Now hear this: new research aims to restore lost hearing

Science News, May 20, 2006 by Christen Brownlee

"We demonstrated the first sign of self-organizing ability in the inner ear. These hair cells took cells that wouldn't have been supporting cells and pushed them into a supporting role," says Kelley.

He and his colleagues are currently studying whether a hair cell's reach extends beyond the supporting cells--for example, whether hair cells can coax other cells to become hair cells.

Kelley's team is also investigating how each hair cell bundle grows to face the same way. "Having a hair cell that's not oriented properly won't work. It's not much better than having no hair cells at all," says Kelley.

Kelley and his colleagues made a breakthrough discovery 3 years ago. They observed that two types of mutant mice with abnormal curls in their tails had randomly oriented hair cell bundles. Eventually, the researchers found that the mice had mutations in one of two genes. The mice called circle-tail had a mutated scribble1 gene, and the loop-tail mice had a mutated vangoghlike2.

When researchers work out the function of these genes, they may discover how hair cells orient themselves, says Kelley.

He adds that each new piece of information about hair cells that researchers acquire puts them closer to rebuilding a fully functional inner ear. "It's not enough to say that we got some function back. We need to be able to say that we can do better with regeneration than people can do with a cochlear implant" he says. Currently the most advanced treatment available for restoring hearing, the device is most effective when implanted in young children.

HAIRY ISSUES Such work will prove invaluable to people who lose their hearing when hair cells die. But for people such as Steyger, who are forced to accept hearing loss as an unavoidable consequence of a life-saving antibiotic, it would be better to prevent the damage in the first place.

Hair cell-killing, or ototoxic, antibiotics are used against a variety of life-threatening infections. These drugs leach from the blood into the cochlear fluid, collect inside hair cells, and at high concentrations, kill them.

It's unclear, says Steyger, how ototoxic drugs get inside hair cells. Some scientists have suggested that most cells take up the drugs through a process called endocytosis. The cells form tiny packets around the chemical to pull it inside.

However, Steyger and his colleagues reported in the June 2005 Hearing Research that hair cells don't use only endocytosis to take in drugs. The researchers worked with an ototoxic drug, called gentamycin, labeled with a red dye. They added the drug to lab dishes of kidney cells, which are less fragile than hair cells but can also be damaged by drugs. The result was surprising: The cells snapped up the dyed drug within seconds.

"Endocytosis is a time-consuming process. It can last from minutes to several hours" says Steyger. "Since we were seeing uptake within 10 seconds after application of the drug, we knew that endocytosis couldn't be the only process involved."

Another study by Steyger and his colleagues gave some hints of how gentamycin entered the cells. When the researchers added a solution rich in calcium ions to kidney cells just before they added the drug, only small amounts of gentamycin made it into the cells.