Boundary region between coexisting lipid phases as initial binding sites for Escherichia coli alpha-hemolysin: a real-time study.

α-Hemolysin (HlyA) is a protein toxin, a member of the pore-forming Repeat in Toxin (RTX) family, secreted by some pathogenic strands of Escherichia coli. The mechanism of action of this toxin seems to involve three stages that ultimately lead to cell lysis: binding, insertion, and oligomerization of the toxin within the membrane. Since the influence of phase segregation on HlyA binding and insertion in lipid membranes is not clearly understood, we explored at the meso- and nanoscale-both in situ and in real-time-the interaction of HlyA with lipid monolayers and bilayers. Our results demonstrate that HlyA could insert into monolayers of dioleoylphosphatidylcholine/sphingomyelin/cholesterol (DOPC/16:0SM/Cho) and DOPC/24:1SM/Cho. The time course for HlyA insertion was similar in both lipidic mixtures. HlyA insertion into DOPC/16:0SM/Cho monolayers, visualized by Brewster-angle microscopy (BAM), suggest an integration of the toxin into both the liquid-ordered and liquid-expanded phases. Atomic-force-microscopy imaging reported that phase boundaries favor the initial binding of the toxin, whereas after a longer time period the HlyA becomes localized into the liquid-disordered (Ld) phases of supported planar bilayers composed of DOPC/16:0SM/Cho. Our AFM images, however, showed that the HlyA interaction does not appear to match the general strategy described for other invasive proteins. We discuss these results in terms of the mechanism of action of HlyA.

Novel evidence for the specific interaction between cholesterol and α-haemolysin of Escherichia coli.

Several toxins that act on animal cells present different, but specific, interactions with cholesterol or sphingomyelin. In the present study we demonstrate that HlyA (α-haemolysin) of Escherichia coli interacts directly with cholesterol. We have recently reported that HlyA became associated with detergent-resistant membranes enriched in cholesterol and sphingomyelin; moreover, toxin oligomerization, and hence haemolytic activity, diminishes in cholesterol-depleted erythrocytes. Considering these results, we studied the insertion process, an essential step in the lytic mechanism, by the monolayer technique, finding that HlyA insertion is favoured in cholesterol- and sphingomyelin-containing membranes. On the basis of this result, we studied the direct interaction with either of the lipids by lipid dot blotting, lysis inhibition and SPR (surface plasmon resonance) assays. The results of the present study demonstrated that an interaction between cholesterol and HlyA exists that seems to favour a conformational state of the protein that allows its correct insertion into the membrane and its further oligomerization to form pores.

Arginine-based surfactants alter the rheological and in-plane structural properties of stratum corneum model membranes.

HYPOTHESIS: Amino acid-based surfactants have been proposed as skin permeation enhancers. In this work, we investigated the potentiality of two arginine-based amphiphiles as permeation enhancers by studying their interaction with stratum corneum (SC) model lipid membranes. EXPERIMENTS: Nα-benzoyl arginine decyl- and dodecylamide were tested in comparison with the classical enhancer, oleic acid, and the non-enhancer, stearic acid. Two complementary approaches were used: lipid monolayers, taken as models of the unit film layer of SC, and atomistic molecular dynamics simulations. FINDINGS: The arginine-based amphiphiles studied were able to be incorporated into the SCM membrane and alter its rheological and structural properties by disordering the lipid chains, enhancing membrane elasticity, and thinning the overall membrane. They also affected the lateral structure of heterogeneous SC membranes at the nanoscale by relaxing and rounding the domain borders. Our work shows that the alteration observed of the overall rheological and structural properties of the SC membranes appears to be a shared ability for several amphiphilic permeation enhancers. Our results encourage future exploration of those amphiphiles as skin permeation enhancers.

Histidine 19 Residue Is Essential for Cell Internalization of Antifungal Peptide SmAPα1-21 Derived from the α-Core of the Silybum marianum Defensin DefSm2-D in Fusarium graminearum.

The synthetic peptide SmAPα1-21 (KLCEKPSKTWFGNCGNPRHCG) derived from DefSm2-D defensin α-core is active at micromolar concentrations against the phytopathogenic fungus Fusarium graminearum and has a multistep mechanism of action that includes alteration of the fungal cell wall and membrane permeabilization. Here, we continued the study of this peptide's mode of action and explored the correlation between the biological activity and its primary structure. Transmission electron microscopy was used to study the ultrastructural effects of SmAPα1-21 in conidial cells. New peptides were designed by modifying the parent peptide SmAPα1-21 (SmAPH19R and SmAPH19A, where His19 was replaced by Arg or Ala, respectively) and synthesized by the Fmoc solid phase method. Antifungal activity was determined against F. graminearum. Membrane permeability and subcellular localization in conidia were studied by confocal laser scanning microscopy (CLSM). Reactive oxygen species (ROS) production was assessed by fluorescence spectroscopy and CLSM. SmAPα1-21 induced peroxisome biogenesis and oxidative stress through ROS production in F. graminearum and was internalized into the conidial cells' cytoplasm. SmAPH19R and SmAPH19A were active against F. graminearum with minimal inhibitory concentrations (MICs) of 38 and 100 µM for SmAPH19R and SmAPH19A, respectively. The replacement of His19 by Ala produced a decrease in the net charge with a significant increase in the MIC, thus evidencing the importance of the positive charge in position 19 of the antifungal peptide. Like SmAPα1-21, SmAP2H19A and SmAP2H19R produced the permeabilization of the conidia membrane and induced oxidative stress through ROS production. However, SmAPH19R and SmAPH19A were localized in the conidia cell wall. The replacement of His19 by Ala turned all the processes slower. The extracellular localization of peptides SmAPH19R and SmAPH19A highlights the role of the His19 residue in the internalization.

Asymmetric bilayers mimicking membrane rafts prepared by lipid exchange: Nanoscale characterization using AFM-Force spectroscopy.

Sphingolipids-enriched rafts domains are proposed to occur in plasma membranes and to mediate important cellular functions. Notwithstanding, the asymmetric transbilayer distribution of phospholipids that exists in the membrane confers the two leaflets different potentials to form lateral domains as next to no sphingolipids are present in the inner leaflet. How the physical properties of one leaflet can influence the properties of the other and its importance on signal transduction across the membrane are questions still unresolved. In this work, we combined AFM imaging and Force spectroscopy measurements to assess domain formation and to study the nanomechanical properties of asymmetric supported lipid bilayers (SLBs) mimicking membrane rafts. Asymmetric SLBs were formed by incorporating N-palmitoyl-sphingomyelin (16:0SM) into the outer leaflet of preformed 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC)/Cholesterol SLBs through methyl-ß-cyclodextrin-mediated lipid exchange. Lipid domains were detected after incorporation of 16:0SM though their phase state varied from gel to liquid ordered (Lo) phase if the procedure was performed at 24 or 37 °C, respectively. When comparing symmetric and asymmetric Lo domains, differences in size and morphology were observed, with asymmetric domains being smaller and more interconnected. Both types of Lo domains showed similar mechanical stability in terms of rupture forces and Young's moduli. Notably, force curves in asymmetric domains presented two rupture events that could be attributed to the sequential rupture of a liquid disordered (Ld) and a Lo phase. Interleaflet coupling in asymmetric Lo domains could also be inferred from those measurements. The experimental approach outlined here would significantly enhance the applicability of membrane models.

Interaction of acylated and unacylated forms of E. coli alpha-hemolysin with lipid monolayers: a PM-IRRAS study.

Uropathogenic strains of Escherichia coli produce virulence factors, such as the protein toxin alpha-hemolysin (HlyA), that enable the bacteria to colonize the host and establish an infection. HlyA is synthetized as a protoxin (ProHlyA) that is transformed into the active form in the bacterial cytosol by the covalent linkage of two fatty-acyl moieties to the polypeptide chain before the secretion of HlyA into the extracellular medium. The aim of this work was to investigate the effect of the fatty acylation of HlyA on protein conformation and protein-membrane interactions. Polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS) experiments were performed at the air-water interface, and lipid monolayers mimicking the outer leaflet of red-blood-cell membranes were used as model systems for the study of protein-membrane interaction. According to surface-pressure measurements, incorporation of the acylated protein into the lipid films was faster than that of the nonacylated form. PM-IRRAS measurements revealed that the adsorption of the proteins to the lipid monolayers induced disorder in the lipid acyl chains and also changed the elastic properties of the films independently of protein acylation. No significant difference was observed between HlyA and ProHlyA in the interaction with the model lipid monolayers; but when these proteins became adsorbed on a bare air-water interface, they adopted different secondary structures. The assumption of the correct protein conformation at a hydrophobic-hydrophilic interface could constitute a critical condition for biologic activity.

[Therapeutic applications of pore-forming lytic toxins: potential use of Escherichia coli alpha-hemolysin]. / Aplicaciones terapéuticas de toxinas líticas formadoras de poros: potencialidades de alpha-hemolisina de Escherichia coli.

Many infectious bacteria export soluble proteins which can damage the plasma membrane of eukaryotic cells. Most often they are directed against leukocytes for the purpose of reducing the immune response of the host. In some cases, these toxins are also hemolytic. It has been proposed that both leukotoxic and hemolytic activities could derive from the pore formation in the membranes of the attacked cells. The study of these molecules is not only important from the point of view of basic studies to determine the mechanism of action, but also for potential application in biotechnology and medicine. These molecules increase the cell susceptibility to chemotherapy and also can be employed to destroy specifically cancer cells. On the other hand, it is possible to incorporate toxin molecules in liposomes, transforming them in to biosensors or as controlled drug delivery systems. This aspect has not been extensively explored in Escherichia coli alpha-hemolysin, in which the presence of different functional and structural domains in this molecule could be taken advantage of.

Aplicaciones terapeuticas de toxinas liticas formadoras de poros: potencialidades de alfa-hemolisina de Escherichia coli / Therapeutic applications of pore-forming lytic toxins: potential use of Escherichia coli alpha-hemolysin

Many infectious bacteria export soluble proteins which can damage the plasma membrane of eukaryotic cells. Most often they are directed against leukocytes for the purpose of reducing the immune response of the host. In some cases, these toxins are also hemolytic. It has been proposed that both leukotoxic and hemolytic activities could derive from the pore formation in the membranes of the attacked cells. The study of these molecules is not only important from the point of view of basic studies to determine the mechanism of action, but also for potential application in biotechnology and medicine. These molecules increase the cell susceptibility to chemotherapy and also can be employed to destroy specifically cancer cells. On the other hand, it is possible to incorporate toxin molecules in liposomes, transforming them in to biosensors or as controlled drug delivery systems. This aspect has not been extensively explored in Escherichia coli alpha-hemolysin, in which the presence of different functional and structural domains in this molecule could be taken advantage of.

Vectores lipidicos. Nuevas estrategias aplicadas a la terapia génica / Lipid vectors. New strategies for gene therapy

A diferencia del resto de las moléculas biológicas, los fosfolípidos son capaces de autoensamblarse espontáneamente. Con ellos es relativamente simples generar estructuras selladas extremadamente estables, de tamaño, forma y empaquetamiento controlables, llamadas liposomas. En este artículo revisaremos el uso de liposomas para generar vectores que mejoren los procesos de transfección en células eucarioticas, tanto in vivo como in vitro. Empleando vectores lipídicos, es potencialmente posible enviar selectivamente un segmento de AND a cualquier sitio del cuerpo, forzarlo a ingresar al interior celular y aun controlar el destino intracelular de la carga transportada. La clave del éxito de la transfección por medio de vectores lipídicos radica en que protegen mecánicamente al AND de la degradación plasmática, ofreciendo a la vez la oportunidad de controlar su biodistribuición, independientemente del tamaño del segmento de AND que se quiera expresar. Asimismo, son no carcinogénicos y pobremente inmunogénicos. Los avances en la química de sintésis de lípidos permitirán construir vectores cada vez más eficientes, que capitan con los altos niveles de transfección de los vectores virales, sumado a las ventajas de extrema versatilidad, facilidad de preparación y bioseguridad propias de la moléculas autoensamblables.