Renal NHE3 and NaPi2 partition into distinct membrane domains

Abstract

Hypertension provokes differential trafficking of the renal proximal tubule Na+/H+ exchanger 3 (NHE3) to the base of the apical microvilli and Na+-Pi cotransporter 2 (NaPi2) to endosomes. The resultant diuresis and natriuresis are key to blood pressure control. We tested the hypothesis that this differential trafficking of NHE3 vs. NaPi2 was associated with partitioning to distinct membrane domains. In anesthetized rats, arterial pressure was increased (104 ± 2 to 142 ± 4 mmHg, 15 min) by arterial constriction and urine output increased 23-fold. Renal membranes were fractionated by cold 1% Triton X-100 extraction then centrifugation through OptiPrep flotation gradients. In controls, 84 ± 9% of NHE3 localized to flotillin-enriched lipid raft domains and 69 ± 5% of NaPi2 localized to transferrin receptor-enriched nonrafts. MyosinVI and dipeptidyl peptidase IV, associated with NHE3 regulation, coenriched in lipid rafts with NHE3, while NHE regulatory factor-1 coenriched in nonrafts with NaPi2. Partitioning was not altered by hypertension. Detergent insoluble membranes were pelleted after detergent extraction. NHE3 detergent insolubility decreased as it redistributed from body (80 ± 10% detergent insoluble) to base (75 ± 3%) of the apical microvilli, while NaPi2 partitioned into more insoluble domains as it moved from the microvilli (45 ± 7% detergent insoluble) to endosomes (82 ± 1%). In conclusion, NHE3 and NaPi2, while both localized to apical microvilli, are segregated into domains: NHE3 to lipid rafts and NaPi2 to nonrafts. These domain properties may play a role in the distinct trafficking patterns observed during elevated pressures: NHE3 remains in rafts and settles to the base of the microvilli while NaPi2 is freely endocytosed.