Aging is associated with a progressive decline of muscle mass, strength, and quality, a condition described as sarcopenia of aging. Despite the significance of skeletal muscle atrophy, the mechanisms responsible for the deterioration of muscle performance are only partially understood. The purpose of this review is to highlight cellular, molecular and biochemical changes that contribute to age-related muscle weakness. Keywordsactin; myosin; aged muscle; enzymatic activity; oxidative modifications SarcopeniaAging is associated with a progressive decline of muscle mass, strength, and quality, a condition described as sarcopenia. These age-related changes are observed in healthy, active adults who are 50 years and older (Hughes et al., 2002). The prevalence of sarcopenia in older adults under the age of 70 years is about 25% and increases to 40% in adults 80 years or older (Baumgartner et al., 1998). Sarcopenia represents a risk factor for frailty, loss of independence, and physical disability (Roubenoff, 2000). Loss of mobility resulting from muscle loss predicts major physical disability and mortality, and is associated with poor quality of life, social needs, and health care needs (Fried and Guralnik, 1997). The economic impact of sarcopenia and its detrimental correlates are immense (Janssen et al., 2004). Thus, understanding the mechanisms leading to muscle dysfunction (e.g., weakness) at advanced age represents a high public health priority. The purpose of this article is to highlight cellular, molecular, and biochemical changes that contribute to age-related muscle dysfunction. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public Access Muscle physiology-short reviewFor skeletal muscle, the control of force has to be accurate and precise with the contractile machinery being switched on and off rapidly to allow for complex coordinated movements. Action potentials initiated at the neuromuscular junction propagate along the length of the fiber and the transverse tubules. As the wave of depolarization passes down the transverse tubules there is an interaction with the sarcoplasmic reticulum that results in the release of calcium, initiating the interaction of actin and myosin and muscle contraction. This process is known as excitation-contraction coupling. Thus, age-related structural changes or chemical modifications in proteins that affect excitation-contraction coupling are likely to influence muscle function.The basic contractile unit of muscle, the myofibril, consists of a linear array of sarcomeres, which contains interdigitating myosin and actin fila...

We tested the hypothesis that low specific tension (force/cross-sectional area) in skeletal muscle from aged animals results from structural changes in myosin that occur with aging. Permeabilized semimembranosus fibers from young adult and aged rats were spin labeled site specifically at myosin SH1 (Cys-707). Electron paramagnetic resonance (EPR) was then used to resolve and quantify the structural states of the myosin head to determine the fraction of myosin heads in the strong-binding (force generating) structural state during maximal isometric contraction. Fibers from aged rats generated 27 +/- 0.8% less specific tension than fibers from younger rats (P < 0.001). EPR spectral analyses showed that, during contraction, 31.6 +/- 2.1% of myosin heads were in the strong-binding structural state in fibers from young adult animals but only 22.1 +/- 1.3% of myosin heads in fibers from aged animals were in that state (P = 0.004). Biochemical assays indicated that the age-related change in myosin structure could be due to protein oxidation, as indicated by a decrease in the number of free cysteine residues. We conclude that myosin structural changes can provide a molecular explanation for age-related decline in skeletal muscle force generation.

To understand the molecular mechanism of oxidation-induced inhibition of muscle contractility, we have studied the effects of hydrogen peroxide on permeabilized rabbit psoas muscle fibers, focusing on changes in myosin purified from these fibers. Oxidation by 5 mM peroxide decreased fiber contractility (isometric force and shortening velocity) without significant changes in the enzymatic activity of myofibrils and isolated myosin. The inhibitory effects were reversed by treating fibers with dithiothreitol. Oxidation by 50 mM peroxide had a more pronounced and irreversible inhibitory effect on fiber contractility and also affected enzymatic activity of myofibrils, myosin, and actomyosin. Peroxide treatment also affected regulation of contractility, resulting in fiber activation in the absence of calcium. Electron paramagnetic resonance of spin-labeled myosin in muscle fibers showed that oxidation increased the fraction of myosin heads in the strong-binding structural state under relaxing conditions (low calcium) but had no effect under activating conditions (high calcium). This change in the distribution of structural states of myosin provides a plausible explanation for the observed changes in both contractile and regulatory functions. Mass spectroscopy analysis showed that 50 mM but not 5 mM peroxide induced oxidative modifications in both isoforms of the essential light chains and in the heavy chain of myosin subfragment 1 by targeting multiple methionine residues. We conclude that 1) inhibition of muscle fiber contractility via oxidation of myosin occurs at high but not low concentrations of peroxide and 2) the inhibitory effects of oxidation suggest a critical and previously unknown role of methionines in myosin function.

Frailty is a clinical syndrome associated with the aging process and adverse outcomes. The purpose of this short report was to initiate the development of a Frailty Index in 27- to 28-month-old C57BL/6 mice that matched the clinical criteria used in humans (weakness, slow walking speed, low activity level, poor endurance). The selected criteria included grip strength, walking speed, physical activity, and endurance. The criteria in mice were evaluated by the inverted-cling grip test, rotarod test, voluntary wheel running, and derived endurance scores. Each criterion had a designated cutoff point (1.5 SD below the cohort mean) to identify the mice with the lowest performance. If a mouse presented with three of the criteria scores below the cutoff points, it was identified as frail. Mild frailty was designated if two criteria were below the cutoff points. In this mouse cohort, one mouse was identified as frail and one was mildly frail. This prevalence of 9% frailty is consistent with the prevalence of frailty in humans at the same survival age. Collectively, our selected criterion, cutoff point, and Frailty Index provide a potential standardized definition for frailty in mice that is consistent with the operational definition of frailty in humans.