Bestrophin Cl- channels are highly permeable to HCO3-.
Bestrophin-1 (Best1) is a Cl(-) channel that is linked to various retinopathies in both humans and dogs. Dysfunction of the Best1 Cl(-) channel has been proposed to cause retinopathy because of altered Cl(-) transport across the retinal pigment epithelium (RPE). In addition to Cl(-), many Cl(-) channels also transport HCO3(-). Because HCO3(-) is physiologically important in pH regulation and in fluid and ion transport across the RPE, we measured the permeability and conductance of bestrophins to HCO3(-) relative to Cl(-). Four human bestrophin homologs (hBest1, hBest2, hBest3, and hBest4) and mouse Best2 (mBest2) were expressed in HEK cells, and the relative HCO3(-) permeability (P HCO3/PCl) and conductance (G HCO3/GCl) were determined. P HCO3/PCl was calculated from the change in reversal potential (Erev) produced by replacing extracellular Cl(-) with HCO3(-). hBest1 was highly permeable to HCO3(-) (P HCO3)/PCl = approximately 0.44). hBest2, hBest4, and mBest2 had an even higher relative HCO3(-) permeability (P HCO3/PCl = 0.6-0.7). All four bestrophins had HCO3(-) conductances that were nearly the same as Cl(-) (G HCO3/GCl = 0.9-1.1). Extracellular Na+ did not affect the permeation of hBest1 to HCO3(-). At physiological HCO3(-) concentration, HCO3(-) was also highly conductive. The hBest1 disease-causing mutations Y85H, R92C, and W93C abolished both Cl(-) and HCO3(-) currents equally. The V78C mutation changed P HCO3/PCl and G HCO3/GCl of mBest2 channels. These results raise the possibility that disease-causing mutations in hBest1 produce disease by altering HCO3(-) homeostasis as well as Cl(-) transport in the retina.
Qu
Zhiqiang
Z
Department of Cell Biology and Center for Neurodegenerative Disease, Emory University School of Medicine, Atlanta, GA 30322-3030, USA. zqu@emory.edu
Hartzell
H Criss
HC
D000818:Animals; D000075002:Bestrophins; D001639:Bicarbonates / Q000378:metabolism*; D002460:Cell Line; D002463:Cell Membrane Permeability; D018118:Chloride Channels / Q000235:genetics / Q000378:metabolism*; D005136:Eye Proteins / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008564:Membrane Potentials; D051379:Mice; D009124:Muscle Proteins / Q000378:metabolism; D009154:Mutation; D012964:Sodium / Q000378:metabolism; D014162:Transfection
C113133:BEST1 protein, human; C465970:BEST2 protein, human; C465972:BEST3 protein, human; C465971:BEST4 protein, human; C497463:Best2 protein, mouse; D000075002:Bestrophins; D001639:Bicarbonates; D018118:Chloride Channels; D005136:Eye Proteins; D009124:Muscle Proteins; D012964:Sodium