Studies on Hydrostatic Pressure Responses to Piezo1 and TRPV4 in the Mouse Lens

The lens has a feedback control system made by transient receptor potential vanilloid 4 (TRPV4) and TRPV1 to sense pressure and activate ion transport to maintain intracellular hydrostatic pressure close to zero at the lens surface. We have recently reported the presence of another mechanosensitive channel, Piezo1, in the mouse lens epithelium. The purpose of the current study was to characterize the response to Piezo1 activation and test whether it is activated by changes in hydrostatic pressure. We also examined the radial pressure gradient following activation of either TRPV4 or Piezo1.

Intracellular hydrostatic pressure in mouse lenses was measured with a microelectrode-manometer system. The pressure was determined at the lens surface, and the radial pressure gradient between the surface and the lens center was also determined.

Activation of Piezo1 by the agonist Yoda1 caused an immediate reduction of hydrostatic pressure in cells near the lens surface. The response was transient, and after ∼25 minutes pressure gradually recovered. Pressure recovery did not occur when TRPV1 was inhibited. TRPV4 activation by the agonist GSK1016790A (GSK) added after Yoda1 caused an additional reduction in pressure, suggesting that Piezo1 and TRPV4 are independent. In keeping with this notion, the inactivation of TRPV4 by the antagonist HC067047 did not prevent the pressure response to Yoda1. In some experiments, the pressure was elevated by subjecting the lens to partial Na/K pump inhibition by strophanthidin and simultaneously inhibiting the TRPV4-dependent feedback recovery loop. Under these conditions, pressure did not recover and remained elevated for over 2 hours even though it was confirmed that the activation of Piezo1 by Yoda1 was able to restore pressure to zero. The findings suggest that lens Piezo1 is not activated by positive pressure. The radial hydrostatic pressure gradient was determined in lenses exposed to Yoda1 in the presence of the TRPV1 antagonist A889425 added to prevent pressure recovery. The slope of the pressure gradient was reduced by ∼50% in the differentiating fiber (DF) region but central and surface pressure were unchanged. Under the same condition, the DF pressure gradient response was similar in lenses exposed to the TRPV4 agonist GSK. The pressure gradient responses to Yoda1 and GSK were absent in lenses subjected to inhibition of protein tyrosine phosphatase 1B (PTP1B) by a selective inhibitor, TCS401.

The lens responds with a negative change of intracellular hydrostatic pressure in cells near the lens surface when Piezo1 is activated. Piezo1 and TRPV4 appear to be independent because they cause additive pressure reduction responses. However, unlike TRPV4, Piezo1 likely plays no role in feedback control of surface pressure because it does not respond to positive pressure changes. When TRPV1-dependent pressure recovery was blocked, activation of either Piezo1 or TRPV4 caused a pressure change that appears consistent with phosphatase-dependent activation of latent Na/K pumps in DF cells.