The Cation Channel TRPV4 Regulates Epithelial Barrier Responses to Lipopolysaccharides

Abstract

TRPV4 is a calcium-permeable non-selective cation channel belonging to the vanilloid subfamily of Transient Receptor Potential proteins. This channel has a rather ubiquitous expression in epithelial cells (skin, airways, endothelium and urothelium), and it is known to be sensitive to a wide variety of physical and chemical stimuli, including heat, arachidonic acid metabolites and synthetic α-phorbol derivatives. We have found that lipopolysaccharide (LPS), an outer membrane component of Gram-negative bacteria, activates TRPV4 in HEK293T cells, independently of the renowned TLR4 pathway. Interestingly, conical LPS from E.coli induced larger TRPV4 response than cylindrical LPS from S. minnesota. Considering the role of TRPV4 in mechanosensation we hypothesized that TRPV4 senses the disturbances induced by insertion of LPS in the plasma membrane. By measuring membrane fluidity using fluorescence probes, we found that conically-shaped LPS from E. coli, but not cylindrical LPS from S. minnesota, causes a membrane phase shift towards gel-like state. This suggests that LPS structural differences are important in its ability to induce mechanical alterations in the membrane. We also found that LPS activates native TRPV4 in the human bronchial epithelial cell line 16HBE. Acute stimulation of TRPV4 in monolayer cultures of 16HBE cells induced a rapid increase of the transepithelial electrical resistance. Additionally, LPS-mediated TRPV4 activation increased colocalization of ZO-1 and Occludin in the plasma membrane. Altogether, these data suggest that LPS induces a TRPV4-dependent reorganization of tight junction proteins, leading to an increase in barrier function. We propose that TRPV4 participates in the orchestration of the innate immune response to the challenge with bacterial endotoxins.