Intra- and Intercellular Distribution of Mitochondrial Probes and Changes after Treatment with MDR Modulators

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 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.

Targeting of mitochondria-endoplasmic reticulum by fluorescent macrocyclic compounds

Background

Useful probes of the intracellular environment that target a specific organelle in order to allow direct observation of the changes in these regions is of high current interest. Macrocyclic ligands have already revealed themselves as important selective hosts in some biological applications, forming stable and specific complexes. Therefore, in this paper, several macrocyclic ligands are evaluated as potential molecular probes.

Methodology

Four polyammonium macrocycles and one macrotricyclic bearing pyridine and phenanthroline chromophores have been synthesised and evaluated as molecular probes. The cytotoxicity of the compounds has been analyzed using human breast cancer cells (MCF-7), non-cancerous human dermal fibroblasts (NHDF) and human adult dermal skin fibroblasts from a breast cancer patient (P14). All the compounds showed low toxicity at concentrations ranging from 10 nM to 10 µM, except for [32]phen₂N₄ which proved to be highly cytotoxic for MCF-7 cells. Flow cytometry studies evidenced that the percentage of apoptotic and necrotic MCF-7 and NHDF cells induced by the compounds is considerably low. Also, flow cytometry analysis showed that some compounds seem to modify the mitochondrial membrane potential (MMP) of the cells. Fluorescence microscopy evidenced that compounds easily cross the plasma membrane (5 min) and accumulated into the mitochondria, as confirmed by co-localization with MitoTracker Green™. The fluorescence images also evidenced an intact mitochondria structure after 48 h. Moreover, reticular staining suggestive of endoplasmic reticulum (ER) localization, in addition to the mitochondrial one, has been found by confocal microscopy.

Conclusion

Our study reveals that compounds Me₂[28]py₂N₆, cryptphen, [16]phenN₂, [30]phen₂N₆, have low toxicity and localize in mitochondria and ER. The ability of these compounds for translocating the cellular membrane (5 min) without special conditioning of the cells or derivatization of the probe, the time-dependent localization (48 h) and the cellular viability provide a proof-of-concept towards their use as promising probes towards biomedical studies.

Measuring mitochondrial function in intact cardiac myocytes

Mitochondria are involved in cellular functions that go beyond the traditional role of these organelles as the power plants of the cell. Mitochondria have been implicated in several human diseases, including cardiac dysfunction, and play a role in the aging process. Many aspects of our knowledge of mitochondria stem from studies performed on the isolated organelle. Their relative inaccessibility imposes experimental difficulties to study mitochondria in their natural environment-the cytosol of intact cells-and has hampered a comprehensive understanding of the plethora of mitochondrial functions. Here we review currently available methods to study mitochondrial function in intact cardiomyocytes. These methods primarily use different flavors of fluorescent dyes and genetically encoded fluorescent proteins in conjunction with high-resolution imaging techniques. We review methods to study mitochondrial morphology, mitochondrial membrane potential, Ca(2+) and Na(+) signaling, mitochondrial pH regulation, redox state and ROS production, NO signaling, oxygen consumption, ATP generation and the activity of the mitochondrial permeability transition pore. Where appropriate we complement this review on intact myocytes with seminal studies that were performed on isolated mitochondria, permeabilized cells, and in whole hearts.

Cystic fibrosis transmembrane regulator inhibitors CFTR(inh)-172 and GlyH-101 target mitochondrial functions, independently of chloride channel inhibition

Two highly potent and selective cystic fibrosis (CF) transmembrane regulator (CFTR) inhibitors have been identified by high-throughput screening: the thiazolidinone CFTR(inh)-172 [3-[(3-trifluoromethyl)phenyl]-5-[(4-carboxyphenyl)methylene]- 2-thioxo-4-thiazolidinone] and the glycine hydrazide GlyH-101 [N-(2-naphthalenyl)-((3,5-dibromo-2,4-dihydroxyphenyl)methylene)glycine hydrazide]. Inhibition of the CFTR chloride channel by these compounds has been suggested to be of pharmacological interest in the treatment of secretory diarrheas and polycystic kidney disease. In addition, functional inhibition of CFTR by CFTR(inh)-172 has been proposed to be sufficient to mimic the CF inflammatory profile. In the present study, we investigated the effects of the two compounds on reactive oxygen species (ROS) production and mitochondrial membrane potential in several cell lines: the CFTR-deficient human lung epithelial IB3-1 (expressing the heterozygous F508del/W1282X mutation), the isogenic CFTR-corrected C38, and HeLa and A549 as non-CFTR-expressing controls. Both inhibitors were able to induce a rapid increase in ROS levels and depolarize mitochondria in the four cell types, suggesting that these effects are independent of CFTR inhibition. In HeLa cells, these events were associated with a decrease in the rate of oxygen consumption, with GlyH-101 demonstrating a higher potency than CFTR(inh)-172. The impact of CFTR inhibitors on inflammatory parameters was also tested in HeLa cells. CFTR(inh)-172, but not GlyH-101, induced nuclear translocation of nuclear factor-kappaB (NF-kappaB). CFTR(inh)-172 slightly decreased interleukin-8 secretion, whereas GlyH-101 induced a slight increase. These results support the conclusion that CFTR inhibitors may exert nonspecific effects regarding ROS production, mitochondrial failure, and activation of the NF-kappaB signaling pathway, independently of CFTR inhibition.

Intracellular distribution of fluorescent probes delivered by vesicles of different lipidic composition

In order to study mechanisms involved in liposome-cell interaction, this work attempted to assess the influence of vesicle composition on the delivery of liposomal content to Hela cells. In particular, to evaluate pH-sensitive properties and cell interaction of the prepared liposomes, the lipid formulations contained cholesterol (Chol) and they were varied by using phosphatidylcholines with different purity degree: soy lecithin (SL; 80% phosphatidylcholine), a commercial mixture of soy phosphatidylcholine (P90; 90% phosphatidylcholine) or dipalmitoylphosphatidylcholine (DPPC; 99% of purity). A second series of liposomes also contained stearylamine (SA). Dehydration-rehydration vesicles (DRV) were prepared and then sonicated to decrease vesicle size. Vesicle-cell interactions and liposomal uptake were examined by fluorescence microscopy using carboxyfluorescein (CF) and phosphatidylethanolamine-dioleoyl-sulforhodamine B (Rho-PE) as fluorescent markers. Fluorescence dequenching assay was used to study the influence of pH on CF release from the liposomal formulations. Liposome adhesion on the cell surface and internalization were strongly dependent on vesicle bilayer composition. SA vesicles were not endocytosed. DPPC/Chol liposomes were endocytosed but did not release their fluorescent content into the cytosol. SL/Chol and P90/Chol formulations displayed a diffuse cytoplasmic fluorescence of liposomal marker.

Cyclosporin A-induced oxidative stress is not the consequence of an increase in mitochondrial membrane potential

Cyclosporin A induces closure of the mitochondrial permeability transition pore. We aimed to investigate whether this closure results in concomitant increases in mitochondrial membrane potential (DeltaPsim) and the production of reactive oxygen species. Fluorescent probes were used to assess DeltaPsim (JC-1, 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethyl-benzimidazolyl-carbocyanine iodide), reactive oxygen species [DCF, 5- (and 6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate, acetyl ester] and [Ca2+][Fluo-3, glycine N-[4-[6-[(acetyloxy)methoxy]-2,7-dichloro-3-oxo-3H-xanthen-9-yl]-2-[2-[2-[bis[2-[(acetyloxy)methoxy]-2-oxyethyl]amino]-5-methylphenoxy]ethoxy]phenyl]-N-[2-[(acetyloxy)methoxy]-2-oxyethyl]-(acetyloxy)methyl ester] in human kidney cells (HK-2 cells) and in a line of human small cell carcinoma cells (GLC4 cells), because these do not express cyclosporin A-sensitive P-glycoprotein. We used transfected GLC4 cells expressing P-glycoprotein as control for GLC4 cells. NIM811 (N-methyl-4-isoleucine-cyclosporin) and PSC833 (SDZ-PSC833) were applied as selective mitochondrial permeability transition pore and P-glycoprotein blockers, respectively. To study the effect of cyclosporin A on mitochondrial function, we isolated mitochondria from fresh pig livers. Cyclosporin A and PSC833 induced a more than two-fold increase in JC-1 fluorescence in HK-2 cells, whereas NIM811 had no effect. None of the three substances induced a significant increase in JC-1 fluorescence in GLC4 cells. Despite this, cyclosporin A, NIM811 and PSC833 induced a 1.5-fold increase in DCF fluorescence (P<0.05) and a two-fold increase in Fluo-3 fluorescence (P<0.05). Studies in isolated mitochondria showed that blockage of mitochondrial permeability transition pores by cyclosporin A affected neither DeltaPsim, ATP synthesis, nor respiration rate. The mitochondrial permeability transition pore blockers cyclosporin A and NIM811, but also the non-mitochondrial permeability transition pore blocker PSC833, induced comparable degrees of reactive oxygen species production and cytosolic [Ca2+]. Neither mitochondria, effects on P-glycoprotein nor inhibition of calcineurin therefore play a role in cyclosporin A-induced oxidative stress and disturbed Ca2+ homeostasis.

Discrimination of live and early apoptotic mononuclear cells by the fluorescent SYTO 16 vital dye

Accurate detection of apoptotic cells is important for the determination of cell viability. The aim of this study was to compare the sensitivity of the cell permeant SYTO 16 fluorescent dye for detecting early apoptotic mononuclear cells (MNCs) in normal donor blood with other apoptosis assays [i.e. Annexin-V, light scatter/7-amino-actinomycin-D (7-AAD) and chloromethyl-X-rosamine (CMXRos)] and to identify critical parameters for optimal SYTO 16 staining. Apoptosis was induced in normal human leukocytes from adult peripheral blood or cord blood, or the Jurkat T-lymphocytic cell line and assessed by fluorescence microscopy and flow cytometry. Dual labelling showed that SYTO 16 detected more apoptotic MNCs compared to Annexin-V. SYTO 16 staining intensity was consistent with the light scatter profiles expected of live, apoptotic and necrotic MNCs and was more objective than light scatter/7-AAD. CMXRos staining required considerable care and may not be a robust marker of apoptotic primary MNCs. For SYTO 16 flow cytometric analysis, the optimal conditions for staining 1x10(6) leukocytes were 4 nM SYTO 16 in the presence of 30 muM verapamil for 25-45 min at 37 degrees C in media containing calcium/magnesium supplemented with protein. A P-glycoprotein inhibitor, such as verapamil, and calcium/magnesium are essential for optimal loading of SYTO 16 into live MNCs and discrimination of apoptotic MNCs in normal blood samples. SYTO 16 is a sensitive, simple, inexpensive 'live cell' method for the discrimination of live, apoptotic and necrotic normal blood MNCs and is more sensitive for detecting apoptosis in these cells than Annexin-V or light scatter/7-AAD.

Characterization of depolarization and repolarization phases of mitochondrial membrane potential fluctuations induced by tetramethylrhodamine methyl ester photoactivation

Depolarization and repolarization phases (D and R phases, respectively) of mitochondrial potential fluctuations induced by photoactivation of the fluorescent probe tetramethylrhodamine methyl ester (TMRM) were analyzed separately and investigated using specific inhibitors and substrates. The frequency of R phases was significantly inhibited by oligomycin and aurovertin (mitochondrial ATP synthase inhibitors), rotenone (mitochondrial complex I inhibitor) and iodoacetic acid (inhibitor of the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase). Succinic acid (mitochondrial complex II substrate, given in the permeable form of dimethyl ester) abolished the rotenone-induced inhibition of R phases. Taken together, these findings indicate that the activity of both respiratory chain and ATP synthase were required for the recovery of the mitochondrial potential. The frequency of D phases prevailed over that of R phases in all experimental conditions, resulting in a progressive depolarization of mitochondria accompanied by NAD(P)H oxidation and Ca2+ influx. D phases were not blocked by cyclosporin A (inhibitor of the permeability transition pore) or o-phenyl-EGTA (a Ca2+ chelator), suggesting that the permeability transition pore was not involved in mitochondrial potential fluctuations.

A flow cytometric assay for simultaneous assessment of drug efflux, proliferation, and apoptosis

BACKGROUND

Proliferative status and multidrug resistance play a key role in determining cell response to chemotherapy. There is a need to develop multiple labeling method that allows simultaneous assessment of multidrug resistant (MDR) phenotype, proliferative status, apoptosis related changes in mitochondrial potential, in chemosensitive and chemoresistant tumor cell populations.

METHODS

A three-color labeling was performed using Hoechst 33342 (DNA), JC1 (mitochondrial potential), and a far red fluorescent membrane intercalating dye: PTIR271 (proliferation).

RESULTS

Combined staining of DNA and mitochondrial potential allows identification of subpopulations expressing and MDR phenotype mediated by P-glycoprotein (Pgp), and, in Pgp negative subpopulations, identification of apoptotic cells and evaluation of cell cycle status in viable cells. Addition of a far red fluorescent membrane intercalating dye, PTIR271, allows simultaneous monitoring of cell division status by dye dilution in both drug sensitive and drug resistant populations.

CONCLUSION

This triple labeling is an interesting method to study the proliferation status of drug sensitivity and drug resistance in viable tumor cells.