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Jackson Michael Baumann, Oleg Yarishkin, Monika Lakk, Felix Vasquez-Chona, David Krizaj; Trabecular meshwork cell swelling and osmoregulation is subserved by TRPV4 and TRPM4 cation channels. Invest. Ophthalmol. Vis. Sci. 2019;60(9):5143.
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© ARVO (1962-2015); The Authors (2016-present)
Trabecular meshwork (TM) plays a pivotal role in maintaining intraocular pressure (IOP). Increases in TM resistance to aqueous humor outflow result in IOP elevation, optic neuropathy and vision loss in hypertensive glaucoma. Outflow resistance is regulated dynamically but mechanisms are unclear. TM cell swelling is a likely mechanism for regulating constriction/relaxation of TM tissue for the purpose of modulating aqueous humor outflow. We hypothesized that mechanosensitive and swelling sensitive ion channels in the transient receptor potential vanilloid (TRPV) and melastatin (TRPM) channel families interact with Na-K ATPases to create ideal ionic gradients conducive to water transport through aquaporins for dose-dependent swelling of TM cells.
Primary human TM cells were isolated from corneoscleral rims and donor eyes. Immunostaining, calcium, sodium, potassium imaging, cell volume assays, whole cell patch-clamp electrophysiology and shRNA cell transfections determined functional expression of TRPV, TRPM and Na-K ATPase isoforms. Filamentous actin was tracked with fluorescent phalloidin. Hypotonic stimulation and pharmacological agonists/antagonists of TRPV4 and TRPM4 were used to quantify the impact of mechanosensitive channels and pumps on cell swelling.
Hypotonic stress evoked dose-dependent transmembrane currents and elevations in intracellular calcium and sodium concentrations and decreased f-actin expression intensity. Immunostaining showed expression of TRPV4 and TRPM4 in cultured and intact TM cells. Rapid and transient intracellular calcium spikes induced by hypotonic stimuli required combined activation of TRPV4 and TRPM4 channels together with slower electrogenic sodium currents that were partially mediated by the Na-K ATPase. Regulatory volume decrease likewise involved TRPM4 and Na-K ATPase activation. Interestingly, hypotonic stimuli also caused cell blebbing which could be a localized compensation for osmotic gradient changes.
These findings identify the interaction between calcium-permeable TRPV4 and calcium-activated TRPM4 channels as a central mechanism in TM osmosensing and volume regulation. We demonstrate that this process involves significant shifts in calcium and sodium regulation and Na-K ATPase activation. This dynamic regulation of TM cell volume could play a role in modulating the dynamics of aqueous humor outflow.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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