Some people lose their hearing because they simply age; some because of too much loud noise. For some, the ability to hear never developed.
Researchers at the Scripps Research Institute in La Jolla, Calif., have discovered a protein that is responsible for one form of genetic deafness. The protein helps turn sound into electrical signals.
The research is of more than just biochemical interest; it may also open a new avenue for possibly giving the sense of hearing to some of those who are born without it. The team, led by Ulrich Mueller, a professor of cell biology, took newborn deaf mice and inserted the protein, called TMHS, into their sensory cells for sound perception, giving the mice some form of hearing. The potential now exists for genetic therapy to insert the genes for the protein into newborn humans and fix malfunctioning cells. The work is published in the Dec. 7, 2013 issue of the journal "Cell".
No one knows how many people suffer from genetic deafness but they surely number in the millions, Mueller said. According to the Centers for Disease Control and Prevention, genetic causes are responsible for half the children born deaf in the U.S. Sixty genes have been identified so far, and there likely are many more to be found. Mueller said that the best guess now is that there are 400-500 genes and proteins responsible for genetic deafness.
Sound is channeled by our outer ear into the ear canal where it strikes the ear drum in the middle ear. The eardrum vibrates, and those vibrations move utilizing a set of delicate bones deeper inside the ear to the cochlea, a spiral structure filled with fluid. The vibration in the bones shakes the fluid in the cochlea. A complex of hair-like cells in the cochlea senses the vibrations in the fluid. "The hair cells have stereocilia, little filaments, projections that stick out from the hair cells," Mueller said. The stereocilia sense the motion. It is at that point, the TMHS protein gets involved. TMHS triggers electrical signals in nerve cells surrounding the hair cells. The signals then travel to the brain and are sensed as sound, Mueller said.
The TMHS protein opens holes in the hair cells called ion channels. "Anything that goes into a cell is controlled by proteins," Mueller said. "The language of the brain is electricity. If you want to send an electric signal, you open the pores in the membrane and let the ion into the cell and that change leads to an electric current."
TMHS is a component of the hair cell’s mechano-transduction machinery and binds to the tip-link component PCDH15 and regulates the tip-link assembly. TMHS regulates transducer channel conductance and is required for adaptation. TMHS is structurally similar to other ion channel regulatory subunits such as TARPs (transmembrane alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor regulatory proteins).