Sequential forward and reverse transport of the Na+ Ca2+ exchanger generates Ca2+ oscillations within mitochondria

Krishna Samanta,Anant B Parekh,Gary R Mirams

doi:10.1038/s41467-017-02638-2

Krishna Samanta, Anant B Parekh + Show 1 more

Open Access

https://doi.org/10.1038/s41467-017-02638-2

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Abstract

Mitochondrial Ca2+ homoeostasis regulates aerobic metabolism and cell survival. Ca2+ flux into mitochondria is mediated by the mitochondrial calcium uniporter (MCU) channel whereas Ca2+ export is often through an electrogenic Na+–Ca2+ exchanger. Here, we report remarkable functional versatility in mitochondrial Na+–Ca2+ exchange under conditions where mitochondria are depolarised. Following physiological stimulation of cell-surface receptors, mitochondrial Na+–Ca2+ exchange initially operates in reverse mode, transporting cytosolic Ca2+ into the matrix. As matrix Ca2+ rises, the exchanger reverts to its forward mode state, extruding Ca2+. Transitions between reverse and forward modes generate repetitive oscillations in matrix Ca2+. We further show that reverse mode Na+–Ca2+ activity is regulated by the mitochondrial fusion protein mitofusin 2. Our results demonstrate that reversible switching between transport modes of an ion exchanger molecule generates functionally relevant oscillations in the levels of the universal Ca2+ messenger within an organelle.

Highlights

Mitochondrial Ca2+ homoeostasis regulates aerobic metabolism and cell survival
Flux through the mitochondrial calcium uniporter (MCU) complex is determined by the prevailing electrochemical Ca2+ gradient[6], with a major factor being the large electrical driving force that arises from the negative potential (~−200 mV) across the inner mitochondrial membrane
Mitochondrial fusion is regulated by dynamin-related protein (Drp1) where outer and inner mitochondrial membrane fusion depend on mitofusin 1 and mitofusin 2, and OPA1, respectively[12]

Summary

Introduction

Ca2+ flux into mitochondria is mediated by the mitochondrial calcium uniporter (MCU) channel whereas Ca2+ export is often through an electrogenic Na+–Ca2+ exchanger. Following physiological stimulation of cell-surface receptors, mitochondrial Na+–Ca2+ exchange initially operates in reverse mode, transporting cytosolic Ca2+ into the matrix. We further show that reverse mode Na+–Ca2+ activity is regulated by the mitochondrial fusion protein mitofusin 2. Our results demonstrate that reversible switching between transport modes of an ion exchanger molecule generates functionally relevant oscillations in the levels of the universal Ca2+ messenger within an organelle. Previous work has shown cytosolic Ca2+ oscillations following stimulation of native cysteinyl leukotriene type I receptors in mast cells with the natural agonist leukotriene C4 are propagated rapidly and faithfully into mitochondria to generate oscillations in matrix Ca2+ Loss of mitochondrial Ca2+ buffering resulted in rundown of cytosolic Ca2+ oscillations, which arose through enhanced Ca2+-dependent inactivation of InsP3 receptors

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