Ubiquinol:cytochrome c oxidoreductase (complex III). Effect of inhibitors on cytochrome b reduction in submitochondrial particles and the role of ubiquinone in complex III

Allergens of the urushiol family promote mitochondrial dysfunction by inhibiting the electron transport at the level of cytochromes b and chemically modify cytochrome c1

Background

Urushiols are pro-electrophilic haptens that cause severe contact dermatitis mediated by CD8⁺ effector T-cells and downregulated by CD4⁺ T-cells. However, the molecular mechanism by which urushiols stimulate innate immunity in the initial stages of this allergic reaction is poorly understood. Here we explore the sub-cellular mechanisms by which urushiols initiate the allergic response.

Results

Electron microscopy observations of mouse ears exposed to litreol (3-n-pentadecyl-10-enyl-catechol]) showed keratinocytes containing swollen mitochondria with round electron-dense inclusion bodies in the matrix. Biochemical analyses of sub-mitochondrial fractions revealed an inhibitory effect of urushiols on electron flow through the mitochondrial respiratory chain, which requires both the aliphatic and catecholic moieties of these allergens. Moreover, urushiols extracted from poison ivy/oak (mixtures of 3-n-pentadecyl-8,11,13 enyl/3-n-heptadecyl-8,11 enyl catechol) exerted a higher inhibitory effect on mitochondrial respiration than did pentadecyl catechol or litreol, indicating that the higher number of unsaturations in the aliphatic chain, stronger the allergenicity of urushiols. Furthermore, the analysis of radioactive proteins isolated from mitochondria incubated with ³H-litreol, indicated that this urushiol was bound to cytochrome c₁. According to the proximity of cytochromes c₁ and b, functional evidence indicated the site of electron flow inhibition was within complex III, in between cytochromes b_L (cyt b₅₆₆) and b_H (cyt b₅₆₂).

Conclusion

Our data provide functional and molecular evidence indicating that the interruption of the mitochondrial electron transport chain constitutes an important mechanism by which urushiols initiates the allergic response. Thus, mitochondria may constitute a source of cellular targets for generating neoantigens involved in the T-cell mediated allergy induced by urushiols.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40659-021-00357-z.

Deferoxamine Protects Stromal/Stem Cells of "Lull pgm System"-Processed Lipoaspirates Against Damages Induced by Mitochondrial Respiration Inhibition

BACKGROUND

The ischemic environment of the receiving area compromises the outcome of autologous fat grafts. The aim of this study was to isolate and expand the stromal vascular fraction from patient lipoaspirates and investigate the gain in cell viability exerted by some protective agents against the blockage of mitochondrial respiration.

METHODS

The aspirates were (1) washed, using the "Lull pgm system," (2) centrifuged and (3) decanted. The corresponding stromal vascular fractions were isolated, and after cell adherence selection, the stromal/stem cell subpopulations were exposed to Antimycin A for 1 h. Then, the protection induced by cell pretreatment with deferoxamine, diazoxide and IGF-1 was evaluated.

RESULTS

The residual cell viability of the "Lull pgm system"-washed samples was greater than that of the centrifuged samples (p < 0.05), and this advantage was maintained during the following 12 days of culture. The administration of 400 μM deferoxamine before Antimycin A treatment increased the number of viable cells from 56.5 to 80.8% (p < 0.05). On the contrary, the pretreatment with 250 μM diazoxide or 0.1 μg/ml IGF-1 did not exert any significant pro-survival action. Echinomycin abolished the positive effect of deferoxamine, suggesting that its protection involved HIF-1α.

CONCLUSIONS

Adipose-derived stromal-stem cells survive the inhibition of mitochondrial respiration better if the lipoaspirate is washed using the "Lull pgm system" rather than centrifuged. Moreover, a significant contribution to cell survival can be obtained by preconditioning stromal-stem cells with deferoxamine. In a clinical perspective, this drug could be safely administered before surgery to patients undergoing autologous fat transfer.

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The oxidative phosphorylation system in mammalian mitochondria

The chapter provides a review of the state of art of the oxidative phosphorylation system in mammalian mitochondria. The sections of the paper deal with: (i) the respiratory chain as a whole: redox centers of the chain and protonic coupling in oxidative phosphorylation (ii) atomic structure and functional mechanism of protonmotive complexes I, III, IV and V of the oxidative phosphorylation system (iii) biogenesis of oxidative phosphorylation complexes: mitochondrial import of nuclear encoded subunits, assembly of oxidative phosphorylation complexes, transcriptional factors controlling biogenesis of the complexes. This advanced knowledge of the structure, functional mechanism and biogenesis of the oxidative phosphorylation system provides a background to understand the pathological impact of genetic and acquired dysfunctions of mitochondrial oxidative phosphorylation.

Development of an assay for Complex I/Complex III of the respiratory chain using solid supported membranes and its application in mitochondrial toxicity screening in drug discovery

Membrane-bound transporter proteins are involved in cell signal transduction and metabolism as well as influencing key pharmacological properties such as drug bioavailability. The functional activity of transporters that belong to the group of electrically active membrane proteins can be directly monitored using the solid-supported membrane-based SURFE(2)R™ technology (SURFace Electrogenic Event Reader; Scientific Devices Heidelberg GmbH, Heidelberg, Germany). The method makes use of membrane fragments or vesicles containing transport proteins adsorbed onto solid-supported membrane-covered electrodes and allows the direct measurement of their activity. This technology has been used to develop a robust screening compatible assay for Complex I/Complex III, key components of the respiratory chain in 96-well microtiter plates. The assay was screened against 1,000 compounds from the ComGenex Lead-like small molecule library to ascertain whether mitochondrial liabilities might be an underlying, although undesirable feature of typical commercial screening libraries. Some 105 hits (compounds exhibiting >50% inhibition of Complex I/Complex III activity at 10 μM) were identified and their activities were subsequently confirmed in duplicate, yielding a confirmation rate of 68%. Analysis of the confirmed hits also provided evidence of structure-activity relationships and two compounds from one structural class were further evaluated in dose-response experiments. This study provides evidence that profiling of compounds for potential mitochondrial liabilities, even at an early stage of drug discovery, may be a necessary additional quality filter that should be considered during the compound screening and profiling cascade.

Mitochondria are a major source of paraquat-induced reactive oxygen species production in the brain

Paraquat (PQ(2+)) is a prototypic toxin known to exert injurious effects through oxidative stress and bears a structural similarity to the Parkinson disease toxicant, 1-methyl-4-pheynlpyridinium. The cellular sources of PQ(2+)-induced reactive oxygen species (ROS) production, specifically in neuronal tissue, remain to be identified. The goal of this study was to determine the involvement of brain mitochondria in PQ(2+)-induced ROS production. Highly purified rat brain mitochondria were obtained using a Percoll density gradient method. PQ(2+)-induced hydrogen peroxide (H(2)O(2)) production was measured by fluorometric and polarographic methods. The production of H(2)O(2) was evaluated in the presence of inhibitors and modulators of the mitochondrial respiratory chain. The results presented here suggest that in the rat brain, (a) mitochondria are a principal cellular site of PQ(2+)-induced H(2)O(2) production, (b) PQ(2+)-induced H(2)O(2) production requires the presence of respiratory substrates, (c) complex III of the electron transport chain is centrally involved in H(2)O(2) production by PQ(2+), and (d) the mechanism by which PQ(2+) generates H(2)O(2) depends on the mitochondrial inner transmembrane potential. These observations were further confirmed by measuring PQ(2+)-induced H(2)O(2) production in primary neuronal cells derived from the midbrain. These findings shed light on the mechanism through which mitochondria may contribute to ROS production by other environmental and endogenous redox cycling agents implicated in Parkinson's disease.

Identification of StiR, the first regulator of secondary metabolite formation in the myxobacterium Cystobacter fuscus Cb f17.1

Myxobacteria are well established as proficient producers of natural products with numerous biological activities. Although some knowledge has been gained regarding the biosynthesis of secondary metabolites in this class of bacteria, almost nothing is known about the underlying regulatory mechanisms. In order to identify regulatory elements, we submitted the argyrin and stigmatellin producer Cystobacter fuscus to a random transposon mutagenesis strategy and screened 1,000 mutants for the occurrence of strains showing remarkably increased or decreased production of these compounds. In addition to the identification of the stigmatellin biosynthetic gene cluster, a novel positive regulator (stiR) of stigmatellin production was identified after transposon recovery. In order to exclude secondary mutagenesis effects, a double cross-over mutagenesis strategy was applied to the strain resulting in the repeated generation of the transposon genotype. This strain was shown to be equally effected in natural product biosynthesis.

Identification and validation of the mitochondrial F1F0-ATPase as the molecular target of the immunomodulatory benzodiazepine Bz-423

Bz-423 is a 1,4-benzodiazepine that suppresses disease in lupus-prone mice by selectively killing pathogenic lymphocytes, and it is less toxic compared to current lupus drugs. Cells exposed to Bz-423 rapidly generate O(2)(-) within mitochondria, and this reactive oxygen species is the signal initiating apoptosis. Phage display screening revealed that Bz-423 binds to the oligomycin sensitivity conferring protein (OSCP) component of the mitochondrial F(1)F(0)-ATPase. Bz-423 inhibited the F(1)F(0)-ATPase in vitro, and reconstitution experiments demonstrated that inhibition was mediated by the OSCP. This target was further validated by generating cells with reduced OSCP expression using RNA interference and studying the sensitivity of these cells to Bz-423. Our findings help explain the efficacy and selectivity of Bz-423 for autoimmune lymphocytes and highlight the OSCP as a target to guide the development of novel lupus therapeutics.

Impact of inhibition of Qo site of mitochondrial complex III with myxothiazol on persistent sodium currents via superoxide and protein kinase C in rat hippocampal CA1 cells

Inhibition of Qo site of mitochondrial complex III under hypoxia has received attention, but its downstream pathways remain unclear. We used Qo site inhibitor myxothiazol to mimic the inhibition of the Qo site of complex III and studied the effects of the inhibition of this site on persistent and transient sodium currents and neuron excitability in rat hippocampal CA1 cells. The results showed myxothiazol apparently increased persistent sodium currents but with a weak effect on transient sodium currents; the effect of myxothiazol on persistent sodium currents was blocked by protein kinase C inhibitor and superoxide scavengers, but not by hydrogen peroxide scavenger and hydroxyl radical formation inhibitor; myxothiazol could increase the activity of protein kinase C and neuron excitability. These results suggest that the inhibition of Qo site of mitochondrial complex III increases persistent sodium currents via superoxide production and protein kinase C activation.

Ubiquinol oxidation in the cytochrome bc1 complex: Reaction mechanism and prevention of short-circuiting

This review is focused on the mechanism of ubiquinol oxidation by the cytochrome bc1 complex (bc1). This integral membrane complex serves as a "hub" in the vast majority of electron transfer chains. The bc1 oxidizes a ubiquinol molecule to ubiquinone by a unique "bifurcated" reaction where the two released electrons go to different acceptors: one is accepted by the mobile redox active domain of the [2Fe-2S] iron-sulfur Rieske protein (FeS protein) and the other goes to cytochrome b. The nature of intermediates in this reaction remains unclear. It is also debatable how the enzyme prevents short-circuiting that could happen if both electrons escape to the FeS protein. Here, I consider a reaction mechanism that (i) agrees with the available experimental data, (ii) entails three traits preventing the short-circuiting in bc1, and (iii) exploits the evident structural similarity of the ubiquinone binding sites in the bc1 and the bacterial photosynthetic reaction center (RC). Based on the latter congruence, it is suggested that the reaction route of ubiquinol oxidation by bc1 is a reversal of that leading to the ubiquinol formation in the RC. The rate-limiting step of ubiquinol oxidation is then the re-location of a ubiquinol molecule from its stand-by site within cytochrome b into a catalytic site, which is formed only transiently, after docking of the mobile redox domain of the FeS protein to cytochrome b. In the catalytic site, the quinone ring is stabilized by Glu-272 of cytochrome b and His-161 of the FeS protein. The short circuiting is prevented as long as: (i) the formed semiquinone anion remains bound to the reduced FeS domain and impedes its undocking, so that the second electron is forced to go to cytochrome b; (ii) even after ubiquinol is fully oxidized, the reduced FeS domain remains docked to cytochrome b until electron(s) pass through cytochrome b; (iii) if cytochrome b becomes (over)reduced, the binding and oxidation of further ubiquinol molecules is hampered; the reason is that the Glu-272 residue is turned towards the reduced hemes of cytochrome b and is protonated to stabilize the surplus negative charge; in this state, this residue cannot participate in the binding/stabilization of a ubiquinol molecule.

Respiratory complex III is required to maintain complex I in mammalian mitochondria

A puzzling observation in patients with oxidative phosphorylation (OXPHOS) deficiencies is the presence of combined enzyme complex defects associated with a genetic alteration in only one protein-coding gene. In particular, mutations in the mtDNA encoded cytochrome b gene are associated either with combined complex I+III deficiency or with only complex III deficiency. We have reproduced the combined complex I+III defect in mouse and human cultured cell models harboring cytochrome b mutations. In both, complex III assembly is impeded and causes a severe reduction in the amount of complex I, not observed when complex III activity was pharmacologically inhibited. Metabolic labeling in mouse cells revealed that complex I was assembled, although its stability was severely hampered. Conversely, complex III stability was not influenced by the absence of complex I. This structural dependence among complexes I and III was confirmed in a muscle biopsy of a patient harboring a nonsense cytochrome b mutation.

Tight binding of inhibitors to bovine bc1 complex is independent of the Rieske protein redox state. Consequences for semiquinone stabilization in the quinol oxidation site

To determine the effect of the redox state of the Rieske protein on ligand binding to the quinol oxidation site of the bc(1) complex, we measured the binding rate constants (k(1)) for stigmatellin and myxothiazol, at different concentrations of decylbenzoquinone or decylbenzoquinol, in the bovine bc(1) complex with the Rieske protein in the oxidized or reduced state. Stigmatellin and myxothiazol bound tightly and competitively with respect to quinone or quinol, independently of the redox state of the Rieske protein. In the oxidized bc(1) complex, the k(1) values for stigmatellin ( approximately 2.6 x 10(6) m(-1)s(-1)) and myxothiazol ( approximately 8 x 10(5) m(-1)s(-1)), and the dissociation constant (K(d)) for quinone, were similar between pH 6.5 and 9, indicating that ligand binding is independent of the protonation state of histidine 161 of the Rieske protein (pK(a) approximately 7.6). Reduction of the Rieske protein increased the k(1) value for stigmatellin and decreased the K(d) value for quinone by 50%, without modifying the k(1) for myxothiazol. These results indicate that reduction of the Rieske protein and protonation of histidine 161 do not induce a strong stabilization of ligand binding to the quinol oxidation site, as assumed in models that propose the existence of a highly stabilized semiquinone as a reaction intermediate during quinol oxidation.

The biosynthesis of the aromatic myxobacterial electron transport inhibitor stigmatellin is directed by a novel type of modular polyketide synthase

Deductions from the molecular analysis of the 65,000-bp stigmatellin biosynthetic gene cluster are reported. The biosynthetic genes (stiA-J) encode an unusual bacterial modular type I polyketide synthase (PKS) responsible for the formation of this aromatic electron transport inhibitor produced by the myxobacterium Stigmatella aurantiaca. Involvement of the PKS gene cluster in stigmatellin biosynthesis is shown using site-directed mutagenesis. One module of the PKS is assumed to be used iteratively during the biosynthetic process, which seems to involve an unusual transacylation of the biosynthetic intermediate from an acyl carrier protein domain back to the preceding ketosynthase domain. Finally, the polyketide chain which is presumably catalyzed by a novel C-terminal domain in StiJ that does not resemble thioesterases, is cyclized and aromatized. The presented results of feeding experiments are in good agreement with the proposed biosynthetic scheme. In contrast to all other PKS type I systems reported to date, each module of StiA-J is encoded on a separate gene. The gene cluster contains a "stand alone" O-methyltransferase and two unusual O-methyltransferase domains embedded in the PKS. In addition, inactivation of a cytochrome P450 monooxygenase-encoding gene involved in post-PKS hydroxylation of the aromatic ring leads to the formation of two novel stigmatellin derivatives.