Protein Tied to Usher Syndrome May Be Hearing’s ‘Missing Link’
A protein associated with a disorder that causes deafness and blindness in
people may be a key to unraveling one of the foremost mysteries of how we hear,
says a study in the June 28 issue of the Journal of Neuroscience. Scientists
with the National Institute on Deafness and Other Communication Disorders (NIDCD),
one of the National Institutes of Health (NIH), and the University of Sussex,
Brighton, United Kingdom, have identified protocadherin-15 as a likely player
in the moment-of-truth reaction in which sound is converted into electrical signals.
(Protocadherin-15 is a protein made by a gene that causes one form of type 1
Usher syndrome, the most common cause of deaf-blindness in humans.) The findings
will not only provide insight into how hearing takes place at the molecular level,
but also may help us figure out why some people temporarily lose their hearing
after being exposed to loud noise, only to regain it a day or two later.
“These findings offer a more precise picture of the complicated processes involved
with our sense of hearing,” says Elias A. Zerhouni, M.D., director of the NIH. “With
roughly 15 percent of American adults reporting some degree of hearing loss,
it is increasingly vital that we continue making inroads into our understanding
of these processes, helping us seek new and better treatments, and opening the
doors to better hearing health for Americans.”
Tapping Your Inner ‘Tip Link’
Researchers have long known that hair cells, small sensory cells in the inner
ear, convert sound energy into electrical signals that travel to the brain,
a process called mechanotransduction. However, the closer one zooms in on the
structures involved, the murkier our understanding becomes. When fluid in the
inner ear is set into motion by vibrations emanating from the bones of the
middle ear, the rippling effect causes bristly structures atop the hair cells
to bump up against an overlying membrane and to deflect. Like seats in a three-row
stadium, the bristles, called stereocilia, are arranged in tiers, with each
lower seat connected to a higher seat by minute, threadlike bridges, or links.
As the stereocilia are deflected, pore-like channels on the surface of the
stereocilia open up, allowing potassium to rush in, and generating an electrical
signal. Because the “tip link” — the link that connects the tip of the shorter
stereocilium to the side of the adjacent, taller stereocilium — must be present
for the channel to function, scientists believe that this structure may be
responsible for opening and closing the channel gate. Researchers suggest that
if they can learn the makeup of the tip link, they’ll be that much closer to
understanding how the gate mechanism operates.
“This research identifies protocadherin-15 to be one of the proteins associated
with the tip link, thus finally answering a question that has been baffling researchers
for years,” says James F. Battey, Jr., M.D., Ph.D., director of the NIDCD. “Thanks
to the collaborative effort among these researchers, we are now at the closest
point we have ever been to understanding the mechanism by which the ear converts
mechanical energy — or energy of motion — into a form of energy that the brain
can recognize as sound.”
NIDCD’s Zubair M. Ahmed, Ph.D., and Thomas B. Friedman, Ph.D., together with
the University of Sussex’s Richard Goodyear, Ph.D., and Guy P. Richardson, Ph.D.,
and others used several lines of evidence to identify a protein that Drs. Goodyear
and Richardson had earlier found to comprise tip links in the inner ears of young
chicks. The protein is referred to as the “tip-link antigen” (TLA) because it
induces the production of special antibodies, which bind to the protein at the
stereocilia tips.
Using mass spectrometry, a laboratory technique that breaks down a substance
into its individual components, the researchers analyzed the makeup of the TLA
and found two peptide sequences that match up to key segments of the protein
protocadherin-15 in humans, mice, and chickens, suggesting that the two proteins
are evolutionarily comparable. Additional experiments using western blot analysis,
a technique that identifies individual proteins in a substance by separating
them from one another by mass and testing how they react to certain antibodies,
demonstrated that the antibody that recognizes protocadherin-15 in mice also
binds to the TLA.
The team also analyzed the amino acid sequences of protocadherin-15 and discovered
four distinct forms — three of which are present in various developmental stages
of the mouse inner ear. The researchers refer to the three alternative forms
found in the inner ear as CD1, CD2, and CD3 because the sequential variations
occur in the protein’s “cytoplasmic domain” — a stretch of amino acids anchored
inside the stereocilium. (The fourth form, referred to as SI, is likely to be
secreted.) With the help of two imaging techniques that use antibodies to label
a targeted protein, the team found that the distribution of protocadherin-15
along the stereocilium varies by form, with the CD3 form stationed only at the
tips of the stereocilia in mature hair cells, while the CD1 form is found along
the lengths of the stereocilia in mature cells, but not at the tips. In contrast,
the CD2 form is expressed along the lengths of stereocilia during hair cell development,
but is not present in mature hair cells.
Finally, the team found that a chemical known to break tip links — called BAPTA — had
no effect on the CD1 and CD2 forms of protocadherin-15 but destroyed the CD3
form. Likewise, just as tip links are known to reappear roughly four hours after
the chemical is removed, the CD3 form returned within four to 24 hours upon removal
of the chemical.
Based on these findings, the researchers conclude that, not only is protocadherin-15
now identified as the tip-link antigen, but it is distributed in a specific way
in relation to the tip-link complex. They propose that the CD3 form of protocadherin-15,
located at the tip of the shorter stereocilium, may link directly or indirectly
to the CD1 form on the adjacent, taller stereocilium. This scenario could help
explain how tip links that are broken in real-life situations, such as from excessive
exposure to loud noise, could cause temporary hearing loss until the link re-establishes
itself and hearing is restored.
In future studies, the scientists plan to delve more deeply into the role that
protocadherin-15 plays in the tip-link complex and whether it interacts with
other components in the formation of the tip link. They also hope to determine
how tip links can be stimulated to re-form, once broken.
The work was supported by the NIDCD and The Wellcome Trust, London, UK. Other
researchers on the project represent the NIH’s National Human Genome Research
Institute, Bethesda, MD; University of Cambridge, UK; Brigham Young University,
Provo, UT; the National Centre of Excellence in Molecular Biology, Lahore, Pakistan;
and the University of Kentucky, Lexington.
![](images/OHC_stereocilia_illustratio.jpg) |
Above: Stereocilia are arranged in three tiers atop a hair cell; Inset: Tip links connecting shorter stereocilia to their taller neighbors
Reproduced with permission from Nature
Reviews Genetics 5, 489–98 copyright 2004 Macmillan Magazines
Ltd. |
NIDCD supports and conducts research and research training on the normal
and disordered processes of hearing, balance, smell, taste, voice, speech and
language and provides health information, based upon scientific discovery,
to the public. For more information about NIDCD programs, see the Web site
at www.nidcd.nih.gov.
The National Institutes of Health (NIH) — The Nation's Medical Research
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