Effects of nystatin on intracellular contents and membrane transport of alkali cations, and cell volume in HeLa cells

The antimicrobial and immunomodulatory effects of Ionophores for the treatment of human infection

Ionophores are a diverse class of synthetic and naturally occurring ion transporter compounds which demonstrate both direct and in-direct antimicrobial properties against a broad panel of bacterial, fungal, viral and parasitic pathogens. In addition, ionophores can regulate the host-immune response during communicable and non-communicable disease states. Although the clinical use of ionophores such as Amphotericin B, Bedaquiline and Ivermectin highlight the utility of ionophores in modern medicine, for many other ionophore compounds issues surrounding toxicity, bioavailability or lack of in vivo efficacy studies have hindered clinical development. The antimicrobial and immunomodulating properties of a range of compounds with characteristics of ionophores remain largely unexplored. As such, ionophores remain a latent therapeutic avenue to address both the global burden of antimicrobial resistance, and the unmet clinical need for new antimicrobial therapies. This review will provide an overview of the broad-spectrum antimicrobial and immunomodulatory properties of ionophores, and their potential uses in clinical medicine for combatting infection.

Ras Diffusion and Interactions with the Plasma Membrane Measured by FRAP Variations

Ras proteins are non-integral membrane proteins, which bind to the plasma membrane by virtue of farnesylation and palmitoylation or a positively charged polybasic cluster at their C-terminus. Their membrane interactions and/or localization to membrane microdomains, which play important roles in signaling, are regulated by their lateral diffusion at the plasma membrane and their ability to exchange between the membrane and the cytoplasm (binding/unbinding kinetics). Here, using N-Ras as an example, we describe the use of variations of fluorescence recovery after photobleaching (FRAP) to measure the dynamics of the association of N-Ras with the plasma membrane of living cells and their dependence on several parameters (cholesterol, clustering of raft proteins, and palmitoylation/depalmitoylation).

Ivermectin, antiviral properties and COVID-19: a possible new mechanism of action

Ivermectin is an antiparasitic drug that has shown also an effective pharmacological activity towards various infective agents, including viruses. This paper proposes an alternative mechanism of action for this drug that makes it capable of having an antiviral action, also against the novel coronavirus, in addition to the processes already reported in literature.

Osmotic Effects Induced by Pore-Forming Agent Nystatin: From Lipid Vesicles to the Cell

The responses of Chinese hamster ovary epithelial cells, caused by the pore-forming agent nystatin, were investigated using brightfield and fluorescence microscopy. Different phenomena, i.e., the detachment of cells, the formation of blebs, the occurrence of “cell-vesicles” and cell ruptures, were observed. These phenomena were compared to those discovered in giant lipid vesicles. A theoretical model, based on the osmotic effects that occur due to the size-discriminating nystatin transmembrane pores in lipid vesicles, was extended with a term that considers the conservation of the electric charge density in order to describe the cell’s behavior. The increase of the cellular volume was predicted and correlated with the observed phenomena.

Ethanol alters cellular activation and CD14 partitioning in lipid rafts

Alcohol consumption interferes with innate immunity. In vivo EtOH administration suppresses cytokine responses induced through Toll-like receptor 4 (TLR4) and inhibits TLR4 signaling. Actually, EtOH exhibits a generalized suppressive effect on signaling and cytokine responses induced by through most TLRs. However, the underlying mechanism remains unknown. RAW264.7 cells were treated with LPS or co-treated with EtOH or with lipid raft-disrupting drugs. TNF-alpha production, IRAK-1 activation, and CD14 partition were evaluated. EtOH or nystatin, a lipid raft-disrupting drug, suppressed LPS-induced production of TNF-alpha. The suppressive effect of EtOH on LPS-induced TNF-alpha production was additive with that of methyl-beta-cyclodextrin (MCD), another lipid raft-disrupting drug. EtOH interfered with IRAK-1 activation, an early TLR4 intracellular signaling event. Cell fractionation analyses show that acute EtOH altered LPS-related partition of CD14, a critical component of the LPS receptor complex. These results suggest a novel mechanism of EtOH action that involves interference with lipid raft clustering induced by LPS. This membrane action of EtOH might be one of the mechanisms by which EtOH acts as a generalized suppressor for TLR signaling.

Shrinkage-induced protein tyrosine phosphorylation in Chinese hamster ovary cells

To investigate the signal transduction of osmotic stress, we examined hypertonicity-induced tyrosine phosphorylations in Chinese hamster ovary cells. Hyperosmosis elicited characteristic phosphotyrosine accumulation in at least 3 proteins (approximately 42, approximately 85, and approximately 120 kDa). The most prominent response occurred in the 85-kDa band (p85) whose phosphorylation was rapid, sustained, apparent already at mild hypertonicity (350 mosM), proportional to the extracellular osmotic concentration, and reversible. Hyperosmotic environment could not induce tyrosine phosphorylation if cell shrinkage was prevented by nystatin and appropriately composed media. Conversely, isotonic shrinkage caused strong tyrosine phosphorylation. Thus, the initial signal is a decrease in cell volume and not an increase in the intra- or extracellular osmotic concentration, or a rise in cytosolic K+ and Cl- levels. Tyrosine phosphorylation of p85 was not due to the hypertonicity-induced protein kinase C-dependent stimulation of the extracellular signal-regulated protein kinase, nor to the activation of stress-activated protein kinases. Tonicity-responsive proteins interacted with Grb2-glutathione S-transferase fusion proteins: the 120-kDa protein complexed with the SH2 and both SH3 domains, whereas p85 associated with the SH2 and the N-terminal SH3 domains of the adapter. Tyrosine phosphorylation of p85 is a sensitive indicator of reduced intracellular hydration and might signify a hitherto unrecognized, early volume-dependent signaling event.

Glucose-dependent, cAMP-mediated ATP efflux from Saccharomyces cerevisiae

Extracellular ATP plays an important role in the physiology of multicellular organisms; however, it is unknown whether unicellular organisms such as yeast also release ATP extracellularly. Experiments are described here which show that Saccharomyces cerevisiae releases ATP to the extracellular fluid. This efflux required glucose and the rate was increased dramatically by the proton ionophores nigericin, monensin, carbonyl cyanide m-chlorophenylhydrazone and carbonyl cyanide p-(trifluoromethoxy)-phenylhydrazone; ATP efflux was also increased by the plasma membrane proton pump inhibitor diethylstilbestrol. The increase in the concentration of extracellular ATP was not due to cell lysis or general disruption of plasma membrane integrity as measured by colony-forming and methylene-blue-staining assays. ATP efflux was strictly correlated with a rise in intracellular cAMP; therefore, the cAMP pathway is likely to be involved in triggering ATP efflux. These results demonstrate that yeast cells release ATP in a regulated manner.

Flow cytometric determination of absolute membrane potential of cells

Membrane potential measurements using fluorescent membrane potential indicator dyes report on relative changes but usually do not result in an absolute value of the measured parameter. The method developed in this paper is based on the assumption that the negatively charged bis-oxonol distributes across the cytoplasmic membrane according to the Nernst equation. It is further supposed that the fluorescence intensity measured from a given stained cell is a single-value function of the intracellular dye concentration. The protocol suggested incorporates the construction of a calibration curve (fluorescence intensity measured from stained cells vs. extracellular dye concentration). This allows the evaluation of the membrane potential in millivolts using fluorescence readings of the cells both in the depolarized state and in the state of interest. Good agreement was found between absolute membrane potential data of human peripheral blood lymphocytes by our method and results of parallel patch clamp measurements.

Hemisodium, a novel selective Na ionophore. Effect on normal human erythrocytes

Hemisodium is a novel Na ionophore that belongs to the class of compounds called cryptands. These compounds possess an electron-rich cavity for binding of cations and are conformationally organized during synthesis to favor the selective binding of one cation over another. In media containing 145 mM NaCl and 5 mM KCl, hemisodium (10(-5) M) increased erythrocyte Na content from 23 to 345 mmol/kg.dry cell solid (dcs) over 4 h and increased water content from 1.8 to 3.5 liter/kg.dcs over the same period. K content decreased somewhat over the same time period, but this fall in K content was prevented entirely by incubation in either low Na media (to prevent net Na entry) or in Cl free media. Thus, the decrease in K content in high NaCl media was due to cell swelling, which activated KCl cotransport, and not due to a direct action of hemisodium on K permeability. Hemisodium-mediated Na transport was conductive, because erythrocyte membrane potential (Vm), determined by diS-C3-5 fluorescence, changed from -9 to +22 mV in high Na media in the presence of hemisodium and DIDS. In cells equilibrated with sulfamate, an anion with low conductive permeability, Vm changed 54 mV per 10-fold change in external Na concentration with the addition of hemisodium. In contrast, a 10-fold change in the external concentration of K, Rb, Cs, or T1 failed to alter Vm in the presence of hemisodium, suggesting a high Na specificity of the ionophore. Na conductance determined from net fluxes increased from 0.04 to 5.2 microS/cm2 with 10 microM hemisodium, and with that concentration the ratio of Na to K conductance was 45:1. Among the Na ionophores available so far, hemisodium appears to have the greatest specificity. Hemisodium may be a valuable tool in membrane transport studies.

Regulatory changes in the K+, Cl- and water contents of HeLa cells incubated in an isosmotic high K(+)-medium

HeLa cells had their normal medium replaced by an isosmotic medium containing 80 mM K+, 70 mM Na+ and 100 microM ouabain. The cellular contents of K+ first increased and then decreased to the original values, that is, the cells showed a regulatory decrease (RVD) in size. The initial increase was not inhibited by various agents except by substitution of medium Cl- with gluconate. In contrast, the regulatory decrease was inhibited strongly by addition of either 1 mM quinine, 10 microM BAPTA-AM without medium Ca2+, or 0.5 mM DIDS, and partly by either 1 mM EGTA without medium Ca2+, 10 microM trifluoperazine, or substitution of medium Cl- with NO3-. Addition of DIDS to the NO3(-)-substituted medium further suppressed the K+ loss but the effect was incomplete. Intracellular Ca2+ showed a transient increase after the medium replacement. These results suggest that the initial increase in cell K+ is a phenomenon related to osmotic water movement toward Donnan equilibrium, whereas the regulatory K+ decrease is caused by K+ efflux through Ca(2+)-dependent K+ channels. The K+ decrease induced a decrease in cellular water, i.e., RVD. The K+ efflux may be more selectively associated with Cl- efflux through DIDS-sensitive channels than the efflux of other anions.

Cholesterol controls the clustering of the glycophospholipid-anchored membrane receptor for 5-methyltetrahydrofolate

The folate receptor is a glycosyl-phosphatidylinositol (GPI)-anchored membrane protein that mediates the delivery of 5-methyltetrahydrofolate to the cytoplasm of MA104 cells. Ordinarily the receptor is sequestered into numerous discrete clusters that are associated with an uncoated pit membrane specialization called a caveola. By using two different methodological approaches, we found that the maintenance of both receptor clusters and caveolae depends upon the presence of cholesterol in the membrane. These results suggest that cholesterol plays a critical role in maintaining the caveola membrane domain and modulates the interaction of GPI-anchored membrane proteins via their phospholipid anchors.

An electrophysiological technique to measure change in hepatocyte water volume

We have applied an electrophysiologic technique (Reuss, L. (1985) Proc. Natl. Acad. Sci. USA 82, 6014) to measure changes in steady-state hepatocyte volume during osmotic stress. Hepatocytes in mouse liver slices were loaded with tetramethylammonium ion (TMA+) during transient exposure of cells to nystatin. Intracellular TMA+ activity (alpha 1TMA) was measured with TMA(+)-sensitive, double-barrelled microelectrodes. Loading hepatocytes with TMA+ did not change their membrane potential (Vm), and under steady-state conditions alpha iTMA remained constant over 4 min in a single impalement. Hyperosmotic solutions (50, 100 and 150 mM sucrose added to media) and hyposmotic solutions (sucrose in media reduced by 50 and 100 mM) increased and decreased alpha iTMA, respectively, which demonstrated transmembrane water movements. The slope of the plot of change in steady-state cell water volume, [(alpha iTMA)0/(alpha iTMA)4min] -1, on the relative osmolality of media, (experimental mosmol/control mosmol) -1, was less predicted for a perfect osmometer. Corresponding measurements of Vm showed that its magnitude increased with hyposmolality and decreased with hyperosmolality. When Ba2+ (2 mM) was present during hyposmotic stress of 0.66 X 286 mosmol (control), cell water volume increased by a factor of 1.44 +/- 0.02 compared with that of hyposmotic stress alone, which increased cell water volume by a factor of only 1.12 +/- 0.02, P less than 0.001. Ba2+ also decreased the hyperpolarization of hyposmotic stress from a factor of 1.62 +/- 0.04 to 1.24 +/- 0.09, P less than 0.01. We conclude that hepatocytes partially regulate their steady-state volume during hypo- and hyperosmotic stress. However, volume regulation during hyposmotic stress diminished along with hyperpolarization of Vm in the presence of K(+)-channel blocker, Ba2+. This shows that variation in Vm during osmotic stress provides an intercurrent, electromotive force for hepatocyte volume regulation.