Ca2+ Signaling by TRPV4 Channels in Respiratory Function and Disease

Abstract

Members of the transient receptor potential (TRP) superfamily are broadly expressed in our body and contribute to multiple cellular functions. Most interestingly, the fourth member of the vanilloid family of TRP channels (TRPV4) serves different partially antagonistic functions in the respiratory system. This review highlights the role of TRPV4 channels in lung fibroblasts, the lung endothelium, as well as the alveolar and bronchial epithelium, during physiological and pathophysiological mechanisms. Data available from animal models and human tissues confirm the importance of this ion channel in cellular signal transduction complexes with Ca²⁺ ions as a second messenger. Moreover, TRPV4 is an excellent therapeutic target with numerous specific compounds regulating its activity in diseases, like asthma, lung fibrosis, edema, and infections.

Keywords: Ca2+ signaling; TRP channels; TRPV4; endothelium; epithelium; lung; respiratory system; signal transduction.

Figure 1

(A) Cartoon representing the structure of a he fourth member of the vanilloid family of transient receptor potential channels (TRPV4) channel with functional domains, as well as activator interaction sites (modified from Reference [31]). See text for details. 4α-PDD, 4α-phorbol 12,13 didecanoate; 5,6 EET, 5,6 epoxyeicosatrienoic acids; A, ankyrin repeat domain (ARD); AA, arachidonic acid; CaM, Ca²⁺/calmodulin binding site; N, N-glycosylation site; P, channel pore; PKA, protein kinase A; PKC, protein kinase C; PM, plasma membrane; PDZ, PDZ binding domain; PRD, proline-rich domain; TRP, TRP-box; S1-6, transmembrane segments 1–6. (B) A selection of the proposed multimerization potential of TRPV4 (V4) proteins to form functional homo- or together with Transient Receptor Potential Classical 1 (TRPC1) (C1) and Transient Receptor Potential Polycystin 2 (TRPP2) (P2) monomers hetero-tetrameric channels (modified from Reference [32]). P, channel pore.

Figure 2

Contraction of primary murine lung fibroblasts (PMLF) in a gel matrix assay (modified from the Doctoral thesis of Jonas Weber (Ludwig-Maximilians-Universität (LMU)-Munich 2020, see http://edoc.ub.uni-muenchen.de/26578/). PMLFs from wild-type (WT) and TRPV4-deficient (TRPV4-/-) mice were seeded in collagen matrices in 6-well plates and transforming growth factor β (TGF-β1) (2 ng/mL, TGF-β1) or solvent (control) was added before release from the well edges (as described in Reference [93]). Diameters of matrices before and after adding TGF-β1/solvent were measured and percent (%) contraction was quantified by calculating differences. Data are shown as means with standard errors of the mean (SEM) of three independent cell isolations (described in Reference [93]). * indicates a p-value of <0.05.

Figure 3

Epithelial mesenchymal transition (EMT) of primary alveolar epithelial cells type 2 (AT2) after application of TGF-β1 quantified by immunostaining of α-smooth muscle actin (α-SMA) and cell nuclei with Hoechst dye (Hoechst). (A) Pictures of representative immunohistochemistry stainings of AT2 cells from wild-type (WT) and TRPV4-deficient (TRPV4-/-) mice by a fluorescence-coupled anti-α-SMA antibody (described in Reference [110]) after application of TGF-β1 (+TGF-β1) or solvent (control). (B) Summary of data. Grey values of AT2 cells from wild-type (WT) and TRPV4-deficient (TRPV4-/-) mice are plotted. Data are shown as means with standard errors of the mean (SEM) of three independent cell isolations (described in Reference [109]). * indicates a p-value of <0.05.

Figure 4

A model for edema formation by different triggers in wild-type (WT) and TRPV4-deficient (TRPV4-/-) lungs. (A) The alveolar capillary membrane with resident cells in the quiescent state (no trigger) in wild-type (WT) mice. (B) TRPV4 channels are expressed in endothelial, AT1 and AT2 cells. Edema formation by pressure, Cl^- and other TRPV4 activators in WT lungs is due to increased endothelial permeability by TRPV4 channels (reviewed in Reference [101]). (C) Stronger edema formation by ischemia-reperfusion (IR) in TRPV4-deficient (TRPV4-/-) mice. IR-induced edema is dependent on acute activation of TRPC6 channels in the vascular endothelium and supported by a chronic loss of barrier function in the alveolar epithelium due to ablation of TRPV4 channels. TRPV4 deficiency results in reduced surfactant protein-C (SP-C) production in AT2 and decreased aquaporin-5 (AQP-5) expression in AT1 cells, which also showed less barrier function and reduced cell migration [109].