Mitochondrial Ca2+ channels: Great unknowns with important functions

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).

Mitochondrial ion channels in aging and related diseases

Transport of materials and information across cellular boundaries, such as plasma, mitochondrial and nuclear membranes, happens mainly through varieties of ion channels and pumps. Various biophysical and biochemical processes play vital roles. The underlying mechanisms and associated phenomenological lipid membrane transports are linked directly or indirectly to the cell health condition. Mitochondrial membranes (mitochondrial outer membrane (MOM) and mitochondrial inner membrane (MIM)) host crucial cellular processes. Their malfunction is often found responsible for the rise of cell-originated diseases, including cancer, Alzheimer's, neurodegenerative disease, etc. A large number of ion channels active across MOM and MIM are known to belong to vital cell-based structures found to be linked directly to cellular signaling. Hence their malfunctions are often found to contribute to abnormalities in intracellular communication, which may even be associated with the rise of various diseases. In this article, the aim is to pinpoint ion channels that are directly or indirectly linked to especially aging and related abnormalities in health conditions. An attempt has been made to address the natural structures of these channels, their mutated conditions, and the ways we may cause interventions in their malfunctioning. The malfunction of ion channel subunits, including especially various proteins, involved directly in channel formation and/or indirectly in channel stabilization, leads to the rise of various channel-specific diseases, which are known as channelopathies. Channelopathies in aging will be discussed briefly. This mini-review may be found as an important reference for drug discovery scientists dealing with aging-related diseases.

Insights on meat quality from combining traditional studies and proteomics

Following a century of major discoveries on the mechanisms determining meat colour and tenderness using traditional scientific methods, further research into complex and interactive factors contributing to variations in meat quality is increasingly being based on data-driven "omics" approaches such as proteomics. Using two recent meta-analyses of proteomics studies on beef colour and tenderness, this review examines how knowledge of the mechanisms and factors underlying variations in these meat qualities can be both confirmed and extended by data-driven approaches. While proteomics seems to overlook some sources of variations in beef toughness, it highlights the role of post-mortem energy metabolism in setting the conditions for development of meat colour and tenderness, and also points to the complex interplay of energy metabolism, calcium regulation and mitochondrial metabolism. In using proteomics as a future tool for explaining variations in meat quality, the need for confirmation by further hypothesis-driven experimental studies of post-hoc explanations of why certain proteins are biomarkers of beef quality in data-driven studies is emphasised.

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

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

HIGHLIGHTS-

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

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

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

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

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

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

Mitochondrial Dysfunction and Synaptic Transmission Failure in Alzheimer's Disease

Alzheimer's disease (AD) is a chronic neurodegenerative disorder, in which multiple risk factors converge. Despite the complexity of the etiology of the disease, synaptic failure is the pathological basis of cognitive impairment, the cardinal sign of AD. Decreased synaptic density, compromised synaptic transmission, and defected synaptic plasticity are hallmark synaptic pathologies accompanying AD. However, the mechanisms by which synapses are injured in AD-related conditions have not been fully elucidated. Mitochondria are a critical organelle in neurons. The pivotal role of mitochondria in supporting synaptic function and the concomitant occurrence of mitochondrial dysfunction with synaptic stress in postmortem AD brains as well as AD animal models seem to lend the credibility to the hypothesis that mitochondrial defects underlie synaptic failure in AD. This concept is further strengthened by the protective effect of mitochondrial medicine on synaptic function against the toxicity of amyloid-β, a key player in the pathogenesis of AD. In this review, we focus on the association between mitochondrial dysfunction and synaptic transmission deficits in AD. Impaired mitochondrial energy production, deregulated mitochondrial calcium handling, excess mitochondrial reactive oxygen species generation and release play a crucial role in mediating synaptic transmission deregulation in AD. The understanding of the role of mitochondrial dysfunction in synaptic stress may lead to novel therapeutic strategies for the treatment of AD through the protection of synaptic transmission by targeting to mitochondrial deficits.

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

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

CALIX[4]ARENES AS MODULATORS OF ENERGY-DEPENDENT CA2+_ACCUMULATION AND FUNCTIONING OF THE ELECTRON TRANSPORT CHAIN IN SMOOTH MUSCLE MITOCHONDRIA

The influence of supramolecular macrocyclic compounds calix[4]arenes (C-97, C-99, C-107) at a concentration of 100 nM in the process of energy-dependent Ca²⁺-transport in isolated mitochondria of smooth muscle, as well as autofluorescence mitochondrial coenzyme NADH, FAD and hydrodynamic diameter of these organelles was investigated. Using Ca²⁺-sensitive fluorescent dye Fluo-4 AM it was shown that the selected calix[4]arenes can suppress energy-dependent accumulation of Ca²⁺ by mitochondria. Accumulation of Ca²⁺ (80 jiM in the medium) accompanied by the growth of the fluorescent probe response from a conventional unit to a value of 1,57±0,04 (n=5). Calix[4]arenes C-97, C-99, C-107 falls fluorescent signal below the 0,88±0,08, 0,92±0,08 and 0,78±0,04 respectively. Thus, the selected calix[4]arenas lead to release of previously accumulated Ca2+ from mitochondria. Under the influence of C-97 and C-99 fluorescent signal from NADH reduced to -0,11±0,02 and -0,12±0,02, respectively, in relation to the reference value - -0,05±0,01 (n=5). Analysis of fluorescence response NADH and FAD in a suspension of isolated mitochondria suggests that the effects of test compounds on the functional activity of the electron transport chain is associated with the initial stimulation of its 1-th complex and subsequent inhibition of Ca²⁺-dependent NAD- containing Krebs cycle dehydrogenases. Along with this, the use of photon correlation spectroscopy to assess changes in the volume of mitochondria (their hydrodynamic diameter) under the action of selected calix[4]arenes has shown that interference with the electron transport chain leads to changes in the osmotic balance between the matrix of the mitochondria and the external environment. The result is the growth of isolated organelles volume. In particular, the hydrodynamic diameter of mitochondria increased by 22±6 % and 34±8 % (n=5) in presence of C-97 or C-99. The conclusion was done about the advisability of further studies of the calyx[4]arenes effect on smooth muscle Ca²⁺-homeostase and mitochondrial bioenergetics in order to find effective modifiers of their func- tional activity.

Rearrangement of MICU1 multimers for activation of MCU is solely controlled by cytosolic Ca(2.)

Mitochondrial Ca²⁺ uptake is a vital process that controls distinct cell and organelle functions. Mitochondrial calcium uptake 1 (MICU1) was identified as key regulator of the mitochondrial Ca²⁺ uniporter (MCU) that together with the essential MCU regulator (EMRE) forms the mitochondrial Ca²⁺ channel. However, mechanisms by which MICU1 controls MCU/EMRE activity to tune mitochondrial Ca²⁺ signals remain ambiguous. Here we established a live-cell FRET approach and demonstrate that elevations of cytosolic Ca²⁺ rearranges MICU1 multimers with an EC₅₀ of 4.4 μM, resulting in activation of mitochondrial Ca²⁺ uptake. MICU1 rearrangement essentially requires the EF-hand motifs and strictly correlates with the shape of cytosolic Ca²⁺ rises. We further show that rearrangements of MICU1 multimers were independent of matrix Ca²⁺ concentration, mitochondrial membrane potential, and expression levels of MCU and EMRE. Our experiments provide novel details about how MCU/EMRE is regulated by MICU1 and an original approach to investigate MCU/EMRE activation in intact cells.

Coherent correlation networks among protein biomarkers of beef tenderness: What they reveal

The development of proteomic biomarkers for meat tenderness remains an important challenge. The present study used Longissimus thoracis (LT) and Semitendinosus (ST) muscles of young bulls of three continental breeds (Aberdeen Angus, Blond d'Aquitaine and Limousin) to i) identify cellular pathways robustly related with meat tenderness, using potential protein biomarkers and ii) describe biochemical mechanisms underlying muscle to meat conversion. Correlation networks reveal robust correlations, i.e. present for at least two breeds, between potential meat tenderness biomarkers. For the two muscles of the three breeds, DJ-1 and Peroxiredoxin 6 were consistently correlated with Hsp20 and μ-calpain, respectively. For the three breeds, μ-calpain was related to Hsp70-8 in the LT muscle. Various correlations were muscle specific. For the three breeds, DJ-1 was correlated with Hsp27 for the ST, and with ENO3 and LDH-B for the LT muscle. Overall, in the LT, more correlations were found between proteins related to the glycolytic pathway and in the ST, with the small Hsps (Hsp20, 27 and αB-crystallin). Hsp70-Grp75 appeared involved in several relevant biological pathways. At the scientific level, results give insights in biological functions involved in meat tenderness. Further studies are needed to confirm the possible use of these biomarkers in the meat industry to improve assurance of good meat qualities.

[Ca2+/H(+)-exchange in myometrium mitochondria]

Using the fluorescent probe Fluo-4 AM the authors have identified Na(+)-independent Ca2+H(+)-exchange in isolated mitochondria of rat myometrium and studied its individual properties. Formation of directional protons gradient in the matrix of mitochondria causes antyporte release of Ca2+, which has been previously accumulated in energetic processes (in the presence of Mg-ATP and succinate). The functioning of Ca2+/H(+)-exchange depends on the proton gradient and is characterized by reversibility, in case of extramitochondria environment alkalization the additional accumulation of Ca2+ by organelles is recorded. Monovalent cations gradients (Na+, K+, Li+) do not cause the release of Ca2+ from mitochondria. Rate of Ca2+/H(+)-exchange is growing in terms of increasing deltapH on the mitochondria membrane and kinetics of deltapH-induced Ca2+ release from the matrix corresponds to the laws of first order reaction. Research of Ca2+/H(+)-exchange some properties in the myometrium mitochondria showed that the above transport process is of electrogenic nature, perhaps it is done in a 1: 1 stechiometry (Hill coefficient on H+ close to 1) and is able to adjust matrix Ca2+ concentration under physiological conditions (pH activation of about 6.9). Thus, in the inner membrane of the myometrium mitochondria the available system of the secondary active Ca(2+)-transport from the matrix of these organelles to myoplasm and the functioning of Ca2+/H(+)-exchanger may underlie this process.

The destiny of Ca(2+) released by mitochondria

Mitochondrial Ca²⁺ is known to regulate diverse cellular functions, for example energy production and cell death, by modulating mitochondrial dehydrogenases, inducing production of reactive oxygen species, and opening mitochondrial permeability transition pores. In addition to the action of Ca²⁺ within mitochondria, Ca²⁺ released from mitochondria is also important in a variety of cellular functions. In the last 5 years, the molecules responsible for mitochondrial Ca²⁺ dynamics have been identified: a mitochondrial Ca²⁺ uniporter (MCU), a mitochondrial Na⁺–Ca²⁺ exchanger (NCLX), and a candidate for a mitochondrial H⁺–Ca²⁺ exchanger (Letm1). In this review, we focus on the mitochondrial Ca²⁺ release system, and discuss its physiological and pathophysiological significance. Accumulating evidence suggests that the mitochondrial Ca²⁺ release system is not only crucial in maintaining mitochondrial Ca²⁺ homeostasis but also participates in the Ca²⁺ crosstalk between mitochondria and the plasma membrane and between mitochondria and the endoplasmic/sarcoplasmic reticulum.

[Ca2+ accumulation study in isolated smooth muscle mitochondria using fluo-4 AM]

The opportunity of Ca2+-sensitive fluorescent dye Fluo-4 AM and spectrofluorimetry method application for the study of energy-dependent Ca2+ accumulation in mitochondria from uterus smooth muscle is proved. It has been found that the presence of mitochondrial preparation increases time-dependent fluorescent response considerably and this effect depends on Ca2+ concentration in the medium. Thus, in these conditions, deesterification active probe is formed which is sensitive to Ca2+. It is shown that the accumulation of calcium ions in mitochondria in the presence of Mg-ATP and succinate depends on exogenous Ca2+ concentration and is characterized by substrate saturating. The apparent activation constant of Ca2+ accumulation is 53.9 +/- 6.9 mM, which corresponds to the physiological concentration of the cation in the cell next to mitochondria. Transit addition of Ca2+-ionophore A23187 to the incubation me- dium caused a rapid release of ionized cation from mitochondria. When proton gradient on the inner mitochondrial membrane is dissipated by protonophore CCCP, in the case of suppressing the generation of the gradient by oligomycin and in the presence of ruthenium red that inhibits Ca2+ mitochondrial accumulation systems, Ca2+ entry is significantly reduced. The results indicate the prospects of using Fluo-4 AM to study the properties of the Ca2+ accumulation system in isolated mitochondria of the myometrium.

Mitochondria-Targeted Antioxidant SS31 Prevents Amyloid Beta-Induced Mitochondrial Abnormalities and Synaptic Degeneration in Alzheimer's Disease

In neuronal systems, the health and activity of mitochondria and synapses are tightly coupled. For this reason, it has been postulated that mitochondrial abnormalities may, at least in part, drive neurodegeneration in conditions such as Alzheimer’s disease (AD). Mounting evidence from multiple Alzheimer’s disease cell and mouse models and postmortem brains suggest that loss of mitochondrial integrity may be a key factor that mediates synaptic loss. Therefore, the prevention or rescue of mitochondrial dysfunction may help delay or altogether prevent AD-associated neurodegeneration. Since mitochondrial health is heavily dependent on antioxidant defenses, researchers have begun to explore the use of mitochondria-targeted antioxidants as therapeutic tools to prevent neurodegenerative diseases. This review will highlight advances made using a model mitochondria-targeted antioxidant peptide, SS31, as a potential treatment for AD.

Calcium signaling in the liver

Intracellular free Ca(2+) ([Ca(2+)]i) is a highly versatile second messenger that regulates a wide range of functions in every type of cell and tissue. To achieve this versatility, the Ca(2+) signaling system operates in a variety of ways to regulate cellular processes that function over a wide dynamic range. This is particularly well exemplified for Ca(2+) signals in the liver, which modulate diverse and specialized functions such as bile secretion, glucose metabolism, cell proliferation, and apoptosis. These Ca(2+) signals are organized to control distinct cellular processes through tight spatial and temporal coordination of [Ca(2+)]i signals, both within and between cells. This article will review the machinery responsible for the formation of Ca(2+) signals in the liver, the types of subcellular, cellular, and intercellular signals that occur, the physiological role of Ca(2+) signaling in the liver, and the role of Ca(2+) signaling in liver disease.

Selective mitochondrial Ca2+ uptake deficit in disease endstage vulnerable motoneurons of the SOD1G93A mouse model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a progressive neurodegenerative disease that targets some somatic motoneuron populations, while others, e.g. those of the oculomotor system, are spared. The pathophysiological basis of this pattern of differential vulnerability, which is preserved in a transgenic mouse model of amyotrophic lateral sclerosis (SOD1(G93A)), and the mechanism of neurodegeneration in general are unknown. Hyperexcitability and calcium dysregulation have been proposed by others on the basis of data from juvenile mice that are, however, asymptomatic. No studies have been done with symptomatic mice following disease progression to the disease endstage. Here, we developed a new brainstem slice preparation for whole-cell patch-clamp recordings and single cell fura-2 calcium imaging to study motoneurons in adult wild-type and SOD1(G93A) mice up to disease endstage. We analysed disease-stage-dependent electrophysiological properties and intracellular Ca(2+) handling of vulnerable hypoglossal motoneurons in comparison to resistant oculomotor neurons. Thereby, we identified a transient hyperexcitability in presymptomatic but not in endstage vulnerable motoneurons. Additionally, we revealed a remodelling of intracellular Ca(2+) clearance within vulnerable but not resistant motoneurons at disease endstage characterised by a reduction of uniporter-dependent mitochondrial Ca(2+) uptake and enhanced Ca(2+) extrusion across the plasma membrane. Our study challenged the notion that hyperexcitability is a direct cause of neurodegeneration in SOD1(G93A) mice, but molecularly identified a Ca(2+) clearance deficit in motoneurons and an adaptive Ca(2+) handling strategy that might be targeted by future therapeutic strategies.

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

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

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

Previous studies have demonstrated several molecularly distinct players involved in mitochondrial Ca²⁺ uptake. In the present study, electrophysiological recordings on mitoplasts that were isolated from HeLa cells were performed in order to biophysically and pharmacologically characterize Ca²⁺ currents across the inner mitochondrial membrane. In mitoplast-attached configuration with 105 mM Ca²⁺ as a charge carrier, three distinct channel conductances of 11, 23, and 80 pS were observed. All types of mitochondrial currents were voltage-dependent and essentially depended on the presence of Ca²⁺ in the pipette. The 23 pS channel exhibited burst kinetics. Though all channels were sensitive to ruthenium red, their sensitivity was different. The 11 and 23 pS channels exhibited a lower sensitivity to ruthenium red than the 80 pS channel. The activities of all channels persisted in the presence of cylosporin A, CGP 37187, various K⁺-channel inhibitors, and Cl⁻ channel blockers disodium 4,4′-diisothiocyanatostilbene-2,2′-disulfonate and niflumic acid. Collectively, our data identified multiple conductances of Ca²⁺ currents in mitoplasts isolated from HeLa cells, thus challenging the dogma of only one unique mitochondrial Ca²⁺ uniporter.

Spatiotemporal correlations between cytosolic and mitochondrial Ca(2+) signals using a novel red-shifted mitochondrial targeted cameleon

The transfer of Ca²⁺ from the cytosol into the lumen of mitochondria is a crucial process that impacts cell signaling in multiple ways. Cytosolic Ca²⁺ ([Ca²⁺]_cyto) can be excellently quantified with the ratiometric Ca²⁺ probe fura-2, while genetically encoded Förster resonance energy transfer (FRET)-based fluorescent Ca²⁺ sensors, the cameleons, are efficiently used to specifically measure Ca²⁺ within organelles. However, because of a significant overlap of the fura-2 emission with the spectra of the cyan and yellow fluorescent protein of most of the existing cameleons, the measurement of fura-2 and cameleons within one given cell is a complex task. In this study, we introduce a novel approach to simultaneously assess [Ca²⁺]_cyto and mitochondrial Ca²⁺ ([Ca²⁺]_mito) signals at the single cell level. In order to eliminate the spectral overlap we developed a novel red-shifted cameleon, D1GO-Cam, in which the green and orange fluorescent proteins were used as the FRET pair. This ratiometric Ca²⁺ probe could be successfully targeted to mitochondria and was suitable to be used simultaneously with fura-2 to correlate [Ca²⁺]_cyto and [Ca²⁺]_mito within same individual cells. Our data indicate that depending on the kinetics of [Ca²⁺]_cyto rises there is a significant lag between onset of [Ca²⁺]_cyto and [Ca²⁺]_mito signals, pointing to a certain threshold of [Ca²⁺]_cyto necessary to activate mitochondrial Ca²⁺ uptake. The temporal correlation between [Ca²⁺]_mito and [Ca²⁺]_cyto as well as the efficiency of the transfer of Ca²⁺ from the cytosol into mitochondria varies between different cell types. Moreover, slow mitochondrial Ca²⁺ extrusion and a desensitization of mitochondrial Ca²⁺ uptake cause a clear difference in patterns of mitochondrial and cytosolic Ca²⁺ oscillations of pancreatic beta-cells in response to D-glucose.

Regulation of presynaptic calcium in a mammalian synaptic terminal

Ca(2+) signaling in synaptic terminals plays a critical role in neurotransmitter release and short-term synaptic plasticity. In the present study, we examined the role of synaptic Ca(2+) handling mechanisms in the synaptic terminals of mammalian rod bipolar cells, neurons that play a pivotal role in the high-sensitivity vision pathway. We found that mitochondria sequester Ca(2+) under conditions of high Ca(2+) load, maintaining intraterminal Ca(2+) near resting levels. Indeed, the effect of the mitochondria was so powerful that the ability to clamp intraterminal Ca(2+) with a somatically positioned whole cell patch pipette was compromised. The plasma membrane Ca(2+)-ATPase (PMCA), but not the Na(+)/Ca(2+) exchanger (NCX) or the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA), was an important regulator of resting Ca(2+). Furthermore, PMCA activity, but not NCX or SERCA activity, was essential for the recovery of Ca(2+) levels following depolarization-evoked Ca(2+) entry. Loss of PMCA function was also associated with impaired restoration of membrane surface area following depolarization-evoked exocytosis. Given its roles in the regulation of intraterminal Ca(2+) at rest and after a stimulus-evoked Ca(2+) rise, the PMCA is poised to modulate luminance coding and adaptation to background illumination in the mammalian rod bipolar cell.

Update on vascular endothelial Ca2+ signalling: A tale of ion channels, pumps and transporters

A monolayer of endothelial cells (ECs) lines the lumen of blood vessels and forms a multifunctional transducing organ that mediates a plethora of cardiovascular processes. The activation of ECs from as state of quiescence is, therefore, regarded among the early events leading to the onset and progression of potentially lethal diseases, such as hypertension, myocardial infarction, brain stroke, and tumor. Intracellular Ca²⁺ signals have long been know to play a central role in the complex network of signaling pathways regulating the endothelial functions. Notably, recent work has outlined how any change in the pattern of expression of endothelial channels, transporters and pumps involved in the modulation of intracellular Ca²⁺ levels may dramatically affect whole body homeostasis. Vascular ECs may react to both mechanical and chemical stimuli by generating a variety of intracellular Ca²⁺ signals, ranging from brief, localized Ca²⁺ pulses to prolonged Ca²⁺ oscillations engulfing the whole cytoplasm. The well-defined spatiotemporal profile of the subcellular Ca²⁺ signals elicited in ECs by specific extracellular inputs depends on the interaction between Ca²⁺ releasing channels, which are located both on the plasma membrane and in a number of intracellular organelles, and Ca²⁺ removing systems. The present article aims to summarize both the past and recent literature in the field to provide a clear-cut picture of our current knowledge on the molecular nature and the role played by the components of the Ca²⁺ machinery in vascular ECs under both physiological and pathological conditions.

Role of calcium and mitochondria in MeHg-mediated cytotoxicity

Methylmercury (MeHg) mediated cytotoxicity is associated with loss of intracellular calcium (Ca²⁺) homeostasis. The imbalance in Ca²⁺ physiology is believed to be associated with dysregulation of Ca²⁺ intracellular stores and/or increased permeability of the biomembranes to this ion. In this paper we summarize the contribution of glutamate dyshomeostasis in intracellular Ca²⁺ overload and highlight the mitochondrial dysfunctions induced by MeHg via Ca²⁺ overload. Mitochondrial disturbances elicited by Ca²⁺ may involve several molecular events (i.e., alterations in the activity of the mitochondrial electron transport chain complexes, mitochondrial proton gradient dissipation, mitochondrial permeability transition pore (MPTP) opening, thiol depletion, failure of energy metabolism, reactive oxygen species overproduction) that could culminate in cell death. Here we will focus on the role of oxidative stress in these phenomena. Additionally, possible antioxidant therapies that could be effective in the treatment of MeHg intoxication are briefly discussed.

Endothelial mitochondria--less respiration, more integration

Lining the inner surface of the circulatory system, the vascular endothelium accomplishes a vast variety of specialized functions. Even slight alterations of these functions are implicated in the development of certain cardiovascular diseases that represent major causes of morbidity and mortality in developed countries. Endothelial mitochondria are essential to the functional integrity of the endothelial cell as they integrate a wide range of cellular processes including Ca²⁺ handling, redox signaling and apoptosis, all of which are closely interrelated. Growing evidence supports the notion that impairment of mitochondrial signaling in the endothelium is an early event and a causative factor in the development of diseases such as atherosclerosis or diabetic complications. In this review, we want to outline the significance of mitochondria in both physiology and pathology of the vascular endothelium.

Studying mitochondrial Ca(2+) uptake - a revisit

Mitochondrial Ca(2+) sequestration is a well-known process that is involved in various physiological and pathological mechanisms. Using isolated suspended mitochondria one unique mitochondrial Ca(2+) uniporter was considered to account ubiquitously for the transfer of Ca(2+) into these organelles. However, by applying alternative techniques for measuring mitochondrial Ca(2+) uptake evidences for molecularly distinct mitochondrial Ca(2+) carriers accumulated recently. Herein we compared different methodical approaches of studying mitochondrial Ca(2+) uptake. Patch clamp technique on mitoplasts from endothelial and HeLa cells revealed the existence of three and two mitoplast Ca(2+) currents (I(CaMito)), respectively. According to their conductance, these channels were named small (s-), intermediate (i-), large (l-) and extra-large (xl-) mitoplast Ca(2+) currents (MCC). i-MCC was found in mitoplasts of both cell types whereas s-MCC and l-MCC or xl-MCC were/was exclusively found in mitoplasts from endothelial cells or HeLa cells. The comparison of mitochondrial Ca(2+) signals, measured either indirectly by sensing extra-mitochondrial Ca(2+) or directly by recording changes of the matrix Ca(2+), showed different Ca(2+) sensitivities of the distinct mitochondrial Ca(2+) uptake routes. Subpopulations of mitochondria with different Ca(2+) uptake capacities in intact endothelial cells could be identified using Rhod-2/AM. In contrast, cells expressing mitochondrial targeted pericam or cameleon (4mtD3cpv) showed homogeneous mitochondrial Ca(2+) signals in response to cell stimulation. The comparison of different experimental approaches and protocols using isolated organelles, permeabilized and intact cells, pointed to cell-type specific and versatile pathways for mitochondrial Ca(2+) uptake. Moreover, this work highlights the necessity of the utilization of multiple technical approaches to study the complexity of mitochondrial Ca(2+) homeostasis.

The effect of OPA1 on mitochondrial Ca²⁺ signaling

The dynamin-related GTPase protein OPA1, localized in the intermembrane space and tethered to the inner membrane of mitochondria, participates in the fusion of these organelles. Its mutation is the most prevalent cause of Autosomal Dominant Optic Atrophy. OPA1 controls the diameter of the junctions between the boundary part of the inner membrane and the membrane of cristae and reduces the diffusibility of cytochrome c through these junctions. We postulated that if significant Ca²⁺ uptake into the matrix occurs from the lumen of the cristae, reduced expression of OPA1 would increase the access of Ca²⁺ to the transporters in the crista membrane and thus would enhance Ca²⁺ uptake. In intact H295R adrenocortical and HeLa cells cytosolic Ca²⁺ signals evoked with K⁺ and histamine, respectively, were transferred into the mitochondria. The rate and amplitude of mitochondrial [Ca²⁺] rise (followed with confocal laser scanning microscopy and FRET measurements with fluorescent wide-field microscopy) were increased after knockdown of OPA1, as compared with cells transfected with control RNA or mitofusin1 siRNA. Ca²⁺ uptake was enhanced despite reduced mitochondrial membrane potential. In permeabilized cells the rate of Ca²⁺ uptake by depolarized mitochondria was also increased in OPA1-silenced cells. The participation of Na⁺/Ca²⁺ and Ca²⁺/H⁺ antiporters in this transport process is indicated by pharmacological data. Altogether, our observations reveal the significance of OPA1 in the control of mitochondrial Ca²⁺ metabolism.

Leucine zipper EF hand-containing transmembrane protein 1 (Letm1) and uncoupling proteins 2 and 3 (UCP2/3) contribute to two distinct mitochondrial Ca2+ uptake pathways

Cytosolic Ca²⁺ signals are transferred into mitochondria over a huge concentration range. In our recent work we described uncoupling proteins 2 and 3 (UCP2/3) to be fundamental for mitochondrial uptake of high Ca²⁺ domains in mitochondria-ER junctions. On the other hand, the leucine zipper EF hand-containing transmembrane protein 1 (Letm1) was identified as a mitochondrial Ca²⁺/H⁺ antiporter that achieved mitochondrial Ca²⁺ sequestration at small Ca²⁺ increases. Thus, the contributions of Letm1 and UCP2/3 to mitochondrial Ca²⁺ uptake were compared in endothelial cells. Knock-down of Letm1 did not affect the UCP2/3-dependent mitochondrial uptake of intracellularly released Ca²⁺ but strongly diminished the transfer of entering Ca²⁺ into mitochondria, subsequently, resulting in a reduction of store-operated Ca²⁺ entry (SOCE). Knock-down of Letm1 and UCP2/3 did neither impact on cellular ATP levels nor the membrane potential. The enhanced mitochondrial Ca²⁺ signals in cells overexpressing UCP2/3 rescued SOCE upon Letm1 knock-down. In digitonin-permeabilized cells, Letm1 exclusively contributed to mitochondrial Ca²⁺ uptake at low Ca²⁺ conditions. Neither the Letm1- nor the UCP2/3-dependent mitochondrial Ca²⁺ uptake was affected by a knock-down of mRNA levels of mitochondrial calcium uptake 1 (MICU1), a protein that triggers mitochondrial Ca²⁺ uptake in HeLa cells. Our data indicate that Letm1 and UCP2/3 independently contribute to two distinct, mitochondrial Ca²⁺ uptake pathways in intact endothelial cells.

Mitochondrial permeability transition in Ca(2+)-dependent apoptosis and necrosis

A variety of stimuli utilize an increase of cytosolic free Ca(2+) concentration as a second messenger to transmit signals, through Ca(2+) release from the endoplasmic reticulum or opening of plasma membrane Ca(2+) channels. Mitochondria contribute to the tight spatiotemporal control of this process by accumulating Ca(2+), thus shaping the return of cytosolic Ca(2+) to resting levels. The rise of mitochondrial matrix free Ca(2+) concentration stimulates oxidative metabolism; yet, in the presence of a variety of sensitizing factors of pathophysiological relevance, the matrix Ca(2+) increase can also lead to opening of the permeability transition pore (PTP), a high conductance inner membrane channel. While transient openings may serve the purpose of providing a fast Ca(2+) release mechanism, persistent PTP opening is followed by deregulated release of matrix Ca(2+), termination of oxidative phosphorylation, matrix swelling with inner membrane unfolding and eventually outer membrane rupture with release of apoptogenic proteins and cell death. Thus, a rise in mitochondrial Ca(2+) can convey both apoptotic and necrotic death signals by inducing opening of the PTP. Understanding the signalling networks that govern changes in mitochondrial free Ca(2+) concentration, their interplay with Ca(2+) signalling in other subcellular compartments, and regulation of PTP has important implications in the fine comprehension of the main biological routines of the cell and in disease pathogenesis.

IP(3) Receptors, Mitochondria, and Ca Signaling: Implications for Aging

The tight interplay between endoplasmic-reticulum-(ER-) and mitochondria-mediated Ca(2+) signaling is a key determinant of cellular health and cellular fate through the control of apoptosis and autophagy. Proteins that prevent or promote apoptosis and autophagy can affect intracellular Ca(2+) dynamics and homeostasis through binding and modulation of the intracellular Ca(2+)-release and Ca(2+)-uptake mechanisms. During aging, oxidative stress becomes an additional factor that affects ER and mitochondrial function and thus their role in Ca(2+) signaling. Importantly, mitochondrial dysfunction and sustained mitochondrial damage are likely to underlie part of the aging process. In this paper, we will discuss the different mechanisms that control intracellular Ca(2+) signaling with respect to apoptosis and autophagy and review how these processes are affected during aging through accumulation of reactive oxygen species.

The IP(3) receptor-mitochondria connection in apoptosis and autophagy

The amount of Ca(2+) taken up in the mitochondrial matrix is a crucial determinant of cell fate; it plays a decisive role in the choice of the cell between life and death. The Ca(2+) ions mainly originate from the inositol 1,4,5-trisphosphate (IP(3))-sensitive Ca(2+) stores of the endoplasmic reticulum (ER). The uptake of these Ca(2+) ions in the mitochondria depends on the functional properties and the subcellular localization of the IP(3) receptor (IP(3)R) in discrete domains near the mitochondria. To allow for an efficient transfer of the Ca(2+) ions from the ER to the mitochondria, structural interactions between IP(3)Rs and mitochondria are needed. This review will focus on the key proteins involved in these interactions, how they are regulated, and what are their physiological roles in apoptosis, necrosis and autophagy. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Uncoupling protein 3 adjusts mitochondrial Ca(2+) uptake to high and low Ca(2+) signals

Uncoupling proteins 2 and 3 (UCP2/3) are essential for mitochondrial Ca²⁺ uptake but both proteins exhibit distinct activities in regard to the source and mode of Ca²⁺ mobilization. In the present work, structural determinants of their contribution to mitochondrial Ca²⁺ uptake were explored. Previous findings indicate the importance of the intermembrane loop 2 (IML2) for the contribution of UCP2/3. Thus, the IML2 of UCP2/3 was substituted by that of UCP1. These chimeras had no activity in mitochondrial uptake of intracellularly released Ca²⁺, while they mimicked the wild-type proteins by potentiating mitochondrial sequestration of entering Ca²⁺. Alignment of the IML2 sequences revealed that UCP1, UCP2 and UCP3 share a basic amino acid in positions 163, 164 and 167, while only UCP2 and UCP3 contain a second basic residue in positions 168 and 171, respectively. Accordingly, mutants of UCP3 in positions 167 and 171/172 were made. In permeabilized cells, these mutants exhibited distinct Ca²⁺ sensitivities in regard to mitochondrial Ca²⁺ sequestration. In intact cells, these mutants established different activities in mitochondrial uptake of either intracellularly released (UCP3^R171,E172) or entering (UCP3^R167) Ca²⁺. Our data demonstrate that distinct sites in the IML2 of UCP3 effect mitochondrial uptake of high and low Ca²⁺ signals.

Mitochondrial dynamics

Mitochondrial dynamics is a key feature for the interaction of mitochondria with other organelles within a cell and also for the maintenance of their own integrity. Four types of mitochondrial dynamics are discussed: Movement within a cell and interactions with the cytoskeleton, fusion and fission events which establish coherence within the chondriome, the dynamic behavior of cristae and their components, and finally, formation and disintegration of mitochondria (mitophagy). Due to these essential functions, disturbed mitochondrial dynamics are inevitably connected to a variety of diseases. Localized ATP gradients, local control of calcium-based messaging, production of reactive oxygen species, and involvement of other metabolic chains, that is, lipid and steroid synthesis, underline that physiology not only results from biochemical reactions but, in addition, resides on the appropriate morphology and topography. These events and their molecular basis have been established recently and are the topic of this review.