SUBLYTIC TERMINAL COMPLEMENT ATTACK ON MYOTUBES DECREASES THE EXPRESSION OF MESSENGER-RNAS ENCODING MUSCLE-SPECIFIC PROTEINS

Activation of inflammatory and cytotoxic complement effecters that inc lude the C5b-9 complex plays an important pathogenic role in myastheni a gravis, an inflammatory autoimmune disease of the muscle. Altered mu scle-specific gene expression has been observed in experimental myasth enic 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 markedl y decreased levels of mRNAs encoding alpha-actin, troponin I slow twit ch isoform, acetylcholine receptor alpha, and muscle aldolase A, where as the heat shock protein 83 mRNA level remained constant, by northern analysis. Because the half-life of the acetylcholine receptor alpha w as estimated to be >8 h, the C5b-9 effect was, in part, due to enhance d mRNA decay. Because C5b-9 also induced c-jun mRNA and reduced the my oD mRNA level, a possible inhibition of muscle gene transcription by C 5b-9 was examined in myotubes transfected with troponin promoter-lucif erase gene constructs. Luciferase activity was reduced to 50% in respo nse to C5b-9 at 2 h. Thus, C5b-9 appears to inhibit the muscle-specifi c gene expression by stimulating mRNA decay and by decreasing the tran scription process. The data also indicate a possible pathogenic role o f C5b-9 in immune-mediated inflammatory muscle disorders in which comp lement activation has been implicated.