Intrinsic and extrinsic factors in muscle aging

Relative quantification of progressive changes in healthy and dysferlin-deficient mouse skeletal muscle proteomes

INTRODUCTION/AIMS

Individuals with dysferlinopathies, a group of genetic muscle diseases, experience delay in the onset of muscle weakness. The cause of this delay and subsequent muscle wasting are unknown, and there are currently no clinical interventions to limit or prevent muscle weakness. To better understand molecular drivers of dysferlinopathies, age-dependent changes in the proteomic profile of skeletal muscle (SM) in wild-type (WT) and dysferlin-deficient mice were identified.

METHODS

Quadriceps were isolated from 6-, 18-, 42-, and 77-wk-old C57BL/6 (WT, Dysf+/+ ) and BLAJ (Dysf-/- ) mice (n = 3, 2 male/1 female or 1 male/2 female, 24 total). Whole-muscle proteomes were characterized using liquid chromatography-mass spectrometry with relative quantification using TMT10plex isobaric labeling. Principle component analysis was utilized to detect age-dependent proteomic differences over the lifespan of, and between, WT and dysferlin-deficient SM. The biological relevance of proteins with significant variation was established using Ingenuity Pathway Analysis.

RESULTS

Over 3200 proteins were identified between 6-, 18-, 42-, and 77-wk-old mice. In total, 46 proteins varied in aging WT SM (p < .01), while 365 varied in dysferlin-deficient SM. However, 569 proteins varied between aged-matched WT and dysferlin-deficient SM. Proteins with significant variation in expression across all comparisons followed distinct temporal trends.

DISCUSSION

Proteins involved in sarcolemma repair and regeneration underwent significant changes in SM over the lifespan of WT mice, while those associated with immune infiltration and inflammation were overly represented over the lifespan of dysferlin-deficient mice. The proteins identified herein are likely to contribute to our overall understanding of SM aging and dysferlinopathy disease progression.

Whey Protein Supplementation Improves Rehabilitation Outcomes in Hospitalized Geriatric Patients: A Double Blinded, Randomized Controlled Trial

Whey protein supplementation (WPS) has been shown to improve functional outcomes in populations that are able to participate in high-intensity resistance training. The purpose of the study was to evaluate the efficacy of WPS on rehabilitation outcomes in a frail, hospitalized elderly population. Men and women (n = 47) were randomly assigned to either a control group or WPS group for the length of their hospital stay. Several functional and serum measures were determined pre- and post-intervention. WPS significantly increased average daily protein intake and was well tolerated. The WPS group exhibited significant improvements in grip strength and knee extensor force over the control group, and a significant positive correlation was found between change in prealbumin and percent-increase knee extensor force. These findings support the use of WPS to improve protein nutritional status and rehabilitation outcomes in a clinical setting involving a frail, elderly population.

Steroidogenesis in Leydig cells: effects of aging and environmental factors

Serum testosterone (TS) levels decrease with aging in both humans and rodents. Using the rat as a model system, it was found that age-related reductions in serum TS were not due to loss of Leydig cells, but rather to the reduced ability of the Leydig cells to produce TS in response to luteinizing hormone (LH). Detailed analyses of the steroidogenic pathway have suggested that two defects along the pathway, LH-stimulated cAMP production and cholesterol transport to and into the mitochondria, are of particular importance in age-related reductions in TS production. Although the mechanisms involved in these defects are far from certain, increasing oxidative stress appears to play a particularly important role. Interestingly, increased oxidative stress also appears to be involved in the suppressive effects of endocrine disruptors on Leydig cell TS production.

Microbial-mammalian cometabolites dominate the age-associated urinary metabolic phenotype in Taiwanese and American populations

Understanding the metabolic processes associated with aging is key to developing effective management and treatment strategies for age-related diseases. We investigated the metabolic profiles associated with age in a Taiwanese and an American population. ¹H NMR spectral profiles were generated for urine specimens collected from the Taiwanese Social Environment and Biomarkers of Aging Study (SEBAS; n = 857; age 54-91 years) and the Mid-Life in the USA study (MIDUS II; n = 1148; age 35-86 years). Multivariate and univariate linear projection methods revealed some common age-related characteristics in urinary metabolite profiles in the American and Taiwanese populations, as well as some distinctive features. In both cases, two metabolites--4-cresyl sulfate (4CS) and phenylacetylglutamine (PAG)--were positively associated with age. In addition, creatine and β-hydroxy-β-methylbutyrate (HMB) were negatively correlated with age in both populations (p < 4 × 10⁻⁶). These age-associated gradients in creatine and HMB reflect decreasing muscle mass with age. The systematic increase in PAG and 4CS was confirmed using ultraperformance liquid chromatography-mass spectrometry (UPLC-MS). Both are products of concerted microbial-mammalian host cometabolism and indicate an age-related association with the balance of host-microbiome metabolism.

A defined long-term in vitro tissue engineered model of neuromuscular junctions

Neuromuscular junction (NMJ) formation, occurring between motoneurons and skeletal muscle, is a complex multistep process involving a variety of signaling molecules and pathways. In vitro motoneuron-muscle co-cultures are powerful tools to study the role of different growth factors, hormones and cellular structures involved in NMJ formation. In this study, a serum-free culture system utilizing defined temporal growth factor application and a non-biological substrate resulted in the formation of robust NMJs. The system resulted in long-term survival of the co-culture and selective expression of neonatal myosin heavy chain, a marker of myotube maturation. NMJ formation was verified by colocalization of dense clusters of acetylcholine receptors visualized using alpha-bungarotoxin and synaptophysin containing vesicles present in motoneuron axonal terminals. This model will find applications in basic NMJ research and tissue engineering applications such as bio-hybrid device development for limb prosthesis and regenerative medicine as well as for high-throughput drug and toxin screening applications.

Stem cell review series: aging of the skeletal muscle stem cell niche

Declining stem cell function during aging contributes to impaired tissue function. Muscle-specific stem cells ('satellite cells') are responsible for generating new muscle in response to injury in the adult. However, aged muscle displays a significant reduction in regenerative abilities and an increased susceptibility to age-related pathologies. This review describes components of the satellite cell niche and addresses how age-related changes in these components impinge on satellite cell function. In particular, we review changes in the key niche elements, the myofiber and the basal lamina that are in intimate contact with satellite cells. We address how these elements are influenced by factors secreted by interstitial cells, cells of the immune system, and cells associated with the vasculature, all of which change with age. In addition, we consider more distant sources of influence on the satellite cell niche that change with age, such as neural-mediated trophic factors and electrical activity and systemic factors present in the circulation. A better understanding of the niche elements and their influence on the satellite cell will facilitate the development of therapeutic interventions aimed at improving satellite cell activity and ultimately tissue response to injury in aged individuals.

Physical activity, inflammation, and muscle loss

Sarcopenia is the degenerative loss of skeletal muscle that occurs naturally in individuals as they age. Although many factors underlie sarcopenia, epidemiological and experimental evidence suggests that low-grade chronic inflammation is an important contributor to its progression. Still, few healthcare professionals have a clear understanding of the profound effects of cytokines on sarcopenia, or how these effects may be counteracted. Interestingly, mounting evidence suggests that along with good diet and vitamin supplementation, this muscle damage can be mitigated with regular physical activity. Without a doubt, exercise is an intervention that reliably counteracts the loss of muscle mass, strength, and power common in our increasingly aged, and pervasively sedentary, population.

Age influences the early events of skeletal muscle regeneration: studies of whole muscle grafts transplanted between young (8 weeks) and old (13-21 months) mice

Injured skeletal muscle generally regenerates less efficiently with age, but little is understood about the effects of ageing on the very early inflammatory and neovascular events in the muscle repair process. This study used a total of 174 whole muscle grafts transplanted within and between young and old mice to analyse the effects of ageing on the early inflammatory response in two strains of mice (BALB/c and SJL/J). There was a very slight delay in the early inflammatory response, and in the appearance of myotubes at day 4 in BALB/c muscle grafted into an old host environment (implicating systemic events). In SJL/J mice, the initial speed of the inflammatory response was slightly delayed with old muscle grafts regardless of host age (implicating muscle-derived factors), while an old host environment transiently affected myogenesis (myotube formation). The slight delays in inflammatory and neovascular responses in old mice did not dramatically impact on the overall formation of new muscle. The neovascular response to injured young and old muscle tissue was further analysed using the corneal micropocket assay. This showed a very clear 1-2 day delay in angiogenesis induced by old versus young BALB/c muscle tissue implanted into the young rat cornea, indicating that new blood vessel formation is at least partly determined by muscle-derived factors. Taken together these results indicate that, while there are slight age-associated delays in inflammation and neovascularisation in response to injured muscle, there is no detrimental effect on myogenesis in the mouse model used in this study.

Pelvic organ prolapse in fibulin-5 knockout mice: pregnancy-induced changes in elastic fiber homeostasis in mouse vagina

Pelvic organ prolapse is strongly associated with a history of vaginal delivery. The mechanisms by which pregnancy and parturition lead to failure of pelvic organ support, however, are not known. Recently, it was reported that mice with null mutations in lysyl oxidase-like 1 (LOXL1) develop pelvic organ prolapse. Elastin is a substrate for lysyl oxidase (LOX) and LOXL1, and LOXL1 interacts with fibulin-5 (FBLN5). Therefore, to clarify the potential role of elastic fiber assembly in the pathogenesis of pelvic organ prolapse, pelvic organ support was characterized in Fbln5-/- mice, and changes in elastic fiber homeostasis in the mouse vagina during pregnancy and parturition were determined. Pelvic organ prolapse in Fbln5-/- mice was remarkably similar to that in primates. The temporal relationship between LOX mRNA and protein, processing of LOXL1 protein, FBLN5 and tropoelastin protein, and desmosine content in the vagina suggest that a burst of elastic fiber assembly and cross linking occurs in the vaginal wall postpartum. Together with the phenotype of Fbln5-/- mice, the results suggest that synthesis and assembly of elastic fibers are crucial for recovery of pelvic organ support after vaginal delivery and that disordered elastic fiber homeostasis is a primary event in the pathogenesis of pelvic organ prolapse in mice.

Modulation of overload-induced inflammation by aging and anabolic steroid administration

Aging can alter the skeletal muscle growth response induced by overload. The initiation of overload induces muscle extracellular matrix expansion, increased cellularity, and inflammatory gene expression, which are all related to processes important for myofiber growth. These remodeling processes are also biological targets of testosterone. It is not certain how aging affects the inflammatory response to functional overload and whether anabolic steroid administration can alter this response. The effect of anabolic steroid administration on inflammatory processes during functional overload is not known. The purpose of this study was to determine if age altered the skeletal muscle inflammatory response at the onset of functional overload and whether anabolic steroid administration would modulate this response in young or older animals. Five-month and 25 month F344 x BRN rats were given nandrolone decanoate (ND) (6 mg/kg bw/wk) or sham injections for 3 weeks, and then the soleus muscle was overloaded (OV) for 3 days by synergist ablation. ND alone induced a 230% increase in ED1(+) cells in 5 month muscle. Three days of OV had no effect on ED1(+) cell number at either age. OV combined with ND induced a 90% increase in ED2(+) cells in 5 month muscle, while there was no effect of either treatment alone at this age. In 25 month muscle, OV induced a 40% increase in ED2(+) cells. Regardless of age, OV induced muscle TNF-alpha mRNA expression (300%) and IL-6 mRNA expression (900%). ND attenuated OV-induced IL-6 mRNA but not TNF-alpha expression in both age groups. The overload induction of IL-1beta mRNA was 3-fold greater in 25 month muscle (1400%), compared to 5 month muscle (400%). ND administration ablated the overload IL-1beta mRNA induction in 25 month muscle. Anabolic steroid administration can suppress inflammatory cytokine gene expression at the onset of overload and this effect is age dependent.

Increases in calmodulin abundance and stabilization of activated inducible nitric oxide synthase mediate bacterial killing in RAW 264.7 macrophages

The rapid activation of macrophages in response to bacterial antigens is central to the innate immune system that permits the recognition and killing of pathogens to limit infection. To understand regulatory mechanisms underlying macrophage activation, we have investigated changes in the abundance of calmodulin (CaM) and iNOS in response to the bacterial cell wall component lipopolysaccharide (LPS) using RAW 264.7 macrophages. Critical to these measurements was the ability to differentiate free iNOS from the CaM-bound (active) form of iNOS associated with nitric oxide generation. We observe a rapid 2-fold increase in CaM abundance during the first 30 min that is blocked by inhibition of either NFkappaB nuclear translocation or protein synthesis. A similar 2-fold increase in the abundance of the complex between CaM and iNOS is observed with the same time dependence. In contrast, there are no detectable increases in the CaM-free (i.e., inactive) form of iNOS within the first 2 h; it remains at a very low abundance during the initial phase of macrophage activation. Increasing cellular CaM levels in stably transfected macrophages results in a corresponding increase in the abundance of the CaM/iNOS complex that promotes effective bacterial killing following infection by Salmonella typhimurium. Thus, LPS-dependent increases in CaM abundance function in the stabilization and activation of iNOS on the rapid time scale associated with macrophage activation and bacterial killing. These results explain how CaM and iNOS coordinately function to form a stable complex that is part of a rapid host response that functions within the first 30 min following bacterial infection to upregulate the innate immune system involving macrophage activation.

Age-related loss of muscle mass and bone strength in mice is associated with a decline in physical activity and serum leptin

The mechanisms underlying age-related loss of muscle and bone tissue are poorly understood but are thought to involve changes in sex hormone status, physical activity, and circulating levels of inflammatory cytokines. This study attempts to develop an animal model useful for evaluating these mechanisms in vivo. Male C57BL/6 mice were included for study at 3, 6, 12, 18, 24, and 29 months of age. Endocortical mineralizing surface, serum leptin, body weight, and percentage of body fat all increased between 6 and 12 months of age as activity level declined. Serum levels of the inflammatory marker IL-6 increased significantly after 12 months of age, following the observed increase in body weight and percent body fat. Hindlimb muscle mass declined significantly between 18 and 24 months of age, both absolutely and relative to total body mass, with a further decline ( approximately 15%) between 24 and 29 months. Loss of muscle mass after 18 months of age was accompanied by a significant increase in bone resorption, as indicated by serum pyridinoline cross-links, and a significant decrease in fat mass, serum leptin, bone strength, bone mineral density, and vertical cage activity. No significant changes in serum testosterone with aging were detected in the mice, as levels were essentially constant between 6 and 29 months. Our data show that mice lose a significant amount of muscle and bone tissue with age, and this loss of musculoskeletal tissue is accompanied by a drop in serum leptin and preceded by a significant decrease in physical activity.

Effect of β-hydroxy-β-methylbutyrate, arginine, and lysine supplementation on strength, functionality, body composition, and protein metabolism in elderly women

OBJECTIVE

With advancing age, there is a gradual loss of muscle mass, strength, and functionality. The current studies were conducted to determine whether a mixture of specific nutrients, arginine and lysine, which support protein synthesis, and beta-hydroxy-beta-methylbutyrate (HMB), which can slow protein breakdown, could blunt the gradual loss of muscle that occurs in the elderly, thus improving strength and functionality.

METHODS

In double-blind studies conducted at two separate sites, women (mean 76.7 y) were randomized to a placebo group (n = 23) or an experimental treatment group (2 g beta-hydroxy-beta-methylbutyrate, 5 g arginine, and 1.5 g lysine daily; n = 27).

RESULTS

After 12 wk, there was a 17% improvement in the "get-up-and-go" functionality test in the experimental group (-2.3 +/- 0.5 s) but no change in the placebo group (0.0 +/- 0.5 s; P = 0.002). The improvement in functionality also was reflected by increased limb circumference, leg strength, and handgrip strength (all P < 0.05) and positive trends in fat-free mass (P = 0.08). Whole-body protein synthesis, estimated with the (15)N-glycine tracer technique over a 24-h free-living period, increased approximately 20% in the experimental treatment group as opposed to the placebo group (P = 0.03).

CONCLUSION

These studies indicated that daily supplementation of beta-hydroxy-beta-methylbutyrate, arginine, and lysine for 12 wk positively alters measurements of functionality, strength, fat-free mass, and protein synthesis, suggesting that the strategy of targeted nutrition has the ability to affect muscle health in elderly women.

Passive stretches protect skeletal muscle of adult and old mice from lengthening contraction-induced injury

We tested the hypothesis that a single bout of training with passive stretches or isometric contractions protects the extensor digitorum longus muscle in old mice from contraction-induced injury. Lengthening contractions produced similar decreases in force (approximately 70%-80%) and numbers of overtly injured fibers (approximately 15%-20%) in adult and old mice, but twofold greater inflammatory cell accumulation above untreated control values in old versus adult mice. For both age groups, prior training with passive stretches improved postinjury force almost twofold compared with untrained muscles and reduced injured fibers by one half. Training with passive stretches or isometric contractions reduced inflammatory cell accumulation following lengthening contractions by as much as two thirds in old mice, but not in adult mice. The data indicate that passive stretches provide some protection against contraction-induced injury in old mice, and that accumulation of inflammatory cells does not correlate strongly with force deficit and number of injured fibers.

Reloading of atrophied rat soleus muscle induces tenascin-C expression around damaged muscle fibers

The hypothesis was tested that mechanical loading, induced by hindlimb suspension and subsequent reloading, affects expression of the basement membrane components tenascin-C and fibronectin in the belly portion of rat soleus muscle. One day of reloading, but not the previous 14 days of hindlimb suspension, led to ectopic accumulation of tenascin-C and an increase of fibronectin in the endomysium of a proportion (8 and 15%) of muscle fibers. Large increases of tenascin-C (40-fold) and fibronectin (7-fold) mRNA within 1 day of reloading indicates the involvement of pretranslational mechanisms in tenascin-C and fibronectin accumulation. The endomysial accumulation of tenascin-C was maintained up to 14 days of reloading and was strongly associated with centrally nucleated fibers. The observations demonstrate that an unaccustomed increase of rat soleus muscle loading causes modification of the basement membrane of damaged muscle fibers through ectopic endomysial expression of tenascin-C.

Change in muscle strength explains accelerated decline of physical function in older women with high interleukin-6 serum levels

OBJECTIVES

To test whether accelerated sarcopenia in older persons with high interleukin (IL)-6 serum levels plays a role in the prospective association between inflammation and disability found in many studies.

DESIGN

Cohort study of older women with moderate to severe disability.

PARTICIPANTS

Six hundred twenty older women from the Women's Health and Aging Study in whom information on baseline IL-6 serum level was available.

MEASUREMENTS

Self-report of functional status, objective measures of walking performance, and knee extensor strength were assessed at baseline and over six semiannual follow-up visits. Potential confounders were baseline age, race, body mass index, smoking, depression, and medical conditions.

RESULTS

At baseline, women with high IL-6 were more often disabled and had lower walking speed. After adjusting for confounders, women in the highest IL-6 tertile (IL-6>3.10 pg/mL) were at higher risk of developing incident mobility disability (risk ratio (RR) = 1.50, 95% confidence interval (CI) = 1.01-2.27), disability in activities of daily living (RR = 1.41, 95% CI = 1.01-1.98), and severe limitation in walking (RR = 1.61, 95% CI = 1.09-2.38) and experienced steeper declines in walking speed (P <.001) than women in the lowest IL-6 tertile (IL-6 < or =1.78 pg/mL). Decline in knee extensor strength was also steeper, but differences across IL-6 tertiles were not significant. After adjusting for change over time in knee extensor strength, the association between high IL-6 and accelerated decline of physical function was no longer statistically significant.

CONCLUSIONS

Older women with high IL-6 serum levels have a higher risk of developing physical disability and experience a steeper decline in walking ability than those with lower levels, which are partially explained by a parallel decline in muscle strength.

Age-dependent effects of repeated immunization with a first generation adenovirus vector on the immune response and transgene expression in young and old rats

We evaluated the kinetics of transgene expression and humoral and cellular immune responses against viral antigens and the product of the reporter gene LacZ in young (4 months) and old (20 months) Wistar rats. Animals received the intramuscular injection of a recombinant E1-deleted human type 5 adenovirus encoding beta-gal (Ad-LacZ) on days 0 and 30. The transgene expression evaluated on day 2 after infection revealed a significantly higher beta-gal activity in young than in old animals (1.9-fold increase, p<0.05). beta-gal expression decreased on day 6, and on day 15 transgene activity was undetectable in muscles from both groups. Ad-LacZ inoculation was repeated on day 30 in both animal groups. However, after the second adenovirus administration, no increase in beta-gal activity was observed. Humoral and cellular immune responses, evaluated after the first and second Ad-LacZ injection, developed with similar kinetics in young and old rats. In particular, the antigen specific antibodies were able to kill adenovirus-infected tumor cells in both complement-dependent cytotoxicity (CDC) and antibody-mediated cell-dependent cytotoxicity (ADCC) assays. Lymphocyte proliferation in response to the in vitro stimulation with specific antigens was significantly lower in old than in young animals whereas no difference was found in cytotoxic T lymphocyte activity against adenovirus-infected tumor cells. Our results demonstrate that repeated immunization with AdCMV.LacZ induces minor age-related differences in immune response which precludes gene expression both in young and old animals.

Generation of reactive oxygen and nitrogen species in contracting skeletal muscle: potential impact on aging

Since the early 1980s biologists have recognized that skeletal muscle generates free radicals. Of particular interest are two closely related redox cascades--reactive oxygen species (ROS) and nitric oxide (NO) derivatives. The ROS cascade is initiated by superoxide anion radicals derived from the mitochondrial electron transport chain, the membrane-associated NAD(P)H oxidase complex, or other sources. NO is produced by two NO synthase isoforms constitutively expressed by muscle fibers. ROS and NO derivatives are produced continually and are detectable in both the cytosolic and extracellular compartments. Production increases during strenuous exercise. Both ROS and NO modulate contractile function. Under basal conditions, low levels of ROS enhance force production. Excessive ROS accumulation inhibits force, for example, during fatiguing exercise. NO inhibits skeletal muscle contraction, an effect that is partially mediated by cyclic GMP as a second messenger. With aging, redox modulation of muscle contraction may be altered by changes in the rates of ROS and NO production, the levels of endogenous antioxidants that buffer ROS and NO, and the sensitivities of regulatory proteins to ROS and NO action. The impact of aging on contractile regulation depends on the relative magnitude of these changes and their net effects on ROS and NO activities at the cellular level.

Muscle adaptation during distraction osteogenesis in skeletally immature and mature rabbits

The lack of adaptation of muscle is thought to be a major source of complications during distraction osteogenesis (DO). Although adaptation to DO varies with the regimen (lengthening rate >1 mm/day and increase in bone length >20%) muscle contractures associated with DO may be a function of age. We tested this idea by subjecting skeletally mature and skeletally immature rabbits to an aggressive regimen of DO (1.4 mm/day with a 20% increase in tibial length). By using immunofluorescence to assess the presence of neonatal myosin heavy chain in sections from the tibialis anterior, we observed that the generation of new muscle tissue in response to DO was vigorous in young animals (27% positive fibers), whereas it was more muted in adult animals (9.9% positive fibers). This adaptive response was associated with a pronounced proliferation of myoblasts in the young but not in the mature animals. Adult tibialis anterior subjected to DO showed a 50% loss in tetanic and twitch tension whereas those in young animals did not. This correlated with partial denervation of adult but not young muscle, as judged by morphological criteria. These experiments indicate that adaptation to DO depends not only on mechanical variables but also on skeletal maturity.

Inflammatory Cytokines in Nonpathological States

During infection, inflammatory cytokines induce regulated changes in the host's internal milieu that create a hostile environment to an invading pathogen. Recent evidence indicates that these cytokines are constitutively produced, their production is increased by environmental stressors other than microbes, and they modulate "normal" physiological processes.

The effect of age on in vitro motility speed of slow myosin extracted from single rat soleus fibres

The effect of age on the motor protein myosin was examined in a novel in vitro motility assay. Myosin was extracted from soleus fibres of young (3-6 month) and old (20-24 month) rats. All fibres expressed the type I myosin heavy chain (MyHC) and the slow isoforms of the myosin light chains (MyLCs). In vitro motility speed was significantly (P < 0.001) faster in the young adult (1.43 +/- 0.23 microm s-1) than in the aged group (1.27 +/- 0.23 microm s-1). The result indicates that the age-related decrease in contractile speed observed in slow fibres may be the effect of a change in the properties of myosin with age.