Theoretical considerations on the role of membrane potential in the regulation of endosomal pH

Abstract

Na+,K(+)-ATPase has been observed to partially inhibit acidification of early endosomes by increasing membrane potential, whereas chloride channels have been observed to enhance acidification in endosomes and lysosomes. However, little theoretical analysis of the ways in which different pumps and channels may interact has been carried out. We therefore developed quantitative models of endosomal pH regulation based on thermodynamic considerations. We conclude that 1) both size and shape of endosomes will influence steady-state endosomal pH whenever membrane potential due to the pH gradient limits proton pumping, 2) steady-state pH values similar to those observed in early endosomes of living cells can occur in endosomes containing just H(+)-ATPases and Na+,K(+)-ATPases when low endosomal buffering capacities are present, and 3) inclusion of active chloride channels results in predicted pH values well below those observed in vivo. The results support the separation of endocytic compartments into two classes, those (such as early endosomes) whose acidification is limited by attainment of a certain membrane potential, and those (such as lysosomes) whose acidification is limited by the attainment of a certain pH. The theoretical framework and conclusions described are potentially applicable to other membrane-enclosed compartments that are acidified, such as elements of the Golgi apparatus.

ClC-3 Is an Intracellular Chloride/Proton Exchanger with Large Voltage-Dependent Nonlinear Capacitance

“…The calculations were performed closely following the methodology introduced by Rybak et al 47 It uses a numerical solution of the equations describing the ionic distribution at equilibrium and is independent of the number of transporters and pumps in the vesicular membrane (see the Supporting Information). The endosomes are modeled as lipid spheres with different diameter and incorporated ATPases that pump protons into the vesicle lumen (Figure 6a).…”

“…As a consequence, the cumulative electrochemical gradient for all ions is limited by the energy provided by the hydrolysis of one molecule of ATP. 47 Proton pumping automatically leads to an increase of the concentration of positive charges (protons) into the vesicular lumen and to more positive luminal electric potential. This creates a negative feedback loop that opposes efficient proton pumping by the V-type ATP-ases.…”

“…(d, e) Estimated effects of the membrane capacitance on intravesicular potential and pH, respectively. Calculations were performed as described by Rybak et al 47 the nonlinear capacitances of ClC-3 by neutralizing the proton glutamate E281. The proton glutamate mutants, 27,28,37,38 resulted in defined alteration of ClC-3 function that closely resembled the effects of the homologous mutations in ClC-4 and ClC-5 (Figures 1, 3, and S1).…”

“…The specialization of the ClC isoforms might represent functional optimization in order to fulfill specific physiological requirements in distinct intracellular compartments. The capacitance of a cellular organelle determines the number of charges necessary to establish a certain membrane potential, 47 and thus plays crucial roles in shaping basic physiological processes like action potential generation and propagation. Computational analysis (Figure 6) suggests that insertion of a ClC protein into the membrane will increase the capacitance and thus limit the electrochemical barrier for protons and accordingly support luminal acidification.…”

Evidence that translocation of anthrax toxin's lethal factor is initiated by entry of its N terminus into the protective antigen channel

“…There is consensus that the interior of the compartment is positive with respect to the cytosol: the same polarity that promotes channel blocking. Estimates of the magnitude of the potential vary widely, however, from ϩ10 to ϩ300 mV (23)(24)(25). Whereas the pore might discharge the transmembrane potential, the N terminus of LF or EF inserted into the prepore might block the pore before it opens.…”