OPA1 promotes pH flashes that spread between contiguous mitochondria without matrix protein exchange

Jaime Santo-Domingo,Damon Poburko,Marta Giacomello,Luca Scorrano,Nicolas Demaurex

doi:10.1038/emboj.2013.124

Abstract

The chemical nature and functional significance of mitochondrial flashes associated with fluctuations in mitochondrial membrane potential is unclear. Using a ratiometric pH probe insensitive to superoxide, we show that flashes reflect matrix alkalinization transients of ∼0.4 pH units that persist in cells permeabilized in ion-free solutions and can be evoked by imposed mitochondrial depolarization. Ablation of the pro-fusion protein Optic atrophy 1 specifically abrogated pH flashes and reduced the propagation of matrix photoactivated GFP (paGFP). Ablation or invalidation of the pro-fission Dynamin-related protein 1 greatly enhanced flash propagation between contiguous mitochondria but marginally increased paGFP matrix diffusion, indicating that flashes propagate without matrix content exchange. The pH flashes were associated with synchronous depolarization and hyperpolarization events that promoted the membrane potential equilibration of juxtaposed mitochondria. We propose that flashes are energy conservation events triggered by the opening of a fusion pore between two contiguous mitochondria of different membrane potentials, propagating without matrix fusion to equilibrate the energetic state of connected mitochondria.

Highlights

Mitochondria are double-membrane organelles that play a central role in cellular energy conversion, lipid metabolism, calcium signalling, and apoptosis
Spontaneous DCm fluctuations are observed in permeabilized cells (Uechi et al, 2006) and in isolated mitochondria, where they are modulated by adenine nucleotides acting from the matrix side (Vergun et al, 2003; Vergun and Reynolds, 2004)
Superoxide flashes coinciding with DCm decreases were reported in individual mitochondria from skeletal muscle and intact beating hearts (Wang et al, 2008; Pouvreau, 2010; Fang et al, 2011; Wei et al, 2011), but the circularly permutated yellow fluorescent protein (cpYFP) probe used was shown to be highly sensitive to pH (Schwarzlander et al, 2011) and was subsequently used to report matrix alkalinization transients in mitochondria from Arabidopsis thaliana root cells (Schwarzlander et al, 2012a)

Summary

Introduction

Mitochondria are double-membrane organelles that play a central role in cellular energy conversion, lipid metabolism, calcium signalling, and apoptosis. A plethora of mechanisms were proposed to trigger the DCm fluctuations: local Ca2 þ elevations (Duchen et al, 1998), opening of the mitochondrial permeability transition pore (mPTP) (Huser and Blatter, 1999; De Giorgi et al, 2000; Zorov et al, 2000; Jacobson and Duchen, 2002), coupling of DCm to the ATP synthase (Thiffault and Bennett, 2005), switching between active and inactive states of oxidative phosphorylation (Buckman and Reynolds, 2001), or opening of a proton-selective channel by matrix alkalinization (Hattori et al, 2005). The superoxide nature of the flashes is disputed, (Muller, 2009), and because cpYFP is pH sensitive (Nagai et al, 2001) several groups have instead proposed that the flashes are transient mitochondrial matrix pH (pHmito) elevations (Azarias and Chatton, 2011; Schwarzlander et al, 2011, 2012a), reviewed in Santo-Domingo and Demaurex (2012) and Schwarzlander et al (2012b)

Methods

Results

Conclusion