Nitrosative modifications of the Ca release complex and actin underlie arthritis-induced muscle weakness

To cite: Yamada T, Fedotovskaya O, Cheng AJ, et al. Ann Rheum Dis Published Online First: [please include Day Month Year] doi:10.1136/ annrheumdis-2013-205007 ABSTRACT Objective Skeletal muscle weakness is a prominent clinical feature in patients with rheumatoid arthritis (RA), but the underlying mechanism(s) is unknown. Here we investigate the mechanisms behind arthritis-induced skeletal muscle weakness with special focus on the role of nitrosative stress on intracellular Ca handling and specific force production. Methods Nitric oxide synthase (NOS) expression, degree of nitrosative stress and composition of the major intracellular Ca release channel (ryanodine receptor 1, RyR1) complex were measured in muscle. Changes in cytosolic free Ca concentration ([Ca]i) and force production were assessed in single-muscle fibres and isolated myofibrils using atomic force cantilevers. Results The total neuronal NOS (nNOS) levels were increased in muscles both from collagen-induced arthritis (CIA) mice and patients with RA. The nNOS associated with RyR1 was increased and accompanied by increased [Ca]i during contractions of muscles from CIA mice. A marker of peroxynitrite-derived nitrosative stress (3-nitrotyrosine, 3-NT) was increased on the RyR1 complex and on actin of muscles from CIA mice. Despite increased [Ca]i, individual CIA muscle fibres were weaker than in healthy controls, that is, force per crosssectional area was decreased. Furthermore, force and kinetics were impaired in CIA myofibrils, hence actin and myosin showed decreased ability to interact, which could be a result of increased 3-NT content on actin. Conclusions Arthritis-induced muscle weakness is linked to nitrosative modifications of the RyR1 protein complex and actin, which are driven by increased nNOS associated with RyR1 and progressively increasing Ca activation.