Asymmetric Hybrid Polymer-Lipid Giant Vesicles as Cell Membrane Mimics

Lipid membrane asymmetry plays an important role in cell function and activity, being for instance a relevant signal of its integrity. The development of artificial asymmetric membranes thus represents a key challenge. In this context, an emulsion-centrifugation method is developed to prepare giant vesicles with an asymmetric membrane composed of an inner monolayer of poly(butadiene)-b-poly(ethylene oxide) (PBut-b-PEO) and outer monolayer of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). The formation of a complete membrane asymmetry is demonstrated and its stability with time is followed by measuring lipid transverse diffusion. From fluorescence spectroscopy measurements, the lipid half-life is estimated to be 7.5 h. Using fluorescence recovery after photobleaching technique, the diffusion coefficient of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(lissamine rhodamine B sulfonyl) (DOPE-rhod, inserted into the POPC leaflet) is determined to be about D = 1.8 ± 0.50 μm2 s-1 at 25 °C and D = 2.3 ± 0.7 μm2 s-1 at 37 °C, between the characteristic values of pure POPC and pure polymer giant vesicles and in good agreement with the diffusion of lipids in a variety of biological membranes. These results demonstrate the ability to prepare a cell-like model system that displays an asymmetric membrane with transverse and translational diffusion properties similar to that of biological cells.

Anionic polymerization of activated oxetane and its copolymerization with ethylene oxide for the synthesis of amphiphilic block copolymers

The anionic polymerization of oxetane in toluene was achieved using a combination of tetraoctylammonium bromide and simple triisobutylaluminum. By its copolymerization with ethylene oxide, di- and triblock copolymers were prepared in one and two steps respectively.

Liposomes in Polymersomes: Multicompartment System with Temperature-Triggered Release

Multicompartmentalization is a key feature of eukaryotic cells, allowing separation and protection of species within the membrane walls. During the last years, several methods have been reported to afford synthetic multicompartment lipidic or polymeric vesicles that mimic biological cells and that allow cascade chemical or enzymatic reactions within their lumen. We hereby report on the preparation and study of liposomes in polymersomes (LiPs) systems. We discuss on the loading and coloading of lipidic nanovesicles made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dipentadecanoyl-sn-glycero-3-phosphocholine (diC15-PC), or 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) inside the lumen of giant poly(butadiene)-b-poly(ethylene oxide) (PBut-b-PEO) polymersomes. These LiPs systems were characterized by confocal microscopy and UV-visible spectroscopy. We further demonstrate that we can achieve controlled sequential release of dyes from diC15-PC and DPPC liposomes at defined temperatures inside the giant PBut-b-PEO polymersomes. This controlled release could be used as a means to initiate cascade reactions on demand in confined microreactors.

Modulation of phase separation at the micron scale and nanoscale in giant polymer/lipid hybrid unilamellar vesicles (GHUVs)

Phase separation in giant polymer/lipid hybrid unilamellar vesicles (GHUVs) has been described over the last few years. However there is still a lack of understanding on the physical and molecular factors governing the phase separation in such systems. Among these parameters it has been suggested that in analogy to multicomponent lipid vesicles hydrophobic mismatches as well as lipid fluidity play a role. In this work, we aim to map a global picture of phase separation and domain formation in the membrane of GHUVs by using various copolymers based on poly(dimethylsiloxane) (PDMS) and poly(ethylene glycol) (PEO) with different architectures (grafted, triblock) and molar masses, combined with phospholipids in the fluid (POPC) or gel state (DPPC) at room temperature. From confocal imaging and fluorescence lifetime imaging microscopy (FLIM) techniques, the phase separation into either micro- or nano-domains within GHUVs was studied. In particular, our systematic studies demonstrate that in addition to the lipid/polymer fraction or the lipid physical state, important factors such as line tension at lipid polymer/lipid boundaries can be finely modulated by the molar mass and the architecture of the copolymer and lead to the formation of stable lipid domains with different sizes and morphologies in such GHUVs.

Effect of BMP-2 from matrices of different stiffnesses for the modulation of stem cell fate

Stem cells cultured on extracellular matrix (ECM) with different stiffnesses have been shown to engage into different lineage commitments. However, in vivo, the components of the ECM are known to bind and strongly interact with growth factors. The effect, on the stem cell fate, of the cooperation between the mechanical properties and the growth factor in the same microenvironment has not yet been investigated. Here, we propose a protocol for mimicking this stem cell microenvironment with an in vitro system. This system consists in grafting (without using a spacer) biomolecules that contain N-termini groups onto hydrogel (poly(acrylamide-co-acrylic acid)) surfaces of various stiffnesses ranging from 0.5 to 70 kPa. First, we demonstrate that the commitment of mesenchymal stem cell populations changes in response to the substrate's rigidity, with myogenic differentiation occurring at 13-17 kPa and osteogenic differentiation at 45-49 kPa. Chemical grafting of soft and stiff matrices with an osteogenic factor (BMP-2(mimetic peptide)) results only in osteogenic differentiation. Also, when grafted on even softer gels (0.5-3.5 kPa), the BMP-2(mimetic peptide) had no effect on the stem cell differentiation. We prove that correct organization of F-actin cytoskeleton due to the mechanical properties of the microenvironment is necessary for BMP-induced smad1/5/8 phosphorylation and nuclear translocation. These results suggest that stem cell differentiation is dictated mechanically, but in the presence of a biochemical factor, the effect of the mechanical factor on stem cell commitment is modified. This can explain the diversity of stem cell behaviors in vivo where different growth factors are sequestrated on the ECM.

Rapid and controlled synthesis of hydrophobic polyethers by monomer activation

A series of polyethers was prepared by monomer-activated anionic polymerization. The combination of an ammonium salt with triisobutylaluminum allowed the polymerization of epoxides containing various aliphatic side chains (1,2-butene oxide (BO), 1,2-hexene oxide (HO), and 1,2-octadecene oxide (ODO)) or a fluorinated group (3,3,3-trifluoro-1,2-epoxypropane (TFEP)). Polymers of molar masses up to 35 000 g/mol with relatively narrow molar mass distribution could be synthesized. Polymer structures were analyzed by 13C NMR, and the thermal behavior was studied by differential scanning calorimetry (DSC). The evolution of hydrophobicity was investigated according to the length and composition of the side chain. Using a long aliphatic side-chain epoxide (C16) imparted crystallinity to the resulting atactic polymer.

Nanogels based on poly(vinyl acetate) for the preparation of patterned porous films

The use of poly(vinyl acetate) (PVAc) nanogels for the fabrication of patterned porous surfaces is described. These nanogels were synthesized by controlled radical cross-linking copolymerization (CRCC) involving a xanthate-mediated reversible addition-fragmentation chain transfer (RAFT) mechanism. This synthesis methodology allowed for the preparation of nanogels based on PVAc with a controlled constitutive chain length and average numbers of chains and cross-links. Solutions of these branched polymers were prepared in THF with a fixed amount of water and spin coated onto a surface of graphite. The surface porosity of corresponding films was observed by atomic force microscopy (AFM). Compared with linear PVAc homologues with a degree of polymerization (DP) sufficiently high to favor the formation of porous structures (DP = 50), a sharper and better defined porosity was observed with nanogels, the constitutive chains of which had the same DP. For nanogels differing only in their cross-link density, the pores were smaller and better defined in the case of the higher cross-link density, suggesting an enhanced stabilization of the water droplets during film formation. To explain these observations, it is postulated that PVAc nanogels can behave as compact particles providing steric stabilization of water droplets, which is referred to as a Pickering effect. The coalescence of water droplets would be better prevented as the cross-link density of the nanogels increases, resulting in a smaller size pore.

Doxorubicin loaded magnetic polymersomes: theranostic nanocarriers for MR imaging and magneto-chemotherapy

Hydrophobically modified maghemite (γ-Fe(2)O(3)) nanoparticles were encapsulated within the membrane of poly(trimethylene carbonate)-b-poly(l-glutamic acid) (PTMC-b-PGA) block copolymer vesicles using a nanoprecipitation process. This formation method gives simple access to highly magnetic nanoparticles (MNPs) (loaded up to 70 wt %) together with good control over the vesicles size (100-400 nm). The simultaneous loading of maghemite nanoparticles and doxorubicin was also achieved by nanoprecipitation. The deformation of the vesicle membrane under an applied magnetic field has been evidenced by small angle neutron scattering. These superparamagnetic hybrid self-assemblies display enhanced contrast properties that open potential applications for magnetic resonance imaging. They can also be guided in a magnetic field gradient. The feasibility of controlled drug release by radio frequency magnetic hyperthermia was demonstrated in the case of encapsulated doxorubicin molecules, showing the viability of the concept of magneto-chemotherapy. These magnetic polymersomes can be used as efficient multifunctional nanocarriers for combined therapy and imaging.