Feeling the burn: Towards a determination of the molecular mechanisms of activation of TRPA1.

摘要

Members of the TRP family of non-selective cation channels are activated in response to different temperature ranges and are implicated in temperature and pain sensation. Many of these "thermoTRPs" also respond to natural compounds, which produce a burning, cooling, or prickling pungency via a sensory modality known as chemesthesis. TRPA1 is activated by noxious cold temperatures and by a variety of botanical compounds such as allyl isothiocyanate (mustard oil, MO) and cinnamaldehyde (cinnamon oil, CA) that convey a burning sensation in the mouth and nose. TRPA1 is also activated by environmental irritants such as acrolein. Recent gene ablation studies in mice have shown that TRPA1 is required for the nocifensive properties of mustard oil and acrolein. How such diverse stimuli activate TRPA1 is not known. I have identified another pungent natural compound found in fresh garlic extracts which activates TRPA1. Allicin, a compound present only in fresh but not baked garlic extracts, activates TRPA1 in vitro and is likely responsible for raw garlic's prickling pungency. Other natural compounds involved in chemesthesis including menthol and camphor (activators of related thermoTRP channels TRPM8 and TRPV3) can inhibit the activation of TRPA1 by cold or cinnamaldehyde. I characterized the promiscuity of various sensory compounds with thermoTRPs using calcium imaging and electrophysiology techniques. Using a combination of calcium imaging, electrophysiology, mutagenesis, and biochemistry, I ascertained the mechanism of activation of TRPA1 by various pungent natural compounds. Direct binding of electrophilic compounds to cysteine residues in TRPA1 causes activation of the channel. This activation is persistent and dependent on cysteine binding. While TRPA1 is not unique in its ability to bind these noxious reactive compounds, it is uniquely activated in response to this binding event, quickly signaling damage via the pain pathway. These studies indicate that TRPA1 plays an important role in sensing cellular damage and producing a pain signal.