Our hearing plays a key role in how we orient ourselves in the world and is critical for balance. There are three canals in our inner ears that are called semicircular canals and they sense our body’s side-to-side movement and tilting movements—other parts of the inner ear communicate to our brain about where our head is when we are still. These movements are sensed when the fluid and hair cells inside each of the canals transmit information via the acoustic nerve to our brains. When we experience inner ear problems we are ultimately experiencing disruptions in how our canals are working, which can pose great problems for our overall sense of balance.
The History of Hearing
Scientists have long understood that these tiny cells in our ear canals are responsible for detecting sound as well as movement, and that they then convert these senses into nerve signals. In fact, the Swedish physician and anatomist Gustaf Retzius provided detailed descriptions of the structure and cellular makeup of the inner ear in the 1800s. Much later in the 1970s, we learned more about the basic sequence of events whereby signals are translated from the inner ear to the brain, as researchers showed that proteins in the membranes of hair cells could open and allow electrically charged ions such as calcium and potassium to enter.
But while we know that these cells exist and their biomechanical messages have been studies, it has been unclear exactly how and where they convert these senses into signals.
New Research on the Protein that Enables Hearing
A recent study conducted by scientific researchers at Harvard Medical School argue that they have ended the long search for the sensor protein that is responsible for hearing and balance. The study was published August 22 in the journal Neuron. In it, scientists discuss the protein TMC1, a protein that was actually discovered in 2002. It has been difficult to parse out the function of the protein, however. This is in part because the protein is located in the inner ear, which is difficult to access because it is located within the densest bone of the human body. It has also been difficult to access because there are simply few auditory cells that scientists could retrieve, dissect, and create images of. For comparison: the human retina has 100 million sensory cells, whereas there are just 16,000 in humans’ inner ear.
Contemporary digital imaging technologies have made it easier to isolate the TMC1 protein, however. A protein is a chain of amino acids, and scientists discovered that individual TMC1 proteins pair up to form minimalistic pores, otherwise called ion channels. This was a structure that scientists could test in mice by monitoring changes in the flow of charged ions moving through the pore in response to sound. In other words, this pore allows sound and head movement to be converted into nerve signals that travel to the brain, which enables hearing and balance. To test which pores were related to hearing, scientists substituted 17 amino acids in the model, one at a time, gauging the ways in which each substitution altered the cells’ overall ability to allow the flow of ions—to respond to sound. It turned out that 11 of these amino acids altered the influx of ions. Importantly, five of them did so quite dramatically; they in fact reduced the flow by up to 80 percent. David Corey and Jeffrey Holt are two of the senior authors of the paper reporting the findings, and they explain that the findings “yield definitive proof that TMC1 is the critical molecular sensor that converts sound and motion into electrical signals the brain can understand.”
These findings can have great effect on the study of hearing loss, as the TMC1 gene is just one of around 150 genes that are associated with genetically-linked hearing loss. With more focused attention on the TMC1 gene, we may be able to spearhead new developments into targeted therapies that may be able to treat hearing loss that results from the gene’s molecular gate (the pore) is obstructed or missing altogether.
“We now know that TMC1 forms the pore that enables sound detection in animals ranging from fish to birds to humans,” Corey said. “It is truly the protein that lets us hear.” With this knowledge, we are surely on the cusp of incredible research that will enable us to further explore hearing loss and to potentially devise targeted therapies to address it.
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