A novel method for measuring intracellular pH and potassium concentration

Pyroptosis is driven by non-selective gasdermin-D pore and its morphology is different from MLKL channel-mediated necroptosis

Necroptosis and pyroptosis are two forms of programmed cell death with a common feature of plasma membrane rupture. Here we studied the morphology and mechanism of pyroptosis in comparison with necroptosis. Different from necroptosis, pyroptosis undergoes membrane blebbing and produces apoptotic body-like cell protrusions (termed pyroptotic bodies) prior to plasma membrane rupture. The rupture in necroptosis is explosion-like, whereas in pyroptosis it leads to flattening of cells. It is known that the execution of necroptosis is mediated by mixed lineage kinase domain-like (MLKL) oligomers in the plasma membrane, whereas gasdermin-D (GSDMD) mediates pyroptosis after its cleavage by caspase-1 or caspase-11. We show that N-terminal fragment of GSDMD (GSDMD-N) generated by caspase cleavage also forms oligomer and migrates to the plasma membrane to kill cells. Both MLKL and GSDMD-N are lipophilic and the N-terminal sequences of both proteins are important for their oligomerization and plasma membrane translocation. Unlike MLKL which forms channels on the plasma membrane that induces influx of selected ions which osmotically swell the cells to burst, GSDMD-N forms non-selective pores and does not rely on increased osmolarity to disrupt cells. Our study reveals the pore-forming activity of GSDMD and channel-forming activity of MLKL determine different ways of plasma membrane rupture in pyroptosis and necroptosis.

A practicable and accurate method to differentiate between intra- and extracellular water of microbial cells

A thermographimetric method which allows for a quick and accurate estimation of intra- and extracellular water of microbial cells is reviewed and improved. Knowledge of these fractions is important for physiological as well as for toxicological investigations. Results of the study indicate that besides the species, nutrient availability and growth conditions affect the intracellular water content. Intra- and extracellular water, dry matter, volume and density of a single cell of Arthrobacter sp. are calculated. There are indications that intracellular compartments of eukaryotes could also be investigated with this method.

Influence of pH on the uptake of 5-fluorouracil into isolated tumour cells

To investigate the possible dependence of 5-fluorouracil (5FU) uptake in tumours on the intra- (pHi) and extracellular (pHe) pH, a pH gradient (deltapH) was imposed across the plasma membrane of ascites tumour cells in vitro, similar to that known to occur in some solid tumours in vivo, by incubation in media of PHe 5-8. A > or = 2:1 (intracellular/extracellular) accumulation of radiolabelled 5FU occurred after 5 min incubation of the cells with 0.5 mM 5FU at pHe of 5.0, 5.5 or 6.0. 5FU metabolism is slow under these conditions, and 5FU uptake was not affected by longer incubations up to 20 min, nor by the absence of a sodium gradient. pHi was estimated from the distribution of the weak acid, 5.5-dimethyl-2,4-oxazolidione ([14C]DMO) across the cell membrane. There was significant correlation between the intracellular/extracellular 5FU ratio and pHe (from pHe 6-8), deltapH and pHi (P < 0.02). Similar results were obtained with HT29 cells. Incubation with a drug that made plasma membranes permeable to H+ significantly decreased 5FU uptake in Lettre cells. The co-transport of 5FU may occur on a proton symport using the proton motive force of the deltapH.

A conserved tryptophan in pneumolysin is a determinant of the characteristics of channels formed by pneumolysin in cells and planar lipid bilayers

Pneumolysin is one of the family of thiol-activatable, cytolytic toxins. Within these toxins the amino acid sequence Trp-Glu-Trp-Trp is conserved. Mutations made in this region of pneumolysin, residues 433-436 inclusive, did not affect cell binding or the formation of toxin oligomers in the target cell membrane. However, the mutations did affect haemolysis, leakage of low-molecular-mass metabolites from Lettre cells and the induction of conductance channels across planar lipid bilayers. Of eight modified pneumolysins examined, Trp-433-->Phe showed the smallest amount of haemolysis or leakage (less than 5% of wild type). Pneumolysin-induced leakage from Lettre cells was sensitive to inhibition by bivalent cations but the extent of inhibition varied depending on the modification. Leakage by the mutant Trp-433-->Phe was least sensitive to cation inhibition. The ion-conducting channels formed across planar lipid bilayers exhibit small (less than 30 pS), medium (30 pS-1 nS) and large (more than 1 nS) conductance steps. Small- and medium-sized channels were preferentially closed by bivalent cations. In contrast with wild-type toxin, which formed predominantly small channels, the modified toxin Trp-433-->Phe formed large channels that were insensitive to cation-induced closure. Polysaccharides of molecular mass more than 15 kDa inhibited haemolysis by wild-type toxin, but polysaccharide of up to 40 kDa did not prevent haemolysis by Trp-433-->Phe. Electron microscopy revealed that Trp-433-->Phe formed oligomeric arc and ring structures with dimensions identical with those of wild-type toxin, and that the ratio of arcs to rings formed was the same for wild-type toxin and the Trp-433-->Phe variant. We conclude that the change Trp-433-->Phe affects channel formation at a point subsequent to binding to the cell membrane and the formation of oligomers, and that the size of arc and ring structures revealed by electron microscopy does not reflect the functional state of the channels.

Perturbation of host-cell membrane is a primary mechanism of HIV cytopathology

Cytopathic viruses injure cells by a number of different mechanisms. The mechanism by which HIV-1 injures T cells was studied by temporally examining host-cell macromolecular syntheses, stages of the cell cycle, and membrane permeability following acute infection. T cells cytopathically infected at an m.o.i. of 1-5 grew normally for 24-72 hr, depending on the cell line, followed by the first manifestation of cell injury, slowing of cell division. At that time significant amounts of unintegrated HIV DNA and p24 core protein became detectable, and acridine orange flow cytometric cell cycle studies demonstrated the presence of fewer cells in the G2/M stage of the cell cycle. There was no change in the frequency of cells in the S-stage, and metabolic pulsing with radioactive precursors demonstrated that host-cell DNA, RNA, and protein syntheses were normal at that time and normal up to the time cells started to die (approximately 24 hr later), when all three decreased. Cellular lipid synthesis, however, was perturbed when cell multiplication slowed, with phospholipid synthesis reduced and neutral lipid synthesis enhanced. Permeability of the host-cell membrane to small molecules, such as Ca2+ and sucrose, was slightly enhanced early postinfection, and by the time of slowing of cell division, host membrane permeability was greatly increased to both Ca2+ and sucrose (Stokes radius 5.2 A) but not to inulin (Stokes radium 20 A). These changes in host-cell membrane permeability and phospholipid synthesis were not observed in acutely infected H9 cells, which are not susceptible to HIV cytopathology. Thus, HIV-1 appeared to predominantly injure T cells by perturbing host-cell membrane permeability and lipid synthesis, which is similar to the cytopathic mechanisms of paramyxoviruses.

Divalent cation-sensitive pores formed by natural and synthetic melittin and by Triton X-100

Leakage of ions and low-molecular-weight metabolites from Lettre cells is induced by synthetic melittin, as effectively as by melittin isolated from bee venom; in each case leakage is inhibited by Ca2+, Zn2+ or H+. Inhibition of leakage by divalent cations is reversible in that Lettre cells incubated with melittin (or with Triton X-100) in the presence of inhibitory amounts of Zn2+, when freed of Zn2+ by EGTA or by centrifugation, begin to leak (in Zn2(+)-sensitive manner). Electrorotation of Lettre cells is altered by melittin, compatible with membrane permeabilization; melittin plus Zn2+ does not alter electrorotation until Zn2+ (and unbound melittin) are removed. Melittin or Triton X-100 added to calcein-loaded liposomes induces leakage of calcein; divalent cations inhibit. Energy transfer between liposome-associated melittin and 2-, 7- or 12-(9-anthroyloxy)stearate (AS) is maximal with 12-AS; addition of Zn2+ has little effect. Circular dichroism spectra of melittin plus liposomes are unaffected by Zn2+. These results show that the formation of divalent cation-sensitive pores is not dependent on the presence of endogenous membrane proteins and that the action of divalent cations is not by displacement of melittin (or Triton) from the lipid bilayer.

Plasma membrane potential of lymphocytes from ataxia telangiectasia patients

The plasma membrane potential of lymphocytes prepared from ataxia telangiectasia (AT) patients and normal subjects was assessed using the optical indicator bis-(3-phenyl-5-oxoisoxazol-4-yl) pentamethineoxonol (oxonol-V). AT lymphocytes had a potential of -46 +/- 9 mV and normal lymphocytes had a potential of -63 +/- 4 mV. The intracellular cation content (Na+ and K+) of AT and normal lymphocytes was similar. AT and normal lymphocytes were both depolarized by extracellular K+ and to a similar extent. This study indicates that one feature characterizing ataxia telangiectasia is a modification of the ability of the lymphocyte cell membrane to sustain a normal membrane potential.

Protection of cells against membrane damage by haemolytic agents: divalent cations and protons act at the extracellular side of the plasma membrane

The protective effect of Ca2+, Zn2+ and H+ against membrane damage induced by different haemolytic agents has been studied by measuring monovalent cation leakage and haemolysis of erythrocytes, and phosphoryl[3H]choline and adenine nucleotide leakage from Lettre cells prelabelled with [3H]choline. The protective effect of Ca2+ and Zn2+ on erythrocytes damaged by Staphylococcus aureus alpha-toxin, Sendai virus or melittin is unaffected by the addition of A23187, even though this ionophore greatly increases the uptake of 45Ca2+ or 65Zn2+. The same result has been found for the protective effect of Zn2+ on Lettre cells damaged by S. aureus alpha-toxin, Sendai virus, melittin or Triton X-100. Leakage of phosphoryl[3H]choline from prelabelled Lettre cells is inhibited if extracellular pH is lowered; lowering the intracellular pH without affecting the extracellular pH, affords little protection. It is concluded that Ca2+, Zn2+ and H+ protect cells against membrane damage induced by haemolytic agents by an action at the extracellular side of the plasma membrane.

Membrane potential and cation content of osteoblast-like cells (UMR 106) assessed by fluorescent dyes

A number of cellular functions have recently been associated with alterations of the membrane potential in non-excitable cells. To assess the electrophysiologic regulation of osteoblast function, a method for measuring the membrane potential (Em) of a rat osteogenic sarcoma cell line (UMR 106) by the voltage-sensitive oxonol dye di-BA-C4(3) was developed. The fluorescent signal of di-BA-C4(3) was calibrated through a null point method using the protonophore FCCP. At null point, Em is equivalent to H+ equilibrium potential, and may be calculated by the Nernst equation. Intracellular pH (pHi) changes induced by the protonophore were monitored using BCECF, a pH-sensitive fluorescent probe. In the presence of FCCP, intracellular pH was found to be linearly correlated to extracellular pH (pHo). Therefore, the value of pHi at null point was extrapolated as well. With this technique, we estimated the plasma membrane potential of the "putative" rat osteoblasts (UMR 106) as -28.3 +/- 4.0 mV (n = 10). This method corrected the 16% overestimation of Em derived from the assumption that pHi does not change during the calibration procedure, as described in previous studies employing pH null point techniques. With null point methods, using BCECF and the carboxylic ionophores nigericin and monensin, intracellular concentrations of potassium and sodium were also measured and found to be 125 +/- 0.7 mM (n = 3) and 24 +/- 5.3 mM (n = 3), respectively. Although the Em of UMR 106 cells was dependent on extracellular potassium concentration, these cells did not behave as a potassium electrode. The sodium/potassium permeability ratio, calculated by the Goldman equation, was estimated at 0.317. This high membrane permeability to sodium may contribute to the genesis of the low plasma membrane potential of UMR 106 cells.

Plasma membrane potential of neutrophils generated by the Na+ pump

The plasma membrane potential of human neutrophils was monitored using the anionic dye oxonol-V. The cells maintain a potential of -75 +/- 17 mV when suspended in physiological saline solutions. The cells are scarcely depolarized by extracellular K+ and the depolarization induced by the chemotactic peptide fMet-Leu-Phe is of similar magnitude for cells suspended in 5 or 155 mM K+. Neutrophils are, however, depolarized by suspension in K+-free media or after treatment with ouabain. Neutrophils catalyse Na+-H+ exchange and possess other electroneutral ion transport systems. We propose that the neutrophil membrane potential is generated by an electrogenic Na+ pump, that osmotic stability is achieved by electroneutral ion transport systems and that electrical stability is maintained by anion leakage. Similar mechanisms may also operate in other biological membranes.

Oxonol dyes as monitors of membrane potential: the effect of viruses and toxins on the plasma membrane potential of animal cells in monolayer culture and in suspension

Optical indicators of the cationic, cyanine and anionic oxonol classes were used to evaluate the plasma membrane potential of animal cells in suspension and in monolayer culture. The optical signals were calibrated by using diffusion potentials either of K+ (in the presence of valinomycin) or of H+ (in the presence of carbonyl cyanide p-trifluoromethoxyphenylhydrazone; FCCP); both classes of dye gave similar values of plasma membrane potential, in the range -40 to -90 mV for different cell types. Addition of haemolytic Sendai virus or Staphylococcus aureus alpha-toxin depolarizes cells and causes them to leak monovalent cations; these effects are antagonized by extracellular Ca2+. Cells infected with vesicular stomatitis or Semliki Forest virus become depolarized during an infectious cycle; infection with other viruses was without affect on plasma membrane potential.

Sequential onset of permeability changes in mouse ascites cells induced by Sendai virus

The addition of haemolytic Sendai virus to cells induces membrane changes in the following sequence: (i) Increased permeability to ions, (ii) increased permeability to low molecular weight metabolites, (iii) increased permeability to proteins. The consequences of an increased permeability to ions are: (a) alteration of membrane potential, (b) net changes in intracellular cations and (c) cell swelling, in that order. Depending on virus: cell ratio, Ca2+ concentration and temperature, it is possible to observe ion leakage without metabolite or protein leakage, and ion and metabolite leakage without protein leakage. A model for the induction of permeability changes is presented.

Chemiluminescence in neutrophils and Lettré cells induced by myxoviruses

Luminol-mediated chemiluminescence in neutrophils is stimulated by Sendai virus and by influenza virus; Lettré cells also exhibit chemiluminescence (less than 10% of that of neutrophils), which is stimulated by Sendai virus and by influenza virus. Virally induced permeability changes are not responsible for chemiluminescence, since (i) extracellular Ca2+ inhibits permeability changes but stimulates chemiluminescence, and (ii) influenza virus, which induces permeability changes at pH 5.3 but not at pH 7.4, induces chemiluminescence at either pH. Other agents [zymosan, N-formyl-L-methionyl-L-leucyl-L-phenylalanine, 4-phorbol 12-myristate 13-acetate (phorbol ester), A23187] likewise induce chemiluminescence in the absence of permeability changes.

The effect of Ca2+ on virus-cell fusion and permeability changes

Sendai virus-mediated permeability changes in Lettre cells or red blood cells are affected by extracellular Ca2+ in the following way: the lag period to onset of permeability changes is lengthened and the subsequent extent of leakage is reduced. Ca2+ neither stimulates nor inhibits fusion of the viral envelope to the plasma membrane of Lettre cells or red blood cells. It is concluded that Ca2+ protects cells against virally-induced permeability changes in a manner not involving membrane fusion.

Plasma membrane potential of Lettré cells does not depend on cation gradients but on pumps

The plasma membrane potential of Lettré cells has been determined with the optical indicator oxonol-V and found to be -57 mV at 37 degrees C (range -20 to -80 mV depending on the physiological condition of the cells). Increasing extracellular K+ does not depolarize cells: even in the presence of 155 mM K+ the potential is -41 mV; membrane potential is also insensitive to the chemical gradient of Na+, Mg2+, Ca2+ or Cl-. Ouabain depolarizes the cells; H+ efflux from cells is stimulated by extracellular Na+. We propose that in Lettré cells the plasma membrane potential is generated by electrogenic cation pumps. The balancing fluxes of Na+ and K+ are mainly through electroneutral cation exchanges (Na+/K+ and Na+/H+) and the magnitude of the potential is limited by organic anion leaks. Such a mechanism may operate in other biological membranes also.

Effect of Ca2+-antagonists on virally-induced cell-permeability changes

Sendai virus-mediated permeability changes in cells are affected by extracellular Ca2+ or Mn2+ as follows: the lag period to onset of permeability changes is lengthened and the subsequent extent of leakage is reduced. Drugs that block Ca2+ action in excitable cells, such as verapamil and prenylamine, and drugs that inhibit the action of calmodulin, such as trifluoperazine and R24571, have an effect opposite to that of Ca2+: lag is shortened and extent of leakage is increased. The concentration at which either type of drug shows 50% of maximal effect is similar to the concentration at which 50% of binding by drug to calmodulin is achieved. It is concluded that calmodulin may be involved in protecting cells against virally-mediated membrane damage; alternatively the action of calmodulin-binding drugs may not be as specific as currently thought.

Common action of certain viruses, toxins, and activated complement: pore formation and its prevention by extracellular Ca2+

Haemolysis by Sendai virus, alpha-toxin, and activated complement is inhibited by high concentrations of divalent cations. In Daudi cells, sublytic amounts of these agents induce the following changes: collapse of surface membrane potential, uptake of Na+ and loss of K+ from cells, and leakage of phosphorylated metabolites from cells. The changes induced by Sendai virus and complement are sensitive to physiological concentrations of extracellular Ca2+. It is concluded that fluctuations in plasma Ca2+ concentration may affect the damaging action of certain pore-forming agents on susceptible cells.