Lipid Droplets and Their Autophagic Turnover via the Raft-Like Vacuolar Microdomains

Although once perceived as inert structures that merely serve for lipid storage, lipid droplets (LDs) have proven to be the dynamic organelles that hold many cellular functions. The LDs’ basic structure of a hydrophobic core consisting of neutral lipids and enclosed in a phospholipid monolayer allows for quick lipid accessibility for intracellular energy and membrane production. Whereas formed at the peripheral and perinuclear endoplasmic reticulum, LDs are degraded either in the cytosol by lipolysis or in the vacuoles/lysosomes by autophagy. Autophagy is a regulated breakdown of dysfunctional, damaged, or surplus cellular components. The selective autophagy of LDs is called lipophagy. Here, we review LDs and their degradation by lipophagy in yeast, which proceeds via the micrometer-scale raft-like lipid domains in the vacuolar membrane. These vacuolar microdomains form during nutrient deprivation and facilitate internalization of LDs via the vacuolar membrane invagination and scission. The resultant intra-vacuolar autophagic bodies with LDs inside are broken down by vacuolar lipases and proteases. This type of lipophagy is called microlipophagy as it resembles microautophagy, the type of autophagy when substrates are sequestered right at the surface of a lytic compartment. Yeast microlipophagy via the raft-like vacuolar microdomains is a great model system to study the role of lipid domains in microautophagic pathways.

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Lecithin:Retinol Acyl Transferase (LRAT) induces the formation of lipid droplets

10.1101/733931 ◽

2019 ◽

Cited By ~ 3

Author(s):

Martijn R. Molenaar ◽

Tsjerk A. Wassenaar ◽

Kamlesh K. Yadav ◽

Alexandre Toulmay ◽

Muriel C. Mari ◽

...

Keyword(s):

Mammalian Cells ◽

Lipid Droplets ◽

Neutral Lipids ◽

Yeast Cells ◽

Acyl Transferase ◽

Hydrophobic Core ◽

Retinyl Ester ◽

Steryl Esters ◽

Retinyl Esters

AbstractLipid droplets are unique and nearly ubiquitous organelles that store neutral lipids in a hydrophobic core, surrounded by a monolayer of phospholipids. The primary neutral lipids are triacylglycerols and steryl esters. It is not known whether other classes of neutral lipids can form lipid droplets by themselves. Here we show that production of retinyl esters by lecithin:retinol acyl transferase (LRAT) in yeast cells, incapable of producing triacylglycerols and steryl esters, causes the formation of lipid droplets. By electron microscopy, these lipid droplets are morphologically indistinguishable from those in wild-type cells. In silico and in vitro experiments confirmed the propensity of retinyl esters to segregate from membranes and to form lipid droplets. The hydrophobic N-terminus of LRAT displays preferential interactions with retinyl esters in membranes and promotes the formation of large retinyl ester-containing lipid droplets in mammalian cells. Our combined data indicate that the molecular design of LRAT is optimally suited to allow the formation of characteristic large lipid droplets in retinyl ester-storing cells.

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Dictyostelium Lipid Droplets Host Novel Proteins

Eukaryotic Cell ◽

10.1128/ec.00182-13 ◽

2013 ◽

Vol 12 (11) ◽

pp. 1517-1529 ◽

Cited By ~ 22

Author(s):

Xiaoli Du ◽

Caroline Barisch ◽

Peggy Paschke ◽

Cornelia Herrfurth ◽

Oliver Bertinetti ◽

...

Keyword(s):

Fatty Acids ◽

Endoplasmic Reticulum ◽

Lipid Droplet ◽

Lipid Droplets ◽

Unsaturated Fatty Acids ◽

Neutral Lipids ◽

Saturated Fatty Acids ◽

Endoplasmic Reticulum Membrane ◽

Hydrophobic Core ◽

Content Type

ABSTRACT Across all kingdoms of life, cells store energy in a specialized organelle, the lipid droplet. In general, it consists of a hydrophobic core of triglycerides and steryl esters surrounded by only one leaflet derived from the endoplasmic reticulum membrane to which a specific set of proteins is bound. We have chosen the unicellular organism Dictyostelium discoideum to establish kinetics of lipid droplet formation and degradation and to further identify the lipid constituents and proteins of lipid droplets. Here, we show that the lipid composition is similar to what is found in mammalian lipid droplets. In addition, phospholipids preferentially consist of mainly saturated fatty acids, whereas neutral lipids are enriched in unsaturated fatty acids. Among the novel protein components are LdpA, a protein specific to Dictyostelium , and Net4, which has strong homologies to mammalian DUF829/Tmem53/NET4 that was previously only known as a constituent of the mammalian nuclear envelope. The proteins analyzed so far appear to move from the endoplasmic reticulum to the lipid droplets, supporting the concept that lipid droplets are formed on this membrane.

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A Molecular Logic Gate Enables Super-Resolved Imaging of Intracellular Lipid Droplets

10.26434/chemrxiv.9784799.v1 ◽

2019 ◽

Author(s):

Adam Eördögh ◽

Carolina Paganini ◽

Dorothea Pinotsi ◽

Paolo Arosio ◽

Pablo Rivera-Fuentes

Keyword(s):

Single Molecule ◽

Lipid Droplets ◽

Neutral Lipids ◽

Logic Gate ◽

Living Cells ◽

Three Dimensions ◽

Single Molecule Imaging ◽

Molecular Logic Gate ◽

Molecular Logic ◽

Cellular Compartments

<div>Photoactivatable dyes enable single-molecule imaging in biology. Despite progress in the development of new fluorophores and labeling strategies, many cellular compartments remain difficult to image beyond the limit of diffraction in living cells. For example, lipid droplets, which are organelles that contain mostly neutral lipids, have eluded single-molecule imaging. To visualize these challenging subcellular targets, it is necessary to develop new fluorescent molecular devices beyond simple on/off switches. Here, we report a fluorogenic molecular logic gate that can be used to image single molecules associated with lipid droplets with excellent specificity. This probe requires the subsequent action of light, a lipophilic environment and a competent nucleophile to produce a fluorescent product. The combination of these requirements results in a probe that can be used to image the boundary of lipid droplets in three dimensions with resolutions beyond the limit of diffraction. Moreover, this probe enables single-molecule tracking of lipids within and between droplets in living cells.</div>

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Interactions of Lipid Droplets with the Intracellular Transport Machinery

International Journal of Molecular Sciences ◽

10.3390/ijms22052776 ◽

2021 ◽

Vol 22 (5) ◽

pp. 2776

Author(s):

Selma Yilmaz Dejgaard ◽

John F. Presley

Keyword(s):

Membrane Trafficking ◽

Lipid Droplets ◽

Intracellular Transport ◽

Neutral Lipids ◽

Small Gtpases ◽

Lipid Phase ◽

Intracellular Organelles ◽

And Function ◽

Lipid Phase Separation ◽

Endocytic Pathways

Historically, studies of intracellular membrane trafficking have focused on the secretory and endocytic pathways and their major organelles. However, these pathways are also directly implicated in the biogenesis and function of other important intracellular organelles, the best studied of which are peroxisomes and lipid droplets. There is a large recent body of work on these organelles, which have resulted in the introduction of new paradigms regarding the roles of membrane trafficking organelles. In this review, we discuss the roles of membrane trafficking in the life cycle of lipid droplets. This includes the complementary roles of lipid phase separation and proteins in the biogenesis of lipid droplets from endoplasmic reticulum (ER) membranes, and the attachment of mature lipid droplets to membranes by lipidic bridges and by more conventional protein tethers. We also discuss the catabolism of neutral lipids, which in part results from the interaction of lipid droplets with cytosolic molecules, but with important roles for both macroautophagy and microautophagy. Finally, we address their eventual demise, which involves interactions with the autophagocytotic machinery. We pay particular attention to the roles of small GTPases, particularly Rab18, in these processes.

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A Decade of Mighty Lipophagy: What We Know and What Facts We Need to Know?

Oxidative Medicine and Cellular Longevity ◽

10.1155/2021/5539161 ◽

2021 ◽

Vol 2021 ◽

pp. 1-18

Author(s):

Muhammad Babar Khawar ◽

Muddasir Hassan Abbasi ◽

Mussarat Rafiq ◽

Naila Naz ◽

Rabia Mehmood ◽

...

Keyword(s):

Cell Death ◽

Lipid Droplets ◽

Lysosomal Enzymes ◽

Lipid Homeostasis ◽

Cellular Lipid ◽

Biological Functions ◽

Lysosomal Function ◽

Selective Autophagy ◽

Cellular Components ◽

Different Levels

Lipids are integral cellular components that act as substrates for energy provision, signaling molecules, and essential constituents of biological membranes along with a variety of other biological functions. Despite their significance, lipid accumulation may result in lipotoxicity, impair autophagy, and lysosomal function that may lead to certain diseases and metabolic syndromes like obesity and even cell death. Therefore, these lipids are continuously recycled and redistributed by the process of selective autophagy specifically termed as lipophagy. This selective form of autophagy employs lysosomes for the maintenance of cellular lipid homeostasis. In this review, we have reviewed the current literature about how lipid droplets (LDs) are recruited towards lysosomes, cross-talk between a variety of autophagy receptors present on LD surface and lysosomes, and lipid hydrolysis by lysosomal enzymes. In addition to it, we have tried to answer most of the possible questions related to lipophagy regulation at different levels. Moreover, in the last part of this review, we have discussed some of the pathological states due to the accumulation of these LDs and their possible treatments under the light of currently available findings.

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Cellular factors required for protection from hyperoxia toxicity in Saccharomyces cerevisiae

Biochemical Journal ◽

10.1042/bj20041914 ◽

2005 ◽

Vol 388 (1) ◽

pp. 93-101 ◽

Cited By ~ 54

Author(s):

Caryn E. OUTTEN ◽

Robert L. FALK ◽

Valeria C. CULOTTA

Keyword(s):

Prolonged Exposure ◽

Reduced Form ◽

Cellular Functions ◽

Mitochondrial Redox State ◽

Cellular Factors ◽

Two Factors ◽

Cytosolic Glutathione ◽

Yeast Deletion Library ◽

Mitochondrial Glutathione ◽

Cellular Components

Prolonged exposure to hyperoxia represents a serious danger to cells, yet little is known about the specific cellular factors that affect hyperoxia stress. By screening the yeast deletion library, we have identified genes that protect against high-O2 damage. Out of approx. 4800 mutants, 84 were identified as hyperoxia-sensitive, representing genes with diverse cellular functions, including transcription and translation, vacuole function, NADPH production, and superoxide detoxification. Superoxide plays a significant role, since the majority of hyperoxia-sensitive mutants displayed cross-sensitivity to superoxide-generating agents, and mutants with compromised SOD (superoxide dismutase) activity were particularly vulnerable to hyperoxia. By comparison, factors known to guard against H2O2 toxicity were poorly represented amongst hyperoxia-sensitive mutants. Although many cellular components are potential targets, our studies indicate that mitochondrial glutathione is particularly vulnerable to hyperoxia damage. During hyperoxia stress, mitochondrial glutathione is more susceptible to oxidation than cytosolic glutathione. Furthermore, two factors that help maintain mitochondrial GSH in the reduced form, namely the NADH kinase Pos5p and the mitochondrial glutathione reductase (Glr1p), are critical for hyperoxia resistance, whereas their cytosolic counterparts are not. Our findings are consistent with a model in which hyperoxia toxicity is manifested by superoxide-related damage and changes in the mitochondrial redox state.

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Intracellular Lipid Droplets: From Structure to Function

Lipid Insights ◽

10.1177/1178635317745518 ◽

2017 ◽

Vol 10 ◽

pp. 117863531774551 ◽

Cited By ~ 13

Author(s):

Stefano Vanni

Keyword(s):

Lipid Droplets ◽

Neutral Lipids ◽

Interfacial Structure ◽

Aqueous Environment ◽

Sterol Esters ◽

Computational Approaches ◽

Main Site ◽

Natural Surfactant ◽

Oil Emulsions ◽

Function Relationship

Lipid droplets (LDs) are unique intracellular organelles that are mainly constituted by neutral lipids (triglycerides, sterol esters). As such they serve as the main site of energy storage in the cell and they are akin to oil emulsions in water. To prevent the direct exposure of the hydrophobic neutral lipids to the aqueous environment of the cytosol, LDs are surrounded by a monolayer of phospholipids that thus behave as a natural surfactant. This interfacial structure is rather unique inside the cell, but a molecular understanding of how the LD structure modulates its functions is still lacking, mainly due to technical challenges in both experimental and computational approaches to investigate oil-in-water emulsions. Recently, we have investigated the structure of LDs using a combination of existing and newly developed computational approaches that are optimized to study oil-water interfaces.1 Our simulations provide a comprehensive molecular characterization of the unique surface properties of LDs, suggesting structure-function relationship in several LD-related metabolic processes.

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Palmitoylation of Apolipoprotein B Is Required for Proper Intracellular Sorting and Transport of Cholesteroyl Esters and Triglycerides

Molecular Biology of the Cell ◽

10.1091/mbc.11.2.721 ◽

2000 ◽

Vol 11 (2) ◽

pp. 721-734 ◽

Cited By ~ 28

Author(s):

Yang Zhao ◽

James B. McCabe ◽

Jean Vance ◽

Luc G. Berthiaume

Keyword(s):

Apolipoprotein B ◽

Neutral Lipids ◽

Low Density Lipoproteins ◽

Low Density ◽

Hydrophobic Core ◽

Wild Type ◽

Very Low Density Lipoproteins ◽

Structural Requirement ◽

Lipoprotein Particles ◽

Intracellular Sorting

Apolipoprotein B (apoB) is an essential component of chylomicrons, very low density lipoproteins, and low density lipoproteins. ApoB is a palmitoylated protein. To investigate the role of palmitoylation in lipoprotein function, a palmitoylation site was mapped to Cys-1085 and removed by mutagenesis. Secreted lipoprotein particles formed by nonpalmitoylated apoB were smaller and denser and failed to assemble a proper hydrophobic core. Indeed, the relative concentrations of nonpolar lipids were three to four times lower in lipoprotein particles containing mutant apoB compared with those containing wild-type apoB, whereas levels of polar lipids isolated from wild-type or mutant apoB lipoprotein particles appeared identical. Palmitoylation localized apoB to large vesicular structures corresponding to a subcompartment of the endoplasmic reticulum, where addition of neutral lipids was postulated to occur. In contrast, nonpalmitoylated apoB was concentrated in a dense perinuclear area corresponding to the Golgi compartment. The involvement of palmitoylation as a structural requirement for proper assembly of the hydrophobic core of the lipoprotein particle and its intracellular sorting represent novel roles for this posttranslational modification.

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Neutral Lipids Are Not a Source of Arachidonic Acid for Lipid Mediator Signaling in Human Foamy Monocytes

Cells ◽

10.3390/cells8080941 ◽

2019 ◽

Vol 8 (8) ◽

pp. 941 ◽

Cited By ~ 3

Author(s):

Carlos Guijas ◽

Miguel A. Bermúdez ◽

Clara Meana ◽

Alma M. Astudillo ◽

Laura Pereira ◽

...

Keyword(s):

Arachidonic Acid ◽

Neutral Lipids ◽

Lipid Mediator ◽

Secretory Product ◽

Cellular Functions ◽

Atherosclerotic Lesions ◽

Free Arachidonic Acid ◽

Phospholipid Pool ◽

Stimulation Of

Human monocytes exposed to free arachidonic acid (AA), a secretory product of endothelial cells, acquire a foamy phenotype which is due to the accumulation of cytoplasmic lipid droplets with high AA content. Recruitment of foamy monocytes to the inflamed endothelium contributes to the development of atherosclerotic lesions. In this work, we investigated the potential role of AA stored in the neutral lipids of foamy monocytes to be cleaved by lipases and contribute to lipid mediator signaling. To this end, we used mass spectrometry-based lipidomic approaches combined with strategies to generate monocytes with different concentrations of AA. Results from our experiments indicate that the phospholipid AA pool in monocytes is stable and does not change upon exposure of the cells to the external AA. On the contrary, the AA pool in triacylglycerol is expandable and can accommodate relatively large amounts of fatty acid. Stimulation of the cells with opsonized zymosan results in the expected decreases of cellular AA. Under all conditions examined, all of the AA decreases observed in stimulated cells were accounted for by decreases in the phospholipid pool; we failed to detect any contribution of the triacylglycerol pool to the response. Experiments utilizing selective inhibitors of phospholipid or triacylglyerol hydrolysis confirmed that the phospholipid pool is the sole contributor of the AA liberated by stimulated cells. Thus, the AA in the triacylglycerol is not a source of free AA for the lipid mediator signaling during stimulation of human foamy monocytes and may be used for other cellular functions.

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In Close Proximity: The Lipid Droplet Proteome and Crosstalk With the Endoplasmic Reticulum

Contact ◽

10.1177/2515256418768996 ◽

2018 ◽

Vol 1 ◽

pp. 251525641876899 ◽

Cited By ~ 5

Author(s):

Kirill Bersuker ◽

James A. Olzmann

Keyword(s):

Mass Spectrometry ◽

Endoplasmic Reticulum ◽

Lipid Droplets ◽

Neutral Lipids ◽

26S Proteasome ◽

High Confidence ◽

Close Proximity ◽

Bona Fide ◽

A Site ◽

High Degree

Lipid droplets (LDs) are conserved, endoplasmic reticulum (ER)-derived organelles that act as a dynamic cellular repository for neutral lipids. Numerous studies have examined the composition of LD proteomes by using mass spectrometry to identify proteins present in biochemically isolated buoyant fractions that are enriched in LDs. Although many bona fide LD proteins were identified, high levels of non-LD proteins that contaminate buoyant fractions complicate the detection of true LD proteins. To overcome this problem, we recently developed a proximity-labeling proteomic method to define high-confidence LD proteomes. Moreover, employing this approach, we discovered that ER-associated degradation impacts the composition of LD proteomes by targeting select LD proteins for clearance by the 26S proteasome as they transit between the ER and LDs. These findings implicate the ER as a site of LD protein degradation and underscore the high degree of crosstalk between ER and LDs.

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