Bernd Fritzsch (corresponding)
According to WHO, 360 million people (or the population of the US) have disabling hearing loss (http://www.who.int/mediacentre/factsheets/fs300/en/). Hearing loss deprives an increasing number of elderly from their accustomed way of communication when they need it most to overcome social isolation and maintain intellectual and emotional engagement in the society. As societies worldwide are ageing, so are the number of elderly with moderate to profound hearing loss. This is accompanied, with a delay of about 10 years, with reduced function in the second ear sensor leading to increase in falling. Falls are the second leading cause of accidental injury death worldwide. About 50% of hearing loss can be prevented with proper precaution but the age related hearing and balance loss might only be delayed. Given the magnitude and projected increase of hearing defects, it is reasonable to look for alternatives to not only prevent but to restore lost hearing. This collection is providing some more recent reviews on possibilities of modern approaches to hearing restoration but also some critical assessment of what is currently possible and what obstacles need to be overcome to turn the hope to restore hearing into a reality.
Without question, the hearing organ of the human ear is the most sophisticated cell assembly in the human body where each cell has to be in the right number and the right degree of differentiation at the right place for the entire hearing organ to function perfectly. One aspect of age related hearing loss is the loss of tiny cells in the hearing organ, called hair cells, needed to convert sound energy into electricity to be conducted to the brain by a set of nerve fibers to be processed to extract meaning out of sound. Obviously, if many or all of these hair cells die, they need to be replaced. Great gains have been made in the last few years to demonstrate that under certain conditions it is possible to restore some of these cells in tissue culture and, in rare cases, in the ear with a limited gain in hearing. However, thus far and for the foreseeable future, a whole restoration is for a number of incompletely understood problems out of question.
For starters, inserting regenerated hair cells into the appropriate place to function as converters for sound goes well beyond the problems of making enough hair cells in the dish to warrant their transplantation into the ear. In a recent paper, Park (2015) highlights some of the clinically relevant problems of the stem cell approach. For example, Park points out the hair cells need to be inserted into a fluid that appears to be extremely toxic for these cells. In fact, many cases of inherited hearing loss are related to alteration of the ionic balance in the ear and disruption of the various compartments that our ionically distinct. In every case, it is clear that loss of hair cells follows the alteration of ionic compositions. Thus it is not surprising that hair cells, exposed on all sides by the unique potassium rich fluid of the inner of the three chambers needed for hearing are rapidly dying without manipulating the potassium rich fluid filling this part of the ear.
Beyond this problem of survival of hair cells generated in the dish when inserted into the ear is problem of where these cells insert. Simply speaking, if hair cells are inserted into areas that do not move in the sound pressure field they cannot contribute to hearing.
Finally, more recent data suggest that perhaps much more is needed than adding a few hair cells to restore a meaningful hearing level. Recent work using a novel approach to disrupt normal development of the hearing organ has generated a mouse model in which nearly all hair cells form but are not normally organized (Jahan et al., 2015). Obviously, one would have expected that at least some hearing is possible in such mice but they turned out to be stone deaf. Translated into the quest to restore hearing, it might be necessary that nearly all hair cells need to be restored before a significant recovery of hearing takes place, arguing very strongly that prevention and repair instead of restoration should be the major target of efforts to help the millions of soon-to-be deaf people worldwide. Toward this end the collection will be updated with articles highlighting prevention and restoration aspects.
Y.-H. Park, J Audiol Otol. 2015 19(2):63-67 https://www.scienceopen.com/document/vid/44bc44ca-ce7c-457d-9d98-69532933ec5a?6
I Jahan, et al. Development 142(16): 2810-2821 doi: 10.1242/dev.123091
Image Credit: Dr. David Furness, Wellcome Images; Immature hair bundle on an inner hair cell in the cochlea. Flickr CC BY NC-ND 2.0