Qu Zhiqiang, Hartzell H Criss
Department of Cell Biology and Center for Neurodegenerative Disease, Emory University School of Medicine, Atlanta, GA 30322-3030, USA.
Am J Physiol Cell Physiol. 2008 Jun;294(6):C1371-7. doi: 10.1152/ajpcell.00398.2007. Epub 2008 Apr 9.
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.
贝斯特罗芬-1（Best1）是一种氯离子通道，与人类和犬类的多种视网膜病变相关。有人提出，Best1氯离子通道功能障碍会因视网膜色素上皮（RPE）跨膜氯离子转运改变而导致视网膜病变。除了氯离子外，许多氯离子通道也能转运碳酸氢根离子。由于碳酸氢根离子在pH调节以及RPE的液体和离子转运中具有重要生理意义，我们测定了贝斯特罗芬对碳酸氢根离子相对于氯离子的通透性和电导。四种人类贝斯特罗芬同源物（hBest1、hBest2、hBest3和hBest4）以及小鼠Best2（mBest2）在人胚肾细胞（HEK细胞）中表达，并测定了相对碳酸氢根离子通透性（P HCO3/PCl）和电导（G HCO3/GCl）。P HCO3/PCl由用碳酸氢根离子替代细胞外氯离子所产生的反转电位（Erev）变化计算得出。hBest1对碳酸氢根离子具有高通透性（P HCO3/PCl约为0.44）。hBest2、hBest4和mBest2具有更高的相对碳酸氢根离子通透性（P HCO3/PCl = 0.6 - 0.7）。所有四种贝斯特罗芬的碳酸氢根离子电导与氯离子几乎相同（G HCO3/GCl = 0.9 - 1.1）。细胞外钠离子不影响hBest1对碳酸氢根离子的通透。在生理碳酸氢根离子浓度下，碳酸氢根离子也具有高导电性。hBest1致病突变Y85H、R92C和W93C同等程度地消除了氯离子和碳酸氢根离子电流。V78C突变改变了mBest2通道的P HCO3/PCl和G HCO3/GCl。这些结果增加了hBest1致病突变通过改变视网膜中碳酸氢根离子稳态以及氯离子转运而导致疾病的可能性。
