Membrane potential of mitochondria measured with an electrode sensitive to tetraphenyl phosphonium and relationship between proton electrochemical potential and phosphorylation potential in steady state

Mitochondrial Membrane Potential (ΔΨ) Fluctuations Associated with the Metabolic States of Mitochondria

The proton electrochemical gradient generated by the respiratory chain activity accounts for over 90% of the available respiratory energy and, as such, its evaluation and accurate measurement regarding total values and fluctuations are an invaluable component of the understanding of mitochondrial function. Consequently, alterations in electric potential across the inner mitochondrial membrane generated by differential protonic accumulation and transport are known as the mitochondrial membrane potential, or Δψ, and are reflective of the functional metabolic status of mitochondria. There are several experimental approaches to measure Δψ, ranging from fluorometric evaluations to electrochemical probes. In this chapter, we describe how Δψ may be evaluated in isolated mitochondria and live cells using electrochemical and fluorescent methods, such as tetraphenylphosphonium (TPP+) and tetramethylrhodamine methyl ester (TMRM), respectively. These methods are dependent on the accumulation of cationic probes within mitochondria, which are assessed by using a TPP+-selective electrode or instruments that measure fluorescence (microplate reader and flow cytometer).

Mitochondria facilitate neuronal differentiation by metabolising nuclear-encoded RNA

Mitochondrial activity directs neuronal differentiation dynamics during brain development. In this context, the long-established metabolic coupling of mitochondria and the eukaryotic host falls short of a satisfactory mechanistic explanation, hinting at an undisclosed facet of mitochondrial function. Here, we reveal an RNA-based inter-organellar communication mode that complements metabolic coupling of host-mitochondria and underpins neuronal differentiation. We show that within minutes of exposure to differentiation cues and activation of the electron transport chain, the mitochondrial outer membrane transiently fuses with the nuclear membrane of neural progenitors, leading to efflux of nuclear-encoded RNAs (neRNA) into the positively charged mitochondrial intermembrane space. Subsequent degradation of mitochondrial neRNAs by Polynucleotide phosphorylase 1 (PNPase) located in the intermembrane space curbs the transcriptomic memory of progenitor cells. Further, acquisition of neRNA by mitochondria leads to a collapse of proton motive force, suppression of ATP production, and a resultant amplification of autophagic flux that attenuates proteomic memory. Collectively, these events force the progenitor cells towards a "tipping point" characterised by emergence of a competing neuronal differentiation program. It appears that neuronal differentiation is a consequence of reprogrammed coupling of metabolomic and transcriptomic landscapes of progenitor cells, with mitochondria emerging as key "reprogrammers" that operate by acquiring and metabolising neRNAs. However, the documented role of mitochondria as "reprogrammers" of differentiation remains to be validated in other neuronal lineages and in vivo.

Fluorescent Nanocable as a Biomedical Tool: Intracellular Self-Assembly Formed by a Natural Product Interconnects and Synchronizes Mitochondria

Self-assembly processes commonly occur in various biological contexts to form functional biological structures. However, the self-assembly of nanofibers within cells by heterologous molecules showing a biological function is rare. In this work, we reported the intracellular formation of fluorescent nanofibers by a natural small molecule, lycobetaine (LBT), which facilitated the direct physical connection between mitochondria and synchronized their membrane potential oscillations. The luminescent properties of LBT enabled the real-time observation of nanofiber formation, while the semiconductive nature of the LBT nanofiber facilitated electrical signal transduction among the connected mitochondria. This study introduces an approach to modulate mitochondrial connectivity within cells using "nano-cables" which facilitate studies on synchronized mitochondrial operations and the underlying mechanisms of drug action.

Neurotransmitter accumulation and Parkinson's disease-like phenotype caused by anion channelrhodopsin opto-controlled astrocytic mitochondrial depolarization in substantia nigra pars compacta

Parkinson's disease (PD) is a mitochondria-related neurodegenerative disease characterized by locomotor deficits and loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNc). Majority of PD research primarily focused on neuronal dysfunction, while the roles of astrocytes and their mitochondria remain largely unexplored. To bridge the gap and investigate the roles of astrocytic mitochondria in PD progression, we constructed a specialized optogenetic tool, mitochondrial-targeted anion channelrhodopsin, to manipulate mitochondrial membrane potential in astrocytes. Utilizing this tool, the depolarization of astrocytic mitochondria within the SNc in vivo led to the accumulation of γ-aminobutyric acid (GABA) and glutamate in SNc, subsequently resulting in excitatory/inhibitory imbalance and locomotor deficits. Consequently, in vivo calcium imaging and interventions of neurotransmitter antagonists demonstrated that GABA accumulation mediated movement deficits of mice. Furthermore, 1 h/day intermittent astrocytic mitochondrial depolarization for 2 weeks triggered spontaneous locomotor dysfunction, α-synuclein aggregation, and the loss of DA neurons, suggesting that astrocytic mitochondrial depolarization was sufficient to induce a PD-like phenotype. In summary, our findings suggest the maintenance of proper astrocytic mitochondrial function and the reinstatement of a balanced neurotransmitter profile may provide a new angle for mitigating neuronal dysfunction during the initial phases of PD.

How and when to measure mitochondrial inner membrane potentials

The scientific literature on mitochondria has increased significantly over the years due to findings that these organelles have widespread roles in the onset and progression of pathological conditions such as metabolic disorders, neurodegenerative and cardiovascular diseases, inflammation, and cancer. Researchers have extensively explored how mitochondrial properties and functions are modified in different models, often using fluorescent inner mitochondrial membrane potential (ΔΨm) probes to assess functional mitochondrial aspects such as protonmotive force and oxidative phosphorylation. This review provides an overview of existing techniques to measure ΔpH and ΔΨm, highlighting their advantages, limitations, and applications. It discusses drawbacks of ΔΨm probes, especially when used without calibration, and conditions where alternative methods should replace ΔΨm measurements for the benefit of the specific scientific objectives entailed. Studies investigating mitochondria and their vast biological roles would be significantly advanced by the understanding of the correct applications as well as limitations of protonmotive force measurements and use of fluorescent ΔΨm probes, adopting more precise, artifact-free, sensitive, and quantitative measurements of mitochondrial functionality.

Thioflavin T indicates mitochondrial membrane potential in mammalian cells

The fluorescent benzothiazole dye thioflavin T (ThT) is widely used as a marker for protein aggregates, most commonly in the context of neurodegenerative disease research and diagnosis. Recently, this same dye was shown to indicate membrane potential in bacteria due to its cationic nature. This finding prompted a question whether ThT fluorescence is linked to the membrane potential in mammalian cells, which would be important for appropriate utilization of ThT in research and diagnosis. Here, we show that ThT localizes into the mitochondria of HeLa cells in a membrane-potential-dependent manner. Specifically, ThT colocalized in cells with the mitochondrial membrane potential indicator tetramethylrhodamine methyl ester (TMRM) and gave similar temporal responses as TMRM to treatment with a protonophore, carbonyl cyanide-4-(trifluoromethoxy) phenylhydrazone (FCCP). Additionally, we found that presence of ThT together with exposure to blue light (λ = 405 nm), but neither factor alone, caused depolarization of mitochondrial membrane potential. This additive effect of the concentration and blue light was recapitulated by a mathematical model implementing the potential-dependent distribution of ThT and its effect on mitochondrial membrane potential through photosensitization. These results show that ThT can act as a mitochondrial membrane potential indicator in mammalian cells, when used at low concentrations and with low blue light exposure. However, it causes dissipation of the mitochondrial membrane potential depending additively on its concentrations and blue light exposure. This conclusion motivates a re-evaluation of ThT's use at micromolar range in live-cell analyses and indicates that this dye can enable future studies on the potential connections between mitochondrial membrane potential dynamics and protein aggregation.

Peptide Power: Mechanistic Insights into the Effect of Mitochondria-Targeted Tetrapeptides on Membrane Electrostatics from Molecular Simulations

Mitochondrial dysfunction is implicated in nine of the ten leading causes of death in the US, yet there are no FDA-approved therapeutics to treat it. Synthetic mitochondria-targeted peptides (MTPs), including the lead compound SS-31, offer promise, as they have been shown to restore healthy mitochondrial function and treat a variety of common diseases. At the cellular level, research has shown that MTPs accumulate strongly at the inner mitochondrial membrane (IMM), slow energy sinks (e.g., proton leaks), and improve ATP production. Modulation of electrostatic fields around the IMM has been implicated as a key aspect in the mechanism of action (MoA) of these peptides; however, molecular and mechanistic details have remained elusive. In this study, we employed all-atom molecular dynamics simulations (MD) to investigate the interactions of four MTPs with lipid bilayers and calculate their effect on structural and electrostatic properties. In agreement with previous experimental findings, we observed the modulation of the membrane surface and dipole potentials by MTPs. The simulations reveal that the MTPs achieve a reduction in the dipole potential by acting to disorder both lipid head groups and water layers proximal to the bilayer surface. We also find that MTPs decrease the bilayer thickness and increase the membrane's capacitance. These changes suggest that MTPs may enhance how much potential energy can be stored across the IMM at a given transmembrane potential difference. The MTPs also displace cations away from the bilayer surface, modulating the surface potential and offering an alternative mechanism for how these MTPs reduce mitochondrial energy sinks like proton leaks and mitigate Ca2+ accumulation stress. In conclusion, this study highlights the therapeutic potential of MTPs and underlines how interactions of MTPs with lipid bilayers serve as a fundamental component of their MoA.

Efficiency of mitochondrial uncoupling by modified butyltriphenylphosphonium cations and fatty acids correlates with lipophilicity of cations: Protonophoric vs leakage mechanisms

In order to determine the share of protonophoric activity in the uncoupling action of lipophilic cations a number of analogues of butyltriphenylphosphonium with substitutions in phenyl rings (C4TPP-X) were studied on isolated rat liver mitochondria and model lipid membranes. An increase in the rate of respiration and a decrease in the membrane potential of isolated mitochondria were observed for all the studied cations, the efficiency of these processes was significantly enhanced in the presence of fatty acids and correlated with the octanol-water partition coefficient of the cations. The ability of C4TPP-X cations to induce proton transport across the lipid membrane of liposomes loaded with a pH-sensitive fluorescent dye increased also with their lipophilicity and depended on the presence of palmitic acid in the liposome membrane. Of all the cations, only butyl[tri(3,5-dimethylphenyl)]phosphonium (C4TPP-diMe) was able to induce proton transport by the mechanism of formation of a cation-fatty acid ion pair on planar bilayer lipid membranes and liposomes. The rate of oxygen consumption by mitochondria in the presence of C4TPP-diMe increased to the maximum values corresponding to conventional uncouplers; for all other cations the maximum uncoupling rates were significantly lower. We assume that the studied cations of the C4TPP-X series, with the exception of C4TPP-diMe at low concentrations, cause nonspecific leak of ions through lipid model and biological membranes which is significantly enhanced in the presence of fatty acids.

Mitochondria in health, disease, and aging

Mitochondria are well known as organelles responsible for the maintenance of cellular bioenergetics through the production of ATP. Although oxidative phosphorylation may be their most important function, mitochondria are also integral for the synthesis of metabolic precursors, calcium regulation, the production of reactive oxygen species, immune signaling, and apoptosis. Considering the breadth of their responsibilities, mitochondria are fundamental for cellular metabolism and homeostasis. Appreciating this significance, translational medicine has begun to investigate how mitochondrial dysfunction can represent a harbinger of disease. In this review, we provide a detailed overview of mitochondrial metabolism, cellular bioenergetics, mitochondrial dynamics, autophagy, mitochondrial damage-associated molecular patterns, mitochondria-mediated cell death pathways, and how mitochondrial dysfunction at any of these levels is associated with disease pathogenesis. Mitochondria-dependent pathways may thereby represent an attractive therapeutic target for ameliorating human disease.

Incorporating a Polyethyleneglycol Linker to Enhance the Hydrophilicity of Mitochondria-Targeted Triphenylphosphonium Constructs

The targeting of bioactive molecules and probes to mitochondria can be achieved by coupling to the lipophilic triphenyl phosphonium (TPP) cation, which accumulates several hundred-fold within mitochondria in response to the mitochondrial membrane potential (Δψm ). Typically, a simple alkane links the TPP to its "cargo", increasing overall hydrophobicity. As it would be beneficial to enhance the water solubility of mitochondria-targeted compounds we explored the effects of replacing the alkyl linker with a polyethylene glycol (PEG). We found that the use of PEG led to compounds that were readily taken up by isolated mitochondria and by mitochondria inside cells. Within mitochondria the PEG linker greatly decreased adsorption of the TPP constructs to the matrix-facing face of the mitochondrial inner membrane. These findings will allow the distribution of mitochondria-targeted TPP compounds within mitochondria to be fine-tuned.

Changes in the mitochondrial membrane potential in endothelial cells can be detected by Raman microscopy

The role of mitochondria goes beyond their capacity to create molecular fuel and includes e.g. the production of reactive oxygen species and the regulation of cell death. In endothelial cells, mitochondria have a significant impact on cellular function under both healthy and pathological conditions. Endothelial dysfunction contributes to the development of various lifestyle diseases and the key players in their pathogenesis are among others vascular inflammation and oxidative stress. The latter is very closely related to mitochondrial dysfunction; however, it is not straightforward. First, because mitochondria are small cellular structures, and second, it requires a sensitive method to follow the subtle biochemical changes. For this purpose, Raman microscopy (RM) was used here, which is considered a high-resolution method and can be applied in situ, usually as a non-labeled technique. In this work, we show that RM can not only locate mitochondria in the cell but also track their functional changes. Moreover, we test if labeling cells with Raman probes (Rp) can improve the specificity and sensitivity of RM (compared to conventional labeled techniques such as fluorescence, and the non-labeled Raman technique). MitoBADY Rp was used to detect changes in mitochondrial membrane potential as an indicator of mitochondrial activity, e.g. hyperpolarization or distortion of the proton gradient in the intermembrane space (depolarization). Thus, we show and compare RM, in the form of a label and non-labeled, to such a subtle cellular analysis.

Mitochondrial Dynamics during Development

Mitochondria are dynamic membrane-bound organelles in eukaryotic cells. These are important for the generation of chemical energy needed to power various cellular functions and also support metabolic, energetic, and epigenetic regulation in various cells. These organelles are also important for communication with the nucleus and other cellular structures, to maintain developmental sequences and somatic homeostasis, and for cellular adaptation to stress. Increasing information shows mitochondrial defects as an important cause of inherited disorders in different organ systems. In this article, we provide an extensive review of ontogeny, ultrastructural morphology, biogenesis, functional dynamics, important clinical manifestations of mitochondrial dysfunction, and possibilities for clinical intervention. We present information from our own clinical and laboratory research in conjunction with information collected from an extensive search in the databases PubMed, EMBASE, and Scopus.

Mitochondrial NAD(P)+ Transhydrogenase: From Molecular Features to Physiology and Disease

Significance: Proton-translocating NAD(P)⁺ transhydrogenase, also known as nicotinamide nucleotide transhydrogenase (NNT), catalyzes a reversible reaction coupling the protonmotive force across the inner mitochondrial membrane and hydride (H^-, a proton plus two electrons) transfer between the mitochondrial pools of NAD(H) and NADP(H). The forward NNT reaction is a source of NADPH in the mitochondrial matrix, fueling antioxidant and biosynthetic pathways with reductive potential. Despite the greater emphasis given to the net forward reaction, the reverse NNT reaction that oxidizes NADPH also occurs in physiological and pathological conditions. Recent Advances: NNT (dys)function has been linked to various metabolic pathways and disease phenotypes. Most of these findings have been based on spontaneous loss-of-function Nnt mutations found in the C57BL/6J mouse strain (Nnt^C57BL/6J mutation) and disease-causing Nnt mutations in humans. The present review focuses on recent advances based on the mouse Nnt^C57BL/6J mutation. Critical Issues: Most studies associating NNT function with disease phenotypes have been based on comparisons between different strains of inbred mice (with or without the Nnt^C57BL/6J mutation), which creates uncertainties over the actual contribution of NNT in the context of other potential genetic modifiers. Future Directions: Future research might contribute to understanding the role of NNT in pathological conditions and elucidate how NNT regulates physiological signaling through its forward and reverse reactions. The importance of NNT in redox balance and tumor cell proliferation makes it a potential target of new therapeutic strategies for oxidative-stress-mediated diseases and cancer. Antioxid. Redox Signal. 36, 864-884.

PET imaging of mitochondrial function in acute doxorubicin-induced cardiotoxicity: a proof-of-principle study

Mitochondrial dysfunction plays a key role in doxorubicin-induced cardiotoxicity (DIC). In this proof-of-principle study, we investigated whether PET mapping of cardiac membrane potential, an indicator of mitochondrial function, could detect an acute cardiotoxic effect of doxorubicin (DOX) in a large animal model. Eight Yucatan pigs were imaged dynamically with [¹⁸F](4-Fluorophenyl)triphenylphosphonium ([¹⁸F]FTPP⁺) PET/CT. Our experimental protocol included a control saline infusion into the left anterior descending coronary artery (LAD) followed by a DOX test infusion of either 1 mg/kg or 2 mg/kg during PET. We measured the change in total cardiac membrane potential (ΔΨ_T), a proxy for the mitochondrial membrane potential, ΔΨ_m, after the saline and DOX infusions. We observed a partial depolarization of the mitochondria following the DOX infusions, which occurred only in myocardial areas distal to the intracoronary catheter, thereby demonstrating a direct association between the exposure of the mitochondria to DOX and a change in ΔΨ_T. Furthermore, doubling the DOX dose caused a more severe depolarization of myocardium in the LAD territory distal to the infusion catheter. In conclusion, [¹⁸F]FTPP⁺ PET-based ΔΨ_T mapping can measure partial depolarization of myocardial mitochondria following intracoronary DOX infusion in a large animal model.

Comprehensive understanding of multiple actions of anticancer drug tamoxifen in isolated mitochondria

Tamoxifen has been widely used in the treatment of estrogen receptor (ER)-positive breast cancer, whereas it also exhibits ER-independent anticancer effects in various cancer cell types. As one of the convincing mechanisms underlying the ER-independent effects, induction of apoptosis through mitochondrial dysfunction has been advocated. However, the mechanism of action of tamoxifen even at the isolated mitochondrial level is not fully understood and remains controversial. Here, we attempted to comprehensively understand tamoxifen's multiple actions in isolated rat liver mitochondria through not only revisiting the actions hitherto reported but also conducting originally designed experiments. Using submitochondrial particles, we found that tamoxifen has potential as an inhibitor of both respiratory complex I and ATP synthase. However, these inhibitory effects were not elicited in intact mitochondria, likely because penetration of tamoxifen across the inner mitochondrial membrane is highly restricted owing to its localized positive charge (-N+H(CH3)2). This restricted penetration may also explain why tamoxifen is unable to function as a protonophore-type uncoupler in mitochondria. Moreover, tamoxifen suppressed opening of the mitochondrial permeability transition pore induced by Ca2+ overload through enhancing phosphate uptake into the matrix. The photoaffinity labeling experiments using a photolabile tamoxifen derivative (pTAM1) indicated that pTAM1 specifically binds to voltage-dependent anion channels (VDACs) 1 and 3, which regulate transport of various substances into mitochondria. The binding of tamoxifen to VDAC1 and/or VDAC3 could be responsible for the enhancement of phosphate uptake. Taking all the results together, we consider the principal impairment of mitochondrial functions caused by tamoxifen.

Membrane Permeability of Modified Butyltriphenylphosphonium Cations

The alkyltriphenylphosphonium (TPP) group is the most widely used vector targeted to mitochondria. Previously, the length of the alkyl linker was varied as well as structural modifications in the TPP phenyl rings to obtain the optimal therapeutic effect of a pharmacophore conjugated with a lipophilic cation. In the present work, we synthesized butyltriphenylphosphonium cations halogenated and methylated in phenyl rings (C₄TPP-X) and measured electrical current through a planar lipid bilayer in the presence of C₄TPP-X. The permeability of C₄TPP-X varied in the range of 6 orders of magnitude and correlates well with the previously measured translocation rate constant for dodecyltriphenylphosphonium analogues. The partition coefficient of the butyltriphenylphosphonium analogues obtained by calculating the difference in the free energy of cation solvation in water and octane using quantum chemical methods correlates well with the permeability values. Using an ion-selective electrode, a lower degree of accumulation of analogues with halogenated phenyl groups was found on isolated mitochondria of rat liver, which is in agreement with their permeability decrease. Our results indicate the translocation of the butyltriphenylphosphonium cations across the hydrophobic membrane core as rate-limiting stage in the permeability process rather than their binding/release to/from the membrane.

High-Performance Near-Infrared Aggregation-Induced Emission Luminogen with Mitophagy Regulating Capability for Multimodal Cancer Theranostics

The construction of intelligent near-infrared (NIR) fluorophores for high specificity to cancer cells and application in multiple therapeutic modalities is crucial for precise cancer diagnostic and therapy. In this study, an aggregation-induced emission-active NIR fluorophore (TACQ) with mitophagy-modulating activity was synthesized and developed for mitochondrial targeting multimodal cancer theranostics. The strengthened push-pull interaction extended the emission of TACQ into the NIR-II region (>1000 nm). Further, the rotor structure and twisted molecular conformation enables nanoaggregates of TACQ to balance the radiative and nonradiative decays to simultaneously exhibit bright NIR emission, high photothermal conversion efficiency (55%), and efficient generation of reactive oxygen species. The lipocationic property of TACQ allows it to selectively accumulate in the mitochondria of cancer cells. TACQ can induce mitophagy and block mitophagic flux facilitating cancer cell apoptosis. Both in vitro and in vivo evaluations revealed that TACQ is an efficient theranostic agent for NIR fluorescence and photothermal imaging-guided synergistic chemo-photothermal and photodynamic therapy.

Augmented Liver Uptake of the Membrane Voltage Sensor Tetraphenylphosphonium Distinguishes Early Fibrosis in a Mouse Model

Mitochondrial (mito-) oxidative phosphorylation (OxPhos) is a critical determinant of cellular membrane potential/voltage. Dysregulation of OxPhos is a biochemical signature of advanced liver fibrosis. However, less is known about the net voltage of the liver in fibrosis. In this study, using the radiolabeled [³H] voltage sensor, tetraphenylphosphonium (TPP), which depends on membrane potential for cellular uptake/accumulation, we determined the net voltage of the liver in a mouse model of carbon tetrachloride (CCl₄)-induced hepatic fibrosis. We demonstrated that the liver uptake of ³H-TPP significantly increased at 4 weeks of CCl₄-administration (6.07 ± 0.69% ID/g, p < 0.05) compared with 6 weeks (4.85 ± 1.47% ID/g) and the control (3.50 ± 0.22% ID/g). Analysis of the fibrosis, collagen synthesis, and deposition showed that the increased ³H-TPP uptake at 4 weeks corresponds to early fibrosis (F1), according to the METAVIR scoring system. Biodistribution data revealed that the ³H-TPP accumulation is significant in the fibrogenic liver but not in other tissues. Mechanistically, the augmentation of the liver uptake of ³H-TPP in early fibrosis concurred with the upregulation of mito-electron transport chain enzymes, a concomitant increase in mito-oxygen consumption, and the activation of the AMPK-signaling pathway. Collectively, our results indicate that mito-metabolic response to hepatic insult may underlie the net increase in the voltage of the liver in early fibrosis.

Bridging the Gap Between Nature and Antioxidant Setbacks: Delivering Gallic Acid to Mitochondria

Research on mitochondria-targeted active molecules became a hot topic in the past decade. Development of mitochondria permeability transition pore (mPTP )-targeting agents with clinical applications is needed not only because of the importance of the target in several diseases but also due to the fact that the current developed molecules have shown poor clinical success. In fact, only a reduced percentage reach mitochondria , effectively preventing pathological mPTP opening. The mitochondrial-targeting strategies should be a promising solution to increase the selectivity of compounds to the mPTP , reducing also their potential side effects. Chemical conjugation of bioactive molecules with a lipophilic cation such as the triphenylphosphonium (TPP ⁺) has been established as a robust strategy to specifically target mitochondria . Phytochemicals such as hydroxybenzoic acids are normal constituents of the human diet. These molecules display beneficial healthy effects, ranging from antioxidant action through diverse mechanisms to modulation of mitochondrial-related apoptotic system, although their therapeutic application is limited due to pharmacokinetic drawbacks. Accordingly, the development of a new antioxidant based on the dietary benzoic acid-gallic acid -is described as well as the demonstration of its mitochondriotropic characteristics.

Mitochondrial energetics with transmembrane electrostatically localized protons: do we have a thermotrophic feature?

Transmembrane electrostatically localized protons (TELP) theory has been recently recognized as an important addition over the classic Mitchell's chemiosmosis; thus, the proton motive force (pmf) is largely contributed from TELP near the membrane. As an extension to this theory, a novel phenomenon of mitochondrial thermotrophic function is now characterized by biophysical analyses of pmf in relation to the TELP concentrations at the liquid-membrane interface. This leads to the conclusion that the oxidative phosphorylation also utilizes environmental heat energy associated with the thermal kinetic energy (kBT) of TELP in mitochondria. The local pmf is now calculated to be in a range from 300 to 340 mV while the classic pmf (which underestimates the total pmf) is in a range from 60 to 210 mV in relation to a range of membrane potentials from 50 to 200 mV. Depending on TELP concentrations in mitochondria, this thermotrophic function raises pmf significantly by a factor of 2.6 to sixfold over the classic pmf. Therefore, mitochondria are capable of effectively utilizing the environmental heat energy with TELP for the synthesis of ATP, i.e., it can lock heat energy into the chemical form of energy for cellular functions.

Conjugation of Natural Triterpenic Acids with Delocalized Lipophilic Cations: Selective Targeting Cancer Cell Mitochondria

Currently, a new line of research on mitochondria-targeted anticancer drugs is actively developing in the field of biomedicine and medicinal chemistry. The distinguishing features of this universal target for anticancer agents include presence of mitochondria in the overwhelming majority, if not all types of transformed cells, crucial importance of these cytoplasmic organelles in energy production, regulation of cell death pathways, as well as generation of reactive oxygen species and maintenance of calcium homeostasis. Hence, mitochondriotropic anticancer mitocan agents, acting through mitochondrial destabilization, have good prospects in cancer therapy. Available natural pentacyclic triterpenoids are considered promising scaffolds for development of new mitochondria-targeted anticancer agents. These secondary metabolites affect the mitochondria of tumor cells and initiate formation of reactive oxygen species. The present paper focuses on the latest research outcomes of synthesis and study of cytotoxic activity of conjugates of pentacyclic triterpenoids with some mitochondria-targeted cationic lipophilic molecules and highlights the advantages of applying them as novel mitocan agents compared to their prototype natural triterpenic acids.

Mitochondrial K+ Transport: Modulation and Functional Consequences

The existence of a K⁺ cycle in mitochondria has been predicted since the development of the chemiosmotic theory and has been shown to be crucial for several cellular phenomena, including regulation of mitochondrial volume and redox state. One of the pathways known to participate in K⁺ cycling is the ATP-sensitive K⁺ channel, MitoK_ATP. This channel was vastly studied for promoting protection against ischemia reperfusion when pharmacologically activated, although its molecular identity remained unknown for decades. The recent molecular characterization of MitoK_ATP has opened new possibilities for modulation of this channel as a mechanism to control cellular processes. Here, we discuss different strategies to control MitoK_ATP activity and consider how these could be used as tools to regulate metabolism and cellular events.

Quantification of Myocardial Mitochondrial Membrane Potential Using PET

PURPOSE OF REVIEW

To present a method enabling in vivo quantification of tissue membrane potential (ΔΨ_T), a proxy of mitochondrial membrane potential (ΔΨ_m), to review the origin and role of ΔΨ_m, and to highlight potential applications of myocardial ΔΨ_T imaging.

RECENT FINDINGS

Radiolabelled lipophilic cations have been used for decades to measure ΔΨ_m in vitro. Using similar compounds labeled with positron emitters and appropriate compartment modeling, this technique now allows in vivo quantification of ΔΨ_T with positron emission tomography. Studies have confirmed the feasibility of measuring myocardial ΔΨ_T in both animals and humans. In addition, ΔΨ_T showed very low variability among healthy subjects, suggesting that this method could allow detection of relatively small pathological changes. In vivo assessment of myocardial ΔΨ_T provides a new tool to study the pathophysiology of cardiovascular diseases and has the potential to serve as a new biomarker to assess disease stage, prognosis, and response to therapy.

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.

Mitochondrial proton leaks and uncoupling proteins

Non-shivering thermogenesis in brown adipose tissue is mediated by uncoupling protein 1 (UCP1), which provides a carefully regulated proton re-entry pathway across the mitochondrial inner membrane operating in parallel to the ATP synthase and allowing respiration, and hence thermogenesis, to be released from the constraints of respiratory control. In the 40 years since UCP1 was first described, an extensive, and frequently contradictory, literature has accumulated, focused on the acute physiological regulation of the protein by fatty acids, purine nucleotides and possible additional factors. The purpose of this review is to examine, in detail, the experimental evidence underlying these proposed mechanisms. Emphasis will be placed on the methodologies employed and their relation to the physiological constraints under which the protein functions in the intact cell. The nature of the endogenous, UCP1-independent, proton leak will also be discussed. Finally, the troubled history of the putative novel uncoupling proteins, UCP2 and UCP3, will be evaluated.

Mechanisms of Energy Transduction by Charge Translocating Membrane Proteins

Life relies on the constant exchange of different forms of energy, i.e., on energy transduction. Therefore, organisms have evolved in a way to be able to harvest the energy made available by external sources (such as light or chemical compounds) and convert these into biological useable energy forms, such as the transmembrane difference of electrochemical potential (Δμ̃). Membrane proteins contribute to the establishment of Δμ̃ by coupling exergonic catalytic reactions to the translocation of charges (electrons/ions) across the membrane. Irrespectively of the energy source and consequent type of reaction, all charge-translocating proteins follow two molecular coupling mechanisms: direct- or indirect-coupling, depending on whether the translocated charge is involved in the driving reaction. In this review, we explore these two coupling mechanisms by thoroughly examining the different types of charge-translocating membrane proteins. For each protein, we analyze the respective reaction thermodynamics, electron transfer/catalytic processes, charge-translocating pathways, and ion/substrate stoichiometries.

Exposure to marine benthic dinoflagellate toxins may lead to mitochondrial dysfunction

Even though marine dinoflagellates are important primary producers, many toxic species may alter the natural equilibrium of aquatic ecosystems and even generate human intoxication incidents, as they are the major causative agents of harmful algal blooms. In order to deepen the knowledge regarding benthic dinoflagellate adverse effects, the present study aims to clarify the influence of Gambierdiscus excentricus strain UNR-08, Ostreopsis cf. ovata strain UNR-03 and Prorocentrum lima strain UNR-01 crude extracts on rat mitochondrial energetic function and permeability transition pore (mPTP) induction. Our results, expressed in number of dinoflagellate cell toxic compounds tested in a milligram of mitochondrial protein, revealed that 934 cells mg prot-1 of G. excentricus, and 7143 cells mg prot-1 of both O. cf. ovata and P. lima negatively affect mitochondrial function, including by decreasing ATP synthesis-related membrane potential variations. Moreover, considerably much lower concentrations of dinoflagellate extracts (117 cells mg prot-1 of G. excentricus, 1429 cells mg prot-1 of O. cf. ovata and 714 cells mg prot-1 of P. lima) produced mPTP-induced swelling in Ca2+-loaded isolated mitochondria. The present study clearly demonstrates the toxicity of G. excentricus, O. cf. ovata and P. lima extracts at the mitochondrial level, which may lead to mitochondrial failure and consequent cell toxicity, and that G. excentricus always provide much more severe effects than O. cf. ovata and P. lima.

The effect of mitochondrial calcium uniporter and cyclophilin D knockout on resistance of brain mitochondria to Ca2+-induced damage

The mitochondrial calcium uniporter (MCU) and cyclophilin D (CyD) are key players in induction of the permeability transition pore (PTP), which leads to mitochondrial depolarization and swelling, the major signs of Ca2+-induced mitochondrial damage. Mitochondrial depolarization inhibits ATP production, whereas swelling results in the release of mitochondrial pro-apoptotic proteins. The extent to which simultaneous deletion of MCU and CyD inhibits PTP induction and prevents damage of brain mitochondria is not clear. Here, we investigated the effects of MCU and CyD deletion on the propensity for PTP induction using mitochondria isolated from the brains of MCU-KO, CyD-KO, and newly created MCU/CyD-double knockout (DKO) mice. Neither deletion of MCU nor of CyD affected respiration or membrane potential in mitochondria isolated from the brains of these mice. Mitochondria from MCU-KO and MCU/CyD-DKO mice displayed reduced Ca2+ uptake and diminished extent of PTP induction. The Ca2+ uptake by mitochondria from CyD-KO mice was increased compared with mitochondria from WT mice. Deletion of CyD prevented mitochondrial swelling and resulted in transient depolarization in response to Ca2+, but it did not prevent Ca2+-induced delayed mitochondrial depolarization. Mitochondria from MCU/CyD-DKO mice did not swell in response to Ca2+, but they did exhibit mild sustained depolarization. Dibucaine, an inhibitor of the Ca2+-activated mitochondrial phospholipase A2, attenuated and bovine serum albumin completely eliminated the sustained depolarization. This suggests the involvement of phospholipase A2 and free fatty acids. Thus, in addition to induction of the classical PTP, alternative deleterious mechanisms may contribute to mitochondrial damage following exposure to elevated Ca2+.

A ketogenic diet combined with exercise alters mitochondrial function in human skeletal muscle while improving metabolic health

Animal data indicate that ketogenic diets are associated with improved mitochondrial function, but human data are lacking. We aimed to characterize skeletal muscle mitochondrial changes in response to a ketogenic diet combined with exercise training in healthy individuals. Twenty-nine physically active adults completed a 12-wk supervised exercise program after self-selection into a ketogenic diet (KD, n = 15) group or maintenance of their habitual mixed diet (MD, n = 14). Measures of metabolic health and muscle biopsies (vastus lateralis) were obtained before and after the intervention. Mitochondria were isolated from muscle and studied after exposure to carbohydrate (pyruvate), fat (palmitoyl-l-carnitine), and ketone (β-hydroxybutyrate+acetoacetate) substrates. Compared with MD, the KD resulted in increased whole body resting fat oxidation (P < 0.001) and decreased fasting insulin (P = 0.019), insulin resistance [homeostatic model assessment of insulin resistance (HOMA-IR), P = 0.022], and visceral fat (P < 0.001). The KD altered mitochondrial function as evidenced by increases in mitochondrial respiratory control ratio (19%, P = 0.009), ATP production (36%, P = 0.028), and ATP/H₂O₂ (36%, P = 0.033) with the fat-based substrate. ATP production with the ketone-based substrate was four to eight times lower than with other substrates, indicating minimal oxidation. The KD resulted in a small decrease in muscle glycogen (14%, P = 0.035) and an increase in muscle triglyceride (81%, P = 0.006). These results expand our understanding of human adaptation to a ketogenic diet combined with exercise. In conjunction with weight loss, we observed altered skeletal muscle mitochondrial function and efficiency, an effect that may contribute to the therapeutic use of ketogenic diets in various clinical conditions, especially those associated with insulin resistance.

Mitochondrial Reversible Changes Determine Diastolic Function Adaptations During Myocardial (Reverse) Remodeling

BACKGROUND

Often, pressure overload-induced myocardial remodeling does not undergo complete reverse remodeling after decreasing afterload. Recently, mitochondrial abnormalities and oxidative stress have been successively implicated in the pathogenesis of several chronic pressure overload cardiac diseases. Therefore, we aim to clarify the myocardial energetic dysregulation in (reverse) remodeling, mainly focusing on the mitochondria.

METHODS

Thirty-five Wistar Han male rats randomly underwent sham or ascending (supravalvular) aortic banding procedure. Echocardiography revealed that banding induced concentric hypertrophy and diastolic dysfunction (early diastolic transmitral flow velocity to peak early-diastolic annular velocity ratio, E/E': sham, 13.6±2.1, banding, 18.5±4.1, P=0.014) accompanied by increased oxidative stress (dihydroethidium fluorescence: sham, 1.6×10⁸±6.1×10⁷, banding, 2.6×10⁸±4.5×10⁷, P<0.001) and augmented mitochondrial function. After 8 to 9 weeks, half of the banding animals underwent overload relief by an aortic debanding surgery (n=10).

RESULTS

Two weeks later, hypertrophy decreased with the decline of oxidative stress (dihydroethidium fluorescence: banding, 2.6×10⁸±4.5×10⁷, debanding, 1.96×10⁸±6.8×10⁷, P<0.001) and diastolic dysfunction improved simultaneously (E/E': banding, 18.5±4.1, debanding, 15.1±1.8, P=0.029). The reduction of energetic demands imposed by overload relief allowed the mitochondria to reduce its activity and myocardial levels of phosphocreatine, phosphocreatine/ATP, and ATP/ADP to normalize in debanding towards sham values (phosphocreatine: sham, 38.4±7.4, debanding, 35.6±8.7, P=0.71; phosphocreatine/ATP: sham, 1.22±0.23 debanding, 1.11±0.24, P=0.59; ATP/ADP: sham, 6.2±0.9, debanding, 5.6±1.6, P=0.66). Despite the decreased mitochondrial area, complex III and V expression increased in debanding compared with sham or banding. Autophagy and mitophagy-related markers increased in banding and remained higher in debanding rats.

CONCLUSIONS

During compensatory and maladaptive hypertrophy, mitochondria become more active. However, as the disease progresses, the myocardial energetic demands increase and the myocardium becomes energy deficient. During reverse remodeling, the concomitant attenuation of cardiac hypertrophy and oxidative stress allowed myocardial energetics, left ventricle hypertrophy, and diastolic dysfunction to recover. Autophagy and mitophagy are probably involved in the myocardial adaptation to overload and to unload. We conclude that these mitochondrial reversible changes underlie diastolic function adaptations during myocardial (reverse) remodeling.

Parabens enhance the calcium-dependent testicular mitochondrial permeability transition: Their relevance on the reproductive capacity in male animals

Parabens, alkyl ester derivatives from p-hydroxybenzoic acid, are extensively used as antimicrobial preservatives. Nonetheless, due to its widespread and massive employment, several studies highlighted the association between parabens and alterations in the reproductive system. This study aimed to relate the adverse effect of the most commonly used parabens in testis mitochondria with male fertility. From all the parabens used, propyl and butyl were the ones that most negatively decreased the respiratory control ratio. In the case of butyl, inhibitions of 20% and 60% were observed, respectively, at the lowest and highest concentration, when compared to the control group. The membrane potential was only significantly affected by propyl (14%) and butyl (31%), and at a concentration of 250 µM. Succinate dehydrogenase, cytochrome c oxidase, and ATPase activities showed a nonsignificant decrease. Cytochrome c reductase, on the other hand, showed statistically significant inhibitions for both propyl (56%) and butylparaben (55%). The susceptibility to the mitochondrial permeability transition pore (MPTP) opening was increased by all parabens, although this increase was markedly significant for propyl and butyl. These results show that the susceptibility of mitochondria to parabens is dependent on the alkyl chain length and parabens hydrophobicity, and the main mitochondrial target is Complex II-III and MPTP. Hence, this study demonstrates the contribution of parabens exposition to the inhibition of testis mitochondrial function and their putative noxious effect on the male reproductive system.

Mutant huntingtin does not cross the mitochondrial outer membrane

Mutant huntingtin (mHTT) is associated with mitochondria, but the exact mitochondrial location of mHTT has not been definitively established. Recently, it was reported that mHTT is present in the intermembrane space and inhibits mitochondrial protein import by interacting with TIM23, a major component of mitochondrial protein import machinery, but evidence for functional ramifications were not provided. We assessed mHTT location using synaptic and nonsynaptic mitochondria isolated from brains of YAC128 mice and subjected to alkali treatment or limited trypsin digestion. Mitochondria were purified either with discontinuous Percoll gradient or with anti-TOM22-conjugated iron microbeads. We also used mitochondria isolated from postmortem brain tissues of unaffected individuals and HD patients. Our results demonstrate that mHTT is located on the cytosolic side of the mitochondrial outer membrane (MOM) but does not cross it. This refutes the hypothesis that mHTT may interact with TIM23 and inhibit mitochondrial protein import. The levels of expression of nuclear-encoded, TIM23-transported mitochondrial proteins ACO2, TUFM, IDH3A, CLPP and mitochondrially encoded and synthesized protein mtCO1 were similar in mitochondria from YAC128 mice and their wild-type littermates as well as in mitochondria from postmortem brain tissues of unaffected individuals and HD patients, supporting the lack of deficit in mitochondrial protein import. Regardless of purification technique, mitochondria from YAC128 and WT mice had similar respiratory activities and mitochondrial membrane potentials. Thus, our data argue against mHTT crossing the MOM and entering into the mitochondrial intermembrane space, making it highly unlikely that mHTT interacts with TIM23 and inhibits protein import in intact mitochondria.

Lipophilic ion aromaticity is not important for permeability across lipid membranes

To clarify the contribution of charge delocalization in a lipophilic ion to the efficacy of its permeation through a lipid membrane, we compared the behavior of alkyl derivatives of triphenylphosphonium, tricyclohexylphosphonium and trihexylphosphonium both in natural and artificial membranes. Exploring accumulation of the lipophilic cations in response to inside-negative membrane potential generation in mitochondria by using an ion-selective electrode revealed similar mitochondrial uptake of butyltricyclohexylphosphonium (C₄TCHP) and butyltriphenylphosphonium (C₄TPP). Fluorescence correlation spectroscopy also demonstrated similar membrane potential-dependent accumulation of fluorescein derivatives of tricyclohexyldecylphosphonium and decyltriphenylphosphonium in mitochondria. The rate constant of lipophilic cation translocation across the bilayer lipid membrane (BLM), measured by the current relaxation method, moderately increased in the following sequence: trihexyltetradecylphosphonium ([P_6,6,6,14]) < triphenyltetradecylphosphonium (C₁₄TPP) < tricyclohexyldodecylphosphonium (C₁₂TCHP). In line with these results, measurements of the BLM stationary conductance indicated that membrane permeability for C₄TCHP is 2.5 times higher than that for C₄TPP. Values of the difference in the free energy of ion solvation in water and octane calculated using the density functional theory and the polarizable continuum solvent model were similar for methyltriphenylphosphonium, tricyclohexylmethylphosphonium and trihexylmethylphosphonium. Our results prove that both cyclic and aromatic moieties are not necessary for lipophilic ions to effectively permeate through lipid membranes.

In vivo quantitative mapping of human mitochondrial cardiac membrane potential: a feasibility study

PURPOSE

Alteration in mitochondrial membrane potential (ΔΨ_m) is an important feature of many pathologic processes, including heart failure, cardiotoxicity, ventricular arrhythmia, and myocardial hypertrophy. We present the first in vivo, non-invasive, assessment of regional ΔΨ_m in the myocardium of normal human subjects.

METHODS

Thirteen healthy subjects were imaged using [¹⁸F]-triphenylphosphonium ([¹⁸F]TPP+) on a PET/MR scanner. The imaging protocol consisted of a bolus injection of 300 MBq followed by a 120-min infusion of 0.6 MBq/min. A 60 min, dynamic PET acquisition was started 1 h after bolus injection. The extracellular space fraction (f_ECS) was simultaneously measured using MR T1-mapping images acquired at baseline and 15 min after gadolinium injection with correction for the subject's hematocrit level. Serial venous blood samples were obtained to calculate the plasma tracer concentration. The tissue membrane potential (ΔΨ_T), a proxy of ΔΨ_m, was calculated from the myocardial tracer concentration at secular equilibrium, blood concentration, and fECS measurements using a model based on the Nernst equation.

RESULTS

In 13 healthy subjects, average tissue membrane potential (ΔΨ_T), representing the sum of cellular membrane potential (ΔΨ_c) and ΔΨ_m, was - 160.7 ± 3.7 mV, in excellent agreement with previous in vitro assessment.

CONCLUSION

In vivo quantification of the mitochondrial function has the potential to provide new diagnostic and prognostic information for several cardiac diseases as well as allowing therapy monitoring. This feasibility study lays the foundation for further investigations to assess these potential roles. Clinical trial identifier: NCT03265431.

The Soluble Adenylyl Cyclase Inhibitor LRE1 Prevents Hepatic Ischemia/Reperfusion Damage Through Improvement of Mitochondrial Function

Hepatic ischemia/reperfusion (I/R) injury is a leading cause of organ dysfunction and failure in numerous pathological and surgical settings. At the core of this issue lies mitochondrial dysfunction. Hence, strategies that prime mitochondria towards damage resilience might prove applicable in a clinical setting. A promising approach has been to induce a mitohormetic response, removing less capable organelles, and replacing them with more competent ones, in preparation for an insult. Recently, a soluble form of adenylyl cyclase (sAC) has been shown to exist within mitochondria, the activation of which improved mitochondrial function. Here, we sought to understand if inhibiting mitochondrial sAC would elicit mitohormesis and protect the liver from I/R injury. Wistar male rats were pretreated with LRE1, a specific sAC inhibitor, prior to the induction of hepatic I/R injury, after which mitochondria were collected and their metabolic function was assessed. We find LRE1 to be an effective inducer of a mitohormetic response based on all parameters tested, a phenomenon that appears to require the activity of the NAD⁺-dependent sirtuin deacylase (SirT3) and the subsequent deacetylation of mitochondrial proteins. We conclude that LRE1 pretreatment leads to a mitohormetic response that protects mitochondrial function during I/R injury.

Mitochondrial impairment and cytotoxicity effects induced by the marine epibenthic dinoflagellate Coolia malayensis

Mitochondria was used to clarify the effects of Coolia malayensis strain UNR-02 crude extract by studying mitochondrial membrane potential (ΔΨ_m) generation and the fluctuations of ΔΨ_m associated with the induction of mitochondrial permeability transition (MPT). The cytoxicity of C. malayensis was also determined using both HepG2 and H9c2(2-1) cells. C. malayensis extract significantly depressed the oxidative phosphorylation efficiency, as was inferred from the perturbations in ΔΨ_m and in the phosphorylative cycle induced by ADP. Increased susceptibility to Ca²⁺-induced MPT was also observed. At the cellular level, the extract significantly decreased cell mass of both cell lines.

Effect of betulin and betulonic acid on isolated rat liver mitochondria and liposomes

The paper considers the effects of plant triterpenoid betulin and its derivative betulonic acid on rat liver mitochondria and liposomes. It was found that betulonic acid and, to a lesser extent, betulin, activate mitochondrial respiration in states 2 and 4 and inhibit ADP- and DNP-stimulated (uncoupled) respiration. The effect of betulonic acid resulted in a significant decrease of the respiratory control and ADP/O ratios and decrease in Δψ. The effects of both compounds were most pronounced in the case of succinate-fueled mitochondrial respiration. This may include both the possible protonophore effect of betulonic acid and the inhibition of respiratory chain complexes by both compounds. Both agents enhanced H₂O₂ production in succinate-fueled mitochondria, while betulonic acid exerted an antioxidant effect with NAD-dependent substrates. Betulin was found to induce mitochondrial aggregation, but had no effect on membrane permeability. A similar pattern was found on liposomes. As revealed by the laurdan generalized polarization (GP) technique, betulin increased laurdan GP in lecithin liposomes, indicating a decrease in membrane fluidity. Measurements of GP as a function of fluorescence excitation wavelength gave an ascending line for high concentrations of betulin, which can be interpreted as phase heterogeneity of the lipid/betulin system. High concentrations of betulin (> 60 mol%) was also demonstrated to cause permeabilization of lecithin liposomes. Betulonic acid was much less effective in inducing the aggregation of mitochondria and liposomes and had no effect on membrane permeability. The possible mechanisms of betulin and betulonic acid effect on rat liver mitochondria and liposomes are discussed.

In-vivo Imaging of Mitochondrial Depolarization of Myocardium With Positron Emission Tomography and a Proton Gradient Uncoupler

BACKGROUND

We recently reported a method using positron emission tomography (PET) and the tracer 18F-labeled tetraphenylphosphonium (18F-TPP+) for mapping the tissue (i.e., cellular plus mitochondrial) membrane potential (ΔΨT) in the myocardium. The purpose of this work is to provide additional experimental evidence that our methods can be used to observe transient changes in the volume of distribution for 18F-TPP+ and mitochondrial membrane potential (ΔΨm).

METHODS

We tested these hypotheses by measuring decreases of 18F-TPP+ concentration elicited when a proton gradient uncoupler, BAM15, is administered by intracoronary infusion during PET scanning. BAM15 is the first proton gradient uncoupler shown to affect the mitochondrial membrane without affecting the cellular membrane potential. Preliminary dose response experiments were conducted in two pigs to determine the concentration of BAM15 infusate necessary to perturb the 18F-TPP+ concentration. More definitive experiments were performed in two additional pigs, in which we administered an intravenous bolus plus infusion of 18F-TPP+ to reach secular equilibrium followed by an intracoronary infusion of BAM15.

RESULTS

Intracoronary BAM15 infusion led to a clear decrease in 18F-TPP+ concentration, falling to a lower level, and then recovering. A second BAM15 infusion reduced the 18F-TPP+ level in a similar fashion. We observed a maximum depolarization of 10 mV as a result of the BAM15 infusion.

SUMMARY

This work provides evidence that the total membrane potential measured with 18F-TPP+ PET is sensitive to temporal changes in mitochondrial membrane potential.

A Comparative Study of the Action of Protonophore Uncouplers and Decoupling Agents as Inducers of Free Respiration in Mitochondria in States 3 and 4: Theoretical and Experimental Approaches

Theoretical and experimental studies have revealed that that in the liver mitochondria an increase in the rate of free respiration in state 3 induced by protonophore uncouplers 2,4-dinitrophenol and сarbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone is equal to or slightly greater than the increase in respiration rate in state 4 induced by these uncouplers. In contrast to these protonophore uncouplers, the decoupler α,ω-tetradecanedioic acid, increasing the rate of respiration in state 4, does not significantly affect the rate of free respiration in state 3. We have proposed quantitative indicators that allow determining the constituent part of the rate of respiration in state 4, associated with the decoupling effect of the uncoupler. Using the example of palmitic acid, we have found out the fundamental possibility of the simultaneous functioning of uncouplers by two mechanisms: as protonophores and as decouplers. The data obtained contradict the delocalized version of Mitchell's chemiosmotic theory, but are in complete agreement with its local version. It can be assumed that the F₀F₁-ATP synthase and nearby respiratory chain complexes form a local zone of coupled respiration and oxidative ATP synthesis (zones of oxidative phosphorylation). The uncoupler-induced stimulation of mitochondrial free respiration of mitochondria in state 3 is mainly due to the return of protons to the matrix in local zones, where the generation of a proton motive force (Δр) by respiratory chain complexes is associated with various transport processes, but not with ATP synthesis (zones of protonophore uncoupling). In contrast, respiratory stimulation in state 4 by decouplers is realized in local zones of oxidative phosphorylation by switching the respiratory chain complexes to the idle mode of operation in the absence of ATP synthesis.

Mitochondrial damage & lipid signaling in traumatic brain injury

Mitochondria are essential for neuronal function because they serve not only to sustain energy and redox homeostasis but also are harbingers of death. A dysregulated mitochondrial network can cascade until function is irreparably lost, dooming cells. TBI is most prevalent in the young and comes at significant personal and societal costs. Traumatic brain injury (TBI) triggers a biphasic and mechanistically heterogenous response and this mechanistic heterogeneity has made the development of standardized treatments challenging. The secondary phase of TBI injury evolves over hours and days after the initial insult, providing a window of opportunity for intervention. However, no FDA approved treatment for neuroprotection after TBI currently exists. With recent advances in detection techniques, there has been increasing recognition of the significance and roles of mitochondrial redox lipid signaling in both acute and chronic central nervous system (CNS) pathologies. Oxidized lipids and their downstream products result from and contribute to TBI pathogenesis. Therapies targeting the mitochondrial lipid composition and redox state show promise in experimental TBI and warrant further exploration. In this review, we provide 1) an overview for mitochondrial redox homeostasis with emphasis on glutathione metabolism, 2) the key mechanisms of TBI mitochondrial injury, 3) the pathways of mitochondria specific phospholipid cardiolipin oxidation, and 4) review the mechanisms of mitochondria quality control in TBI with consideration of the roles lipids play in this process.

The Evaluation of Mitochondrial Membrane Potential Using Fluorescent Dyes or a Membrane-Permeable Cation (TPP+) Electrode in Isolated Mitochondria and Intact Cells

The proton electrochemical gradient generated by respiratory chain activity accounts for over 90% of all available ATP and, as such, its evaluation and accurate measurements regarding its total values and fluctuations is an invaluable component in the understanding of mitochondrial functions. Consequently, alterations in electric potential across the inner mitochondrial membrane generated by differential protonic accumulations and transport are known as the mitochondrial membrane potential, or Δψ, and are reflective of the functional metabolic status of mitochondria. There are several experimental approaches to measure Δψ, ranging from fluorometric evaluations to electrochemical probes. Here we discuss the advantages and disadvantages of several of these methods, ranging from one that is dependent on the movement of a particular ion (tetraphenylphosphonium (TPP⁺) with a selective electrode) to the selection of a fluorescent dye from various types to achieve the same goal. The evaluation of the accumulation and movements of TPP⁺ across the inner mitochondrial membrane, or the fluorescence of accumulated dye particles, is a sensitive and accurate method of evaluating the Δψ in respiring mitochondria (either isolated or still inside the cell).

Aged garlic extract and its constituent, S-allyl-L-cysteine, induce the apoptosis of neuroblastoma cancer cells due to mitochondrial membrane depolarization

Aged garlic extract (AGE) has been demonstrated to have therapeutic properties in tumors; however its mechanisms of action have not yet been fully elucidated. A previous study revealed that AGE exerts an anti-proliferative effect on a panel of both sensitive [wild-type (WT)] and multidrug-resistant (MDR) human cancer cells. Following treatment of the cells with AGE, cytofluorimetric analysis revealed the occurrence of dose-dependent mitochondrial membrane depolarization (MMD). In this study, in order to further clarify the mechanisms of action of AGE, the effects of AGE on mitochondria isolated from rat liver mitochondria (RLM) were also examined. AGE induced an effect on the components of the electrochemical gradient (Δµ_H⁺), mitochondrial membrane potential (ΔΨ_m) and mitochondrial electrochemical gradient (ΔpH_m). The mitochondrial membrane dysfunctions of RLM induced by AGE, namely the decrease in both membrane potential and chemical gradient were associated with a higher oxidation of both the endogenous glutathione and pyridine nucleotide content. To confirm the anti-proliferative effects of AGE, experiments were performed on the human neuroblastoma (NB) cancer cells, SJ-N-KP and the MYCN-amplified IMR5 cells, using its derivative S-allyl-L-cysteine (SAC), with the aim of providing evidence of the anticancer activity of this compound and its possible molecular mechanism as regards the induction of cytotoxicity. Following treatment of the cells with SAC at 20 mM, cell viability was determined by MTT assay and apoptosis was detected by flow cytometry, using Annexin V-FITC labeling. The percentages of cells undergoing apoptosis was found to be 48.0% in the SJ-N-KP and 50.1% in the IMR5 cells. By cytofluorimetric analysis, it was suggested that the target of SAC are the mitochondria. Mitochondrial activity was examined by labeling the cells with the probe, 5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethylimidacarbocyanine iodide (JC-1). Following treatment with SAC at 50 mM, both NB cell lines exhibited a marked increase in MMD. On the whole, the findings of this study indicate that both natural products, AGE and SAC, cause cytotoxicity to tumor cells via the induction of mitochondrial permeability transition (MPT).

Mutant huntingtin fails to directly impair brain mitochondria

Although the mechanisms by which mutant huntingtin (mHtt) results in Huntington's disease (HD) remain unclear, mHtt-induced mitochondrial defects were implicated in HD pathogenesis. The effect of mHtt could be mediated by transcriptional alterations, by direct interaction with mitochondria, or by both. In the present study, we tested a hypothesis that mHtt directly damages mitochondria. To test this hypothesis, we applied brain cytosolic fraction from YAC128 mice, containing mHtt, to brain non-synaptic and synaptic mitochondria from wild-type mice and assessed mitochondrial respiration with a Clark-type oxygen electrode, membrane potential and Ca2+ uptake capacity with tetraphenylphosphonium (TPP+ )- and Ca2+ -sensitive electrodes, respectively, and, reactive oxygen species production with Amplex Red assay. The amount of mHtt bound to mitochondria following incubation with mHtt-containing cytosolic fraction was greater than the amount of mHtt bound to brain mitochondria isolated from YAC128 mice. Despite mHtt binding to wild-type mitochondria, no abnormalities in mitochondrial functions were detected. This is consistent with our previous results demonstrating the lack of defects in brain mitochondria isolated from R6/2 and YAC128 mice. This, however, could be because of partial loss of mitochondrially bound mHtt during the isolation procedure. Consequently, we increased the amount of mitochondrially bound mHtt by incubating brain non-synaptic and synaptic mitochondria isolated from YAC128 mice with mHtt-containing cytosolic fraction. Despite the enrichment of YAC128 brain mitochondria with mHtt, mitochondrial functions (respiration, membrane potential, reactive oxygen species production, Ca2+ uptake capacity) remained unchanged. Overall, our results suggest that mHtt does not directly impair mitochondrial functions, arguing against the involvement of this mechanism in HD pathogenesis. Open Science: This manuscript was awarded with the Open Materials Badge For more information see: https://cos.io/our-services/open-science-badges/.

The Mitochondrial Targets of Neuroprotective Drug Vinpocetine on Primary Neuron Cultures, Brain Capillary Endothelial Cells, Synaptosomes, and Brain Mitochondria

Vinpocetine is considered as neuroprotectant drug and used for treatment of brain ischemia and cognitive deficiencies for decades. A number of enzymes, channels and receptors can bind vinpocetine, however the mechanisms of many effects' are still not clear. The present study investigated the effects of vinpocetine from the mitochondrial bioenergetic aspects. In primary brain capillary endothelial cells the purinergic receptor-stimulated mitochondrial Ca²⁺ uptake and efflux were studied. Vinpocetine exerted a partial inhibition on the mitochondrial calcium efflux. In rodent brain synaptosomes vinpocetine (30 μM) inhibited respiration in uncoupler stimulated synaptosomes and decreased H₂O₂ release from the nerve terminals in resting and in complex I inhibited conditions, respectively. In isolated rat brain mitochondria using either complex I or complex II substrates leak respiration was stimulated, but ADP-induced respiration was inhibited by vinpocetine. The stimulation of oxidation was associated with a small extent of membrane depolarization. Mitochondrial H₂O₂ production was inhibited by vinpocetine under all conditions investigated. The most pronounced effects were detected with the complex II substrate succinate. Vinpocetine also mitigated both Ca²⁺-induced mitochondrial Ca²⁺-release and Ca²⁺-induced mitochondrial swelling. It lowered the rate of mitochondrial ATP synthesis, while increasing ATPase activity. These results indicate more than a single mitochondrial target of this vinca alkaloid. The relevance of the affected mitochondrial mechanisms in the anti ischemic effect of vinpocetine is discussed.