Functional TRPV4 channels are expressed in human airway smooth muscle cells

Abstract

Hypotonic stimulation induces airway constriction in normal and asthmatic airways. However, the osmolarity sensor in the airway has not been characterized. TRPV4 (also known as VR-OAC, VRL-2, TRP12, OTRPC4), an osmotic-sensitive cation channel in the transient receptor potential (TRP) channel family, was recently cloned. In the present study, we show that TRPV4 mRNA was expressed in cultured human airway smooth muscle cells as analyzed by RT-PCR. Hypotonic stimulation induced Ca2+influx in human airway smooth muscle cells in an osmolarity-dependent manner, consistent with the reported biological activity of TRPV4 in transfected cells. In cultured muscle cells, 4α-phorbol 12,13-didecanoate (4-αPDD), a TRPV4 ligand, increased intracellular Ca2+level only when Ca2+was present in the extracellular solution. The 4-αPDD-induced Ca2+response was inhibited by ruthenium red (1 μM), a known TRPV4 inhibitor, but not by capsazepine (1 μM), a TRPV1 antagonist, indicating that 4-αPDD-induced Ca2+response is mediated by TRPV4. Verapamil (10 μM), an L-type voltage-gated Ca2+channel inhibitor, had no effect on the 4-αPDD-induced Ca2+response, excluding the involvement of L-type Ca2+channels. Furthermore, hypotonic stimulation elicited smooth muscle contraction through a mechanism dependent on membrane Ca2+channels in both isolated human and guinea pig airways. Hypotonicity-induced airway contraction was not inhibited by the L-type Ca2+channel inhibitor nifedipine (1 μM) or by the TRPV1 inhibitor capsazepine (1 μM). We conclude that functional TRPV4 is expressed in human airway smooth muscle cells and may act as an osmolarity sensor in the airway.

TRP channels in airway smooth muscle as therapeutic targets

“…Two members of the TRPV family, TRPV2 and TRPV4, when expressed in heterologous systems have been shown to activate in response to hypotonic shock (Muraki et al 2003;Liedtke et al 2000;Strotmann et al 2000). Jia et al (2004) report that the message for both TRPV2 and V4 are present in primary cultured human bronchial smooth muscle cells (HBSMC) and that hypotonicity-induced constriction of human bronchi is dependent upon the presence of extracellular Ca 2+ , a finding that can be recapitulated in guinea-pig airways. Their study shows hypotonicity-induced increases in Ca 2+ in cultured HBSMC, an effect that can be mimicked by the phorbol derivative 4alpha-phorbol 12,13-didecanoate (4α-PDD), reported to be a TRPV4 activator (Watanabe et al 2002).…”

“…Whereas the TRPCs are predominantly activated as a result of Gq-coupled receptor occupation or store depletion, the members of the TRPV family can be activated by diverse stimuli including protons, irritants, lipids, mechanical stimuli, noxious heat and changes in cell volume (Gunthorpe et al 2002). It is perhaps the latter property, responsiveness to changes in cell volume, that led Jia et al (2004) to look at the expression and function of TRPV channels in airway smooth muscle (Table 1).…”

Human TRPV4 Channel Splice Variants Revealed a Key Role of Ankyrin Domains in Multimerization and Trafficking

“…TRPV4 shows multiple modes of activation and regulatory sites, enabling it to respond to various stimuli, including osmotic cell swelling (2)(3)(4)(5), mechanical stress (6 -8), heat (9), acidic pH (7), endogenous ligands (10), high viscous solutions (11), and synthetic agonists such as 4␣-phorbol 12,13-didecanoate (4␣-PDD) (12). TRPV4 mRNA is expressed in a broad range of tissues (13), although functional tests have only been carried out in a few: endothelial (14), epithelial (5,11,15), smooth muscle (16), keratinocytes (17), and DRG neurons (18). An alternative splice variant lacking the seventh exon has also been cloned from human aortic endothelial cells (19), although this variant was not further investigated because of its unresponsiveness to hypotonic stimuli.…”

Is TRPV1 a useful target in respiratory diseases?

“…Jia and co-workers showed a hypotonicity-induced airway contraction that was independent of tachykinin-containing sensory nerves in isolated intact human and guinea pig airways [82]. From their work, a direct action on airway A c c e p t e d m a n u s c r i p t 11 smooth muscle via TRPV4 excitation followed by Ca 2+ influx may be suggested.…”