Identification of the mechanotransduction channel of cochlear hair cells
Final Report Abstract
Cochlear hair cells perceive sound by mechanical deflection of a specified structure called the stereociliary hair bundle. This leads to the opening of a Ca2+ conducting ion channel and ultimately to the transduction of the mechanical stimulus into an electrical signal that is ultimately processed in the brain. Even though in the past numerous genes have been identified to be involved in this mechanoelectrical transduction the molecular identity of the actual transduction channel remained elusive. We investigated two related genes TMC1 and TMC2 and their function in hair cells. Previously it was shown that mutations in the TMC1 gene are the cause for nonsyndromic autosomal dominant and recessive hearing loss in humans while for TMC2 there are no known human mutations linked to deafness. Beside this there was no information whatsoever about the disease mechanisms as well as localization of the two proteins. Since both proteins share structural similarities with known ion channels we hypothesized that they form the ion conducting pore of the mechanotransduction channel in cochlear hair cells. We introduce a new technique to manipulate hair cells by electroporation. We electroporate shRNAi expression vectors to knock down TMC1 and TMC2 proteins and show that TMC1 and TMC2 are essential for proper mechanotransduction in hair cells and that they can functionally compensate each other. We hypothesize that by forming heteromultimeric channels with diverse properties they could account for the tonotopic change in the mechanocurrent along the cochlear length. Heterologously in hair cells expressed tagged versions of TMC1 and TMC2 both can localize to the stereocilia bundle where the mechanotransduction complex is located. Furthermore we show in vitro that TMC2 can interact with pcdh15 a protein of the lower tip link density where the mechanotransduction channel is localized. Our findings strongly suggest that TMC1 and TMC2 are essential components of the mechanotransduction machinery if not the pore forming subunit of the mechanotransduction channel.
