MICU1 controls both the threshold and cooperative activation of the mitochondrial Ca²⁺ uniporter

Ca2+ Fluxes and Cancer

Ca²⁺ ions are key second messengers in both excitable and non-excitable cells. Owing to the rather pleiotropic nature of Ca²⁺ transporters and other Ca²⁺-binding proteins, however, Ca²⁺ signaling has attracted limited attention as a potential target of anticancer therapy. Here, we discuss cancer-associated alterations of Ca²⁺ fluxes at specific organelles as we identify novel candidates for the development of drugs that selectively target Ca²⁺ signaling in malignant cells.

Coupled transmembrane mechanisms control MCU-mediated mitochondrial Ca2+ uptake

Ca2+ uptake by mitochondria regulates bioenergetics, apoptosis, and Ca2+ signaling. The primary pathway for mitochondrial Ca2+ uptake is the mitochondrial calcium uniporter (MCU), a Ca2+-selective ion channel in the inner mitochondrial membrane. MCU-mediated Ca2+ uptake is driven by the sizable inner-membrane potential generated by the electron-transport chain. Despite the large thermodynamic driving force, mitochondrial Ca2+ uptake is tightly regulated to maintain low matrix [Ca2+] and prevent opening of the permeability transition pore and cell death, while meeting dynamic cellular energy demands. How this is accomplished is controversial. Here we define a regulatory mechanism of MCU-channel activity in which cytoplasmic Ca2+ regulation of intermembrane space-localized MICU1/2 is controlled by Ca2+-regulatory mechanisms localized across the membrane in the mitochondrial matrix. Ca2+ that permeates through the channel pore regulates Ca2+ affinities of coupled inhibitory and activating sensors in the matrix. Ca2+ binding to the inhibitory sensor within the MCU amino terminus closes the channel despite Ca2+ binding to MICU1/2. Conversely, disruption of the interaction of MICU1/2 with the MCU complex disables matrix Ca2+ regulation of channel activity. Our results demonstrate how Ca2+ influx into mitochondria is tuned by coupled Ca2+-regulatory mechanisms on both sides of the inner mitochondrial membrane.

The structure of the MICU1-MICU2 complex unveils the regulation of the mitochondrial calcium uniporter

The MICU1-MICU2 heterodimer regulates the mitochondrial calcium uniporter (MCU) and mitochondrial calcium uptake. Herein, we present two crystal structures of the MICU1-MICU2 heterodimer, in which Ca²⁺ -free and Ca²⁺ -bound EF-hands are observed in both proteins, revealing both electrostatic and hydrophobic interfaces. Furthermore, we show that MICU1 interacts with EMRE, another regulator of MCU, through a Ca²⁺ -dependent alkaline groove. Ca²⁺ binding strengthens the MICU1-EMRE interaction, which in turn facilitates Ca²⁺ uptake. Conversely, the MICU1-MCU interaction is favored in the absence of Ca²⁺ , thus inhibiting the channel activity. This Ca²⁺ -dependent switch illuminates how calcium signals are transmitted from regulatory subunits to the calcium channel and the transition between gatekeeping and activation channel functions. Furthermore, competition with an EMRE peptide alters the uniporter threshold in resting conditions and elevates Ca²⁺ accumulation in stimulated mitochondria, confirming the gatekeeper role of the MICU1-MICU2 heterodimer. Taken together, these structural and functional data provide new insights into the regulation of mitochondrial calcium uptake.

Discovery of EMRE in fungi resolves the true evolutionary history of the mitochondrial calcium uniporter

Calcium (Ca²⁺) influx into mitochondria occurs through a Ca²⁺-selective uniporter channel, which regulates essential cellular processes in eukaryotic organisms. Previous evolutionary analyses of its pore-forming subunits MCU and EMRE, and gatekeeper MICU1, pinpointed an evolutionary paradox: the presence of MCU homologs in fungal species devoid of any other uniporter components and of mt-Ca²⁺ uptake. Here, we trace the mt-Ca²⁺ uniporter evolution across 1,156 fully-sequenced eukaryotes and show that animal and fungal MCUs represent two distinct paralogous subfamilies originating from an ancestral duplication. Accordingly, we find EMRE orthologs outside Holoza and uncover the existence of an animal-like uniporter within chytrid fungi, which enables mt-Ca²⁺ uptake when reconstituted in vivo in the yeast Saccharomyces cerevisiae. Our study represents the most comprehensive phylogenomic analysis of the mt-Ca²⁺ uptake system and demonstrates that MCU, EMRE, and MICU formed the core of the ancestral opisthokont uniporter, with major implications for comparative structural and functional studies.

The mitochondrial calcium uptake system, crucial for cellular processes, evolved in ancient eukaryotes. Here, authors perform a phylogenomic analysis across 1,156 eukaryotes, and show that previously identified animal and fungal genes in this system originated from an ancestral duplication.

Altered Organelle Calcium Transport in Ovarian Physiology and Cancer

Calcium levels have a huge impact on the physiology of the female reproductive system, in particular, of the ovaries. Cytosolic calcium levels are influenced by regulatory proteins (i.e., ion channels and pumps) localized in the plasmalemma and/or in the endomembranes of membrane-bound organelles. Imbalances between plasma membrane and organelle-based mechanisms for calcium regulation in different ovarian cell subtypes are contributing to ovarian pathologies, including ovarian cancer. In this review, we focused our attention on altered calcium transport and its role as a contributor to tumor progression in ovarian cancer. The most important proteins described as contributing to ovarian cancer progression are inositol trisphosphate receptors, ryanodine receptors, transient receptor potential channels, calcium ATPases, hormone receptors, G-protein-coupled receptors, and/or mitochondrial calcium uniporters. The involvement of mitochondrial and/or endoplasmic reticulum calcium imbalance in the development of resistance to chemotherapeutic drugs in ovarian cancer is also discussed, since Ca2+ channels and/or pumps are nowadays regarded as potential therapeutic targets and are even correlated with prognosis.

Structural insights into the Ca2+-dependent gating of the human mitochondrial calcium uniporter

Mitochondrial Ca2+ uptake is mediated by an inner mitochondrial membrane protein called the mitochondrial calcium uniporter. In humans, the uniporter functions as a holocomplex consisting of MCU, EMRE, MICU1 and MICU2, among which MCU and EMRE form a subcomplex and function as the conductive channel while MICU1 and MICU2 are EF-hand proteins that regulate the channel activity in a Ca2+-dependent manner. Here, we present the EM structures of the human mitochondrial calcium uniporter holocomplex (uniplex) in the presence and absence of Ca2+, revealing distinct Ca2+ dependent assembly of the uniplex. Our structural observations suggest that Ca2+ changes the dimerization interaction between MICU1 and MICU2, which in turn determines how the MICU1-MICU2 subcomplex interacts with the MCU-EMRE channel and, consequently, changes the distribution of the uniplex assemblies between the blocked and unblocked states.

Structures reveal gatekeeping of the mitochondrial Ca2+ uniporter by MICU1-MICU2

The mitochondrial calcium uniporter is a Ca2+-gated ion channel complex that controls mitochondrial Ca2+ entry and regulates cell metabolism. MCU and EMRE form the channel while Ca2+-dependent regulation is conferred by MICU1 and MICU2 through an enigmatic process. We present a cryo-EM structure of an MCU-EMRE-MICU1-MICU2 holocomplex comprising MCU and EMRE subunits from the beetle Tribolium castaneum in complex with a human MICU1-MICU2 heterodimer at 3.3 Å resolution. With analogy to how neuronal channels are blocked by protein toxins, a uniporter interaction domain on MICU1 binds to a channel receptor site comprising MCU and EMRE subunits to inhibit ion flow under resting Ca2+ conditions. A Ca2+-bound structure of MICU1-MICU2 at 3.1 Å resolution indicates how Ca2+-dependent changes enable dynamic response to cytosolic Ca2+ signals.

Exploring the In Vivo Role of the Mitochondrial Calcium Uniporter in Brown Fat Bioenergetics

The mitochondrial calcium uniporter has been proposed to coordinate the organelle’s energetics with calcium signaling. Uniporter current has previously been reported to be extremely high in brown adipose tissue (BAT), yet it remains unknown how the uniporter contributes to BAT physiology. Here, we report the generation and characterization of a mouse model lacking Mcu, the pore forming subunit of the uniporter, specifically in BAT (BAT-Mcu-KO). BAT-Mcu-KO mice lack uniporter-based calcium uptake in BAT mitochondria but exhibit unaffected cold tolerance, diet-induced obesity, and transcriptional response to cold in BAT. Unexpectedly, we found in wild-type animals that cold powerfully activates the ATF4-dependent integrated stress response (ISR) in BAT and up-regulates circulating FGF21 and GDF15, raising the hypothesis that the ISR partly underlies the pleiotropic effects of BAT on systemic metabolism. Our study demonstrates that the uniporter is largely dispensable for BAT thermogenesis and demonstrates activation of the ISR in BAT in response to cold.

Flicker et al. generate a mouse lacking mitochondrial calcium uniporter activity in brown fat. They show that the uniporter is dispensable for brown fat bioenergetics. Unexpectedly, they find that in wild type animals, cold stress induces ATF4 signaling in normal brown fat, suggesting a mechanism for cold-induced GDF15 and FGF21 elevation.

Apoptosis as the main type of cell death induced by calcium electroporation in rhabdomyosarcoma cells

Calcium electroporation (CaEP) has been previously reported as an effective method of rhabdomyosarcoma cells reduction. CaEP causes temporary cell membrane permeabilization with simultaneous calcium ions influx. A rapid influx of calcium ions leads to mitochondrial overload by Ca²⁺, loss of mitochondrial membrane potential causing cytochrome c release, caspase cascade activation and, as a consequence, cell death. This study was conducted on two cell lines: normal muscle cells (C2C12) and rhabdomyosarcoma cells (RD), which showed different cellular responses to CaEP. Our study defined apoptosis as the main cell death type occurring after CaEP in RD cells. Increased activity of caspase 3/7, Parp-1 and cleaved Parp-1 were proven in the case of RD cells. RD cells compartment rearrangement was observed in the time-lapse by holotomographic microscopy (HTM). C2C12 cells were less sensitive to electroporation and increased Ca²⁺ concentration, and viability was maintained at the level of control cells, only slight changes in pro-apoptotic factors were observed. The results reveal CaEP as a promising therapeutic approach in cancers which develop from muscle tissue.

Mitochondrial ion channels as targets for cardioprotection

Acute myocardial infarction (AMI) and the heart failure (HF) that often result remain the leading causes of death and disability worldwide. As such, new therapeutic targets need to be discovered to protect the myocardium against acute ischaemia/reperfusion (I/R) injury in order to reduce myocardial infarct (MI) size, preserve left ventricular function and prevent the onset of HF. Mitochondrial dysfunction during acute I/R injury is a critical determinant of cell death following AMI, and therefore, ion channels in the inner mitochondrial membrane, which are known to influence cell death and survival, provide potential therapeutic targets for cardioprotection. In this article, we review the role of mitochondrial ion channels, which are known to modulate susceptibility to acute myocardial I/R injury, and we explore their potential roles as therapeutic targets for reducing MI size and preventing HF following AMI.

Skd3 (human ClpB) is a potent mitochondrial protein disaggregase that is inactivated by 3-methylglutaconic aciduria-linked mutations

Cells have evolved specialized protein disaggregases to reverse toxic protein aggregation and restore protein functionality. In nonmetazoan eukaryotes, the AAA+ disaggregase Hsp78 resolubilizes and reactivates proteins in mitochondria. Curiously, metazoa lack Hsp78. Hence, whether metazoan mitochondria reactivate aggregated proteins is unknown. Here, we establish that a mitochondrial AAA+ protein, Skd3 (human ClpB), couples ATP hydrolysis to protein disaggregation and reactivation. The Skd3 ankyrin-repeat domain combines with conserved AAA+ elements to enable stand-alone disaggregase activity. A mitochondrial inner-membrane protease, PARL, removes an autoinhibitory peptide from Skd3 to greatly enhance disaggregase activity. Indeed, PARL-activated Skd3 solubilizes α-synuclein fibrils connected to Parkinson's disease. Human cells lacking Skd3 exhibit reduced solubility of various mitochondrial proteins, including anti-apoptotic Hax1. Importantly, Skd3 variants linked to 3-methylglutaconic aciduria, a severe mitochondrial disorder, display diminished disaggregase activity (but not always reduced ATPase activity), which predicts disease severity. Thus, Skd3 is a potent protein disaggregase critical for human health.

The Function of Mitochondrial Calcium Uniporter at the Whole-Cell and Single Mitochondrion Levels in WT, MICU1 KO, and MICU2 KO Cells

Mitochondrial Ca2+ ([Ca2+]M) uptake through its Ca2+ uniporter (MCU) is central to many cell functions such as bioenergetics, spatiotemporal organization of Ca2+ signals, and apoptosis. MCU activity is regulated by several intrinsic proteins including MICU1, MICU2, and EMRE. While significant details about the role of MICU1, MICU2, and EMRE in MCU function have emerged recently, a key challenge for the future experiments is to investigate how these regulatory proteins modulate mitochondrial Ca2+ influx through MCU in intact cells under pathophysiological conditions. This is further complicated by the fact that several variables affecting MCU function change dynamically as cell functions. To overcome this void, we develop a data-driven model that closely replicates the behavior of MCU under a wide range of cytosolic Ca2+ ([Ca2+]C), [Ca2+]M, and mitochondrial membrane potential values in WT, MICU1 knockout (KO), and MICU2 KO cells at the single mitochondrion and whole-cell levels. The model is extended to investigate how MICU1 or MICU2 KO affect mitochondrial function. Moreover, we show how Ca2+ buffering proteins, the separation between mitochondrion and Ca2+-releasing stores, and the duration of opening of Ca2+-releasing channels affect mitochondrial function under different conditions. Finally, we demonstrate an easy extension of the model to single channel function of MCU.

Is MCU dispensable for normal heart function?

The uptake of Ca2+ into mitochondria is thought to be an important signal communicating the need for increased energy production. However, dysregulated uptake leading to mitochondrial Ca2+ overload can trigger opening of the mitochondrial permeability transition pore and potentially cell death. Thus mitochondrial Ca2+ entry is regulated via the activity of a Ca2+-selective channel known as the mitochondrial calcium uniporter. The last decade has seen enormous momentum in the discovery of the molecular identities of the multiple proteins comprising the uniporter. Increasing numbers of studies in cultured cells and animal models have provided insight into how disruption of uniporter proteins affects mitochondrial Ca2+ regulation and impacts tissue function and physiology. This review aims to summarize some of these recent findings, particularly in the context of the heart.

Structural Mechanisms of Store-Operated and Mitochondrial Calcium Regulation: Initiation Points for Drug Discovery

Calcium (Ca2+) is a universal signaling ion that is essential for the life and death processes of all eukaryotes. In humans, numerous cell stimulation pathways lead to the mobilization of sarco/endoplasmic reticulum (S/ER) stored Ca2+, resulting in the propagation of Ca2+ signals through the activation of processes, such as store-operated Ca2+ entry (SOCE). SOCE provides a sustained Ca2+ entry into the cytosol; moreover, the uptake of SOCE-mediated Ca2+ by mitochondria can shape cytosolic Ca2+ signals, function as a feedback signal for the SOCE molecular machinery, and drive numerous mitochondrial processes, including adenosine triphosphate (ATP) production and distinct cell death pathways. In recent years, tremendous progress has been made in identifying the proteins mediating these signaling pathways and elucidating molecular structures, invaluable for understanding the underlying mechanisms of function. Nevertheless, there remains a disconnect between using this accumulating protein structural knowledge and the design of new research tools and therapies. In this review, we provide an overview of the Ca2+ signaling pathways that are involved in mediating S/ER stored Ca2+ release, SOCE, and mitochondrial Ca2+ uptake, as well as pinpoint multiple levels of crosstalk between these pathways. Further, we highlight the significant protein structures elucidated in recent years controlling these Ca2+ signaling pathways. Finally, we describe a simple strategy that aimed at applying the protein structural data to initiating drug design.

Structure and mechanism of the mitochondrial Ca2+ uniporter holocomplex

Mitochondria take up Ca²⁺ through the mitochondrial calcium uniporter complex to regulate energy production, cytosolic Ca²⁺ signalling and cell death^1,2. In mammals, the uniporter complex (uniplex) contains four core components: the pore-forming MCU protein, the gatekeepers MICU1 and MICU2, and an auxiliary subunit, EMRE, essential for Ca²⁺ transport^3-8. To prevent detrimental Ca²⁺ overload, the activity of MCU must be tightly regulated by MICUs, which sense changes in cytosolic Ca²⁺ concentrations to switch MCU on and off^9,10. Here we report cryo-electron microscopic structures of the human mitochondrial calcium uniporter holocomplex in inhibited and Ca²⁺-activated states. These structures define the architecture of this multicomponent Ca²⁺-uptake machinery and reveal the gating mechanism by which MICUs control uniporter activity. Our work provides a framework for understanding regulated Ca²⁺ uptake in mitochondria, and could suggest ways of modulating uniporter activity to treat diseases related to mitochondrial Ca²⁺ overload.

Nuclear-mitochondrial communication involving miR-181c plays an important role in cardiac dysfunction during obesity

AIMS

In cardiomyocytes, there is microRNA (miR) in the mitochondria that originates from the nuclear genome and matures in the cytoplasm before translocating into the mitochondria. Overexpression of one such miR, miR-181c, can lead to heart failure by stimulating reactive oxygen species (ROS) production and increasing mitochondrial calcium level ([Ca²⁺]_m). Mitochondrial calcium uptake 1 protein (MICU1), a regulatory protein in the mitochondrial calcium uniporter complex, plays an important role in regulating [Ca²⁺]_m. Obesity results in miR-181c overexpression and a decrease in MICU1. We hypothesize that lowering miR-181c would protect against obesity-induced cardiac dysfunction.

METHODS AND RESULTS

We used an in vivo mouse model of high-fat diet (HFD) for 18 weeks and induced high lipid load in H9c2 cells with oleate-conjugated bovine serum albumin in vitro. We tested the cardioprotective role of lowering miR-181c by using miR-181c/d^-/- mice (in vivo) and AntagomiR against miR-181c (in vitro). HFD significantly upregulated heart levels of miR-181c and led to cardiac hypertrophy in wild-type mice, but not in miR-181c/d^-/- mice. HFD also increased ROS production and pyruvate dehydrogenase activity (a surrogate for [Ca²⁺]_m), but the increases were alleviated in miR-181c/d^-/- mice. Moreover, miR-181c/d^-/- mice fed a HFD had higher levels of MICU1 than did wild-type mice fed a HFD, attenuating the rise in [Ca²⁺]_m. Overexpression of miR-181c in neonatal ventricular cardiomyocytes (NMVM) caused increased ROS production, which oxidized transcription factor Sp1 and led to a loss of Sp1, thereby slowing MICU1 transcription. Hence, miR-181c increases [Ca²⁺]_m through Sp1 oxidation and downregulation of MICU1, suggesting that the cardioprotective effect of miR-181c/d^-/- results from inhibition of Sp1 oxidation.

CONCLUSION

This study has identified a unique nuclear-mitochondrial communication mechanism in the heart orchestrated by miR-181c. Obesity-induced overexpression of miR-181c increases [Ca²⁺]_m via downregulation of MICU1 and leads to cardiac injury. A strategy to inhibit miR-181c in cardiomyocytes can preserve cardiac function during obesity by improving mitochondrial function. Altering miR-181c expression may provide a pharmacologic approach to improve cardiomyopathy in individuals with obesity/type 2 diabetes.

Developmental and Tumor Angiogenesis Requires the Mitochondria-Shaping Protein Opa1

While endothelial cell (EC) function is influenced by mitochondrial metabolism, the role of mitochondrial dynamics in angiogenesis, the formation of new blood vessels from existing vasculature, is unknown. Here we show that the inner mitochondrial membrane mitochondrial fusion protein optic atrophy 1 (OPA1) is required for angiogenesis. In response to angiogenic stimuli, OPA1 levels rapidly increase to limit nuclear factor kappa-light-chain-enhancer of activated B cell (NFκB) signaling, ultimately allowing angiogenic genes expression and angiogenesis. Endothelial Opa1 is indeed required in an NFκB-dependent pathway essential for developmental and tumor angiogenesis, impacting tumor growth and metastatization. A first-in-class small molecule-specific OPA1 inhibitor confirms that EC Opa1 can be pharmacologically targeted to curtail tumor growth. Our data identify Opa1 as a crucial component of physiological and tumor angiogenesis.

OPA1 and Angiogenesis: Beyond the Fusion Function

In this issue of Cell Metabolism, Herkenne et al. (2020) show that the mitochondrial fusion protein OPA1 promotes angiogenesis independent of its function in mitochondrial dynamics, identifying a key new therapeutic target to prevent vascular growth during development and tumor formation.

The debate continues - What is the role of MCU and mitochondrial calcium uptake in the heart?

Since the identification of the mitochondrial calcium uniporter (MCU) in 2011, several studies utilizing genetic models have attempted to decipher the role of mitochondrial calcium uptake in cardiac physiology. Confounding results in various mutant mouse models have led to an ongoing debate regarding the function of MCU in the heart. In this review, we evaluate and discuss the totality of evidence for mitochondrial calcium uptake in the cardiac stress response and highlight recent reports that implicate MCU in the control of homeostatic cardiac metabolism and function. This review concludes with a discussion of current gaps in knowledge and remaining experiments to define how MCU contributes to contractile function, cell death, metabolic regulation, and heart failure progression.

Variable Assembly of EMRE and MCU Creates Functional Channels with Distinct Gatekeeping Profiles

MCU is a Ca2+-selective channel that mediates mitochondrial Ca2+ influx. The human channel contains tetrameric pore-forming MCU, regulatory subunits MICU1/2, and EMRE that is required both for channel function and MICU1/2-mediated Ca2+ regulation. A structure of MCU with EMRE revealed a 4:4 stoichiometry, but the stoichiometry in vivo is unknown. Expression of tagged EMRE and MCU at a 1:10 ratio in cells lacking EMRE and MCU restored channel activity but not full channel gatekeeping. Increasing EMRE expression enhanced gatekeeping, raising the cytoplasmic Ca2+ concentration ([Ca2+]c) threshold for channel activation. MCU-EMRE concatemers creating channels with 1EMRE:4MCU restored Ca2+ uptake in cells, whereas cells expressing concatemers that enforced a 4EMRE:4MCU stoichiometry demonstrated enhanced channel gatekeeping. Concatemers enforcing 2EMRE/4MCU recapitulated the activity, gatekeeping, and size of endogenous channels. Thus, MCU does not require four EMRE, with most endogenous channels containing two, but complexes with 1-4 EMRE have activity with full or partial gatekeeping.

Mitochondrial pyruvate and fatty acid flux modulate MICU1-dependent control of MCU activity

The tricarboxylic acid (TCA) cycle converts the end products of glycolysis and fatty acid β-oxidation into the reducing equivalents NADH and FADH2 Although mitochondrial matrix uptake of Ca2+ enhances ATP production, it remains unclear whether deprivation of mitochondrial TCA substrates alters mitochondrial Ca2+ flux. We investigated the effect of TCA cycle substrates on MCU-mediated mitochondrial matrix uptake of Ca2+, mitochondrial bioenergetics, and autophagic flux. Inhibition of glycolysis, mitochondrial pyruvate transport, or mitochondrial fatty acid transport triggered expression of the MCU gatekeeper MICU1 but not the MCU core subunit. Knockdown of mitochondrial pyruvate carrier (MPC) isoforms or expression of the dominant negative mutant MPC1R97W resulted in increased MICU1 protein abundance and inhibition of MCU-mediated mitochondrial matrix uptake of Ca2+ We also found that genetic ablation of MPC1 in hepatocytes and mouse embryonic fibroblasts resulted in reduced resting matrix Ca2+, likely because of increased MICU1 expression, but resulted in changes in mitochondrial morphology. TCA cycle substrate-dependent MICU1 expression was mediated by the transcription factor early growth response 1 (EGR1). Blocking mitochondrial pyruvate or fatty acid flux was linked to increased autophagy marker abundance. These studies reveal a mechanism that controls the MCU-mediated Ca2+ flux machinery and that depends on TCA cycle substrate availability. This mechanism generates a metabolic homeostatic circuit that protects cells from bioenergetic crisis and mitochondrial Ca2+ overload during periods of nutrient stress.

Insights Into the Role of Mitochondrial Ion Channels in Inflammatory Response

Mitochondria are the source of many pro-inflammatory signals that cause the activation of the immune system and generate inflammatory responses. They are also potential targets of pro-inflammatory mediators, thus triggering a severe inflammatory response cycle. As mitochondria are a central hub for immune system activation, their dysfunction leads to many inflammatory disorders. Thus, strategies aiming at regulating mitochondrial dysfunction can be utilized as a therapeutic tool to cure inflammatory disorders. Two key factors that determine the structural and functional integrity of mitochondria are mitochondrial ion channels and transporters. They are not only important for maintaining the ionic homeostasis of the cell, but also play a role in regulating reactive oxygen species generation, ATP production, calcium homeostasis and apoptosis, which are common pro-inflammatory signals. The significance of the mitochondrial ion channels in inflammatory response is still not clearly understood and will need further investigation. In this article, we review the different mechanisms by which mitochondria can generate the inflammatory response as well as highlight how mitochondrial ion channels modulate these mechanisms and impact the inflammatory processes.

Mitochondrial calcium uniporter as a potential therapeutic strategy for Alzheimer's disease

Alzheimer's disease (AD), a neurodegenerative disorder, is the leading cause of dementia in the world whose aetiology is still unclear. AD was always related to ageing though there have been instances where people at an early age also succumb to this disease. With medical advancements, the mortality rate has significantly reduced which also makes people more prone to AD. AD is rare, yet the prominent disease has been widely studied with several hypotheses trying to understand the workings of its onset. The most recent and popular hypothesis in AD is the involvement of mitochondrial dysfunction and calcium homeostasis in the development of the disease though their exact roles are not known. With the sudden advent of the mitochondrial calcium uniporter (MCU), many previously known pathological hallmarks of AD may be better understood. Several studies have shown the effect of excess calcium in mitochondria and the influence of MCU complex in mitochondrial function. In this article, we discuss the possible involvement of MCU in AD by linking the uniporter to mitochondrial dysfunction, calcium homeostasis, reactive oxygen species, neurotransmitters and the hallmarks of AD - amyloid plaque formation and tau tangle formation.

Mitochondrial ion channels in pancreatic β-cells: Novel pharmacological targets for the treatment of Type 2 diabetes

Pancreatic beta‐cells are central regulators of glucose homeostasis. By tightly coupling nutrient sensing and granule exocytosis, beta‐cells adjust the secretion of insulin to the circulating blood glucose levels. Failure of beta‐cells to augment insulin secretion in insulin‐resistant individuals leads progressively to impaired glucose tolerance, Type 2 diabetes, and diabetes‐related diseases. Mitochondria play a crucial role in β‐cells during nutrient stimulation, linking the metabolism of glucose and other secretagogues to the generation of signals that promote insulin secretion. Mitochondria are double‐membrane organelles containing numerous channels allowing the transport of ions across both membranes. These channels regulate mitochondrial energy production, signalling, and cell death. The mitochondria of β‐cells express ion channels whose physio/pathological role is underappreciated. Here, we describe the mitochondrial ion channels identified in pancreatic β‐cells, we further discuss the possibility of targeting specific β‐cell mitochondrial channels for the treatment of Type 2 diabetes, and we finally highlight the evidence from clinical studies.

LINKED ARTICLES

This article is part of a themed issue on Cellular metabolism and diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.10/issuetoc

The Mitochondrial Calcium Uniporter Interacts with Subunit c of the ATP Synthase of Trypanosomes and Humans

Mitochondrial Ca2+ transport mediated by the uniporter complex (MCUC) plays a key role in the regulation of cell bioenergetics in both trypanosomes and mammals. Here we report that Trypanosoma brucei MCU (TbMCU) subunits interact with subunit c of the mitochondrial ATP synthase (ATPc), as determined by coimmunoprecipitation and split-ubiquitin membrane-based yeast two-hybrid (MYTH) assays. Mutagenesis analysis in combination with MYTH assays suggested that transmembrane helices (TMHs) are determinants of this specific interaction. In situ tagging, followed by immunoprecipitation and immunofluorescence microscopy, revealed that T. brucei ATPc (TbATPc) coimmunoprecipitates with TbMCUC subunits and colocalizes with them to the mitochondria. Blue native PAGE and immunodetection analyses indicated that the TbMCUC is present together with the ATP synthase in a large protein complex with a molecular weight of approximately 900 kDa. Ablation of the TbMCUC subunits by RNA interference (RNAi) significantly increased the AMP/ATP ratio, revealing the downregulation of ATP production in the cells. Interestingly, the direct physical MCU-ATPc interaction is conserved in Trypanosoma cruzi and human cells. Specific interaction between human MCU (HsMCU) and human ATPc (HsATPc) was confirmed in vitro by mutagenesis and MYTH assays and in vivo by coimmunoprecipitation. In summary, our study has identified that MCU complex physically interacts with mitochondrial ATP synthase, possibly forming an MCUC-ATP megacomplex that couples ADP and Pi transport with ATP synthesis, a process that is stimulated by Ca2+ in trypanosomes and human cells.IMPORTANCE The mitochondrial calcium uniporter (MCU) is essential for the regulation of oxidative phosphorylation in mammalian cells, and we have shown that in Trypanosoma brucei, the etiologic agent of sleeping sickness, this channel is essential for its survival and infectivity. Here we reveal that that Trypanosoma brucei MCU subunits interact with subunit c of the mitochondrial ATP synthase (ATPc). Interestingly, the direct physical MCU-ATPc interaction is conserved in T. cruzi and human cells.

PINK1/Parkin Mediated Mitophagy, Ca2+ Signalling, and ER-Mitochondria Contacts in Parkinson's Disease

Endoplasmic reticulum (ER)–mitochondria contact sites are critical structures for cellular function. They are implicated in a plethora of cellular processes, including Ca²⁺ signalling and mitophagy, the selective degradation of damaged mitochondria. Phosphatase and tensin homolog (PTEN)-induced kinase (PINK) and Parkin proteins, whose mutations are associated with familial forms of Parkinson’s disease, are two of the best characterized mitophagy players. They accumulate at ER–mitochondria contact sites and modulate organelles crosstalk. Alterations in ER–mitochondria tethering are a common hallmark of many neurodegenerative diseases including Parkinson’s disease. Here, we summarize the current knowledge on the involvement of PINK1 and Parkin at the ER–mitochondria contact sites and their role in the modulation of Ca²⁺ signalling and mitophagy.

Calcium Signaling in Cardiomyocyte Function

Rhythmic increases in intracellular Ca²⁺ concentration underlie the contractile function of the heart. These heart muscle-wide changes in intracellular Ca²⁺ are induced and coordinated by electrical depolarization of the cardiomyocyte sarcolemma by the action potential. Originating at the sinoatrial node, conduction of this electrical signal throughout the heart ensures synchronization of individual myocytes into an effective cardiac pump. Ca²⁺ signaling pathways also regulate gene expression and cardiomyocyte growth during development and in pathology. These fundamental roles of Ca²⁺ in the heart are illustrated by the prevalence of altered Ca²⁺ homeostasis in cardiovascular diseases. Indeed, heart failure (an inability of the heart to support hemodynamic needs), rhythmic disturbances, and inappropriate cardiac growth all share an involvement of altered Ca²⁺ handling. The prevalence of these pathologies, contributing to a third of all deaths in the developed world as well as to substantial morbidity makes understanding the mechanisms of Ca²⁺ handling and dysregulation in cardiomyocytes of great importance.

Lactate Efflux From Intervertebral Disc Cells Is Required for Maintenance of Spine Health

Maintenance of glycolytic metabolism is postulated to be required for health of the spinal column. In the hypoxic tissues of the intervertebral disc and glycolytic cells of vertebral bone, glucose is metabolized into pyruvate for ATP generation and reduced to lactate to sustain redox balance. The rise in intracellular H⁺ /lactate concentrations are balanced by plasma-membrane monocarboxylate transporters (MCTs). Using MCT4 null mice and human tissue samples, complemented with genetic and metabolic approaches, we determine that H⁺ /lactate efflux is critical for maintenance of disc and vertebral bone health. Mechanistically, MCT4 maintains glycolytic and tricarboxylic acid (TCA) cycle flux and intracellular pH homeostasis in the nucleus pulposus compartment of the disc, where hypoxia-inducible factor 1α (HIF-1α) directly activates an intronic enhancer in SLC16A3. Ultimately, our results provide support for research into lactate as a diagnostic biomarker for chronic, painful, disc degeneration. © 2019 American Society for Bone and Mineral Research.

Structure of the MICU1-MICU2 heterodimer provides insights into the gatekeeping threshold shift

Mitochondrial calcium uptake proteins 1 and 2 (MICU1 and MICU2) mediate mitochondrial Ca2+ influx via the mitochondrial calcium uniporter (MCU). Its molecular action for Ca2+ uptake is tightly controlled by the MICU1-MICU2 heterodimer, which comprises Ca2+ sensing proteins which act as gatekeepers at low [Ca2+] or facilitators at high [Ca2+]. However, the mechanism underlying the regulation of the Ca2+ gatekeeping threshold for mitochondrial Ca2+ uptake through the MCU by the MICU1-MICU2 heterodimer remains unclear. In this study, we determined the crystal structure of the apo form of the human MICU1-MICU2 heterodimer that functions as the MCU gatekeeper. MICU1 and MICU2 assemble in the face-to-face heterodimer with salt bridges and me-thio-nine knobs stabilizing the heterodimer in an apo state. Structural analysis suggests how the heterodimer sets a higher Ca2+ threshold than the MICU1 homodimer. The structure of the heterodimer in the apo state provides a framework for understanding the gatekeeping role of the MICU1-MICU2 heterodimer.

EMRE is essential for mitochondrial calcium uniporter activity in a mouse model

The mitochondrial calcium uniporter is widely accepted as the primary route of rapid calcium entry into mitochondria, where increases in matrix calcium contribute to bioenergetics but also mitochondrial permeability and cell death. Hence, regulation of uniporter activity is critical to mitochondrial homeostasis. The uniporter subunit EMRE is known to be an essential regulator of the channel-forming protein MCU in cell culture, but EMRE's impact on organismal physiology is less understood. Here we characterize a mouse model of EMRE deletion and show that EMRE is indeed required for mitochondrial calcium uniporter function in vivo. EMRE-/- mice are born less frequently; however, the mice that are born are viable, healthy, and do not manifest overt metabolic impairment, at rest or with exercise. Finally, to investigate the role of EMRE in disease processes, we examine the effects of EMRE deletion in a muscular dystrophy model associated with mitochondrial calcium overload.

Mitochondrial Calcium Regulation of Redox Signaling in Cancer

Calcium (Ca²⁺) uptake into the mitochondria shapes cellular Ca²⁺ signals and acts as a key effector for ATP generation. In addition, mitochondria-derived reactive oxygen species (mROS), produced as a consequence of ATP synthesis at the electron transport chain (ETC), modulate cellular signaling pathways that contribute to many cellular processes. Cancer cells modulate mitochondrial Ca²⁺ ([Ca²⁺]m) homeostasis by altering the expression and function of mitochondrial Ca²⁺ channels and transporters required for the uptake and extrusion of mitochondrial Ca²⁺. Regulated elevations in [Ca²⁺]m are required for the activity of several mitochondrial enzymes, and this in turn regulates metabolic flux, mitochondrial ETC function and mROS generation. Alterations in both [Ca²⁺]m and mROS are hallmarks of many tumors, and elevated mROS is a known driver of pro-tumorigenic redox signaling, resulting in the activation of pathways implicated in cellular proliferation, metabolic alterations and stress-adaptations. In this review, we highlight recent studies that demonstrate the interplay between [Ca²⁺]m and mROS signaling in cancer.

The mRNA Decay Factor CAR-1/LSM14 Regulates Axon Regeneration via Mitochondrial Calcium Dynamics

mRNA decay factors regulate mRNA turnover by recruiting non-translating mRNAs and targeting them for translational repression and mRNA degradation. How mRNA decay pathways regulate cellular function in vivo with specificity is poorly understood. Here, we show that C. elegans mRNA decay factors, including the translational repressors CAR-1/LSM14 and CGH-1/DDX6, and the decapping enzymes DCAP-1/DCP1, function in neurons to differentially regulate axon development, maintenance, and regrowth following injury. In neuronal cell bodies, CAR-1 fully colocalizes with CGH-1 and partially colocalizes with DCAP-1, suggesting that mRNA decay components form at least two types of cytoplasmic granules. Following axon injury in adult neurons, loss of CAR-1 or CGH-1 results in increased axon regrowth and growth cone formation, whereas loss of DCAP-1 or DCAP-2 results in reduced regrowth. To determine how CAR-1 inhibits regrowth, we analyzed mRNAs bound to pan-neuronally expressed GFP::CAR-1 using a crosslinking and immunoprecipitation-based approach. Among the putative mRNA targets of CAR-1, we characterized the roles of micu-1, a regulator of the mitochondrial calcium uniporter MCU-1, in axon injury. We show that loss of car-1 results increased MICU-1 protein levels, and that enhanced axon regrowth in car-1 mutants is dependent on micu-1 and mcu-1. Moreover, axon injury induces transient calcium influx into axonal mitochondria, dependent on MCU-1. In car-1 loss-of-function mutants and in micu-1 overexpressing animals, the axonal mitochondrial calcium influx is more sustained, which likely underlies enhanced axon regrowth. Our data uncover a novel pathway that controls axon regrowth through axonal mitochondrial calcium uptake.

Imaging Mitochondrial Functions: from Fluorescent Dyes to Genetically-Encoded Sensors

Mitochondria are multifunctional organelles that are crucial to cell homeostasis. They constitute the major site of energy production for the cell, they are key players in signalling pathways using secondary messengers such as calcium, and they are involved in cell death and redox balance paradigms. Mitochondria quickly adapt their dynamics and biogenesis rates to meet the varying energy demands of the cells, both in normal and in pathological conditions. Therefore, understanding simultaneous changes in mitochondrial functions is crucial in developing mitochondria-based therapy options for complex pathological conditions such as cancer, neurological disorders, and metabolic syndromes. To this end, fluorescence microscopy coupled to live imaging represents a promising strategy to track these changes in real time. In this review, we will first describe the commonly available tools to follow three key mitochondrial functions using fluorescence microscopy: Calcium signalling, mitochondrial dynamics, and mitophagy. Then, we will focus on how the development of genetically-encoded fluorescent sensors became a milestone for the understanding of these mitochondrial functions. In particular, we will show how these tools allowed researchers to address several biochemical activities in living cells, and with high spatiotemporal resolution. With the ultimate goal of tracking multiple mitochondrial functions simultaneously, we will conclude by presenting future perspectives for the development of novel genetically-encoded fluorescent biosensors.

MicroRNA-20b Promotes Cardiac Hypertrophy by the Inhibition of Mitofusin 2-Mediated Inter-organelle Ca2+ Cross-Talk

MicroRNA (miRNA) and mitofusin-2 (Mfn2) are important in the development of cardiac hypertrophy, but the target relationship and mechanism associated with Ca²⁺ handling between SR and mitochondria under hypertrophic condition is not established. Mfn2 expression, Mfn2-mediated interorganelle Ca²⁺ cross-talk, and target regulation by miRNA-20b (miR-20b) were evaluated using animal/cellular hypertrophic models with state-of-the-art techniques. The results demonstrated that Mfn2 was downregulated and miR-20b was upregulated upon the target binding profile under hypertrophic condition. Our data showed that miR-20b induced cardiac hypertrophy that was reversed by recombinant adeno-associated virus vector 9 (rAAV9)-anti-miR-20b or miR-20b antisense inhibitor (AMO-20b). The deleterious action of miR-20b on Mfn2 expression/function and mitochondrial ATP synthesis was observed and reversed by rAAV9-anti-miR-20b or AMO-20b. The targeted regulation of miR-20b on Mfn2 was confirmed by luciferase reporter and miRNA-masking. Importantly, the facts that mitochondrial calcium uniporter (MCU) activation by Spermine increased the cytosolic Ca²⁺ into mitochondria, manifested as enhanced histamine-mediated Ca²⁺ release from mitochondrial, suggesting that Ca²⁺ reuptake/buffering capability of mitochondria to cytosolic Ca²⁺ is injured by miR-20b-mediated Mfn2 signaling, by which leads cytosolic Ca²⁺ overload and cardiac hypertrophy through Ca²⁺ signaling pathway. In conclusion, pro-hypertonic miR-20b plays crucial roles in cardiac hypertrophy through downregulation of Mfn2 and cytosolic Ca²⁺ overload by weakening the buffering capability of mitochondria.

The Role of Mitochondrial Calcium Signaling in the Pathophysiology of Cancer Cells

The pioneering work of Richard Altman on the presence of mitochondria in cells set in motion a field of research dedicated to uncovering the secrets of the mitochondria. Despite limitations in studying the structure and function of the mitochondria, advances in our understanding of this organelle prompted the development of potential treatments for various diseases, from neurodegenerative conditions to muscular dystrophy and cancer. As the powerhouses of the cell, the mitochondria represent the essence of cellular life and as such, a selective advantage for cancer cells. Much of the function of the mitochondria relies on Ca²⁺ homeostasis and the presence of effective Ca²⁺ signaling to maintain the balance between mitochondrial function and dysfunction and subsequently, cell survival. Ca²⁺ regulates the mitochondrial respiration rate which in turn increases ATP synthesis, but too much Ca²⁺ can also trigger the mitochondrial apoptosis pathway; however, cancer cells have evolved mechanisms to modulate mitochondrial Ca²⁺ influx and efflux in order to sustain their metabolic demand and ensure their survival. Therefore, targeting the mitochondrial Ca²⁺ signaling involved in the bioenergetic and apoptotic pathways could serve as potential approaches to treat cancer patients. This chapter will review the role of Ca²⁺ signaling in mediating the function of the mitochondria and its involvement in health and disease with special focus on the pathophysiology of cancer.

Cancer-Related Increases and Decreases in Calcium Signaling at the Endoplasmic Reticulum-Mitochondria Interface (MAMs)

Endoplasmic reticulum (ER)-mitochondria regions are specialized subdomains called also mitochondria-associated membranes (MAMs). MAMs allow regulation of lipid synthesis and represent hubs for ion and metabolite signaling. As these two organelles can module both the amplitude and the spatiotemporal patterns of calcium (Ca²⁺) signals, this particular interaction controls several Ca²⁺-dependent pathways well known for their contribution to tumorigenesis, such as metabolism, survival, sensitivity to cell death, and metastasis. Mitochondria-mediated apoptosis arises from mitochondrial Ca²⁺ overload, permeabilization of the mitochondrial outer membrane, and the release of mitochondrial apoptotic factors into the cytosol. Decreases in Ca²⁺ signaling at the ER-mitochondria interface are being studied in depth as failure of apoptotic-dependent cell death is one of the predominant characteristics of cancer cells. However, some recent papers that linked MAMs Ca²⁺ crosstalk-related upregulation to tumor onset and progression have aroused the interest of the scientific community.In this review, we will describe how different MAMs-localized proteins modulate the effectiveness of Ca²⁺-dependent apoptotic stimuli by causing both increases and decreases in the ER-mitochondria interplay and, specifically, by modulating Ca²⁺ signaling.

The Puzzling Role of Neuron-Specific PMCA Isoforms in the Aging Process

The aging process is a physiological phenomenon associated with progressive changes in metabolism, genes expression, and cellular resistance to stress. In neurons, one of the hallmarks of senescence is a disturbance of calcium homeostasis that may have far-reaching detrimental consequences on neuronal physiology and function. Among several proteins involved in calcium handling, plasma membrane Ca²⁺-ATPase (PMCA) is the most sensitive calcium detector controlling calcium homeostasis. PMCA exists in four main isoforms and PMCA2 and PMCA3 are highly expressed in the brain. The overall effects of impaired calcium extrusion due to age-dependent decline of PMCA function seem to accumulate with age, increasing the susceptibility to neurotoxic insults. To analyze the PMCA role in neuronal cells, we have developed stable transfected differentiated PC12 lines with down-regulated PMCA2 or PMCA3 isoforms to mimic age-related changes. The resting Ca²⁺ increased in both PMCA-deficient lines affecting the expression of several Ca²⁺-associated proteins, i.e., sarco/endoplasmic Ca²⁺-ATPase (SERCA), calmodulin, calcineurin, GAP43, CCR5, IP₃Rs, and certain types of voltage-gated Ca²⁺ channels (VGCCs). Functional studies also demonstrated profound changes in intracellular pH regulation and mitochondrial metabolism. Moreover, modification of PMCAs membrane composition triggered some adaptive processes to counterbalance calcium overload, but the reduction of PMCA2 appeared to be more detrimental to the cells than PMCA3.

MICU1 Confers Protection from MCU-Dependent Manganese Toxicity

The mitochondrial calcium uniporter is a highly selective ion channel composed of species- and tissue-specific subunits. However, the functional role of each component still remains unclear. Here, we establish a synthetic biology approach to dissect the interdependence between the pore-forming subunit MCU and the calcium-sensing regulator MICU1. Correlated evolutionary patterns across 247 eukaryotes indicate that their co-occurrence may have conferred a positive fitness advantage. We find that, while the heterologous reconstitution of MCU and EMRE in vivo in yeast enhances manganese stress, this is prevented by co-expression of MICU1. Accordingly, MICU1 deletion sensitizes human cells to manganese-dependent cell death by disinhibiting MCU-mediated manganese uptake. As a result, manganese overload increases oxidative stress, which can be effectively prevented by NAC treatment. Our study identifies a critical contribution of MICU1 to the uniporter selectivity, with important implications for patients with MICU1 deficiency, as well as neurological disorders arising upon chronic manganese exposure.

miR-181c Activates Mitochondrial Calcium Uptake by Regulating MICU1 in the Heart

Background

Translocation of miR‐181c into cardiac mitochondria downregulates the mitochondrial gene, mt‐COX1. miR‐181c/d^−/− hearts experience less oxidative stress during ischemia/reperfusion (I/R) and are protected against I/R injury. Additionally, miR‐181c overexpression can increase mitochondrial matrix Ca²⁺ ([Ca²⁺]_m), but the mechanism by which miR‐181c regulates [Ca²⁺]_m is unknown.

Methods and Results

By RNA sequencing and analysis, here we show that hearts from miR‐181c/d^−/− mice overexpress nuclear‐encoded Ca²⁺ regulatory and metabolic pathway genes, suggesting that alterations in miR‐181c and mt‐COX1 perturb mitochondria‐to‐nucleus retrograde signaling and [Ca²⁺]_m regulation. Quantitative polymerase chain reaction validation of transcription factors that are known to initiate retrograde signaling revealed significantly higher Sp1 (specificity protein) expression in the miR‐181c/d^−/− hearts. Furthermore, an association of Sp1 with the promoter region of MICU1 was confirmed by chromatin immunoprecipitation‐quantitative polymerase chain reaction and higher expression of MICU1 was found in the miR‐181c/d^−/− hearts. Conversely, downregulation of Sp1 by small interfering RNA decreased MICU1 expression in neonatal mouse ventricular myocytes. Changes in PDH activity provided evidence for a change in [Ca²⁺]_m via the miR‐181c/MICU1 axis. Moreover, this mechanism was implicated in the pathology of I/R injury. When MICU1 was knocked down in the miR‐181c/d^−/− heart by lentiviral expression of a short‐hairpin RNA against MICU1, cardioprotective effects against I/R injury were abrogated. Furthermore, using an in vitro I/R model in miR‐181c/d^−/− neonatal mouse ventricular myocytes, we confirmed the contribution of both Sp1 and MICU1 in ischemic injury.

Conclusions

miR‐181c regulates mt‐COX1, which in turn regulates MICU1 expression through the Sp1‐mediated mitochondria‐to‐nucleus retrograde pathway. Loss of miR‐181c can protect the heart from I/R injury by modulating [Ca²⁺]_m through the upregulation of MICU1.

Light-driven activation of mitochondrial proton-motive force improves motor behaviors in a Drosophila model of Parkinson's disease

Mitochondrial degeneration is considered one of the major causes of Parkinson's disease (PD). Improved mitochondrial functions are expected to be a promising therapeutic strategy for PD. In this study, we introduced a light-driven proton transporter, Delta-rhodopsin (dR), to Drosophila mitochondria, where the mitochondrial proton-motive force (Δp) and mitochondrial membrane potential are maintained in a light-dependent manner. The loss of the PD-associated mitochondrial gene CHCHD2 resulted in reduced ATP production, enhanced mitochondrial peroxide production and lower Ca2+-buffering activity in dopaminergic (DA) terminals in flies. These cellular defects were improved by the light-dependent activation of mitochondrion-targeted dR (mito-dR). Moreover, mito-dR reversed the pathology caused by the CHCHD2 deficiency to suppress α-synuclein aggregation, DA neuronal loss, and elevated lipid peroxidation in brain tissue, improving motor behaviors. This study suggests the enhancement of Δp by mito-dR as a therapeutic mechanism that ameliorates neurodegeneration by protecting mitochondrial functions.

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.

Overexpression of Mitochondrial Calcium Uniporter Causes Neuronal Death

Neurodegenerative diseases are a large and heterogeneous group of disorders characterized by selective and progressive death of specific neuronal subtypes. In most of the cases, the pathophysiology is still poorly understood, although a number of hypotheses have been proposed. Among these, dysregulation of Ca²⁺ homeostasis and mitochondrial dysfunction represent two broadly recognized early events associated with neurodegeneration. However, a direct link between these two hypotheses can be drawn. Mitochondria actively participate to global Ca²⁺ signaling, and increases of [Ca²⁺] inside organelle matrix are known to sustain energy production to modulate apoptosis and remodel cytosolic Ca²⁺ waves. Most importantly, while mitochondrial Ca²⁺ overload has been proposed as the no-return signal, triggering apoptotic or necrotic neuronal death, until now direct evidences supporting this hypothesis, especially in vivo, are limited. Here, we took advantage of the identification of the mitochondrial Ca²⁺ uniporter (MCU) and tested whether mitochondrial Ca²⁺ signaling controls neuronal cell fate. We overexpressed MCU both in vitro, in mouse primary cortical neurons, and in vivo, through stereotaxic injection of MCU-coding adenoviral particles in the brain cortex. We first measured mitochondrial Ca²⁺ uptake using quantitative genetically encoded Ca²⁺ probes, and we observed that the overexpression of MCU causes a dramatic increase of mitochondrial Ca²⁺ uptake both at resting and after membrane depolarization. MCU-mediated mitochondrial Ca²⁺ overload causes alteration of organelle morphology and dysregulation of global Ca²⁺ homeostasis. Most importantly, MCU overexpression in vivo is sufficient to trigger gliosis and neuronal loss. Overall, we demonstrated that mitochondrial Ca²⁺ overload is per se sufficient to cause neuronal cell death both in vitro and in vivo, thus highlighting a potential key step in neurodegeneration.

Mitochondrial calcium exchange links metabolism with the epigenome to control cellular differentiation

Fibroblast to myofibroblast differentiation is crucial for the initial healing response but excessive myofibroblast activation leads to pathological fibrosis. Therefore, it is imperative to understand the mechanisms underlying myofibroblast formation. Here we report that mitochondrial calcium (mCa2+) signaling is a regulatory mechanism in myofibroblast differentiation and fibrosis. We demonstrate that fibrotic signaling alters gating of the mitochondrial calcium uniporter (mtCU) in a MICU1-dependent fashion to reduce mCa2+ uptake and induce coordinated changes in metabolism, i.e., increased glycolysis feeding anabolic pathways and glutaminolysis yielding increased α-ketoglutarate (αKG) bioavailability. mCa2+-dependent metabolic reprogramming leads to the activation of αKG-dependent histone demethylases, enhancing chromatin accessibility in loci specific to the myofibroblast gene program, resulting in differentiation. Our results uncover an important role for the mtCU beyond metabolic regulation and cell death and demonstrate that mCa2+ signaling regulates the epigenome to influence cellular differentiation.

Cytoplasmic and Mitochondrial Calcium Signaling: A Two-Way Relationship

Intracellular Ca²⁺ signals are well organized in all cell types, and trigger a variety of vital physiological processes. The temporal and spatial characteristics of cytosolic Ca²⁺ increases are mainly governed by the fluxes of this ion across the membrane of the endoplasmic/sarcoplasmic reticulum and the plasma membrane. However, various Ca²⁺ transporters also allow for Ca²⁺ exchanges between the cytoplasm and mitochondria. Increases in mitochondrial Ca²⁺ stimulate the production of ATP, which allows the cells to cope with the increased energy demand created by the stimulus. Less widely appreciated is the fact that Ca²⁺ handling by mitochondria also shapes cytosolic Ca²⁺ signals. Indeed, the frequency, amplitude, and duration of cytosolic Ca²⁺ increases can be altered by modifying the rates of Ca²⁺ transport into, or from, mitochondria. In this review, we focus on the interplay between mitochondria and Ca²⁺ signaling, highlighting not only the consequences of cytosolic Ca²⁺ changes on mitochondrial Ca²⁺, but also how cytosolic Ca²⁺ dynamics is controlled by modifications of the Ca²⁺-handling properties and the metabolism of mitochondria.

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.

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.

The crystal structure of MICU2 provides insight into Ca2+ binding and MICU1-MICU2 heterodimer formation

The mitochondrial calcium uniporter (MCU) complex mediates the uptake of Ca2+ into mitochondria. Its activity is regulated by a heterodimer of MICU1 and MICU2, two EF-hand-containing proteins that act as the main gatekeeper of the uniporter. Herein we report the crystal structure of human MICU2 at 1.96 Å resolution. Our structure reveals a dimeric architecture of MICU2, in which each monomer adopts the canonical two-lobe structure with a pair of EF-hands in each lobe. Both Ca2+ -bound and Ca2+ -free EF-hands are observed in our structure. Moreover, we characterize the interaction sites within the MICU2 homodimer, as well as the MICU1-MICU2 heterodimer in both Ca2+ -free and Ca2+ -bound conditions. Glu242 in MICU1 and Arg352 in MICU2 are crucial for apo heterodimer formation, while Phe383 in MICU1 and Glu196 in MICU2 significantly contribute to the interaction in the Ca2+ -bound state. Based on our structural and biochemical analyses, we propose a model for MICU1-MICU2 heterodimer formation and its conformational transition from apo to a more compact Ca2+ -bound state, which expands our understanding of this co-regulatory mechanism critical for MCU's mitochondrial calcium uptake function.

Impaired mitochondrial calcium efflux contributes to disease progression in models of Alzheimer's disease

Impairments in neuronal intracellular calcium (_iCa²⁺) handling may contribute to Alzheimer’s disease (AD) development. Metabolic dysfunction and progressive neuronal loss are associated with AD progression, and mitochondrial calcium (_mCa²⁺) signaling is a key regulator of both of these processes. Here, we report remodeling of the _mCa²⁺ exchange machinery in the prefrontal cortex of individuals with AD. In the 3xTg-AD mouse model impaired _mCa²⁺ efflux capacity precedes neuropathology. Neuronal deletion of the mitochondrial Na⁺/Ca²⁺ exchanger (NCLX, Slc8b1 gene) accelerated memory decline and increased amyloidosis and tau pathology. Further, genetic rescue of neuronal NCLX in 3xTg-AD mice is sufficient to impede AD-associated pathology and memory loss. We show that _mCa²⁺ overload contributes to AD progression by promoting superoxide generation, metabolic dysfunction and neuronal cell death. These results provide a link between the calcium dysregulation and metabolic dysfunction hypotheses of AD and suggest _mCa²⁺ exchange as potential therapeutic target in AD.

Dysregulation of intracellular calcium is reported in Alzheimer’s disease. Here the authors show that loss of the mitochondrial Na⁺ /Ca²⁺ exchanger, NCLX – primary route of mitochondrial calcium efflux, precedes neuronal pathology in experimental models and contributes to Alzheimer’s disease progression.