Protein turnover in atrophying muscle: from nutritional intervention to microarray expression analysis

Microgravity induces proteomics changes involved in endoplasmic reticulum stress and mitochondrial protection

On Earth, biological systems have evolved in response to environmental stressors, interactions dictated by physical forces that include gravity. The absence of gravity is an extreme stressor and the impact of its absence on biological systems is ill-defined. Astronauts who have spent extended time under conditions of minimal gravity (microgravity) experience an array of biological alterations, including perturbations in cardiovascular function. We hypothesized that physiological perturbations in cardiac function in microgravity may be a consequence of alterations in molecular and organellar dynamics within the cellular milieu of cardiomyocytes. We used a combination of mass spectrometry-based approaches to compare the relative abundance and turnover rates of 848 and 196 proteins, respectively, in rat neonatal cardiomyocytes exposed to simulated microgravity or normal gravity. Gene functional enrichment analysis of these data suggested that the protein content and function of the mitochondria, ribosomes, and endoplasmic reticulum were differentially modulated in microgravity. We confirmed experimentally that in microgravity protein synthesis was decreased while apoptosis, cell viability, and protein degradation were largely unaffected. These data support our conclusion that in microgravity cardiomyocytes attempt to maintain mitochondrial homeostasis at the expense of protein synthesis. The overall response to this stress may culminate in cardiac muscle atrophy.

Seasonal oxidative capacity of skeletal muscles in hibernating Daurian ground squirrels (Spermophilus dauricus)

We investigated the mechanism of high oxidative capacity of skeletal muscles in hibernating Daurian ground squirrels (Spermophilus dauricus Brandt, 1843). Myoglobin (Mb) levels, as well as citrate synthase and lactate dehydrogenase (LDH) activities, were measured by spectrophotometry. Mb content in the soleus (SOL) muscle lasted from the beginning of hibernation to spring. Mb content in SOL was 87% higher in the hibernating group than in the summer group. Mb content in the extensor digitorum longus (EDL) muscle stayed at similar levels during the different periods of the year. Citrate synthase activity in SOL was 30% higher in the hibernating group than in the summer group. Meanwhile, citrate synthase activity in EDL did not change during hibernation. LDH activity in SOL was not different between the hibernating group and the summer active group, whereas LDH activity in EDL increased significantly (up to 11%) in the 2 days arousal after hibernation group compared with the hibernating group. We conclude that high oxidative capacity is provided by increased oxygen storage capacity of slow-twitch muscle fibers rather than from fast-twitch muscle fibers in hibernating animals.

Chronic hindlimb suspension unloading markedly decreases turnover rates of skeletal and cardiac muscle proteins and adipose tissue triglycerides

We previously showed that a single bolus of "doubly-labeled" water ((2)H2 (18)O) can be used to simultaneously determine energy expenditure and turnover rates (synthesis and degradation) of tissue-specific lipids and proteins by modeling labeling patterns of protein-bound alanine and triglyceride-bound glycerol (Bederman IR, Dufner DA, Alexander JC, Previs SF. Am J Physiol Endocrinol Metab 290: E1048-E1056, 2006). Using this novel method, we quantified changes in the whole body and tissue-specific energy balance in a rat model of simulated "microgravity" induced by hindlimb suspension unloading (HSU). After chronic HSU (3 wk), rats exhibited marked atrophy of skeletal and cardiac muscles and significant decrease in adipose tissue mass. For example, soleus muscle mass progressively decreased 11, 43, and 52%. We found similar energy expenditure between control (90 ± 3 kcal · kg(-1)· day(-1)) and hindlimb suspended (81 ± 6 kcal/kg day) animals. By comparing food intake (∼ 112 kcal · kg(-1) · day(-1)) and expenditure, we found that animals maintained positive calorie balance proportional to their body weight. From multicompartmental fitting of (2)H-labeling patterns, we found significantly (P < 0.005) decreased rates of synthesis (percent decrease from control: cardiac, 25.5%; soleus, 70.3%; extensor digitorum longus, 44.9%; gastrocnemius, 52.5%; and adipose tissue, 39.5%) and rates of degradation (muscles: cardiac, 9.7%; soleus, 52.0%; extensor digitorum longus, 27.8%; gastrocnemius, 37.4%; and adipose tissue, 50.2%). Overall, HSU affected growth of young rats by decreasing the turnover rates of proteins in skeletal and cardiac muscles and adipose tissue triglycerides. Specifically, we found that synthesis rates of skeletal and cardiac muscle proteins were affected to a much greater degree compared with the decrease in degradation rates, resulting in large negative balance and significant tissue loss. In contrast, we found a small decrease in adipose tissue triglyceride synthesis paired with a large decrease in degradation, resulting in smaller negative energy balance and loss of fat mass. We conclude that HSU in rats differentially affects turnover of muscle proteins vs. adipose tissue triglycerides.

Severe burn and disuse in the rat independently adversely impact body composition and adipokines

Introduction

Severe trauma is accompanied by a period of hypermetabolism and disuse. In this study, a rat model was used to determine the effects of burn and disuse independently and in combination on body composition, food intake and adipokines.

Methods

Male rats were assigned to four groups 1) sham ambulatory (SA), 2) sham hindlimb unloaded (SH), 3) 40% total body surface area full thickness scald burn ambulatory (BA) and 4) burn and hindlimb unloaded (BH). Animals designated to the SH and BH groups were placed in a tail traction system and their hindlimbs unloaded. Animals were followed for 14 days. Plasma, urine, fecal and tissue samples were analyzed.

Results

SA had a progressive increase in body mass (BM), SH and BA no change and BH a reduction. Compared to SA, BM was reduced by 10% in both SH and BA and by 17% when combined in BH. Compared to SA, all groups had reductions in lean and fat body mass with BH being greater. The decrease in lean mass was associated with the rate of urinary corticosterone excretion. The loss in fat mass was associated with decreases in plasma leptin and adiponectin and an increase in ghrelin. Following the acute response to injury, BH had a greater food intake per 100 g BM. Food intake was associated with the levels of leptin, adiponectin and ghrelin.

Conclusions

The effects of the combination of burn and disuse in this animal model were additive, therefore in assessing metabolic changes with severe trauma both injury and disuse should be considered. Furthermore, the observed changes in adipokines, corticosterone and ghrelin provide insights for interventions to attenuate the hypermetabolic state following injury, possibly reducing catabolism and muscle loss and subsequent adverse effects on recovery and function.

Resveratrol reverses monocrotaline-induced pulmonary vascular and cardiac dysfunction: a potential role for atrogin-1 in smooth muscle

Arterial remodeling contributes to elevated pulmonary artery (PA) pressures and right ventricular hypertrophy seen in pulmonary hypertension (PH). Resveratrol, a sirtuin-1 (SIRT1) pathway activator, can prevent the development of PH in a commonly used animal model, but it is unclear whether it can reverse established PH pathophysiology. Furthermore, atrophic ubiquitin ligases, such as atrogin-1 and MuRF-1, are known to be induced by SIRT1 activators but have not been characterized in hypertrophic vascular disease. Therefore, we hypothesized that monocrotaline (MCT)-induced PH would attenuate atrophy pathways in the PA while, conversely, SIRT1 activation (resveratrol) would reverse indices of PH and restore atrophic gene expression. Thus, we injected Sprague-Dawley rats with MCT (50 mg/kg i.p.) or saline at Day 0, and then treated with oral resveratrol or sildenafil from days 28-42 post-MCT injection. Oral resveratrol attenuated established MCT-induced PH indices, including right ventricular systolic pressure, right ventricular hypertrophy, and medial thickening of intrapulmonary arteries. Resveratrol also normalized PA atrogin-1 mRNA expression, which was significantly reduced by MCT. In cultured human PA smooth muscle cells (hPASMC), resveratrol significantly inhibited PDGF-stimulated proliferation and cellular hypertrophy, which was also associated with improvements in atrogin-1 levels. In addition, SIRT1 inhibition augmented hPASMC proliferation, as assessed by DNA mass, and suppressed atrogin mRNA expression. These findings demonstrate an inverse relationship between indices of PH and PA atrogin expression that is SIRT1 dependent and may reflect a novel role for SIRT1 in PASMCs opposing cellular hypertrophy and proliferation.

Inhibition of atrogin-1/MAFbx expression by adenovirus-delivered small hairpin RNAs attenuates muscle atrophy in fasting mice

Atrogin-1 or muscle atrophy F-box (MAFbx) is a major atrophy-related E3 ubiquitin ligase highly expressed in skeletal muscle during muscle atrophy and other disease states such as sepsis, cancer cachexia, and fasting. In this paper, we report experiments inhibiting MAFbx activity in fasting mice and in the skeletal myoblast cell line C2C12 via an adenovirus-mediated small hairpin RNA (shRNA) expression system in order to assess its suitability as a therapeutic target. Our results demonstrated that downregulation of MAFbx by shRNAs attenuated muscle loss induced by fasting in mice. Furthermore, we showed that when MAFbx expression was blocked both in cells and in fasting mice, the level of a myogenic factor, MyoD, was upregulated; whereas a muscle negative regulator, growth differentiation factor (GDF)-8 (myostatin), was suppressed. Our results also suggested that lower levels of MAFbx could also enhance muscle cell differentiation that corresponded to the reduced expression of GDF-8 and the increased level of MyoD. Taken together, the present study showed that MAFbx could be a potential molecular target for treating muscle atrophy.

One-year follow-up in a child with McArdle disease: exercise is medicine

A 9-year-old boy with McArdle disease, who demonstrated remarkable recovery of objectively measured exercise tolerance after 1 year of follow-up, during which he pursued age-appropriate physical activities. The patient presented 1 year previously with severe myalgia, muscle weakness, proteinuria, hematuria, hyperthermia, and elevated creatine kinase levels after noncompetitive swimming. At that time, he reported a 3-year history of general myalgia and poor exercise tolerance. He was diagnosed with McArdle disease by both biochemical and genetic methods. Subsequently he performed a maximal exercise test and was prescribed a return to age-appropriate physical activity (protected by a pre-exercise dietary consumption of approximately 20 g carbohydrate). At 1-year follow up, he reported no subsequent acute clinical episodes, no general problems with exercise either at school or in ordinary activities, a virtual normalization of serum creatine kinase levels, and a 14% increase in body mass-adjusted peak oxygen uptake (from 18.8 to 21.8 mL O2/kg/min). The results suggest that, with protection by increasing pre-exercise blood glucose with carbohydrate ingestion, a substantially normal lifestyle may be possible in some children with McArdle disease.

Favorable responses to acute and chronic exercise in McArdle patients

OBJECTIVE

This study reports acute exercise responses in a large (N = 46) series of patients with McArdle disease and responses to exercise training in a smaller (n = 9) set of patients.

DESIGN

Patients were studied during both incremental and steady-state cycle ergometer exercise, using cardiopulmonary testing, and the patients were compared with age- and gender-matched controls.

SETTING

The study was performed in a university setting (clinical exercise physiology laboratory).

PARTICIPANTS

The 46 patients showed common features of McArdle disease. They were definitively diagnosed by histochemistry, biochemistry, and/or molecular genetic analysis. The 46 controls were healthy, sedentary individuals.

INTERVENTION

Nine patients were studied before and after an 8-month supervised aerobic exercise training program (including five weekly sessions of walking and/or cycling exercise with a duration no greater than 60 minutes).

MAIN OUTCOME MEASUREMENTS

The main indicators of exercise capacity that we measured were peak power output, peak oxygen uptake (VO2peak), and ventilatory threshold (VT).

RESULTS

Exercise capacity (peak power output, 35% control; VO2peak, 44% control; VT, 66% control) was markedly depressed in the patients. The patients who trained improved peak power output (25%), VO2peak (44%), and VT (27%), with no evidence of negative outcomes from training. Although not achieving normal values, the response to training put the patients into the lower limit of normal controls.

CONCLUSIONS

Under carefully controlled conditions, patients with McArdle disease may perform acute exercise safely, and they may respond favorably to training. This may offer an additional therapeutic option to help normalize the lifestyles of these patients.

Acetyl-l-carnitine feeding to unloaded rats triggers in soleus muscle the coordinated expression of genes involved in mitochondrial biogenesis

The expressional profile of mitochondrial transcripts and of genes involved in the mitochondrial biogenesis pathway induced by ALCAR daily supplementation in soleus muscle of control and unloaded 3-month-old rats has been analyzed. It has been found that ALCAR treatment is able to upregulate the expression level of mitochondrial transcripts (COX I, ATP6, ND6, 16 S rRNA) in both control and unloaded animals. Interestingly, ALCAR feeding to unloaded rats resulted in the increase of transcript level for master factors involved in mitochondrial biogenesis (PGC-1alpha, NRF-1, TFAM). It also prevented the unloading-induced downregulation of mRNA levels for kinases able to transduce metabolic (AMPK) and neuronal stimuli (CaMKIIbeta) into mitochondrial biogenesis. No significant effect on the expressional level of such genes was found in control ALCAR-treated rats. In addition, ALCAR feeding was able to prevent the loss of mitochondrial protein content due to unloading condition. Correlation analysis revealed a strong coordination in the expression of genes involved in mitochondrial biogenesis only in ALCAR-treated suspended animals, supporting a differentiated effect of ALCAR treatment in relation to the loading state of the soleus muscle. In conclusions, we demonstrated the ability of ALCAR supplementation to promote only in soleus muscle of hindlimb suspended rats an orchestrated expression of genes involved in mitochondrial biogenesis, which might counteract the unloading-induced metabolic changes, preventing the loss of mitochondrial proteins.

Proteomics and genomics of microgravity

Many serious adverse physiological changes occur during spaceflight. In the search for underlying mechanisms and possible new countermeasures, many experimental tools and methods have been developed to study microgravity caused physiological changes, ranging from in vitro bioreactor studies to spaceflight investigations. Recently, genomic and proteomic approaches have gained a lot of attention; after major scientific breakthroughs in the fields of genomics and proteomics, they are now widely accepted and used to understand biological processes. Understanding gene and/or protein expression is the key to unfolding the mechanisms behind microgravity-induced problems and, ultimately, finding effective countermeasures to spaceflight-induced alterations. Significant progress has been made in identifying the genes/proteins responsible for these changes. Although many of these genes and/or proteins were observed to be either upregulated or downregulated, however, no large-scale genomics and proteomics studies have been published so far. This review aims at summarizing the current status of microgravity-related genomics and proteomics studies and stimulating large-scale proteomics and genomics research activities.

Metabolic consequences of muscle disuse atrophy

In response to decreased usage, skeletal muscle undergoes an adaptive reductive remodeling. This adaptive response has been found with disuse during human spaceflight, rat spaceflight, rat hind-limb unloading, bed rest, and aging. The reductive remodeling of skeletal muscle with disuse is largely independent of the reason for the disuse. The process involves more than a transition from slow to fast myosin fiber types. There are associated metabolic changes including a fuel shift toward glycolysis, decreased capacity for fat oxidation, and energy substrate accumulation in the atrophied muscles. Glycolysis is very effective for high-intensity short-duration acute activities, but if sustained output is needed, an energy profile where fat use is favored rather than compromised is desirable. For astronauts, there is a need to maintain as much functional capacity as possible during spaceflight for extravehicular activities. The shift toward increased activity of the glycolytic enzymes in atrophied muscle is accommodated by an increase in gluconeogenic capacity in the liver.

Effect of hind limb muscle unloading on liver metabolism of rats

In response to decreased use, skeletal muscle undergoes an adaptive reductive remodeling. There is a shift in fiber types from slow twitch to fast twitch fiber types. Associated with muscle unloading is an increased reliance on carbohydrate metabolism for energy. The hind limb suspended (HLS) rat model was used as the experimental model to determine whether skeletal muscle unloading had any impact on the liver. We used a combination of actual enzyme assays and microarray mRNA expression to address this question. The GenMAPP program was used to identify altered metabolic pathways. We found that the major changes in the liver with HLS were increases in the expression of genes involved in the generation of energy fuels for export, specifically gluconeogenesis and lipogenesis. The expression of mRNA was increased (P<0.05) for three of the four enzymes involved in the regulation of gluconeogenesis pathway (pyruvate carboxylase (PC), phosphoenolpyruvate carboxykinase (PEPCK), and glucose-6-phosphatase (G-6-Pase). Actual assay of enzymatic activity, in micromol . min(-1) . mg protein(-1) showed G-6-Pase (0.14+0.01 vs 0.17+0.01 P<0.05), fructose 1,6, bisphophosphatase (0.048+0.002 vs 0.054+0.002, P<0.07), and PEPCK (0.031+0.002 vs 0.038+0.012 (P<0.05) to be increased. We conclude that 1) atrophied muscle is not the only tissue to be affected by HLS, as there is also a response by the liver; and 2) the major changes in liver substrate metabolism induced by HLS appear to be limited to glucose and triglyceride production. The increase in glycolytic capacity in disused muscle is paralleled by an increase in glucogenic capacity by the liver.

Protein abnormality in denervated skeletal muscles from patients with brachial injury

A proteomic analysis was performed to compare protein expression between normal sternocleidomastoid muscle and denervated muscle. Two-dimensional electrophoresis (2-DE) of muscle proteins showed that 26 proteins among about 800 spots in 2-DE gel displayed a decrease and 6 proteins an increase in expression in muscles with denervation atrophy compared to normal controls; the identified proteins that were abnormally expressed could be generally grouped together as metabolic proteins, chaperone proteins, and contractile-apparatus proteins. The significance of these altered proteins is discussed. In particular, the decrease in hD54 may reduce the activity of transmembrane signaling in atrophied muscle, while the disregulation of DnaJC 1 showed a possible role of molecular chaperones in the functional recovery of atrophied muscles.