Two mechanisms by which fluorescent oxonols indicate membrane potential in human red blood cells

Liposome-Based Nanoencapsulation of a Mitochondria-Stapling Photosensitizer for Efficient Photodynamic Therapy

Mitochondria-targeting photodynamic therapy (PDT) can block mitochondrial function and trigger the inherent proapoptotic cascade signal of mitochondria, which has been considered to have the potential to amplify the efficiency of PDT. However, the dynamic change of mitochondrial membrane potential (MMP) makes most cationic photosensitizers easily fall off from the mitochondria, which greatly limits the efficiency of PDT. Here, we have developed a smart liposome encapsulation method based on a mitochondria-stapling photosensitizer for efficient theranostic photodynamic therapy. The stapling photosensitizer can be covalently bound inside mitochondria via two reaction sites without a falloff effect, regardless of the change of MMP. As a result, the liposome-based nanophotosensitizer showed a high efficiency of PDT (IC₅₀ = 0.98 μM) under 630 nm light. At the same time, the nanophotosensitizer had fluorescence imaging-guided ability to monitor abnormal mitochondrial morphology during PDT. Importantly, the results of mice experiments also showed that the liposome-based nanophotosensitizer possessed excellent antitumor PDT activity because the released photosensitizer can stay inside mitochondria during the whole process of PDT.

Controllable Coumarin-Based NIR Fluorophores: Selective Subcellular Imaging, Cell Membrane Potential Indication, and Enhanced Photodynamic Therapy

Fluorescent materials with advanced functionalities provide powerful tools to visualize subcellular microstructures and monitor subcellular dynamic functions, which significantly boost our understanding of complex biological phenomena and manipulation of biological behaviors. However, realization of diverse biological applications from a single molecular backbone is still a challenging endeavor. In this contribution, a series of coumarin-based cationic fluorophores (Cou-n, n = 1-4) with near-infrared emission (675 nm) and large Stokes shifts (110 nm) have been developed. Considering their excellent biocompatibilities and alkyl chain-dependent lipophilicities, Cou-1 and Cou-3 could selectively and ultrafast (<30 s) stain the cell plasma membrane and mitochondria in a washing-free manner, respectively. Meanwhile, Cou-1 could sensitively respond to the change of the plasma membrane potential, which enabled Cou-1 to successfully indicate the cell passage number. Taking advantage of specific mitochondria targeting as well as efficient singlet oxygen generation, Cou-3 exhibited enhanced photodynamic therapy (PDT) effect for tumor inhibition in vivo. In addition, the suitable lipophilicity of Cou-3 aided it in selectively imaging Gram-positive bacteria and efficiently killing bacteria by the PDT process.

Mechanism of response of potential-sensitive dyes studied by time-resolved fluorescence

The mechanism of response of two potential-sensitive dyes, diOC(2)(5) (3,3'-diethyloxadicarbocyanine iodide) and oxonol V (bis-[3-phenyl-5-oxoisoxazol-4-yl]pentamethine oxonol), were studied by using steady-state and time-resolved fluorescence techniques. The lipid concentration dependence of the Deltapsi (membrane potential)-induced change in total fluorescence intensity was quite different for these two dyes. Time-resolved fluorescence measurements showed that the fluorescence decay of these dyes in membranes could be resolved into at least three exponentials. Deltapsi-induced changes in the levels of these three populations were also measured under a variety of conditions. In the case of diOC(2)(5) an inside negative Deltapsi increased the levels of the bound forms. This shows that diOC(2)(5) responds to Deltapsi mainly by an "on-off" mechanism whereby Deltapsi perturbs the membrane-water partition coefficient of the dye. The Deltapsi-induced changes approached zero when the dye was totally membrane bound. In contrast, the Deltapsi-induced response of oxonol V increased with increased membrane binding. An inside negative Deltapsi decreased the level of the bound form with a longer lifetime. This shows that the mechanism of response of oxonol V is a Deltapsi-induced shift in the equilibrium between bound forms of the dye.

Effect of pluronic p85 on amino acid transport in bovine brain microvessel endothelial cells

A synthetic amphiphilic block copolymer Pluronic P85 (P85) was shown to be among the most potent inhibitors of Pgp efflux system in the blood-brain barrier (BBB) and capable of enhancing delivery of Pgp substrates to the brain. The purpose of this work is to evaluate the effects of P85 on amino acid transport in BBB. Primary bovine brain microvessel endothelial cells (BBMEC) grown on membrane inserts were used as an in vitro BBB model. Expression of amino acid transporters, like large neutral amino acid transporter 1, cationic amino acid transporter 1, and small neutral amino acid transporter 1, were confirmed by reverse transcriptase polymerase chain reaction. Effects of P85 on amino acid transporters were examined using their substrates: (3)H-phenylalanine, (3)H-lysine, and (3)H-methylaminoisobutyric acid, respectively. BBMEC permeability studies were carried out in apical (AP) to basolateral (BL) and BL to AP directions. P85 added at the AP side had little, if any, effect on AP to BL ("blood to brain") transport for all examined amino acids in BBMEC monolayers. However, 0.1% P85 added at the BL side significantly increased the BL to AP transport of these substrates. Furthermore, the effective concentrations of P85 were also shown to induce plasma membrane depolarization and increase intracellular sodium concentration in BBMEC, which can contribute to the effects of the copolymer on the energy-dependent transport systems. All together, despite profound effects on transport system(s) at the brain side of cell monolayers, P85 had no effect on AP to BL transport of amino acids in brain microvessel endothelial cell model.

Evaluating the validity of blood-based membrane potential changes for the identification of bipolar disorder I

OBJECTIVE

The objective of this study was to develop a diagnostic blood test for bipolar disorder I using membrane potentials as biological markers.

METHODS

We measured the fluorescence intensity of a dye sensitive to membrane potential in whole blood samples from bipolar I, unipolar, schizophrenic patients, and psychiatrically normal controls. Patients were diagnosed through structured clinical interviews according to DSM-IV. Both the t-test and logistic regression analysis were used to analyze the data.

RESULTS

The membrane potential as indicated by the fluorescence intensity of the membrane potential dye in blood cells drawn from patients with bipolar disorder I was significantly different from the blood cells drawn from unipolar and schizophrenic patients, and from psychiatrically normal controls (P<0.001). The specificity and sensitivity were determined to be 0.88 and 0.78 respectively which compared well with the state of the art diagnostic techniques for other diseases. Logistic regression analysis revealed that the membrane potential was a reliable predictor which could be used as a diagnostic marker for bipolar I.

CONCLUSIONS

These results indicate that the membrane potential of blood cells can be used as a diagnostic marker to augment the DSM-IV diagnosis of bipolar disorder I. Expanded clinical trials are needed to establish this technique for general use.

Monitoring of the proton electrochemical gradient in reconstituted vesicles: quantitative measurements of both transmembrane potential and intravesicular pH by ratiometric fluorescent probes

Proteoliposomes carrying reconstituted yeast plasma membrane H(+)-ATPase in their lipid membrane or plasma membrane vesicles are model systems convenient for studying basic electrochemical processes involved in formation of the proton electrochemical gradient (Deltamicro(H) (+)) across the microbial or plant cell membrane. Deltapsi- and pH-sensitive fluorescent probes were used to monitor the gradients formed between inner and outer volume of the reconstituted vesicles. The Deltapsi-sensitive fluorescent ratiometric probe oxonol VI is suitable for quantitative measurements of inside-positive Deltapsi generated by the reconstituted H(+)-ATPase. Its Deltapsi response can be calibrated by the K(+)/valinomycin method and ratiometric mode of fluorescence measurements reduces undesirable artefacts. In situ pH-sensitive fluorescent probe pyranine was used for quantitative measurements of pH inside the proteoliposomes. Calibration of pH-sensitive fluorescence response of pyranine entrapped inside proteoliposomes was performed with several ionophores combined in order to deplete the gradients passively formed across the membrane. Presented model system offers a suitable tool for simultaneous monitoring of both components of the proton electrochemical gradient, Deltapsi and DeltapH. This approach should help in further understanding how their formation is interconnected on biomembranes and even how transport of other ions is combined to it.

Substrates Induce Conformational Changes in Human Anion Exchanger 1 (hAE1) as Observed by Fluorescence Resonance Energy Transfer

The one-for-one exchange of Cl(-) and HCO(3)(-) ions is catalyzed by human erythrocyte anion exchanger 1 (hAE1) through a ping-pong mechanism whereby the protein exists in two main conformations, with the single anion-binding site exposed at either the cytoplasmic (inner) side (E(i)) or the extracellular side (E(o)), with interconversion between the two states being possible only after anion binding. Steady-state and time-resolved resonance energy transfer (FRET) techniques were used to determine the distance of the binding site for diTBA (bis-(1,3-diethylthiobarbituric acid)trimethine oxonol), a high affinity fluorescent oxonol inhibitor of hAE1, from a benchmark site (probably Lys-430) labeled by external fluorescein maleimide (FM). Using red cell ghost membranes, energy transfer distances were measured in media containing different anions between FM as the donor, covalently attached to one monomer, and diTBA as the acceptor, reversibly bound to the adjacent monomer of a hAE1 dimer. Energy transfer increased significantly in chloride or bicarbonate buffers relative to conditions where no transportable anions were present, that is, in citrate buffer. These differences in transfer efficiencies were interpreted in light of the conformational distributions of hAE1 in various buffers and the possible effects of diTBA itself on the distribution. The analysis indicates that the diTBA binding site comes closer to the FM site by approximately 7 A in chloride buffer as compared to that in citrate (or equivalent changes in diTBA orientation occur) because of the effects of anion binding. This provides the first direct physical evidence for structural changes in hAE1 induced by substrates.

Cell-penetrating peptides: A comparative membrane toxicity study

Cell-penetrating peptides (CPPs) constitute a new class of delivery vectors with high pharmaceutical potential. However, the abilities of these peptides to translocate through cell membranes can be accompanied by toxic effects resulting from membrane perturbation at higher peptide concentrations. Therefore, we investigated membrane toxicity of five peptides with well-documented cell-penetrating properties, pAntp(43-58), pTAT(48-60), pVEC(615-632), model amphipathic peptide (MAP), and transportan 10, on two human cancer cell lines, K562 (erythroleukemia) and MDA-MB-231 (breast cancer), as well as on immortalized aortic endothelial cells. We studied the effects of these five peptides on the leakage of lactate dehydrogenase and on the fluorescence of plasma membrane potentiometric dye bis-oxonol. In all cell lines, pAntp(43-58), pTAT(48-60), and pVEC(615-632) induced either no leakage or low leakage of lactate dehydrogenase, accompanied by modest changes in bis-oxonol fluorescence. MAP and transportan 10 caused significant leakage; in K562 and MDA-MB-231 cells, 40% of total lactate dehydrogenase leaked out during 10 min exposure to 10 microM of transportan 10 and MAP, accompanied by a significant increase in bis-oxonol fluorescence. However, none of the CPPs tested had a hemolytic effect on bovine erythrocytes comparable to mastoparan 7. The toxicity profiles presented in the current study are of importance when selecting CPPs for different applications.

Ratiometric fluorescence measurements of membrane potential generated by yeast plasma membrane H(+)-ATPase reconstituted into vesicles

Potential-sensitive fluorescent probes oxonol V and oxonol VI were employed for monitoring membrane potential (Delta(psi)) generated by the Schizosaccharomyces pombe plasma membrane H(+)-ATPase reconstituted into vesicles. Oxonol VI was used for quantitative measurements of the Delta(psi) because its response to membrane potential changes can be easily calibrated, which is not possible with oxonol V. However, oxonol V has a superior sensitivity to Delta(psi) at very low concentration of reconstituted vesicles, and thus it is useful for testing quality of the reconstitution. Oxonol VI was found to be a good emission-ratiometric probe. We have shown that the reconstituted H(+)-ATPase generates Delta(psi) of about 160 mV on the vesicle membrane. The generated Delta(psi) was stable at least over tens of minutes. An influence of the H(+) membrane permeability on the Delta(psi) buildup was demonstrated by manipulating the H(+) permeability with the protonophore CCCP. Ratiometric measurements with oxonol VI thus offer a promising tool for studying processes accompanying the yeast plasma membrane H(+)-ATPase-mediated Delta(psi) buildup.

Estimation of membrane potential deltapsi in reconstituted plasma membrane vesicles using a numerical model of oxonol VI distribution

A model of membrane potential-dependent distribution of oxonol VI to estimate the electrical potential difference deltapsi across Schizosaccharomyces pombe plasma membrane vesicles (PMV) has been developed. deltapsi was generated by the H+-ATPase reconstituted in the PMV. The model treatment was necessary since the usual calibration of the dye fluorescence changes by diffusion potentials (K+ + valinomycin) failed. The model allows for fitting of fluorescence changes at different vesicle and dye concentrations, yielding deltapsi in ATP-energized PMV of 80 mV. The described model treatment to estimate deltapsi may be applicable for other reconstituted membrane systems.

Slow fluorescent indicators of membrane potential: a survey of different approaches to probe response analysis

Basic tenets related to the use of three main classes of potentiometric redistribution fluorescent dyes (carbocyanines, oxonols, and rhodamines) are discussed in detail. They include the structure/function relationship, formation of nonfluorescent (H-type) and fluorescent (J-type) dimers and higher aggregates, probe partitioning between membranes and medium and binding to membranes and intracellular components (with attendant changes in absorption and emission spectra, fluorescence quantum yield and lifetime). The crucial importance of suitable probe-to-cell concentration ratio and selection of optimum monitored fluorescence wavelength is illustrated in schematic diagrams and possible artifacts or puzzling results stemming from faulty experimental protocol are pointed out. Special attention is paid to procedures used for probe-response calibration (potential clamping by potassium in the presence of valinomycin, use of gramicidin D in combination with N-methylglucamine, activation of Ca-dependent K-channels by A23187, the null-point technique). Among other problems treated are dye toxicity, interaction with mitochondria and other organelles, and possible effects of intracellular pH and the quantity of cytosolic proteins and/or RNA on probe response. Individual techniques using redistribution dyes (fluorescence measurements in cuvettes, flow cytometry and microfluorimetry of individual cells including fluorescence confocal microscopy) are discussed in terms of reliability, limitations and drawbacks, and selection of suitable probes. Up-to-date examples of application of slow dyes illustrate the broad range of problems in which these probes can be used.

An oxonol dye is the most potent known inhibitor of band 3-mediated anion exchange

When cells are acutely exposed to the oxonol dye, bis(1,3-dibutylbarbituric acid)pentamethine oxonol (diBA), at 0 degrees C, the concentration that gives half inhibition of Cl- exchange (IC50) is 0.146 +/- 0.013 microM (n = 12) initially, but the inhibition increases with time. These characteristics indicate that a rapid initial binding is followed by a slow conformational change that makes the binding tighter. If diBA is allowed to equilibrate with band 3, the IC50 is only 1.05 +/- 0.13 nM (n = 5), making diBA a more potent inhibitor than 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), for which the IC50 under similar conditions is 31 +/- 6 nM [T. Janas, P. J. Bjerrum, J. Brahm, and J. O. Wieth. Am. J. Physiol. 257 (Cell Physiol. 26): C601-C606, 1989]. Inhibition by diBA is very slowly reversible at 0 degrees C (t1/2 > 50 h), but the effect is more readily reversible at higher temperatures. DiBA competes with 4,4'-dinitrostilbene-2,2'-disulfonate (DNDS) for inhibition, suggesting an external site of action. In contrast to DIDS and DNDS, however, increasing Cl- concentrations do not decrease the inhibitory effect of diBA, indicating that the inhibition is not competitive. Thus diBA may be useful for investigating conformational changes during anion exchange and for stopping transport without preventing substrate binding. However, when diBA and other oxonols are used to sense membrane potential, they may have undesirable side effects on anion transport processes.

Anomalous response of oxonol-V to membrane potential in mitochondrial proton pumps

The response of the fluorescent membrane potential probe oxonol-V (bis[3-phenyl 5 oxoisoxazol-4-yl]pentamethine oxonol) in submitochondrial particles (SMP) was dependent upon whether the potential (inside positive) was generated by active proton pumps or by valinomycin-aided passive K+ influx. The fluorescence intensity showed a decrease in the former case and an increase in the latter situation. This anomalous behavior was not observed with other similar anionic probes. Gradual inhibition of proton pumping activity showed that the difference in the response of oxonol-V is not due to possible difference in the magnitude of membrane potential generated in these two situations. In the presence of membrane permeant anions such as TPB- (tetraphenyl boron) or chlorate, the direction of response of oxonol-V fluorescence was the same in both situations. Time-resolved fluorescence of the oxonol-V-SMP system showed three populations: one free form (fluorescence lifetime approximately 60 ps) and two SMP-bound forms (lifetimes of 0.45 ns and 1.4 ns). A fourth population was created during the action of proton pumps. The shorter lifetime (approximately 250 ps) of this new bound form suggest this population to be an aggregated form. This population was absent during the action of proton pumps in the presence of TPB- or chlorate. These results suggest the creation of a charge separated state during the action of proton pumps. The decrease in fluorescence intensity could be the result of aggregation of oxonol-V around the positive end of a proton pump existing in a dipole or charge separated state.

Small lipid-soluble cations are not membrane voltage probes for Neurospora or Saccharomyces

Small lipid-soluble cations, such as tetraphenylphosphonium (TPP+) and tetraphenylarsonium (TPA+) are frequently used as probes of membrane voltage (delta psi, or Vm) for small animal cells, organelles, and vesicles. Because much controversy has accompanied corresponding measurements on 'walled' eukaryotic cells (plants, fungi), we studied their transport and relation to Vm in the large-celled fungus Neurospora crassa-where Vm can readily be determined with microelectrodes-as well as in the most commonly used model eukaryotic cell, the yeast Saccharomyces cerevisiae. We found no reasonable conditions under which the distribution of TPP+ or TPA+, between the cytoplasm (i) and extracellular solution (o), can serve to estimate Vm, even roughly, in either of these organisms. When applied at probe concentrations (i.e., < or = 100 microM, which did not depolarize the cells nor deplete ATP), TPP+ stabilized at ratios (i/o) below 30 in both organisms. That would imply apparent Vm values positive to -90 mV, in the face of directly measured Vm values (in Neurospora) negative to -180 mV. When applied at moderate or high concentrations (1-30 mM), TPP+ and TPA+ induced several phases of depolarization and changes of membrane resistance (Rm), as well as depletion of cytoplasmic energy stores. Only the first phase depolarization, occurring within the perfusion-turnover time and accompanied by a nearly proportionate decline of Rm, could have resulted from TPP+ or TPA+ currents per se. And the implied currents were small. Repeated testing, furthermore, greatly reduced the depolarizing effects of these lipid-soluble ions, implicating an active cellular response to decrease membrane permeability.

WW-781, a potent reversible inhibitor of red cell Cl- flux, binds to band 3 by a two-step mechanism

WW-781 ([3-methyl-1-p-sulfophenyl-5-pyrazolone-(4)]-[1,3-dibutylbarbit uric acid]-pentamethine oxonol), a fluorescent dye that has been used for measuring membrane potentials by optical methods, inhibits human red blood cell Cl- exchange, which is mediated by the membrane protein known as band 3 or capnophorin. The inhibition is slowly reversible upon removal of WW-781 from the medium, with a half time of approximately 4.7 min in 150 mM Cl- medium at 0 degrees C. The mechanism of inhibition by WW-781 involves a two-step binding reaction. WW-781 binds rapidly to band 3 to form an initial complex, which can also rapidly dissociate. Formation of this initial complex is followed by the much slower formation of a second complex (with a rate constant of approximately 1.1 min-1), probably involving a protein conformational change, through which WW-781 is more tightly bound to band 3. At low concentrations, WW-781 inhibits Cl- exchange with a stoichiometry of 1 WW-781 molecule per band 3 monomer, suggesting that under these conditions the binding of WW-781 is highly selective for the band 3 protein.

A theoretical description of non-steady-state diffusion of hydrophobic ions across lipid vesicle membranes including effects of ion-ion interactions in the aqueous phase

A theoretical model of hydrophobic ion diffusion across vesicular membranes is presented, which is based upon activated rate theory. The model is applicable to the sudden addition of hydrophobic ions to a vesicle suspension, for example in a stopped-flow experiment. The time course of diffusion is calculated by numerical integration of differential rate equations for the ion concentrations and electrical potential differences across the membrane. The model utilizes the three-capacitor model of the membrane and an extended Debye-Hückel theory, taking into account non-neutrality on each side of the membrane. At low ionic strengths good agreement is found between the infinite time diffusion potential and the equilibrium Nernst potential. At large excess of inert electrolyte discrepancies are found, but under such conditions the membrane potential is negligible due to screening.

An adsorption isotherm for the interaction of membrane-permeable hydrophobic ions with lipid vesicles

An adsorption isotherm is presented which describes the association of membrane-permeable hydrophobic ions with lipid vesicles. The theory is based upon the Langmuir adsorption isotherm, but is has been significantly extended to take into account diffusion of ions across the membrane and dissociation into the intravesicular space as well as electrical effects due to the build up of boundary potentials within the membrane. The boundary potentials are calculated according to the three-capacitor model of the lipid membrane. In contrast to the Gouy-Chapman theory, which is only applicable when the charged group is located directly at the membrane-solution interface, the theory presented here allows the charge to be located at any position within the membrane.