title: Ion transport function of SLC4A11 in corneal endothelium.
abstract: OBJECTIVE
  Mutations in SLC4A11, a member of the SLC4 superfamily of bicarbonate transporters, give rise to corneal endothelial cell dystrophies. SLC4A11 is a putative Na⁺ borate and Na⁺:OH⁻ transporter. Therefore we ask whether SLC4A11 in corneal endothelium transports borate (B[OH]₄⁻), bicarbonate (HCO3⁻), or hydroxyl (OH⁻) anions coupled to Na⁺.
  
  
  METHODS
  SLC4A11 expression in cultured primary bovine corneal endothelial cells (BCECs) was determined by semiquantitative PCR, SDS-PAGE/Western blotting, and immunofluorescence staining. Ion transport function was examined by measuring intracellular pH (pHi) or Na⁺ ([Na⁺](i)) in response to Ringer solutions with/without B(OH)₄⁻ or HCO₃⁻ after overexpressing or small interfering RNA (siRNA) silencing of SLC4A11.
  
  
  RESULTS
  SLC4A11 is localized to the basolateral membrane in BCEC. B(OH)₄⁻ (2.5-10 mM) in bicarbonate-free Ringer induced a rapid small acidification (0.01 pH unit) followed by alkalinization (0.05-0.1 pH unit), consistent with diffusion of boric acid into the cell followed by B(OH)₄⁻. However, the rate of B(OH)₄⁻-induced pHi change was unaffected by overexpression of SLC4A11. B(OH)₄⁻ did not induce significant changes in resting [Na⁺(i)] or the amplitude and rate of acidification caused by Na⁺ removal. siRNA-mediated knockdown of SLC4A11 (∼70%) did not alter pHi responses to CO₂/HCO₃⁻-rich Ringer, Na⁺-free induced acidification, or the rate of Na⁺ influx in the presence of bicarbonate. However, in the absence of bicarbonate, siSLC4A11 knockdown significantly decreased the rate (43%) and amplitude (48%) of acidification due to Na⁺ removal and recovery (53%) upon add-back. Additionally, the rate of acid recovery following NH₄⁺ prepulse was decreased significantly (27%) by SLC4A11 silencing.
  
  
  CONCLUSIONS
  In corneal endothelium, SLC4A11 displays robust Na⁺-coupled OH⁻ transport, but does not transport B(OH)₄⁻ or HCO₃⁻.
authors.affiliation: School of Optometry, Indiana University, Bloomington, Indiana, USA.
authors.forename: Supriya S
authors.initials: SS
authors.lastname: Jalimarada
authors.forename: Diego G
authors.initials: DG
authors.lastname: Ogando
authors.forename: Eranga N
authors.initials: EN
authors.lastname: Vithana
authors.forename: Joseph A
authors.initials: JA
authors.lastname: Bonanno
mesh_terms: D000818:Animals; D027321:Anion Transport Proteins / Q000502:physiology*; D017920:Antiporters / Q000502:physiology*; D001485:Basement Membrane / Q000378:metabolism; D001639:Bicarbonates / Q000378:metabolism*; D001692:Biological Transport; D015153:Blotting, Western; D001881:Borates / Q000378:metabolism*; D002417:Cattle; D002478:Cells, Cultured; D004591:Electrophoresis, Polyacrylamide Gel; D004728:Endothelium, Corneal / Q000166:cytology / Q000378:metabolism*; D019084:Fluorescent Antibody Technique, Indirect; D015870:Gene Expression; D020868:Gene Silencing / Q000502:physiology; D005822:Genetic Vectors; D006863:Hydrogen-Ion Concentration; D006878:Hydroxides / Q000378:metabolism*; D017136:Ion Transport; D012333:RNA, Messenger / Q000378:metabolism; D034741:RNA, Small Interfering / Q000235:genetics; D060888:Real-Time Polymerase Chain Reaction; D012964:Sodium / Q000378:metabolism; D014162:Transfection
chemical_list: D027321:Anion Transport Proteins; D017920:Antiporters; D001639:Bicarbonates; D001881:Borates; D006878:Hydroxides; D012333:RNA, Messenger; D034741:RNA, Small Interfering; C425809:SLC4A11 protein, human; C031356:hydroxide ion; D012964:Sodium
keywords: SLC4A11; bicarbonate; borate transport; intracellular Na+; intracellular pH