Regulation of Schwann cell phenotype by sublytic terminal complement complexes

Abstract

The complement cascade consists of effector and regulatory proteins that mediate humoral immunity and inflammation. Complement activation signals cell death but in sublytic concentrations, C5b-9 ((or terminal complement complexes (TCC)) activates cells without cytolysis. During peripheral nerve injury, sublytic TCC insertion in Schwann cell (SchC)/myelin membrane mediates membrane splitting and vesiculation, but with continued SchC survival. Resulting loss of SchC/axonal contact profoundly alters SchC phenotype through decreased expression of critical myelin proteins, and robust proliferation. We propose that sublytic TCC stimulate these phenotypic changes. We examined whether sublytic TCC downregulated expression of the myelin genes protein zero (Po) and myelin basic protein (MBP). Terminally-differentiated SchC passaged in vitro expressed high levels of Po and MBP mRNAs and protein. Po and MBP mRNAs were downregulated 60-80% over 6 hours by sublytic TCC. Sublytic TCC induced a rapid, but incomplete, degradation of an otherwise stable Po mRNA. Expression of a Po promoter/luciferase reporter construct transiently transfected into SchC was reduced 70% by TCC at 6 hours. TCC also differentially regulated the expression of c-jun and SCIP, two transcription factors that repress Po expression. We then investigated whether sublytic TCC stimulated proliferation and survival. SchC incubated in a serum-free defined medium for 24 hours synchronized 90% of SchC into G1/G0 phase of the cell cycle. TCC drove 49% of SchC into S phase, and also induced DNA synthesis. Furthermore, TCC induced proliferation by doubling SchC numbers. These effects were mediated by signaling pathways involving Gi proteins, ERK, PKC, p70 S6 kinase, but not PKA. Finally, we examined whether TCC could prevent SchC apoptosis. Incubation in defined medium caused 40-50% SchC death within 24 hours that was circumvented by the addition of beta-neuregulin. Likewise, TCC dramatically reduced the number of apoptotic cells to 5%, and mediated its effects through G proteins, ERK, and PKC pathways. Our data collectively suggest that sublytic TCC, although contributing to demyelination by downregulating myelin gene expression, also promote apoptotic rescue and mitosis, critical in repair and remyelination of peripheral nerve during inflammation, trauma, and Wallerian degeneration.