Neuroprotective effects of the complement terminal pathway during demyelination: implications for oligodendrocyte survival

Abstract

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system that is mediated by activated lymphocytes, macrophages/microglia, and complement. In MS, the myelin-forming oligodendrocytes (OLGs) are the targets of the immune attack. Experimental evidence indicates that C5b-9 plays a role in demyelination during the acute phase of experimental allergic encephalomyelitis (EAE). Terminal complement C5b-9 complexes are capable of protecting OLGs from apoptosis. During chronic EAE complement C5 promotes axonal preservation, remyelination and provides protection from gliosis. These findings indicate that the activation of complement and C5b-9 assembly can also have protective roles during demyelination.

Figure 1. Complement activation pathways and assembly of the terminal pathway

The classical pathway is initiated by the binding of the C1 complex to antibody bound by antigen, leading to the formation of the C4b2a enzyme complex, known as the C3 convertase. The lectin pathway is activated by the binding of either MBL or ficolin and MAPS1, 2, and 3, respectively, to an array of mannose groups on the surface of bacterial cells and the generation of C3 convertase of the classical pathway. The alternative pathway is initiated by hydrolyzed C3 and factor B and the subsequent formation of the alternative pathway C3 convertase, C3bBb. Generation of C3 convertase allows the formation of the C5 convertase enzyme, which initiates the formation of the C5b-9 terminal complement complex. The complement system is regulated at several levels: CD55, CR1, CD46, C4bp, and factors I and H regulate the activity of the C3 convertase and C5 convertase, and other proteins such as CD59 block the final assembly of the pores by preventing the binding of C9. The S protein/vitronectin binds to C5b-7 and leads to the formation of a cytolytically inactive SC5b-9 complex.

Figure 2. Impact of cytokines on OLGs

Activated mycroglia, T-lymphocytes, and macrophages present in the CNS during MS produce various cytokines and mediators (TNFα, IFNγ, FasL and TRAIL) that induce OLG cell death. Cytokines belonging to the TNF receptor superfamily and IFNγ produced by inflammatory cells induce caspase activation and also deregulate the balance between the pro-apoptotic and anti-apoptotic members of the Bcl-2 family of proteins. This deregulation leads to mitochondrial dysfunction and induction of OLG cell death. Cell death then contributes to the decrease in OLGs number seen in MS lesions and ultimately leads to increased demyelination and axonal injury.

Figure 3. Signaling pathways that are activated by C5b-9 and involved in OLG survival

Insertion of the sublytic C5b-9 into the cell membrane leads to G-protein activation, followed by the activation of PI3K and Akt. Activation of PI3K induces up-regulation of FLIP and inhibition of caspase-8 processing, thereby inhibiting Fas-induced apoptosis in OLGs. Inhibition of caspase-8 processing is also responsible for the inhibition of Bid cleavage. Akt is able to induce the phosphorylation of Bad and Bim and protects the mitochondria from pore formation by Bax/Bak and cytochrome c release. All these signaling pathways result in the inhibition of apoptosis and OLGs survival.

Figure 4. Expression of TGF-β33 and CTGF in chronic EAE

Expression of TGF-β3 and CTGF was examined by western blotting. Spinal cords from C5-s and C5-d mice with chronic EAE were lysed in RIPA buffer and fractionated on 10% PAGE gels, followed by transfer to nitrocellulose. Increased levels of TGF-β3 and CTGF were found in C5-d mice.