Scientist have long believed that the protein structures which make hearing possible remained static throughout a lifetime—but new research out of Case Western Reserve University School of Medicine suggests that the opposite is actually true.
Hearing happens when sound waves strike bundles of cells inside the inner ear, which translate the sound energy into brain signals. These bundles are made of individual cells known as stereocilia—hair-like projections that, like many brain cells, stay with us our entire lives.
Which, of course, can be problematic. For example, if you listen to music that is too loud, the stereocilia get whipped by the soundwaves, often damaging them permanently—or so we thought.
Potential for repair
However, as reported in the cover paper of November 17th’s Cell Reports, the researchers at Case Western have discovered that the proteins within each hair cell change and circulate, possibly allowing for repair and renewal of each stereocilium—a potential game-changer for hearing loss therapy in the future.
The scientists used tiny animals known as zebrafish—some of the most important animal models for the study of human genomics—to examine the motion of proteins within live ear cells.
“What was surprising in our research with zebrafish is that proteins move so rapidly, implying that protein movement may be required to maintain the integrity of hair bundles in the inner ear,” explained senior author Brian McDermott, PhD, associate professor of Otolaryngology-Head and Neck Surgery at Case Western Reserve University School of Medicine, in a statement.
“Our research tells us that constant movement, replacement and adjustment among proteins in the inner ear’s hair bundles serve a maintenance and even repair function.”
This constant, surprisingly fast motion could explain just how stereocilia can last a lifetime without being entirely destroyed. The researchers were able to view it live using a confocal microscope on the inner ears of baby zebrafish. Baby zebrafish are completely transparent—meaning the workings within could be seen without dissection.
A lifetime of hearing
“We made movies of the secret inner workings of the hair bundle in a live animal, and what is happening in the ear is amazing and unexpected,” McDermott said.
The scientists discovered that within these stereocilia cells, proteins that important for cell movement—known as actin and myosin—move throughout the cells over the course of a few hours. However, a different kind of protein known as fascin 2b moves even faster—in just a few seconds. All of which means that the proteins that control the structure of stereocilia are far from stationary, turning over quickly and likely renewing many of the cells over time.
“It was once thought that most everything within the stereocilia was relatively immobile and static,” McDermott said. “We found that the constant, dynamic movement likely contributes to the permanency of the hair bundle structure to last a lifetime, or 70 to 90 years in human terms.”
As to how a hair cell might repair itself, that what McDermott’s team is looking to figure out next.
“No one has shown that stereocilia heal themselves, so next we will find out if there is a repair mechanism,” he said. “People go deaf from damage to stereocilia in the inner ear. If we figure out ways of manipulating protein dynamics within stereocilia to heal this damage, we would likely be able to diminish hearing loss in humans.”
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Image credit: Thinkstock
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