Mitochondrial membrane potential: evidence from studies with a fluorescent probe

Determining the bacterial cell biology of Planctomycetes

Bacteria of the phylum Planctomycetes have been previously reported to possess several features that are typical of eukaryotes, such as cytosolic compartmentalization and endocytosis-like macromolecule uptake. However, recent evidence points towards a Gram-negative cell plan for Planctomycetes, although in-depth experimental analysis has been hampered by insufficient genetic tools. Here we develop methods for expression of fluorescent proteins and for gene deletion in a model planctomycete, Planctopirus limnophila, to analyse its cell organization in detail. Super-resolution light microscopy of mutants, cryo-electron tomography, bioinformatic predictions and proteomic analyses support an altered Gram-negative cell plan for Planctomycetes, including a defined outer membrane, a periplasmic space that can be greatly enlarged and convoluted, and an energized cytoplasmic membrane. These conclusions are further supported by experiments performed with two other Planctomycetes, Gemmata obscuriglobus and Rhodopirellula baltica. We also provide experimental evidence that is inconsistent with endocytosis-like macromolecule uptake; instead, extracellular macromolecules can be taken up and accumulate in the periplasmic space through unclear mechanisms.

Selective Intracellular Delivery of Recombinant Arginine Deiminase (ADI) Using pH-Sensitive Cell Penetrating Peptides To Overcome ADI Resistance in Hypoxic Breast Cancer Cells

Arginine depletion strategies, such as pegylated recombinant arginine deiminase (ADI-PEG20), offer a promising anticancer treatment. Many tumor cells have suppressed expression of a key enzyme, argininosuccinate synthetase 1 (ASS1), which converts citrulline to arginine. These tumor cells become arginine auxotrophic, as they can no longer synthesize endogenous arginine intracellularly from citrulline, and are therefore sensitive to arginine depletion therapy. However, since ADI-PEG20 only depletes extracellular arginine due to low internalization, ASS1-expressing cells are not susceptible to treatment since they can synthesize arginine intracellularly. Recent studies have found that several factors influence ASS1 expression. In this study, we evaluated the effect of hypoxia, frequently encountered in many solid tumors, on ASS1 expression and its relationship to ADI-resistance in human MDA-MB-231 breast cancer cells. It was found that MDA-MB-231 cells developed ADI resistance in hypoxic conditions with increased ASS1 expression. To restore ADI sensitivity as well as achieve tumor-selective delivery under hypoxia, we constructed a pH-sensitive cell penetrating peptide (CPP)-based delivery system to carry ADI inside cells to deplete both intra- and extracellular arginine. The delivery system was designed to activate the CPP-mediated internalization only at the mildly acidic pH (6.5-7) associated with the microenvironment of hypoxic tumors, thus achieving better selectivity toward tumor cells. The pH sensitivity of the CPP HBHAc was controlled by recombinant fusion to a histidine-glutamine (HE) oligopeptide, generating HBHAc-HE-ADI. The tumor distribution of HBHAc-HE-ADI was comparable to ADI-PEG20 in a mouse xenograft model of human breast cancer cells in vivo. In addition, HBHAc-HE-ADI showed increased in vitro cellular uptake in cells incubated in a mildly acidic pH (hypoxic conditions) compared to normal pH (normoxic conditions), which correlated with pH-sensitive in vitro cytotoxicity in hypoxic MDA-MB-231 and human prostate cancer PC3 cells. Together, we conclude that the HBHAc-HE-based peptide delivery offers a useful means to overcome hypoxia-induced resistance to ADI in breast cancer cells, and to target the mildly acidic tumor microenvironment.

Central role of mitochondrial injury in the pathogenesis of acute pancreatitis

Acute pancreatitis is an inflammatory disease with no specific treatment. One of the main reasons behind the lack of specific therapy is that the pathogenesis of acute pancreatitis is poorly understood. During the development of acute pancreatitis, the disease-inducing factors can damage both cell types of the exocrine pancreas, namely the acinar and ductal cells. Because damage of either of the cell types can contribute to the inflammation, it is crucial to find common intracellular mechanisms that can be targeted by pharmacological therapies. Despite the many differences, recent studies revealed that the most common factors that induce pancreatitis cause mitochondrial damage with the consequent breakdown of bioenergetics, that is, ATP depletion in both cell types. In this review, we summarize our knowledge of mitochondrial function and damage within both pancreatic acinar and ductal cells. We also suggest that colloidal ATP delivery systems for pancreatic energy supply may be able to protect acinar and ductal cells from cellular damage in the early phase of the disease. An effective energy delivery system combined with the prevention of further mitochondrial damage may, for the first time, open up the possibility of pharmacological therapy for acute pancreatitis, leading to reduced disease severity and mortality.

Early safety assessment using cellular systems biology yields insights into mechanisms of action

The integration of high-content screening (HCS) readers with organ-specific cell models, panels of functional biomarkers, and advanced informatics is a powerful approach to identifying the toxic liabilities of compounds early in the development process and forms the basis of "early safety assessment." This cellular systems biology (CSB) approach (CellCiphr profile) has been used to integrate rodent and human cellular hepatic models with panels of functional biomarkers measured at multiple time points to profile both the potency and specificity of the cellular toxicological response. These profiles also provide initial insights on the mechanism of the toxic response. The authors describe here mechanistic assay profiles designed to further dissect the toxic mechanisms of action and elucidate subtle effects apparent in subpopulations of cells. They measured 8 key mechanisms of toxicity with multiple biomarker feature measurements made simultaneously in populations of living primary hepatocytes and HepG2 cells. Mining the cell population response from these mechanistic profiles revealed the concentration dependence and nature of the heterogeneity of the response, as well as relationships between the functional responses. These more detailed mechanistic profiles define differences in compound activities that are not apparent in the average population response. Because cells and tissues encounter wide ranges of drug doses in space and time, these mechanistic profiles build on the CellCiphr profile and better reflect the complexity of the response in vivo.

Minimal models of electric potential oscillations in non-excitable membranes

Sustained oscillations in the membrane potential have been observed in a variety of cellular and subcellular systems, including several types of non-excitable cells and mitochondria. For the plasma membrane, these electrical oscillations have frequently been related to oscillations in intracellular calcium. For the inner mitochondrial membrane, in several cases the electrical oscillations have been attributed to modifications in calcium dynamics. As an alternative, some authors have suggested that the sustained oscillations in the mitochondrial membrane potential induced by some metabolic intermediates depends on the direct effect of internal protons on proton conductance. Most theoretical models developed to interpret oscillations in the membrane potential integrate several transport and biochemical processes. Here we evaluate whether three simple dynamic models may constitute plausible representations of electric oscillations in non-excitable membranes. The basic mechanism considered in the derivation of the models is based upon evidence obtained by Hattori et al. for mitochondria and assumes that an ionic species (i.e., the proton) is transported via passive and active transport systems between an external and an internal compartment and that the ion affects the kinetic properties of transport by feedback regulation. The membrane potential is incorporated via its effects on kinetic properties. The dynamic properties of two of the models enable us to conclude that they may represent alternatives enabling description of the generation of electrical oscillations in membranes that depend on the transport of a single ionic species.

Compound MMH01 possesses toxicity against human leukemia and pancreatic cancer cells

MMH01 is a compound isolated from Antrodia cinnamomea. MMH01 markedly inhibited growth of human leukemia U937 and pancreatic cancer BxPC3 cells. It resulted in distinct patterns of cell cycle distribution in U937 (G2/M, sub-G1 and polyploidy) and BxPC3 cells (G0/G1 and sub-G1). The modes of cell death in U937 cells include apoptosis and mitotic catastrophe, whereas apoptosis-associated events or necrosis in BxPC3 cells. Neither mitochondrial membrane permeabilization nor caspase dependence was noted. Proteins involving mitotic catastrophe-associated cell death such as cyclin B1 and checkpoint kinase 2 were activated in U937 cells. Only slight to moderate viability inhibition was noted to human monocytes, the normal counterpart of these myeloid leukemic cells. In conclusion, MMH01 possesses cytotoxicity against human leukemia and pancreatic cancer cells.

Incorporation of transmembrane hydroxide transport into the chemiosmotic theory

A cornerstone of textbook bioenergetics is that oxidative ATP synthesis in mitochondria requires, in normal conditions of internal and external pH, a potential difference (delta psi) of well over 100 mV between the aqueous compartments that the energy-transducing membrane separates. Measurements of delta psi inferred from diffusion of membrane-permeant ions confirm this, but those using microelectrodes consistently find no such delta psi--a result ostensibly irreconcilable with the chemiosmotic theory. Transmembrane hydroxide transport necessarily accompanies mitochondrial ATP synthesis, due to the action of several carrier proteins; this nullifies some of the proton transport by the respiratory chain. Here, it is proposed that these carriers' structure causes the path of this "lost" proton flow to include a component perpendicular to the membrane but within the aqueous phases, so maintaining a steady-state proton-motive force between the water at each membrane surface and in the adjacent bulk medium. The conflicting measurements of delta psi are shown to be consistent with the response of this system to its chemical environment.

Effects on mitochondrial K flux of pH, K concentration, and N-ethyl maleimide

Based on published evidence that cation transport in mitochondria is not significantly dependent on a membrane potential, it is suggested that the process of mitochondrial cation transport may be nonelectrogenic. These experiments focused on the possibility that K+ flux into rat liver mitochondria may be directly coupled, via an energy-linked carrier mechanism, to OH- influx or H+ efflux. The dependence of the unidirectional K+ influx on the external K+ concentration indicates involvement of a saturable mechanism. Increasing the external pH from 7.0 to 8.0 increases the apparent V max of the K+ influx without significantly altering the apparent Km for K+. The pH dependence is greater in the presence of N-ethyl maleimide, a known inhibitor of the mitochondrial Pi/OH- exchange mechanism. N-Ethyl maleimide decreases the apparent V max at pH 7.0 and increases it at pH 8.0. Evidence indicates that both N-ethyl maleimide and a high external Pi concentration may stimulate the K+ influx at alkaline external pH (8.0) by preventing net exchanges between endogenous Pi and external OH-. An apparent first-order dependence of the K+ influx on the external OH- concentration is observed in the presence of N-ethyl maleimide. These results are consistent with a possible role of external OH- as a cosubstrate of the K+ transport mechanism.

Effects of the cyanine dye 3,3'-dipropylthiocarbocyanine on mitochondrial energy conservation

Mitochondrial respiration and oxidative phosphorylation were inhibited by the membrane potential probe 3,3'-dipropylthiocarbocyanine [diS-C3-(5)]. Evidence is presented that suggests that the dye acts as both an inhibitor of electron transport and an uncoupler of oxidative phosphorylation.

Use of dyes to estimate the electrical potential of the mitochondrial membrane

A number of cationic or anionic fluorescent dyes were investigated as possible monitors of the membrane potential of rat liver mitochondria, and giant mitochondria isolated from the liver of mice maintained on a diet containing cuprizone. The fluorescence of four dyes (8-anilino-1-naphthalenesulfonic acid, merocyanine 540, 3,3'-dipropyl-thiocarbocyanine, and bis[1,3-dibutylbarbituric acid-(5)]-pentamethine oxonol) was found to respond appropriately to changes in an apparent K+ diffusion potential. Generally, valinomycin-induced K+ diffusion potentials as calculated using the Nernst equation were used to calibrate the dependence of the fluorescence on the membrane potential. The appropriateness of this approach was verified for two dyes using microelectrodes in giant mitochondria. The apparent membrane potential change induced by the addition of succinate was variable but was very low and generally less than 60 mV in magnitude. The results are consistent with the notion that a large membrane potential is not established upon the initiation of metabolism and that the membrane potential does not play a significant role in the observed ADP phosphorylation.

A quantitative resolution of the spectra of a membrane potential indicator, diS-C3-(5), bound to cell components and to red blood cells

Cationic cyanine dyes have been widely used to measure electrical potentials of red blood cells and other membrane preparations. A quantitative analysis of the binding of the most extensively studied of these dyes, diS-C3-(5), to red blood cells and their constituents is presented here. Absorption spectra were recorded for the dye in suspensions of isolated red cell membranes and in solutions of cell lysate. The dependence of the spectra on the concentrations of dye and cell constituents shows that the dye binds to these membranes as monomers with an absorbance maximum at 670 nm instead of 650 nm as for free aqueous dye and the dye binds to oxyhaemoglobin partly as monomer but primarily as dimer, with absorbance maxima ca. 670 and 595 nm, respectively. Quantitative estimates are derived for all binding constants and extinction coefficients. These estimates are applied to suspensions of whole cells to predict the dye binding, absorbance spectra, and calibration curves of binding and fluorescence vs. membrane voltage. Satisfactory agreement is found with binding and absorbance data for whole cells at zero membrane potential and with the binding and fluorescence data reported by Hladky and Rink (J. Physiol. (London) 263:287, 1976) for cells driven to positive and negative potentials using valinomycin. The marked tendency of oxyhaemoglobin to bind dye as dimer is not shared by some other proteins tested, including deoxyhaemoglobin and oxymyoglobin.

Optical probes of membrane potential

There are two basically different mechanisms for the fluorescence and absorption changes of merocyanine, cyanine and oxonol dyes. The permeant dyes (cyanine and oxonol dyes, with delocalized charges) work by a potential-dependent accumulation mechanism. These dyes show large (up to 80%) fluorescence and absorption changes with suspensions of cells, and the changes are complete in seconds. The impermeant dyes (merocyanine dyes, with localized charges) and the permeant dyes also show optical changes that take place in fractions of milliseconds. The rapid optical changes are relatively small (less than or equal to 5 X 10(-3)) but can often be easily detected in experiments with single cells. The rapid, nonaccumulative, optical changes result from membrane-localized dye movements. Cyanine dye-absorption changes occur because of a potential-dependent partition of dye between the membrane and the adjacent aqueous region at the high dye-concentration side of the membrane. Dimers and larger aggregates are formed in the aqueous region during the change. Merocyanine dyes may also work by the same mechanism. DiS-C3-(5) is presently the best dye for measuring membrane potentials of cells, organelles, and vesicles in suspension, but several other cyanines work nearly as well (P.J. Sims, A.S. Waggoner, C.-H. Wang, J.F. Hoffman, Biochemistry 13:3315, 1974). For each system, the ratio of dye to membrane must be varied until the optimum fluorescence change is found. A separate calibration curve must be obtained for each system. For measuring fluorescence and/or absorption changes in single cells, merocyanine 540 and diBA-C4-(5) work well but produce some photodynamic damage with high intensity illumination. A rhodanine merocyanine (WW-375) gives very large absorption changes and does not damage tissue during strong illumination. As the mechanisms of the optical changes are worked out, it should be possible to design and synthesize more sensitive, less toxic dyes that are easier to calibrate. And, as the mechanisms of the optical changes are worked out, these dyes may be useful for studying the structure and dynamics of excitable membranes.

Light-induced membrane potential and pH gradient in Halobacterium halobium envelope vesicles

Illumination of envelope vesicles prepared from Halobacterium halobium cells causes translocation of protons from inside to outside, due to the light-induced cycling of bacteriorhodopsin. This process results in a pH gradient across the membranes, an electrical potential, and the movements of K+ and Na+. The electrical potential was estimated by following the fluorescence of a cyanine dye, 3,3'-dipentyloxadicarbocyanine. Illumination of H. halobium vesicles resulted in a rapid, reversible decrease of the dye fluorescence, by as much as 35%. This effect was not seen in nonvesicular patches of purple membrane. Observation of maximal fluorescence decreases upon ilumination of vesicles required an optimal dye/membrane protein ratio. The pH optimum for the lightinduced fluorescence decrease was 6.0. The decrease was linear with actinic light intensity up to about 4 X 10(5) ergs cn-2 s-1. Valinomycin, gramicidin, and triphenylmethylphosphonium ion all abolished the fluorescence changes. However, the light-induced pH change was enhanced by these agents. Conversely, buffered vesicles showed no pH change but gave the same or larger fluorescence changes. Thus, we have identified the fluorescence decrease with a light-induced membrane potential, inside negative. By using valinomycin-K+-induced membrane potentials, we calibrated the fluorescence decrease with calculated Nernst diffusion potentials. We found a linear dependence between potential and fluorescence decrease of 3 mV/%, up to 90 mV. When the envelope vesicles were illuminated, the total proton-motive force generated was dependent on the presence of Na+ and K+ and their concentration gradients across the membrane. In general, K+ appeared to be more permeable than Na+ and, thus, permitted development of greater pH gradients and lower electrical potentials. By calculating the total proton-motive force from the sum of the pH and potential terms, we found that the vesicles can produce proton-motive forces near--200 mV.

The state of energization of the membrane of Escherichia coli as affected by physiological conditions and colicin K

The bacterial protein colicin K, when added to sensitive Escherichia coli in the presence of 3,3'-dihexyloxacarbocyanine, cuases a doubling in fluorescence of the probe. Glucose and oxygen cause a decreased fluorescence while anoxia and cyanide cause a rise in fluorescence. These results in conjunction with the work of other laboratories suggest that colicin K causes a depolarization of the transmembrane electrical potential. Fluorescence in the absence of colicin K was relatively independent of KCl, NaCl, and MgCl2 concentrations below 0.1 M. Although colicin K caused rapid efflux of the K+ analogue 86Rb+, the fluorescence rise was only partially blocked by 0.13 M KCl. The level of fluorescence caused by the action of colcin K was inversely proportional to the logarithm of the concentration of MgCl2 over the range of 2 muM to 4 mM. This suggests that a Nernst electrochemical potential for an anion can counteract a membrane depolarization caused by colcin. After colcin K action, the fluorescence of the carbocyanine could be further increased by anoxia or cyanide. The distribution of the weak base dimethyloxazolidinedione indicated that the pH in the interior of aerobic E. coli supplied with lactate was alkaline by 0.1 unit and unaffected by colicin. These results suggest that colicin K does not completely depolarize the membrane potential and does not interfere with the component of membrane energization generated by electron transport. Colicin K does not act as a cationophore. The partial depolarization of the membrane may account for the inhibition of active solute transport caused by colicin K.

The application of fluorescent probes in membrane studies

Fluorescence has been used in biochemical studies for many years but only recently has the information content and the practical applicability of the fluorescence method been fully realized.

Following the early studies of Newton (1954) and Weber (11954) and after the initial utilization of fluorescent probes by Chance and coworkers (Azziet al.1969) and Tasakiet al.(1968), in the study of membranes, the use of fluorescence to provide structural information at microscopic or molecular levels in biological membranes has become widespread. widespread. The application of the fluorescence technique to biological systems has progressed parallel to the development of a theoretical basis for fluorescence data interpretation and the synthesis of a large number of fluorescent probes, organic molecules having fluorescence characteristics that are dependent on their environment.

Donnan potential of rabbit skeletal muscle myofibrils I: electrofluorochromometric detection of potential

The fluorescence of the dye CC-6 [(3-hexyl-2-(3-hexyl-2-benzoxazolinylidene)-1-propenyl)-benzoxazolium iodide] has been shown to indicate Donnan potentials in rabbit skeletal muscle myofibrils. These results are in agreement with previously published work in which the potentials were measured with microelectrodes on glycerol-extraced muscle fibers. The magnitude of the Donnan potential of the myofibrils has been shown to be dependent on the state (rigor or relaxed) of the system.

The effect of azide on phototaxis in Chlamydomonas reinhardi

Phototaxis in Chlamydomonas reinhardi was specifically inhibited by azide. The effect of azide was rapid and reversible, and did not depend upon the intensity of actinic light. Under conditions of completely inhibited phototaxis, azide had no effect on the number of motile cells in the population or on the rate of motility. The effect was not related to changes in oxygen uptake or cellular ATP concentration. Apparently, a cellular component or process specifically involved in phototaxis is inactivated by azide.