Tissue Plasminogen Activator: Mechanistic Studies in Steroid-Induced Glaucoma

Abstract

As the second leading cause of blindness worldwide, following cataracts, glaucoma poses a heavy societal burden with a projection of 80 million cases by 2020. Glaucoma is a group of optic neuropathies that can lead to vision loss. Exposure to steroids has been linked to the development ofiatrogenic open angle glaucoma in humans; however, the exact causal mechanism has not been characterized. Previous studies have shown that steroid exposure causes a significant reduction of tissue plasminogen activator (tPA) levels in the trabecular meshwork (TM) tissue of animal models and human cells, which is linked to a reduction in outflow facility (OF) and an increase in intraocular pressure (IOP). We have further shown that OF is significantly reduced in tPA deficient mice (PlatKO). We sought to elucidate the molecular pathways involving tPA in steroid-induced glaucoma to determine potential routes of intervention. tPA is a serine protease that may participate in aqueous humor outflow regulation in the anterior chamber angle at the TM. We hypothesized that tPA functions as both a proteolytic enzyme and receptor ligand to alter OF. We administered enzymatically active and enzymatically inactive forms of tPA to steroid (triamcinolone acetonide) exposed C57BL/6J mice to study these functions in isolation. We observed an equivalent attenuation of steroid-induced OF reduction following both treatments, indicating that tPA’s proteolytic action is not required. These treatments were also effective in improving the phenotypic OF reduction seen inTg-MYOCY437Hmice, a genetic model of open angle glaucoma. Furthermore, these results were recapitulated in an in vitro biomimetic model utilizing human TM (HTM)cells exposed to the steroid prednisolone acetate. 3 An investigation of the downstream route by which tPA induces OF enhancement centered on the transcriptional regulation of matrix metalloproteinase (MMP) expression. Specifically, MMP-9 was found to play an essential role as the effects of enzymatically active and enzymatically inactive tPA were prevented in MMP-9deficient mice (Mmp-9KO). We sought to identify the tPA receptor which serves as an intermediary to MMP expression and OF regulation. Through the use of pharmacologic inhibitors on HTM cells treated with enzymatically inactive tPA, we determined that the LRP-1 NMDAR co-receptor system is involved. These studies provide novel insights to the mechanism of tPA-dependent OF modulation at the level of the TM. The results enhance our understanding of the pathophysiology of steroid-induced glaucoma and can be utilized in the development of novel therapeutics, which may also be applicable to other forms of open angle glaucoma.