Membrane interactions and biological activity of antimicrobial peptides from Australian scorpion

UyCT peptides are antimicrobial peptides isolated from the venom of the Australian scorpion. The activity of the UyCT peptides against Gram positive and Gram negative bacteria and red blood cells was determined. The membrane interactions of these peptides were evaluated by dye release (DR) of the fluorophore calcein from liposomes and isothermal titration calorimetry (ITC); and their secondary structure was determined by circular dichroism (CD). Three different lipid systems were used to mimic red blood cells, Escherichia coli and Staphylococcus aureus membranes. UyCT peptides exhibited broad spectrum antimicrobial activity with low MIC for S. aureus and multi-drug resistant Gram negative strains. Peptide combinations showed some synergy enhancing their potency but not hemolytic activity. The UyCT peptides adopted a helical structure in lipid environments and DR results confirmed that the mechanism of action is by disrupting the membrane. ITC data indicated that UyCT peptides preferred prokaryotic rather than eukaryotic membranes. The overall results suggest that UyCT peptides could be pharmaceutical leads for the treatment of Gram negative multiresistant bacterial infections, especially against Acinetobacter baumanni, and candidates for peptidomimetics to enhance their potency and minimize hemolysis. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.     

Forest progression modes in littoral Congo, Central Atlantic Africa

Aim To understand the persistence of a forest–savanna mosaic in places where rainfall data suggest that forest take‐over should take place. To study the various modes of forest encroachment, and the role of human activities to hamper it. Location Data were collected on several forest–savanna ecotones in the coastal region of the Republic of Congo. The sites were chosen to illustrate the differing principal modes of forest expansion, corresponding to different levels of anthropic pressure. Methods The study sites were situated on five transects perpendicular to the ecotone (total sampled area: 1.7 ha) and 10 forest clumps in savanna (with diameters from 3 to 20 m). Along the transects botanical identification, diameter measurement and cartography were performed, while leaf area index was measured at a high resolution (every metre) along two of them. Collected data were analysed using a continuous quantification approach, which is much more useful than classical quadrat analysis. Time calibration of progression rates was performed using a simple model of the growth of the characteristic pioneer species, Aucoumea klaineana. Results The two main different modes are reflected in different successional patterns. The edge diffusion is slow (its rate is evaluated to c. 1 m year^?1) and is characterized by a progressive increase in large‐diameter tree density and shade‐tolerant tree density away from the ecotone. Conversely, savanna to forest phase transition by coalescence of clumps exhibits high tree density remnants distributed in established forest. The composition of these remnants is compatible with that of the forest clumps in savannas. Main conclusions Three functional groups of pioneer trees are distinguished: some occupy the edge (edge pioneer), others establish clumps of forest in savanna (clump pioneers) and the longer‐living A. klaineana ensures the transition to ‘mature’ forest. The two different observed patterns (linear edge progression and clump coalescence) can be understood with the use of a model of forest–savanna dynamics, ‘FORSAT’. The two control parameters are the annual rainfall and the frequency of man‐made fires in each savanna. In particular, an increase in the fire frequency can lead to a shift from the coalescence regime to the edge progression one.     

Disentanglement of heterogeneous dynamics in mixed lipid systems

Static phosphorous NMR has been a powerful technique for the study of model supramolecular phospholipid structures. Application to natural lipid bilayers with complex compositions, however, has been severely limited by the difficulty in deconvoluting overlapping broad lineshapes. We demonstrate a solution to this problem, using a global fit to a few slow magic-angle spinning spectra, in combination with an adaptation of Boltzmann statistics maximum entropy. The method provides a model-free means to characterize a heterogeneous mix of lipid dynamics via a distribution of ³¹P chemical shift anisotropies. It is used here to identify clear changes in membrane dynamics of a phosphatidylethanolamine and phosphatidylglycerol mixture, mimicking an Escherichia coli membrane upon addition of just 2% of the antimicrobial peptide maculatin 1.1. This illustration opens the prospect for investigation of arbitrarily complex natural lipid systems, important in many areas of biophysical chemistry and biomedicine.     

Catechin treatment improves cerebrovascular flow-mediated dilation and learning abilities in atherosclerotic mice

Severe dyslipidemia and the associated oxidative stress could accelerate the age-related decline in cerebrovascular endothelial function and cerebral blood flow (CBF), leading to neuronal loss and impaired learning abilities. We hypothesized that a chronic treatment with the polyphenol catechin would prevent endothelial dysfunction, maintain CBF responses, and protect learning abilities in atherosclerotic (ATX) mice. We treated ATX (C57Bl/6-LDLR(-/-)hApoB(+/+); 3 mo old) mice with catechin (30 mg · kg(-1) · day(-1)) for 3 mo, and C57Bl/6 [wild type (WT), 3 and 6 mo old] mice were used as controls. ACh- and flow-mediated dilations (FMD) were recorded in pressurized cerebral arteries. Basal CBF and increases in CBF induced by whisker stimulation were measured by optical coherence tomography and Doppler, respectively. Learning capacities were evaluated with the Morris water maze test. Compared with 6-mo-old WT mice, cerebral arteries from 6-mo-old ATX mice displayed a higher myogenic tone, lower responses to ACh and FMD, and were insensitive to NOS inhibition (P < 0.05), suggesting endothelial dysfunction. Basal and increases in CBF were lower in 6-mo-old ATX than WT mice (P < 0.05). A decline in the learning capabilities was also observed in ATX mice (P < 0.05). Catechin 1) reduced cerebral superoxide staining (P < 0.05) in ATX mice, 2) restored endothelial function by reducing myogenic tone, improving ACh- and FMD and restoring the sensitivity to nitric oxide synthase inhibition (P < 0.05), 3) increased the changes in CBF during stimulation but not basal CBF, and 4) prevented the decline in learning abilities (P < 0.05). In conclusion, catechin treatment of ATX mice prevents cerebrovascular dysfunctions and the associated decline in learning capacities.     

Contribution of social isolation, restraint, and hindlimb unloading to changes in hemodynamic parameters and motion activity in rats

The most accepted animal model for simulation of the physiological and morphological consequences of microgravity on the cardiovascular system is one of head-down hindlimb unloading. Experimental conditions surrounding this model include not only head-down tilting of rats, but also social and restraint stresses that have their own influences on cardiovascular system function. Here, we studied levels of spontaneous locomotor activity, blood pressure, and heart rate during 14 days under the following experimental conditions: cage control, social isolation in standard rat housing, social isolation in special cages for hindlimb unloading, horizontal attachment (restraint), and head-down hindlimb unloading. General activity and hemodynamic parameters were continuously monitored in conscious rats by telemetry. Heart rate and blood pressure were both evaluated during treadmill running to reveal cardiovascular deconditioning development as a result of unloading. The main findings of our work are that: social isolation and restraint induced persistent physical inactivity, while unloading in rats resulted in initial inactivity followed by normalization and increased locomotion after one week. Moreover, 14 days of hindlimb unloading showed significant elevation of blood pressure and slight elevation of heart rate. Hemodynamic changes in isolated and restrained rats largely reproduced the trends observed during unloading. Finally, we detected no augmentation of tachycardia during moderate exercise in rats after 14 days of unloading. Thus, we concluded that both social isolation and restraint, as an integral part of the model conditions, contribute essentially to cardiovascular reactions during head-down hindlimb unloading, compared to the little changes in the hydrostatic gradient.     

The lipid network

Natural cell membranes are composed of a remarkable variety of lipids, which provide specific biophysical properties to support membrane protein function. An improved understanding of this complexity of membrane composition may also allow the design of membrane active drugs. Crafting a relevant model of a cell membrane with controlled composition is becoming an art, with the ability to reveal the molecular mechanisms of biological processes and lead to better treatment of pathologies. By matching physiological observations from in vivo experiments to high-resolution information, more easily obtained from in vitro studies, complex interactions at the lipid interface are determined. The role of the lipid network in biological membranes is, therefore, the subject of increasing attention.     

Interactions of a synthetic Leu–Lys-rich antimicrobial peptide with phospholipid bilayers

The interaction of the synthetic antimicrobial peptide P5 (KWKKLLKKPLLKKLLKKL-NH₂) with model phospholipid membranes was studied using solid-state NMR and circular dichroism (CD) spectroscopy. P5 peptide had little secondary structure in buffer, but addition of large unilamellar vesicles (LUV) composed of dimyristoylphosphatidylcholine (DMPC) increased the β-sheet content to ~20%. Addition of negatively charged LUV, DMPC–dimyristoylphosphatidylglycerol (DMPG) 2:1, led to a substantial (~40%) increase of the α-helical conformation. The peptide structure did not change significantly above and below the phospholipid phase transition temperature. P5 peptide interacted differently with DMPC bilayers with deuterated acyl chains (d₅₄-DMPC) and mixed d₅₄-DMPC–DMPG bilayers, used to mimic eukaryotic and prokaryotic membranes, respectively. In DMPC vesicles, P5 peptide had no significant interaction apart from slightly perturbing the upper region of the lipid acyl chain with minimum effect at the terminal methyl groups. By contrast, in the DMPC–DMPG vesicles the peptide increased disorder throughout the entire acyl chain of DMPC in the mixed bilayer. P5 promoted disordering of the headgroup of neutral membranes, observed by ³¹P NMR. However, no perturbations in the T ₁ relaxation nor the T ₂- values were observed at 30°C, although a slight change in the dynamics of the headgroup at 20°C was noticeable compared with peptide-free vesicles. However, the P5 peptide caused similar perturbations of the headgroup of negatively charged vesicles at both temperatures. These data correlate with the non-haemolytic activity of the P5 peptide against red blood cells (neutral membranes) while inhibiting bacterial growth (negatively charged membranes).     

Signaling network triggers and membrane physical properties control the actin cytoskeleton-driven isotropic phase of cell spreading

Cell spreading is regulated by signaling from the integrin receptors that activate intracellular signaling pathways to control actin filament regulatory proteins. We developed a hybrid model of whole-cell spreading in which we modeled the integrin signaling network as ordinary differential equations in multiple compartments, and cell spreading as a three-dimensional stochastic model. The computed activity of the signaling network, represented as time-dependent activity levels of the actin filament regulatory proteins, is used to drive the filament dynamics. We analyzed the hybrid model to understand the role of signaling during the isotropic phase of fibroblasts spreading on fibronectin-coated surfaces. Simulations showed that the isotropic phase of spreading depends on integrin signaling to initiate spreading but not to maintain the spreading dynamics. Simulations predicted that signal flow in the absence of Cdc42 or WASP would reduce the spreading rate but would not affect the shape evolution of the spreading cell. These predictions were verified experimentally. Computational analyses showed that the rate of spreading and the evolution of cell shape are largely controlled by the membrane surface load and membrane bending rigidity, and changing information flow through the integrin signaling network has little effect. Overall, the plasma membrane acts as a damper such that only ∼5% of the actin dynamics capability is needed for isotropic spreading. Thus, the biophysical properties of the plasma membrane can condense varying levels of signaling network activities into a single cohesive macroscopic cellular behavior.