Lipid droplet

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Lipid droplets, also referred to as lipid bodies, oil bodies or adiposomes, [1] are lipid-rich cellular organelles that regulate the storage and hydrolysis of neutral lipids and are found largely in the adipose tissue. [2] They also serve as a reservoir for cholesterol and acyl-glycerols for membrane formation and maintenance. Lipid droplets are found in all eukaryotic organisms and store a large portion of lipids in mammalian adipocytes. Initially, these lipid droplets were considered to merely serve as fat depots, but since the discovery in the 1990s of proteins in the lipid droplet coat that regulate lipid droplet dynamics and lipid metabolism, lipid droplets are seen as highly dynamic organelles that play a very important role in the regulation of intracellular lipid storage and lipid metabolism. The role of lipid droplets outside of lipid and cholesterol storage has recently begun to be elucidated and includes a close association to inflammatory responses through the synthesis and metabolism of eicosanoids and to metabolic disorders such as obesity, cancer, [3] [4] and atherosclerosis. [5] In non-adipocytes, lipid droplets are known to play a role in protection from lipotoxicity by storage of fatty acids in the form of neutral triacylglycerol, which consists of three fatty acids bound to glycerol. Alternatively, fatty acids can be converted to lipid intermediates like diacylglycerol (DAG), ceramides and fatty acyl-CoAs. These lipid intermediates can impair insulin signaling, which is referred to as lipid-induced insulin resistance and lipotoxicity. [6] Lipid droplets also serve as platforms for protein binding and degradation. Finally, lipid droplets are known to be exploited by pathogens such as the hepatitis C virus, the dengue virus and Chlamydia trachomatis among others. [7] [8]

Contents

Cells need to adjust the size and structure of their organelles to keep up with growth and changing environmental conditions. To do this, they either make new phospholipids—the main components of organelle membranes—or modify their fatty acid (FA) content. Fatty acids are also used to produce triacylglycerols (TGs), which store energy in structures called lipid droplets. [9] The synthesis of triacylglycerols (TG) can be occurred through two different enzyme pathways. Diacylglycerol Acyltransferases (DGATs) like Dga1 are enzymes found in most eukaryotes. They add an acyl group from a fatty acid that has been activated with coenzyme A (FA-CoA) to diacylglycerol (DG), forming TG. Phospholipid-Diacylglycerol Acyltransferases (PDATs) are enzymes primarily found in fungi, microalgae, and plants. PDATs like Lro1 in yeast transfer a fatty acid directly from a phospholipid to DG to form TG. Moreover, Lro1 couple TG synthesis with the deacylation of membrane phospholipids (PL), resulting in the formation of TG and lysophospholipids (LPL). [10]

When nutrients become available, the yeast cells enter the exponential growth phase (EXP) to grow quickly. It has been shown that during the EXP phase, Lro1-GFP is localized in the endoplasmic reticulum (ER) to synthesize triacylglycerols (TG), which are essential for phospholipid synthesis. [9] However, when nutrients become scarce, the cells enter the post-diauxic shift (PDS) phase and Lro1-GFP no longer is in the ER. Instead, it moves to a specific area of the nuclear envelope. This relocation suggests a shift in Lro1's role, possibly in response to the stress of nutrient depletion. Furthermore, this movement is influenced by signals from the cell cycle and nutrient availability, and it stops when the nucleus grows larger. [9]

Two approaches were used to investigate whether Lro1 can access the inner nuclear membrane (INM). [9] In yeast, the ability of integral membrane proteins to move from the endoplasmic reticulum (ER) to the inner nuclear membrane (INM) is restricted by the size of their cytosolic domains. Proteins with cytosolic domains larger than 90 kDa cannot pass through the nuclear pore complex into the INM. It has been shown that when the Lro1's N-terminal domain was enlarged with one, two, or three copies of the maltose-binding protein (MBP), its ability to target the nucleolus was significantly reduced. This suggests that Lro1 normally resides at the INM, but when its N-domain becomes too large, it can no longer pass through the nuclear pore complex to reach the INM and these larger proteins are likely being degraded. [9]

The anchor-away technique was used as the second approach. The researchers fused the INM protein Heh1 with FK506 binding protein (FKBP12) to serve as an anchor at the INM and interact with other proteins. Lro1 was fused to GFP for visualization and the FKBP12-rapamycin-binding (FRB) domain. This fusion enables Lro1 to interact with Heh1 at the INM in the presence of rapamycin. FRB-GFP construct contains GFP fused to the FRB domain but without Lro1, to show the specific effects of Lro1's presence and used as a control. [9] Upon the addition of rapamycin, FRB-GFP quickly (within 30 minutes) changed from a diffuse distribution to a ring-like pattern, indicating that it had been recruited to the INM by Heh1-FKBP12. This ring-like localization confirmed that the INM anchor (Heh1) is accessible to FRB-GFP. In the strain expressing FRB-Lro1-GFP, rapamycin treatment caused a loss of Lro1's cortical ER localization and its accumulation at a perinuclear ring, which is characteristic of INM proteins. This suggests that Lro1, via its N-domain, can indeed associate with the INM. In contrast, when Lro1's N-terminal domain was enlarged by adding 3xMBP, the fusion protein retained its localization at the cortical ER even after rapamycin treatment. [9]

The nuclear membrane near the nucleolus tends to expand when there's excess phospholipid synthesis. It has been investigated that whether the protein Lro1 is catalytically active to produce triacylglycerol (TG) in this specific membrane area. [9] To test Lro1's activity, the researchers expressed it in a yeast strain that couldn't produce any neutral lipids on its own. They did this by deleting four key enzymes involved in lipid production: the DG acyltransferases (LRO1 and DGA1) and the steryl acyltransferases (ARE1 and ARE2). This mutant strain, called "4D," lacks neutral lipids and lipid droplets (LDs), making it ideal for studying Lro1's function. [9] The mutant "4D" yeast cells cannot survive under nutrient-poor conditions because they cannot make triacylglycerol (TG) or lipid droplets (LDs), which are essential for survival during this phase. When Lro1 is reintroduced as the only enzyme capable of producing TG, it rescues the cells, allowing them to survive better during the stationary phase by forming lipid droplets. [9] Moreover, Lro1 with a mutation in the conserved lipase motif cannot perform its catalytic function, meaning it cannot produce TG. As a result, these cells also fail to survive in PDS, similar to the cells without any functional Lro1, demonstrating that the catalytic activity of Lro1 is crucial for survival and LD formation. [9]

These findings [9] suggest that Lro1's activity in the nucleus creates a local site for TG synthesis, which helps reshape the nuclear membrane as needed. [9]

Structure

Lipid droplets are composed of a neutral lipid core consisting mainly of triacylglycerols (TAGs) and cholesteryl esters surrounded by a phospholipid monolayer. [2] The surface of lipid droplets is decorated by a number of proteins which are involved in the regulation of lipid metabolism. [2] The first and best-characterized family of lipid droplet coat proteins is the perilipin protein family, consisting of five proteins. These include perilipin 1 (PLIN1), perilipin 2 (PLIN2/ ADRP), [11] perilipin 3 (PLIN3/ TIP47), perilipin 4 (PLIN4/ S3-12) and perilipin 5 (PLIN5/ OXPAT/ LSDP5/ MLDP). [12] [13] [14] Proteomics studies have elucidated the association of many other families of proteins to the lipid surface including proteins involved in membrane trafficking, vesicle docking, endocytosis and exocytosis. [15] Analysis of the lipid composition of lipid droplets has revealed the presence of a diverse set of phospholipid species; [16] phosphatidylcholine and phosphatidylethanolamine are the most abundant, followed by phosphatidylinositol.

Lipid droplets vary greatly in size, ranging from 20 to 40 nm to 100 um. [17] In adipocytes, lipid bodies tend to be larger and they may compose the majority of the cell, while in other cells they may only be induced under certain conditions and are considerably smaller in size.

Lro1 is a type II integral membrane protein. The N-terminal domain facing the cytoplasm or nucleoplasm and containing a short basic region (RKRR). The larger luminal domain contains the catalytic PDAT domain, which is located within the lumen of the endoplasmic reticulum (ER). The N-terminal domain of Lro1 along with the transmembrane segment showed intranuclear localization with clear enrichment at the nucleolus. [9] When the K/R residues in the N-terminal domain are mutated to alanines, the nucleolar enrichment is partially compromised but not completely lost, indicating that other regions of Lro1 are also contributing to its proper localization. Furthermore, the N-terminal domain of Lro1 was replaced with 4 IgG binding domains of Protein A (4xIgGb). This replacement leads to a loss of localization at both the nucleolus and ER during the PDS phase. [9]

Formation

Lipid droplets bud off the membrane of the endoplasmic reticulum. Initially, a lens is formed by accumulation of TAGs between the two layers of its phospholipid membrane. Nascent lipid droplets may grow by diffusion of fatty acids, endocytosis of sterols, or fusion of smaller lipid droplets through the aid of SNARE proteins. [17] The budding of lipid droplets is promoted by an asymmetric accumulation of phospholipids that decrease surface tension towards the cytosol. [18] Lipid droplets have also been observed to be created by the fission of existing lipid droplets, though this is thought to be less common than de novo formation. [19]

Lipid droplets visualized with label-free live-cell imaging Lipid droplets.gif
Lipid droplets visualized with label-free live-cell imaging

The formation of lipid droplets from the endoplasmic reticulum begins with the synthesis of the neutral lipids to be transported. The manufacture of TAGs from diacylglycerol (by the addition of a fatty acyl chain) is catalyzed by the DGAT proteins, though the extent to which these and other proteins are required depends on cell type. [20] Neither DGAT1 nor DGAT2 is singularly essential for TAG synthesis or droplet formation, though mammalian cells lacking both cannot form lipid droplets and have severely stunted TAG synthesis. DGAT1, which seems to prefer exogenous fatty acid substrates, is not essential for life; DGAT2, which seems to prefer endogenously synthesized fatty acids, is. [19]

In non-adipocytes, lipid storage, lipid droplet synthesis and lipid droplet growth can be induced by various stimuli including growth factors, long-chain unsaturated fatty acids (including oleic acid and arachidonic acid), oxidative stress and inflammatory stimuli such bacterial lipopolysaccharides, various microbial pathogens, platelet-activating factor, eicosanoids, and cytokines. [21]

An example is the endocannabinoids that are unsaturated fatty acid derivatives, which mainly are considered to be synthesised “on demand” from phospholipid precursors residing in the cell membrane, but may also be synthesised and stored in intracellular lipid droplets and released from those stores under appropriate conditions. [22]

It is possible to observe the formation of lipid droplets, live and label-free, using label-free live-cell imaging.

See also

Related Research Articles

<span class="mw-page-title-main">Lipid</span> Substance of biological origin that is soluble in nonpolar solvents

Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins, monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing energy, signaling, and acting as structural components of cell membranes. Lipids have applications in the cosmetic and food industries, and in nanotechnology.

<span class="mw-page-title-main">Lipolysis</span> Metabolism involving breakdown of lipids

Lipolysis is the metabolic pathway through which lipid triglycerides are hydrolyzed into a glycerol and free fatty acids. It is used to mobilize stored energy during fasting or exercise, and usually occurs in fat adipocytes. The most important regulatory hormone in lipolysis is insulin; lipolysis can only occur when insulin action falls to low levels, as occurs during fasting. Other hormones that affect lipolysis include leptin, glucagon, epinephrine, norepinephrine, growth hormone, atrial natriuretic peptide, brain natriuretic peptide, and cortisol.

<span class="mw-page-title-main">Phosphatidylinositol</span> Signaling molecule

Phosphatidylinositol or inositol phospholipid is a biomolecule. It was initially called "inosite" when it was discovered by Léon Maquenne and Johann Joseph von Scherer in the late 19th century. It was discovered in bacteria but later also found in eukaryotes, and was found to be a signaling molecule.

Fatty acid metabolism consists of various metabolic processes involving or closely related to fatty acids, a family of molecules classified within the lipid macronutrient category. These processes can mainly be divided into (1) catabolic processes that generate energy and (2) anabolic processes where they serve as building blocks for other compounds.

<span class="mw-page-title-main">Glycerophospholipid</span> Class of lipids

Glycerophospholipids or phosphoglycerides are glycerol-based phospholipids. They are the main component of biological membranes in eukaryotic cells. They are a type of lipid, of which its composition affects membrane structure and properties. Two major classes are known: those for bacteria and eukaryotes and a separate family for archaea.

<span class="mw-page-title-main">Perilipin-1</span> Protein in humans

Perilipin, also known as lipid droplet-associated protein, perilipin 1, or PLIN, is a protein that, in humans, is encoded by the PLIN gene. The perilipins are a family of proteins that associate with the surface of lipid droplets. Phosphorylation of perilipin is essential for the mobilization of fats in adipose tissue.

Lipid metabolism is the synthesis and degradation of lipids in cells, involving the breakdown and storage of fats for energy and the synthesis of structural and functional lipids, such as those involved in the construction of cell membranes. In animals, these fats are obtained from food and are synthesized by the liver. Lipogenesis is the process of synthesizing these fats. The majority of lipids found in the human body from ingesting food are triglycerides and cholesterol. Other types of lipids found in the body are fatty acids and membrane lipids. Lipid metabolism is often considered the digestion and absorption process of dietary fat; however, there are two sources of fats that organisms can use to obtain energy: from consumed dietary fats and from stored fat. Vertebrates use both sources of fat to produce energy for organs such as the heart to function. Since lipids are hydrophobic molecules, they need to be solubilized before their metabolism can begin. Lipid metabolism often begins with hydrolysis, which occurs with the help of various enzymes in the digestive system. Lipid metabolism also occurs in plants, though the processes differ in some ways when compared to animals. The second step after the hydrolysis is the absorption of the fatty acids into the epithelial cells of the intestinal wall. In the epithelial cells, fatty acids are packaged and transported to the rest of the body.

<span class="mw-page-title-main">Hormone-sensitive lipase</span> Enzyme

Hormone-sensitive lipase (EC 3.1.1.79, HSL), also previously known as cholesteryl ester hydrolase (CEH), sometimes referred to as triacylglycerol lipase, is an enzyme that, in humans, is encoded by the LIPE gene, and catalyzes the following reaction:

<span class="mw-page-title-main">Phosphatidate phosphatase</span>

The enzyme phosphatidate phosphatase (PAP, EC 3.1.3.4) is a key regulatory enzyme in lipid metabolism, catalyzing the conversion of phosphatidate to diacylglycerol:

<span class="mw-page-title-main">Perilipin-2</span> Protein-coding gene in the species Homo sapiens

Adipose differentiation-related protein, also known as perilipin 2, ADRP or adipophilin, is a protein which belongs to the perilipin (PAT) family of cytoplasmic lipid droplet (CLD)–binding proteins. In humans it is encoded by the ADFP gene. This protein surrounds the lipid droplet along with phospholipids and is involved in assisting the storage of neutral lipids within the lipid droplets.

<span class="mw-page-title-main">Adipose triglyceride lipase</span> Mammalian protein found in Homo sapiens

Adipose triglyceride lipase, also known as patatin-like phospholipase domain-containing protein 2 and ATGL, is an enzyme that in humans is encoded by the PNPLA2 gene. ATGL catalyses the first reaction of lipolysis, where triacylglycerols are hydrolysed to diacylglycerols.

Fat globules are individual pieces of intracellular fat in human cell biology. The lipid droplet's function is to store energy for the organism's body and is found in every type of adipocytes. They can consist of a vacuole, droplet of triglyceride, or any other blood lipid, as opposed to fat cells in between other cells in an organ. They contain a hydrophobic core and are encased in a phospholipid monolayer membrane. Due to their hydrophobic nature, lipids and lipid digestive derivatives must be transported in the globular form within the cell, blood, and tissue spaces.

Spherosomes, also called lipid droplets or oleosomes are small cell organelles bounded by a single membrane which take part in storage and synthesis of lipids.

<span class="mw-page-title-main">Sec14</span>

Sec14 is a cytosolic protein found in yeast which plays a role in the regulation of several cellular functions, specifically those related to intracellular transport. Encoded by the Sec14 gene, Sec14p may transport phosphatidylinositol and phosphatidylcholine produced in the endoplasmic reticulum and the Golgi body to other cellular membranes. Additionally, Sec14p potentially plays a role in the localization of lipid raft proteins. Sec14p is an essential gene in yeast, and is homologous in function to phosphatidylinositol transfer protein in mammals. A conditional mutant with non-functional Sec14p presents with Berkeley bodies and deficiencies in protein secretion.

<span class="mw-page-title-main">Lipotoxicity</span> Metabolic disorder in which lipids accumulate in non-fat tissue

Lipotoxicity is a metabolic syndrome that results from the accumulation of lipid intermediates in non-adipose tissue, leading to cellular dysfunction and death. The tissues normally affected include the kidneys, liver, heart and skeletal muscle. Lipotoxicity is believed to have a role in heart failure, obesity, and diabetes, and is estimated to affect approximately 25% of the adult American population.

<span class="mw-page-title-main">Diglyceride</span> Type of fat derived from glycerol and two fatty acids

A diglyceride, or diacylglycerol (DAG), is a glyceride consisting of two fatty acid chains covalently bonded to a glycerol molecule through ester linkages. Two possible forms exist, 1,2-diacylglycerols and 1,3-diacylglycerols. Diglycerides are natural components of food fats, though minor in comparison to triglycerides. DAGs can act as surfactants and are commonly used as emulsifiers in processed foods. DAG-enriched oil has been investigated extensively as a fat substitute due to its ability to suppress the accumulation of body fat; with total annual sales of approximately USD 200 million in Japan since its introduction in the late 1990s till 2009.

<span class="mw-page-title-main">FITM2</span> Protein-coding gene in the species Homo sapiens

Fat storage-inducing transmembrane protein 2 is a protein that in humans is encoded by the FITM2 gene. It plays a role in fat storage. Its location is 20q13.12 and it contains 2 exons. It is also a member of the FIT protein family that has been conserved throughout evolution. Conserved from Saccharomyces cerevisiae to humans is the capability to take fat and store it as cytoplasmic triglyceride droplets. While FIT proteins facilitate the segregation of triglycerides (TGs) into cytosolic lipid droplets, they are not involved in triglyceride biosynthesis. In mammals, both FIT2 and FIT1 from the same family are present, embedded in the wall of the endoplasmic reticulum (ER) where they regulate lipid droplet formation in the cytosol. In S. cerevisiae, it also plays a role in the metabolism of phospholipids. These TGs are in the cytoplasm, encapsulated by a phospholipid monolayer in configurations or organelles that have been given many different names including lipid particles, oil bodies, adiposomes, eicosasomes, and most prevalent in scientific research – lipid droplets.

<span class="mw-page-title-main">Perilipin-4</span> Protein-coding gene in the species Homo sapiens

Perilipin 4, also known as S3-12, is a protein that in humans is encoded by the PLIN4 gene on chromosome 19. It is highly expressed in white adipose tissue, with lower expression in heart, skeletal muscle, and brown adipose tissue. PLIN4 coats lipid droplets in adipocytes to protect them from lipases. The PLIN4 gene may be associated with insulin resistance and obesity risk.

Seipin is a homo-oligomeric integral membrane protein in the endoplasmic reticulum (ER) that concentrates at junctions with cytoplasmic lipid droplets (LDs). Alternatively, seipin can be referred to as Berardinelli–Seip congenital lipodystrophy type 2 protein (BSCL2), and it is encoded by the corresponding gene of the same name, i.e. BSCL2. At protein level, seipin is expressed in cortical neurons in the frontal lobes, as well as motor neurons in the spinal cord. It is highly expressed in areas like the brain, testis and adipose tissue. Seipin's function is still unclear but it has been localized close to lipid droplets, and cells knocked out in seipin have anomalous droplets. Hence, recent evidence suggests that seipin plays a crucial role in lipid droplet biogenesis.

UBXD8 is a protein in the Ubiquitin regulatory X (UBX) domain-containing protein family. The UBX domain contains many eukaryotic proteins that have similarities in amino acid sequence to the tiny protein modifier ubiquitin. UBXD8 engages in a molecular interaction with p97, a protein that is essential for the degradation of membrane proteins associated with the endoplasmic reticulum (ER) through the proteasome. Ubxd8 possesses a UBA domain, alongside the UBX domain, that could interact with polyubiquitin chains. Additionally, it possesses a UAS domain of undetermined function, and this protein is used as a protein sensor that detects long chain unsaturated fatty acids (FAs), having a vital function in regulating the balance of Fatty Acids within cells to maintain cellular homeostasis.

References

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  2. 1 2 3 Mobilization and cellular uptake of stored fats and triacylglycerol (with Animation)
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  21. ↑ Melo, R. C. N.; D'Avila, H.; Wan, H.-C.; Bozza, P. T.; Dvorak, A. M.; Weller, P. F. (23 March 2011). "Lipid Bodies in Inflammatory Cells: Structure, Function, and Current Imaging Techniques". Journal of Histochemistry & Cytochemistry. 59 (5): 540–556. doi:10.1369/0022155411404073. PMC   3201176 . PMID   21430261.
  22. ↑ Ayakannu, Thangesweran; Taylor, Anthony H.; Marczylo, Timothy H.; Willets, Jonathon M.; Konje, Justin C. (2013). "The Endocannabinoid System and Sex Steroid Hormone-Dependent Cancers". International Journal of Endocrinology. 2013: 259676. doi: 10.1155/2013/259676 . ISSN   1687-8337. PMC   3863507 . PMID   24369462.
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