Cyclic Mechanical Stress and Trabecular Meshwork Cell Contractility

Abstract

Purpose Ocular pulse decreases outflow facility of perfused anterior segments. However, the mechanism by which conventional outflow tissues respond to cyclic intraocular pressure oscillations is unknown. The purpose of the present study was to examine responses of trabecular meshwork (TM) cells to cyclic biomechanical stress in the presence and absence of compounds known to affect cell contractility. Methods To model flow in the juxtacanalicular region of the TM and to measure changes in transendothelial flow, human TM cell monolayers on permeable filters were perfused at a constant flow rate until reaching a stable baseline pressure and then were exposed to cyclic stress with an average amplitude of 2.7 mm Hg peak to peak at a 1-Hz frequency for 2 hours in the presence or absence of compounds known to affect cell contractility (isoproterenol, Y27632, pilocarpine, and nifedipine). Pressure was recorded continuously. Immunocytochemistry staining was used to determine filamentous actin stress fiber content, whereas Western blot analysis was used to measure the extent of myosin light chain (p-MLC) phosphorylation and ratio of filamentous to globular actin. Results Human TM cells respond to cyclic pressure oscillations by increasing mean intrachamber pressure (decreasing hydraulic conductivity) (126.13% ± 2.4%; P < 0.05), a response blocked in the presence of Y27632, a rho-kinase inhibitor (101.35 ± 0.59; P = 0.234), but not isoproterenol, pilocarpine, or nifedipine. Although mechanical stress appeared to have no effect, Y27632 decreased phosphorylated myosin light chain, filamentous/globular actin ratio, and stress fiber formation in TM cells. Conclusions Human TM cells respond to cyclic mechanical stress by increasing intrachamber pressure. Pulse-mediated effects are blocked by Y27632, implicating a role for Rho-kinase-mediated signaling and cellular contractility in ocular pulse-associated changes in outflow facility.

Intraocular Pressure Regulation: Findings of Pulse-Dependent Trabecular Meshwork Motion Lead to Unifying Concepts of Intraocular Pressure Homeostasis

“…4 Such findings are consistent with the concept that cyclic mechanical stresses function to confer a physiologic benefit important to IOP homeostasis. These studies point to the need to improve our understanding of the linkage between cyclic stresses, TM motion, and IOP regulatory homeostasis.…”

“…[15][16][17] Pressure-dependent, tensionally integrated In the aqueous outflow system, cyclic changes in IOP alter conventional aqueous outflow in an ex vivo model 3 ; a synchronous increase in cellularity has lead to the proposal that cyclic changes act to alter cellular contractile mechanisms. 4 In addition, mechanical stresses lead to alterations in gene expression [18][19][20][21] as well as changes in cytoskeletal networks 22,23 and modulation of signal transduction. 24 The composition of the ECM in the trabecular beams and juxtacanalicular space is subject to modulation by mechanical stretching.…”

Intraocular Pressure Homeostasis: Maintaining Balance in a High-Pressure Environment

“…51,[53][54][55][56][57] Since an ocular pulse of 2-3 mm Hg could perturb the TM, there is some logic to evaluating cyclic mechanical stretching. Studies showing a TM displacement due to an ocular pulse have been conducted recently.…”

“…Cyclic stretch and IOP have been assessed, but these tend to reduce facility rather than increase it. 57,60 However, a pressure or stretch increase that occurred on a cyclic background, could conceptually produce different responses than would be observed with an equivalent sustained noncyclic stretch. 61 The group of genes with expression profiles changed by TM cell cyclic stretching (15% stretching at 1 cycle per second for 6 h), shows some similarity, but also comprises some unique genes compared with those found with sustained stretching.…”

Pulsatile motion of the trabecular meshwork in healthy human subjects quantified by phase-sensitive optical coherence tomography

“…For example, cyclic changes in IOP alter conventional aqueous outflow in an ex vivo model [26] and alter TM cell contractility in TM cell monolayers [27]. Mechanical stresses also lead to alterations in gene expression [28][29][30][31][32] as well as changes in cytoskeletal networks [28,33,34] and modulation of signal transduction [35,36].…”