Sphingosine-1-phosphate effects on conventional outflow physiology

Abstract

Glaucoma is the leading cause of irreversible blindness worldwide with the most prevalent form, primary open-angle glaucoma (POAG), accounting for the vast majority of glaucoma cases. The main risk-factor for POAG is an elevated intraocular pressure (IOP), and is due to an increased resistance to aqueous humor outflow in the conventional outflow pathway at the juxtacanalicular region of the trabecular meshwork (TM) and the inner wall of Schlemm’s canal (SC). Reducing elevated IOP is the most effective method to prevent further loss of vision in glaucoma; therefore, it is important to understand how outflow resistance is regulated in the conventional outflow pathway in order to find effective methods to reduce ocular hypertension. Sphingosine-1-phosphate (S1P) is an endogenous lipid that reduces outflow facility in porcine eyes, thereby increasing resistance. S1P plays a major role in affecting cell migration, endothelial permeability, and junctional formation, processes that are intimately linked and regulated by cytoskeletal dynamics. Due to S1P’s known effect of decreasing endothelial permeability in vascular endothelial cells, the overall hypothesis of this dissertation is that the S1P-induced decrease in outflow facility occurs through a mechanism that involves S1P receptor activation in SC cells. The results from the studies within this dissertation demonstrate the expression of the S1P₁₋₃ receptor subtypes in SC and TM cells and a decrease of outflow facility by S1P in perfused human eyes. Additionally, S1P promotes F-actin formation and myosin light chain (MLC) phosphorylation at the SC cell cortex. The S1P-promoted MLC phosphorylation in both SC and TM cells, in addition to the S1P-induced decrease of outflow facility in porcine and human eyes, were blocked by the S1P₂ antagonist JTE-013. Results from these studies demonstrate S1P to actively regulate actomyosin dynamics in the cells of the outflow pathway through the S1P₂ receptor. S1P₂ also mediates the S1P-induced increase in outflow resistance. Therefore, S1P₂ is a novel pharmacological target in the conventional outflow pathway to reduce elevated IOP in glaucoma patients.