laboratoire de physique statistique
laboratoire de physique statistique


Conserved Amphipathic Helices Mediate Lipid Droplet Targeting of Perilipins 1-3 - Rowe, Emily R. and Mimmack, Michael L. and Barbosa, Antonio D. and Haider, Afreen and Isaac, Iona and Ouberai, Myriam M. and Thiam, Abdou Rachid and Patel, Satish and Saudek, Vladimir and Siniossoglou, Symeon and Savage, David B.

Abstract : Perilipins (PLINs) play a key role in energy storage by orchestrating the activity of lipases on the surface of lipid droplets. Failure of this activity results in severe metabolic disease in humans. Unlike all other lipid droplet-associated proteins, PLINs localize almost exclusively to the phospholipid monolayer surrounding the droplet. To understand how they sense and associate with the unique topology of the droplet surface, we studied the localization of human PLINs in Saccharomyces cerevisiae, demonstrating that the targeting mechanism is highly conserved and that 11-mer repeat regions are sufficient for droplet targeting. Mutations designed to disrupt folding of this region into amphipathic helices (AHs) significantly decreased lipid droplet targeting in vivo and in vitro. Finally, we demonstrated a substantial increase in the helicity of this region in the presence of detergent micelles, which was prevented by an AH-disrupting missense mutation. We conclude that highly conserved 11-mer repeat regions of PLINs target lipid droplets by folding into AHs on the droplet surface, thus enabling PLINs to regulate the interface between the hydrophobic lipid core and its surrounding hydrophilic environment.
The physics of lipid droplet nucleation, growth and budding - Thiam, Abdou Rachid and Foret, Lionel

Abstract : Lipid droplets (Ws) are intracellular oil-in-water emulsion droplets, covered by a phospholipid monolayer and mainly present in the cytosol. Despite their important role in cellular metabolism and growing number of newly identified functions, LD formation mechanism from the endoplasmic reticulum remains poorly understood. To form a LD, the oil molecules synthesized in the ER accumulate between the monolayer leaflets and induce deformation of the membrane. This formation process works through three steps: nucleation, growth and budding, exactly as in phase separation and dewetting phenomena. These steps involve sequential biophysical membrane remodeling mechanisms for which we present basic tools of statistical physics, membrane biophysics, and soft matter science underlying them. We aim to highlight relevant factors that could control LD formation size, site and number through this physics description. An emphasis will be given to a currently underestimated contribution of the molecular interactions between lipids to favor an energetically costless mechanism of LD formation. (C) 2016 Elsevier B.V. All rights reserved.
Seipin is required for converting nascent to mature lipid droplets - Wang, Huajin and Becuwe, Michel and Housden, Benjamin E. and Chitraju, Chandramohan and Porras, Ashley J. and Graham, Morven M. and Liu, Xinran N. and Thiam, Abdou Rachid and Savage, David B. and Agarwal, Anil K. and Garg, Abhimanyu and Olarte, Maria-Jesus and Lin, Qingqing and Froehlich, Florian and Hannibal-Bach, Hans Kristian and Upadhyayula, Srigokul and Perrimon, Norbert and Kirchhausen, Tomas and Ejsing, Christer S. and Walther, Tobias C. and Farese, Jr., Robert V.
ELIFE 5 (2016) 

Abstract : How proteins control the biogenesis of cellular lipid droplets (LDs) is poorly understood. Using Drosophila and human cells, we show here that seipin, an ER protein implicated in LD biology, mediates a discrete step in LD formation-the conversion of small, nascent LDs to larger, mature LDs. Seipin forms discrete and dynamic foci in the ER that interact with nascent LDs to enable their growth. In the absence of seipin, numerous small, nascent LDs accumulate near the ER and most often fail to grow. Those that do grow prematurely acquire lipid synthesis enzymes and undergo expansion, eventually leading to the giant LDs characteristic of seipin deficiency. Our studies identify a discrete step of LD formation, namely the conversion of nascent LDs to mature LDs, and define a molecular role for seipin in this process, most likely by acting at ER-LD contact sites to enable lipid transfer to nascent LDs.
Stability of C12Ej Bilayers Probed with Adhesive Droplets - Astafyeva, Ksenia and Urbach, Wladimir and Garroum, Nabil and Taulier, Nicolas and Thiam, Abdou R.
LANGMUIR 316791-6796 (2015) 

Abstract : The stability of model surfactant bilayers from the poly(ethylene glycol) mono-n-dodecyl ether (C12Ej) family was probed. The surfactant bilayers were formed by the adhesion of emulsion droplets. We generated C12Ej bilayers by forming water-in-oil (w/o) emulsions with saline water droplets, covered by the surfactant, in a silicone and octane oil mixture. Using microfluidics, we studied the stability of those bilayers. C12E1 allowed only short-lived bilayers whereas C12E2 bilayers were stable over a wide range of oil mixtures. At high C12E2 concentration, a two-phase region was displayed in the phase diagram: bilayers formed by the adhesion of two water droplets and Janus-like particles consisting of adhering aqueous and amphiphilic droplets. C12E8 and C12E25 did not mediate bilayer formation and caused phase inversion leading to o/w emulsion. With intermediate C12E4 and C12E5 surfactants, both w/o and o/w emulsions were unstable. We provided the titration of the C12E2 bilayer with C12E4 and C12E5 to study and predict their stability behavior.
The Energy of COPI for Budding Membranes - Thiam, Abdou Rachid and Pincet, Frederic
PLOS ONE 10 (2015) 

Abstract : As a major actor of cellular trafficking, COPI coat proteins assemble on membranes and locally bend them to bud 60 nm-size coated particles. Budding requires the energy of the coat assembly to overcome the one necessary to deform the membrane which primarily depends on the bending modulus and surface tension, gamma. Using a COPI-induced oil nano-droplet formation approach, we modulated the budding of nanodroplets using various amounts and types of surfactant. We found a Heaviside-like dependence between the budding efficiency and.: budding was only dependent on. and occurred beneath 1.3 mN/m. With the sole contribution of. to the membrane deformation energy, we assessed that COPI supplies similar to 1500 k(B)T for budding particles from membranes, which is consistent with common membrane deformation energies. Our results highlight how a simple remodeling of the composition of membranes could mechanically modulate budding in cells.
Protein Crowding Is a Determinant of Lipid Droplet Protein Composition - Kory, Nora and Thiam, Abdou-Rachid and Farese, Jr., Robert V. and Walther, Tobias C.
DEVELOPMENTAL CELL 34351-363 (2015) 

Abstract : Lipid droplets (LDs) are lipid storage organelles that grow or shrink, depending on the availability of metabolic energy. Proteins recruited to LDs mediate many metabolic functions, including phosphatidylcholine and triglyceride synthesis. How the LD protein composition is tuned to the supply and demand for lipids remains unclear. We show that LDs, in contrast to other organelles, have limited capacity for protein binding. Consequently, macromolecular crowding plays a major role in determining LD protein composition. During lipolysis, when LDs and their surfaces shrink, some, but not all, proteins become displaced. In vitro studies show that macromolecular crowding, rather than changes in monolayer lipid composition, causes proteins to fall off the LD surface. As predicted by a crowding model, proteins compete for binding to the surfaces of LDs. Moreover, the LD binding affinity determines protein localization during lipolysis. Our findings identify protein crowding as an important principle in determining LD protein composition.
Single-step microfluidic fabrication of soft monodisperse polyelectrolyte microcapsules by interfacial complexation - Kaufman, Gilad and Boltyanskiy, Rostislav and Nejati, Siamak and Thiam, Abdou R. and Loewenberg, Michael and Dufresne, Eric R. and Osuji, Chinedum O.
LAB ON A CHIP 143494-3497 (2014) 

Abstract : Common methods for fabrication of polyelectrolyte microcapsules rely on a multi-step process. We propose a single-step approach to generate polyelectrolyte microcapsules with 1-2 m shells based on polyelectrolyte complexation across a water/oil droplet interface and study the effect of parameters controlling the polyelectrolyte complexation on shell thickness.
Arf1/COPI machinery acts directly on lipid droplets and enables their connection to the ER for protein targeting - Wilfling, Florian and Thiam, Abdou Rachid and Olarte, Maria-Jesus and Wang, Jing and Beck, Rainer and Gould, Travis J. and Allgeyer, Edward S. and Pincet, Frederic and Bewersdorf, Joerg and Farese, Jr., Robert V. and Walther, Tobias C.
ELIFE 3 (2014) 

Abstract : Lipid droplets (LDs) are ubiquitous organelles that store neutral lipids, such as triacylglycerol (TG), as reservoirs of metabolic energy and membrane precursors. The Arf1/COPI protein machinery, known for its role in vesicle trafficking, regulates LD morphology, targeting of specific proteins to LDs and lipolysis through unclear mechanisms. Recent evidence shows that Arf1/COPI can bud nano-LDs (similar to 60 nm diameter) from phospholipid-covered oil/water interfaces in vitro. We show that Arf1/COPI proteins localize to cellular LDs, are sufficient to bud nano-LDs from cellular LDs, and are required for targeting specific TG-synthesis enzymes to LD surfaces. Cells lacking Arf1/COPI function have increased amounts of phospholipids on LDs, resulting in decreased LD surface tension and impairment to form bridges to the ER. Our findings uncover a function for Arf1/COPI proteins at LDs and suggest a model in which Arf1/COPI machinery acts to control ER-LD connections for localization of key enzymes of TG storage and catabolism.
COPI buds 60-nm lipid droplets from reconstituted water-phospholipid-triacylglyceride interfaces, suggesting a tension clamp function - Thiam, Abdou Rachid and Antonny, Bruno and Wang, Jing and Delacotte, Jerome and Wilfling, Florian and Walther, Tobias C. and Beck, Rainer and Rothman, James E. and Pincet, Frederic

Abstract : Intracellular trafficking between organelles is achieved by coat protein complexes, coat protomers, that bud vesicles from bilayer membranes. Lipid droplets are protected by a monolayer and thus seem unsuitable targets for coatomers. Unexpectedly, coat protein complex I (COPI) is required for lipid droplet targeting of some proteins, suggesting a possible direct interaction between COPI and lipid droplets. Here, we find that COPI coat components can bud 60-nm triacylglycerol nanodroplets from artificial lipid droplet (LD) interfaces. This budding decreases phospholipid packing of the monolayer decorating the mother LD. As a result, hydrophobic triacylglycerol molecules become more exposed to the aqueous environment, increasing LD surface tension. In vivo, this surface tension increase may prime lipid droplets for reactions with neighboring proteins or membranes. It provides a mechanism fundamentally different from transport vesicle formation by COPI, likely responsible for the diverse lipid droplet phenotypes associated with depletion of COPI subunits.
The biophysics and cell biology of lipid droplets - Thiam, Abdou Rachid and Farese, Jr., Robert V. and Walther, Tobias C.

Abstract : Lipid droplets are intracellular organelles that are found in most cells, where they have fundamental roles in metabolism. They function prominently in storing oil-based reserves of metabolic energy and components of membrane lipids. Lipid droplets are the dispersed phase of an oil-in-water emulsion in the aqueous cytosol of cells, and the importance of basic biophysical principles of emulsions for lipid droplet biology is now being appreciated. Because of their unique architecture, with an interface between the dispersed oil phase and the aqueous cytosol, specific mechanisms underlie their formation, growth and shrinkage. Such mechanisms enable cells to use emulsified oil when the demands for metabolic energy or membrane synthesis change. The regulation of the composition of the phospholipid surfactants at the surface of lipid droplets is crucial for lipid droplet homeostasis and protein targeting to their surfaces.