Hudspeth studies the tiny bundles of hair, buried deep inside our ears, that translate sound waves generated by mechanical forces — the drawing of a bow across strings, the crashing of a car through a window — into electrical signals that can be processed by the brain. New research in his lab has now identified a gene critical to the process by which these hairs develop.

To better understand hearing, Hudspeth’s Laboratory of Sensory Neuroscience has long studied bullfrogs, whose extra-large ear hairs are well suited for analysis. In recent years, however, Hudspeth’s team has also been focusing on zebrafish, which are excellent for genetic studies. The scientists wanted to find a deaf fish that would lead them to key genes and molecules involved in hearing.

Six years ago, Hudspeth, who is the F. M. Kirby Professor and director of the F. M. Kirby Center for Sensory Neuronscience, and Catherine Starr, then a graduate student in Hudspeth’s lab, began the painstaking process of screening thousands of genetically mutated zebrafish. To find out whether or not the fish could hear, Starr repeatedly tapped on the sides of their tanks. Normal zebrafish will swim away from the sound. Starr was looking for a fish that did not respond.

When she eventually found one, Starr paired the deaf fish with a normal fish, and then used genetic mapping techniques to identify a mutant gene in deaf offspring. Because the mutant gene is similar to a human gene called Choroideremia, Starr named her fish gene choroideremia, or chm.

To confirm she’d found the right gene, Starr injected the RNA product of the chm gene into fertilized fish eggs. She found that 50 percent of genetically mutant fish, which would normally have ended up deaf, were “rescued” by the chm RNA. Like normal fish, they quickly swam away when their container was tapped.

Starr also created chm knockout zebrafish by injecting modified anti-chm RNA snippets called morpholinos into eggs of normal zebrafish. The offspring responded poorly to sound and had fewer hair cells in their hearing organs.

Starr’s zebrafish are the first animal models for studying a human disorder called choroideremia, but the immediate implications are less for hearing, and more for sight. “In humans, none of the patients with mutations in this gene had hearing problems,” Starr says.

Mutations to the human Choroideremia gene lead instead to a disorder characterized by degeneration of the choroid and retina, portions of the eye that provide nourishment and light sensitivity. People with the genetically inherited disorder progressively lose their vision, beginning with a ring of irregular sight that gradually expands both toward the center and toward the periphery of their fields of view.

The human Choroideremia gene codes for Rab escort protein 1 (REP1), a well-studied molecule that plays a key role in the transport of other proteins to their intended destinations within cells. “The next step is to find out how REP1 affects hearing and vision,” Others in Hudspeth’s lab are investigating that, as well as related questions. Former graduate student James Kappler, for instance, has examined the physical differences between Starr’s deaf fish and fish with normal hearing. He immersed live zebrafish larvae in a fluorescent dye that is absorbed by hair cells in their hearing organs, called neuromasts. His findings: normal zebrafish larvae show 30 spots representing fluorescent neuromasts on one side of the body, while mutant larvae showed only two.

Meanwhile, Avani Sinha, a high school student, looked at the hearing organs of larvae under a scanning electron microscope and saw that normal larvae had about 15 hair cells in each neuromast while mutant neuromasts had only one or two, if any.

“Some people who are deaf have no hair cells in their ears. Some develop hair cells fine, but the ionic balance in their ear is off, so the cells can’t signal,” says Starr. “The same is true of fish.”

In fact, there is another deaf fish in Hudspeth’s lab that can’t hear even though it has hairs in its hearing organs. “That mutant is in the works,” Starr says.

July 16, 2004