Characterization of distinct single-channel properties of Ca²⁺ inward currents in mitochondria

MCU-independent Ca2+ uptake mediates mitochondrial Ca2+ overload and necrotic cell death in a mouse model of Duchenne muscular dystrophy

Mitochondrial Ca2+ overload can mediate mitochondria-dependent cell death, a major contributor to several human diseases. Indeed, Duchenne muscular dystrophy (MD) is driven by dysfunctional Ca2+ influx across the sarcolemma that causes mitochondrial Ca2+ overload, organelle rupture, and muscle necrosis. The mitochondrial Ca2+ uniporter (MCU) complex is the primary characterized mechanism for acute mitochondrial Ca2+ uptake. One strategy for preventing mitochondrial Ca2+ overload is deletion of the Mcu gene, the pore forming subunit of the MCU-complex. Conversely, enhanced MCU-complex Ca2+ uptake is achieved by deleting the inhibitory Mcub gene. Here we show that myofiber-specific Mcu deletion was not protective in a mouse model of Duchenne MD. Specifically, Mcu gene deletion did not reduce muscle histopathology, did not improve muscle function, and did not prevent mitochondrial Ca2+ overload. Moreover, myofiber specific Mcub gene deletion did not augment Duchenne MD muscle pathology. Interestingly, we observed MCU-independent Ca2+ uptake in dystrophic mitochondria that was sufficient to drive mitochondrial permeability transition pore (MPTP) activation and skeletal muscle necrosis, and this same type of activity was observed in heart, liver, and brain mitochondria. These results demonstrate that mitochondria possess an uncharacterized MCU-independent Ca2+ uptake mechanism that is sufficient to drive MPTP-dependent necrosis in MD in vivo.

Mitochondrial Dysfunction in Cardiac Arrhythmias

Electrophysiological and structural disruptions in cardiac arrhythmias are closely related to mitochondrial dysfunction. Mitochondria are an organelle generating ATP, thereby satisfying the energy demand of the incessant electrical activity in the heart. In arrhythmias, the homeostatic supply-demand relationship is impaired, which is often accompanied by progressive mitochondrial dysfunction leading to reduced ATP production and elevated reactive oxidative species generation. Furthermore, ion homeostasis, membrane excitability, and cardiac structure can be disrupted through pathological changes in gap junctions and inflammatory signaling, which results in impaired cardiac electrical homeostasis. Herein, we review the electrical and molecular mechanisms of cardiac arrhythmias, with a particular focus on mitochondrial dysfunction in ionic regulation and gap junction action. We provide an update on inherited and acquired mitochondrial dysfunction to explore the pathophysiology of different types of arrhythmias. In addition, we highlight the role of mitochondria in bradyarrhythmia, including sinus node dysfunction and atrioventricular node dysfunction. Finally, we discuss how confounding factors, such as aging, gut microbiome, cardiac reperfusion injury, and electrical stimulation, modulate mitochondrial function and cause tachyarrhythmia.

MCU proteins dominate in vivo mitochondrial Ca2+ uptake in Arabidopsis roots

Ca2+ signaling is central to plant development and acclimation. While Ca2+-responsive proteins have been investigated intensely in plants, only a few Ca2+-permeable channels have been identified, and our understanding of how intracellular Ca2+ fluxes is facilitated remains limited. Arabidopsis thaliana homologs of the mammalian channel-forming mitochondrial calcium uniporter (MCU) protein showed Ca2+ transport activity in vitro. Yet, the evolutionary complexity of MCU proteins, as well as reports about alternative systems and unperturbed mitochondrial Ca2+ uptake in knockout lines of MCU genes, leave critical questions about the in vivo functions of the MCU protein family in plants unanswered. Here, we demonstrate that MCU proteins mediate mitochondrial Ca2+ transport in planta and that this mechanism is the major route for fast Ca2+ uptake. Guided by the subcellular localization, expression, and conservation of MCU proteins, we generated an mcu triple knockout line. Using Ca2+ imaging in living root tips and the stimulation of Ca2+ transients of different amplitudes, we demonstrated that mitochondrial Ca2+ uptake became limiting in the triple mutant. The drastic cell physiological phenotype of impaired subcellular Ca2+ transport coincided with deregulated jasmonic acid-related signaling and thigmomorphogenesis. Our findings establish MCUs as a major mitochondrial Ca2+ entry route in planta and link mitochondrial Ca2+ transport with phytohormone signaling.

Mitochondrial calcium exchange in physiology and disease

The uptake of calcium into and extrusion of calcium from the mitochondrial matrix is a fundamental biological process that has critical effects on cellular metabolism, signaling, and survival. Disruption of mitochondrial calcium (mCa2+) cycling is implicated in numerous acquired diseases such as heart failure, stroke, neurodegeneration, diabetes, and cancer and is genetically linked to several inherited neuromuscular disorders. Understanding the mechanisms responsible for mCa2+ exchange therefore holds great promise for the treatment of these diseases. The past decade has seen the genetic identification of many of the key proteins that mediate mitochondrial calcium uptake and efflux. Here, we present an overview of the phenomenon of mCa2+ transport and a comprehensive examination of the molecular machinery that mediates calcium flux across the inner mitochondrial membrane: the mitochondrial uniporter complex (consisting of MCU, EMRE, MICU1, MICU2, MICU3, MCUB, and MCUR1), NCLX, LETM1, the mitochondrial ryanodine receptor, and the mitochondrial permeability transition pore. We then consider the physiological implications of mCa2+ flux and evaluate how alterations in mCa2+ homeostasis contribute to human disease. This review concludes by highlighting opportunities and challenges for therapeutic intervention in pathologies characterized by aberrant mCa2+ handling and by summarizing critical unanswered questions regarding the biology of mCa2+ flux.

Sarcoplasmic reticulum-mitochondria communication; implications for cardiac arrhythmia

Sudden cardiac death due to ventricular tachyarrhythmias remains the major cause of mortality in the world. Heart failure, diabetic cardiomyopathy, old age-related cardiac dysfunction and inherited disorders are associated with enhanced propensity to malignant cardiac arrhythmias. Both defective mitochondrial function and abnormal intracellular Ca²⁺ homeostasis have been established as the key contributing factors in the pathophysiology and arrhythmogenesis in these conditions. This article reviews current advances in understanding of bidirectional control of ryanodine receptor-mediated sarcoplasmic reticulum Ca²⁺ release and mitochondrial function, and how defects in crosstalk between these two organelles increase arrhythmic risk in cardiac disease.

The pore-forming subunit MCU of the mitochondrial Ca2+ uniporter is required for normal glucose-stimulated insulin secretion in vitro and in vivo in mice

AIMS/HYPOTHESIS

Mitochondrial oxidative metabolism is central to glucose-stimulated insulin secretion (GSIS). Whether Ca²⁺ uptake into pancreatic beta cell mitochondria potentiates or antagonises this process is still a matter of debate. Although the mitochondrial Ca²⁺ importer (MCU) complex is thought to represent the main route for Ca²⁺ transport across the inner mitochondrial membrane, its role in beta cells has not previously been examined in vivo.

METHODS

Here, we inactivated the pore-forming subunit of the MCU, encoded by Mcu, selectively in mouse beta cells using Ins1^Cre-mediated recombination. Whole or dissociated pancreatic islets were isolated and used for live beta cell fluorescence imaging of cytosolic or mitochondrial Ca²⁺ concentration and ATP production in response to increasing glucose concentrations. Electrophysiological recordings were also performed on whole islets. Serum and blood samples were collected to examine oral and i.p. glucose tolerance.

RESULTS

Glucose-stimulated mitochondrial Ca²⁺ accumulation (p< 0.05), ATP production (p< 0.05) and insulin secretion (p< 0.01) were strongly inhibited in beta cell-specific Mcu-null (βMcu-KO) animals, in vitro, as compared with wild-type (WT) mice. Interestingly, cytosolic Ca²⁺ concentrations increased (p< 0.001), whereas mitochondrial membrane depolarisation improved in βMcu-KO animals. βMcu-KO mice displayed impaired in vivo insulin secretion at 5 min (p< 0.001) but not 15 min post-i.p. injection of glucose, whilst the opposite phenomenon was observed following an oral gavage at 5 min. Unexpectedly, glucose tolerance was improved (p< 0.05) in young βMcu-KO (<12 weeks), but not in older animals vs WT mice.

CONCLUSIONS/INTERPRETATION

MCU is crucial for mitochondrial Ca²⁺ uptake in pancreatic beta cells and is required for normal GSIS. The apparent compensatory mechanisms that maintain glucose tolerance in βMcu-KO mice remain to be established.

Calcium Signaling and Reactive Oxygen Species in Mitochondria

In heart failure, alterations of Na⁺ and Ca²⁺ handling, energetic deficit, and oxidative stress in cardiac myocytes are important pathophysiological hallmarks. Mitochondria are central to these processes because they are the main source for ATP, but also reactive oxygen species (ROS), and their function is critically controlled by Ca²⁺ During physiological variations of workload, mitochondrial Ca²⁺ uptake is required to match energy supply to demand but also to keep the antioxidative capacity in a reduced state to prevent excessive emission of ROS. Mitochondria take up Ca²⁺ via the mitochondrial Ca²⁺ uniporter, which exists in a multiprotein complex whose molecular components were identified only recently. In heart failure, deterioration of cytosolic Ca²⁺ and Na⁺ handling hampers mitochondrial Ca²⁺ uptake and the ensuing Krebs cycle-induced regeneration of the reduced forms of NADH (nicotinamide adenine dinucleotide) and NADPH (nicotinamide adenine dinucleotide phosphate), giving rise to energetic deficit and oxidative stress. ROS emission from mitochondria can trigger further ROS release from neighboring mitochondria termed ROS-induced ROS release, and cross talk between different ROS sources provides a spatially confined cellular network of redox signaling. Although low levels of ROS may serve physiological roles, higher levels interfere with excitation-contraction coupling, induce maladaptive cardiac remodeling through redox-sensitive kinases, and cell death through mitochondrial permeability transition. Targeting the dysregulated interplay between excitation-contraction coupling and mitochondrial energetics may ameliorate the progression of heart failure.

Cannabinoids and Cardiovascular System

Cannabinoids influence cardiovascular variables in health and disease via multiple mechanisms. The chapter covers the impact of cannabinoids on cardiovascular function in physiology and pathology and presents a critical analysis of the proposed signalling pathways governing regulation of cardiovascular function by endogenously produced and exogenous cannabinoids. We know that endocannabinoid system is overactivated under pathological conditions and plays both a protective compensatory role, such as in some forms of hypertension, atherosclerosis and other inflammatory conditions, and a pathophysiological role, such as in disease states associated with excessive hypotension. This chapter focuses on the mechanisms affecting hemodynamics and vasomotor effects of cannabinoids in health and disease states, highlighting mismatches between some studies. The chapter will first review the effects of marijuana smoking on cardiovascular system and then describe the impact of exogenous cannabinoids on cardiovascular parameters in humans and experimental animals. This will be followed by analysis of the impact of cannabinoids on reactivity of isolated vessels. The article critically reviews current knowledge on cannabinoid induction of vascular relaxation by cannabinoid receptor-dependent and -independent mechanisms and dysregulation of vascular endocannabinoid signaling in disease states.

Why don't mice lacking the mitochondrial Ca2+ uniporter experience an energy crisis?

Current dogma holds that the heart balances energy demand and supply effectively and sustainably by sequestering enough Ca²⁺ into mitochondria during heartbeats to stimulate metabolic enzymes in the tricarboxylic acid (TCA) cycle and electron transport chain (ETC). This process is called excitation-contraction-bioenergetics (ECB) coupling. Recent breakthroughs in identifying the mitochondrial Ca²⁺ uniporter (MCU) and its associated proteins have opened up new windows for interrogating the molecular mechanisms of mitochondrial Ca²⁺ homeostasis regulation and its role in ECB coupling. Despite remarkable progress made in the past 7 years, it has been surprising, almost disappointing, that germline MCU deficiency in mice with certain genetic background yields viable pups, and knockout of the MCU in adult heart does not cause lethality. Moreover, MCU deficiency results in few adverse phenotypes, normal performance, and preserved bioenergetics in the heart at baseline. In this review, we briefly assess the existing literature on mitochondrial Ca²⁺ homeostasis regulation and then we consider possible explanations for why MCU-deficient mice are spared from energy crises under physiological conditions. We propose that MCU and/or mitochondrial Ca²⁺ may have limited ability to set ECB coupling, that other mitochondrial Ca²⁺ handling mechanisms may play a role, and that extra-mitochondrial Ca²⁺ may regulate ECB coupling. Since the heart needs to regenerate a significant amount of ATP to assure the perpetuation of heartbeats, multiple mechanisms are likely to work in concert to match energy supply with demand.

Deletion of mitochondrial calcium uniporter incompletely inhibits calcium uptake and induction of the permeability transition pore in brain mitochondria

Ca²⁺ influx into mitochondria is mediated by the mitochondrial calcium uniporter (MCU), whose identity was recently revealed as a 40-kDa protein that along with other proteins forms the mitochondrial Ca²⁺ uptake machinery. The MCU is a Ca²⁺-conducting channel spanning the inner mitochondrial membrane. Here, deletion of the MCU completely inhibited Ca²⁺ uptake in liver, heart, and skeletal muscle mitochondria. However, in brain nonsynaptic and synaptic mitochondria from neuronal somata/glial cells and nerve terminals, respectively, the MCU deletion slowed, but did not completely block, Ca²⁺ uptake. Under resting conditions, brain MCU-KO mitochondria remained polarized, and in brain MCU-KO mitochondria, the electrophoretic Ca²⁺ ionophore ETH129 significantly accelerated Ca²⁺ uptake. The residual Ca²⁺ uptake in brain MCU-KO mitochondria was insensitive to inhibitors of mitochondrial Na⁺/Ca²⁺ exchanger and ryanodine receptor (CGP37157 and dantrolene, respectively), but was blocked by the MCU inhibitor Ru360. Respiration of WT and MCU-KO brain mitochondria was similar except that for mitochondria that oxidized pyruvate and malate, Ca²⁺ more strongly inhibited respiration in WT than in MCU-KO mitochondria. Of note, the MCU deletion significantly attenuated but did not completely prevent induction of the permeability transition pore (PTP) in brain mitochondria. Expression level of cyclophilin D and ATP content in mitochondria, two factors that modulate PTP induction, were unaffected by MCU-KO, whereas ADP was lower in MCU-KO than in WT brain mitochondria. Our results suggest the presence of an MCU-independent Ca²⁺ uptake pathway in brain mitochondria that mediates residual Ca²⁺ influx and induction of PTP in a fraction of the mitochondrial population.

The mitochondrial calcium uniporter in the heart: energetics and beyond

Ca²⁺ and mitochondria are inextricably linked to cardiac function and dysfunction. Ca²⁺ is central to cardiac excitation-contraction coupling and stimulates mitochondrial energy production to fuel contraction. Under pathological conditions of dysregulated Ca²⁺ cycling, mitochondrial Ca²⁺ overload activates cellular death pathways. Thus, in the cardiomyocyte, the mitochondrial Ca²⁺ microdomain is where contraction, energy and death collide. A key component of mitochondrial Ca²⁺ signalling is the mitochondrial Ca²⁺ uniporter complex (uniplex), an inner membrane Ca²⁺ transporter and major pathway of mitochondrial Ca²⁺ entry. Once known only as the unidentified target for ruthenium red and related compounds, in recent years, the uniplex has evolved into a complex multiprotein assembly. The identification of the molecular constituents of the uniplex has made possible the generation of targeted genetic models to interrogate uniplex function in vivo. This review will summarize our current understanding of the molecular structure of the uniplex, its impact on mitochondrial energetics and cardiac physiology, its contribution to cardiomyocyte death, and its expanding roles in cardiac biology.

The Role of Mitochondria in the Activation/Maintenance of SOCE: The Contribution of Mitochondrial Ca2+ Uptake, Mitochondrial Motility, and Location to Store-Operated Ca2+ Entry

In most cell types, the depletion of internal Ca²⁺ stores triggers the activation of Ca²⁺ entry. This crucial phenomenon is known since the 1980s and referred to as store-operated Ca²⁺ entry (SOCE). With the discoveries of the stromal-interacting molecules (STIMs) and the Ca²⁺-permeable Orai channels as the long-awaited molecular constituents of SOCE, the role of mitochondria in controlling the activity of this particular Ca²⁺ entry pathway is kind of buried in oblivion. However, the capability of mitochondria to locally sequester Ca²⁺ at sites of Ca²⁺ release and entry was initially supposed to rule SOCE by facilitating the Ca²⁺ depletion of the endoplasmic reticulum and removing entering Ca²⁺ from the Ca²⁺-inhibitable channels, respectively. Moreover, the central role of these organelles in controlling the cellular energy metabolism has been linked to the activity of SOCE. Nevertheless, the exact molecular mechanisms by which mitochondria actually determine SOCE are still pretty obscure. In this essay we describe the complexity of the mitochondrial Ca²⁺ uptake machinery and its regulation, molecular components, and properties, which open new ways for scrutinizing the contribution of mitochondria to SOCE. Moreover, data concerning the variability of the morphology and cellular distribution of mitochondria as putative determinants of SOCE activation, maintenance, and termination are summarized.

Fine tuning of cytosolic Ca (2+) oscillations

Ca ²⁺ oscillations, a widespread mode of cell signaling, were reported in non-excitable cells for the first time more than 25 years ago. Their fundamental mechanism, based on the periodic Ca ²⁺ exchange between the endoplasmic reticulum and the cytoplasm, has been well characterized. However, how the kinetics of cytosolic Ca ²⁺ changes are related to the extent of a physiological response remains poorly understood. Here, we review data suggesting that the downstream targets of Ca ²⁺ are controlled not only by the frequency of Ca ²⁺ oscillations but also by the detailed characteristics of the oscillations, such as their duration, shape, or baseline level. Involvement of non-endoplasmic reticulum Ca ²⁺ stores, mainly mitochondria and the extracellular medium, participates in this fine tuning of Ca ²⁺ oscillations. The main characteristics of the Ca ²⁺ exchange fluxes with these compartments are also reviewed.

UCP3 Regulates Single-Channel Activity of the Cardiac mCa1

Mitochondrial Ca(2+) uptake (mCa(2+) uptake) is thought to be mediated by the mitochondrial Ca(2+) uniporter (MCU). UCP2 and UCP3 belong to a superfamily of mitochondrial ion transporters. Both proteins are expressed in the inner mitochondrial membrane of the heart. Recently, UCP2 was reported to modulate the function of the cardiac MCU related channel mCa1. However, the possible role of UCP3 in modulating cardiac mCa(2+) uptake via the MCU remains inconclusive. To understand the role of UCP3, we analyzed cardiac mCa1 single-channel activity in mitoplast-attached single-channel recordings from isolated murine cardiac mitoplasts, from adult wild-type controls (WT), and from UCP3 knockout mice (UCP3(-/-)). Single-channel registrations in UCP3(-/-) confirmed a murine voltage-gated Ca(2+) channel, i.e., mCa1, which was inhibited by Ru360. Compared to WT, mCa1 in UCP3(-/-) revealed similar single-channel characteristics. However, in UCP3(-/-) the channel exhibited decreased single-channel activity, which was insensitive to adenosine triphosphate (ATP) inhibition. Our results suggest that beyond UCP2, UCP3 also exhibits regulatory effects on cardiac mCa1/MCU function. Furthermore, we speculate that UCP3 might modulate previously described inhibitory effects of ATP on mCa1/MCU activity as well.

The regulation of neuronal mitochondrial metabolism by calcium

Calcium signalling is fundamental to the function of the nervous system, in association with changes in ionic gradients across the membrane. Although restoring ionic gradients is energetically costly, a rise in intracellular Ca(2+) acts through multiple pathways to increase ATP synthesis, matching energy supply to demand. Increasing cytosolic Ca(2+) stimulates metabolite transfer across the inner mitochondrial membrane through activation of Ca(2+) -regulated mitochondrial carriers, whereas an increase in matrix Ca(2+) stimulates the citric acid cycle and ATP synthase. The aspartate-glutamate exchanger Aralar/AGC1 (Slc25a12), a component of the malate-aspartate shuttle (MAS), is stimulated by modest increases in cytosolic Ca(2+) and upregulates respiration in cortical neurons by enhancing pyruvate supply into mitochondria. Failure to increase respiration in response to small (carbachol) and moderate (K(+) -depolarization) workloads and blunted stimulation of respiration in response to high workloads (veratridine) in Aralar/AGC1 knockout neurons reflect impaired MAS activity and limited mitochondrial pyruvate supply. In response to large workloads (veratridine), acute stimulation of respiration occurs in the absence of MAS through Ca(2+) influx through the mitochondrial calcium uniporter (MCU) and a rise in matrix [Ca(2+) ]. Although the physiological importance of the MCU complex in work-induced stimulation of respiration of CNS neurons is not yet clarified, abnormal mitochondrial Ca(2+) signalling causes pathology. Indeed, loss of function mutations in MICU1, a regulator of MCU complex, are associated with neuromuscular disease. In patient-derived MICU1 deficient fibroblasts, resting matrix Ca(2+) is increased and mitochondria fragmented. Thus, the fine tuning of Ca(2+) signals plays a key role in shaping mitochondrial bioenergetics.

Enjoy the Trip: Calcium in Mitochondria Back and Forth

In the last 5 years, most of the molecules that control mitochondrial Ca(2+) homeostasis have been finally identified. Mitochondrial Ca(2+) uptake is mediated by the Mitochondrial Calcium Uniporter (MCU) complex, a macromolecular structure that guarantees Ca(2+) accumulation inside mitochondrial matrix upon increases in cytosolic Ca(2+). Conversely, Ca(2+) release is under the control of the Na(+)/Ca(2+) exchanger, encoded by the NCLX gene, and of a H(+)/Ca(2+) antiporter, whose identity is still debated. The low affinity of the MCU complex, coupled to the activity of the efflux systems, protects cells from continuous futile cycles of Ca(2+) across the inner mitochondrial membrane and consequent massive energy dissipation. In this review, we discuss the basic principles that govern mitochondrial Ca(2+) homeostasis and the methods used to investigate the dynamics of Ca(2+) concentration within the organelles. We discuss the functional and structural role of the different molecules involved in mitochondrial Ca(2+) handling and their pathophysiological role.

By Regulating Mitochondrial Ca2+-Uptake UCP2 Modulates Intracellular Ca2+

Introduction

The possible role of UCP2 in modulating mitochondrial Ca²⁺-uptake (mCa²⁺-uptake) via the mitochondrial calcium uniporter (MCU) is highly controversial.

Methods

Thus, we analyzed mCa²⁺-uptake in isolated cardiac mitochondria, MCU single-channel activity in cardiac mitoplasts, dual Ca²⁺-transients from mitochondrial ((Ca²⁺)m) and intracellular compartment ((Ca²⁺)c) in the whole-cell configuration in cardiomyocytes of wild-type (WT) and UCP2^-/- mice.

Results

Isolated mitochondria showed a Ru360 sensitive mCa²⁺-uptake, which was significantly decreased in UCP2^-/- (229.4±30.8 FU vs. 146.3±23.4 FU, P<0.05). Single-channel registrations confirmed a Ru360 sensitive voltage-gated Ca²⁺-channel in mitoplasts, i.e. mCa1, showing a reduced single-channel activity in UCP2^-/- (Po,total: 0.34±0.05% vs. 0.07±0.01%, P<0.05). In UCP2^-/- cardiomyocytes (Ca²⁺)m was decreased (0.050±0.009 FU vs. 0.021±0.005 FU, P<0.05) while (Ca²⁺)c was unchanged (0.032±0.002 FU vs. 0.028±0.004 FU, P>0.05) and transsarcolemmal Ca²⁺-influx was inhibited suggesting a possible compensatory mechanism. Additionally, we observed an inhibitory effect of ATP on mCa²⁺-uptake in WT mitoplasts and (Ca²⁺)m of cardiomyocytes leading to an increase of (Ca²⁺)c while no ATP dependent effect was observed in UCP2^-/-.

Conclusion

Our results indicate regulatory effects of UCP2 on mCa²⁺-uptake. Furthermore, we propose, that previously described inhibitory effects on MCU by ATP may be mediated via UCP2 resulting in changes of excitation contraction coupling.

Interplay Between Intracellular Ca(2+) Oscillations and Ca(2+)-stimulated Mitochondrial Metabolism

Oscillations of cytosolic Ca(2+) concentration are a widespread mode of signalling. Oscillatory spikes rely on repetitive exchanges of Ca(2+) between the endoplasmic reticulum (ER) and the cytosol, due to the regulation of inositol 1,4,5-trisphosphate receptors. Mitochondria also sequester and release Ca(2+), thus affecting Ca(2+) signalling. Mitochondrial Ca(2+) activates key enzymes involved in ATP synthesis. We propose a new integrative model for Ca(2+) signalling and mitochondrial metabolism in electrically non-excitable cells. The model accounts for (1) the phase relationship of the Ca(2+) changes in the cytosol, the ER and mitochondria, (2) the dynamics of mitochondrial metabolites in response to cytosolic Ca(2+) changes, and (3) the impacts of cytosol/mitochondria Ca(2+) exchanges and of mitochondrial metabolism on Ca(2+) oscillations. Simulations predict that as expected, oscillations are slowed down by decreasing the rate of Ca(2+) efflux from mitochondria, but also by decreasing the rate of Ca(2+) influx through the mitochondrial Ca(2+) uniporter (MCU). These predictions were experimentally validated by inhibiting MCU expression. Despite the highly non-linear character of Ca(2+) dynamics and mitochondrial metabolism, bioenergetics were found to be robust with respect to changes in frequency and amplitude of Ca(2+) oscillations.

UCP2 modulates single-channel properties of a MCU-dependent Ca(2+) inward current in mitochondria

The mitochondrial Ca(2+) uniporter is a highly Ca(2+)-selective protein complex that consists of the pore-forming mitochondrial Ca(2+) uniporter protein (MCU), the scaffolding essential MCU regulator (EMRE), and mitochondrial calcium uptake 1 and 2 (MICU1/2), which negatively regulate mitochondrial Ca(2+) uptake. We have previously reported that uncoupling proteins 2 and 3 (UCP2/3) are also engaged in the activity of mitochondrial Ca(2+) uptake under certain conditions, while the mechanism by which UCP2/3 facilitates mitochondrial Ca(2+) uniport remains elusive. This work was designed to investigate the impact of UCP2 on the three distinct mitochondrial Ca(2+) currents found in mitoplasts isolated from HeLa cells, the intermediate- (i-), burst- (b-) and extra-large (xl-) mitochondrial/mitoplast Ca(2+) currents (MCC). Using the patch clamp technique on mitoplasts from cells with reduced MCU and EMRE unveiled a very high affinity of MCU for xl-MCC that succeeds that for i-MCC, indicating the coexistence of at least two MCU/EMRE-dependent Ca(2+) currents. The manipulation of the expression level of UCP2 by either siRNA-mediated knockdown or overexpression changed exclusively the open probability (NPo) of xl-MCC by approx. 38% decrease or nearly a 3-fold increase, respectively. These findings confirm a regulatory role of UCP2 in mitochondrial Ca(2+) uptake and identify UCP2 as a selective modulator of just one distinct MCU/EMRE-dependent mitochondrial Ca(2+) inward current.

The Mitochondrial Calcium Uniporter Selectively Matches Metabolic Output to Acute Contractile Stress in the Heart

In the heart, augmented Ca(2+) fluxing drives contractility and ATP generation through mitochondrial Ca(2+) loading. Pathologic mitochondrial Ca(2+) overload with ischemic injury triggers mitochondrial permeability transition pore (MPTP) opening and cardiomyocyte death. Mitochondrial Ca(2+) uptake is primarily mediated by the mitochondrial Ca(2+) uniporter (MCU). Here, we generated mice with adult and cardiomyocyte-specific deletion of Mcu, which produced mitochondria refractory to acute Ca(2+) uptake, with impaired ATP production, and inhibited MPTP opening upon acute Ca(2+) challenge. Mice lacking Mcu in the adult heart were also protected from acute ischemia-reperfusion injury. However, resting/basal mitochondrial Ca(2+) levels were normal in hearts of Mcu-deleted mice, and mitochondria lacking MCU eventually loaded with Ca(2+) after stress stimulation. Indeed, Mcu-deleted mice were unable to immediately sprint on a treadmill unless warmed up for 30 min. Hence, MCU is a dedicated regulator of short-term mitochondrial Ca(2+) loading underlying a "fight-or-flight" response that acutely matches cardiac workload with ATP production.

Structure and function of the mitochondrial calcium uniporter complex

The mitochondrial calcium uniporter (MCU) is the critical protein of the inner mitochondrial membrane mediating the electrophoretic Ca²⁺ uptake into the matrix. It plays a fundamental role in the shaping of global calcium signaling and in the control of aerobic metabolism as well as apoptosis. Two features of mitochondrial calcium signaling have been known for a long time: i) mitochondrial Ca²⁺ uptake widely varies among cells and tissues, and ii) channel opening strongly relies on the extramitochondrial Ca²⁺ concentration, with low activity at resting [Ca²⁺] and high capacity as soon as calcium signaling is activated. Such complexity requires a specialized molecular machinery, with several primary components can be variably gathered together in order to match energy demands and protect from toxic stimuli. In line with this, MCU is now recognized to be part of a macromolecular complex known as the MCU complex. Our understanding of the structure and function of the MCU complex is now growing promptly, revealing an unexpected complexity that highlights the pleiotropic role of mitochondrial Ca²⁺ signals. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

Molecular mechanism of mitochondrial calcium uptake

Mitochondrial calcium uptake plays a critical role in various cellular functions. After half a century of extensive studies, the molecular components and important regulators of the mitochondrial calcium uptake complex have been identified. However, the mechanism by which these protein molecules interact with one another and coordinate to regulate calcium passage through mitochondrial membranes remains elusive. Here, we summarize recent progress in the structural and functional characterization of these important protein molecules, which are involved in mitochondrial calcium uptake. In particular, we focus on the current understanding of the molecular mechanism underlying calcium through two mitochondrial membranes. Additionally, we provide a new perspective for future directions in investigation and molecular intervention.

Assessment of mitochondrial Ca²⁺ uptake

Mitochondrial Ca(2+) uptake regulates mitochondrial function and contributes to cell signaling. Accordingly, quantifying mitochondrial Ca(2+) signals and elaborating the mechanisms that accomplish mitochondrial Ca(2+) uptake are essential to gain our understanding of cell biology. Here, we describe the benefits and drawbacks of various established old and new techniques to assess dynamic changes of mitochondrial Ca(2+) concentration ([Ca(2+)]mito) in a wide range of applications.

The mitochondrial calcium uniporter: mice can live and die without it

Calcium is of critical importance to mitochondrial and cell function, and calcium signaling is highly localized in the cell. When stimulated, mitochondria are capable of rapidly taking up calcium, affecting both matrix energetics within mitochondria and shaping the amplitude and frequency of cytosolic calcium "waves". During pathological conditions a large increase in mitochondrial calcium levels is thought to activate the mitochondrial permeability transition pore, resulting in cell death. The protein responsible for mitochondrial calcium uptake, the mitochondrial calcium uniporter (MCU), was identified in 2011 and its molecular elucidation has stimulated and invigorated research in this area. MCU knockout mice have been created, a variety of other regulators have been identified, and a disease phenotype in humans has been attributed to the loss of a uniporter regulator. In the three years since its molecular elucidation, further research into the MCU has revealed a complex uniporter, and raised many questions about its physiologic and pathologic cell roles. This article is part of a Special Issue entitled "Mitochondria: From Basic Mitochondrial Biology to Cardiovascular Disease".

Role of β-hydroxybutyrate, its polymer poly-β-hydroxybutyrate and inorganic polyphosphate in mammalian health and disease

We provide a comprehensive review of the role of β-hydroxybutyrate (β-OHB), its linear polymer poly-β-hydroxybutyrate (PHB), and inorganic polyphosphate (polyP) in mammalian health and disease. β-OHB is a metabolic intermediate that constitutes 70% of ketone bodies produced during ketosis. Although ketosis has been generally considered as an unfavorable pathological state (e.g., diabetic ketoacidosis in type-1 diabetes mellitus), it has been suggested that induction of mild hyperketonemia may have certain therapeutic benefits. β-OHB is synthesized in the liver from acetyl-CoA by β-OHB dehydrogenase and can be used as alternative energy source. Elevated levels of PHB are associated with pathological states. In humans, short-chain, complexed PHB (cPHB) is found in a wide variety of tissues and in atherosclerotic plaques. Plasma cPHB concentrations correlate strongly with atherogenic lipid profiles, and PHB tissue levels are elevated in type-1 diabetic animals. However, little is known about mechanisms of PHB action especially in the heart. In contrast to β-OHB, PHB is a water-insoluble, amphiphilic polymer that has high intrinsic viscosity and salt-solvating properties. cPHB can form non-specific ion channels in planar lipid bilayers and liposomes. PHB can form complexes with polyP and Ca(2+) which increases membrane permeability. The biological roles played by polyP, a ubiquitous phosphate polymer with ATP-like bonds, have been most extensively studied in prokaryotes, however polyP has recently been linked to a variety of functions in mammalian cells, including blood coagulation, regulation of enzyme activity in cancer cells, cell proliferation, apoptosis and mitochondrial ion transport and energy metabolism. Recent evidence suggests that polyP is a potent activator of the mitochondrial permeability transition pore in cardiomyocytes and may represent a hitherto unrecognized key structural and functional component of the mitochondrial membrane system.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

Structural and mechanistic insights into MICU1 regulation of mitochondrial calcium uptake

Mitochondrial calcium uptake is a critical event in various cellular activities. Two recently identified proteins, the mitochondrial Ca(2+) uniporter (MCU), which is the pore-forming subunit of a Ca(2+) channel, and mitochondrial calcium uptake 1 (MICU1), which is the regulator of MCU, are essential in this event. However, the molecular mechanism by which MICU1 regulates MCU remains elusive. In this study, we report the crystal structures of Ca(2+)-free and Ca(2+)-bound human MICU1. Our studies reveal that Ca(2+)-free MICU1 forms a hexamer that binds and inhibits MCU. Upon Ca(2+) binding, MICU1 undergoes large conformational changes, resulting in the formation of multiple oligomers to activate MCU. Furthermore, we demonstrate that the affinity of MICU1 for Ca(2+) is approximately 15-20 μM. Collectively, our results provide valuable details to decipher the molecular mechanism of MICU1 regulation of mitochondrial calcium uptake.

Metabolism-secretion coupling and mitochondrial calcium activities in clonal pancreatic β-cells

Pancreatic β-cells are the only cells capable of lowering blood glucose by secreting insulin. The β-cell continuously adjusts its secretory activity to substrate availability in order to keep blood glucose levels within the physiological range--a process called metabolism-secretion coupling. Glucose is readily taken up by the β-cell and broken down into intermediates that fuel oxidative metabolism inside the mitochondria to generate ATP. The resulting increase in the ATP/ADP ratio causes closure of plasma membrane KATP channels, thereby depolarizing the cell and triggering the opening of voltage-gated Ca²⁺ channels. Consequential oscillations of cytosolic Ca²⁺ not only mediate the exocytosis of insulin granules but are also relayed to other subcellular compartments including the mitochondria, where Ca²⁺ is required to accelerate mitochondrial metabolism in response to nutrient stimulation. The mitochondrial Ca²⁺ uptake machinery plays a fundamental role in this feed-forward mechanism that guarantees sustained insulin secretion and, thus, represents a promising therapeutic target for type 2 diabetes.

Mitochondrial Ca(2+) uniporter (MCU)-dependent and MCU-independent Ca(2+) channels coexist in the inner mitochondrial membrane

A protein referred to as CCDC109A and then renamed to mitochondrial calcium uniporter (MCU) has recently been shown to accomplish mitochondrial Ca²⁺ uptake in different cell types. In this study, we investigated whole-mitoplast inward cation currents and single Ca²⁺ channel activities in mitoplasts prepared from stable MCU knockdown HeLa cells using the patch-clamp technique. In whole-mitoplast configuration, diminution of MCU considerably reduced inward Ca²⁺ and Na⁺ currents. This was accompanied by a decrease in occurrence of single channel activity of the intermediate conductance mitochondrial Ca²⁺ current (i-MCC). However, ablation of MCU yielded a compensatory 2.3-fold elevation in the occurrence of the extra large conductance mitochondrial Ca²⁺ current (xl-MCC), while the occurrence of bursting currents (b-MCC) remained unaltered. These data reveal i-MCC as MCU-dependent current while xl-MCC and b-MCC seem to be rather MCU-independent, thus, pointing to the engagement of at least two molecularly distinct mitochondrial Ca²⁺ channels.

Calculation of ion currents across the inner membrane of functionally intact mitochondria

Mitochondrial ion transport systems play a central role in cell physiology. Rates of Ca (2+) and K(+) transport across the inner mitochondrial membrane have been derived from the measurement of ion accumulation over time within functional isolated mitochondria or mitochondria of cultured cells. Alternatively, the electrical currents generated by ionic flux have been directly measured in purified and swollen mitochondrial samples (mitoplasts) or reconstituted channels, and typically range from 1 pA to several 100s pA. However, the direct electrophysiological approach necessarily requires extensive processing of the mitochondria prior to measurement, which can only be performed on isolated mitoplasts. To compare rates of mitochondrial ion transport measured in electrophysiological experiments to those measured in intact mitochondria and cells, we converted published rates of mitochondrial ion uptake into units of ionic current. We estimate that for monovalent ions, uptake by intact mitochondria at the rate of 1 nmol ∙ mg(-1) protein ∙ min(-1) is equivalent to 0.2 fA of current per whole single mitochondrion (0.4 fA for divalent ions). In intact mitochondria, estimated rates of electrogenic cation uptake are limited to 1-100 fA of integral current per single mitochondrion. These estimates are orders of magnitude lower than the currents through mitochondrial channels directly measured via patch-clamp or artificial lipid bilayer approaches.

Role of polyhydroxybutyrate in mitochondrial calcium uptake

Polyhydroxybutyrate (PHB) is a biological polymer which belongs to the class of polyesters and is ubiquitously present in all living organisms. Mammalian mitochondrial membranes contain PHB consisting of up to 120 hydroxybutyrate residues. Roles played by PHB in mammalian mitochondria remain obscure. It was previously demonstrated that PHB of the size similar to one found in mitochondria mediates calcium transport in lipid bilayer membranes. We hypothesized that the presence of PHB in mitochondrial membrane might play a significant role in mitochondrial calcium transport. To test this, we investigated how the induction of PHB hydrolysis affects mitochondrial calcium transport. Mitochondrial PHB was altered enzymatically by targeted expression of bacterial PHB hydrolyzing enzyme (PhaZ7) in mitochondria of mammalian cultured cells. The expression of PhaZ7 induced changes in mitochondrial metabolism resulting in decreased mitochondrial membrane potential in HepG2 but not in U87 and HeLa cells. Furthermore, it significantly inhibited mitochondrial calcium uptake in intact HepG2, U87 and HeLa cells stimulated by the ATP or by the application of increased concentrations of calcium to the digitonin permeabilized cells. Calcium uptake in PhaZ7 expressing cells was restored by mimicking calcium uniporter properties with natural electrogenic calcium ionophore - ferutinin. We propose that PHB is a previously unrecognized important component of the mitochondrial calcium uptake system.

Molecularly distinct routes of mitochondrial Ca2+ uptake are activated depending on the activity of the sarco/endoplasmic reticulum Ca2+ ATPase (SERCA)

The transfer of Ca²⁺ across the inner mitochondrial membrane is an important physiological process linked to the regulation of metabolism, signal transduction, and cell death. While the definite molecular composition of mitochondrial Ca²⁺ uptake sites remains unknown, several proteins of the inner mitochondrial membrane, that are likely to accomplish mitochondrial Ca²⁺ fluxes, have been described: the novel uncoupling proteins 2 and 3, the leucine zipper-EF-hand containing transmembrane protein 1 and the mitochondrial calcium uniporter. It is unclear whether these proteins contribute to one unique mitochondrial Ca²⁺ uptake pathway or establish distinct routes for mitochondrial Ca²⁺ sequestration. In this study, we show that a modulation of Ca²⁺ release from the endoplasmic reticulum by inhibition of the sarco/endoplasmatic reticulum ATPase modifies cytosolic Ca²⁺ signals and consequently switches mitochondrial Ca²⁺ uptake from an uncoupling protein 3- and mitochondrial calcium uniporter-dependent, but leucine zipper-EF-hand containing transmembrane protein 1-independent to a leucine zipper-EF-hand containing transmembrane protein 1- and mitochondrial calcium uniporter-mediated, but uncoupling protein 3-independent pathway. Thus, the activity of sarco/endoplasmatic reticulum ATPase is significant for the mode of mitochondrial Ca²⁺ sequestration and determines which mitochondrial proteins might actually accomplish the transfer of Ca²⁺ across the inner mitochondrial membrane. Moreover, our findings herein support the existence of distinct mitochondrial Ca²⁺ uptake routes that might be essential to ensure an efficient ion transfer into mitochondria despite heterogeneous cytosolic Ca²⁺ rises.