It's easy to find connections between the senses of touch and hearing: just run a finger along the teeth of a comb. Closely spaced teeth produce a higher sound and faster vibrations than distant ones. These senses are also linked at the scales of cells and molecules, and the groups of Thomas Jentsch at the FMP and MDC  and Gary Lewin at the MDC have just discovered a new connection. A protein that is needed for  people to hear also helps them to discriminate between low and high frequency vibrations through their sense of touch. People who inherit mutations in a molecule called KCNQ4 perceive low-frequency sounds and vibrations much more strongly than individuals with a functioning form of KCNQ4 and who have normal hearing . The work is reported in the Nov. 20 issue of Nature Neuroscience.
KCNQ4 sits in the membranes of certain types of sensory nerves. It functions as a channel that allows positively charged potassium ions to pass through the membrane into cells. Thomas' lab has discovered a number of such channels and investigated their functions throughout the nervous system. There are a lot of them, and the same channel may do different things depending on the type of nerve in which it is found. Sometimes, for example, ion channels serve to  excite a nerve to trigger an electrochemical signal to the brain; in other cases they may help restore the cell to its resting state, resetting it to receive a new signal. The functions of ion channels are so important that disturbing them often leads to a disease.
Several years ago, the Jentsch lab identified KCNQ4 and showed that it is mutated in people who suffer from a particular form of deafness. They found the channel in mechanosensitive cells in the inner ear, in a structure called the cochlea. These sensory cells have microscopic hairs that vibrate under the pressure caused by specific frequencies of sound. KCNQ4 helps in processing electrical signals that sound produces in these ‘hair cells.’ If a person inherits a mutation that prevents the channel from working, the cells eventually die. People with these mutations display a slowly progressing hearing loss that begins at at the high frequency end of the sound spectrum.
"Sound stimuli are transmitted along a pathway of nerves to the brain, and you find KCNQ4 along this route," Thomas says. "The nerves extend from the ear to the brainstem, where you have a routing station for sensory information. For example, the brainstem has a structure called the trigeminal ganglia which receives touch, vibration and pain information from the skin of the face. Since KCNQ4 is also found in these cells, we wondered whether it plays a role in this type of perception as well. We investigated cells of the dorsal root ganglia, which receive similar information from the skin of the rest of the body. Indeed, we found KCNQ4 in about 10 percent of the cells of these ganglia. This percentage and the size of the cell bodies of the nerves suggested that they perceive mechanical stimuli."
Gary's lab has been studying mechanosensitive neurons of these ganglia for many years. These cells extend processes that lie under the skin and sense stimuli such as pain, pressure, and vibrations, then transmit signals via the dorsal root ganglia and on to the brain. The cells having KCNQ4 turned out to have a particular function. They are known as rapidly adapting, low-threshold mechanoreceptors, or RAMS, which means that they quickly readjust to a resting state after passing along a dynamic signal. This means that upon static pressure, they tune down the strength of a signal, and the effects were found in cells that sense low vibrations.
"Thomas’ lab had already developed strains of mice which either totally lack this channel, or have exactly the same mutation they had found in deaf patients," Gary says. "They had used these mice previously to look for the mechanism of deafness. Now we could use them to study their perception of vibrations. We looked at both the way nerves functioned and the animals' behavior. Interestingly, the mutation didn't kill the nerves, as it does in the ear. Instead, losing KCNQ4 made them fire more frequently and at faster intervals than in animals with a working version of the protein. These signals are so strong, in fact, that they may make it harder to tell the difference between low and high frequency vibrations.."
Thomas’ lab showed that the KCNQ4 channel is present right in the nerve endings that detect vibrations, and  scientists from both labs travelled to Spain and the Netherlands to investigate the sense of vibration in families affected by this form of hearing loss. Just as in the mice, the people were more sensitive to lower frequencies.
It's interesting, the scientists say, that both sensory systems depend on pressure. In hearing, changes in air pressure cause vibrations in the liquid environment of the ear. This shakes the hairs in the cochlea and stimulates nerves. In the skin, vibrations also trigger signals through changes in pressure on cells. KCNQ4 potassium channels are important for both senses. It's yet another example of a basic principle: evolution has built different parts of the body using common sets of genes, which then weave together in new ways to produce distinct, very refined systems such as touch and hearing.  

- Russ Hodge

Reference:
Heidenreich M, Lechner SG, Vardanyan V, Wetzel C, Cremers CW, De Leenheer EM, Aránguez G, Moreno-Pelayo MA, Jentsch TJ, Lewin GR. KCNQ4 K+ channels tune mechanoreceptors for normal touch sensation in mouse and man. Nat Neurosci. 2011 Nov 20.