Antimicrobial Peptides Increase Line Tension in Raft-Forming Lipid Membranes

The formation of phase separated membrane domains is believed to be essential for the function of the cell. The precise composition and physical properties of lipid bilayer domains play crucial roles in regulating protein activity and governing cellular processes. Perturbation of the domain structure in human cells can be related to neurodegenerative diseases and cancer. Lipid rafts are also believed to be essential in bacteria, potentially serving as targets for antibiotics. An important question is how the membrane domain structure is affected by bioactive and therapeutic molecules, such as surface-active peptides, which target cellular membranes. Here we focus on antimicrobial peptides (AMPs), crucial components of the innate immune system, to gain insights into their interaction with model lipid membranes containing domains. Using small-angle neutron/X-ray scattering (SANS/SAXS), we show that the addition of several natural AMPs (indolicidin, LL-37, magainin II, and aurein 2.2) causes substantial growth and restructuring of the domains, which corresponds to increased line tension. Contrast variation SANS and SAXS results demonstrate that the peptide inserts evenly in both phases, and the increased line tension can be related to preferential and concentration dependent thinning of the unsaturated membrane phase. We speculate that the lateral restructuring caused by the AMPs may have important consequences in affecting physiological functions of real cells. This work thus shines important light onto the complex interactions and lateral (re)organization in lipid membranes, which is relevant for a molecular understanding of diseases and the action of antibiotics.

Small-angle X-ray and neutron scattering applied to lipid-based nanoparticles: Recent advancements across different length scales

Lipid-based nanoparticles (LNPs), ranging from nanovesicles to non-lamellar assemblies, have gained significant attention in recent years, as versatile carriers for delivering drugs, vaccines, and nutrients. Small-angle scattering methods, employing X-rays (SAXS) or neutrons (SANS), represent unique tools to unveil structure, dynamics, and interactions of such particles on different length scales, spanning from the nano to the molecular scale. This review explores the state-of-the-art on scattering methods applied to unveil the structure of lipid-based nanoparticles and their interactions with drugs and bioactive molecules, to inform their rational design and formulation for medical applications. We will focus on complementary information accessible with X-rays or neutrons, ranging from insights on the structure and colloidal processes at a nanoscale level (SAXS) to details on the lipid organization and molecular interactions of LNPs (SANS). In addition, we will review new opportunities offered by Time-resolved (TR)-SAXS and -SANS for the investigation of dynamic processes involving LNPs. These span from real-time monitoring of LNPs structural evolution in response to endogenous or external stimuli (TR-SANS), to the investigation of the kinetics of lipid diffusion and exchange upon interaction with biomolecules (TR-SANS). Finally, we will spotlight novel combinations of SAXS and SANS with complementary on-line techniques, recently enabled at Large Scale Facilities for X-rays and neutrons. This emerging technology enables synchronized multi-method investigation, offering exciting opportunities for the simultaneous characterization of the structure and chemical or mechanical properties of LNPs.

Studying lipid flip-flop in asymmetric liposomes using 1H NMR and TR-SANS

The specific spatial and temporal distribution of lipids in membranes play a crucial role in determining the biochemical and biophysical properties of the system. In nature, the asymmetric distribution of lipids is a dynamic process with ATP-dependent lipid transporters maintaining asymmetry, and passive transbilayer diffusion, that is, flip-flop, counteracting it. In this chapter, two probe-free techniques, 1H NMR and time-resolved small angle neutron scattering, are described in detail as methods of investigating lipid flip-flop rates in synthetic liposomes that have been generated with an asymmetric bilayer composition.

Recent Progress in Solution Structure Studies of Photosynthetic Proteins Using Small-Angle Scattering Methods

Utilized for gaining structural insights, small-angle neutron and X-ray scattering techniques (SANS and SAXS, respectively) enable an examination of biomolecules, including photosynthetic pigment-protein complexes, in solution at physiological temperatures. These methods can be seen as instrumental bridges between the high-resolution structural information achieved by crystallography or cryo-electron microscopy and functional explorations conducted in a solution state. The review starts with a comprehensive overview about the fundamental principles and applications of SANS and SAXS, with a particular focus on the recent advancements permitting to enhance the efficiency of these techniques in photosynthesis research. Among the recent developments discussed are: (i) the advent of novel modeling tools whereby a direct connection between SANS and SAXS data and high-resolution structures is created; (ii) the employment of selective deuteration, which is utilized to enhance spatial selectivity and contrast matching; (iii) the potential symbioses with molecular dynamics simulations; and (iv) the amalgamations with functional studies that are conducted to unearth structure-function relationships. Finally, reference is made to time-resolved SANS/SAXS experiments, which enable the monitoring of large-scale structural transformations of proteins in a real-time framework.

Accelerating Lipid Flip-Flop at Low Concentrations: A General Mechanism for Membrane Binding Peptides

We report a physicochemical investigation of the lipid transport properties of model lipid membranes in the presence of the antimicrobial peptide indolicidin through comparisons of experimental SANS/SAXS scattering techniques to fully atomistic molecular dynamics simulations. In agreement with the experiment, we show that upon peripheral binding of the peptides, even at low concentrations, lipid flip-flop dynamics is greatly accelerated. Computer modeling elucidates the interplay between structural changes and lipid dynamics induced by peptides and proposes a mechanism for the mode of action of antimicrobial peptides, assessing the major role of entropy for the catalysis of the flipping events. The mechanism introduced here is universal for all peptides with preferential peripheral binding to the membrane as it does not depend on the specific amino acid sequence.

Small-angle scattering techniques for peptide and peptide hybrid nanostructures and peptide-based biomaterials

The use of small-angle scattering (SAS) in the study of the self-assembly of peptides and peptide conjugates (lipopeptides, polymer-peptide conjugates and others) is reviewed, highlighting selected research that illustrates different methods and analysis techniques. Both small-angle x-ray scattering (SAXS) and small-angle neutron scattering (SANS) are considered along with examples that exploit their unique capabilities. For SAXS, this includes the ability to perform rapid measurements enabling high throughput or fast kinetic studies and measurements under dilute conditions. For SANS, contrast variation using H2O/D2O mixtures enables the study of peptides interacting with lipids and TR-SANS (time-resolved SANS) studies of exchange kinetics and/or peptide-induced structural changes. Examples are provided of studies measuring form factors of different self-assembled structures (micelles, fibrils, nanotapes, nanotubes etc) as well as structure factors from ordered phases (lyotropic mesophases), peptide gels and hybrid materials such as membranes formed by mixing peptides with polysaccharides or peptide/liposome mixtures. SAXS/WAXS (WAXS: wide-angle x-ray scattering) on peptides and peptide hybrids is also discussed, and the review concludes with a perspective on potential future directions for research in the field.

Small-Angle Neutron Scattering for Studying Lipid Bilayer Membranes

Small-angle neutron scattering (SANS) is a powerful tool for studying biological membranes and model lipid bilayer membranes. The length scales probed by SANS, being from 1 nm to over 100 nm, are well-matched to the relevant length scales of the bilayer, particularly when it is in the form of a vesicle. However, it is the ability of SANS to differentiate between isotopes of hydrogen as well as the availability of deuterium labeled lipids that truly enable SANS to reveal details of membranes that are not accessible with the use of other techniques, such as small-angle X-ray scattering. In this work, an overview of the use of SANS for studying unilamellar lipid bilayer vesicles is presented. The technique is briefly presented, and the power of selective deuteration and contrast variation methods is discussed. Approaches to modeling SANS data from unilamellar lipid bilayer vesicles are presented. Finally, recent examples are discussed. While the emphasis is on studies of unilamellar vesicles, examples of the use of SANS to study intact cells are also presented.

Molecular Transport and Growth of Lipid Vesicles Exposed to Antimicrobial Peptides

It is well-known that lipids constituting the cytoplasmic membrane undergo continuous reorganization to maintain the appropriate composition important for the integrity of the cell. The transport of lipids is controlled by mainly membrane proteins, but also spontaneous lipid transport between leaflets, lipid "flip-flop", occurs. These processes do not only occur spontaneously under equilibrium, but also promote structural rearrangements, morphological transitions, and growth processes. It has previously been shown that intravesicular lipid "flip-flop" and intervesicular lipid exchange under equilibrium can be deduced indirectly from contrast variation time-resolved small-angle neutron scattering (TR-SANS) where the molecules are "tagged" using hydrogen/deuterium (H/D) substitution. In this work, we show that this technique can be extended to simultaneously detect changes in the growth and the lipid "flip-flop" and exchange rates induced by a peptide additive on lipid vesicles consisting of DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine), d-DMPC (1,2-dimyristoyl-d54-sn-glycero-3-phosphocholine), DMPG (1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol)), and small amounts of DMPE-PEG (1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000]). Changes in the overall size were independently monitored using dynamic light scattering (DLS). We find that the antimicrobial peptide, indolicidin, accelerates lipid transport and additionally induces limited vesicular growth. Moreover, in TR-SANS experiments using partially labeled lipid mixtures to separately study the kinetics of the lipid components, we show that, whereas peptide addition affects both lipids similarly, DMPG exhibits faster kinetics. We find that vesicular growth is mainly associated with peptide-mediated lipid reorganization that only slightly affects the overall exchange kinetics. This is confirmed by a TR-SANS experiment of vesicles preincubated with peptide showing that after pre-equilibration the kinetics are only slightly slower.

Time-Resolved SANS to Measure Monomer Inter-Bilayer Exchange and Intra-Bilayer Translocation

The monomeric exchange kinetics of sub-micron particles provide insight into their stability and dynamism. Traditional techniques used to measure the intra- and inter-particle exchange often require monitoring the transfer of bulky and perturbing fluorescent labels. Time-resolved small angle neutron scattering (TR-SANS) overcomes these flaws by isotope labeling, allowing for the monomeric exchange rate determination of unperturbed, stress-free particles. Here, we describe TR-SANS in detail and novel applications of the technique.

Time-resolved SANS reveals pore-forming peptides cause rapid lipid reorganization

Time-resolved SANS showed alamethicin and melittin promote DMPC lipid vesicle mixing and perturb DMPC kinetics in similar ways.