Institut Charles Sadron MCUBE
Research
Ongoing and foreseen research in the Membranes and Sof Matter Physics Team are presented below:
- Giant Unilamellar Vesicles (GUV) formation
Our recent successes in developing new, easier still more powerful methods of preparing giant unilamellar vesicles have rendered possible to routinely work with a large spectra of lipid compositions in a variety of sugar or electrolyte solutions, in the presence of many bio-relevant molecules.
As a developer of gel-growing methods for GUVs, the group has acquired a core expertise in this field.
- Lipid Oxidation
Our seminal contributions towards the understanding of lipid oxidation have brought us into a leading position in this field. We will pursue our long-term effort into three main directions: i) at the fundamental level, the challenge that we will address is to understand and control photo-induced oxidation effects beyond hydroperoxidation, ii) this requires the development of new families of photosensitizers as well as new strategies for modulating the interactions between the oxidizing species and the target lipid sites, often the unsaturated bonds, iii) results from our research can have an impact in the development of new tools and strategies for Photodynamic Therapy.
We started efforts towards the vectorization of photosensitizer molecules by Cell Penetrating Peptides. The team was the first to visualize membrane oxidation effects under optical microscopy.
- Lipid Phase Transitions
Modeling lipid bilayers with realistic, biologically relevant compositions is a daunting challenge. First, the number of different lipid molecules is large, especially considering the many products arising from oxidation. Second, the lipid membrane is a place of dissipative processes generating composition gradients and leaflet asymmetry. One can therefore see these lipid mixtures as complex and adaptive 2D solvents for the embedded transmembrane and peripheral proteins standing in or close to the membrane. There are a number of issues that can be addressed at this level like the lateral phase coexistence in many component membranes, and wetting/capillary effects around heterogeneous inclusions. Our project is to bridge the gap between numerical simulations, typically MD of coarse-grained lipid systems, and macroscopic thermodynamic models of regular solutions with effective Landau-Ginsburg models accounting for non-ideal mixing.
-Lipid membranes out-of-equilibrium
A variety of external stress fields (mechanical, electrical…) or internal activity are ubiquitous in the practical situations under which lipid bilayers are assembled, transformed or utilized. Under these conditions the state of the membrane cannot be described by equilibrium statistical physics. Understanding far from equilibrium lipid and bilayer dynamics is one of the strongest challenges laying ahead for this field.
Biolubrication: the lubrication boundary of articular cartilage is a typical practical situation where these concepts come to play. The challenge here resides in the possibility of explaining and reproducing the conditions that lead to the robustness and low friction of this bio-lubricating contact. Supported stacks of lipid bilayers have been shown to provide a biomimetic system that displays some of the sought lubricating properties. In these systems, we will study the bilayer at different length-scales to understand the hydrodynamics of sheared confined layers and the bilayer friction with a solid polymer substrate.
Active membrane: transmembrane proteins have a pivotal role in a large number of cellular processes and provide essential functions of the cell membranes such as inter- and intra-cellular signalling, transfer of substances inside and outside the cell, the transfer of ions through the membrane, energy transfer, cell homeostasis. By itself membranes exhibit thermal fluctuations, but membrane protein activity leads to out-of-equilibrium fluctuations, which break the fluctuation-dissipation theorem. Active fluctuations have been widely described theoretically, but less is known on the experimental point of view. First experiments on bacteriorhodopsin (BR), a light-activated proton pump reconstituted in Giant Unilamellar Vesicles, were performed by the group of Patrica Bassereau (Curie Institut), by using Micropipette aspiration and refined microscopy analysis. Despite these experiments show clear evidence for the magnification of shape fluctuations when proteins are activated, the experimental technique accesses only micrometer scale information and did not provide a measurement of the fluctuation spectrum. A complete understanding of the mechanism requires a fine characterization of the fluctuation spectrum at submicron length scales, which we seek to do by studying the fluctuations of active supported membranes through off-specular x-ray scattering experiments.
- Nano- and active- particles interacting with membranes
Nano-objects have a peculiar affinity for cell membranes, and sometimes are able to translocate through it. Their small dimensions, which, in many cases, are of the order of the membrane barrier thickness, but also other factors such as the hydrophobic-hydrophilic balance, are expected to control the interactions. We pursue efforts to understand the fundamental mechanisms of membrane nanoparticle interactions along three routes: i) we develop fast new methods for optical and cryo-TEM (with the EM platform) ii) we finely tune the composition of the lipid membranes, by adding charges, cholesterol or negative curvature lipids, thus approaching compositions of relevant living systems or formulations and iii) we screen different nano-objects (silica, polystyrene, silver, gold and other nanoparticles, cell penetrating peptides with different cargos, cyclodextrins …) in order to explore the relevant parameter space.
A new activity has also started on the interactions between self-propelled Janus colloids and GUVs. Active motion and phoretic interactions result into new dynamics such as transport of GUV by active particles and orbital motion of active colloids around GUVs.
- Polymer-membrane interactions
In the living realm cell membranes interact permanently with a wide variety of polymers. On the one hand, biological polymers as peptides, DNA, proteins, to name only a few, populate biological fluids. On the other hand, nowadays way of life produces an incredibly broad range of synthetic macromolecules, that we are suspected to breath, ingest, or simply be in contact with. Two examples of how understanding polymer-membrane interactions would be of strong benefit are i) in the drug delivery domain, where unravelling how the HIV virus infects cells has already brought to light the existence and the high potential of cell-penetrating peptides, and ii) in public health problem, where up-to-date studies attempt to clarify the sensitivity of cells to nanoparticles. Our team is interested in various aspects of polymer-lipid membrane interactions. We have developed a quantitative method to quantify the adsorption of fluorescently labelled cell-penetrating peptides (CPPs) on a lipid membrane. Using this method we have shown that CPP-cargo macromolecules recover affinity for a membrane under temperature driven self-assembly into micellar structures (CPP decorating corona), thanks to the induced local increase of CPP surface density. Recently, we have shown that low molecular mass polystyrene incorporates into membranes, modifying strongly the transition gel-fluid temperature Tm, acting similarly to cholesterol in binary, saturated+unsaturated lipid membranes.
- Undulating structures
The experimental and theoretical expertise developed by our core activity in lipid membranes is relevant also for other areas of Soft Matter. In particular, mathematical methods developed to understand the influence of thermal fluctuations on the mechanical properties of bilayer stacks can potentially be translated to describe macroscopic undulating systems such as natural and composite bundles of nearly aligned fibers, as for instance in hair tresses and ponytails, glass or metallic wools. We have recently computed the compression modulus of the fiber stacks combining self-consistent mean-field treatment and numerical simulations in two dimensions. We have also investigated both experimentally and numerically the compression law of fiber bundles of various fiber systems combining both mechanical measurement and image analysis of the shape of the fibers.
- Wetting, adhesion and drying
Wetting of several systems such as drops, particles, functional surfaces and giant vesicles are investigated in several aspects. The adhesion of functionalized GUVs on glass surfaces is used for instance to monitor changes in lipid area upon oxidation, or to cover and engulf grafted nucleic acid molecules.
Dynamics of colloidal particles partially engulfed by membranes of GUVs or partially immersed at planar fluid interfaces are also investigated in order to elucidate the role of wetting on particle diffusions and particle transfer across membranes and interfaces.
Finally, our group is also involved in modelling the drying of colloidal solutions, a complex situation involving diffusion of interacting particles and surfactants; evaporation induced flows, particle deformation and coalescence, and emerging elastic stresses in the deposited film.