Sequential forward and reverse transport of the Na+ Ca2+ exchanger generates Ca2+ oscillations within mitochondria

Modes of Mitochondrial Reactive Oxygen Species Production in Inflammation

Background: Inflammation is one of the most important pathways in innate immunity and its relationship with redox biology is becoming increasingly clear in the last decades. However, the specific redox modes and pathways by which inflammation is produced are not yet well defined. Significance: In this review, we provide a general explanation of the reactive oxygen species (ROS) production and quenching modes occurring in mammalian mitochondria, as well as a summary of the most recent advances in mitochondrial redox biology and bioenergetics regarding sodium (Na+) homeostasis. In addition, we provide a collection of examples in which several inflammatory pathways have been associated with specific modes of either mitochondrial ROS production or quenching. Innovation: The role of Na+ in mitochondrial biology is being developed. Since its discovery as a second messenger, the research of its role in the immune system has emerged. Now, the role of Na+ in mitochondrial bioenergetics has recently been identified, which owns unprecedented applications. The potential implication of Na+ in inflammatory mechanisms grows as its role does not only cover ROS production and respiration but also the control through the management of mitochondrial membrane potential. Future directions: Na+ is becoming relevant for mitochondrial biology. Thus, processes regarding mitochondrial bioenergetics, redox state, or metabolism may probably need to include the study of Na+ in their road map. Some of these pathways are involved in inflammation and more are possibly to come. This review is expected to serve as a bridge between both fields. Antioxid. Redox Signal. 00, 000-000.

Calcium-mediated regulation of mitophagy: implications in neurodegenerative diseases

Calcium signaling plays a pivotal role in diverse cellular processes through precise spatiotemporal regulation and interaction with effector proteins across distinct subcellular compartments. Mitochondria, in particular, act as central hubs for calcium buffering, orchestrating energy production, redox balance and apoptotic signaling, among others. While controlled mitochondrial calcium uptake supports ATP synthesis and metabolic regulation, excessive accumulation can trigger oxidative stress, mitochondrial membrane permeabilization, and cell death. Emerging findings underscore the intricate interplay between calcium homeostasis and mitophagy, a selective type of autophagy for mitochondria elimination. Although the literature is still emerging, this review delves into the bidirectional relationship between calcium signaling and mitophagy pathways, providing compelling mechanistic insights. Furthermore, we discuss how disruptions in calcium homeostasis impair mitophagy, contributing to mitochondrial dysfunction and the pathogenesis of common neurodegenerative diseases.

Mfn2 induces NCLX-mediated calcium release from mitochondria

Mfn2 is a mitochondrial outer membrane fusion protein with the additional role of tethering mitochondria to the ER. Here, we describe a novel connection between Mfn2 and calcium release from mitochondria. We show that Mfn2 controls the mitochondrial inner membrane sodium-calcium exchange protein NCLX, which is a major source for calcium release from mitochondria. This discovery was made with the fungal toxin Phomoxanthone (PXA), which induces calcium release from mitochondria. PXA-induced calcium release is blocked by a chemical inhibitor of NCLX, while NCLX and Mfn2 deletions both also prevent PXA-induced calcium release. CETSA experiments show that PXA directly targets Mfn2, which likely controls NCLX through physical interactions since co-immunoprecipitation and proximity ligation assays show increased association between Mfn2 and NCLX upon treatment with PXA. Interactions between Mfn2 and NCLX also increase when cells are treated with mitochondrial ROS-inducing conditions, such as oligomycin treatment of respiring cells, while the interactions do not increase in Oma1 -/- cells. It seems likely that opening of cristae by Oma1-mediated cleavage of Opa1 promotes translocation of NCLX from cristae to the rim where it can come into contact with Mfn2 thus promoting PXA-induced calcium release from mitochondria. These results therefore delineate a pathway that connects ROS produced inside mitochondria with calcium release and signaling in the cytosol.

Dysregulation of calcium homeostasis in cancer and its role in chemoresistance

Globally, cancer, as a major public health concern, poses a severe threat to people's well-being. Advanced and specialized therapies can now cure the majority of people with early-stage cancer. However, emerging resistance to traditional and novel chemotherapeutic drugs remains a serious issue in clinical medicine. Chemoresistance often leads to cancer recurrence, metastasis, and increased mortality, accounting for 90% of chemotherapy failures. Thus, it is important to understand the molecular mechanisms of chemoresistance and find novel therapeutic approaches for cancer treatment. Among the several factors responsible for chemoresistance, calcium (Ca2+) dysregulation plays a significant role in cancer progression and chemoresistance. Therefore, targeting this derailed Ca2+ signalling for cancer therapy has become an emerging research area. Of note, the Ca2+ signal and its proteins are a multifaceted and potent tool by which cells achieve specific outcomes. Depending on cell survival needs, Ca2+ is either upregulated or downregulated in both chemosensitive and chemoresistant cancer cells. Consequently, the appropriate treatment should be selected based on Ca2+ signalling dysregulation. This review discusses the role of Ca2+ in cancer cells and the targeting of Ca2+ channels, pumps, and exchangers. Furthermore, we have emphasised the role of Ca2+ in chemoresistance and therapeutic strategies. In conclusion, targeting Ca2+ signalling is a multifaceted process. Methods such as site-specific drug delivery, target-based drug-designing, and targeting two or more Ca2+ proteins simultaneously may be explored; however, further clinical studies are essential to validate Ca2+ blockers' anti-cancer efficacy.

Calcium's Role and Signaling in Aging Muscle, Cellular Senescence, and Mineral Interactions

Calcium research, since its pivotal discovery in the early 1800s through the heating of limestone, has led to the identification of its multi-functional roles. These include its functions as a reducing agent in chemical processes, structural properties in shells and bones, and significant role in cells relating to this review: cellular signaling. Calcium signaling involves the movement of calcium ions within or between cells, which can affect the electrochemical gradients between intra- and extracellular membranes, ligand binding, enzyme activity, and other mechanisms that determine cell fate. Calcium signaling in muscle, as elucidated by the sliding filament model, plays a significant role in muscle contraction. However, as organisms age, alterations occur within muscle tissue. These changes include sarcopenia, loss of neuromuscular junctions, and changes in mineral concentration, all of which have implications for calcium's role. Additionally, a field of study that has gained recent attention, cellular senescence, is associated with aging and disturbed calcium homeostasis, and is thought to affect sarcopenia progression. Changes seen in calcium upon aging may also be influenced by its crosstalk with other minerals such as iron and zinc. This review investigates the role of calcium signaling in aging muscle and cellular senescence. We also aim to elucidate the interactions among calcium, iron, and zinc across various cells and conditions, ultimately deepening our understanding of calcium signaling in muscle aging.

Calcium-binding protein parvalbumin in the spinal cord and dorsal root ganglia

Parvalbumin is one of the calcium-binding proteins. In the spinal cord, it is mainly expressed in inhibitory neurons; in the dorsal root ganglia, it is expressed in proprioceptive neurons. In contrast to in the brain, weak systematization of parvalbumin-expressing neurons occurs in the spinal cord. The aim of this paper is to provide a systematic review of parvalbumin-expressing neuronal populations throughout the spinal cord and the dorsal root ganglia of mammals, regarding their mapping, co-expression with some functional markers. The data reviewed are mostly concerning rodentia species because they are predominantly presented in literature.

Mitochondrial Damage and Hypertension: Another Dark Side of Sodium Excess

PURPOSE OF REVIEW

Essential or primary hypertension (HT) is a worldwide health problem with no definitive cure. Although the exact pathogenesis of HT is not known, genetic factors, increased renin-angiotensin and sympathetic system activity, endothelial dysfunction, oxidative stress, and inflammation play a role in its development. Environmental factors such as sodium intake are also important for BP regulation, and excess sodium intake in the form of salt (NaCl, sodium chloride) increases blood pressure in salt-sensitive people. Excess salt intake increases extracellular volume, oxidative stress, inflammation, and endothelial dysfunction. Recent evidence suggests that increased salt intake also disturbs mitochondrial function both structurally and functionally which is important as mitochondrial dysfunction is associated with HT. In the current review, we have summarized the experimental and clinical data regarding the impact of salt intake on mitochondrial structure and function.

RECENT FINDINGS

Excess salt intake damage mitochondrial structure (e.g., shorter mitochondria with less cristae, increased mitochondrial fission, increased mitochondrial vacuolization). Functionally, high salt intake impairs mitochondrial oxidative phosphorylation and electron transport chain, ATP production, mitochondrial calcium homeostasis, mitochondrial membrane potential, and mitochondrial uncoupling protein function. Excess salt intake also increases mitochondrial oxidative stress and modifies Krebs cycle protein expressions. Studies have shown that high salt intake impairs mitochondrial structure and function. These maladaptive mitochondrial changes facilitate the development of HT especially in salt-sensitive individuals. High salt intake impairs many functional and structural components of mitochondria. These mitochondrial alterations along with increased salt intake promote the development of hypertension.

NLRP14 Safeguards Calcium Homeostasis via Regulating the K27 Ubiquitination of Nclx in Oocyte-to-Embryo Transition

Sperm-induced Ca2+ rise is critical for driving oocyte activation and subsequent embryonic development, but little is known about how lasting Ca2+ oscillations are regulated. Here it is shown that NLRP14, a maternal effect factor, is essential for keeping Ca2+ oscillations and early embryonic development. Few embryos lacking maternal NLRP14 can develop beyond the 2-cell stage. The impaired developmental potential of Nlrp14-deficient oocytes is mainly caused by disrupted cytoplasmic function and calcium homeostasis due to altered mitochondrial distribution, morphology, and activity since the calcium oscillations and development of Nlrp14-deficient oocytes can be rescued by substitution of whole cytoplasm by spindle transfer. Proteomics analysis reveal that cytoplasmic UHRF1 (ubiquitin-like, containing PHD and RING finger domains 1) is significantly decreased in Nlrp14-deficient oocytes, and Uhrf1-deficient oocytes also show disrupted calcium homeostasis and developmental arrest. Strikingly, it is found that the mitochondrial Na+ /Ca2+ exchanger (NCLX) encoded by Slc8b1 is significantly decreased in the Nlrp14mNull oocyte. Mechanistically, NLRP14 interacts with the NCLX intrinsically disordered regions (IDRs) domain and maintain its stability by regulating the K27-linked ubiquitination. Thus, the study reveals NLRP14 as a crucial player in calcium homeostasis that is important for early embryonic development.

Store-Operated Ca2+ Entry as a Putative Target of Flecainide for the Treatment of Arrhythmogenic Cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is a genetic disorder that may lead patients to sudden cell death through the occurrence of ventricular arrhythmias. ACM is characterised by the progressive substitution of cardiomyocytes with fibrofatty scar tissue that predisposes the heart to life-threatening arrhythmic events. Cardiac mesenchymal stromal cells (C-MSCs) contribute to the ACM by differentiating into fibroblasts and adipocytes, thereby supporting aberrant remodelling of the cardiac structure. Flecainide is an Ic antiarrhythmic drug that can be administered in combination with β-adrenergic blockers to treat ACM due to its ability to target both Nav1.5 and type 2 ryanodine receptors (RyR2). However, a recent study showed that flecainide may also prevent fibro-adipogenic differentiation by inhibiting store-operated Ca2+ entry (SOCE) and thereby suppressing spontaneous Ca2+ oscillations in C-MSCs isolated from human ACM patients (ACM C-hMSCs). Herein, we briefly survey ACM pathogenesis and therapies and then recapitulate the main molecular mechanisms targeted by flecainide to mitigate arrhythmic events, including Nav1.5 and RyR2. Subsequently, we describe the role of spontaneous Ca2+ oscillations in determining MSC fate. Next, we discuss recent work showing that spontaneous Ca2+ oscillations in ACM C-hMSCs are accelerated to stimulate their fibro-adipogenic differentiation. Finally, we describe the evidence that flecainide suppresses spontaneous Ca2+ oscillations and fibro-adipogenic differentiation in ACM C-hMSCs by inhibiting constitutive SOCE.

Signaling pathways and intervention for therapy of type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) represents one of the fastest growing epidemic metabolic disorders worldwide and is a strong contributor for a broad range of comorbidities, including vascular, visual, neurological, kidney, and liver diseases. Moreover, recent data suggest a mutual interplay between T2DM and Corona Virus Disease 2019 (COVID-19). T2DM is characterized by insulin resistance (IR) and pancreatic β cell dysfunction. Pioneering discoveries throughout the past few decades have established notable links between signaling pathways and T2DM pathogenesis and therapy. Importantly, a number of signaling pathways substantially control the advancement of core pathological changes in T2DM, including IR and β cell dysfunction, as well as additional pathogenic disturbances. Accordingly, an improved understanding of these signaling pathways sheds light on tractable targets and strategies for developing and repurposing critical therapies to treat T2DM and its complications. In this review, we provide a brief overview of the history of T2DM and signaling pathways, and offer a systematic update on the role and mechanism of key signaling pathways underlying the onset, development, and progression of T2DM. In this content, we also summarize current therapeutic drugs/agents associated with signaling pathways for the treatment of T2DM and its complications, and discuss some implications and directions to the future of this field.

Acute Downregulation but Not Genetic Ablation of Murine MCU Impairs Suppressive Capacity of Regulatory CD4 T Cells

By virtue of mitochondrial control of energy production, reactive oxygen species (ROS) generation, and maintenance of Ca²⁺ homeostasis, mitochondria play an essential role in modulating T cell function. The mitochondrial Ca²⁺ uniporter (MCU) is the pore-forming unit in the main protein complex mediating mitochondrial Ca²⁺ uptake. Recently, MCU has been shown to modulate Ca²⁺ signals at subcellular organellar interfaces, thus fine-tuning NFAT translocation and T cell activation. The mechanisms underlying this modulation and whether MCU has additional T cell subpopulation-specific effects remain elusive. However, mice with germline or tissue-specific ablation of Mcu did not show impaired T cell responses in vitro or in vivo, indicating that 'chronic' loss of MCU can be functionally compensated in lymphocytes. The current work aimed to specifically investigate whether and how MCU influences the suppressive potential of regulatory CD4 T cells (Treg). We show that, in contrast to genetic ablation, acute siRNA-mediated downregulation of Mcu in murine Tregs results in a significant reduction both in mitochondrial Ca²⁺ uptake and in the suppressive capacity of Tregs, while the ratios of Treg subpopulations and the expression of hallmark transcription factors were not affected. These findings suggest that permanent genetic inactivation of MCU may result in compensatory adaptive mechanisms, masking the effects on the suppressive capacity of Tregs.

Mitochondrial Ca2+ handling as a cell signaling hub: lessons from astrocyte function

Astrocytes are a heterogenous population of macroglial cells spread throughout the central nervous system with diverse functions, expression signatures, and intricate morphologies. Their subcellular compartments contain a distinct range of mitochondria, with functional microdomains exhibiting widespread activities, such as controlling local metabolism and Ca2+ signaling. Ca2+ is an ion of utmost importance, both physiologically and pathologically, and participates in critical central nervous system processes, including synaptic plasticity, neuron-astrocyte integration, excitotoxicity, and mitochondrial physiology and metabolism. The mitochondrial Ca2+ handling system is formed by the mitochondrial Ca2+ uniporter complex (MCUc), which mediates Ca2+ influx, and the mitochondrial Na+/Ca2+ exchanger (NCLX), responsible for most mitochondrial Ca2+ efflux, as well as additional components, including the mitochondrial permeability transition pore (mtPTP). Over the last decades, mitochondrial Ca2+ handling has been shown to be key for brain homeostasis, acting centrally in physiopathological processes such as astrogliosis, astrocyte-neuron activity integration, energy metabolism control, and neurodegeneration. In this review, we discuss the current state of knowledge regarding the mitochondrial Ca2+ handling system molecular composition, highlighting its impact on astrocytic homeostasis.

Mitofusin 1 and 2 differentially regulate mitochondrial function underlying Ca2+ signaling and proliferation in rat aortic smooth muscle cells

Mitochondria play a substantial role in cytosolic Ca²⁺ buffering and energy metabolism. We recently demonstrated that mitofusin 2 (Mfn2) regulated Ca²⁺ signaling by tethering mitochondria and sarcoplasmic reticulum (SR), and thus, facilitated mitochondrial function and the proliferation of vascular smooth muscle cells (VSMCs). However, the physiological role of mitofusin 1 (Mfn1) on Ca²⁺ signaling and mitochondrial function remains unclear. Herein, the roles of Mfn1 and Mfn2 in mitochondrial function underlying Ca²⁺ signaling, ATP production, and cell proliferation were examined in rat aortic smooth muscle A10 cells. Following an arginine vasopressin-induced increase in cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt), Mfn2 siRNA (siMfn2) reduced cytosolic Ca²⁺ removal and mitochondrial Ca²⁺ uptake. However, Mfn1 siRNA (siMfn1) attenuated mitochondrial Ca²⁺ uptake, facilitated Ca²⁺ removal from mitochondria, and resulted in increased [Ca²⁺]_cyt, which was mediated by the downregulation of mitochondrial Ca²⁺ uniporter (MCU) expression and the upregulation of mitochondrial Na⁺/Ca²⁺ exchanger (NCLX) expression. Furthermore, siMfn1 increased the mitochondrial membrane potential, ATP production by adenine nucleotide translocase (ANT), and cell proliferation, whereas siMfn2 exhibited the opposite responses. In conclusion, Mfn1 modulates the expressions of MCU, NCLX, and ANT, and Mfn2 tethers mitochondria to SR, which demonstrates their different mitochondrial functions for Ca²⁺ signaling, ATP production, and the proliferation of VSMCs.

CKII Control of Axonal Plasticity Is Mediated by Mitochondrial Ca2+ via Mitochondrial NCLX

Mitochondrial Ca²⁺ efflux by NCLX is a critical rate-limiting step in mitochondria signaling. We previously showed that NCLX is phosphorylated at a putative Casein Kinase 2 (CKII) site, the serine 271 (S271). Here, we asked if NCLX is regulated by CKII and interrogated the physiological implications of this control. We found that CKII inhibitors down-regulated NCLX-dependent Ca²⁺ transport activity in SH-SY5Y neuronal cells and primary hippocampal neurons. Furthermore, we show that the CKII phosphomimetic mutants on NCLX inhibited (S271A) and constitutively activated (S271D) NCLX transport, respectively, rendering it insensitive to CKII inhibition. These phosphomimetic NCLX mutations also control the allosteric regulation of NCLX by mitochondrial membrane potential (ΔΨm). Since the omnipresent CKII is necessary for modulating the plasticity of the axon initial segment (AIS), we interrogated, in hippocampal neurons, if NCLX is required for this process. Similarly to WT neurons, NCLX-KO neurons can exhibit homeostatic plasticity following M-channel block. However, while WT neurons utilize a CKII-sensitive distal relocation of AIS Na⁺ and Kv7 channels to decrease their intrinsic excitability, we did not observe such translocation in NCLX-KO neurons. Thus, our results indicate that NCLX is regulated by CKII and is a crucial link between CKII signaling and fast neuronal plasticity.

Essential role of the mitochondrial Na+/Ca2+ exchanger NCLX in mediating PDE2-dependent neuronal survival and learning

Impaired phosphodiesterase (PDE) function and mitochondrial Ca2+ (i.e., [Ca2+]m) lead to multiple health syndromes by an unknown pathway. Here, we fluorescently monitor robust [Ca2+]m efflux mediated by the mitochondrial Na+/Ca2+ exchanger NCLX in hippocampal neurons sequentially evoked by caffeine and depolarization. Surprisingly, neuronal depolarization-induced Ca2+ transients alone fail to evoke strong [Ca2+]m efflux in wild-type (WT) neurons. However, pre-treatment with the selective PDE2 inhibitor Bay 60-7550 effectively rescues [Ca2+]m efflux similarly to caffeine. Moreover, PDE2 acts by diminishing mitochondrial cAMP, thus promoting NCLX phosphorylation at its PKA site. We find that the protection of neurons against excitotoxic insults, conferred by PDE2 inhibition in WT neurons, is NCLX dependent. Finally, the administration of Bay 60-7550 enhances new object recognition in WT, but not in NCLX knockout (KO), mice. Our results identify a link between PDE and [Ca2+]m signaling that may provide effective therapy for cognitive and ischemic syndromes.

Control of Ca2+ and metabolic homeostasis by the Na+/Ca2+ exchangers (NCXs) in health and disease

Spatial and temporal control of calcium (Ca²⁺) levels is essential for the background rhythms and responses of living cells to environmental stimuli. Whatever other regulators a given cellular activity may have, localized and wider scale Ca²⁺ events (sparks, transients, and waves) are hierarchical determinants of fundamental processes such as cell contraction, excitability, growth, metabolism and survival. Different cell types express specific channels, pumps and exchangers to efficiently generate and adapt Ca²⁺ patterns to cell requirements. The Na⁺/Ca²⁺ exchangers (NCXs) in particular contribute to Ca²⁺ homeostasis by buffering intracellular Ca²⁺ loads according to the electrochemical gradients of substrate ions - i.e., Ca²⁺ and sodium (Na⁺) - and under a dynamic control of redundant regulatory processes. An interesting feature of NCX emerges from the strict relationship that connects transporter activity with cell metabolism: on the one hand NCX operates under constant control of ATP-dependent regulatory processes, on the other hand the ion fluxes generated through NCX provide mechanistic support for the Na⁺-driven uptake of glutamate and Ca²⁺ influx to fuel mitochondrial respiration. Proof of concept evidence highlights therapeutic potential of preserving a timed and balanced NCX activity in a growing rate of diseases (including excitability, neurodegenerative, and proliferative disorders) because of an improved ability of stressed cells to safely maintain ion gradients and mitochondrial bioenergetics. Here, we will summarize and review recent works that have focused on the pathophysiological roles of NCXs in balancing the two-way relationship between Ca²⁺ signals and metabolism.

The Short-Term Opening of Cyclosporin A-Independent Palmitate/Sr2+-Induced Pore Can Underlie Ion Efflux in the Oscillatory Mode of Functioning of Rat Liver Mitochondria

Mitochondria are capable of synchronized oscillations in many variables, but the underlying mechanisms are still unclear. In this study, we demonstrated that rat liver mitochondria, when exposed to a pulse of Sr2+ ions in the presence of valinomycin (a potassium ionophore) and cyclosporin A (a specific inhibitor of the permeability transition pore complex) under hypotonia, showed prolonged oscillations in K+ and Sr2+ fluxes, membrane potential, pH, matrix volume, rates of oxygen consumption and H2O2 formation. The dynamic changes in the rate of H2O2 production were in a reciprocal relationship with the respiration rate and in a direct relationship with the mitochondrial membrane potential and other indicators studied. The pre-incubation of mitochondria with Ca2+(Sr2+)-dependent phospholipase A2 inhibitors considerably suppressed the accumulation of free fatty acids, including palmitic and stearic acids, and all spontaneous Sr2+-induced cyclic changes. These data suggest that the mechanism of ion efflux from mitochondria is related to the opening of short-living pores, which can be caused by the formation of complexes between Sr2+(Ca2+) and endogenous long-chain saturated fatty acids (mainly, palmitic acid) that accumulate due to the activation of phospholipase A2 by the ions. A possible role for transient palmitate/Ca2+(Sr2+)-induced pores in the maintenance of ion homeostasis and the prevention of calcium overload in mitochondria under pathophysiological conditions is discussed.

Curcumin and NCLX inhibitors share anti-tumoral mechanisms in microsatellite-instability-driven colorectal cancer

BACKGROUND AND AIMS

Recent evidences highlight a role of the mitochondria calcium homeostasis in the development of colorectal cancer (CRC). To overcome treatment resistance, we aimed to evaluate the role of the mitochondrial sodium-calcium-lithium exchanger (NCLX) and its targeting in CRC. We also identified curcumin as a new inhibitor of NCLX.

METHODS

We examined whether curcumin and pharmacological compounds induced the inhibition of NCLX-mediated mitochondrial calcium (mtCa2+) extrusion, the role of redox metabolism in this process. We evaluated their anti-tumorigenic activity in vitro and in a xenograft mouse model. We analyzed NCLX expression and associations with survival in The Cancer Genome Atlas (TCGA) dataset and in tissue microarrays from 381 patients with microsatellite instability (MSI)-driven CRC.

RESULTS

In vitro, curcumin exerted strong anti-tumoral activity through its action on NCLX with mtCa2+ and reactive oxygen species overload associated with a mitochondrial membrane depolarization, leading to reduced ATP production and apoptosis. NCLX inhibition with pharmacological and molecular approaches reproduced the effects of curcumin. NCLX inhibitors decreased CRC tumor growth in vivo. Both transcriptomic analysis of TCGA dataset and immunohistochemical analysis of tissue microarrays demonstrated that higher NCLX expression was associated with MSI status, and for the first time, NCLX expression was significantly associated with recurrence-free survival.

CONCLUSIONS

Our findings highlight a novel anti-tumoral mechanism of curcumin through its action on NCLX and mitochondria calcium overload that could benefit for therapeutic schedule of patients with MSI CRC.

Induction of Paraptosis by Cyclometalated Iridium Complex-Peptide Hybrids and CGP37157 via a Mitochondrial Ca2+ Overload Triggered by Membrane Fusion between Mitochondria and the Endoplasmic Reticulum

We previously reported that a cyclometalated iridium (Ir) complex-peptide hybrid (IPH) 4 functionalized with a cationic KKKGG peptide unit on the 2-phenylpyridine ligand induces paraptosis, a relatively newly found programmed cell death, in cancer cells (Jurkat cells) via the direct transport of calcium (Ca²⁺) from the endoplasmic reticulum (ER) to mitochondria. Here, we describe that CGP37157, an inhibitor of a mitochondrial sodium (Na⁺)/Ca²⁺ exchanger, induces paraptosis in Jurkat cells via intracellular pathways similar to those induced by 4. The findings allow us to suggest that the induction of paraptosis by 4 and CGP37157 is associated with membrane fusion between mitochondria and the ER, subsequent Ca²⁺ influx from the ER to mitochondria, and a decrease in the mitochondrial membrane potential (ΔΨ_m). On the contrary, celastrol, a naturally occurring triterpenoid that had been reported as a paraptosis inducer in cancer cells, negligibly induces mitochondria-ER membrane fusion. Consequently, we conclude that the paraptosis induced by 4 and CGP37157 (termed paraptosis II herein) proceeds via a signaling pathway different from that of the previously known paraptosis induced by celastrol, a process that negligibly involves membrane fusion between mitochondria and the ER (termed paraptosis I herein).

Sensing Intra‐ and Extra‐Cellular Ca2+ in the Islet of Langerhans

Calcium ions (Ca2+) take part in intra‐ and inter‐cellular signaling to mediate cellular functions. Sensing this ubiquitous messenger is instrumental in disentangling the specific functions of cellular sub‐compartments and/or intercellular communications. In this review, the authors first describe intra‐ and inter‐cellular Ca2+ signaling in relation to insulin secretion from the pancreatic islets, and then outline the development of diverse sensors, for example, chemically synthesized indicators, genetically encoded proteins, and ion‐selective microelectrodes, for intra‐ and extra‐cellular sensing of Ca2+. Particular emphasis is placed on emerging approaches in this field, such as low‐affinity Ca2+ indicators and unique Ca2+‐responsive composite materials. The authors conclude by remarking on the challenges and opportunities for further developments in this field, which may facilitate a more comprehensive understanding of Ca2+ signaling within and outside the islets, and its relevance in health and disease.

Physiological and Pathophysiological Roles of Mitochondrial Na+-Ca2+ Exchanger, NCLX, in Hearts

It has been over 10 years since SLC24A6/SLC8B1, coding the Na⁺/Ca²⁺/Li⁺ exchanger (NCLX), was identified as the gene responsible for mitochondrial Na⁺-Ca²⁺ exchange, a major Ca²⁺ efflux system in cardiac mitochondria. This molecular identification enabled us to determine structure–function relationships, as well as physiological/pathophysiological contributions, and our understandings have dramatically increased. In this review, we provide an overview of the recent achievements in relation to NCLX, focusing especially on its heart-specific characteristics, biophysical properties, and spatial distribution in cardiomyocytes, as well as in cardiac mitochondria. In addition, we discuss the roles of NCLX in cardiac functions under physiological and pathophysiological conditions—the generation of rhythmicity, the energy metabolism, the production of reactive oxygen species, and the opening of mitochondrial permeability transition pores.

Impact of mitochondria on local calcium release in murine sinoatrial nodal cells

Roles of mitochondria in sinoatrial nodal cells (SANCs) have not been fully clarified. We have previously demonstrated that mitochondrial Ca²⁺ efflux through the Na⁺-Ca²⁺ exchanger, NCXm, modulates sarcoplasmic reticulum (SR) Ca²⁺ content and automaticity of HL-1 cardiomyocytes. In this study, we extended this line of investigation to clarify the spatial and functional association between mitochondria and local calcium release (LCR) from the SR in murine SANCs. High-speed two dimensional (2D) and confocal line-scan imaging of SANCs revealed that LCRs in the early phase of the Ca²⁺ transient cycle length (CL) appeared with a higher probability near mitochondria. Although LCR increased toward the late phase of CL, no significant difference was noted in the occurrence of late LCRs near and distant from mitochondria. LCRs, especially in the late phase of CL, induced temporal and spatial heterogeneity of the Ca²⁺ transient amplitude. Attenuating mitochondrial Ca²⁺ efflux using an NCXm inhibitor, CGP-37157 (1 μM), reduced the amplitude, duration and size of LCR. It also attenuated early LCR occurrence, and simultaneously prolonged LCR period and CL. Additionally, CGP-37157 reduced caffeine-induced Ca²⁺ transient. Therefore, the inhibitory effect on LCR was attributable to the reduction of the SR Ca²⁺ content through NCXm inhibition. No obvious off-target effects of 1 μM CGP-37157 were found on T- and L-type voltage-gated Ca²⁺ currents and hyperpolarization-activated inward current. Taken together, these results suggest that mitochondria are involved in LCR generation by modulating the SR Ca²⁺ content through NCXm-mediated Ca²⁺ efflux in murine SANCs.

The Molecular Effects of Dietary Acid Load on Metabolic Disease (The Cellular PasaDoble: The Fast-Paced Dance of pH Regulation)

Metabolic diseases are becoming more common and more severe in populations adhering to western lifestyle. Since metabolic conditions are highly diet and lifestyle dependent, it is suggested that certain diets are the cause for a wide range of metabolic dysfunctions. Oxidative stress, excess calcium excretion, inflammation, and metabolic acidosis are common features in the origins of most metabolic disease. These primary manifestations of "metabolic syndrome" can lead to insulin resistance, diabetes, obesity, and hypertension. Further complications of the conditions involve kidney disease, cardiovascular disease, osteoporosis, and cancers. Dietary analysis shows that a modern "Western-style" diet may facilitate a disruption in pH homeostasis and drive disease progression through high consumption of exogenous acids. Because so many physiological and cellular functions rely on acid-base reactions and pH equilibrium, prolonged exposure of the body to more acids than can effectively be buffered, by chronic adherence to poor diet, may result in metabolic stress followed by disease. This review addresses relevant molecular pathways in mammalian cells discovered to be sensitive to acid - base equilibria, their cellular effects, and how they can cascade into an organism-level manifestation of Metabolic Syndromes. We will also discuss potential ways to help mitigate this digestive disruption of pH and metabolic homeostasis through dietary change.

Mitochondrial TFAM as a Signaling Regulator between Cellular Organelles: A Perspective on Metabolic Diseases

Tissues actively involved in energy metabolism are more likely to face metabolic challenges from bioenergetic substrates and are susceptible to mitochondrial dysfunction, leading to metabolic diseases. The mitochondria receive signals regarding the metabolic states in cells and transmit them to the nucleus or endoplasmic reticulum (ER) using calcium (Ca2+) for appropriate responses. Overflux of Ca2+ in the mitochondria or dysregulation of the signaling to the nucleus and ER could increase the incidence of metabolic diseases including insulin resistance and type 2 diabetes mellitus. Mitochondrial transcription factor A (Tfam) may regulate Ca2+ flux via changing the mitochondrial membrane potential and signals to other organelles such as the nucleus and ER. Since Tfam is involved in metabolic function in the mitochondria, here, we discuss the contribution of Tfam in coordinating mitochondria-ER activities for Ca2+ flux and describe the mechanisms by which Tfam affects mitochondrial Ca2+ flux in response to metabolic challenges.

CaMKII activation in early diabetic hearts induces altered sarcoplasmic reticulum-mitochondria signaling

Prediabetic myocardium, induced by fructose-rich diet (FRD), is prone to increased sarcoplasmic reticulum (SR)-Ca²⁺ leak and arrhythmias due to increased activity of the Ca²⁺/calmodulin protein kinase II (CaMKII). However, little is known about the role of SR-mitochondria microdomains, mitochondrial structure, and mitochondrial metabolisms. To address this knowledge gap we measured SR-mitochondrial proximity, intracellular Ca²⁺, and mitochondrial metabolism in wild type (WT) and AC3-I transgenic mice, with myocardial-targeted CaMKII inhibition, fed with control diet (CD) or with FRD. Confocal images showed significantly increased spontaneous Ca²⁺ release events in FRD vs. CD WT cardiomyocytes. [³H]-Ryanodine binding assay revealed higher [³H]Ry binding in FRD than CD WT hearts. O₂ consumption at State 4 and hydrogen peroxide (H₂O₂) production rate were increased, while respiratory control rate (RCR) and Ca²⁺ retention capacity (CRC) were decreased in FRD vs. CD WT isolated mitochondria. Transmission Electron Microscopy (TEM) images showed increased proximity at the SR-mitochondria microdomains, associated with increased tethering proteins, Mfn2, Grp75, and VDAC in FRD vs. CD WT. Mitochondria diameter was decrease and roundness and density were increased in FRD vs. CD WT specimens. The fission protein, Drp1 was significantly increased while the fusion protein, Opa1 was unchanged in FRD vs. CD WT hearts. These differences were prevented in AC3-I mice. We conclude that SR-mitochondria microdomains are subject to CaMKII-dependent remodeling, involving SR-Ca²⁺ leak and mitochondria fission, in prediabetic mice induced by FRD. We speculate that CaMKII hyperactivity induces SR-Ca²⁺ leak by RyR2 activation which in turn increases mitochondria Ca²⁺ content due to the enhanced SR-mitochondria tethering, decreasing CRC.

Effects of various calcium transporters on mitochondrial Ca2+ changes and oocyte maturation

Ca2+ participates in many important cellular processes, but the underlying mechanisms are still poorly understood, especially during oocyte maturation. First, we confirmed that calcium in the culture medium was essential for oocyte maturation. Next, various inhibitors of Ca2+ channels were applied to investigate their roles in mitochondrial Ca2+ changes and oocyte maturation. Our results showed that Trmp7, Orai, T-type Ca2+ channels and Na+ /Ca2+ exchanger complex (NCLX) were important for oocyte maturation. Trmp7 inhibition delayed germinal vesicle breakdown. Orai and NCLX inhibition significantly weakened the distribution of mitochondrial Ca2+ around the nucleus compared to the Ctrl group. Interestingly, even T-type Ca2+ channels-specific inhibitor Mibefradil blocked germinal vesicle breakdown; mitochondrial Ca2+ surrounding the nucleus still was maintained at a high level without spindle formation. Two calcium transporter inhibitors, Thapsigargin and Ruthenium Red, which have been confirmed to inhibit oocyte activation, did not significantly affect oocyte maturation. Increasing the knowledge of calcium transport may provide a basis to build on for improving oocyte in vitro maturation in human assisted reproduction clinics.

An integrative view on vacuolar pH homeostasis in Arabidopsis thaliana: Combining mathematical modeling and experimentation

The acidification of plant vacuoles is of great importance for various physiological processes, as a multitude of secondary active transporters utilize the proton gradient established across the vacuolar membrane. Vacuolar-type H⁺ -translocating ATPases and a pyrophosphatase are thought to enable vacuoles to accumulate protons against their electrochemical potential. However, recent studies pointed to the ATPase located at the trans-Golgi network/early endosome (TGN/EE) to contribute to vacuolar acidification in a manner not understood as of now. Here, we combined experimental data and computational modeling to test different hypotheses for vacuolar acidification mechanisms. For this, we analyzed different models with respect to their ability to describe existing experimental data. To better differentiate between alternative acidification mechanisms, new experimental data have been generated. By fitting the models to the experimental data, we were able to prioritize the hypothesis in which vesicular trafficking of Ca²⁺ /H⁺ -antiporters from the TGN/EE to the vacuolar membrane and the activity of ATP-dependent Ca²⁺ -pumps at the tonoplast might explain the residual acidification observed in Arabidopsis mutants defective in vacuolar proton pump activity. The presented modeling approach provides an integrative perspective on vacuolar pH regulation in Arabidopsis and holds potential to guide further experimental work.

Sprinkling salt on mitochondria: The metabolic and pathophysiological roles of mitochondrial Na+ signaling mediated by NCLX

NCLX, the mitochondrial Na+/Ca2+ transporter is a key player in Ca2+ signaling. However, its role in Na+ signaling is poorly understood. In this review we focus on Na+ signaling by NCLX, and discuss recent physiological and pathophysiological roles attributed to the Na+ influx into mitochondria.

Astrocytic mitochondria in adult mouse brain slices show spontaneous calcium influx events with unique properties

Astrocytes govern critical aspects of brain function via spontaneous calcium signals in their soma and processes. A significant proportion of these spontaneous astrocytic calcium events are associated with mitochondria, however, the extent, sources, or kinetics of astrocytic mitochondrial calcium influx have not been studied in the adult mouse brain. To measure calcium influx into astrocytic mitochondria in situ, we generated an adeno-associated virus (AAV) with the astrocyte-specific GfaABC₁D promoter driving expression of the genetically encoded calcium indicator, GCaMP6f tagged to mito7, a mitochondrial matrix targeted signal sequence. Using this construct, we observed AAV-mediated expression of GCaMP6f in adult mouse astrocytic mitochondria that co-localized with MitoTracker deep red (MTDR) in the dorsolateral striatum (DLS) and in the hippocampal stratum radiatum (HPC). Astrocytic mitochondria co-labeled with MTDR and GCaMP6f displayed robust, spontaneous calcium influx events in situ, with subcellular differences in calcium influx kinetics between somatic, branch, and branchlet mitochondria, and inter-regional differences between mitochondria in DLS and HPC astrocytes. Calcium influx into astrocytic mitochondria was strongly dependent on endoplasmic reticulum calcium stores, but did not require the mitochondrial calcium uniporter, MCU. Exposure to either glutamate, D1 or D2 dopamine receptor agonists increased calcium influx in some mitochondria, while simultaneously decreasing calcium influx in other mitochondria from the same astrocyte. These findings show that astrocytic mitochondria possess unique properties with regard to their subcellular morphology, mechanisms of calcium influx, and responses to neurotransmitter receptor agonists. Our results have important implications for understanding the role of astrocytic mitochondria during pathological processes.

Structural and Functional Characterization of a Nav1.5-Mitochondrial Couplon

RATIONALE

The cardiac sodium channel NaV1.5 has a fundamental role in excitability and conduction. Previous studies have shown that sodium channels cluster together in specific cellular subdomains. Their association with intracellular organelles in defined regions of the myocytes, and the functional consequences of that association, remain to be defined.

OBJECTIVE

To characterize a subcellular domain formed by sodium channel clusters in the crest region of the myocytes and the subjacent subsarcolemmal mitochondria.

METHODS AND RESULTS

Through a combination of imaging approaches including super-resolution microscopy and electron microscopy we identified, in adult cardiac myocytes, a NaV1.5 subpopulation in close proximity to subjacent subsarcolemmal mitochondria; we further found that subjacent subsarcolemmal mitochondria preferentially host the mitochondrial NCLX (Na+/Ca2+ exchanger). This anatomic proximity led us to investigate functional changes in mitochondria resulting from sodium channel activity. Upon TTX (tetrodotoxin) exposure, mitochondria near NaV1.5 channels accumulated more Ca2+ and showed increased reactive oxygen species production when compared with interfibrillar mitochondria. Finally, crosstalk between NaV1.5 channels and mitochondria was analyzed at a transcriptional level. We found that SCN5A (encoding NaV1.5) and SLC8B1 (which encode NaV1.5 and NCLX, respectively) are negatively correlated both in a human transcriptome data set (Genotype-Tissue Expression) and in human-induced pluripotent stem cell-derived cardiac myocytes deficient in SCN5A.

CONCLUSIONS

We describe an anatomic hub (a couplon) formed by sodium channel clusters and subjacent subsarcolemmal mitochondria. Preferential localization of NCLX to this domain allows for functional coupling where the extrusion of Ca2+ from the mitochondria is powered, at least in part, by the entry of sodium through NaV1.5 channels. These results provide a novel entry-point into a mechanistic understanding of the intersection between electrical and structural functions of the heart.

The journey of Ca2+ through the cell - pulsing through the network of ER membrane contact sites

Calcium is the third most abundant metal on earth, and the fundaments of its homeostasis date back to pre-eukaryotic life forms. In higher organisms, Ca²⁺ serves as a cofactor for a wide array of (enzymatic) interactions in diverse cellular contexts and constitutes the most important signaling entity in excitable cells. To enable responsive behavior, cytosolic Ca²⁺ concentrations are kept low through sequestration into organellar stores, particularly the endoplasmic reticulum (ER), but also mitochondria and lysosomes. Specific triggers are then used to instigate a local release of Ca²⁺ on demand. Here, communication between organelles comes into play, which is accomplished through intimate yet dynamic contacts, termed membrane contact sites (MCSs). The field of MCS biology in relation to cellular Ca²⁺ homeostasis has exploded in recent years. Taking advantage of this new wealth of knowledge, in this Review, we invite the reader on a journey of Ca²⁺ flux through the ER and its associated MCSs. New mechanistic insights and technological advances inform the narrative on Ca²⁺ acquisition and mobilization at these sites of communication between organelles, and guide the discussion of their consequences for cellular physiology.

A calcium optimum for cytotoxic T lymphocyte and natural killer cell cytotoxicity

Cytotoxic T lymphocytes (CTL) and natural killer (NK) cells are required for host defense. They destroy malignant target cells like cancer cells. Among metal cations, Ca²⁺ plays a prescinded role for CTL and NK cytotoxicity as it is the only cation used as ubiquitous second messenger. Measuring intracellular Ca²⁺ concentrations [Ca²⁺]_int in single cells has greatly changed our understanding of Ca²⁺ signaling. Yet, comparing the role of Ca²⁺ in the pre-[Ca²⁺]_int and [Ca²⁺]_int measurement era reveals that even in the pre-[Ca²⁺]_int measurement era (before 1980), the functions of Ca²⁺ and some other metal cations for the cytotoxic immune response were well established. It was even shown that Ca²⁺ influx across the plasma membrane but not Ca²⁺ release from intracellular sources is relevant for lymphocyte cytotoxicity and that very little Ca²⁺ is needed for efficient lymphocyte cytotoxicity against cancer cells. In the [Ca²⁺]_int measurement era after 1980, many of the important findings were better and more quantitatively refined and in addition the molecules important for Ca²⁺ transport were defined. The unexpected finding that there is a Ca²⁺ optimum of CTL and NK cell cytotoxicity deserves some attention and may be important for anti-cancer therapy.

Mitochondrial Ca2 + Is Related to Mitochondrial Activity and Dynamic Events in Mouse Oocytes

Mitochondrial energy insufficiency is strongly associated with oocyte activation disorders. Ca²⁺, especially that in the mitochondrial matrix, plays a pivotal role in mitochondrial energy supplementation, but the underlying mechanisms are still only poorly understood. An encoded mitochondrial matrix Ca²⁺ probe (Mt-GCaMP6s) was introduced to observe mitochondrial Ca²⁺ ([Ca²⁺]_m) dynamic changes during oocyte maturation and activation. We found that active mitochondria surrounding the nucleus showed a higher [Ca²⁺]_m than those distributed in the cortex during oocyte maturation. During oocyte partheno-activation, the patterns of Ca²⁺ dynamic changes were synchronous in the cytoplasm and mitochondria. Such higher concentration of mitochondrial matrix Ca²⁺ was closely related to the distribution of mitochondrial calcium uptake (MICU) protein. We further showed that higher [Ca²⁺]_m mitochondria around the chromosomes in oocytes might have a potential role in stimulating mitochondrial energy for calmodulin-responsive oocyte spindle formation, while synchronizing Ca²⁺ functions in the cytoplasm and nuclear area are important for oocyte activation.

Regulation of Cell Death by Mitochondrial Transport Systems of Calcium and Bcl-2 Proteins

Mitochondria represent the fundamental system for cellular energy metabolism, by not only supplying energy in the form of ATP, but also by affecting physiology and cell death via the regulation of calcium homeostasis and the activity of Bcl-2 proteins. A lot of research has recently been devoted to understanding the interplay between Bcl-2 proteins, the regulation of these interactions within the cell, and how these interactions lead to the changes in calcium homeostasis. However, the role of Bcl-2 proteins in the mediation of mitochondrial calcium homeostasis, and therefore the induction of cell death pathways, remain underestimated and are still not well understood. In this review, we first summarize our knowledge about calcium transport systems in mitochondria, which, when miss-regulated, can induce necrosis. We continue by reviewing and analyzing the functions of Bcl-2 proteins in apoptosis. Finally, we link these two regulatory mechanisms together, exploring the interactions between the mitochondrial Ca2+ transport systems and Bcl-2 proteins, both capable of inducing cell death, with the potential to determine the cell death pathway-either the apoptotic or the necrotic one.

The Physiological and Pathological Roles of Mitochondrial Calcium Uptake in Heart

Calcium ion (Ca²⁺) plays a critical role in the cardiac mitochondria function. Ca²⁺ entering the mitochondria is necessary for ATP production and the contractile activity of cardiomyocytes. However, excessive Ca²⁺ in the mitochondria results in mitochondrial dysfunction and cell death. Mitochondria maintain Ca²⁺ homeostasis in normal cardiomyocytes through a comprehensive regulatory mechanism by controlling the uptake and release of Ca²⁺ in response to the cellular demand. Understanding the mechanism of modulating mitochondrial Ca²⁺ homeostasis in the cardiomyocyte could bring new insights into the pathogenesis of cardiac disease and help developing the strategy to prevent the heart from damage at an early stage. In this review, we summarized the latest findings in the studies on the cardiac mitochondrial Ca²⁺ homeostasis, focusing on the regulation of mitochondrial calcium uptake, which acts as a double-edged sword in the cardiac function. Specifically, we discussed the dual roles of mitochondrial Ca²⁺ in mitochondrial activity and the impact on cardiac function, the molecular basis and regulatory mechanisms, and the potential future research interest.

Total Matrix Ca2+ Modulates Ca2+ Efflux via the Ca2+/H+ Exchanger in Cardiac Mitochondria

Mitochondrial Ca²⁺ handling is accomplished by balancing Ca²⁺ uptake, primarily via the Ru360-sensitive mitochondrial calcium uniporter (MCU), Ca²⁺ buffering in the matrix and Ca²⁺ efflux mainly via Ca²⁺ ion exchangers, such as the Na⁺/Ca²⁺ exchanger (NCLX) and the Ca²⁺/H⁺ exchanger (CHE). The mechanism of CHE in cardiac mitochondria is not well-understood and its contribution to matrix Ca²⁺ regulation is thought to be negligible, despite higher expression of the putative CHE protein, LETM1, compared to hepatic mitochondria. In this study, Ca²⁺ efflux via the CHE was investigated in isolated rat cardiac mitochondria and permeabilized H9c2 cells. Mitochondria were exposed to (a) increasing matrix Ca²⁺ load via repetitive application of a finite CaCl₂ bolus to the external medium and (b) change in the pH gradient across the inner mitochondrial membrane (IMM). Ca²⁺ efflux at different matrix Ca²⁺ loads was revealed by inhibiting Ca²⁺ uptake or reuptake with Ru360 after increasing number of CaCl₂ boluses. In Na⁺-free experimental buffer and with Ca²⁺ uptake inhibited, the rate of Ca²⁺ efflux and steady-state free matrix Ca²⁺ [mCa²⁺]_ss increased as the number of administered CaCl₂ boluses increased. ADP and cyclosporine A (CsA), which are known to increase Ca²⁺ buffering while maintaining a constant [mCa²⁺]_ss, decreased the rate of Ca²⁺ efflux via the CHE, with a significantly greater decrease in the presence of ADP. ADP also increased Ca²⁺ buffering rate and decreased [mCa²⁺]_ss. A change in the pH of the external medium to a more acidic value from 7.15 to 6.8∼6.9 caused a twofold increase in the Ca²⁺ efflux rate, while an alkaline change in pH from 7.15 to 7.4∼7.5 did not change the Ca²⁺ efflux rate. In addition, CHE activation was associated with membrane depolarization. Targeted transient knockdown of LETM1 in permeabilized H9c2 cells modulated Ca²⁺ efflux. The results indicate that Ca²⁺ efflux via the CHE in cardiac mitochondria is modulated by acidic buffer pH and by total matrix Ca²⁺. A mechanism is proposed whereby activation of CHE is sensitive to changes in both the matrix Ca²⁺ buffering system and the matrix free Ca²⁺ concentration.

Interrelation between ROS and Ca2+ in aging and age-related diseases

Calcium (Ca2+) and reactive oxygen species (ROS) are versatile signaling molecules coordinating physiological and pathophysiological processes. While channels and pumps shuttle Ca2+ ions between extracellular space, cytosol and cellular compartments, short-lived and highly reactive ROS are constantly generated by various production sites within the cell. Ca2+ controls membrane potential, modulates mitochondrial adenosine triphosphate (ATP) production and affects proteins like calcineurin (CaN) or calmodulin (CaM), which, in turn, have a wide area of action. Overwhelming Ca2+ levels within mitochondria efficiently induce and trigger cell death. In contrast, ROS comprise a diverse group of relatively unstable molecules with an odd number of electrons that abstract electrons from other molecules to gain stability. Depending on the type and produced amount, ROS act either as signaling molecules by affecting target proteins or as harmful oxidative stressors by damaging cellular components. Due to their wide range of actions, it is little wonder that Ca2+ and ROS signaling pathways overlap and impact one another. Growing evidence suggests a crucial implication of this mutual interplay on the development and enhancement of age-related disorders, including cardiovascular and neurodegenerative diseases as well as cancer.

Effects of mitochondria-associated Ca2+ transporters suppression on oocyte activation

Oocyte activation deficiency leads to female infertility. [Ca²⁺ ]_i oscillations are required for mitochondrial energy supplement transition from the resting to the excited state, but the underlying mechanisms are still very little known. Three mitochondrial Ca²⁺ channels, Mitochondria Calcium Uniporter (MCU), Na⁺ /Ca²⁺ Exchanger (NCLX) and Voltage-dependent Ca²⁺ Channel (VDAC), were deactivated by inhibitors RU360, CGP37157 and Erastin, respectively. Both Erastin and CGP37157 inhibited mitochondrial activity significantly while attenuating [Ca²⁺ ]_i and [Ca²⁺ ]_m oscillations, which caused developmental block of pronuclear formation. Thus, NCLX and VDAC are two mitochondria-associated Ca²⁺ transporter proteins regulating oocyte activation, which may be used as potential targets to treat female infertility. SIGNIFICANCE OF THE STUDY: NCLX and VDAC are two mitochondria-associated Ca²⁺ transporter proteins regulating oocyte activation.

Calcium Permeable Channels in Cancer Hallmarks

Cancer, the second cause of death worldwide, is characterized by several common criteria, known as the “cancer hallmarks” such as unrestrained cell proliferation, cell death resistance, angiogenesis, invasion and metastasis. Calcium permeable channels are proteins present in external and internal biological membranes, diffusing Ca²⁺ ions down their electrochemical gradient. Numerous physiological functions are mediated by calcium channels, ranging from intracellular calcium homeostasis to sensory transduction. Consequently, calcium channels play important roles in human physiology and it is not a surprise the increasing number of evidences connecting calcium channels disorders with tumor cells growth, survival and migration. Multiple studies suggest that calcium signals are augmented in various cancer cell types, contributing to cancer hallmarks. This review focuses in the role of calcium permeable channels signaling in cancer with special attention to the mechanisms behind the remodeling of the calcium signals. Transient Receptor Potential (TRP) channels and Store Operated Channels (SOC) are the main extracellular Ca²⁺ source in the plasma membrane of non-excitable cells, while inositol trisphosphate receptors (IP₃R) are the main channels releasing Ca²⁺ from the endoplasmic reticulum (ER). Alterations in the function and/or expression of these calcium channels, as wells as, the calcium buffering by mitochondria affect intracellular calcium homeostasis and signaling, contributing to the transformation of normal cells into their tumor counterparts. Several compounds reported to counteract several cancer hallmarks also modulate the activity and/or the expression of these channels including non-steroidal anti-inflammatory drugs (NSAIDs) like sulindac and aspirin, and inhibitors of polyamine biosynthesis, like difluoromethylornithine (DFMO). The possible role of the calcium permeable channels targeted by these compounds in cancer and their action mechanism will be discussed also in the review.

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.

Knockout of the Mitochondrial Calcium Uniporter Strongly Suppresses Stimulus-Metabolism Coupling in Pancreatic Acinar Cells but Does Not Reduce Severity of Experimental Acute Pancreatitis

Acute pancreatitis is a frequent disease that lacks specific drug treatment. Unravelling the molecular mechanisms of acute pancreatitis is essential for the development of new therapeutics. Several inducers of acute pancreatitis trigger sustained Ca²⁺ increases in the cytosol and mitochondria of pancreatic acinar cells. The mitochondrial calcium uniporter (MCU) mediates mitochondrial Ca²⁺ uptake that regulates bioenergetics and plays an important role in cell survival, damage and death. Aberrant Ca²⁺ signaling and mitochondrial damage in pancreatic acinar cells have been implicated in the initiation of acute pancreatitis. The primary aim of this study was to assess the involvement of the MCU in experimental acute pancreatitis. We found that pancreatic acinar cells from MCU^-/- mice display dramatically reduced mitochondrial Ca²⁺ uptake. This is consistent with the drastic changes of stimulus-metabolism coupling, manifested by the reduction of mitochondrial NADH/FAD⁺ responses to cholecystokinin and in the decrease of cholecystokinin-stimulated oxygen consumption. However, in three experimental models of acute pancreatitis (induced by caerulein, taurolithocholic acid 3-sulfate or palmitoleic acid plus ethanol), MCU knockout failed to reduce the biochemical and histological changes characterizing the severity of local and systemic damage. A possible explanation of this surprising finding is the redundancy of damaging mechanisms activated by the inducers of acute pancreatitis.

Membrane current evoked by mitochondrial Na+-Ca2+ exchange in mouse heart

The electrogenicity of mitochondrial Na+-Ca2+ exchange (NCXm) had been controversial and no membrane current through it had been reported. We succeeded for the first time in recording NCXm-mediated currents using mitoplasts derived from mouse ventricle. Under conditions that K+, Cl-, and Ca2+ uniporter currents were inhibited, extra-mitochondrial Na+ induced inward currents with 1 μM Ca2+ in the pipette. The half-maximum concentration of Na+ was 35.6 mM. The inward current was diminished without Ca2+ in the pipette, and was augmented with 10 μM Ca2+. The Na+-induced inward currents were largely inhibited by CGP-37157, an NCXm blocker. However, the reverse mode of NCXm, which should be detected as an outward current, was hardly induced by extra-mitochondrial application of Ca2+ with Na+ in the pipette. It was concluded that NCXm is electrogenic. This property may be advantageous for facilitating Ca2+ extrusion from mitochondria, which has large negative membrane potential.

Allosteric Regulation of NCLX by Mitochondrial Membrane Potential Links the Metabolic State and Ca2+ Signaling in Mitochondria

Calcium is a key regulator of mitochondrial function under both normal and pathological conditions. The mechanisms linking metabolic activity to mitochondrial Ca²⁺ signaling remain elusive, however. Here, by monitoring mitochondrial Ca²⁺ transients while manipulating mitochondrial membrane potential (ΔΨ_m), we found that mild fluctuations in ΔΨ_m, which do not affect Ca²⁺ influx, are sufficient to strongly regulate NCLX, the major efflux pathway of Ca²⁺ from the mitochondria. Phosphorylation of NCLX or expression of phosphomimicking mutant (S258D) rescued NCLX activity from ΔΨ_m-driven allosteric inhibition. By screening ΔΨ_m sensitivity of NCLX mutants, we also identified amino acid residues that, through functional interaction with Ser258, control NCLX regulation. Finally, we find that glucose-driven ΔΨ_m changes in pancreatic β-cells control mitochondrial Ca²⁺ signaling primarily via NCLX regulation. Our results identify a feedback control between metabolic activity and mitochondrial Ca²⁺ signaling and the "safety valve" NCLX phosphorylation that can rescue Ca²⁺ efflux in depolarized mitochondria.

Quantitative determination of cellular [Na+] by fluorescence lifetime imaging with CoroNaGreen

Meyer et al. establish the suitability of the sodium-sensitive indicator dye CoroNaGreen for fluorescence lifetime imaging inside cells. This approach represents a valuable tool for quantitative and dynamic determination of intracellular sodium concentrations independent of dye concentration.

Fluorescence lifetime imaging microscopy (FLIM) with fluorescent ion sensors enables the measurement of ion concentrations based on the detection of photon emission events after brief excitation with a pulsed laser source. In contrast to intensity-based imaging, it is independent of dye concentration, photobleaching, or focus drift and has thus been successfully employed for quantitative analysis of, e.g., calcium levels in different cell types and cellular microdomains. Here, we tested the suitability of CoroNaGreen for FLIM-based determination of sodium concentration ([Na⁺]) inside cells. In vitro measurements confirmed that fluorescence lifetimes of CoroNaGreen (CoroNaFL) increased with increasing [Na⁺]. Moreover, CoroNaFL was largely independent of changes in potassium concentration or viscosity. Changes in pH slightly affected FL in the acidic range (pH ≤ 5.5). For intracellular determination of [Na⁺], HEK293T cells were loaded with the membrane-permeable form of CoroNaGreen. Fluorescence decay curves of CoroNaGreen, derived from time-correlated single-photon counting, were approximated by a bi-exponential decay. In situ calibrations revealed a sigmoidal dependence of CoroNaFL on [Na⁺] between 0 and 150 mM, exhibiting an apparent K_d of ∼80 mM. Based on these calibrations, a [Na⁺] of 17.6 mM was determined in the cytosol. Cellular nuclei showed a significantly lower [Na⁺] of 13.0 mM, whereas [Na⁺] in perinuclear regions was significantly higher (26.5 mM). Metabolic inhibition or blocking the Na⁺/K⁺-ATPase by removal of extracellular K⁺ caused significant [Na⁺] increases in all cellular subcompartments. Using an alternative approach for data analysis (“Ratio FLIM”) increased the temporal resolution and revealed a sequential response to K⁺ removal, with cytosolic [Na⁺] increasing first, followed by the nucleus and finally the perinuclear regions. Taken together, our results show that CoroNaGreen is suitable for dynamic, FLIM-based determination of intracellular [Na⁺]. This approach thus represents a valuable tool for quantitative determination of [Na⁺] and changes thereof in different subcellular compartments.

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.

The Ionotropic P2X4 Receptor has Unique Properties in the Heart by Mediating the Negative Chronotropic Effect of ATP While Increasing the Ventricular Inotropy

Background: Mounting evidence indicate that reducing the sinoatrial node (SAN) activity may be a useful therapeutic strategy to control of heart failure. Purines, like ATP and its metabolite adenosine, consistently reduce the SAN spontaneous activity leading to negative cardiac chronotropy, with variable effects on the force of myocardial contraction (inotropy). Apart from adenosine A₁ receptors, the human SAN expresses high levels of ATP-sensitive ionotropic P2X4 receptors (P2X4R), yet their cardiac role is unexplored. Methods: Here, we investigated the activity of P2 purinoceptors on isolated spontaneously beating atria (chronotropy) and on 2 Hz-paced right ventricular (RV, inotropy) strips from Wistar rats. Results: ATP (pEC ₅₀ = 4.05) and its stable analogue ATPγS (pEC ₅₀ = 4.69) concentration-dependently reduced atrial chronotropy. Inhibition of ATP breakdown into adenosine by NTPDases with POM-1 failed to modify ATP-induced negative chronotropy. The effect of ATP on atrial rate was attenuated by a broad-spectrum P2 antagonist, PPADS, as well as by 5-BDBD, which selectively blocks the P2X4R subtype; however, no effect was observed upon blocking the A₁ receptor with DPCPX. The P2X4R positive allosteric modulator, ivermectin, increased the negative chronotropic response of ATP. Likewise, CTP, a P2X agonist that does not generate adenosine, replicated the P2X4R-mediated negative chronotropism of ATP. Inhibition of the Na⁺/Ca²⁺ exchanger (NCX) with KB-R7943 and ORM-10103, but not blockage of the HCN channel with ZD7288, mimicked the effect of the P2X4R blocker, 5-BDBD. In paced RV strips, ATP caused a mild negative inotropic effect, which magnitude was 2 to 3-fold increased by 5-BDBD and KB-R7943. Immunofluorescence confocal microscopy studies confirm that cardiomyocytes of the rat SAN and RV co-express P2X4R and NCX1 proteins. Conclusions: Data suggest that activation of ATP-sensitive P2X4R slows down heart rate by reducing the SAN activity while increasing the magnitude of ventricular contractions. The mechanism underlying the dual effect of ATP in the heart may involve inhibition of intracellular Ca²⁺-extrusion by bolstering NCX function in the reverse mode. Thus, targeting the P2X4R activation may create novel well-tolerated heart-rate lowering drugs with potential benefits in patients with deteriorated ventricular function.