The Intracellular Na+/H+ Exchanger NHE7 Effects a Na+-Coupled, but Not K+-Coupled Proton-Loading Mechanism in Endocytosis

Vesicular H+-ATPases and ClC-chloride transporters are described to acidify intracellular compartments, which also express the highly conserved Na+/H+ exchangers NHE6, NHE7, and NHE9. Mutations of these exchangers cause autism-spectrum disorders and neurodegeneration. NHE6, NHE7, and NHE9 are hypothesized to exchange cytosolic K+ for H+ and alkalinize vesicles, but this notion has remained untested in K+ because their intracellular localization prevents functional measurements. Using protonkilling techniques, we selected a cell line that expresses wild-type NHE7 at the plasma membrane, enabling measurement of the exchanger's transport parameters. We found that NHE7 transports Li+ and Na+, but not K+, is nonreversible in physiological conditions and is constitutively activated by cytosolic H+. Therefore, NHE7 acts as a proton-loading transporter rather than a proton leak. NHE7 mediates an acidification of intracellular vesicles that is additive to that of V-ATPases and that accelerates endocytosis. This study reveals an unexpected function for vesicular Na+/H+ exchangers and provides clues for understanding NHE-linked neurological disorders.