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C5b-9 Terminal Complement Complex Protects Oligodendrocytes from Death by Regulating Bad Through Phosphatidylinositol 3-Kinase/Akt Pathway¹

Lucian Soane^*, Hyun-Jun Cho^*, Florin Niculescu, Horea Rus^*, and Moon L. Shin^2,*

Departments of ^* Pathology and Neurology and Division of Rheumatology and Clinical Immunology, Department of Medicine, University of Maryland School of Medicine, Baltimore, MD 21201

Apoptosis of oligodendrocytes is induced by serum growth factor deprivation. We showed that oligodendrocytes and progenitor cells respond to serum withdrawal by a rapid decline of Bcl-2 mRNA expression and caspase-3-dependent apoptotic death. Sublytic assembly of membrane-inserted terminal complement complexes consisting of C5b, C6, C7, C8, and C9 proteins (C5b-9) inhibits caspase-3 activation and apoptotic death of oligodendrocytes. In this study, we examined an involvement of the mitochondria in oligodendrocyte apoptosis and the role of C5b-9 on this process. Decreased phosphatidylinositol 3-kinase and Akt activities occurred in association with cytochrome c release and caspase-9 activation when cells were placed in defined medium. C5b-9 inhibited the mitochondrial pathway of apoptosis in oligodendrocytes, as shown by decreased cytochrome c release and inhibition of caspase-9 activation. Phosphatidylinositol 3-phosphate kinase and Akt activities were also induced by C5b-9, and the phosphatidylinositol 3-phosphate kinase inhibitor LY294002 reversed the protective effect of C5b-9. Phosphatidylinositol 3-phosphate kinase activity was also responsible for the phosphorylation of Bad at Ser¹¹² and Ser¹³⁶. This phosphorylation resulted in dissociation of Bad from the Bad/Bcl-x_L complex in a Gi-dependent manner. The mitochondrial pathway of oligodendrocyte apoptosis is, therefore, inhibited by C5b-9 through post-translational regulation of Bad. This mechanism may be involved in the promotion of oligodendrocyte survival in inflammatory demyelinating disorders affecting the CNS.

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