Mitochondrial Diagnostics: A Discovery-Based Biochemical Platform for Phenotyping Human Peripheral Blood Cell Mitochondria

In vitro experiments using permeabilized cells and/or isolated mitochondria represent a powerful biochemical tool for elucidating the role of the mitochondrion in driving disease. Such analyses have routinely been utilized across multiple scientific fields to shed valuable insight on mitochondrial-linked pathologies. The present chapter is intended to serve as a methodological blueprint for comprehensively phenotyping peripheral blood cell mitochondria. While primarily adapted for peripheral blood cells, the protocols outlined herein could easily be made amenable to most all cell types with minimal modifications.

Complex II subunit SDHD is critical for cell growth and metabolism, which can be partially restored with a synthetic ubiquinone analog

Background

Succinate dehydrogenase (Complex II) plays a dual role in respiration by catalyzing the oxidation of succinate to fumarate in the mitochondrial Krebs cycle and transferring electrons from succinate to ubiquinone in the mitochondrial electron transport chain (ETC). Mutations in Complex II are associated with a number of pathologies. SDHD, one of the four subunits of Complex II, serves by anchoring the complex to the inner-membrane and transferring electrons from the complex to ubiquinone. Thus, modeling SDHD dysfunction could be a valuable tool for understanding its importance in metabolism and developing novel therapeutics, however no suitable models exist.

Results

Via CRISPR/Cas9, we mutated SDHD in HEK293 cells and investigated the in vitro role of SDHD in metabolism. Compared to the parent HEK293, the knockout mutant HEK293ΔSDHD produced significantly less number of cells in culture. The mutant cells predictably had suppressed Complex II-mediated mitochondrial respiration, but also Complex I-mediated respiration. SDHD mutation also adversely affected glycolytic capacity and ATP synthesis. Mutant cells were more apoptotic and susceptible to necrosis. Treatment with the mitochondrial therapeutic idebenone partially improved oxygen consumption and growth of mutant cells.

Conclusions

Overall, our results suggest that SDHD is vital for growth and metabolism of mammalian cells, and that respiratory and growth defects can be partially restored with treatment of a ubiquinone analog. This is the first report to use CRISPR/Cas9 approach to construct a knockout SDHD cell line and evaluate the efficacy of an established mitochondrial therapeutic candidate to improve bioenergetic capacity.

Photobiomodulation Response From 660 nm is Different and More Durable Than That From 980 nm

BACKGROUND AND OBJECTIVES

Photobiomodulation (PBM) therapy uses light at various wavelengths to stimulate wound healing, grow hair, relieve pain, and more-but there is no consensus about optimal wavelengths or dosimetry. PBM therapy works through putative, wavelength-dependent mechanisms including direct stimulation of mitochondrial respiration, and/or activation of transmembrane signaling channels by changes in water activity. A common wavelength used in the visible red spectrum is ~660 nm, whereas recently ~980 nm is being explored and both have been proposed to work via different mechanisms. We aimed to gain more insight into identifying treatment parameters and the putative mechanisms involved.

STUDY DESIGN/MATERIALS AND METHODS

Fluence-response curves were measured in cultured keratinocytes and fibroblasts exposed to 660 or 980 nm from LED sources. Metabolic activity was assessed using the MTT assay for reductases. ATP production, a major event triggered by PBM therapy, was assessed using a luminescence assay. To measure the role of mitochondria, we used an ELISA to measure COX-1 and SDH-A protein levels. The respective contributions of cytochrome c oxidase and ATP synthase to the PBM effects were gauged using specific inhibitors.

RESULTS

Keratinocytes and fibroblasts responded differently to exposures at 660 nm (red) and 980 nm (NIR). Although 980 nm required much lower fluence for cell stimulation, the resulting increase in ATP levels was short-term, whereas 660 nm stimulation elevated ATP levels for at least 24 hours. COX-1 protein levels were increased following 660 nm treatment but were unaffected by 980 nm. In fibroblasts, SDH-A levels were affected by both wavelengths, whereas in keratinocytes only 660 nm light impacted SDH-A levels. Inhibition of ATP synthase nearly completely abolished the effects of both wavelengths on ATP synthesis. Interestingly, inhibiting cytochrome c oxidase did not prevent the rise in ATP levels in response to PBM treatment.

CONCLUSION

To the best of our knowledge, this is the first demonstration of differing kinetics in response to PBM therapy at red versus NIR wavelength. We also found cell-type-specific differences in PBM therapy response to the two wavelengths studied. These findings confirm that different response pathways are involved after 660 and 980 nm exposures and suggest that 660 nm causes a more durable response. © 2021 Wiley Periodicals LLC.

The photosensitiser azure A disrupts mitochondrial bioenergetics through intrinsic and photodynamic effects

Azure A (AA) is a cationic molecule of the class of phenothiazines that has been applied in vitro as a photosensitising agent in photodynamic antimicrobial chemotherapy. It is a di-demethylated analogue of methylene blue (MB), which has been demonstrated to be intrinsically and photodynamically highly active on mitochondrial bioenergetics. However, as far as we know, there are no studies about the photodynamic effects of AA on mammalian mitochondria. Therefore, this investigation aimed to characterise the intrinsic and photodynamic acute effects of AA (0.540 μM) on isolated rat liver mitochondria, isolated hepatocytes, and isolated perfused rat liver. The effects of AA were assessed by evaluating several parameters of mitochondrial bioenergetics, oxidative stress, cell viability, and hepatic energy metabolism. The photodynamic effects of AA were assessed under simulated hypoxic conditions, a suitable way for mimicking the microenvironment of hypoxic solid tumour cells. AA interacted with the mitochondria and, upon photostimulation (10 min of light exposure), produced toxic amounts of reactive oxygen species (ROS), which damaged the organelle, as demonstrated by the high levels of lipid peroxidation and protein carbonylation. The photostimulated AA also depleted the GSH pool, which could compromise the mitochondrial antioxidant defence. Bioenergetically, AA photoinactivated the complexes I, II, and IV of the mitochondrial respiratory chain and the F1FO-ATP synthase complex, sharply inhibiting the oxidative phosphorylation. Upon photostimulation (10 min of light exposure), AA reduced the efficiency of mitochondrial energy transduction and oxidatively damaged lipids in isolated hepatocytes but did not decrease the viability of cells. Despite the useful photobiological properties, AA presented noticeable dark toxicity on mitochondrial bioenergetics, functioning predominantly as an uncoupler of oxidative phosphorylation. This harmful effect of AA was evidenced in isolated hepatocytes, in which AA diminished the cellular ATP content. In this case, the cells exhibited signs of cell viability reduction in the presence of high AA concentrations, but only after a long time of incubation (at least 90 min). The impairments on mitochondrial bioenergetics were also clearly manifested in intact perfused rat liver, in which AA diminished the cellular ATP content and stimulated the oxygen uptake. Consequently, gluconeogenesis and ureogenesis were strongly inhibited, whereas glycogenolysis and glycolysis were stimulated. AA also promoted the release of cytosolic and mitochondrial enzymes into the perfusate concomitantly with inhibition of oxygen consumption. In general, the intrinsic and photodynamic effects of AA were similar to those of MB, but AA caused some distinct effects such as the photoinactivation of the complex IV of the mitochondrial respiratory chain and a diminution of the ATP levels in the liver. It is evident that AA has the potential to be used in mitochondria-targeted photodynamic therapy, even under low oxygen concentrations. However, the fact that AA directly disrupts mitochondrial bioenergetics and affects several hepatic pathways that are linked to ATP metabolism, along with its ability to perturb cellular membranes and its little potential to reduce cell viability, could result in significant adverse effects especially in long-term treatments.

No Direct Postconditioning Effect of Poloxamer 188 on Mitochondrial Function after Ischemia Reperfusion Injury in Rat Isolated Hearts

Myocardial infarction is a leading cause for morbidity and mortality worldwide. The only viable treatment for the ischemic insult is timely reperfusion, which further exacerbates myocardial injury. Maintaining mitochondrial function is crucial in preserving cardiomyocyte function in ischemia reperfusion (IR) injury. Poloxamer (P) 188 has been shown to improve cardiac IR injury by improving cellular and mitochondrial function. The aim of this study was to show if P188 postconditioning has direct protective effects on mitochondrial function in the heart. Langendorff prepared rat hearts were subjected to IR injury ex-vivo and reperfused for 10 min with 1 mM P188 vs. vehicle. Cardiac mitochondria were isolated with 1 mM P188 vs. 1 mM polyethylene glycol (PEG) vs. vehicle by differential centrifugation. Mitochondrial function was assessed by adenosine triphosphate synthesis, oxygen consumption, and calcium retention capacity. Mitochondrial function decreased significantly after ischemia and showed mild improvement with reperfusion. P188 did not improve mitochondrial function in the ex-vivo heart, and neither further P188 nor PEG induced direct mitochondrial protection after IR injury in this model.

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

Effect of crista morphology on mitochondrial ATP output: A computational study

Folding of the mitochondrial inner membrane (IM) into cristae greatly increases the ATP-generating surface area, S _IM, per unit volume but also creates diffusional bottlenecks that could limit reaction rates inside mitochondria. This study explores possible effects of inner membrane folding on mitochondrial ATP output, using a mathematical model for energy metabolism developed by the Jafri group and two- and three-dimensional spatial models for mitochondria, implemented on the Virtual Cell platform. Simulations demonstrate that cristae are micro-compartments functionally distinct from the cytosol. At physiological steady states, standing gradients of ADP form inside cristae that depend on the size and shape of the compartments, and reduce local flux (rate per unit area) of the adenine nucleotide translocase. This causes matrix ADP levels to drop, which in turn reduces the flux of ATP synthase. The adverse effects of membrane folding on reaction fluxes increase with crista length and are greater for lamellar than tubular crista. However, total ATP output per mitochondrion is the product of flux of ATP synthase and S _IM which can be two-fold greater for mitochondria with lamellar than tubular cristae, resulting in greater ATP output for the former. The simulations also demonstrate the crucial role played by intracristal kinases (adenylate kinase, creatine kinase) in maintaining the energy advantage of IM folding.

Complex I protein NDUFS2 is vital for growth, ROS generation, membrane integrity, apoptosis, and mitochondrial energetics

Complex I is the largest and most intricate of the protein complexes of mitochondrial electron transport chain (ETC). This L-shaped enzyme consists of a peripheral hydrophilic matrix domain and a membrane-bound orthogonal hydrophobic domain. The interfacial region between these two arms is known to be critical for binding of ubiquinone moieties and has also been shown to be the binding site of Complex I inhibitors. Knowledge on specific roles of the ETC interfacial region proteins is scarce due to lack of knockout cell lines and animal models. Here we mutated nuclear encoded NADH dehydrogenase [ubiquinone] iron-sulfur protein 2 (NDUFS2), one of three protein subunits of the interfacial region, in a human embryonic kidney cell line 293 using a CRISPR/Cas9 procedure. Disruption of NDUFS2 significantly decreased cell growth in medium, Complex I specific respiration, glycolytic capacity, ATP pool and cell-membrane integrity, but significantly increased Complex II respiration, ROS generation, apoptosis, and necrosis. Treatment with idebenone, a clinical benzoquinone currently being investigated in other indications, partially restored growth, ATP pool, and oxygen consumption of the mutant. Overall, our results suggest that NDUFS2 is vital for growth and metabolism of mammalian cells, and respiratory defects of NDUFS2 dysfunction can be partially corrected with treatment of an established mitochondrial therapeutic candidate. This is the first report to use CRISPR/Cas9 approach to construct a knockout NDUFS2 cell line and use the constructed mutant to evaluate the efficacy of a known mitochondrial therapeutic to enhance bioenergetic capacity.

Inhibition of Mitochondrial ATP Synthesis and Regulation of Oxidative Stress Based on {SbW8 O30 } Determined by Single-Cell Proteomics Analysis

The 10-nuclear heteroatom cluster modified {SbW₈ O₃₀ } was successfully synthesized and exhibited inhibitory activity (IC₅₀ =0.29 μM). Based on proteomics analysis, Na₄ Ni₂ Sb₂ W₂ -SbW₈ inhibited ATP production by affecting the expression of 16 related proteins, hindering metabolic functions in vivo and cell proliferation due to reactive oxygen species (ROS) stress. In particular, the low expression of FAD/FMN-binding redox enzymes (relative expression ratio of the experimental group to the control=0.43843) could be attributed to the redox mechanism of Na₄ Ni₂ Sb₂ W₂ -SbW₈ , which was consistent with the effect of polyoxometalates (POMs) and FMN-binding proteins on ATP formation. An electrochemical study showed that Na₄ Ni₂ Sb₂ W₂ -SbW₈ combined with FMN to form Na₄ Ni₂ Sb₂ W₂ -SbW₈ -2FMN complex through a one-electron process of the W atoms. Na₄ Ni₂ Sb₂ W₂ -SbW₈ acted as catalase and glutathione peroxidase to protect the cell from ROS stress, and the inhibition rates were 63.3 % at 1.77 μM of NADPH and 86.06 % at 10.62 μM of 2-hydroxyterephthalic acid. Overall, our results showed that POMs can be specific oxidative/antioxidant regulatory agents.

Mitochondriopathies as a Clue to Systemic Disorders-Analytical Tools and Mitigating Measures in Context of Predictive, Preventive, and Personalized (3P) Medicine

The mitochondrial respiratory chain is the main site of reactive oxygen species (ROS) production in the cell. Although mitochondria possess a powerful antioxidant system, an excess of ROS cannot be completely neutralized and cumulative oxidative damage may lead to decreasing mitochondrial efficiency in energy production, as well as an increasing ROS excess, which is known to cause a critical imbalance in antioxidant/oxidant mechanisms and a "vicious circle" in mitochondrial injury. Due to insufficient energy production, chronic exposure to ROS overproduction consequently leads to the oxidative damage of life-important biomolecules, including nucleic acids, proteins, lipids, and amino acids, among others. Different forms of mitochondrial dysfunction (mitochondriopathies) may affect the brain, heart, peripheral nervous and endocrine systems, eyes, ears, gut, and kidney, among other organs. Consequently, mitochondriopathies have been proposed as an attractive diagnostic target to be investigated in any patient with unexplained progressive multisystem disorder. This review article highlights the pathomechanisms of mitochondriopathies, details advanced analytical tools, and suggests predictive approaches, targeted prevention and personalization of medical services as instrumental for the overall management of mitochondriopathy-related cascading pathologies.

Pervasive Mitonuclear Coadaptation Underlies Fast Development in Interpopulation Hybrids of a Marine Crustacean

Cellular energy production requires coordinated interactions between genetic components from the nuclear and mitochondrial genomes. This coordination results in coadaptation of interacting elements within populations. Interbreeding between divergent gene pools can disrupt coadapted loci and result in hybrid fitness breakdown. While specific incompatible loci have been detected in multiple eukaryotic taxa, the extent of the nuclear genome that is influenced by mitonuclear coadaptation is not clear in any species. Here, we used F₂ hybrids between two divergent populations of the copepod Tigriopus californicus to examine mitonuclear coadaptation across the nuclear genome. Using developmental rate as a measure of fitness, we found that fast-developing copepods had higher ATP synthesis capacity than slow developers, suggesting variation in developmental rates is at least partly associated with mitochondrial dysfunction. Using Pool-seq, we detected strong biases for maternal alleles across 7 (of 12) chromosomes in both reciprocal crosses in high-fitness hybrids, whereas low-fitness hybrids showed shifts toward the paternal population. Comparison with previous results on a different hybrid cross revealed largely different patterns of strong mitonuclear coadaptation associated with developmental rate. Our findings suggest that functional coadaptation between interacting nuclear and mitochondrial components is reflected in strong polygenic effects on this life-history phenotype, and reveal that molecular coadaptation follows independent evolutionary trajectories among isolated populations.

Potential Effects of Poloxamer 188 on Rat Isolated Brain Mitochondria after Oxidative Stress In Vivo and In Vitro

Outcome after cerebral ischemia is often dismal. Reperfusion adds significantly to the ischemic injury itself. Therefore, new strategies targeting ischemia/reperfusion (I/R) injury are critically needed. Poloxamer (P)188, an amphiphilic triblock copolymer, is a highly promising pharmacological therapeutic as its capability to insert into injured cell membranes has been reported to protect against I/R injury in various models. Although mitochondrial function particularly profits from P188 treatment after I/R, it remains unclear if this beneficial effect occurs directly or indirectly. Here, rat isolated brain mitochondria underwent oxidative stress in vivo by asphyxial cardiac arrest or in vitro by the addition of hydrogen peroxide (H₂O₂) after isolation. Mitochondrial function was assessed by adenosine triphosphate synthesis, oxygen consumption, and calcium retention capacity. Both asphyxia and H₂O₂ exposure significantly impaired mitochondrial function. P188 did not preserve mitochondrial function after either injury mechanism. Further research is indicated.

A Cerium Vanadate Nanozyme with Specific Superoxide Dismutase Activity Regulates Mitochondrial Function and ATP Synthesis in Neuronal Cells

Nanoparticles that functionally mimic the activity of metal-containing enzymes (metallo-nanozymes) are of therapeutic importance for treating various diseases. However, it is still not clear whether such nanozymes can completely substitute the function of natural enzymes in living cells. In this work, we show for the first time that a cerium vanadate (CeVO₄ ) nanozyme can substitute the function of superoxide dismutase 1 and 2 (SOD1 and SOD2) in the neuronal cells even when the natural enzyme is down-regulated by specific gene silencing. The nanozyme prevents the mitochondrial damage in SOD1- and SOD2-depleted cells by regulating the superoxide levels and restores the physiological levels of the anti-apoptotic Bcl-2 family proteins. Furthermore, the nanozyme effectively prevents the mitochondrial depolarization, leading to a significant improvement in the cellular levels of ATP under oxidative stress.

Targeting breast cancer metabolism with a novel inhibitor of mitochondrial ATP synthesis

Inhibitors of mitochondrial respiration and ATP synthesis may promote the selective killing of respiration-competent cancer cells that are critical for tumor progression. We previously reported that CADD522, a small molecule inhibitor of the RUNX2 transcription factor, has potential for breast cancer treatment. In the current study, we show that CADD522 inhibits mitochondrial oxidative phosphorylation by decreasing the mitochondrial oxygen consumption rate (OCR) and ATP production in human breast cancer cells in a RUNX2-independent manner. The enzyme activity of mitochondrial ATP synthase was inhibited by CADD522 treatment. Importantly, results from cellular thermal shift assays that detect drug-induced protein stabilization revealed that CADD522 interacts with both α and β subunits of the F1-ATP synthase complex. Differential scanning fluorimetry also demonstrated interaction of α subunits of the F1-ATP synthase to CADD522. These results suggest that CADD522 might target the enzymatic F1 subunits in the ATP synthase complex. CADD522 increased the levels of intracellular reactive oxygen species (ROS), which was prevented by MitoQ, a mitochondria-targeted antioxidant, suggesting that cancer cells exposed to CADD522 may elevate ROS from mitochondria. CADD522-increased mitochondrial ROS levels were enhanced by exogenously added pro-oxidants such as hydrogen peroxide or tert-butyl hydroperoxide. Conversely, CADD522-mediated cell growth inhibition was blocked by N-acetyl-l-cysteine, a general ROS scavenger. Therefore, CADD522 may exert its antitumor activity by increasing mitochondrial driven cellular ROS levels. Collectively, our data suggest in vitro proof-of-concept that supports inhibition of mitochondrial ATP synthase and ROS generation as contributors to the effectiveness of CADD522 in suppression of tumor growth.

Multistimuli Sensing Adhesion Unit for the Self-Positioning of Minimal Synthetic Cells

Cells have the ability to sense different environmental signals and position themselves accordingly in order to support their survival. Introducing analogous capabilities to the bottom-up assembled minimal synthetic cells is an important step for their autonomy. Here, a minimal synthetic cell which combines a multistimuli sensitive adhesion unit with an energy conversion module is reported, such that it can adhere to places that have the right environmental parameters for ATP production. The multistimuli sensitive adhesion unit senses light, pH, oxidative stress, and the presence of metal ions and can regulate the adhesion of synthetic cells to substrates in response to these stimuli following a chemically coded logic. The adhesion unit is composed of the light and redox responsive protein interaction of iLID and Nano and the pH sensitive and metal ion mediated binding of protein His-tags to Ni²⁺ -NTA complexes. Integration of the adhesion unit with a light to ATP conversion module into one synthetic cell allows it to adhere to places under blue light illumination, non-oxidative conditions, at neutral pH and in the presence of metal ions, which are the right conditions to synthesize ATP. Thus, the multistimuli responsive adhesion unit allows synthetic cells to self-position and execute their functions.

The Rnf complex from the acetogenic bacterium Acetobacterium woodii: Purification and characterization of RnfC and RnfB

rnf genes are widespread in anaerobic bacteria and hypothesized to encode a respiratory enzyme that couples exergonic reduction of NAD with reduced ferredoxin as a reductant to vectorial ion (Na⁺, H⁺) translocation across the cytoplasmic membrane. However, despite its importance for the physiology of these bacteria, little is known about the subunit composition and the function of subunits. Here, we have purified the entire Rnf complex from the acetogen Acetobacterium woodii or after its production in Escherichia coli. These studies revealed covalently bound flavin in RnfB and RnfD. Unfortunately, the complex did not catalyze electron transfer from reduced ferredoxin to NAD. We, therefore, concentrated on the two cytosolic subunits RnfC and RnfB. RnfC was produced in E. coli, purified and shown to have 8.3 mol iron and 8.6 mol sulfur per mol of the subunit, consistent with the presence of two [4Fe-4S] centers, which were verified by EPR analysis. Flavins could not be detected, but RnfC catalyzed NADH-dependent FMN reduction. These data confirm RnfC as NADH-binding subunit and FMN as an intermediate in the electron transport chain. RnfB could only be produced as a fusion to the maltose-binding protein. It contained 25 mol iron and 26 mol sulfur, consistent with the predicted six [4Fe4S] centers. The FeS centers in RnfB were reduced with reduced ferredoxin as reductant. These data are consistent with RnfB as the ferredoxin-binding subunit of the complex.

Increasing mitochondrial ATP synthesis with butyrate normalizes ADP and contractile function in metabolic heart disease

Metabolic heart disease (MHD), which is strongly associated with heart failure with preserved ejection fraction, is characterized by reduced mitochondrial energy production and contractile performance. In this study, we tested the hypothesis that an acute increase in ATP synthesis, via short chain fatty acid (butyrate) perfusion, restores contractile function in MHD. Isolated hearts of mice with MHD due to consumption of a high fat high sucrose (HFHS) diet or on a control diet (CD) for 4 months were studied using 31 P NMR spectroscopy to measure high energy phosphates and ATP synthesis rates during increased work demand. At baseline, HFHS hearts had increased ADP and decreased free energy of ATP hydrolysis (ΔG~ATP ), although contractile function was similar between the two groups. At high work demand, the ATP synthesis rate in HFHS hearts was reduced by over 50%. Unlike CD hearts, HFHS hearts did not increase contractile function at high work demand, indicating a lack of contractile reserve. However, acutely supplementing HFHS hearts with 4mM butyrate normalized ATP synthesis, ADP, ΔG~ATP and contractile reserve. Thus, acute reversal of depressed mitochondrial ATP production improves contractile dysfunction in MHD. These findings suggest that energy starvation may be a reversible cause of myocardial dysfunction in MHD, and opens new therapeutic opportunities.

Comparative Transcriptome Analysis Reveals the Potential Mechanism of Abortion in Tobacco sua-Cytoplasmic Male Sterility

sua-CMS (cytoplasmic male sterility) is the only male sterile system in tobacco breeding, but the mechanism of abortion is unclear. Cytological characteristics show that abortion in the sua-CMS line msZY occurs before the differentiation of sporogenous cells. In this study, a comparative transcriptomic analysis was conducted on flower buds at the abortion stage of msZY and its male fertile control ZY. A total of 462 differentially expressed genes were identified in msZY and ZY, which were enriched via protein processing in the endoplasmic reticulum (ER), oxidative phosphorylation, photosynthesis, and circadian rhythm-plant by Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses. Most genes were downregulated in the ER stress pathway, heat-shock protein family, F1F0-ATPase encoding by the mitochondrial genome, and differentiation of stamens. Genes in the programmed cell death (PCD) pathway were upregulated in msZY. The transcriptome results were consistent with those of qRT-PCR. Ultrastructural and physiological analyses indicted active vacuole PCD and low ATP content in msZY young flower buds. We speculated that PCD and a deficiency in ATP synthesis are essential for the abortion of sua-CMS. This study reveals the potential mechanism of abortion of tobacco sua-CMS.

The decline in photosynthetic rate upon transfer from high to low light is linked to the slow kinetics of chloroplast ATP synthase in Bletilla striata

Upon a sudden transition from high to low light, the rate of CO₂ assimilation (A_N) in some plants first decreases to a low level before gradually becoming stable. However, the underlying mechanisms remain controversial. The activity of chloroplast ATP synthase (g_H⁺) is usually depressed under high light when compared with low light. Therefore, we hypothesize that upon a sudden transfer from high to low light, the relatively low g_H⁺ restricts ATP synthesis and thus causes a reduction in A_N. To test this hypothesis, we measured gas exchange, chlorophyll fluorescence, P700 redox state, and electrochromic shift signals in Bletilla striata (Orchidaceae). After the transition from saturating to lower irradiance, A_N and ETRII decreased first to a low level and then gradually increased to a stable value. Within the first seconds after transfer from high to low light, g_H⁺ was maintained at low levels. During further exposure to low light, g_H⁺ gradually increased to a stable value. Interestingly, a tight positive relationship was found between g_H⁺ and ETRII. These results suggested that upon a sudden transition from high to low light, A_N was restricted by g_H⁺ at the step of ATP synthesis. Taken together, we propose that the decline in A_N upon sudden transfer from high to low light is linked to the slow kinetics of chloroplast ATP synthase.

An update of the chemiosmotic theory as suggested by possible proton currents inside the coupling membrane

Understanding how biological systems convert and store energy is a primary purpose of basic research. However, despite Mitchell's chemiosmotic theory, we are far from the complete description of basic processes such as oxidative phosphorylation (OXPHOS) and photosynthesis. After more than half a century, the chemiosmotic theory may need updating, thanks to the latest structural data on respiratory chain complexes. In particular, up-to date technologies, such as those using fluorescence indicators following proton displacements, have shown that proton translocation is lateral rather than transversal with respect to the coupling membrane. Furthermore, the definition of the physical species involved in the transfer (proton, hydroxonium ion or proton currents) is still an unresolved issue, even though the latest acquisitions support the idea that protonic currents, difficult to measure, are involved. Moreover, F_oF₁-ATP synthase ubiquitous motor enzyme has the peculiarity (unlike most enzymes) of affecting the thermodynamic equilibrium of ATP synthesis. It seems that the concept of diffusion of the proton charge expressed more than two centuries ago by Theodor von Grotthuss is to be taken into consideration to resolve these issues. All these uncertainties remind us that also in biology it is necessary to consider the Heisenberg indeterminacy principle, which sets limits to analytical questions.

Mitochondrial Costs of Being Hot: Effects of Acute Thermal Change on Liver Bioenergetics in Toads (Bufo bufo)

Global climatic warming is predicted to drive extreme thermal events, especially in temperate terrestrial environments. Hence, describing how physiological parameters are affected by acute temperature changes would allow us to understand the energy management of organisms facing such non-predictable and constraining events. As mitochondria play a key role in the conversion of energy from food into ATP but also produce harmful reactive oxygen species, the understanding of its functioning is crucial to determine the proximal causes of potential decline in an animal's performance. Here we studied the effects of acute temperature changes (between 20 and 30°C) on mitochondrial respiration, ATP synthesis rate, oxidative phosphorylation efficiency (ATP/O), and H2O2 generation in isolated liver mitochondria of a terrestrial ectotherm, the common toad (Bufo bufo). Using succinate as the respiratory substrate, we found that the mitochondrial rates of oxygen consumption, ATP synthesis, and H2O2 generation increased as the temperature increased, being 65, 52, and 66% higher at 30°C than at 20°C, respectively. We also found that the mitochondrial coupling efficiency (ATP/O) decreased, while the oxidative cost of ATP production (H2O2/ATP ratio) increased. The present results further indicate that between 40 and 60% of temperature effects on mitochondrial ATP production and H2O2 generation was at minima driven by an action on the oxidative capacity of the mitochondria. These results suggest that B. bufo may need to allocate extra energy to maintain ATP production and protect cells from oxidative stress, reducing the energy allocable performances.

Kinetic coupling of the respiratory chain with ATP synthase, but not proton gradients, drives ATP production in cristae membranes

Mitochondria have a characteristic ultrastructure with invaginations of the inner membrane called cristae that contain the protein complexes of the oxidative phosphorylation system. How this particular morphology of the respiratory membrane impacts energy conversion is currently unknown. One proposed role of cristae formation is to facilitate the establishment of local proton gradients to fuel ATP synthesis. Here, we determined the local pH values at defined sublocations within mitochondria of respiring yeast cells by fusing a pH-sensitive GFP to proteins residing in different mitochondrial subcompartments. Only a small proton gradient was detected over the inner membrane in wild type or cristae-lacking cells. Conversely, the obtained pH values did barely permit ATP synthesis in a reconstituted system containing purified yeast F₁F₀ ATP synthase, although, thermodynamically, a sufficiently high driving force was applied. At higher driving forces, where robust ATP synthesis was observed, a P-side pH value of 6 increased the ATP synthesis rate 3-fold compared to pH 7. In contrast, when ATP synthase was coreconstituted with an active proton-translocating cytochrome oxidase, ATP synthesis readily occurred at the measured, physiological pH values. Our study thus reveals that the morphology of the inner membrane does not influence the subcompartmental pH values and is not necessary for robust oxidative phosphorylation in mitochondria. Instead, it is likely that the dense packing of the oxidative phosphorylation complexes in the cristae membranes assists kinetic coupling between proton pumping and ATP synthesis.

Runting and Stunting Syndrome Is Associated With Mitochondrial Dysfunction in Sex-Linked Dwarf Chicken

Runting and stunting syndrome (RSS) in chicken are commonly known as “frozen chicken.” The disease is characterized by lower body weight and slow growth and the incidence rate is widely 5%–20% in sex-linked dwarf (SLD) chickens. However, the etiology of RSS in chickens has plagued researchers for several decades. In this study, histopathology studies demonstrated that the hepatocytes of the RSS chickens contain many mitochondria with damaged and outer and inner membrane along with vacuolar hydropic degeneration. No mtDNA mutation was detected, but our microarray data showed that RSS chickens exhibited abnormal expression of genes, many of which are involved in oxidative phosphorylation (OXPHOS) and fatty acid metabolism. In particular, nuclear gene IGF2BP3 was upregulated in RSS chickens' liver cells. The abnormal expression of these genes is likely to impair the OXPHOS, resulting in reduced ATP synthesis in the hepatocytes of the RSS chickens, which may in turn leads to poor weight gain and retarded growth or stunting of chicks. Our findings suggest that mitochondria dysfunction rather than chronic inflammation is responsible for the reduced growth and RSS in SLD chickens. Mutations in GHR have been shown to compromise mitochondrial function in SLD chickens. Since the mitochondrial damage in the RSS chicken is more severe, we suggest that extra genes are likely to be affected to exacerbate the phenotype.

Characterization of Two Polyphosphate Kinase 2 Enzymes Used for ATP Synthesis

Enzymes used for adenosine triphosphate (ATP) synthesis play important roles in energy-dependent cascade reactions in vitro. In this study, two novel polyphosphate kinase 2 (PPK2) enzymes, HbPPK2 from Hydrogenophilaceae bacterium and NdPPK2 from Nocardioides dokdonensis, were characterized for ATP synthesis with the substrate polyphosphate (polyP). The optimum temperature and pH of both purified HbPPK2 and NdPPK2 were 30 °C and 6.5. HbPPK2 and NdPPK2 retained 30% and 14% of the initial activity at 30 °C for 12 h, respectively, whereas the presence of polyP significantly enhanced the stability of enzymes. The two PPK2s preferentially catalyzed the long-chain polyP hexametaphosphate as the phosphate donor. Adenosine monophosphate could not be used by HbPPK2 and NdPPK2 to synthesize ATP, indicating that they belonged to the class I subfamily of PPK2. HbPPK2 was used for ATP regeneration to produce glutathione by a two-enzyme cascade in vitro. 47.1 ± 0.4 mM glutathione was synthesized with a productivity of 13.5 ± 0.1 mM/h. ATP was regenerated approximately 471 times in the system within 3.5 h. HbPPK2 showed potential application for ATP regeneration in cascade reaction.