Mitochondria provide chemical energy for endoergonic reactions in the form of ATP, and their activity must meet cellular energy requirements, but the mechanisms that link organelle performance to ATP ...levels are poorly understood. Here we confirm the existence of a protein complex localized in mitochondria that mediates ATP-dependent potassium currents (that is, mitoK
). We show that-similar to their plasma membrane counterparts-mitoK
channels are composed of pore-forming and ATP-binding subunits, which we term MITOK and MITOSUR, respectively. In vitro reconstitution of MITOK together with MITOSUR recapitulates the main properties of mitoK
. Overexpression of MITOK triggers marked organelle swelling, whereas the genetic ablation of this subunit causes instability in the mitochondrial membrane potential, widening of the intracristal space and decreased oxidative phosphorylation. In a mouse model, the loss of MITOK suppresses the cardioprotection that is elicited by pharmacological preconditioning induced by diazoxide. Our results indicate that mitoK
channels respond to the cellular energetic status by regulating organelle volume and function, and thereby have a key role in mitochondrial physiology and potential effects on several pathological processes.
Whether the mitochondrial permeability transition pore (PTP), also called mitochondrial megachannel (MMC), originates from the F‐ATP synthase is a matter of controversy. This hypothesis is supported ...both by site‐directed mutagenesis of specific residues of F‐ATP synthase affecting regulation of the PTP/MMC and by deletion of specific subunits causing dramatic changes in channel conductance. In contrast, human cells lacking an assembled F‐ATP synthase apparently display persistence of the PTP. We discuss recent data that shed new light on this controversy, supporting the conclusion that the PTP/MMC originates from a Ca2+‐dependent conformational change in F‐ATP synthase allowing its reversible transformation into a high‐conductance channel.
Voltage-dependent anion-selective channels (VDAC) are pore-forming proteins located in the outer mitochondrial membrane. Three isoforms are encoded by separate genes in mammals (VDAC1-3). These ...proteins play a crucial role in the cell, forming the primary interface between mitochondrial and cellular metabolisms. Research on the role of VDACs in the cell is a rapidly growing field, but the function of VDAC3 remains elusive. The high-sequence similarity between isoforms suggests a similar pore-forming structure. Electrophysiological analyzes revealed that VDAC3 works as a channel; however, its gating and regulation remain debated. A comparison between VDAC3 and VDAC1-2 underlines the presence of a higher number of cysteines in both isoforms 2 and 3. Recent mass spectrometry data demonstrated that the redox state of VDAC3 cysteines is evolutionarily conserved. Accordingly, these residues were always detected as totally reduced or partially oxidized, thus susceptible to disulfide exchange. The deletion of selected cysteines significantly influences the function of the channel. Some cysteine mutants of VDAC3 exhibited distinct kinetic behavior, conductance values and voltage dependence, suggesting that channel activity can be modulated by cysteine reduction/oxidation. These properties point to VDAC3 as a possible marker of redox signaling in the mitochondrial intermembrane space. Here, we summarize our current knowledge about VDAC3 predicted structure, physiological role and regulation, and possible future directions in this research field.
Mitochondrial calcium accumulation was recently shown to depend on a complex composed of an inner-membrane channel (MCU and MCUb) and regulatory subunits (MICU1, MCUR1, and EMRE). A fundamental ...property of MCU is low activity at resting cytosolic Ca2+ concentrations, preventing deleterious Ca2+ cycling and organelle overload. Here we demonstrate that these properties are ensured by a regulatory heterodimer composed of two proteins with opposite effects, MICU1 and MICU2, which, both in purified lipid bilayers and in intact cells, stimulate and inhibit MCU activity, respectively. Both MICU1 and MICU2 are regulated by calcium through their EF-hand domains, thus accounting for the sigmoidal response of MCU to Ca2+ in situ and allowing tight physiological control. At low Ca2+, the dominant effect of MICU2 largely shuts down MCU activity; at higher Ca2+, the stimulatory effect of MICU1 allows the prompt response of mitochondria to Ca2+ signals generated in the cytoplasm.
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•MICU1 and MICU2 form an obligate heterodimer•MICU1 enhances MCU opening•MICU2 acts as an MCU gatekeeper
Mitochondrial calcium uptake is rapid in response to calcium signaling. Patron et al. demonstrate that this response is not intrinsic to the mitochondrial calcium uniporter (MCU) but rather is dependent on a disulfide-mediated dimer of MCU interactors (MICU1 and MICU2). At low Ca2+, MICU2 shuts down MCU activity, but after Ca2+ stimulation, MICU2 inhibition is released and MICU1 enhances MCU opening.
Plasma membrane potassium channels importantly contribute to maintain ion homeostasis across the cell membrane. The view is emerging that also those residing in intracellular membranes play pivotal ...roles for the coordination of correct cell function. In this review we critically discuss our current understanding of the nature and physiological tasks of potassium channels in organelle membranes in both animal and plant cells, with a special emphasis on their function in the regulation of photosynthesis and mitochondrial respiration. In addition, the emerging role of potassium channels in the nuclear membranes in regulating transcription will be discussed. The possible functions of endoplasmic reticulum-, lysosome- and plant vacuolar membrane-located channels are also referred to. Altogether, experimental evidence obtained with distinct channels in different membrane systems points to a possible unifying function of most intracellular potassium channels in counterbalancing the movement of other ions including protons and calcium and modulating membrane potential, thereby fine-tuning crucial cellular processes. This article is part of a Special Issue entitled ‘EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2–7, 2016’, edited by Prof. Paolo Bernardi.
•Potassium channels are present in the nucleus, mitochondria, ER , lysosomes, chloroplasts, vacuoles and peroxisomes.•Several intracellular potassium channels have multiple localization within the cell.•K+channels in intracellular membranes fine-tune physiological processes by allowing counterion fluxes and by altering Δψ.
The field of mitochondrial ion channels has undergone a rapid development during the last three decades, due to the molecular identification of some of the channels residing in the outer and inner ...membranes. Relevant information about the function of these channels in physiological and pathological settings was gained thanks to genetic models for a few, mitochondria‐specific channels. However, many ion channels have multiple localizations within the cell, hampering a clear‐cut determination of their function by pharmacological means. The present review summarizes our current knowledge about the ins and outs of mitochondrial ion channels, with special focus on the channels that have received much attention in recent years, namely, the voltage‐dependent anion channels, the permeability transition pore (also called mitochondrial megachannel), the mitochondrial calcium uniporter and some of the inner membrane‐located potassium channels. In addition, possible strategies to overcome the difficulties of specifically targeting mitochondrial channels versus their counterparts active in other membranes are discussed, as well as the possibilities of modulating channel function by small peptides that compete for binding with protein interacting partners. Altogether, these promising tools along with large‐scale chemical screenings set up to identify new, specific channel modulators will hopefully allow us to pinpoint the actual function of most mitochondrial ion channels in the near future and to pharmacologically affect important pathologies in which they are involved, such as neurodegeneration, ischaemic damage and cancer.
Linked Articles
This article is part of a themed section on Mitochondrial Pharmacology: Featured Mechanisms and Approaches for Therapy Translation. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.22/issuetoc
Skeletal muscle is a dynamic organ, characterized by an incredible ability to rapidly increase its rate of energy consumption to sustain activity. Muscle mitochondria provide most of the ATP required ...for contraction via oxidative phosphorylation. Here we found that skeletal muscle mitochondria express a unique MCU complex containing an alternative splice isoform of MICU1, MICU1.1, characterized by the addition of a micro-exon that is sufficient to greatly modify the properties of the MCU. Indeed, MICU1.1 binds Ca2+ one order of magnitude more efficiently than MICU1 and, when heterodimerized with MICU2, activates MCU current at lower Ca2+ concentrations than MICU1-MICU2 heterodimers. In skeletal muscle in vivo, MICU1.1 is required for sustained mitochondrial Ca2+ uptake and ATP production. These results highlight a novel mechanism of the molecular plasticity of the MCU Ca2+ uptake machinery that allows skeletal muscle mitochondria to be highly responsive to sarcoplasmic Ca2+ responses.
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•Skeletal muscle mitochondria express a unique MCU complex conserved in vertebrates•The complex prevalently contains the alternative splice isoform of MICU1, MICU1.1•MICU1.1-MICU2 heterodimers diminish the Ca2+ activation threshold of MCU•In skeletal muscle, MICU1.1 is required to amplify ATP supply for contraction
Vecellio Reane et al. identify a unique skeletal muscle-specific mitochondrial Ca2+ uniporter complex containing an alternative splice isoform of MICU1, MICU1.1, that profoundly modifies the properties of mitochondrial Ca2+ uptake. MICU1.1 is required for providing sufficient levels of mitochondrial Ca2+ to sustain oxidative ATP production for resistance and strenuous exercise.
The size of the light-induced proton motive force (pmf) across the thylakoid membrane of chloroplasts is regulated in response to environmental stimuli. Here, we describe a component of the thylakoid ...membrane, the two-pore potassium (K⁺) channel TPK3, which modulates the composition of the pmf through ion counterbalancing. Recombinant TPK3 exhibited potassium-selective channel activity sensitive to Ca²⁺ and H⁺. In Arabidopsis plants, the channel is found in the thylakoid stromal lamellae. Arabidopsis plants silenced for the TPK3 gene display reduced growth and altered thylakoid membrane organization. This phenotype reflects an impaired capadty to generate a normal pmf, which results in reduced CO₂ assimilation and deficient nonphotochemical dissipation of excess absorbed light. Thus, the TPK3 channel manages the pmf necessary to convert photochemical energy into physiological functions.
Wnt signaling is an important pathway mainly active during embryonic development and controlling cell proliferation. This regulatory pathway is aberrantly activated in several human diseases. Ion ...channels are known modulators of several important cellular functions ranging from the tuning of the membrane potential to modulation of intracellular pathways, in particular the influence of ion channels in Wnt signaling regulation has been widely investigated. This review will discuss the known links between ion channels and canonical Wnt signaling, focusing on their possible roles in human metabolic diseases, neurological disorders, and cancer.
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