It was a matter of life or death. As 14-month-old Peter Steyger lay in a hospital bed stricken with bacterial meningitis, his parents were faced with a critical decision. Doctors could rescue the toddler with intravenous doses of the antibiotic streptomycin. However, that lifesaving treatment could have a lifelong consequence. For Steyger’s parents, the choice was an easy one. Their son received the antibiotic and lived to tell the tale. However, Steyger, who’s now in his early 40s, suffered hearing loss that he says has affected virtually every sector of his life, including his choice of career.
“What I’m doing [in my research] is explaining why I’m deaf,” says Steyger, who investigates hearing at Oregon Health and Science University in Portland. “That’s what drives me.”
Steyger’s research focuses on how drugs such as streptomycin enter and kill hair cells, the sensory cells in the inner ear that are pivotal to hearing. Each hair cell has a bundle of hairlike extensions that jut out from the inside surface of the cochlea, a snail-shaped coil of tissue deep in the inner ear. As sounds pass into the ear, their vibrations rumble cochlear fluid, which pushes the hair cells back and forth. These cells translate each sway into an electrical signal that travels through nerve fibers to the brain. There, the brain decodes these signals as hearing.
Normal aging and life’s daily cacophony cause some of the 16,000 hair cells that each person is born with to eventually wear out and die. The death of hair cells causes most instances of acquired hearing loss, although deafness can also result from other causes, such as glitches in nerves that connect the ear to the brain.
Traditionally, scientists have considered people’s hair cells—and good hearing—to be irreplaceable. However, genetics research, work with stem cells, and studies of the delicate architecture of the inner ear now suggest that it may be possible to replace lost hair cells and thus restore hearing. And Steyger’s research on how certain drugs damage hair cells may someday prevent others from facing a silent future.
“[Hearing loss] really has a profound effect on society,” says Jeffrey Corwin, who investigates hair cell regeneration at the University of Virginia in Charlottesville. “Now, I think it’s only a matter of when—not if—we or others in the field will succeed in getting the ability to bring hair cells back.”
Hair and now
Hair cell death doesn’t equal deafness for all animals. Most vertebrates—including sharks, chickens, and frogs—can grow new hair cells when old ones die. But at some point in evolution, Corwin notes, all mammals lost this innate replacement plan. Researchers haven’t completely figured out how other vertebrates regenerate hair cells in adulthood.
Mammalian hair cell growth during development, however, is fast and furious. For nearly a decade, Corwin and his team have been investigating which genes have different inner ear activity in the embryo than they do later in life. These genes, he surmises, may hold clues for prompting hair cells to grow.
“Early on, some genes really jumped out,” Corwin says. The genes that he and his team were most excited about seemed to regulate how inner ear cells divide or which of various cell types they became.
Eventually, he and his colleague Zheng-Yi Chen of Harvard University narrowed their focus to the gene called Retinoblastoma1 (Rb1). This gene is associated with a type of eye cancer, but the scientists found that Rb1 also turns on in the inner ear as mammals near birth. Sure enough, when Chen, Corwin, and their collaborators eliminated Rb1 in the inner ears of mice, the rodents’ hair cells continued to multiply long into adulthood. The team reported its results in the Feb. 18, 2005 Science.
Other scientists have had similarly promising results by examining some well-studied genes. For example, researchers have known since the late 1990s that a gene called Atoh1 (also Math1) seems to be critical early in development for prompting immature cells to become hair cells.
Yehoash Raphael of the University of Michigan in Ann Arbor and his colleagues reported in the March 2005 Nature Medicine that when they inserted Atoh1 in deafened adult guinea pigs’ ears, the animals regrew fully functioning hair cells (SN: 2/19/05, p. 115: Hearing Repaired: Gene therapy restores guinea pigs’ hearing). The research suggests that gene therapy may be a feasible treatment for hearing loss.
However, researchers estimate that hundreds or thousands of yet-undiscovered genes participate in hair cell formation. Chen, Corwin, and their team have already identified some prospective candidates.
Gone but not forgotten
Another approach to replenishing hair cells in the inner ear is to cultivate and transplant stem cells. Unlike most cells in the body, which have become set in their ways, stem cells act like wild cards. They can morph into other types of cells (SN: 4/2/05, p. 218: Full Stem Ahead). For example, stem cells in bone marrow can make several types of blood cells. Researchers have looked in the ear for stem cells that might produce new hair cells.
Stefan Heller of Stanford University and his colleagues reported 3 years ago that they had located hair cell-making stem cells in the vestibular organ, a part of the inner ear that’s located near the cochlea. The hair cells in the vestibular organ aren’t used to hear but to sense acceleration changes to help an animal keep its balance.
Other researchers had previously reported that the vestibular organ has a limited capacity to regenerate hair cells. Heller says that the newfound stem cells are probably responsible for this effect.
He and his team are currently working to determine whether harvesting these stem cells from the vestibular organ and transplanting them into the cochlea could replace hair cells lost there. Heller also announced last November at the Society for Neuroscience Meeting in Washington, D.C., that his team has found evidence that the adult cochlea itself harbors stem cells. “There’s probably some regenerative capacity that exists in the cochlea, but we think it’s much more hidden, much harder to unlock,” says Heller. “We’re taking cues from our previous study to try to find whether we can identify these stem cells in the cochlea and unlock their potential.”
Following a pattern
Even if scientists succeed in regenerating hair cells in the cochlea, it’s unclear whether simply restoring these cells would return hearing to an animal that’s lost it. According to Matthew Kelley of the National Institute on Deafness and Other Communication Disorders (NIDCD) in Bethesda, Md., the exact positions of hair cells are critical to whether they function effectively.
Unlike other vertebrates’ hair cells, which spring up haphazardly across a wide swath of the cochlea, hair cells in people and other mammals grow in an extremely orderly fashion. A line of inner hair cells, which transmit sound signals to the brain, snakes from the cochlea’s base through each of its spiral turns. This line is flanked on one side by three rows of outer hair cells, which seem to amplify the sound waves that reach the inner cells.
The hairs in the bundles that top both the inner and outer hair cells are stacked like staircases, and all the stacks face in the same direction. Surrounding each hair cell are four supporting cells, which recycle the ions that hair cells use to send their electrical messages.
Researchers have long suspected that this strict order underlies mammals’ capacity to hear better than other vertebrates do. “But how these cells sort and arrange themselves in such a perfect pattern remains a very intriguing question,” notes Kelley.
Two years ago, Kelley and his colleagues reported that the hair cells themselves seem to recruit surrounding support cells. The researchers employed Atoh1, the same gene that Raphael used in his study of deaf guinea pigs. The team slipped Atoh1 into cochlear tissue called the greater epithelial ridge, which doesn’t normally have any hair or supporting cells. The researchers weren’t surprised to see hair cells begin to sprout. However, within several days, they noticed that cells around each hair cell were morphing into supporting cells.
“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.
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.
This suggests that ototoxic drugs could enter cells through ion channels, pores through which calcium and other ions cross the cell membrane, the team concluded in the June 2005 Hearing Research.
Steyger’s research has now moved from cells growing in the lab to those in living bullfrogs, chicks, guinea pigs, and mice. This March, the group reported in Hearing Research that it has tracked the dye-tagged antibiotic as it collected in and killed hair cells and kidney cells in living animals.
By studying animals, scientists may develop a drug to give before or with ototoxic drugs to close the ion channels and prevent the drugs from collecting in and killing the cells, Steyger says.
He adds: “Prevention is better than a cure—a stitch in time saves nine, or so they say.”