Role of calcium ions in the regulation of intramitochondrial metabolism. Effects of Na+, Mg2+ and ruthenium red on the Ca2+-stimulated oxidation of oxoglutarate and on pyruvate dehydrogenase activity in intact rat heart mitochondria

Mitochondrial Calcium Regulation of Cardiac Metabolism in Health and Disease

Oxidative phosphorylation is regulated by mitochondrial calcium (Ca2+) in health and disease. In physiological states, Ca2+ enters via the mitochondrial Ca2+ uniporter and rapidly enhances NADH and ATP production. However, maintaining Ca2+ homeostasis is critical: insufficient Ca2+ impairs stress adaptation, and Ca2+ overload can trigger cell death. In this review, we delve into recent insights further defining the relationship between mitochondrial Ca2+ dynamics and oxidative phosphorylation. Our focus is on how such regulation affects cardiac function in health and disease, including heart failure, ischemia-reperfusion, arrhythmias, catecholaminergic polymorphic ventricular tachycardia, mitochondrial cardiomyopathies, Barth syndrome, and Friedreich's ataxia. Several themes emerge from recent data. First, mitochondrial Ca2+ regulation is critical for fuel substrate selection, metabolite import, and matching of ATP supply to demand. Second, mitochondrial Ca2+ regulates both the production and response to reactive oxygen species (ROS), and the balance between its pro- and antioxidant effects is key to how it contributes to physiological and pathological states. Third, Ca2+ exerts localized effects on the electron transport chain (ETC), not through traditional allosteric mechanisms but rather indirectly. These effects hinge on specific transporters, such as the uniporter or the Na+/Ca2+ exchanger, and may not be noticeable acutely, contributing differently to phenotypes depending on whether Ca2+ transporters are acutely or chronically modified. Perturbations in these novel relationships during disease states may either serve as compensatory mechanisms or exacerbate impairments in oxidative phosphorylation. Consequently, targeting mitochondrial Ca2+ holds promise as a therapeutic strategy for a variety of cardiac diseases characterized by contractile failure or arrhythmias.

Unravelling the complexity of the mitochondrial Ca2+ uniporter: regulation, tissue specificity, and physiological implications

Calcium (Ca2+) signalling acts a pleiotropic message within the cell that is decoded by the mitochondria through a sophisticated ion channel known as the Mitochondrial Ca2+ Uniporter (MCU) complex. Under physiological conditions, mitochondrial Ca2+ signalling is crucial for coordinating cell activation with energy production. Conversely, in pathological scenarios, it can determine the fine balance between cell survival and death. Over the last decade, significant progress has been made in understanding the molecular bases of mitochondrial Ca2+ signalling. This began with the elucidation of the MCU channel components and extended to the elucidation of the mechanisms that regulate its activity. Additionally, increasing evidence suggests molecular mechanisms allowing tissue-specific modulation of the MCU complex, tailoring channel activity to the specific needs of different tissues or cell types. This review aims to explore the latest evidence elucidating the regulation of the MCU complex, the molecular factors controlling the tissue-specific properties of the channel, and the physiological and pathological implications of mitochondrial Ca2+ signalling in different tissues.

Hepatic glucagon action: beyond glucose mobilization

Glucagon's ability to promote hepatic glucose production has been known for over a century, with initial observations touting this hormone as a diabetogenic agent. However, glucagon receptor agonism [when balanced with an incretin, including glucagon-like peptide 1 (GLP-1) to dampen glucose excursions] is now being developed as a promising therapeutic target in the treatment of metabolic diseases, like metabolic dysfunction-associated steatotic disease/metabolic dysfunction-associated steatohepatitis (MASLD/MASH), and may also have benefit for obesity and chronic kidney disease. Conventionally regarded as the opposing tag-team partner of the anabolic mediator insulin, glucagon is gradually emerging as more than just a "catabolic hormone." Glucagon action on glucose homeostasis within the liver has been well characterized. However, growing evidence, in part thanks to new and sensitive "omics" technologies, has implicated glucagon as more than just a "glucose liberator." Elucidation of glucagon's capacity to increase fatty acid oxidation while attenuating endogenous lipid synthesis speaks to the dichotomous nature of the hormone. Furthermore, glucagon action is not limited to just glucose homeostasis and lipid metabolism, as traditionally reported. Glucagon plays key regulatory roles in hepatic amino acid and ketone body metabolism, as well as mitochondrial turnover and function, indicating broader glucagon signaling consequences for metabolic homeostasis mediated by the liver. Here we examine the broadening role of glucagon signaling within the hepatocyte and question the current dogma, to appreciate glucagon as more than just that "catabolic hormone."

Fueling the heartbeat: Dynamic regulation of intracellular ATP during excitation-contraction coupling in ventricular myocytes

The heart beats approximately 100,000 times per day in humans, imposing substantial energetic demands on cardiac muscle. Adenosine triphosphate (ATP) is an essential energy source for normal function of cardiac muscle during each beat, as it powers ion transport, intracellular Ca2+ handling, and actin-myosin cross-bridge cycling. Despite this, the impact of excitation-contraction coupling on the intracellular ATP concentration ([ATP]i) in myocytes is poorly understood. Here, we conducted real-time measurements of [ATP]i in ventricular myocytes using a genetically encoded ATP fluorescent reporter. Our data reveal rapid beat-to-beat variations in [ATP]i. Notably, diastolic [ATP]i was <1 mM, which is eightfold to 10-fold lower than previously estimated. Accordingly, ATP-sensitive K+ (KATP) channels were active at physiological [ATP]i. Cells exhibited two distinct types of ATP fluctuations during an action potential: net increases (Mode 1) or decreases (Mode 2) in [ATP]i. Mode 1 [ATP]i increases necessitated Ca2+ entry and release from the sarcoplasmic reticulum (SR) and were associated with increases in mitochondrial Ca2+. By contrast, decreases in mitochondrial Ca2+ accompanied Mode 2 [ATP]i decreases. Down-regulation of the protein mitofusin 2 reduced the magnitude of [ATP]i fluctuations, indicating that SR-mitochondrial coupling plays a crucial role in the dynamic control of ATP levels. Activation of β-adrenergic receptors decreased [ATP]i, underscoring the energetic impact of this signaling pathway. Finally, our work suggests that cross-bridge cycling is the largest consumer of ATP in a ventricular myocyte during an action potential. These findings provide insights into the energetic demands of EC coupling and highlight the dynamic nature of ATP concentrations in cardiac muscle.

Mrs2-mediated mitochondrial magnesium uptake is essential for the regulation of MCU-mediated mitochondrial Ca2+ uptake and viability

Mitochondrial Ca2+ uptake is essential in regulating bioenergetics, cell death, and cytosolic Ca2+ transients. Mitochondrial Calcium Uniporter (MCU) mediates the mitochondrial Ca2+ uptake. Though MCU regulation by MICUs is unequivocally established, there needs to be more knowledge of whether divalent cations regulate MCU. Here, we set out to understand the mitochondrial matrix Mg2+-dependent regulation of MCU activity. We showed that decreased matrix [Mg2+] is associated with increased MCU activity and significantly prompted mitochondrial permeability transition pore opening. Our findings support the critical role of mMg2+ in regulating MCU activity.

Supralinear Dependence of the IP3 Receptor-to-Mitochondria Local Ca2+ Transfer on the Endoplasmic Reticulum Ca2+ Loading

Calcium signal propagation from endoplasmic reticulum (ER) to mitochondria regulates a multitude of mitochondrial and cell functions, including oxidative ATP production and cell fate decisions. Ca2+ transfer is optimal at the ER-mitochondrial contacts, where inositol 1,4,5-trisphosphate (IP3) receptors (IP3R) can locally expose the mitochondrial Ca2+ uniporter (mtCU) to high [Ca2+] nanodomains. The Ca2+ loading state of the ER (Ca2 + ER) can vary broadly in physiological and pathological scenarios, however, the correlation between Ca2 + ER and the local Ca2+ transfer is unclear. Here, we studied IP3-induced Ca2+ transfer to mitochondria at different Ca2 + ER in intact and permeabilized RBL-2H3 cells via fluorescence measurements of cytoplasmic [Ca2+] ([Ca2+]c) and mitochondrial matrix [Ca2+] ([Ca2+]m). Preincubation of intact cells in high versus low extracellular [Ca2+] caused disproportionally greater increase in [Ca2+]m than [Ca2+]c responses to IP3-mobilizing agonist. Increasing Ca2 + ER by small Ca2+ boluses in suspensions of permeabilized cells supralinearly enhanced the mitochondrial Ca2+ uptake from IP3-induced Ca2+ release. The IP3-induced local [Ca2+] spikes exposing the mitochondrial surface measured using a genetically targeted sensor appeared to linearly correlate with Ca2 + ER, indicating that amplification happened in the mitochondria. Indeed, overexpression of an EF-hand deficient mutant of the mtCU gatekeeper MICU1 reduced the cooperativity of mitochondrial Ca2+ uptake. Interestingly, the IP3-induced [Ca2+]m signal plateaued at high Ca2 + ER, indicating activation of a matrix Ca2+ binding/chelating species. Mitochondria thus seem to maintain a "working [Ca2+]m range" via a low-affinity and high-capacity buffer species, and the ER loading steeply enhances the IP3R-linked [Ca2+]m signals in this working range.

Calcium-Dependent Interaction of Nitric Oxide Synthase with Cytochrome c Oxidase: Implications for Brain Bioenergetics

Targeted nitric oxide production is relevant for maintaining cellular energy production, protecting against oxidative stress, regulating cell death, and promoting neuroprotection. This study aimed to characterize the putative interaction of nitric-oxide synthase with mitochondrial proteins. The primary finding of this study is that cytochrome c oxidase (CCO) subunit IV (CCOIV) is associated directly with NOS in brain mitochondria when calcium ions are present. The matrix side of CCOIV binds to the N-terminus of NOS, supported by the abrogation of the binding by antibodies towards the N-terminus of NOS. Evidence supporting the interaction between CCOIV and NOS was provided by the coimmunoprecipitation of NOS from detergent-solubilized whole rat brain mitochondria with antibodies to CCOIV and the coimmunoprecipitation of CCOIV from crude brain NOS preparations using antibodies to NOS. The CCOIV domain that interacts with NOS was identified using a series of overlapping peptides derived from the primary sequence of CCOIV. As calcium ions not only activate NOS, but also facilitate the docking of NOS to CCOIV, this study points to a dynamic mechanism of controlling the bioenergetics by calcium changes, thereby adapting bioenergetics to cellular demands.

Exercise metabolism and adaptation in skeletal muscle

Viewing metabolism through the lens of exercise biology has proven an accessible and practical strategy to gain new insights into local and systemic metabolic regulation. Recent methodological developments have advanced understanding of the central role of skeletal muscle in many exercise-associated health benefits and have uncovered the molecular underpinnings driving adaptive responses to training regimens. In this Review, we provide a contemporary view of the metabolic flexibility and functional plasticity of skeletal muscle in response to exercise. First, we provide background on the macrostructure and ultrastructure of skeletal muscle fibres, highlighting the current understanding of sarcomeric networks and mitochondrial subpopulations. Next, we discuss acute exercise skeletal muscle metabolism and the signalling, transcriptional and epigenetic regulation of adaptations to exercise training. We address knowledge gaps throughout and propose future directions for the field. This Review contextualizes recent research of skeletal muscle exercise metabolism, framing further advances and translation into practice.

Beyond the TCA cycle: new insights into mitochondrial calcium regulation of oxidative phosphorylation

While mitochondria oxidative phosphorylation is broadly regulated, the impact of mitochondrial Ca2+ on substrate flux under both physiological and pathological conditions is increasingly being recognized. Under physiologic conditions, mitochondrial Ca2+ enters through the mitochondrial Ca2+ uniporter and boosts ATP production. However, maintaining Ca2+ homeostasis is crucial as too little Ca2+ inhibits adaptation to stress and Ca2+ overload can trigger cell death. In this review, we discuss new insights obtained over the past several years expanding the relationship between mitochondrial Ca2+ and oxidative phosphorylation, with most data obtained from heart, liver, or skeletal muscle. Two new themes are emerging. First, beyond boosting ATP synthesis, Ca2+ appears to be a critical determinant of fuel substrate choice between glucose and fatty acids. Second, Ca2+ exerts local effects on the electron transport chain indirectly, not via traditional allosteric mechanisms. These depend critically on the transporters involved, such as the uniporter or the Na+-Ca2+ exchanger. Alteration of these new relationships during disease can be either compensatory or harmful and suggest that targeting mitochondrial Ca2+ may be of therapeutic benefit during diseases featuring impairments in oxidative phosphorylation.

Mitochondrial Magnesium is the cationic rheostat for MCU-mediated mitochondrial Ca2+ uptake

Calcium (Ca2+) uptake by mitochondria is essential in regulating bioenergetics, cell death, and cytosolic Ca2+ transients. Mitochondrial Calcium Uniporter (MCU) mediates the mitochondrial Ca2+ uptake. MCU is a heterooligomeric complex with a pore-forming component and accessory proteins required for channel activity. Though MCU regulation by MICUs is unequivocally established, there needs to be more knowledge of whether divalent cations regulate MCU. Here we set out to understand the mitochondrial matrix Mg2+-dependent regulation of MCU activity. We showed Mrs2 as the authentic mammalian mitochondrial Mg2+ channel using the planar lipid bilayer recordings. Using a liver-specific Mrs2 KO mouse model, we showed that decreased matrix [Mg2+] is associated with increased MCU activity and matrix Ca2+ overload. The disruption of Mg2+dependent MCU regulation significantly prompted mitochondrial permeability transition pore opening-mediated cell death during tissue IR injury. Our findings support a critical role for mMg2+ in regulating MCU activity and attenuating mCa2+ overload.

Amino acids and the changing face of the α-cell

Glucagon has long been defined by its glucogenic action and as a result α-cells have been characterised based largely on their interaction with glucose. Recent findings have challenged this preconception, bringing to the fore the significant role glucagon plays in amino acid breakdown and underlining the importance of amino acids in glucagon secretion. The challenge that remains is defining the mechanism that underlie these effects - understanding which amino acids are most important, how they act on the α-cell and how their actions integrate with other fuels such as glucose and fatty acids. This review will describe the current relationship between amino acids and glucagon and how we can use this knowledge to redefine the α-cell.

Acquired disorders of mitochondrial metabolism and dynamics in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is an orphan disease of the cardiopulmonary unit that reflects an obstructive pulmonary vasculopathy and presents with hypertrophy, inflammation, fibrosis, and ultimately failure of the right ventricle (RVF). Despite treatment using pulmonary hypertension (PH)-targeted therapies, persistent functional impairment reduces the quality of life for people with PAH and death from RVF occurs in approximately 40% of patients within 5 years of diagnosis. PH-targeted therapeutics are primarily vasodilators and none, alone or in combination, are curative. This highlights a need to therapeutically explore molecular targets in other pathways that are involved in the pathogenesis of PAH. Several candidate pathways in PAH involve acquired mitochondrial dysfunction. These mitochondrial disorders include: 1) a shift in metabolism related to increased expression of pyruvate dehydrogenase kinase and pyruvate kinase, which together increase uncoupled glycolysis (Warburg metabolism); 2) disruption of oxygen-sensing related to increased expression of hypoxia-inducible factor 1α, resulting in a state of pseudohypoxia; 3) altered mitochondrial calcium homeostasis related to impaired function of the mitochondrial calcium uniporter complex, which elevates cytosolic calcium and reduces intramitochondrial calcium; and 4) abnormal mitochondrial dynamics related to increased expression of dynamin-related protein 1 and its binding partners, such as mitochondrial dynamics proteins of 49 kDa and 51 kDa, and depressed expression of mitofusin 2, resulting in increased mitotic fission. These acquired mitochondrial abnormalities increase proliferation and impair apoptosis in most pulmonary vascular cells (including endothelial cells, smooth muscle cells and fibroblasts). In the RV, Warburg metabolism and induction of glutaminolysis impairs bioenergetics and promotes hypokinesis, hypertrophy, and fibrosis. This review will explore our current knowledge of the causes and consequences of disordered mitochondrial function in PAH.

The Therapeutic Strategies Targeting Mitochondrial Metabolism in Cardiovascular Disease

Cardiovascular disease (CVD) is a group of systemic disorders threatening human health with complex pathogenesis, among which mitochondrial energy metabolism reprogramming has a critical role. Mitochondria are cell organelles that fuel the energy essential for biochemical reactions and maintain normal physiological functions of the body. Mitochondrial metabolic disorders are extensively involved in the progression of CVD, especially for energy-demanding organs such as the heart. Therefore, elucidating the role of mitochondrial metabolism in the progression of CVD is of great significance to further understand the pathogenesis of CVD and explore preventive and therapeutic methods. In this review, we discuss the major factors of mitochondrial metabolism and their potential roles in the prevention and treatment of CVD. The current application of mitochondria-targeted therapeutic agents in the treatment of CVD and advances in mitochondria-targeted gene therapy technologies are also overviewed.

A controversial issue: Can mitochondria modulate cytosolic calcium and contraction of skeletal muscle fibers?

Mitochondria are characterized by a high capacity to accumulate calcium thanks to the electrochemical gradient created by the extrusion of protons in the respiratory chain. Thereby calcium can enter crossing the inner mitochondrial membrane via MCU complex, a high-capacity, low-affinity transport mechanism. Calcium uptake serves numerous purposes, among them the regulation of three dehydrogenases of the citric cycle, apoptosis via permeability transition, and, in some cell types, modulation of cytosolic calcium transients. This Review is focused on mitochondrial calcium uptake in skeletal muscle fibers and aims to reanalyze its functional impact. In particular, we ask whether mitochondrial calcium uptake is relevant for the control of cytosolic calcium transients and therefore of contractile performance. Recent data suggest that this may be the case, at least in particular conditions, as modified expression of MCU complex subunits or of proteins involved in mitochondrial dynamics and ablation of the main cytosolic calcium buffer, parvalbumin.

Mitochondrial Ca2+ uptake by the MCU facilitates pyramidal neuron excitability and metabolism during action potential firing

Neuronal activation is fundamental to information processing by the brain and requires mitochondrial energy metabolism. Mitochondrial Ca²⁺ uptake by the mitochondrial Ca²⁺ uniporter (MCU) has long been implicated in the control of energy metabolism and intracellular Ca²⁺ signalling, but its importance to neuronal function in the brain remains unclear. Here, we used in situ electrophysiology and two-photon imaging of mitochondrial Ca²⁺, cytosolic Ca²⁺, and NAD(P)H to test the relevance of MCU activation to pyramidal neuron Ca²⁺ signalling and energy metabolism during action potential firing. We demonstrate that mitochondrial Ca²⁺ uptake by the MCU is tuned to enhanced firing rate and the strength of this relationship varied between neurons of discrete brain regions. MCU activation promoted electron transport chain activity and chemical reduction of NAD⁺ to NADH. Moreover, Ca²⁺ buffering by mitochondria attenuated cytosolic Ca²⁺ signals and thereby reduced the coupling between activity and the slow afterhyperpolarization, a ubiquitous regulator of excitability. Collectively, we demonstrate that the MCU is engaged by accelerated spike frequency to facilitate neuronal activity through simultaneous control of energy metabolism and excitability. As such, the MCU is situated to promote brain functions associated with high frequency signalling and may represent a target for controlling excessive neuronal activity.

Citrin mediated metabolic rewiring in response to altered basal subcellular Ca2+ homeostasis

In contrast to long-term metabolic reprogramming, metabolic rewiring represents an instant and reversible cellular adaptation to physiological or pathological stress. Ca2+ signals of distinct spatio-temporal patterns control a plethora of signaling processes and can determine basal cellular metabolic setting, however, Ca2+ signals that define metabolic rewiring have not been conclusively identified and characterized. Here, we reveal the existence of a basal Ca2+ flux originating from extracellular space and delivered to mitochondria by Ca2+ leakage from inositol triphosphate receptors in mitochondria-associated membranes. This Ca2+ flux primes mitochondrial metabolism by maintaining glycolysis and keeping mitochondria energized for ATP production. We identified citrin, a well-defined Ca2+-binding component of malate-aspartate shuttle in the mitochondrial intermembrane space, as predominant target of this basal Ca2+ regulation. Our data emphasize that any manipulation of this ubiquitous Ca2+ system has the potency to initiate metabolic rewiring as an instant and reversible cellular adaptation to physiological or pathological stress.

Enhanced resistance to Ca2+-induced mitochondrial permeability transition in the long-lived red-footed tortoise Chelonoidis carbonaria

The interaction between supraphysiological cytosolic Ca2+ levels and mitochondrial redox imbalance mediates the mitochondrial permeability transition (MPT). The MPT is involved in cell death, diseases and aging. This study compared the liver mitochondrial Ca2+ retention capacity and oxygen consumption in the long-lived red-footed tortoise (Chelonoidis carbonaria) with those in the rat as a reference standard. Mitochondrial Ca2+ retention capacity, a quantitative measure of MPT sensitivity, was remarkably higher in tortoises than in rats. This difference was minimized in the presence of the MPT inhibitors ADP and cyclosporine A. However, the Ca2+ retention capacities of tortoise and rat liver mitochondria were similar when both MPT inhibitors were present simultaneously. NADH-linked phosphorylating respiration rates of tortoise liver mitochondria represented only 30% of the maximal electron transport system capacity, indicating a limitation imposed by the phosphorylation system. These results suggested underlying differences in putative MPT structural components [e.g. ATP synthase, adenine nucleotide translocase (ANT) and cyclophilin D] between tortoises and rats. Indeed, in tortoise mitochondria, titrations of inhibitors of the oxidative phosphorylation components revealed a higher limitation of ANT. Furthermore, cyclophilin D activity was approximately 70% lower in tortoises than in rats. Investigation of critical properties of mitochondrial redox control that affect MPT demonstrated that tortoise and rat liver mitochondria exhibited similar rates of H2O2 release and glutathione redox status. Overall, our findings suggest that constraints imposed by ANT and cyclophilin D, putative components or regulators of the MPT pore, are associated with the enhanced resistance to Ca2+-induced MPT in tortoises.

Substrate- and Calcium-Dependent Differential Regulation of Mitochondrial Oxidative Phosphorylation and Energy Production in the Heart and Kidney

Mitochondrial dehydrogenases are differentially stimulated by Ca2+. Ca2+ has also diverse regulatory effects on mitochondrial transporters and other enzymes. However, the consequences of these regulatory effects on mitochondrial oxidative phosphorylation (OxPhos) and ATP production, and the dependencies of these consequences on respiratory substrates, have not been investigated between the kidney and heart despite the fact that kidney energy requirements are second only to those of the heart. Our objective was, therefore, to elucidate these relationships in isolated mitochondria from the kidney outer medulla (OM) and heart. ADP-induced mitochondrial respiration was measured at different CaCl2 concentrations in the presence of various respiratory substrates, including pyruvate + malate (PM), glutamate + malate (GM), alpha-ketoglutarate + malate (AM), palmitoyl-carnitine + malate (PCM), and succinate + rotenone (SUC + ROT). The results showed that, in both heart and OM mitochondria, and for most complex I substrates, Ca2+ effects are biphasic: small increases in Ca2+ concentration stimulated, while large increases inhibited mitochondrial respiration. Furthermore, significant differences in substrate- and Ca2+-dependent O2 utilization towards ATP production between heart and OM mitochondria were observed. With PM and PCM substrates, Ca2+ showed more prominent stimulatory effects in OM than in heart mitochondria, while with GM and AM substrates, Ca2+ had similar biphasic regulatory effects in both OM and heart mitochondria. In contrast, with complex II substrate SUC + ROT, only inhibitory effects on mitochondrial respiration was observed in both the heart and the OM. We conclude that the regulatory effects of Ca2+ on mitochondrial OxPhos and ATP synthesis are biphasic, substrate-dependent, and tissue-specific.

Caffeine promotes angiogenesis through modulating endothelial mitochondrial dynamics

Caffeine induces multiple vascular effects. In this study we investigated the angiogenic effect of physiological concentrations of caffeine with focus on endothelial cell behaviors (migration and proliferation) during angiogenesis and its mitochondrial and bioenergetic mechanisms. We showed that caffeine (10-50 μM) significantly enhanced angiogenesis in vitro, evidenced by concentration-dependent increases in tube formation, and migration of human umbilical vein endothelial cells (HUVECs) without affecting cell proliferation. Caffeine (50 μM) enhanced endothelial migration via activation of cAMP/PKA/AMPK signaling pathway, which was mimicked by cAMP analog 8-Br-cAMP, and blocked by PKA inhibitor H89, adenylate cyclase inhibitor SQ22536 or AMPK inhibitor compound C. Furthermore, caffeine (50 μM) induced significant mitochondrial shortening through the increased phosphorylation of mitochondrial fission protein dynamin-related protein 1 (Drp1) in HUVECs, which increased its activity to regulate mitochondrial fission. Pharmacological blockade of Drp1 by Mdivi-1 (10 μM) or disturbance of mitochondrial fission by Drp1 silencing markedly suppressed caffeine-induced lamellipodia formation and endothelial cell migration. Moreover, we showed that caffeine-induced mitochondrial fission led to accumulation of more mitochondria in lamellipodia regions and augmentation of mitochondrial energetics, both of which were necessary for cell migration. In a mouse model of hindlimb ischemia, administration of caffeine (0.05% in 200 mL drinking water daily, for 14 days) significantly promoted angiogenesis and perfusion as well as activation of endothelial AMPK signaling in the ischemic hindlimb. Taken together, caffeine induces mitochondrial fission through cAMP/PKA/AMPK signaling pathway. Mitochondrial fission is an integral process in caffeine-induced endothelial cell migration by altering mitochondrial distribution and energetics.

Mitochondrial Bioenergetics and Turnover during Chronic Muscle Disuse

Periods of muscle disuse promote marked mitochondrial alterations that contribute to the impaired metabolic health and degree of atrophy in the muscle. Thus, understanding the molecular underpinnings of muscle mitochondrial decline with prolonged inactivity is of considerable interest. There are translational applications to patients subjected to limb immobilization following injury, illness-induced bed rest, neuropathies, and even microgravity. Studies in these patients, as well as on various pre-clinical rodent models have elucidated the pathways involved in mitochondrial quality control, such as mitochondrial biogenesis, mitophagy, fission and fusion, and the corresponding mitochondrial derangements that underlie the muscle atrophy that ensues from inactivity. Defective organelles display altered respiratory function concurrent with increased accumulation of reactive oxygen species, which exacerbate myofiber atrophy via degradative pathways. The preservation of muscle quality and function is critical for maintaining mobility throughout the lifespan, and for the prevention of inactivity-related diseases. Exercise training is effective in preserving muscle mass by promoting favourable mitochondrial adaptations that offset the mitochondrial dysfunction, which contributes to the declines in muscle and whole-body metabolic health. This highlights the need for further investigation of the mechanisms in which mitochondria contribute to disuse-induced atrophy, as well as the specific molecular targets that can be exploited therapeutically.

Disturbances in Calcium Homeostasis Promotes Skeletal Muscle Atrophy: Lessons From Ventilator-Induced Diaphragm Wasting

Mechanical ventilation (MV) is often a life-saving intervention for patients in respiratory failure. Unfortunately, a common and undesired consequence of prolonged MV is the development of diaphragmatic atrophy and contractile dysfunction. This MV-induced diaphragmatic weakness is commonly labeled “ventilator-induced diaphragm dysfunction” (VIDD). VIDD is an important clinical problem because diaphragmatic weakness is a major risk factor for the failure to wean patients from MV; this inability to remove patients from ventilator support results in prolonged hospitalization and increased morbidity and mortality. Although several processes contribute to the development of VIDD, it is clear that oxidative stress leading to the rapid activation of proteases is a primary contributor. While all major proteolytic systems likely contribute to VIDD, emerging evidence reveals that activation of the calcium-activated protease calpain plays a required role. This review highlights the signaling pathways leading to VIDD with a focus on the cellular events that promote increased cytosolic calcium levels and the subsequent activation of calpain within diaphragm muscle fibers. In particular, we discuss the emerging evidence that increased mitochondrial production of reactive oxygen species promotes oxidation of the ryanodine receptor/calcium release channel, resulting in calcium release from the sarcoplasmic reticulum, accelerated proteolysis, and VIDD. We conclude with a discussion of important and unanswered questions associated with disturbances in calcium homeostasis in diaphragm muscle fibers during prolonged MV.

Mitochondrial Ca2+ in heart failure: Not enough or too much?

Ca²⁺ serves as a ubiquitous second messenger mediating a variety of cellular processes including electrical excitation, contraction, gene expression, secretion, cell death and others. The identification of the molecular components of the mitochondrial Ca²⁺ influx and efflux pathways has created a resurgent interest in the regulation of mitochondrial Ca²⁺ balance and its physiological and pathophysiological roles. While the pace of discovery has quickened with the availability of new cellular and animal models, many fundamental questions remain to be answered regarding the regulation and functional impact of mitochondrial Ca²⁺ in health and disease. This review highlights several experimental observations pertaining to key aspects of mitochondrial Ca²⁺ homeostasis that remain enigmatic, particularly whether mitochondrial Ca²⁺ signaling is depressed or excessive in heart failure, which will determine the optimal approach to therapeutic intervention.

Metabolite regulation of the mitochondrial calcium uniporter channel

Calcium (Ca2+) is known to stimulate mitochondrial bioenergetics through the modulation of TCA cycle dehydrogenases and electron transport chain (ETC) complexes. This is hypothesized to be an essential pathway of energetic control to meet cellular ATP demand. While regulatory mechanisms of mitochondrial calcium uptake have been reported, it remains unknown if metabolite flux itself feedsback to regulate mitochondrial calcium (mCa2+) uptake. This hypothesis was recently tested by Nemani et al. (Sci. Signal. 2020) where the authors report that TCA cycle substrate flux regulates the mitochondrial calcium uniporter channel gatekeeper, mitochondrial calcium uptake 1 (MICU1), gene transcription in an early growth response protein 1 (EGR1) dependent fashion. They posit this is a regulatory feedback mechanism to control ionic homeostasis and mitochondrial bioenergetics with changing fuel availability. Here, we provide a historical overview of mitochondrial calcium exchange and comprehensive appraisal of these results in the context of recent literature and discuss possible regulatory pathways of mCa2+ uptake and mitochondrial bioenergetics.

Is Mitochondrial Dysfunction a Common Root of Noncommunicable Chronic Diseases?

Mitochondrial damage is implicated as a major contributing factor for a number of noncommunicable chronic diseases such as cardiovascular diseases, cancer, obesity, and insulin resistance/type 2 diabetes. Here, we discuss the role of mitochondria in maintaining cellular and whole-organism homeostasis, the mechanisms that promote mitochondrial dysfunction, and the role of this phenomenon in noncommunicable chronic diseases. We also review the state of the art regarding the preclinical evidence associated with the regulation of mitochondrial function and the development of current mitochondria-targeted therapeutics to treat noncommunicable chronic diseases. Finally, we give an integrated vision of how mitochondrial damage is implicated in these metabolic diseases.

Glutamate Transporters and Mitochondria: Signaling, Co-compartmentalization, Functional Coupling, and Future Directions

In addition to being an amino acid that is incorporated into proteins, glutamate is the most abundant neurotransmitter in the mammalian CNS, the precursor for the inhibitory neurotransmitter γ-aminobutyric acid, and one metabolic step from the tricarboxylic acid cycle intermediate α-ketoglutarate. Extracellular glutamate is cleared by a family of Na⁺-dependent transporters. These transporters are variably expressed by all cell types in the nervous system, but the bulk of clearance is into astrocytes. GLT-1 and GLAST (also called EAAT2 and EAAT1) mediate this activity and are extremely abundant proteins with their expression enriched in fine astrocyte processes. In this review, we will focus on three topics related to these astrocytic glutamate transporters. First, these transporters co-transport three Na⁺ ions and a H⁺ with each molecule of glutamate and counter-transport one K⁺; they are also coupled to a Cl^- conductance. The movement of Na⁺ is sufficient to cause profound astrocytic depolarization, and the movement of H⁺ is linked to astrocytic acidification. In addition, the movement of Na⁺ can trigger the activation of Na⁺ co-transporters (e.g. Na⁺-Ca²⁺ exchangers). We will describe the ways in which these ionic movements have been linked as signals to brain function and/or metabolism. Second, these transporters co-compartmentalize with mitochondria, potentially providing a mechanism to supply glutamate to mitochondria as a source of fuel for the brain. We will provide an overview of the proteins involved, discuss the evidence that glutamate is oxidized, and then highlight some of the un-resolved issues related to glutamate oxidation. Finally, we will review evidence that ischemic insults (stroke or oxygen/glucose deprivation) cause changes in these astrocytic mitochondria and discuss the ways in which these changes have been linked to glutamate transport, glutamate transport-dependent signaling, and altered glutamate metabolism. We conclude with a broader summary of some of the unresolved issues.

Tracking intra- and inter-organelle signaling of mitochondria

Mitochondria are as highly specialized organelles and masters of the cellular energy metabolism in a constant and dynamic interplay with their cellular environment, providing adenosine triphosphate, buffering Ca²⁺ and fundamentally contributing to various signaling pathways. Hence, such broad field of action within eukaryotic cells requires a high level of structural and functional adaptation. Therefore, mitochondria are constantly moving and undergoing fusion and fission processes, changing their shape and their interaction with other organelles. Moreover, mitochondrial activity gets fine-tuned by intra- and interorganelle H⁺ , K⁺ , Na⁺ , and Ca²⁺ signaling. In this review, we provide an up-to-date overview on mitochondrial strategies to adapt and respond to, as well as affect, their cellular environment. We also present cutting-edge technologies used to track and investigate subcellular signaling, essential to the understanding of various physiological and pathophysiological processes.

MCUB Regulates the Molecular Composition of the Mitochondrial Calcium Uniporter Channel to Limit Mitochondrial Calcium Overload During Stress

BACKGROUND

The mitochondrial calcium uniporter (mtCU) is an ≈700-kD multisubunit channel residing in the inner mitochondrial membrane required for mitochondrial Ca²⁺ (_mCa²⁺) uptake. Here, we detail the contribution of MCUB, a paralog of the pore-forming subunit MCU, in mtCU regulation and function and for the first time investigate the relevance of MCUB to cardiac physiology.

METHODS

We created a stable MCUB knockout cell line (MCUB^-/-) using CRISPR-Cas9n technology and generated a cardiac-specific, tamoxifen-inducible MCUB mutant mouse (CAG-CAT-MCUB x MCM; MCUB-Tg) for in vivo assessment of cardiac physiology and response to ischemia/reperfusion injury. Live-cell imaging and high-resolution spectrofluorometery were used to determine intracellular Ca²⁺ exchange and size-exclusion chromatography; blue native page and immunoprecipitation studies were used to determine the molecular function and impact of MCUB on the high-molecular-weight mtCU complex.

RESULTS

Using genetic gain- and loss-of-function approaches, we show that MCUB expression displaces MCU from the functional mtCU complex and thereby decreases the association of mitochondrial calcium uptake 1 and 2 (MICU1/2) to alter channel gating. These molecular changes decrease MICU1/2-dependent cooperative activation of the mtCU, thereby decreasing _mCa²⁺ uptake. Furthermore, we show that MCUB incorporation into the mtCU is a stress-responsive mechanism to limit _mCa²⁺ overload during cardiac injury. Indeed, overexpression of MCUB is sufficient to decrease infarct size after ischemia/reperfusion injury. However, MCUB incorporation into the mtCU does come at a cost; acute decreases in _mCa²⁺ uptake impair mitochondrial energetics and contractile function.

CONCLUSIONS

We detail a new regulatory mechanism to modulate mtCU function and _mCa²⁺ uptake. Our results suggest that MCUB-dependent changes in mtCU stoichiometry are a prominent regulatory mechanism to modulate _mCa²⁺ uptake and cellular physiology.

Elucidation of the Molecular Mechanism Underlying Lippia citriodora(Lim.)-Induced Relaxation and Anti-Depression

Lippia citriodora ethanolic extract (VEE) and verbascoside (Vs), a phenypropanoid glycoside, have been demonstrated to exert relaxant and anxiolytic properties. However, the molecular mechanisms behind their effects are still unclear. In this work, we studied the effects and action mechanisms of VEE and Vs in vivo and in vitro, on human neurotypic SH-SY5Y cells.TST was conducted on mice treated orally with VEE (25, 50 and 100 mg/Kg), Vs (2.5 and 5 mg/Kg), Bupropion (20 mg/Kg) and Milli-Q water. Higher dose of VEE-treated mice showed an increase of immobility time compared to control groups, indicating an induction of relaxation. This effect was found to be induced by regulation of genes playing key roles in calcium homeostasis (calcium channels), cyclic AMP (cAMP) production and energy metabolism. On the other hand, low doses of VEE and Vs showed an antidepressant-like effect and was confirmed by serotonin, noradrenalin, dopamine and BDNF expressions. Finally, VEE and Vsenhancedcell viability, mitochondrial activity and calcium uptake in vitro confirming in vivo findings. Our results showed induction of relaxation and antidepressant-like effects depending on the administered dose of VEE and Vs, through modulation of cAMP and calcium.

Mitochondrial Ca2+ concentrations in live cells: quantification methods and discrepancies

Intracellular Ca²⁺ signaling controls numerous cellular functions. Mitochondria respond to cytosolic Ca²⁺ changes by adapting mitochondrial functions and, in some cell types, shaping the spatiotemporal properties of the cytosolic Ca²⁺ signal. Numerous methods have been developed to specifically and quantitatively measure the mitochondrial-free Ca²⁺ concentrations ([Ca²⁺ ]_m ), but there are still significant discrepancies in the calculated absolute values of [Ca²⁺ ]_m in stimulated live cells. These discrepancies may be due to the distinct properties of the methods used to measure [Ca²⁺ ]_m , the calcium-free/bound ratio, and the cell-type and stimulus-dependent Ca²⁺ dynamics. Critical processes happening in the mitochondria, such as ATP generation, ROS homeostasis, and mitochondrial permeability transition opening, depend directly on the [Ca²⁺ ]_m values. Thus, precise determination of absolute [Ca²⁺ ]_m values is imperative for understanding Ca²⁺ signaling. This review summarizes the reported calibrated [Ca²⁺ ]_m values in many cell types and discusses the discrepancies among these values. Areas for future research are also proposed.

Quantitative Proteomic Analysis to Understand the Mechanisms of Zinc Oxide Nanoparticle Toxicity to Daphnia pulex (Crustacea: Daphniidae): Comparing with Bulk Zinc Oxide and Zinc Salt

The widespread use of zinc oxide nanoparticles (ZnO NPs) has resulted in their release to the environment. There has been concern about the ecotoxicity of ZnO NPs, but little is known about their toxic mechanisms. In the present study, we conducted acute toxicity tests to show that ZnO NPs are more toxic to the freshwater crustacean Daphnia pulex compared to bulk ZnO or ZnSO₄·7H₂O. To provide an integrated and quantitative insights into the toxicity of ZnO NPs, we conducted isobaric tags for relative and absolute quantitation (iTRAQ) proteomic analysis, which detected 262, 331, and 360 differentially expressed proteins (DEPs) in D. pulex exposed to ZnO NPs, bulk ZnO, and ZnSO₄·7H₂O, respectively. Among the DEPs, 224 were shared among the three treatments. These proteins were related to energy metabolism, oxidative stress, and endoplasmic reticulum stress. The three forms of Zn all caused D. pulex to downregulate Chitinase expression, disrupt Ca²⁺ homeostasis, and reduce expression of digestive enzymes. Nevertheless, 29 proteins were expressed only in the ZnO NP treatment. In particular, histone (H3) and ribosomal proteins (L13) were obviously influenced under ZnO NP treatment. However, increased expression levels of h3 and l13 genes were not induced only in ZnO NP treatment, they were sensitive to Zn ions under the same exposure concentration. These results indicate that the three zinc substances have a similar mode of action and that released zinc ions are the main contributor to ZnO NP toxicity to D. pulex under a low concentration. Further investigation is needed to clarify whether a small proportion of DEPs or higher bioavailability cause ZnO NPs to be more toxic compared to bulk ZnO or ionic zinc.

Slow Ca2+ Efflux by Ca2+/H+ Exchange in Cardiac Mitochondria Is Modulated by Ca2+ Re-uptake via MCU, Extra-Mitochondrial pH, and H+ Pumping by FOF1-ATPase

Mitochondrial (m) Ca²⁺ influx is largely dependent on membrane potential (ΔΨ_m), whereas mCa²⁺ efflux occurs primarily via Ca²⁺ ion exchangers. We probed the kinetics of Ca²⁺/H⁺ exchange (CHE_m) in guinea pig cardiac muscle mitochondria. We tested if net mCa²⁺ flux is altered during a matrix inward H⁺ leak that is dependent on matrix H⁺ pumping by ATP_m hydrolysis at complex V (F_OF₁-ATPase). We measured [Ca²⁺]_m, extra-mitochondrial (e) [Ca²⁺]_e, ΔΨ_m, pH_m, pH_e, NADH, respiration, ADP/ATP ratios, and total [ATP]_m in the presence or absence of protonophore dinitrophenol (DNP), mitochondrial uniporter (MCU) blocker Ru360, and complex V blocker oligomycin (OMN). We proposed that net slow influx/efflux of Ca²⁺ after adding DNP and CaCl₂ is dependent on whether the ΔpH_m gradient is/is not maintained by reciprocal outward H⁺ pumping by complex V. We found that adding CaCl₂ enhanced DNP-induced increases in respiration and decreases in ΔΨ_m while [ATP]_m decreased, ΔpH_m gradient was maintained, and [Ca²⁺]_m continued to increase slowly, indicating net mCa²⁺ influx via MCU. In contrast, with complex V blocked by OMN, adding DNP and CaCl₂ caused larger declines in ΔΨ_m as well as a slow fall in pH_m to near pH_e while [Ca²⁺]_m continued to decrease slowly, indicating net mCa²⁺ efflux in exchange for H⁺ influx (CHE_m) until the ΔpH_m gradient was abolished. The kinetics of slow mCa²⁺ efflux with slow H⁺ influx via CHE_m was also observed at pH_e 6.9 vs. 7.6 by the slow fall in pH_m until ΔpH_m was abolished; if Ca²⁺ reuptake via the MCU was also blocked, mCa²⁺ efflux via CHE_m became more evident. Of the two components of the proton electrochemical gradient, our results indicate that CHE_m activity is driven largely by the ΔpH_m chemical gradient with H⁺ leak, while mCa²⁺ entry via MCU depends largely on the charge gradient ΔΨ_m. A fall in ΔΨ_m with excess mCa²⁺ loading can occur during cardiac cell stress. Cardiac cell injury due to mCa²⁺ overload may be reduced by temporarily inhibiting F_OF₁-ATPase from pumping H⁺ due to ΔΨ_m depolarization. This action would prevent additional slow mCa²⁺ loading via MCU and permit activation of CHE_m to mediate efflux of mCa²⁺.

HIGHLIGHTS-

We examined how slow mitochondrial (m) Ca²⁺ efflux via Ca²⁺/H⁺ exchange (CHE_m) is triggered by matrix acidity after a rapid increase in [Ca²⁺]_m by adding CaCl₂ in the presence of dinitrophenol (DNP) to permit H⁺ influx, and oligomycin (OMN) to block H⁺ pumping via F_OF₁-ATP synthase/ase (complex V).

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Declines in ΔΨ_m and pH_m after DNP and added CaCl₂ were larger when complex V was blocked.

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[Ca²⁺]_m slowly increased despite a fall in ΔΨ_m but maintained pH_m when H⁺ pumping by complex V was permitted.

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[Ca²⁺]_m slowly decreased and external [Ca²⁺]_e increased with declines in both ΔΨ_m and pH_m when complex V was blocked.

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ATP_m hydrolysis supports a falling pH_m and redox state and promotes a slow increase in [Ca²⁺]_m.

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After rapid Ca²⁺ influx due to a bolus of CaCl₂, slow mCa²⁺ efflux by CHE_m occurs directly if pH_e is low.

Assessing Calcium-Stimulated Mitochondrial Bioenergetics Using the Seahorse XF96 Analyzer

The development of fluorescence-based oxygen sensors coupled with microplate-based assays for quantitative bioenergetics analyses enables screening multiple experimental conditions at once with small biological material and in a timely manner. In this chapter, we outline detailed protocols and practical tips to design and perform controlled measurements of (a) respiratory and glycolytic metabolism of intact cells, (b) substrate-dependent respiration in permeabilized cells and isolated mitochondria, and (c) calcium-dependent regulation of mitochondrial bioenergetics with Seahorse XF Flux Analyzers.

Mitochondrial calcium transport and the redox nature of the calcium-induced membrane permeability transition

Mitochondria possess a Ca²⁺ transport system composed of separate Ca²⁺ influx and efflux pathways. Intramitochondrial Ca²⁺ concentrations regulate oxidative phosphorylation, required for cell function and survival, and mitochondrial redox balance, that participates in a myriad of signaling and damaging pathways. The interaction between Ca²⁺ accumulation and redox imbalance regulates opening and closing of a highly regulated inner membrane pore, the membrane permeability transition pore (PTP). In this review, we discuss the regulation of the PTP by mitochondrial oxidants, reactive nitrogen species, and the interactions between these species and other PTP inducers. In addition, we discuss the involvement of mitochondrial redox imbalance and PTP in metabolic conditions such as atherogenesis, diabetes, obesity and in mtDNA stability.

Kidney-targeted inhibition of protein kinase C-α ameliorates nephrotoxic nephritis with restoration of mitochondrial dysfunction

To investigate the role of protein kinase C-α (PKC-α) in glomerulonephritis, the capacity of PKC-α inhibition to reverse the course of established nephrotoxic nephritis (NTN) was evaluated. Nephritis was induced by a single injection of nephrotoxic serum and after its onset, a PKC-α inhibitor was administered either systemically or by targeted glomerular delivery. By day seven, all mice with NTN had severe nephritis, whereas mice that received PKC-α inhibitors in either form had minimal evidence of disease. To further understand the underlying mechanism, label-free shotgun proteomic analysis of the kidney cortexes were performed, using quantitative mass spectrometry. Ingenuity pathway analysis revealed 157 differentially expressed proteins and mitochondrial dysfunction as the most modulated pathway. Functional protein groups most affected by NTN were mitochondrial proteins associated with respiratory processes. These proteins were down-regulated in the mice with NTN, while their expression was restored with PKC-α inhibition. This suggests a role for proteins that regulate oxidative phosphorylation in recovery. In cultured glomerular endothelial cells, nephrotoxic serum caused a decrease in mitochondrial respiration and membrane potential, mitochondrial morphologic changes and an increase in glycolytic lactic acid production; all normalized by PKC-α inhibition. Thus, PKC-α has a critical role in NTN progression, and the results implicate mitochondrial processes through restoring oxidative phosphorylation, as an essential mechanism underlying recovery. Importantly, our study provides additional support for targeted therapy to glomeruli to reverse the course of progressive disease.

Impaired Bioenergetics in Mutant Mitochondrial DNA Determines Cell Fate During Seizure-Like Activity

Mutations in genes affecting mitochondrial proteins are increasingly recognised in patients with epilepsy, but the factors determining cell fate during seizure activity in these mutations remain unknown. Fluorescent dye imaging techniques were applied to fibroblast cell lines from patients suffering from common mitochondrial mutations and to age-matched controls. Using live cell imaging techniques in fibroblasts, we show that fibroblasts with mutations in the mitochondrial genome had reduced mitochondrial membrane potential and NADH pools and higher redox indices, indicative of respiratory chain dysfunction. Increasing concentrations of ferutinin, a Ca²⁺ ionophore, led to oscillatory Ca²⁺ signals in fibroblasts resembling dynamic Ca²⁺ changes that occur during seizure-like activity. Co-monitoring of mitochondrial membrane potential (ΔΨ_m) changes induced by ferutinin showed accelerated membrane depolarisation and cell collapse in fibroblasts with mutations in the mitochondrial genome when compared to controls. Ca²⁺ flash photolysis using caged Ca²⁺ confirmed impaired Ca²⁺ handling in fibroblasts with mitochondrial mutations. Findings indicate that intracellular Ca²⁺ levels cannot be compensated during periods of hyperexcitability, leading to Ca²⁺ overload and subsequent cell death in mitochondrial diseases.

The emerging role of immune dysfunction in mitochondrial diseases as a paradigm for understanding immunometabolism

Immunometabolism aims to define the role of intermediary metabolism in immune cell function, with bioenergetics and the mitochondria recently taking center stage. To date, the medical literature on mitochondria and immune function extols the virtues of mouse models in exploring this biologic intersection. While the laboratory mouse has become a standard for studying mammalian biology, this model comprises part of a comprehensive approach. Humans, with their broad array of inherited phenotypes, serve as a starting point for studying immunometabolism; specifically, patients with mitochondrial disease. Using this top-down approach, the mouse as a model organism facilitates further exploration of the consequences of mutations involved in mitochondrial maintenance and function. In this review, we will discuss the emerging phenotype of immune dysfunction in mitochondrial disease as a model for understanding the role of the mitochondria in immune function in available mouse models.

The role of the mitochondria and the endoplasmic reticulum contact sites in the development of the immune responses

Mitochondria and endoplasmic reticulum (ER) contact sites (MERCs) are dynamic modules enriched in subset of lipids and specialized proteins that determine their structure and functions. The MERCs regulate lipid transfer, autophagosome formation, mitochondrial fission, Ca2+ homeostasis and apoptosis. Since these functions are essential for cell biology, it is therefore not surprising that MERCs also play a critical role in organ physiology among which the immune system stands by its critical host defense function. This defense system must discriminate and tolerate host cells and beneficial commensal microorganisms while eliminating pathogenic ones in order to preserve normal homeostasis. To meet this goal, the immune system has two lines of defense. First, the fast acting but unspecific innate immune system relies on anatomical physical barriers and subsets of hematopoietically derived cells expressing germline-encoded receptors called pattern recognition receptors (PRR) recognizing conserved motifs on the pathogens. Second, the slower but very specific adaptive immune response is added to complement innate immunity. Adaptive immunity relies on another set of specialized cells, the lymphocytes, harboring receptors requiring somatic recombination to be expressed. Both innate and adaptive immune cells must be activated to phagocytose and process pathogens, migrate, proliferate, release soluble factors and destroy infected cells. Some of these functions are strongly dependent on lipid transfer, autophagosome formation, mitochondrial fission, and Ca2+ flux; this indicates that MERCs could regulate immunity.

Mitochondrial Permeability Transition Pore and Calcium Handling

The opening of a large conductance channel in the inner mitochondrial membrane, known as the mitochondrial permeability transition pore (PTP), has been shown to be a primary mediator of cell death in the heart subjected to ischemia-reperfusion injury. Inhibitors of the PTP have been shown to reduce cardiac ischemia-reperfusion injury in many animal models. Furthermore, most cardioprotective strategies appear to reduce ischemic cell death either by reducing the triggers for the opening of the PTP, such as reducing calcium overload or reactive oxygen species, or by inhibiting PTP modulators. This chapter will focus on key issues in the study of the PTP and provide some methods for measuring PTP opening in isolated mitochondria.

MicroRNA-138 and MicroRNA-25 Down-regulate Mitochondrial Calcium Uniporter, Causing the Pulmonary Arterial Hypertension Cancer Phenotype

RATIONALE

Pulmonary arterial hypertension (PAH) is an obstructive vasculopathy characterized by excessive pulmonary artery smooth muscle cell (PASMC) proliferation, migration, and apoptosis resistance. This cancer-like phenotype is promoted by increased cytosolic calcium ([Ca²⁺]_cyto), aerobic glycolysis, and mitochondrial fission.

OBJECTIVES

To determine how changes in mitochondrial calcium uniporter (MCU) complex (MCUC) function influence mitochondrial dynamics and contribute to PAH's cancer-like phenotype.

METHODS

PASMCs were isolated from patients with PAH and healthy control subjects and assessed for expression of MCUC subunits. Manipulation of the pore-forming subunit, MCU, in PASMCs was achieved through small interfering RNA knockdown or MCU plasmid-mediated up-regulation, as well as through modulation of the upstream microRNAs (miRs) miR-138 and miR-25. In vivo, nebulized anti-miRs were administered to rats with monocrotaline-induced PAH.

MEASUREMENTS AND MAIN RESULTS

Impaired MCUC function, resulting from down-regulation of MCU and up-regulation of an inhibitory subunit, mitochondrial calcium uptake protein 1, is central to PAH's pathogenesis. MCUC dysfunction decreases intramitochondrial calcium ([Ca²⁺]_mito), inhibiting pyruvate dehydrogenase activity and glucose oxidation, while increasing [Ca²⁺]_cyto, promoting proliferation, migration, and fission. In PAH PASMCs, increasing MCU decreases cell migration, proliferation, and apoptosis resistance by lowering [Ca²⁺]_cyto, raising [Ca²⁺]_mito, and inhibiting fission. In normal PASMCs, MCUC inhibition recapitulates the PAH phenotype. In PAH, elevated miRs (notably miR-138) down-regulate MCU directly and also by decreasing MCU's transcriptional regulator cAMP response element-binding protein 1. Nebulized anti-miRs against miR-25 and miR-138 restore MCU expression, reduce cell proliferation, and regress established PAH in the monocrotaline model.

CONCLUSIONS

These results highlight miR-mediated MCUC dysfunction as a unifying mechanism in PAH that can be therapeutically targeted.

Activation of Transient Receptor Potential Melastatin Subtype 8 Attenuates Cold-Induced Hypertension Through Ameliorating Vascular Mitochondrial Dysfunction

Background

Environmental cold‐induced hypertension is common, but how to treat cold‐induced hypertension remains an obstacle. Transient receptor potential melastatin subtype 8 (TRPM8) is a mild cold‐sensing nonselective cation channel that is activated by menthol. Little is known about the effect of TRPM8 activation by menthol on mitochondrial Ca²⁺ homeostasis and the vascular function in cold‐induced hypertension.

Methods and Results

Primary vascular smooth muscle cells from wild‐type or Trpm8^−/− mice were cultured. In vitro, we confirmed that sarcoplasmic reticulum–resident TRPM8 participated in the regulation of cellular and mitochondrial Ca²⁺ homeostasis in the vascular smooth muscle cells. TRPM8 activation by menthol antagonized angiotensin II induced mitochondrial respiratory dysfunction and excess reactive oxygen species generation by preserving pyruvate dehydrogenase activity, which hindered reactive oxygen species–triggered Ca²⁺ influx and the activation of RhoA/Rho kinase pathway. In vivo, long‐term noxious cold stimulation dramatically increased vasoconstriction and blood pressure. The activation of TRPM8 by dietary menthol inhibited vascular reactive oxygen species generation, vasoconstriction, and lowered blood pressure through attenuating excessive mitochondrial reactive oxygen species mediated the activation of RhoA/Rho kinase in a TRPM8‐dependent manner. These effects of menthol were further validated in angiotensin II–induced hypertensive mice.

Conclusions

Long‐term dietary menthol treatment targeting and preserving mitochondrial function may represent a nonpharmaceutical measure for environmental noxious cold–induced hypertension.

The Involvement of Mg2+ in Regulation of Cellular and Mitochondrial Functions

Mg²⁺ is an essential mineral with pleotropic impacts on cellular physiology and functions. It acts as a cofactor of several important enzymes, as a regulator of ion channels such as voltage-dependent Ca²⁺ channels and K⁺ channels and on Ca²⁺-binding proteins. In general, Mg²⁺ is considered as the main intracellular antagonist of Ca²⁺, which is an essential secondary messenger initiating or regulating a great number of cellular functions. This review examines the effects of Mg²⁺ on mitochondrial functions with a particular focus on energy metabolism, mitochondrial Ca²⁺ handling, and apoptosis.

Regulation of ATP production: dependence on calcium concentration and respiratory state

Nanomolar free calcium enhances oxidative phosphorylation. However, the effects over a broad concentration range, at different respiratory states, or on specific energy substrates are less clear. We examined the action of varying [Ca²⁺] over respiratory states ranging 4 to 3 on skeletal muscle mitochondrial respiration, potential, ATP production, and H₂O₂ production using ADP recycling to clamp external [ADP]. Calcium at 450 nM enhanced respiration in mitochondria energized by the complex I substrates, glutamate/malate (but not succinate), at [ADP] of 4-256 µM, but more substantially at intermediate respiratory states and not at all at state 4. Using varied [Ca²⁺], we found that the stimulatory effects on respiration and ATP production were most prominent at nanomolar concentrations, but inhibitory at 10 µM or higher. ATP production decreased more than respiration at 10 µM calcium. However, potential continued to increase up to 10 µM; suggesting a calcium-induced inability to utilize potential for phosphorylation independent of opening of the mitochondrial permeability transition pore (MTP). This effect of 10 µM calcium was confirmed by direct determination of ATP production over a range of potential created by differing substrate concentrations. Consistent with past reports, nanomolar [Ca²⁺] had a stimulatory effect on utilization of potential for phosphorylation. Increasing [Ca²⁺] was positively and continuously associated with H₂O₂ production. In summary, the stimulatory effect of calcium on mitochondrial function is substrate dependent and most prominent over intermediate respiratory states. Calcium stimulates or inhibits utilization of potential for phosphorylation dependent on concentration with inhibition at higher concentration independent of MTP opening.

Mitochondrial Mechanosensor Microdomains in Cardiovascular Disorders

The cardiomyocytes populating the 'working myocardium' are highly organized and such organization ranges from macroscale (e.g. the geometrical rod shape) to microscale (dyad/t-tubules) domains. This meticulous level of organization is imperative for assuring the normal and physiological pump-function of the heart. In the pathological cardiac tissue, the domains-related architecture is partially lost, resulting in morphological, electrical and metabolic remodeling and promoting cardiovascular diseases including heart failure and arrhythmias. Indeed, arrhythmogenesis during heart failure is a major clinical problem. Arrhythmias have been extensively studied from an electrical etiology, but only recently, physiologists and scientists have focused their attention on cellular and subcellular mechanosensors. We and others have investigated whether the nanoscale mechanosensitive properties of cardiomyocytes from failing hearts have a bearing upon the initiation of abnormal electrical activity. This chapter highlights the recent findings in the field, especially the role of mitochondria function and alignment in failing cardiomyocytes interrogated via nanomechanical stimuli.

Catalytic Coupling of Oxidative Phosphorylation, ATP Demand, and Reactive Oxygen Species Generation

Competing models of mitochondrial energy metabolism in the heart are highly disputed. In addition, the mechanisms of reactive oxygen species (ROS) production and scavenging are not well understood. To deepen our understanding of these processes, a computer model was developed to integrate the biophysical processes of oxidative phosphorylation and ROS generation. The model was calibrated with experimental data obtained from isolated rat heart mitochondria subjected to physiological conditions and workloads. Model simulations show that changes in the quinone pool redox state are responsible for the apparent inorganic phosphate activation of complex III. Model simulations predict that complex III is responsible for more ROS production during physiological working conditions relative to complex I. However, this relationship is reversed under pathological conditions. Finally, model analysis reveals how a highly reduced quinone pool caused by elevated levels of succinate is likely responsible for the burst of ROS seen during reperfusion after ischemia.

Open-Loop Control of Oxidative Phosphorylation in Skeletal and Cardiac Muscle Mitochondria by Ca(2.)

In cardiac muscle, mitochondrial ATP synthesis is driven by demand for ATP through feedback from the products of ATP hydrolysis. However, in skeletal muscle at higher workloads there is an apparent contribution of open-loop stimulation of ATP synthesis. Open-loop control is defined as modulation of flux through a biochemical pathway by a moiety, which is not a reactant or a product of the biochemical reactions in the pathway. The role of calcium, which is known to stimulate the activity of mitochondrial dehydrogenases, as an open-loop controller, was investigated in isolated cardiac and skeletal muscle mitochondria. The kinetics of NADH synthesis and respiration, feedback from ATP hydrolysis products, and stimulation by calcium were characterized in isolated mitochondria to test the hypothesis that calcium has a stimulatory role in skeletal muscle mitochondria not apparent in cardiac mitochondria. A range of respiratory states were obtained in cardiac and skeletal muscle mitochondria utilizing physiologically relevant concentrations of pyruvate and malate, and flux of respiration, NAD(P)H fluorescence, and rhodamine 123 fluorescence were measured over a range of extra mitochondrial calcium concentrations. We found that under these conditions calcium stimulates NADH synthesis in skeletal muscle mitochondria but not in cardiac mitochondria.

Impact of intracellular ion channels on cancer development and progression

Cancer research is nowadays focused on the identification of possible new targets in order to try to develop new drugs for curing untreatable tumors. Ion channels have emerged as "oncogenic" proteins, since they have an aberrant expression in cancers compared to normal tissues and contribute to several hallmarks of cancer, such as metabolic re-programming, limitless proliferative potential, apoptosis-resistance, stimulation of neo-angiogenesis as well as cell migration and invasiveness. In recent years, not only the plasma membrane but also intracellular channels and transporters have arisen as oncological targets and were proposed to be associated with tumorigenesis. Therefore, the research is currently focusing on understanding the possible role of intracellular ion channels in cancer development and progression on one hand and, on the other, on developing new possible drugs able to modulate the expression and/or activity of these channels. In a few cases, the efficacy of channel-targeting drugs in reducing tumors has already been demonstrated in vivo in preclinical mouse models.

Expression of the mitochondrial calcium uniporter in cardiac myocytes improves impaired mitochondrial calcium handling and metabolism in simulated hyperglycemia

Diabetic cardiomyopathy is associated with metabolic changes, including decreased glucose oxidation (Gox) and increased fatty acid oxidation (FAox), which result in cardiac energetic deficiency. Diabetic hyperglycemia is a pathophysiological mechanism that triggers multiple maladaptive phenomena. The mitochondrial Ca²⁺ uniporter (MCU) is the channel responsible for Ca²⁺ uptake in mitochondria, and free mitochondrial Ca²⁺ concentration ([Ca²⁺]_m) regulates mitochondrial metabolism. Experiments with cardiac myocytes (CM) exposed to simulated hyperglycemia revealed reduced [Ca²⁺]_m and MCU protein levels. Therefore, we investigated whether returning [Ca²⁺]_m to normal levels in CM by MCU expression could lead to normalization of Gox and FAox with no detrimental effects. Mouse neonatal CM were exposed for 72 h to normal glucose [5.5 mM glucose + 19.5 mM mannitol (NG)], high glucose [25 mM glucose (HG)], or HG + adenoviral MCU expression. Gox and FAox, [Ca²⁺]_m, MCU levels, pyruvate dehydrogenase (PDH) activity, oxidative stress, mitochondrial membrane potential, and apoptosis were assessed. [Ca²⁺]_m and MCU protein levels were reduced after 72 h of HG. Gox was decreased and FAox was increased in HG, PDH activity was decreased, phosphorylated PDH levels were increased, and mitochondrial membrane potential was reduced. MCU expression returned these parameters toward NG levels. Moreover, increased oxidative stress and apoptosis were reduced in HG by MCU expression. We also observed reduced MCU protein levels and [Ca²⁺]_m in hearts from type 1 diabetic mice. Thus we conclude that HG-induced metabolic alterations can be reversed by restoration of MCU levels, resulting in return of [Ca²⁺]_m to normal levels.

Mitochondrial Ca(2+) uptake in skeletal muscle health and disease

Muscle uses Ca²⁺ as a messenger to control contraction and relies on ATP to maintain the intracellular Ca²⁺ homeostasis. Mitochondria are the major sub-cellular organelle of ATP production. With a negative inner membrane potential, mitochondria take up Ca²⁺ from their surroundings, a process called mitochondrial Ca²⁺ uptake. Under physiological conditions, Ca²⁺ uptake into mitochondria promotes ATP production. Excessive uptake causes mitochondrial Ca²⁺ overload, which activates downstream adverse responses leading to cell dysfunction. Moreover, mitochondrial Ca²⁺ uptake could shape spatio-temporal patterns of intracellular Ca²⁺ signaling. Malfunction of mitochondrial Ca²⁺ uptake is implicated in muscle degeneration. Unlike non-excitable cells, mitochondria in muscle cells experience dramatic changes of intracellular Ca²⁺ levels. Besides the sudden elevation of Ca²⁺ level induced by action potentials, Ca²⁺ transients in muscle cells can be as short as a few milliseconds during a single twitch or as long as minutes during tetanic contraction, which raises the question whether mitochondrial Ca²⁺ uptake is fast and big enough to shape intracellular Ca²⁺ signaling during excitation-contraction coupling and creates technical challenges for quantification of the dynamic changes of Ca²⁺ inside mitochondria. This review focuses on characterization of mitochondrial Ca²⁺ uptake in skeletal muscle and its role in muscle physiology and diseases.

Glucagon: acute actions on hepatic metabolism

Type 2 diabetes mellitus is the result of impaired systemic control of glucose homeostasis, in part through the dysregulation of the hormone glucagon. Glucagon acts on the liver to increase glucose production through alterations in hepatic metabolism, and reducing the elevated glucagon signalling in diabetic patients is an attractive strategy for the treatment of hyperglycaemia. Here we review the actions of the hormone in the liver, focusing on the acute alterations of metabolic pathways. This review summarises a presentation given at the 'Novel data on glucagon' symposium at the 2015 annual meeting of the EASD. It is accompanied by two other reviews on topics from this symposium (by Mona Abraham and Tony Lam, DOI: 10.1007/s00125-016-3950-3 , and by Young Lee and colleagues, DOI: 10.1007/s00125-016-3965-9 ) and an overview by the Session Chair, Isabel Valverde (DOI: 10.1007/s00125-016-3946-z ).