Yehoash Raphael, Kresge Hearing Research Institute, University of Michigan
THE SCIENCE OF HAIR CELL REGENERATION
Tiny cells called hair cells are located in the inner ears of birds and mammals, and are required for normal hearing. They are called hair cells because they actually resemble the hairs on your head. These cells convert sound information into electrical signals that are sent to the brain. Once hair cells die in mammals - including humans - hearing loss is permanent.
Many types of commonly acquired hearing loss are the result of damage or death to these delicate hair cells, including noise-induced hearing loss, presbycusis (age-related hearing loss), and ototoxic hearing loss (hearing loss that occurs after a patient is exposed to certain life-saving but toxic medications).
In the late 1980s, both Dr. Edwin Rubel (then at the University of Virginia and now at the University of Washington) and Dr. Douglas Cotanche (then at the University of Pennsylvania and now at the Harvard School of Public Policy) discovered that even after chickens' hair cells had been deliberately destroyed in their labs, the cells grew back. Dr. Cotanche's work was funded by Hearing Health Foundation.
Recent research has shown several promising leads that may overcome the lack of spontaneous recovery in the mammal. For example, mammals with an absent P27 gene display intracochlear cellular differentiation and the appearance of new hair-like cells after injury. Gene therapy with Atoh1 (MATH1) has resulted in patchy prototype hair cell formation in the mouse. Insertion of stem cells into the mouse ear has been followed by the appearance in several different areas of the cochlea of new cells that have assumed some of the local characteristics of the region. These research results underscore the readiness of the field for an integrated effort to find the right combination of factors that will result in a functioning inner ear.
Today, about a dozen laboratories in the United States are working on ways to translate to humans what we already know about hair cell regeneration in chickens. One approach is to stimulate existing stem-like cells in the human inner ear to regenerate hair cells, by delivering molecules that stimulate new hair cell production. Studies in animals that spontaneously regenerate hair cells, such as birds and fish, are unveiling important molecules needed to reach this goal.
A second approach is to transplant stem-like cells into the damaged ear that can give rise to new hair cells. Different labs are working on different pieces of the puzzle; some are working on gene therapies, some are working on stem cell therapies, and others are working on possible ways to integrate both approaches. When these labs are successful at solving the puzzle of regeneration, there will be - for the first time ever - a biological cure for hearing loss. Roughly 90 percent of tinnitus cases occur with an underlying hearing loss. Hence, a cure for hearing loss could also equal a cure for tinnitus.
And Hearing Health Foundation is at the center of this important work. Please click here to donate now.