Ion transport function of SLC4A11 in corneal endothelium

Abstract

Purpose. 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-. © 2013 The Association for Research in Vision and Ophthalmology, Inc.