BBA - Bioenergetics (v.1787, #11)

Mitochondrial calcium in health and disease by Andrew P. Halestrap (1289-1290).

Mitochondria produce around 92% of the ATP used in the typical animal cell by oxidative phosphorylation using energy from their electrochemical proton gradient. Intramitochondrial free Ca2+ concentration ([Ca2+]m) has been found to be an important component of control of the rate of this ATP production. In addition, [Ca2+]m also controls the opening of a large pore in the inner mitochondrial membrane, the permeability transition pore (PTP), which plays a role in mitochondrial control of programmed cell death or apoptosis. Therefore, [Ca2+]m can control whether the cell has sufficient ATP to fulfill its functions and survive or is condemned to death. Ca2+ is also one of the most important second messengers within the cytosol, signaling changes in cellular response through Ca2+ pulses or transients. Mitochondria can also sequester Ca2+ from these transients so as to modify the shape of Ca2+ signaling transients or control their location within the cell. All of this is controlled by the action of four or five mitochondrial Ca2+ transport mechanisms and the PTP. The characteristics of these mechanisms of Ca2+ transport and a discussion of how they might function are described in this paper.
Keywords: Mitochondria; Calcium uptake; Calcium efflux; Calcium signaling; Permeability transition; Reactive oxygen species;

Studies in Bristol in the 1960s and 1970s, led to the recognition that four mitochondrial dehydrogenases are activated by calcium ions. These are FAD-glycerol phosphate dehydrogenase, pyruvate dehydrogenase, NAD-isocitrate dehydrogenase and oxoglutarate dehydrogenase. FAD-glycerol phosphate dehydrogenase is located on the outer surface of the inner mitochondrial membrane and is influenced by changes in cytoplasmic calcium ion concentration. The other three enzymes are located within mitochondria and are regulated by changes in mitochondrial matrix calcium ion concentration. These and subsequent studies on purified enzymes, mitochondria and intact cell preparations have led to the widely accepted view that the activation of these enzymes is important in the stimulation of the respiratory chain and hence ATP supply under conditions of increased ATP demand in many stimulated mammalian cells. The effects of calcium ions on FAD-isocitrate dehydrogenase involve binding to an EF-hand binding motif within this enzyme but the binding sites involved in the effects of calcium ions on the three intramitochondrial dehydrogenases remain to be fully established. It is also emphasised in this article that these three dehydrogenases appear only to be regulated by calcium ions in vertebrates and that this raises some interesting and potentially important developmental issues.
Keywords: Calcium ion; Mitochondria; Vertebrate; Dehydrogenase; ATP supply; Calcium binding;

Measurements of mitochondrial calcium in vivo by Tullio Pozzan; Rüdiger Rudolf (1317-1323).
Mitochondria play a pivotal role in intracellular Ca2+ signalling by taking up and releasing the ion upon specific conditions. In order to do so, mitochondria depend on a number of factors, such as the mitochondrial membrane potential and spatio-temporal constraints. Whereas most of the basic principles underlying mitochondrial Ca2+ handling have been successfully deciphered over the last 50 years using assays based on in vitro preparations of mitochondria or cultured cells, we have only just started to understand the actual physiological relevance of these processes in the whole animal. Recent advancements in imaging and genetically encoded sensor technologies have allowed us to visualise mitochondrial Ca2+ transients in live mice. These studies used either two-photon microscopy or bioluminescence imaging of cameleon or aequorin-GFP Ca2+ sensors, respectively. Both methods revealed a consistent picture of Ca2+ uptake into mitochondria under physiological conditions even during very short-lasting elevations of cytosolic Ca2+ levels. The big future challenge is to understand the functional impact of such Ca2+ signals on the physiology of the observed tissue as well as of the whole organism. To that end, the development of multiparametric in vivo approaches will be mandatory.
Keywords: Aequorin; Cameleon; GFP; Microscopy; Mouse; Rhod-2;

Mitochondrial Ca2+ transport was initially considered important only in buffering of cytosolic Ca2+ by acting as a “sink” under conditions of Ca2+ overload. The main regulator of ATP production was considered to be the relative concentrations of high energy phosphates. However, work by Denton and McCormack in the 1970s and 1980s showed that free intramitochondrial Ca2+ ([Ca2+]m) activated dehydrogenase enzymes in mitochondria, leading to increased NADH and hence ATP production. This leads them to propose a scheme, subsequently termed a “parallel activation model” whereby increases in energy demand, such as hormonal stimulation or increased workload in muscle, produced an increase in cytosolic [Ca2+] that was relayed by the mitochondrial Ca2+ transporters into the matrix to give an increase in [Ca2+]m. This then stimulated energy production to meet the increased energy demand. With the development of methods for measuring [Ca2+]m in living cells that proved [Ca2+]m changed over a dynamic physiological range rather than simply soaking up excess cytosolic [Ca2+], this model has now gained widespread acceptance. However, work by ourselves and others using targeted probes to measure changes in both [Ca2+] and [ATP] in different cell compartments has revealed variations in the interrelationships between these two in different tissues, suggesting that metabolic regulation by Ca2+ is finely tuned to the demands and function of the individual organ.
Keywords: Mitochondria; Calcium; Heart; Cardiomyocyte; Pancreas; Liver; ATP; Luciferase; Aequorin;

The heart is capable of balancing the rate of mitochondrial ATP production with utilization continuously over a wide range of activity. This results in a constant phosphorylation potential despite a large change in metabolite turnover. The molecular mechanisms responsible for generating this energy homeostasis are poorly understood. The best candidate for a cytosolic signaling molecule reflecting ATP hydrolysis is Ca2+. Since Ca2+ initiates and powers muscle contraction as well as serves as the primary substrate for SERCA, Ca2+ is an ideal feed-forward signal for priming ATP production. With the sarcoplasmic reticulum to cytosolic Ca2+ gradient near equilibrium with the free energy of ATP, cytosolic Ca2+ release is exquisitely sensitive to the cellular energy state providing a feedback signal. Thus, Ca2+ can serve as a feed-forward and feedback regulator of ATP production. Consistent with this notion is the correlation of cytosolic and mitochondrial Ca2+ with work in numerous preparations as well as the localization of mitochondria near Ca2+ release sites. How cytosolic Ca2+ signaling might regulate oxidative phosphorylation is a focus of this review. The relevant Ca2+ sensitive sites include several dehydrogenases and substrate transporters together with a post-translational modification of F1-FO-ATPase and cytochrome oxidase. Thus, Ca2+ apparently activates both the generation of the mitochondrial membrane potential as well as utilization to produce ATP. This balanced activation extends the energy homeostasis observed in the cytosol into the mitochondria matrix in the never resting heart.
Keywords: Dehydrogenase; F1-FO-ATPase; Membrane potential; Oxidative phosphorylation; NADH; Oxygen consumption; Sarcoplasmic reticulum; Starling effect; Energy homeostasis; Aralar; Citrin;

Ca2+ transfer from the ER to mitochondria: When, how and why by Rosario Rizzuto; Saverio Marchi; Massimo Bonora; Paola Aguiari; Angela Bononi; Diego De Stefani; Carlotta Giorgi; Sara Leo; Alessandro Rimessi; Roberta Siviero; Erika Zecchini; Paolo Pinton (1342-1351).
The heterogenous subcellular distribution of a wide array of channels, pumps and exchangers allows extracellular stimuli to induce increases in cytoplasmic Ca2+ concentration ([Ca2+]c) with highly defined spatial and temporal patterns, that in turn induce specific cellular responses (e.g. contraction, secretion, proliferation or cell death). In this extreme complexity, the role of mitochondria was considered marginal, till the direct measurement with targeted indicators allowed to appreciate that rapid and large increases of the [Ca2+] in the mitochondrial matrix ([Ca2+]m) invariably follow the cytosolic rises. Given the low affinity of the mitochondrial Ca2+ transporters, the close proximity to the endoplasmic reticulum (ER) Ca2+-releasing channels was shown to be responsible for the prompt responsiveness of mitochondria. In this review, we will summarize the current knowledge of: i) the mitochondrial and ER Ca2+ channels mediating the ion transfer, ii) the structural and molecular foundations of the signaling contacts between the two organelles, iii) the functional consequences of the [Ca2+]m increases, and iv) the effects of oncogene-mediated signals on mitochondrial Ca2+ homeostasis. Despite the rapid progress carried out in the latest years, a deeper molecular understanding is still needed to unlock the secrets of Ca2+ signaling machinery.
Keywords: Mitochondria; Calcium; Endoplasmic reticulum; Cell death; Apoptosis; MAM;

SR/ER–mitochondrial local communication: Calcium and ROS by György Csordás; György Hajnóczky (1352-1362).
Mitochondria form junctions with the sarco/endoplasmic reticulum (SR/ER), which support signal transduction and biosynthetic pathways and affect organellar distribution. Recently, these junctions have received attention because of their pivotal role in mediating calcium signal propagation to the mitochondria, which is important for both ATP production and mitochondrial cell death. Many of the SR/ER–mitochondrial calcium transporters and signaling proteins are sensitive to redox regulation and are directly exposed to the reactive oxygen species (ROS) produced in the mitochondria and SR/ER. Although ROS has been emerging as a novel signaling entity, the redox signaling of the SR/ER–mitochondrial interface is yet to be elucidated. We describe here possible mechanisms of the mutual interaction between local Ca2+ and ROS signaling in the control of SR/ER–mitochondrial function.
Keywords: Superoxide anion; H2O2; IP3 receptor; Ryanodine receptor; SERCA; Bioenergetics; Apoptosis;

Calcium regulation of mitochondria motility and morphology by Danny V. Jeyaraju; Giulia Cisbani; Luca Pellegrini (1363-1373).
In the Fifties, electron microscopy studies on neuronal cells showed that mitochondria typically cluster at synaptic terminals, thereby introducing the concept that proper mitochondria trafficking and partitioning inside the cell could provide functional support to the execution of key physiological processes. Today, the notion that a central event in the life of every eukaryotic cell is to configure, maintain, and reorganize the mitochondrial network at sites of high energy demand in response to environmental and cellular cues is well established, and the challenge ahead is to define the underlying molecular mechanisms and regulatory pathways. Recent pioneering studies have further contributed to place mitochondria at the center of the cell biology by showing that the machinery governing remodeling of mitochondria shape and structure regulates the functional output of the organelle as the powerhouse of the cell, the gateway to programmed cell death, and the platform for Ca2+ signaling. Thus, a raising issue is to identify the cues integrating mitochondria trafficking and dynamics into cell physiology and metabolism. Given the versatile function of calcium as a second messenger and of the role of mitochondria as a major calcium store, evidences are emerging linking Ca2+ transients to the modulation of mitochondrial activities. This review focuses on calcium as a switch controlling mitochondria motility and morphology in steady state, stressed, and pathological conditions.
Keywords: Mitochondria; Calcium; Motility; Membrane dynamic; Fusion; Fission; Signaling; Cristae; Apoptosis; Metabolism; Neurodegeneration; Neurological syndrome; Rhomboid; Parl; Opa1; Drp1; PKA; Calcineurin; CaMK;

Modulation of calcium signalling by mitochondria by Ciara Walsh; Stephanie Barrow; Svetlana Voronina; Michael Chvanov; Ole H. Petersen; Alexei Tepikin (1374-1382).
In this review we will attempt to summarise the complex and sometimes contradictory effects that mitochondria have on different forms of calcium signalling. Mitochondria can influence Ca2+ signalling indirectly by changing the concentration of ATP, NAD(P)H, pyruvate and reactive oxygen species — which in turn modulate components of the Ca2+ signalling machinery i.e. buffering, release from internal stores, influx from the extracellular solution, uptake into cellular organelles and extrusion by plasma membrane Ca2+ pumps. Mitochondria can directly influence the calcium concentration in the cytosol of the cell by importing Ca2+ via the mitochondrial Ca2+ uniporter or transporting Ca2+ from the interior of the organelle into the cytosol by means of Na+/Ca2+ or H+/Ca2+ exchangers. Considerable progress in understanding the relationship between Ca2+ signalling cascades and mitochondrial physiology has been accumulated over the last few years due to the development of more advanced optical techniques and electrophysiological approaches.
Keywords: Calcium signalling; ATP; ROS; IP3 receptor; Ryanodine receptor; Store-operated Ca2+ entry;

Regulation of plasma membrane calcium fluxes by mitochondria by Nicolas Demaurex; Damon Poburko; Maud Frieden (1383-1394).
The role of mitochondria in cell signaling is becoming increasingly apparent, to an extent that the signaling role of mitochondria appears to have stolen the spotlight from their primary function as energy producers. In this chapter, we will review the ionic basis of calcium handling by mitochondria and discuss the mechanisms that these organelles use to regulate the activity of plasma membrane calcium channels and transporters.
Keywords: Calcium; Mitochondria; Ion channel; Endoplasmic reticulum; Cell signaling;

Mitochondrial calcium and the permeability transition in cell death by John J. Lemasters; Tom P. Theruvath; Zhi Zhong; Anna-Liisa Nieminen (1395-1401).
Dysregulation of Ca2+ has long been implicated to be important in cell injury. A Ca2+-linked process important in necrosis and apoptosis (or necrapoptosis) is the mitochondrial permeability transition (MPT). In the MPT, large conductance permeability transition (PT) pores open that make the mitochondrial inner membrane abruptly permeable to solutes up to 1500 Da. The importance of Ca2+ in MPT induction varies with circumstance. Ca2+ overload is sufficient to induce the MPT. By contrast after ischemia–reperfusion to cardiac myocytes, Ca2+ overload is the consequence of bioenergetic failure after the MPT rather than its cause. In other models, such as cytotoxicity from Reye-related agents and storage-reperfusion injury to liver grafts, Ca2+ appears to be permissive to MPT onset. Lastly in oxidative stress, increased mitochondrial Ca2+ and ROS generation act synergistically to produce the MPT and cell death. Thus, the exact role of Ca2+ for inducing the MPT and cell death depends on the particular biologic setting.
Keywords: Calcium; Cyclosporin A; Mitochondrial permeability transition; Necrapoptosis; Oxidative stress;

The role of the mitochondrial permeability transition pore in heart disease by Andrew P. Halestrap; Philippe Pasdois (1402-1415).
Like Dr. Jeckyll and Mr. Hyde, mitochondria possess two distinct persona. Under normal physiological conditions they synthesise ATP to meet the energy needs of the beating heart. Here calcium acts as a signal to balance the rate of ATP production with ATP demand. However, when the heart is overloaded with calcium, especially when this is accompanied by oxidative stress, mitochondria embrace their darker side, and induce necrotic cell death of the myocytes. This happens acutely in reperfusion injury and chronically in congestive heart failure. Here calcium overload, adenine nucleotide depletion and oxidative stress combine forces to induce the opening of a non-specific pore in the mitochondrial membrane, known as the mitochondrial permeability transition pore (mPTP). The molecular nature of the mPTP remains controversial but current evidence implicates a matrix protein, cyclophilin-D (CyP-D) and two inner membrane proteins, the adenine nucleotide translocase (ANT) and the phosphate carrier (PiC). Inhibition of mPTP opening can be achieved with inhibitors of each component, but targeting CyP-D with cyclosporin A (CsA) and its non-immunosuppressive analogues is the best described. In animal models, inhibition of mPTP opening by either CsA or genetic ablation of CyP-D provides strong protection from both reperfusion injury and congestive heart failure. This confirms the mPTP as a promising drug target in human cardiovascular disease. Indeed, the first clinical trials have shown CsA treatment improves recovery after treatment of a coronary thrombosis with angioplasty.
Keywords: Mitochondrial permeability transition pore; Heart; Ischaemia; Reperfusion; Reactive oxygen specie; Calcium; Adenine nucleotide translocase; Cyclophilin-D; Cyclosporin A; Mitochondrial phosphate carrier;

The ability of isolated brain mitochondria to accumulate, store and release calcium has been extensively characterized. Extrapolation to the intact neuron led to predictions that the in situ mitochondria would reversibly accumulate Ca2+ when the concentration of the cation in the vicinity of the mitochondria rose above the ‘set-point’ at which uptake and efflux were in balance, storing Ca2+ as a complex with phosphate, and slowly releasing the cation when plasma membrane ion pumps lowered the cytoplasmic free Ca2+. Excessive accumulation of the cation was predicted to lead to activation of the permeability transition, with catastrophic consequences for the neuron. Each of these predictions has been confirmed with intact neurons, and there is convincing evidence for the permeability transition in cellular Ca2+ overload associated with glutamate excitotoxicity and stroke, while the neurodegenerative disease in which possible defects in mitochondrial Ca2+ handling have been most intensively investigated is Huntington's Disease. In this brief review evidence that mitochondrial Ca2+ transport is relevant to neuronal survival in these conditions will be discussed.
Keywords: Mitochondria; Brain; Calcium; Excitotoxicity; Permeability transition; Huntington's Disease;