True Identity of Pivotal Hearing Structure Is Revealed
Editor: Think hearing is simple? Then you might want to think again.
Oh, and read this article. Turns out it's more complex than scientists
thought!
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September 2007
Our ability to hear is made possible by way of a Rube Goldberg-style
process in which sound vibrations entering the ear shake and jostle a
successive chain of structures until, lo and behold, they are converted
into electrical signals that can be interpreted by the brain. Exactly how
the electrical signal is generated has been the subject of ongoing
research interest.
In a study published in the September 6 issue of the journal Nature,
researchers have shed new light on the hearing process by identifying two
key proteins that join together at the precise location where energy of
motion is turned into electrical impulses. The discovery, described by
some scientists as one of the holy grails of the field, was made by
researchers at the National Institute on Deafness and Other Communication
Disorders (NIDCD), one of the National Institutes of Health (NIH), and the
Scripps Research Institute in La Jolla, CA.
"This team has helped solve one of the lingering mysteries of the
field," says James F. Battey, Jr., M.D., Ph.D., director of the NIDCD.
"The better we understand the pivotal point at which a person is able to
discern sound, the closer we are to developing more precise therapies for
treating people with hearing loss, a condition that affects roughly 32.5
million people in the United States alone."
When a noise occurs, such as a car honking or a person laughing, sound
vibrations entering the ear first bounce against the eardrum, causing it
to vibrate. This, in turn, causes three bones in the middle ear to
vibrate, amplifying the sound. Vibrations from the middle ear set fluid in
the inner ear, or cochlea, into motion and a traveling wave to form along
a membrane running down its length. Sensory cells (called hair cells)
sitting atop the membrane "ride the wave" and in doing so, bump up against
an overlying membrane. When this happens, bristly structures protruding
from their tops (called stereocilia) deflect, or tilt to one side. The
tilting of the stereocilia cause pore-sized channels to open up, ions to
rush in, and an electrical signal to be generated that travels to the
brain, a process called mechanoelectrical transduction.
Most scientists believe that the channel gates are opened and closed by
microscopic bridges-called "tip links"-that connect shorter stereocilia to
taller ones positioned behind them. If scientists could determine what the
tip links are made of, they'd be one step closer to understanding what
causes the channel gates to open. This is no easy feat, however, because
stereocilia are extremely small, scarce, and difficult to handle. Several
proteins had been reported to occur at the tip link in earlier studies,
but results have been conflicting to this point.
Cadherin 23 and Protocadherin 15 Unite to Form Tip Link
Using three lines of evidence, NIDCD scientists Hirofumi Sakaguchi,
M.D., Ph.D., Joshua Tokita, and Bechara Kachar, M.D., together with Piotr
Kazmierczak and Ulrich Müller, Ph.D., of Scripps Research Institute, and
other collaborators have demonstrated that two proteins associated with
hearing loss-cadherin 23 (CDH23) and protocadherin 15 (PCDH15)-unite and
adhere to one another to form the tip link. Mutations in CDH23 are known
to cause one form of Usher syndrome as well as a nonsyndromic recessive
form of deafness, and mutations in PCDH15 are responsible for another form
of Usher syndrome. (A syndrome is a disease or disorder that has more than
one feature or symptom, while the term "nonsyndromic" refers to a disease
or disorder that is not associated with other inherited characteristics.)
Usher syndrome is the most common cause of deaf-blindness in humans.
"Cadherin 23 and protocadherin 15 have been implicated in a variety of
forms of late- and early-onset deafness, and a whole range of mutations
can produce different outcomes," says NIDCD's Kachar, a co-senior
investigator on the study. "Now that we know how these two proteins
interact at the tip link, we can perhaps predict how different types of
hearing loss can take place depending on where a mutation is located."
Three Lines of Evidence
The researchers first created antibodies that would bind to and label
short segments on the CDH23 and PCDH15 proteins in the inner ears of rats
and guinea pigs. (Both proteins were identified at the tip link,
respectively, in earlier studies.) Using green fluorescence and electron
microscopy studies, they showed that CDH23 was located on the side of the
taller stereocilium and PCDH15 was present on the tip of the shorter one,
with their loose ends overlapping in between. The researchers were able to
identify both proteins, while earlier studies had not, because they
removed an obstacle to the antibody-binding process: calcium. Under normal
conditions, CDH23 and PCDH15 are studded with calcium ions, which prevent
antibodies from binding to the targeted sites. When calcium was removed
through the addition of a chemical known as BAPTA, both labels became
visible.
Next, the researchers built a structure resembling a tip link by
expressing the CDH23 and PCDH15 proteins in the laboratory and watching
how they interacted. When conditions were right, the two proteins wound
themselves tightly together from one end to the other in a configuration
that mirrored a naturally occurring tip link. The results were surprising,
since the scientific consensus had been that these proteins would not
interact at all. As with normal tip links, the structure thrived in
calcium concentrations that paralleled those found in fluid of the inner
ear, while a drastic reduction in calcium disrupted the structure.
Lastly, the scientists found that one mutation of PCDH15 that causes
one form of deafness inhibited the interaction of the two proteins,
leading them to conclude that the mutation reduces the adhesive properties
of the two proteins and prevents the formation of the tip link. In a
second mutation of PCDH15, the tip link was not destroyed; the scientists
suggested that the deafness is not likely caused by the breakup of the tip
link but by interference with its mechanical properties.
Knowing precisely the composition and configuration of the tip link,
scientists can now explore how these proteins interact with other
components to form the rest of the transduction machinery. In addition,
scientists can study how new treatments might be developed to address the
breaking up of tip links through environmental factors, such as loud
noise.
"Now that we understand what the tip link is made of and what
conditions are required to assemble it," says Kachar, "we can study what
it might take to rejoin tip links as a possible method for restoring
hearing in people with some forms of hearing loss that may have resulted
from disruption of the tip link."
Funding of the study was principally provided by the NIDCD. Other NIH
institutes and centers that contributed funding were the National
Institute of General Medical Sciences (NIGMS), the National Institute of
Arthritis and Musculoskeletal and Skin Diseases (NIAMS), and the National
Center for Research Resources (NCRR).
The 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
Agency-includes 27 Institutes and Centers and is a component of the U.S.
Department of Health and Human Services. It is the primary federal agency
for conducting and supporting basic, clinical, and translational medical
research, and it investigates the causes, treatments, and cures for both
common and rare diseases. For more information about NIH and its programs,
visit www.nih.gov.
