cochlear hair cells

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.

Hearing loss is a problem that affects almost a fifth of the world’s population, can cause feelings of isolation, and is closely correlated with dementia. Unfortunately, despite these drastic numbers and the fact that hearing loss can greatly worsen one’s quality of life, there is no way to reverse the effects of hearing loss. Or is there?

Neuroscientists at the Del Monte Institute for Neuroscience at the University of Rochester Medical Center believe they may have come up with just the solution. The most common cause of hearing problems is the damage of cochlear hair cells in the ear, which detect sound waves and allow mammals to hear. Sound wave detection from inner cochlear hair cells make up about 95% of the auditory nerve’s signal to the brain, and outer cochlear hair cells amplify sound vibrations. However, although mammals are unable to regenerate these cells, fish and birds can, allowing them to fix any hearing loss that they may encounter.

With this new study, the scientists discovered that the activating the ERBB2 growth gene, a gene that allows cochlear hair cell growth in birds and fish, triggers “a cascading series of cellular events by which cochlear support cells began to multiply and activate other neighboring stem cells to become new sensory hair cells.”

Furthermore, the scientists tested cells with/without the ERBB2 growth gene in mice, and they found that cells with the ERBB2 gene stimulated stem cell-like growth by inducing the expression of many proteins. One such protein was SPP1, which signals through the CD44 receptor, a receptor found in cochlear hair cells. This response signals mitosis, a process that we learned about in AP Biology, which is the process by which cells duplicate and, thus, grow.

The cell cycle, or the cycle of processes of a cell’s life, consists of four main phases: gap 1 (cell growth), synthesis (DNA replication), gap 2 (cell growth & organelle duplication), and cell division. In order for a cell to advance onto the next phase of the cycle, it must first pass checkpoints that affirm that the cell has completed the previous phases and is ready to move on. However, for cells that do not grow regularly, such as neurons and cochlear hair cells, they enter a phase between gap 1 and synthesis, called gap 0. In this phase, cells exit the regular cell cycle and cease to duplicate unless they receive a signal to do otherwise. This is why the SPP1 protein causes cell growth: because it gives the cochlear hair cells the signal to exit the gap 0 phase, continue on with the cell cycle, and duplicate, allowing the recovery of hearing.

Growth of the cochlear hair cells would allow mammals, including, eventually, humans, to regain their hearing after suffering from hearing loss. Scientists plan to continue researching and experimenting with this newfound information and hope to one day use this knowledge to reverse hearing loss at any stage of a person’s life. Just think about someone you know who has hearing loss: a grandparent? An uncle? A friend? Or maybe even you? This new research about the ERBB2 gene could heal the way they listen to music, watch TV, have a conversation, and live their everyday life.

BioQuakes

AP Biology class blog for discussing current research in Biology

Tag: cochlear hair cells

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