Genetic dissection of renal mechanisms of systemicpH homeostasis in man and mouse.

摘要

We have used a genome-wide linkage approach to determine that mutations in the gene ATP6B1 cause an autosomal recessive form of distal renal tubular acidosis (dRTA), a clinical condition in humans characterized by metabolic acidosis and an inappropriately alkaline urine. ATP6B1 encodes the B1-subunit of the vacuolar-type H+-ATPase, an energy-dependent proton (H+) pump expressed at the apical surfaces of acid-secreting intercalated cells (ICs) of the renal cortical (CCD) and medullary collecting duct. The phenotype of dRTA patients demonstrates that the B1-subunit, and by interference, apical H+-ATPase, is essential for normal distal nephron H+ secretion on a standard human diet. As sensorineural hearing loss cosegregates with dRTA in most kindreds harboring ATP6B1 mutations, our demonstration of B1-subunit expression in cochlea and endolymphatic sac suggests a local role for this protein in auditory physiology, perhaps in pH regulation of the inner ear fluid endolymph.; To further genetically dissect B1-subunit dependent physiological processes, we generated mice devoid of functional B1-subunit (Atp6b1−/− mice). On a standard diet, these mice produce an abnormally alkaline urine, and yet are not acidotic, probably due to the net alkali load of the rodent diet. However, following acid challenge, metabolic acidosis is more severe and urinary acidification is incomplete in Atp6b1−/− mice, demonstrating that the B1-subunit does play an important role in the murine response to systemic acid challenge. Furosemide infusion fails to induce a significant urinary acidification in Atp6b1−/− mice, confirming that their acidification defect localizes to the distal nephron, and demonstrating that electrogenic H+ secretion in the CCD is normally mediated by B1-subunit containing H+ pumps. Polarized expression of some H+-ATPase subunits persists in Atp6b1−/− CCD, suggesting that the H +-ATPase complex may be partially assembled in these cells. However, plasma membrane H+-ATPase activity is absent in isolated Atp6b1−/− CCD ICs, suggesting that all plasma membrane H+-ATPase activity is normally mediated by pumps containing the B1-subunit, rather than another B-subunit isoform. Our examination of the pathophysiology arising in humans and mice genetically-devoid of functional H+-ATPase B1-subunit thus demonstrates an important role for this gene product in the renal regulation of systemic pH homeostasis in both species.