Proceedings of The Physiological Society

Mitochondria: Form and function (London, UK) (2017) Proc Physiol Soc 38, SA01

Research Symposium

Calcium and beyond: cation channels in the inner mitochondrial membrane

D. De Stefani1

1. Department of Biomedical Sciences, University of Padova, Padova, PD, Italy.

The electron flow through the mitochondrial electron transport chain is coupled to proton pumping out of the organelle matrix, thus generating a large membrane potential. The potential energy is stored as capacitance and used for different activities, including ATP synthesis, heat production and cation transport, essentially of Ca2+ and K+. Among these functions, the latter has been historically poorly appreciated, mainly because of the lack of molecular information on mitochondrial channels mediating Ca2+ and K+ entry. In the case of Ca2+, the deadlock was suddenly broken few years ago thanks to the identification of the key component of mitochondrial Ca2+ uptake machinery, the Mitochondrial Calcium Uniporter (MCU). Two features of mitochondrial calcium signaling have been known for a long time: i) mitochondrial Ca2+ uptake widely varies among cells and tissues, and ii) channel opening relies on the extramitochondrial Ca2+ concentration, with low activity at resting [Ca2+] and a steep activation as soon as cytoplasmic [Ca2+] rises. This sigmoidal relationship prevents on one hand mitochondrial Ca2+ overload and ion vicious cycling in resting cells, and on the other hand it concurrently ensures a prompt response to cellular stimulation that leads to an increase in energy production. This complexity requires a specialized and highly dynamic molecular machinery, with several primary components that can be variably gathered together in order to match cellular energy demands and protect from death stimuli. In line with this, MCU is now recognized to be part of a macromolecular structure known as the MCU complex that can include at least MCUb, EMRE, MICU1 and its isoforms. The ongoing elucidation of the identity and the genuine function of the MCU complex components is now providing the molecular understanding of the biophysical properties of mitochondrial Ca2+ uptake. Here I will first review our current understanding of the MCU complex and then present data on a novel regulator of organelle Ca2+ handling named MICU3, an EF-hand containing protein localized in the mitochondrial inter membrane space and preferentially expressed in neurons. I'll show that MICU3 forms a disulfide bond-mediated dimer with MICU1, but not with MICU2, and it acts as a highly-potent enhancer of MCU-dependent mitochondrial calcium uptake. Accordingly, silencing of MICU3 in primary cortical neurons significantly decreases mitochondrial calcium uptake. Surprisingly, downregulation of MICU3 also suppresses cytosolic calcium spiking induced by activation of synaptic NMDA receptors, thus suggesting that mitochondria actively participate in the decoding of synaptic activity. In the case of K+, the situation is more enigmatic. Seven different K+ channels have been described in the IMM, but their molecular identity is still debated, especially in the case of ATP-sensitive K+ channels. KATP are indeed widely distributed ion channels acting as sensors of cellular metabolism. They are present in the plasma membrane (pmKATP), where they couple cell excitability with energy availability. They are also reported to be located at the level of intracellular membranes, in particular in mitochondria, but in this context even their own existence is still a matter of debate. Here, I will describe a mitochondria localized protein complex that mediates ATP-sensitive potassium currents, the so-called mitoKATP. Similarly to its plasma membrane counterpart, the mitoKATP is composed by a pore-forming subunit, named mitoK, together with an ATP-sensitive subunit, named mitoSUR. In vitro reconstitution of mitoK and mitoSUR recapitulates the main electrophysiological properties and pharmacological profile of the mitoKATP. This channel is normally closed at physiological ATP concentrations, thus preventing cations to enter the mitochondrial matrix and the consequent dissipation of mitochondrial membrane potential (ΔΨm). The overexpression of the channel forming subunit alone (mitoK) causes loss of ΔΨm, decrease of mitochondrial Ca2+ uptake, organelle fragmentation and disruption of cristae, in line with an increased cation permeability of the inner mitochondrial membrane. However, the concomitant overexpression of the mitoSUR subunit restores the correct channel gating and rescues organelle dysfunction. Conversely, mitoK ablation causes mitochondrial dysfunction characterized by instability of mitochondrial membrane potential accompanied by a decrease of oxidative performance. Overall, our data suggest that the mitoKATP is a novel player in the regulation of mitochondrial physiology with a potential impact on many pathological processes such as ischemia-reperfusion injury and ageing.

Where applicable, experiments conform with Society ethical requirements