A Brain Cell in Your Ear

Deep inside our ears, there's a whole lotta wigglin' going on. With these microscopic wiggles, we hear. Without them, we'd be deaf.

The wiggling is done by the tops of "hair cells" which are covered by bristles that wave in the winds generated by a sound, be it the coo of a baby or the screech of an oncoming car. Hair cells turn the sound-induced to and fro into electricity, which is then sent to the brain, creating our sense of hearing.

But the hair cells we're born with are all we've got, and damage to them—either through loud noises like jackhammers or rock concerts, or through slower deteriorations from aging—is permanent. This means hearing loss is inevitable for a lot of us.

But a recent study published in the Proceedings of the National Academy of Sciences suggests that other cells may be coaxed to stand in for damaged hair cells. The key to restoring hearing might lie within our own brains.

Understudies in the brain

Figuring out how to replace damaged hair cells has occupied the careers of many scientists, who've pursued a number of clever strategies. Some have tried using stem cells, those primordial cells that, with proper handling, can be turned into different cell types. Others have looked toward birds, which do replace their hair cells. Still others have tried to bypass broken hair cells entirely with tiny machines called cochlear implants.

The new tack of this study is to start with cells that already look a little bit like hair cells, the idea being that this would make it easier for them to assume their new role.

"There might be some hidden treasures in the brain," says Ebenezer Yamoah, Ph.D., senior author of the study, which took place at the University of California at Davis.

Yamoah and his team found promising hair-cell replacements lining a fluid-containing compartment called a "ventricle" in brains of mice and humans. These cells in the ventricle lining have hair-like appendages that dangle down into the cerebral-spinal fluid, which circulates throughout the brain, helping to keep it healthy. Not only did these dangly parts remind the scientists of the hairs on top of hair cells, but they found that the ventricle cells contained some of the same proteins found in hair cells.

To explore whether they could stand in for hair cells, the scientists transplanted some of them into tissue taken from the inner ear, where hair cells normally live. Yamoah says they were "incorporated"—meaning rather than floating away or dying off, they dug in and took their place on the hair-cell stage.

In another test, the scientists put these ventricle cells into a petri dish with another cell type known as a spiral ganglion neuron. Normally these neurons touch hair cells, receiving their electrical impulses and passing them on to the brain. In the dish, these cells achieved the same type of neural handshake, and transmitted workable signals between them.

Seeds of hearing

Despite these similarities, Yamoah does not think these cells come out of the ventricle ready to go as hair cells. Instead, he surmises that the cells have to learn their new role once they are placed in the inner ear. In the neural equivalent of "When in Rome, do as the Romans do," the transplanted brain cells adapt to the culture around them, learning to become hair cells from different chemical and electrical signals in the inner ear.

"Once you transfer them into the inner ear, the extrinsic environment will sculpt them, will shape their function," says Yamoah.

And while looking and acting like a hair cell is promising, the real test will be whether these cells can really wiggle in the winds of sound and restore hearing.

Yamoah says those experiments are now underway in deaf mice, which have had their original hair cells removed. The scientists will "re-seed" the inner ears of the mice with the ventricle cells to see if they can restore any hearing. This will show whether these cells are reliable understudies or useless imposters.

As for feasibility in humans, Yamoah says that neurosurgeons do have the ability to go into the brain and take out these ventricle cells. Like any brain surgery, it's complicated, but the outcome—what he calls a "biological implant"—might be preferable to the engineered cochlear implant available to people now, which destroys all hair cells.

A biological implant could be a more refined way of restoring hearing, providing new cells only where needed, and leaving the working ones alone. This may be five years out, Yamoah ventures.

And just as an Italian would have an easier time of passing himself off as someone from Rome than, say, a Texan would, starting with cells that already resemble hair cells may hasten the recovery of hearing.

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After spending 15 years in the lab doing her own neuroscience research, Michele Solis is now putting her Ph.D. to work as a science writer. Her work covers a variety of topics including autism, linguistics, and animal communication. She contributes regularly to the Autism Speaks, Simons Foundation, and Crosscut Web sites.