Abstract
Purpose :
Ocular hypertension (OHT) induces pathology of retinal ganglion cells (RGCs) leading to axonal degeneration and soma loss. Previous reports using a model of sustained OHT indicated that while somas were completely protected by limiting the function of the proapoptotic BAX protein, axonal degeneration was only delayed. In an effort to examine a more clinically relevant paradigm, we tested if reduced BAX levels (to mimic a therapeutic designed to suppress BAX function) provided protection to RGC axons when OHT was alleviated (to mimic IOP-lowering therapy).
Methods :
Experiments were conducted on C57BL/6J mice that were heterozygous (HET) for a functional Bax gene, which reduces protein levels by 50% and yields significant protection to RGC somas. These mice were compared to wild type (WT) littermates. Mice were subjected to a controlled elevated intraocular pressure (CEI) procedure by raising IOP to 60 mm Hg for 4 hrs before being returned to normal. During CEI, mice were monitored for changes in physiological parameters including O2 levels, heart rate, body temperature, and blood pressure. Axon damage was assessed histologically 10 days after CEI. In a separate group of mice, anterograde transport was measured 3 months after CEI by intravitreal injection of CTB-Alexa488. BAX-dependent catabolism in RGC axons was evaluated in isolated optic nerves of naïve WT mice exposed to the BH3-mimetic ABT-737 for 8 hrs including staining for activated caspase 3.
Results :
WT and Bax HET mice showed no physiological differences during CEI. After 10 days, 9/15 WT mice exhibited some level of axonal damage (P=0.0019 relative to contralateral eyes) compared to 3/14 Bax HET mice (P=0.34). After 3 months, WT mice exhibited significantly less anterograde transport to the superior colliculus compared to both naïve and Bax HET animals (P<0.0001). Evaluation of BAX-dependent catabolism ex vivo showed that ABT-737 was able to induce BAX translocation in RGC axons, elicit the activation of caspase 3, and stimulate axon degeneration.
Conclusions :
Targeting BAX early, combined with IOP-lowering therapy, may enhance axon resilience in some forms of glaucoma. The mechanism for protection is likely suppression of early BAX-dependent catabolism within the axon.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.