Ear's electrical missing link found

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BY BRYN NELSON
STAFF WRITER

October 14, 2004


A tiny portion of the inner ear has apparently ensured that we're wired for sound.

Researchers have long sought the missing link in the ear's conversion of sound waves into electrical signals that the brain can recognize as distinct sounds. In a study published yesterday in an online edition of the journal Nature, Harvard Medical School neurobiologist David Corey and 15 collaborators found a prime candidate in a tiny channel located at the tips of the inner ear's unusual hair cells - a channel shared among humans, mice, fish and even fruit flies.


"It's something we've been looking for, for 20 years," said Corey.

The gene for the newly identified channel, known as TRPA1, adds an important suspect to the list of three dozen genes linked to hearing loss, and the discovery may aid the search for treatments for some of the more than 300 known forms of inherited deafness. Corey said his group is already collaborating with other Harvard researchers to screen families with inherited deafness for defects in the TRPA1 gene.

The remarkable, almost Rube Goldberg-like progression of sound through the human ear leads to a snail-like structure in the inner ear known as the cochlea.

Lining the cochlea's inward spiral is a very narrow and very long ribbon of hair cells. Scientists have named these hair cells for their tufts of up to 300 microscopic cilia that sway back and forth in response to vibrations, almost like sea anemones waving in the ocean. Tiny strings linking the hairs together loosen and tense in response to the movement, Corey said, effectively opening and closing small channels.

When open, the channels allow ions like potassium to rush in and create an electrical voltage, creating a signal that zips from connected neurons to the brain, which then perceives the signal as sound.

But researchers couldn't pinpoint the agent of the transformation from sound vibrations to nerve impulses until the completed DNA sequences of fruit flies, mice and humans revealed candidates in a family of related ion channels. In mice and frogs, molecular probes revealed that one candidate protein localized to the tips of the hair cell's cilia, where scientists knew the mystery channel ought to be. And when the researchers blocked production of that protein, TRPA1, hair cells in both zebrafish and mice no longer responded to mechanical vibrations.

Dr. John S. Oghalai, an assistant professor of otolaryngology at Baylor College of Medicine in Houston who wasn't involved in the study, hailed it as "a major discovery."

Although researchers suspected that the inner ear's hair cells contained such a channel, Oghalai said its identification could mark the beginning of a new search for inherited deafness treatments. Since the channel acts as a sensor that converts vibrations to electrical signals, Oghalai said it also could prove "revolutionary" for mechanical engineering applications that require a similar conversion process.
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