Adipophilin-enriched domains in the ER membrane are sites of lipid droplet biogenesis

Lipid droplets finally get a little R-E-S-P-E-C-T

Long underappreciated as important cellular organelles, lipid droplets are finally being recognized as dynamic structures with a complex and interesting biology. In light of this newfound respect, we discuss emerging views on lipid droplet biology and speculate on the major advances to come.

A novel function of lipid droplets in regulating longevity

Growing evidence supports the view that LDs (lipid droplets) are dynamic organelles that can serve both as an intracellular signalling compartment and as an organizing platform orchestrating many vital processes in eukaryotic cells. It has become clear that the LDs-confined deposition and lipolytic degradation of neutral lipids define longevity in multicellular eukaryotic organisms and yeast. We summarize the evidence in support of the essential role that LDs play in longevity regulation and propose several molecular mechanisms by which these dynamic organellar compartments control the aging process in multicellular eukaryotes and yeast.

Lipid droplet growth by fusion: insights from freeze-fracture imaging

An understanding of how lipid droplets grow in the cell is important to current human health issues. Homotypic fusion of small lipid droplets to create larger ones is one proposed mechanism though the evidence for this process continues to be debated. By applying the technique of freeze-fracture electron microscopy to cells that have been stimulated to accumulate lipid droplets, we here present images which suggest that at least some large lipid droplets may indeed result from amalgamation of multiple smaller ones. These visual data add significantly to the notion that fusion contributes to lipid droplet growth.

Conditions of endoplasmic reticulum stress stimulate lipid droplet formation in Saccharomyces cerevisiae

LDs (lipid droplets) are cellular organelles which can be found in nearly all eukaryotic cells. Despite their importance in cell biology, the mechanism underlying LD biogenesis remains largely unknown. In the present study we report that conditions of ER (endoplasmic reticulum) stress stimulate LD formation in Saccharomyces cerevisiae. We found that LDs accumulated in yeast mutants with compromised protein glycosylation or ER-associated protein degradation. Moreover, tunicamycin and Brefeldin A, agents which induce ER stress, were found to stimulate LD formation. In contrast, the restoration of protein glycosylation reduced LD accumulation. Interestingly, enhanced neutral lipids synthesis and LD formation under conditions of ER stress was not dependent on Ire1p. Lastly, we demonstrated that the absence of LDs did not compromise cell viability under ER stress. Our results suggest that although more LDs are produced, LDs are not essential to cell survival under ER stress.

Hydrophobic and basic domains target proteins to lipid droplets

In recent years, progress in the study of the lateral organization of the plasma membrane has led to the proposal that mammalian cells use two different organelles to store lipids: intracellular lipid droplets (LDs) and plasma membrane caveolae. Experimental evidence suggests that caveolin (CAV) may act as a sensitive lipid-organizing molecule that physically connects these two lipid-storing organelles. Here, we determine the sequences necessary for efficient sorting of CAV to LDs. We show that targeting is a process cooperatively mediated by two motifs. CAV's central hydrophobic domain (Hyd) anchors CAV to the endoplasmic reticulum (ER). Next, positively charged sequences (Pos-Seqs) mediate sorting of CAVs into LDs. Our findings were confirmed by identifying an equivalent, non-conserved but functionally interchangeable Pos-Seq in ALDI, a bona fide LD-resident protein. Using this information, we were able to retarget a cytosolic protein and convert it to an LD-resident protein. Further studies suggest three requirements for targeting via this mechanism: the positive charge of the Pos-Seq, physical proximity between Pos-Seq and Hyd and a precise spatial orientation between both motifs. The study uncovers remarkable similarities with the signals that target proteins to the membrane of mitochondria and peroxisomes.

Modulation of myocellular fat stores: lipid droplet dynamics in health and disease

Storage of fatty acids as triacylglycerol (TAG) occurs in almost all mammalian tissues. Whereas adipose tissue is by far the largest storage site of fatty acids as TAG, subcellular TAG-containing structures—referred to as lipid droplets (LD)—are also present in other tissues. Until recently, LD were considered inert storage sites of energy dense fats. Nowadays, however, LD are increasingly considered dynamic functional organelles involved in many intracellular processes like lipid metabolism, vesicle trafficking, and cell signaling. Next to TAG, LD also contain other neutral lipids such as diacylglycerol. Furthermore, LD are coated by a monolayer of phospholipids decorated with a variety of proteins regulating the delicate balance between LD synthesis, growth, and degradation. Disturbances in LD-coating proteins may result in disequilibrium of TAG synthesis and degradation, giving rise to insulin-desensitizing lipid intermediates, especially in insulin-responsive tissues like skeletal muscle. For a proper and detailed understanding, more information on processes and players involved in LD synthesis and degradation is necessary. This, however, is hampered by the fact that research on LD dynamics in (human) muscle is still in its infancy. A rapidly expanding body of knowledge on LD dynamics originates from studies in other tissues and other species. Here, we aim to review the involvement of LD-coating proteins in LD formation and degradation (LD dynamics) and to extrapolate this knowledge to human skeletal muscle and to explore the role of LD dynamics in myocellular insulin sensitivity.

Induced translocation of glycosylphosphatidylinositol-anchored proteins from lipid droplets to adiposomes in rat adipocytes

BACKGROUND AND PURPOSE

Adipocytes release membrane vesicles called adiposomes, which harbor the glycosylphosphatidylinositol-anchored proteins (GPI proteins), Gce1 and CD73, after induction with palmitate, H(2)O(2) and the sulphonylurea drug glimepiride. The role of lipid droplets (LD) in trafficking of GPI proteins from detergent-insoluble, glycolipid-enriched, plasma membrane microdomains (DIGs) to adiposomes in rat adipocytes was studied.

EXPERIMENTAL APPROACH

Redistribution of Gce1 and CD73 was followed by pulse-chase and long-term labelling, western blot analysis and activity determinations with subcellular fractions and cell-free systems exposed to palmitate, H(2)O(2) and glimepiride.

KEY RESULTS

In response to these signals, Gce1 and CD73 disappeared from DIGs, then transiently appeared in LD and finally were released into adiposomes from small, and, more efficiently, large adipocytes. From DIGs to LD, Gce1 and CD73 were accompanied by cholesterol. Cholesterol depletion from DIGs or LD caused accumulation at DIGs or accelerated loss from LD and release into adiposomes, respectively, of the GPI proteins. Blockade of translocation of Gce1, CD73, caveolin-1 and perilipin-A from DIGs to LD blocked LD biogenesis and long term-accumulation of LD interfered with induced release of the GPI proteins into adiposomes. GPI protein release was up-regulated upon long term-depletion of LD. Adiposomes were released by a DIGs-based cell-free system, but only in presence of LD.

CONCLUSIONS

GPI proteins are translocated from DIGs to LD prior to their release into adiposomes, which is regulated by cholesterol, LD content and LD biogenesis. This detour may serve to transfer information about the LD content and to control lipolysis/esterification between large and small adipocytes via GPI protein-harbouring adiposomes.

Targeting sequences of UBXD8 and AAM-B reveal that the ER has a direct role in the emergence and regression of lipid droplets

Lipid droplets are sites of neutral lipid storage thought to be actively involved in lipid homeostasis. A popular model proposes that droplets are formed in the endoplasmic reticulum (ER) by a process that begins with the deposition of neutral lipids between the membrane bilayer. As the droplet grows, it becomes surrounded by a monolayer of phospholipid derived from the outer half of the ER membrane, which contains integral membrane proteins anchored by hydrophobic regions. This model predicts that for an integral droplet protein inserted into the outer half of the ER membrane to reach the forming droplet, it must migrate in the plane of the membrane to sites of lipid accumulation. Here, we report the results of experiments that directly test this hypothesis. Using two integral droplet proteins that contain unique hydrophobic targeting sequences (AAM-B and UBXD8), we present evidence that both proteins migrate from their site of insertion in the ER to droplets that are forming in response to fatty acid supplementation. Migration to droplets occurs even when further protein synthesis is inhibited or dominant-negative Sar1 blocks transport to the Golgi complex. Surprisingly, when droplets are induced to disappear from the cell, both proteins return to the ER as the level of neutral lipid declines. These data suggest that integral droplet proteins form from and regress to the ER as part of a cyclic process that does not involve traffic through the secretory pathway.

Ultrastructural analysis of lipid incorporation in the embryonic silkworm, Bombyx mori

Insect eggs store many lipid droplets as an energy source for embryonic development. We previously reported that lipid droplets are incorporated into embryos in three steps in the silkworm, Bombyx mori. The midgut plays important roles in lipid incorporation during the second and third steps, whereas the manner of lipid incorporation during the first step is still unknown. In this study, we focused on how lipids were incorporated into the embryo in the first step, compared with the mechanisms used in the second step, by means of transmission electron microscopy using the high-pressure freezing and freeze substitution method. At the beginning of the first step (blastoderm formation stage), some lipid droplets were observed in each cell of the embryonic tissues. Lipid droplets were seen to be derived from the oocyte peripheral cytoplasm by superficial cleavage. At the end of the first step (late appendage formation stage), some lipid droplets were attached to the elongated rough endoplasmic reticulum (rER). It seemed that formation of the lipid droplets occurred in embryonic cells at the end of the first step, because the rER is the site of biogenesis of lipid droplets. The incorporation of lipid droplets in the first step may be subdivided into two stages: the blastoderm formation stage and the subsequent stage before blastokinesis.

Regulation of HSD1 in seeds of Arabidopsis thaliana

The hydroxysteroid dehydrogenase HSD1, identified in the proteome of oil bodies from mature Arabidopsis seeds, is encoded by At5g50600 and At5g50700, two gene copies anchored on a duplicated region of chromosome 5. Using a real-time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) approach, the accumulation of HSD1 mRNA was shown to be specifically and highly induced in oil-accumulating tissues of maturing seeds. HSD1 mRNA disappeared during germination. The activity of HSD1 promoter and the localization of HSD1 transcripts by in situ hybridization were consistent with this pattern. A complementary set of molecular and genetic analyses showed that HSD1 is a target of LEAFY COTYLEDON2, a transcriptional regulator able to bind the promoter of HSD1. Immunoblot analyses and immunolocalization experiments using anti-AtHSD1 antibodies established that the pattern of HSD1 deposition faithfully reflected mRNA accumulation. At the subcellular level, the study of HSD1:GFP fusion proteins showed the targeting of HSD1 to the surface of oil bodies. Transgenic lines overexpressing HSD1 were then obtained to test the importance of proper transcriptional regulation of HSD1 in seeds. Whereas no impact on oil accumulation could be detected, transgenic seeds exhibited lower cold and light requirements to break dormancy, germinate and mobilize storage lipids. Interestingly, overexpressors of HSD1 over-accumulated HSD1 protein in seeds but not in vegetative organs, suggesting that post-transcriptional regulations exist that prevent HSD1 accumulation in tissues deprived of oil bodies.

Formation of milk lipids: a molecular perspective

Lipids, primarily triglycerides, are major milk constituents of most mammals, providing a large percentage of calories, essential fatty acids and bioactive lipids required for neonatal growth and development. To meet the caloric and nutritional demands of newborns, the mammary glands of most species have evolved an enormous capacity to synthesize and secrete large quantities of lipids during lactation. Significant information exists regarding the physiological regulation of lipid metabolism in the mammary gland from the study of dairy animals. However, detailed understanding of the molecular mechanisms regulating milk lipid formation is only now coming into focus through advances in mouse genetics, global analysis of mammary gland gene expression, organelle protein properties and the cell biology of lipid metabolism.

Regulation of de novo fatty acid synthesis in maturing oilseeds of Arabidopsis

As a Brassicaceae, Arabidopsis thaliana constitutes an excellent model system to investigate oil biosynthesis in seeds. Extensive tools for the genetic and molecular dissection of this model species are now available. Together with analytical procedures adapted to its tiny seeds, these tools have allowed major advances in isolating and characterising the factors that participate in the metabolic and developmental control of seed filling. Once the biochemical pathways producing storage lipids, namely triacylglycerols, were elucidated, the question of the regulation of this metabolic network has arisen. The coordinated up regulation of genes encoding enzymes of the fatty acid biosynthetic pathway observed at the onset of seed maturation suggests that the pathway may be subjected to a system of global transcriptional regulation. This has been further established by the study of master regulators of the maturation program like LEAFY COTYLEDON2 and the characterisation of the WRINKLED1 transcription factor. These factors have been shown to participate in a regulatory cascade controlling the induction of the genes involved in fatty acid biosynthesis at the onset of the maturation phase. Although much remains to be elucidated, the framework of the regulatory system controlling fatty acid biosynthesis in Arabidopsis seeds is coming into focus.

Storage oil hydrolysis during early seedling growth

Storage oil breakdown plays an important role in the life cycle of many plants by providing the carbon skeletons that support seedling growth immediately following germination. This metabolic process is initiated by lipases (EC: 3.1.1.3), which catalyze the hydrolysis of triacylglycerols (TAGs) to release free fatty acids and glycerol. A number of lipases have been purified to near homogeneity from seed tissues and analysed for their in vitro activities. Furthermore, several genes encoding lipases have been cloned and characterised from plants. However, only recently has data been presented to establish the molecular identity of a lipase that has been shown to be required for TAG breakdown in seeds. In this review we briefly outline the processes of TAG synthesis and breakdown. We then discuss some of the biochemical literature on seed lipases and describe the cloning and characterisation of a lipase called SUGAR-DEPENDENT1, which is required for TAG breakdown in Arabidopsis thaliana seeds.

The MAP1-LC3 conjugation system is involved in lipid droplet formation

Lipid droplets (LDs) are ubiquitous in eukaryotic cells, while excess free fatty acids and glucose in plasma are converted to triacylglycerol (TAG) and stored as LDs. However, the mechanism for the generation and growth of LDs in cells is largely unknown. We show here that the LC3 lipidation system essential for macroautophagy is involved in LD formation. LD formation accompanied by accumulation of TAG induced by starvation was largely suppressed in the hepatocytes that cannot execute autophagy. Under starvation conditions, LDs in addition to autophagosomes were abundantly formed in the cytoplasm of these tissue cells. Moreover, LC3 was localized on the surface of LDs and LC3-II (lipidation form) was fractionated to a perilipin (LD marker)-positive lipid fraction from the starved liver. Taken together, these results indicate that the LC3 conjugation system is critically involved in lipid metabolism via LD formation.

TIP47, a lipid cargo protein involved in macrophage triglyceride metabolism

OBJECTIVE

Uptake of lipids by macrophages (MPhi) leads to lipid droplet accumulation and foam cell formation. The PAT family proteins are implicated in lipid droplet formation, but the precise function of the 47-kDa tail interacting protein (TIP47), a member of this family, is poorly defined. The present study was performed to determine the function of TIP47 in MPhi lipid metabolism.

METHODS AND RESULTS

Freeze-fracture cytochemistry demonstrates that TIP47 is present in the plasma membrane of MPhi and is aggregated into clusters when the cells are incubated with oleate. Suppression of adipophilin levels using siRNA knockdown leads to migration of TIP47 from a cytoplasmic pool to the lipid droplet. Further, reduction of TIP47 decreases triglyceride levels, whereas raising TIP47 levels by expression of EGFP-TIP47 shows the opposite effect.

CONCLUSION

Our results show that the TIP47 protein levels directly correlate with triglyceride levels. We propose that TIP47 may act as a carrier protein for free fatty acids and in this way participates in conversion of MPhi into foam cells.

Effect of intracellular lipid droplets on cytosolic Ca2+ and cell death during ischaemia-reperfusion injury in cardiomyocytes

Lipid droplets (LD) consist of accumulations of triacylglycerols and have been proposed to be markers of ischaemic but viable tissue. Previous studies have described the presence of LD in myocardium surviving an acute coronary occlusion. We investigated whether LD may be protective against cell death secondary to ischaemia-reperfusion injury. The addition of oleate-bovine serum albumin complex to freshly isolated adult rat cardiomyocytes or to HL-1 cells resulted in the accumulation of intracellular LD detectable by fluorescence microscopy, flow cytometry and (1)H-nuclear magnetic resonance spectroscopy. Simulated ischaemia-reperfusion of HL-1 cells (respiratory inhibition at pH 6.4 followed by 30 min of reperfusion) resulted in significant cell death (29.7+/-2.6% of total lactate dehydrogenase release). However, cell death was significantly attenuated in cells containing LD (40% reduction in LDH release compared with control cells, P=0.02). The magnitude of LD accumulation was inversely correlated (r(2)=0.68, P=0.0003) with cell death. The protection associated with intracellular LD was not a direct effect of the fatty acids used to induce their formation, because oleate added 30 min before ischaemia, during ischaemia or during reperfusion did not form LD and did not protect against cell death. Increasing the concentration of free oleate during reperfusion progressively decreased the protection afforded by LD. HL-1 cells labelled with fluo-4, a Ca(2+)-sensitive fluorochrome, fluorescence within LD areas increased more throughout simulated ischaemia and reperfusion than in the cytosolic LD-free areas of the same cells. As a consequence, cells with LD showed less cytosolic Ca(2+) overload than control cells. These results suggest that LD exert a protective effect during ischaemia-reperfusion by sequestering free fatty acids and Ca(2+).

Leukocyte lipid bodies - Biogenesis and functions in inflammation

Lipid body accumulation within leukocytes is a common feature in both clinical and experimental infectious, neoplasic and other inflammatory conditions. Here, we will review the contemporary evidence related to the biogenesis and structure of leukocyte lipid bodies (also known as lipid droplets) as inflammatory organelles. Studies of leukocyte lipid bodies are providing functional, ultrastructural and protein compositional evidences that lipid bodies are not solely storage depots of neutral lipid. Over the past years substantial progresses have been made to demonstrate that lipid body biogenesis is a highly regulated process, that culminate in the compartmentalization of a specific set of proteins and lipids, that place leukocyte lipid bodies as inducible cytoplasmic organelles with roles in cell signaling and activation, regulation of lipid metabolism, membrane trafficking and control of the synthesis and secretion of inflammatory mediators. Pertinent to the roles of lipid bodies in inflammation and cell signaling, enzymes involved in eicosanoid synthesis are localized at lipid bodies and lipid bodies are sites for eicosanoid generation. Collectively, lipid bodies in leukocytes are emerging as critical regulators of different inflammatory diseases, key markers of leukocyte activation and attractive targets for novel anti-inflammatory therapies.

Endolysosomal phospholipidosis and cytosolic lipid droplet storage and release in macrophages

This review summarizes the current knowledge of endolysosomal and cytoplasmic lipid storage in macrophages induced by oxidized LDL (Ox-LDL), enzymatically degraded LDL (E-LDL) and other atherogenic lipoprotein modifications, and their relation to the adapter protein 3 (AP-3) dependent ABCA1 and ABCG1 cellular lipid efflux pathways. We compare endolysosomal lipid storage caused either through drug induced phospholipidosis, inheritable endolysosomal and cytosolic lipid storage disorders and Ox-LDL or E-LDL induced phagosomal uptake and cytosolic lipid droplet storage in macrophages. Ox-LDL is resistant to rapid endolysosomal hydrolysis and is trapped within the endolysosomal compartment generating lamellar bodies which resemble the characteristics of phospholipidosis. Various inherited lysosomal storage diseases including sphingolipidosis, glycosphingolipidosis and cholesterylester storage diseases also present a phospholipidosis phenotype. In contrast E-LDL resembling coreless unesterified cholesterol enriched LDL-particles, with a multilamellar, liposome-like structure, lead to rapid phagosomal degradation and cytosolic lipid droplet accumulation. As a consequence the uptake of E-LDL through type I and type II phagocytosis leads to increased lipid droplet formation and moderate upregulation of ABCA1 and ABCG1 while uptake of Ox-LDL leads to a rapid expansion of the lysosomal compartment and a pronounced upregulation of the ABCA1/ABCG1/AP-3 lipid efflux pathway.

The life of lipid droplets

Lipid droplets are the least characterized of cellular organelles. Long considered simple lipid storage depots, these dynamic and remarkable organelles have recently been implicated in many biological processes, and we are only now beginning to gain insights into their fascinating lives in cells. Here we examine what we know of the life of lipid droplets. We review emerging data concerning their cellular biology and present our thoughts on some of the most salient questions for investigation.

Biogenesis of cytoplasmic lipid droplets: from the lipid ester globule in the membrane to the visible structure

The cytoplasmic lipid droplet (CLD) and very low-density lipoprotein are generated from the lipid ester synthesized in the endoplasmic reticulum. The lipid ester deposited between the two membrane leaflets is supposed to bulge toward the cytoplasm to make a nascent CLD, but its size must be below the resolution limit of conventional techniques and the detectable CLD should only form after acquisition of additional lipid esters. The CLD is different from vesicular organelles in that the internal content is highly hydrophobic and the shape is invariably spherical. Due to its unique characteristics, quantitative discordance between the surface and the volume may occur in the growth and/or involution processes of the CLD. The possibility that these processes may give rise to the structural and functional diversities of the CLD is discussed.

Triglyceride containing lipid droplets and lipid droplet-associated proteins

PURPOSE OF REVIEW

Cytosolic lipid droplets are now recognized as dynamic organelles. This review summarizes our current understanding of the mechanisms involved in the formation of lipid droplets, the importance of lipid droplet-associated proteins and the link between lipid droplet accumulation and development of insulin resistance.

RECENT FINDINGS

Lipid droplets are formed as primordial droplets and they increase in size by fusion. This fusion process requires the alpha-soluble N-ethylmaleimide-sensitive factor adaptor protein receptor SNAP23, which is also involved in the insulin-dependent translocation of a glucose transporter to the plasma membrane. Recent data suggest that SNAP23 is the link between increased lipid droplet accumulation and development of insulin resistance. Lipid droplets also form tight interactions with other organelles. Furthermore, additional lipid droplet-associated proteins have been identified and shown to play a role in droplet assembly and turnover, and in sorting and trafficking events.

SUMMARY

Recent studies have identified a number of key proteins that are involved in the formation and turnover of lipid droplets, and SNAP23 has been identified as a link between accumulation of lipid droplets and development of insulin resistance. Further understanding of lipid droplet biology could indicate potential therapeutic targets to prevent accumulation of lipid droplets and associated complications.

Molecular mechanisms and therapeutic targets in steatosis and steatohepatitis

Steatosis of the liver may arise from a variety of conditions, but the molecular basis for lipid droplet formation is poorly understood. Although a certain amount of lipid storage may even be hepatoprotective, prolonged lipid storage can result in an activation of inflammatory reactions and loss of metabolic competency. Apart from drug-induced steatosis, certain metabolic disorders associated with obesity, insulin resistance, and hyperlipidemia give also rise to nonalcoholic fatty liver diseases (NAFLD). It is noteworthy that advanced stages of nonalcoholic hepatic steatosis and steatohepatitis (NASH) result ultimately in fibrosis and cirrhosis. In this regard, the lipid droplets (LDs) have been discovered to be metabolically highly active structures that play major roles in lipid transport, sorting, and signaling cascades. In particular, LDs maintain a dynamic communication with the endoplasmic reticulum (ER) and the plasma membrane via sphingolipid-enriched domains of the plasma membrane-the lipid rafts. These microdomains frequently harbor receptor tyrosine kinases and other signaling molecules and connect extracellular events with intracellular signaling cascades. Here, we review recent knowledge on the molecular mechanisms of drug and metabolically induced hepatic steatosis and its progression to steatohepatitis (NASH). The contribution of cytokines and other signaling molecules, as well as activity of nuclear receptors, lipids, transcription factors, and endocrine mediators toward cellular dysfunction and progression of steatotic liver disease to NASH is specifically addressed, as is the cross-talk of different cell types in the pathogenesis of NAFLD. Furthermore, we provide an overview of recent therapeutic approaches in NASH therapy and discuss new as well as putative targets for pharmacological interventions.

Fusion of oil bodies in endosperm of oat grains

Few microscopical studies have been made on lipid storage in oat grains, with variable results as to the extent of lipid accumulation in the starchy endosperm. Grains of medium- and high-lipid oat (Avena sativa L.) were studied at two developmental stages and at maturity, by light microscopy using different staining methods, and by scanning and transmission electron microscopy. Discrete oil bodies occurred in the aleurone layer, scutellum and embryo. In contrast, oil bodies in the starchy endosperm often had diffuse boundaries and fused with each other and with protein vacuoles during grain development, forming a continuous oil matrix between the protein and starch components. The different microscopical methods were confirmative to each other regarding the coalescence of oil bodies, a phenomenon probably correlated with the reduced amount of oil-body associated proteins in the endosperm. This was supported experimentally by SDS-PAGE separation of oil-body proteins and immunoblotting and immunolocalization with antibodies against a 16 kD oil-body protein. Much more oil-body proteins per amount of oil occurred in the embryo and scutellum than in the endosperm. Immunolocalization of 14 and 16 kD oil-body associated proteins on sectioned grains resulted in more heavy labeling of the embryo, scutellum and aleurone layer than the rest of the endosperm. Observations on the appearance of oil bodies at an early stage of development pertain to the prevailing hypotheses of oil-body biogenesis.

Lipid droplets as dynamic organelles connecting storage and efflux of lipids

Neutral lipids are stored in the cytosol in so-called lipid droplets. These are dynamic organelles with neutral lipids as the core surrounded by a monolayer of amphipathic lipids (phospholipids and cholesterol) and specific proteins (PAT proteins and proteins involved in the turnover of lipids and in the formation and trafficking of the droplets). Lipid droplets are formed at microsomal membranes as primordial droplets with a diameter of 0.1-0.4 microm and increase in size by fusion. In this article, we review the assembly and fusion of lipid droplets, and the processes involved in the secretion of triglycerides. Triglycerides are secreted from cells by two principally different processes. In the mammary gland, lipid droplets interact with specific regions of the plasma membrane and bud off with an envelope consisting of the membrane, to form milk globules. In the liver and intestine, very low-density lipoproteins (VLDL) and chylomicrons are secreted by using the secretory pathway of the cell. Finally, we briefly review the importance of lipid droplets in the development of insulin resistance and atherosclerosis.

Lipid droplet-organelle interactions; sharing the fats

Lipid droplets (LDs) are key cellular organelles involved in lipid storage and mobilisation. While the major signalling cascades and many of the regulators of lipolysis have been identified, the cellular interactions involved in lipid mobilisation and release remain largely undefined. In non-adipocytes, LDs are small, mobile and interact with other cellular compartments. In contrast, adipocytes primarily contain very large, immotile LDs. The striking morphological differences between LDs in adipocytes and non-adipocytes suggest that key differences must exist in the manner in which LDs in different cell types interact with other organelles. Recent studies have highlighted the complexity of LD interactions, which can be both homotypic, with each other, and heterotypic, with other organelles. The molecules involved in these interactions are also now emerging, including Rab proteins, key regulators of membrane traffic, and caveolin, an integral membrane protein providing a functional link between the cell surface and LDs. Here we summarise recent insights into the cell biology of the LD particularly focussing on the homotypic and heterotypic interactions in both adipocytes and non-adipocytes. We speculate that these interactions may involve inter-organelle membrane contact sites or a hemi-fusion type mechanism to facilitate lipid transfer.

Storage reserve accumulation in Arabidopsis: metabolic and developmental control of seed filling

In the life cycle of higher plants, seed development is a key process connecting two distinct sporophytic generations. Seed development can be divided into embryo morphogenesis and seed maturation. An essential metabolic function of maturing seeds is the deposition of storage compounds that are mobilised to fuel post-germinative seedling growth. Given the importance of seeds for food and animal feed and considering the tremendous interest in using seed storage products as sustainable industrial feedstocks to replace diminishing fossil reserves, understanding the metabolic and developmental control of seed filling constitutes a major focus of plant research. Arabidopsis thaliana is an oilseed species closely related to the agronomically important Brassica oilseed crops. The main storage compounds accumulated in seeds of A. thaliana consist of oil stored as triacylglycerols (TAGs) and seed storage proteins (SSPs). Extensive tools developed for the molecular dissection of A. thaliana development and metabolism together with analytical and cytological procedures adapted for very small seeds have led to a good description of the biochemical pathways producing storage compounds. In recent years, studies using these tools have shed new light on the intricate regulatory network controlling the seed maturation process. This network involves sugar and hormone signalling together with a set of developmentally regulated transcription factors. Although much remains to be elucidated, the framework of the regulatory system controlling seed filling is coming into focus.

Group IVA phospholipase A2 is necessary for the biogenesis of lipid droplets

Lipid droplets (LD) are organelles present in all cell types, consisting of a hydrophobic core of triacylglycerols and cholesteryl esters, surrounded by a monolayer of phospholipids and cholesterol. This work shows that LD biogenesis induced by serum, by long-chain fatty acids, or the combination of both in CHO-K1 cells was prevented by phospholipase A(2) inhibitors with a pharmacological profile consistent with the implication of group IVA cytosolic phospholipase A(2) (cPLA(2)alpha). Knocking down cPLA(2)alpha expression with short interfering RNA was similar to pharmacological inhibition in terms of enzyme activity and LD biogenesis. A Chinese hamster ovary cell clone stably expressing an enhanced green fluorescent protein-cPLA(2)alpha fusion protein (EGFP-cPLA(2)) displayed higher LD occurrence under basal conditions and upon LD induction. Induction of LD took place with concurrent phosphorylation of cPLA(2)alpha at Ser(505). Transfection of a S505A mutant cPLA(2)alpha showed that phosphorylation at Ser(505) is key for enzyme activity and LD formation. cPLA(2)alpha contribution to LD biogenesis was not because of the generation of arachidonic acid, nor was it related to neutral lipid synthesis. cPLA(2)alpha inhibition in cells induced to form LD resulted in the appearance of tubulo-vesicular profiles of the smooth endoplasmic reticulum, compatible with a role of cPLA(2)alpha in the formation of nascent LD from the endoplasmic reticulum.

GFP-tagged proteins visualized by freeze-fracture immuno-electron microscopy: a new tool in cellular and molecular medicine

GFP-tagging is widely used as a molecular tool to localize and visualize the trafficking of proteins in cells but interpretation is frequently limited by the low resolution afforded by fluorescence light microscopy. Although complementary thin-section immunogold electron microscopic techniques go some way in aiding interpretation, major limitations, such as relatively poor structural preservation of membrane systems, low labelling efficiency and the two-dimensional nature of the images, remain. Here we demonstrate that the electron microscopic technique freeze-fracture replica immunogold labelling overcomes these disadvantages and can be used to define, at high resolution, the precise location of GFP-tagged proteins in specific membrane systems and organelles of the cell. Moreover, this technique provides information on the location of the protein within the phospholipid bilayer, potentially providing insight into mis-orientation of tagged proteins compared to their untagged counterparts. Complementary application of the freeze-fracture replica immunogold labelling technique alongside conventional fluorescence microscopy is seen as a novel and valuable approach to verification, clarification and extension of the data obtained using fluorescent-tagged proteins. The application of this approach is illustrated by new findings on PAT-family proteins tagged with GFP transfected into fibroblasts from patients with Niemann-Pick type C disease.

Cell biology of lipid droplets

Lipid storage has attracted much attention in the past years, both by the broader public and the biomedical scientific community. Driven by concerns about the obesity epidemic that affects most industrialized countries and even substantial parts of the population in less and least developed countries, work from researchers of many disciplines has shed light on the genetics, the physiology, and the cellular mechanisms of fat accumulation. This review focuses on the actual organelle of fat deposition, the lipid droplet (LD), and on the recent progress in mechanistic understanding of processes like LD biogenesis, LD growth and degradation, protein targeting to LDs and LD fusion.

Lipid droplets: a classic organelle with new outfits

Lipid droplets are depots of neutral lipids that exist virtually in any kind of cell. Recent studies have revealed that the lipid droplet is not a mere lipid blob, but a major contributor not only to lipid homeostasis but also to diverse cellular functions. Because of the unique structure as well as the functional importance in relation to obesity, steatosis, and other prevailing diseases, the lipid droplet is now reborn as a brand new organelle, attracting interests from researchers of many disciplines.

Hepatitis C virus core protein induces lipid droplet redistribution in a microtubule- and dynein-dependent manner

Attachment of hepatitis C virus (HCV) core protein to lipid droplets (LDs) is linked to release of infectious progeny from infected cells. Core progressively coats the entire LD surface from a unique site on the organelle, and this process coincides with LD aggregation around the nucleus. We demonstrate that LD redistribution requires only core protein and is accompanied by reduced abundance of adipocyte differentiation-related protein (ADRP) on LD surfaces. Using small hairpin RNA technology, we show that knock down of ADRP has a similar phenotypic effect on LD redistribution. Hence, ADRP is crucial to maintain a disperse intracellular distribution of LDs. From additional experimental evidence, LDs are associated with microtubules and aggregate principally around the microtubule-organizing centre in HCV-infected cells. Disrupting the microtubule network or microinjecting anti-dynein antibody prevented core-mediated LD redistribution. Moreover, microtubule disruption reduced virus titres, implicating transport networks in virus assembly and release. We propose that the presence of core on LDs favours their movement towards the nucleus, possibly to increase the probability of interaction between sites of HCV RNA replication and virion assembly.

Cellular cholesterol trafficking and compartmentalization

Cholesterol is an essential structural component in the cell membranes of most vertebrates. The biophysical properties of cholesterol and the enzymology of cholesterol metabolism provide the basis for how cells handle cholesterol and exchange it with one another. A tightly controlled--but only partially characterized--network of cellular signalling and lipid transfer systems orchestrates the functional compartmentalization of this lipid within and between organellar membranes. This largely dictates the exchange of cholesterol between tissues at the whole body level. Increased understanding of these processes and their integration at the organ systems level provides fundamental insights into the physiology of cholesterol trafficking.

In vitro formation of a novel type of membrane vesicles containing Dpm1p: putative transport vesicles for lipid droplets in budding yeast

A novel type of membrane vesicles was formed in vitro from microsomes of Saccharomyces cerevisiae, which carries Dpm1p, an enzyme involved in dolichol-sugar synthesis, but not a typical secretory cargo. While COPII vesicles formed in vitro were sedimentable by centrifugation at 200,000g(max) for 15 min, the novel vesicles were not. However, they were sedimented by additional centrifugation at the same speed for 1 h. Immunoelectron microscopy showed that the Dpm1p-containing vesicles had small vesicular/saccular structures of around 40-50 nm in diameter. The addition of glycerol-3-phosphate and oleoyl-CoA, substrates for lipid biosynthesis, significantly enhanced the efficiency of vesicle budding in an ATP-dependent fashion. Dpm1p was localized to lipid droplets as well as endoplasmic reticulum. Fluorescence microscopy further showed that Dpm1p-GFP was present in restricted subregions in isolated lipid droplets. The possibility that the vesicles were intermediates from the endoplasmic reticulum to lipid droplets was examined, and their possible role is discussed.

Hepatitis C virus core protein, lipid droplets and steatosis

Lipid droplets are intracellular organelles involved not only in lipid storage but also in cell signalling and the regulation of intracellular vesicular trafficking. Recent basic studies have suggested that interactions between hepatitis C virus (HCV) core protein and lipid droplets are required for the HCV infection cycle. In infected cells, the HCV core protein is associated with the surface of lipid droplets and the endoplasmic reticulum membranes closely surrounding these droplets, and its self-assembly drives virion budding. This interaction also seems to be directly linked to a virus-induced steatosis, which involves the deposition of triglycerides in the liver and contributes to the progression of fibrosis in patients with chronic hepatitis C. Many clinical studies have reported that virus-induced steatosis is significantly more severe with HCV genotype 3 than with other genotypes, and this phenomenon has been modelled in recent basic studies based on the production of HCV core proteins of various genotypes in vitro. The association of HCV core protein with lipid droplets seems to play a central role in HCV pathogenesis and morphogenesis, suggesting that virus-induced steatosis may be essential for the viral life cycle.

Lipid droplets in lipogenesis and lipolysis

Organisms store energy for later use during times of nutrient scarcity. Excess energy is stored as triacylglycerol in lipid droplets during lipogenesis. When energy is required, the stored triacylglycerol is hydrolyzed via activation of lipolytic pathways. The coordination of lipid storage and utilization is regulated by the perilipin family of lipid droplet coat proteins [perilipin, adipophilin/adipocyte differentiation-related protein (ADRP), S3-12, tail-interacting protein of 47 kilodaltons (TIP47), and myocardial lipid droplet protein (MLDP)/oxidative tissues-enriched PAT protein (OXPAT)/lipid storage droplet protein 5 (LSDP5)]. Lipid droplets are dynamic and heterogeneous in size, location, and protein content. The proteins that coat lipid droplets change during lipid droplet biogenesis and are dependent upon multiple factors, including tissue-specific expression and metabolic state (basal vs. lipogenic vs. lipolytic). New data suggest that proteins previously implicated in vesicle trafficking, including Rabs, soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNAREs), and motor and cytoskeletal proteins, likely orchestrate the movement and fusion of lipid droplets. Thus, rather than inert cytoplasmic inclusions, lipid droplets are now appreciated as dynamic organelles that are critical for management of cellular lipid stores. That much remains to be discovered is suggested by the recent identification of a novel lipase [adipocyte triglyceride lipase (ATGL)] and lipase regulator [Comparative Gene Identification-58 (CGI-58)], which has led to reconsideration of the decades-old model of lipolysis. Future discovery likely will be driven by the exploitation of model organisms and by human genetic studies.

Recent advances in freeze-fracture electron microscopy: the replica immunolabeling technique

Freeze-fracture electron microscopy is a technique for examining the ultrastructure of rapidly frozen biological samples by transmission electron microscopy. Of a range of approaches to freeze-fracture cytochemistry that have been developed and tried, the most successful is the technique termed freeze-fracture replica immunogold labeling (FRIL). In this technique, samples are frozen, fractured and replicated with platinum-carbon as in standard freeze fracture, and then carefully treated with sodium dodecylsulphate to remove all the biological material except a fine layer of molecules attached to the replica itself. Immunogold labeling of these molecules permits their distribution to be seen superimposed upon high resolution planar views of membrane structure. Examples of how this technique has contributed to our understanding of lipid droplet biogenesis and function are discussed.

Topology of molecular machines of the endoplasmic reticulum: a compilation of proteomics and cytological data

The endoplasmic reticulum (ER) is a key organelle of the secretion pathway involved in the synthesis of both proteins and lipids destined for multiple sites within and without the cell. The ER functions to both co- and post-translationally modify newly synthesized proteins and lipids and sort them for housekeeping within the ER and for transport to their sites of function away from the ER. In addition, the ER is involved in the metabolism and degradation of specific xenobiotics and endogenous biosynthetic products. A variety of proteomics studies have been reported on different subcompartments of the ER providing an ER protein dictionary with new data being made available on many protein complexes of relevance to the biology of the ER including the ribosome, the translocon, coatomer proteins, cytoskeletal proteins, folding proteins, the antigen-processing machinery, signaling proteins and proteins involved in membrane traffic. This review examines proteomics and cytological data in support of the presence of specific molecular machines at specific sites or subcompartments of the ER.

Freeze-fracture cytochemistry in cell biology

The term freeze-fracture cytochemistry embraces a series of techniques which share the goal of chemical identification of the structural components viewed in freeze-fracture replicas. As one of the major features of freeze fracture is its ability to provide planar views of membranes, a major emphasis in freeze-fracture cytochemistry is to identify integral membrane proteins, study their spatial organization in the membrane plane, and examine their role in dynamic cellular processes. Effective techniques in freeze-fracture cytochemistry, of wide application in cell biology, are now available. These include fracture-label, label fracture, and the freeze-fracture replica immunolabeling technique (FRIL). In fracture-label, samples are frozen and fractured, thawed for labeling, and finally processed for viewing either by critical-point drying and platinum-carbon replication or by thin-section electron microscopy. Label-fracture involves immunogold labeling a cell suspension, processing as for standard freeze-fracture replication, and then examining the replica without removal of the cellular components. Of greatest versatility, however, is the FRIL technique, in which samples are frozen, fractured, and replicated with platinum-carbon as in standard freeze fracture, and then carefully treated with sodium dodecylsulphate (SDS) to remove all the biological material except a fine layer of molecules attached to the replica itself. Immunogold labeling of these molecules permits the distribution of identified components to be viewed superimposed upon high resolution planar views of replicated membrane structure, for both the plasma membrane and intracellular membranes in cells and tissues. Examples of how these techniques have contributed to our understanding of cardiovascular cell function in health and disease are discussed.

Engineering plant oils as high-value industrial feedstocks for biorefining: the need for underpinning cell biology research

Plant oils represent renewable sources of long-chain hydrocarbons that can be used as both fuel and chemical feedstocks, and genetic engineering offers an opportunity to create further high-value specialty oils for specific industrial uses. While many genes have been identified for the production of industrially important fatty acids, expression of these genes in transgenic plants has routinely resulted in a low accumulation of the desired fatty acids, indicating that significantly more knowledge of seed oil production is required before any future rational engineering designs are attempted. Here, we provide an overview of the cellular features of fatty acid desaturases, the so-called diverged desaturases, and diacylglycerol acyltransferases, three sets of enzymes that play a central role in determining the types and amounts of fatty acids that are present in seed oil, and as such, the final application and value of the oil. Recent studies of the intracellular trafficking, assembly and regulation of these enzymes have provided new insights to the mechanisms of storage oil production, and suggest that the compartmentalization of enzyme activities within specific regions or subdomains of the ER may be essential for both the synthesis of novel fatty acid structures and the channeling of these important fatty acids into seed storage oils.

Evolutionarily conserved gene family important for fat storage

The ability to store fat in the form of cytoplasmic triglyceride droplets is conserved from Saccharomyces cerevisiae to humans. Although much is known regarding the composition and catabolism of lipid droplets, the molecular components necessary for the biogenesis of lipid droplets have remained obscure. Here we report the characterization of a conserved gene family important for lipid droplet formation named fat-inducing transcript (FIT). FIT1 and FIT2 are endoplasmic reticulum resident membrane proteins that induce lipid droplet accumulation in cell culture and when expressed in mouse liver. shRNA silencing of FIT2 in 3T3-LI adipocytes prevents accumulation of lipid droplets, and depletion of FIT2 in zebrafish blocks diet-induced accumulation of lipid droplets in the intestine and liver, highlighting an important role for FIT2 in lipid droplet formation in vivo. Together these studies identify and characterize a conserved gene family that is important in the fundamental process of storing fat.

Milk lipoprotein membranes and their imperative enzymes

There are two main sources of lipoprotein membranes in milk: the relatively well-defined milk fat globule membrane (MFGM) that covers the milk fat globules, and the much less attended lipoprotein source, in the form of vesicles floating in the milk serum. We challenge the common view that the milk serum lipoprotein membrane (MSLM) is secondly derived from the MFGM and present a different view suggesting that it represents Golgi-derived vesicles that are released intact to milk. The potential role of enzymes attached to the MSLM and MFGM is considered in detail for select ubiquitously expressed enzymes.

The lipodystrophy protein seipin is found at endoplasmic reticulum lipid droplet junctions and is important for droplet morphology

Lipodystrophy is a disorder characterized by a loss of adipose tissue often accompanied by severe hypertriglyceridemia, insulin resistance, diabetes, and fatty liver. It can be inherited or acquired. The most severe inherited form is Berardinelli-Seip Congenital Lipodystrophy Type 2, associated with mutations in the BSCL2 gene. BSCL2 encodes seipin, the function of which has been entirely unknown. We now report the identification of yeast BSCL2/seipin through a screen to detect genes important for lipid droplet morphology. The absence of yeast seipin results in irregular lipid droplets often clustered alongside proliferated endoplasmic reticulum (ER); giant lipid droplets are also seen. Many small irregular lipid droplets are also apparent in fibroblasts from a BSCL2 patient. Human seipin can functionally replace yeast seipin, but a missense mutation in human seipin that causes lipodystrophy, or corresponding mutations in the yeast gene, render them unable to complement. Yeast seipin is localized in the ER, where it forms puncta. Almost all lipid droplets appear to be on the ER, and seipin is found at these junctions. Therefore, we hypothesize that seipin is important for droplet maintenance and perhaps assembly. In addition to detecting seipin, the screen identified 58 other genes whose deletions cause aberrant lipid droplets, including 2 genes encoding proteins known to activate lipin, a lipodystrophy locus in mice, and 16 other genes that are involved in endosomal-lysosomal trafficking. The genes identified in our screen should be of value in understanding the pathway of lipid droplet biogenesis and maintenance and the cause of some lipodystrophies.