Molecules in the Ear That Convert Sound Into Brain
Signals
Recently Scientists Identify Molecules in the Ear That
Convert Sound Into Brain Signals.
For scientists who study the genetics of hearing and
deafness, finding the exact genetic machinery in the inner ear that responds to
sound waves and converts them into electrical impulses, the language of the
brain, has been something of a holy grail.
Now this quest has come to fruition. Scientists at The
Scripps Research Institute (TSRI) in La Jolla, CA, have identified a critical
component of this ear-to-brain conversion -- a protein called TMHS. This
protein is a component of the so-called mechanotransduction channels in the
ear, which convert the signals from mechanical sound waves into electrical
impulses transmitted to the nervous system.
"Scientists have been trying for decades to identify
the proteins that form mechanotransduction channels," said Ulrich Mueller,
PhD, a professor in the Department of Cell Biology and director of the Dorris
Neuroscience Center at TSRI who led the new study, described in the December 7,
2012 issue of the journal Cell.
Not only have the scientists finally found a key protein
in this process, but the work also suggests a promising new approach toward
gene therapy. In the laboratory, the scientists were able to place functional
TMHS into the sensory cells for sound perception of newborn deaf mice,
restoring their function. "In some forms of human deafness, there may be a
way to stick these genes back in and fix the cells after birth," said
Mueller.
TMHS appears to be the direct link between the
spring-like mechanism in the inner ear that responds to sound and the machinery
that shoots electrical signals to the brain. When the protein is missing in
mice, these signals are not sent to their brains and they cannot perceive
sound.
Specific genetic forms of this protein have previously
been found in people with common inherited forms of deafness, and this
discovery would seem to be the first explanation for how these genetic
variations account for hearing loss.
Many
Different Structures
The physical basis for hearing and mechanotransduction
involves receptor cells deep in the ear that collect vibrations and convert
them into electrical signals that run along nerve fibers to areas in the brain
where they are interpreted as sound.
This basic mechanism evolved far back in time, and
structures nearly identical to the modern human inner ear have been found in
the fossilized remains of dinosaurs that died 120 million years ago.
Essentially all mammals today share the same form of inner ear.
What happens in hearing is that mechanical vibration
waves traveling from a sound source hit the outer ear, propagate down the ear
canal into the middle ear and strike the eardrum. The vibrating eardrum moves a
set of delicate bones that communicate the vibrations to a fluid-filled spiral
in the inner ear known as the cochlea. When the bones move, they compress a
membrane on one side of the cochlea and cause the fluid inside to move.
Inside the cochlea are specialized "hair" cells
that have symmetric arrays of extensions known as stereocilia protruding out
from their surface. The movement of the fluid inside the cochlea causes the
stereocilia to move, and this movement causes proteins known as ion channels to
open. The opening of these channels is a signal monitored by sensory neurons
surrounding the hair cells, and when those neurons sense some threshold level
of stimulation, they fire, communicating electrical signals to the auditory
cortex of the brain.
Because hearing involves so many different structures,
there are hundreds and hundreds of underlying genes involved -- and many ways
in which it can be disrupted.
Hair cells form in the inner ear canal long before birth,
and people must live with a limited number of them. They never propagate
throughout life, and many if not most forms of deafness are associated with
defects in hair cells that ultimately lead to their loss. Many genetic forms of
deafness emerge when hair cells lack the ability to transduce sound waves into
electric signals.
Over the years, Mueller and other scientists have
identified dozens of genes linked to hearing loss -- some from genetic studies
involving deaf people and others from studies in mice, which have inner ears
that are remarkably similar to humans.
A
Clearer Picture
What has been lacking, however, is a complete mechanistic
picture. Scientists have known many of the genes implicated in deafness, but
not how they account for the various forms of hearing loss. With the discovery
of the relevance of TMHS, however, the picture is becoming clearer.
TMHS turns out to play a role in a molecular complex
called the tip link, which several years ago was discovered to cap the
stereocilia protruding out of hair cells. These tip links connect the tops of
neighboring stereocilia, bundling them together, and when they are missing the
hair cells become splayed apart.
But the tip links do more than just maintain the
structure of these bundles. They also house some of the machinery crucial for
hearing -- the proteins that physically receive the force of a sound wave and
transduce it into electrical impulses by regulating the activity of ion
channels. Previously, Mueller's laboratory identified the molecules that form
the tip links, but the ion channels and the molecules that connect the tip link
to the ion channels remained elusive. For years, scientists have eagerly sought
the exact identity of the proteins responsible for this process, said Mueller.
In their new study, Mueller and his colleagues showed
that TMHS is one of the lynchpins of this process, where it is a subunit of the
ion channel that directly binds to the tip link. When the TMHS protein is
missing, otherwise completely normal hair cells lose their ability to send
electrical signals.
The scientists demonstrated this using a laboratory technique
that emulates hearing with cells in the test tube. Vibrations deflected off the
cells mimic sound, and the cells can be probed to see if they can transduce the
vibrations in electrical signals -- as they would in the body if the cells were
then trying to send signals to the brain. What they showed is that without
TMHS, this ability disappears.
"We can now start to understand how organisms
convert mechanical signals to electrical signals, which are the language of the
brain,"̈
said Mueller.
In addition to Mueller, the article "TMHS is an
Integral Component of the Mechanotransduction Machinery of Cochlear Hair
Cells" is authored by Wei Xiong (first author), Nicolas Grillet, Heather
M. Elledge, Thomas F.J. Wagner, Bo Zhao, Kenneth R. Johnson and Piotr Kazmierczak.
This work was funded with support from the National
Institutes of Health (DC005965, DC007704), the Dorris Neuroscience Center, the
Skaggs Institute for Chemical Biology and the Bundy Foundation.
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