Uncoupling caspase 8-mediated-apotosis from caspase 8-mediated-inflammation in glaucoma

Abstract

Purpose : Retinal ganglion cell (RGC) apoptosis is the common endpoint in glaucoma and both the intrinsic and extrinsic apoptotic pathways have been implicated in human and experimental models of glaucoma. However, caspase 8, which is central to the extrinsic pathway of apoptosis has been linked to both RGC apoptosis, as well as glial activation, and neuroinflammation. The recent development of a caspase 8 mutant mouse (Casp8^DA/DA) in which a point mutation in the auto-cleavage site blocks caspase 8-mediated apoptosis but does not block caspase 8-mediated inflammation, allows us to uncouple the two pathways and determine the extent to which caspase 8-mediated inflammation and/or apoptosis contributes to the death of RGCs in glaucoma.

Methods : Intracameral injection of magnetic microbeads (control: saline) was used to elevate the intraocular pressure (IOP) in three groups of mice: WT mice (positive control, RGC apoptosis); Fas deficient Fas^lpr mice (negative control, no RGC apoptosis); Casp8^DA/DA mutant mice (experimental, no extrinsic apoptosis). IOP was monitored by rebound tonometry. At 5 weeks post microbead injection, visual acuity was measured by optomotor reflex (OMR) and RGC function was assessed by pattern ERG (pERG). Retina and optic nerves were processed for RGC and axon quantification.

Results : Rebound tonometry showed equal elevation of IOP in microbead-injected WT, Fas^lpr, and Casp8 ^DA/DA mice as compared to saline controls. At 5 weeks post microbead injection, as compared to saline controls, a significant reduction in both visual acuity and pERG was observed in WT mice (pos. control), which correlated with a significant loss of RGCs and axons. By contrast, no significant reduction in visual acuity or pERG, nor loss of RGCs and axons was observed in Fas^lpr mice (neg. control). The Casp8^DA/DA mice, in which the extrinsic apoptotic pathway is blocked, displayed a significant reduction in visual acuity and pERG equal to that observed in WT mice and, as in WT mice, this loss of function correlated with a significant loss of RGCs and axons.

Conclusions : Our data shows that the caspase 8-mediated extrinsic pathway of apoptosis is not required for the death of RGCs and loss of visual function in a microbead-induced mouse model of glaucoma, indicating that caspase 8-mediated inflammation, but not apoptosis is the driving force in the development of glaucoma.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.