Role of Glypicans in Trabecular Meshwork Function, IOP, and Glaucoma

Duke University, Durham NC

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Abstract

PROJECT SUMMARY Ocular hypertension (OHT) arising from impaired aqueous humor (AH) outflow through the trabecular meshwork (TM) is a major risk factor for glaucoma (optic nerve atrophy), one of the leading causes of blindness globally. Increased contractile activity, cell adhesive interactions, and accumulation of extracellular matrix within the TM are well-recognized to elevate intraocular pressure (IOP), with pharmacological suppression of these cellular attributes being proven to lower IOP in glaucoma patients. Despite these advancements, our understanding of the prominent upstream molecular mechanisms regulating TM cell actomyosin cytoskeletal organization and cell adhesive interactions is very limited. Therefore, filling this knowledge gap is imperative for the identification of new class of molecular targets for development of efficacious IOP lowering agents with reduced adverse effects. Towards this overarching goal, we recently made multiple novel observations, including 1) the identification of molecular parallels in cell adhesive characteristics between TM cells and kidney podocytes, two cell types which share the common functional attributes of filtration and barrier activity, 2) identification of previously undescribed proteins increasing actin cytoskeletal contraction and crosslinking including glypican-4 (GPC4, a cell surface heparan sulfate proteoglycan) in human TM cells treated with dexamethasone (Dex), 3) identification of GPC4 as a potentially prominent upstream molecule regulating actin cytoskeletal organization by activating Wnt5/PCP signaling in Dex and TGF-β2 treated TM cells, 4) detection of robustly elevated levels of GPC4 in the AH of primary open-angle glaucoma patients, 5) elevation of IOP in mice with increased expression of human GPC4 and finally, 6) inability of active TGF-β2 to elevate IOP in GPC4 null mice. Based on these collective new findings, we hypothesize that ocular hypertensive agent-mediated increases in glypican (e.g. GPC4) levels in the AH outflow pathway triggers dysregulation of the Wnt and growth factor signaling activities that in turn impair AH outflow and elevate IOP, via increased Rho GTPase activity, actomyosin contraction, cell adhesive interactions, and the barrier activity of TM. Using human TM cells and donor eyes, and gene targeted mouse models to test the proposed hypothesis, this study will address: 1. Whether augmented GPC4 expression and secretion dysregulates the Wnt and related (e.g. Hedgehog and TGF-β) signaling pathways to stimulate TM cell contractile activity and adhesion, and barrier activity in the presence of ocular hypertensive agents. 2. Whether GPC4 induced OHT is mediated primarily by activation of Wnt5/PCP signaling in the TM and 3. Whether increased GPC4 levels play a pathological role in POAG, and glucocorticoid- and TGF-β2- induced ocular hypertension by dysregulating the Wnt signaling pathways. Completion of these novel studies is expected to uncover, for the first time, the role of glypicans (e.g. GPC4) in; a) homeostasis of IOP, b) etiology of ocular hypertension, and generate data to support c) identification of new molecular targets for lowering IOP and enable d) development of a new ocular hypertensive mouse model.