title: Vestibular hair bundles control pH with (Na+, K+)/H+ exchangers NHE6 and NHE9.
abstract: In hair cells of the inner ear, robust Ca2+/H+ exchange mediated by plasma-membrane Ca2+-ATPase would rapidly acidify mechanically sensitive hair bundles without efficient removal of H+. We found that, whereas the basolateral membrane of vestibular hair cells from the frog saccule extrudes H+ via an Na+-dependent mechanism, bundles rapidly remove H+ in the absence of Na+ and HCO3(-), even when the soma is acidified. K+ was fully effective and sufficient for H+ removal; in contrast, Rb+ failed to support pH recovery. Na+/H+-exchanger isoform 1 (NHE1) was present on hair-cell soma membranes and was likely responsible for Na+-dependent H+ extrusion. NHE6 and NHE9 are organellar isoforms that can appear transiently on plasma membranes and have been proposed to mediate K+/H+ exchange. We identified NHE6 in a subset of hair bundles; NHE9 was present in all bundles. Heterologous expression of these isoforms in yeast strains lacking endogenous exchangers conferred pH-dependent tolerance to high levels of KCl and NaCl. NHE9 preferred cations in the order K+, Na+ >> Rb+, consistent with the relative efficacies of these ions in promoting pH recovery in hair bundles. Electroneutral K+/H+ exchange, which we propose is performed by NHE9 in hair bundles, exploits the high-K+ endolymph, responds only to pH imbalance across the bundle membrane, is unaffected by the +80 mV endocochlear potential, and uses mechanisms already present in the ear for K+ recycling. This mechanism allows the hair cell to remove H+ generated by Ca2+ pumping without ATP hydrolysis in the cell.
authors.affiliation: Oregon Hearing Research Center and Vollum Institute, Oregon Health & Science University, Portland, Oregon 97239, USA.
authors.forename: Jennifer K
authors.initials: JK
authors.lastname: Hill
authors.forename: Christopher L
authors.initials: CL
authors.lastname: Brett
authors.forename: Anthony
authors.initials: A
authors.lastname: Chyou
authors.forename: Laura M
authors.initials: LM
authors.lastname: Kallay
authors.forename: Masao
authors.initials: M
authors.lastname: Sakaguchi
authors.forename: Rajini
authors.initials: R
authors.lastname: Rao
authors.forename: Peter G
authors.initials: PG
authors.lastname: Gillespie
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D019556:COS Cells; D020013:Calcium Signaling / Q000502:physiology; D000252:Calcium-Transporting ATPases / Q000502:physiology; D002522:Chlorocebus aethiops; D005452:Fluoresceins / Q000032:analysis; D005456:Fluorescent Dyes / Q000032:analysis; D005816:Genetic Complementation Test; D006199:Hair Cells, Auditory, Inner / Q000737:chemistry / Q000502:physiology*; D018069:Hair Cells, Vestibular / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport / Q000502:physiology; D008565:Membrane Proteins / Q000235:genetics / Q000502:physiology*; D008969:Molecular Sequence Data; D038761:Photobleaching; D053499:Plasma Membrane Calcium-Transporting ATPases / Q000502:physiology; D011188:Potassium / Q000502:physiology*; D021381:Protein Transport; D011522:Protons; D011892:Rana catesbeiana; D012235:Rhodamines / Q000032:analysis; D012441:Saccharomyces cerevisiae / Q000235:genetics; D012964:Sodium / Q000502:physiology*; D017923:Sodium-Hydrogen Exchangers / Q000235:genetics / Q000502:physiology*; D014162:Transfection
chemical_list: D005452:Fluoresceins; D005456:Fluorescent Dyes; D008565:Membrane Proteins; C503961:NHE6 protein, mouse; C514888:NHE9 protein, mouse; D011522:Protons; D012235:Rhodamines; D017923:Sodium-Hydrogen Exchangers; C094271:seminaphthofluorescein-calcein; D012964:Sodium; D053499:Plasma Membrane Calcium-Transporting ATPases; C506959:Atp2b2 protein, mouse; D000252:Calcium-Transporting ATPases; D011188:Potassium