Pre-existing bilayer stresses modulate triglyceride accumulation in the ER versus lipid droplets

A Coarse-Grained SPICA Makeover for Solvated and Bare Sodium and Chloride Ions

Aqueous ionic solutions are pivotal in various scientific domains due to their natural prevalence and vital roles in biological and chemical processes. Molecular dynamics has emerged as an effective methodology for studying the dynamic behavior of these systems. While all-atomistic models have made significant strides in accurately representing and simulating these ions, the challenge persists in achieving precise models for coarse-grained (CG) simulations. Our study introduces two optimized models for sodium and chloride ions within the nonpolarizable surface property fitting coarse-grained force field (SPICA-FF) framework. The two models represent solvated ions, such as the original FF model, and unsolvated or bare ions. The nonbonded Lennard-Jones interactions were reparameterized to faithfully reproduce bulk properties, including density and surface tension, in sodium chloride solutions at varying concentrations. Notably, these optimized models replicate experimental surface tensions at high ionic strengths, a property not well-captured by the ions of the original model in the SPICA-FF. The optimized unsolvated model also proved successful in reproducing experimental osmotic pressure. Additionally, the newly reparameterized ion models capture hydrophobic interactions within sodium chloride solutions and show qualitative agreement when modeling structural changes in phospholipid bilayers, aligning with experimental observations. For aqueous solutions, these optimized models promise a more precise representation of the ion behavior.

Dynamical modelling of lipid droplet formation suggests a key function of membrane phospholipids

Cells store triacylglycerol (TAG) within lipid droplets (LDs). A dynamic model describing complete LD formation at the endoplasmic reticulum (ER) membrane does not yet exist. A biochemical-biophysical model of LD synthesis is proposed. It describes the time-dependent accumulation of TAG in the ER membrane as the formation of a potential LD (pLD) bounded by spherical caps of the inner and outer monolayers of the membrane. The expansion rate of the pLD depends on the TAG supply, the elastic properties of the ER membrane, and the recruitment of phospholipids (PLs) to the cap-covering monolayers. Model simulations provided the following insights: (a) Marginal differences in the surface tension of the cap monolayers are sufficient to fully drive the expansion of the pLD towards the cytosol or lumen. (b) Selective reduction of PL supply to the luminal monolayer ensures stable formation of cytosolic LDs, irrespective of variations in the elasto-mechanical properties of the ER membrane. (c) The rate of TAG supply to the cytosolic monolayer has a major effect on the size and maturation time of LDs but has no significant effect on the TAG export per individual LD. The recruitment of additional PLs to the cap monolayers of pLDs critically controls the budding direction, size, and maturation time of LDs. The ability of cells to acquire additional LD initiation sites appears to be key to coping with acutely high levels of potentially toxic free fatty acids.

Learning Force Field Parameters from Differentiable Particle-Field Molecular Dynamics

We develop ∂-HylleraasMD (∂-HyMD), a fully end-to-end differentiable molecular dynamics software based on the Hamiltonian hybrid particle-field formalism, and use it to establish a protocol for automated optimization of force field parameters. ∂-HyMD is templated on the recently released HylleraaasMD software, while using the JAX autodiff framework as the main engine for the differentiable dynamics. ∂-HyMD exploits an embarrassingly parallel optimization algorithm by spawning independent simulations, whose trajectories are simultaneously processed by reverse mode automatic differentiation to calculate the gradient of the loss function, which is in turn used for iterative optimization of the force-field parameters. We show that parallel organization facilitates the convergence of the minimization procedure, avoiding the known memory and numerical stability issues of differentiable molecular dynamics approaches. We showcase the effectiveness of our implementation by producing a library of force field parameters for standard phospholipids, with either zwitterionic or anionic heads and with saturated or unsaturated tails. Compared to the all-atom reference, the force field obtained by ∂-HyMD yields better density profiles than the parameters derived from previously utilized gradient-free optimization procedures. Moreover, ∂-HyMD models can predict with good accuracy properties not included in the learning objective, such as lateral pressure profiles, and are transferable to other systems, including triglycerides.

Bioenergy feedstock production in Chlamydomonas reinhardtii (microalgae) cultivated under mixotrophic growth with cellulose hydrolysate from agricultural waste

In the present study, cellulose purified from finger millet agricultural waste is subjected to enzymatic hydrolysis, and the hydrolysate (predominantly glucose) is used as a carbon source supplement in the media for the mixotrophic growth of Chlamydomonas reinhardtii. Interestingly, a switch between excess starch production and excess lipid (triacylglycerols, TAG) production occurs by a small change in hydrolysate concentration in the media. Starch production increased 4.5-fold with respect to the photoautotrophic control, with a glucose concentration of 3 mg/mL in the media after hydrolysate addition. This culture had TAG production enhancement by 1.5-fold. However, mixotrophic cultivation with 4 mg/mL glucose concentration in the media with hydrolysate addition resulted in TAG productivity enhancement by 4.2-fold compared to control and starch amount increase of 1.3-fold. The organic carbon source (glucose) and the inorganic carbon source (citrate ions) in the hydrolysate together played a role in this delicate switching between starch and lipid pathways. Proteins, starch, and TAG molecules are analyzed in the microalgal cells grown under different conditions with FTIR spectroscopy, a rapid, high-throughput method of biomolecular estimation. High-resolution single-cell AFM studies of the cell wall structure reveal enhanced corrugations in surface morphology during mixotrophic growth with cellulose hydrolysate, illustrating an adaptive mechanism with improved mechanical stress management. Lipid droplet morphology at the single-cell level points to two distinct mechanisms of lipid accumulation: one in which the lipids are segregated as droplets, and the other in which lipid molecules are uniformly dispersed in the cytosol as unresolved, ultra-small droplets. The present study therefore analyzes both the bulk and the single-cell level changes when cellulose hydrolysate is used as a carbon source for Chlamydomonas reinhardtii mixotrophic cultivation, which serves a four-fold purpose: value from waste, fixation of atmospheric CO2, production of lipids for biodiesel, and starch for bioethanol.

Lipid Droplets and Neurodegeneration

Energy metabolism in the brain has been considered one of the critical research areas of neuroscience for ages. One of the most vital parts of brain metabolism cascades is lipid metabolism, and fatty acid plays a crucial role in this process. The fatty acid breakdown process in mitochondria undergoes through a conserved pathway known as β-oxidation where acetyl-CoA and shorter fatty acid chains are produced along with a significant amount of energy molecule. Further, the complete breakdown of fatty acids occurs when they enter the mitochondrial oxidative phosphorylation. Cells store energy as neutral lipids in organelles known as Lipid Droplets (LDs) to prepare for variations in the availability of nutrients. Fatty acids are liberated by lipid droplets and are transported to various cellular compartments for membrane biogenesis or as an energy source. Current research shows that LDs are important in inflammation, metabolic illness, and cellular communication. Lipid droplet biology in peripheral organs like the liver and heart has been well investigated, while the brain's LDs have received less attention. Recently, there has been increased awareness of the existence and role of these dynamic organelles in the central nervous system, mainly connected to neurodegeneration. In this review, we discussed the role of beta-oxidation and lipid droplet formation in the oxidative phosphorylation process, which directly affects neurodegeneration through various pathways.

FIT2 proteins and lipid droplet emergence, an interplay between phospholipid synthesis, surface tension, and membrane curvature

Lipid droplets (LDs) serve as intracellular compartments primarily dedicated to the storage of metabolic energy in the form of neutral lipids. The processes that regulate and control LD biogenesis are being studied extensively and are gaining significance due to their implications in major metabolic disorders, including type 2 diabetes and obesity. A protein of particular interest is Fat storage-Inducing Transmembrane 2 (FIT2), which affects the emergence step of LD biogenesis. Instead of properly emerging towards the cytosol, LDs in FIT2-deficient cells remain embedded within the membrane of the endoplasmic reticulum (ER). In vitro studies revealed the ability of FIT2 to bind both di- and triacylglycerol (DAG/TAG), key players in lipid storage, and its activity to cleave acyl-CoA. However, the translation of these in vitro functions to the observed embedding of LDs in FIT2 deficient cells remains to be established. To understand the role of FIT2 in vivo, we discuss the parameters that affect LD emergence. Our focus centers on the role that membrane curvature and surface tension play in LD emergence, as well as the impact that the lipid composition exerts on these key parameters. In addition, we discuss hypotheses on how FIT2 could function locally to modulate lipids at sites of LD emergence.

Giant organelle vesicles to uncover intracellular membrane mechanics and plasticity

Tools for accessing and studying organelles remain underdeveloped. Here, we present a method by which giant organelle vesicles (GOVs) are generated by submitting cells to a hypotonic medium followed by plasma membrane breakage. By this means, GOVs ranging from 3 to over 10 µm become available for micromanipulation. GOVs are made from organelles such as the endoplasmic reticulum, endosomes, lysosomes and mitochondria, or in contact with one another such as giant mitochondria-associated ER membrane vesicles. We measure the mechanical properties of each organelle-derived GOV and find that they have distinct properties. In GOVs procured from Cos7 cells, for example, bending rigidities tend to increase from the endoplasmic reticulum to the plasma membrane. We also found that the mechanical properties of giant endoplasmic reticulum vesicles (GERVs) vary depending on their interactions with other organelles or the metabolic state of the cell. Lastly, we demonstrate GERVs' biochemical activity through their capacity to synthesize triglycerides and assemble lipid droplets. These findings underscore the potential of GOVs as valuable tools for studying the biophysics and biology of organelles.

Molecular dynamics simulations of intracellular lipid droplets: a new tool in the toolbox

Lipid droplets (LDs) are ubiquitous intracellular organelles with a central role in multiple lipid metabolic pathways. However, identifying correlations between their structural properties and their biological activity has proved challenging, owing to their unique physicochemical properties as compared with other cellular membranes. In recent years, molecular dynamics (MD) simulations, a computational methodology allowing the accurate description of molecular assemblies down to their individual components, have been demonstrated to be a useful and powerful approach for studying LD structural and dynamical properties. In this short review, we attempt to highlight, as comprehensively as possible, how MD simulations have contributed to our current understanding of multiple molecular mechanisms involved in LD biology.

A unifying mechanism for seipin-mediated lipid droplet formation

Lipid droplets (LDs) are dynamic organelles essential for cellular lipid homeostasis. Assembly of LDs occurs in the endoplasmic reticulum (ER), and the conserved ER membrane protein seipin emerged as a key player in this process. Here, we review recent advances provided by structural, biochemical, and in silico analysis that revealed mechanistic insights into the molecular role of the seipin complexes and led to an updated model for LD biogenesis. We further discuss how other ER components cooperate with seipin during LD biogenesis. Understanding the molecular mechanisms underlying seipin-mediated LD assembly is important to uncover the fundamental aspects of lipid homeostasis and organelle biogenesis and to provide hints on the pathogenesis of lipid storage disorders.

Zooming into lipid droplet biology through the lens of electron microscopy

Electron microscopy (EM), in its various flavors, has significantly contributed to our understanding of lipid droplets (LD) as central organelles in cellular metabolism. For example, EM has illuminated that LDs, in contrast to all other cellular organelles, are uniquely enclosed by a single phospholipid monolayer, revealed the architecture of LD contact sites with different organelles, and provided near-atomic resolution maps of key enzymes that regulate neutral lipid biosynthesis and LD biogenesis. In this review, we first provide a brief history of pivotal findings in LD biology unveiled through the lens of an electron microscope. We describe the main EM techniques used in the context of LD research and discuss their current capabilities and limitations, thereby providing a foundation for utilizing suitable EM methodology to address LD-related questions with sufficient level of structural preservation, detail, and resolution. Finally, we highlight examples where EM has recently been and is expected to be instrumental in expanding the frontiers of LD biology.

Liposome-Mediated Anti-Viral Drug Delivery Across Blood-Brain Barrier: Can Lipid Droplet Target Be Game Changers?

Lipid droplets (LDs) are subcellular organelles secreted from the endoplasmic reticulum (ER) that play a major role in lipid homeostasis. Recent research elucidates additional roles of LDs in cellular bioenergetics and innate immunity. LDs activate signaling cascades for interferon response and secretion of pro-inflammatory cytokines. Since balanced lipid homeostasis is critical for neuronal health, LDs play a crucial role in neurodegenerative diseases. RNA viruses enhance the secretion of LDs to support various phases of their life cycle in neurons which further leads to neurodegeneration. Targeting the excess LD formation in the brain could give us a new arsenal of antiviral therapeutics against neuroviruses. Liposomes are a suitable drug delivery system that could be used for drug delivery in the brain by crossing the Blood-Brain Barrier. Utilizing this, various pharmacological inhibitors and non-coding RNAs can be delivered that could inhibit the biogenesis of LDs or reduce their sizes, reversing the excess lipid-related imbalance in neurons. Liposome-Mediated Antiviral Drug Delivery Across Blood-Brain Barrier. Developing effective antiviral drug is challenging and it doubles against neuroviruses that needs delivery across the Blood-Brain Barrier (BBB). Lipid Droplets (LDs) are interesting targets for developing antivirals, hence targeting LD formation by drugs delivered using Liposomes can be game changers.

Binding of perilipin 3 to membranes containing diacylglycerol is mediated by conserved residues within its PAT domain

Perilipins (PLINs) constitute an evolutionarily conserved family of proteins that specifically associate with the surface of lipid droplets (LDs). These proteins function in LD biogenesis and lipolysis and help to stabilize the surface of LDs. PLINs are typically composed of three different protein domains. They share an N-terminal PAT domain of unknown structure and function, a central region containing 11-mer repeats that form amphipathic helices, and a C-terminal domain that adopts a 4-helix bundle structure. How exactly these three distinct domains contribute to PLIN function remains to be determined. Here, we show that the N-terminal PAT domain of PLIN3 binds diacylglycerol (DAG), the precursor to triacylglycerol, a major storage lipid of LDs. PLIN3 and its PAT domain alone bind liposomes with micromolar affinity and PLIN3 binds artificial LDs containing low concentrations of DAG with nanomolar affinity. The PAT domain of PLIN3 is predicted to adopt an amphipathic triangular shaped structure. In silico ligand docking indicates that DAG binds to one of the highly curved regions within this domain. A conserved aspartic acid residue in the PAT domain, E86, is predicted to interact with DAG, and we found that its substitution abrogates high affinity binding of DAG as well as DAG-stimulated association with liposome and artificial LDs. These results indicate that the PAT domain of PLINs harbor specific lipid-binding properties that are important for targeting these proteins to the surface of LDs and to ER membrane domains enriched in DAG to promote LD formation.

Concept of lipid droplet biogenesis

Lipid droplets (LD) are functionally conserved fat storage organelles found in all cell types. LDs have a unique structure comprising of a hydrophobic core of neutral lipids (fat), triacylglycerol (TAG) and cholesterol esters (CE) surrounded by a phospholipid monolayer. LD surface is decorated by a multitude of proteins and enzymes rendering this compartment functional. Accumulating evidence suggests that LDs originate from discrete ER-subdomains, demarcated by the lipodystrophy protein seipin, however, the mechanisms of which are not well understood. LD biogenesis factors together with biophysical properties of the ER membrane orchestrate spatiotemporal regulation of LD nucleation and growth at specific ER subdomains in response to metabolic cues. Defects in LD formation manifests in several human pathologies, including obesity, lipodystrophy, ectopic fat accumulation, and insulin resistance. Here, we review recent advances in understanding the molecular events during initial stages of eukaryotic LD assembly and discuss the critical role of factors that ensure fidelity of this process.

Mechanisms of Lipid Droplet Accumulation in Steatotic Liver Diseases

The steatotic diseases of metabolic dysfunction-associated steatotic liver disease (MASLD), alcohol-associated liver disease (ALD), and chronic hepatitis C (HCV) account for the majority of liver disease prevalence, morbidity, and mortality worldwide. While these diseases have distinct pathogenic and clinical features, dysregulated lipid droplet (LD) organelle biology represents a convergence of pathogenesis in all three. With increasing understanding of hepatocyte LD biology, we now understand the roles of LD proteins involved in these diseases but also how genetics modulate LD biology to either exacerbate or protect against the phenotypes associated with steatotic liver diseases. Here, we review the history of the LD organelle and its biogenesis and catabolism. We also review how this organelle is critical not only for the steatotic phenotype of liver diseases but also for their advanced phenotypes. Finally, we summarize the latest attempts and challenges of leveraging LD biology for therapeutic gain in steatotic diseases. In conclusion, the study of dysregulated LD biology may lead to novel therapeutics for the prevention of disease progression in the highly prevalent steatotic liver diseases of MASLD, ALD, and HCV.

Structural basis for triacylglyceride extraction from mycobacterial inner membrane by MFS transporter Rv1410

Mycobacterium tuberculosis is protected from antibiotic therapy by a multi-layered hydrophobic cell envelope. Major facilitator superfamily (MFS) transporter Rv1410 and the periplasmic lipoprotein LprG are involved in transport of triacylglycerides (TAGs) that seal the mycomembrane. Here, we report a 2.7 Å structure of a mycobacterial Rv1410 homologue, which adopts an outward-facing conformation and exhibits unusual transmembrane helix 11 and 12 extensions that protrude ~20 Å into the periplasm. A small, very hydrophobic cavity suitable for lipid transport is constricted by a functionally important ion-lock likely involved in proton coupling. Combining mutational analyses and MD simulations, we propose that TAGs are extracted from the core of the inner membrane into the central cavity via lateral clefts present in the inward-facing conformation. The functional role of the periplasmic helix extensions is to channel the extracted TAG into the lipid binding pocket of LprG.

Lipid droplets control mitogenic lipid mediator production in human cancer cells

OBJECTIVES

Polyunsaturated fatty acids (PUFAs) are structural components of membrane phospholipids and precursors of oxygenated lipid mediators with diverse functions, including the control of cell growth, inflammation and tumourigenesis. However, the molecular pathways that control the availability of PUFAs for lipid mediator production are not well understood. Here, we investigated the crosstalk of three pathways in the provision of PUFAs for lipid mediator production: (i) secreted group X phospholipase A2 (GX sPLA2) and (ii) cytosolic group IVA PLA2 (cPLA2α), both mobilizing PUFAs from membrane phospholipids, and (iii) adipose triglyceride lipase (ATGL), which mediates the degradation of triacylglycerols (TAGs) stored in cytosolic lipid droplets (LDs).

METHODS

We combined lipidomic and functional analyses in cancer cell line models to dissect the trafficking of PUFAs between membrane phospholipids and LDs and determine the role of these pathways in lipid mediator production, cancer cell proliferation and tumour growth in vivo.

RESULTS

We demonstrate that lipid mediator production strongly depends on TAG turnover. GX sPLA2 directs ω-3 and ω-6 PUFAs from membrane phospholipids into TAG stores, whereas ATGL is required for their entry into lipid mediator biosynthetic pathways. ATGL controls the release of PUFAs from LD stores and their conversion into cyclooxygenase- and lipoxygenase-derived lipid mediators under conditions of nutrient sufficiency and during serum starvation. In starving cells, ATGL also promotes the incorporation of LD-derived PUFAs into phospholipids, representing substrates for cPLA2α. Furthermore, we demonstrate that the built-up of TAG stores by acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1) is required for the production of mitogenic lipid signals that promote cancer cell proliferation and tumour growth.

CONCLUSION

This study shifts the paradigm of PLA2-driven lipid mediator signalling and identifies LDs as central lipid mediator production hubs. Targeting DGAT1-mediated LD biogenesis is a promising strategy to restrict lipid mediator production and tumour growth.

Structural and thermodynamic properties of bulk triglycerides and triglyceride/water mixtures reproduced using a polarizable coarse-grained model

Triglycerides (TGs) play important roles in renewable energies, food production, medicine, and metabolism in organisms. Here, we developed a novel coarse-grained (CG) force field (FF) for triglycerides to reproduce both the structural and thermodynamic properties of bulk TGs, TG/air interfaces, and TG/water mixtures using molecular dynamics (MD) simulations. We rigorously optimized the bonded and nonbonded force parameters between the CG beads of TGs and nonbonded force parameters between TG beads and polarizable CG water beads by employing an efficient meta-multilinear interpolation parameterization algorithm recently developed by us. This CG FF performs very well in reproducing the percolating network of the TG bulk phase self-assembled in water and a variety of molecular conformations predicted by all-atom MD simulations. More importantly, it also correctly reproduces multiple experimentally measurable macroscopic thermodynamic properties, including the density and surface tensions of both the TG/air and TG/water interfaces. This paves the way for studying more complicated systems involving TGs on a large scale.

Ostwald Ripening of Triacylglycerol Droplets Embedded in Glass-Supported Phospholipid Bilayers

Lipid droplets are fat storage organelles that consist of a neutral lipid core surrounded by a phospholipid monolayer. Because of their important biological functions, reconstituting model lipid droplets in synthetic phospholipid membranes is of great interest. In the present study, we investigated the incorporation of triacylglycerol droplets into glass-supported phospholipid bilayers by using fluorescence microscopy. We adsorbed triolein emulsions onto a glass surface that was partially covered with planar bilayers. After adsorption, triolein droplets were found to be immobilized in the bilayer membrane. The volume of each bound droplet varied over time. Large droplets grew, whereas small droplets shrank. Additionally, data on fluorescence recovery after photobleaching obtained for a phospholipid probe indicate that phospholipids on and near triolein droplets were fully mobile. Furthermore, photobleaching data obtained for a triacylglycerol probe indicate that triolein molecules diffused between different droplets along the planar bilayer. These results demonstrate Ostwald ripening, where triolein molecules in a small droplet dissolved in the bilayer, diffused laterally, and eventually bound to the interfaces of larger droplets. We investigated the ripening rate by using the average of the cube root of the fluorescence emission obtained for individual droplets. The ripening slowed after the addition of trilinolein to the triolein phase. Finally, we investigated the time dependence of the size distributions of the triolein droplets. The distribution was initially nearly unimodal and subsequently became bimodal.

Perilipin 3 promotes the formation of membrane domains enriched in diacylglycerol and lipid droplet biogenesis proteins

Lipid droplets (LDs) serve as intracellular stores of energy-rich neutral lipids. LDs form at discrete sites in the endoplasmic reticulum (ER) and they remain closely associated with the ER during lipogenic growth and lipolytic consumption. Their hydrophobic neutral lipid core is covered by a monolayer of phospholipids, which harbors a specific set of proteins. This LD surface is coated and stabilized by perilipins, a family of soluble proteins that specifically target LDs from the cytosol. We have previously used chimeric fusion proteins between perilipins and integral ER membrane proteins to test whether proteins that are anchored to the ER bilayer could be dragged onto the LD monolayer. Expression of these chimeric proteins induces repositioning of the ER membrane around LDs. Here, we test the properties of membrane-anchored perilipins in cells that lack LDs. Unexpectedly, membrane-anchored perilipins induce expansion and vesiculation of the perinuclear membrane resulting in the formation of crescent-shaped membrane domains that harbor LD-like properties. These domains are stained by LD-specific lipophilic dyes, harbor LD marker proteins, and they transform into nascent LDs upon induction of neutral lipid synthesis. These ER domains are enriched in diacylglycerol (DAG) and in ER proteins that are important for early steps of LD biogenesis, including seipin and Pex30. Formation of the domains in vivo depends on DAG levels, and we show that perilipin 3 (PLIN3) binds to liposomes containing DAG in vitro. Taken together, these observations indicate that perilipin not only serve to stabilize the surface of mature LDs but that they are likely to exert a more active role in early steps of LD biogenesis at ER subdomains enriched in DAG, seipin, and neutral lipid biosynthetic enzymes.

Structural insights into perilipin 3 membrane association in response to diacylglycerol accumulation

Lipid droplets (LDs) are dynamic organelles that contain an oil core mainly composed of triglycerides (TAG) that is surrounded by a phospholipid monolayer and LD-associated proteins called perilipins (PLINs). During LD biogenesis, perilipin 3 (PLIN3) is recruited to nascent LDs as they emerge from the endoplasmic reticulum. Here, we analyze how lipid composition affects PLIN3 recruitment to membrane bilayers and LDs, and the structural changes that occur upon membrane binding. We find that the TAG precursors phosphatidic acid and diacylglycerol (DAG) recruit PLIN3 to membrane bilayers and define an expanded Perilipin-ADRP-Tip47 (PAT) domain that preferentially binds DAG-enriched membranes. Membrane binding induces a disorder to order transition of alpha helices within the PAT domain and 11-mer repeats, with intramolecular distance measurements consistent with the expanded PAT domain adopting a folded but dynamic structure upon membrane binding. In cells, PLIN3 is recruited to DAG-enriched ER membranes, and this requires both the PAT domain and 11-mer repeats. This provides molecular details of PLIN3 recruitment to nascent LDs and identifies a function of the PAT domain of PLIN3 in DAG binding.

Saccharomyces cerevisiae Δ9-desaturase Ole1 forms a supercomplex with Slc1 and Dga1

Biosynthesis of the various lipid species that compose cellular membranes and lipid droplets depends on the activity of multiple enzymes functioning in coordinated pathways. The flux of intermediates through lipid biosynthetic pathways is regulated to respond to nutritional and environmental demands placed on the cell necessitating that there be extensive flexibility in pathway activity and organization. This flexibility can in part be achieved through the organization of biosynthetic enzymes into metabolon supercomplexes. However, the composition and organization of such supercomplexes remains unclear. Here, we identified protein-protein interactions between acyltransferases Sct1, Gpt2, Slc1, Dga1 and the Δ9 acyl-CoA desaturase Ole1 in Saccharomyces cerevisiae. We further determined that a subset of these acyltransferases interact with each other without Ole1 acting as a scaffold. We show that truncated versions of Dga1 lacking the carboxyl-terminal 20 amino acid residues are non-functional and unable to bind Ole1. Furthermore, charged-to-alanine scanning mutagenesis revealed that a cluster of charged residues near the carboxyl-terminus were required for the interaction with Ole1. Mutation of these charged residues disrupted the interaction between Dga1 and Ole1, but allowed Dga1 to retain catalytic activity and to induce lipid droplet formation. These data support the formation of a complex of acyltransferases involved in lipid biosynthesis that interacts with Ole1, the sole acyl-CoA desaturase in S. cerevisiae, that can channel unsaturated acyl-chains toward phospholipid or triacylglycerol synthesis. This desaturasome complex may provide the architecture that allows for the necessary flux of de novo synthesized unsaturated acyl-CoA to phospholipid or triacylglycerol synthesis as demanded by cellular requirements.

Lipid droplet biogenesis and functions in health and disease

Ubiquitous yet unique, lipid droplets are intracellular organelles that are increasingly being recognized for their versatility beyond energy storage. Advances uncovering the intricacies of their biogenesis and the diversity of their physiological and pathological roles have yielded new insights into lipid droplet biology. Despite these insights, the mechanisms governing the biogenesis and functions of lipid droplets remain incompletely understood. Moreover, the causal relationship between the biogenesis and function of lipid droplets and human diseases is poorly resolved. Here, we provide an update on the current understanding of the biogenesis and functions of lipid droplets in health and disease, highlighting a key role for lipid droplet biogenesis in alleviating cellular stresses. We also discuss therapeutic strategies of targeting lipid droplet biogenesis, growth or degradation that could be applied in the future to common diseases, such as cancer, hepatic steatosis and viral infection.

Cholesterol esters form supercooled lipid droplets whose nucleation is facilitated by triacylglycerols

Cellular cholesterol can be metabolized to its fatty acid esters, cholesteryl esters (CEs), to be stored in lipid droplets (LDs). With triacylglycerols (TGs), CEs represent the main neutral lipids in LDs. However, while TG melts at ~4 °C, CE melts at ~44 °C, raising the question of how CE-rich LDs form in cells. Here, we show that CE forms supercooled droplets when the CE concentration in LDs is above 20% to TG and, in particular, liquid-crystalline phases when the fraction of CEs is above 90% at 37 °C. In model bilayers, CEs condense and nucleate droplets when the CE/phospholipid ratio reaches over 10-15%. This concentration is reduced by TG pre-clusters in the membrane that thereby facilitate CE nucleation. Accordingly, blocking TG synthesis in cells is sufficient to strongly dampen CE LD nucleation. Finally, CE LDs emerged at seipins, which cluster and nucleate TG LDs in the ER. However, when TG synthesis is inhibited, similar numbers of LDs are generated in the presence and absence of seipin, suggesting that seipin controls CE LD formation via its TG clustering capacity. Our data point to a unique model whereby TG pre-clusters, favorable at seipins, catalyze the nucleation of CE LDs.

Seipin-still a mysterious protein?

Cells store excess energy in the form of lipid droplets (LDs), a specialized sub-compartment of the endoplasmic reticulum (ER) network. The lipodystrophy protein seipin is a key player in LD biogenesis and ER-LD contact site maintenance. Recent structural and in silico studies have started to shed light on the molecular function of seipin as a LD nucleator in early LD biogenesis, whilst new cell biological work implies a role for seipin in ER-mitochondria contact sites and calcium metabolism. In this minireview, I discuss recent insights into the molecular function of seipin.

Bilayer-Embedded Lipid Droplets Coated with Perilipin-2 Display a Pancake Shape

Lipid droplets (LD) are organelles localized in the membrane of the endoplasmic reticulum (ER) that play an important role in many biological functions. Free LDs that have been released from the ER membrane and are present in the cytosol resemble an oil-in-water emulsion. The surface of an LD is coated with a phospholipid monolayer, and the core of an LD is composed of neutral lipids. Adipose differentiation-related protein (ADRP), also known as perilipin-2, is a protein that surrounds the LD, together with the phospholipid monolayer. ADRP molecules are involved in assisting in the storage of neutral lipids within LDs. In this article, we focus our interest on the influence of ADRP molecules on the 3D shape of bilayer-embedded LDs and the diffusion of phospholipids in the monolayer covering LDs. For this study, we employed two different microfluidic setups: one to produce and explore bilayer-embedded LDs and a second one to mimic the surface of a single LD. Using the first setup, we demonstrate that ADRP molecules stay preferentially localized on the surfaces of bilayer-embedded LDs, and we study their 3D-shape in the presence of ADRP. Using the second setup, we performed FRAP experiments to measure the phospholipid diffusion on a model LD surface as a function of the ADRP concentration. Although the presence of proteins on the LD surface minimally affects the phospholipid and protein motility, ADRP appears to have a significant effect on the 3D structure of LDs embedded in the bilayer.

Phospholipids diffusion on the surface of model lipid droplets

Lipid droplets (LD) are organelles localized in the membrane of the Endoplasmic Reticulum (ER) that play an important role in metabolic functions. They consist of a core of neutral lipids surrounded by a monolayer of phosphoplipids and proteins resembling an oil-in-water emulsion droplet. Many studies have focused on the biophysical properties of these LDs. However, despite numerous efforts, we are lacking information on the mobility of phospholipids on the LDs surface, although they may play a key role in the protein distribution. In this article, we developed a microfluidic setup that allows the formation of a triolein-buffer interface decorated with a phospholipid monolayer. Using this setup, we measured the motility of phospholipid molecules by performing Fluorescent Recovery After Photobleaching (FRAP) experiments for different lipidic compositions. The results of the FRAP measurements reveal that the motility of phospholipids is controlled by the monolayer packing decorating the interface.

Membrane Lipid Reshaping Underlies Oxidative Stress Sensing by the Mitochondrial Proteins UCP1 and ANT1

Oxidative stress and ROS are important players in the pathogenesis of numerous diseases. In addition to directly altering proteins, ROS also affects lipids with negative intrinsic curvature such as phosphatidylethanolamine (PE), producing PE adducts and lysolipids. The formation of PE adducts potentiates the protonophoric activity of mitochondrial uncoupling proteins, but the molecular mechanism remains unclear. Here, we linked the ROS-mediated change in lipid shape to the mechanical properties of the membrane and the function of uncoupling protein 1 (UCP1) and adenine nucleotide translocase 1 (ANT1). We show that the increase in the protonophoric activity of both proteins occurs due to the decrease in bending modulus in lipid bilayers in the presence of lysophosphatidylcholines (OPC and MPC) and PE adducts. Moreover, MD simulations showed that modified PEs and lysolipids change the lateral pressure profile of the membrane in the same direction and by the similar amplitude, indicating that modified PEs act as lipids with positive intrinsic curvature. Both results indicate that oxidative stress decreases stored curvature elastic stress (SCES) in the lipid bilayer membrane. We demonstrated that UCP1 and ANT1 sense SCES and proposed a novel regulatory mechanism for the function of these proteins. The new findings should draw the attention of the scientific community to this important and unexplored area of redox biochemistry.

Triglyceride lipolysis triggers liquid crystalline phases in lipid droplets and alters the LD proteome

Lipid droplets (LDs) are reservoirs for triglycerides (TGs) and sterol-esters (SEs), but how these lipids are organized within LDs and influence their proteome remain unclear. Using in situ cryo-electron tomography, we show that glucose restriction triggers lipid phase transitions within LDs generating liquid crystalline lattices inside them. Mechanistically this requires TG lipolysis, which decreases the LD's TG:SE ratio, promoting SE transition to a liquid crystalline phase. Molecular dynamics simulations reveal TG depletion promotes spontaneous TG and SE demixing in LDs, additionally altering the lipid packing of the PL monolayer surface. Fluorescence imaging and proteomics further reveal that liquid crystalline phases are associated with selective remodeling of the LD proteome. Some canonical LD proteins, including Erg6, relocalize to the ER network, whereas others remain LD-associated. Model peptide LiveDrop also redistributes from LDs to the ER, suggesting liquid crystalline phases influence ER-LD interorganelle transport. Our data suggests glucose restriction drives TG mobilization, which alters the phase properties of LD lipids and selectively remodels the LD proteome.

ORP5 and ORP8 orchestrate lipid droplet biogenesis and maintenance at ER-mitochondria contact sites

Lipid droplets (LDs) are the primary organelles of lipid storage, buffering energy fluctuations of the cell. They store neutral lipids in their core that is surrounded by a protein-decorated phospholipid monolayer. LDs arise from the endoplasmic reticulum (ER). The ER protein seipin, localizing at ER-LD junctions, controls LD nucleation and growth. However, how LD biogenesis is spatially and temporally coordinated remains elusive. Here, we show that the lipid transfer proteins ORP5 and ORP8 control LD biogenesis at mitochondria-associated ER membrane (MAM) subdomains, enriched in phosphatidic acid. We found that ORP5/8 regulates seipin recruitment to these MAM-LD contacts, and their loss impairs LD biogenesis. Importantly, the integrity of ER-mitochondria contact sites is crucial for ORP5/8 function in regulating seipin-mediated LD biogenesis. Our study uncovers an unprecedented ORP5/8 role in orchestrating LD biogenesis and maturation at MAMs and brings novel insights into the metabolic crosstalk between mitochondria, ER, and LDs at the membrane contact sites.

Seipin concentrates distinct neutral lipids via interactions with their acyl chain carboxyl esters

Lipid droplets (LDs) are essential for cellular lipid homeostasis by storing diverse neutral lipids (NLs), such as triacylglycerol (TAG), steryl esters (SE), and retinyl esters (RE). A proper assembly of TAG-containing LDs at the ER requires Seipin, a conserved protein often mutated in lipodystrophies. Here, we show that the yeast Seipin Sei1 and its partner Ldb16 also promote the storage of other NL in LDs. Importantly, this role of Sei1/Ldb16 is evolutionarily conserved as expression of human-Seipin restored normal SE-containing LDs in yeast Seipin mutants. As in the case of TAG, the formation of SE-containing LDs requires interactions between hydroxyl-residues in human Seipin or yeast Ldb16 with NL carboxyl esters. These findings provide a universal mechanism for Seipin-mediated LD formation and suggest a model for how Seipin distinguishes NLs from aliphatic phospholipid acyl chains in the center of the membrane bilayer.

Interplay between Lipid Metabolism, Lipid Droplets, and DNA Virus Infections

Lipid droplets (LDs) are cellular organelles rich in neutral lipids such as triglycerides and cholesterol esters that are coated by a phospholipid monolayer and associated proteins. LDs are known to play important roles in the storage and availability of lipids in the cell and to serve as a source of energy reserve for the cell. However, these structures have also been related to oxidative stress, reticular stress responses, and reduced antigen presentation to T cells. Importantly, LDs are also known to modulate viral infection by participating in virus replication and assembly. Here, we review and discuss the interplay between neutral lipid metabolism and LDs in the replication cycle of different DNA viruses, identifying potentially new molecular targets for the treatment of viral infections.

Membrane shaping proteins, lipids, and cytoskeleton: Recipe for nascent lipid droplet formation

Lipid droplets (LDs) are ubiquitous, neutral lipidorganelles that act as hubs of metabolic processes. LDs are structurally unique with a hydrophobic core that mainly consists of neutral lipids, sterol esters, and triglycerides, enclosed within a phospholipid monolayer. Nascent LD formation begins with the accumulation of neutral lipids in the endoplasmic reticulum (ER) bilayer. The ER membrane proteins such as seipin, LDAF1, FIT, and MCTPs are reported to play an important role in the formation of nascent LDs. As the LDs grow, they unmix from the highly charged ER membrane to form mature LDs. LD biogenesis is an exciting, emerging research area, and herein, we discuss the recent progress in our understanding of the formation of eukaryotic nascent LDs. We focus on the role of ER membrane shaping proteins such as reticulons and reticulon-like proteins, membrane lipids, and cytoskeleton proteins such as septin in the formation of nascent LDs.

Production, Biosynthesis, and Commercial Applications of Fatty Acids From Oleaginous Fungi

Oleaginous fungi (including fungus-like protists) are attractive in lipid production due to their short growth cycle, large biomass and high yield of lipids. Some typical oleaginous fungi including Galactomyces geotrichum, Thraustochytrids, Mortierella isabellina, and Mucor circinelloides, have been well studied for the ability to accumulate fatty acids with commercial application. Here, we review recent progress toward fermentation, extraction, of fungal fatty acids. To reduce cost of the fatty acids, fatty acid productions from raw materials were also summarized. Then, the synthesis mechanism of fatty acids was introduced. We also review recent studies of the metabolic engineering strategies have been developed as efficient tools in oleaginous fungi to overcome the biochemical limit and to improve production efficiency of the special fatty acids. It also can be predictable that metabolic engineering can further enhance biosynthesis of fatty acids and change the storage mode of fatty acids.

Seipin transmembrane segments critically function in triglyceride nucleation and lipid droplet budding from the membrane

Lipid droplets (LDs) are organelles formed in the endoplasmic reticulum (ER) to store triacylglycerol (TG) and sterol esters. The ER protein seipin is key for LD biogenesis. Seipin forms a cage-like structure, with each seipin monomer containing a conserved hydrophobic helix and two transmembrane (TM) segments. How the different parts of seipin function in TG nucleation and LD budding is poorly understood. Here, we utilized molecular dynamics simulations of human seipin, along with cell-based experiments, to study seipin’s functions in protein–lipid interactions, lipid diffusion, and LD maturation. An all-atom simulation indicates that seipin TM segment residues and hydrophobic helices residues located in the phospholipid tail region of the bilayer attract TG. Simulating larger, growing LDs with coarse-grained models, we find that the seipin TM segments form a constricted neck structure to facilitate conversion of a flat oil lens into a budding LD. Using cell experiments and simulations, we also show that conserved, positively charged residues at the end of seipin’s TM segments affect LD maturation. We propose a model in which seipin TM segments critically function in TG nucleation and LD growth.

Seipin collaborates with the ER membrane to control the sites of lipid droplet formation

Most cells store metabolic energy in lipid droplets (LDs). LDs are composed of a hydrophobic core, covered by a phospholipid monolayer, and functionalized by a specific set of proteins. Formation of LDs takes place in the endoplasmic reticulum (ER), where neutral lipid biosynthetic enzymes are located. Recent evidence indicate that this process is confined to specific ER subdomains, where proteins meet to initiate LD assembly. The lipodystrophy protein Seipin, is emerging as a major coordinator of LD biogenesis. Seipin forms a large oligomeric toroidal structure, which traps neutral lipids to promote LD nucleation. Here, we discuss the role of LD biogenesis factors that associate with Seipin to assemble functional LDs.

Melatonin Regulates Lipid Metabolism in Porcine Cumulus-Oocyte Complexes via the Melatonin Receptor 2

Previous studies suggest that the inclusion of melatonin (MTn) in in vitro maturation protocols improves the developmental competence of oocytes by scavenging reactive oxygen species (ROS). However, the molecular mechanisms integrating melatonin receptor (MT)-mediated lipid metabolism and redox signaling during in vitro cumulus-oocyte complex (COC) development still remain unclear. Here, we aimed to elucidate the potential role of MTn receptors in lipid metabolic adjustments during in vitro porcine COC development. We observed that MTn-mediated Gsα-cAMP/PKA signaling facilitated lipolysis primarily through the MT2 receptor and subsequently increased fatty acid (FA) release by hydrolyzing intracellular triglycerides (TGs) in cumulus cells. Furthermore, CD36 was a critical FA transporter that transported available FAs from cumulus cells to oocytes and promoted de novo TG synthesis in the latter. In addition, MTn regulated lipogenesis and intracellular lipolysis to maintain lipid homeostasis and limit ROS production, thereby supporting oocyte cytoplasmic maturation and the subsequent embryo development. Taken together, these findings provide insight into the possible mechanism integrating MT2-mediated lipid homeostasis and redox signaling, which limits ROS production during in vitro COC development. Therefore, understanding the dynamics of the interactions between lipid homeostasis and redox signaling driven by MT2 is necessary in order to predict drug targets and the effects of therapeutics used to improve female reproductive health.

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.

Lipid Droplets Embedded in a Model Cell Membrane Create a Phospholipid Diffusion Barrier

Lipid droplets (LDs) are ubiquitous, cytoplasmic fat storage organelles that originate from the endoplasmic reticulum (ER) membrane. They are composed of a core of neutral lipids surrounded by a phospholipid monolayer. Proteins embedded into this monolayer membrane adopt a monotopic topology and are crucial for regulated lipid storage and consumption. A key question is, which collective properties of protein-intrinsic and lipid-mediated features determine spatio-temporal protein partitioning between phospholipid bilayer and LD monolayer membranes. To address this question, a freestanding phospholipid bilayer with physiological lipidic composition is produced using microfluidics and micrometer-sized LDs are dispersed around the bilayer that spontaneously insert into the bilayer. Using confocal microscopy, the 3D geometry of the reconstituted LDs is determined with high spatial resolution. The micrometer-sized bilayer-embedded LDs present a characteristic lens shape that obeys predictions from equilibrium wetting theory. Fluorescence recovery after photobleaching measurements reveals the existence of a phospholipid diffusion barrier at the monolayer-bilayer interface. Coarse-grained molecular dynamics simulation reveals lipid specific density distributions along the pore rim, which may rationalize the diffusion barrier. The lipid diffusion barrier between the LD covering monolayer and the bilayer may be a key phenomenon influencing protein partitioning between the ER membrane and LDs in living cells.

IQGAP1 scaffolding links phosphoinositide kinases to cytoskeletal reorganization

IQGAP1 is a multidomain scaffold protein that coordinates the direction and impact of multiple signaling pathways by scaffolding its various binding partners. However, the spatial and temporal resolution of IQGAP1 scaffolding remains unclear. Here, we use fluorescence imaging and correlation methods that allow for real-time live-cell changes in IQGAP1 localization and complex formation during signaling. We find that IQGAP1 and PIPKIγ interact on both the plasma membrane and in cytosol. Epidermal growth factor (EGF) stimulation, which can initiate cytoskeletal changes, drives the movement of the cytosolic pool toward the plasma membrane to promote cytoskeletal changes. We also observe that a significant population of cytosolic IQGAP1-PIPKIγ complexes localize to early endosomes, and in some instances form aggregated clusters which become highly mobile upon EGF stimulation. Our imaging studies show that PIPKIγ and PI3K bind simultaneously to IQGAP1, which may accelerate conversion of PI4P to PI(3,4,5)P₃ that is required for cytoskeletal changes. Additionally, we find that IQGAP1 is responsible for PIPKIγ association with two proteins associated with cytoskeletal changes, talin and Cdc42, during EGF stimulation. These results directly show that IQGAP1 provides a physical link between phosphoinositides (through PIPKIγ), focal adhesion formation (through talin), and cytoskeletal reorganization (through Cdc42) upon EGF stimulation. Taken together, our results support the importance of IQGAP1 in regulating cell migration by linking phosphoinositide lipid signaling with cytoskeletal reorganization.

Biogenesis and Breakdown of Lipid Droplets in Pathological Conditions

Lipid droplets (LD) have long been considered as mere fat drops; however, LD have lately been revealed to be ubiquitous, dynamic and to be present in diverse organelles in which they have a wide range of key functions. Although incompletely understood, the biogenesis of eukaryotic LD initiates with the synthesis of neutral lipids (NL) by enzymes located in the endoplasmic reticulum (ER). The accumulation of NL leads to their segregation into nanometric nuclei which then grow into lenses between the ER leaflets as they are further filled with NL. The lipid composition and interfacial tensions of both ER and the lenses modulate their shape which, together with specific ER proteins, determine the proneness of LD to bud from the ER toward the cytoplasm. The most important function of LD is the buffering of energy. But far beyond this, LD are actively integrated into physiological processes, such as lipid metabolism, control of protein homeostasis, sequestration of toxic lipid metabolic intermediates, protection from stress, and proliferation of tumours. Besides, LD may serve as platforms for pathogen replication and defense. To accomplish these functions, from biogenesis to breakdown, eukaryotic LD have developed mechanisms to travel within the cytoplasm and to establish contact with other organelles. When nutrient deprivation occurs, LD undergo breakdown (lipolysis), which begins with the LD-associated members of the perilipins family PLIN2 and PLIN3 chaperone-mediated autophagy degradation (CMA), a specific type of autophagy that selectively degrades a subset of cytosolic proteins in lysosomes. Indeed, PLINs CMA degradation is a prerequisite for further true lipolysis, which occurs via cytosolic lipases or by lysosome luminal lipases when autophagosomes engulf portions of LD and target them to lysosomes. LD play a crucial role in several pathophysiological processes. Increased accumulation of LD in non-adipose cells is commonly observed in numerous infectious diseases caused by intracellular pathogens including viral, bacterial, and parasite infections, and is gradually recognized as a prominent characteristic in a variety of cancers. This review discusses current evidence related to the modulation of LD biogenesis and breakdown caused by intracellular pathogens and cancer.

Role of diacylglycerol O-acyltransferase (DGAT) isoforms in bovine hepatic fatty acid metabolism

Fatty acid accumulation in hepatocytes induced by high concentrations of fatty acids due to lipolysis and the associated oxidative damage they cause occur most frequently after calving. Because of their role in esterification of fatty acids, diacylglycerol acyltransferase isoforms (DGAT1 and DGAT2) could play a role in the susceptibility of dairy cows to develop fatty liver. To gain mechanistic insights, we performed in vivo and in vitro analyses using liver biopsies or isolated primary hepatocytes. The in vivo study (n = 5 cows/group) involved healthy cows [average liver triacylglycerol (TAG) = 0.78%; 0.58 to 0.93%, ratio of triglyceride weight to wet liver weight] or cows diagnosed with fatty liver (average TAG = 7.60%; 5.31 to 10.54%). In vitro, hepatocytes isolated from 3 healthy female calves (1 d old, 44 to 53 kg) were challenged with (fatty acids) or without (control) a 1.2 mM mixture of fatty acids in an attempt to induce metabolic stress. Furthermore, hepatocytes were treated with DGAT1 inhibitor or DGAT2 inhibitor for 2 h followed by a challenge with (DGAT1 inhibitor + fatty acids or DGAT2 inhibitor + fatty acids) or without (DGAT1 inhibitor or DGAT2 inhibitor) the 1.2 mM mixture of fatty acids for 12 h. Data analysis of liver biopsies was compared using a 2-tailed unpaired Student's t-test. Data from calf hepatocyte treatment comparisons were assessed by one-way ANOVA, and multiplicity for each experiment was adjusted by the Holm's procedure. Data indicated that both fatty liver and in vitro challenge with fatty acids were associated with greater mRNA and protein abundance of SREBF1, FASN, DGAT1, and DGAT2. In contrast, mRNA and protein abundance of CPT1A and very low-density lipoprotein synthesis-related proteins MTTP and APOB were markedly lower. However, compared with fatty acid challenge alone, DGAT1 inhibitor + fatty acids led to greater mRNA and protein abundance of CPT1A and APOB, and greater mRNA abundance of SREBF1 and MTTP. Furthermore, this treatment led to lower mRNA abundance of FASN and DGAT2 and TAG concentrations. Compared with fatty acid challenge alone, DGAT2 inhibitor + fatty acids led to greater mRNA and protein abundance of CPT1A, MTTP, and APOB, and lower mRNA and protein abundance of SREBF1 and FASN. In addition, compared with control and fatty acids, there was greater protein abundance of GRP78 and PERK in both DGAT1 and DGAT2 inhibitor with or without fatty acids. Furthermore, compared with control and fatty acids, reactive oxygen species concentrations in the DGAT1 inhibitor with or without fatty acid group was greater. Overall, data suggested that DGAT1 is particularly relevant in the context of hepatocyte TAG synthesis from exogenous fatty acids. Disruption of both DGAT1 and DGAT2 altered lipid homeostasis, channeling fatty acids toward oxidation and generation of reactive oxygen species. Both DGAT isoforms play a role in promoting fatty acid storage into TAG and lipid droplets to protect hepatocytes from oxidative damage.

The Bak core dimer focuses triacylglycerides in the membrane

Apoptosis, the intrinsic programmed cell death process, is mediated by the Bcl-2 family members Bak and Bax. Activation via formation of symmetric core dimers and oligomerization on the mitochondrial outer membrane (MOM) leads to permeabilization and cell death. Although this process is linked to the MOM, the role of the membrane in facilitating such pores is poorly understood. We recently described Bak core domain dimers, revealing lipid binding sites and an initial role of lipids in oligomerization. Here we describe simulations that identified localized clustering and interaction of triacylglycerides (TAGs) with a minimized Bak dimer construct. Coalescence of TAGs occurred beneath this Bak dimer, mitigating dimer-induced local membrane thinning and curvature in representative coarse-grain MOM and model membrane systems. Furthermore, the effects observed as a result of coarse-grain TAG cluster formation was concentration dependent, scaling from low physiological MOM concentrations to those found in other organelles. We find that increasing the TAG concentration in liposomes mimicking the MOM decreased the ability of activated Bak to permeabilize these liposomes. These results suggest that the presence of TAGs within a Bak-lipid membrane preserves membrane integrity and is associated with reduced membrane stress, suggesting a possible role of TAGs in Bak-mediated apoptosis.

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.

Recharging your fats: CHARMM36 parameters for neutral lipids triacylglycerol and diacylglycerol

Neutral lipids (NLs) are an abundant class of cellular lipids. They are characterized by the total lack of charged chemical groups in their structure, and, as a consequence, they play a major role in intracellular lipid storage. NLs that carry a glycerol backbone, such as triacylglycerols (TGs) and diacylglycerols (DGs), are also involved in the biosynthetic pathway of cellular phospholipids, and they have recently been the subject of numerous structural investigations by means of atomistic molecular dynamics simulations. However, conflicting results on the physicochemical behavior of NLs were observed depending on the nature of the atomistic force field used. Here, we show that current phospholipid-derived CHARMM36 parameters for DGs and TGs cannot adequately reproduce interfacial properties of these NLs because of excessive hydrophilicity at the glycerol-ester region. By following a CHARMM36-consistent parameterization strategy, we develop improved parameters for both TGs and DGs that are compatible with both cutoff-based and particle mesh Ewald schemes for the treatment of Lennard-Jones interactions. We show that our improved parameters can reproduce interfacial properties of NLs and their behavior in more complex lipid assemblies. We discuss the implications of our findings in the context of intracellular lipid storage and NLs’ cellular activity.

Mechanism of lipid droplet formation by the yeast Sei1/Ldb16 Seipin complex

Lipid droplets (LDs) are universal lipid storage organelles with a core of neutral lipids, such as triacylglycerols, surrounded by a phospholipid monolayer. This unique architecture is generated during LD biogenesis at endoplasmic reticulum (ER) sites marked by Seipin, a conserved membrane protein mutated in lipodystrophy. Here structural, biochemical and molecular dynamics simulation approaches reveal the mechanism of LD formation by the yeast Seipin Sei1 and its membrane partner Ldb16. We show that Sei1 luminal domain assembles a homooligomeric ring, which, in contrast to other Seipins, is unable to concentrate triacylglycerol. Instead, Sei1 positions Ldb16, which concentrates triacylglycerol within the Sei1 ring through critical hydroxyl residues. Triacylglycerol recruitment to the complex is further promoted by Sei1 transmembrane segments, which also control Ldb16 stability. Thus, we propose that LD assembly by the Sei1/Ldb16 complex, and likely other Seipins, requires sequential triacylglycerol-concentrating steps via distinct elements in the ER membrane and lumen.

Sterols are required for the coordinated assembly of lipid droplets in developing seeds

Lipid droplets (LDs) are intracellular organelles critical for energy storage and lipid metabolism. They are typically composed of an oil core coated by a monolayer of phospholipids and proteins such as oleosins. The mechanistic details of LD biogenesis remain poorly defined. However, emerging evidence suggest that their formation is a spatiotemporally regulated process, occurring at specific sites of the endoplasmic reticulum defined by a specific set of lipids and proteins. Here, we show that sterols are required for formation of oleosin-coated LDs in Arabidopsis. Analysis of sterol pathway mutants revealed that deficiency in several ∆⁵-sterols accounts for the phenotype. Importantly, mutants deficient in these sterols also display reduced LD number, increased LD size and reduced oil content in seeds. Collectively, our data reveal a role of sterols in coordinating the synthesis of oil and oleosins and their assembly into LDs, highlighting the importance of membrane lipids in regulating LD biogenesis.

Lipid droplet biogenesis originates at the endoplasmic reticulum and is defined by a specific set of lipids and proteins. Here, the authors show that sterols play an important role in coordinating oil and oleosin biosynthesis for the formation of lipid droplets in plant leaves and seeds.

Seipin: harvesting fat and keeping adipocytes healthy

Seipin is a key protein in the assembly of cytoplasmic lipid droplets (cLDs) and their maintenance at endoplasmic reticulum (ER)-LD junctions; the absence of seipin results in generalized lipodystrophy. How seipin mediates LD dynamics and prevents lipodystrophy are not well understood. New evidence suggests that seipin attracts triglyceride monomers from the ER to sites of droplet formation. By contrast, seipin may not be directly involved in the assembly of nuclear LDs and may actually suppress their formation at a distance. Seipin promotes adipogenesis, but lipodystrophy may also involve postadipogenic effects. We hypothesize that among these are a cycle of runaway lipolysis and lipotoxicity caused by aberrant LDs, resulting in a depletion of fat stores and a failure of adipose and other cells to thrive.

Physical Characterization of Triolein and Implications for Its Role in Lipid Droplet Biogenesis

Lipid droplets (LDs) are neutral lipid-storing organelles surrounded by a phospholipid (PL) monolayer. At present, how LDs are formed in the endoplasmic reticulum (ER) bilayer is poorly understood. In this study, we present a revised all-atom (AA) triolein (TG) model, the main constituent of the LD core, and characterize its properties in a bilayer membrane to demonstrate the implications of its behavior in LD biogenesis. In bilayer simulations, TG resides at the surface, adopting PL-like conformations (denoted in this work as SURF-TG). Free energy sampling simulation results estimate the barrier for TG relocating from the bilayer surface to the bilayer center to be ∼2 kcal/mol in the absence of an oil lens. SURF-TG is able to modulate membrane properties by increasing PL ordering, decreasing bending modulus, and creating local negative curvature. The other neutral lipid, dioleoyl-glycerol (DAG), also reduces the membrane bending modulus and populates negative curvature regions. A phenomenological coarse-grained (CG) model is also developed to observe larger-scale SURF-TG-mediated membrane deformation. CG simulations confirm that TG nucleates between the bilayer leaflets at a critical concentration when SURF-TG is evenly distributed. However, when one monolayer contains more SURF-TG, the membrane bends toward the other leaflet, followed by TG nucleation if a concentration is higher than the critical threshold. The central conclusion of this study is that SURF-TG is a negative curvature inducer, as well as a membrane modulator. To this end, a model is proposed in which the accumulation of SURF-TG in the luminal leaflet bends the ER bilayer toward the cytosolic side, followed by TG nucleation.

Investigating the structural properties of hydrophobic solvent-rich lipid bilayers

In vitro reconstitutions of lipid membranes have proven to be an indispensable tool to rationalize their molecular complexity and to understand their role in countless cellular processes. However, amongst the various techniques used to reconstitute lipid bilayers in vitro, several approaches are not solvent-free, but rather contain residual hydrophobic solvents in between the two bilayer leaflets, generally as a consequence of the procedure used to generate the bilayer. To what extent the presence of these hydrophobic solvents modifies bilayer properties with respect to native, solvent-free, conditions remains an open question that has important implications for the appropriate interpretation of numerous experimental observations. Here, we thorouhgly characterize hydrophobic solvent-rich lipid bilayers using atomistic molecular dynamics simulations. Our data indicate that while the presence of hydrophobic solvents at high concentrations, such as hexadecane, has a significant effect on membrane thickness, their effects on surface properties, membrane order and lateral stress are quite moderate. Our results corroborate the validity of in vitro approaches as model systems for the investigations of biological membranes but raise a few cautionary aspects that must be considered when investigating specific membrane properties.