Increased Cytoskeletal Stiffness of Schlemm's Canal Endothelial Cells in Glaucoma

Abstract

The elevated intraocular pressure characteristic of glaucoma has been associated with a decreased pore density in the cells of the inner wall of Schlemm's canal (SC). SC cells form these pores in response to a transcellular pressure drop, facilitating the flow of aqueous humor across this endothelium. We hypothesize that impaired pore formation in glaucoma is due to an elevated stiffness of SC cells.Atomic force microscopy (AFM) measurements of elastic modulus were performed using pyramidal or spherical (4.5 or 10 µm) tips on SC cells isolated from 6 healthy and 5 glaucomatous human eyes. Using finite element modeling and AFM experiments with latrunculin-A (an F-actin depolymerizing agent), we previously showed that sharp pyramidal tips characterize the cortex stiffness while larger, spherical tips characterize the stiffness of the subcortical cytoskeleton. The geometry of an SC cell was reconstructed based on electron microscopy images and used to model cell deformation under pressure (3-6 mmHg).When probed with spherical tips, the modulus of glaucomatous cells (1.36±0.14 kPa) was significantly higher (p<0.02) than that of healthy cells (0.89±0.10 kPa). No significant difference was detected between healthy and glaucomatous cells using sharp tips (7.40±1.33 vs. 7.99±0.97 kPa). The higher modulus measured on glaucomatous cells using spherical tips suggest that the altered stiffness is likely in the subcortical cytoskeleton and not in the cell cortex. Preliminary studies using finite element modeling predict a 34-40% decrease in cell deformation solely due to the increased cell stiffness measured on glaucomatous cells. This is consistent with our hypothesis that cell deformation is likely a precursor to the pressure-driven pore formation process and that increased cell stiffness may inhibit this process.Acknowledgements: BrightFocus Foundation, NIH EY019696, NIH T32 EY007128.