In the search for a molecule that might form the hair cell channel, the researchers turned to a family of ion channels known as TRPs, or transient receptor potential channels. "We thought the TRP family was a likely place to look for this channel, because many other TRP channels are involved in sensory transduction," Corey said. "In mammalian pheromone receptors, insect vision, hearing in flies, or touch in worms there were a lot of other TRP channels that seemed to be sensory." In addition, the selectivity and conductance of TRPs corresponded to what was already known about the unidentified hair cell channel.
TRPA1 was a particularly good candidate within this family, Corey said, because it has an architecture similar to a TRP channel that is mechanosensory in the fruit fly a molecule known as NOMPC, identified by HHMI investigator Charles S. Zuker. In other parts of the nervous system, however, the TRPA1 channel is activated by such stimuli as painfully cold temperatures and pungent chemicals like mustard, cinnamon oil, and wasabi, which seemed incompatible with mechanical activation in the ear.
The distribution of TRPA1 was one of the first clues that TRPA1 might indeed be the channel the researchers were searching for. They found that the TRPA1 gene was expressed in the inner ear of the mouse, including in the hair cells. Not only was TRPA1 in the right place to be the channel they were searching for, Corey said, it was also there at the right time. When the researchers tested for TRPA1 gene expression in developing mouse embryos, they found that the gene became active when the embryo was about 16 days old just a day before hair cells become mechanically sensitive.
Next, the scientists looked for TRPA1 within the hair cell. In frogs and mice, a fluorescently-tagged TRPA1 antibody
Contact: Jennifer Michalowski
Howard Hughes Medical Institute