Journal Club 2013/12/26

Mol Pharmacol-2013-Spahn-mol.113.088997

Abstract

Transient Receptor Potential Vanilloid 1 (TRPV1) is a nonselective ligand-gated cation channel responding to noxious heat, protons, and chemicals like capsaicin. TRPV1 is expressed in sensory neurons and plays a critical role in pain associated with tissue injury, inflammation or nerve lesions. Transient Receptor Potential Ankyrin 1 (TRPA1) is co- expressed with TRPV1. It is activated by compounds that cause a burning sensation (e.g. mustard oil) and, indirectly, by components of the inflammatory milieu eliciting nociceptor excitation and pain hypersensitivity. Previous studies indicate an interaction of TRPV1 and TRP A1 signaling pathways. Here we sought to examine the molecular mechanisms underlying such interactions in nociceptive neurons. We first excluded physical interactions of both channels using radioligand binding studies. By microfluorimetry, electrophysiological experiments, cAMP measurements, and site-directed mutagenesis we found a sensitization of TRPV1 after TRPA1 stimulation with mustard oil in a calcium- and cAMP/PKA-dependent manner. TRPA1 stimulation enhanced TRPV1 phosphorylation via the putative PKA phosphorylation site serine 116. We also detected calcium-sensitive increased TRPV1 activity after TRPA1 activation in dorsal root ganglion (DRG) neurons. The inhibition of TRPA1 by HC-030031 after its initial stimulation and the calcium-insensitive TRPA1 mutant D477A still showed increased capsaicin-induced TRPV1 activity excluding an additive TRPA1 current after TRPV1 stimulation. Our study shows sensitization of TRPV1 via activation of TRPA1, which involves adenylyl cyclase, increased cAMP, subsequent translocation and activation of PKA, and phosphorylation of TRPV1 at PKA phosphorylation residues. This suggests that cross-sensitization of TRP channels contributes to enhanced pain sensitivity in inflamed tissues.