Volume sensing in the transient receptor potential vanilloid 4 ion channel is cell type–specific and mediated by an N-terminal volume-sensing domain

Abstract

Edited by George M. Carman Many retinal diseases are associated with pathological cell swelling, but the underlying etiology remains to be established. A key component of the volume-sensitive machinery, the transient receptor potential vanilloid 4 (TRPV4) ion channel, may represent a sensor and transducer of cell swelling, but the molecular link between the swelling and TRPV4 activation is unresolved. Here, our results from experiments using electrophysiology, cell volumetric measurements, and fluorescence imaging conducted in murine retinal cells and Xenopus oocytes indicated that cell swelling in the physiological range activated TRPV4 in Müller glia and Xenopus oocytes, but required phospholipase A 2 (PLA 2) activity exclusively in Müller cells. Volumedependent TRPV4 gating was independent of cytoskeletal rearrangements and phosphorylation. Our findings also revealed that TRPV4-mediated transduction of volume changes is dependent by its N terminus, more specifically by its distal-most part. We conclude that the volume sensitivity and function of TRPV4 in situ depend critically on its functional and cell type-specific interactions.

Polymodal Sensory Transduction in Mouse Corneal Epithelial Cells

“…77 Our recent study suggests that differential responsiveness to osmotic stress involves binding of the N-terminus to cytoskeletal/membrane structures that mimic PLA2-bound protein conformations of the channel. 29 Further studies are required to delineate the molecular determinants that control how the CEC TRPV4 channels respond to extrinsic and intrinsic stressors, and whether lack of activation by strain involves masking of the N-terminal region by PACSIN SH3 domains. 90 The present study identifies the TRPV4-hemichannel-ATP signaling axis as a potential modulator of paracrine, mechanically, and hypotonically induced corneal excitation.…”

“…We recently found that such activations can be augmented by water fluxes through aquaporin channels, which drive time-dependent facilitation of TRPV4 activation via the N-terminal domain of the protein. 29 , 36 Overactivation of this mechanism in CECs, which utilize AQP5 to regulate corneal thickness and water transport across the epithelial barrier, 79 could contribute to edema by triggering the positive feedback loop between cell swelling, calcium overloads, purinergic excitation, and fluid dysregulation. 80 , 81 The transient, HC-06-resistant component of hypotonicity-evoked [Ca 2+ ] i increases remains to be identified but could involve release of calcium from internal stores or activation of recently identified OSCA/TMEM63 channels.…”

“… 40 , 84 – 86 The simplest explanation, that TRPV4 activation reflects the unique lipid and signaling aspects of the CEC microenvironment, is supported by the remarkable spectrum of TRPV4 activation mechanisms across cells and tissues. 26 , 28 , 29 Phospholipase A2 activation is required for TRPV4 gating in glia, nonpigmented cells of the ciliary body, and recombinant HEK293 cells but not neurons and heterologously expressing oocytes, 20 , 26 – 28 whereas membrane stretching in response to positive pressure activates TRPV4 in glia and oocytes but not chondrocytes. 26 , 85 , 87 The structural determinants that regulate the differential responsiveness remain to be fully elucidated, with existing evidence indicating multisite regulation: residues from S2-S3 and S4-S5 linkers encode its sensitivity to eicosanoids, 88 TM3-4 regions control responses to agonists and heat, 89 whereas CYP450 enzymes regulate responsiveness to polyunsaturated lipids.…”

“…TRPV4 has been identified as a key regulator of mechanically evoked cellular volume regulation, barrier permeability and immune interactions in lung, bladder, esophagus, skin, gut, kidney, choroid plexus, retinal pigment, and ciliary body epithelia. [17][18][19][20][21][22][23] TRPV4-mediated calcium influx has been linked to cytoskeletal reorganization, cell adhesion, formation of focal adhesions, and adenosine triphosphate (ATP)-dependent modulation of afferent excitation in epithelia, 5,19,24,25 yet mechanisms of TRPV4 gating are both celltype and context-specific, [26][27][28][29] and the properties of TRPV4 activation in native CECs have not been conclusively established. In this study, we employed imaging, electrophysiological, and molecular approaches to characterize calcium regulation and ATP release in CECs exposed to TRPV4 agonists, strain, volume, and temperature increases.…”

Mechanically induced cytoskeletal remodeling in trabecular meshwork cells requires TRPV4 - Rho signaling interactions

“…Interestingly, pretreatment with FAK or β1 integrin inhibitors suppressed sensitization of TRPV4 channels by mechanical stressors (Baratchi et al, 2017). Another clue is TRPV4 activation by arachidonic acid, which is produced in response to stretch and might enhance TM sensitivity to weak hydrodynamic loading (23,93,96).…”