The B1 H+-ATPase (Atp6v1b1) Subunit in Non–Type A Intercalated Cells is Required for Driving Pendrin Activity and the Renal Defense Against Alkalosis

Abstract

Significance Statement In the kidney, the B1 H+-ATPase subunit is mostly expressed in intercalated cells (IC). Its importance in acid-secreting type A ICs is evident in patients with inborn distal renal tubular acidosis and ATP6V1B1 mutations. However, the protein is also highly expressed in alkali-secreting non–type A ICs where its function is incompletely understood. We demonstrate in Atp6v1b1 knock out mice that the B1 subunit is critical for the renal response to defend against alkalosis during an alkali load or chronic furosemide treatment. These findings highlight the importance of non–type A ICs in maintaining acid–base balance in response to metabolic challenges or commonly used diuretics. Background Non–type A ICs in the collecting duct system express the luminal Cl−/HCO3 − exchanger pendrin and apical and/or basolateral H+-ATPases containing the B1 subunit isoform. Non–type A ICs excrete bicarbonate during metabolic alkalosis. Mutations in the B1 subunit (ATP6V1B1) cause distal renal tubular acidosis due to its role in acid secretory type A ICs. The function of B1 in non–type A ICs has remained elusive. Methods We examined the responses of Atp6v1b1 −/− and Atp6v1b1 +/+ mice to an alkali load and to chronic treatment with furosemide. Results An alkali load or 1 week of furosemide resulted in a more pronounced hypokalemic alkalosis in male ATP6v1b1 −/− versus Atp6v1b1 +/+ mice that could not be compensated by respiration. Total pendrin expression and activity in non–type A ICs of ex vivo microperfused cortical collecting ducts were reduced, and β 2-adrenergic stimulation of pendrin activity was blunted in ATP6v1b1 −/− mice. Basolateral H+-ATPase activity was strongly reduced, although the basolateral expression of the B2 isoform was increased. Ligation assays for H+-ATPase subunits indicated impaired assembly of V0 and V1 H+-ATPase domains. During chronic furosemide treatment, ATP6v1b1 −/− mice also showed polyuria and hyperchloremia versus Atp6v1b1 +/+ . The expression of pendrin, the water channel AQP2, and subunits of the epithelial sodium channel ENaC were reduced. Conclusions Our data demonstrate a critical role of H+-ATPases in non–type A ICs function protecting against alkalosis and reveal a hitherto unrecognized need of basolateral B1 isoform for a proper H+-ATPase complexes assembly and ability to be stimulated.