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.

Lymphatic regulator PROX1 determines Schlemm’s canal integrity and identity

“…In this study, we confirmed that Klf4 is upregulated in flow-exposed dermal BECs and LECs in vitro and in developing SC ECs in vivo (28,54). Moreover, previous studies indicated that even if the current of aqueous humor is low, the presence of AHO itself is enough to actively modulate the behaviors of SC ECs, which is in agreement with our data (15). We demonstrated that Prox1 and Klf4 expressions in SC are simultaneously modulated in a temporal manner and are associated with AHO levels during developmental stage and adulthood.…”

“…SC controls the ocular tissue fluid homeostasis and tissue pressure by functioning as a major draining channel. Furthermore, the junction between SC ECs and trabecular meshwork (TM) is composed of a specialized tethering structure, which is responsible for providing an appropriate pressure gradient across the inner wall of an SC to maintain the integrities of SC ECs, thereby providing a tissue pressure-sensing/responding mechanism, such as anchoring filaments for LVs (14)(15)(16).…”

Rho kinase inhibitors for glaucoma treatment - Review

“…An increased resistance to flow in the conventional pathway (which carries up to 90% of the total aqueous humor out of the eye) is predominantly responsible for the elevated IOP in many types of glaucoma. In primary open-angle glaucoma (POAG), the most common type, increased outflow resistance has been classically associated with the juxtacanalicular portion of the TM mediated through the extracellular matrix (ECM) as well as in the endothelial-lined SC (21,22) . Morphologically, the TM is a complex tissue consisting of TM cells, ECM, and empty spaces, which the aqueous humor runs through.…”

Bioactive Lysophospholipids: Role in Regulation of Aqueous Humor Outflow and Intraocular Pressure in the Context of Pathobiology and Therapy of Glaucoma

“…84,90 In addition, LPA has been reported to increase SC cell stiffness and intracellular calcium levels. 19,42 Interestingly, myocilininduced anti or de-adhesive interactions in TM cells have been found to be suppressed in the presence of LPA. 91,92 LPA has been demonstrated to increase SC cell permeability barrier activity by following the diffusion of horse radish peroxidase using the transwell cell culture system.…”

“…[13][14][15][16][17][18] It is also widely believed that changes such as tissue stiffness due to altered cellular contraction, oxidative damage, and altered metabolic activity of TM tissue are associated with increased resistance to AH outflow and elevated IOP. 13,[19][20][21][22][23] Little is known, however, about the cellular and molecular mechanisms that drive the increase in resistance to AH outflow and trigger the associated changes in glaucomatous eyes.…”