One important unanswered question in glaucoma is whether apoptosis of retinal ganglion cells (RGCs) and axon degeneration are two separate events mediated by molecularly distinct pathways, or whether they are linked and triggered through a common pathway. Answering this question is critical to developing effective neuroprotective therapies that protect both axons and RGC somas. In support of the idea there are two distinct pathways that mediate axon degeneration and RGC apoptosis, it was demonstrated that deletion of the pro- apoptotic BAX gene blocked apoptosis of RGCs in a mouse model of glaucoma, but did not prevent axon degeneration. There is accumulating evidence that axon degeneration is triggered by axon damage in the optic nerve head, which precedes RGC apoptosis and is linked to microglial activation and neurotoxic inflammation. Therefore, it is possible that blocking inflammation could prevent axon degeneration and subsequent death of RGCs, leaving open the possibility that there is a shared pathway linking axon degeneration and RGC apoptosis. In support of the idea that inflammation may be the shared pathway that links axon degeneration and RGC apoptosis, our laboratory demonstrated previously that blocking the Fas/Fas ligand (FasL) signaling pathway prevents axon degeneration and RGC apoptosis in both inducible and chronic mouse models of glaucoma and this protection coincided with an inhibition of microglia activation and induction of inflammatory mediators. However, since Fas triggers apoptotic and inflammatory pathways, the question remains as to whether blocking inflammation alone would prevent axon degeneration and subsequent RGC apoptosis. The challenge to resolving this issue is the inability to uncouple Fas-mediated inflammation from Fas-mediated apoptosis. However, Fas signaling requires activation of caspase-8 for both apoptosis and inflammation and recent studies examining the molecular mechanism of caspase-8 activation discovered an additional auto- cleavage step that is required for the induction of caspase-8-mediated apoptosis but not required for induction of caspase-8 mediated inflammation. Using this information, a mutant mouse was created (Casp8DA/DA) in which a point mutation in the auto-cleavage site blocked caspase-8-mediated apoptosis, but did not block caspase-8-mediated inflammation. We will use these mice to definitively determine how caspase-8-mediated inflammation and/or apoptosis triggers axon degeneration and RGC apoptosis in glaucoma. We hypothesize that elevated IOP triggers caspase-8-mediated inflammation that acts as a common pathway triggering axon degeneration and RGC apoptosis. This hypothesis will be tested in two aims: (1) Determine whether caspase- 8-mediated apoptosis contributes to axon degeneration and death of RGCs in a microbead-induced mouse model of glaucoma and (2) Demonstrate that Fas-mediated activation of microglia mediates neurotoxic inflammation, axon degeneration, and death of RGCs in the microbead-induced mouse model of glaucoma.
Glaucoma is a chronic disease that leads to loss of vision following damage to the optic nerve and death of specific cells in the retina, called ?retinal ganglion cells?. Our research has demonstrated that inflammation plays an important role in the development of glaucoma. Our current work is focused on determining whether inflammation is a shared pathway that can be targeted to protect both the optic nerve and retinal ganglion cells.
