Cordat Emmanuelle, Reithmeier Reinhart A F
Department of Biochemistry, Rm. 5216, Medical Sciences Bldg., Univ. of Toronto, Toronto, Ontario M5S 1A8, Canada.
Am J Physiol Renal Physiol. 2006 Dec;291(6):F1354-61. doi: 10.1152/ajprenal.00015.2006. Epub 2006 Jul 18.
Kidney AE1 (kAE1) is a glycoprotein responsible for the electroneutral exchange of chloride for bicarbonate, promoting the reabsorption of bicarbonate into the blood by alpha-intercalated cells of the collecting tubule. Mutations occurring in the gene encoding kAE1 can induce defects in urinary acidification resulting in distal renal tubular acidosis (dRTA). We expressed two kAE1 dRTA mutants, A858D, a mild dominant mutation, and DeltaV850, a recessive mutation, in epithelial Madin-Darby canine kidney (MDCK) cells. Individuals heterozygous with wild-type (WT) kAE1 either did not display any symptoms of dRTA (DeltaV850/WT) or displayed a mild incomplete form of dRTA (A858D/WT), while compound heterozygotes (DeltaV850/A858D) had dRTA. We found that the A858D mutant was slightly impaired in the endoplasmic reticulum (ER) exit but could target to the basolateral membrane of polarized MDCK cells. Despite an altered binding to an inhibitor affinity resin, anion transport assays showed that the A858D mutant was functional at the cell surface. The DeltaV850 mutant showed altered binding to the affinity resin but was predominantly retained in the ER, resulting in undetectable AE1 expression at the basolateral membrane. When coexpressed in MDCK cells, the WT protein, and to a lesser extent the A858D mutant, enhanced the cell surface expression of the DeltaV850 mutant. The DeltaV850 mutant also affected the cell surface expression of the A858D mutant. Compound heterozygous (A858D/DeltaV850) patients likely possess a decreased amount of functional anion exchangers at the basolateral membrane of their alpha-intercalated cells, resulting in impaired bicarbonate transport into the blood and defective acid transport into the urine.
肾性阴离子交换蛋白1（kAE1）是一种糖蛋白，负责氯离子与碳酸氢根的电中性交换，促进集合小管α闰细胞将碳酸氢根重吸收回血液。编码kAE1的基因突变可导致尿酸化缺陷，从而引起远端肾小管酸中毒（dRTA）。我们在犬肾上皮细胞（MDCK）中表达了两种kAE1 dRTA突变体，即轻度显性突变A858D和隐性突变ΔV850。野生型（WT）kAE1的杂合个体要么未表现出任何dRTA症状（ΔV850/WT），要么表现出轻度不完全形式的dRTA（A858D/WT），而复合杂合子（ΔV850/A858D）则患有dRTA。我们发现，A858D突变体在内质网（ER）输出方面略有受损，但能够靶向极化MDCK细胞的基底外侧膜。尽管与抑制剂亲和树脂的结合发生了改变，但阴离子转运试验表明，A858D突变体在细胞表面具有功能。ΔV850突变体与亲和树脂的结合发生了改变，但主要保留在内质网中，导致基底外侧膜上无法检测到AE1表达。当在MDCK细胞中共表达时，WT蛋白以及程度较轻的A858D突变体增强了ΔV850突变体的细胞表面表达。ΔV850突变体也影响了A858D突变体的细胞表面表达。复合杂合子（A858D/ΔV850）患者的α闰细胞基底外侧膜上功能性阴离子交换体的数量可能减少，导致碳酸氢根向血液中的转运受损以及酸向尿液中的转运缺陷。
