New Findings on the Pathogenesis of Distal Renal Tubular Acidosis

Abstract

Background: Distal renal tubular acidosis (dRTA) is characterized by an impairment of the urinary acidification process in the distal nephron. Complete or incomplete metabolic acidosis coupled with inappropriately alkaline urine are the hallmarks of this condition. Genetic forms of dRTA are caused by loss of function mutations of either SLC4A1, encoding the AE1 anion exchanger, or ATP6V1B1 and ATP6V0A4, encoding for the B1 and a4 subunits of the vH⁺ATPase, respectively. These genes are crucial for the function of A-type intercalated cells (A-IC) of the distal nephron.

Summary: Alterations of acid-base homeostasis are variably associated with hypokalemia, hypercalciuria, nephrocalcinosis or nephrolithiasis, and a salt-losing phenotype. Here we report the diagnostic test and the underlying physiopathological mechanisms. The molecular mechanisms identified so far can explain the defect in acid secretion, but do not explain all clinical features. We review the latest experimental findings on the pathogenesis of dRTA, reporting mechanisms that are instrumental for the clinician and potentially inspiring a novel therapeutic strategy.

Key message: Primary dRTA is usually intended as a single-cell disease because the A-IC are mainly affected. However, novel evidence shows that different cell types of the nephron may contribute to the signs and symptoms, moving the focus from a single-cell towards a renal disease.

Keywords: AE1; Intercalated cells; Metabolic acidosis; Renal tubular acidosis; vH+ATPase.

Fig. 1

Schematic representation of the novel identified mechanisms of diseases caused by R589H AE1 mutation. On the left, a normal A-IC is represented, while on the right, an A-IC expressing the R589H AE1 mutation is shown. We hypothesized that reduced expression of R589H AE1 leads to alkalinization of intracellular pH because of intracellular bicarbonate retention. This leads to actin cytoskeleton depolymerization and thus to intracellular retention of vH⁺ATPase.

Fig. 2

Epithelial cell involvement in experimental models of dRTA secondary to vH⁺ATPase dysfunction. ATP6V0A4 knockout (KO) mice present with signs of dysfunction of proximal tubule cells, while ATP6V1B1 KO mice show an indirect involvement of cortical and medullary principal cells secondary to A-IC dysfunction. Finally, genetic ablation of ATP6AP2 shows a critical role of medullary thick ascending limb and principal cells together with A-IC. This latter vH⁺ATPase subunit has not been identified yet in humans as a cause of dRTA.