Two Rhesus protein ammonia transporters team up to eliminate ammonium into urine.

RENAL AMMONIAGENESIS SERVES the de novo synthesis of bicarbonate consumed by metabolism and the excretion of acid. Ammonium is eventually excreted into urine after accumulation in the medullary interstitium. The process of ammonium excretion was long thought to be a mostly passive process mediated by simple diffusion of NH3 and NH4 and driven by urinary acidification and consequent trapping of ammonium in urine. The discovery of Rhesus proteins capable of transporting NH3 has revolutionized our view on renal ammonium excretion. The kidney expresses two members of this family, Rh B Glycoprotein (RhBG) and Rh G Glycoprotein (RhCG), and whereas RhCG is found both in apical and basolateral membranes of cells along the collecting duct, RhBG is restricted to the basolateral membrane of the same cells. New work presented in a recent issue of the American Journal of PhysiologyRenal Physiology (7) demonstrates that combined deletion of both transporters causes a more severe defect in urinary ammonium excretion than single deletion of each transporter, suggesting that both transporters cooperate in final urinary elimination of ammonium. Renal ammoniagenesis is critical for the maintenance of acid-base homeostasis and the renal defense against acidosis. Proximal tubular glutamine metabolism can yield two molecules of NH3 and HCO3 allowing the buffering of acids generated by metabolism (5). The synthesis of ammonia involves several steps catalyzed by mitochondrial phosphatedependent glutaminase and glutamate dehydrogenase and fuels -ketoglutarate into gluconeogenesis where phospho-enol pyruvate carboxy kinase (PEPCK) acts as a key enzyme. At physiological pH, most of the NH3 will bind a proton and thereby contribute to the elimination of acid as well as to the reabsorption of bicarbonate. A major fraction of NH4 excreted into the proximal tubule lumen is reabsorbed by the Na-K-2Cl cotransporter (NKCC2) in the thick ascending limb of the loop of Henle and accumulated in the medullary interstitium, possibly by binding to sulfatides (9). The final excretion of ammonium into urine occurs at the level of the collecting duct system. NH3/NH4 is taken up from the interstitium and secreted into urine where NH3 recombines with protons secreted by H -ATPases (and H -K -ATPases), and is finally trapped as NH4 . Ammonium to be excreted from the interstitium into urine must pass two plasma membranes, the basolateral and luminal membranes of cells lining the collecting duct (11). In vitro microperfusion experiments demonstrated that passage of ammonium across the basolateral membrane can be mediated by Na -K -2Cl -cotransporters and Na -K -ATPases where NH4 substitutes for potassium. However, mice lacking the NKCC1 cotransporter have no problem excreting ammonium into urine, suggesting that either NKCC1, the only Na -K -2Cl cotransporter expressed in the collecting duct, is not involved in ammonium transport in vivo, or that its absence can be fully compensated (12, 13). The discovery that members of the Rhesus family of membrane proteins, RhAG, RhBG, and RhCG, can transport NH3/ NH4 suggested that these proteins may participate in renal and extrarenal ammonium transport (8). RhAG is absent from the kidney, whereas RhBG and RhCG are highly expressed in the kidney collecting duct. RhCG is found in all type A intercalated cells (Fig. 1) and segment-specific cells of the initial collecting duct system at both the luminal and basolateral membranes. RhBG is found in the same cells as RhCG but exclusively at the basolateral membrane (14). The functional relevance of these two proteins was first revealed in a series of mouse studies with complete or partial deletions of Rhbg or Rhcg KO (2). These studies showed that complete deletion of RhCG leads to a strong reduction in urinary ammonium excretion and severe metabolic acidosis. Mice with partial deletion suffer also from reduced ammonium excretion (14). Moreover, microperfusion experiments demonstrated that luminal NH3 permeability was drastically reduced and that total transepithelial NH3 transport abolished (2, 3). Thus Rhcg is absolutely required for urinary ammonium excretion by the collecting duct and is likely the only NH3 permeation pathway in the luminal membrane. But what about the basolateral uptake pathway? In the case of RhBG, two different mouse models have been generated, a complete and a partial knockout (KO). The total Rhbg KO exhibits no impairment of urinary ammonium excretion; normal ability to regulate systemic acid-base homeostasis and to transport NH3 or NH4 across the basolateral membrane was observed (4). In contrast, a mouse model with partial Rhbg deletion had mildly reduced urinary ammonium excretion (1). A major difference between both reports was the amount of acid loaded, which was much higher in the partial Rhbg KO mouse model, suggesting that Rhbg may become limiting only under very extreme conditions. Now, in a report published in a recent issue of the American Journal of Physiology-Renal Physiology, a mouse model lacking both Rhcg and Rhbg is presented (7). Deletion was achieved with Ksp-Cre mice driven by the cadherin promoter, which has no or only little activity in the late distal convoluted tubule and connecting tubule, leaving Rhbg and Rhcg expression intact in these early parts of the collecting duct system. Nevertheless, these mice excrete 30–50% less ammonium than their wild-type controls. This relatively mild phenotype may be due to a massive compensatory increase in key molecules of proximal tubular ammoniagenesis such as PDG and PEPCK as well as the NHE3 Na /H exchanger. UnfortuAddress for reprint requests and other correspondence: C. A Wagner, Institute of Physiology, Univ. of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland (e-mail: [email protected]). Am J Physiol Renal Physiol 306: F721–F723, 2014; doi:10.1152/ajprenal.00681.2013. Editorial Focus