Identification of ubiquitin ligases required for skeletal muscle atrophy

Skeletal muscle adapts to decreases in activity and load by undergoing atrophy. To identify candidate molecular mediators of muscle atrophy, we performed transcript profiling. Although many genes were up-regulated in a single rat model of atrophy, only a small subset was universal in all atrophy models. Two of these genes encode ubiquitin ligases: Muscle RING Finger 1 (MuRF1), and a gene we designate Muscle Atrophy F-box (MAFbx), the latter being a member of the SCF family of E3 ubiquitin ligases. Overexpression of MAFbx in myotubes produced atrophy, whereas mice deficient in either MAFbx or MuRF1 were found to be resistant to atrophy. These proteins are potential drug targets for the treatment of muscle atrophy.

Low relative skeletal muscle mass (sarcopenia) in older persons is associated with functional impairment and physical disability

OBJECTIVES

To establish the prevalence of sarcopenia in older Americans and to test the hypothesis that sarcopenia is related to functional impairment and physical disability in older persons.

DESIGN

Cross-sectional survey.

SETTING

Nationally representative cross-sectional survey using data from the Third National Health and Nutrition Examination Survey (NHANES III).

PARTICIPANTS

Fourteen thousand eight hundred eighteen adult NHANES III participants aged 18 and older.

MEASUREMENTS

The presence of sarcopenia and the relationship between sarcopenia and functional impairment and disability were examined in 4,504 adults aged 60 and older. Skeletal muscle mass was estimated from bioimpedance analysis measurements and expressed as skeletal muscle mass index (SMI = skeletal muscle mass/body mass x 100). Subjects were considered to have a normal SMI if their SMI was greater than -one standard deviation above the sex-specific mean for young adults (aged 18-39). Class I sarcopenia was considered present in subjects whose SMI was within -one to -two standard deviations of young adult values, and class II sarcopenia was present in subjects whose SMI was below -two standard deviations of young adult values.

RESULTS

The prevalence of class I and class II sarcopenia increased from the third to sixth decades but remained relatively constant thereafter. The prevalence of class I (59% vs 45%) and class II (10% vs 7%) sarcopenia was greater in the older (> or = 60 years) women than in the older men (P <.001). The likelihood of functional impairment and disability was approximately two times greater in the older men and three times greater in the older women with class II sarcopenia than in the older men and women with a normal SMI, respectively. Some of the associations between class II sarcopenia and functional impairment remained significant after adjustment for age, race, body mass index, health behaviors, and comorbidity.

CONCLUSIONS

Reduced relative skeletal muscle mass in older Americans is a common occurrence that is significantly and independently associated with functional impairment and disability, particularly in older women. These observations provide strong support for the prevailing view that sarcopenia may be an important and potentially reversible cause of morbidity and mortality in older persons.

Sarcopenia: origins and clinical relevance

This presentation reflects on the origins of the term sarcopenia. The Greek roots of the word are sarx for flesh and penia for loss. The term actually describes important changes in body composition and related functions. Clearly defining sarcopenia will allow investigators to appropriately classify patients and examine underlying pathogenic mechanisms and will allow funding agencies to appropriately target research funds to a taxonomically distinct syndrome.

Age-associated changes in skeletal muscles and their effect on mobility: an operational diagnosis of sarcopenia

Sarcopenia, the reduction of muscle mass and strength that occurs with aging, is widely considered one of the major causes of disability in older persons. Surprisingly, criteria that may help a clinician to identify persons with impaired muscle function are still lacking. Using data from a large representative sample of the general population, we examined how muscle function and calf muscle area change with aging and affect mobility in men and women free of neurological conditions. We tested several putative indicators of sarcopenia, including knee extension isometric torque, handgrip, lower extremity muscle power, and calf muscle area. For each indicator, sarcopenia was considered to be present when the measure was >2 SDs below the mean. For all four measures, the prevalence of sarcopenia increased with age, both in men and women. The age-associated gradient in prevalence was maximum for muscle power and minimum for calf-muscle area. However, lower extremity muscle power was no better than knee-extension torque or handgrip in the early identification of poor mobility, defined either as walking speed <0.8 m/s or inability to walk at least 1 km without difficulty and without developing symptoms. Optimal cutoff values that can be used in the clinical practice to identify older persons with poor mobility were developed. The findings of the study lay the basis for a cost-effective, clinical marker of sarcopenia based on a measure of isometric handgrip strength. Our findings should be verified in a longitudinal study.

Muscle mass, muscle strength, and muscle fat infiltration as predictors of incident mobility limitations in well-functioning older persons

BACKGROUND

Lower muscle mass has been correlated with poor physical function; however, no studies have examined this relationship prospectively. This study aims to investigate whether low muscle mass, low muscle strength, and greater fat infiltration into the muscle predict incident mobility limitation.

METHODS

Our study cohort included 3075 well-functioning black and white men and women aged 70-79 years participating in the Health, Aging, and Body Composition study. Participants were followed for 2.5 years. Muscle cross-sectional area and muscle tissue attenuation (a measure of fat infiltration) were measured by computed tomography at the mid-thigh, and knee extensor strength by using a KinCom dynamometer. Incident mobility limitation was defined as two consecutive self-reports of any difficulty walking one-quarter mile or climbing 10 steps.

RESULTS

Mobility limitations were developed by 22.3% of the men and by 31.8% of the women. Cox's proportional hazards models, adjusting for demographic, lifestyle, and health factors, showed a hazard ratio of 1.90 [95% confidence interval (CI), 1.27-2.84] in men and 1.68 (95% CI, 1.23-2.31) in women for the lowest compared to the highest quartile of muscle area (p <.01 for trend). Results for muscle strength were 2.02 (95% CI, 1.39-2.94) and 1.91 (95% CI, 1.41-2.58), p <.001 trend, and for muscle attenuation were 1.91 (95% CI, 1.31-2.83) and 1.68 (95% CI, 1.20-2.35), p <.01 for trend. When included in one model, only muscle attenuation and muscle strength independently predicted mobility limitation (p < .05). Among men and women, associations were similar for blacks and whites.

CONCLUSION

Lower muscle mass (smaller cross-sectional thigh muscle area), greater fat infiltration into the muscle, and lower knee extensor muscle strength are associated with increased risk of mobility loss in older men and women. The association between low muscle mass and functional decline seems to be a function of underlying muscle strength.

Sarcopenia: alternative definitions and associations with lower extremity function

OBJECTIVES

To compare two sarcopenia definitions and examine the relationship between them and lower extremity function and other health related factors using data from the baseline examination of the Health Aging and Body Composition (Health ABC) Study.

DESIGN

Observational cohort study.

SETTING

Two U.S. communities in Memphis, Tennessee, and Pittsburgh, Pennsylvania.

PARTICIPANTS

Participants were aged 70 to 79 (N=2984, 52% women, 41% black).

MEASUREMENTS

Participants were assessed using dual energy x-ray absorptiometry and were classified as sarcopenic using two different approaches to adjust lean mass for body size: appendicular lean mass divided by height-squared (aLM/ht2) and appendicular lean mass adjusted for height and body fat mass (residuals).

RESULTS

These methods differed substantially in the classification of individuals as being sarcopenic, especially those who were more obese. The former method was highly correlated with body mass index and identified fewer overweight or obese individuals as sarcopenic. In both men and women, none of the obese group would be considered sarcopenic using the aLM/ht2 method, compared with 11.5% of men and 21.0% of women using the residuals method. In men, both classifications of sarcopenia were associated with smoking, poorer health, lower activity, and impaired lower extremity function. Fewer associations with health factors were noted in women, but the classification based on both height and fat mass was more strongly associated with lower extremity functional limitations (odds ratio (OR)=0.9, 95% confidence interval (CI)=0.7-1.2 for low kg/ht2; OR=1.9, 95% CI=1.4-2.5 for lean mass adjusted for height and fat mass).

CONCLUSION

These findings suggest that fat mass should be considered in estimating prevalence of sarcopenia in women and in overweight or obese individuals.

Body composition in healthy aging

Health risks in elderly people cannot be evaluated simply in conventional terms of body fatness or fat distribution. Elderly people have less muscle and bone mass, expanded extracellular fluid volumes, and reduced body cell mass compared to younger adults. These nonfat components of body composition play critical roles, influencing cognitive and physical functional status, nutritional and endocrine status, quality of life, and comorbidity in elderly people. Different patterns of "disordered body composition" have different relationships to these outcomes and may require different, tailored approaches to treatment that combine various exercise regimens and dietary supplements with hormone replacement or appetite-stimulating drugs. Skeletal muscle atrophy, or "sarcopenia," is highly prevalent in the elderly population, increases with age, and is strongly associated with disability, independent of morbidity. Elders at greatest risk are those who are simultaneously sarcopenic and obese. The accurate identification of sarcopenic obesity requires precise methods of simultaneously measuring fat and lean components, such as dual-energy X-ray absorptiometry.

Inflammatory markers and loss of muscle mass (sarcopenia) and strength

PURPOSE

The objective of this study was to investigate whether high levels of serum interleukin (IL)-6, C-reactive protein (CRP), and alpha1-antichymotrypsin (ACT) were associated with the loss of muscle strength or muscle mass (sarcopenia) in older persons.

SUBJECTS

The study included 986 men and women of the Longitudinal Aging Study Amsterdam, with a mean age of 74.6 years (standard deviation 6.2).

METHODS

Grip strength (n = 986) and appendicular muscle mass (n = 328, using dual-energy x-ray absorptiometry) were obtained in 1995 and 1996 and repeated after a 3-year follow-up. Loss of muscle strength was defined as a loss of grip strength greater than 40%, and sarcopenia was defined as a loss of muscle mass greater than 3%, approximating the lowest 15% of the study sample.

RESULTS

Multiple linear and logistic regression analyses revealed that higher levels of IL-6 were associated with greater decline in muscle strength, which decreased by -3.21 kg (standard error 0.81) per standard deviation increase in log-transformed IL-6. After adjustment for confounders, including sociodemographic, health, and lifestyle factors, high IL-6 (>5 pg/mL) and high CRP (>6.1 mug/mL) were associated with a 2 to 3-fold greater risk of losing greater than 40% of muscle strength. Persons with high levels of ACT (>181% of the normal human pooled plasma) were 40% less likely to experience loss of muscle strength and tended (P = .07) to have a smaller decline in muscle mass compared with those in the lowest quartile of ACT. No consistent associations of IL-6 and CRP with sarcopenia were found.

CONCLUSION

The findings of this prospective, population-based study suggest that higher levels of IL-6 and CRP increase the risk of muscle strength loss, whereas higher levels of ACT decrease the risk of muscle strength loss in older men and women.

The molecular basis of skeletal muscle atrophy

Skeletal muscle atrophy attributable to muscular inactivity has significant adverse functional consequences. While the initiating physiological event leading to atrophy seems to be the loss of muscle tension and a good deal of the physiology of muscle atrophy has been characterized, little is known about the triggers or the molecular signaling events underlying this process. Decreases in protein synthesis and increases in protein degradation both have been shown to contribute to muscle protein loss due to disuse, and recent work has delineated elements of both synthetic and proteolytic processes underlying muscle atrophy. It is also becoming evident that interactions among known proteolytic pathways (ubiquitin-proteasome, lysosomal, and calpain) are involved in muscle proteolysis during atrophy. Factors such as TNF-alpha, glucocorticoids, myostatin, and reactive oxygen species can induce muscle protein loss under specified conditions. Also, it is now apparent that the transcription factor NF-kappaB is a key intracellular signal transducer in disuse atrophy. Transcriptional profiles of atrophying muscle show both up- and downregulation of various genes over time, thus providing further evidence that there are multiple concurrent processes involved in muscle atrophy. The purpose of this review is to synthesize our current understanding of the molecular regulation of muscle atrophy. We also discuss how ongoing work should uncover more about the molecular underpinnings of muscle wasting, particularly that due to disuse.

Sarcopenic obesity: a new category of obesity in the elderly

BACKGROUND AND AIM

In elderly patients, age-related changes in body composition, as well as the increased prevalence of obesity, determine a combination of excess weight and reduced muscle mass or strength, recently defined as sarcopenic obesity (SO). This review examines the main studies regarding sarcopenic obesity in the elderly.

DATA SYNTHESIS

Definition of SO necessarily combines those of sarcopenia and obesity. The prevalence of sarcopenia and SO increases with age. Muscle and fat mass are strongly interconnected from a pathogenetic point of view. A better understanding of the mechanisms which lead from loss of muscle mass to fat gain or vice versa from fat gain to muscle loss seems to be crucial. Recent data suggest that peptides produced by adipose tissue may play an important role in the pathophysiology of SO, thus more research is needed to better characterize this new area. Obesity and sarcopenia in the elderly may potentiate each other maximizing their effects on disability, morbidity and mortality. Identifying elderly subjects with SO should be mandatory; effective treatment of sarcopenia and SO may attenuate its clinical impact.

CONCLUSION

The concept of SO may help to clarify the relationship between obesity, morbidity and mortality in the elderly.

Leg muscle mass and composition in relation to lower extremity performance in men and women aged 70 to 79: the health, aging and body composition study

OBJECTIVES

The loss of muscle mass with aging, or sarcopenia, is hypothesized to be associated with the deterioration of physical function. Our aim was to determine whether low leg muscle mass and greater fat infiltration in the muscle were associated with poor lower extremity performance (LEP).

DESIGN

A cross-sectional study, using baseline data of the Health, Aging and Body Composition study (1997/98).

SETTING

Medicare beneficiaries residing in ZIP codes from the metropolitan areas surrounding Pittsburgh, Pennsylvania, and Memphis, Tennessee.

PARTICIPANTS

Three thousand seventy-five well-functioning black and white men and women aged 70 to 79.

MEASUREMENTS

Two timed tests (6-meter walk and repeated chair stands) were used to measure LEP. Muscle cross-sectional area and muscle tissue attenuation (indicative of fat infiltration) were obtained from computed tomography scans at the midthigh. Body fat was assessed using dual-energy x-ray absorptiometry.

RESULTS

Blacks had greater muscle mass and poorer LEP than whites. Black women had greater fat infiltration into the muscle than white women. After adjustment for clinic site, age, height, and total body fat, smaller muscle area was associated with poorer LEP in all four race-gender groups. (Regression coefficients, expressed per standard deviation (+/-55 cm2) of muscle area, were 0.658 and 0.519 in white and black men and 0.547 and 0.435 in white and black women, respectively, P <.01.) In addition, reduced muscle attenuation-indicative of greater fat infiltration into the muscle-was associated with poorer LEP, independent of total body fat and muscle area. (Regression coefficients per standard deviation (= 7 Hounsfield Units) of muscle attenuation were 0.292 and 0.224 in white and black men, and 0.193 and 0.159 in white and black women, respectively, P <.05). The most important body composition components related to LEP were muscle area in men and total body fat in women. Results were similar after additional adjustment for lifestyle factors and health status. No interactions between race and muscle area (P>.7) or between race and muscle attenuation (P>.2) were observed.

CONCLUSION

Smaller midthigh muscle area and greater fat infiltration in the muscle are associated with poorer LEP in well-functioning older men and women.

Signaling in muscle atrophy and hypertrophy

Muscle performance is influenced by turnover of contractile proteins. Production of new myofibrils and degradation of existing proteins is a delicate balance, which, depending on the condition, can promote muscle growth or loss. Protein synthesis and protein degradation are coordinately regulated by pathways that are influenced by mechanical stress, physical activity, availability of nutrients, and growth factors. Understanding the signaling that regulates muscle mass may provide potential therapeutic targets for the prevention and treatment of muscle wasting in metabolic and neuromuscular diseases.

IL-6-induced skeletal muscle atrophy

Chronic, low-level elevation of circulating interleukin (IL)-6 is observed in disease states as well as in many outwardly healthy elderly individuals. Increased plasma IL-6 is also observed after intense, prolonged exercise. In the context of skeletal muscle, IL-6 has variously been reported to regulate carbohydrate and lipid metabolism, increase satellite cell proliferation, or cause muscle wasting. In the present study, we used a rodent local infusion model to deliver modest levels of IL-6, comparable to that present after exercise or with chronic low-level inflammation in the elderly, directly into a single target muscle in vivo. The aim of this study was to examine the direct effects of IL-6 on skeletal muscle in the absence of systemic changes in this cytokine. Data included cellular and molecular markers of cytokine and growth factor signaling (phosphorylation and mRNA content) as well as measurements to detect muscle atrophy. IL-6 infusion resulted in muscle atrophy characterized by a preferential loss of myofibrillar protein (-17%). IL-6 induced a decrease in the phosphorylation of ribosomal S6 kinase (-60%) and STAT5 (-33%), whereas that of STAT3 was increased approximately twofold. The changes seen in the IL-6-infused muscles suggest alterations in the balance of growth factor-related signaling in favor of a more catabolic profile. This suggests that downregulation of growth factor-mediated intracellular signaling may be a mechanism contributing to the development of muscle atrophy induced by elevated IL-6.

AGE-RELATED CHANGES IN THE STRUCTURE AND FUNCTION OF SKELETAL MUSCLES

1. For animals of all ages, during activation of skeletal muscles and the subsequent contraction, the balance between the force developed by the muscle and the external load determines whether the muscle shortens, remains at fixed length (isometric) or is lengthened. With maximum activation, the force developed is least during shortening, intermediate when muscle length is fixed and greatest during lengthening contractions. During lengthening contractions, when force is high, muscles may be injured by the contractions. 2. 'Frailty' and 'failure to thrive' are most frequently observed in elderly, physically inactive people. A 'frail' person is defined as one of small stature, with muscles that are atrophied, weak and easily fatigued. The condition of 'failure to thrive' is typified by a lack of response to well-designed programmes of nutrition and physical activity. 3. With ageing, skeletal muscle atrophy in humans appears to be inevitable. A gradual loss of muscle fibres begins at approximately 50 years of age and continues such that by 80 years of age, approximately 50% of the fibres are lost from the limb muscles that have been studied. For both humans and rats, the observation that the timing and magnitude of the loss of motor units is similar to that for muscle fibres suggests that the mechanism responsible for the loss of fibres and the loss of whole motor units is the same. The degree of atrophy of the fibres that remain is largely dependent on the habitual level of physical activity of the individual. 4. 'Master athletes' maintain a high level of fitness throughout their lifespan. Even among master athletes, performance of marathon runners and weight lifters declines after approximately 40 years of age, with peak levels of performance decreased by approximately 50% by 80 years of age. The success of the master athletes and of previously sedentary elderly who undertake well-designed, carefully administered training programmes provide dramatic evidence that age-associated atrophy, weakness and fatigability can be slowed, but not halted.

Sarcopenia: current concepts

Sarcopenia, the loss of muscle mass and strength with age, is becoming recognized as a major cause of disability and morbidity in the elderly population. Sarcopenia is part of normal aging and does not require a disease to occur, although muscle wasting is accelerated by chronic diseases. Sarcopenia is thought to have multiple causes, although the relative importance of each is not clear. Neurological, metabolic, hormonal, nutritional, and physical-activity-related changes with age are likely to contribute to the loss of muscle mass. In this review, we discuss current concepts of the pathogenesis, treatment, and prevention of sarcopenia.

Atrophy of the soleus muscle by hindlimb unweighting

The unweighting model is a unique whole animal model that will permit the future delineation of the mechanism(s) by which gravity maintains contractile mass in postural (slow-twitch) skeletal muscle. Since the origination of the model of rodent hindlimb unweighting almost one decade ago, about half of the 59 refereed articles in which this model was utilized have been published in the Journal of Applied Physiology. Thus the purpose of this review is to provide, for those researchers with an interest in the hindlimb unweighting model, a summation of the data derived from this model to data and hopefully to stimulate research interest in aspects of the model for which data are lacking. The stress response of the animal to hindlimb unweighting is transient, minimal in magnitude, and somewhat variable. After 1 wk of unweighting, the animal exhibits no chronic signs of stress. The atrophy of the soleus muscle, a predominantly slow-twitch muscle, is emphasized because unweighting preferentially affects it compared with other calf muscles, which are mainly fast-twitch muscles. The review considers the following information about the unweighted soleus muscle: electromyogram activity, amount and type of protein lost, capillarization, oxidative capacity, glycolytic enzyme activities, fiber cross section, contractile properties, glucose uptake, sensitivity to insulin, protein synthesis and degradation rates, glucocorticoid receptor numbers, responses of specific mRNAs, and changes in metabolite concentrations.

Skeletal muscle is a primary target of SOD1G93A-mediated toxicity

The antioxidant enzyme superoxide dismutase 1 (SOD1) is a critical player of the antioxidative defense whose activity is altered in several chronic diseases, including amyotrophic lateral sclerosis. However, how oxidative insult affects muscle homeostasis remains unclear. This study addresses the role of oxidative stress on muscle homeostasis and function by the generation of a transgenic mouse model expressing a mutant SOD1 gene (SOD1(G93A)) selectively in skeletal muscle. Transgenic mice developed progressive muscle atrophy, associated with a significant reduction in muscle strength, alterations in the contractile apparatus, and mitochondrial dysfunction. The analysis of molecular pathways associated with muscle atrophy revealed that accumulation of oxidative stress served as signaling molecules to initiate autophagy, one of the major intracellular degradation mechanisms. These data demonstrate that skeletal muscle is a primary target of SOD1(G93A) -mediated toxicity and disclose the molecular mechanism whereby oxidative stress triggers muscle atrophy.

Disruption of dynein/dynactin inhibits axonal transport in motor neurons causing late-onset progressive degeneration

To test the hypothesis that inhibition of axonal transport is sufficient to cause motor neuron degeneration such as that observed in amyotrophic lateral sclerosis (ALS), we engineered a targeted disruption of the dynein-dynactin complex in postnatal motor neurons of transgenic mice. Dynamitin overexpression was found to disassemble dynactin, a required activator of cytoplasmic dynein, resulting in an inhibition of retrograde axonal transport. Mice overexpressing dynamitin demonstrate a late-onset progressive motor neuron degenerative disease characterized by decreased strength and endurance, motor neuron degeneration and loss, and denervation of muscle. Previous transgenic mouse models of ALS have shown abnormalities in microtubule-based axonal transport. In this report, we describe a mouse model that confirms the critical role of disrupted axonal transport in the pathogenesis of motor neuron degenerative disease.

Smad2 and 3 transcription factors control muscle mass in adulthood

Loss of muscle mass occurs in a variety of diseases, including cancer, chronic heart failure, aquired immunodeficiency syndrome, diabetes, and renal failure, often aggravating pathological progression. Preventing muscle wasting by promoting muscle growth has been proposed as a possible therapeutic approach. Myostatin is an important negative modulator of muscle growth during myogenesis, and myostatin inhibitors are attractive drug targets. However, the role of the myostatin pathway in adulthood and the transcription factors involved in the signaling are unclear. Moreover, recent results confirm that other transforming growth factor-beta (TGF-beta) members control muscle mass. Using genetic tools, we perturbed this pathway in adult myofibers, in vivo, to characterize the downstream targets and their ability to control muscle mass. Smad2 and Smad3 are the transcription factors downstream of myostatin/TGF-beta and induce an atrophy program that is muscle RING-finger protein 1 (MuRF1) independent. Furthermore, Smad2/3 inhibition promotes muscle hypertrophy independent of satellite cells but partially dependent of mammalian target of rapamycin (mTOR) signaling. Thus myostatin and Akt pathways cross-talk at different levels. These findings point to myostatin inhibitors as good drugs to promote muscle growth during rehabilitation, especially when they are combined with IGF-1-Akt activators.

Mechanisms of IGF-1-Mediated Regulation of Skeletal Muscle Hypertrophy and Atrophy

Insulin-like growth factor-1 (IGF-1) is a key growth factor that regulates both anabolic and catabolic pathways in skeletal muscle. IGF-1 increases skeletal muscle protein synthesis via PI3K/Akt/mTOR and PI3K/Akt/GSK3β pathways. PI3K/Akt can also inhibit FoxOs and suppress transcription of E3 ubiquitin ligases that regulate ubiquitin proteasome system (UPS)-mediated protein degradation. Autophagy is likely inhibited by IGF-1 via mTOR and FoxO signaling, although the contribution of autophagy regulation in IGF-1-mediated inhibition of skeletal muscle atrophy remains to be determined. Evidence has suggested that IGF-1/Akt can inhibit muscle atrophy-inducing cytokine and myostatin signaling via inhibition of the NF-κΒ and Smad pathways, respectively. Several miRNAs have been found to regulate IGF-1 signaling in skeletal muscle, and these miRs are likely regulated in different pathological conditions and contribute to the development of muscle atrophy. IGF-1 also potentiates skeletal muscle regeneration via activation of skeletal muscle stem (satellite) cells, which may contribute to muscle hypertrophy and/or inhibit atrophy. Importantly, IGF-1 levels and IGF-1R downstream signaling are suppressed in many chronic disease conditions and likely result in muscle atrophy via the combined effects of altered protein synthesis, UPS activity, autophagy, and muscle regeneration.

Oxidative stress and disuse muscle atrophy

Skeletal muscle inactivity is associated with a loss of muscle protein and reduced force-generating capacity. This disuse-induced muscle atrophy results from both increased proteolysis and decreased protein synthesis. Investigations of the cell signaling pathways that regulate disuse muscle atrophy have increased our understanding of this complex process. Emerging evidence implicates oxidative stress as a key regulator of cell signaling pathways, leading to increased proteolysis and muscle atrophy during periods of prolonged disuse. This review will discuss the role of reactive oxygen species in the regulation of inactivity-induced skeletal muscle atrophy. The specific objectives of this article are to provide an overview of muscle proteases, outline intracellular sources of reactive oxygen species, and summarize the evidence that connects oxidative stress to signaling pathways contributing to disuse muscle atrophy. Moreover, this review will also discuss the specific role that oxidative stress plays in signaling pathways responsible for muscle proteolysis and myonuclear apoptosis and highlight gaps in our knowledge of disuse muscle atrophy. By presenting unresolved issues and suggesting topics for future research, it is hoped that this review will serve as a stimulus for the expansion of knowledge in this exciting field.

Loss of skeletal muscle mass in aging: Examining the relationship of starvation, sarcopenia and cachexia

A loss of body weight or skeletal muscle mass is common in older persons and is a harbinger of poor outcome. Involuntary weight loss can be categorized into three primary etiologies of starvation, sarcopenia, and cachexia. Starvation results in a loss of body fat and non-fat mass due to inadequate intake of protein and energy. Sarcopenia is associated with a reduction in muscle mass and strength occurring with normal aging, associated with a reduction in motor unit number and atrophy of muscle fibers, especially the type IIa fibers. The loss of muscle mass with aging is clinically important because it leads to diminished strength and exercise capacity. Cachexia is widely recognized as severe wasting accompanying disease states such as cancer or immunodeficiency disease, but does not have a universally accepted definition. The key clinical question is whether these changes in body composition are distinct entities or represent an interdependent continuum. The importance of defining the distinction lies in developing a targeted therapeutic approach to skeletal muscle loss and muscle strength in older persons. Failure to distinguish among these causes of skeletal muscle loss often results in frustration over the clinical response to therapeutic interventions.