Proc. Nati. Acad. Sci. USA Vol. 86, pp. 539-543, January 1989 Cell Biology A possible role for Na',K+-ATPase in regulating ATP-dependent endosome acidification RENATE FUCHS, SANDRA SCHMID, AND IRA MELLMAN* Department of Cell Biology, Yale University School of Medicine, 333 Cedar Street, P.O. Box 3333, New Haven, CT 06510 Communicated by Robert W. Berliner, September 16, 1988 ABSTRACT Endosomes maintain a slightly acidic internal (CHO) cells by free flow electrophoresis, we have recently pH, which is directly responsible for their ability to ensure demonstrated that, even in vitro, kinetically "early" endo- proper sorting of incoming receptors and ligands during somes-i.e., those involved in rapid receptor recycling-are endocytosis. At least two distinct subpopulations of endosomes less capable of ATP-dependent acidification than are "late" can be distinguished, designated "early" and "late" on the endosomes, a distinct subpopulation associated with the basis of their kinetics of labeling with endocytic tracers. The transport of internalized macromolecules to lysosomes (10). subpopulations differ not only in their functions (rapid receptor Since the factors that regulate endosomal pH remain recycling and transport to lysosomes, respectively) but also in unknown, we have sought to investigate the possible basis for their capacities for acidification in intact cells and in vitro. To pH regulation in early and late endosomes and lysosomes. investigate the possible basis for pH regulation in endosomes, Our results indicate that ATP-dependent proton transport in we have studied the transport properties and ion permeabilities endosomes is electrogenic and is accompanied by the gen- of early and late endosomes isolated from Chinese hamster eration of an interior-positive membrane potential that op- ovary cells. Using endosomes selectively labeled with pH- poses continued proton translocation. Specifically in early sensitive endocytic tracers, we found that ATP-dependent endosomes, acidification appears to be further modulated by acidification is electrogenic, being accompanied by the gener- a second electrogenic ion transport system-namely, the ation of an interior-positive membrane potential which opposes Na',K+-ATPase, which is present in the same membrane. further acidification. While membrane potential and, conse- Since the activity ofthe Na' ,K+-ATPase would also result in quently, acidification was controlled by the influx of permeant the generation ofan interior-positive potential, its presence in anions and efflux of protons and alkali cations, acidification early endosomes from CHO cells might explain the limited was further modulated in Na' and K+-containing buffers by capacity of this subpopulation for acidification in vitro and in the ouabain- and vanadate-sensitive Na',K+-ATPase, which vivo. Indeed, recent measurements ofendosome acidification appears to be a functional component of the endosomal in intact cells have suggested that early endosome acidifica- membrane. The data suggest that electrogenic Na' transport tion is enhanced in cells grown in the Na',K+-ATPase due to Na',K+-ATPase activity contributes to the interior- inhibitor ouabain (11). positive membrane potential, thereby reducing ATP-depen- dent H' transport. Importantly, inhibition of acidification by Na',K+-ATPase activity was found only in early endosomes, MATERIALS AND METHODS consistent with their limited acidification capacity relative to Cells and Cell Culture. CHO cells were maintained in late endosomes and lysosomes. suspension culture in a-MEM supplemented with 5% fetal calf serum (J. R. Scientific, Woodland, CA) and penicillin/ It is now well established that acidic pH in endocytic streptomycin. For experiments, cells were plated in complete organelles plays a critical role in maintaining the orderly medium on 15-cm tissue culture dishes (Falcon) 2-3 days traffic and processing of receptors and ligands internalized before use, after which time the monolayers were slightly during endocytosis (for review, see ref. 1). This is particularly subconfluent. true in endosomes, where the low pH facilitates the disso- Endosome Labeling. Endosomes were labeled with either ciation of many receptor-ligand complexes and, accordingly, of two pH-sensitive endocytic tracers, fluorescein isothio- the recycling offree receptors back to the cell surface. While cyanate (FITC)-conjugated dextran (FITC-dextran) (Mr, endosomes, like lysosomes, are known to lower their internal 70,000; Sigma) (2) or FITC-conjugated transfemn (FITC- pH by an ATP-driven proton pump (2, 3), the mechanisms Tfn) prepared as described (12). For labeling with FITC-Tfn, that regulate acidification in either organelle are unclear. which selectively labels early endosomes (10, 12), the mono- Lysosomes are typically the most acidic organelle in mam- layers were washed three times with phosphate-buffered malian cells and generally maintain their internal pH at 4.7- saline (PBS), incubated in serum-free medium for 30 min at 4.8 (4). Endosomes exhibit a greater range of pH values that 37TC, and then in serum-free medium containing FITC-Tfn can vary between 5.0 and 6.5 within a single cell (5, 6). (20 ,ug/ml) for an additional 30-60 min. Inclusion of uncon- Given the role played by endosomal pH in regulating jugated (250 ktg/ml) during uptake decreased the amount of endocytic membrane traffic, it is likely that the observed cell-associated FITC to undetectable levels, indicating that variations in pH are functionally significant. In intact cells, FITC-Tfn endocytosis was specific for the Tfn receptor. For material internalized during endocytosis encounters endo- some experiments, surface-bound FITC-Tfn was removed somes of increasingly acidic pH en route to lysosomes (3, 5- prior to harvest by incubation in pH 5 and then in pH 7 9). This gradient in pH reflects the existence of distinct medium containing the iron chelator desferoxamine (12). subpopulations of endosomes that differ not only in their Labeling with FITC-dextran (5-10 mg/ml) was performed capacity for acidification but also in composition and function in complete medium at 370C for 5 min to label early endo- (10). Using endosomes isolated from Chinese hamster ovary somes, 5 min followed by a 10-min chase in FITC- dextran-free medium to label late endosomes, and 5 min The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" Abbreviations: FITC, fluorescein isothiocyanate; Tfn, transferrin. in accordance with 18 U.S.C. §1734 solely to indicate this fact. *To whom reprint requests should be addressed. 539 Downloaded by guest on September 25, 2021 540 Cell Biology: Fuchs et al. Proc. Natl. Acad. Sci. USA 86 (1989) followed by a 45-min chase to label lysosomes, as described tion. Fig. 1A illustrates the ATP-dependent acidification of (10). After the labeling period, the monolayers were washed endosomes from CHO cells equilibrated in isosmotic KCl several times with cold PBS and harvested with a rubber buffer. Addition of 2.5 mM ATP caused a rapid quenching of scraper. FITC fluorescence reflecting intravesicular acidification. Isolation of Enriched Endosome Fractions. FITC-Tfn (or When Cl- was replaced with a relatively impermeant organic FITC-dextran)-labeled cells were washed three times by anion (gluconate), acidification was partially inhibited (Fig. centrifugation (350 x g, 5 min at 40C) and resuspended in cold 1B). This suggests that external Cl- helps to dissipate an PBS. The final pellet was suspended in cold TEA buffer (0.25 interior-positive membrane potential formed by the electro- M sucrose/10 mM triethanolamine/10 mM NaOAc/1 mM genic translocation of protons into the endosome. Indeed, EDTA/1 mM phenylmethylsulfonyl fluoride, pH 7.4) and acidification in potassium gluconate buffer was stimulated by disrupted with a stainless steel Dounce homogenizer (Kontes). the K+ ionophore valinomycin (Fig. 1B) by permitting the Enriched endosomal fractions were prepared from the post- rapid efflux of internal K+ down the electrical gradient. nuclear supernatants by centrifugation in discontinuous su- Addition of valinomycin to endosomes in KCI buffer had no crose gradients (40,000 rpm, 1.5 hr, 40C; Beckman SW40 rotor) effect (Fig. 1A). Thus, ATP-driven endosome acidification is (12). The gradients were formed by adjusting the postnuclear electrogenic and, like acidification of isolated lysosome (16, supernatant to 1.15 M sucrose and overlaying with 1 M, 0.86 17), coated vesicle (18, 19), and Golgi fractions (20), it is M, and 0.25 M sucrose in TEA buffer; most of the FITC- partly dependent on the membrane's permeability to Cl-. containing endosomes were recovered at the 0.86/1.0 M In addition, the endosomes were found to have a significant interface. As described (10), cell fractionation by free flow conductance for H', which could also be involved in regu- electrophoresis and Percoll density-gradient centrifugation lating H' accumulation via an electrogenic ATPase. Follow- confirmed that under the labeling conditions used, the en- ing acidification of KCI-equilibrated endosomes, removal of docytic tracers were confined to early and/or late endosomes ATP (by adding hexokinase and glucose) resulted in a as indicated. dissipation of the transmembrane pH gradient (Fig. 1C). The Cell-Free Assay of ATP-Dependent Acidification. ATP- rate of H' efflux after removal of ATP (ti,, -5.8 min) was dependent acidification of isolated endosomes was deter- almost half