House dust mite potentiates capsaicin-evoked Ca²⁺ transients in mouse pulmonary sensory neurons via activation of protease-activated receptor-2

House dust mite (HDM) is a major source of allergen in house dust and has been suggested to be involved in the pathogenesis of asthma. In this study, we aimed to investigate whether HDM can modulate the sensitivity of pulmonary sensory neurons and, if so, to elucidate the underlying mechanism. Fura-2-based ratiometric Ca²⁺ imaging was carried out to determine the effect of HDM extract on the capsaicin-evoked Ca²⁺ transient in mouse vagal pulmonary sensory neurons. Pretreatment with HDM (50 μg ml^-1, 5 min) significantly enhanced the Ca²⁺ transient evoked by capsaicin in these neurons isolated from wild-type mice. This potentiating effect of HDM was not antagonized by E-64, a selective cysteine protease inhibitor, but was completely prevented by AEBSF, a specific serine protease inhibitor. In addition, the potentiating effect of HDM on capsaicin-evoked Ca²⁺ transient was absent in the pulmonary sensory neurons isolated from protease-activated receptor-2 (PAR₂) knockout mice. Furthermore, the sensitizing effect of HDM was completely abolished by U73122, a phosholipase C inhibitor, or chelerythrine, a protein kinase C inhibitor. In summary, our results demonstrate that HDM, mainly through its serine protease activity, potentiates capsaicin-evoked Ca²⁺ transient in mouse pulmonary sensory neurons via the activation of PAR₂ and the phosholipase C-protein kinase C intracellular transduction cascade.