Quantitative measurement of mitochondrial membrane potential in cultured cells: calcium-induced de- and hyperpolarization of neuronal mitochondria

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.

Collapsin response mediator protein 4 enhances the radiosensitivity of colon cancer cells through calcium‑mediated cell signaling

Radiation therapy is an effective treatment against various types of cancer, but some radiation‑resistant cancer cells remain a major therapeutic obstacle; thus, understanding radiation resistance mechanisms is essential for cancer treatment. In this study, we established radiation‑resistant colon cancer cell lines and examined the radiation‑induced genetic changes associated with radiation resistance. Using RNA‑sequencing analysis, collapsin response mediator protein 4 (<em>CRMP4</em>) was identified as the candidate gene associated with radiation sensitivity. When cells were exposed to radiation, intracellular Ca²⁺ influx, collapse of mitochondrial membrane potential, and cytochrome c release into the cytosol were increased, followed by apoptosis induction. Radiation treatment‑ or Ca²⁺ ionophore A23187‑induced apoptosis was significantly inhibited in <em>CRMP4</em>‑deficient cells, including radiation‑resistant or <em>CRMP4</em>‑shRNA cell lines. Furthermore, treatment of <em>CRMP4</em>‑deficient cells with low levels (&lt;5 µM) of BAPTA‑AM, a Ca²⁺ chelator, resulted in radiation resistance. Conversely, Ca²⁺ deficiency induced by a high BAPTA‑AM concentration (&gt;10 µM) resulted in higher cell death in the <em>CRMP4</em>‑depleted cells compared to <em>CRMP4</em>‑expressing control cells. Our results suggest that <em>CRMP4</em> plays an important role in Ca²⁺‑mediated cell death pathways under radiation exposure and that CRMP4 may be a therapeutical target for colon cancer treatment.

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.

Uncoupling proteins in the mitochondrial defense against oxidative stress

Oxidative stress is a major component of most major retinal diseases. Many extrinsic anti-oxidative strategies have been insufficient at counteracting one of the predominant intrinsic sources of reactive oxygen species (ROS), mitochondria. The proton gradient across the inner mitochondrial membrane is a key driving force for mitochondrial ROS production, and this gradient can be modulated by members of the mitochondrial uncoupling protein (UCP) family. Of the UCPs, UCP2 shows a widespread distribution and has been shown to uncouple oxidative phosphorylation, with concomitant decreases in ROS production. Genetic studies using transgenic and knockout mice have documented the ability of increased UCP2 activity to provide neuroprotection in models of a number of diseases, including retinal diseases, indicating that it is a strong candidate for a therapeutic target. Molecular studies have identified the structural mechanism of action of UCP2 and have detailed the ways in which its expression and activity can be controlled at the transcriptional, translational and posttranslational levels. These studies suggest a number of ways in control of UCP2 expression and activity can be used therapeutically for both acute and chronic conditions. The development of such therapeutic approaches will greatly increase the tools available to combat a broad range of serious retinal diseases.

Regulating Mitochondrial pH with Light and Implications for Chemoresistance

Chemoresistance is one of the major challenges for cancer treatment, more recently ascribed to defective mitochondrial outer membrane permeabilization (MOMP), significantly diminishing chemotherapeutic agent-induced apoptosis. A boron-dipyrromethene (BODIPY) chromophore-based triarylsulfonium photoacid generator (BD-PAG) was used to target mitochondria with the aim to regulate mitochondrial pH and further depolarize the mitochondrial membrane. Cell viability assays demonstrated the relative biocompatibility of BD-PAG in the dark while live cell imaging suggested high accumulation in mitochondria. Specific assays indicated that BD-PAG is capable of regulating mitochondrial pH with significant effects on mitochondrial membrane depolarization. Therapeutic tests using chlorambucil in combination with BD-PAG revealed a new strategy in chemoresistance suppression.

Quantitative analysis of mitochondrial membrane potential heterogeneity in unsynchronized and synchronized cancer cells

Mitochondrial membrane potential (ΔΨm) is a global indicator of mitochondrial function. Previous reports on heterogeneity of ΔΨm were qualitative or semiquantitative. Here, we quantified intercellular differences in ΔΨm in unsynchronized human cancer cells, cells synchronized in G1, S, and G2, and human fibroblasts. We assessed ΔΨm using a two-pronged microscopy approach to measure relative fluorescence of tetramethylrhodamine methyl ester (TMRM) and absolute values of ΔΨm. We showed that ΔΨm is more heterogeneous in cancer cells compared to fibroblasts, and it is maintained throughout the cell cycle. The effect of chemical inhibition of the respiratory chain and ATP synthesis differed between basal, low and high ΔΨm cells. Overall, our results showed that intercellular heterogeneity of ΔΨm is mainly modulated by intramitochondrial factors, it is independent of the ΔΨm indicator and it is not correlated with intercellular heterogeneity of plasma membrane potential or the phases of the cell cycle.

A bioluminescent probe for longitudinal monitoring of mitochondrial membrane potential

Mitochondrial membrane potential (ΔΨm) is a universal selective indicator of mitochondrial function and is known to play a central role in many human pathologies, such as diabetes mellitus, cancer and Alzheimer's and Parkinson's diseases. Here, we report the design, synthesis and several applications of mitochondria-activatable luciferin (MAL), a bioluminescent probe sensitive to ΔΨm, and partially to plasma membrane potential (ΔΨp), for non-invasive, longitudinal monitoring of ΔΨm in vitro and in vivo. We applied this new technology to evaluate the aging-related change of ΔΨm in mice and showed that nicotinamide riboside (NR) reverts aging-related mitochondrial depolarization, revealing another important aspect of the mechanism of action of this potent biomolecule. In addition, we demonstrated application of the MAL probe for studies of brown adipose tissue (BAT) activation and non-invasive in vivo assessment of ΔΨm in animal cancer models, opening exciting opportunities for understanding the underlying mechanisms and for discovery of effective treatments for many human pathologies.

Total Cellular ATP Production Changes With Primary Substrate in MCF7 Breast Cancer Cells

Cancer growth is predicted to require substantial rates of substrate catabolism and ATP turnover to drive unrestricted biosynthesis and cell growth. While substrate limitation can dramatically alter cell behavior, the effects of substrate limitation on total cellular ATP production rate is poorly understood. Here, we show that MCF7 breast cancer cells, given different combinations of the common cell culture substrates glucose, glutamine, and pyruvate, display ATP production rates 1.6-fold higher than when cells are limited to each individual substrate. This increase occurred mainly through faster oxidative ATP production, with little to no increase in glycolytic ATP production. In comparison, non-transformed C2C12 myoblast cells show no change in ATP production rate when substrates are limited. In MCF7 cells, glutamine allows unexpected access to oxidative capacity that pyruvate, also a strictly oxidized substrate, does not. Pyruvate, when added with other exogenous substrates, increases substrate-driven oxidative ATP production, by increasing both ATP supply and demand. Overall, we find that MCF7 cells are highly flexible with respect to maintaining total cellular ATP production under different substrate-limited conditions, over an acute (within minutes) timeframe that is unlikely to result from more protracted (hours or more) transcription-driven changes to metabolic enzyme expression. The near-identical ATP production rates maintained by MCF7 and C2C12 cells given single substrates reveal a potential difficulty in using substrate limitation to selectively starve cancer cells of ATP. In contrast, the higher ATP production rate conferred by mixed substrates in MCF7 cells remains a potentially exploitable difference.

Forces, fluxes, and fuels: tracking mitochondrial metabolism by integrating measurements of membrane potential, respiration, and metabolites

Assessing mitochondrial function in cell-based systems is a central component of metabolism research. However, the selection of an initial measurement technique may be complicated given the range of parameters that can be studied and the need to define the mitochondrial (dys)function of interest. This methods-focused review compares and contrasts the use of mitochondrial membrane potential measurements, plate-based respirometry, and metabolomics and stable isotope tracing. We demonstrate how measurements of 1) cellular substrate preference, 2) respiratory chain activity, 3) cell activation, and 4) mitochondrial biogenesis are enriched by integrating information from multiple methods. This manuscript is meant to serve as a perspective to help choose which technique might be an appropriate initial method to answer a given question, as well as provide a broad "roadmap" for designing follow-up assays to enrich datasets or resolve ambiguous results.

Monitoring and modeling of lymphocytic leukemia cell bioenergetics reveals decreased ATP synthesis during cell division

The energetic demands of a cell are believed to increase during mitosis, but the rates of ATP synthesis and consumption during mitosis have not been quantified. Here, we monitor mitochondrial membrane potential of single lymphocytic leukemia cells and demonstrate that mitochondria hyperpolarize from the G2/M transition until the metaphase-anaphase transition. This hyperpolarization was dependent on cyclin-dependent kinase 1 (CDK1) activity. By using an electrical circuit model of mitochondria, we quantify mitochondrial ATP synthesis rates in mitosis from the single-cell time-dynamics of mitochondrial membrane potential. We find that mitochondrial ATP synthesis decreases by approximately 50% during early mitosis and increases back to G2 levels during cytokinesis. Consistently, ATP levels and ATP synthesis are lower in mitosis than in G2 in synchronized cell populations. Overall, our results provide insights into mitotic bioenergetics and suggest that cell division is not a highly energy demanding process.

ATP drives most cellular processes, although ATP production and consumption levels during mitosis remain unreported. Here, the authors combine metabolic measurements and modeling to quantify ATP levels and synthesis dynamics, revealing that ATP synthesis and consumption are lowered during mitosis.

Charged pore-lining residues are required for normal channel kinetics in the eukaryotic mechanosensitive ion channel MSL1

Mechanosensitive (MS) ion channels are widespread mechanisms for cellular mechanosensation that can be directly activated by increasing membrane tension. The well-studied MscS family of MS ion channels is found in bacteria, archaea, and plants. MscS-Like (MSL)1 is localized to the inner mitochondrial membrane of Arabidopsis thaliana, where it is required for normal mitochondrial responses to oxidative stress. Like Escherichia coli MscS, MSL1 has a pore-lining helix that is kinked. However, in MSL1 this kink is comprised of two charged pore-lining residues, R326 and D327. Using single-channel patch-clamp electrophysiology in E. coli, we show that altering the size and charge of R326 and D327 leads to dramatic changes in channel kinetics. Modest changes in gating pressure were also observed while no effects on channel rectification or conductance were detected. MSL1 channel variants had differing physiological function in E. coli hypoosmotic shock assays, without clear correlation between function and particular channel characteristics. Taken together, these results demonstrate that altering pore-lining residue charge and size disrupts normal channel state stability and gating transitions, and led us to propose the “sweet spot” model. In this model, the transition to the closed state is facilitated by attraction between R326 and D327 and repulsion between R326 residues of neighboring monomers. In the open state, expansion of the channel reduces inter-monomeric repulsion, rendering open state stability influenced mainly by attractive forces. This work provides insight into how unique charge-charge interactions can be combined with an otherwise conserved structural feature to help modulate MS channel function.

Mitochondria Are Fundamental for the Emergence of Metazoans: On Metabolism, Genomic Regulation, and the Birth of Complex Organisms

Out of many intracellular bacteria, only the mitochondria and chloroplasts abandoned their independence billions of years ago and became endosymbionts within the host eukaryotic cell. Consequently, one cannot grow eukaryotic cells without their mitochondria, and the mitochondria cannot divide outside of the cell, thus reflecting interdependence. Here, we argue that such interdependence underlies the fundamental role of mitochondrial activities in the emergence of metazoans. Several lines of evidence support our hypothesis: (a) Differentiation and embryogenesis rely on mitochondrial function; (b) mitochondrial metabolites are primary precursors for epigenetic modifications (such as methyl and acetyl), which are critical for chromatin remodeling and gene expression, particularly during differentiation and embryogenesis; and (c) mitonuclear coregulation adapted to accommodate both housekeeping and tissue-dependent metabolic needs. We discuss the evolution of the unique mitochondrial genetic system, mitochondrial metabolites, mitonuclear coregulation, and their critical roles in the emergence of metazoans and in human disorders.

Selective Delivery of Dicarboxylates to Mitochondria by Conjugation to a Lipophilic Cation via a Cleavable Linker

Many mitochondrial metabolites and bioactive molecules contain two carboxylic acid moieties that make them unable to cross biological membranes. Hence, there is considerable interest in facilitating the uptake of these molecules into cells and mitochondria to modify or report on their function. Conjugation to the triphenylphosphonium (TPP) lipophilic cation is widely used to deliver molecules selectively to mitochondria in response to the membrane potential. However, permanent attachment to the cation can disrupt the biological function of small dicarboxylates. Here, we have developed a strategy using TPP to release dicarboxylates selectively within mitochondria. For this, the dicarboxylate is attached to a TPP compound via a single ester bond, which is then cleaved by intramitochondrial esterase activity, releasing the dicarboxylate within the organelle. Leaving the second carboxylic acid free also means mitochondrial uptake is dependent on the pH gradient across the inner membrane. To assess this strategy, we synthesized a range of TPP monoesters of the model dicarboxylate, malonate. We then tested their mitochondrial accumulation and ability to deliver malonate to isolated mitochondria and to cells, in vitro and in vivo. A TPP–malonate monoester compound, TPP₁₁–malonate, in which the dicarboxylate group was attached to the TPP compound via a hydrophobic undecyl link, was most effective at releasing malonate within mitochondria in cells and in vivo. Therefore, we have developed a TPP–monoester platform that enables the selective release of bioactive dicarboxylates within mitochondria.

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.

On the physiological and cellular homeostasis of ascorbate

Recent interest in the role of ascorbate in crucial metabolic processes is driven by the growing number of medical reports that show beneficial effects of ascorbate supplementation for maintaining general well-being and recovery from a variety of medical conditions. The effect of ascorbate on the local body environment highly depends on its local concentration; at low concentrations it can cause the reduction of reactive oxygen and facilitate activities of enzymes, while at high concentrations it generates free radicals by reducing ferric ions. Ascorbate serving as an electron donor assists the iron-containing proteins and the iron transfer between various aqueous compartments. These functions require effective and adjustable mechanisms responsible for ascorbate biodistribution. In the paper we propose a new biophysical model of ascorbate redistribution between various aqueous body compartments. It combines recent experimental evidence regarding the ability of ascorbate to cross the lipid bilayer by unassisted diffusion, with active transport by well-characterized sodium vitamin C transporter (SVCT) membrane proteins. In the model, the intracellular concentration of ascorbate is maintained by the balance of two opposing fluxes: fast active and slow passive transport. The model provides a mechanistic understanding of ascorbate flux across the epidermal barrier in the gut as well as the role of astrocytes in ascorbate recycling in the brain. In addition, ascorbate passive diffusion across biological membranes, which depends on membrane electric potentials and pH gradients, provides the rationale for the correlation between ascorbate distribution and the transfer of iron ions inside a cell. The proposed approach provides, for the first time, a mechanistic account of processes leading to ascorbate physiological and cellular distribution, which helps to explain numerous experimental and clinical observations.

Natural products and phytochemical nanoformulations targeting mitochondria in oncotherapy: an updated review on resveratrol

Mitochondria are intracellular organelles with two distinct membranes, known as an outer mitochondrial membrane and inner cell membrane. Originally, mitochondria have been derived from bacteria. The main function of mitochondria is the production of ATP. However, this important organelle indirectly protects cells by consuming oxygen in the route of energy generation. It has been found that mitochondria are actively involved in the induction of the intrinsic pathways of apoptosis. So, there have been efforts to sustain mitochondrial homeostasis and inhibit its dysfunction. Notably, due to the potential role of mitochondria in the stimulation of apoptosis, this organelle is a promising target in cancer therapy. Resveratrol is a non-flavonoid polyphenol that exhibits significant pharmacological effects such as antioxidant, anti-diabetic, anti-inflammatory and anti-tumor. The anti-tumor activity of resveratrol may be a consequence of its effect on mitochondria. Multiple studies have investigated the relationship between resveratrol and mitochondria, and it has been demonstrated that resveratrol is able to significantly enhance the concentration of reactive oxygen species, leading to the mitochondrial dysfunction and consequently, apoptosis induction. A number of signaling pathways such as sirtuin and NF-κB may contribute to the mitochondrial-mediated apoptosis by resveratrol. Besides, resveratrol shifts cellular metabolism from glycolysis into mitochondrial respiration to induce cellular death in cancer cells. In the present review, we discuss the possible interactions between resveratrol and mitochondria, and its potential application in cancer therapy.

Chronic imaging of mitochondria in the murine cerebral vasculature using in vivo two-photon microscopy

Mitochondria are important regulators of cerebral vascular function in health and disease, but progress in understanding their roles has been hindered by methodological limitations. We report the first in vivo imaging of mitochondria specific to the cerebral endothelium in real time in the same mouse for extended periods. Mice expressing Dendra2 fluorescent protein in mitochondria (mito-Dendra2) in the cerebral vascular endothelium were generated by breeding PhAM-floxed and Tie2-Cre mice. We used mito-Dendra2 expression, cranial window implantation, and two-photon microscopy to visualize mitochondria in the cerebral vascular endothelium of mice. Immunohistochemistry and mitochondrial staining were used to confirm the localization of the mitochondrial signal to endothelial cells and the specificity of mito-Dendra2 to mitochondria. Mito-Dendra2 and Rhodamine B-conjugated dextran allowed simultaneous determinations of mitochondrial density, vessel diameters, area, and mitochondria-to-vessel ratio in vivo, repeatedly, in the same mouse. Endothelial expression of mito-Dendra2 was confirmed in vitro on brain slices and aorta. In addition, we observed an overlapping mito-Dendra2 and Chromeo mitochondrial staining of cultured brain microvascular endothelial cells. Repeated imaging of the same location in the cerebral microcirculation in the same mouse demonstrated stability of mito-Dendra2. While the overall mitochondrial signal was stable over time, mitochondria within the same endothelial cell were mobile. In conclusion, our results indicate that the mito-Dendra2 signal and vascular parameters are suitable for real-time and longitudinal examination of mitochondria in vivo in the cerebral vasculature of mice.NEW & NOTEWORTHY We introduce an innovative in vivo approach to study mitochondria in the cerebral circulation in their physiological environment by demonstrating the feasibility of long-term imaging and three-dimensional reconstruction. We postulate that the appropriate combination of Cre/Lox system and two-photon microscopy will contribute to a better understanding of the role of mitochondria in not only endothelium but also the different cell types of the cerebral circulation.

Assessment of Enrichment of Human Mesenchymal Stem Cells Based on Plasma and Mitochondrial Membrane Potentials

Background: Human mesenchymal stem cells (hMSCs) are utilized preclinically and clinically as a candidate cell therapy for a wide range of inflammatory and degenerative diseases. Despite promising results in early clinical trials, consistent outcomes with hMSC-based therapies have proven elusive in many of these applications. In this work, we attempt to address this limitation through the design of a stem cell therapy to enrich hMSCs for desired electrical and ionic properties with enhanced stemness and immunomodulatory/regenerative capacity. Materials and Methods: In this study, we sought to develop initial protocols to achieve electrically enriched hMSCs (EE-hMSCs) with distinct electrical states and assess the potential relationship with respect to hMSC state and function. We sorted hMSCs based on fluorescence intensity of tetramethylrhodamine ethyl ester (TMRE) and investigated phenotypic differences between the sorted populations. Results: Subpopulations of EE-hMSCs exhibit differential expression of genes associated with senescence, stemness, immunomodulation, and autophagy. EE-hMSCs with low levels of TMRE, indicative of depolarized membrane potential, have reduced mRNA expression of senescence-associated markers, and increased mRNA expression of autophagy and immunomodulatory markers relative to EE-hMSCs with high levels of TMRE (hyperpolarized). Conclusions : This work suggests that the utilization of EE-hMSCs may provide a novel strategy for cell therapies, enabling live cell enrichment for distinct phenotypes that can be exploited for different therapeutic outcomes.

Interferon-Gamma Stimulated Murine Macrophages In Vitro: Impact of Ionic Composition and Osmolarity and Therapeutic Implications

Background: Injections of osmolytes are promising immunomodulatory treatments for medical benefit, although the rationale and underlying mechanisms are often lacking. The goals of the present study were twofold: (1) to clarify the anti-inflammatory role of the potassium ion and (2) to begin to decouple the effects that ionic strength, ionic species, and osmolarity have on macrophage biology. Materials and Methods: RAW 264.7 murine macrophages were encapsulated in three-dimensional, poly(ethylene glycol) diacrylate hydrogels and activated with interferon-gamma to yield M(IFN). Gene and protein profiles were made of M(IFN) exposed to different hyperosmolar treatments (80 mM potassium gluconate, 80 mM sodium gluconate, and 160 mM sucrose). Results: Relative to M(IFN), all hyperosmolar treatments suppressed expression of pro-inflammatory markers (nitric oxide synthase-2 [NOS-2], tumor necrosis factor-alpha, monocyte chemoattractant protein-1 [MCP-1]) and increased messenger RNA (mRNA) expression of the pleiotropic and angiogenic markers interleukin-6 (IL-6) and vascular endothelial growth factor-A (VEGF), respectively. Ionic osmolytes also demonstrated a greater level of change compared to the nonionic treatments, with mRNA levels of IL-6 the most significantly affected. M(IFN) exposed to K+ exhibited the lowest levels of NOS-2 and MCP-1, and this ion limited IL-6 release induced by osmolarity. Conclusion: Cumulatively, these data suggest that osmolyte composition, ionic strength, and osmolarity are all parameters that can influence therapeutic outcomes. Future work is necessary to further decouple and mechanistically understand the influence that these biophysical parameters have on cell biology, including their impact on other macrophage functions, intracellular osmolyte composition, and cellular and organellular membrane potentials.

Functional imaging of a model unicell: Spironucleus vortens as an anaerobic but aerotolerant flagellated protist

Advances in optical microscopy are continually narrowing the chasm in our appreciation of biological organization between the molecular and cellular levels, but many practical problems are still limiting. Observation is always limited by the rapid dynamics of ultrastructural modifications of intracellular components, and often by cell motility: imaging of the unicellular protist parasite of ornamental fish, Spironucleus vortens, has proved challenging. Autofluorescence of nicotinamide nucleotides and flavins in the 400-580 nm region of the visible spectrum, is the most useful indicator of cellular redox state and hence vitality. Fluorophores emitting in the red or near-infrared (i.e., phosphors) are less damaging and more penetrative than many routinely employed fluors. Mountants containing free radical scavengers minimize fluorophore photobleaching. Two-photon excitation provides a small focal spot, increased penetration, minimizes photon scattering and enables extended observations. Use of quantum dots clarifies the competition between endosomal uptake and exosomal extrusion. Rapid motility (161 μm/s) of the organism makes high resolution of ultrastructure difficult even at high scan speeds. Use of voltage-sensitive dyes determining transmembrane potentials of plasma membrane and hydrogenosomes (modified mitochondria) is also hindered by intracellular motion and controlled anesthesia perturbs membrane organization. Specificity of luminophore binding is always questionable; e.g. cationic lipophilic species widely used to measure membrane potentials also enter membrane-bounded neutral lipid droplet-filled organelles. This appears to be the case in S. vortens, where Coherent Anti-Stokes Raman Scattering (CARS) micro-spectroscopy unequivocally images the latter and simultaneous provides spectral identification at 2840 cm^-1. Secondary Harmonic Generation highlights the highly ordered structure of the flagella.

Intermittent Hypoxia Augments Pulmonary Vasoconstrictor Reactivity through PKCβ/Mitochondrial Oxidant Signaling

Pulmonary vasoconstriction resulting from intermittent hypoxia (IH) contributes to pulmonary hypertension (pHTN) in patients with sleep apnea (SA), although the mechanisms involved remain poorly understood. Based on prior studies in patients with SA and animal models of SA, the objective of this study was to evaluate the role of PKCβ and mitochondrial reactive oxygen species (mitoROS) in mediating enhanced pulmonary vasoconstrictor reactivity after IH. We hypothesized that PKCβ mediates vasoconstriction through interaction with the scaffolding protein PICK1 (protein interacting with C kinase 1), activation of mitochondrial ATP-sensitive potassium channels (mitoK_ATP), and stimulated production of mitoROS. We further hypothesized that this signaling axis mediates enhanced vasoconstriction and pHTN after IH. Rats were exposed to IH or sham conditions (7 h/d, 4 wk). Chronic oral administration of the antioxidant Tempol or the PKCβ inhibitor LY-333531 abolished IH-induced increases in right ventricular systolic pressure and right ventricular hypertrophy. Furthermore, scavengers of O₂^- or mitoROS prevented enhanced PKCβ-dependent vasoconstrictor reactivity to endothelin-1 in pulmonary arteries from IH rats. In addition, this PKCβ/mitoROS signaling pathway could be stimulated by the PKC activator PMA in pulmonary arteries from control rats, and in both rat and human pulmonary arterial smooth muscle cells. These responses to PMA were attenuated by inhibition of mitoK_ATP or PICK1. Subcellular fractionation and proximity ligation assays further demonstrated that PKCβ acutely translocates to mitochondria upon stimulation and associates with PICK1. We conclude that a PKCβ/mitoROS signaling axis contributes to enhanced vasoconstriction and pHTN after IH. Furthermore, PKCβ mediates pulmonary vasoconstriction through interaction with PICK1, activation of mitoK_ATP, and subsequent mitoROS generation.

L-type Ca2+ channel-mediated Ca2+ influx adjusts neuronal mitochondrial function to physiological and pathophysiological conditions

L-type voltage-gated Ca2+ channels (LTCCs) are implicated in neurodegenerative processes and cell death. Accordingly, LTCC antagonists have been proposed to be neuroprotective, although this view is disputed, because intentional LTCC activation can also have beneficial effects. LTCC-mediated Ca2+ influx influences mitochondrial function, which plays a crucial role in the regulation of cell viability. Hence, we investigated the effect of modulating LTCC-mediated Ca2+ influx on mitochondrial function in cultured hippocampal neurons. To activate LTCCs, neuronal activity was stimulated by increasing extracellular K+ or by application of the GABAA receptor antagonist bicuculline. The activity of LTCCs was altered by application of an agonistic (Bay K8644) or an antagonistic (isradipine) dihydropyridine. Our results demonstrated that activation of LTCC-mediated Ca2+ influx affected mitochondrial function in a bimodal manner. At moderate stimulation strength, ATP synthase activity was enhanced, an effect that involved Ca2+-induced Ca2+ release from intracellular stores. In contrast, high LTCC-mediated Ca2+ loads led to a switch in ATP synthase activity to reverse-mode operation. This effect, which required nitric oxide, helped to prevent mitochondrial depolarization and sustained increases in mitochondrial Ca2+ Our findings indicate a complex role of LTCC-mediated Ca2+ influx in the tuning and maintenance of mitochondrial function. Therefore, the use of LTCC inhibitors to protect neurons from neurodegeneration should be reconsidered carefully.

Defined extracellular ionic solutions to study and manipulate the cellular resting membrane potential

All cells possess an electric potential across their plasma membranes and can generate and receive bioelectric signals. The cellular resting membrane potential (RMP) can regulate cell proliferation, differentiation and apoptosis. Current approaches to measure the RMP rely on patch clamping, which is technically challenging, low-throughput and not widely available. It is therefore critical to develop simple strategies to measure, manipulate and characterize the RMP. Here, we present a simple methodology to study the RMP of non-excitable cells and characterize the contribution of individual ions to the RMP using a voltage-sensitive dye. We define protocols using extracellular solutions in which permeable ions (Na+, Cl- and K+) are substituted with non-permeable ions [N-Methyl-D-glucamine (NMDG), gluconate, choline, SO42-]. The resulting RMP modifications were assessed with both patch clamp and a voltage sensitive dye. Using an epithelial and cancer cell line, we demonstrate that the proposed ionic solutions can selectively modify the RMP and help determine the relative contribution of ionic species in setting the RMP. The proposed method is simple and reproducible and will make the study of bioelectricity more readily available to the cell biology community.This article has an associated First Person interview with the first author of the paper.

Optical changes in THP-1 macrophage metabolism in response to pro- and anti-inflammatory stimuli reported by label-free two-photon imaging

Temporal changes in macrophage metabolism are likely crucial to their role in inflammatory diseases. Label-free two-photon excited fluorescence (TPEF) and fluorescence lifetime imaging microscopy are well suited to track dynamic changes in macrophage metabolism. We performed TPEF imaging of human macrophages following either pro- or an anti-inflammatory stimulation. Two endogenous fluorophores, NAD(P)H and FAD, coenzymes involved in key metabolic pathways, provided contrast. We used the corresponding intensity images to determine the optical redox ratio of FAD to FAD + NAD(P)H. We also analyzed the intensity fluctuation patterns within NAD(P)H TPEF images to determine mitochondrial clustering patterns. Finally, we acquired NAD(P)H TPEF lifetime images to assess the relative levels of bound NAD(P)H. Our studies indicate that the redox ratio increases, whereas mitochondrial clustering decreases in response to both pro- and anti-inflammatory stimuli; however, these changes are enhanced in pro-inflammatory macrophages. Interestingly, we did not detect any significant changes in the corresponding NAD(P)H bound fraction. A combination of optical metabolic metrics could be used to classify pro- and anti-inflammatory macrophages with high accuracy. Contributions from alterations in different metabolic pathways may explain our findings, which highlight the potential of label-free two-photon imaging to assess nondestructively macrophage functional state.

Visualization of Sirtuin 4 Distribution between Mitochondria and the Nucleus, Based on Bimolecular Fluorescence Self-Complementation

Mitochondrial sirtuins (Sirts) control important cellular processes related to stress. Despite their regulatory importance, however, the dynamics and subcellular distributions of Sirts remain debatable. Here, we investigate the subcellular localization of sirtuin 4 (Sirt4), a sirtuin variant with a mitochondrial targeting sequence (MTS), by expressing Sirt4 fused to the superfolder green fluorescent protein (Sirt4-sfGFP) in HeLa and pancreatic β-cells. Super resolution fluorescence microscopy revealed the trapping of Sirt4-sfGFP to the outer mitochondrial membrane (OMM), possibly due to slow mitochondrial import kinetics. In many cells, Sirt4-sfGFP was also present within the cytosol and nucleus. Moreover, the expression of Sirt4-sfGFP induced mitochondrial swelling in HeLa cells. In order to bypass these effects, we applied the self-complementing split fluorescent protein (FP) technology and developed mito-STAR (mitochondrial sirtuin 4 tripartite abundance reporter), a tripartite probe for the visualization of Sirt4 distribution between mitochondria and the nucleus in single cells. The application of mito-STAR proved the importation of Sirt4 into the mitochondrial matrix and demonstrated its localization in the nucleus under mitochondrial stress conditions. Moreover, our findings highlight that the self-complementation of split FP is a powerful technique to study protein import efficiency in distinct cellular organelles.

Inhibition of Histone Deacetylase 6 Protects Hippocampal Cells Against Mitochondria-mediated Apoptosis in a Model of Severe Oxygen-glucose Deprivation

Background:

Histone deacetylase (HDAC) 6 inhibitors have demonstrated significant protective effects in traumatic injuries. However, their roles in neuroprotection and underlying mechanisms are poorly understood. This study sought to investigate the neuroprotective effects of Tubastatin A (Tub-A), an HDAC6 inhibitor, during oxygenglucose deprivation (OGD) in HT22 hippocampal cells.

Methods:

HT22 hippocampal cells were exposed to OGD. Cell viability and cytotoxicity were assessed by cell counting kit-8 (CCK-8) and lactate dehydrogenase (LDH) release assay. Cellular apoptosis was assessed by Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay. Mitochondria membrane potential was detected using JC-1 dye. Expressions of acetylated α-tubulin, α-tubulin, cytochrome c, VDAC, Bax, Bcl- 2, cleaved caspase 3, phosphorylated Akt, Akt, phosphorylated GSK3β and GSK3β were analyzed by Western blot analysis.

Results:

Tub-A induced acetylation of α-tubulin, demonstrating appropriate efficacy. Tub-A significantly increased cell viability and attenuated LDH release after exposure to OGD. Furthermore, Tub-A treatment blunted the increase in TUNEL-positive cells following OGD and preserved the mitochondrial membrane potential. Tub-A also attenuated the release of cytochrome c from the mitochondria into the cytoplasm and suppressed the ratio of Bax/Bcl-2 and cleaved caspase 3. This was mediated, in part, by the increased phosphorylation of Akt and GSK3β signaling pathways.

Conclusion:

HDAC 6 inhibition, using Tub-A, protects against OGD-induced injury in HT22 cells by modulating Akt/GSK3β signaling and inhibiting mitochondria-mediated apoptosis.

Effects of prolonged type 2 diabetes on mitochondrial function in cerebral blood vessels

One of the major characteristics of hyperglycemic states such as type 2 diabetes is increased reactive oxygen species (ROS) generation. Since mitochondria are a major source of ROS, it is vital to understand the involvement of these organelles in the pathogenesis of ROS-mediated conditions. Therefore, we investigated mitochondrial function and ROS production in cerebral blood vessels of 21-wk-old Zucker diabetic fatty obese rats and their lean controls. We have previously shown that in the early stages of insulin resistance, and short periods of type 2 diabetes mellitus, only mild differences exist in mitochondrial function. In the present study, we examined mitochondrial respiration, mitochondrial protein expression, and ROS production in large-surface cerebral arteries. We used 21-wk-old animals exposed to peak glucose levels for 7 wk and compared them with our previous studies on younger diabetic animals. We found that the same segments of mitochondrial respiration (basal respiration and proton leak) were diminished in diabetic groups as they were in younger diabetic animals. Levels of rattin, a rat humanin analog, tended to decrease in the diabetic group but did not reach statistical significance (P = 0.08). Other mitochondrial proteins were unaffected, which might indicate the existence of compensatory mechanisms with extension of this relatively mild form of diabetes. Superoxide levels were significantly higher in large cerebral vessels of diabetic animals compared with the control group. In conclusion, prolonged dietary diabetes leads to stabilization, rather than deterioration, of metabolic status in the cerebral circulation, despite continued overproduction of ROS.NEW & NOTEWORTHY We have characterized for the first time the dynamics of mitochondrial function during the progression of type 2 diabetes mellitus with regard to mitochondrial respiration, protein expression, and reactive oxygen species production. In addition, this is the first measurement of rattin levels in the cerebral vasculature, which could potentially lead to novel treatment options.

Studies of Jatrogossone A as a Reactive Oxygen Species Inducer in Cancer Cellular Models

Natural products continue to provide a platform to study biological systems. A bioguided study of cancer cell models led us to a new member of the jatrophane natural products from Jatropha gossypiifolia, which was independently identified and characterized as jatrogossone A (1). Purification and structure elucidation was performed by column chromatography and high-performance liquid chromatography-mass spectrometry and NMR techniques, and the structure was confirmed via X-ray crystallography. The unique molecular scaffold of jatrogossone A prompted an evaluation of its mode of action. Cytotoxicity assays demonstrated that jatrogossone A displays selective antiproliferative activity against cancer cell models in the low micromolar range with a therapeutic window. Jatrogossone A (1) affects mitochondrial membrane potential (ΔΨm) in a time- and dose-dependent manner. This natural product induces radical oxygen species (ROS) selectively in cancer cellular models, with minimal ROS induction in noncancerous cells. Compound 1 induces ROS in the mitochondria, as determined by colocalization studies, and it induces mitophagy. It promotes also in vitro cell death by causing cell arrest at the G2/M stage, caspase (3/7) activation, and PARP-1 cleavage. The combined findings provide a potential mechanism by which 1 relies on upregulation of mitochondrial ROS to potentiate cytotoxic effects through intracellular signaling.

Towards pharmacological treatment screening of cardiomyocyte cells using Si nanowire FETs

Silicon nanowires (Si NWs) are the most promising candidates for recording biological signals due to improved interfacing properties with cells and the possibility of high-speed transduction of biochemical signals into detectable electrical responses. The recording of extracellular action potentials (APs) from cardiac cells is important for fundamental studies of AP propagation features reflecting cell activity and the influence of pharmacological substances on the signal. We applied a novel approach of using fabricated Si NW field-effect transistors (FETs) in combination with fluorescent marker techniques to evaluate the functional activity of cardiac cells. Extracellular AP signal recording from HL-1 cardiomyocytes was demonstrated. This method was supplemented by studies of the pharmacological effects of stimulations using noradrenaline (NorA) as a modulator of functional activity on a cellular and subcellular levels, which were also tested using fluorescent marker techniques. The role of calcium alteration and membrane potential were revealed using Fluo-4 AM and tetramethylrhodamine, methyl ester, perchlorate (TMRM) fluorescent dyes. In addition, chemical treatment with sodium dodecyl sulfate (SDS) solutions was tested. The results obtained demonstrate positive prospects for AP monitoring in different treatments for studies related to a wide range of myocardial diseases using lab-on-chip technology.

From nutritional wealth to autophagy: In vivo metabolic dynamics in the cytosol, mitochondrion and shuttles of IgG producing CHO cells

To fulfil the optimization needs of current biopharmaceutical processes the knowledge how to improve cell specific productivities is of outmost importance. This requires a detailed understanding of cellular metabolism on a subcellular level inside compartments such as cytosol and mitochondrion. Using IgG1 producing Chinese hamster ovary (CHO) cells, a pioneering protocol for compartment-specific metabolome analysis was applied. Various production-like growth conditions ranging from ample glucose and amino acid supply via moderate to severe nitrogen limitation were investigated in batch cultures. The combined application of quantitative metabolite pool analysis, ¹³C tracer studies and non-stationary flux calculations revealed that Pyr/H⁺ symport (MPC1/2) bore the bulk of the mitochondrial transport under ample nutrient supply. Glutamine limitation induced the concerted adaptation of the bidirectional Mal/aKG (OGC) and the Mal/HPO₄^2- antiporter (DIC), even installing completely reversed shuttle fluxes. As a result, NADPH and ATP formation were adjusted to cellular needs unraveling the key role of cytosolic malic enzyme for NADPH production. Highest cell specific IgG1 productivities were closely correlated to a strong mitochondrial malate export according to the anabolic demands. The requirement to install proper NADPH supply for optimizing the production of monoclonal antibodies is clearly outlined. Interestingly, it was observed that mitochondrial citric acid cycle activity was always maintained enabling constant cytosolic adenylate energy charges at physiological levels, even under autophagy conditions.

Mitochondrial Heterogeneity

Cell-to-cell heterogeneity drives a range of (patho)physiologically important phenomena, such as cell fate and chemotherapeutic resistance. The role of metabolism, and particularly of mitochondria, is increasingly being recognized as an important explanatory factor in cell-to-cell heterogeneity. Most eukaryotic cells possess a population of mitochondria, in the sense that mitochondrial DNA (mtDNA) is held in multiple copies per cell, where the sequence of each molecule can vary. Hence, intra-cellular mitochondrial heterogeneity is possible, which can induce inter-cellular mitochondrial heterogeneity, and may drive aspects of cellular noise. In this review, we discuss sources of mitochondrial heterogeneity (variations between mitochondria in the same cell, and mitochondrial variations between supposedly identical cells) from both genetic and non-genetic perspectives, and mitochondrial genotype-phenotype links. We discuss the apparent homeostasis of mtDNA copy number, the observation of pervasive intra-cellular mtDNA mutation (which is termed "microheteroplasmy"), and developments in the understanding of inter-cellular mtDNA mutation ("macroheteroplasmy"). We point to the relationship between mitochondrial supercomplexes, cristal structure, pH, and cardiolipin as a potential amplifier of the mitochondrial genotype-phenotype link. We also discuss mitochondrial membrane potential and networks as sources of mitochondrial heterogeneity, and their influence upon the mitochondrial genome. Finally, we revisit the idea of mitochondrial complementation as a means of dampening mitochondrial genotype-phenotype links in light of recent experimental developments. The diverse sources of mitochondrial heterogeneity, as well as their increasingly recognized role in contributing to cellular heterogeneity, highlights the need for future single-cell mitochondrial measurements in the context of cellular noise studies.

(+)-Catechin inhibits heart mitochondrial complex I and nitric oxide synthase: functional consequences on membrane potential and hydrogen peroxide production

The aim was to study thein vitroeffect of nM to low μM concentration of (+)-catechin on the enzymatic activities of mitochondrial complex I and mtNOS, as well as the consequences on the membrane potential and H₂O₂production rate.

Mitochondria modulate programmed neuritic retraction

Neuritic retraction in the absence of overt neuronal death is a shared feature of normal aging and neurodegenerative disorders, but the intracellular mechanisms modulating this process are not understood. We propose that cumulative distal mitochondrial protein damage results in impaired protein import, leading to mitochondrial dysfunction and focal activation of the canonical apoptosis pathway in neurites. This is a controlled process that may not lead to neuronal death and, thus, we term this phenomenon "neuritosis." Consistent with our hypothesis, we show that in primary cerebrocortical neurons, mitochondrial distance from the soma correlates with increased mitochondrial protein damage, PINK1 accumulation, reactive oxygen species production, and decreased mitochondrial membrane potential and depolarization threshold. Furthermore, we demonstrate that the distance-dependent mitochondrial membrane potential gradient exists in vivo in mice. We demonstrate that impaired distal mitochondria have a lower threshold for focal/nonlethal neuritic caspase-3 activation in normal neurons that is exacerbated in aging, stress, and neurodegenerative conditions, thus delineating a fundamental mechanistic underpinning for synaptic vulnerability.

High glycolytic activity of tumor cells leads to underestimation of electron transport system capacity when mitochondrial ATP synthase is inhibited

This study sought to elucidate how oligomycin, an ATP synthase blocker, leads to underestimation of maximal oxygen consumption rate (_maxOCR) and spare respiratory capacity (SRC) in tumor cells. T98G and U-87MG glioma cells were titrated with the protonophore CCCP to induce _maxOCR. The presence of oligomycin (0.3-3.0 µg/mL) led to underestimation of _maxOCR and a consequent decrease in SRC values of between 25% and 40% in medium containing 5.5 or 11 mM glucose. The inhibitory effect of oligomycin on CCCP-induced _maxOCR did not occur when glutamine was the metabolic substrate or when the glycolytic inhibitor 2-deoxyglucose was present. ATP levels were reduced and ADP/ATP ratios increased in cells treated with CCCP, but these changes were minimized when oligomycin was used to inhibit reverse activity of ATP synthase. Exposing digitonin-permeabilized cells to exogenous ATP, but not ADP, resulted in partial inhibition of CCCP-induced _maxOCR. We conclude that underestimation of _maxOCR and SRC in tumor cells when ATP synthase is inhibited is associated with high glycolytic activity and that the glycolytic ATP yield may have an inhibitory effect on the metabolism of respiratory substrates and cytochrome c oxidase activity. Under CCCP-induced _maxOCR, oligomycin preserves intracellular ATP by inhibiting ATP synthase reverse activity.

VDAC electronics: 5. Mechanism and computational model of hexokinase-dependent generation of the outer membrane potential in brain mitochondria

Glycolysis plays a key role in brain energy metabolism. The initial and rate-limiting step of brain glycolysis is catalyzed mainly by hexokinase I (HKI), the majority of which is bound to the mitochondrial outer membrane (MOM), mostly through the mitochondrial inter-membrane contact sites formed by the voltage-dependent anion channel (VDAC, outer membrane) and the adenine nucleotide translocator (ANT, inner membrane). Earlier, we proposed a mechanism for the generation of the mitochondrial outer membrane potential (OMP) as a result of partial application of the inner membrane potential (IMP) to MOM through the electrogenic ANT-VDAC-HK inter-membrane contact sites. According to this previous mechanism, the Gibbs free energy of the hexokinase reaction might modulate the generated OMP (Lemeshko, Biophys. J., 2002). In the present work, a new computational model was developed to perform thermodynamic estimations of the proposed mechanism of IMP-HKI-mediated generation of OMP. The calculated OMP was high enough to electrically regulate MOM permeability for negatively charged metabolites through free, unbound VDACs in MOM. On the other hand, the positive-inside polarity of OMP generated by the IMP-HKI-mediated mechanism is expected to protect mitochondria against elevated concentrations of cytosolic Ca²⁺. This computational analysis suggests that metabolically-dependent generation of OMP in the brain mitochondria, controlled by many factors that modulate VDAC1-HKI interaction, VDAC's voltage-gating properties and permeability, might represent one of the physiological mechanisms of regulation of the brain energy metabolism and of neuronal death resistance, and might also be involved in various neurodegenerative disorders, such as Alzheimer's disease.

Mitochondrial membrane potential

The mitochondrial membrane potential (ΔΨm) generated by proton pumps (Complexes I, III and IV) is an essential component in the process of energy storage during oxidative phosphorylation. Together with the proton gradient (ΔpH), ΔΨm forms the transmembrane potential of hydrogen ions which is harnessed to make ATP. The levels of ΔΨm and ATP in the cell are kept relatively stable although there are limited fluctuations of both these factors that can occur reflecting normal physiological activity. However, sustained changes in both factors may be deleterious. A long-lasting drop or rise of ΔΨm vs normal levels may induce unwanted loss of cell viability and be a cause of various pathologies. Among other factors, ΔΨm plays a key role in mitochondrial homeostasis through selective elimination of dysfunctional mitochondria. It is also a driving force for transport of ions (other than H+) and proteins which are necessary for healthy mitochondrial functioning. We propose additional potential mechanisms for which ΔΨm is essential for maintenance of cellular health and viability and provide recommendations how to accurately measure ΔΨm in a cell and discuss potential sources of artifacts.

Metabolic activation-driven mitochondrial hyperpolarization predicts insulin secretion in human pancreatic beta-cells

Mitochondrial metabolism plays a central role in insulin secretion in pancreatic beta-cells. Generation of protonmotive force and ATP synthesis from glucose-originated pyruvate are critical steps in the canonical pathway of glucose-stimulated insulin secretion. Mitochondrial metabolism is intertwined with pathways that are thought to amplify insulin secretion with mechanisms distinct from the canonical pathway, and the relative importance of these two pathways is controversial. Here I show that glucose-induced mitochondrial membrane potential (MMP) hyperpolarization is necessary for, and predicts, the rate of insulin secretion in primary cultured human beta-cells. When glucose concentration is elevated, increased metabolism results in a substantial MMP hyperpolarization, as well as in increased rates of ATP synthesis and turnover marked by faster cell respiration. Using modular kinetic analysis I explored what properties of cellular energy metabolism enable a large glucose-induced change in MMP in human beta-cells. I found that an ATP-dependent pathway activates glucose or substrate oxidation, acting as a positive feedback in energy metabolism. This activation mechanism is essential for concomitant fast respiration and high MMP, and for a high magnitude glucose-induced MMP hyperpolarization and therefore for insulin secretion.

Osteoblast-like MC3T3-E1 Cells Prefer Glycolysis for ATP Production but Adipocyte-like 3T3-L1 Cells Prefer Oxidative Phosphorylation

Mesenchymal stromal cells (MSCs) are early progenitors that can differentiate into osteoblasts, chondrocytes, and adipocytes. We hypothesized that osteoblasts and adipocytes utilize distinct bioenergetic pathways during MSC differentiation. To test this hypothesis, we compared the bioenergetic profiles of preosteoblast MC3T3-E1 cells and calvarial osteoblasts with preadipocyte 3T3L1 cells, before and after differentiation. Differentiated MC3T3-E1 osteoblasts met adenosine triphosphate (ATP) demand mainly by glycolysis with minimal reserve glycolytic capacity, whereas nondifferentiated cells generated ATP through oxidative phosphorylation. A marked Crabtree effect (acute suppression of respiration by addition of glucose, observed in both MC3T3-E1 and calvarial osteoblasts) and smaller mitochondrial membrane potential in the differentiated osteoblasts, particularly those incubated at high glucose concentrations, indicated a suppression of oxidative phosphorylation compared with nondifferentiated osteoblasts. In contrast, both nondifferentiated and differentiated 3T3-L1 adipocytes met ATP demand primarily by oxidative phosphorylation despite a large unused reserve glycolytic capacity. In sum, we show that nondifferentiated precursor cells prefer to use oxidative phosphorylation to generate ATP; when they differentiate to osteoblasts, they gain a strong preference for glycolytic ATP generation, but when they differentiate to adipocytes, they retain the strong preference for oxidative phosphorylation. Unique metabolic programming in mesenchymal progenitor cells may influence cell fate and ultimately determine the degree of bone formation and/or the development of marrow adiposity. © 2018 American Society for Bone and Mineral Research.

Secretome of Differentiated PC12 Cells Restores the Monocrotophos-Induced Damages in Human Mesenchymal Stem Cells and SHSY-5Y Cells: Role of Autophagy and Mitochondrial Dynamics

A perturbed cellular homeostasis is a key factor associated with xenobiotic exposure resulting in various ailments. The local cellular microenvironment enriched with secretory components aids in cell-cell communication that restores this homeostasis. Deciphering the underlying mechanism behind this restorative potential of secretome could serve as a possible solution to many health hazards. We, therefore, explored the protective efficacy of the secretome of differentiated PC12 cells with emphasis on induction of autophagy and mitochondrial biogenesis. Monocrotophos (MCP), a widely used neurotoxic organophosphate, was used as the test compound at sublethal concentration. The conditioned medium (CM) of differentiated PC12 cells comprising of their secretome restored the cell viability, oxidative stress and apoptotic cell death in MCP-challenged human mesenchymal stem cells and SHSY-5Y, a human neuroblastoma cell line. Delving further to identify the underlying mechanism of this restorative effect we observed a marked increase in the expression of autophagy markers LC3, Beclin-1, Atg5 and Atg7. Exposure to autophagy inhibitor, 3-methyladenine, led to a reduced expression of these markers with a concomitant increase in the expression of pro-apoptotic caspase-3. Besides that, the increased mitochondrial fission in MCP-exposed cells was balanced with increased fusion in the presence of CM facilitated by AMPK/SIRT1/PGC-1α signaling cascade. Mitochondrial dysfunctions are strongly associated with autophagy activation and as per our findings, cellular secretome too induces autophagy. Therefore, connecting these three potential apices can be a major breakthrough in repair and rescue of xenobiotic-damaged tissues and cells.

Romo1 is a mitochondrial nonselective cation channel with viroporin-like characteristics

Romo1 regulates mitochondrial reactive oxygen species production and acts as an essential redox sensor in mitochondrial dynamics. Lee et al. demonstrate that Romo1 is a unique mitochondrial ion channel with viroporin-like characteristics that distinguish Romo1 from other known eukaryotic ion channels.

Reactive oxygen species (ROS) modulator 1 (Romo1) is a nuclear-encoded mitochondrial inner membrane protein known to regulate mitochondrial ROS production and to act as an essential redox sensor in mitochondrial dynamics. Although its physiological roles have been studied for a decade, the biophysical mechanisms that explain these activities of Romo1 are unclear. In this study, we report that Romo1 is a unique mitochondrial ion channel that differs from currently identified eukaryotic ion channels. Romo1 is a highly conserved protein with structural features of class II viroporins, which are virus-encoded nonselective cation channels. Indeed, Romo1 forms a nonselective cation channel with its amphipathic helical transmembrane domain necessary for pore-forming activity. Notably, channel activity was specifically inhibited by Fe²⁺ ions, an essential transition metal ion in ROS metabolism. Using structural bioinformatics, we designed an experimental data–guided structural model of Romo1 with a rational hexameric structure. We propose that Romo1 establishes a new category of viroporin-like nonselective cation channel in eukaryotes.

The role of the ATPase inhibitor factor 1 (IF1) in cancer cells adaptation to hypoxia and anoxia

The physiological role of the mitochondrial ATP synthase complex is to generate ATP through oxidative phosphorylation. Indeed, the enzyme can reverse its activity and hydrolyze ATP under ischemic conditions, as shown in isolated mitochondria and in mammalian heart and liver. However, what occurs when cancer cells experience hypoxia or anoxia has not been well explored. In the present study, we investigated the bioenergetics of cancer cells under hypoxic/anoxic conditions with particular emphasis on ATP synthase, and the conditions driving it to work in reverse. In this context, we further examined the role exerted by its endogenous inhibitor factor, IF1, that it is overexpressed in cancer cells. Metabolic and bioenergetic analysis of cancer cells exposed to severe hypoxia (down to 0.1% O2) unexpectedly showed that Δψm is preserved independently of the presence of IF1 and that ATP synthase still phosphorylates ADP though at a much lower rate than in normoxia. However, when we induced an anoxia-mimicking condition by collapsing ΔμΗ+ with the FCCP uncoupler, the IF1-silenced clones only reversed the ATP synthase activity hydrolyzing ATP in order to reconstitute the electrochemical proton gradient. Notably, in cancer cells IF1 overexpression fully prevents ATP synthase hydrolytic activity activation under uncoupling conditions. Therefore, our results suggest that IF1 overexpression promotes cancer cells survival under temporary anoxic conditions by preserving cellular ATP despite mitochondria dysfunction.

Quantitative in vivo mapping of myocardial mitochondrial membrane potential

BACKGROUND

Mitochondrial membrane potential (ΔΨm) arises from normal function of the electron transport chain. Maintenance of ΔΨm within a narrow range is essential for mitochondrial function. Methods for in vivo measurement of ΔΨm do not exist. We use 18F-labeled tetraphenylphosphonium (18F-TPP+) to measure and map the total membrane potential, ΔΨT, as the sum of ΔΨm and cellular (ΔΨc) electrical potentials.

METHODS

Eight pigs, five controls and three with a scar-like injury, were studied. Pigs were studied with a dynamic PET scanning protocol to measure 18F-TPP+ volume of distribution, VT. Fractional extracellular space (fECS) was measured in 3 pigs. We derived equations expressing ΔΨT as a function of VT and the volume-fractions of mitochondria and fECS. Seventeen segment polar maps and parametric images of ΔΨT were calculated in millivolts (mV).

RESULTS

In controls, mean segmental ΔΨT = -129.4±1.4 mV (SEM). In pigs with segmental tissue injury, ΔΨT was clearly separated from control segments but variable, in the range -100 to 0 mV. The quality of ΔΨT maps was excellent, with low noise and good resolution. Measurements of ΔΨT in the left ventricle of pigs agree with previous in in-vitro measurements.

CONCLUSIONS

We have analyzed the factors affecting the uptake of voltage sensing tracers and developed a minimally invasive method for mapping ΔΨT in left ventricular myocardium of pigs. ΔΨT is computed in absolute units, allowing for visual and statistical comparison of individual values with normative data. These studies demonstrate the first in vivo application of quantitative mapping of total tissue membrane potential, ΔΨT.

Fluorescence Measurement of Mitochondrial Membrane Potential Changes in Cultured Cells

The mitochondrial membrane potential is the dominant component of the proton-motive force that is the potential term in the proton circuit linking electron transport to ATP synthesis and other energy-dependent mitochondrial processes. Cationic fluorescent probes have been used for many years to detect gross qualitative changes in mitochondrial membrane potentials in intact cell culture. In this chapter I describe how these fluorescence signals may be used to obtain a semiquantitative measure of changes in mitochondrial membrane potential.

Guidelines on experimental methods to assess mitochondrial dysfunction in cellular models of neurodegenerative diseases

Neurodegenerative diseases are a spectrum of chronic, debilitating disorders characterised by the progressive degeneration and death of neurons. Mitochondrial dysfunction has been implicated in most neurodegenerative diseases, but in many instances it is unclear whether such dysfunction is a cause or an effect of the underlying pathology, and whether it represents a viable therapeutic target. It is therefore imperative to utilise and optimise cellular models and experimental techniques appropriate to determine the contribution of mitochondrial dysfunction to neurodegenerative disease phenotypes. In this consensus article, we collate details on and discuss pitfalls of existing experimental approaches to assess mitochondrial function in in vitro cellular models of neurodegenerative diseases, including specific protocols for the measurement of oxygen consumption rate in primary neuron cultures, and single-neuron, time-lapse fluorescence imaging of the mitochondrial membrane potential and mitochondrial NAD(P)H. As part of the Cellular Bioenergetics of Neurodegenerative Diseases (CeBioND) consortium (www.cebiond.org), we are performing cross-disease analyses to identify common and distinct molecular mechanisms involved in mitochondrial bioenergetic dysfunction in cellular models of Alzheimer’s, Parkinson’s, and Huntington’s diseases. Here we provide detailed guidelines and protocols as standardised across the five collaborating laboratories of the CeBioND consortium, with additional contributions from other experts in the field.

Cytotoxic effects of the proton pump inhibitor omeprazole on the non-target marine microalga Tetraselmis suecica

Omeprazole (OMP) is one of the most commonly used drugs for the treatment of gastro-intestinal disorders. Although it is daily consumed in high quantities and commonly detected in waters worldwide, relatively little is known about its ecotoxicity. The aim of this study was to evaluate the potential acute toxicity of increasing concentrations of OMP on the marine microalga Tetraselmis suecica analysing several cytotoxicity biomarkers by flow cytometry after 24h of exposure. Results showed that OMP caused a decrease in growth and autofluorescence, an increase in cellular volume and intracellular complexity, hyperpolarization of cytoplasmic and mitochondrial membranes and intracellular acidification. In addition, large amounts of reactive oxygen species (ROS) were generated which resulted in a decrease in the percentage of the viable population. However, the viable population showed an increase in their metabolic activity as an early response to overcome the stress. In conclusion, OMP may affect proton pumps in non-target organisms such as microalgae; it disturbed pH homeostasis and provoked an early accumulation of ROS that resulted in a rapid cell death in cells exposed to the highest concentration assayed.

Rapid immunopurification of mitochondria for metabolite profiling and absolute quantification of matrix metabolites

Mitochondria carry out numerous metabolic reactions that are critical to cellular homeostasis. Here we present a protocol for interrogating mitochondrial metabolites and measuring their matrix concentrations. Our workflow uses high-affinity magnetic immunocapture to rapidly purify HA-tagged mitochondria from homogenized mammalian cells in ∼12 min. These mitochondria are extracted with methanol and water. Liquid chromatography and mass spectrometry (LC/MS) is used to determine the identities and mole quantities of mitochondrial metabolites using authentic metabolite standards and isotopically labeled internal standards, whereas the corresponding mitochondrial matrix volume is determined via immunoblotting, confocal microscopy of intact cells, and volumetric analysis. Once all values have been obtained, the matrix volume is combined with the aforementioned mole quantities to calculate the matrix concentrations of mitochondrial metabolites. With shortened isolation times and improved mitochondrial purity when compared with alternative methods, this LC/MS-compatible workflow allows for robust profiling of mitochondrial metabolites and serves as a strategy generalizable to the study of other mammalian organelles. Once all the necessary reagents have been prepared, quantifying the matrix concentrations of mitochondrial metabolites can be accomplished within a week.

Anti-angiogenic drugs: direct anti-cancer agents with mitochondrial mechanisms of action

Components of the mitochondrial electron transport chain have recently gained much interest as potential therapeutic targets. Since mitochondria are essential for the supply of energy that is required for both angiogenic and tumourigenic activity, targeting the mitochondria represents a promising potential therapeutic approach for treating cancer. Here we investigate the established anti-angiogenesis drugs combretastatin A4, thalidomide, OGT 2115 and tranilast that we hypothesise are able to exert a direct anti-cancer effect in the absence of vasculature by targeting the mitochondria. Drug cytotoxicity was measured using the MTT assay. Mitochondrial function was measured in intact isolated mitochondria using polarography, fluorimetry and enzymatic assays to measure mitochondrial oxygen consumption, membrane potential and complex I-IV activities respectively. Combretastatin A4, OGT 2115 and tranilast were both shown to decrease mitochondrial oxygen consumption. OGT 2115 and tranilast decreased mitochondrial membrane potential and reduced complex I activity while combretastatin A4 and thalidomide did not. OGT 2115 inhibited mitochondrial complex II-III activity while combretastatin A4, thalidomide and tranilast did not. Combretastatin A4, thalidomide and OGT 2115 induced bi-phasic concentration-dependent increases and decreases in mitochondrial complex IV activity while tranilast had no evident effect. These data demonstrate that combretastatin A4, thalidomide, OGT 2115 and tranilast are all mitochondrial modulators. OGT 2115 and tranilast are both mitochondrial inhibitors capable of eliciting concentration-dependent reductions in cell viability by decreasing mitochondrial membrane potential and oxygen consumption.

Drosophila MIC60/mitofilin conducts dual roles in mitochondrial motility and crista structure

Mitochondria are crucial organelles for providing energy for a cell. It is known that MIC60/mitofilin is important for maintaining mitochondrial structure in dissociated cells; however, its physiological roles in an intact animal are less clear. In this study, we unravel the functional consequences of deleting MIC60/mitofilin in fruit flies.

MIC60/mitofilin constitutes a hetero-oligomeric complex on the inner mitochondrial membranes to maintain crista structure. However, little is known about its physiological functions. Here, by characterizing Drosophila MIC60 mutants, we define its roles in vivo. We discover that MIC60 performs dual functions to maintain mitochondrial homeostasis. In addition to its canonical role in crista membrane structure, MIC60 regulates mitochondrial motility, likely by influencing protein levels of the outer mitochondrial membrane protein Miro that anchors mitochondria to the microtubule motors. Loss of MIC60 causes loss of Miro and mitochondrial arrest. At a cellular level, loss of MIC60 disrupts synaptic structure and function at the neuromuscular junctions. The dual roles of MIC60 in both mitochondrial crista structure and motility position it as a crucial player for cellular integrity and survival.