Gerbil experiments may assist deaf to hear

BBC News (http://www.bbc.co.uk/news/health-19570024) reported that scientific researchers in the United Kingdom hope to treat deafness, in the future, with stem cells. And the gerbil may provide an answer.

Deaf gerbils were partially able to hear (45% recovery of hearing loss) when the nerves in their ears were rebuilt. In the journal Nature, the UK study reported a huge step forward in treating deafness after stem cells were used to restore hearing in gerbils, for the first time. The nerves which relay sounds into the brain were rebuilt, thus providing the gerbils with partial hearing.

However, treating humans is still a long way off. The report indicated that it would be a drastic shift from being unable to hear traffic to hearing a conversation. Although the news is encouraging, the use of stem cells to restore nerves in the ear is an exact technique that may not help the majority of people with hearing loss.

About 10% of people with profound hearing loss have damaged nerve cells that are unable to pick up hearing signals. This happens deep inside the inner ear where vibrations move tiny hairs and this movement creates an electrical signal that passes to the brain. Hair cells are cells in the ear that transform sound signals into electrical signals that can be processed by the nervous system and the stria vascularis is a structure in the ear that maintains the electrical potential and potassium concentration of the fluid surrounding the hair cells at healthy levels. Hence, the aim of researchers at the University of Sheffield was to replace the damaged nerve cells (called spiral ganglion neurons) in deaf gerbils with new ones. The researchers have converted embryonic stem cells into the early versions of hair cells. The difficult part is injecting them into the ear to restore hearing because they need to be in the exact place and pointing in exactly the right direction.

Researchers used stem cells from a human embryo, which are capable of becoming any other type of cell in the human body from nerve to skin, muscle to kidney. They added chemicals to the stem cells that converted them into cells similar to the spiral ganglion neurons. These were then delicately injected into the inner ears of 18 deaf gerbils. Over 10 weeks the gerbils' hearing improved. On average 45% of their hearing range was restored by the end of the study. About a third of the gerbils responded really well to treatment with some regaining up to 90% of their hearing, while just under a third barely responded at all.

Gerbils were used because they are able to hear a similar range of sounds to people, unlike mice which hear higher-pitched sounds. Researchers detected the improvement in hearing by measuring brainwaves. If this became a treatment in humans then the effect would need to be shown over a much longer term than the 10 week gerbil study. There are also questions around the safety and ethics of stem cell treatments which would need to be addressed.

Andrew Tan, research associate at the Institute of Neuroscience, University of Oregon, reported that gerbils living in a quiet environment do, on average, develop hearing loss that is more severe at high frequencies. However, there is wide variability in the hearing loss. The most obvious anatomical change in such animals is in the stria vascularis. Although there is a correlation between stria vascularis changes and hearing loss in these animals, it is unknown whether the stria vascularis changes actually cause the hearing loss. Since these gerbils lived in a quiet environment, the hearing loss and stria vascularis changes are not due to noise but, presumably, to genetically-programmed changes – i.e. genetic variability. 

People living in quiet environments also have hearing loss. Since the anatomical changes most commonly reported in the ears of old people are in the stria vascularis, such hearing loss is often attributed to changes in the stria vascularis, but not everyone agrees. Noise causes even worse high-frequency hearing loss. But even then, different people exposed to the same loud noise will not have the same hearing loss. The people with greater hearing loss could have different genes that render their ears more susceptible to damage by noise. Noise-induced high-frequency hearing loss seems to be generally attributed to high-frequency hair-cell loss, but it may also involve other structures of the ear, Tan reports.

Professor Dave Moore, the director of the Medical Research Council's Institute of Hearing Research in Nottingham, told the BBC: "It is a big moment. It really is a major development." However, he cautioned that there will still be difficulties repeating the gerbil experiment in people. The biggest difficulty is getting into the part of the inner ear where the stem cells will provide the most benefit. “It's extremely tiny and very difficult to get to and that will be a really formidable undertaking," he said. Moore said using stem cells to repair the hairs was "almost an impossible task" and that the far-fetched concept of growing and transplanting a replacement ear seemed a more likely solution.

(https://www.hhmi.org/askascientist/answers/when_humans_lose_their_hearing_by_age_trauma_noise_exposure_etc_why_do_they_lose_more_sensiti.html).