COMPLEMENT-MEDIATED NEUROTOXICITY IS REGULATED BY HOMOLOGOUS RESTRICTION

The ability of beta-amyloid peptides to activate the classical complem ent cascade and the presence of various complement proteins including the membrane attack complex (C5b-9) on dystrophic neurites in Alzheime r's disease brains, raises the possibility that the complement system may contribute to this neurodegenerative disorder. To address this iss ue, we have studied the effect of complement activation on nerve growt h factor (NGF)-differentiated rat pheochromocytoma PC12 cells, and on retinoic acid (RA)-differentiated human neuroblastoma SH-SY5Y cells. A lthough incubation of both cell types with human serum resulted in act ivation of complement, as indicated by iC3b formation, only PC12 but n ot SH-SY5Y cells were killed by human serum treatment. In contrast, he at-inactivated serum (56 degrees C, 45 min) was not neurotoxic. On SH- SY5Y cells, both PCR amplification and immunocytochemistry demonstrate d the presence of CD59, a glycosylphosphatidylinositol-anchored protei n that restricts homologous complement activation by inhibiting the fo rmation of the membrane attack complex. The presence of CD59 probably accounts for the inability of human complement to lyse the human cell lines. Indeed, removal of glycosulphosphatidylinositol (GPI)-anchored proteins with phosphatidylinositol-specific phospholipase C (PI-PLC) r endered SH-SY5Y cells vulnerable to complement attack and eventually l ed to serum-mediated cell death. Reconstituted C5b-9 was also toxic to both PC12 and PI-PLC-pretreated SH-SY5Y cells. These observations sug gest that complement activation can cause neuronal cell death and that this process is regulated by homologous restriction.