Aging-sensitive cellular and molecular mechanisms associated with skeletal muscle hypertrophy.

Sarcopenia is an age-related loss of muscle mass and strength. The aged can increase various measures of muscle size and strength in response to resistance exercise (RE), but this may not normalize specific tension. In rats, aging reduces the hypertrophy response and impairs regeneration. In this study, we measured cellular and molecular markers, indicative of muscle hypertrophy, that also respond to acute increases in loading. Comparing 6- and 30-mo-old rats, the aims were to 1) determine whether these markers are altered with age and 2) identify age-sensitive responses to acute RE. The muscles of old rats exhibited sarcopenia involving a deficit in contractile proteins and decreased force generation. The RNA-to-protein ratio was higher in the old muscles, suggesting a decrease in translational efficiency. There was evidence of reduced signaling via components downstream from the insulin/insulin-like growth factor (IGF)-I receptors in old muscles. The mRNA levels of myostatin and suppressor of cytokine signaling 2, negative regulators of muscle mass, were lower in old muscles but did not decrease following RE. RE induced increases in the mRNAs for IGF-I, mechano-growth factor, cyclin D1, and suppressor of cytokine signaling 3 were similar in old and young muscles. RE induced phosphorylation of the IGF-I receptor, and Akt increased in young but not old muscles, whereas that of S6K1 was similar for both. The results of this study indicate that a number of components of intracellular signaling pathways are sensitive to age. As a result, key anticatabolic responses appear to be refractory to the stimuli provided by RE.     

Ionic and molecular mechanisms of beta-amyloid-induced depolarization of the mouse skeletal muscle fibres

Excess production and accumulation of beta-amyloid peptide (betaAP) are central for pathogenesis of Alzheimer's disease. Numerous studies showed that betaAP possessed wide range of toxic effects on neurons, however the mechanism of betaAP influence on another types of excitable cells, for example, skeletal muscle fibres, is unknown. In electrophysiological experiments on the mouse diaphragm, we found for the first time that betaAP (25-35 fragment, 10-6 M) disturbs the processes of the resting membrane potential generation in muscle fibres, leading to depolarization by two mechanisms: 1) inhibition of Na+,K(+)-ATPase, which leads to loss of impact of this pump to the resting membrane potential; 2) increase of membrane cationic permeability due to formation of "amyloid" channels blocked with Zn2+ ions. Our results significantly broaden current understanding of mechanisms of motor disturbances and skeletal muscle pathology in Alzheimer's disease, inclusion body myositis and other betaAP-related disorders.     

Neural control of aging skeletal muscle

Functional and structural decline in the neuromuscular system with aging has been recognized as a cause of impairment in physical performance and loss of independence in the elderly. Alterations in spinal cord motor neurones and at the neuromuscular junction have been identified as evidence of denervation in skeletal muscles from aging mammals, including humans. However, the reciprocal influences of neurones on gene expression in muscle and of muscle on age-related neurodegeneration are poorly understood, and, as a result, interventions aimed at delaying or preventing degeneration of the neural component in aging muscle have been largely unsuccessful. The present article discusses the evidence for neural influence on age-related impairments of skeletal muscle, including a role in excitation-contraction uncoupling. The role of nerves in regulating the trophic actions of insulin-like growth factor-1 (IGF-1) and other neurotrophic factors is considered as a novel influence on the effects of aging on the neuromuscular junction. A better understanding of nerve-muscle interactions will allow for more rational interventions in the aging neuromuscular system.     

Vitamins and aging: pathways to NAD+ synthesis

Recent genetic evidence reveals additional salvage pathways for NAD(+) synthesis. In this issue, Belenky et al. (2007) report that nicotinamide riboside, a new NAD(+) precursor, regulates Sir2 deacetylase activity and life span in yeast. The ability of nicotinamide riboside to enhance life span does not depend on calorie restriction.     

Manipulation of a nuclear NAD+ salvage pathway delays aging without altering steady-state NAD+ levels

Yeast deprived of nutrients exhibit a marked life span extension that requires the activity of the NAD(+)-dependent histone deacetylase, Sir2p. Here we show that increased dosage of NPT1, encoding a nicotinate phosphoribosyltransferase critical for the NAD(+) salvage pathway, increases Sir2-dependent silencing, stabilizes the rDNA locus, and extends yeast replicative life span by up to 60%. Both NPT1 and SIR2 provide resistance against heat shock, demonstrating that these genes act in a more general manner to promote cell survival. We show that Npt1 and a previously uncharacterized salvage pathway enzyme, Nma2, are both concentrated in the nucleus, indicating that a significant amount of NAD(+) is regenerated in this organelle. Additional copies of the salvage pathway genes, PNC1, NMA1, and NMA2, increase telomeric and rDNA silencing, implying that multiple steps affect the rate of the pathway. Although SIR2-dependent processes are enhanced by additional NPT1, steady-state NAD(+) levels and NAD(+)/NADH ratios remain unaltered. This finding suggests that yeast life span extension may be facilitated by an increase in the availability of NAD(+) to Sir2, although not through a simple increase in steady-state levels. We propose a model in which increased flux through the NAD(+) salvage pathway is responsible for the Sir2-dependent extension of life span.     

Mitochondrial Ca2+ uptake in skeletal muscle health and disease

Muscle uses Ca²⁺ as a messenger to control contraction and relies on ATP to maintain the intracellular Ca²⁺ homeostasis. Mitochondria are the major sub-cellular organelle of ATP production. With a negative inner membrane potential, mitochondria take up Ca²⁺ from their surroundings, a process called mitochondrial Ca²⁺ uptake. Under physiological conditions, Ca²⁺ uptake into mitochondria promotes ATP production. Excessive uptake causes mitochondrial Ca²⁺ overload, which activates downstream adverse responses leading to cell dysfunction. Moreover, mitochondrial Ca²⁺ uptake could shape spatio-temporal patterns of intracellular Ca²⁺ signaling. Malfunction of mitochondrial Ca²⁺ uptake is implicated in muscle degeneration. Unlike non-excitable cells, mitochondria in muscle cells experience dramatic changes of intracellular Ca²⁺ levels. Besides the sudden elevation of Ca²⁺ level induced by action potentials, Ca²⁺ transients in muscle cells can be as short as a few milliseconds during a single twitch or as long as minutes during tetanic contraction, which raises the question whether mitochondrial Ca²⁺ uptake is fast and big enough to shape intracellular Ca²⁺ signaling during excitation-contraction coupling and creates technical challenges for quantification of the dynamic changes of Ca²⁺ inside mitochondria. This review focuses on characterization of mitochondrial Ca²⁺ uptake in skeletal muscle and its role in muscle physiology and diseases.     

Antigenic determinants of acylphosphatase from porcine skeletal muscle

Analysis of the quantitative precipitin reaction of acylphosphatase from porcine skeletal muscle with rabbit antiserum indicated the presence of at least two antigenic determinants on the porcine enzyme molecule. Immunological cross-reactivities of acylphosphatases from equine and rabbit skeletal muscles were examined. In double immunodiffusion with the antiserum, the precipitin lines of the porcine and equine enzymes completely fused, while the rabbit enzyme gave no precipitin line. The reaction between the 125I-labeled porcine enzyme and its antibody was inhibited to the same extent by the porcine and equine enzymes, but not by the rabbit enzyme. The three enzymes were similar in net charge and molecular weight on polyacrylamide gel electrophoreses. No conformational difference among the three enzymes was observed in their circular dichroism spectra. The amino acid composition of the rabbit enzyme differed from those of the porcine and equine enzymes in the contents of Glu, Gly, Lys, and Arg. Differences in the sequence of the rabbit enzyme from that of the porcine enzyme were investigated by comparison of the peptide maps of the tryptic peptides of the two enzymes. Four peptides of the rabbit enzyme were located at different positions from those of the porcine enzyme. Three of the four peptides from both enzymes were sequenced and all the tryptic peptides of both enzymes were characterized by amino acid analysis. The tryptic peptides of rabbit enzyme were tentatively aligned on the basis of their amino acid compositions and sequence homologies, compared with the corresponding peptides of the porcine enzyme. Among five amino acid residues of the porcine enzyme, Arg-4, Asp-28, Arg-31, Glu-56, and Ile-68, which are replaced in the rabbit enzyme, Arg-4 and Asp-28 are considered to be included in the antigenic determinants.     

Injury and adaptive mechanisms in skeletal muscle

Work-related musculoskeletal disorders (MSD) are a major concern in the United States. Overexertion and repetitive motion injuries dominate reporting of lost-time MSD incidents. Over the past three decades, there has been much study on contraction-induced skeletal muscle injury. The effect of the biomechanical loading signature that includes velocity, range of motion, the number of repetitions, force, work-rest cycle, and exposure duration has been studied. More recently, the effect of aging on muscle injury susceptibility and regeneration has been studied. This review will focus on contraction-induced skeletal muscle injury, the effects of repetitions, range of motion, work-rest cycles, and aging on injury susceptibility and regenerative and adaptive pathways. The different physiological phenomena responsive to overt muscle injury versus adaptation will be distinguished. The inherent capability of skeletal muscle to adapt to mechanical loading, given the appropriate exposure signature will also be discussed. Finally, we will submit that repeated high-intensity mechanical loading is a desirable means to attenuate the effects of sarcopenia, and may be the most effective and appealing mode of physical activity to counteract the effects often observed with musculo-skeletal dysfunction in the workplace.     

NAD+ metabolism in health and disease

Nicotinamide adenine dinucleotide (NAD(+)) is both a coenzyme for hydride-transfer enzymes and a substrate for NAD(+)-consuming enzymes, which include ADP-ribose transferases, poly(ADP-ribose) polymerases, cADP-ribose synthases and sirtuins. Recent results establish protective roles for NAD(+) that might be applicable therapeutically to prevent neurodegenerative conditions and to fight Candida glabrata infection. In addition, the contribution that NAD(+) metabolism makes to lifespan extension in model systems indicates that therapies to boost NAD(+) might promote some of the beneficial effects of calorie restriction. Nicotinamide riboside, the recently discovered nucleoside precursor of NAD(+) in eukaryotic systems, might have advantages as a therapy to elevate NAD(+) without inhibiting sirtuins, which is associated with high-dose nicotinamide, or incurring the unpleasant side-effects of high-dose nicotinic acid.     

Skewed macrophage polarization in aging skeletal muscle

Skeletal muscle aging is a major cause of disability and frailty in the elderly. The progressive impairment of skeletal muscle function with aging was recently linked to a disequilibrium between damage and repair. Macrophages participate in muscle tissue repair, first as pro‐inflammatory M1 subtype and then as anti‐inflammatory M2 subtype. However, information on the presence of macrophages in skeletal muscle is still sporadic and the effect of aging on macrophage phenotype remains unknown. In this study, we sought to characterize the polarization status of macrophages in skeletal muscle of persons across a wide range of ages. We found that most macrophages in human skeletal muscle are M2, and that this number increased with advancing age. On the contrary, M1 macrophages declined with aging, making the total number of macrophages invariant with older age. Notably, M2 macrophages colocalized with increasing intermuscular adipose tissue (IMAT) in aging skeletal muscle. Similarly, aged BALB/c mice showed increased IMAT and M2 macrophages in skeletal muscle, accompanied by slightly increased collagen protein production. Collectively, we report that polarization of macrophages to the major M2 subtype is associated with IMAT and propose that increased M2 in aged skeletal muscle may impact upon muscle metabolism associated with aging.     

MICU3 regulates mitochondrial Ca2+-dependent antioxidant response in skeletal muscle aging

Age-related loss of skeletal muscle mass and function, termed sarcopenia, could impair the quality of life in the elderly. The mechanisms involved in skeletal muscle aging are intricate and largely unknown. However, more and more evidence demonstrated that mitochondrial dysfunction and apoptosis also play an important role in skeletal muscle aging. Recent studies have shown that mitochondrial calcium uniporter (MCU)-mediated mitochondrial calcium affects skeletal muscle mass and function by affecting mitochondrial function. During aging, we observed downregulated expression of mitochondrial calcium uptake family member3 (MICU3) in skeletal muscle, a regulator of MCU, which resulted in a significant reduction in mitochondrial calcium uptake. However, the role of MICU3 in skeletal muscle aging remains poorly understood. Therefore, we investigated the effect of MICU3 on the skeletal muscle of aged mice and senescent C2C12 cells induced by d-gal. Downregulation of MICU3 was associated with decreased myogenesis but increased oxidative stress and apoptosis. Reconstitution of MICU3 enhanced antioxidants, prevented the accumulation of mitochondrial ROS, decreased apoptosis, and increased myogenesis. These findings indicate that MICU3 might promote mitochondrial Ca²⁺ homeostasis and function, attenuate oxidative stress and apoptosis, and restore skeletal muscle mass and function. Therefore, MICU3 may be a potential therapeutic target in skeletal muscle aging.Subject terms: Ageing, Calcium and phosphate metabolic disorders     

Proteolytic signaling mechanisms in skeletal muscles in patients with alcohol-induced muscle disease

Chronic alcoholic myopathy is one of the most numerous and profound manifestations of chronic alcohol intoxication. This disease is characterized by the pronounced atrophy of the locomotor muscles, which involves fibers expressing predominantly type II (fast) myosin isoforms. In early experiments with rats receiving alcohol and studies of patients, the impairment of the anabolic intracellular signaling pathways and decrease in protein synthesis rate were shown. We were the first to analyze the signaling pathways involved in the pathogenesis of alcoholic myopathy with different fiber atrophy levels. At the early stages of pathogenesis, we observed also a sufficient increase in mRNA of E3 ubiquitin ligases. However, the ubiquitinylation level was not altered in patients as compared to the control subjects. This phenomenon could be related to an increased expression of heat shock proteins known for their protective action.