Lipid droplets were observed and described in the XIXth century, and named as liposomes for many years. They were also known as lipid bodies, fat bodies, oil bodies, spherosomes and adiposomes.
Most animal cells store lipids as lipid droplets scattered in the cytoplasm. They can be also found in plant cells, and even in yeast and bacteria. In fact, lipid droplets were first studied in plant seed cells. Although the majority of cells may content lipid droplets, adipocytes are animal cells specialized in producing and keeping lipid drops. There are two main types of adipocytes. Unilocular adipocytes (white fat cells) contain one very large lipid droplet that occupies most of the cytoplasm. The main function of white fat is to store and release energy when needed. Multilocular adipocytes (brown fat cells) contain multiple small lipid droplets in the cytoplasm and are specialized in producing heat. In animals, after the fat tissue, the second place where most fat is stored is the liver. There are many lipid droplets in the enterocytes after the digestive process. In these cells, the triacylglycerols are exported as chylomicrons. In plant seed, there are cells specialized in storing energetic material as lipid droplets.
Lipid droplets perform many functions that may depend on the cell type. Fat of lipid droplets is used to synthesize lipids of the cell membranes and to produce energy. Fat is a much better source of energy than glycogen. A less known function is to prevent lipotoxicity by removing fatty acids from the bloodstream. A high amount of circulating fatty acids may produce toxic molecules in the catabolic pathways. Lipids are also needed for synthesizing steroid hormones. In some animal species, adipocytes with their large lipid droplets form a thick layer under the skin that provides thermal insulation. In the cell, lipid droplets are associated with protein degradation centers, may be protecting mitochondria and endoplasmic reticulum from oxidative stress.
Lipid droplets are rounded and clear organelles located in the cytoplasm (Figure 1). They are visualized at light microscopy after staining with liposoluble dyes such as Sudan black. Lipid droplets are variable in size and number depending on the cell type and physiological state of the cell. Many cells contain small lipid droplets, from 100 to 200 nm, whereas adipocytes of white fat may contain lipid droplets up to 200 µm in diameter. The number and size may change quickly in the same cell (Figure 2).
Lipid droplets are composed of neutral lipids surrounded by a membrane monolayer of amphipathic lipids with associated proteins. The lipid droplet is the only cell organelle limited by one lipid monolayer. It contains neutral lipids, mainly triacylglycerols and cholesterol esters, in a variable proportion depending on the cell type. For example, adipocytes have lipid droplets with a majority of triacylglycerols, whereas macrophages show a higher proportion of cholesterol esters. The membrane monolayer mostly consists of phospholipids (most are phosphatidylcholine, but also phosphatidylethanolamine and phosphatidylinositol), and cholesterol. There are almost no sphingolipids. Many proteins with a variety of functions are inserted in this monolayer.
In animal cells, about 50 types of proteins are estimated to be associated with lipid droplets. They can be classified according to their functions: structural, enzymes, involved in vesicular trafficking, signaling, and those that use lipid droplet surface to carry out functions unrelated to the lipid droplet itself. Many of these proteins are also found in other cell compartments. For example, triglyceride synthesizing enzymes are found in the endoplasmic reticulum membranes and in the surface of the lipid droplets. Other proteins may use the lipid droplets as a transient storing place. For example, histones in the fruit fly embryos are temporarily stored in lipid droplets for a later production of a massive amount of nuclei. Some viruses use the lipid droplet surfaces as a platform for the final assembling of virus particles.
In animal cells, perilipins are a family of proteins associated with lipid droplets. In plant cells, protein associated with lipid droplets are oleosins (structural proteins that can cross the membrane monolayer and are in contact with triglycerides), enzymes (caleosin, dioxygenase, stereoleosin, which are related to cell stress), and proteins associated with the hydrophobic segments of the membrane monolayer. There are also proteins specialized in synthesizing or degrading lipids. The more abundant protein is oleosin, which may cover most of the lipid droplet surface, and contains an amino acid chain inserted among the triacylglycerols. The other proteins are usually attached to the lipid droplet surface and have a very short hydrophobic amino acid chain or they do not have it at all.
Lipid droplets are initially formed by the accumulation of esterified lipids between the two monolayers of the endoplasmic reticulum membrane (Figure 3). Resident proteins of the endoplasmic reticulum synthesize these lipids. When a critical size is reached, the depot of lipids is released to the cytosol as a lipid droplet surrounded by a superficial monolayer coming from the endoplamic reticulum membrane. Lipid droplets may grow by synthesizing new lipids or by fusing with other lipid droplets. Proteins inserted in the delimiting monolayer may be synthesized in the endoplasmic reticulum or in the cytosol. New lipid droplets may also be produced by strangulation of large lipid droplets. There are two mechanisms to remove lipid droplets from the cytoplasm: by the activity lipases (lipid degrading enzymes), which are found in the surface of the lipid droplets, and by autophagy.
Some proteins, like Pex 30, can be found in the endoplasmic reticulum regions that form lipid droplets. Curiously, these proteins are also found in those regions of the endoplasmic reticulum that form peroxisomes. Thus, the peroxisomes and lipid droplets may share some molecular machinery during their formation.
In some species, lipid droplets always remain physically connected to the endoplasmic reticulum, as in yeasts, but in mammals they are mainly free in the cytosol. These physical contacts between lipid droplets and endoplasmic reticulum are thought to behave as bridges for endoplasmic enzymes that travel to lipid droplets for fatty acid degradation. However, lipid droplets also make physical contacts with mitochondria, lysosomes, peroxisomes, endosomes and nuclear envelope. In yeast, the physical contact between lipid droplets and endoplasmic reticular seems to be permanent. Sometimes, endosome are observed covering lipid droplets, which may lead to lipid droplet degradation in lysosomes by autophagy.
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