Interactions of membrane-active peptides with thick, neutral, nonzwitterionic bilayers

Possible Role of Bent Structure of Methylated Lithocholic Acid on Artificial and Plasma Membranes

Bile acids form micelles that are essential for the absorption of dietary lipids. However, excessive bile acid micelles can disrupt the plasma membrane by removing phospholipids, resulting in cell death. We hypothesized that the bent geometrical structure of the steroid scaffold of bile acids decreases the lipid order (similar to unsaturated phospholipids with cis double bonds), disrupting the plasma membrane. Here, lithocholic acid (LCA), a bile acid, was methylated to prevent micellization. Methylated lithocholic acid (Me-LCA) was mixed with a thin phase-separated lipid bilayer comprising 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and cholesterol (Chol). Me-LCA was not localized in the DPPC-rich rigid phase but localized in the DOPC-rich fluid phase, and excess Me-LCA did not affect the phase separation. Me-LCA is distributed in the plasma and organelle membranes. However, Me-LCA with bent structure did not affect the membrane properties, membrane fluidity, and hydrophobicity of liposomes composed of DOPC, DPPC, and Chol and also did not affect the proliferation of cells.

Melittin Induces Local Order Changes in Artificial and Biological Membranes as Revealed by Spectral Analysis of Laurdan Fluorescence

Antimicrobial peptides (AMPs) are a class of molecules widely used in applications on eukaryotic and prokaryotic cells. Independent of the peptide target, all of them need to first pass or interact with the plasma membrane of the cells. In order to have a better image of the peptide action mechanism with respect to the particular features of the membrane it is necessary to better understand the changes induced by AMPs in the membranes. Laurdan, a lipid membrane probe sensitive to polarity changes in the environment, is used in this study for assessing changes induced by melittin, a well-known peptide, both in model and natural lipid membranes. More importantly, we showed that generalized polarization (GP) values are not always efficient or sufficient to properly characterize the changes in the membrane. We proved that a better method to investigate these changes is to use the previously described log-normal deconvolution allowing us to infer other parameters: the difference between the relative areas of elementary peak (ΔS_r), and the ratio of elementary peaks areas (R_s). Melittin induced a slight decrease in local membrane fluidity in homogeneous lipid membranes. The addition of cholesterol stabilizes the membrane more in the presence of melittin. An opposite response was observed in the case of heterogeneous lipid membranes in cells, the local order of lipids being diminished. R_S proved to be the most sensitive parameter characterizing the local membrane order, allowing us to distinguish among the responses to melittin of both classes of membrane we investigated (liposomes and cellular membranes). Molecular simulation of the melittin pore in homogeneous lipid bilayer suggests that lipids are more closely packed in the proximity of the melittin pore (a smaller area per lipid), supporting the experimental observation.

Simple surface functionalization of polymersomes using non-antibacterial peptide anchors

Background

Hollow vesicles formed from block copolymers, so-called polymersomes, have been extensively studied in the last decade for their various applications in drug delivery, in diagnostics and as nanoreactors. The immobilization of proteins on the polymersomes’ surface can aid in cell targeting, lead to functional biosensors or add an additional reaction space for multistep syntheses. In almost all surface functionalization strategies to date, a chemical pre-conjugation of the polymer with a reactive group or ligand and the functionalization of the protein are required. To avoid chemical pre-conjugation, we investigated the simple and quick functionalization of preformed poly(2-methyloxazoline)-poly(dimethylsiloxane)-poly(2-methyloxazoline) (PMOXA-PDMS-PMOXA) polymersomes through the spontaneous insertion of four hydrophobic, non-antibacterial peptide anchors into the membrane to display enhanced green fluorescent protein (eGFP) on the polymersomes’ surface.

Results

Three of the four hydrophobic peptides, the transmembrane domains of a eukaryotic cytochrome b₅, of the viral lysis protein L and of the yeast syntaxin VAM3 could be recombinantly expressed as soluble eGFP-fusion proteins and spontaneously inserted into the polymeric membrane. Characterization of the surface functionalization revealed that peptide insertion was linearly dependent on the protein concentration and possible at a broad temperature range of 4–42 °C. Up to 2320 ± 280 eGFP molecules were immobilized on a single polymersome, which is in agreement with the calculated maximum loading capacity. The peptide insertion was stable without disrupting membrane integrity as shown in calcein leakage experiments and the functionalized polymersomes remained stable for at least 6 weeks.

Conclusion

The surface functionalization of polymersomes with hydrophilic proteins can be mediated by several peptide anchors in a spontaneous process at extremely mild insertion conditions and without the need of pre-conjugating polymers.

Electronic supplementary material

The online version of this article (doi:10.1186/s12951-016-0205-x) contains supplementary material, which is available to authorized users.

Aquaporin-Based Biomimetic Polymeric Membranes: Approaches and Challenges

In recent years, aquaporin biomimetic membranes (ABMs) for water separation have gained considerable interest. Although the first ABMs are commercially available, there are still many challenges associated with further ABM development. Here, we discuss the interplay of the main components of ABMs: aquaporin proteins (AQPs), block copolymers for AQP reconstitution, and polymer-based supporting structures. First, we briefly cover challenges and review recent developments in understanding the interplay between AQP and block copolymers. Second, we review some experimental characterization methods for investigating AQP incorporation including freeze-fracture transmission electron microscopy, fluorescence correlation spectroscopy, stopped-flow light scattering, and small-angle X-ray scattering. Third, we focus on recent efforts in embedding reconstituted AQPs in membrane designs that are based on conventional thin film interfacial polymerization techniques. Finally, we describe some new developments in interfacial polymerization using polyhedral oligomeric silsesquioxane cages for increasing the physical and chemical durability of thin film composite membranes.

Combined cancer photothermal-chemotherapy based on doxorubicin/gold nanorod-loaded polymersomes

Gold nanorods (GNRs) are well known in photothermal therapy based on near-infrared (NIR) laser absorption of the longitudinal plasmon band. Herein, we developed an effective stimulus system -- GNRs and doxorubicin co-loaded polymersomes (P-GNRs-DOX) -- to facilitate co-therapy of photothermal and chemotherapy. DOX can be triggered to release once the polymersomes are corrupted under local hyperthermic condition of GNRs induced by NIR laser irradiation. Also, the cytotoxicity of GNRs caused by the residual cetyltrimethylacmmonium bromide (CTAB) was reduced by shielding the polymersomes. The GNRs-loaded polymersomes (P-GNRs) can be efficiently taken up by the tumor cells. The distribution of the nanomaterial was imaged by IR-820 and quantitatively analyzed by ICP-AES. We studied the ablation of tumor cells in vitro and in vivo, and found that co-therapy offers significantly improved therapeutic efficacy (tumors were eliminated without regrowth.) compared with chemotherapy or photothermal therapy alone. By TUNEL immunofluorescent staining of tumors after NIR laser irradiation, we found that the co-therapy showed more apoptotic tumor cells than the other groups. Furthermore, the toxicity study by pathologic examination of the heart tissues demonstrated a lower systematic toxicity of P-GNRs-DOX than free DOX. Thus, the chemo-photothermal treatment based on polymersomes loaded with DOX and GNRs is a useful strategy for maximizing the therapeutic efficacy and minimizing the dosage-related side effects in the treatment of solid tumors.

Natural channel protein inserts and functions in a completely artificial, solid-supported bilayer membrane

Reconstitution of membrane proteins in artificial membrane systems creates a platform for exploring their potential for pharmacological or biotechnological applications. Previously, we demonstrated amphiphilic block copolymers as promising building blocks for artificial membranes with long-term stability and tailorable structural parameters. However, the insertion of membrane proteins has not previously been realized in a large-area, stable, and solid-supported artificial membrane. Here, we show the first, preliminary model of a channel membrane protein that is functionally incorporated in a completely artificial polymer, tethered, solid-supported bilayer membrane (TSSBM). Unprecedented ionic transport characteristics that differ from previous results on protein insertion into planar, free-standing membranes, are identified. Our findings mark a change in understanding protein insertion and ion flow within natural channel proteins when inserted in an artificial TSSBM, thus holding great potential for numerous applications such as drug screening, trace analyzing, and biosensing.

Membrane thickness and the mechanism of action of the short peptaibol trichogin GA IV

Trichogin GA IV (GAIV) is an antimicrobial peptide of the peptaibol family, like the extensively studied alamethicin (Alm). GAIV acts by perturbing membrane permeability. Previous data have shown that pore formation is related to GAIV aggregation and insertion in the hydrophobic core of the membrane. This behavior is similar to that of Alm and in agreement with a barrel-stave mechanism, in which transmembrane oriented peptides aggregate to form a channel. However, while the 19-amino acid long Alm has a length comparable to the membrane thickness, GAIV comprises only 10 amino acids, and its helix is about half the normal bilayer thickness. Here, we report the results of neutron reflectivity measurements, showing that GAIV inserts in the hydrophobic region of the membrane, causing a significant thinning of the bilayer. Molecular dynamics simulations of GAIV/membrane systems were also performed. For these studies we developed a novel approach for constructing the initial configuration, by embedding the short peptide in the hydrophobic core of the bilayer. These calculations indicated that in the transmembrane orientation GAIV interacts strongly with the polar phospholipid headgroups, drawing them towards its N- and C-termini, inducing membrane thinning and becoming able to span the bilayer. Finally, vesicle leakage experiments demonstrated that GAIV activity is significantly higher with thinner membranes, becoming similar to that of Alm when the bilayer thickness is comparable to its size. Overall, these data indicate that a barrel-stave mechanism of pore formation might be possible for GAIV and for similarly short peptaibols despite their relatively small size.

VISUALIZING SOFT MATTER: MESOSCOPIC SIMULATIONS OF MEMBRANES, VESICLES AND NANOPARTICLES

Biological membranes have properties and behavior that emerge from the propagation of the molecular characteristics of their components across many scales. Artificial smart materials, such as drug delivery vehicles and nanoparticles, often rely on modifying naturally-occurring soft matter, such as polymers and lipid vesicles, so that they possess useful behavior. Mesoscopic simulations allow in silico experiments to be easily and cheaply performed on complex, soft materials requiring as input only the molecular structure of the constituents at a coarse-grained level. They can therefore act as a guide to experimenters prior to performing costly assays. Additionally, mesoscopic simulations provide the only currently feasible window on the length and time scales relevant to important biophysical processes such as vesicle fusion. We describe here recent work using Dissipative Particle Dynamics simulations to explore the structure and behavior of amphiphilic membranes, the fusion of vesicles, and the interactions between rigid nanoparticles and soft surfaces.

Polymersome surface decoration by an EGFP fusion protein employing Cecropin A as peptide "anchor"

Polymer based nanocompartments have potential applications in synthetic biology, medicine (drug release) and industrial biotechnology (chiral nanoreactors, multistep syntheses, selective product recovery). A step towards the aforementioned goals is the polymer membrane functionalization through covalent bonding of chemical anchors or insertion of proteins/peptides, to obtain specific properties like recognition, catalytic activity and facilitated diffusion, mimicking the complexity of a biological membrane. The use of genetic engineering techniques widens the possible applications of peptides and proteins specifically designed for polymer membrane interactions. A fusion protein (CecEGFP) based on the antibacterial peptide Cecropin A and the EGFP (Enhanced Green Fluorescent Protein) was designed, expressed and biophysically characterized. CecEGFP interaction with the tri-block copolymer PIB-PEG-PIB (PIB=polyisobutylene, PEG=polyethylene glycol) based polymersome membrane was analyzed by circular dichroism as well as EGFP and Trp fluorescence measurements. Results proved that Cecropin A is usable as a "membrane surface anchor" for water soluble proteins, as it inserts into the polymer membrane. The aim and novelty of this study is within the design of fusion proteins specifically developed for polymer membrane interactions. The use of amphiphilic Cecropin A "anchoring" water soluble proteins to the polymersome surface, avoids chemical coupling between polymers and proteins.

Emerging Applications of Polymersomes in Delivery: from Molecular Dynamics to Shrinkage of Tumors

Polymersomes are self-assembled shells of amphiphilic block copolymers that are currently being developed by many groups for fundamental insights into the nature of self-assembled states as well as for a variety of potential applications. While recent reviews have highlighted distinctive properties - particularly stability - that are strongly influenced by both copolymer type and polymer molecular weight, here we first review some of the more recent developments in computational molecular dynamics (MD) schemes that lend insight into assembly. We then review polymersome loading, in vivo stealthiness, degradation-based disassembly for controlled release, and even tumor-shrinkage in vivo. Comparisons of polymersomes with viral capsids are shown to encompass and inspire many aspects of current designs.

Infectious Disease: Connecting Innate Immunity to Biocidal Polymers

Infectious disease is a critically important global healthcare issue. In the U.S. alone there are 2 million new cases of hospital-acquired infections annually leading to 90,000 deaths and 5 billion dollars of added healthcare costs. Couple these numbers with the appearance of new antibiotic resistant bacterial strains and the increasing occurrences of community-type outbreaks, and clearly this is an important problem. Our review attempts to bridge the research areas of natural host defense peptides (HDPs), a component of the innate immune system, and biocidal cationic polymers. Recently discovered peptidomimetics and other synthetic mimics of HDPs, that can be short oligomers as well as polymeric macromolecules, provide a unique link between these two areas. An emerging class of these mimics are the facially amphiphilic polymers that aim to emulate the physicochemical properties of HDPs but take advantage of the synthetic ease of polymers. These mimics have been designed with antimicrobial activity and, importantly, selectivity that rivals natural HDPs. In addition to providing some perspective on HDPs, selective mimics, and biocidal polymers, focus is given to the arsenal of biophysical techniques available to study their mode of action and interactions with phospholipid membranes. The issue of lipid type is highlighted and the important role of negative curvature lipids is illustrated. Finally, materials applications (for instance, in the development of permanently antibacterial surfaces) are discussed as this is an important part of controlling the spread of infectious disease.

The computational route from bilayer membranes to vesicle fusion

Biological membranes are examples of 'smart' materials whose properties and behaviour emerge from the propagation across many scales of the molecular characteristics of their constituents. Artificial smart materials, such as drug delivery vehicles and biosensors, often rely on modifying naturally occurring soft matter, such as polymers and lipid vesicles, so that they possess useful behaviour. However, the complexity of natural membranes, both in their static properties, exemplified in their phase behaviour, and in their dynamic properties, as in the kinetics of their formation and interactions, hinders their rational modification. Mesoscopic simulations, such as dissipative particle dynamics (DPD), allow in silico experiments to be easily and cheaply performed on complex, soft materials requiring as input only the molecular structure of the constituents at a coarse-grained level. They can therefore act as a guide to experimenters prior to performing costly assays. Additionally, mesoscopic simulations provide the only currently feasible window on the length- and timescales relevant to important biophysical processes such as vesicle fusion. We review here the development of computational models of bilayer membranes, and in particular the use of mesoscopic simulations to follow the molecular rearrangements that occur during membrane fusion.

Phase behavior of mixed Langmuir monolayers from amphiphilic block copolymers and an antimicrobial peptide

The behavior of binary monolayers from PMOXA-PDMS-PMOXA triblock copolymers and alamethicin, an antimicrobial peptide, was investigated in the context of formation of novel biocomposite nanostructured materials. The properties of mixed monolayers were studied by surface pressure-area isotherms and Brewster angle imaging. As reported previously, functionality of alamethicin relies on its aggregation properties in lipid mono- and bilayers. This is also the case in polymer matrixes, however, here the mixing properties differ from lipid-peptide systems due to the polymers' structural specificity. The peptide influence on the polymer films is provided in detail for the first time, and supported by the compressibility data to asses the elastic properties of such composite membranes.