Factors and processes involved in membrane potential build-up in yeast: diS-C3(3) assay

Effect of preliminary stresses on the induction of viable but non-culturable Escherichia coli O157:H7 NCTC 12900 and Staphylococcus aureus ATCC 6538

As a novel non-thermal pasteurization technology, high pressure carbon dioxide (HPCD) has been used in food processing. However, it could induce microorganisms into a viable but nonculturable (VBNC) state, posing a potential risk to food safety and public health. In this study, we attempted to investigate the effect of various preliminary stresses including cold, heat, osmosis, acidity and oxidation on HPCD-induced VBNC formation. The results indicated that there was no effect of preliminary stresses on VBNC Staphylococcus aureus induction. However, heat, acidity and long-term (24 h) cultivation preadaptation could significantly increase the VBNC E. coli production induced by HPCD. Transcriptome analysis revealed that genes involved in ATP production were significantly decreased in these three stress-treated cells, and further ATP levels determination revealed that the ATP levels of the cell were significantly decreased after heat, acidity and long-term cultivation preadaptation, implying the decrease of ATP level caused by these stresses might be the reason for increasing VBNC production. To further study the relationship between ATP level and VBNC cell ratios after preadaptation. We artificially decreased the ATP levels, and detect their VBNC ratios after HPCD treatment. We found that with the ATP concentration decreasing after exposure to carbonyl cyanide m-chlorophenyl hydrazine (CCCP), the VBNC ratios were increased after HPCD treatment, indicating that the ATP contents were highly negatively correlated with VBNC ratios. This study proved that the preadaptation of heat, acidity and long-term cultivation could promote VBNC induction by decreasing the intracellular ATP level. In general, the obtained result gave the instruction about the storage environment for food materials, helped to further develop and optimize the HPCD processing to prevent VBNC formation and accelerate the development of HPCD technology in food industry.

Extracellular pH decline introduced by high pressure carbon dioxide is a main factor inducing bacteria to enter viable but non-culturable state

High pressure carbon dioxide (HPCD) has been used in food processing as a non-thermal pasteurization technology. However, the potential of HPCD to induce viable but non-culturable (VBNC) cells limits its application. The objective of this study was to explore the roles of extracellular pH of 3 (pH_ex3) and high pressure (HP) of HPCD during VBNC induction and the underlying molecular mechanism. By using the model organism Escherichia coli O157:H7, we found that the combined effects of pH_ex3 and HP could mimic the effect of HPCD for VBNC induction. Further investigation of the individual effect of pH_ex3 and HP on VBNC induction revealed that pH_ex3 could induce a higher proportion of VBNC cells with a slower induction rate compared with HPCD, whereas HP was unable to induce VBNC formation. Notably, the cells treated by pH_ex3 and HPCD had similar morphological changes, and VBNC cells induced by pH_ex3 and HPCD had similar stress resistance characteristics. These results strongly indicated that pH_ex3 introduced by HPCD was a main factor for VBNC induction. Additionally, we found that HP played the role in accelerating VBNC formation in the process of HPCD treatment. Transcriptomic analysis revealed 85, 263 and 529 differentially expressed genes (DEGs) for HP-, pH_ex3- and HPCD-treated cells compared with untreated ones. 59 DEGs shared by pH_ex3 and HPCD treatment might be responsible for VBNC induction, and they were mainly involved in cellular transport and localization.

Whole-Genome Approach to Understanding the Mechanism of Action of a Histatin 5-Derived Peptide

The incidence of opportunistic fungal infections that threaten immunocompromised patients, along with the limited arsenal of antifungal drugs, calls for renewed efforts to develop novel antifungal therapies. Antimicrobial peptides have garnered interest as potential therapeutics. Among naturally occurring peptides, histatin 5 is a well-characterized 24-amino-acid peptide with strong antifungal activity. Our lab has identified a smaller histatin derivative, KM29, with stronger activity against multiple Candida spp., prompting us to investigate its fungicidal mechanism. A genetic screen was developed to test the Saccharomyces cerevisiae genomewide deletion collection for mutants with increased or decreased peptide sensitivity. The goal was to identify genes that would reveal insights into the mechanism of action of KM29, to be assessed in Candida albicans Several biological processes yielded increased sensitivity, with endosomal transport and vacuolar function appearing at high frequencies. Among the pathways involved in increased resistance, mitochondrial function showed the highest normalized genome frequency; hence, we focused on characterizing this pathway. KM29 localizes to mitochondria, and the killing activity depends on a functional electron transport chain. In addition, KM29 triggered reactive oxygen species (ROS) production, which was responsible for some cell death but insufficient to account for the complete killing activity. In agreement with this finding, we found that KM29 induced mitochondrial fragmentation and a mild loss of mitochondrial membrane potential. Furthermore, respiratory mutants exhibited severely diminished KM29 uptake. We confirmed this behavior in a C. albicans respiratory mutant. Taking our findings together, this work delineates the mitochondrial functions associated with KM29 fungicidal activity and provides additional pathways for further characterization in Candida spp.

Identification of Antifungal H+-ATPase Inhibitors with Effect on Plasma Membrane Potential

The plasma membrane H⁺-ATPase (Pma1) is an essential fungal protein and a proposed target for new antifungal medications. The compounds in a small-molecule library containing ∼191,000 commercially available compounds were screened for their ability to inhibit Saccharomyces cerevisiae plasma membranes containing Pma1. The overall hit rate was 0.2%, corresponding to 407 compounds. These hit compounds were further evaluated for ATPase selectivity and broad-spectrum antifungal activity. Following this work, one Pma1 inhibitor series based on compound 14 and analogs was selected for further evaluation. This compound series was able to depolarize the membrane and inhibit extracellular acidification in intact fungal cells concomitantly with a significant increase in intracellular ATP levels. Collectively, we suggest that these effects may be a common feature of Pma1 inhibitors. Additionally, the work uncovered a dual mechanism for the previously identified cationic peptide BM2, revealing fungal membrane disruption, in addition to Pma1 inhibition. The methods presented here provide a solid platform for the evaluation of Pma1-specific inhibitors in a drug development setting. The present inhibitors could serve as a starting point for the development of new antifungal agents with a novel mode of action.

Detection of inhibitors of Candida albicans Cdr transporters using a diS-C3(3) fluorescence

Candida albicans is a major cause of opportunistic and life-threatening, systemic fungal infections. Hence new antifungal agents, as well as new methods to treat fungal infections, are still needed. The application of inhibitors of drug-efflux pumps may increase the susceptibility of C. albicans to drugs. We developed a new fluorescence method that allows the in vivo activity evaluation of compounds inhibiting of C. albicans transporters. We show that the potentiometric dye 3,3′-dipropylthiacarbocyanine iodide diS-C₃(3) is pumped out by both Cdr1 and Cdr2 transporters. The fluorescence labeling with diS-C₃(3) enables a real-time observation of the activity of C. albicans Cdr1 and Cdr2 transporters. We demonstrate that enniatin A and beauvericin show different specificities toward these transporters. Enniatin A inhibits diS-C₃(3) efflux by Cdr1 while beauvericin inhibits both Cdr1p and Cdr2p.

Complementary Methods of Processing diS-C3(3) Fluorescence Spectra Used for Monitoring the Plasma Membrane Potential of Yeast: Their Pros and Cons

Carbocyanine dye diS-C3(3) was repeatedly employed in monitoring the plasma membrane potential of yeast and other living cells. Four methods of measuring and evaluating probe fluorescence signal were used in different studies, based on following fluorescence parameters: fluorescence intensity emitted within a certain spectral interval, F(580)/F(560) fluorescence emission ratio, wavelength of emission spectrum maximum, and the ratio of respective fluorescence intensities corresponding to the diS-C3(3) bound to cytosolic macromolecules and remaining dissolved in the aqueous cell medium (i.e., unbound, or free). Here we show that data corresponding to the three latter spectral assessments of diS-C3(3) accumulation in cells is mutually convertible, which means that their alternative use cannot lead to ambiguities in the interpretation of the results of biological experiments. On the other hand, experiments based on the effortless measurements of fluorescence intensities should be interpreted cautiously because controversial results can be obtained, depending on the particular choice of cell-to-dye concentration ratio and emission wavelength.

Alcohols are inhibitors of Saccharomyces cerevisiae multidrug-resistance pumps Pdr5p and Snq2p

The effect of alcohols on cell membrane proteins has originally been assumed to be mediated by their primary action on membrane lipid matrix. Many studies carried out later on both animal and yeast cells have revealed that ethanol and other alcohols inhibit the functions of various membrane channels, receptors and solute transport proteins, and a direct interaction of alcohols with these membrane proteins has been proposed. Using our fluorescence diS-C3 (3) diagnostic assay for multidrug-resistance pump inhibitors in a set of isogenic yeast Pdr5p and Snq2p mutants, we found that n-alcohols (from ethanol to hexanol) variously affect the activity of both pumps. Beginning with propanol, these alcohols have an inhibitory effect that increases with increasing length of the alcohol acyl chain. While ethanol does not exert any inhibitory effect at any of the concentration used (up to 3%), hexanol exerts a strong inhibition at 0.1%. The alcohol-induced inhibition of MDR pumps was detected even in cells whose membrane functional and structural integrity were not compromised. This supports a notion that the inhibitory action does not necessarily involve only changes in the lipid matrix of the membrane but may entail a direct interaction of the alcohols with the pump proteins.

3-Hydroxybutyrate methyl ester as a potential drug against Alzheimer's disease via mitochondria protection mechanism

Alzheimer's disease (AD) is induced by many reasons, including decreased cellular utilization of glucose and brain cell mitochondrial damages. Degradation product of microbially synthesized polyhydroxybutyrate (PHB), namely, 3-hydroxybutyrate (3HB), can be an alternative to glucose during sustained hypoglycemia. In this study, the derivative of 3HB, 3-hydroxybutyrate methyl ester (HBME), was used by cells as an alternative to glucose. HBME inhibited cell apoptosis under glucose deprivation, rescued activities of mitochondrial respiratory chain complexes that were impaired in AD patients and decreased the generation of ROS. Meanwhile, HBME stabilized the mitochondrial membrane potential. In vivo studies showed that HBME crossed the blood brain barrier easier compared with charged 3HB, resulting in a better bioavailability. AD mice treated with HBME performed significantly better (p < 0.05) in the Morris water maze compared with other groups, demonstrating that HBME has a positive in vivo pharmaceutical effect to improve the spatial learning and working memory of mice. A reduced amyloid-β deposition in mouse brains after intragastric administration of HBME was also observed. Combined with the in vitro and in vivo results, HBME was proposed to be a drug candidate against AD, its working mechanism appeared to be mediated by various effects of protecting mitochondrial damages.

Fluorescence method for determining the mechanism and speed of action of surface-active drugs on yeast cells

New antifungal agents are needed to treat life-threatening fungal infections, particularly with the development of resistance. Surface-active antifungals have the advantages of minimizing host toxicity and the emergence of drug resistance. We have developed a time-dependent drug exposure assay that allows us to rapidly investigate the mechanism of surface-active antifungal drug action. The assay uses a multidrug pump-deficient strain of Saccharomyces cerevisiae and the potentiometric dye 3,3′-dipropylthiacarbocyanine iodide [diS-C3(3)] and can assess whether cells are depolarized, hyperpolarized, or permeabilized by drug exposure. In this work, we investigated the mechanisms of action of five surface-active compounds: SDS, nystatin, amphotericin B, octenidine dihydrochloride, and benzalkonium chloride. The diS-C3(3) time-dependent drug exposure assay can be used to identify the mechanisms of action of a wide range of drugs. It is a fast and cost-effective method for screening drugs to determine their lowest effective concentrations.

Cell ATP level of Saccharomyces cerevisiae sensitively responds to culture growth and drug-inflicted variations in membrane integrity and PDR pump activity

Cellular ATP level in Saccharomyces cerevisiae was measured during culture growth of strain US50-18C overproducing all major PDR pumps and its isogenic mutants variously deleted in these pumps. It was found to be inversely proportional to the intensity of cell metabolism during different growth phases and to the activity of PDR pumps, which are thus among major ATP consumers in the cells. The ATP level was increased when membrane integrity was affected by 0.5% butanol, and further increased by compound 23.1, a semisynthetic phenol lipid derivative that acts as inhibitor of Pdr5p and Snq2p pumps. The magnitude of increase in cell ATP caused by inhibition of Pdr5p pump by compound 23.1 and the Pdr5p pump inhibitor FK506 used for comparison reflects the activity and hence the energy demand of the pump. The rise in cell ATP caused by different PDR pump inhibitors can be thus used as an indicator of pump activity and the potency of the inhibitor.

Synchronous plasma membrane electrochemical potential oscillations during yeast colony development and aging

Microorganisms that survive in natural environments form organized multicellular communities, biofilms and colonies with specific properties. During stress and nutrient limitation, slow growing and senescent cells in such communities retain vital processes by maintaining plasma membrane integrity and retaining the ability to generate transmembrane electrochemical gradients. We report the use of a Saccharomyces cerevisiae colonial model to show that population growth in a multicellular community depends on nutrient diffusion and that resting cells start to accumulate from the beginning of the second acidic phase of colony development. Despite differentiation of colony members, synchronous transmembrane potential oscillation was detected in the organized colony. The electrochemical membrane potential periodically oscillated at frequencies between those for circadian to infradian rhythms during colony aging and transiently decreased at time points previously linked with rebuilding of yeast metabolism. Despite extensive decreases in the intracellular ATP concentration and in the amount and activity of the plasma membrane proton pump during nutrient limited growth and colony aging, the transmembrane electrochemical potential appeared to be maintained above a level critical for population survival.

Energy depletion protects Candida albicans against antimicrobial peptides by rigidifying its cell membrane

Inhibitors of the energy metabolism, such as sodium azide and valinomycin, render yeast cells completely resistant against the killing action of a number of cationic antimicrobial peptides, including the salivary antimicrobial peptide Histatin 5. In this study the Histatin 5-mediated killing of the opportunistic yeast Candida albicans was used as a model system to comprehensively investigate the molecular basis underlying this phenomenon. Using confocal and electron microscopy it was demonstrated that the energy poison azide reversibly blocked the entry of Histatin 5 at the level of the yeast cell wall. Azide treatment hardly induced depolarization of the yeast cell membrane potential, excluding it as a cause of the lowered sensitivity. In contrast, the diminished sensitivity to Histatin 5 of energy-depleted C. albicans was restored by increasing the fluidity of the membrane using the membrane fluidizer benzyl alcohol. Furthermore, rigidification of the membrane by incubation at low temperature or in the presence of the membrane rigidifier Me(2)SO increased the resistance against Histatin 5, while not affecting the energy charge of the cell. In line, azide induced alterations in the physical state of the interior of the lipid bilayer. These data demonstrate that changes in the physical state of the membrane underlie the increased resistance to antimicrobial peptides.

DiS-C3(3) monitoring of in vivo mitochondrial membrane potential in C. elegans

The mitochondrial respiratory chain plays a crucial role in cellular and organismal health. In addition to being the major source of energy for most cells, mitochondrial respiratory chain function regulates or modulates redox and metabolite homeostasis, apoptosis and the generation of reactive oxygen species. In order to measure the relative in vivo mitochondrial membrane potential of different strains of the nematode, Caenorhabditis elegans, we have developed a fluorescence assay using the cationic, lipophilic carbocyanine dye, diS-C(3)(3). We demonstrate that two complex I-deficient mutants have significantly lower mitochondrial membrane potentials in vivo than wild type animals. Our fluorescence assay will enable us to better dissect and understand the complex phenotypic consequences of mitochondrial dysfunction.

Activity of yeast multidrug resistance pumps during growth is controlled by carbon source and the composition of growth-depleted medium: DiS-C3(3) fluorescence assay

Like other tested wild-type strains (DTXII and IL-125-2B), exponential glucose- and/or fructose-grown cells of Saccharomyces cerevisiae BY4742 exhibit the previously described high activity of Pdr5p and Snq2p pumps (measured as export of the potentiometric fluorescent probe diS-C3(3)). Upon saccharide depletion from the medium the pump activity in these cells, which differ from other strains in having a lower membrane potential, sharply drops to a very low level similar to that found in cells grown on ethanol or glycerol. This negligible pump activity in respiring cells thus appears to have a universal character. Addition of glucose or fructose to respiring BY4742 cells grown to low culture densities restores multidrug resistance pump activity due partly to pump synthesis in pre-existing cells and partly to the high pump activity of newly grown cells; no such pump activity boost occurs when the sugar is added to high-density cultures of ethanol-grown or post-diauxic glucose-grown cells, even if these cultures are diluted to low density by their original growth-depleted medium. A strong sugar-induced increase in pump activity is found solely if respiring cells from high-density cultures are resuspended in fresh YPD or YPE medium before sugar addition. Its absence in respiring cells suspended in growth-depleted medium reflects an as yet unidentified effect of the composition of the growth-exhausted medium (depletion of some components and/or accumulation of extracellular metabolites during yeast growth) on sugar-induced pump activity rise.

Glucose-induced MDR pump resynthesis in respiring yeast cells depends on nutrient level

Glucose addition to a stationary culture of wild-type Saccharomyces cerevisiae BY4742 cells with zero activity of MDR pumps resuspended in a fresh medium causes pump resynthesis (measured as pump-effected diS-C3(3) efflux). In a stationary culture in its original growth medium, this glucose-induced pump resynthesis fails to occur due to depletion of essential nutrients or to extracellular metabolites produced by cells during growth. Direct pump inactivation by metabolites is excluded since exponential cells with high MDR pump activity cultured in a medium with high concentration of extracellular metabolites retain this activity for at least 2 h. The metabolites also do not affect pump synthesis on the level of gene expression as addition of concentrated growth medium or an amino acid mixture to stationary cells in spent growth medium restores glucose-induced pump synthesis. The block of MDR pump synthesis is therefore due to the lack of essential nutrients in spent medium.

A novel pathway determining multidrug sensitivity in Schizosaccharomyces pombe

In this study, we show that a mutation isolated during a screen for determinants of chemosensitivity in S. pombe results in loss of function of a previously uncharacterized protein kinase now named Hal4. Hal4 shares sequence homology to Hal4 and Hal5 in S. cerevisiae, and previous evidence indicates that these kinases positively regulate the major potassium transporter Trk1,2 and thereby maintain the plasma membrane potential. Disruption of this ion homeostasis pathway results in a hyperpolarized membrane and a concomitant increased sensitivity to cations. We demonstrate that a mutation in hal4+ results in hyperpolarization of the plasma membrane. In addition to the original selection agent, the hal4-1 mutant is sensitive to a variety of chemotherapeutic agents and stress-inducing compounds. Furthermore, this wider chemosensitive phenotype is also displayed by corresponding mutants in S. cerevisiae, and in a trk1deltatrk2delta double deletion mutant in S. pombe. We propose that this pathway and its role in regulating the plasma membrane potential may act as a pleiotropic determinant of sensitivity to chemotherapeutic agents.

Impact of the growth phase on the activity of multidrug resistance pumps and membrane potential of S. cerevisiae: effect of pump overproduction and carbon source

The potentiometric fluorescence probe diS-C3(3) is expelled from S. cerevisiae by ABC pumps Pdr5 and Snq2 and can conveniently be used for studying their performance. The activity of these pumps in a strain with wild-type PDR1 allele was shown to drop sharply on glucose depletion from the medium and then again at the end of the diauxic shift when the cells are adapted to growth on respiratory substrates. The presence of the PDR1-3 allele causing pump overproduction prevented this second drop and the pump activity typical for diauxic cells was largely retained. Growth phase-dependent changes of membrane potential measured by the same probe in pump-free mutants included a Deltapsi drop in the late exponential and diauxic growth phase, indicating lowered activity of H+ -ATPase. Suppression of activity of both ABC pumps and H+ -ATPase obviously signifies cell transition to an energy-saving mode. Challenging respiration-adapted cells with glucose showed a novel feature of yeast ABC pumps--a strong dependence of pump activity on the type of the carbon source.

Effect of biocides on S. cerevisiae: relationship between short-term membrane affliction and long-term cell killing

The long-term action of recommended (RC) and near-recommended concentrations of several commercial biocides (Lonzabac 12.100, Genamin CS302D, benzalkonium chloride and 2-phenoxyethanol) on cells of S. cerevisiae wild-type strain DTXII was described using plating tests while short-term effects were determined using the potentiometric fluorescent probe diS-C3(3) that detects both changes in membrane potential and impairment of membrane integrity. A 2-d plating of cells exposed to 0.5xRC of benzalkonium chloride and Genamin CS302D for 15 min showed a complete long-term cell killing, with 2-phenoxyethanol the killing was complete only at 2xRC and Lonzabac caused complete killing at RC but not at 0.5xRC. The diS-C3(3) fluorescence assay performed immediately after a 10-min biocide exposure revealed several concentration-dependent modes of action: Lonzabac at 0.5xRC caused a mere depolarization, higher concentrations causing gradually increasing cell damage; benzalkonium chloride and Genamin CS302D rapidly damaged the membrane of some cells and depolarized the rest whereas 2-phenoxyethanol, which had the lowest effect in the plating test, produced a concentration-dependent fraction of cells with impaired membranes. Cell staining slightly increased during the diS-C3(3) assay; addition of a protonophore showed that part of the remaining undamaged cells retained their membrane potential. Comparison of short-term and long-term data implies that membrane depolarization alone is not sufficient for complete long-term killing of yeast cells under the action of a biocide unless it is accompanied by perceptible impairment of membrane integrity. The results show that the diS-C3(3) fluorescence assay, which reflects the short-term effects of a biocide on cell membranes, can be successfully used to assess the microbicidal efficiency of biocides.

Inhibition of sodium/proton exchange by a Rab-GTPase-activating protein regulates endosomal traffic in yeast

Endosomal Na+/H+ exchangers are important for salt and osmotolerance, vacuolar pH regulation, and endosomal trafficking. We show that the C terminus of yeast Nhx1 interacts with Gyp6, a GTPase-activating protein for the Ypt/Rab family of GTPases, and that Gyp6 colocalizes with Nhx1 in the endosomal/prevacuolar compartment (PVC). The gyp6 null mutant exhibits novel phenotypes consistent with loss of negative regulation of Nhx1, including increased tolerance to hygromycin, increased vacuolar pH, and decreased plasma membrane potential. In contrast, overexpression of Gyp6 increases sensitivity to hygromycin, decreases vacuolar pH, and results in a slight missorting of vacuolar carboxypeptidase Y to the cell surface. We conclude that Gyp6 is a negative regulator of Nhx1-dependent trafficking out of the PVC. Taken together with its GTPase-activating protein-dependent role as a negative regulator of Ypt6-mediated retrograde traffic to the Golgi, we propose that Gyp6 coordinates upstream and downstream events in the PVC to Golgi pathway. Our findings provide a possible molecular link between intraendosomal pH and regulation of vesicular trafficking.

Monitoring the kinetics and performance of yeast membrane ABC transporters by diS-C3(3) fluorescence

Kinetic features (initial start-up phase, drug pumping velocity and efficiency as dependent on drug concentration and growth phase) of yeast plasma membrane multidrug resistance ABC pumps were studied by monitoring the uptake of the fluorescent potentiometric dye diS-C3(3), which has been found to be expelled from the cells by these pumps. The monitoring was done with Saccharomyces cerevisiae mutants AD1-8 and AD1-3 deleted in different ABC pumps, and in their pump-competent parent strain US50-18C overexpressing transcriptional activators Pdr1p and Pdr3p. On addition to the cells, diS-C3(3) is expelled by the Pdr5p, Yor1p and Snq2p pumps with overlapping substrate specificity. The pump action can be assessed as a difference between the dye uptake curve for pump-competent and pump-deleted cells. The pump-mediated dye efflux, which shows an initial lag of various lengths, maintains a certain residual intracellular dye level. In the absence of external glucose the dye efflux ability of the pumps depends on the growth phase; late exponential and stationary cells can maintain the export for tens of minutes, whereas exponential cells keep up the pump action for limited time periods. This may reflect an insufficient number of pump molecules in the membrane or an effect of insufficient pump energization from endogenous sources. This effect is not mediated by changes in membrane potential because lowered membrane potential caused by inhibition of the plasma membrane H+-ATPase does not affect the pump action.

Use of synchronously excited fluorescence to assess the accumulation of membrane potential probes in yeast cells

Evaluation of emission spectra of fluorescent probes used for the monitoring of membrane potential in microbial cells can be greatly facilitated by using synchronously excited spectroscopy (SES). This method permits the suppression of undesirable spectrum components (contributions due to scattered light or cell autofluorescence) and leads to considerable increase in monitored emission intensity and to narrowing of spectral peaks. It allows an efficient fractional decomposition of the probe fluorescence spectra into their free and bound dye fluorescence components. The usefulness of the method was tested by monitoring the accumulation of the fluorescent membrane potential probe diS-C3(3) in yeast cells, which serves as a qualitative measure of the membrane potential.

A high-throughput assay for mitochondrial membrane potential in permeabilized yeast cells

A fluorometric assay for mitochondrial membrane potential in permeabilized yeast cells has been developed. This method involves permeabilizing the plasma membrane and measuring the distribution of a mitochondrial membrane potential sensitive probe 3,3'-dipropylthiadicarbocyanine iodide (DiSC(3)(5); DiSC(3)). In permeabilized cells, DiSC(3) fluorescence decreased when introduced into energized mitochondria and increased three- to sixfold when the mitochondrial membrane potential was dissipated by the chemical uncoupler carbonylcyanide m-chlorophenyl hydrazone. Plasma membrane potential was abolished by permeabilization, as shown by a lack of polarization of the plasma membrane induced by K(+) and glucose. Uncoupling protein 1 (UCP1), a mitochondrial H(+) transporter, was used as a model for method validation. The fluorescence intensity responded vigorously to specific modulators in UCP1-expressing cells. This method has been adapted as a high-throughput assay to screen for modulators of mitochondrial membrane potential.

Factors underlying membrane potential-dependent and -independent fluorescence responses of potentiometric dyes in stressed cells: diS-C(3)(3) in yeast

The redistribution fluorescent dye diS-C(3)(3) responds to yeast plasma membrane depolarisation or hyperpolarisation by Delta psi-dependent outflow from or uptake into the cells, reflected in changes in the fluorescence maximum lambda(max) and fluorescence intensity. Upon membrane permeabilisation the dye redistributes between the cell and the medium in a purely concentration-dependent manner, which gives rise to Delta psi-independent fluorescence responses that may mimic Delta psi-dependent blue or red shift in lambda(max). These lambda(max) shifts after cell permeabilisation depend on probe and ion concentrations inside and outside the cells at the moment of permeabilisation and reflect (a) permeabilisation-induced Delta psi collapse, (b) changing probe binding capacity of cell constituents (inverse to the ambient ionic strength) and (c) hampering of probe equilibration by the poorly permeable cell wall. At low external ion concentrations, cell permeabilisation causes ion outflow and probe influx (hyperpolarisation-like red shift in lambda(max)) caused by an increase in the probe-binding capacity of the cell interior and, in the case of heat shock, protein denaturation unmasking additional probe-binding sites. At high external ion levels minimising net ion efflux and at high intracellular probe concentrations at the moment of permeabilisation, the Delta psi collapse causes a blue lambda(max) shift mimicking an apparent depolarisation.

Effect of killer toxin K1 on yeast membrane potential reported by the diS-C3(3) probe reflects strain- and physiological state-dependent variations

The rate and extent of uptake of the fluorescent probe diS-C3(3) reporting on membrane potential in S. cerevisiae is affected by the strain under study, cell-growth phase, starvation and by the concentration of glucose both in the growth medium and in the monitored cell suspension under non-growth conditions. Killer toxin K1 brings about changes in membrane potential. In all types of cells tested, viz. in glucose-supplied stationary or exponential cells of the killer-sensitive strain S6/1 or a conventional strain RXII, or in glucose-free exponential cells of both strains, both active and heat-inactivated toxin slow down the potential-dependent uptake of diS-C3(3) into the cells. This may reflect "clogging" of pores in the cell wall that hinders, but does not prevent, probe passage to the plasma membrane and its equilibration. The clogging effect of heat-inactivated toxin is stronger than that exerted by active toxin. In susceptible cells, i.e. in exponential-phase glucose-supplied cells of the sensitive strain S6/1, this phase of probe uptake retardation is followed by an irreversible red shift in probe fluorescence maximum lambda max indicating damage to membrane integrity and cell permeabilization. A similar fast red shift in lambda max signifying lethal cell damage was found in heat-killed or nystatin-treated cells.