Researchers supported by the NIDCD have shown for the first time that a drug can be used to grow sensory hair cells in the inner ear. Using a mouse model, researchers were able to partially restore hearing in mouse ears that had been damaged by traumatic noise. The discovery could potentially lead to future treatments to reverse some types of hearing loss in humans. Overall, an estimated 36 million adult Americans report hearing loss. The study appears in the January 10, 2013 issue of Neuron.
The work is significant, says Matthew Kelley, Ph.D., chief of the Laboratory of Cochlear Development at the NIDCD (who was not involved in the study). "This is the first study to show that noise-induced hearing loss can be ameliorated using a pharmacological treatment."
The loss of sensory hair cells in the cochlea is one of the most common causes of sensorineural hearing loss (hearing loss caused by problems in the inner ear, rather than in the auditory processing system that leads from the ear to the brain). We can lose hair cells for a number of reasons, such as chronic exposure to noise, aging, disease, or drugs that are specifically toxic to them. Unlike birds or fish, however, mammals aren't able to naturally regrow hair cells when they are lost. Because there are only 15,000 hair cells in each of our ears, just the loss of a thousand is enough to make a noticeable difference in how well we hear.
In this study, Albert Edge, Ph.D. and his colleagues at the Harvard Medical School and Massachusetts Eye and Ear injected a drug into the cochlea (a spiral shaped organ in the inner ear that shelters hair cells) of the deaf mice. The drug inhibited an enzyme called gamma secretase, which suppresses a cell signaling system called Notch. During development, Notch is known to keep stem cells in the inner ear from turning into hair cells. But by blocking the Notch pathway, the drug encouraged cells supporting and surrounding the hair cells to turn into new hair cells, which led to a small improvement in hearing.
The findings are exciting, but like all scientific discoveries need to be put into context to be fully understood. "While we think we have a pretty good idea of the biological pathway that’s changed when gamma-secretase is blocked," says Dr. Kelley, "we can’t be one hundred percent sure that this pathway is actually responsible for the recovery that's observed here." Further work by Dr. Edge and his colleagues should be able to establish the essential role that gamma-secretase plays in controlling hair cell growth in the inner ear. In addition, since the improvement in hearing was observed in young mice, it will be especially important to know if the same mechanism can drive hair cell regeneration in older ears. This is where the tactic might have its most practical application—helping reverse age-related or noise-induced hearing loss in adults.