A possible role for Na+,K+-ATPase in regulating ATP-dependent endosome acidification.

Abstract

Endosomes maintain a slightly acidic internal pH, which is directly responsible for their ability to ensure proper sorting of incoming receptors and ligands during endocytosis. At least two distinct subpopulations of endosomes can be distinguished, designated "early" and "late" on the basis of their kinetics of labeling with endocytic tracers. The subpopulations differ not only in their functions (rapid receptor recycling and transport to lysosomes, respectively) but also in their capacities for acidification in intact cells and in vitro. To investigate the possible basis for pH regulation in endosomes, we have studied the transport properties and ion permeabilities of early and late endosomes isolated from Chinese hamster ovary cells. Using endosomes selectively labeled with pHsensitive endocytic tracers, we found that ATP-dependent acidification is electrogenic, being accompanied by the generation of an interior-positive membrane potential which opposes further acidification. While membrane potential and, consequently, acidification was controlled by the influx of permeant anions and efflux of protons and alkali cations, acidification was further modulated in Na' and K+-containing buffers by the ouabain-and vanadate-sensitive Na',K+-ATPase, which appears to be a functional component of the endosomal membrane. The data suggest that electrogenic Na' transport due to Na',K+-ATPase activity contributes to the interiorpositive membrane potential, thereby reducing ATP-dependent H' transport. Importantly, inhibition of acidification by Na',K+-ATPase activity was found only in early endosomes, consistent with their limited acidification capacity relative to late endosomes and lysosomes.It is now well established that acidic pH in endocytic organelles plays a critical role in maintaining the orderly traffic and processing of receptors and ligands internalized during endocytosis (for review, see ref. 1). This is particularly true in endosomes, where the low pH facilitates the dissociation of many receptor-ligand complexes and, accordingly, the recycling of free receptors back to the cell surface. While endosomes, like lysosomes, are known to lower their internal pH by an ATP-driven proton pump (2, 3), the mechanisms that regulate acidification in either organelle are unclear. Lysosomes are typically the most acidic organelle in mammalian cells and generally maintain their internal pH at 4.7-4.8 (4). Endosomes exhibit a greater range of pH values that can vary between 5.0 and 6.5 within a single cell (5, 6).Given the role played by endosomal pH in regulating endocytic membrane traffic, it is likely that the observed variations in pH are functionally significant. In intact cells, material internalized during endocytosis encounters endosomes of increasingly acidic pH en route to lysosomes (3,(5)(6)(7)(8)(9). This gradient in pH reflects the existence of distinct subpopulations of endosomes that differ not only in their capacity for acidification but also in composition and function (10). Using endosomes isolated fr...

Stochastic Model of Acidification, Activation of Hemagglutinin and Escape of Influenza Viruses from an Endosome

“…However, Equation (6) alone is not sufficient to account for endosomal maturation, because the final pH ∞ [12] and the permeability L decreases with the endosomal maturation [19] as they depend on time, as we analyse below.…”

“….). The ionic concentrations inside endosome are tightly regulated by channels, exchangers and leak and in particular, by raising the interior-positive membrane potential, Na-K ATPase exchangers have been proposed to limit the acidification of early compared to late endosomes [19].…”

“…where ǫ = 1/n s and we use the Wentzel-Kramers-Brillouin (WKB) expansion of the mean first passage time (MFPT) τ (c) of the scaled number of protonated sites x(t, c) to the (unknown) critical threshold 0 < x T = ǫn T < ǫn s [11,14,[29][30][31] (22) Replacing the transition rates r(x, c) and l(x) by their expressions (19) in Equation (22), we obtain that (see Section 4)…”

Endocytosis in health and disease—a thematic issue dedicated to Renate Fuchs

“…Almost 100 years later, it was shown that endosomes maintain an acidic internal pH as the result of the action of a membrane proton pump and that this acidification was required for proper endosomal sorting function. Renate Fuchs was among the first researchers who investigated the mechanism of endosomal acidification and demonstrated that subpopulations of endosomes maintain distinct pH values [2].…”

“…An overview of the known endocytic pathways and subcompartments is given in the article "Endocytic pathways and endosomal trafficking: a primer" contributed by Sarah R. Elkin, Ashley M. Lakoduk, and Sandra Schmid from the UT Southwestern Medical Center, Dallas, Texas, USA. Sandy Schmid and Renate Fuchs met at Yale University and collaborated on identification of endosome subpopulations and their acidification [2,3].…”