Hair cells, plasma membrane Ca2+ ATPase and deafness

Hearing relies on the ability of the inner ear to convert sound waves into electrical signals. The main actors in this process are hair cells. Their stereocilia contain a number of specific proteins and a scaffold of actin molecules. They are organized in bundles by tip-link filaments composed of cadherin 23 and protocadherin 15. The bundle is deflected by sound waves leading to the opening of mechano-transduction channels and to the influx of K(+) and Ca(2+) into the stereocilia. Cadherin 23 and the plasma membrane calcium ATPase isoform 2 (PMCA2) are defective in human and murine cases of deafness. While the involvement of cadherin 23 in deafness/hearing could be expected due to its structural role in the tip-links, that of PMCA2 has been discovered only recently. This review will summarize the structural and functional characteristics of hair cells, focusing on the proteins whose mutations may lead to a deafness phenotype.