You may not know it, but there's a miniature piano-like keyboard inside each of your ears -- only this keyboard is composed of approximately 16,000 tiny hair cells and, instead of playing notes from low to high, they help you hear the range of frequencies in the sounds you hear every day. Damage to these hair cells cause hearing loss and, until recently, scientists believed once these hair cells were damaged they could not be repaired.
But now researchers from the National Institutes of Health (NIH) have uncovered a molecule that may change all of that. The molecule is a protein known as Bmp7, which is produced during embryonic development and helps the hair cells find their position on the tonotopic map. The tonotopic map is the spatial arrangement of where sound is perceived, transmitted or received in the inner ear.
The study was led by Zoe F. Mann, Ph.D. and Matthew W. Kelley, Ph.D. of the Laboratory of Cochlear Development at the National Institute of Deafness and Other Communication Disorders (NIDCD) in collaboration with scientists from the University of Virginia (UVa) School of Medicine, Charlottesville, and Imperial College in London. The American Hearing Research Foundation provided additional support. Their findings were published in the May 20, 2014 issue of Nature Communications.
“The findings could open doors to therapies that take advantage of Bmp7’s navigational talents to direct the formation of regenerated sensory cells that are tuned to respond to a specific frequency,” says James F. Battey, Jr., M.D., Ph.D., director of NIDCD. “Since many forms of hearing loss are limited to specific frequencies, this approach could lead to replacement sensory cells that are tailored to individual needs.”
These hair cells, which are spread across the flat surface known as the basilar membrane and rolled up like a carpet and tucked neatly inside the snail-shaped cochlea in the inner ear, are designed to listen for a distinct frequency. Those responsible for detecting low frequencies are at one end and those responsible for detecting high frequencies are at the opposite end of the axis. As these hair cells are damaged, so is our ability to hear the frequency they were designed to detect. This type of hearing loss is known as sensorineural hearing loss and can be caused by the aging process, excessive exposure to noise and certain medications.
Using information they received from studying basilar papilla in six-day old chick embryos, the researchers discovered Bmp7 promotes the development of low-frequency-sensing hair cells. They determined that during embryonic development, Bmp7 is strongest at the low-frequency end of the map. As the levels decrease along the length of the basilar papilla map, there is a gradual change towards tuning to higher frequencies.
This discovery is good news, especially for two of the most common types of sensorineural hearing loss caused by old age and exposure to excessive noise.
Those with presbycusis, or age-related hearing loss, usually lose hearing in the higher frequencies first. That means while they may not be able to hear the bird chirping outside their window, they may be able to hear the bass pounding in the car stereo down the street. According to the NIDCD, 30-35 percent of adults between the ages of 65-75 have presbycusis. The percentage increases to 40-50 percent for those over the age of 75.
Loud noise exposure can also damage these hair cells. Noise-induced hearing loss (NIHL) usually occurs over a long period of time. Those with NIHL may hear sounds as muffled or distorted and have trouble hearing conversations in person or on television. According to the NIH, approximately 26 million Americans between the ages of 20-69 have hearing loss and as many as 16 percent of teens age 12-19 have reported hearing loss that may have been caused by NIHL
Because Bmp7 is present in the inner ear of mammals, scientists intend to further their research using mice in the hopes of understanding this protein's precise role in patterning parts of the auditory system.
“The entire auditory system is assembled according to individual frequencies,” said Dr. Kelley. “Complex sounds like music or speech that consist of many different frequencies are split into individual frequencies in the ear, processed through separate channels, and then reassembled in the brain. By revealing the part played by Bmp7 in patterning hair cells in the inner ear, we may have uncovered a broader role for the molecule in the auditory system as a whole.”