The Role of Transient Receptor Potential A1 and G Protein-Coupled Receptor 39 in Zinc-Mediated Acute and Chronic Itch in Mice

Itching is a common symptom of many skin or systemic diseases and has a negative impact on the quality of life. Zinc, one of the most important trace elements in an organism, plays an important role in the regulation of pain. Whether and how zinc regulates itching is largely unclear. Herein, we explored the role of Zn2+ in the regulation of acute and chronic itch in mice. It is found that intradermal injection (i.d.) of Zn2+ dose-dependently induced acute itch and transient receptor potential A1 (TRPA1) participated in Zn2+-induced acute itch in mice. Moreover, the pharmacological analysis showed the involvement of histamine, mast cells, opioid receptors, and capsaicin-sensitive C-fibers in Zn2+-induced acute itch in mice. Systemic administration of Zn2+ chelators, such as N,N,N′,N′-Tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), pyrithione, and clioquinol were able to attenuate both acute itch and dry skin-induced chronic itch in mice. Quantitative polymerase chain reaction (Q-PCR) analysis showed that the messenger RNA (mRNA) expression levels of zinc transporters (ZIPs and ZnTs) significantly changed in the dorsal root ganglia (DRG) under dry skin-induced chronic itch condition in mice. Activation of extracellular signal-regulated kinase (ERK) pathway was induced in the DRG and skin by the administration of zinc or under dry skin condition, which was inhibited by systemic administration of Zn2+ chelators. Finally, we found that the expression of GPR39 (a zinc-sensing GPCR) was significantly upregulated in the dry skin mice model and involved in the pathogenesis of chronic itch. Together, these results indicated that the TRPA1/GPR39/ERK axis mediated the zinc-induced itch and, thus, targeting zinc signaling may be a promising strategy for anti-itch therapy.

Essential Oils from Monarda fistulosa: Chemical Composition and Activation of Transient Receptor Potential A1 (TRPA1) Channels

Little is known about the pharmacological activity of Monarda fistulosa L. essential oils. To address this issue, we isolated essential oils from the flowers and leaves of M. fistulosa and analyzed their chemical composition. We also analyzed the pharmacological effects of M. fistulosa essential oils on transient receptor potential (TRP) channel activity, as these channels are known targets of various essential oil constituents. Flower (MEOFl) and leaf (MEOLv) essential oils were comprised mainly of monoterpenes (43.1% and 21.1%) and oxygenated monoterpenes (54.8% and 77.7%), respectively, with a high abundance of monoterpene hydrocarbons, including p-cymene, γ-terpinene, α-terpinene, and α-thujene. Major oxygenated monoterpenes of MEOFl and MEOLv included carvacrol and thymol. Both MEOFl and MEOLv stimulated a transient increase in intracellular free Ca2+ concentration ([Ca2+]i) in TRPA1 but not in TRPV1 or TRPV4-transfected cells, with MEOLv being much more effective than MEOFl. Furthermore, the pure monoterpenes carvacrol, thymol, and β-myrcene activated TRPA1 but not the TRPV1 or TRPV4 channels, suggesting that these compounds represented the TRPA1-activating components of M. fistulosa essential oils. The transient increase in [Ca2+]i induced by MEOFl/MEOLv, carvacrol, β-myrcene, and thymol in TRPA1-transfected cells was blocked by a selective TRPA1 antagonist, HC-030031. Although carvacrol and thymol have been reported previously to activate the TRPA1 channels, this is the first report to show that β-myrcene is also a TRPA1 channel agonist. Finally, molecular modeling studies showed a substantial similarity between the docking poses of carvacrol, thymol, and β-myrcene in the binding site of human TRPA1. Thus, our results provide a cellular and molecular basis to explain at least part of the therapeutic properties of these essential oils, laying the foundation for prospective pharmacological studies involving TRP ion channels.

Targeted millisecond-scale activation of cells using non-invasive Sonogenetics

Our understanding of the nervous system has been fundamentally advanced by light- and small molecule-sensitive proteins that can be used to modify neuronal excitability. However, optogenetics requires invasive instrumentation while chemogenetics lacks temporal control. Here, we identify a candidate channel that confers sensitivity to non-invasive ultrasound on millisecond timescales. Using a functional screen, we find that human Transient Receptor Potential A1 (hsTRPA1) increases ultrasound-evoked intracellular calcium levels and membrane potentials. Ultrasound, but not agonist, -evoked, gating of hsTRPA1, requires the N-terminal tip region, intact actin cytoskeleton, and cholesterol, implicating these features in the sonogenetic mechanism. We then use calcium imaging and electrophysiology to confirm that ultrasound-evoked responses of primary neurons are potentiated by hsTRPA1. We also show that unilateral expression of hsTRPA1 in mouse layer V motor cortical neurons leads to ultrasound-evoked contralateral limb responses to ultrasound delivered through an intact skull. Finally, ultrasound induces c-fos in hsTRPA1-expressing neurons, suggesting that our approach can be used for targeted activation of neural circuits. Together, our results demonstrate that hsTRPA1-based sonogenetics can effectively and non-invasively modulate neurons within the intact mammalian brain, a method that could be extended to other cell types across species.

Non-electrophilic TRPA1 agonists, menthol, carvacrol and clotrimazole, open epithelial tight junctions via TRPA1 activation

Activation of the transient receptor potential A1 channel (TRPA1) by electrophilic agonists was reported to induce the opening of tight junctions (TJs). Because compounds that increase TJ permeability can be paracellular permeability enhancers, we investigated the effect of non-electrophilic TRPA1 activators, including food ingredients (menthol and carvacrol) and medication (clotrimazole), on epithelial permeability. We show that all three compounds induced increase of the permeability of fluorescein isothiocyanate-conjugated dextran (4 kDa) and decrease of transepithelial electrical resistance, accompanied by Ca2+ influx and cofilin activation in epithelial MDCK II monolayers. These phenotypes were attenuated by pretreatment of a TRPA1 antagonist, suggesting TRPA1-mediated opening of TJs. These results suggest that non-electrophilic TRPA1 activators with established safety can be utilized to regulate epithelial barriers.

Products of oxidative stress and transient receptor potential ankyrin A1 expression in the brainstem after lung ischemia–reperfusion injury

Lung ischemia–reperfusion injury is a common clinical concern. As the injury occurs, the pulmonary afferent nerves play a key role in regulating respiratory functions under pathophysiological conditions. The present study was to examine products of oxidative stress and expression of transient receptor potential A1 in the commissural nucleus of the solitary tract after lung ischemia–reperfusion injury; and further to determine molecular mediators linking to activation of oxidative stress and transient receptor potential ankyrin A1. A rat model of lung ischemia–reperfusion injury was used. Enzyme-linked immunosorbent assay and western blot analysis were employed to examine products of oxidative stress (i.e. 8-isoprostaglandin F2α and 8-hydroxy-2′-deoxyguanosine), and expression of transient receptor potential A1, Nrf2-antioxidant response element, and NADPH oxidase. 8-isoprostaglandin F2α and 8-hydroxy-2′-deoxyguanosine were amplified in the commissural nucleus of the solitary tract of lung ischemia–reperfusion injury rats, accompanied with downregulation of Nrf2-antioxidant response element, and upregulation of NOX4 and transient receptor potential A1. Blocking NADPH oxidase (subtype NOX4) decreased products of oxidative stress in the commissural nucleus of the solitary tract and attenuated upregulation of transient receptor potential A1 induced by lung ischemia–reperfusion injury. Our data revealed specific signaling pathways by which lung ischemia–reperfusion injury impairs Nrf2-antioxidant response and activates oxidative stress in the brainstem thereby leading to amplification of transient receptor potential A1 receptor likely via products of oxidative stress. Data suggest the abnormalities in the pulmonary afferent signals at the brainstem level which is likely to affect respiratory functions as lung ischemia–reperfusion injury occurs.