Interaction of skeletal muscle with the complement system: Skeletal muscle activates the alternative complement pathway and activation of terminal complement proteins down-regulates mRNAs encoding muscle-specific proteins

Abstract

Through immunochemical studies of skeletal muscle affected by myasthenia gravis (MG), the terminal complement complex (TCC) made of C5b-9 has been localized near the neuromuscular junction. In the passive transfer model for MG, activation of complement by anti-acetylcholine receptor (AChR) antibodies and subsequent generation of TCC plays a critical role in the development of muscle dysfunction. We have explored the interaction of complement with skeletal muscle in order to understand the underlying mechanisms responsible for complement-mediated muscle injury that may occur in MG. We have found that primary rat myotubes activate the complement system in the absence of antibody. This activation was found to be through the alternative pathway of complement activation. In a homologous system, alternative pathway activation also takes place which leads to the full activation of the cascade up to C9. Thus, activation of complement by skeletal muscle cells can generate C5b-9 complexes. Myotubes derived from the C2C12 cell line and, to a lesser extent, rat primary myotubes were found to produce Factor H, a soluble inhibitor of the alternative pathway activation of the complement system. Furthermore, we have investigated the biological effect of TCC on the expression of skeletal muscle specific proteins in C2C12 myotubes. Activation of complement by anti-AChR IgG with C7 deficient human serum reconstituted with C7 to form C5b-9 on C2 myotubes reduced the level of mRNAs encoding muscle-specific proteins which include troponin I, {dollar}\alpha{dollar}-actin, and the {dollar}\alpha{dollar} subunit of AChR. This reduction appeared mainly due to a decrease in the half life of these mRNA species. The expression of mRNA encoding HSP 83 was unchanged, thus allowing its use as a control for the amount of RNA in each lane. {dollar}\beta{dollar}-actin mRNA levels were either unchanged or slightly increased upon C5b-9 treatment. c-jun expression also increased during the first 30 minutes of treatment. Thus, down-regulation of muscle-specific mRNAs by C5b-9 could have a profound effect on muscle cell activities.