Bulging and budding of lipid droplets from symmetric and asymmetric membranes: competition between membrane elastic energy and interfacial energy

Molecular mechanisms and energetics of lipid droplet formation and directional budding

The formation and budding of lipid droplets (LDs) are known to be governed by the LD size and by membrane tensions in the endoplasmic reticulum (ER) bilayer and LD-monolayers. Using coarse-grained simulations of an LD model, we first show that ER-embedded LDs of different sizes can form through a continuous transition from wide LD lenses to spherical LDs at a fixed LD size. The ER tendency to relax its bilayer modulates the transition via a subtle interplay between the ER and LD lipid densities. By calculating the energetic landscape of the LD transition, we demonstrate that this size-independent transition is regulated by the mechanical force balance of ER and LD-tensions, independent from membrane bending and line tension whose energetic contributions are negligible according to our calculations. Our findings explain experimental observation of stable LDs of various shapes. We then propose a novel mechanism for directional LD budding where the required membrane asymmetry is provided by the exchange of lipids between the LD-monolayers. Remarkably, we demonstrate that this budding process is energetically neutral. Consequently, LD budding can proceed by a modest energy input from proteins or other driving agents. We obtain equal lipid densities and membrane tensions in LD-monolayers throughout budding. Our findings indicate that unlike LD formation, LD budding by inter-monolayer lipid exchange is a tension-independent process.

Asymmetric Lipid Bilayers and Potassium Channels Embedded Therein in the Contact Bubble Bilayer

Cell membranes are highly intricate systems comprising numerous lipid species and membrane proteins, where channel proteins, lipid molecules, and lipid bilayers, as continuous elastic fabric, collectively engage in multi-modal interplays. Owing to the complexity of the native cell membrane, studying the elementary processes of channel-membrane interactions necessitates a bottom-up approach starting from forming simplified synthetic membranes. This is the rationale for establishing an in vitro membrane reconstitution system consisting of a lipid bilayer with a defined lipid composition and a channel molecule. Recent technological advancements have facilitated the development of asymmetric membranes, and the contact bubble bilayer (CBB) method allows single-channel current recordings under arbitrary lipid compositions in asymmetric bilayers. Here, we present an experimental protocol for the formation of asymmetric membranes using the CBB method. The KcsA potassium channel is a prototypical model channel with huge structural and functional information and thus serves as a reporter of membrane actions on the embedded channels. We demonstrate specific interactions of anionic lipids in the inner leaflet. Considering that the local lipid composition varies steadily in cell membranes, we `present a novel lipid perfusion technique that allows rapidly changing the lipid composition while monitoring the single-channel behavior. Finally, we demonstrate a leaflet perfusion method for modifying the composition of individual leaflets. These techniques with custom synthetic membranes allow for variable experiments, providing crucial insights into channel-membrane interplay in cell membranes.

Area difference between monolayers facilitates budding of lipid droplets from vesicles

Lipid droplets (LDs) are intracellular organelles that play a central role in cellular lipid balance and energy homeostasis. Though extensive experimental studies have been carried out on LD biogenesis, relatively little is known about the mechanical interaction between LDs and vesicles, and in particular effects of area difference between vesicle leaflets on LD evolution are not theoretically rationalized. Here we theoretically explore how the monolayer area difference regulates the budding and morphological evolution of an LD embedded in the vesicle membrane. It is shown that both the monolayer area difference and interfacial energy strength, attributed to the LD-membrane contact, facilitate the LD budding with the confined LD evolving from a bulge to a spherical protrusion. The budding direction is towards the monolayer with more phospholipids. Outward and inward budding phase diagrams are established with respect to the interfacial energy strength and area ratio between the outer and inner monolayers. Moreover, the osmotic pressure of the vesicle promotes the LD budding at a small monolayer area difference and inhibits the budding at a relatively large monolayer area difference.

Lens Nucleation and Droplet Budding in a Membrane Model for Lipid Droplet Biogenesis

Lipid droplet biogenesis comprises the emergence of cytosolic lipid droplets with a typical diameter 0.1-5 μm via synthesis of fat in the endoplasmatic reticulum, the formation of membrane-embedded lenses, and the eventual budding of lenses into solution as droplets. Lipid droplets in cells are increasingly being viewed as highly dynamic organelles with multiple functions in cell physiology. However, the mechanism of droplet formation in cells remains poorly understood, partly because their formation involves the rapid transformation of transient lipid structures that are difficult to capture. Thus, the development of controlled experimental systems that model lipid biogenesis is highly relevant for an enhanced mechanistic understanding. Here we prepare and characterize triolein (TO) lenses in a multilamellar spin-coated phosphatidylcholine (POPC) film and determine the lens nucleation threshold to 0.25-0.5% TO. The TO lens shapes are characterized by atomic force microscopy (AFM) including their mean cap angle ⟨α⟩ = 27.3° and base radius ⟨a⟩ = 152.7 nm. A cross-correlation analysis of corresponding AFM and fluorescence images confirms that TO is localized to lenses. Hydration of the lipid/lens film induces the gel to fluid membrane phase transition and makes the lenses more mobile. The budding of free droplets into solution from membrane lenses is detected by observing a change in motion from confined wiggling to ballistic motion of droplets in solution. The results confirm that droplet budding can occur spontaneously without being facilitated by proteins. The developed model system provides a controlled platform for testing mechanisms of lipid droplet biogenesis in vitro and addressing questions related to lens formation and droplet budding by quantitative image analysis.

Computational Studies of Lipid Droplets

Lipid droplets (LDs) are intracellular organelles whose primary function is energy storage. Known to emerge from the endoplasmic reticulum (ER) bilayer, LDs have a unique structure with a core consisting of neutral lipids, triacylglycerol (TG) or sterol esters (SE), surrounded by a phospholipid (PL) monolayer and decorated by proteins that come and go throughout their complex lifecycle. In this Feature Article, we review recent developments in computational studies of LDs, a rapidly growing area of research. We highlight how molecular dynamics (MD) simulations have provided valuable molecular-level insight into LD targeting and LD biogenesis. Additionally, we review the physical properties of TG from different force fields compared with experimental data. Possible future directions and challenges are discussed.

Key Factors Governing Initial Stages of Lipid Droplet Formation

Lipid droplets (LDs) are neutral lipid storage organelles surrounded by a phospholipid (PL) monolayer. LD biogenesis from the endoplasmic reticulum is driven by phase separation of neutral lipids, overcoming surface tension and membrane deformation. However, the core biophysics of the initial steps of LD formation remains relatively poorly understood. Here, we use a tunable, phenomenological coarse-grained model to study triacylglycerol (TG) nucleation in a bilayer membrane. We show that PL rigidity has a strong influence on TG lensing and membrane remodeling: when membrane rigidity increases, TG clusters remain more planar with high anisotropy but a minor degree of phase nucleation. This finding is confirmed by advanced sampling simulations that calculate nucleation free energy as a function of the degree of nucleation and anisotropy. We also show that asymmetric tension, controlled by the number of PL molecules on each membrane leaflet, determines the budding direction. A TG lens buds in the direction of the monolayer containing excess PL molecules to allow for better PL coverage of TG, consistent with the reported experiments. Finally, two governing mechanisms of the LD growth, Ostwald ripening and merging, are observed. Taken together, this study characterizes the interplay between two thermodynamic quantities during the initial LD phases, the TG bulk free energy and membrane remodeling energy.

Origin of gradients in lipid density and surface tension between connected lipid droplet and bilayer

We combined theory and experiments to depict physical parameters modulating the phospholipid (PL) density and tension equilibrium between a bilayer and an oil droplet in contiguity. This situation is encountered during a neutral lipid (NL) droplet formation in the endoplasmic reticulum. We set up macroscopic and microscopic models to uncover free parameters and the origin of molecular interactions controlling the PL densities of the droplet monolayer and the bilayer. The established physical laws and predictions agreed with experiments performed with droplet-embedded vesicles. We found that the droplet monolayer is always by a few percent (∼10%) less packed with PLs than the bilayer. Such a density gradient arises from PL-NL interactions on the droplet, which are lower than PL-PL trans interactions in the bilayer, i.e., interactions between PLs belonging to different leaflets of the bilayer. Finally, despite the pseudo-surface tension for the water/PL acyl chains in the bilayer being higher than the water/NL surface tension, the droplet monolayer always has a higher surface tension than the bilayer because of its lower PL density. Thus, a PL density gradient is mandatory to maintain the mechanical and thermodynamic equilibrium of the droplet-bilayer continuity. Our study sheds light on the origin of the molecular interactions responsible for the unique surface properties of lipid droplets compared with cellular bilayer membranes.

Bulging-to-Budding Transition of Lipid Droplets Confined within Vesicle Membranes

Lipid droplets (LDs) are intracellular organelles that act as reservoirs for energy homeostasis and phospholipid balance between supply and consumption. In comparison with extensive studies on LD biogenesis from a biological viewpoint, little is known about the mechanical interaction between LDs and vesicles. Here we perform a systematic theoretical study on the budding and morphological evolution of an artificial LD embedded within the lipid membrane of a pressurized vesicle. It is found that LD bulging and budding depend on the bending rigidity and spontaneous curvature of the vesicle membrane, LD-vesicle interfacial interaction energy strength and size ratio, and osmotic pressure of the vesicle. Beyond critical interfacial interaction strength, the embedded LD undergoes a discontinuous shape transition from a lens-shaped bulge to a spherical protrusion connecting to the nearly spherical vesicle lumen via an infinitesimally small monolayer neck. Moreover, a positive monolayer spontaneous curvature promotes budding transition. As the vesicle becomes smaller, higher cost of the monolayer stretching energy is required for an LD to achieve budding transition. Budding phase diagrams distinguishing the embedded and budding states of the LD-vesicle complex accounting for osmotic pressure and interfacial interaction strength are established with the budding transition boundary displaying a nonmonotonic feature. Our results reveal how embedded LDs overcome soft membrane confinement and protrude, and provide fundamental insights into the clustering of nanoparticles between vesicle monolayers.