Uncovering the inner ears’ noise defense mechanism

Posted by:gulf onDecember 8, 2015

New research has identified a new way in which the inner ear can protect itself against the damaging effects of noise.

 Noise-induced hearing loss is the second most common cause of deafness, after age-related hearing loss. It is caused by exposure to loud sounds, and is an especial hazard for those who work in noisy environments, such as factory workers and people in the military, and for those who spend a lot of time in nightclubs or at rock concerts.

The ear’s defence mechanisms against noise damage

The ear has a number of ways in which it tries to protect itself from damaging levels of noise. One is the middle ear reflex, which reduces the level of sound getting into the cochlea when it’s too loud. Another is the efferent system, the network of nerves that allow the brain to talk to the ear, and can control how much the outer hair cells in the cochlea amplify the sound signal to the brain.

As well as this, the sensory cells of the inner ear have their own ways of mitigating the damaging effects of noise. Loud noise causes levels of free radicals, or ‘reactive oxygen species’ (ROS), to build up in the hair cells – if they’re not cleared, they will damage proteins and other cellular structures, which can eventually lead to the cell’s death. As hair cells can’t be replaced, this damage is permanent and if enough hair cells die, it will cause permanent hearing loss. To counter this, hair cells and other cells in the cochlea have developed a robust antioxidant defense mechanism. These antioxidants clear the ROS from the cells before they can cause any damage, but when noise gets too loud, even these processes can be overwhelmed. It’s likely that people who have less well-functioning antioxidant systems, perhaps through a genetic cause, may be at a higher risk of developing noise-induced hearing loss.

And researchers in France have published new findings that suggest exactly that, especially for people with the DFNB59 type of deafness.

DFNB59 causes hypersensitivity to noise

People who have the DFNB59 form of deafness all have mutations in the same gene, which produces a protein called ‘pejvakin’. Pejvakin means ‘echo’ in Persian, and is so-called because it was first discovered in a Persian family who have this kind of hearing loss.

DFNB59 is an unusual type of hearing loss with varying severity and presentations. People who have this kind of deafness may have moderate hearing loss, but it could also be much more severe. The hearing loss in some people with DFNB59 is due to damage to the auditory nerve; in others, it is due to damage to the hair cells. In some people, their hearing loss may be progressive, getting worse with time. The variety of different presentations of hearing loss suggests that another factor, possibly an environmental one, is also involved. And in this case, it seems to be noise.

The researchers studied both people with DFNB59 deafness and mice in which the pejvakin gene was deleted, and they found that both groups were unusually sensitive to noise damage to their hearing, even at sound levels which would not normally be considered to be damaging. This suggests that pejvakin has a role in protecting the cochlea from noise.

Pejvakin is involved in antioxidant defence

The researchers studied the mice which lacked pejvakin in more detail, and found that levels of ROS were abnormally high (known as oxidative stress) in the cochlea, compared with mice that were able to produce pejvakin. This pointed to pejvakin having a role in one of the antioxidant defence mechanisms in the inner ear – in its absence, ROS were able to build up to damaging levels and cause permanent noise-induced hearing loss. 
Further study linked pejvakin to structures within the hair cells called peroxisomes. Peroxisomes are found in nearly all cells, and they play a variety of roles within those cells, including reducing oxidative stress. In response to high levels of ROS, peroxisomes can replicate themselves (or proliferate), in order to better cope with the oxidative stress. However, in the mice lacking pejvakin, peroxisome proliferation fails; if anything, the numbers of peroxisomes within the hair cells falls during oxidative stress, and the peroxisomes that are present often look abnormal. Pejvakin therefore seems to be necessary for this process to occur correctly, which is supported by the researchers’ observation that using gene therapy to restore pejkavin in the inner ears of these mice also partially restored the peroxisome response and protected against noise to some degree. These findings also indicate that peroxisomes are a crucial part of the inner ear’s defence mechanism against noise.

What does this mean?

This research suggests that gene therapy might one day be a useful treatment for people with DFNB59 deafness; more research is now needed into the feasibility of this approach. In addition, it highlights that people with DFNB59 deafness are especially susceptible to noise-induced hearing loss, and should take steps to avoid noisy situations and protect themselves as much as possible from noise. Finally, the observation that peroxisomes are important in the inner ear’s own defence against noise could lead to the identification of new targets for the development of drugs which can prevent noise-induced hearing loss.

Source: http://www.actiononhearingloss.org.uk/community/blogs/our-guest-bl5564og/uncovering-the-inner-ears-noise-defence-mechanism.aspx

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