Retinal Ganglion Cell Loss and Heat Shock Protein 27 and 72 Induction After Acute Intraocular Pressure Elevation in Rats

Abstract

Purpose:: To determine the effect of ischemia on the expression of inducible heat shock protein (HSP) 27 and 72 on retinal ganglion cells (RGCs) in acute intraocular pressure (IOP) elevation in rat.

Methods:: RGCs were labeled with fluorogold application to both superior colliculi. Ischemia was then achieved in anesthetized Sprague Dawley rats by transient elevation of IOP by infusing normal saline (0.9%) into the anterior chamber. Retinal blanching was observed for 30 (group 1: n=5) or 90 (group 2: n=2) minutes. Another animal served as normal control. Subsequent to 1 or 2 weeks of IOP elevation, animals were sacrificed and perfused. Both eyes were enucleated immediately and the retinas hemisected. RGCs were counted in one half of the retina while immunohistochemistry for HSP 27 and 72 (not HSC70) was performed in the other half. Using 5-6 sections, the mean number of HSP positive RGCs was calculated. RGC damage were graded from 0 (no loss) to 4 (total loss).

Results:: There was none to severe loss of RGCs after 1 week and 2 weeks of IOP elevation. There was a grade 0 loss of RGCs in 10 eyes, grade 1 in 1 eye, grade 2 in 1 eye and grade 3 in 2 eyes, and none with grade 4. HSP 27 was detected only in eyes with elevated IOP and RGC damage at 1 week (mean: 6.3-15 cells) and 2 weeks (mean: 4.5-7.2 cells) in comparison to eyes without RGC damage at 1 week (mean: 0.17-0.3 cells) and 2 weeks (mean: 0 cells) and the control (mean: 0-0.3 cells). HSP 72 was detected at higher levels (mean: 2.2-7.3 cells) in the experimental eyes compared to the contralateral eye (mean: 1-2.7 cells) and the control (mean: 0.2-1.8 cells), however, there was no consistent association with HSP 72 positive cells and RGC loss.

Conclusions:: HSP 27 and 72 can be induced in RGCs of rats following acute IOP elevation. The dynamics of the HSP 27 and 72 induction appear different. While HSP 27 was not induced at either time period in the absence of RGC loss, HSP 72 induction was more apparent, indicating that it may be induced early after ischemia. Our results suggest that the HSP 27 induction is a specific response to the ischemia-reperfusion injury and following RGC loss.

Keywords: ganglion cells • stress response • apoptosis/cell death