Mechanical responsiveness of the endothelial cell of Schlemm's canal: scope, variability and its potential role in controlling aqueous humour outflow

Abstract

Primary open-angle glaucoma is associated with elevated intraocular pressure, which in turn is believed to result from impaired outflow of aqueous humour. Aqueous humour outflow passes mainly through the trabecular meshwork (TM) and then through pores formed in the endothelium of Schlemm's canal (SC), which experiences a basal-to-apical pressure gradient. This gradient dramatically deforms the SC endothelial cell and potentially contributes to the formation of those pores. However, mechanical properties of the SC cell are poorly defined. Using optical magnetic twisting cytometry and traction force microscopy, here we characterize the mechanical properties of primary cultures of the human SC cell, and for the first time, the scope of their changes in response to pharmacological agents that are known to modulate outflow resistance. Lysophosphatidic acid, sphingosine-1-phosphate (S1P) and thrombin caused an increase in cell stiffness by up to 200 per cent, whereas in most cell strains, exposure to latrunculin A, isoproterenol, dibutryl cyclic-AMP or Y-27632 caused a decrease in cell stiffness by up to 80 per cent, highlighting that SC cells possess a remarkably wide contractile scope. Drug responses were variable across donors. S1P, for example, caused 200 per cent stiffening in one donor strain but only 20 per cent stiffening in another. Isoproterenol caused dose-dependent softening in three donor strains but little or no response in two others, a finding mirrored by changes in traction forces and consistent with the level of expression of β 2 -adrenergic receptors. Despite donor variability, those drugs that typically increase outflow resistance systematically caused cell stiffness to increase, while in most cases, those drugs that typically decrease outflow resistance caused cell stiffness to decrease. These findings establish the endothelial cell of SC as a reactive but variable mechanical component of the aqueous humour outflow pathway. Although the mechanism and locus of increased outflow resistance remain unclear, these data suggest the SC endothelial cell to be a modulator of outflow resistance.