Ionophore properties of monensin derivatives studied on human erythrocytes by 23Na NMR and K+ and H+ potentiometry: relationship with antimicrobial and antimalarial activities

In-cell NMR: Why and how?

NMR spectroscopy has been applied to cells and tissues analysis since its beginnings, as early as 1950. We have attempted to gather here in a didactic fashion the broad diversity of data and ideas that emerged from NMR investigations on living cells. Covering a large proportion of the periodic table, NMR spectroscopy permits scrutiny of a great variety of atomic nuclei in all living organisms non-invasively. It has thus provided quantitative information on cellular atoms and their chemical environment, dynamics, or interactions. We will show that NMR studies have generated valuable knowledge on a vast array of cellular molecules and events, from water, salts, metabolites, cell walls, proteins, nucleic acids, drugs and drug targets, to pH, redox equilibria and chemical reactions. The characterization of such a multitude of objects at the atomic scale has thus shaped our mental representation of cellular life at multiple levels, together with major techniques like mass-spectrometry or microscopies. NMR studies on cells has accompanied the developments of MRI and metabolomics, and various subfields have flourished, coined with appealing names: fluxomics, foodomics, MRI and MRS (i.e. imaging and localized spectroscopy of living tissues, respectively), whole-cell NMR, on-cell ligand-based NMR, systems NMR, cellular structural biology, in-cell NMR… All these have not grown separately, but rather by reinforcing each other like a braided trunk. Hence, we try here to provide an analytical account of a large ensemble of intricately linked approaches, whose integration has been and will be key to their success. We present extensive overviews, firstly on the various types of information provided by NMR in a cellular environment (the "why", oriented towards a broad readership), and secondly on the employed NMR techniques and setups (the "how", where we discuss the past, current and future methods). Each subsection is constructed as a historical anthology, showing how the intrinsic properties of NMR spectroscopy and its developments structured the accessible knowledge on cellular phenomena. Using this systematic approach, we sought i) to make this review accessible to the broadest audience and ii) to highlight some early techniques that may find renewed interest. Finally, we present a brief discussion on what may be potential and desirable developments in the context of integrative studies in biology.

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.

Salinomycin Inhibits Influenza Virus Infection by Disrupting Endosomal Acidification and Viral Matrix Protein 2 Function

Screening of chemical libraries with 2,000 synthetic compounds identified salinomycin as a hit against influenza A and B viruses, with 50% effective concentrations ranging from 0.4 to 4.3 μM in cells. This compound is a carboxylic polyether ionophore that exchanges monovalent ions for protons across lipid bilayer membranes. Monitoring the time course of viral infection showed that salinomycin blocked nuclear migration of viral nuclear protein (NP), the most abundant component of the viral ribonucleoprotein (vRNP) complex. It caused cytoplasmic accumulation of NP, particularly within perinuclear endosomes, during virus entry. This was primarily associated with failure to acidify the endosomal-lysosomal compartments. Similar to the case with amantadine (AMT), proton channel activity of viral matrix protein 2 (M2) was blocked by salinomycin. Using purified retroviral Gag-based virus-like particles (VLPs) with M2, it was proved that salinomycin directly affects the kinetics of a proton influx into the particles but in a manner different from that of AMT. Notably, oral administration of salinomycin together with the neuraminidase inhibitor oseltamivir phosphate (OSV-P) led to enhanced antiviral effect over that with either compound used alone in influenza A virus-infected mouse models. These results provide a new paradigm for developing antivirals and their combination therapy that control both host and viral factors.IMPORTANCE Influenza virus is a main cause of viral respiratory infection in humans as well as animals, occasionally with high mortality. Circulation of influenza viruses resistant to the matrix protein 2 (M2) inhibitor, amantadine, is highly prevalent. Moreover, the frequency of detection of viruses resistant to the neuraminidase inhibitors, including oseltamivir phosphate (OSV-P) or zanamivir, is also increasing. These issues highlight the need for discovery of new antiviral agents with different mechanisms. Salinomycin as the monovalent cation-proton antiporter exhibited consistent inhibitory effects against influenza A and B viruses. It plays multifunctional roles by blocking endosomal acidification and by inactivating the proton transport function of M2, the key steps for influenza virus uncoating. Notably, salinomycin resulted in marked therapeutic effects in influenza virus-infected mice when combined with OSV-P, suggesting that its chemical derivatives could be developed as an adjuvant antiviral therapy to treat influenza infections resistant or less sensitive to existing drugs.

FT-IR, 1H, 13C NMR, ESI-MS and semiempirical investigation of the structures of Monensin phenyl urethane complexes with the sodium cation

In this paper three forms of phenyl urethane of Monensin i.e. its acid form (H-MU) and its 1:1 complex with NaClO4 (H-MU-Na) and its sodium salt (Na-MU) were obtained and their structures were studied by FT-IR, (1)H and (13)C NMR, ESI MS and PM5 methods. The FT-IR data of Na-MU complexes demonstrate that the C=O urethane group is not engaged in the complexation of the sodium cation. However spectroscopic studies of H-MU-Na complex show that the structure in which this C=O urethane groups participate in the complexation is also present, but it is in the minority. The PM5 semiempirical calculations allow visualisation of all structures and determination of the hydrogen bond parameters.

Structure and antimicrobial properties of monensin A and its derivatives: summary of the achievements

In this paper structural and microbiological studies on the ionophorous antibiotic monensin A and its derivatives have been collected. Monensin A is an ionophore which selectively complexes and transports sodium cation across lipid membranes, and therefore it shows a variety of biological properties. This antibiotic is commonly used as coccidiostat and nonhormonal growth promoter. The paper focuses on both the latest and earlier achievements concerning monensin A antimicrobial activity. The activities of monensin derivatives, including modifications of hydroxyl groups and carboxyl group, are also presented.

Reinvestigation of the structure of monensin A phenylurethane sodium salt based on X-ray crystallographic and spectroscopic studies, and its activity against hospital strains of methicillin-resistant S. epidermidis and S. aureus

Monensin A phenylurethane sodium salt (MON-UR1-Na) crystals were studied by the X-ray, NMR, FT-IR and PM5 semi-empirical methods. The X-ray data show that the compound forms a pseudocyclic structure, stabilized by three intramolecular hydrogen bonds, and the sodium cation coordinated by five oxygen atoms in the hydrophilic sphere. The NMR and FT-IR data demonstrate that this pseudocyclic structure is also conserved in CH(2)Cl(2) solution. This structure of MON-UR1-Na is significantly different than the ones previously proposed by Westley et al. and Tanaka et al. The semi-empirical calculations of the MON-UR1-Na structures indicate that the one of the crystal is the most energetically favorable one. Other parameters, such as the size, chemical and biological nature of the urethane substituent, and especially the free carbonyl urethane group, may have a role in the biological activity of MON-UR1-Na. The in vitro microbiological tests provide evidence that MON-UR1-Na shows higher antibacterial activity against human pathogenic bacteria, including antibiotic-resistant Staphylococcus aureus and Staphylococcus epidermidis than the parent unmodified antibiotic-Monensin A.

A cation counterflux supports lysosomal acidification

The profound luminal acidification essential for the degradative function of lysosomes requires a counter-ion flux to dissipate an opposing voltage that would prohibit proton accumulation. It has generally been assumed that a parallel anion influx is the main or only counter-ion transport that enables acidification. Indeed, defective anion conductance has been suggested as the mechanism underlying attenuated lysosome acidification in cells deficient in CFTR or ClC-7. To assess the individual contribution of counter-ions to acidification, we devised means of reversibly and separately permeabilizing the plasma and lysosomal membranes to dialyze the cytosol and lysosome lumen in intact cells, while ratiometrically monitoring lysosomal pH. Replacement of cytosolic Cl(-) with impermeant anions did not significantly alter proton pumping, while the presence of permeant cations in the lysosomal lumen supported acidification. Accordingly, the lysosomes were found to acidify to the same pH in both CFTR- and ClC-7-deficient cells. We conclude that cations, in addition to chloride, can support lysosomal acidification and defects in lysosomal anion conductance cannot explain the impaired microbicidal capacity of CF phagocytes.

Inhibition of Plasmodium sporozoites infection by targeting the host cell

There is a great need of new drugs against malaria because of the increasing spread of parasite resistance against the most commonly used drugs in the field. We found that monensin, a common veterinary antibiotic, has a strong inhibitory effect in Plasmodium berghei and Plasmodium yoelii sporozoites hepatocyte infection in vitro. Infection of host cells by another apicomplexan parasite with a similar mechanism of host cell invasion, Toxoplasma tachyzoites, was also inhibited. Treatment of mice with monensin abrogates liver infection with P. berghei sporozoites in vivo. We also found that at low concentrations monensin inhibits the infection of Plasmodium sporozoites by rendering host cells resistant to infection, rather than having a direct effect on sporozoites. Monensin effect is targeted to the initial stages of parasite invasion of the host cell with little or no effect on development, suggesting that this antibiotic affects an essential host cell component that is required for Plasmodium sporozoite invasion.

In vitro antimalarial activity of hyperforin, a prenylated acylphloroglucinol. A structure–activity study

The antimalarial activity of hyperforin, a phenol-like compound that can be easily absorbed orally, and a series of derivatives variously modified on the cyclohexatrienone system was investigated. Hyperforin was active against Plasmodium falciparum with an IC(50) value in the micromolar range, and the activity was not critically dependent on either its phenol-like sensitivity to autooxidation or the presence of unsaturation on the prenyl residues. Related phloroglucinols like the hop beta-acids and the enantiomers of usnic acid showed only marginal activity, suggesting that hyperforin is a new antimalarial chemotype.

Callipeltin A: sodium ionophore effect and tension development in vascular smooth muscle

Callipeltin A is a cyclic depsidecapeptide isolated from the marine sponges Callipelta sp. and Latrunculia sp. that has been previously shown to increase the force of contraction of guinea-pig atria through the inhibition of Na+/Ca2+ exchanger (NCX). We investigated the effect of callipeltin A on guinea-pig aortic rings contracted by procedures that activate NCX in "calcium entry mode". Callipeltin A did not inhibit these contractions. Resting aorta responded to callipeltin A with a remarkable contraction that was concentration-dependent (EC50 0.44microM). This contraction was not inhibited by the calcium channel blocker verapamil and was not mediated by the activation of alpha-adrenergic or endothelin-1 receptors. Pre-incubation of aortic rings with 0.5mM amiloride, an inhibitor of NCX, completely prevented callipeltin A-induced contraction. Furthermore, callipeltin A (EC50 0.51microM) increased Na+ efflux of Na-loaded erythrocytes. 1H and 13C NMR resonances of callipeltin A revealed small but significant changes in the titration with K+ and Na+ salts. It is suggested that the effect of callipeltin A on cardiac and vascular preparations is linked to a Na-ionophore action.

Interaction between dopamine and its transporter: role of intracellular sodium ions and membrane potential

The present study addresses the effect of intracellular Na(+) and membrane potential on the binding of dopamine (DA) to the dopamine transporter (DAT). Perforation of plasma membranes of DAT-expressing cells with gramicidin diminished DA uptake and decreased the potency (increases K(i)) of DA in inhibiting the binding of cocaine analog [(3)H]2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane (CFT). It also compromised the ability of external Na(+) to reduce DA K(i). No substantial effect on DA K(i) was observed upon gramicidin treatment in Na(+)-free buffer, membrane depolarization with high [K(+)](o), or elevation of [Na(+)](i) with monensin under non-depolarizing conditions. Elevation of DA K(i) was greater at more positive potentials when [Na(+)](i) was raised to a similar level, or at higher [Na(+)](i) when the membrane was depolarized to a similar level. In cells expressing D313N DAT, DA K(i) was significantly higher but less sensitive to gramicidin than that in wild-type (WT) cells. In contrast, DA K(i) in cell-free membranes was insensitive to Na(+), gramicidin, and D313N mutation. The data suggest that (i) intracellular Na(+) plays a role in affecting the external access to DA binding sites at DAT on depolarized plasma membranes of cells, and (ii) access to DA binding sites in cell-free membranes may occur from the intracellular side of the membrane. Unlike DA binding, CFT binding to both cells and membranes was sensitive to Na(+) and D313N mutation but insensitive to gramicidin, consistent with exclusively external access to sites that are different from but conformationally linked to those for DA.

Solid state fermentation of broiler litter for production of biocontrol agents

Several varieties of heat-sterilized broiler litter with 60% (wet basis, wb) moisture content were substrate in solid-state fermentations to produce biocontrol agents. Litter varieties included litter produced by one flock of broilers from medicated and non-medicated controlled rations, and litter produced by two flocks and four flocks on a single application of bedding material from medicated commercial sources. Litter preparations were inoculated with monocultures of Bacillus thuringiensis serovar japonensis strain Buibui, a pathogen of Japanese beetle larvae (Popillia japonica), or Pseudomonas fluorescens 2-79. B. thuringiensis did not grow in unextracted 1-flock litter nor in water extracted litter, but grew in methanol extracted litter to 5 x 10(10) cell forming units (CFU)/g litter (dry weight, dw) and a spore count of 1 x 10(10) CFU/g litter (dw). B. thuringiensis also grew in unprocessed 2-flock and 4-flock litter, achieving cell counts of 3 x 10(9) and 1 x 10(9) CFU/g litter (dw), respectively, and spore counts of 1 x 10(9) CFU/g litter (dw). P. fluorescens grew in medicated 1-flock litter with no extraction to a cell density greater than 4 x 10(11) CFU/g litter (dw). Bioassays in soil containing over 0.5% (db) litter fermented with B. thuringiensis resulted in over 90% mortality in 21 days for first instars of Japanese beetle when compared to a control treatment using compost without fermented litter. The investigations demonstrate that bacterial biocontrol agents produced via solid substrate fermentations using broiler poultry litter have potential in biocontrol applications in the soil environment.

Cationomycin and monensin partition between serum proteins and erythrocyte membrane: consequences for Na+ and K+ transport and antimalarial activities

The ionophore properties of cationomycin and monensin were studied on human erythrocytes by measuring Na+ influx by 23Na NMR and concomitant K+ efflux by potentiometry in the presence of increasing amounts of serum. Both ion currents (Na+ or K+) decreased linearly with the reciprocal of serum amount. The serum effects on ion currents were stronger with cationomycin than with monensin. Assuming this decreased transport activity was due to drug binding to serum proteins, a partition coefficient between the protein and the membrane phase was determined for each ionophore by using a novel model. This partition coefficient is about 30 times higher for cationomycin than for monensin; the same result was obtained with purified human serum albumin, indicating that albumin may be the major ionophore binding protein of serum. In parallel, we also measured IC50 for 50% in vitro growth inhibition of Plasmodium falciparum, the agent of malaria. In the presence of increasing serum concentrations, the antimalarial activity was decreased for both ionophores. Serum effect was less severe for monensin than for cationomycin, in agreement with the weaker interaction of monensin with proteins as shown from the partition coefficient values. A correlation was established between the ion transport currents (sodium and potassium) and the IC50 measured on P. falciparum in the presence of the various concentrations of serum. The relative value of the ion transport currents (expressed as percentage of control in absence of serum) can be indicative of the ionophore unbound fraction in the medium.

Structure, dynamics, and topological orientation of the polyether, ionophore antibiotic monensin, in a micellar environment

The structure and dynamics of the ionophoric antibiotic monensin in the presence of micelles have been determined. The conformation of monensin was derived from 50 nuclear Overhauser enhancement (NOE) derived distance restraints and metric-matrix based distance geometry calculations. The conformation was further refined with extensive NOE restrained molecular dynamics simulations carried out in a biphasic simulation cell. From the addition of doxylstearate and monitoring of the induced relaxation of the nmr signals, the relative topological orientation of the molecule within the micelle was ascertained. The results indicate two dihedral angles that act as hinge regions allowing the molecule to adopt a wide range of conformations. Considering the biological activity of monensin, i.e., the capture and transport of cations across cell membranes, an open and closed form of monensin have been postulated. The identification of these hinge regions, which are only observed in the membrane-like environment of the detergent micelles, provides insight into the mechanism of action and can serve as targets for modification to alter the biological profile of monensin.