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

Cells store energy in the form of neutral lipids (NLs) packaged into micrometer-sized organelles named lipid droplets (LDs). These structures emerge from the endoplasmic reticulum (ER) at sites marked by the protein seipin, but the mechanisms regulating their biogenesis remain poorly understood. Using a combination of molecular simulations, yeast genetics, and fluorescence microscopy, we show that interactions between lipids’ acyl-chains modulate the propensity of NLs to be stored in LDs, in turn preventing or promoting their accumulation in the ER membrane. Our data suggest that diacylglycerol, which is enriched at sites of LD formation, promotes the packaging of NLs into LDs, together with ER-abundant lipids, such as phosphatidylethanolamine. On the opposite end, short and saturated acyl-chains antagonize fat storage in LDs and promote accumulation of NLs in the ER. Our results provide a new conceptual understanding of LD biogenesis in the context of ER homeostasis and function.

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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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Mdm1 maintains endoplasmic reticulum homeostasis by spatially regulating lipid droplet biogenesis

The Journal of Cell Biology ◽

10.1083/jcb.201808119 ◽

2019 ◽

Vol 218 (4) ◽

pp. 1319-1334 ◽

Cited By ~ 38

Author(s):

Hanaa Hariri ◽

Natalie Speer ◽

Jade Bowerman ◽

Sean Rogers ◽

Gang Fu ◽

...

Keyword(s):

Fatty Acids ◽

Endoplasmic Reticulum ◽

Lipid Droplet ◽

Lipid Droplets ◽

Fatty Acyl ◽

Nutrient Depletion ◽

Coenzyme A Ligase ◽

Response To Stress ◽

Acyl Coenzyme A ◽

Er Homeostasis

Lipid droplets (LDs) serve as cytoplasmic reservoirs for energy-rich fatty acids (FAs) stored in the form of triacylglycerides (TAGs). During nutrient stress, yeast LDs cluster adjacent to the vacuole/lysosome, but how this LD accumulation is coordinated remains poorly understood. The ER protein Mdm1 is a molecular tether that plays a role in clustering LDs during nutrient depletion, but its mechanism of function remains unknown. Here, we show that Mdm1 associates with LDs through its hydrophobic N-terminal region, which is sufficient to demarcate sites for LD budding. Mdm1 binds FAs via its Phox-associated domain and coenriches with fatty acyl–coenzyme A ligase Faa1 at LD bud sites. Consistent with this, loss of MDM1 perturbs free FA activation and Dga1-dependent synthesis of TAGs, elevating the cellular FA level, which perturbs ER morphology and sensitizes yeast to FA-induced lipotoxicity. We propose that Mdm1 coordinates FA activation adjacent to the vacuole to promote LD production in response to stress, thus maintaining ER homeostasis.

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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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The yeast lipin orthologue Pah1p is important for biogenesis of lipid droplets

The Journal of Cell Biology ◽

10.1083/jcb.201010111 ◽

2011 ◽

Vol 192 (6) ◽

pp. 1043-1055 ◽

Cited By ~ 167

Author(s):

Oludotun Adeyo ◽

Patrick J. Horn ◽

SungKyung Lee ◽

Derk D. Binns ◽

Anita Chandrahas ◽

...

Keyword(s):

Endoplasmic Reticulum ◽

Neutral Lipid ◽

Strong Evidence ◽

Lipid Droplets ◽

Neutral Lipids ◽

Glucose Medium ◽

Wild Type ◽

Lipid Levels ◽

Total Neutral Lipid ◽

A Site

Lipins are phosphatidate phosphatases that generate diacylglycerol (DAG). In this study, we report that yeast lipin, Pah1p, controls the formation of cytosolic lipid droplets. Disruption of PAH1 resulted in a 63% decrease in droplet number, although total neutral lipid levels did not change. This was accompanied by an accumulation of neutral lipids in the endoplasmic reticulum (ER). The droplet biogenesis defect was not a result of alterations in neutral lipid ratios. No droplets were visible in the absence of both PAH1 and steryl acyltransferases when grown in glucose medium, even though the strain produces as much triacylglycerol as wild type. The requirement of PAH1 for normal droplet formation can be bypassed by a knockout of DGK1. Nem1p, the activator of Pah1p, localizes to a single punctum per cell on the ER that is usually next to a droplet, suggesting that it is a site of droplet assembly. Overall, this study provides strong evidence that DAG generated by Pah1p is important for droplet biogenesis.

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A Unique Junctional Interface at Contact Sites Between the Endoplasmic Reticulum and Lipid Droplets

Frontiers in Cell and Developmental Biology ◽

10.3389/fcell.2021.650186 ◽

2021 ◽

Vol 9 ◽

Author(s):

Vineet Choudhary ◽

Roger Schneiter

Keyword(s):

Endoplasmic Reticulum ◽

Lipid Droplets ◽

Neutral Lipids ◽

Close Contact ◽

Lipid Storage ◽

Metabolic Energy ◽

Lipid Monolayer ◽

Storage Diseases ◽

Contact Sites ◽

Ordered Assembly

Lipid droplets (LDs) constitute compartments dedicated to the storage of metabolic energy in the form of neutral lipids. LDs originate from the endoplasmic reticulum (ER) with which they maintain close contact throughout their life cycle. These ER–LD junctions facilitate the exchange of both proteins and lipids between these two compartments. In recent years, proteins that are important for the proper formation of LDs and localize to ER–LD junctions have been identified. This junction is unique as it is generally believed to invoke a transition from the ER bilayer membrane to a lipid monolayer that delineates LDs. Proper formation of this junction requires the ordered assembly of proteins and lipids at specialized ER subdomains. Without such a well-ordered assembly of LD biogenesis factors, neutral lipids are synthesized throughout the ER membrane, resulting in the formation of aberrant LDs. Such ectopically formed LDs impact ER and lipid homeostasis, resulting in different types of lipid storage diseases. In response to starvation, the ER–LD junction recruits factors that tether the vacuole to these junctions to facilitate LD degradation. In addition, LDs maintain close contacts with peroxisomes and mitochondria for metabolic channeling of the released fatty acids toward beta-oxidation. In this review, we discuss the function of different components that ensure proper functioning of LD contact sites, their role in lipogenesis and lipolysis, and their relation to lipid storage diseases.

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The Impact of Endoplasmic Reticulum-Associated Protein Modifications, Folding and Degradation on Lung Structure and Function

Frontiers in Physiology ◽

10.3389/fphys.2021.665622 ◽

2021 ◽

Vol 12 ◽

Author(s):

Emily M. Nakada ◽

Rui Sun ◽

Utako Fujii ◽

James G. Martin

Keyword(s):

Endoplasmic Reticulum ◽

Er Stress ◽

Unfolded Protein ◽

Lung Structure ◽

Selective Translation ◽

The Unfolded Protein Response ◽

And Function ◽

Protein Response ◽

Er Homeostasis ◽

The Impact

The accumulation of unfolded/misfolded proteins in the endoplasmic reticulum (ER) causes ER stress and induces the unfolded protein response (UPR) and other mechanisms to restore ER homeostasis, including translational shutdown, increased targeting of mRNAs for degradation by the IRE1-dependent decay pathway, selective translation of proteins that contribute to the protein folding capacity of the ER, and activation of the ER-associated degradation machinery. When ER stress is excessive or prolonged and these mechanisms fail to restore proteostasis, the UPR triggers the cell to undergo apoptosis. This review also examines the overlooked role of post-translational modifications and their roles in protein processing and effects on ER stress and the UPR. Finally, these effects are examined in the context of lung structure, function, and disease.

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Multiple C2 domain-containing transmembrane proteins promote lipid droplet biogenesis and growth at specialized ER subdomains

Molecular Biology of the Cell ◽

10.1091/mbc.e20-09-0590 ◽

2021 ◽

pp. mbc.E20-09-0590

Author(s):

Amit S. Joshi ◽

Joey V. Ragusa ◽

William A. Prinz ◽

Sarah Cohen

Keyword(s):

Endoplasmic Reticulum ◽

Lipid Droplets ◽

Transmembrane Domain ◽

Neutral Lipids ◽

Transmembrane Proteins ◽

C2 Domain ◽

General Role ◽

C2 Domains ◽

Contact Sites ◽

Er Membrane

Lipid droplets (LDs) are neutral lipid-containing organelles enclosed in a single monolayer of phospholipids. LD formation begins with the accumulation of neutral lipids within the bilayer of the endoplasmic reticulum (ER) membrane. It is not known how the sites of formation of nascent LDs in the ER membrane are determined. Here we show that multiple C2 domain-containing transmembrane proteins, MCTP1 and MCTP2, are at sites of LD formation in specialized ER subdomains. We show that the transmembrane domain (TMD) of these proteins is similar to a reticulon homology domain. Like reticulons, these proteins tubulate the ER membrane and favor highly curved regions of the ER. Our data indicate that the MCTP TMDs promote LD biogenesis, increasing LD number. MCTPs co-localize with seipin, a protein involved in LD biogenesis, but form more stable microdomains in the ER. The MCTP C2 domains bind charged lipids and regulate LD size, likely by mediating ER-LD contact sites. Together, our data indicate that MCTPs form microdomains within ER tubules that regulate LD biogenesis, size, and ER-LD contacts. Interestingly, MCTP punctae colocalized with other organelles as well, suggesting that these proteins may play a more general role in linking tubular ER to organelle contact sites. [Media: see text] [Media: see text]

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Intracellular communication between lipid droplets and peroxisomes: the Janus face of PEX19

Biological Chemistry ◽

10.1515/hsz-2018-0125 ◽

2018 ◽

Vol 399 (7) ◽

pp. 741-749 ◽

Cited By ~ 5

Author(s):

Bianca Schrul ◽

Wolfgang Schliebs

Keyword(s):

Metabolic Pathways ◽

Lipid Droplets ◽

Environmental Changes ◽

Neutral Lipids ◽

Nutrient Supply ◽

Metabolic Energy ◽

Central Question ◽

And Function ◽

Metabolic Output

Abstract In order to adapt to environmental changes, such as nutrient availability, cells have to orchestrate multiple metabolic pathways, which are catalyzed in distinct specialized organelles. Lipid droplets (LDs) and peroxisomes are both endoplasmic reticulum (ER)-derived organelles that fulfill complementary functions in lipid metabolism: Upon nutrient supply, LDs store metabolic energy in the form of neutral lipids and, when energy is needed, supply fatty acids for oxidation in peroxisomes and mitochondria. How these organelles communicate with each other for a concerted metabolic output remains a central question. Here, we summarize recent insights into the biogenesis and function of LDs and peroxisomes with emphasis on the role of PEX19 in these processes.

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Motility Plays an Important Role in the Lifetime of Mammalian Lipid Droplets

International Journal of Molecular Sciences ◽

10.3390/ijms22083802 ◽

2021 ◽

Vol 22 (8) ◽

pp. 3802

Author(s):

Yi Jin ◽

Zhuqing Ren ◽

Yanjie Tan ◽

Pengxiang Zhao ◽

Jian Wu

Keyword(s):

Signal Transduction ◽

Endoplasmic Reticulum ◽

Lipid Droplet ◽

Molecular Basis ◽

Lipid Droplets ◽

Neutral Lipids ◽

Future Research ◽

Biological Processes ◽

Cellular Processes ◽

Resistance To Stress

The lipid droplet is a kind of organelle that stores neutral lipids in cells. Recent studies have found that in addition to energy storage, lipid droplets also play an important role in biological processes such as resistance to stress, immunity, cell proliferation, apoptosis, and signal transduction. Lipid droplets are formed at the endoplasmic reticulum, and mature lipid droplets participate in various cellular processes. Lipid droplets are decomposed by lipase and lysosomes. In the life of a lipid droplet, the most important thing is to interact with other organelles, including the endoplasmic reticulum, mitochondria, peroxisomes, and autophagic lysosomes. The interaction between lipid droplets and other organelles requires them to be close to each other, which inevitably involves the motility of lipid droplets. In fact, through many microscopic observation techniques, researchers have discovered that lipid droplets are highly dynamic organelles that move quickly. This paper reviews the process of lipid droplet motility, focusing on explaining the molecular basis of lipid droplet motility, the factors that regulate lipid droplet motility, and the influence of motility on the formation and decomposition of lipid droplets. In addition, this paper also proposes several unresolved problems for lipid droplet motility. Finally, this paper makes predictions about the future research of lipid droplet motility.

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Seipin accumulates and traps diacylglycerols and triglycerides in its ring-like structure

10.1101/2020.10.27.357079 ◽

2020 ◽

Author(s):

Valeria Zoni ◽

Wataru Shinoda ◽

Stefano Vanni

Keyword(s):

Molecular Dynamics ◽

Endoplasmic Reticulum ◽

Molecular Dynamics Simulations ◽

Lipid Droplets ◽

Neutral Lipids ◽

Health Concern ◽

Intracellular Organelles ◽

Dynamics Simulations ◽

Precise Role

AbstractLipid droplets (LD) are intracellular organelles responsible for lipid storage, and they emerge from the endoplasmic reticulum (ER) upon the accumulation of neutral lipids, mostly triglycerides (TG), between the two leaflets of the ER membrane. LD biogenesis takes place at ER sites that are marked by the protein seipin, which subsequently recruits additional proteins to catalyse LD formation. Deletion of seipin, however, does not abolish LD biogenesis, and its precise role in controlling LD assembly remains unclear. Here we use molecular dynamics simulations to investigate the molecular mechanism through which seipin promotes LD formation. We find that seipin clusters TG molecules inside its unconventional ring-like oligomeric structure, and that both its luminal and transmembrane regions contribute to this process. Diacylglycerol, the precursor of TG, also clusters inside the seipin oligomer, in turn promoting TG accumulation. Our results suggest that seipin remodels the membrane of specific ER sites to prime them for LD biogenesis.Significance statementMetabolic disorders related to aberrant fat accumulation, including lipodystrophy and obesity, are a particularly serious health concern. In cells, fat accumulates in intracellular organelles, named lipid droplets (LDs). LDs form in the endoplasmic reticulum, where triglycerides, the most abundant form of fat, is produced. The Bernardinelli-Seip congenital lipodystrophy type 2 protein, seipin, has been identified as a key regulator of LD formation, but its mechanism of action remains debated and its molecular details mostly obscure. Here, we use molecular dynamics simulations to investigate the mechanism of seipin. We find that seipin can cluster and trap both triglycerides and its precursor, diacylglycerol. Our results suggest that seipin organizes the lipid composition of specific ER sites to prime them for LD biogenesis.

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