TRPV4 channels are essential for alveolar epithelial barrier function as protection from lung edema

Abstract

Ischemia/reperfusion-induced edema (IRE), one of the most significant causes of mortality after lung transplantation, can be mimicked ex vivo in isolated perfused mouse lungs (IPL). Transient receptor potential vanilloid 4 (TRPV4) is a nonselective cation channel studied in endothelium; however, its role in the lung epithelium remains elusive. Here, we show enhanced IRE in TRPV4-deficient (TRPV4 –/– ) IPL compared with that of WT controls, indicating a protective role of TRPV4 in maintenance of the alveolar epithelial barrier. By immunohistochemistry, mRNA profiling, and electrophysiological characterization, we detected TRPV4 in bronchial epithelium, alveolar epithelial type I (ATI), and alveolar epithelial type II (ATII) cells. Genetic ablation of TRPV4 resulted in reduced expression of the water-conducting aquaporin-5 (AQP-5) channel in ATI cells. Migration of TRPV4 –/– ATI cells was reduced, and cell barrier function was impaired. Analysis of isolated primary TRPV4 –/– ATII cells revealed a reduced expression of surfactant protein C, and the TRPV4 activator GSK1016790A induced increases in current densities only in WT ATII cells. Moreover, TRPV4 –/– lungs of adult mice developed significantly larger mean chord lengths and altered lung function compared with WT lungs. Therefore, our data illustrate essential functions of TRPV4 channels in alveolar epithelial cells and in protection from edema formation.

Ca2+ Signaling by TRPV4 Channels in Respiratory Function and Disease

“…Although association of water conducting AQP-5 channels with and regulation by TRPV4 proteins has already been described [113,114], an AQP-5-deficient mouse model showed no differences in the formation of pulmonary edema and iso-osmolar fluid transport from the alveolar space [115]. Importantly, AT2 cells of TRPV4-/-mice showed a decreased production of pro-surfactant protein C, a precursor of surfactant protein-C (SP-C), secreted from these cells and older mice exhibited emphysema-like changes in their lung structure [109]. Next to facilitating gas exchange, surfactant is also important for protection of the alveolar epithelium from chronic micro-injuries.…”

“…Moreover, in a process called epithelial mesenchymal transition (EMT), AT2 cells transform to mesenchymal cells, which express high amounts of α-smooth muscle actin (α-SMA) and may [107] or may not [108] be involved in wound healing during lung fibrosis. TRPV4 mRNA is expressed in AT2 cells and GSK1016790A (see Table 1) increased basal currents in WT AT2 cells but not in cells from TRPV4-/-mice [109]. While differentiation to AT1 cells was not changed in AT2 cells from TRPV4-deficient mice [109], EMT was significantly reduced in TRPV4-/-AT2 cells compared to WT control cells (Figure 3).…”

“…TRPV4 mRNA is expressed in AT2 cells and GSK1016790A (see Table 1) increased basal currents in WT AT2 cells but not in cells from TRPV4-/-mice [109]. While differentiation to AT1 cells was not changed in AT2 cells from TRPV4-deficient mice [109], EMT was significantly reduced in TRPV4-/-AT2 cells compared to WT control cells (Figure 3). Ischemia-reperfusion (IR)-induced lung injury, which results in alveolar damage, lung edema and hypoxemia remains a significant cause of primary graft failure after lung transplantation [111].…”

“…As these data are in clear contrast to the function of TRPV4 channels in the endothelium (see above), TRPV4 function in the alveolar epithelial barrier was carefully analyzed. Deletion of TRPV4 channels in AT1 cells inhibited aquaporin-5 (AQP-5) expression at the plasma membrane and reduced cell migration, as well as barrier function [109]. Although association of water conducting AQP-5 channels with and regulation by TRPV4 proteins has already been described [113,114], an AQP-5-deficient mouse model showed no differences in the formation of pulmonary edema and iso-osmolar fluid transport from the alveolar space [115].…”

“…All these data support a model of increased edema formation by endothelial TRPV4 channels induced by mechanical ventilation and application of toxicants (see above), while deletion of TRPV4 proteins in the alveolar epithelium and activation of TRPC6 channels in the endothelium results in significantly higher edema formation by IR (see Figure 4). Most interestingly, edema formation by these chronic changes in the TRPV4-/-epithelium are partly reversed in TRPC6/TRPV4-double deficient lungs [109], as TRPC6 ablation reduces IR-induced lung edema formation by a loss of permeability in the pulmonary endothelium [117]. Therefore, TRPV4 serves different partly antagonistic function in the formation of lung edema dependent on the tissue, where the channel is expressed (Figure 4).…”