The renaissance of mitochondrial pH

Abstract

The generation of a proton gradient across the inner mitochondrial membrane (IMM) is an essential energy conservation event that couples the oxidation of carbohydrates and fat to the synthesis of ATP. Studies in isolated mitochondria have established that the chemical gradient for protons (pH m) and the mitochondrial membrane potential ( m) contribute independently to the proton-motive force (p) that drives the synthesis of ATP. Because  m contributes most of the p and can be easily measured in intact cells with fluorescent dyes, most studies ignore the contribution of pH m and only record changes in  m to track the metabolic state of mitochondria. pH m , however, drives the fluxes of metabolic substrates required for mitochondrial respiration and the activity of electroneutral ion exchangers that maintain mitochondria osmolarity and volume, and recent studies indicate that the mitochondrial pH (pH mito) plays an important and underappreciated role in physiological and pathological situations such as apoptosis, neurotransmission, and insulin secretion. In this Perspective, we discuss the putative roles of the pH mito and review the different techniques used to measure pH mito and pH m in isolated mitochondria and in intact cells, focusing on our recent results obtained with genetically encoded pH-sensitive indicators. These measurements have revealed that the pH mito is in dynamic equilibrium with the cytosolic pH and that spontaneous pH mito elevations coinciding with  m drops occur in single mitochondria. Unlike the "superoxide flashes" reported with a pH-sensitive circularly permuted YFP (cpYFP), these "pH flashes" preserve the p during spontaneous fluctuations in  m ; therefore, we propose that the flashes are energy conservation events that reflect the intrinsic properties of the mitochon-drial proton circuit. Correspondence to Nicolas Demaurex: N i c o l a s. D e m a u r e x @ u n i g e. c h Abbreviations used in this paper: CHX, Ca 2+-H + exchanger; cpYFP, circularly permuted YFP; p, proton-motive force; pH m , chemical gradient for protons;  m , mitochondrial membrane potential; IMM, inner mitochondrial membrane; IMS, intermembrane space; KHX, K +-H + exchanger; mPTP, permeability transition pore; NHX, Na +-H + exchanger; OXPHOS, oxidative phosphorylation; pH IMS , pH within the IMS; pH mito , mitochondrial pH; PiC, P i-H + phosphate cotransporter; ROS, reactive oxygen species; UCP, uncoupling protein.