Researchers at Harvard Medical School and their colleagues report in the October 13 Nature advanced on-line edition that they have identified a protein deep in the inner ear that they believe translates sound into the nerve impulses used by the brain.
"People have been looking for this protein for a decade," says David Corey, HMS professor of neurobiology and an investigator of the Howard Hughes Medical Institute. Other protein candidates have been nominated, but this is "the strongest evidence yet that this protein is the hair-cell transduction channel," says Corey, lead author of the paper.
The discovery could help scientists investigate normal hearing and inherited forms of deafness, which typically involve other protein pieces of the same acoustic apparatus, says Corey, also co-director of the HMS Center for Hereditary Deafness.
"This is the most important molecule in the ear," said Peter Gillespie, a neurobiologist at Oregon Health & Science University who recently has helped identify important parts connecting to either side of the channel. "This channel is the jewel everyone would like to find. Identifying it is getting at the real kernel of how the inner ear works."
The protein, TRPA1 (pronounced TRIP-AY-ONE), is located at the tips of specialized cilia on hair cells of the inner ear. Scientists believe the protein forms pores that open and close in sync with sound waves, allowing ions to flow into the cells and to transform the vibrations into electric signals. The same protein channel also may help people distinguish between tones of different frequencies.
Sound travels through the auditory system like a message relayed through the jungle from drum to drum. Snippets of conversation or the roar of a leaf blower are collected by the fleshy outer part of the ear and funneled inside where a delicate percussion section vibrates, taps and shivers.
The key elements in converting sound into nerve impulses are the bundles of cilia that protrude from the tops of hair cells and give them their name. Inside the cochlea, the stiff cilia bend at their bases when the pulsing sound waves push against them thousands of times a second. Small protein strings called tip links connect the tip of each cilium with its taller neighbor. (Six months ago, other researchers discovered the molecular identity of the tip links.) With each vibration, the bending cilia pull on the links connecting them, yanking open the channels to allow ions to flood into the cilia. The resulting voltage change activates the conversion of sound to a nerve signal. Then, the cilia bend back and ion channels snap shut.
"Hair cells convert a mechanical stimulus into an electrical signal with molecular, strings, springs and levers," Corey says. "It's cell biology, but it's wonderfully mechanical as well."
In their paper, Corey and his colleagues present evidence that the mysterious ion channel is actually TRPA1. The TRP proteins are a trendy new family of ion channels involved in sensory perception. Different TRP proteins help insects see and hear, mammals taste and sense heat and pheromones. A small clan known as TRPN help fruit flies sense touch and fish hear.