Biomechanical Characterization Of SC Cells

Progress Updates

Aqueous humor is produced by the ciliary body cells behind the iris (in the posterior chamber) and flows into the front of the eye (into the anterior chamber) and then out of the eye through a specialized pathway. However, in glaucoma, there is a problem, or a “resistance” to the outflow of this aqueous humor through this pathway, causing increased intraocular pressure and eventually leading to damage of the optic nerve. The cells of the inner wall endothelium of Schlemm's Canal (SC) may be involved in the increased aqueous humor outflow resistance characteristic of primary open angle glaucoma. SC is a circular structure in the eye that collects aqueous humor from the anterior chamber and delivers it into the bloodstream. The mechanical properties of SC cells determine the magnitude of the transcellular pressure gradient they can support, and may indirectly regulate the outflow resistance of this cell layer.

The elevated intraocular pressure characteristic of glaucoma is caused by an elevated flow resistance of aqueous humor out of the eye. A variety of drugs that affect this flow resistance have been shown to alter the cytoskeleton of cells in the pathway through which the aqueous humor flows. In particular, high cell stiffness is associated with high flow resistance. Dr. Johnson’s team used atomic force microscopy (AFM; a cell measurement and imaging machine) to characterize the mechanical properties of the SC cells in this flow pathway. By using a variety of different tip sizes in their AFM studies to probe cell stiffness, they discovered that these cells have a very stiff cell cortex (a specialized support layer on the inner face of the cell’s outer envelope) and a much more compliant cytoplasm (the jelly-like material that fills the cell) underneath the cell cortex. This was an important finding not only for these Schlemm's canal cells, but an important observation for all endothelial cells.

The team then examined the stiffness of these two cellular regions comparing cells from healthy human eyes to those with glaucoma. They found that while the stiffness of the cortex of glaucomatous SC cells was not significantly altered from that of cells from normal eyes, the cytoplasm of glaucomatous SC cells was much stiffer than that of normal eyes.

This is potentially quite an important finding. This not only indicates that the mechanical stiffness of glaucomatous SC cells may be different than that in normal cells, but indicates that this abnormality appears to exist in the cytoplasm underling the cortex. This finding likely implicates the cytoplasm in the pore formation process. If confirmed in more cell lines, it would give a firm target for pharmacological treatment of this devastating disease of sight.