PMCA2w/a Splice Variant: A Key Regulator of Hair Cell Mechano-transduction Machinery

Sensory hair cells of the inner ear detect sound stimuli, inertial or gravi- tational forces. These mechanical inputs cause deflection of the cell stereociliary bundle and activate a small number of cation-selective mechano-transduction (MET) channels that admit K+ and Ca2+ ions into the cytoplasm. Stereociliary Ca2+ levels are homeostatically regulated by an unusual splicing isoform (w/a) of plasma membrane calcium-pump isoform 2 (PMCA2w/a), ablation or missense mutations of which cause deafness and loss of balance in humans and mice. At variance with other PMCA2 isoforms, PMCA2w/a expressed in CHO transfectants increases only marginally its activity in response to a rapid increase of the cytoplasmic free Ca2+ concentration ([Ca2+]c). In this expression system, deafness-related mutations of PMCA2w/a decrease the pump ability to extrude Ca2+ both at steady state and in response to a [Ca2+]c rise. Consistent with these findings, mouse strains in which the pump is genetically ablated or mutated show hearing impairment correlated with defects in homeostatic regulation of stereociliary Ca2+, decreased sensitivity of the MET channels to hair bundle displacement, and morphological abnormalities in the organ of Corti. These results highlight a critical role played by PMCA2w/a in the control of hair cell function and survival and provide mechanistic insight into the etiology of deafness and vestibular disorders.