Cirsiliol and Quercetin Inhibit ATP Synthesis and Decrease the Energy Balance in Methicillin-Resistant Staphylococcus aureus (MRSA) and Methicillin-Resistant Staphylococcus epidermidis (MRSE) Strains Isolated from Patients

Polyphenols have attracted attention in the fight against antibiotic-resistant bacteria, as they show antibacterial action. Considering that polyphenols inhibit F1Fo-ATP synthase (ATP synthase) and that bacteria need a constant energy production to maintain their homeostasis, we evaluated the effect of two flavones, cirsiliol (tri-hy-droxy-6,7-dimethoxyflavone) and quercetin (3,3,4,5,7-pentahydroxyflavone), on energy production and intracellular ATP content in a methicillin-resistant Staphylococcus aureus (MRSA) strain and a methicillin-resistant Staphylococcus epidermidis (MRSE) strain isolated from patients, comparing the results to those obtained by treating the bacteria with oligomycin, a specific ATP synthase Fo moiety inhibitor. Real-time quantitative ATP synthesis and total ATP content of permeabilized Gram-positive bacteria were assayed by luminometry. The results showed that cirsiliol and quercetin inhibited ATP synthase and decreased the intracellular ATP levels in both strains, although the effect was higher in MRSE. In addition, while cirsiliol and quercetin acted immediately after the treatment, oligomycin inhibited ATP synthesis only after 30 min of incubation, suggesting that the different responses may depend on the different permeability of the bacterial wall to the three molecules. Thus, cirsiliol and quercetin could be considered potential additions to antibiotics due to their ability to target ATP synthase, against which bacteria cannot develop resistance.

Proliferating Astrocytes in Primary Culture Do Not Depend upon Mitochondrial Respiratory Complex I Activity or Oxidative Phosphorylation

Understanding the role of astrocytes in the development of the nervous system and neurodegenerative disorders implies a necessary knowledge of the oxidative metabolism of proliferating astrocytes. The electron flux through mitochondrial respiratory complexes and oxidative phosphorylation may impact the growth and viability of these astrocytes. Here, we aimed at assessing to which extent mitochondrial oxidative metabolism is required for astrocyte survival and proliferation. Primary astrocytes from the neonatal mouse cortex were cultured in a physiologically relevant medium with the addition of piericidin A or oligomycin at concentrations that fully inhibit complex I-linked respiration and ATP synthase, respectively. The presence of these mitochondrial inhibitors for up to 6 days in a culture medium elicited only minor effects on astrocyte growth. Moreover, neither the morphology nor the proportion of glial fibrillary acidic protein-positive astrocytes in culture was affected by piericidin A or oligomycin. Metabolic characterization of the astrocytes showed a relevant glycolytic metabolism under basal conditions, despite functional oxidative phosphorylation and large spare respiratory capacity. Our data suggest that astrocytes in primary culture can sustainably proliferate when their energy metabolism relies only on aerobic glycolysis since their growth and survival do not require electron flux through respiratory complex I or oxidative phosphorylation.

Relative Importance of Different Elements of Mitochondrial Oxidative Phosphorylation in Maintaining the Barrier Integrity of Retinal Endothelial Cells: Implications for Vascular-Associated Retinal Diseases

PURPOSE

Mitochondrial dysfunction is central to breaking the barrier integrity of retinal endothelial cells (RECs) in various blinding eye diseases such as diabetic retinopathy and retinopathy of prematurity. Therefore, we aimed to investigate the role of different mitochondrial constituents, specifically those of oxidative phosphorylation (OxPhos), in maintaining the barrier function of RECs.

METHODS

Electric cell-substrate impedance sensing (ECIS) technology was used to assess in real time the role of different mitochondrial components in the total impedance (Z) of human RECs (HRECs) and its components: capacitance (C) and the total resistance (R). HRECs were treated with specific mitochondrial inhibitors that target different steps in OxPhos: rotenone for complex I, oligomycin for complex V (ATP synthase), and FCCP for uncoupling OxPhos. Furthermore, data were modeled to investigate the effects of these inhibitors on the three parameters that govern the total resistance of cells: Cell-cell interactions (R_b), cell-matrix interactions (α), and cell membrane permeability (Cm).

RESULTS

Rotenone (1 µM) produced the greatest reduction in Z, followed by FCCP (1 µM), whereas no reduction in Z was observed after oligomycin (1 µM) treatment. We then further deconvoluted the effects of these inhibitors on the R_b, α, and C_m parameters. Rotenone (1 µM) completely abolished the resistance contribution of R_b, as the R_b became zero immediately after the treatment. Secondly, FCCP (1 µM) eliminated the resistance contribution of R_b only after 2.5 h and increased C_m without a significant effect on α. Lastly, of all the inhibitors used, oligomycin had the lowest impact on R_b, as evidenced by the fact that this value became similar to that of the control group at the end of the experiment without noticeable effects on C_m or α.

CONCLUSION

Our study demonstrates the differential roles of complex I, complex V, and OxPhos coupling in maintaining the barrier functionality of HRECs. We specifically showed that complex I is the most important component in regulating HREC barrier integrity. These observed differences are significant since they could serve as the basis for future pharmacological and gene expression studies aiming to improve the activity of complex I and thereby provide avenues for therapeutic modalities in endothelial-associated retinal diseases.

The Action of the Pulsed Electric Field of the Subnanosecond Range on Human Tumor Cells

The action of the pulsed electric field of the subnanosecond range on Jurkat, HEK 293, and U-87 MG human cell lines was studied. The cells were treated in a waveguide in 0.18 ml electrodeless Teflon cuvettes. The electric field strength in the cell culture medium was ~2 kV/cm, the pulse duration was ~1 ns, the leading edge was 150 ps, the frequency was 100 Hz, and the treatment time was 5 min. According to estimates, the change of the transmembrane potential during the pulse was ~20 mV and we assume that it was insufficient for electroporation. Jurkat and HEK 293 cells appeared to be more resistant to the treatment than U-87 MG cells. We have observed that the impulses with the above-mentioned parameters can cause a noticeable change in the mitochondrial activity of U-87 MG cells. © 2022 Bioelectromagnetics Society.

β-amyloid monomers drive up neuronal aerobic glycolysis in response to energy stressors

Research on cerebral glucose metabolism has shown that the aging brain experiences a fall of aerobic glycolysis, and that the age-related loss of aerobic glycolysis may accelerate Alzheimer’s disease pathology. In the healthy brain, aerobic glycolysis, namely the use of glucose outside oxidative phosphorylation, may cover energy demand and increase neuronal resilience to stressors at once. Currently, the drivers of aerobic glycolysis in neurons are unknown. We previously demonstrated that synthetic monomers of β-amyloid protein (Aβ) enhance glucose uptake in neurons, and that endogenous Aβ is required for depolarization-induced glucose uptake in cultured neurons. In this work, we show that cultured cortical neurons increased aerobic glycolysis in response to the inhibition of oxidative phosphorylation by oligomycin or to a kainate pulse. Such an increase was prevented by blocking the endogenous Aβ tone and re-established by the exogenous addition of synthetic Aβ monomers. The activity of mitochondria-bound hexokinase-1 appeared to be necessary for monomers-stimulated aerobic glycolysis during oxidative phosphorylation blockade or kainate excitation. Our data suggest that, through Aβ release, neurons coordinate glucose uptake with aerobic glycolysis in response to metabolic stressors. The implications of this new finding are that the age-related drop in aerobic glycolysis and the susceptibility to Alzheimer’s disease could be linked to factors interfering with release and functions of Aβ monomers.

The flexibility of metabolic interactions between chloroplasts and mitochondria in Nicotiana tabacum leaf

To examine the effect of mitochondrial function on photosynthesis, wild-type and transgenic Nicotiana tabacum with varying amounts of alternative oxidase (AOX) were treated with different respiratory inhibitors. Initially, each inhibitor increased the reduction state of the chloroplast electron transport chain, most severely in AOX knockdowns and least severely in AOX overexpressors. This indicated that the mitochondrion was a necessary sink for photo-generated reductant, contributing to the 'P700 oxidation capacity' of photosystem I. Initially, the Complex III inhibitor myxothiazol and the mitochondrial ATP synthase inhibitor oligomycin caused an increase in photosystem II regulated non-photochemical quenching not evident with the Complex III inhibitor antimycin A (AA). This indicated that the increased quenching depended upon AA-sensitive cyclic electron transport (CET). Following 12 h with oligomycin, the reduction state of the chloroplast electron transport chain recovered in all plant lines. Recovery was associated with large increases in the protein amount of chloroplast ATP synthase and mitochondrial uncoupling protein. This increased the capacity for photophosphorylation in the absence of oxidative phosphorylation and enabled the mitochondrion to act again as a sink for photo-generated reductant. Comparing the AA and myxothiazol treatments at 12 h showed that CET optimized photosystem I quantum yield, depending upon the P700 oxidation capacity. When this capacity was too high, CET drew electrons away from other sinks, moderating the P700+ amount. When P700 oxidation capacity was too low, CET acted as an electron overflow, moderating the amount of reduced P700. This study reveals flexible chloroplast-mitochondrion interactions able to overcome lesions in energy metabolism.

Transient phases of OXPHOS inhibitor resistance reveal underlying metabolic heterogeneity in single cells

Cell-to-cell heterogeneity in metabolism plays an unknown role in physiology and pharmacology. To functionally characterize cellular variability in metabolism, we treated cells with inhibitors of oxidative phosphorylation (OXPHOS) and monitored their responses with live-cell reporters for ATP, ADP/ATP, or activity of the energy-sensing kinase AMPK. Across multiple OXPHOS inhibitors and cell types, we identified a subpopulation of cells resistant to activation of AMPK and reduction of ADP/ATP ratio. This resistant state persists transiently for at least several hours and can be inherited during cell divisions. OXPHOS inhibition suppresses the mTORC1 and ERK growth signaling pathways in sensitive cells, but not in resistant cells. Resistance is linked to a multi-factorial combination of increased glucose uptake, reduced protein biosynthesis, and G0/G1 cell-cycle status. Our results reveal dynamic fluctuations in cellular energetic balance and provide a basis for measuring and predicting the distribution of cellular responses to OXPHOS inhibition.

Ca2+ as cofactor of the mitochondrial H+ -translocating F1 FO -ATP(hydrol)ase

The mitochondrial F₁ F_O -ATPase in the presence of the natural cofactor Mg²⁺ acts as the enzyme of life by synthesizing ATP, but it can also hydrolyze ATP to pump H⁺ . Interestingly, Mg²⁺ can be replaced by Ca²⁺ , but only to sustain ATP hydrolysis and not ATP synthesis. When Ca²⁺ inserts in F₁ , the torque generation built by the chemomechanical coupling between F₁ and the rotating central stalk was reported as unable to drive the transmembrane H⁺ flux within F_O . However, the failed H⁺ translocation is not consistent with the oligomycin-sensitivity of the Ca²⁺ -dependent F₁ F_O -ATP(hydrol)ase. New enzyme roles in mitochondrial energy transduction are suggested by recent advances. Accordingly, the structural F₁ F_O -ATPase distortion driven by ATP hydrolysis sustained by Ca²⁺ is consistent with the permeability transition pore signal propagation pathway. The Ca²⁺ -activated F₁ F_O -ATPase, by forming the pore, may contribute to dissipate the transmembrane H⁺ gradient created by the same enzyme complex.

A cyclopeptide and three oligomycin-class polyketides produced by an underexplored actinomycete of the genus Pseudosporangium

Aside from the well-studied conventional actinomycetes such as Streptomyces, the less investigated genera of actinomycetes also represent a promising source of natural products. Genome mining indicated that members of the underexplored genus Pseudosporangium, from which no secondary metabolites have been reported to date, may harbor the biosynthetic machinery for the formation of novel natural products. The strain RD062863, that is available at a public culture collection, was obtained and subjected to metabolite analysis, which resulted in the discovery of a novel cyclopeptide, pseudosporamide (1), along with three new oligomycin-class polyketides, pseudosporamicins A–C (2–4). The unusual structure of compound 1, featured by a biaryl-bond bridging across a tripeptide scaffold, N-acetyl-ʟ-Tyr-ʟ-Pro-ʟ-Trp, was determined by a combination of spectroscopic analyses, chemical derivatization, ECD calculation, and DFT-based theoretical chemical shift calculation, revealing the presence of an (S_a)-axial chirality around the biaryl bond. Compounds 2–4 lacked hydroxylation on the side chain of the spiroacetal rings, which showed clear contrast to other oligomycin congeners and related polyketides with ring-truncation or expansion. The new macrolides 2–4 displayed potent antimicrobial activity against the Gram-positive bacterium Kocuria rhizohpila and the plant pathogenic fungus Glomerella cingulata. All compounds showed moderate cytotoxicity against P388 murine leukemia cells with IC₅₀ values in the micromolar to submicromolar ranges. These results exemplified the validity of phylogeny-focused strain selection combined with biosynthetic gene-directed genome mining for the efficient discovery of new natural products.

The occurrence and control of nitric oxide generation by the plant mitochondrial electron transport chain

The plant mitochondrial electron transport chain (ETC) is bifurcated such that electrons from ubiquinol are passed to oxygen via the usual cytochrome path or through alternative oxidase (AOX). We previously showed that knockdown of AOX in transgenic tobacco increased leaf concentrations of nitric oxide (NO), implying that an activity capable of generating NO had been effected. Here, we identify the potential source of this NO. Treatment of leaves with antimycin A (AA, Qi -site inhibitor of Complex III) increased NO amount more than treatment with myxothiazol (Myxo, Qo -site inhibitor) despite both being equally effective at inhibiting respiration. Comparison of nitrate-grown wild-type with AOX knockdown and overexpression plants showed a negative correlation between AOX amount and NO amount following AA. Further, Myxo fully negated the ability of AA to increase NO amount. With ammonium-grown plants, neither AA nor Myxo strongly increased NO amount in any plant line. When these leaves were supplied with nitrite alongside the AA or Myxo, then the inhibitor effects across lines mirrored that of nitrate-grown plants. Hence the ETC, likely the Q-cycle of Complex III generates NO from nitrite, and AOX reduces this activity by acting as a non-energy-conserving electron sink upstream of Complex III.

Structural Analysis of the Streptomyces avermitilis CYP107W1-Oligomycin A Complex and Role of the Tryptophan 178 Residue

CYP107W1 from Streptomyces avermitilis is a cytochrome P450 enzyme involved in the biosynthesis of macrolide oligomycin A. A previous study reported that CYP107W1 regioselectively hydroxylated C12 of oligomycin C to produce oligomycin A, and the crystal structure of ligand free CYP107W1 was determined. Here, we analyzed the structural properties of the CYP107W1-oligomycin A complex and characterized the functional role of the Trp178 residue in CYP107W1. The crystal structure of the CYP107W1 complex with oligomycin A was determined at a resolution of 2.6 Å. Oligomycin A is bound in the substrate access channel on the upper side of the prosthetic heme mainly by hydrophobic interactions. In particular, the Trp178 residue in the active site intercalates into the large macrolide ring, thereby guiding the substrate into the correct binding orientation for a productive P450 reaction. A Trp178 to Gly mutation resulted in the distortion of binding titration spectra with oligomycin A, whereas binding spectra with azoles were not affected. The Gly178 mutant's catalytic turnover number for the 12-hydroxylation reaction of oligomycin C was highly reduced. These results indicate that Trp178, located in the open pocket of the active site, may be a critical residue for the productive binding conformation of large macrolide substrates.

Novel features on the regulation by mitochondria of calcium and secretion transients in chromaffin cells challenged with acetylcholine at 37°C

From experiments performed at room temperature, we know that the buffering of Ca²⁺ by mitochondria contributes to the shaping of the bulk cytosolic calcium transient ([Ca²⁺]_c) and secretion transients of chromaffin cells stimulated with depolarizing pulses. We also know that the mitochondrial Ca²⁺ transporters and the release of catecholamine are faster at 37°C with respect to room temperature. Therefore, we planned this investigation to gain further insight into the contribution of mitochondrial Ca²⁺ buffering to the shaping of [Ca²⁺]_c and catecholamine release transients, using some novel experimental conditions that have not been yet explored namely: (1) perifusion of bovine chromaffin cells (BCCs) with saline at 37°C and their repeated challenging with the physiological neurotransmitter acetylcholine (ACh); (2) separate blockade of mitochondrial Ca²⁺ uniporter (mCUP) with Ru360 or the mitochondrial Na⁺/Ca²⁺ exchanger (mNCX) with CGP37157; (3) full blockade of the mitochondrial Ca²⁺ cycling (mCC) by the simultaneous inhibition of the mCUP and the mNCX. Ru360 caused a pronounced delay of [Ca²⁺]_c clearance and augmented secretion. In contrast, CGP37157 only caused a tiny delay of [Ca²⁺]_c clearance and a mild decrease in secretion. The mCC resulting in continued Ca²⁺ uptake and its release back into the cytosol was interrupted by combined Ru360 + CGP37157 (Ru/CGP), the protonophore carbonyl cyanide‐p‐trifluoromethoxyphenylhydrazone, or combined oligomycin + rotenone (O/R); these three treatments caused a mild but sustained elevation of basal [Ca²⁺]_c that, however, was not accompanied by a parallel increase in basal secretion. Nevertheless, all treatments caused a pronounced augmentation of ACh‐induced secretion, with minor changes of the ACh‐induced [Ca²⁺]_c transients. Combined Ru/CGP did not alter the resting membrane potential in current‐clamped cells. Additionally, Ru/CGP did not increase basal [Ca²⁺]_c near subplasmalemmal sites and caused a mild decrease in the size of the readily releasable vesicle pool. Our results provide new functional features in support of the view that in BCCs there are two subpopulations of mitochondria, M1 underneath the plasmalemma nearby exocytotic sites and M2 at the core cell nearby vesicle transport sites. While M1 serves to shape the ACh‐elicited exocytotic response through its efficient Ca²⁺ removal by the mCUP, M2 shapes the lower [Ca²⁺]_c elevations required for new vesicle supply to the exocytotic machinery, from the large reserve vesicle pool at the cell core. The mCUP of the M1 pool seems to play a more prominent role in controlling the ACh responses, in comparison with the mNCX.

Regulation by mitochondria of exocytosis at 37°C.

Targeting deficiencies in mitochondrial respiratory complex I and functional uncoupling exerts anti-seizure effects in a genetic model of temporal lobe epilepsy and in a model of acute temporal lobe seizures

Mitochondria actively participate in neurotransmission by providing energy (ATP) and maintaining normative concentrations of reactive oxygen species (ROS) in both presynaptic and postsynaptic elements. In human and animal epilepsies, ATP-producing respiratory rates driven by mitochondrial respiratory complex (MRC) I are reduced, antioxidant systems are attenuated and oxidative damage is increased. We report that MRCI-driven respiration and functional uncoupling (an inducible antioxidant mechanism) are reduced and levels of H2O2 are elevated in mitochondria isolated from KO mice. Experimental impairment of MRCI in WT hippocampal slices via rotenone reduces paired-pulse ratios (PPRs) at mossy fiber-CA3 synapses (resembling KO PPRs), and exacerbates seizure-like events in vitro. Daily treatment with AATP [a combination therapy composed of ascorbic acid (AA), alpha-tocopherol (T), sodium pyruvate (P) designed to synergistically target mitochondrial impairments] improved mitochondrial functions, mossy fiber PPRs, and reduced seizure burden index (SBI) scores and seizure incidence in KO mice. AATP pretreatment reduced severity of KA-induced seizures resulting in 100% protection from the severe tonic-clonic seizures in WT mice. These data suggest that restoration of bioenergetic homeostasis in the brain may represent a viable anti-seizure target for temporal lobe epilepsy.

Dual actions of the metabolic inhibitor, sodium azide on K(ATP) channel currents in the rat CRI-G1 insulinoma cell line

1. The effects of various inhibitors of the mitochondrial electron transport chain on the activity of ATP-sensitive K+ channels were examined in the Cambridge rat insulinoma G1 (CRI-G1) cell line using a combination of whole cell and single channel recording techniques. 2. Whole cell current clamp recordings, with 5 mM ATP in the pipette, demonstrate that the mitochondrial uncoupler sodium azide (3 mM) rapidly hyperpolarizes CRI-G1 cells with a concomitant increase in K+ conductance. This is due to activation of K(ATP) channels as the sulphonylurea tolbutamide (100 microM) completely reversed the actions of azide. Other inhibitors of the mitochondrial electron transport chain, rotenone (10 microM) or oligomycin (2 microM) did not hyperpolarize CRI-G1 cells or increase K+ conductance. 3. In cell-attached recordings, bath application of 3 mM sodium azide (in the absence of glucose) resulted in a rapid increase in K(ATP) channel activity, an action readily reversible by tolbutamide (100 microM). Application of sodium azide (3 mM), in the presence of Mg-ATP, to the intracellular surface of excised inside-out patches also increased K(ATP) channel activity, in a reversible manner. 4. In contrast, rotenone (10 microM) or oligomycin (2 microM) did not increase K(ATP) channel activity in either cell-attached, in the absence of glucose, or inside-out membrane patch recordings. 5. Addition of sodium azide (3 mM) to the intracellular surface of inside-out membrane patches in the presence of Mg-free ATP or the non-hydrolysable analogue 5'-adenylylimidodiphosphate (AMP-PNP) inhibited, rather than increased, K(ATP) channel activity. 6. In conclusion, sodium azide, but not rotenone or oligomycin, directly activates K(ATP) channels in CRI-G1 insulin secreting cells. This action of azide is similar to that reported previously for diazoxide.

A soluble mitochondrial ATP synthetase complex catalyzing ATP-phosphate and ATP-ADP exchange

The highly purified soluble ATP synthetase complex from mitochondria, containing energy-transfer Factor A (the terminal ADP phosphorylation enzyme of oxidative phosphorylation) and Factor D, catalyzes ATP-P(i) and ATP-ADP exchange reactions. The ATP-P(i) exchange activity is inhibited by low concentrations of the uncouplers of oxidative phosphorylation, oligomycin and p-chloromercnriphenylsulfonate. It is stimulated threefold by dithiothreitol and is Mg(++) dependent. Antiserum to coupling factor 1 (F(1)) also inhibits the ATP-P(i) exchange. The ATP-ADP exchange activity appears to be greater than the ATP-P(i) exchange activity. The results suggest that the nonphosphorylated high-energy intermediate (X approximately C), and possibly the phosphorylated intermediate (X approximately P), are formed on the synthetase. Sites of uncoupler and oligomycin action reside in the terminal ATP synthetase.