The most common cause of hearing loss is the damage and loss of cells that grow the hairs inside the inner ear. These cells are aptly named cochlear hair cells. Repetitive exposure to loud environments, such as construction zones, concerts, or military bases can damage these cells, which, until recently, were thought to be irreplaceable. Normally, these cells enter the G0 phase after initial development ends when the organism is mature, which makes them similar to the brain cells we learned about in class. When a cell is in the G0 phase, it is frozen in the cell cycle, so the cell does not proceed through mitosis. This means that once the organism is done growing, there is no replacement of the damaged cells, as no cells are dividing.
In the animal kingdom, however, these cells are known to regenerate. Birds and fish have a mechanism which relies on a gene called ERBB2. The artificial expression of this gene in mammals has also been proven to trigger cell growth in a trial led by Jingyuan Zhang, PhD. They found that activating the ERBB2 gene triggered a cascading series of cellular responses which made the active cochlear hair cells multiply as well as trigger stem cells to become cochlear hair cells.
The research found that the activation of the ERBB2 gene caused stem-cell like development through the expression of a few proteins. The most important protein to this process, SPP1, signals the CD44 receptor, which exists on cochlear hair cells. The theory is that because these receptors are triggered, they somehow promote mitosis in the cells. The promotion of mitosis, the process of cell division in the cell cycle, would mean that these cells could be reproduced and the damaged cells could be replaced by new cells.
When this process was tested in adult mice, this cascade happened as previously shown in growing mice, meaning that the possibility of the development of new cochlear hair cells is possible in mature mammals, it just needs to be stimulated correctly.
The next step in the research is to determine whether or not these new cochlear hair cells are functioning mechanically. I don’t know about you, but I would maybe not stop wearing my earmuffs to use a jackhammer if I were you.