Date Published: October 06, 2011
Publisher: Blackwell Publishing Ltd
Author(s): Giorgos K Sakellariou, Deborah Pye, Aphrodite Vasilaki, Lea Zibrik, Jesus Palomero, Tabitha Kabayo, Francis McArdle, Holly Van Remmen, Arlan Richardson, James G Tidball, Anne McArdle, Malcolm J Jackson.
Mice lacking Cu,Zn superoxide dismutase (SOD1) show accelerated, age-related loss of muscle mass. Lack of SOD1 may lead to increased superoxide, reduced nitric oxide (NO), and increased peroxynitrite, each of which could initiate muscle fiber loss. Single muscle fibers from flexor digitorum brevis of wild-type (WT) and Sod1−/− mice were loaded with NO-sensitive (4-amino-5-methylamino-2′,7′-difluorofluorescein diacetate, DAF-FM) and superoxide-sensitive (dihydroethidium, DHE) probes. Gastrocnemius muscles were analyzed for SOD enzymes, nitric oxide synthases (NOS), and 3-nitrotyrosine (3-NT) content. A lack of SOD1 did not increase superoxide availability at rest because no increase in ethidium or 2-hydroxyethidium (2-HE) formation from DHE was seen in fibers from Sod1−/− mice compared with those from WT mice. Fibers from Sod1−/− mice had decreased NO availability (decreased DAF-FM fluorescence), increased 3-NT in muscle proteins indicating increased peroxynitrite formation and increased content of peroxiredoxin V (a peroxynitrite reductase), compared with WT mice. Muscle fibers from Sod1−/− mice showed substantially reduced generation of superoxide in response to contractions compared with fibers from WT mice. Inhibition of NOS did not affect DHE oxidation in fibers from WT or Sod1−/− mice at rest or during contractions, but transgenic mice overexpressing nNOS showed increased DAF-FM fluorescence and reduced DHE oxidation in resting muscle fibers. It is concluded that formation of peroxynitrite in muscle fibers is a major effect of lack of SOD1 in Sod1−/− mice and may contribute to fiber loss in this model, and that NO regulates superoxide availability and peroxynitrite formation in muscle.
The loss of muscle mass and strength that occurs during aging contributes to frailty and loss of independence (Marcell, 2003). By the age of 70, the cross-sectional area of skeletal muscle is reduced by 25–30%, and muscle strength is reduced by 30–40% (Porter et al., 1995). Oxidative damage has been claimed to be involved in the loss of tissue function that occurs during aging, and skeletal muscle of old rodents contains increased amounts of the products of oxidative damage to biomolecules such as lipid, DNA, and proteins in comparison with young or adult rodents (e.g. Sastre et al., 2003; Broome et al., 2006; Vasilaki et al., 2007). An increase in superoxide generation has been implicated in the process of aging in skeletal muscle and other tissues (Melov et al., 2000; Sastre et al., 2003). Mice lacking Cu,Zn superoxide dismutase (SOD1) show an accelerated, age-related loss of skeletal muscle mass associated with significant changes in muscle structure and contractility and potentially provide a useful model to study the role of a chronic oxidative stress in loss of skeletal muscle (Muller et al., 2006).
Superoxide and NO are the primary reactive oxygen and nitrogen species generated within skeletal muscle both at rest and during contractile activity. While further reaction of both species is well described with the generation of secondary ROS such as hydrogen peroxide and hydroxyl radicals from superoxide (Jackson, 2008) and interaction of NO with targets such as guanylate cyclase (Halliwell & Gutteridge, 2007), the reaction of superoxide with NO has been described in simple chemical and some biological systems (Beckman & Koppenol, 1996), but the functional effects of this interaction in skeletal muscle have not been defined.