St. Jude Study Solves Mystery of Mammalian Ears

Editor: Scientists at St. Jude Children's Research Hospital have answered a long-standing question about mammalian hair cells. Here's their press release.

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Protein motor in cochlea hair cells dominates the process of sound amplification in the mammalian ear, while movement of the cilia atop those cells dominates the response in non-mammals

A 30-year scientific debate over how specialized cells in the inner ear amplify sound in mammals appears to have been settled more in favor of bouncing cell bodies rather than vibrating, hair-like cilia, according to investigators at St. Jude Children's Research Hospital.

The finding could explain why dogs, cats, humans and other mammals have such sensitive hearing and the ability to discriminate among frequencies. The work also highlights the importance of basic hearing research in studies into the causes of deafness. A report on this work appears in the advanced online issue of "Proceedings of the National Academy of Science."

"Our discovery helps explain the mechanics of hearing and what might be going wrong in some forms of deafness," said Jian Zuo, Ph.D., the paper's senior author and associate member of the St. Jude Department of Developmental Neurobiology. "There are a variety of causes of hearing loss, including side effects of chemotherapy for cancer."

The long-standing argument centers around outer hair cells, which are rod- shaped cells that respond to sound waves. Located in the fluid-filled part of the inner ear called the cochlea, these outer hair cells sport tufts of hair- like cilia that project into the fluid. The presence of outer hair cells makes mammalian hearing more than 100 times better than it would be if the cells were absent.

In mammals, the rod-shaped body of the outer hair cell contracts and then vibrates in response to the sound waves, amplifying the sound. While both mammals and non-mammals have cilia on their outer hair cells, only mammalian outer hair cells have prestin, a protein "motor" that drives this cellular contraction. This contraction pulls the tufts of cilia downward, maximizing the force of their vibration. In mammals, both the cilia and the cell itself vibrate. Thus far the question has been whether the cilia are the main engine of sound amplification in both mammals and non-mammals.

In the study, Zuo and his team conducted a complex series of studies that showed in mammals that the role of somatic mobility driven by prestin is not simply to modify the response of the outer hair cells' cilia to incoming sound waves in the cochlea fluid. Instead, somatic motility itself appears to dominate the amplification process in the mammalian cochlea, while the cilia dominate amplification in non-mammals.

Other authors of this study include Jiangang Gao, Xudong Wu and Manish Patel (St. Jude); Xiang Wang, Shuping Jia and David He (Creighton University, Omaha, Neb.); Sal Aguinaga, Kristin Huynh, Keiji Matsuda, Jing Zheng, MaryAnn Cheatham and Peter Dallos (Northwestern University, Evanston, Ill.).

This work was supported in part by ALSAC, The Hugh Knowles Center and the National Institutes of Health.

St. Jude Children's Research Hospital