Sublytic Terminal Complement Attack on Myotubes Decreases the Expression of mRNAs Encoding Muscle‐Specific Proteins

Activation of inflammatory and cytotoxic complement effectors that include the C5b‐9 complex plays an important pathogenic role in myasthenia gravis, an inflammatory autoimmune disease of the muscle. Altered muscle‐specific gene expression has been observed in experimental myasthenic rats. In this study, we have examined the effect of sublytic C5b‐9 on myotubes differentiated from C2C12 myoblasts, by generating C5b‐9 with C7‐deficient serum with or without C7. Within 2 h, C7‐deficient serum plus C7, compared with C7‐deficient serum alone, induced markedly decreased levels of mRNAs encoding α‐actin, troponin I slow twitch isoform, acetylcholine receptor α, and muscle aldolase A, whereas the heat shock protein 83 mRNA level remained constant, by northern analysis. Because the half‐life of the acetylcholine receptor α was estimated to be >8 h, the C5b‐9 effect was, in part, due to enhanced mRNA decay. Because C5b‐9 also induced c‐ jun mRNA and reduced the myoD mRNA level, a possible inhibition of muscle gene transcription by C5b‐9 was examined in myotubes transfected with troponin promoter‐luciferase gene constructs. Luciferase activity was reduced to 50% in response to C5b‐9 at 2 h. Thus, C5b‐9 appears to inhibit the muscle‐specific gene expression by stimulating mRNA decay and by decreasing the transcription process. The data also indicate a possible pathogenic role of C5b‐9 in immune‐mediated inflammatory muscle disorders in which complement activation has been implicated.