Interaction of the bisbiguanides chlorhexidine and alexidine with phospholipid vesicles: evidence for separate modes of action

Strains of Providencia stuartii with demonstrated resistance towards chlorhexidine did not show such resistance towards either of the related biguanide antiseptics, alexidine or vantocil. Alexidine promoted a significantly faster alteration in the permeability of Escherichia coli cell membranes towards various metal cations than chlorhexidine. Differential thermal analysis of various mixed lipid vesicles and pure phospholipid vesicles showed alexidine to share with vantocil the property of producing lipid phase separation and domain formation. It is suggested that the nature of the end-group on the biguanides affects the ability to produce lipid domains in cell membranes and this this might, in turn, affect activity and resistance patterns observed.     

A comparative study of the bactericidal and growth inhibitory activities of the bisbiguanides alexidine and chlorhexidine

A comparative study of the growth inhibitory and bactericidal activities of two related bisbiguanide antiseptics, alexidine and chorhexidine is reported. Whilst overall bactericidal activities and MICs were similar, alexidine was more rapid in its action and it is suggested that (1) it might possess additional targets at the cell envelope to chlorhexidine, and (2) the nature of the interaction at the cytoplasmic membrane for the two compounds differs.     

A comparative study of the bactericidal and growth inhibitory activities of the bisbiguanides alexidine and chlorhexidine

A comparative study of the growth inhibitory and bactericidal activities of two related bisbiguanide antiseptics, alexidine and chorhexidine is reported. Whilst overall bactericidal activities and MICs were similar, alexidine was more rapid in its action and it is suggested that (1) it might possess additional targets at the cell envelope to chlorhexidine, and (2) the nature of the interaction at the cytoplasmic membrane for the two compounds differs.     

Impact on S. aureus and E. coli Membranes of Treatment with Chlorhexidine and Alcohol Solutions: Insights from Molecular Simulations and Nuclear Magnetic Resonance

Membranes form the first line of defence of bacteria against potentially harmful molecules in the surrounding environment. Understanding the protective properties of these membranes represents an important step towards development of targeted anti-bacterial agents such as sanitizers. Use of propanol, isopropanol and chlorhexidine can significantly decrease the threat imposed by bacteria in the face of growing anti-bacterial resistance via mechanisms that include membrane disruption. Here we have employed molecular dynamics simulations and nuclear magnetic resonance to explore the impact of chlorhexidine and alcohol on the S. aureus cell membrane, as well as the E. coli inner and outer membranes. We identify how sanitizer components partition into these bacterial membranes, and show that chlorhexidine is instrumental in this process.     

The role of blood cell membrane lipids on the mode of action of HIV-1 fusion inhibitor sifuvirtide

Sifuvirtide is a gp41 based peptide that inhibits HIV-1 fusion with the host cells and is currently under clinical trials. Previous studies showed that sifuvirtide partitions preferably to saturated phosphatidylcholine lipid membranes, instead of fluid-phase lipid vesicles. We extended the study to the interaction of the peptide with circulating blood cells, by using the dipole potential sensitive probe di-8-ANEPPS. Sifuvirtide decreased the dipole potential of erythrocyte and lymphocyte membranes in a concentration dependent manner, demonstrating its interaction. Also, the lipid selectivity of the peptide towards more rigid phosphatidylcholines was confirmed based on the dipole potential variations. Overall, the interaction of the peptide with the cell membranes is a contribution of different lipid preferences that presumably directs the peptide towards raft-like domains where the receptors are located, facilitating the reach of the peptide to its molecular target, the gp41 in its pre-fusion conformation.     

Investigation of toroidal pore and oligomerization by melittin using transmission electron microscopy

We studied the effects of melittin on various cell wall components and vesicles of various lipid compositions. To interact with the cytoplasmic membrane, melittin must traverse the cell wall, which is composed of oligosaccharides. Here, we found that melittin had a strong affinity for chitin, peptidoglycan, and lipopolysaccharide. We further examined the influence of lipid compositions on the lysis of the membranes by melittin. The result showed that melittin bound better to negatively charged than to zwitterionic lipid vesicles but was more potent at inducing leakage from zwitterionic lipid vesicles. Our studies further indicated that the oligomeric state of melittin varied between tetramers and octamers during the formation of toroidal pores. Dextran leakage experiments confirmed the formation and dimension of these toroidal pores. Finally, transmission electron microscopy revealed that melittin formed pores via peptide oligomerization by the toroidal pore-forming mechanism. The toroidal pores composed of 7-8 nm diameter rings that encircled 3.5-4.5 nm diameter cavities on zwitterionic lipid vesicles.     

Inhibitory and lethal effects of chlorhexidine and a polymeric biguanide on some strains of Providencia stuartii

The effects of chlorhexidine diacetate and vantocil IB on the viability of Providencia stuartii strains are described. Exposure of Prov. stuartii strains to different concentrations of chlorhexidine in broth culture resulted in a decrease in viability over the first 6 h, followed by regrowth. During incubation, bacteria adhered to the surface of the culture vessel and multiplied despite the presence of a bactericidal concentration of the drug in the medium. It is concluded that the phenomenon of 'regrowth' results from adhesion to glass containers and the subsequent dispersal of some of these cells into the culture medium.     

Direct evidence of vinculin tail-lipid membrane interaction in beta-sheet conformation

The focal adhesion protein vinculin (1066 residues) plays an important role in cell adhesion and migration. The interaction between vinculin and lipid membranes is necessary to ensure these processes. There are three putative lipid-membrane interaction sites located at the vinculin tail domain two that form amphipathic alpha-helices (residues 935-978 and 1020-1040) and one that remains unstructured (residues 1052-1066) during crystallization. In this work, the structural and biochemical properties of the last 21 residues of the vinculin tail domain were investigated. Differential scanning calorimetry was performed in the presence of lipid vesicles consisting of dimyristoyl-l-α-phosphatidylcholine and dimyristoyl-l-α-phosphatidylglycerol at various molar ratios. The results demonstrate that this peptide inserts into lipid vesicle membranes. Examining the secondary structure of this peptide by molecular dynamics simulations and circular dichroism spectroscopy, we show that it adopts an antiparallel beta sheet backbone geometry that could ensure the association with lipid vesicles.     

HIV-1 Fusion Inhibitor Peptides Enfuvirtide and T-1249 Interact with Erythrocyte and Lymphocyte Membranes

Enfuvirtide and T-1249 are two HIV-1 fusion inhibitor peptides that bind to gp41 and prevent its fusogenic conformation, inhibiting viral entry into host cells. Previous studies established the relative preferences of these peptides for membrane model systems of defined lipid compositions. We aimed to understand the interaction of these peptides with the membranes of erythrocytes and peripheral blood mononuclear cells. The peptide behavior toward cell membranes was followed by di-8-ANEPPS fluorescence, a lipophilic probe sensitive to the changes in membrane dipole potential. We observed a fusion inhibitor concentration-dependent decrease on the membrane dipole potential. Quantitative analysis showed that T-1249 has an approximately eight-fold higher affinity towards cells, when compared with enfuvirtide. We also compared the binding towards di-8-ANEPPS labeled lipid vesicles that model cell membranes and obtained concordant results. We demonstrated the distinct enfuvirtide and T-1249 membranotropism for circulating blood cells, which can be translated to a feasible in vivo scenario. The enhanced interaction of T-1249 with cell membranes correlates with its higher efficacy, as it can increase and accelerate the drug binding to gp41 in its pre-fusion state.     

Inhibitors of V-ATPase proton transport reveal uncoupling functions of tether linking cytosolic and membrane domains of V0 subunit a (Vph1p)

Vacuolar ATPases (V-ATPases) are important for many cellular processes, as they regulate pH by pumping cytosolic protons into intracellular organelles. The cytoplasm is acidified when V-ATPase is inhibited; thus we conducted a high-throughput screen of a chemical library to search for compounds that acidify the yeast cytosol in vivo using pHluorin-based flow cytometry. Two inhibitors, alexidine dihydrochloride (EC(50) = 39 μM) and thonzonium bromide (EC(50) = 69 μM), prevented ATP-dependent proton transport in purified vacuolar membranes. They acidified the yeast cytosol and caused pH-sensitive growth defects typical of V-ATPase mutants (vma phenotype). At concentrations greater than 10 μM the inhibitors were cytotoxic, even at the permissive pH (pH 5.0). Membrane fractions treated with alexidine dihydrochloride and thonzonium bromide fully retained concanamycin A-sensitive ATPase activity despite the fact that proton translocation was inhibited by 80-90%, indicating that V-ATPases were uncoupled. Mutant V-ATPase membranes lacking residues 362-407 of the tether of Vph1p subunit a of V(0) were resistant to thonzonium bromide but not to alexidine dihydrochloride, suggesting that this conserved sequence confers uncoupling potential to V(1)V(0) complexes and that alexidine dihydrochloride uncouples the enzyme by a different mechanism. The inhibitors also uncoupled the Candida albicans enzyme and prevented cell growth, showing further specificity for V-ATPases. Thus, a new class of V-ATPase inhibitors (uncouplers), which are not simply ionophores, provided new insights into the enzyme mechanism and original evidence supporting the hypothesis that V-ATPases may not be optimally coupled in vivo. The consequences of uncoupling V-ATPases in vivo as potential drug targets are discussed.     

Lipid vesicle-cell interactions. II. Induction of cell fusion

The ability of lipid vesicles of simple composition (lecithin, lysolecithin, and stearylamine) to induce cells of various types to fuse has been investigated. One in every three or four cells in monolayer cultures can be induced to fuse with a vesicle dose of about 100 per cell. At such dosages and for exposures of 15 min to 1 h, vesicles have essentially no effect on cell viability. Under anaerobic conditions, these cells lyse rather than fuse. Avian erythrocytes are readily fused with lipid vesicles in the presence of dextran. Fusion indices increase linearly with the zeta potential of the vesicles (increasing stearylamine content), indicating that contact between vesicle and cell membrane is required. Fusion indices increase sublinearly with increasing lysolecithin content. Divalent cations increase fusion indices at high vesicle doses. The data presented are consistent with the hypothesis that cell fusion occurs via simultaneous fusion of a vesicle with two adhering cell membranes.     

Small-angle neutron scattering studies of the effects of amphotericin B on phospholipid and phospholipid-sterol membrane structure

Small-angle neutron scattering (SANS) studies have been performed to study the structural changes induced in the membranes of vesicles prepared (by thin film evaporation) from phospholipid and mixed phospholipid-sterol mixtures, in the presence of different concentrations and different aggregation states of the anti-fungal drug, amphotericin B (AmB). In the majority of the experiments reported, the lipid vesicles were prepared with the drug added directly to the lipid dispersions dissolved in solvents favouring either AmB monomers or aggregates, and the vesicles then sonicated to a mean size of ~100 nm. Experiments were also performed, however, in which micellar dispersions of the drug were added to pre-formed lipid and lipid-sterol vesicles. The vesicles were prepared using the phospholipid palmitoyloleoylphosphatidylcholine (POPC), or mixtures of this lipid with either 30 mol% cholesterol or 30 mol% ergosterol. Analyses of the SANS data show that irrespective of the AmB concentration or aggregation state, there is an increase in the membrane thickness of both the pure POPC and the mixed POPC-sterol vesicles-in all cases amounting to ~4 ?. The structural changes induced by the drug's insertion into the model fungal cell membranes (as mimicked by POPC-ergosterol vesicles) are thus the same as those resulting from its insertion into the model mammalian cell membranes (as mimicked by POPC-cholesterol vesicles). It is concluded that the specificity of AmB for fungal versus human cells does not arise because of (static) structural differences between lipid-cholesterol-AmB and lipid-ergosterol-AmB membranes, but more likely results from differences in the kinetics of their transmembrane pore formation and/or because of enthalpic differences between the two types of sterol-AmB complexes.     

The dependence of lipid asymmetry upon phosphatidylcholine acyl chain structure

Lipid asymmetry, the difference in inner and outer leaflet lipid composition, is an important feature of biomembranes. By utilizing our recently developed MβCD-catalyzed exchange method, the effect of lipid acyl chain structure upon the ability to form asymmetric membranes was investigated. Using this approach, SM was efficiently introduced into the outer leaflet of vesicles containing various phosphatidylcholines (PC), but whether the resulting vesicles were asymmetric (SM outside/PC inside) depended upon PC acyl chain structure. Vesicles exhibited asymmetry using PC with two monounsaturated chains of >14 carbons; PC with one saturated and one unsaturated chain; and PC with phytanoyl chains. Vesicles were most weakly asymmetric using PC with two 14 carbon monounsaturated chains or with two polyunsaturated chains. To define the origin of this behavior, transverse diffusion (flip-flop) of lipids in vesicles containing various PCs was compared. A correlation between asymmetry and transverse diffusion was observed, with slower transverse diffusion in vesicles containing PCs that supported lipid asymmetry. Thus, asymmetric vesicles can be prepared using a wide range of acyl chain structures, but fast transverse diffusion destroys lipid asymmetry. These properties may constrain acyl chain structure in asymmetric natural membranes to avoid short or overly polyunsaturated acyl chains.     

Vesicles from mixtures of bipolar archaebacterial lipids with egg phosphatidylcholine

Bipolar lipids from the membranes of archaebacterium Caldariella acidophila can form small unilamellar liposomes, when sonicated from lipid mixtures containing at least 25 mol% egg phosphatidylcholine. With increasing contents of archaebacterial lipid the inner radius of highly sonicated vesicles increases (from approx. 90 Å to approx. 160 Å) concomitant with an enhanced asymmetric distribution of the phosphatidylcholine molecules towards the outer face of the lipid bilayer membranes.     

EFFECTS OF PUROMYCIN ON THE STRUCTURE OF RAT INTESTINAL EPITHELIAL CELLS DURING FAT ABSORPTION

This report provides information on the morphology of rat intestinal epithelial cells during fat absorption. In addition, the role of protein metabolism in this process has been evaluated by blocking its synthesis with puromycin and studying the fine structure of mucosal cells from rats at various times after fat intubation. The results indicate that SER-derived vesicles, containing fat droplets, migrate from the apical cytoplasm of the absorptive cell and fuse with saccules or vacuoles of the Golgi complex. Arguments are made that the Golgi complex is important in completing chylomicron formation and in providing appropriate enveloping membranes for the chylomicron. Such membranes may be necessary for Golgi vacuoles to fuse with the lateral cell membranes and release chylomicra. Puromycin treatment causes the absorptive cell to accumulate increased quantities of lipid that are devoid of membrane during fat absorption. In addition, puromycin-treated cells contain much less RER and Golgi membranes are strikingly decreased in number. In this paper we discuss the consequences of these abnormalities and suggest that continued protein synthesis by the RER is required in order to generate Golgi membranes. If such membranes are absent the cell's ability to discarge chylomicra is impaired and lipid accumulates.     

The Interaction of Equine Lysozyme:Oleic Acid Complexes with Lipid Membranes Suggests a Cargo Off-Loading Mechanism

The normal function of equine lysozyme (EL) is the hydrolysis of peptidoglycan residues of bacterial cell walls. EL is closely related to α-lactalbumins with respect to sequence and structure and further possesses the calcium binding site of α-lactalbumins. Recently, EL multimeric complexes with oleic acids (ELOAs) were shown to possess tinctorial and morphological properties, similar to amyloidal aggregates, and to be cytotoxic. ELOA's interactions with phospholipid membranes appear to be central to its biological action, similar to human α-lactalbumin made lethal to tumor cells. Here, we describe the interaction of ELOA with phospholipid membranes. Confocal scanning laser microscopy shows that ELOA, but not native EL, accumulates on the surface of giant unilamellar vesicles, without inducing significant membrane permeability. Quartz crystal microbalance with dissipation data indicated an essentially non-disruptive binding of ELOA to supported lipid bilayers, leading to formation of highly dissipative and “soft” lipid membrane; at higher concentrations of ELOA, the lipid membrane desorbs from the surface probably as bilayer sheets of vesicles. This membrane rearrangement occurred to a similar extent when free oleic acid (OA) was added, but not when free OA was removed from ELOA by prior incubation with bovine serum albumin, emphasizing the role of OA in this process. NMR data indicated an equilibrium between free and bound OA, which shifts towards free OA as ELOA is progressively diluted, indicating that OA is relatively loosely bound. Activity measurements together with fluorescence spectroscopy and circular dichroism suggested a conversion of ELOA towards a more native-like state on interaction with lipid membranes, although complete refolding was not observed. Altogether, these results suggest that ELOA may act as an OA carrier and facilitate OA transfer to the membrane. ELOA's properties illustrate that protein folding variants may possess specific functional properties distinct from the native protein.     

Vesicles mimicking normal and cancer cell membranes exhibit differential responses to the cell-penetrating peptide Pep-1

The cell-penetrating peptide (CPP) Pep-1 presents a great potential in drug delivery due to its intrinsic property to cross plasma membrane. However, its mechanism of entry into the cell remains unresolved. In this study, we compare the selectivity of Pep-1 towards vesicles mimicking normal and cancer cell membranes. The interaction was performed in a wide range of peptide-to-lipid molar ratios using infrared (IR), fluorescence, scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques. At low peptide concentration, fluorescence experiments show that lipid-phosphatidylserine (PS) seems to enable Pep-1 translocation into cancer cell membrane as evidenced by the blue shift of its maximal emission wavelength. DSC data show that Pep-1 induces segregation of lipids. At high peptide concentration, IR data indicate that the interaction of Pep-1 is relatively stronger with normal cell membrane than with cancer cell membrane through the phosphate groups, while the interaction is weaker with normal cell membrane than with cancer cell membrane through the carbonyl groups. TGA and DSC data reveal that vesicles of normal cell membrane are thermally more stable than vesicles of cancer cell membrane. This suggests that the additional lipid PS included in cancer cell membrane has a destabilizing effect on the membrane structure. SEM images reveal that Pep-1 form superstructures including spherical particles and fibrils in the presence of both model membranes. PS seems to enhance peptide transport across cellular membranes. The biophysical techniques in this study provide valuable insights into the properties of CPPs in drug delivery systems.     

Cholesterol in Negatively Charged Lipid Bilayers Modulates the Effect of the Antimicrobial Protein Granulysin

The release of granulysin, a 9-kDa cationic protein, from lysosomal granules of cytotoxic T lymphocytes and natural killer cells plays an important role in host defense against microbial pathogens. Granulysin is endocytosed by the infected target cell via lipid rafts and kills subsequently intracellular bacteria. The mechanism by which granulysin binds to eukaryotic and prokaryotic cells but lyses only the latter is not well understood. We have studied the effect of granulysin on large unilamellar vesicles (LUVs) and supported bilayers with prokaryotic and eukaryotic lipid mixtures or model membranes with various lipid compositions and charges. Binding of granulysin to bilayers with negative charges, as typically found in bacteria and lipid rafts of eukaryotic cells, was shown by immunoblotting. Fluorescence release assays using LUV revealed an increase in permeability of prokaryotic, negatively charged and lipid raft-like bilayers devoid of cholesterol. Changes in permeability of these bilayers could be correlated to defects of various sizes penetrating supported bilayers as shown by atomic force microscopy. Based on these results, we conclude that granulysin causes defects in negatively charged cholesterol-free membranes, a membrane composition typically found in bacteria. In contrast, granulysin is able to bind to lipid rafts in eukaryotic cell membranes, where it is taken up by the endocytotic pathway, leaving the cell intact.     

Fatty acid-albumin complexes and the determination of the transport of long chain free fatty acids across membranes

Understanding the mechanism that governs the transport of long chain free fatty acids (FFA) across lipid bilayers is critical for understanding transport across cell membranes. Conflicting results have been reported for lipid vesicles; most investigators report that flip-flop occurs within the resolution time of the method (<5 ms) and that dissociation from the membrane is rate limiting, while other studies find that flip-flop is rate limiting and on the order of seconds. We have reinvestigated this problem and find that the methods used in studies reporting rapid flip-flop have not been interpreted correctly. We find that accurate information about transport of FFA across lipid vesicles requires that FFA be delivered to the vesicles as complexes with albumin (BSA). For example, we find that stopped-flow mixing of uncomplexed FFA with small unilamellar vesicles (SUV) containing pyranine yields the very fast influx rates reported previously (>100 s(-1)). However, these influx rates increase linearly with lipid vesicle concentration and can therefore not, as previously interpreted, represent flip-flop. In contrast, measurements of influx rates in SUV and giant unilamellar vesicles performed with oleate-BSA complexes reveal no dependence on vesicle concentration and yield influx rate constants of approximately 4 and approximately 0.5 s(-1), respectively. Rate constants for efflux and dissociation were determined from the transfer of oleate from vesicles to BSA and reveal similar influx and efflux but dissociation rate constants that are approximately 5-10-fold greater. We conclude that flip-flop is rate limiting for transport of FFA across lipid vesicles and slows with an increasing radius of curvature. These results, in contrast to those reporting that flip-flop is extremely fast, indicate that the lipid bilayer portion of biological membranes may present a significant barrier to transport of FFA across cell membranes.