Folding of the apolipoprotein A1 driven by the salt concentration as a possible mechanism to improve cholesterol trapping

The folding of the cholesterol trapping apolipoprotein A1 in aqueous solution at increasing ionic strength is studied using atomically detailed molecular dynamics simulations. We calculate various structural properties to characterize the conformation of the protein, such as the radius of gyration, the radial distribution function and the end to end distance. Additionally we report information using tools specifically tailored for the characterization of proteins, such as the mean smallest distance matrix and the Ramachandran plot. We find that two qualitatively different configurations of this protein are preferred, one where the protein is extended, and one where it forms loops or closed structures. It is argued that the latter promote the association of the protein with cholesterol and other fatty acids.Comment: 14 pages, 6 figures. To appear in "Selected Topics of Computational and Experimental Fluid Mechanics", Springer, J. Klapp, G. Ru\'iz, A. Medina, A. L\'opez & L. Di G. Sigalotti (eds.), 201

Divalent Metal Ion Triggered Activity of a Synthetic Antimicrobial in Cardiolipin Membranes

One member of a prototypical class of antimicrobial oligomers was used to study pore formation in cardiolipin rich membranes. Using both vesicle dye-leakage assays and small angle x-ray scattering, bilayer remodeling was studied. The results indicate that the presence of negative intrinsic curvature (NIC) lipids is essential for pore formation by this class of molecules: In Gram-positive bacteria, cardiolipin and divalent metal cations like Ca(2+) and Mg(2+) are needed. This is completely consistent with the role of phosphatidylethanolamine (PE) lipid in Gram-negative bacteria, where antimicrobial activity is dependent on the negative intrinsic curvature of PE, rather than a specific interaction with PE

Alternating hemiplegia syndrome: Electroencephalogram, brain mapping, and brain perfusion SPECT scan study in a Chinese girl

A 3-year-old Chinese girl with alternating hemiplegia syndrome failed to respond to anticonvulsants, antimigrainous drugs, and calcium channel blockers. She made a complete remission with a 4-week course of steroid, and relapsed after steroid withdrawal. Electroencephalogram and brain mapping during the hemiplegic attack showed unilateral high-voltage sharp slow-wave discharges in the temporo-occipital region contralateral to the hemiplegic side and diffuse high-voltage slowing during attacks of quadriplegia or other clinical manifestation such as dullness, lethargy, or yawning. Brain perfusion single photon emission computed tomographic (SPECT) scan study during the attack showed decreased uptake in the temporoparietal region contralateral to the hemiplegic side and in the ipsilateral basal ganglia, whereas the perfusion was normal between attacks. Electroencephalogram background activity was improved while the child was in clinical remission with steroid treatment. Computed tomographic and magnetic resonance imaging scans of the brain were normal. Carotid angiogram failed to show any structural or dynamic changes of the carotid arteries. The possible mechanism underlying alternating hemiplegia syndrome might be transient and reversible cerebral ischemia with high-voltage slow-wave discharges shown in the electroencephalogram and decreased perfusion in SPECT scan.link_to_subscribed_fulltex

Multicellular self-organization of P. aeruginosa due to interactions with secreted trails

Guided movement in response to slowly diffusing polymeric trails provides a unique mechanism for self-organization of some microorganisms. To elucidate how this signaling route leads to microcolony formation, we experimentally probe the trajectory and orientation of Pseudomonas aeruginosa that propel themselves on a surface using type IV pili motility appendages, which preferentially attach to deposited exopolysaccharides. We construct a stochastic model by analyzing single-bacterium trajectories, and show that the resulting theoretical prediction for the many-body behavior of the bacteria is in quantitative agreement with our experimental characterization of how cells explore the surface via a power law strategy

Helical antimicrobial peptides assemble into protofibril scaffolds that present ordered dsDNA to TLR9.

Amphiphilicity in ɑ-helical antimicrobial peptides (AMPs) is recognized as a signature of potential membrane activity. Some AMPs are also strongly immunomodulatory: LL37-DNA complexes potently amplify Toll-like receptor 9 (TLR9) activation in immune cells and exacerbate autoimmune diseases. The rules governing this proinflammatory activity of AMPs are unknown. Here we examine the supramolecular structures formed between DNA and three prototypical AMPs using small angle X-ray scattering and molecular modeling. We correlate these structures to their ability to activate TLR9 and show that a key criterion is the AMP's ability to assemble into superhelical protofibril scaffolds. These structures enforce spatially-periodic DNA organization in nanocrystalline immunocomplexes that trigger strong recognition by TLR9, which is conventionally known to bind single DNA ligands. We demonstrate that we can "knock in" this ability for TLR9 amplification in membrane-active AMP mutants, which suggests the existence of tradeoffs between membrane permeating activity and immunomodulatory activity in AMP sequences

Synthetic antimicrobial, oligomers induce a composition-dependent topological transition in membranes

Abstract: Antimicrobial peptides (AMPs) are cationic amphiphiles that comprise a key component of innate immunity. Synthetic analogues of AMPs, such as the family of phenylene ethynylene antimicrobial oligomers (AMOs), recently demonstrated broad-spectrum antimicrobial activity, but the underlying molecular mechanism is unknown. Homologues in this family can be inactive, specifically active against bacteria, or nonspecifically active against bacteria and eukaryotic cells. Using synchrotron small-angle X-ray scattering (SAXS), we show that observed antibacterial activity correlates with an AMO-induced topological transition of small unilamellar vesicles into an inverted hexagonal phase, in which hexagonal arrays of 3.4-nm water channels defined by lipid tubes are formed. Polarized and fluorescence microscopy show that AMO-treated giant unilamellar vesicles remain intact, instead of reconstructing into a bulk 3D phase, but are selectively permeable to encapsulated macromolecules that are smaller than 3.4 nm. Moreover, AMOs with different activity profiles require different minimum threshold concentrations of phosphoethanolamine (PE) lipids to reconstruct the membrane. Using ternary membrane vesicles composed of DOPG:DOPE:DOPC with a charge density fixed at typical bacterial values, we find that the inactive AMO cannot generate the inverted hexagonal phase even when DOPE completely replaces DOPC. The specifically active AMO requires a threshold ratio of DOPE:DOPC ) 4:1, and the nonspecifically active AMO requires a drastically lower threshold ratio of DOPE:DOPC ) 1.5:1. Since most gram-negative bacterial membranes have more PE lipids than do eukaryotic membranes, our results imply that there is a relationship between negative-curvature lipids such as PE and antimicrobial hydrophobicity that contributes to selective antimicrobial activity