Novel contact sites between lipid droplets, early endosomes, and the endoplasmic reticulum

Lipid droplets (LDs) are storage organelles consisting of a neutral lipid core surrounded by a phospholipid monolayer and a set of LD-specific proteins. Most LD components are synthesized in the endoplasmic reticulum (ER), an organelle that is often physically connected with LDs. How LD identity is established while maintaining biochemical and physical connections with the ER is not known. Here, we show that the yeast seipin Fld1, in complex with the ER membrane protein Ldb16, prevents equilibration of ER and LD surface components by stabilizing the contact sites between the two organelles. In the absence of the Fld1/Ldb16 complex, assembly of LDs results in phospholipid packing defects leading to aberrant distribution of lipid-binding proteins and abnormal LDs. We propose that the Fld1/Ldb16 complex facilitates the establishment of LD identity by acting as a diffusion barrier at the ER–LD contact sites.

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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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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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Endoplasmic Reticulum-Vacuole Contact Sites “Bloom” With Stress-Induced Lipid Droplets

Contact ◽

10.1177/2515256418756112 ◽

2018 ◽

Vol 1 ◽

pp. 251525641875611 ◽

Cited By ~ 2

Author(s):

W. Mike Henne ◽

Hanaa Hariri

Keyword(s):

Endoplasmic Reticulum ◽

Recent Work ◽

Nutrient Availability ◽

Lipid Droplets ◽

Nutrient Sources ◽

Contact Sites ◽

Long Term Storage ◽

Specific Proteins ◽

Term Storage

Lipid droplets (LDs) serve as specialized cytoplasmic organelles that harbor energy-rich lipids for long-term storage and may be mobilized as nutrient sources during extended starvation. How cells coordinate LD biogenesis and utilization in response to fluctuations in nutrient availability remains poorly understood. Here, we discuss our recent work revealing how yeast spatially organize LD budding at organelle contacts formed between the endoplasmic reticulum and yeast vacuole/lysosome (sites known as nucleus-vacuole junctions [NVJs]). During times of imminent nutrient exhaustion, we observe blooms of stress-induced LDs surrounding the NVJ and find that this LD clustering is regulated by NVJ-resident protein Mdm1. We also discuss several emerging studies revealing specific proteins that demarcate a subpopulation of NVJ-associated LDs. Collectively, these studies reveal a previously unappreciated role for the spatial compartmentalization of LDs at organelle contacts and highlight an important role for interorganellar cross talk in LD dynamics under times of nutritional stress.

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PxdA interacts with the DipA phosphatase to regulate endosomal hitchhiking of peroxisomes

Molecular Biology of the Cell ◽

10.1091/mbc.e20-08-0559 ◽

2021 ◽

pp. mbc.E20-08-0559

Author(s):

John Salogiannis ◽

Jenna R. Christensen ◽

Livia D. Songster ◽

Adriana Aguilar-Maldonado ◽

Nandini Shukla ◽

...

Keyword(s):

Endoplasmic Reticulum ◽

Lipid Droplets ◽

Molecular Mechanisms ◽

Ustilago Maydis ◽

Adaptor Proteins ◽

Early Endosome ◽

Alternative Mode ◽

Ribonucleoprotein Complexes ◽

Early Endosomes ◽

Mode Of Transport

In canonical microtubule-based transport, adaptor proteins link cargos to dynein and kinesin motors. Recently, an alternative mode of transport known as ‘hitchhiking’ was discovered, where cargos achieve motility by hitching a ride on already-motile cargos, rather than attaching to a motor protein. Hitchhiking has been best-studied in two filamentous fungi, Aspergillus nidulans and Ustilago maydis. In U. maydis, ribonucleoprotein complexes, peroxisomes, lipid droplets, and endoplasmic reticulum hitchhike on early endosomes. In A. nidulans, peroxisomes hitchhike using a putative molecular linker, PxdA, which associates with early endosomes. However, whether other organelles use PxdA to hitchhike on early endosomes is unclear, as are the molecular mechanisms that regulate hitchhiking. Here we find that the proper distribution of lipid droplets, mitochondria and pre-autophagosomes do not require PxdA, suggesting that PxdA is a peroxisome-specific molecular linker. We identify two new pxdA alleles, including a point mutation (R2044P) that disrupts PxdA's ability to associate with early endosomes and reduces peroxisome movement. We also identify a novel regulator of peroxisome hitchhiking, the phosphatase DipA. DipA co-localizes with early endosomes and its early endosome-association relies on PxdA. Together, our data suggest that PxdA and the DipA phosphatase are specific regulators of peroxisome hitchhiking on early endosomes. [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text]

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Human VPS13A is associated with multiple organelles and influences mitochondrial morphology and lipid droplet motility

eLife ◽

10.7554/elife.43561 ◽

2019 ◽

Vol 8 ◽

Cited By ~ 29

Author(s):

Wondwossen M Yeshaw ◽

Marianne van der Zwaag ◽

Francesco Pinto ◽

Liza L Lahaye ◽

Anita IE Faber ◽

...

Keyword(s):

Endoplasmic Reticulum ◽

Lipid Droplet ◽

Mammalian Cells ◽

Lipid Droplets ◽

Neurodegenerative Disorder ◽

Close Contact ◽

Mitochondrial Morphology ◽

Published Data ◽

Membrane Contact Sites ◽

Contact Sites

The VPS13A gene is associated with the neurodegenerative disorder Chorea Acanthocytosis. It is unknown what the consequences are of impaired function of VPS13A at the subcellular level. We demonstrate that VPS13A is a peripheral membrane protein, associated with mitochondria, the endoplasmic reticulum and lipid droplets. VPS13A is localized at sites where the endoplasmic reticulum and mitochondria are in close contact. VPS13A interacts with the ER residing protein VAP-A via its FFAT domain. Interaction with mitochondria is mediated via its C-terminal domain. In VPS13A-depleted cells, ER-mitochondria contact sites are decreased, mitochondria are fragmented and mitophagy is decreased. VPS13A also localizes to lipid droplets and affects lipid droplet motility. In VPS13A-depleted mammalian cells lipid droplet numbers are increased. Our data, together with recently published data from others, indicate that VPS13A is required for establishing membrane contact sites between various organelles to enable lipid transfer required for mitochondria and lipid droplet related processes.

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Seipin and Nem1 establish discrete ER subdomains to initiate yeast lipid droplet biogenesis

The Journal of Cell Biology ◽

10.1083/jcb.201910177 ◽

2020 ◽

Vol 219 (7) ◽

Cited By ~ 7

Author(s):

Vineet Choudhary ◽

Ola El Atab ◽

Giulia Mizzon ◽

William A. Prinz ◽

Roger Schneiter

Keyword(s):

Endoplasmic Reticulum ◽

Lipid Droplet ◽

Lipid Droplets ◽

Fat Storage ◽

Contact Sites ◽

Yeast Lipid

Lipid droplets (LDs) are fat storage organelles that originate from the endoplasmic reticulum (ER). Relatively little is known about how sites of LD formation are selected and which proteins/lipids are necessary for the process. Here, we show that LDs induced by the yeast triacylglycerol (TAG)-synthases Lro1 and Dga1 are formed at discrete ER subdomains defined by seipin (Fld1), and a regulator of diacylglycerol (DAG) production, Nem1. Fld1 and Nem1 colocalize to ER–LD contact sites. We find that Fld1 and Nem1 localize to ER subdomains independently of each other and of LDs, but both are required for the subdomains to recruit the TAG-synthases and additional LD biogenesis factors: Yft2, Pex30, Pet10, and Erg6. These subdomains become enriched in DAG. We conclude that Fld1 and Nem1 are both necessary to recruit proteins to ER subdomains where LD biogenesis occurs.

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ORP5 localizes to ER–lipid droplet contacts and regulates the level of PI(4)P on lipid droplets

The Journal of Cell Biology ◽

10.1083/jcb.201905162 ◽

2019 ◽

Vol 219 (1) ◽

Cited By ~ 17

Author(s):

Ximing Du ◽

Linkang Zhou ◽

Yvette Celine Aw ◽

Hoi Yin Mak ◽

Yanqing Xu ◽

...

Keyword(s):

Endoplasmic Reticulum ◽

Membrane Protein ◽

Lipid Droplets ◽

Lipid Synthesis ◽

Integral Membrane Protein ◽

Normal Cells ◽

Bilayer Membranes ◽

Contact Sites ◽

Evolutionarily Conserved ◽

Phosphatidylinositol 4

Lipid droplets (LDs) are evolutionarily conserved organelles that play important roles in cellular metabolism. Each LD is enclosed by a monolayer of phospholipids, distinct from bilayer membranes. During LD biogenesis and growth, this monolayer of lipids expands by acquiring phospholipids from the endoplasmic reticulum (ER) through nonvesicular mechanisms. Here, in a mini-screen, we find that ORP5, an integral membrane protein of the ER, can localize to ER–LD contact sites upon oleate loading. ORP5 interacts with LDs through its ligand-binding domain, and ORP5 deficiency enhances neutral lipid synthesis and increases the size of LDs. Importantly, there is significantly more phosphatidylinositol-4-phosphate (PI(4)P) and less phosphatidylserine (PS) on LDs in ORP5-deficient cells than in normal cells. The increased presence of PI(4)P on LDs in ORP5-deficient cells requires phosphatidylinositol 4-kinase 2-α. Our results thus demonstrate the existence of PI(4)P on LDs and suggest that LD-associated PI(4)P may be primarily used by ORP5 to deliver PS to LDs.

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Getting a handle on lipid droplets: Insights into ER–lipid droplet tethering

The Journal of Cell Biology ◽

10.1083/jcb.201902160 ◽

2019 ◽

Vol 218 (4) ◽

pp. 1089-1091 ◽

Cited By ~ 6

Author(s):

Truc B. Nguyen ◽

James A. Olzmann

Keyword(s):

Endoplasmic Reticulum ◽

Lipid Metabolism ◽

Lipid Droplet ◽

Lipid Droplets ◽

Human Cells ◽

Membrane Contact Sites ◽

Contact Sites ◽

Cell Biol ◽

Membrane Contact

Lipid droplets (LDs) are hubs for lipid metabolism that form membrane contact sites with multiple organelles. In this issue, Hariri et al. (2019. J. Cell Biol. https://doi.org/10.1083/jcb.201808119) reveal the functions of Mdm1-mediated endoplasmic reticulum (ER)–LD tethering in yeast and Datta et al. (2019. J. Cell Biol. https://doi.org/10.1083/jcb.201808133) identify a role for the Mdm1 orthologue, Snx14, as an ER–LD tether that regulates lipid metabolism in human cells.

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