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FK506 (tacrolimus) is an immunosuppressant widely used as an ointment in the treatment of atopic dermatitis. However, local application of FK506 can evoke burning sensations in atopic dermatitis patients, and its mechanisms are unknown. In this study, we found that FK506 activates transient receptor potential ankyrin 1 (TRPA1) channels. In Ca2+-imaging experiments, increases in intracellular Ca2+ concentrations ([Ca2+]i) by FK506 were observed in HEK293T cells expressing hTRPA1 or hTRPM8. FK506-induced currents were observed in HEK293T cells expressing hTRPA1 or mTRPA1, but less or not at all in cells expressing hTRPV1 or hTRPM8 using a patch-clamp technique. FK506 also evoked single-channel opening of hTRPA1 in an inside-out configuration. FK506-induced [Ca2+]i increases were also observed in TRPA1-expressing mouse primary sensory neurons. Furthermore, injection of FK506 evoked licking or biting behaviors and these behaviors were almost abolished in TRPA1 knockout mice. These results indicate that FK506 might cause pain sensations through TRPA1 activation.

Hypotonic solutions can cause painful sensations in nasal and ocular mucosa through molecular mechanisms that are not entirely understood. We clarified the ability of human TRPA1 (hTRPA1) to respond to physical stimulus, and evaluated the response of hTRPA1 to cell swelling under hypotonic conditions. Using a Ca2+-imaging method, we found that modulation of AITC-induced hTRPA1 activity occurred under hypotonic conditions. Moreover, cell swelling in hypotonic conditions evoked single-channel activation of hTRPA1 in a cell-attached mode when the patch pipette was attached after cell swelling under hypotonic conditions, but not before swelling. Single-channel currents activated by cell swelling were also inhibited by a known hTRPA1 blocker. Since pre-application of thapsigargin or pretreatment with the calcium chelator BAPTA did not affect the single-channel activation induced by cell swelling, changes in intracellular calcium concentrations are likely not related to hTRPA1 activation induced by physical stimuli.

The placenta is required to transport calcium (Ca2+) from mother to fetus during fetal bone mineralization. In an attempt to clarify the molecular basis of Ca2+ entry for this transport, we identified TRPM6 as a candidate for apical Ca2+ entry pathway. TRPM6 mRNA increased during the last 4 days of pregnancy, coinciding with fetal bone mineralization in mice. TRPM6 mRNA and protein was localized in the trophoblasts in labyrinth where the maternal–fetal Ca2+ transport occurs. In patch-clamp recordings, we observed TRPM6/TRPM7-like currents in mouse trophoblasts after starting fetal bone mineralization but not before mineralization. Plasma membrane Ca2+ permeability was significantly increased in TRPM6/TRPM7 expressed HEK293 cells under physiological Mg2+ and ATP concentration but not in TRPM6 or TRPM7 homomer-expressing cells. These results suggest that TRPM6 is functionally expressed in mouse placental trophoblasts, implicating in maternal–fetal Ca2+ transport likely with TRPM7, which might enable to sustain fetal bone mineralization.

Transient receptor potential (TRP) channels play important roles in thermal, chemical, and mechanical sensation in various tissues. In this study, we investigated the differences in urothelial TRP channels between normal urothelial cells and bladder cancer cells. TRPV2 and TRPM7 expression levels and TRPV2 activator-induced intracellular Ca(2+) increases were significantly higher, whereas TRPV4 expression and TRPV4 activator-induced intracellular Ca(2+) increases were significantly lower in mouse bladder cancer (MBT-2) cells compared to normal mouse urothelial cells. The proliferation rate of MBT-2 cells overexpressing dominant-negative TRPV2 was significantly increased. In contrast, treatment with TRPV2 activators significantly decreased the proliferation rate. TRPM7-overexpressing MBT-2 cells proliferated more slowly, as compared to mock-transfected cells. Moreover, expression of dominant-negative TRPV2 significantly decreased plasma membrane Ca(2+) permeability of MBT-2 cells as compared to that in mock-transfected cells. Increases in the expression of TRPV2 mRNA, immunoreactivity, and TRPV2 activator-induced intracellular Ca(2+) were also observed in T24 human bladder cancer cells. These results suggested that TRPV2 and TRPM7 were functionally expressed in bladder cancer cells and served as negative regulators of bladder cancer cell proliferation, most likely to prevent excess mechanical stresses.

The article Hypotonicity-induced cell swelling activates TRPA1, written by Fumitaka Fujita, Kunitoshi Uchida, Yasunori Takayama, Yoshiro Suzuki, Masayuki Takaishi and Makoto Tominaga, was originally published electronically on the publisher’s internet portal (currently SpringerLink) on 16 June 2017 without open access.

Transient receptor potential vanilloid 1 (TRPV1) is activated by elevated temperature (>42 °C), and it has been reported that cold temperature decreases capsaicin-induced TRPV1 activity. In contrast, transient receptor potential melastatin 8 (TRPM8) is activated by low temperatures and menthol, and heat stimulation suppresses menthol-evoked TRPM8 currents. These findings suggest that the effects of specific agents on TRPV1 and TRPM8 channels are intricately interrelated. We examined the effects of menthol on human (h)TRPV1 and of capsaicin on hTRPM8. hTRPV1 currents activated by heat and capsaicin were inhibited by menthol, whereas hTRPM8 currents activated by cold and menthol were similarly inhibited by capsaicin. An in vivo sensory irritation test showed that menthol conferred an analgesic effect on the sensory irritation evoked by a capsaicin analogue. These results indicate that in our study the agonists of TRPV1 and TRPM8 interacted with both of these channels and suggest that the anti-nociceptive effects of menthol can be partially explained by this phenomenon. Electronic supplementary material The online version of this article (doi:10.1007/s12576-015-0427-y) contains supplementary material, which is available to authorized users.