Activation of Plasmin Proteolytic System in Glaucoma and the Neuroprotective Role of TrkB Receptor Agonists

Abstract

Purpose: Involvement of proteases has been implicated in remodelling of the extracellular matrix (ECM) in the optic nerve head (ONH) leading to its excavation and an increase in cup to disc ratio in glaucoma. Plasmin proteolytic activity in particular has been implicated in imparting damage to the retina and retinal ganglion cells (RGCs). We have shown previously that Tropomyosin related kinase B (TrkB) receptor activation is neuroprotective for the RGCs in glaucoma. Here, we compared the activity and expression profile of plasmin in normal and glaucomatous conditions and evaluated the effects of TrkB receptor agonist- 7,8 dihydroxyflavone (7,8DHF) treatment on the ECM degradation in glaucoma.

Methods: Ocular hypertension was induced unilaterally in adult Sprague-Dawley rats by repetitive injection of microspheres (weekly) into the anterior chamber of the eye (n=16). Intraocular pressure was monitored using a rebounded tonometer (iCare). One set of rats was treated with a TrkB receptor agonist-7,8 DHF (5mg/kg, i.p, weekly) (n=8). After 20 weeks, rats were sacrificed, perfused with saline, and the ONH region of the retina was collected. Plasmin activity was measured using specific ELISA kits and the expression levels determined by immunohistochemistry and western blotting.

Results: Our results indicate that the plasmin activity was significantly higher in the ONH of rat eyes exposed to experimental glaucoma compared to the contralateral controls (p<0.001). Plasmin expression was primarily observed to be localised around the ONH region in retinal flat mounts and was not altered to any considerable extent in the high IOP exposed retinas. Treatment with 7,8 DHF lead to a significant decline in the plasmin proteolytic activity (p<0.001) and a reduction in the collagen degradation products in the ONH in glaucomatous eyes.

Conclusions: This study demonstrates for the first time activation of plasmin proteolytic activity as a pathological change in glaucoma in vivo. Further investigations may provide insights into the failure of a molecular regulatory mechanism which would normally regulate the plasmin activity in the retina. Administration of 7,8DHF significantly reduced both the plasmin activation as well as degradation of ECM proteins in the ONH.

Keywords: 531 ganglion cells • 615 neuroprotection • 543 growth factors/growth factor receptors