Interaction of complement system and microglia activation in retina and optic nerve in a NMDA damage model

Abstract

It is known that intravitreally injected N-methyl-d-aspartate (NMDA) leads to fast retina and optic nerve degeneration and can directly activate microglia. Here, we analyzed the relevance for microglia related degenerating factors, the proteins of the complement system, at a late stage in the NMDA damage model. Therefore, different doses of NMDA (0 (PBS), 20, 40, 80 nmol) were intravitreally injected in rat eyes. Proliferative and activated microglia/macrophages (MG/Mϕ) were found in retina and optic nerve 2 weeks after NMDA injection. All three complement pathway proteins were activated in retinas after 40 and 80 nmol NMDA treatment. 80 nmol NMDA injection also lead to more numerous depositions of complement factors C3 and membrane attack complex (MAC) in retina and MAC in optic nerve. Additionally, more MAC+ depositions were detected in optic nerves of the 40 nmol NMDA group.In this NMDA model, the retina is first affected followed by optic nerve damage. However, we found initiating complement processes in the retina, while more deposits of the terminal complex were present 2 weeks after NMDA injection in the optic nerve. The complement system can be activated in waves and possibly a second wave is still on-going in the retina, while the first activation wave is in the final phase in the optic nerve. Only the damaged tissues showed microglia activation as well as proliferation and an increase of complement proteins. Interestingly, the microglia/macrophages (MG/Mϕ) in this model were closely connected with the inductors of the classical and lectin pathway, but not with the alternative pathway. However, all three initiating complement pathways were upregulated in the retina. The alternative pathway seems to be triggered by other mechanisms in this NMDA model. Our study showed an ongoing interaction of microglia and complement proteins in a late stage of a degenerative process.