TRPV4 Channel is an Osmosensor and Mechanosensor in Proximal Tubule Cells

Abstract

The proximal tubule (PT) is the first segment of the nephron and accounts for reabsorption of water and solutes filtered by the glomerulus. In addition, the proximal tubule plays a pivotal role in the endocytosis to reabsorb and metabolize proteins and other substances in glomerular filtrates. Recent studies showed that the PT functions can be modulated by mechanical forces, namely the shear stress on the apical surface and the radial stretch on tubular lumen. Since TRPV4 channel is mainly expressed at the basolateral side of the tubule, we hypothesized that TRPV4 could play a role as stretch-activated channel rather than a shear stress sensor. Patch clamp and Ca2+ imaging experiments showed that TRPV4 channel is expressed in cultured proximal tubule cells and can be activated by the specific agonist GSK1016790A and by hypotonic solution. Moreover, mechanical stretch induced by uniaxial strain in silicone stretchable chambers activated a large Ca2+ influx that was significantly reduced in Trpv4-/- PT cells. Notably, no TRPV4-dependent Ca2+ entry was observed in response to flow-induced cell deformation. Furthermore, we observed that the uptake of FITC-labeled albumin in PT cells, treated with GSK1016790A or mechanically stimulated in stretchable chambers, was significantly reduced in Trpv4-/- PT cells. Finally, we injected angiotensin II in mice, using osmotic minipumps, in order to challenge PT endocytotic activity and increase the proteinuria of mice. Our data showed a massive increase in diuresis of mice. Interestingly, we observed an increase in albuminuria in wild-type mice, and this effect was significantly higher in Trpv4-/- mice. In contrast, glucose and phosphate urinary levels, which undergo PT reabsorption as well, were not significantly affected. Taken together, these results support the view that TRPV4 channel is a mechanosensor in PT cells and modulates their endocytotic activity.