Dynamical membrane curvature instability controlled by intermonolayer friction

International audienceWe study a dynamical curvature instability caused by a local chemical modification of a phospholipid membrane. In our experiments, a basic solution is microinjected close to a giant unilamellar vesicle, which induces a local chemical modification of some lipids in the external monolayer of the membrane. This modification causes a local deformation of the vesicle, which then relaxes. We present a theoretical description of this instability, taking into account both the change of the equilibrium lipid density and the change of the spontaneous membrane curvature induced by the chemical modification. We show that these two types of changes of the membrane properties yield different dynamics. In contrast, it is impossible to distinguish them when studying the equilibrium shape of a vesicle subjected to a global modification. In our model, the longest relaxation timescale is related to the intermonolayer friction, which plays an important part when there is a change in the equilibrium density in one monolayer. We compare our experimental results to the predictions of our model by fitting the measured time evolution of the deformation height to the solution of our dynamical equations. We obtain good agreement between theory and experiments. Our fits enable us to estimate the intermonolayer friction coefficient, yielding values that are consistent with previous measurements

Interplay of packing and flip-flop in local bilayer deformation. How phosphatidylglycerol could rescue mitochondrial function in a cardiolipin-deficient yeast mutant

In a previous work, we have shown that a spatially localized transmembrane pH gradient, produced by acid micro-injection near the external side of cardiolipin-containing giant unilamellar vesicles, leads to the formation of tubules that retract after the dissipation of this gradient. These tubules have morphologies similar to mitochondrial cristae. The tubulation effect is due to direct phospholipid packing modification in the outer leaflet that is promoted by protonation of cardiolipin headgroups. Here we compare the case of cardiolipin-containing giant unilamellar vesicles with that of phosphatidylglycerol-containing giant unilamellar vesicles. Local acidification also promotes formation of tubules in the latter. However, compared to cardiolipin-containing giant unilamellar vesicles the tubules are longer, exhibit a visible pearling and have a much longer lifetime after acid micro-injection is stopped. We attribute these differences to an additional mechanism that increases monolayer surface imbalance, namely inward PG flip-flop promoted by the local transmembrane pH-gradient. Simulations using a fully non-linear membrane model as well as geometrical calculations are in agreement with this hypothesis. Interestingly, among yeast mutants deficient in cardiolipin biosynthesis, only the crd1-null mutant, which accumulates phosphatidylglycerol, displays significant mitochondrial activity. Our work provides a possible explanation of such a property and further emphasizes the salient role of specific lipids in mitochondrial function.Comment: 28 pages, 10 figure

Lipid membrane deformation in response to a local pH modification: theory and experiments

We study the deformation of a lipid membrane in response to a local pH modification. Experimentally, a basic solution is microinjected close to a giant unilamellar vesicle. A local deformation appears in the zone of the membrane that is closest to the micropipette, and relaxes when the injection is stopped. A theoretical description of this phenomenon is provided. It takes fully into account the spatiotemporal evolution of the concentration of hydroxide ions during and after the microinjection, as well as the linear dynamics of the membrane. This description applies to a local injection of any substance that reacts reversibly with the membrane lipids. We compare experimental data obtained in the domain of small deformations to the results of our linear description, and we obtain a good agreement between theory and experiments. In addition, we present direct experimental observations of the pH profile on the membrane during and after the microinjection, using pH-sensitive fluorescent lipids.Comment: 11 pages, 8 figure

Un modèle avancé de cellule synthétique minimale Titre en anglais : An advanced model of a minimalistic synthetic cell

International audienceCréer artificiellement des cellules qui possèdent les propriétés minimales de la vie : voilà une tâche ambitieuse que de nombreuses équipes de recherche à travers le monde se sont fixée [1-3]. Les différentes approches envisagées-pré-biotique, semi-synthétique ou supramoléculaire-se basent toutes sur un même constat : même le plus simple organisme unicellulaire existant aujourd'hui est extrêmement complexe et possède une telle diversité et richesse de comportement que tout modèle peut être seulement très approximatif. Cependant, cette complexité est-elle vraiment nécessaire à la «vie» de la cellule ou est-il possible en laboratoire de construire quelque chose de beaucoup plus simple, à partir d'un nombre limité de composants, ayant les caractéristiques de la vie ? Cette démarche bottom-up (partir d'éléments simples pour construire un système dynamique complexe) utilisée pour l'élaboration de ces systèmes chimiques autopoïétiques [4] nécessite au préalable de définir ce que l'on entend par «vie». Il est communément accepté aujourd'hui qu'un assemblage moléculaire local est «vivant» si : (1) il est capable de se régénérer; (2) il peut s'auto-reproduire; (3) il est capable d'évolution. Trois composants critiques nécessaires à l'élaboration d'une telle cellule minimale ont été identifiés [2] : une substance porteuse de l'information génétique (ARN ou ADN), un catalyseur et un compartiment clos semi-perméable (une vésicule par exemple)

ADSORPTION DE PROTEINES A L'INTERFACE AIR/SOLUTION HYDROALCOOLIQUE. APPLICATION AU CHAMPAGNE

PARIS-AgroParisTech Centre Paris (751052302) / SudocSudocFranceF

Lipid vesicles -development and applications for studding membrane heterogeneity and interactions

International audienceVesicles are closed membranes of spherical shape separating a water compartment from the bulk water in which the vesicle is suspended. Cell-sized vesicles (5–50 μm in diameter), or giant unilamellar vesicles, GUV (10–100 μm in diameter) are quite efficiently prepared by the liposome electroformation method. Recently the method was used for preparation of GUVs containing raft-like domains visible under optical microscope. Raft-like containing large unilamellar vesicles (LUV–0.1−1 μm in diameter) can be prepared by extrusion through polycarbonate filters. The raft-like domain containing GUVs and LUVs permeated studies of the role of membrane heterogeneity and compartimentalisation in the bilayer membrane interactions. Some questions regarding cell structure and functions were evoked: the phospholipase A2 (PLA2) activity, the mechanisms of cholesterol (Chol) extraction from membrane by the high density lipoproteins (HDL), preparation of detergent resistant membranes (DRM) by detergent extraction. This chapter presents a review of our experimental and theoretical early and most recent results

Mitochondrial Cristae Architecture and Functions: Lessons from Minimal Model Systems

International audienceMitochondria are known as the powerhouse of eukaryotic cells. Energy production occurs in specific dynamic membrane invaginations in the inner mitochondrial membrane called cristae. Although the integrity of these structures is recognized as a key point for proper mitochondrial function, less is known about the mechanisms at the origin of their plasticity and organization, and how they can influence mitochondria function. Here, we review the studies which question the role of lipid membrane composition based mainly on minimal model systems

How to extract selectively the l o -phase domains from large unilamellar vesicles with Triton X-100?

International audienceThe structural transition stages induced as a result of interaction at 4 °C of the Triton X-100 with large unilamellar vesicles (LUVs) were studied by means of a sucrose flotation procedure similar to that used to isolate biological detergent resistant membranes (DRMs). Flotation of lipid structures after centrifugation was determined on the basis of the [1α, 2α (n)-3H]-cholesterol content of each fraction of a 40–35–5% sucrose density gradient. We measured the amount of Triton X-100 insoluble floating fractions (TIFFs) for different lipid compositions of large unilamellar vesicles and different effective detergent to lipid ratios. At 4 °C and for two-component lipid membrane (PC/SM 2:1,mol/mol), an effective detergent to lipid ratio of 50 is necessary to complete membrane solubilization. When liquid-ordered and liquid-disordered phase domains coexist in the vesicle membrane (PC/SM/Chol 53:27:20, mol/mol), complete solubilization occurs at higher effective detergent to lipid ratio. This is consistent with a higher resistance of the liquid-ordered phase to detergent extraction. Nevertheless, in the case of heterogeneous (lo/ld phase) vesicles, and for a range of effective detergent to lipid ratios promoting incomplete solubilization, we detected in TIFF intermediate density structures which did not exist for two-component lipid membranes (PC/SM 2:1, mol/mol). We interpreted these results in relation with recent findings of our group and propose a mechanism for heterogeneous large unilamellar vesicle solubilization. We show that for lipid bilayers exhibiting lo/ld phase co-existence, a specific effective detergent to lipid ratio allowing the isolation of pre-existing tightly packed ordered domains can be found, but, in any case, certain amount of the detergent is presented in floating fraction membranes

Tracking of non-rigid structures evolving on 3D surfaces by compensating perspective view deformations

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Amyloid β and the failure to form mitochondrial cristae. A biomimetic study involving artificial membranes

International audienceAlzheimer's disease (AD) is a degenerative disease of the central nervous system which causes irreversible damage to neuron structure and function. The main hypothesis concerning the cause of AD is excessive accumulation of amyloid-β peptides (Aβ). There has recently been a surge in studies on neuronal morphological and functional pathologies related to Aβ-induced mitochondrial dysfunctions and morphological alternations. What is the relation between the accumulation of Aβ in mitochondria, decreased production of ATP, and the large number of mitochondria with broken or scarce cristae observed in AD patients' neurons? The problem is complex, as it is now widely recognized that mitochondria function determines mitochondrial inner membrane (IM) morphology and, conversely, that IM morphology can influence mitochondrial functions. In our previous work, we designed an artificial mitochondrial IM, a minimal model system (giant unilamellar vesicle) mimicking the IM. We showed experimentally that modulation of the local pH gradient at the membrane level of cardiolipin-containing vesicles induces dynamic membrane invaginations similar to the mitochondrial cristae. In the present work we show, using our artificial IM, that Aβ renders the membrane unable to support the formation of cristae-like structures when local pH gradient occurs, leading to the failure of this cristae-like morphology. Fluorescent probe studies suggest that the dramatic change of membrane mechanical properties is due to Aβ-induced lipid bilayer dehydration, increased ordering of lipids, loss of membrane fluidity, and possibly to Aβ-induced changes in dynamic friction between the two leaflets of the lipid membrane