Phospholipid-Dependent Functions of a Macrocyclic Analogue of the Ion-Channel-Forming Antibiotic Gramicidin A.

An ion-channel-forming natural peptide, gramicidin A (1), exhibits potent antimicrobial activity against Gram-positive bacteria, although medical applications are limited to topical use due to its mammalian cytotoxicity. We recently reported that the artificial macrocyclic analogue 2 provides a promising starting point for developing new ion-channel-based systemic antibacterial agents because of its low mammalian cytotoxicity compared to that of the parent 1. To dissect the molecular factors involved in the species selectivity of 2, we evaluated the ion transport activities, phospholipid affinities, and conformational properties of 1 and 2 using various compositions of phospholipids. A combination of lipid dot blot assays and circular dichroism (CD) analysis with H+/Na+ exchange assays revealed that the higher H+/Na+ exchange activity of 2 than that of 1 in liposomes containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) or 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG) is attributable to its higher affinity towards the phospholipids than that of 1. Notably, we also discovered that 2 showed weaker H+/Na+ exchange activity in liposomes containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylethanolamine (POPE). CD analysis of 2 in liposomes indicated that the weak H+/Na+ exchange activity is induced by disturbance of the ion-conducting ß6.3-helical conformation in the POPE-containing lipid bilayer. These results suggest that the POPE-induced attenuation of the ion-conducting activity of 2 contributes to the alleviation of undesirable mammalian cytotoxicity of 2 compared to that of 1.

Structure, toxicity and antibiotic activity of gramicidin S and derivatives.

Development of new antibiotics is declining whereas antibiotic resistance is rising, heralding a post-antibiotic era. Antimicrobial peptides such as gramicidin S (GS), exclusively topically used due to its hemolytic side-effect, could still be interesting as therapeutic compounds. By modifying the amino-acid composition of GS, we synthesized GS analogues. We now show that derivative VK7 has a lower MIC (7.8-31.2 µg/ml, median 15.6 µg/ml) against strains of multi-drug resistant (MDR) Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa than GS has (3.9-62.5 µg/ml, median 31.3 µg/ml). Low MICs for both VK7 and GS were observed for Staphylococcus aureus and Enterococcus faecium. VK7 showed reduced haemolysis and less lactate dehydrogenase release. All compounds were fully bactericidal at MIC values. Modification of GS enables production of novel derivatives potentially useful for systemic treatment of human infections.

Interaction of polypeptide antibiotic gramicidin S with platelets.

Gramicidin S (GS) is a cyclic decapeptide antibiotic active against both Gram-positive and Gram-negative bacteria as well as against several pathogenic fungi. However, clinical application of GS is limited because of GS hemolytic activity. The large number of GS analogues with potentially attenuated hemolytic activity has been developed over the last two decades. For all new GS derivatives, the antimicrobial test is accompanied with the hemolytic activity assay. At the same time, neither GS nor its analogues were tested against other blood cells. In the present work, the effects of GS on platelets and platelet aggregates have been studied. GS interaction with platelets is concentration dependent and leads either to platelet swelling or platelet shape change. Effect of GS on platelets is independent of platelet aggregation mechanism. GS induces disaggregation of platelet aggregates formed in the presence of aggregation agonists. The rate of the GS interaction with platelet membranes depends on membrane lipid mobility and significantly increases with temperature. The interaction of GS with the platelet membranes depends strongly on the state of the membrane lipids. Factors affecting the membrane lipids (temperature, lipid peroxidation and ionising irradiation) modify GS interaction with platelets. Our results show that GS is active not only against erythrocytes but also against other blood cells (platelets). The estimated numbers of GS molecules per 1 µm2 of a blood cell required to induce erythrocyte hemolysis and disaggregation of platelet aggregates are comparable. This must be considered when developing new antimicrobial GS analogues with improved hemolytic properties.

Development of Therapeutic Gramicidin S Analogues Bearing Plastic ß,γ-Diamino Acids.

Gramicidin S (GS), one of the most widely investigated antimicrobial peptides (AMPs), is known for its robust antimicrobial activity. However, it is restricted to topical application due to undesired hemolytic activity. With the aim of obtaining nontoxic GS analogues, we describe herein a molecular approach in which the native GS ß-turn region is replaced by synthetic ß,γ-diamino acids (ß,γ-DiAAs). Four ß,γ-DiAA diastereomers were employed to mimic the ß-turn structure to afford GS analogues GS3-6, which exhibit diminished hemolytic activity. A comparative structural study demonstrates that the (ßR,γS)-DiAA is the most-stable ß-turn mimic. To further improve the therapeutic index (e. g., high antibacterial activity and low hemolytic activity) and to extend the molecular diversity, GS5 and GS6 were used as structural scaffolds to introduce additional hydrophobic or hydrophilic groups. We show that GS6K, GS6F and GS display comparable antibacterial activity, and GS6K and GS6F have significantly decreased toxicity. Moreover, antibacterial mechanism studies suggest that GS6K kills bacteria mainly through the disruption of the membrane.

[4,4'-D-diaminopropionic acid]gramicidin S: a synthetic gramicidin S analog with antimicrobial activity against Gram-negative bacteria.

Gramicidin S is especially active against Gram-positive bacteria; e.g., Staphylococcus aureus. An analog, [4,4'-D-diaminopropionic acid]gramicidin S, which contains D-diaminopropionic acid residues instead of D-phenylalanine residues, has been synthesized. This analog is active against some of the Gram-negative bacteria; e.g., Escherichia coli and Salmonella typhosa. Activities of several related analogs are discussed.

[Investigations of the ototoxicity of dicortinef (ear drops)]. / Badania ototoksycznosci preparatu Dicortinef (krople do uszu).

The authors investigated the ototoxic influence of Dicortinef in laboratory animals. Their examinations were performed on 15 guinea pigs (weighing 210-380 g.) after application of this medicine to the fenestra rotunda. The harmful effect of Dicortinef was expressed by the characteristic fall in the microphonic potential (PM) and potential of the acoustic nerve (AP).

Nature of the neurotoxic membrane actions of amyloid-ß on hippocampal neurons in Alzheimer's disease.

The mechanism by which amyloid-ß (Aß) produces brain dysfunction in patients with Alzheimer's disease is largely unknown. According to previous studies, Aß might share perforating properties with gramicidin, a well-accepted membrane-disrupting peptide. Therefore, we hypothesize that the key steps leading to synaptotoxicity by Aß and gramicidin involve peptide aggregation, pore formation, and calcium dysregulation. Here, we show that Aß and gramicidin form aggregates enriched in ß-sheet structures using electron microscopy, and Thioflavin and Congo Red staining techniques. Also, we found that Aß and gramicidin display fairly similar actions in hippocampal cell membranes, i.e. inducing Ca(2+) entry and synaptoxicity characterized by the loss of synaptic proteins and a decrease in neuronal viability. These effects were not observed in a Ca(2+) free solution, indicating that both Aß and gramicidin induce neurotoxicity by a Ca(2+)-dependent mechanism. Using combined perforated patch clamp and imaging recordings, we found that only Aß produced a perforation that progressed from a small (Cl(-)-selective pore) to a larger perforation that allowed the entry of fluorescent molecules. Therefore, based on these results, we propose that the perforation at the plasma membrane by Aß is a dynamic process that is critical in producing neurotoxicity similar to that found in the brains of AD patients.

The effects of organic pesticides on inner membrane permeability in Escherichia coli ML35.

We have tested whether some pesticides might cause inner membrane leakage in ML35 Escherichia coli cells, which express beta-galactosidase (lacZ; EC 3.2.1.23) constitutively but lack the permease (lacY) required for substrate entry. The activity of beta-galactosidase (indicative of substrate leakage through the inner membrane) was increased by various concentrations of pesticides, including the organometallic fungicides maneb and mancozeb, the insecticide Thiodan, and the herbicide Ally, as well as by antibiotics such as ampicillin, gramicidin D, and the calcium ionophore A23187. The enzyme activity was increased by up to approximately 30% when the E. coli ML35 strain was exposed to various concentrations (between 50 and 250 ppm) of both fungicides. Thiodan had only a slight effect on beta-galactosidase activity (increase of 12.8%), whereas, among the antibiotics, the calcium ionophore at 20 microg/ml caused a significant increase in enzyme activity by up to 61.8%. This effect is similar to that of sodium dodecyl sulfate, used as positive control ( approximately 70% increase). Accumulation of maneb and mancozeb by bacterial cells was also studied taking advantage of their metal content and using atomic absorption spectrophotometry. In parallel with the increase in enzyme activity, both fungicides accumulated in the cells as a function of their concentration. Time course experiments (3, 6, and 9 h) of fungicide accumulation and of bacterial growth at various pesticide concentrations were also carried out. Maneb seems to inhibit the bacterial growth better than mancozeb. In addition, maneb uptake increases with time up to 9 h at all tested concentrations, whereas the accumulation of mancozeb is similar at all the exposure times tested. This indicates a different uptake and/or metabolizing strategy by E. coli cells for the two fungicides.

A comparison of in vitro toxicity and antifungal efficacy of membrane-active drugs after liposome encapsulation.

The membrane-active ionophores were observed to possess antifungal activity against Candida albicans 336 and were toxic to human erythrocytes. Liposome encapsulation of these drugs significantly reduced their toxicity to erythrocytes but resulted in the loss of their antifungal potency. These results are compared with membrane-active polyenes which maintained their antifungal activity after encapsulation into liposomes. Liposomal-ionophores, however, showed antifungal activity along with low concentrations of Amphotericin B indicating the presence of synergism between these drugs.

Cyclic amphipathic peptide-DNA complexes mediate high-efficiency transfection of adherent mammalian cells.

A DNA transfection protocol has been developed that makes use of the cyclic cationic amphipathic peptide gramicidin S and dioleoyl phosphatidylethanolamine. The DNA complex is formed by mixing gramicidin S with DNA at a 1:1 charge ratio and then adding phosphatidylethanolamine at a lipid/peptide molar ratio of 5:1. The complex mediates rapid association of DNA with cells and leads to transient expression levels of beta-galactosidase ranging from 1 to 30% of the transfected cells, with long-term expression being about an order of magnitude lower. The respective roles of peptide and phospholipid are not yet resolved but optimal transfection requires both the cyclic peptide and the hexagonal phase-competent phospholipid PtdEtn. Transfection in CV-1 cells is not affected by lysomotrophic agents, which suggests that DNA entry into the cell is via the plasma membrane. This technique that is simple, economical, and reproducible mediates transfection levels up to 20-fold higher than cationic liposomes in adherent mammalian cells.

Synergistic antifungal activity and reduced toxicity of liposomal amphotericin B combined with gramicidin S or NF.

Amphotericin B (AmpB) disrupts membrane integrity by binding to sterols in fungal and mammalian cell membranes. The gramicidins, which form pores in all membranes but exhibit poor antifungal activity, are too toxic to mammalian cells to be used systemically. This study demonstrated synergistic antifungal activity of free and liposomal forms of AmpB when combined with the free and liposomal forms of gramicidin S and gramicidin NF against five Candida strains. In vitro erythrocyte lysis was prevented by using the liposomal forms of all drugs tested alone or in combination. Presumably, AmpB increases accessibility of the fungal cell membrane to the gramicidins, while liposome encapsulation decreases the rate of transfer of the drugs to the mammalian cell membrane. Liposome encapsulation of inactive or toxic drugs, used in combination with liposomal AmpB, may give new life to drugs previously believed to be inactive or too toxic for therapeutic consideration.

Gramicidin toxicity in NG108-15 cells: protective effects of acetamidine and guanidine.

Studies were conducted using a novel in vitro approach to investigate the efficacy of acetamidine hydrochloride (ACE) and guanidine hydrochloride (GUAN), previously shown to block gramicidin D (GRAM) channels in artificial membranes, in preventing the toxic effects of GRAM in NG108-15 (neuroblastoma x glioma hybrid) cells. Specifically, intracellular microelectrode techniques were employed to examine changes in membrane resting potential (Vm) and input resistance (Rin). At 1 micromol/L, ACE significantly reduced loss of Vm induced by 1 or 10 microg/ml GRAM, although higher concentrations of ACE did not afford enhanced antagonism. GUAN, in contrast, produced a concentration-dependent antagonism of GRAM-induced Vm and Rin loss, with high concentrations (10 or 100 micromol/L) completely preventing diminutions in both Vm and Rin. In control cells superfused without GRAM, ACE produced a direct, concentration-dependent reduction in Vm and Rin, whereas GUAN hyperpolarized NG108-15 cells but did not alter Rin. These data represent the initial demonstration of the reversal of GRAM toxicity in an intact cell system.

Probing the interaction of the multidrug-resistance phenotype with the polypeptide ionophore gramicidin D via functional channel formation.

It has been proposed that the multidrug resistance (MDR) transporter, P-glycoprotein (P-170), may be physiologically involved in the transport of polypeptides. As a step towards understanding the interaction of P-170 with polypeptides, we isolated various gramicidin-D-resistant mammalian cell lines. Gramicidin D is a hydrophobic pentadecapeptide ionophore that forms proton and alkali metal cation-permeable channels in lipid bilayers. Gramicidin-D-resistant cells displayed a prominent MDR gene amplification, P-170 overexpression, reduced drug accumulation, and consequent resistance to MDR-type cytotoxic agents. Modulators of the MDR phenotype, including verapamil, reserpine and quinidine, rendered these cells sensitive to gramicidin D. Using these cell lines, we established an assay that probes for the intra-membranal interaction between P-170 and gramicidin D. Gramicidin-D channel formation was followed by cellular accumulation of 86Rb+. Ionophore-resistant cells, and other MDR cells, did not show an appreciable increase in 86Rb+ influx rates, in the presence of increasing gramicidin-D concentrations. In contrast, parental cells displayed a dose-dependent increase in the 86Rb+ influx rates. Interestingly, in the absence of serum, gramicidin-D-resistant cells resumed the wild-type, ionophore-dose-dependent increase in 86Rb+ influx rates. MDR modulators caused a resumption of channel formation in ionophore-resistant cells. We conclude that acquisition of the MDR phenotype is an efficient means of cellular protection against gramicidin D. Hence, a new approach is offered in which P-170 interaction with gramicidin D is quantitatively followed by a rapid assessment of the biological activity (i.e. channel formation) of the substrate itself. Possible mechanisms of P-170 interaction with free ionophore monomers, and membrane-associated gramicidin D are discussed.