Human Disuse Atrophy Research Articles

Declines in muscle protein synthesis account for short-term muscle disuse atrophy in humans in the absence of increased muscle protein breakdown.

BackgroundWe determined the short‐term (i.e. 4 days) impacts of disuse atrophy in relation to muscle protein turnover [acute fasted‐fed muscle protein synthesis (MPS)/muscle protein breakdown (MPB) and integrated MPS/estimated MPB].MethodsHealthy men (N = 9, 22 ± 2 years, body mass index 24 ± 3 kg m−2) underwent 4 day unilateral leg immobilization. Vastus lateralis (VL) muscle thickness (MT) and extensor strength and thigh lean mass (TLM) were measured. Bilateral VL muscle biopsies were collected on Day 4 at t = −120, 0, 90, and 180 min to determine integrated MPS, estimated MPB, acute fasted‐fed MPS (l‐[ring‐13C6]‐phe), and acute fasted tracer decay rate representative of MPB (l‐[15N]‐phe and l‐[2H8]‐phe). Protein turnover cell signalling was measured by immunoblotting.ResultsImmobilization decreased TLM [pre: 7477 ± 1196 g, post: 7352 ± 1209 g (P < 0.01)], MT [pre: 2.67 ± 0.50 cm, post: 2.55 ± 0.51 cm (P < 0.05)], and strength [pre: 260 ± 43 N m, post: 229 ± 37 N m (P < 0.05)] with no change in control legs. Integrated MPS decreased in immob vs. control legs [control: 1.55 ± 0.21% day−1, immob: 1.29 ± 0.17% day−1 (P < 0.01)], while tracer decay rate (i.e. MPB) (control: 0.02 ± 0.006, immob: 0.015 ± 0.015) and fractional breakdown rate (FBR) remained unchanged [control: 1.44 ± 0.51% day−1, immob: 1.73 ± 0.35% day−1 (P = 0.21)]. Changes in MT correlated with those in MPS but not FBR. MPS increased in the control leg following feeding [fasted: 0.043 ± 0.012% h−1, fed: 0.065 ± 0.017% h−1 (P < 0.05)] but not in immob [fasted: 0.034 ± 0.014% h−1, fed: 0.049 ± 0.023% h−1 (P = 0.09)]. There were no changes in markers of MPB with immob (P > 0.05).ConclusionsHuman skeletal muscle disuse atrophy is driven by declines in MPS, not increases in MPB. Pro‐anabolic therapies to mitigate disuse atrophy would likely be more effective than therapies aimed at attenuating protein degradation.

Soleus muscle stability in wild hibernating black bears.

Based on studies of fast skeletal muscles, hibernating black and brown bears resist skeletal muscle atrophy during months of reduced physical activity and not feeding. The present study examined atrophy sparing in the slow soleus muscle, known to be highly prone to disuse atrophy in humans and other mammals. We demonstrated histochemically that the black bear soleus is rich in slow fibers, averaging 84.0 ± 6.6%. The percentages of slow fibers in fall (87.3 ± 4.9%) and during hibernation (87.1 ± 5.6%) did not differ ( P = 0.3152) from summer. The average fiber cross-sectional area to body mass ratio (48.6 ± 11.7 µm2/kg) in winter hibernating bears was not significantly different from that of summer (54.1 ± 11.8 µm2/kg, P = 0.4186) and fall (47.0 ± 9.7 µm2/kg, P = 0.9410) animals. The percentage of single hybrid fibers containing both slow and fast myosin heavy chains, detected biochemically, increased from 2.6 ± 3.8% in summer to 24.4 ± 24.4% ( P = 0.0244) during hibernation. The shortening velocities of individual hybrid fibers remained unchanged from that of pure slow and fast fibers, indicating low content of the minority myosins. Slow and fast fibers in winter bears exhibited elevated specific tension (kN/m2; 22%, P = 0.0161 and 11%, P = 0.0404, respectively) and maintained normalized power. The relative stability of fiber type percentage and size, fiber size-to-body mass ratio, myosin heavy chain isoform content, shortening velocity, power output, and elevated specific tension during hibernation validates the ability of the black bear to preserve the biochemical and performance characteristics of the soleus muscle during prolonged hibernation.

Disuse skeletal muscle atrophy in humans. Proteomic and molecular adaptations

Disuse skeletal muscle atrophy in humans. Proteomic and molecular adaptations

Interventional strategies to combat muscle disuse atrophy in humans: focus on neuromuscular electrical stimulation and dietary protein.

Numerous situations, such as the recovery from illness or rehabilitation after injury, necessitate a period of muscle disuse in otherwise healthy individuals. Even a few days of immobilization or bed rest can lead to substantial loss of skeletal muscle tissue and compromise metabolic health. The decline in muscle mass is attributed largely to a decline in postabsorptive and postprandial muscle protein synthesis rates. Reintroduction of some level of muscle contraction by the application of neuromuscular electrical stimulation (NMES) can augment both postabsorptive and postprandial muscle protein synthesis rates and, as such, prevent or attenuate muscle loss during short-term disuse in various clinical populations. Whereas maintenance of habitual dietary protein consumption is a prerequisite for muscle mass maintenance, supplementing dietary protein above habitual intake levels does not prevent muscle loss during disuse in otherwise healthy humans. Combining the anabolic properties of physical activity (or surrogates) with appropriate nutritional support likely further increases the capacity to preserve skeletal muscle mass during a period of disuse. Therefore, effective interventional strategies to prevent or alleviate muscle disuse atrophy should include both exercise (mimetics) and appropriate nutritional support.

CrossTalk proposal: The dominant mechanism causing disuse muscle atrophy is decreased protein synthesis.

CrossTalk proposal: The dominant mechanism causing disuse muscle atrophy is decreased protein synthesis.

Little change in markers of protein breakdown and oxidative stress in humans in immobilization-induced skeletal muscle atrophy

A number of studies in rodents suggest that disuse atrophy results from a large increase in proteolysis affected by, or accompanying, increased oxidative stress. Little information is available, however, about the effects of immobilization on markers of muscle protein breakdown and oxidative stress in humans. Therefore, the purpose of this investigation was to measure markers of breakdown or oxidative stress in subjects who underwent 14 days of knee-brace-mediated immobilization. Vastus lateralis samples taken from 21 young subjects before, and 2 days and 14 days after, single leg immobilization were measured for ubiquitin-protein conjugates, caspase 3/7 activity, the 14-kDa caspase-3 cleaved actin fragment, 4-hydroxy-2-nonenal (4-HNE) adducts, and protein carbonyls. Quadriceps cross-sectional area decreased by 5.7% +/- 1.1% (p < 0.0001) following immobilization. Ubiquitin-protein conjugates were elevated at 2 days of immobilization (12%, p < 0.05) but were not different from baseline at 14 days. Levels of the 14-kDa actin fragment and caspase 3/7 activity did not change over the immobilization period. The oxidative stress markers, 4-HNE adducts and protein carbonyls, did not change at any time point. These static measures of breakdown and oxidative modification suggest that a small increase in protein ubiquitination occurs early (2 days), but elevations in ubiquitinated or oxidatively modified proteins are not sustained during the later phase (14 days) of uncomplicated disuse atrophy in humans, suggesting that these pathways are not playing a major role in simple disuse-induced atrophic loss of protein mass.

Facts, noise and wishful thinking: muscle protein turnover in aging and human disuse atrophy

Surprisingly little is known about the mechanisms of muscle atrophy with aging and disuse in human beings, in contrast to rodents, from which much has been extrapolated to explain the human condition. However, this extrapolation is likely unwarranted because the time course, extent of wasting, muscle fiber involvement and alterations of muscle protein turnover are all quite different in rodent and human muscle. Furthermore, there is little evidence that static indices of protein turnover represent dynamic changes and may be misleading. With disuse there are reductions in the rate of muscle protein synthesis (MPS) large enough to explain the atrophic loss of muscle protein without a concomitant increase in proteolysis. In aging, there is no evidence that there are marked alterations in basal muscle protein turnover in healthy individuals but instead the ability to maintain muscle after feeding is compromised. This anabolic resistance is evident with physical inactivity, which exacerbates the inability to maintain muscle mass with aging. The main conclusion of this review is that in uncomplicated, non-inflammatory disuse atrophy, the facilitative change causing loss of muscle mass is a depression of MPS, exacerbated by anabolic resistance during feeding, with possible adaptive depressions, rather than increases, of muscle proteolysis.

Newton's force as countermeasure for disuse atrophy

the observation that disuse leads to muscle atrophy is an old one, but we still lack a good mechanistic description of what is going on to cause wasting in conditions as diverse as bed rest and spaceflight. In fact, much of the current descriptions of the superficial mechanisms of atrophy come from

Reliability of results and interpretation of measures of 3-methylhistidine in muscle interstitium as marker of muscle proteolysis

TO THE EDITOR: We were interested in the contribution by Tesch and colleagues to the evidence base concerning the events subsequent to acute immobilization in human muscle. Tesch and colleagues (14) used a microdialysis technique applied to human muscle in an attempt to measure an index of myofibrillar proteolysis 72 h after immobilization using the unilateral leg suspension technique. The authors discuss their results as though they had good evidence that the technique they used produces results that are sufficiently firm to be the basis of conjecture about mechanisms of protein balance in muscle after immobilization. We demur and propose that they are on very shaky ground. First, we wonder about the size of the effect reported. The authors claim it to be 44%, but calculation of the change shows it to be actually 29% with the SEs sufficiently large that they almost span the difference; SDs must overlap. Significance at the 5% level can only barely have been reached. Are the authors convinced that they have not ignored a type I error? The authors (14) begin their explanation of the advantages of the dialysis technique using 3-methylhistidine (3MeHis) by describing the gold standard method, i.e., the stable isotopelabeled amino acid tracer dilution method, as cumbersome and possibly unjustified in study of otherwise healthy subjects. We find this notion strange since Ferrando and colleagues (6, 12) have used the technique in many studies of how immobilization affects muscle protein turnover subsequent to disuse. We have also used the techniques in a wide variety of studies of proteolysis in healthy and infirm subjects (1, 2, 10, 11, 15) with no problems and, we would propose, producing results with a greater degree of certainty and biological appropriateness than those obtained by the 3MeHis dialysis technique.

Reply from Tyler Barker and Maret G. Traber

We thank the authors for their interest and Letter to the Editor regarding our recent Journal Club article (Barker & Traber, 2007). The authors of the letter emphasize the wealth of information concerning the dynamic measures of protein turnover in human muscle during immobilization that lead to disuse atrophy, a point that is well taken and appreciated by us. We would also like to thank the authors for their succinct review identifying the significance of muscle protein synthesis on muscle mass in humans. While it is well known that muscle mass is governed by the balance between protein synthesis and degradation, it would be of great importance to identify the initiating event(s) or factor(s) that alter this balance and cause atrophy following limb disuse in humans. Furthermore, it is probably necessary to consider that there may be differences in responses from normal healthy individuals who have experienced a traumatic injury compared with those who have an illness, especially influenced by increased inflammation, when investigating the temporal regulation of the dynamics that govern muscle mass. We agree with the authors' comment that there is very little direct support in humans regarding the influence of oxidative stress on muscle disuse atrophy. The intent of our conclusion was to stimulate the application of acquired scientific knowledge in experimental animals to humans for appropriate study design and hypothesis testing. Whether there is a local increase in oxidative stress prior to or during limb disuse atrophy in humans is unknown, and is the subject of our current research protocols. Due to space limitations, we cited only one paper describing vitamin E amelioration of hindlimb suspension-induced muscle atrophy in rats (Servais et al. 2007). However, other papers have demonstrated the potential influence of vitamin E supplementation on the amelioration of atrophy following muscle disuse (Appell et al. 1997; Kondo et al. 1992b); in contrast, other experimenters have not found any vitamin E benefit on muscle mass (Koesterer et al. 2002). In addition, several excellent reviews have identified an association between oxidative stress and muscle atrophy (Powers et al. 2005, 2007; Reid et al. 2005). What made the findings by Servais et al. (2007) intriguing to us, was that vitamin E supplementation concurrently reduced muscle atrophy and various proteolytic pathways during limb disuse in rats. Furthermore, a variety of antioxidant supplements and/or experimental treatment methods that reduce oxidative stress in diverse animal models demonstrate that when the increase in oxidative stress is reduced during disuse (i.e. hindlimb suspension or mechanical ventilation) there is also a decrease in muscle atrophy (Kondo et al. 1992a; Ikemoto et al. 2002; Betters et al. 2004; Arbogast et al. 2006; McClung et al. 2007; Suzuki et al. 2007). We agree that evidence demonstrating that oxidative stress is the initiating cause of muscle atrophy in humans would be very useful from both the clinical and ‘subclinical’ standpoint. However, as the authors point out, there is no evidence to date on human muscle disuse atrophy or whether there is an increase in oxidative stress, assessed either by increases in oxidative stress biomarkers or by changes in antioxidant concentrations in immobilized muscle. However, the increases in metallothionein and atrogin-1/MAFbx gene expression in humans (Urso et al. 2006, 2007) during disuse atrophy are provocative. The increase in metallothionein in both human (Urso et al. 2007, 2006) and animal (Kondo et al. 1992b) disuse studies suggests a common mechanism because metallothioneins are induced by oxidative stress and display antioxidant properties due in part to their ability to both scavenge reactive oxygen species and minimize the availability of pro-oxidant transition metals (Miles et al. 2000). The increase in atrogin-1/MAFbx during limb disuse in humans (Jones et al. 2004; Ogawa et al. 2006; Urso et al. 2006, 2007; Chen et al. 2007) further implicates oxidative stress because myotubes exposed to H2O2 have also been found to increase atrogin-1/MAFbx mRNA (Li et al. 2003). This finding is important because it identifies a discernable factor (i.e. oxidative stress) that may be initially responsible for the increase in a key enzyme in the ubiquitin proteasome pathway. This may probably occur through or in collaboration with the TNF-α/NF–κB pathway that is sensitive to reactive oxygen species (Li et al. 1998, 1999). Thus, a number of recent findings suggest that an imbalance in the responses to oxidative stress may be critical in the initiation of pathological responses to muscle immobilization. The authors' letter regarding our recent Journal Club article (Barker & Traber, 2007) is appreciated not only for their scientific advice regarding the extrapolations from studies conducted in rodents and applied to humans, but also for their obvious expertise in muscle protein turnover. We believe that oxidative stress may be one of many causative links or ‘upstream driver’ in human muscle atrophy. But, this assertion remains an hypothesis to be tested.

Transcriptional pathways associated with skeletal muscle disuse atrophy in humans

Disuse atrophy is a common clinical phenomenon that significantly impacts muscle function and activities of daily living. The purpose of this study was to implement genome-wide expression profiling to identify transcriptional pathways associated with muscle remodeling in a clinical model of disuse. Skeletal muscle biopsies were acquired from the medial gastrocnemius in patients with an ankle fracture and from healthy volunteers subjected to 4-11 days of cast immobilization. We identified 277 misregulated transcripts in immobilized muscles of patients, of which the majority were downregulated. The most broadly affected pathways were involved in energy metabolism, mitochondrial function, and cell cycle regulation. We also found decreased expression in genes encoding proteolytic proteins, calpain-3 and calpastatin, and members of the myostatin and IGF-I pathway. Only 26 genes showed increased expression in immobilized muscles, including apolipoprotein (APOD) and leptin receptor (LEPR). Upregulation of APOD (5.0-fold, P < 0.001) and LEPR (5.7-fold, P < 0.05) was confirmed by quantitative RT-PCR and immunohistochemistry. In addition, atrogin-1/MAFbx was found to be 2.4-fold upregulated (P < 0.005) by quantitative RT-PCR. Interestingly, 96% of the transcripts differentially regulated in immobilized limbs also showed the same trend of change in the contralateral legs of patients but not the contralateral legs of healthy volunteers. Information obtained in this study complements findings in animal models of disuse and provides important feedback for future clinical studies targeting the restoration of muscle function following limb disuse in humans.

Molecular Signatures Associated with Skeletal Muscle Disuse Atrophy in Humans

Molecular Signatures Associated with Skeletal Muscle Disuse Atrophy in Humans

Fine structure of muscle in human disuse atrophy: significance of proximal muscle involvement in muscle disorders.

The universal occurrence of weakness of skeletal musculature on disuse, however produced, and the paucity of published reports on the fine structural changes in human disuse atrophy, prompted the present investigation. The quadriceps muscle of a leg immobilized in plaster cast (for fracture) and of the opposite non-immobilized limb was biopsied in four adult males, after periods of immobilization from 50 to 75 days. These 8 muscle specimens were examined for histopathological changes, and muscle fibre diameters were measured by micrometry from paraffin sections. The histograms revealed a larger proportion of small fibres (less than 20 micron) and a smaller proportion of large fibres (greater than 40 micron) in the immobilized limb compared to the opposite. Thus, light microscopy showed only atrophic changes. This was confirmed by electronmicroscopy, where atrophy of few to several muscle fibres was seen in the form of loss of myofibrils, collapse and folding of the basement membrane and prominence of glycogen or muscle nuclei. The atrophic change was more severe in the immobilized limbs, but it was also noticeable in all the non-immobilized limbs. Degenerative changes, especially disorganization and breakdown of myofibrils, and fragmentation of plasma membrane, were also seen in occasional atrophied muscle fibres, again more frequently in the immobilized limb. Lipofuscin was often found accumulated in muscle fibres and occasionally in endothelial cells of intramuscular blood vessels; the latter showed prominent pinocytotic vesicles or thickened basement membrane. It is concluded that both atrophy and degeneration of fibres of proximal muscles can occur as non-specific consequences of disuse of the limb in man, that degeneration is a latter and more severe change, that muscles even of the non-immobilized leg are subjected to disuse atrophy during bed-rest, and that the proximal muscles in man seem to have a natural susceptibility to atrophy and degeneration in any muscular disorders.