Published On August 3, 2016
DEEP INSIDE OUR EARS, IN A REGION CALLED THE COCHLEA, hair bundles are neatly arranged like hundreds of tiny antennae. Sound waves around us cause these to move, and that in turn creates signals for the brain to process. Each bundle is made up of several dozen long, thin projections packed together like pencils in a pencil holder, and the bundles are kept tidy by tiny proteins that look like staples.
All vertebrate animals use such bundles at the top of hair cells, in a wide range of variations, to sense their environments. And damage to those hair bundles occurs regularly. In humans, ear-splitting noises can literally rip apart the staples that hold hair bundles together. Unless the body can repair them quickly, the cells die, and can no longer perform their signature task.
One animal, it turns out, excels at such repair. “The sea anemone has an unrivaled ability to repair its hair cells,” says Glen Watson, a biology professor at the University of Louisiana at Lafayette, who has studied the animals for three decades. And researchers are hopeful that these aquatic invertebrates may offer a clue about how to repair hair bundles in human ears.
For the sea anemone, hair bundles are critical to survival. The creatures hunt with microscopic weapons called stinging cells that use a hair bundle as radar. Just as the bundles in our ears detect frequencies of sound, sea anemone hair bundles detect vibrations of swimming zooplankton.
The stimulated hair bundles act as an early warning system that predisposes the anemone to attack. When prey bumps into a tentacle, the contact trips a biochemical switch that generates a whopping 150 atmospheres of pressure inside the stinging cell. The pressure releases a poisoned dart that explodes with a rate of acceleration 5 million times that of gravity. It’s one of the fastest known motions in any animal.
A sea anemone’s hair bundles are often damaged during the struggle to subdue prey. But not only do they repair themselves, they do it surprisingly fast.
Watson discovered that the secret to anemones’ hair bundle repair is a cocktail of proteins contained in a mucus that they release into the water around them. Contact with those proteins hastens hair bundle repair for anemones from about four hours to just eight minutes.
Watson and his team found that the tonic also works on other aquatic creatures. When they put anemone proteins on blind cave fish, it sped damaged hair bundle repair from nine days to one hour.
In a study just released in Journal of Experimental Biology, Watson and his coauthors, Pei-Ciao Tang and Karen Müller Smith, have now tested whether the anemone repair proteins could make the taxonomic leap to mammals. They soaked damaged mouse cochlea in media enriched with the anemone repair proteins. After an hour, damaged hair bundles regained their normal appearance. They also measured the repaired cells’ behavior, using a fluorescent dye that enters hair bundles when hair cell channels are properly functioning—the brighter the glow, the greater the cell performance. Results were nearly identical to that of healthy controls.
Thirty-seven repair proteins have been identified in the anemone mucus. The major components include something called a heat shock protein (HSP70), which is known to assist misfolded proteins in regaining their proper shape, and a large proteasome complex (20S), which breaks down damaged proteins. In the absence of either of those proteins or protein complexes, hair bundle repair doesn’t happen.
The researchers believe that the repair proteins help the hair cells replace or refold a protein critical to the “staple” part of hair bundles, cadherin-23. Cadherins as a class of protein are often found holding cells together.
But it will be some time before researchers find how to test the substance on humans. The inner ear is remote, for good anatomical reason, keeping the delicate complex out of harm’s way. That remoteness, however, makes it difficult to apply treatments directly.
While researchers puzzle out the next steps toward a possible therapy, Watson’s research raises another question: If the biochemical tools to repair hair bundles already exist, why doesn’t the human body put them to use?
Watson speculates that here, too, the remote location of the cochlea may play a role. During evolution, when hair cells moved inside a protected space, they were less prone to physical damage. Over time, our biochemical tools to repair hair bundles grew rusty. It wasn’t until recently in human history, with the advent of guns, machinery and earbuds, that deafening sounds became a constant danger.
“It underscores this idea of evolution, that once nature creates a successful way to do something it doesn’t need to make a new one as it goes along,” Watson says. “It’s mind-blowing that the tool— repair proteins—from the anemone still fits the broken part in a mammalian hair cell to make the machinery work,” Watson says.
If researchers can translate Watsons’ research into human treatment, the benefits could be enormous. Approximately 26 million Americans suffer from noise-induced hearing loss. While the repair proteins wouldn’t be able to restore long-dead cells, they would offer great promise for fixing living cells soon after damage—something particularly important for soldiers who suffer hearing loss on the battlefield.
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