Abstract
Purpose:
To characterize the molecular mechanism of mechanosensing and cytoskeletal remodeling homeostasis in the trabecular meshwork (TM). Intraocular pressure (IOP)-dependent cytoskeletal remodeling is believed to contribute to the etiology of glaucoma yet it is not known how TM cells detect changes in IOP or how mechanical stimuli are transduced into changes in intracellular signaling, contractility and TM dysfunction.
Methods:
RT-PCR and Western blots were used to determine transcript and protein levels in human TM (hTM) cells. Cell area, volume and calcium concentration [Ca2+]i were measured following cell loading with the calcium-insensitive dye calcein and the calcium indicator Fura-2. Stretch-sensitivity was tested by exposing hTM cells to defined amounts of cyclic uniaxial or biaxial substrate stretch. Stretch-induced transmembrane currents were recorded directly with the high-speed pressure clamp technique. Contractility was measured in bovine TM using isometric force transducers.
Results:
Pressure steps or ramps evoked inward and secondary outward currents in hTM that were sensitive to TRP channel antagonists. Consistent with this, hTM expressed known mechanosensitive TRP isoform transcripts and protein, dominated by Trpv4. Hypotonic stimulation (HTS) and substrate stretch induced influx of calcium that was sensitive to BAPTA and TRPV4 blockers, and led to led to the formation of actin stress fibers and upregulation of zyxin. Stretch-dependent remodeling was associated with compensatory changes in the organization of the mechanosensory actin complex tethered to focal adhesions. Accordingly, selective TRPV4 agonists induced large and sustained [Ca2+]i elevations in hTM cells and in cells isolated from trabeculectomized human tissue as well as contractions in strips of intact bovine TM tissue.
Conclusions:
Our results suggest that TRPV4 mediates the intrinsic mechanosensitivity of TM cells and that TRPV4-mediated calcium influx plays a key role in TM cytoskeletal remodeling. This is likely to be an important mechanism through which the conventional outflow pathway senses and responds to IOP.