Biomechanical regulation of permeability in Schlemm’s canal endothelium with respect to glaucoma

Abstract

Glaucoma is the leading cause of irreversible blindness worldwide and is associated with elevated intraocular pressure caused by increased aqueous humor outflow resistance. The majority of outflow resistance is located near the inner wall endothelium of Schlemm’s canal (SC) where aqueous humor passes through micron-sized pores to cross the endothelium. The basal-to- apical direction of aqueous humor filtration pushes SC cells away from their supporting basement membrane, imposing large biomechanical strain on them and leading to the formation of dome- shaped cellular outpouchings known as giant vacuoles (GVs). Our overarching hypothesis is that the demanding biomechanical environment of the inner wall provides local cues that regulate pore formation and filtration across SC endothelium. Four studies examined this hypothesis. The first study demonstrated that pore density increases with biomechanical strain applied to SC cells cultured on elastomeric membranes. The second study demonstrated that inner wall pores co-localise with regions of greater filtration across the inner wall. The third study developed a fluorescent assay to investigate pore formation in cultured SC cells, and experiments using this assay indicated that glaucomatous SC cells may have impaired pore-forming ability in response to strain. The fourth study examined the cytoskeleton and three-dimensional ultrastructure of the inner wall and showed that vimentin intermediate filaments colocalize with GVs, possibly provid- ing internal support to allow inner wall cells to withstand the demanding mechanical environment of the inner wall. Taken together, these studies reveal that pore formation is a mechanosensitive process that allows the inner wall to function as a self-regulating filter by modulating its own porosity in re- sponse to local biomechanical cues arising from basal-to-apical filtration across the endothelium. This process appears altered in glaucoma, contributing to impaired pore formation and elevated outflow resistance characteristic of the disease. Pores and intermediate filaments are therefore potential targets for future glaucoma therapies.