Next we monitored intracellular Ca2+ signals induced by menthol in single TRPM8-HEK293 cells co-expressing scPPX1. Figure 2 (A–C) shows representative experiments, where 50 and 500 μM menthol were added to single cells in the
presence of 1.8 mM Ca2+. Menthol-evoked Ca2+ signals were observed as an increase in the fluorescence intensity ratio of fura-2 (340/380). We found that menthol-induced intracellular Ca2+ signals were significantly inhibited (p≤0.005) in
cells with co-expressed scPPX1 (0.156}0.085, n = 9) in comparison to the controlcells (0.9}0.2, n = 6) (Fig. 2A–C). Further we performed analogous measurements in F-11 neuronal cells that were derived from rat dorsal root ganglion neurons.F-11 cells are used as a model for DRG neurons to study sensory TRP channels in a system resembling their native environment [36]. In our experiments, F-11 cells were transiently transfected with TRPM8 (0.4 μg) alone or together with scPPX1 (0.4 μg), and menthol-induced Ca2+ signals were subsequently analyzed (Fig. 2D–F). Similarly to the HEK cells expression system, we found that menthol-evoked Ca2+-entry was significantly inhibited (p≤0.005) in F-11 cells withco-expressed scPPX1 (0.105}0.029, n = 12) in comparison to the control cellsexpressing TRPM8 channels alone (0.85}0.119, n = 11) (Fig. 2D–F).