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SummarySWEETs play important roles in intercellular sugar transport. Induction of SWEET sugar transporters by transcription activator-like effectors (TALe) of Xanthomonas ssp. is a key factor for bacterial leaf blight (BLB) infection of rice, cassava and cotton. Here, we identified the so far unknown OsSWEET11b with roles in male fertility and BLB susceptibility in rice. While single ossweet11a or b mutants were fertile, double mutants were sterile. Since clade III SWEETs can transport gibberellin (GA), a key hormone for rice spikelet fertility, sterility and BLB susceptibility might be explained by GA transport deficiencies. However, in contrast to the Arabidopsis homologs, OsSWEET11b did not mediate detectable GA transport. Fertility and susceptibility must therefore depend on SWEET11b-mediated sucrose transport. Ectopic induction of OsSWEET11b by designer TALe enables TALe-free Xanthomonas oryzae pv. oryzae (Xoo) to cause disease, identifying OsSWEET11b as a BLB susceptibility gene and demonstrating that the induction of host sucrose uniporter activity is key to virulence of Xoo. Notably, only three of now six clade III SWEETs are targeted by known Xoo strains from Asia and Africa. The identification of OsSWEET11b has relevance in the context of fertility and for protecting rice against emerging Xoo strains that evolve TALes to exploit OsSWEET11b.

Mitochondria take up Ca2+ through the mitochondrial calcium uniporter complex to regulate energy production, cytosolic Ca2+ signalling and cell death1,2. In mammals, the uniporter complex (uniplex) contains four core components: the pore-forming MCU protein, the gatekeepers MICU1 and MICU2, and an auxiliary subunit, EMRE, essential for Ca2+ transport3-8. To prevent detrimental Ca2+ overload, the activity of MCU must be tightly regulated by MICUs, which sense changes in cytosolic Ca2+ concentrations to switch MCU on and off9,10. Here we report cryo-electron microscopic structures of the human mitochondrial calcium uniporter holocomplex in inhibited and Ca2+-activated states. These structures define the architecture of this multicomponent Ca2+-uptake machinery and reveal the gating mechanism by which MICUs control uniporter activity. Our work provides a framework for understanding regulated Ca2+ uptake in mitochondria, and could suggest ways of modulating uniporter activity to treat diseases related to mitochondrial Ca2+ overload.

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

Ca2+ions have distinct roles in the outer segment, cell body, and synaptic terminal of photoreceptors. We tested the hypothesis that distinct Ca2+domains are maintained by Ca2+uptake into mitochondria. Serial block face scanning electron microscopy of zebrafish cones revealed that nearly 100 mitochondria cluster at the apical side of the inner segment, directly below the outer segment. The endoplasmic reticulum surrounds the basal and lateral surfaces of this cluster, but does not reach the apical surface or penetrate into the cluster. Using genetically encoded Ca2+sensors, we found that mitochondria take up Ca2+when it accumulates either in the cone cell body or outer segment. Blocking mitochondrial Ca2+uniporter activity compromises the ability of mitochondria to maintain distinct Ca2+domains. Together, our findings indicate that mitochondria can modulate subcellular functional specialization in photoreceptors.SIGNIFICANCE STATEMENTCa2+homeostasis is essential for the survival and function of retinal photoreceptors. Separate pools of Ca2+regulate phototransduction in the outer segment, metabolism in the cell body, and neurotransmitter release at the synaptic terminal. We investigated the role of mitochondria in compartmentalization of Ca2+. We found that mitochondria form a dense cluster that acts as a diffusion barrier between the outer segment and cell body. The cluster is surprisingly only partially surrounded by the endoplasmic reticulum, a key mediator of mitochondrial Ca2+uptake. Blocking the uptake of Ca2+by mitochondria causes redistribution of Ca2+throughout the cell. Our results show that mitochondrial Ca2+uptake in photoreceptors is complex and plays an essential role in normal function.

Mitochondria accumulate significant amounts of calcium when cytosolic calcium is elevated above the resting levels of 50-100 nM during signaling events. This calcium uptake is primarily mediated by a macromolecular protein assembly called mitochondrial calcium uniporter (MCU) that resides in the mitochondrial inner membrane. In 2004, we applied patch-clamp technique for the first time to record calcium currents from the mitochondrial inner membrane and proved unequivocally that MCU is a highly selective calcium channel. This chapter describes how patch-clamp technique can be applied to record the Ca2+ uniporter currents from the mitochondrial inner membrane, isolation of mitochondria from the heart tissue, and preparation of mitoplast using French Press. We also discuss advantages of patch-clamp technique as compared to other methods of determining mitochondrial uniporter activity and important considerations in applying patch-clamp technique to such a small subcellular organelle. With small variations in the bath and pipette solution composition, the same methodology can be applied to study any other currents (e.g., H+ or Cl-) from the mitochondrial inner membrane.

Background and Purpose Despite the importance of mitochondrial Ca2+ to metabolic regulation and cell physiology, little is known about the mechanisms that regulate Ca2+ entry into the mitochondria. Accordingly, we established a system to determine the role of the mitochondrial Ca2+ uniporter in an isolated heart model, at baseline and during increased workload following β-adrenoceptor stimulation. Experimental Approach Cardiac contractility, oxygen consumption and intracellular Ca2+ transients were measured in ex vivo perfused murine hearts. Ru360 and spermine were used to modify mitochondrial Ca2+ uniporter activity. Changes in mitochondrial Ca2+ content and energetic phosphate metabolite levels were determined. Key Results The addition of Ru360, a selective inhibitor of the mitochondrial Ca2+ uniporter, induced progressively and sustained negative inotropic effects that were dose-dependent with an EC50 of 7 μM. Treatment with spermine, a uniporter agonist, showed a positive inotropic effect that was blocked by Ru360. Inotropic stimulation with isoprenaline elevated oxygen consumption (2.7-fold), Ca2+-dependent activation of pyruvate dehydrogenase (5-fold) and mitochondrial Ca2+ content (2.5-fold). However, in Ru360-treated hearts, this parameter was attenuated. In addition, β-adrenoceptor stimulation in the presence of Ru360 did not affect intracellular Ca2+ handling, PKA or Ca2+/calmodulin-dependent PK signalling. Conclusions and Implications Inhibition of the mitochondrial Ca2+ uniporter decreases β-adrenoceptor response, uncoupling between workload and production of energetic metabolites. Our results support the hypothesis that the coupling of workload and energy supply is partly dependent on mitochondrial Ca2+ uniporter activity.

The mitochondrial calcium uniporter is a highly selective calcium channel distributed broadly across eukaryotes but absent in the yeast Saccharomyces cerevisiae. The molecular components of the human uniporter holocomplex (uniplex) have been identified recently. The uniplex consists of three membrane-spanning subunits--mitochondrial calcium uniporter (MCU), its paralog MCUb, and essential MCU regulator (EMRE)--and two soluble regulatory components--MICU1 and its paralog MICU2. The minimal components sufficient for in vivo uniporter activity are unknown. Here we consider Dictyostelium discoideum (Dd), a member of the Amoebazoa outgroup of Metazoa and Fungi, and show that it has a highly simplified uniporter machinery. We show that D. discoideum mitochondria exhibit membrane potential-dependent calcium uptake compatible with uniporter activity, and also that expression of DdMCU complements the mitochondrial calcium uptake defect in human cells lacking MCU or EMRE. Moreover, expression of DdMCU in yeast alone is sufficient to reconstitute mitochondrial calcium uniporter activity. Having established yeast as an in vivo reconstitution system, we then reconstituted the human uniporter. We show that coexpression of MCU and EMRE is sufficient for uniporter activity, whereas expression of MCU alone is insufficient. Our work establishes yeast as a powerful in vivo reconstitution system for the uniporter. Using this system, we confirm that MCU is the pore-forming subunit, define the minimal genetic elements sufficient for metazoan and nonmetazoan uniporter activity, and provide valuable insight into the evolution of the uniporter machinery.

The mitochondrial uniporter is a selective Ca(2+) channel regulated by MICU1, an EF hand-containing protein in the organelle's intermembrane space. MICU1 physically associates with and is co-expressed with a paralog, MICU2. To clarify the function of MICU1 and its relationship to MICU2, we used gene knockout (KO) technology. We report that HEK-293T cells lacking MICU1 or MICU2 lose a normal threshold for Ca(2+) intake, extending the known gating function of MICU1 to MICU2. Expression of MICU1 or MICU2 mutants lacking functional Ca(2+)-binding sites leads to a striking loss of Ca(2+) uptake, suggesting that MICU1/2 disinhibit the channel in response to a threshold rise in [Ca(2+)]. MICU2's activity and physical association with the pore require the presence of MICU1, though the converse is not true. We conclude that MICU1 and MICU2 are nonredundant and together set the [Ca(2+)] threshold for uniporter activity.

Producción Científica, The mitochondrial calcium uniporter complex is essential for calcium (Ca2+) uptake into mitochondria of all mammalian tissues, where it regulates bioenergetics, cell death, and Ca2+ signal transduction. Despite its involvement in several human diseases, we currently lack pharmacological agents for targeting uniporter activity. Here we introduce a high-throughput assay that selects for human MCU-specific small-molecule modulators in primary drug screens. Using isolated yeast mitochondria, reconstituted with human MCU, its essential regulator EMRE, and aequorin, and exploiting a D-lactate- and mannitol/sucrose-based bioenergetic shunt that greatly minimizes false-positive hits, we identify mitoxantrone out of more than 600 clinically approved drugs as a direct selective inhibitor of human MCU. We validate mitoxantrone in orthogonal mammalian cell-based assays, demonstrating that our screening approach is an effective and robust tool for MCU-specific drug discovery and, more generally, for the identification of compounds that target mitochondrial functions., Ministerio de Economía, Industria y Competitividad (Project BFU2014-53469P), Instituto de Saludo Carlos III (grant RD16/0011/0003)

Background and Purpose: Despite the importance of mitochondrial Ca2+ to metabolic regulation and cell physiology, little is known about the mechanisms that regulate Ca2+ entry into the mitochondria. Accordingly, we established a system to determine the role of the mitochondrial Ca2+ uniporter in an isolated heart model, at baseline and during increased workload following β-adrenoceptor stimulation. Experimental Approach: Cardiac contractility, oxygen consumption and intracellular Ca2+ transients were measured in ex vivo perfused murine hearts. Ru360 and spermine were used to modify mitochondrial Ca2+ uniporter activity. Changes in mitochondrial Ca2+ content and energetic phosphate metabolite levels were determined. Key Results: The addition of Ru360, a selective inhibitor of the mitochondrial Ca2+ uniporter, induced progressively and sustained negative inotropic effects that were dose-dependent with an EC50 of 7 μM. Treatment with spermine, a uniporter agonist, showed a positive inotropic effect that was blocked by Ru360. Inotropic stimulation with isoprenaline elevated oxygen consumption (2.7-fold), Ca2+-dependent activation of pyruvate dehydrogenase (5-fold) and mitochondrial Ca2+ content (2.5-fold). However, in Ru360-treated hearts, this parameter was attenuated. In addition, β-adrenoceptor stimulation in the presence of Ru360 did not affect intracellular Ca2+ handling, PKA or Ca2+/calmodulin-dependent PK signalling. Conclusions and Implications: Inhibition of the mitochondrial Ca2+ uniporter decreases β-adrenoceptor response, uncoupling between workload and production of energetic metabolites. Our results support the hypothesis that the coupling of workload and energy supply is partly dependent on mitochondrial Ca2+ uniporter activity., Centro de Investigaciones Cardiovasculares

Recent studies have revealed MCU as the pore forming domain and MICU1 as a critical Ca2+-sensitive regulator of the mitochondrial Ca2+ uniporter. However, the mechanism of the complex Ca2+ dependence of the uniporter activity remains elusive. Our previous studies showed that prolonged down-regulation of MICU1 in HeLa cells causes lower threshold and decreased cooperativity of mitochondrial Ca2+ uptake. To study the functional significance of the effects of MICU1 we used hepatocytes harvested from the liver of mice exposed to in vivo silencing (4 weeks). Silencing of MICU1 or MCU resulted in >80% decrease in their respective mRNA levels. Silencing of MICU1 caused a leftward-shifted dose response and decreased cooperativity of mitochondrial Ca2+ uptake in both permeabilized and intact hepatocytes. By contrast, silencing of MCU resulted in slower Ca2+ uptake in the entire range of Ca2+ concentrations without change in threshold. Mitochondrial respiration and cellular ATP content were unaffected in media containing both glycolytic and mitochondrial fuels in either MICU1 or MCU-deficient hepatocytes. However, silencing of MICU1 caused an augmented loss of ATP when the cells were confined to oxidative metabolism and an enhanced sensitivity to mitochondrial Ca2+ overload and permeabilization. During stimulation with vasopressin, a Ca2+ mobilizing hormone, both MICU1 and MCU-deficient cells displayed an attenuated mitochondrial matrix [Ca2+] increase and stimulation of respiration. Collectively, these results show that keeping the gate of MCU closed by MICU1 at low [Ca2+] is required to maintain healthy mitochondria, and MICU1-mediated control of MCU (cooperativity?) is required to support the propagation of short-lasting calcium spikes and oscillations to the mitochondria and the ensuing physiological stimulation of oxidative metabolism.

The mitochondrial uniporter is a highly selective calcium channel present broadly in eukaryotes, but absent in Saccharomyces cerevisiae. Therefore, we used yeast as a reconstitution system to identify the minimal components sufficient for in vivo uniporter activity. First, we considered Dictyostelium discoideum and showed that it has a highly simplified uniporter machinery: the expression of DdMCU, a single transmembrane component alone is sufficient to reconstitute mitochondrial calcium uniporter activity. Second, to establish human uniporter activity, the coexpression of MCU and - the animal specific protein - EMRE is necessary, whereas expression of MCU alone is insufficient. Our work established yeast as a powerful in vivo reconstitution system for the uniporter to study the evolution and function of this channel.

Mitochondrial volume regulation depends on K+ movement across the inner membrane and a mitochondrial Ca2+-dependent K+ channel (mitoK(Ca)) reportedly contributes to mitochondrial K+ uniporter activity. Here we utilize a novel K(Ca) channel activator, NS11021, to examine the role of mitoK(Ca) in regulating mitochondrial function by measuring K+ flux, membrane potential (DeltaPsi(m)), light scattering, and respiration in guinea pig heart mitochondria. K+ uptake and the influence of anions were assessed in mitochondria loaded with the K+ sensor PBFI by adding either the chloride (KCl), acetate (KAc), or phosphate (KH2PO4) salts of K+ to energized mitochondria in a sucrose-based medium. K+ fluxes saturated at approximately 10 mM for each salt, attaining maximal rates of 172+/-17, 54+/-2.4, and 33+/-3.8 nmol K+/min/mg in KCl, KAc, or KH2PO4, respectively. NS11021 (50 nM) increased the maximal K+ uptake rate by 2.5-fold in the presence of KH2PO4 or KAc and increased mitochondrial volume, with little effect on DeltaPsi(m). In KCl, NS11021 increased K+ uptake by only 30% and did not increase volume. The effects of NS11021 on K+ uptake were inhibited by the K(Ca) toxins charybdotoxin (200 nM) or paxilline (1 microM). Fifty nanomolar of NS11021 increased the mitochondrial respiratory control ratio (RCR) in KH2PO4, but not in KCl; however, above 1 microM, NS11021 decreased RCR and depolarized DeltaPsi(m). A control compound lacking K(Ca) activator properties did not increase K+ uptake or volume but had similar nonspecific (toxin-insensitive) effects at high concentrations. The results indicate that activating K+ flux through mitoK(Ca) mediates a beneficial effect on energetics that depends on mitochondrial swelling with maintained DeltaPsi(m).

We previously reported that uncoupling Ca2+-loaded mitochondria in the presence of [ethylenebis(oxyethylenenitrilo)]tetraacetic acid (EGTA) produces a partial expression of the permeability transition. From this and related observations, it was proposed that the absence of external free Ca2+ is inhibitory to reverse activity of the Ca2+ uniporter (Igbavboa, U., and Pfeiffer, D.R. (1988) J. Biol. Chem. 263, 1405-1412). By using Sr2+-instead of Ca2+-loaded mitochondria, the transition is avoided upon treatment with EGTA plus uncoupler, and inhibition of reverse uniport activity can be observed directly. In the presence of physiological Mg2+ concentrations, reverse uniport of Sr2+ is eliminated by external EGTA following a brief period of rapid activity. It is proposed that binding of Mg2+ rather than Sr2+ (Ca2+) at an external site is responsible for the inhibition. Regulation at the external site is modified by the size of the Sr2+ load. EGTA, in the presence of Mg2+, does not inhibit the reverse uniport-dependent release of Sr2+ from mitoplasts. The inhibitory effect can be recovered by adding back the soluble components obtained as the intermembrane space fraction following removal of the outer membrane. The soluble factor could be a regulatory subunit which contains the external cation binding site. Adjustments to uniporter activity due to regulation by the binding site and/or the soluble factor may be slow and may be significant in determining how mitochondria respond to rapid Ca2+ transients in vivo.

The role of regucalcin, which is a regulatory protein in intracellular signaling, in the regulation of Ca2+-ATPase activity in the mitochondria of brain tissues was investigated. The addition of regucalcin (10−10 to 10−8 M), which is a physiologic concentration in rat brain tissues, into the enzyme reaction mixture containing 25 µM calcium chloride caused a significant increase in Ca2+-ATPase activity, while it did not significantly change in Mg2+-ATPase activity. The effect of regucalcin (10−9 M) in increasing mitochondrial Ca2+-ATPase activity was completely inhibited in the presence of ruthenium red (10−7 M) or lanthanum chloride (10−7 M), both of which are inhibitors of mitochondrial uniporter activity. Whether the effect of regucalcin is modulated in the presence of calmodulin or dibutyryl cyclic AMP (DcAMP) was examined. The effect of regucalcin (10−9 M) in increasing Ca2+-ATPase activity was not significantly enhanced in the presence of calmodulin (2.5 µg/ml) which significantly increased the enzyme activity. DcAMP (10−6 to 10−4 M) did not have a significant effect on Ca2+-ATPase activity. The effect of regucalcin (10−9 M) in increasing Ca2+-ATPase activity was not seen in the presence of DcAMP (10−4 M). Regucalcin levels were significantly increased in the brain tissues or the mitochondria obtained from regucalcin transgenic (RC TG) rats. The mitochondrial Ca2+-ATPase activity was significantly increased in RC TG rats as compared with that of wild-type rats. This study demonstrates that regucalcin has a role in the regulation of Ca2+-ATPase activity in the brain mitochondria of rats. J. Cell. Biochem. 104: 795–804, 2008. © 2008 Wiley-Liss, Inc.

Many viruses have evolved mechanisms to alter mitochondrial function. The hepatitis C virus (HCV) produces a viral core protein that targets to mitochondria and increases Ca2+-dependent ROS production. The aim of this study was to determine whether core's effects are mediated by changes in mitochondrial Ca2+ uptake. Core expression caused enhanced mitochondrial Ca2+ uptake in response to ER Ca2+ release induced by thapsigargin or ATP. It also increased mitochondrial superoxide production and mitochondrial permeability transition (MPT). Incubating mouse liver mitochondria with an HCV core (100 ng/mg) in vitro increased Ca2+ entry rate by approximately 2-fold. Entry was entirely inhibited by the mitochondrial Ca2+ uniporter inhibitor, Ru-360, but not influenced by an Na+/Ca2+ exchanger inhibitor or ROS scavengers. These results indicate that core directly increases mitochondrial Ca2+ uptake via a primary effect on the uniporter. This enhanced the ability of mitochondria to sequester Ca2+ in response to ER Ca2+ release, and increased mitochondrial ROS production and MPT. Thus, the mitochondrial Ca2+ uniporter is a newly identified target for viral modification of cell function.

Introduction: Altered mitochondrial Ca2+ (MCa2+) uptake has been implicated in graft dysfunction following cold ischemia (CI), when excessive cytosolic Ca2+ enters mitochondria through the MCa2+ uniporter. The actual mechanisms governing uniporter activity are unknown. Ca2+ uptake by the cell is regulated by Phospholipase C (PLC). We therefore hypothesized that PLC exists in mitochondria and regulates MCa2+ uptake following CI. Methods: Rat livers were perfused with UW solution and harvested. Half was homogenized immediately; the other half was first subjected to 24 hour cold storage in UW. Mitochondria were isolated and incubated in a buffer containing 1mM ATP (known [ATP] during ischemia) and 0.2 μM 45Ca2+. Ruthenium Red (RR, [10 μM]) was used as a negative control. A selective PLC inhibitor, U-73122, and its inactive analog, U-73343, were added to determine their effects on MCa2+ uptake. Western blots were performed using anti-PLC antibodies. Mitochondrial transmembrane potential (MΔψ) was evaluated using Mitotracker Red fluorescence. Results: 1) Western blot confirmed the presence of PLC in isolated mitochondria and mitochondrial membranes. 2) MCa2+ uptake was significantly increased after 24 hour cold storage in UW. 3) U-73122 and RR, but not U-73343, significantly and dose-dependently decreased Ca2+ uptake in mitochondria from both ischemic and non-ischemic livers, without affecting MΔψ. Conclusions: These data demonstrate for the first time that PLC exists in liver mitochondria. The effects of U-73122 indicate that PLC is essential for MCa2+ uptake during both physiologic and CI conditions. Inhibition of mitochondrial PLC therefore represents a novel target in the study of CI damage during liver transplantation. Download : Download high-res image (120KB) Download : Download full-size image

Administration of methoxamine (10 microM, 2 min) to perfused rat hearts increased the rate at which subsequently isolated mitochondria accumulated Ca2+. Methoxamine did not change significantly the development of delta phi with time or the basal rates of Ca2+ flux on inhibition of the uniporter with Ruthenium Red. With 200 microM-Pi, the rates of Ca2+ uptake at constant delta phi were unaffected by the small variations in endogenous [Pi] between mitochondrial preparations, and were also unaffected by changes in internal Ca2+ over the approximate range 8-43 nmol of Ca2+/mg. At low internal Ca2+ (about 8 nmol/mg of protein) the rates of Ca2+ uptake at constant delta phi were unaffected by addition of 200 microM-Pi. Under these conditions, the uniporter activity and the uniporter conductance were increased by 38-40% by methoxamine pretreatment. The endogenous Ca2+ content of mitochondria from control heart was about 1.8 nmol of Ca2+/mg of protein. Perfusion with agonist increased the Ca2+ content as follows: 10 microM-methoxamine (2 min), 48%; 1 microM-isoprenaline (2 min), 100%; 1 microM-adrenaline (2 min), 140%. The implications of the data for the adrenergic control of oxidative metabolism by intramitochondrial Ca2+ is discussed.

This study investigates the effects of adrenergic agonists and mitochondrial energy state on the activities of the Ca2+ transport systems of female rat liver mitochondria. Tissue perfusion with the alpha-adrenergic agonist phenylephrine and with adrenaline, but not with the beta-adrenergic agonist isoprenaline, induced significant activation of the uniporter and the respiratory chain. Uniporter activation was evident under two sets of experimental conditions that excluded influences of delta psi, i.e., at high delta psi, where uniporter activity was delta psi independent, and at low delta psi, where uniporter conductance was measured. Preincubation of mitochondria with extracts from phenylephrine-perfused tissue quantitatively reproduced uniporter activation when comparison was made with mitochondria treated similarly with extracts from tissue perfused without agonist. Similar, but more extensive, data were obtained with heart mitochondria pretreated with extracts from hearts perfused with the alpha-adrenergic agonist methoxamine. Phenylephrine did not affect Ca2+ efflux mediated by the Na+-Ca2+ carrier or the Na+-independent system. In contrast, the liver mitochondrial Na+-Ca2+ carrier was activated by tissue perfusion with isoprenaline; the Na+-independent system was unaffected. Na+-Ca2+ carrier activation was not associated with any change in a number of basic bioenergetic parameters. It is concluded that the Ca2+ transport systems of liver mitochondria may be controlled in an opposing manner by alpha-adrenergic agonists (promotion of Ca2+ influx) and beta-adrenergic agonists (promotion of Ca2+ efflux). At delta psi values greater than 110 mV, the Na+-independent system was activated by increase in delta psi; the uniporter and Na+-Ca2+ carrier activities were insensitive to delta psi changes in this range.(ABSTRACT TRUNCATED AT 250 WORDS)

Summary The uptake of cytoplasmic calcium into mitochondria is critical for a variety of physiological processes, including calcium buffering, metabolism, and cell survival. Here, we demonstrate that inhibiting the mitochondrial calcium uniporter in the Drosophila mushroom body neurons (MBn)—a brain region critical for olfactory memory formation—causes memory impairment without altering the capacity to learn. Inhibiting uniporter activity only during pupation impaired adult memory, whereas the same inhibition during adulthood was without effect. The behavioral impairment was associated with structural defects in MBn, including a decrease in synaptic vesicles and an increased length in the axons of the αβ MBn. Our results reveal an in vivo developmental role for the mitochondrial uniporter complex in establishing the necessary structural and functional neuronal substrates for normal memory formation in the adult organism.

Liver mitochondria take up Ca2+ by the Ca2+ uniporter, whereas at steady state efflux is believed to occur mainly by means of a ruthenium red-insensitive Ca2+/2H+ antiporter. The latter activity was studied in respiration-inhibited mitochondria in the presence of ruthenium red and was measured as Ca2+ uptake following acidification of the matrix by addition of nigericin, which catalyzes K+/H+ exchange. Ca2+ uptake was stimulated by protonophorous uncoupling agents and inhibited by increasing the concentration of ruthenium red. However, the rates were always smaller than those obtained by addition of valinomycin instead of nigericin. This indicates that under these conditions, Ca2+ fluxes are not mediated by a Ca2+/2H+ antiporter but by residual uniporter activity.