Binding domain of oligomycin on Na(+),K(+)-ATPase

Anoxia-mediated calcium release through the mitochondrial permeability transition pore silences NMDA receptor currents in turtle neurons

Mammalian neurons are anoxia sensitive and rapidly undergo excitotoxic cell death when deprived of oxygen, mediated largely by Ca(2+) entry through over-activation of N-methyl-d-aspartate receptors (NMDARs). This does not occur in neurons of the anoxia-tolerant western painted turtle, where a decrease in NMDAR currents is observed with anoxia. This decrease is dependent on a modest rise in cytosolic [Ca(2+)] ([Ca(2+)]c) that is mediated by release from the mitochondria. The aim of this study was to determine whether the mitochondrial permeability transition pore (mPTP) is involved in NMDAR silencing through release of mitochondrial Ca(2+). Opening the mPTP during normoxia with atractyloside decreased NMDAR currents by releasing mitochondrial Ca(2+), indicated by an increase in Oregon Green fluorescence. Conversely, the mPTP blocker cyclosporin A prevented the anoxia-mediated increase in [Ca(2+)]c and reduction in NMDAR currents. Mitochondrial membrane potential (Ψm) was determined using rhodamine-123 fluorescence and decreased with the onset of anoxia in a time frame that coincided with the increase in [Ca(2+)]c. Activation of mitochondrial ATP-sensitive potassium (mK(+)ATP) channels also releases mitochondrial Ca(2+) and we show that activation of mK(+)ATP channels during normoxia with diazoxide leads to Ψm depolarization and inhibition with 5-hydroxydecanoic acid blocked anoxia-mediated Ψm depolarization. Ψm does not collapse during anoxia but rather reaches a new steady-state level that is maintained via ATP hydrolysis by the F1-F0 ATPase, as inhibition with oligomycin depolarizes Ψm further than the anoxic level. We conclude that anoxia activates mK(+)ATP channels, which leads to matrix depolarization, Ca(2+) release via the mPTP, and ultimately silencing of NMDARs.

Localization of Na(+)/K(+)-ATPase in silkworm brain: a possible mechanism for protection of Na(+)/K(+)-ATPase from Ca(2+)

In mammalian blood, the Na(+) concentration is higher than the K(+) concentration, whereas in hemolymph of lepidopterous insects, the K(+) concentration is higher than the Na(+) concentration. Na(+)/K(+)-ATPase regulates Na(+) and K(+) concentrations in mammalian blood. Therefore, the absence of Na(+)/K(+)-ATPase in lepidopterous insects might be expected. However, we have observed that Na(+)/K(+)-ATPase is abundant in nerve tissues of larvae of silkworm, a lepidopterous insect. Furthermore, we found that silkworm Na(+)/K(+)-ATPase was completely inhibited by 3 mM Ca(2+)in vitro (Homareda, 2010), although the Ca(2+) concentration is very high (30-50 mM) in the hemolymph of silkworm larvae. To investigate the reason why silkworm Na(+)/K(+)-ATPase is not inhibited by Ca(2+)in vivo, we observed the localization of Na(+)/K(+)-ATPase in nerve tissues using immunohistochemical techniques. Na(+)/K(+)-ATPase was distributed in the cortex and neuropile but not in the perineurium of the silkworm brain, while plasma membrane Ca(2+)-ATPase appeared to distribute in the perineurium as well as in the cortex and neuropile. These results support a possibility that neuronal Na(+)/K(+)-ATPase is protected from a high Ca(2+) concentration by the blood-brain barrier consisting of perineurial glial cells with plasma membrane Ca(2+)-ATPase.

Contribution of intracellular ATP to cisplatin resistance of tumor cells

Decreased cellular accumulation of cisplatin is a frequently observed mechanism of resistance to the drug. Beside passive diffusion, several cellular proteins using ATP hydrolysis as an energy source are assumed to be involved in cisplatin transport in and out of the cell. This investigation aimed at clarifying the contribution of intracellular ATP as an indicator of energy-dependent transport to cisplatin resistance using the A2780 human ovarian adenocarcinoma cell line and its cisplatin-resistant variant A2780cis. Depletion of intracellular ATP with oligomycin significantly decreased cellular platinum accumulation (measured by flameless atomic absorption spectrometry) in sensitive but not in resistant cells, and did not affect cisplatin efflux in both cell lines. Inhibition of Na(+),K(+)-ATPase with ouabain reduced platinum accumulation in A2780 cells but to a lesser extent compared with oligomycin. Western blot analysis revealed lower expression of Na(+),K(+)-ATPase α(1) subunit in resistant cells compared with sensitive counterparts. The basal intracellular ATP level (determined using a bioluminescence-based assay) was significantly higher in A2780cis cells than in A2780 cells. Our results highlight the importance of ATP-dependent transport, among other processes mediated by Na(+),K(+)-ATPase, for cisplatin influx in sensitive cells. Cellular platinum accumulation in resistant cells is reduced and less dependent on energy sources, which may partly result from Na(+),K(+)-ATPase downregulation. Our data suggest the involvement of other ATP-dependent processes beside those regulated by Na(+),K(+)-ATPase. Higher basal ATP level in cisplatin-resistant cells, which appears to be a consequence of enhanced mitochondrial ATP production, may represent a survival mechanism established during development of resistance.

A quantitative assay for lysosomal acidification rates in human osteoclasts

The osteoclast initiates resorption by creating a resorption lacuna. The ruffled border surrounding the lacunae arises from exocytosis of lysosomes. To dissolve the inorganic phase of the bone, the vacuolar adenosine triphosphatase, located in the ruffled border, pumps protons into the resorption lacunae. The electroneutrality of the lacunae is maintained by chloride transport through the chloride-proton antiporter chloride channel 7. Inhibition of either proton or chloride transport prevents bone resorption. The aims of this study were to validate the human osteoclastic microsome- based influx assay with respect to lysosomal acidification and assess whether it is a reliable test of a compound's ability to inhibit acidification. Investigated were the expression levels of the lysosomal acidification machinery, the activation of the assay by adenosine triphosphate, H(+) and Cl(-) dependency, the effect of valinomycin, inhibitor sensitivity, and the ion profile of the human osteoclast microsomes. The expression level of chloride channel 7 was increased in the human osteoclastic microsomes compared with whole osteoclasts. Acid influx was induced by 1.25 mM adenosine triphosphate. Further 1.1 μM valinomycin increased the acid influx by 129%. Total abrogation of acid influx was observed using both H(+) and Cl(-) ionophores. Finally, investigation of the anion profile demonstrated that Cl(-) and Br(-) are the preferred anions for the transporter. In conclusion, the acid influx assay based on microsomes from human osteoclasts is a useful tool for detection of inhibitors of the osteoclastic acidification machinery, and thus may aid the identification of effective drugs for osteoporosis that target the acid secretion by osteoclasts.

Properties of silkworm Na+/K+-ATPase

The high Na(+) and low K(+) concentrations in mammalian blood are maintained by Na(+)/K(+)-ATPase. In contrast, the K(+) concentration is higher than the Na(+) concentration in the hemolymph of the silkworm Bombyx mori, a Lepidopterous insect. Although Na(+)/K(+)-ATPase, therefore, appears not to be in silkworm, we confirmed the presence of Na(+)/K(+)-ATPase in nerve tissues of silkworm but not in skeletal muscle or the dorsal vessel. The enzymatic properties of silkworm Na(+)/K(+)-ATPase were characterized in detail and compared with those of dog Na(+)/K(+)-ATPase. Silkworm Na(+)/K(+)-ATPase had a much lower affinity for K(+) and a somewhat higher affinity for Na(+) than dog Na(+)/K(+)-ATPase. The optimal temperature of silkworm Na(+)/K(+)-ATPase activity was lower than that of dog Na(+)/K(+)-ATPase. The optimal Mg(2+) concentration, pH and sensitivities to Ca(2+) and ouabain, a specific inhibitor of Na(+)/K(+)-ATPase, of the two ATPases were identical. These results indicate that the enzymatic properties of the silkworm Na(+)/K(+)-ATPase are suitable for its growth, despite the differences between dog and silkworm Na(+)/K(+)-ATPases. Antisera raised against dog Na(+)/K(+)-ATPase recognized only the α-subunit of silkworm Na(+)/K(+)-ATPase.

The F(0)F(1)-ATP synthase complex contains novel subunits and is essential for procyclic Trypanosoma brucei

The mitochondrial F₀F₁ ATP synthase is an essential multi-subunit protein complex in the vast majority of eukaryotes but little is known about its composition and role in Trypanosoma brucei, an early diverged eukaryotic pathogen. We purified the F₀F₁ ATP synthase by a combination of affinity purification, immunoprecipitation and blue-native gel electrophoresis and characterized its composition and function. We identified 22 proteins of which five are related to F₁ subunits, three to F₀ subunits, and 14 which have no obvious homology to proteins outside the kinetoplastids. RNAi silencing of expression of the F₁ α subunit or either of the two novel proteins showed that they are each essential for the viability of procyclic (insect stage) cells and are important for the structural integrity of the F₀F₁-ATP synthase complex. We also observed a dramatic decrease in ATP production by oxidative phosphorylation after silencing expression of each of these proteins while substrate phosphorylation was not severely affected. Our procyclic T. brucei cells were sensitive to the ATP synthase inhibitor oligomycin even in the presence of glucose contrary to earlier reports. Hence, the two novel proteins appear essential for the structural organization of the functional complex and regulation of mitochondrial energy generation in these organisms is more complicated than previously thought.

African trypanosomes (Trypanosoma brucei and related subspecies) are unicellular parasites that cause the devastating disease of African sleeping sickness in man and nagana in livestock. Both of these diseases are lethal, killing thousands of people each year and causing major economical complications in the developing world, thus affecting the lives of millions. Furthermore, available drugs are obsolete, difficult to administer and have many undesirable side-effects. Therefore, there is a reinvigorated effort to design new drugs against these parasites. From the pharmacological perspective, unique metabolic processes and protein complexes with singular structure, composition and essential function are of particular interest. One such remarkable protein complex is the mitochondrial F₀F₁-ATP synthase/ATPase. Here we show that F₀F₁-ATP synthase complex is essential for viability of procyclic T. brucei cells and it possesses unique and novel subunits. The three F₀F₁-ATP synthase subunits that were tested were shown to be crucial for the structural integrity of the F₀F₁-ATP synthase complex and its activities. The compositional and functional characterization of the F₀F₁-ATP synthase in T. brucei represents a major step towards deciphering the unique and essential properties of the respiratory chain of both an early diverged eukaryote and a lethal human parasite.

Bz-423 superoxide signals apoptosis via selective activation of JNK, Bak, and Bax

Bz-423 is a proapoptotic 1,4-benzodiazepine with potent therapeutic properties in murine models of lupus and psoriasis. Bz-423 modulates the F(1)F(0)-ATPase, inducing the formation of superoxide within the mitochondrial respiratory chain, which then functions as a second messenger initiating apoptosis. Herein, we report the signaling pathway activated by Bz-423 in mouse embryonic fibroblasts containing knockouts of key apoptotic proteins. Bz-423-induced superoxide activates cytosolic ASK1 and its release from thioredoxin. A mitogen-activated protein kinase cascade follows, leading to the specific phosphorylation of JNK. JNK signals activation of Bax and Bak which then induces mitochondrial outer membrane permeabilization to cause the release of cytochrome c and a commitment to apoptosis. The response of these cells to Bz-423 is critically dependent on both superoxide and JNK activation as antioxidants and the JNK inhibitor SP600125 prevents Bax translocation, cytochrome c release, and cell death. These results demonstrate that superoxide generated from the mitochondrial respiratory chain as a consequence of a respiratory transition can signal a sequential and specific apoptotic response. Collectively, these data suggest that the selectivity of Bz-423 observed in vivo results from cell-type specific differences in redox balance and signaling by ASK1 and Bcl-2 proteins.

Study on cardioactive effects of brazilein

Brazilein (6a,7-dihydro-3,6a,10-trihydroxy-benz[b]indeno[1,2-d]pyran-9(6H)-one) is a compound obtained in a large amount from Caesalpinia sappan ethanol extracts with a high purity of about 98%. In isolated cardiac tissues, we found that brazilein exhibited a positive inotropic action in a concentration-dependent manner with little effect on heart rate and coronary perfusion. To study its possible mode of action, isolated rat hearts were treated with propranolol. This treatment did not alter the cardiotonic effect of brazilein, suggesting that this effect does not involve stimulation of beta-adrenoceptors. On the other hand, an analysis of the interaction between Na(+),K(+)-ATPase and brazilein was carried out. Albino guinea pig erythrocytes (mainly alpha1-Na(+),K(+)-ATPase isoforms) enriched with Na(+),K(+)-ATPase isoforms were utilized to compare the inhibition promoted by brazilein with that of classical inhibitors such as the cardiac glycoside deslanoside. Analysis of inhibition curves revealed that unlike deslanoside, brazilein had a relatively low affinity for erythrocyte isoforms and failed to completely inhibit the Na(+),K(+)-ATPase activity. The extent of the maximum inhibition rate was about 50%. The inhibitory effect of brazilein was not antagonized by 10 mmol/l K(+), as observed with deslanoside. Electrocardiogram research in vivo showed that brazilein did not induce the ventricular arrhythmias observed with deslanoside, suggesting that brazilein might have a less adverse effect and higher therapeutic index than cardiac glycosides. In light of all the above-mentioned observations, it can be concluded that brazilein, a molecule with a non-steroidal skeleton, produced its positive inotropic effect through inhibiting Na(+),K(+)-ATPase and could thus serve as a structural paradigm to develop new inotropic drugs.

Stimulation of p-nitrophenylphosphatase activity of Na+/K+-ATPase by NaCl with oligomycin or ATP

It is known that the addition of NaCl with oligomycin or ATP stimulates ouabain-sensitive and K+-dependent p-nitrophenylphosphatase (pNPPase) activity of Na+/K+-ATPase. We investigated the mechanism of the stimulation. The combination of oligomycin and NaCl increased the affinity of pNPPase activity for K+. When the ratio of Na+ to Rb+ was 10 in the presence of oligomycin, Rb+-binding and pNPPase activity reached a maximal level and Na+ was occluded. Phosphorylation of Na+/K+-ATPase by p-nitrophenylphosphate (pNPP) was not affected by oligomycin. Because oligomycin stabilizes the Na+-occluded E1 state of Na+/K+-ATPase, it seemed that the Na+-occluded E1 state increased the affinity of the phosphoenzyme formed from pNPP for K+. On the other hand, the combination of ATP and NaCl also increased the affinity of pNPPase for K+ and activated ATPase activity. Both activities were affected by the ligand conditions. Oligomycin noncompetitively affected the activation of pNPPase by NaCl and ATP. Nonhydrolyzable ATP analogues could not substitute for ATP. As NaE1P, which is the high-energy phosphoenzyme formed from ATP with Na+, is also the Na+-occluded E1 state, it is suggested that the Na+-occluded E1 state increases the affinity of the phosphoenzyme from pNPP for K+ through the interaction between alpha subunits. Therefore, membrane-bound Na+/K+-ATPase would function as at least an (alphabeta)2-diprotomer with interacting alpha subunits at the phosphorylation step.

Kinetic characterization of P-type membrane ATPase from Streptococcus mutans

The proton translocating membrane ATPase of oral streptococci has been implicated in cytoplasmatic pH regulation, acidurance and cariogenicity. Studies have confirmed that Streptococcus mutans is the most frequently detected species in dental caries. A P-type ATPase that can act together with F(1)F(o)-ATPase in S. mutans membrane has been recently described. The main objective of this work is to characterize the kinetic of ATP hydrolysis of this P-type ATPase. The optimum pH for ATP hydrolysis is around 6.0. The dependence of P-type ATPase activity on ATP concentration reveals high (K(0.5)=0.27 mM) and low (K(0.5)=3.31 mM) affinity sites for ATP, exhibiting positive cooperativity and a specific activity of about 74 U/mg. Equimolar concentrations of ATP and magnesium ions display a behavior similar to that described for ATP concentration in Mg(2+) saturating condition (high affinity site, K(0.5)=0.10 mM, and low affinity site, K(0.5)=2.12 mM), exhibiting positive cooperativity and a specific activity of about 68 U/mg. Sodium, potassium, ammonium, calcium and magnesium ions stimulate the enzyme, showing a single saturation curve, all exhibiting positive cooperativities, whereas inhibition of ATPase activity is observed for zinc ions and EDTA. The kinetic characteristics reveal that this ATPase belongs to type IIIA, like the ones found in yeast and plants.

Ouabain Binding to Na+,K+-ATPase Relaxes Cell Attachment and Sends a SpecificSignal (NACos) to the Nucleus

Abstract. In previous work we described a "P-->A mechanism" that transduces occupancy of the pump ( P) by ouabain into changes in phosphorylation, stimulation of mitogen-activated protein kinase (MAPK), and endocytosis of cell-cell- and cell-substrate-attaching molecules ( A), thereby causing a release of the cell from the monolayer. In the present work we try to understand the mechanism of this effect; whether, in order to trigger the P-->A mechanism, ouabain should block the pumping activity of Na(+),K(+)-ATPase as pump, or whether it would suffice that the drug occupies this enzyme as a receptor. We assay a series of drugs known to act on the pump, such as ouabain, digoxin, digitoxin, palytoxin, oligomycin, strophanthidin, neothyoside-A, proscillaridin-A, etc. We gauge their ability to block the pump by measuring the K(+) content in the cells, and their ability to detach the cells from the monolayer by determining the amount of protein remaining in the culturing well. None of the drugs tested was able to cause detachment without stopping the pump. Ouabain also enhances phosphorylation, yet pump inhibition and signal transduction do not seem to be intimately associated in a causal chain, but to occur simultaneously. To investigate the response of the site of cell attachment, we analyze the position of beta-catenin by fluorescence confocal microscopy, and find that this adherent junction-associated molecule is sent to the nucleus, where it is known to act as a transcriptional cofactor.

Inhibition and stimulation of phospholipid scrambling activity. Consequences for lipid asymmetry, echinocytosis, and microvesiculation of erythrocytes

An increase of the intracellular Ca(2+) concentration in erythrocytes is known to activate rapid nonspecific bidirectional translocation of membrane-inserted phospholipid probes and to decrease the asymmetric distribution of endogenous membrane phospholipids. These scrambling effects are now shown to be suppressed by pretreatment of cells with the essentially impermeable reagents 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and 2,4,6-trinitrobenzenesulfonate. The inhibitory effects are no longer observed during renewed activation of scrambling following a first transient activation by Ca(2+). Assuming the involvement of the human scramblase, this suggests a conformational alteration of this protein during activation by Ca(2+). Marked suppression of scrambling activity is also observed in cells pretreated with the disulfide reducing agent dithioerythritol which can be reverted by the SH oxidizing agent diamide. This indicates the importance of intramolecular and/or intersubunit disulfide bonds for the function of the scramblase. On the other hand, treatment of cells with the SH reagents N-ethylmaleimide and phenylarsine oxide enhances Ca(2+)-activated scrambling and diminution of asymmetry of membrane phospholipids. This suggests an allosteric connection of several protein SH groups to the translocation mechanism. The inhibitors retain their strong suppressive effects. Besides covalent modification, addition of oligomycin highly stimulates and addition of clotrimazole suppresses the Ca(2+)-activated translocation. No evidence for a role of the ATP-binding cassette transporter ABCA1 in the Ca(2+)-activated outward translocation is obtained. Suppression of phospholipid scrambling by dithioerythritol inhibits Ca(2+)-induced spheroechinocytosis and reduces the extent of subsequent microvesiculation. Scrambling of endogenous phospholipids is proposed to induce echinocytosis and to have only a stimulatory effect on microvesiculation.