By using genetic tools in mice, researchers from John Hopkins have identified a pair of proteins that accurately control when hair cells (sound-detecting cells) form in the inner ear – a discovery that could help identify future therapies in restoring hearing in people with irreversible deafness.
‘Scientists in our field have long been looking for the molecular signals that trigger the formation of the hair cells that sense and transmit sound,’ says Angelika Doetzlhofer, Associate Professor of Neuroscience at the Johns Hopkins University School of Medicine. ‘These hair cells are a major player in hearing loss, and knowing more about how they develop will help us figure out ways to replace hair cells that are damaged.’
In the mammalian ear, sound vibrations travel through the hollow, spiral structure known as the cochlea. In the inside of cochlea are two types of sound-detecting cells, inner and outer hair cells, which transmit sound information to the brain.
Deafness due to exposure to loud noises or certain viral infections arises from damage to hair cells. As human hair cells cannot regenerate, hair-cell damage-induced hearing loss is likely permanent.
Hair cells are ‘born’ at the outermost part of the cochlea, where precursor cells start transforming into hair cells. Precursor cells along the spiral shape of the cochlea turn into hair cells along a wave of transformation that stops when it reaches the inner part of the cochlea. Doetzlhofer and her team went in search of molecular cues along the cochlear spiral.
The pattern of two proteins, Activin A and follistatin, stood out from the rest. Levels of Activin A increased where precursor cells were turning into hair cells. Follistatin, however, appeared to have the converse behaviour of Activin A. Its levels were low in the outermost part of the cochlea when precursor cells were first starting to transform into hair cells and high at the innermost part of the cochlea's spiral. Activin A appeared to move in a wave inward, while follistatin moved in a wave outward.
‘In nature, we knew that Activin A and follistatin work in opposite ways to regulate cells,’ says Doetzlhofer. ‘And so, it seems, based on our findings like in the ear, the two proteins perform a balancing act on precursor cells to control the orderly formation of hair cells along the cochlear spiral.’
Doetzlhofer notes that her research in hair cell development, although fundamental, has potential applications to treat deafness caused by damaged hair cells: ‘We are interested in how hair cells evolved because it's an interesting biological question,’ she says. ‘But we also want to use that knowledge to improve or develop new treatment strategies for hearing loss.’