The higher level of organization of the oxidative phosphorylation system: mitochondrial supercomplexes

Electron transport system supercomplexes affect reactive-oxygen species production and respiration in both a hibernator (Ictidomys tridecemlineatus) and a nonhibernator (Rattus norvegicus)

Across many taxa, the complexes of the electron transport system associate with each other within the inner mitochondrial membrane to form supercomplexes (SCs). These SCs are thought to confer some selective advantage, such as increasing cellular respiratory capacity or decreasing the production of damaging reactive oxygen species (ROS). In this study, we investigate the relationship between supercomplex abundance and performance of liver mitochondria isolated from rats that do not hibernate and hibernating ground squirrels in which metabolism fluctuates substantially. We quantified the abundance of SCs (respirasomes (SCs containing CI, CIII, and CIV) or SCs containing CIII and CIV) and examined the relationship with state 3 (OXPHOS) and state 4 (LEAK) respiration rate, as well as net ROS production. We found that, in rats, state 3 and 4 respiration rate correlated negatively with respirasome abundance, but positively with CIII/CIV SC abundance. Despite the greater range of respiration rates in different hibernation stages, these relationships were similar in ground squirrels. This is, to our knowledge, the first report of differential effects of supercomplex types on mitochondrial respiration and ROS production.

An Experimental Approach to Address the Functional Relationship between Antioxidant Enzymes and Mitochondrial Respiratory Complexes

Mitochondrial dysfunction and cytosolic oxidative stress are pathological biomarkers interlinked in several chronic diseases and cellular toxicity promoted by high-energy radiation or xenobiotics. Thus, assessing the activities of the mitochondrial redox chain complexes and the cytosolic antioxidant enzymes in the same cell culture system is a valuable approach to addressing the challenge of chronic diseases or unveiling the molecular mechanisms underlying the toxicity of physical and chemical stress agents. The present article gathers the experimental procedures to obtain, from isolated cells, a mitochondria-free cytosolic fraction and a mitochondria-rich fraction. Furthermore, we describe the methodologies to evaluate the activity of the main antioxidant enzymes in the mitochondria-free cytosolic fraction (superoxide dismutase, catalase, glutathione reductase and glutathione peroxidase), and the activity of the individual mitochondrial complexes I, II and IV, as well as the conjugated activity of complexes I-III and complexes II-III in the mitochondria-rich fraction. The protocol to test the citrate synthase activity was also considered and used to normalize complexes. The procedures were optimized within an experimental setup to allow that each condition to be tested only requires sampling of one T-25 flask of cells 2D cultured, as the typical results presented and discussed here.

Influence of Maternal Exercise on Glucose and Lipid Metabolism in Offspring Stem Cells: ENHANCED by Mom

CONTEXT

Recent preclinical data suggest exercise during pregnancy can improve the metabolic phenotype not only of the mother, but of the developing offspring as well. However, investigations in human offspring are lacking.

OBJECTIVE

To characterize the effect of maternal aerobic exercise on the metabolic phenotype of the offspring's mesenchymal stem cells (MSCs).

DESIGN

Randomized controlled trial.

SETTING

Clinical research facility.

PATIENTS

Healthy female adults between 18 and 35 years of age and ≤ 16 weeks' gestation.

INTERVENTION

Mothers were randomized into 1 of 2 groups: aerobic exercise (AE, n = 10) or nonexercise control (CTRL, n = 10). The AE group completed 150 minutes of weekly moderate-intensity exercise, according to American College of Sports Medicine guidelines, during pregnancy, whereas controls attended stretching sessions.

MAIN OUTCOME MEASURES

Following delivery, MSCs were isolated from the umbilical cord of the offspring and metabolic tracer and immunoblotting experiments were completed in the undifferentiated (D0) or myogenically differentiated (D21) state.

RESULTS

AE-MSCs at D0 had an elevated fold-change over basal in insulin-stimulated glycogen synthesis and reduced nonoxidized glucose metabolite (NOGM) production (P ≤ 0.05). At D21, AE-MSCs had a significant elevation in glucose partitioning toward oxidation (oxidation/NOGM ratio) compared with CTRL (P ≤ 0.05). Immunoblot analysis revealed elevated complex I expression in the AE-MSCs at D21 (P ≤ 0.05). Basal and palmitate-stimulated lipid metabolism was similar between groups at D0 and D21.

CONCLUSIONS

These data provide evidence of a programmed metabolic phenotype in human offspring with maternal AE during pregnancy.

Role of Copper on Mitochondrial Function and Metabolism

Copper is essential for life processes like energy metabolism, reactive oxygen species detoxification, iron uptake, and signaling in eukaryotic organisms. Mitochondria gather copper for the assembly of cuproenzymes such as the respiratory complex IV, cytochrome c oxidase, and the antioxidant enzyme superoxide dismutase 1. In this regard, copper plays a role in mitochondrial function and signaling involving bioenergetics, dynamics, and mitophagy, which affect cell fate by means of metabolic reprogramming. In mammals, copper homeostasis is tightly regulated by the liver. However, cellular copper levels are tissue specific. Copper imbalances, either overload or deficiency, have been associated with many diseases, including anemia, neutropenia, and thrombocytopenia, as well as tumor development and cancer aggressivity. Consistently, new pharmacological developments have been addressed to reduce or exacerbate copper levels as potential cancer therapies. This review goes over the copper source, distribution, cellular uptake, and its role in mitochondrial function, metabolic reprograming, and cancer biology, linking copper metabolism with the field of regenerative medicine and cancer.

Non-energy mechanism of phosphocreatine on the protection of cell survival

If mitochondrial energy availability or oxidative metabolism is altered, patients will suffer from insufficient energy supply Phosphocreatine (PCr) not only acts as an energy carrier, but also acts as an antioxidant and defensive agent to maintain the integrity and stability of the membrane, to maintain ATP homeostasis through regulating mitochondrial respiration. Meanwhile, PCr can enhance calcium balance and reduce morphological pathological changes, ultimately, PCr helps to reduce apoptosis. On the other aspect, the activities of ATP synthase and MitCK play a crucial role in the maintenance of cellular energy metabolic function. It is interesting to note, PCr not only rises the activities of ATP synthase as well as MitCK, but also promotes these two enzymatic reactions. Additionally, PCr can also inhibit mitochondrial permeability transition in a concentration-dependent manner, prevent ROS and CytC from spilling into the cytoplasm, thereby inhibit the release of proapoptotic factors caspase-3 and caspase-9, and eventually, effectively prevent LPS-induced apoptosis of cells. Understandably, PCr prevents the apoptosis caused by abnormal mitochondrial energy metabolism and has a protective role in a non-energy manner. Moreover, recent studies have shown that PCr protects cell survival through PI3K/Akt/eNOS, MAPK pathway, and inhibition of Ang II-induced NF-κB activation. Furthermore, PCr antagonizes oxidative stress through the activation of PI3K/Akt/GSK3b intracellular pathway, PI3K/AKT-PGC1α signaling pathway, while through the promotion of SIRT3 expression to maintain normal cell metabolism. Interestingly, PCr results in delaying the time to enter pathological metabolism through the delayed activation of AMPK pathway, which is different from previous studies, now we propose the hypothesis that the "miRNA-JAK2/STAT3 -CypD pathway" may take part in protecting cells from apoptosis, PCr may be further be involved in the dynamic relationship between CypD and STAT3. Furthermore, we believe that PCr and CypD would be the central link to maintain cell survival and maintain cell stability and mitochondrial repair under the mitochondrial dysfunction caused by oxidative stress. This review provides the modern progress knowledge and views on the molecular mechanism and molecular targets of PCr in a non-energy way.

Mitochondria and the permeability transition pore in cancer metabolic reprogramming

Mitochondria are a major source of ATP provision as well as cellular suicidal weapon store. Accumulating evidences demonstrate that mitochondrial bioenergetics, biosynthesis and signaling are important mediators of tumorigenesis. Metabolic plasticity enables cancer cell reprogramming to cope with cellular and environmental alterations, a process requires mitochondria biology. Mitochondrial metabolism emerges to be a promising arena for cancer therapeutic targets. The permeability transition pore (PTP) participates in physiological Ca²⁺ and ROS homeostasis as well as cell death depending on the open state. The hypothesis that PTP forms from F-ATP synthase provides clues to the potential collaborative role of mitochondrial respiration and PTP in regulating cancer cell fate and metabolic reprogramming.

Mitochondrial Dysfunction in Mitochondrial Medicine: Current Limitations, Pitfalls, and Tomorrow

Until recently restricted to hereditary mitochondrial diseases, mitochondrial dysfunction is now recognized as a key player and strategic factor in the pathophysiological of many human diseases, ranging from the metabolism, vascular, cardiac, and neurodegenerative diseases to cancer. Because of their participation in a myriad of cellular functions and signaling pathways, precisely identifying the cause of mitochondrial "dysfunctions" can be challenging and requires robust and controlled techniques. Initially limited to the analysis of the respiratory chain functioning, these analytical techniques now enlarge to the analyses of mitochondrial and cellular metabolism, based on metabolomic approaches.Here, we address the methods used to assay mitochondrial dysfunction, with a highlight on the techniques used in diagnosis on tissues and cells derived from patients, the information they provide, and their strength and weakness.Targeting mitochondrial dysfunction by various strategies is a huge challenge, requires robust methods of evaluation, and should be able to take into consideration the mitochondria dynamics and localization. The future of mitochondrial medicine is strongly related to a perfect comprehension of its dysfunction.

Intellectual disability-associated factor Zbtb11 cooperates with NRF-2/GABP to control mitochondrial function

Zbtb11 is a conserved transcription factor mutated in families with hereditary intellectual disability. Its precise molecular and cellular functions are currently unknown, precluding our understanding of the aetiology of this disease. Using a combination of functional genomics, genetic and biochemical approaches, here we show that Zbtb11 plays essential roles in maintaining the homeostasis of mitochondrial function. Mechanistically, we find Zbtb11 facilitates the recruitment of nuclear respiratory factor 2 (NRF-2) to its target promoters, activating a subset of nuclear genes with roles in the biogenesis of respiratory complex I and the mitoribosome. Genetic inactivation of Zbtb11 resulted in a severe complex I assembly defect, impaired mitochondrial respiration, mitochondrial depolarisation, and ultimately proliferation arrest and cell death. Experimental modelling of the pathogenic human mutations showed these have a destabilising effect on the protein, resulting in reduced Zbtb11 dosage, downregulation of its target genes, and impaired complex I biogenesis. Our study establishes Zbtb11 as an essential mitochondrial regulator, improves our understanding of the transcriptional mechanisms of nuclear control over mitochondria, and may help to understand the aetiology of Zbtb11-associated intellectual disability.

Towards understanding the evolutionary dynamics of mtDNA

Historically, mtDNA was considered a selectively neutral marker that was useful for estimating the population genetic history of the maternal lineage. Over time there has been an increasing appreciation of mtDNA and mitochondria in maintaining cellular and organismal health. Beyond energy production, mtDNA and mitochondria have critical cellular roles in signalling. Here we briefly review the structure of mtDNA and the role of the mitochondrion in energy production. We then discuss the predictions that can be obtained from quaternary structure modelling and focus on mitochondrial complex I. Complex I is the primary entry point for electrons into the electron transport system is the largest respiratory complex of the chain and produces about 40% of the proton flux used to synthesize ATP. A focus of the review is Drosophila's utility as a model organism to study the selective advantage of specific mutations. However, we note that the incorporation of insights from a multitude of systems is necessary to fully understand the range of roles that mtDNA has in organismal fitness. We speculate that dietary changes can illicit stress responses that influence the selective advantage of specific mtDNA mutations and cause spatial and temporal fluctuations in the frequencies of mutations. We conclude that developing our understanding of the roles mtDNA has in determining organismal fitness will enable increased evolutionary insight and propose we can no longer assume it is evolving as a strictly neutral marker without testing this hypothesis.

Mitochondrial Complex I Is an Essential Player in LPS-Induced Preconditioning in Differentiated PC12 Cells

Preconditioning (PC) as a protective strategy against noxious insults can decline cell death and apoptosis. It has been approved that mitochondria play a key role in PC mechanism. The critical role of complex I (CI) in oxidative phosphorylation machinery and intracellular ROS production, particularly in the brain, accentuates its possible role in PC-induced neuroprotection. Here, differentiated PC12 cells were preconditioned with ultra-low dose LPS (ULD, 3 μg/mL) prior to exposure to high concentration of LPS (HD, 750 μg/mL). Our results showed that HD LPS treatment reduces cell viability and CI activity, and intensifies expression of cleaved caspase 3 compared to the control group. Intriguingly, PC induction resulted in enhancement of cell viability and CI activity and reduction of caspase3 cleavage compared to HD LPS group. In order to explore the role of CI in PC, we combined the ULD LPS with rotenone, a CI inhibitor. Following rotenone administration, cell viability significantly reduced while caspase3 cleavage increased compared to PC induction group. Taken together, cell survival and reduction of apoptosis followed by PC can be at least partially attributed to the preservation of mitochondrial CI function.

Effect of respiratory inhibitors on mitochondrial complexes and ADP/ATP translocators in the Triticum aestivum roots

Effective functioning of the mitochondrial complexes of the oxidative phosphorylation (OXPHOS) system is necessary for ATP synthesis. The OXPHOS complexes exist both as individual forms and supercomplexes, whose formation and stability are supported by specific protein and lipid factors. In this paper, we report on the types and activities of OXPHOS complexes and supercomplexes from wheat (Triticum aestivum L.) root mitochondria analyzed by blue native polyacrylamide gel electrophoresis (BN-PAGE). The activity of OXPHOS complexes decreased when a mixture of rotenone, an inhibitor of complex I, and antimycin A, an inhibitor of complex III (R + AA) was applied to the BN-PAGE gels. By contrast, the types and activities of the OXPHOS complexes and supercomplexes did not change when they were isolated from the R + AA treated roots. However, the amount of the mitochondrial membrane-bound low molecular mass proteins in these roots markedly increased. The proteins were identified as ANT1 and ANT2 (ADP/ATP translocators) and ABA 8'-hydroxylase. We suggest that these low molecular mass proteins contribute to fine control mechanisms that stabilize mitochondrial supercomplexes and help to overcome an inhibitor-induced energy deficit by enhancing ADP/ATP transfer and ultimately improving the supply of ATP.

Mitochondrial respiration of complex II is not lower than that of complex I in mouse skeletal muscle

Skeletal muscle (SKM) requires a large amount of energy, which is produced mainly by mitochondria, for their daily functioning. Of the several mitochondrial complexes, it has been reported that the dysfunction of complex II is associated with several diseases, including myopathy. However, the degree to which complex II contributes to ATP production by mitochondria remains unknown. As complex II is not included in supercomplexes, which are formed to produce ATP efficiently, we hypothesized that complex II-linked respiration was lower than that of complex I. In addition, differences in the characteristics of complex I and II activity suggest that different factors might regulate their function. The isolated mitochondria from gastrocnemius muscle was used for mitochondrial respiration measurement and immunoblotting in male C57BL/6J mice. Student paired t-tests were performed to compare means between two groups. A univariate linear regression model was used to determine the correlation between mitochondrial respiration and proteins. Contrary to our hypothesis, complex II-linked respiration was not significantly less than complex I-linked respiration in SKM mitochondria (complex I vs complex II, 3402 vs 2840 pmol/[s × mg]). Complex I-linked respiration correlated with the amount of complex I incorporated in supercomplexes (r = 0.727, p < 0.05), but not with the total amount of complex I subunits. In contrast, complex II-linked respiration correlated with the total amount of complex II (r = 0.883, p < 0.05), but not with the amount of each complex II subunit. We conclude that both complex I and II play important roles in mitochondrial respiration and that the assembly of both supercomplexes and complex II is essential for the normal functioning of complex I and II in mouse SKM mitochondria.

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Complex II-linked respiration was comparable to complex I-linked respiration in isolated skeletal muscle mitochondria.

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Complex I-linked respiration correlated with the amount of complex I incorporated in supercomplexes, but not with the complex I subunit.

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Complex II-linked respiration correlated with the amount of complex II, but not with the SDH subunit.

Cross-talk between guanidinoacetate neurotoxicity, memory and possible neuroprotective role of creatine

Guanidinoacetate Methyltransferase deficiency is an inborn error of metabolism that results in decreased creatine and increased guanidinoacetate (GAA) levels. Patients present neurological symptoms whose mechanisms are unclear. We investigated the effects of an intrastriatal administration of 10 μM of GAA (0.02 nmol/striatum) on energy metabolism, redox state, inflammation, glutamate homeostasis, and activities/immunocontents of acetylcholinesterase and Na⁺,K⁺-ATPase, as well as on memory acquisition. The neuroprotective role of creatine was also investigated. Male Wistar rats were pretreated with creatine (50 mg/kg) or saline for 7 days underwenting stereotactic surgery. Forty-eight hours after surgery, the animals (then sixty-days-old) were divided into groups: Control, GAA, GAA + Creatine, and Creatine. Experiments were performed 30 min after intrastriatal infusion. GAA decreased SDH, complexes II and IV activities, and ATP levels, but had no effect on mitochondrial mass/membrane potential. Creatine totally prevented SDH and complex II, and partially prevented COX and ATP alterations. GAA increased dichlorofluorescein levels and decreased superoxide dismutase and catalase activities. Creatine only prevented catalase and dichlorofluorescein alterations. GAA increased cytokines, nitrites levels and acetylcholinesterase activity, but not its immunocontent. Creatine prevented such effects, except nitrite levels. GAA decreased glutamate uptake, but had no effect on the immunocontent of its transporters. GAA decreased Na⁺,K⁺-ATPase activity and increased the immunocontent of its α3 subunit. The performance on the novel object recognition task was also impaired. Creatine partially prevented the changes in glutamate uptake and Na⁺,K⁺-ATPase activity, and completely prevented the memory impairment. This study helps to elucidate the protective effects of creatine against the damage caused by GAA.

Protective role of (Bronco-T) against formaldehyde induced antioxidant, oxidative and histopathological changes in lung of male Wistar rats

The present study was sought to evaluate the oxidative, antioxidant status and histopathological changes by the acute chronic exposure of formaldehyde. Bronco-T a poly-herbal formulation treatment, changes the oxidative, antioxidant status and histopathology of rat lungs with antioxidant and regenerative property. In this experiment thirty adult male albino Wister rats were used for the study and subdivided in to five groups consist of 6 rats for each group. Group-I served as control and the other 4 groups such as II, III, IV and V are considered as experimental. The control and treatment rats are maintained for 21 days of experimental period. Experimental rats are exposed to 40 percent formaldehyde for 1 h treated with Bronco-T and salbutamol. In the present investigation, the formaldehyde exposed rats a series of free radical chain reactions were grimly provoked, the evaluation of antioxidant enzymes (SOD, CAT), other enzymes oxidative enzymes (G-6-PDH, SDH) and (ALT, ALAT and LDH) were measured. A clear assertive imbalance between oxidation and anti-oxidation status was critically observed, and oxidative stress was clearly exacerbated in lung tissue leading to altrations in architecture of lung histopathology. Oral gavage Bronco-T exhibits a beneficial action by bringing normal architecture in lung tissue of formaldehyde inhaled rats with antioxidant properties. Bronco-T treatment may be a suitable remedy for formalin occupational diseases.

Integration of demand and supply sides in the ATP energy economics of cells

The central aspects of the energy economics of living cells revolve around the synthesis and utilization of molecules of adenosine triphosphate (ATP). Current descriptions of cell metabolism and its regulation in most textbooks of biochemistry assume that enzymes and transporters behave in the same way in isolation and in a cell. Calculations of the mechanistic or maximal P/O ratios in oxidative phosphorylation by mammalian cells generally consider only the supply side of the problem without linking to ATP-demand processes. The purpose of this article is to calculate the mechanistic P/O ratio by integration of the supply and demand sides of ATP reactions. The mechanistic stoichiometry calculated from an integrated approach is compared with that obtained from the standard model that considers only ATP supply. After accounting for leaks, slips, and other losses, the actual or operative P/O calculated by the integrated method is found to be in good agreement with the experimental values of the P/O ratio determined in mitochondria for both succinate and NADH-linked respiratory substrates. The thermodynamic consequences of these results and the biological implications are discussed. An integrated model of oxidative phosphorylation that goes beyond the chemiosmotic theory is presented, and a solution to the longstanding fundamental problem of respiratory control is found.

Mitochondrial localization, import, and mitochondrial function of the androgen receptor

Nuclear localization of androgen receptor (AR) directs transcriptional regulation of a host of genes, referred to as genomic signaling. Additionally, nonnuclear or nongenomic activities of the AR have long been described, but understanding of these activities remains elusive. Here, we report that AR is imported into and localizes to mitochondria and has a novel role in regulating multiple mitochondrial processes. Employing complementary experimental approaches of AR knockdown in AR-expressing cells and ectopic AR expression in AR-deficient cells, we demonstrate an inverse relationship between AR expression and mitochondrial DNA (mtDNA) content and transcription factor A, mitochondrial (TFAM), a regulator of mtDNA content. We show that AR localizes to mitochondria in prostate tissues and cell lines and is imported into mitochondria in vitro We also found that AR contains a 36-amino-acid-long mitochondrial localization sequence (MLS) capable of targeting a passenger protein (GFP) to the mitochondria and that deletion of the MLS abolishes the import of AR into the mitochondria. Ectopic AR expression reduced the expression of oxidative phosphorylation (OXPHOS) subunits. Interestingly, AR also controlled translation of mtDNA-encoded genes by regulating expression of multiple nuclear DNA-encoded mitochondrial ribosomal proteins. Consistent with these observations, OXPHOS supercomplexes were destabilized, and OXPHOS enzymatic activities were reduced in AR-expressing cells and restored upon AR knockdown. Moreover, mitochondrial impairment induced AR expression and increased its translocation into mitochondria. We conclude that AR localizes to mitochondria, where it controls multiple mitochondrial functions and mitonuclear communication. Our studies also suggest that mitochondria are novel players in nongenomic activities of AR.

The OXPHOS supercomplex assembly factor HIG2A responds to changes in energetic metabolism and cell cycle

HIG2A promotes cell survival under hypoxia and mediates the assembly of complex III and complex IV into respiratory chain supercomplexes. In the present study, we show that human HIGD2A and mouse Higd2a gene expressions are regulated by hypoxia, glucose, and the cell cycle-related transcription factor E2F1. The latter was found to bind the promoter region of HIGD2A. Differential expression of the HIGD2A gene was found in C57BL/6 mice in relation to tissue and age. Besides, the silencing of HIGD2A evidenced the modulation of mitochondrial dynamics proteins namely, OPA1 as a fusion protein increases, while FIS1, a fission protein, decreases. Besides, the mitochondrial membrane potential (ΔΨm) increased. The protein HIG2A is localized in the mitochondria and nucleus. Moreover, we observed that the HIG2A protein interacts with OPA1. Changes in oxygen concentration, glucose availability, and cell cycle regulate HIGD2A expression. Alterations in HIGD2A expression are associated with changes in mitochondrial physiology.

Cysteine Supplementation May be Beneficial in a Subgroup of Mitochondrial Translation Deficiencies

BACKGROUND

Mitochondrial encephalomyopathies are severe, relentlessly progressive conditions and there are very few effective therapies available to date. We have previously suggested that in two rare forms of reversible mitochondrial disease (reversible infantile respiratory chain deficiency and reversible infantile hepatopathy) supplementation with L-cysteine can improve mitochondrial protein synthesis, since cysteine is required for the 2-thiomodification of mitochondrial tRNAs.

OBJECTIVES

We studied whether supplementation with L-cysteine or N-acetyl-cysteine (NAC) results in any improvement of the mitochondrial function in vitro in fibroblasts of patients with different genetic forms of abnormal mitochondrial translation.

METHODS

We studied in vitro in fibroblasts of patients carrying the common m.3243A>G and m.8344A>G mutations or autosomal recessive mutations in genes affecting mitochondrial translation, whether L-cysteine or N-acetyl-cysteine supplementation have an effect on mitochondrial respiratory chain function.

RESULTS

Here we show that supplementation with L-cysteine, but not with N-acetyl-cysteine partially rescues the mitochondrial translation defect in vitro in fibroblasts of patients carrying the m.3243A>G and m.8344A>G mutations. In contrast, N-acetyl-cysteine had a beneficial effect on mitochondrial translation in TRMU and MTO1 deficient fibroblasts.

CONCLUSIONS

Our results suggest that L-cysteine or N-acetyl-cysteine supplementation may be a potential treatment for selected subgroups of patients with mitochondrial translation deficiencies. Further studies are needed to explore the full potential of cysteine supplementation as a treatment for patients with mitochondrial disease.

Effects of tonic immobility and corticosterone on mitochondria metabolism in pectoralis major muscle of broiler chickens

Tonic immobility (TI), which can be divided into short (STI) or long (LTI) duration, is a trait related to fear and stress response. In a previous study, we found that in broilers that LTI phenotype and chronic corticosterone (CORT) administration caused retarded growth and lower muscle weight compared with their control counterparts. The aim of this study is to determine whether the mitochondrial DNA (mtDNA) copy number and mitochondrial oxidative phosphorylation (OXPHOS), the vital factors involved in regulating energy homeostasis, have been changed by LTI or CORT treatment. The results showed that STI broilers had higher mtDNA copy number and cytochrome c oxidase (COX) enzyme activity compared with LTI broilers. Analysis of mtDNA-encoded OXPHOS genes revealed that the mRNA expression of the COX subunit 1, 2, NADH dehydrogenase (ND) subunits 1, 3 and 6, were also increased in STI broilers compared with LTI broilers. Regarding the transcriptional regulation of mtDNA-encoded OXPHOS genes, no difference was found in the methylation of the mitochondria control region between the TI phenotypes or the CORT treatments. The PGC-1α protein level was higher in STI broilers, but the av uncoupling proteins, did not show significant difference at the protein level between TI phenotypes. These results suggest that the mitochondrial function in pectoralis major muscle of STI broilers is better than that of LTI counterparts. However, chronic CORT administration did not affect the mitochondrial metabolism, indicating the mitochondrial insensitivity to CORT treatment in pectoralis major muscle.

The Mitochondrial Permeability Transition Pore and ATP Synthase

Mitochondrial ATP generation by oxidative phosphorylation combines the stepwise oxidation by the electron transport chain (ETC) of the reducing equivalents NADH and FADH₂ with the generation of ATP by the ATP synthase. Recent studies show that the ATP synthase is not only essential for the generation of ATP but may also contribute to the formation of the mitochondrial permeability transition pore (PTP). We present a model, in which the PTP is located within the c-subunit ring in the F_o subunit of the ATP synthase. Opening of the PTP was long associated with uncoupling of the ETC and the initiation of programmed cell death. More recently, it was shown that PTP opening may serve a physiologic role: it can transiently open to regulate mitochondrial signaling in mature cells, and it is open in the embryonic mouse heart. This review will discuss how the ATP synthase paradoxically lies at the center of both ATP generation and cell death.

Mitochondrial cytochrome c oxidase: catalysis, coupling and controversies

Mitochondrial cytochrome c oxidase is a member of a diverse superfamily of haem-copper oxidases. Its mechanism of oxygen reduction is reviewed in terms of the cycle of catalytic intermediates and their likely chemical structures. This reaction cycle is coupled to the translocation of protons across the inner mitochondrial membrane in which it is located. The likely mechanism by which this occurs, derived in significant part from studies of bacterial homologues, is presented. These mechanisms of catalysis and coupling, together with current alternative proposals of underlying mechanisms, are critically reviewed.

Being right on Q: shaping eukaryotic evolution

Reactive oxygen species (ROS) formation by mitochondria is an incompletely understood eukaryotic process. I proposed a kinetic model [BioEssays (2011) 33, 88–94] in which the ratio between electrons entering the respiratory chain via FADH₂ or NADH (the F/N ratio) is a crucial determinant of ROS formation. During glucose breakdown, the ratio is low, while during fatty acid breakdown, the ratio is high (the longer the fatty acid, the higher is the ratio), leading to higher ROS levels. Thus, breakdown of (very-long-chain) fatty acids should occur without generating extra FADH₂ in mitochondria. This explains peroxisome evolution. A potential ROS increase could also explain the absence of fatty acid oxidation in long-lived cells (neurons) as well as other eukaryotic adaptations, such as dynamic supercomplex formation. Effective combinations of metabolic pathways from the host and the endosymbiont (mitochondrion) allowed larger varieties of substrates (with different F/N ratios) to be oxidized, but high F/N ratios increase ROS formation. This might have led to carnitine shuttles, uncoupling proteins, and multiple antioxidant mechanisms, especially linked to fatty acid oxidation [BioEssays (2014) 36, 634–643]. Recent data regarding peroxisome evolution and their relationships with mitochondria, ROS formation by Complex I during ischaemia/reperfusion injury, and supercomplex formation adjustment to F/N ratios strongly support the model. I will further discuss the model in the light of experimental findings regarding mitochondrial ROS formation.

1,25-Dihydroxyvitamin D3 prevents deleterious effects of homocysteine on mitochondrial function and redox status in heart slices

Because homocysteine (Hcy) is a risk factor for cardiovascular disease, and vitamin D deficiency can contribute to cardiovascular pathologies. In the present study, we tested the hypothesis that Hcy could impair energy metabolism, mitochondrial function, and redox status in heart slices of Wistar rats and that 1,25-dihydroxivitamin D₃ (calcitriol) treatment could prevent such effects. Heart slices were first pretreated with 3 different concentrations of calcitriol (50, 100, and 250nmol/L) for 30minutes at 37°C, after which Hcy was added to promote deleterious effects on metabolism. After 1 hour of incubation, the samples were washed, homogenized, and stored at -80°C before analysis. The results showed that Hcy caused changes in energy metabolism (respiratory chain enzymes), mitochondrial function, and cell viability. Homocysteine also induced oxidative stress, increasing lipid peroxidation, reactive oxygen species generation, and protein damage. An imbalance in antioxidant enzymes was also observed. Calcitriol (50nmol/L) reverted the effect of Hcy on the parameters tested, except for the immunocontent of catalase. Both treatments (calcitriol and Hcy) did not alter the vitamin D receptor immunocontent, which combined with the fact that our ex vivo model is acute, suggesting that the beneficial effect of calcitriol occurs directly through antioxidative mechanisms and not via gene expression. In this study, we show that Hcy impairs mitochondrial function and induces changes in the redox status in heart slices, which were reverted by calcitriol. These findings suggest that calcitriol may be a preventive/therapeutic strategy for complications caused by Hcy.

Mitochondrial protein interactome elucidated by chemical cross-linking mass spectrometry

Mitochondrial protein interactions and complexes facilitate mitochondrial function. These complexes range from simple dimers to the respirasome supercomplex consisting of oxidative phosphorylation complexes I, III, and IV. To improve understanding of mitochondrial function, we used chemical cross-linking mass spectrometry to identify 2,427 cross-linked peptide pairs from 327 mitochondrial proteins in whole, respiring murine mitochondria. In situ interactions were observed in proteins throughout the electron transport chain membrane complexes, ATP synthase, and the mitochondrial contact site and cristae organizing system (MICOS) complex. Cross-linked sites showed excellent agreement with empirical protein structures and delivered complementary constraints for in silico protein docking. These data established direct physical evidence of the assembly of the complex I-III respirasome and enabled prediction of in situ interfacial regions of the complexes. Finally, we established a database and tools to harness the cross-linked interactions we observed as molecular probes, allowing quantification of conformation-dependent protein interfaces and dynamic protein complex assembly.

Maternal High Fat Diet Alters Skeletal Muscle Mitochondrial Catalytic Activity in Adult Male Rat Offspring

A maternal high-fat (HF) diet during pregnancy can lead to metabolic compromise, such as insulin resistance in adult offspring. Skeletal muscle mitochondrial dysfunction is one mechanism contributing to metabolic impairments in insulin resistant states. Therefore, the present study aimed to investigate whether mitochondrial dysfunction is evident in metabolically compromised offspring born to HF-fed dams. Sprague-Dawley dams were randomly assigned to receive a purified control diet (CD; 10% kcal from fat) or a high fat diet (HFD; 45% kcal from fat) for 10 days prior to mating, throughout pregnancy and during lactation. From weaning, all male offspring received a standard chow diet and soleus muscle was collected at day 150. Expression of the mitochondrial transcription factors nuclear respiratory factor-1 (NRF1) and mitochondrial transcription factor A (mtTFA) were downregulated in HF offspring. Furthermore, genes encoding the mitochondrial electron transport system (ETS) respiratory complex subunits were suppressed in HF offspring. Moreover, protein expression of the complex I subunit, NDUFB8, was downregulated in HF offspring (36%), which was paralleled by decreased maximal catalytic linked activity of complex I and III (40%). Together, these results indicate that exposure to a maternal HF diet during development may elicit lifelong mitochondrial alterations in offspring skeletal muscle.

The mitochondrial permeability transition pore in AD 2016: An update

Over the past 30years the mitochondrial permeability transition - the permeabilization of the inner mitochondrial membrane due to the opening of a wide pore - has progressed from being considered a curious artifact induced in isolated mitochondria by Ca(2+) and phosphate to a key cell-death-inducing process in several major pathologies. Its relevance is by now universally acknowledged and a pharmacology targeting the phenomenon is being developed. The molecular nature of the pore remains to this day uncertain, but progress has recently been made with the identification of the FOF1 ATP synthase as the probable proteic substrate. Researchers sharing this conviction are however divided into two camps: these believing that only the ATP synthase dimers or oligomers can form the pore, presumably in the contact region between monomers, and those who consider that the ring-forming c subunits in the FO sector actually constitute the walls of the pore. The latest development is the emergence of a new candidate: Spastic Paraplegia 7 (SPG7), a mitochondrial AAA-type membrane protease which forms a 6-stave barrel. This review summarizes recent developments of research on the pathophysiological relevance and on the molecular nature of the mitochondrial permeability transition pore. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

Mitochondrial Actions of Thyroid Hormone

The hypermetabolic effects of thyroid hormones (THs), the major endocrine regulators of metabolic rate, are widely recognized. Although, the cellular mechanisms underlying these effects have been extensively investigated, much has yet to be learned about how TH regulates diverse cellular functions. THs have a profound impact on mitochondria, the organelles responsible for the majority of cellular energy production, and several studies have been devoted to understand the respective importance of the nuclear and mitochondrial pathways for organelle activity. During the last decades, several new aspects of both THs (i.e., metabolism, transport, mechanisms of action, and the existence of metabolically active TH derivatives) and mitochondria (i.e., dynamics, respiratory chain organization in supercomplexes, and the discovery of uncoupling proteins other than uncoupling protein 1) have emerged, thus opening new perspectives to the investigation of the complex relationship between thyroid and the mitochondrial compartment. In this review, in the light of an historical background, we attempt to point out the present findings regarding thyroid physiology and the emerging recognition that mitochondrial dynamics as well as the arrangement of the electron transport chain in mitochondrial cristae contribute to the mitochondrial activity. We unravel the genomic and nongenomic mechanisms so far studied as well as the effects of THs on mitochondrial energetics and, principally, uncoupling of oxidative phosphorylation via various mechanisms involving uncoupling proteins. The emergence of new approaches to the question as to what extent and how the action of TH can affect mitochondria is highlighted. © 2016 American Physiological Society. Compr Physiol 6:1591-1607, 2016.

Axin is expressed in mitochondria and suppresses mitochondrial ATP synthesis in HeLa cells

Many recent studies have revealed that axin is involved in numerous cellular functions beyond the negative regulation of β-catenin-dependent Wnt signaling. Previously, an association of ectopic axin with mitochondria was observed. In an effort to investigate the relationship between axin and mitochondria, we found that axin expression suppressed cellular ATP production, which was more apparent as axin expression levels increased. Also, mitochondrial expression of axin was observed using two axin-expressing HeLa cell models: doxycycline-inducible ectopic axin expression (HeLa-axin) and axin expression enhanced by long-term treatment with XAV939 (HeLa-XAV). In biochemical analysis, axin is associated with oxidative phosphorylation (OXPHOS) complex IV and is involved in defects in the assembly of complex IV-containing supercomplexes. Functionally, axin expression reduced the activity of OXPHOS complex IV and the oxygen consumption rate (OCR), suggesting axin-mediated mitochondrial dysfunction. Subsequent studies using various inhibitors of Wnt signaling showed that the reduction in cellular ATP levels was weaker in cases of ICAT protein expression and treatment with iCRT3 or NSC668036 compared with XAV939 treatment, suggesting that XAV939 treatment affects ATP synthesis in addition to suppressing Wnt signaling activity. Axin-mediated regulation of mitochondrial function may be an additional mechanism to Wnt signaling for regulation of cell growth.

The Impact of Reproductive Technologies on Stallion Mitochondrial Function

The traditional assessment of stallion sperm comprises evaluation of sperm motility and membrane integrity and identification of abnormal morphology of the spermatozoa. More recently, the progressive introduction of flow cytometry is increasing the number of tests available. However, compared with other sperm structures and functions, the evaluation of mitochondria has received less attention in stallion andrology. Recent research indicates that sperm mitochondria are key structures in sperm function suffering major changes during biotechnological procedures such as cryopreservation. In this paper, mitochondrial structure and function will be reviewed in the stallion, when possible specific stallion studies will be discussed, and general findings on mammalian mitochondrial function will be argued when relevant. Especial emphasis will be put on their role as source of reactive oxygen species and in their role regulating sperm lifespan, a possible target to investigate with the aim to improve the quality of frozen-thawed stallion sperm. Later on, the impact of current sperm technologies, principally cryopreservation, on mitochondrial function will be discussed pointing out novel areas of research interest with high potential to improve current sperm technologies.

Differential Expression of Genes that Control Respiration Contribute to Thermal Adaptation in Redband Trout (Oncorhynchus mykiss gairdneri)

Organisms can adapt to local environmental conditions as a plastic response or become adapted through natural selection on genetic variation. The ability to adapt to increased water temperatures will be of paramount importance for many fish species as the climate continues to warm and water resources become limited. Because increased water temperatures will reduce the dissolved oxygen available for fish, we hypothesized that adaptation to low oxygen environments would involve improved respiration through oxidative phosphorylation (OXPHOS). To test this hypothesis, we subjected individuals from two ecologically divergent populations of inland (redband) rainbow trout (Oncorhynchus mykiss gairdneri) with historically different temperature regimes (desert and montane) and their F1 progeny to diel cycles of temperature stress and then examined gene expression data for 80 nuclear- and mitochondrial-encoded OXPHOS subunits that participate in respiration. Of the 80 transcripts, 7 showed ≥ 2-fold difference in expression levels in gill tissue from desert fish under heat stress whereas the montane fish had none and the F1 only had one differentially expressed gene. A structural analysis of the proteins encoded by those genes suggests that the response could coordinate the formation of supercomplexes and oligomers. Supercomplexes may increase the efficiency of respiration because complexes I, III, and IV are brought into close proximity and oligomerization of complex V alters the macrostructure of mitochondria to improve respiration. Significant differences in gene expression patterns in response to heat stress in a common environment indicate that the response was not due to plasticity but had a genetic basis.

Exogenous Superoxide Dismutase Mimetic Without Scavenging H2O2 Causes Photoreceptor Damage in a Rat Model for Oxygen-Induced Retinopathy

PURPOSE

Frequent, brief intermittent episodes of hypoxia (IH) during hyperoxia increase reactive oxygen species in the immature retina with compromised antioxidant systems, thus leading to oxygen-induced retinopathy (OIR). We examined the hypothesis that early exposure to a mimetic of superoxide dismutase (SOD), the first line of defense against oxidative stress, will decrease IH-induced reactive oxygen species (ROS) and prevent severe OIR in our rat model.

METHODS

To test this hypothesis, newborn rats (P0) were exposed to IH consisting of alternating cycles of 50% O₂ with brief hypoxia (12% O₂) until P14 during which they were treated with a single daily intraperitoneal (IP) dose of MnTBAP (a SOD mimetic) at 1.0, 5.0, or 10.0 mg/kg on P0, P1, and P2. A saline-treated group served as vehicle controls. Groups were analyzed following IH at P14 or allowed to recover in room air (RA) until P21. Control littermates were raised in RA with all conditions identical except for inspired O₂. Ocular assessment of OIR severity, oxidative stress, angiogenesis, antioxidant activity, and oxidative phosphorylation (OXPHOS) were conducted at P14 and P21.

RESULTS

Collectively, the data show increased oxidative stress and angiogenesis with MnTBAP, which was associated with photoreceptor damage, retinal characteristics consistent with severe OIR, and changes in genes regulating OXPHOS.

CONCLUSIONS

In the setting of IH, the use of exogenous SOD mimetics must be combined with H₂O₂ scavengers in order to prevent photoreceptor damage and severe OIR.

Respiratory electron transfer pathways in plant mitochondria

The respiratory electron transport chain (ETC) couples electron transfer from organic substrates onto molecular oxygen with proton translocation across the inner mitochondrial membrane. The resulting proton gradient is used by the ATP synthase complex for ATP formation. In plants, the ETC is especially intricate. Besides the "classical" oxidoreductase complexes (complex I-IV) and the mobile electron transporters cytochrome c and ubiquinone, it comprises numerous "alternative oxidoreductases." Furthermore, several dehydrogenases localized in the mitochondrial matrix and the mitochondrial intermembrane space directly or indirectly provide electrons for the ETC. Entry of electrons into the system occurs via numerous pathways which are dynamically regulated in response to the metabolic state of a plant cell as well as environmental factors. This mini review aims to summarize recent findings on respiratory electron transfer pathways in plants and on the involved components and supramolecular assemblies.

Oxidative imbalance and anxiety disorders

The oxidative imbalance appears to have an important role in anxiety development. Studies in both humans and animals have shown a strong correlation between anxiety and oxidative stress. In humans, for example, the increased malondialdehyde levels and discrepancies in antioxidant enzymes in erythrocytes have been observed. In animals, several studies also show that anxiety-like behavior is related to the oxidative imbalance. Moreover, anxiety-like behavior can be caused by pharmacological-induced oxidative stress. Studies using knockout or overexpression of antioxidant enzymes have shown a relationship between anxiety-like behavior and oxidative stress. Related factors of oxidative stress that could influence anxious behavior are revised, including impaired function of different mitochondrial proteins, inflammatory cytokines, and neurotrophic factors. It has been suggested that a therapy specifically focus in reducing reactive species production may have a beneficial effect in reducing anxiety. However, the neurobiological pathways underlying the effect of oxidative stress on anxiety symptoms are not fully comprehended. The challenge now is to identify the oxidative stress mechanisms likely to be involved in the induction of anxiety symptoms. Understanding these pathways could help to clarify the neurobiology of the anxiety disorder and provide tools for new discovery in therapies and preventive strategies.

Adaptive responses of mitochondria to mild copper deprivation involve changes in morphology, OXPHOS remodeling and bioenergetics

Copper is an essential cofactor of complex IV of the electron transfer chain, and it is directly involved in the generation of mitochondrial membrane potential. Its deficiency induces the formation of ROS, large mitochondria and anemia. Thus, there is a connection between copper metabolism and bioenergetics, mitochondrial dynamics and erythropoiesis. Copper depletion might end in cellular apoptosis or necrosis. However, before entering into those irreversible processes, mitochondria may execute a series of adaptive responses. Mitochondrial adaptive responses (MAR) may involve multiple and diverse mechanisms for preserving cell life, such as mitochondrial dynamics, OXPHOS remodeling and bioenergetics output. In this study, a mild copper deficiency was produced in an animal model through intraperitoneal injections of bathocuproine disulfonate in order to study the MAR. Under these conditions, a new type of mitochondrial morphology was discovered in the liver. Termed the "butternut squash" mitochondria, it coexisted with normal and swollen mitochondria. Western blot analyses of mitochondrial dynamics proteins showed an up-regulation of MFN-2 and OPA1 fusion proteins. Furthermore, isolated liver mitochondria displayed OXPHOS remodeling through a decrease in supercomplex activity with a concomitant increase at an individual level of complexes I and IV, higher respiratory rates at complex I and II levels, higher oligomycin-insensitive respiration, and lower respiratory control ratio values when compared to the control group. As expected, total ATP and ATP/ADP values were not significantly different, since animal's health was not compromised. As a whole, these results describe a compensatory and adaptive response of metabolism and bioenergetics under copper deprivation.

Divergent functions of the Arabidopsis mitochondrial SCO proteins: HCC1 is essential for COX activity while HCC2 is involved in the UV-B stress response

The two related putative cytochrome c oxidase (COX) assembly factors HCC1 and HCC2 from Arabidopsis thaliana are Homologs of the yeast Copper Chaperones Sco1p and Sco2p. The hcc1 null mutation was previously shown to be embryo lethal while the disruption of the HCC2 gene function had no obvious effect on plant development, but increased the expression of stress-responsive genes. Both HCC1 and HCC2 contain a thioredoxin domain, but only HCC1 carries a Cu-binding motif also found in Sco1p and Sco2p. In order to investigate the physiological implications suggested by this difference, various hcc1 and hcc2 mutants were generated and analyzed. The lethality of the hcc1 knockout mutation was rescued by complementation with the HCC1 gene under the control of the embryo-specific promoter ABSCISIC ACID INSENSITIVE 3. However, the complemented seedlings did not grow into mature plants, underscoring the general importance of HCC1 for plant growth. The HCC2 homolog was shown to localize to mitochondria like HCC1, yet the function of HCC2 is evidently different, because two hcc2 knockout lines developed normally and exhibited only mild growth suppression compared with the wild type (WT). However, hcc2 knockouts were more sensitive to UV-B treatment than the WT. Complementation of the hcc2 knockout with HCC2 rescued the UV-B-sensitive phenotype. In agreement with this, exposure of wild-type plants to UV-B led to an increase of HCC2 transcripts. In order to corroborate a function of HCC1 and HCC2 in COX biogenesis, COX activity of hcc1 and hcc2 mutants was compared. While the loss of HCC2 function had no significant effect on COX activity, the disruption of one HCC1 gene copy was enough to suppress respiration by more than half compared with the WT. Therefore, we conclude that HCC1 is essential for COX function, most likely by delivering Cu to the catalytic center. HCC2, on the other hand, seems to be involved directly or indirectly in UV-B-stress responses.

Experimental lung injury promotes alterations in energy metabolism and respiratory mechanics in the lungs of rats: prevention by exercise

In the present study we investigated the effects of lung injury on energy metabolism (succinate dehydrogenase, complex II, cytochrome c oxidase, and ATP levels), respiratory mechanics (dynamic and static compliance, elastance and respiratory system resistance) in the lungs of rats, as well as on phospholipids in bronchoalveolar lavage fluid. The protective effect of physical exercise on the alterations caused by lung injury, including lung edema was also evaluated. Wistar rats were submitted to 2 months of physical exercise. After this period the lung injury was induced by intratracheal instillation of lipopolysaccharide. Adult Wistar rats were submitted to 2 months of physical exercise and after this period the lung injury was induced by intratracheal instillation of lipopolysaccharide in dose 100 μg/100 g body weight. The sham group received isotonic saline instillation. Twelve hours after the injury was performed the respiratory mechanical and after the rats were decapitated and samples were collected. The rats subjected to lung injury presented a decrease in activities of the enzymes of the electron transport chain and ATP levels in lung, as well as the formation of pulmonary edema. A decreased lung dynamic and static compliance, as well as an increase in respiratory system resistance, and a decrease in phospholipids content were observed. Physical exercise was able to totally prevent the decrease in succinate dehydrogenase and complex II activities and the formation of pulmonary edema. It also partially prevented the increase in respiratory system resistance, but did not prevent the decrease in dynamic and static compliance, as well as in phospholipids content. These findings suggest that the mitochondrial dysfunction may be one of the important contributors to lung damage and that physical exercise may be beneficial in this pathology, although it did not prevent all changes present in lung injury.

The cytochrome c oxidase and its mitochondrial function in the whiteleg shrimp Litopenaeus vannamei during hypoxia

Cytochrome c oxidase (COX), which is located in the inner membrane of mitochondria, is a key constituent of the electron transport chain that catalyzes the reduction of oxygen. The Pacific whiteleg shrimp Litopenaeus vannamei is constantly exposed to hypoxic conditions, which affects both the central metabolism and the mitochondrial function. The purpose of this study was to isolate shrimp mitochondria, identify the COX complex and to evaluate the effect of hypoxia on the shrimp mitochondrial function and in the COX activity. A 190 kDa protein was identified as COX by immunodetection techniques. The effect of hypoxia was confirmed by an increase in the shrimp plasma L-lactate concentration. COX activity, mitochondrial oxygen uptake and protein content were reduced under hypoxic conditions, and gradually restored as hypoxia continued, this suggests an adaptive mitochondrial response and a highly effective COX enzyme. Both mitochondrial oxygen uptake and COX activity were completely inhibited by KCN and sodium azide, suggesting that COX is the unique oxidase in L. vannamei mitochondria.

Diffusion of molecules and macromolecules in thylakoid membranes

The survival and fitness of photosynthetic organisms is critically dependent on the flexible response of the photosynthetic machinery, harbored in thylakoid membranes, to environmental changes. A central element of this flexibility is the lateral diffusion of membrane components along the membrane plane. As demonstrated, almost all functions of photosynthetic energy conversion are dependent on lateral diffusion. The mobility of both small molecules (plastoquinone, xanthophylls) as well as large protein supercomplexes is very sensitive to changes in structural boundary conditions. Knowledge about the design principles that govern the mobility of photosynthetic membrane components is essential to understand the dynamic response of the photosynthetic machinery. This review summarizes our knowledge about the factors that control diffusion in thylakoid membranes and bridges structural membrane alterations to changes in mobility and function. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.

Three nucleus-encoded subunits of mitochondrial cytochrome c oxidase of the whiteleg shrimp Litopenaeus vannamei: cDNA characterization, phylogeny and mRNA expression during hypoxia and reoxygenation

The mitochondrial cytochrome c oxidase (COX) catalyzes the reduction of oxygen to water playing a key role in the respiratory chain and ATP synthesis. The nucleus-encoded COX subunits do not participate in catalysis, but some are known to play a role in the expression, assembly and activity of the enzyme. Since hypoxia continuously affects the shrimp environment, it is important to study COX to understand their ability to deal with low oxygen levels. The goal of this research was to characterize the complementary DNA (cDNA) sequences of three nucleus-encoded subunits -coxIV, coxVa, and coxVb- and to evaluate the shrimp COX response to hypoxia by measuring their gene expression. The cDNA sequence of coxIV consisted of 532bp, which encodes a 17.47kDa protein, while coxVa cDNA consisted of 460bp and coded a protein of 17.11kDa, and the coxVb coding sequence consisted of 364bp encoding a 13.74kDa protein. Shrimp subunits do not have isoforms, and they are not differentially expressed during hypoxia, as observed in mammals. Coordinated changes were detected in the mRNA amounts of nuclear and mitochondrial subnits; these changes, at the transcriptional level, are suggested to be controlled through transcriptional factors Sp1 and NRF2.

Rotary ATPases: models, machine elements and technical specifications

Rotary ATPases are molecular rotary motors involved in biological energy conversion. They either synthesize or hydrolyze the universal biological energy carrier adenosine triphosphate. Recent work has elucidated the general architecture and subunit compositions of all three sub-types of rotary ATPases. Composite models of the intact F-, V- and A-type ATPases have been constructed by fitting high-resolution X-ray structures of individual subunits or sub-complexes into low-resolution electron densities of the intact enzymes derived from electron cryo-microscopy. Electron cryo-tomography has provided new insights into the supra-molecular arrangement of eukaryotic ATP synthases within mitochondria and mass-spectrometry has started to identify specifically bound lipids presumed to be essential for function. Taken together these molecular snapshots show that nano-scale rotary engines have much in common with basic design principles of man made machines from the function of individual “machine elements” to the requirement of the right “fuel” and “oil” for different types of motors.

Metabolic control analysis of cellular respiration in situ in intraoperational samples of human breast cancer

The aim of this study was to analyze quantitatively cellular respiration in intraoperational tissue samples taken from human breast cancer (BC) patients. We used oxygraphy and the permeabilized cell techniques in combination with Metabolic Control Analysis (MCA) to measure a corresponding flux control coefficient (FCC). The activity of all components of ATP synthasome, and respiratory chain complexes was found to be significantly increased in human BC cells in situ as compared to the adjacent control tissue. FCC(s) were determined upon direct activation of respiration with exogenously-added ADP and by titrating the complexes with their specific inhibitors to stepwise decrease their activity. MCA showed very high sensitivity of all complexes and carriers studied in human BC cells to inhibition as compared to mitochondria in normal oxidative tissues. The sum of FCC(s) for all ATP synthasome and respiratory chain components was found to be around 4, and the value exceeded significantly that for normal tissue (close to 1). In BC cells, the key sites of the regulation of respiration are Complex IV (FCC = 0.74), ATP synthase (FCC = 0.61), and phosphate carrier (FCC = 0.60); these FCC(s) exceed considerably (~10-fold) those for normal oxidative tissues. In human BC cells, the outer mitochondrial membrane is characterized by an increased permeability towards adenine nucleotides, the mean value of the apparent K(m) for ADP being equal to 114.8 ± 13.6 μM. Our data support the two-compartment hypothesis of tumor metabolism, the high sum of FCC(s) showing structural and functional organization of mitochondrial respiratory chain and ATP synthasome as supercomplexes in human BC.

Lack of cytochrome c in Arabidopsis decreases stability of Complex IV and modifies redox metabolism without affecting Complexes I and III

We studied the role of cytochrome c (CYTc), which mediates electron transfer between Complexes III and IV, in cellular events related with mitochondrial respiration, plant development and redox homeostasis. We analyzed single and double homozygous mutants in both CYTc-encoding genes from Arabidopsis: CYTC-1 and CYTC-2. While individual mutants were similar to wild-type, knock-out of both genes produced an arrest of embryo development, showing that CYTc function is essential at early stages of plant development. Mutants in which CYTc levels were extremely reduced respective to wild-type had smaller rosettes with a pronounced decrease in parenchymatic cell size and an overall delay in development. Mitochondria from these mutants had lower respiration rates and a relative increase in alternative respiration. Furthermore, the decrease in CYTc severely affected the activity and the amount of Complex IV, without affecting Complexes I and III. Reactive oxygen species levels were reduced in these mutants, which showed induction of genes encoding antioxidant enzymes. Ascorbic acid levels were not affected, suggesting that a small amount of CYTc is enough to support its normal synthesis. We postulate that, in addition to its role as an electron carrier between Complexes III and IV, CYTc influences Complex IV levels in plants, probably reflecting a role of this protein in Complex IV stability. This double function of CYTc most likely explains why it is essential for plant survival.

Understanding mitochondrial complex I assembly in health and disease

Complex I (NADH:ubiquinone oxidoreductase) is the largest multimeric enzyme complex of the mitochondrial respiratory chain, which is responsible for electron transport and the generation of a proton gradient across the mitochondrial inner membrane to drive ATP production. Eukaryotic complex I consists of 14 conserved subunits, which are homologous to the bacterial subunits, and more than 26 accessory subunits. In mammals, complex I consists of 45 subunits, which must be assembled correctly to form the properly functioning mature complex. Complex I dysfunction is the most common oxidative phosphorylation (OXPHOS) disorder in humans and defects in the complex I assembly process are often observed. This assembly process has been difficult to characterize because of its large size, the lack of a high resolution structure for complex I, and its dual control by nuclear and mitochondrial DNA. However, in recent years, some of the atomic structure of the complex has been resolved and new insights into complex I assembly have been generated. Furthermore, a number of proteins have been identified as assembly factors for complex I biogenesis and many patients carrying mutations in genes associated with complex I deficiency and mitochondrial diseases have been discovered. Here, we review the current knowledge of the eukaryotic complex I assembly process and new insights from the identification of novel assembly factors. This article is part of a Special Issue entitled: Biogenesis/Assembly of Respiratory Enzyme Complexes.

Thymosin β₄ knockdown disrupts mitochondrial functions of SW480 human colon cancer cells

Thymosin β(4) (Tβ(4)), overexpressed in various tumors, has been shown to be involved in cellular anti-oxidation. Reactive oxygen species (ROS) function as signaling molecules and play certain roles in tumor progression. To assess the anti-oxidative role of endogenous Tβ(4) in tumor cells, its expression in SW480 cells was knocked down by a shRNA, which induced significant increases of ROS. Interestingly, some cristae-lost and several electron-dense mitochondria appeared in cells with Tβ(4) knockdown that was accompanied by a marked decline of the membrane potential of these organelles. Strikingly, while the ATP and lactate levels in SW480 cells were notably elevated by Tβ(4) downregulation, this treatment significantly diminished the mitochondrial DNA copy number and protein levels of several subunits of the electron transport complexes. Finally, immunofluorescent staining results suggested the presence of Tβ(4) in mitochondria. To the best of our knowledge, this is the first report to demonstrate that Tβ(4) knockdown can disrupt the morphology and some crucial functions of mitochondria in human colorectal carcinoma (CRC) cells.

Oxygen radicals shaping evolution: why fatty acid catabolism leads to peroxisomes while neurons do without it: FADH₂/NADH flux ratios determining mitochondrial radical formation were crucial for the eukaryotic invention of peroxisomes and catabolic tissue differentiation

Oxygen radical formation in mitochondria is a highly important, but incompletely understood, attribute of eukaryotic cells. I propose a kinetic model in which the ratio between electrons entering the respiratory chain via FADH₂ or NADH is a major determinant in radical formation. During the breakdown of glucose, this ratio is low; during fatty acid breakdown, this ratio is much higher. The longer the fatty acid, the higher the ratio and the higher the level of radical formation. This means that very long chain fatty acids should be broken down without generation of FADH₂ for mitochondria. This is accomplished in peroxisomes, thus explaining their role and evolution. The model explains many recent observations regarding radical formation by the respiratory chain. It also sheds light on the reasons for the lack of neuronal fatty acid (beta-) oxidation and for beneficial aspects of unsaturated fatty acids. Last but not least, it has very important implications for all models describing eukaryotic origins.

Variation of flux control coefficient of cytochrome c oxidase and of the other respiratory chain complexes at different values of protonmotive force occurs by a threshold mechanism

The metabolic control analysis was applied to digitonin-permeabilized HepG2 cell line to assess the flux control exerted by cytochrome c oxidase on the mitochondrial respiration. Experimental conditions eliciting different energy/respiratory states in mitochondria were settled. The results obtained show that the mitochondrial electrochemical potential accompanies a depressing effect on the control coefficient exhibited by the cytochrome c oxidase. Both the components of the protonmotive force, i.e. the voltage (ΔΨ(m)) and the proton (ΔpH(m)) gradient, displayed a similar effect. Quantitative estimation of the ΔΨ(m) unveiled that the voltage-dependent effect on the control coefficient of cytochrome c oxidase takes place sharply in a narrow range of membrane potential from 170-180 to 200-210mV consistent with the physiologic transition from state 3 to state 4 of respiration. Extension of the metabolic flux control analysis to the NADH dehydrogenase and bc(1) complexes of the mitochondrial respiratory chain resulted in a similar effect. A mechanistic model is put forward whereby the respiratory chain complexes are proposed to exist in a voltage-mediated threshold-controlled dynamic equilibrium between supercomplexed and isolated states.