Abstract
Purpose: :
Despite the identification of elevated intraocular pressure (IOP) as a major risk factor for the loss of retinal ganglion cells (RGC) in Primary Open-Angle Glaucoma (POAG), the mechanisms underlying IOP-mediated RGC loss are poorly understood. Therefore, the purpose of this study is to investigate whether serine-proteases (tissue plasminogen activator [tPA] and urokinase plasminogen activator [uPA]) play a role in RGC loss.
Methods: :
To elevate IOP, hypertonic saline (1.9 M) was injected unilaterally into the episcleral veins of brown Norway rat eyes. Four weeks after hypertonic saline injection, IOP was measured using a tonopen. tPA and uPA levels in retinal protein extracts were determined by zymography assays, and their localization in retinal cross sections was determined by immunohistochemistry. Effect of elevated IOP on RGC loss was determined by retrograde labeling of RGCs with hydroxystilbamidine. Effect of a serine protease inhibitor, tPA-STOP, on RGC loss was also determined by retrograde labeling.
Results: :
Four weeks after hypertonic saline injection into 60 eyes, 39 eyes developed 30-38 mm Hg pressure (high-IOP eyes), while 16 eyes developed < 20 mm Hg pressure (low IOP-eyes), compared to 8-10 mm pressure in control eyes (n=10). tPA and uPA levels were increased in retinal protein extracts from high IOP-elevated eyes (three-folds), but not from low-IOP elevated eyes or control eyes. Elevated levels of tPA and uPA were localized in the ganglion cell layer and promoted significant loss of RGCs in high IOP-elevated eyes (50.66 +/-15.61% reduction, compared to controls, p<0.05), but not in low IOP-elevated eyes (98.2+/- 11%). Intravitreal injection of tPA-STOP, an inhibitor of tPA and uPA proteolytic activity, into high IOP-elevated rat eyes significantly reduced RGC loss (86+/-13.3%, p<0.05), when compared to phosphate buffered saline-injected rat eyes (52.20 +/- 11%, p<0.05).
Conclusions: :
The results obtained in this study suggest that tPA and uPA play a direct role in RGC loss. Furthermore, the results presented in this study suggest that designing of protease-based therapies may be helpful adjunct therapy to prevent IOP-mediated vision loss.
Keywords: intraocular pressure • neuroprotection • ganglion cells