Lipid droplet

Everyday, cells within the human body rely on the metabolic energy stores found in lipid droplets to survive. Sufficient levels of energy reserves are found and kept in specialized cells called adipocytes (or fat cells), which are crucial for human survival. Unlike other cells, adipocytes are specialized for storage of metabolic energy reserves, and as such, an abundance of lipid droplets are typically found within them. During times of starvation, these lipid droplet reserves decrease within adipocytes and are scarcely found. However, sustained caloric excess (over-eating) stimulates the growth of these lipid droplet reserves as they accumulate excess lipids from caloric surplus. Caloric excess stimulates not only the expansion of lipid droplets, but also the expansion of adipocytes (or fat cells), in a process known as adipose hypertrophy. In humans, excess lipid droplet stores are associated with significant health issues, including increased risk of chronic conditions such as Type 2 diabetes, heart disease, stroke, high blood pressure, high cholesterol, atherosclerosis, and many cancers (e.g., endometrial, colon, and breast cancers). Generally, obesity also leads to musculoskeletal issues such as osteoarthritis, sleep apnea, liver disease, gallbladder disease, kidney problems, infertility, pregnancy complications, and depression. Conversely, a lack of lipid droplet reserves (or fat) as seen in conditions like anorexia lead to a variety of serious health complications, such as anemia, heart failure, bone loss (osteoporosis), muscle wasting, vitamin deficiencies, stomach problems, and kidney disease. Serious health risks increase as a person's weight approaches either extreme (too high or too low), and as such, weight must be taken seriously to ensure a healthy life. == Structure ==

Notably, lipid droplets (LDs) bear a unique structure relative to all other cellular organelles. LDs emerge from the endoplasmic reticulum, where many remain continuous within cytoplasmic leaflets of the endoplasmic reticulum phospholipid bilayer itself. LDs thus bear a phospholipid monolayer, which envelopes a highly dynamic core of hydrophobic neutral lipids. While all LDs are known to share this structure, the behavior and morphologies of these organelles are both diverse and extremely dynamic. LD surface proteins are known to regulate and dictate several aspects of the LD life-cycle, including LD budding, growth, turnover, and interaction with other cellular organelles, such as mitochondria. Inherently, the LD proteome is highly dynamic and represents a key area of interest in modern lipid research. The first and best-characterized family of lipid droplet associated proteins is the perilipin protein family, consisting of five proteins. These include perilipin 1 (PLIN1), perilipin 2 (PLIN2), perilipin 3 (PLIN3), perilipin 4 (PLIN4) and perilipin 5 (PLIN5). Proteomics studies have elucidated the association of many other families of proteins to the lipid surface, including those involved in membrane trafficking, vesicle docking, endocytosis and exocytosis. Lipid droplet core At their core, lipid droplets (LDs) contain a highly dynamic, hydrophobic deposit of neutral lipids, such as triacylglycerols (TAGs) and cholesteryl esters (CEs). In most cells, metabolic energy is stored in the form of fatty acids (FAs), which are the building blocks of TAGs or fat. Lipid droplets are the only cellular compartment dedicated to the storage of TAGs and other neutral lipids, making these organelles crucial for both energy storage functions and the aversion of lipotoxicity. In adipocytes (or fat cells), TAGs are the predominant component of the LD core. However, in other cell types, various ratios of TAGs and CEs are found in the LD core. Demarcation between TAGs and CEs within the LD core has not been observed by conventional electron microscopy (EM) alone, although segregation within the core may exist in certain circumstances. and island-like fracture faces seen by freeze-fracture electron microscopy. Membrane-like structures have also been observed in the LD core in more specialized cell types. Analysis of the lipid composition of lipid droplets has revealed the presence of a diverse set of phospholipid species; phosphatidylcholine and phosphatidylethanolamine are the most abundant, followed by phosphatidylinositol. Lipid droplet heterogeneity Generally, LD heterogeneity refers to observable differences in LD size, abundance, distribution, location, core lipid composition, or proteome composition about the organelle. Discrete combinations of these features are thought to shape general functional differences between LD populations, many of which may be present across different cell types, as well as within the same cell. Of all factors used to characterize specific LD types, the surface proteome has proven most useful. Features associated with a specific LD type are largely determined by the proteins on the LD surface, many of which facilitate LD growth or shrinkage such as lipid enzymes, as well as scaffolding proteins and factors that mediate interactions with other organelles. LD heterogeneity is best characterized within cells of a given cell type, where it may thus reflect relative changes in cellular metabolic state or become indicative of physiological disease. However, LD heterogeneity within a single cell likely reflects the discrete functions of different LD subpopulations in metabolic homeostasis, in both health and disease. Differences across cell types Canonically, the core function of the LD as a lipid storage depot is conserved across cell types and species. However, cellular identity (and thus specialization) determines the threshold for LD utility and thus caps LD heterogeneity by cell type. == Lipid droplet biogenesis ==

Lipid droplet (LD) biogenesis begins at the membrane of the endoplasmic reticulum (ER). Despite over a decade of modern research, the process of LD formation has yet to be fully understood. LD biogenesis is triggered by the accumulation of neutral lipids within the membrane of the endoplasmic reticulum, which occurs in response to elevated dietary carbohydrate or lipid intake. In simplest terms, lipid droplets form should the rate of neutral lipid synthesis at the ER exceed the ER membrane's capacity to accommodate those lipids, causing them to phase-separate and bud into lipid droplets. To date, LD assembly appears to follow a single robust mechanism, regardless of the type of neutral lipid involved. Above a critical concentration, neutral lipids spontaneously phase-separate in vitro, demixing from phospholipids in the surrounding bilayer and coalescing into a neutral lipid lens. Loss-of-function studies have revealed that this ring-like assembly is crucial for Seipin function. Conventional electron microscopy tomography experiments provided the first in vivo evidence for the lens model of LD biogenesis, where lens-like structures of approximately 50 nm in diameter were observed in yeast, following the induction of TAG biosynthesis. Lipid droplet budding As the lipid droplet (LD) continues to grow and bud away from the endoplasmic reticulum (ER) bilayer, it remains attached through a small membrane stalk or hairpin-like structure. FIT proteins, also known as fat storage-inducing transmembrane (FITM) proteins, are an evolutionarily conserved family of proteins known for their namesake promotion of lipid storage. The role of these proteins in facilitating LD formation proves crucial, as loss of FIT proteins halts LD budding, thereby inducing an accumulation of neutral lipid lenses within the ER bilayer. Lipid droplet growth and maturation Further expansion of the nascent lipid droplet (LD) relies on the continuous phospholipid supply from the ER to the LD monolayer via the membrane stalk, which maintains the connection of the two organelles. During periods of rapid LD expansion, synthesis of new phospholipids is necessary to maintain phospholipid homeostasis about the LD monolayer, which requires relocalization of several enzymes involved in this process to the LD surface. Critically, the ER-residential protein seipin ensures the fidelity of LD growth and maturation via stabilization of the membrane stalk that connects the LD to the ER bilayer. During LD biogenesis, seipin protomers assemble into a decameric, cage-like structure at sites of LD assembly, forming a stable ring of luminal domains at the cage floor with transmembrane domains at the cage sides and top. Reattachment appears to utilize components of the COPI coatomer complex, which is usually known for its roles in retrograde Golgi-to-ER trafficking. Protein regulation of lipid droplet formation Though LD biogenesis can be viewed as a biophysical process driven purely by thermodynamic principles, individual stages of LD formation within cells are highly regulated by several protein machineries. Seipin marks sites of LD biogenesis, and each cytoplasmic LD appears to associate with at least one seipin focus. Recently, the results of several structural studies suggest a role for seipin in the promotion of TAG aggregation and facilitation of neutral lipid lens formation at defined sites about the ER bilayer. Loss of seipin is not sufficient for ablation of LD biogenesis, but rather results in aberrant LD biogenesis and produces organelles with abnormal morphology, protein composition, and functions. Several studies have observed seipin preventing spontaneous coalescence of lenses at random sites throughout the ER bilayer, Other interacting proteins such as lipid droplet assembly factor 1 (LDAF1) may also modulate seipin's ability to bind to TAG. LDAF1 reportedly binds seipin, forming an ~600 kDa oligomeric complex that copurifies with TAG. During LD biogenesis, LDAF1-seipin complexes have been observed at sites of lens formation, where re-localization of LDAF1 to the ER bilayer recruits seipin and promotes LD formation at these sites. As LDs coalesce within the bilayer and begin to bud away from the ER, LDAF1 dissociates from seipin and moves onto the LD surface. Interestingly, loss of LDAF1 produces similar albeit milder aberrations to LD biogenesis, in comparison to loss of seipin. To date, the available data support a model in which the formation of neutral lipid lenses catalyzed by seipin begins to expand at sites marked by the seipin/LDAF1 complex. The long-chain-fatty-acid-CoA ligase 3 (ACSL3) enzyme also localizes to these early sites of LD formation, and its catalytic activity may contribute in part to the rapid synthesis of lipids that fuels further lens growth. == Protein targeting to lipid droplets ==

Mechanisms by which the large and highly dynamic set of proteins traffic to the lipid droplet (LD) surface have been an area of intense modern research. Unlike proteins targeting other cellular organelles, LD proteins have no distinct targeting signal or localization sequence embedded within their amino acid sequence. As such, polytopic membrane proteins can never be class I proteins, nor will they enter the LD surface via the ERTOLD pathway. Certain class I proteins display an affinity for triacylglycerol (TAG) about their hairpin loop, which drives their concentration at the LD surface. Conversely, some class I proteins may be actively retained on the ER surface through protein-protein interactions. Selective degradation of certain hairpin-containing proteins in the ER results in their effective accumulation at the LD surface. Although these examples and putative mechanisms offer insights as to the regulation of class I protein partitioning between the ER and LD surface, a complete mechanistic understanding of this process has yet to be uncovered. Several imaging studies have revealed that some class I proteins localize to LDs during the earliest stages of LD biogenesis, while other proteins target LDs much later. By contrast, other class I proteins were shown to be initially rejected at the Seipin gate and appeared to follow another distinct route, which depends on machinery typically associated with anterograde vesicular traffic in the early secretory pathway. Most frequently, class II proteins are found bound to LDs via amphipathic alpha-helices (AHs), in which hydrophobic and polar residues partition to opposite sides of the helix. Interestingly, the specificity of AH-containing proteins for the LD monolayer appears to be caused by phospholipid packing defects and the higher surface tension of the LD monolayer relative to the phospholipid bilayer of all other cellular organelles. However, not all AH-containing LD proteins behave as class II proteins. Comparison of multiple AHs suggests that class I behavior is favored for AHs with a reduced number of charged residues along the polar face, possibly due to the greater allowance for extensive interactions with the phospholipid side chains. Conversely, class II behavior is favored for AHs with a greater presence of charged residues, as such reduces the axial hydrophobic surface of the AH. == Organelle–lipid droplet contacts ==

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. 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. ==Gallery==

File:Lipid droplets stained in murine microglia cells.png| File:Nardia scalaris Blattzellen IMG 8353.jpg| ==See also==