralis muscle biopsy, which was analyzed for NKA α1-, α2-, α3-, β1-, β2-, and β3-isoform gene expression (real-time RT-PCR), protein abundance (immunoblotting), and NKA total content ([3H]ouabain binding sites). The Elderly had lower peak oxygen consumption (−36.7%, P = 0.000), strength (−36.3%, P = 0.001), NKA α2- (−24.4%, 11.9 ± 4.4 vs. 9.0 ± 2.7 arbitrary units, P = 0.049), and NKA β3-protein abundance (−23.0%, P = 0.041) than Young. The β3-mRNA was higher in Elderly compared with Young ( P = 0.011). No differences were observed between groups for other NKA isoform mRNA or protein abundance, or for [3H]ouabain binding site content. Thus skeletal muscle in elderly individuals was characterized by decreased NKA α2- and β3-protein abundance, but unchanged α1 abundance and [3H]ouabain binding. The latter was likely caused by reduced α2 abundance with aging, preventing an otherwise higher [3H]ouabain binding that might occur with a greater membrane density in smaller muscle fibers. Further study is required to verify reduced muscle NKA α2 with aging and possible contributions to impaired exercise capability and daily living activities.

Early inflammatory and myogenic responses to resistance exercise in the elderly

INTRODUCTION AND METHODS

This study compared changes in myokine and myogenic genes following resistance exercise (3 sets of 12 repetitions of maximal unilateral knee extension) in 20 elderly men (67.8 ± 1.0 years) and 15 elderly women (67.2 ± 1.5 years).

RESULTS

Monocyte chemotactic protein (MCP)-1, macrophage inhibitory protein (MIP)-1β, interleukin (IL)-6 and MyoD mRNA increased significantly (P < 0.05), whereas myogenin and myostatin mRNA decreased significantly after exercise in both groups. Macrophage-1 (Mac-1) and MCP-3 mRNA did not change significantly after exercise in either group. MIP-1β, Mac-1 and myostatin mRNA were significantly higher before and after exercise in men compared with women. In contrast, MCP-3 and myogenin mRNA were significantly higher before and after exercise in the women compared with the men.

CONCLUSIONS

In elderly individuals, gender influences the mRNA expression of certain myokines and growth factors, both at rest and after resistance exercise. These differences may influence muscle regeneration following muscle injury.

Inflammatory markers in skeletal muscle of older adults

Older adults have an increase in circulating markers of inflammation. The current study examined whether there is an increase in the expression of inflammatory markers within the vastus lateralis, a major locomotive muscle, of older adults, and if so, whether the reduction in muscle strength and aerobic capacity in older adults is related to increased muscle inflammation. Skeletal muscle biopsies were taken from older adults (n = 17, 67 ± 1.6 years) and young individuals (n = 16, 24 ± 0.6 years) under resting and fasting conditions. Muscle was analyzed for mRNA levels of intracellular inflammatory molecules (MCP1, TNFα and IL-1β) and total cellular protein abundance of cytokines, chemokines and kinases (IL-6, IL-8, MCP1, TNFα, p65 (NF-κB), JNK1/2 and STAT3). MCP1 expression was significantly higher (p < 0.05; 50 %, mRNA and 40 %, protein) in elderly than younger participants, as was IL-8 (4 %). No detectable difference in kinase protein expression was observed for STAT3, JNK or p65 (NF-κB), TNFα or IL-6. Muscle strength was lower in the elderly compared to the young group (1.55 ± 0.17 vs. 2.56 ± 0.13 Nm/kg, p < 0.001). The elderly group also had a significantly lower VO(2peak) compared to the young group (24.9 ± 1.9 vs. 39.3 ± 1.9, p < 0.001), but muscle strength and VO(2peak) were not correlated with the examined inflammatory markers. Older adults have increased MCP1 (mRNA and protein abundance) and IL-8 (protein abundance) and also reduced muscle strength and VO(2peak). However, the reduction in muscle strength and VO(2peak) was not related to the increase in muscle inflammatory markers in this cohort.

Resistance exercise increases NF-κB activity in human skeletal muscle

Intense resistance exercise causes a significant inflammatory response. NF-κB has been identified as a prospective key transcription factor mediating the postexercise inflammatory response. The purpose of this study was to determine whether a single bout of intense resistance exercise regulates NF-κB signaling in human skeletal muscle. Muscle biopsy samples were obtained from the vastus lateralis of five recreationally active, but not strength-trained, males (21.9 ± 1.3 yr) prior to, and at 2 and 4 h following, a single bout of intense resistance exercise. A further five subjects (4 males, 1 female) (23 ± 0.89 yr) were recruited as a nonexercise control group to examine the effect of the muscle biopsy protocol on key markers of skeletal muscle inflammation. Protein levels of IκBα and phosphorylated NF-κB (p65), as well as the mRNA expression of inflammatory myokines monocyte chemoattractant protein 1 (MCP-1), IL-6, and IL-8 were measured. Additionally, NF-κB (p65) DNA binding to the promoter regions of MCP-1, IL-6, and IL-8 was investigated. IκBα protein levels decreased, while p-NF-κB (p65) protein levels increased 2 h postexercise and returned to near-baseline levels by 4-h postexercise. Immunohistochemical data verified these findings, illustrating an increase in p-NF-κB (p65) protein levels, and nuclear localization at 2 h postexercise. Furthermore, NF-κB DNA binding to MCP-1, IL-6, and IL-8 promoter regions increased significantly 2 h postexercise as did mRNA levels of these myokines. No significant change was observed in the nonexercise control group. These novel data provide evidence that intense resistance exercise transiently activates NF-κB signaling in human skeletal muscle during the first few hours postexercise. These findings implicate NF-κB in the transcriptional control of myokines known to be central to the postexercise inflammatory response.

Association between skeletal muscle inflammatory markers and walking pattern in people with knee osteoarthritis

OBJECTIVE

Patients with knee osteoarthritis (OA) are characterized by increased muscle inflammation and altered gait. We investigated the association between proinflammatory mediators in the vastus lateralis and physical function and gait in patients with knee OA.

METHODS

Nineteen patients with knee OA underwent gait analysis, assessment of self-reported pain and physical function (Western Ontario and McMaster Universities Osteoarthritis Index [WOMAC]), and a muscle biopsy that was taken during their knee replacement surgery. Muscle was analyzed for cellular protein inflammatory mediators, interleukin-6, monocyte chemotactic protein 1 (MCP-1), p65 NF-κB, signal transducer and activator of transcription 3 (STAT-3), and JNK-1. Sagittal plane knee function, including early stance knee range of motion (ROM) and knee sagittal plane impulse, was measured using a motion analysis system. Pearson's correlation was used to assess relationships between selected variables.

RESULTS

Significant positive correlations were found between MCP-1 and self-perceived stiffness, physical function, and the total WOMAC score (P < 0.05). MCP-1 was also negatively correlated with early stance knee ROM (r = -0.52, P = 0.023). Reduced velocity was associated with elevated levels of p65 NF-κB and STAT-3 (P < 0.05). Knee sagittal plane impulse was negatively correlated with JNK-1 (P = 0.02), indicating reduction in knee impulse with an increased level of JNK-1.

CONCLUSION

Increased levels of several proinflammatory mediators were correlated with altered knee function during walking as well as greater physical disability and slower gait velocity. Identification of the cellular and molecular mechanisms associated with muscle inflammation is important to better understand the underlying mechanism responsible for altered gait and function in patients with knee OA.

Impact of SOCS3 overexpression on human skeletal muscle development in vitro

The Janus kinase/signal transducers and activators of transcription (JAK/STAT) signaling cascade has been identified as a crucial factor for myogenesis. The STAT3 isoform is essential for satellite cell migration and myogenic differentiation as it mediates the expression of muscle specific myogenic factors. The SOCS (suppressors of cytokine signaling) family of proteins down-regulates STAT activation. Primary human skeletal muscle cells were isolated and cultured to investigate the effect of SOCS3 adenoviral overexpression on myotube maturation. It was demonstrated that STAT3 inhibition did not influence myotube development or survival. Moreover, SOCS3 overexpression enhances the mRNA expression of downstream targets of STAT3, c-FOS and VEGF. These increases were correlated with enhanced mRNA expression of genes associated with muscle maturation and hypertrophy. Thus SOCS3 influences myoblast differentiation and SOCS3 may be significant in regulating the activity of genes previously identified as transcriptionally regulated by STAT3.