SNARE proteins mediate fusion between cytosolic lipid droplets and are implicated in insulin sensitivity

Models of lipid droplets growth and fission in adipocyte cells

Lipid droplets (LD) are spherical cellular inclusion devoted to lipids storage. It is well known that excessive accumulation of lipids leads to several human worldwide diseases like obesity, type 2 diabetes, hepatic steatosis and atherosclerosis. LDs' size range from fraction to one hundred of micrometers in adipocytes and is related to the lipid content, but their growth is still a puzzling question. It has been suggested that LDs can grow in size due to the fusion process by which a larger LD is obtained by the merging of two smaller LDs, but these events seems to be rare and difficult to be observed. Many other processes are thought to be involved in the number and growth of LDs, like the de novo formation and the growth through additional neutral lipid deposition in pre-existing droplets. Moreover the number and size of LDs are influenced by the catabolism and the absorption or interaction with other organelles. The comprehension of these processes could help in the confinement of the pathologies related to lipid accumulation. In this study the LDs' size distribution, number and the total volume of immature (n=12), mature (n=12, 10-days differentiated) and lipolytic (n=12) 3T3-L1 adipocytes were considered. More than 11,000 LDs were measured in the 36 cells after Oil Red O staining. In a previous work Monte Carlo simulations were used to mimic the fusion process alone between LDs. We found that, considering the fusion as the only process acting on the LDs, the size distribution in mature adipocytes can be obtained with numerical simulation starting from the size distribution in immature cells provided a very high rate of fusion events. In this paper Monte Carlo simulations were developed to mimic the interaction between LDs taking into account many other processes in addition to fusion (de novo formation and the growth through additional neutral lipid deposition in pre-existing droplets) in order to reproduce the LDs growth and we also simulated the catabolism (fission and the decrease through neutral lipid exit from pre-existing droplets) to reproduce their size reduction observed in lipolytic conditions. The results suggest that each single process, considered alone, can not be considered the only responsible for the size variation observed, but more than one of them, playing together, can quite well reproduce the experimental data.

The Causative Gene in Chanarian Dorfman Syndrome Regulates Lipid Droplet Homeostasis in C. elegans

AMP-activated kinase (AMPK) is a key regulator of many cellular mechanisms required for adjustment to various stresses induced by the changing environment. In C. elegans dauer larvae AMPK-null mutants expire prematurely due to hyperactive Adipose Triglyceride Lipase (ATGL-1) followed by rapid depletion of triglyceride stores. We found that the compromise of one of the three C. elegans orthologues of human cgi-58 significantly improves the survival of AMPK-deficient dauers. We also provide evidence that C. elegans CGI-58 acts as a co-activator of ATGL-1, while it also functions cooperatively to maintain regular lipid droplet structure. Surprisingly, we show that it also acts independently of ATGL-1 to restrict lipid droplet coalescence by altering the surface abundance and composition of long chain (C20) polyunsaturated fatty acids (PUFAs). Our data reveal a novel structural role of CGI-58 in maintaining lipid droplet homeostasis through its effects on droplet composition, morphology and lipid hydrolysis; a conserved function that may account for some of the ATGL-1-independent features unique to Chanarin-Dorfman Syndrome.

Chanarin-Dorfman Syndrome (CDS) is a rare metabolic disease characterized by an abnormal accumulation of lipids in various tissues and organs due to a failure in lipid breakdown. Characteristic clinical features exhibited by affected patients include scaly skin (ichthyosis), enlarged liver, blurred vision among others. CDS is caused by mutation of the cgi-58 gene, which is essential for lipid breakdown, but may also have additional cellular functions. Here, we demonstrate that in C. elegans CGI-58 acts both as a key player in lipid breakdown, but it is also required to maintain the barrier that defines the size, shape and catalytic efficacy of the major lipid storage site-the lipid droplets. We provide a genetically tractable animal model of CDS that reproduces many of the defects observed in affected CDS individuals.

Role of Rab GTPases and their interacting proteins in mediating metabolic signalling and regulation

The vesicular transport pathways, which shuttle materials to and from the cell surface and within the cell, and the metabolic (growth factor and nutrient) signalling pathways, which integrate a variety of extracellular and intracellular signals to mediate growth, proliferation or survival, are both important for cellular physiology. There is evidence to suggest that the transport and metabolic signalling pathways intersect-vesicular transport can affect the regulation of metabolic signals and vice versa. The Rab family GTPases regulate the specificity of vesicular transport steps in the cell. Together with their interacting proteins, Rabs would likely constitute the points of intersection between vesicular transport and metabolic signalling pathways. Examples of these points would include growth factor signalling, glucose and lipid metabolism, as well as autophagy. Many of these processes involve mechanistic/mammalian target of rapamycin (mTOR) complex 1 (mTORC1) in downstream cascades, or are regulated by TORC signalling. A general functionality of the vesicular transport processes controlled by the Rabs is also important for spatial and temporal regulation of the transmission of metabolic signals between the cell surface and the nucleus. In other cases, specific Rabs and their interacting proteins are known to function in recruiting metabolism-related proteins to target membranes, or may compete with other factors in the TORC signalling pathway as a means of metabolic regulation. We review and discuss herein examples of how Rabs and their interacting proteins can mediate metabolic signalling and regulation in cells.

N-terminus of seed caleosins is essential for lipid droplet sorting but not for lipid accumulation

Caleosin, a calcium-binding protein associated with plant lipid droplets, stimulates lipid accumulation when heterologously expressed in Saccharomyces cerevisiae. Accumulated lipids are stored in cytoplasmic lipid droplets that are stabilised by incorporated caleosin. We designed a set of mutants affecting putative crucial sites for caleosin function and association with lipid droplets, i.e. the N-terminus, the EF-hand motif and the proline-knot motif. We investigated the effect of introduced mutations on caleosin capacity to initiate lipid accumulation and on caleosin sorting within cell as well as on its association with lipid droplets. Our results strongly suggest that the N-terminal domain is essential for proper protein sorting and targeting to lipid droplets but not for enhancing lipid accumulation.

Regulation of lipid droplet dynamics in Saccharomyces cerevisiae depends on the Rab7-like Ypt7p, HOPS complex and V1-ATPase

It has now been clearly shown that lipid droplets (LDs) play a dynamic role in the cell. This was reinforced by LD proteomics which suggest that a significant number of trafficking proteins are associated with this organelle. Using microscopy, we showed that LDs partly co-localize with the vacuole in S. cerevisiae. Immunoblot experiments confirmed the association of the vacuolar Rab GTPase Rab7-like Ypt7p with LDs. We observed an increase in fatty acid content and LD number in ypt7Δ mutant and also changes in LD morphology and intra LD fusions, revealing a direct role for Ypt7p in LD dynamics. Using co-immunoprecipitation, we isolated potential Ypt7p partners including, Vma13p, the H subunit of the V1 part of the vacuolar (H+) ATPase (V-ATPase). Deletion of the VMA13 gene, as well as deletion of three other subunits of the V1 part of the V-ATPase, also increased the cell fatty acid content and LD number. Mutants of the Homotypic fusion and vacuole protein sorting (HOPS) complex showed similar phenotypes. Here, we demonstrated that LD dynamics and membrane trafficking between the vacuole and LDs are regulated by the Rab7-like Ypt7p and are impaired when the HOPS complex and the V1 domain of the V-ATPase are defective.

The exocyst complex regulates free fatty acid uptake by adipocytes

The exocyst is an octameric molecular complex that drives vesicle trafficking in adipocytes, a rate-limiting step in insulin-dependent glucose uptake. This study assessed the role of the exocyst complex in regulating free fatty acid (FFA) uptake by adipocytes. Upon differentiating into adipocytes, 3T3-L1 cells acquire the ability to incorporate extracellular FFAs in an insulin-dependent manner. A kinetic assay using fluoresceinated FFA (C12 dodecanoic acid) uptake allows the real-time monitoring of FFA internalization by adipocytes. The insulin-dependent uptake of C12 dodecanoic acid by 3T3-L1 adipocytes is mediated by Akt and phosphatidylinositol 3 (PI3)-kinase. Gene silencing of the exocyst components Exo70 and Sec8 significantly reduced insulin-dependent FFA uptake by adipocytes. Consistent with the roles played by Exo70 and Sec8 in FFA uptake, mCherry-tagged Exo70 and HA-tagged Sec8 partially colocalize with lipid droplets within adipocytes, suggesting their active roles in the development of lipid droplets. Tubulin polymerization was also found to regulate FFA uptake in collaboration with the exocyst complex. This study demonstrates a novel role played by the exocyst complex in the regulation of FFA uptake by adipocytes.

Protein kinase STK25 regulates hepatic lipid partitioning and progression of liver steatosis and NASH

Nonalcoholic fatty liver disease (NAFLD) is the most common form of liver disease, and 10% to 20% of NAFLD patients progress to nonalcoholic steatohepatitis (NASH). The molecular pathways controlling progression to NAFLD/NASH remain poorly understood. We recently identified serine/threonine protein kinase 25 (STK25) as a regulator of whole-body insulin and glucose homeostasis. This study investigates the role of STK25 in liver lipid accumulation and NASH. Stk25 transgenic mice challenged with a high-fat diet displayed a dramatic increase in liver steatosis and hepatic insulin resistance compared to wild-type siblings. Focal fibrosis, hepatocellular damage, and inflammation were readily seen in transgenic but not wild-type livers. Transgenic livers displayed reduced β-oxidation and triacylglycerol secretion, while lipid uptake and synthesis remained unchanged. STK25 was associated with lipid droplets, colocalizing with the main hepatic lipid droplet-coating protein adipose differentiation-related protein, the level of which was increased 3.8 ± 0.7-fold in transgenic livers (P < 0.01), while a key hepatic lipase, adipose triacylglycerol lipase, was translocated from the lipid droplets surface to the cytoplasm, providing the likely mechanism underlying the effect of STK25. In summary, STK25 is a lipid droplet-associated protein that promotes NAFLD through control of lipid release from the droplets for β-oxidation and triacylglycerol secretion. STK25 also drives pathogenesis of NASH.

Dynamics of the lipid droplet proteome of the Oleaginous yeast rhodosporidium toruloides

Lipid droplets (LDs) are ubiquitous organelles that serve as a neutral lipid reservoir and a hub for lipid metabolism. Manipulating LD formation, evolution, and mobilization in oleaginous species may lead to the production of fatty acid-derived biofuels and chemicals. However, key factors regulating LD dynamics remain poorly characterized. Here we purified the LDs and identified LD-associated proteins from cells of the lipid-producing yeast Rhodosporidium toruloides cultured under nutrient-rich, nitrogen-limited, and phosphorus-limited conditions. The LD proteome consisted of 226 proteins, many of which are involved in lipid metabolism and LD formation and evolution. Further analysis of our previous comparative transcriptome and proteome data sets indicated that the transcription level of 85 genes and protein abundance of 77 proteins changed under nutrient-limited conditions. Such changes were highly relevant to lipid accumulation and partially confirmed by reverse transcription-quantitative PCR. We demonstrated that the major LD structure protein Ldp1 is an LD marker protein being upregulated in lipid-rich cells. When overexpressed in Saccharomyces cerevisiae, Ldp1 localized on the LD surface and facilitated giant LD formation, suggesting that Ldp1 plays an important role in controlling LD dynamics. Our results significantly advance the understanding of the molecular basis of lipid overproduction and storage in oleaginous yeasts and will be valuable for the development of superior lipid producers.

Automated Image Processing for Spatially Resolved Analysis of Lipid Droplets in Cultured 3T3-L1 Adipocytes

Cellular hypertrophy of adipose tissue underlies many of the proposed proinflammatory mechanisms for obesity-related diseases. Adipose hypertrophy results from an accumulation of esterified lipids (triglycerides) into membrane-enclosed intracellular lipid droplets (LDs). The coupling between adipocyte metabolism and LD morphology could be exploited to investigate biochemical regulation of lipid pathways by monitoring the dynamics of LDs. This article describes an image processing method to identify LDs based on several distinctive optical and morphological characteristics of these cellular bodies as they appear under bright-field. The algorithm was developed against images of 3T3-L1 preadipocyte cultures induced to differentiate into adipocytes. We show that the calculated lipid volumes are in excellent agreement with enzymatic assay data on total intracellular triglyceride content. We also demonstrate that the image processing method can efficiently characterize the highly heterogeneous spatial distribution of LDs in a culture by showing that differentiation occurs in distinct clusters separated by regions of nearly undifferentiated cells. Prospectively, the LD detection method described in this work could be applied to time-lapse data collected with simple visible light microscopy equipment to quantitatively investigate LD dynamics.

Spatiotemporal dynamics of triglyceride storage in unilocular adipocytes

Real-time fluorescence microscopy is used to investigate the trafficking of metabolizable fluorescent fatty acid in unilocular adipocytes from adipose tissue of nonhuman primates. The study reveals novel cell biological features that may contribute to the mechanism of adipocyte hypertrophy.

The spatiotemporal dynamics of triglyceride (TG) storage in unilocular adipocytes are not well understood. Here we applied ex vivo technology to study trafficking and metabolism of fluorescent fatty acids in adipose tissue explants. Live imaging revealed multiple cytoplasmic nodules surrounding the large central lipid droplet (cLD) of unilocular adipocytes. Each cytoplasmic nodule harbors a series of closely associated cellular organelles, including micro–lipid droplets (mLDs), mitochondria, and the endoplasmic reticulum. Exogenously added free fatty acids are rapidly adsorbed by mLDs and concurrently get esterified to TG. This process is greatly accelerated by insulin. mLDs transfer their content to the cLD, serving as intermediates that mediate packaging of newly synthesized TG in the large interior of a unilocular adipocyte. This study reveals novel cell biological features that may contribute to the mechanism of adipocyte hypertrophy.

Hepatitis C virus and lipid droplets: finding a niche

Hepatitis C virus (HCV) causes serious liver disease in chronically infected individuals. Infectious virions are released from hepatocytes as lipoprotein complexes, indicating that the virus interacts with very low density lipoprotein (VLDL) assembly to propagate. The primary source of lipid for incorporation into VLDL is cytoplasmic lipid droplets (LDs). This organelle is targeted by two virus-encoded proteins as part of a process essential for virion morphogenesis. Moreover, LDs regulate infection. A common condition in HCV-infected individuals is steatosis, characterized by an accumulation of LDs. The mechanisms underlying development of steatosis include direct effects of the virus on lipid metabolism. This review reveals new insights into HCV infection and a further twist to the growing list of functions performed by LDs.

ARFGAP1 is dynamically associated with lipid droplets in hepatocytes

The ARF GTPase Activating Protein 1 (ARFGAP1) associates mainly with the cytosolic side of Golgi cisternal membranes where it participates in the formation of both COPI and clathrin-coated vesicles. In this study, we show that ARFGAP1 associates transiently with lipid droplets upon addition of oleate in cultured cells. Also, that addition of cyclic AMP shifts ARFGAP1 from lipid droplets to the Golgi apparatus and that overexpression and knockdown of ARFGAP1 affect lipid droplet formation. Examination of human liver tissue reveals that ARFGAP1 is found associated with lipid droplets at steady state in some but not all hepatocytes.

Rab8a-AS160-MSS4 regulatory circuit controls lipid droplet fusion and growth

Rab GTPases, by targeting to specific membrane compartments, play essential roles in membrane trafficking. Lipid droplets (LDs) are dynamic subcellular organelles whose growth is closely linked to obesity and hepatic steatosis. Fsp27 is shown to be required for LD fusion and growth by enriching at LD-LD contact sites. Here, we identify Rab8a as a direct interactor and regulator of Fsp27 in mediating LD fusion in adipocytes. Knockdown of Rab8a in the livers of ob/ob mice results in the accumulation of smaller LDs and lower hepatic lipid levels. Surprisingly, it is the GDP-bound form of Rab8a that exhibits fusion-promoting activity. We further discover AS160 as the GTPase activating protein (GAP) for Rab8a, which forms a ternary complex with Fsp27 and Rab8a to positively regulate LD fusion. MSS4 antagonizes Fsp27-mediated LD fusion activity through Rab8a. Our results have thus revealed a mechanistic signaling circuit controlling LD fusion and fatty liver formation.

Mitochondrial proteases act on STARD3 to activate progesterone synthesis in human syncytiotrophoblast

BACKGROUND

STARD1 transports cholesterol into mitochondria of acutely regulated steroidogenic tissue. It has been suggested that STARD3 transports cholesterol in the human placenta, which does not express STARD1. STARD1 is proteolytically activated into a 30-kDa protein. However, the role of proteases in STARD3 modification in the human placenta has not been studied.

METHODS

Progesterone determination and Western blot using anti-STARD3 antibodies showed that mitochondrial proteases cleave STARD3 into a 28-kDa fragment that stimulates progesterone synthesis in isolated syncytiotrophoblast mitochondria. Protease inhibitors decrease STARD3 transformation and steroidogenesis.

RESULTS

STARD3 remained tightly bound to isolated syncytiotrophoblast mitochondria. Simultaneous to the increase in progesterone synthesis, STARD3 was proteolytically processed into four proteins, of which a 28-kDa protein was the most abundant. This protein stimulated mitochondrial progesterone production similarly to truncated-STARD3. Maximum levels of protease activity were observed at pH7.5 and were sensitive to 1,10-phenanthroline, which inhibited steroidogenesis and STARD3 proteolytic cleavage. Addition of 22(R)-hydroxycholesterol increased progesterone synthesis, even in the presence of 1,10-phenanthroline, suggesting that proteolytic products might be involved in mitochondrial cholesterol transport.

CONCLUSION

Metalloproteases from human placental mitochondria are involved in steroidogenesis through the proteolytic activation of STARD3. 1,10-Phenanthroline inhibits STARD3 proteolytic cleavage. The 28-kDa protein and the amino terminal truncated-STARD3 stimulate steroidogenesis in a comparable rate, suggesting that both proteins share similar properties, probably the START domain that is involved in cholesterol binding.

GENERAL SIGNIFICANCE

Mitochondrial proteases are involved in syncytiotrophoblast-cell steroidogenesis regulation. Understanding STARD3 activation and its role in progesterone synthesis is crucial to getting insight into its action mechanism in healthy and diseased syncytiotrophoblast cells.

Liver-restricted Repin1 deficiency improves whole-body insulin sensitivity, alters lipid metabolism, and causes secondary changes in adipose tissue in mice

Replication initiator 1 (Repin1) is a zinc finger protein highly expressed in liver and adipose tissue and maps within a quantitative trait locus (QTL) for body weight and triglyceride (TG) levels in the rat. The QTL has further been supported as a susceptibility locus for dyslipidemia and related metabolic disorders in congenic and subcongenic rat strains. Here, we elucidated the role of Repin1 in lipid metabolism in vivo. We generated a liver-specific Repin1 knockout mouse (LRep1(-/-)) and systematically characterized the consequences of Repin1 deficiency in the liver on body weight, glucose and lipid metabolism, liver lipid patterns, and protein/mRNA expression. Hyperinsulinemic-euglycemic clamp studies revealed significantly improved whole-body insulin sensitivity in LRep1(-/-) mice, which may be due to significantly lower TG content in the liver. Repin1 deficiency causes significant changes in potential downstream target molecules including Cd36, Pparγ, Glut2 protein, Akt phosphorylation, and lipocalin2, Vamp4, and Snap23 mRNA expression. Mice with hepatic deletion of Repin1 display secondary changes in adipose tissue function, which may be mediated by altered hepatic expression of lipocalin2 or chemerin. Our findings indicate that Repin1 plays a role in insulin sensitivity and lipid metabolism by regulating key genes of glucose and lipid metabolism.

Microorganism lipid droplets and biofuel development

Lipid droplet (LD) is a cellular organelle that stores neutral lipids as a source of energy and carbon. However, recent research has emerged that the organelle is involved in lipid synthesis, transportation, and metabolism, as well as mediating cellular protein storage and degradation. With the exception of multi-cellular organisms, some unicellular microorganisms have been observed to contain LDs. The organelle has been isolated and characterized from numerous organisms. Triacylglycerol (TAG) accumulation in LDs can be in excess of 50% of the dry weight in some microorganisms, and a maximum of 87% in some instances. These microorganisms include eukaryotes such as yeast and green algae as well as prokaryotes such as bacteria. Some organisms obtain carbon from CO2 via photosynthesis, while the majority utilizes carbon from various types of biomass. Therefore, high TAG content generated by utilizing waste or cheap biomass, coupled with an efficient conversion rate, present these organisms as bio-tech ‘factories’ to produce biodiesel. This review summarizes LD research in these organisms and provides useful information for further LD biological research and microorganism biodiesel development. [BMB Reports 2013; 46(12): 575-581]

Comparative proteomic study reveals 17β-HSD13 as a pathogenic protein in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is characterized by a massive accumulation of lipid droplets (LDs). The aim of this study was to determine the function of 17β-hydroxysteroid dehydrogenase-13 (17β-HSD13), one of our newly identified LD-associated proteins in human subjects with normal liver histology and simple steatosis, in NAFLD development. LDs were isolated from 21 human liver biopsies, including 9 cases with normal liver histology (group 1) and 12 cases with simple steatosis (group 2). A complete set of LD-associated proteins from three liver samples of group 1 or group 2 were determined by 2D LC-MS/MS. By comparing the LD-associated protein profiles between subjects with or without NAFLD, 54 up-regulated and 35 down-regulated LD-associated proteins were found in NAFLD patients. Among them, 17β-HSD13 represents a previously unidentified LD-associated protein with a significant up-regulation in NAFLD. Because the 17β-HSD family plays an important role in lipid metabolism, 17β-HSD13 was selected for validating the proteomic findings and exploring its role in the pathogenesis of NAFLD. Increased hepatic 17β-HSD13 and its LD surface location were confirmed in db/db (diabetic) and high-fat diet-fed mice. Adenovirus-mediated hepatic overexpression of human 17β-HSD13 induced a fatty liver phenotype in C57BL/6 mice, with a significant increase in mature sterol regulatory element-binding protein 1 and fatty acid synthase levels. The present study reports an extensive set of human liver LD proteins and an array of proteins differentially expressed in human NAFLD. We also identified 17β-HSD13 as a pathogenic protein in the development of NAFLD.

Automated identification of subcellular organelles by coherent anti-stokes Raman scattering

Coherent anti-Stokes Raman scattering (CARS) is an emerging tool for label-free characterization of living cells. Here, unsupervised multivariate analysis of CARS datasets was used to visualize the subcellular compartments. In addition, a supervised learning algorithm based on the "random forest" ensemble learning method as a classifier, was trained with CARS spectra using immunofluorescence images as a reference. The supervised classifier was then used, to our knowledge for the first time, to automatically identify lipid droplets, nucleus, nucleoli, and endoplasmic reticulum in datasets that are not used for training. These four subcellular components were simultaneously and label-free monitored instead of using several fluorescent labels. These results open new avenues for label-free time-resolved investigation of subcellular components in different cells, especially cancer cells.

Rab proteins implicated in lipid storage and mobilization

Abnormal intracellular accumulation or transport of lipids contributes greatly to the pathogenesis of human diseases. In the liver, excess accumulation of triacylglycerol (TG) leads to fatty liver disease encompassing steatosis, steatohepatitis and fibrosis. This places individuals at risk of developing cirrhosis, hepatocellular carcinoma or hepatic decompensation and also contributes to the emergence of insulin resistance and dyslipidemias affecting many other organs. Excessive accumulation of TG in adipose tissue contributes to insulin resistance as well as to the release of cytokines attracting leucocytes leading to a pro-inflammatory state. Pathological accumulation of cholesteryl ester (CE) in macrophages in the arterial wall is the progenitor of atherosclerotic plaques and heart disease. Overconsumption of dietary fat, cholesterol and carbohydrates explains why these diseases are on the increase yet offers few clues for how to prevent or treat individuals. Dietary regimes have proven futile and barring surgery, no realistic alternatives are at hand as effective drugs are few and not without side effects. Overweight and obesity-related diseases are no longer restricted to the developed world and as such, constitute a global problem. Development of new drugs and treatment strategies are a priority yet requires as a first step, elucidation of the molecular pathophysiology underlying each associated disease state. The lipid droplet (LD), an up to now overlooked intracellular organelle, appears at the heart of each pathophysiology linking key regulatory and metabolic processes as well as constituting the site of storage of both TGs and CEs. As the molecular machinery and mechanisms of LDs of each cell type are being elucidated, regulatory proteins used to control various cellular processes are emerging. Of these and the subject of this review, small GTPases belonging to the Rab protein family appear as important molecular switches used in the regulation of the intracellular trafficking and storage of lipids.

Recent insights into the molecular pathophysiology of lipid droplet formation in hepatocytes

Triacyglycerols are a major energy reserve of the body and are normally stored in adipose tissue as lipid droplets (LDs). The liver, however, stores energy as glycogen and digested triglycerides in the form of fatty acids. In stressed condition such as obesity, imbalanced nutrition and drug induced liver injury hepatocytes accumulate excess lipids in the form of LDs whose prolonged storage leads to disease conditions most notably non-alcoholic fatty liver disease (NAFLD). Fatty liver disease has become a major health burden with more than 90% of obese, nearly 70% of overweight and about 25% of normal weight patients being affected. Notably, research in recent years has shown LD as highly dynamic organelles for maintaining lipid homeostasis through fat storage, protein sorting and other molecular events studied in adipocytes and other cells of living organisms. This review focuses on the molecular events of LD formation in hepatocytes and the importance of cross talk between different cell types and their signalling in NAFLD as to provide a perspective on molecular mechanisms as well as possibilities for different therapeutic intervention strategies.

Milk secretion: The role of SNARE proteins

During lactation, polarized mammary epithelial secretory cells (MESCs) secrete huge quantities of the nutrient molecules that make up milk, i.e. proteins, fat globules and soluble components such as lactose and minerals. Some of these nutrients are only produced by the MESCs themselves, while others are to a great extent transferred from the blood. MESCs can thus be seen as a crossroads for both the uptake and the secretion with cross-talks between intracellular compartments that enable spatial and temporal coordination of the secretion of the milk constituents. Although the physiology of lactation is well understood, the molecular mechanisms underlying the secretion of milk components remain incompletely characterized. Major milk proteins, namely caseins, are secreted by exocytosis, while the milk fat globules are released by budding, being enwrapped by the apical plasma membrane. Prolactin, which stimulates the transcription of casein genes, also induces the production of arachidonic acid, leading to accelerated casein transport and/or secretion. Because of their ability to form complexes that bridge two membranes and promote their fusion, SNARE (Soluble N-ethylmaleimide-Sensitive Factor Attachment Protein Receptor) proteins are involved in almost all intracellular trafficking steps and exocytosis. As SNAREs can bind arachidonic acid, they could be the effectors of the secretagogue effect of prolactin in MESCs. Indeed, some SNAREs have been observed between secretory vesicles and lipid droplets suggesting that these proteins could not only orchestrate the intracellular trafficking of milk components but also act as key regulators for both the coupling and coordination of milk product secretion in response to hormones.

Controlled particle collision leads to direct observation of docking and fusion of lipid droplets in an optical trap

As an intracellular organelle, phospholipid-coated lipid droplets have shown increasing importance due to their expanding biological functions other than the lipid storage. The growing biological significance necessitates a close scrutiny on lipid droplets, which have been proposed to mature in a cell through processes such as fusion. Unlike phospholipid vesicles that are well-known to fuse through docking and hemifusion steps, little is known on the fusion of lipid droplets. Herein, we used laser tweezers to capture two micrometer-sized 1,2,3-trioleoylglycerol (triolein) droplets coated with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) that closely resemble intracellular lipid droplets. We started the fusion processes by a well-controlled collision between the two lipid droplets in phosphate buffer at pH 7.4. By monitoring the change in the pathway of a trapping laser that captures the collided lipid droplets, docking and physical fusion events were clearly distinguished for the first time and their lifetimes were determined with a resolution of 10 μs after postsynchronization analysis. Our method revealed that the rate-limiting docking process is affected by anions according to a Hofmeister series, which sheds light on the important role of interfacial water shedding during the process. During the physical fusion, the kinetics between bare triolein droplets is faster than lipid droplets, suggesting that breaking of phospholipid coating is involved in the process. This scenario was further supported by direct observation of a short-lived hemifusion state with ∼46 ms lifetime in POPC-coated lipid droplets, but not in bare triolein droplets.

Endothelial fatty acid transport: role of vascular endothelial growth factor B

Dietary lipids present in the circulation have to be transported through the vascular endothelium to be utilized by tissue cells, a vital mechanism that is still poorly understood. Vascular endothelial growth factor B (VEGF-B) regulates this process by controlling the expression of endothelial fatty acid transporter proteins (FATPs). Here, we summarize research on the role of the vascular endothelium in nutrient transport, with emphasis on VEGF-B signaling.

Refurbishing the plasmodesmal chamber: a role for lipid bodies?

Lipid bodies (LBs) are universal constituents of both animal and plant cells. They are produced by specialized membrane domains at the tubular endoplasmic reticulum (ER), and consist of a core of neutral lipids and a surrounding monolayer of phospholipid with embedded amphipathic proteins. Although originally regarded as simple depots for lipids, they have recently emerged as organelles that interact with other cellular constituents, exchanging lipids, proteins and signaling molecules, and shuttling them between various intracellular destinations, including the plasmamembrane (PM). Recent data showed that in plants LBs can deliver a subset of 1,3-β-glucanases to the plasmodesmal (PD) channel. We hypothesize that this may represent a more general mechanism, which complements the delivery of glycosylphosphatidylinositol (GPI)-anchored proteins to the PD exterior via the secretory pathway. We propose that LBs may contribute to the maintenance of the PD chamber and the delivery of regulatory molecules as well as proteins destined for transport to adjacent cells. In addition, we speculate that LBs deliver their cargo through interaction with membrane domains in the cytofacial side of the PM.

Integrated omics study delineates the dynamics of lipid droplets in Rhodococcus opacus PD630

Rhodococcus opacus strain PD630 (R. opacus PD630), is an oleaginous bacterium, and also is one of few prokaryotic organisms that contain lipid droplets (LDs). LD is an important organelle for lipid storage but also intercellular communication regarding energy metabolism, and yet is a poorly understood cellular organelle. To understand the dynamics of LD using a simple model organism, we conducted a series of comprehensive omics studies of R. opacus PD630 including complete genome, transcriptome and proteome analysis. The genome of R. opacus PD630 encodes 8947 genes that are significantly enriched in the lipid transport, synthesis and metabolic, indicating a super ability of carbon source biosynthesis and catabolism. The comparative transcriptome analysis from three culture conditions revealed the landscape of gene-altered expressions responsible for lipid accumulation. The LD proteomes further identified the proteins that mediate lipid synthesis, storage and other biological functions. Integrating these three omics uncovered 177 proteins that may be involved in lipid metabolism and LD dynamics. A LD structure-like protein LPD06283 was further verified to affect the LD morphology. Our omics studies provide not only a first integrated omics study of prokaryotic LD organelle, but also a systematic platform for facilitating further prokaryotic LD research and biofuel development.

Rab GTPases associate with isolated lipid droplets (LDs) and show altered content after ethanol administration: potential role in alcohol-impaired LD metabolism

BACKGROUND

Alcoholic liver disease is manifested by the presence of fatty liver, primarily due to accumulation of hepatocellular lipid droplets (LDs). The presence of membrane-trafficking proteins (e.g., Rab GTPases) with LDs indicates that LDs may be involved in trafficking pathways known to be altered in ethanol (EtOH) damaged hepatocytes. As these Rab GTPases are crucial regulators of protein trafficking, we examined the effect EtOH administration has on hepatic Rab protein content and association with LDs.

METHODS

Male Wistar rats were pair-fed Lieber-DeCarli diets for 5 to 8 weeks. Whole liver and isolated LD fractions were analyzed. Identification of LDs and associated Rab proteins was performed in frozen liver or paraffin-embedded sections followed by immunohistochemical analysis.

RESULTS

Lipid accumulation was characterized by larger LD vacuoles and increased total triglyceride content in EtOH-fed rats. Rabs 1, 2, 3d, 5, 7, and 18 were analyzed in postnuclear supernatant (PNS) as well as LDs. All of the Rabs were found in the PNS, and Rabs 1, 2, 5, and 7 did not show alcohol-altered content, while Rab 3d content was reduced by over 80%, and Rab 18 also showed EtOH-induced reduction in content. Rab 3d was not found to associate with LDs, while all other Rabs were found in the LD fractions, and several showed an EtOH-related decrease (Rabs 2, 5, 7, 18). Immunohistochemical analysis revealed the enhanced content of a LD-associated protein, perilipin 2 (PLIN2) that was paralleled with an associated decrease of Rab 18 in EtOH-fed rat sections.

CONCLUSIONS

Chronic EtOH feeding was associated with increased PLIN2 and altered Rab GTPase content in enriched LD fractions. Although mechanisms driving these changes are not established, further studies on intracellular protein trafficking and LD biology after alcohol administration will likely contribute to our understanding of fatty liver disease.

Long-term live cell microscopy studies of lipid droplet fusion dynamics in adipocytes

During the adipogenic differentiation process of mesenchymal stem cells, lipid droplets (LDs) grow slowly by transferring lipids between each other. Recent findings hint at the possibility that a fusion pore is involved. In this study, we analyze lipid transfer data obtained in long-term label-free microscopy studies in the framework of a Hagen-Poiseuille model. The data obtained show a LD fusion process in which the lipid transfer directionality depends on the size difference between LDs, whereas the respective rates depend on the size difference and additionally on the diameter of the smaller LDs. For the data analysis, the viscosity of the transferred material has to be known. We demonstrate that a viscosity-dependent molecular rotor dye can be used to measure LD viscosities in live cells. On this basis, we calculate the diameter of a putative lipid transfer channel which appears to have a direct dependence on the diameter of the smaller of the two participating LDs.

γ-synuclein is a novel player in the control of body lipid metabolism

Synucleins are a family of homologous, predominantly neuronal proteins known for their involvement in synaptic transmission and neurodegeneration. γ-synuclein is predominantly localized in axons and presynaptic terminals of selected populations of peripheral and central neurons but is also highly expressed in human white adipose tissue (WAT) and increased in obesity. We have recently shown that γ-synuclein is nutritionally regulated in murine adipocytes while its loss protects mice from high fat diet (HFD)-induced obesity and associated metabolic complications. This protection was coupled with increased adipocyte lipolysis, lipid oxidation, and energy expenditure in HFD-fed γ-synuclein-null mutant compared with wild-type mice. Cellular studies suggest that relocalization of ATGL to the lipid droplet in γ-synuclein-deficient adipocytes may contribute to increased lipolysis in these cells. Loss of γ-synuclein in adipocytes also attenuates the assembly of SNARE complexes, an important component of lipid droplet fusion machinery, possibly due to reduced chaperoning of SNAP-23 to the assembling SNARE complex by γ-synuclein. Together our data suggests that not only is γ-synuclein a novel regulator of lipid handling in adipocytes but also that the deficiency of this protein has a significant effect on whole body energy expenditure.

Inhibition of HCV replication by oxysterol-binding protein-related protein 4 (ORP4) through interaction with HCV NS5B and alteration of lipid droplet formation

Hepatitis C virus (HCV) RNA replication involves complex interactions among the 3’x RNA element within the HCV 3’ untranslated region, viral and host proteins. However, many of the host proteins remain unknown. In this study, we devised an RNA affinity chromatography /2D/MASS proteomics strategy and identified nine putative 3’ X-associated host proteins; among them is oxysterol-binding protein-related protein 4 (ORP4), a cytoplasmic receptor for oxysterols. We determined the relationship between ORP4 expression and HCV replication. A very low level of constitutive ORP4 expression was detected in hepatocytes. Ectopically expressed ORP4 was detected in the endoplasmic reticulum and inhibited luciferase reporter gene expression in HCV subgenomic replicon cells and HCV core expression in JFH-1-infected cells. Expression of ORP4S, an ORP4 variant that lacked the N-terminal pleckstrin-homology domain but contained the C-terminal oxysterol-binding domain also inhibited HCV replication, pointing to an important role of the oxysterol-binding domain in ORP4-mediated inhibition of HCV replication. ORP4 was found to associate with HCV NS5B and its expression led to inhibition of the NS5B activity. ORP4 expression had little effect on intracellular lipid synthesis and secretion, but it induced lipid droplet formation in the context of HCV replication. Taken together, these results demonstrate that ORP4 is a negative regulator of HCV replication, likely via interaction with HCV NS5B in the replication complex and regulation of intracellular lipid homeostasis. This work supports the important role of lipids and their metabolism in HCV replication and pathogenesis.

Cytoplasmic dynein heavy chain: the servant of many masters

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The cytoplasmic dynein complex is the main retrograde motor in all eukaryotic cells.

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This complex is built around a dimer of cytoplasmic dynein heavy chains (DYNC1H1).

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Mouse DYNC1H1 mutants have sensory defects, but motor defects have been controversial.

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Now human DYNC1H1 mutations with sensory, motor, and cognitive deficits are being found.

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The study of these mutations will give us new insight into DYNC1H1 function in the nervous system.

Cytoplasmic dynein is the main retrograde motor in all eukaryotic cells. This complex comprises different subunits assembled on a cytoplasmic dynein heavy chain 1 (DYNC1H1) dimer. Cytoplasmic dynein is particularly important for neurons because it carries essential signals and organelles from distal sites to the cell body. In the past decade, several mouse models have helped to dissect the numerous functions of DYNC1H1. Additionally, several DYNC1H1 mutations have recently been found in human patients that give rise to a broad spectrum of developmental and midlife-onset disorders. Here, we discuss the effects of mutations of mouse and human DYNC1H1 and how these studies are giving us new insight into the many critical roles DYNC1H1 plays in the nervous system.

Plin2 inhibits cellular glucose uptake through interactions with SNAP23, a SNARE complex protein

Although a link between excess lipid storage and aberrant glucose metabolism has been recognized for many years, little is known what role lipid storage droplets and associated proteins such as Plin2 play in managing cellular glucose levels. To address this issue, the influence of Plin2 on glucose uptake was examined using 2-NBD-Glucose and [(3)H]-2-deoxyglucose to show that insulin-mediated glucose uptake was decreased 1.7- and 1.8-fold, respectively in L cell fibroblasts overexpressing Plin2. Conversely, suppression of Plin2 levels by RNAi-mediated knockdown increased 2-NBD-Glucose uptake several fold in transfected L cells and differentiated 3T3-L1 cells. The effect of Plin2 expression on proteins involved in glucose uptake and transport was also examined. Expression of the SNARE protein SNAP23 was increased 1.6-fold while levels of syntaxin-5 were decreased 1.7-fold in Plin2 overexpression cells with no significant changes observed in lipid droplet associated proteins Plin1 or FSP27 or with the insulin receptor, GLUT1, or VAMP4. FRET experiments revealed a close proximity of Plin2 to SNAP23 on lipid droplets to within an intramolecular distance of 51 Å. The extent of targeting of SNAP23 to lipid droplets was determined by co-localization and co-immunoprecipitation experiments to show increased partitioning of SNAP23 to lipid droplets when Plin2 was overexpressed. Taken together, these results suggest that Plin2 inhibits glucose uptake by interacting with, and regulating cellular targeting of SNAP23 to lipid droplets. In summary, the current study for the first time provides direct evidence for the role of Plin2 in mediating cellular glucose uptake.

Tracing evolutionary footprints to identify novel gene functional linkages

Systematic determination of gene function is an essential step in fully understanding the precise contribution of each gene for the proper execution of molecular functions in the cell. Gene functional linkage is defined as to describe the relationship of a group of genes with similar functions. With thousands of genomes sequenced, there arises a great opportunity to utilize gene evolutionary information to identify gene functional linkages. To this end, we established a computational method (called TRACE) to trace gene footprints through a gene functional network constructed from 341 prokaryotic genomes. TRACE performance was validated and successfully tested to predict enzyme functions as well as components of pathway. A so far undescribed chromosome partitioning-like protein ro03654 of an oleaginous bacteria Rhodococcus sp. RHA1 (RHA1) was predicted and verified experimentally with its deletion mutant showing growth inhibition compared to RHA1 wild type. In addition, four proteins were predicted to act as prokaryotic SNARE-like proteins, and two of them were shown to be localized at the plasma membrane. Thus, we believe that TRACE is an effective new method to infer prokaryotic gene functional linkages by tracing evolutionary events.

Ectopic lipid storage and insulin resistance: a harmful relationship

Obesity increases the risk of metabolic diseases, including insulin resistance and type 2 diabetes, as well as cardiovascular disease. In addition to lipid accumulation in adipose tissue, obesity is associated with increased lipid storage in ectopic tissues, such as skeletal muscle and liver. Furthermore, lipid accumulation in the heart may result in cardiac dysfunction and heart failure. It has recently been demonstrated that intracellular lipid accumulation in ectopic tissues leads to pathological responses and impaired insulin signalling. Here, we will review the current understanding of how lipid storage and lipid droplet physiology affect the risk of developing metabolic diseases.

Muscle-specific function of the centronuclear myopathy and Charcot-Marie-Tooth neuropathy-associated dynamin 2 is required for proper lipid metabolism, mitochondria, muscle fibers, neuromuscular junctions and peripheral nerves

The ubiquitously expressed large GTPase Dynamin 2 (DNM2) plays a critical role in the regulation of intracellular membrane trafficking through its crucial function in membrane fission, particularly in endocytosis. Autosomal-dominant mutations in DNM2 cause tissue-specific human disorders. Different sets of DNM2 mutations are linked to dominant intermediate Charcot-Marie-Tooth neuropathy type B, a motor and sensory neuropathy affecting primarily peripheral nerves, or autosomal-dominant centronuclear myopathy (CNM) presenting with primary damage in skeletal muscles. To understand the underlying disease mechanisms, it is imperative to determine to which degree the primary affected cell types require DNM2. Thus, we used cell type-specific gene ablation to examine the consequences of DNM2 loss in skeletal muscle cells, the major relevant cell type involved in CNM. We found that DNM2 function in skeletal muscle is required for proper mouse development. Skeletal muscle-specific loss of DNM2 causes a reduction in muscle mass and in the numbers of muscle fibers, altered muscle fiber size distributions, irregular neuromuscular junctions (NMJs) and isolated degenerating intramuscular peripheral nerve fibers. Intriguingly, a lack of muscle-expressed DNM2 triggers an increase of lipid droplets (LDs) and mitochondrial defects. We conclude that loss of DNM2 function in skeletal muscles initiates a chain of harmful parallel and serial events, involving dysregulation of LDs and mitochondrial defects within altered muscle fibers, defective NMJs and peripheral nerve degeneration. These findings provide the essential basis for further studies on DNM2 function and malfunction in skeletal muscles in health and disease, potentially including metabolic diseases such as diabetes.

Perilipin1 promotes unilocular lipid droplet formation through the activation of Fsp27 in adipocytes

Mature white adipocytes contain a characteristic unilocular lipid droplet. However, the molecular mechanisms underlying unilocular lipid droplet formation are poorly understood. We previously showed that Fsp27, an adipocyte-specific lipid droplet-associated protein, promotes lipid droplet growth by initiating lipid exchange and transfer. Here, we identify Perilipin1 (Plin1), another adipocyte-specific lipid droplet-associated protein, as an Fsp27 activator. Plin1 interacts with the CIDE-N domain of Fsp27 and markedly increases Fsp27-mediated lipid exchange, lipid transfer and lipid droplet growth. Functional cooperation between Plin1 and Fsp27 is required for efficient lipid droplet growth in adipocytes, as depletion of either protein impairs lipid droplet growth. The CIDE-N domain of Fsp27 forms homodimers and disruption of CIDE-N homodimerization abolishes Fsp27-mediated lipid exchange and transfer. Interestingly, Plin1 can restore the activity of CIDE-N homodimerization-defective mutants of Fsp27. We thus uncover a novel mechanism underlying lipid droplet growth and unilocular lipid droplet formation that involves the cooperative action of Fsp27 and Plin1 in adipocytes.

Adipocytes store lipid in spherical droplets whose size is determined by lipid exchange and transfer proteins. Sun et al. show that Perilipin1 promotes the growth of lipid droplets by activating the lipid transfer protein Fsp27.

[The lipid droplet: a new organelle?]

Lipid droplets (LD) are depots of neutral lipids that exist virtually in all cells. Until recently, they were considered to be in the same category as glycogen granules, simple inert storage sites for energy. There is now increasing evidence that LD interact dynamically with different organelles, probably as means of providing these organelles with lipids for their membrane expansion. However, most of the mechanisms driving LD biogenesis, growth and intracellular movement remain unknown. Recent data suggest that LD remain functionally connected to the endoplasmic reticulum (ER) membrane and represent specialized ER domains rather than independent organelles. Nevertheless, they represent important cellular structures for which dysfunctions may lead to human diseases such as lypodystrophies or neurodegenerative diseases.

Imaging of neutral lipids by oil red O for analyzing the metabolic status in health and disease

Excess lipid accumulation in peripheral tissues is a key feature of many metabolic diseases. Therefore, techniques for imaging and quantifying lipids in various tissues are important for understanding and evaluating the overall metabolic status of a research subject. Here we present a protocol that detects neutral lipids and lipid droplet (LD) morphology by oil red O (ORO) staining of sections from frozen tissues. The method allows for easy estimation of tissue lipid content and distribution using only basic laboratory and computer equipment. Furthermore, the procedure described here is well suited for the comparison of different metabolically challenged animal models. As an example, we include data on muscular and hepatic lipid accumulation in diet-induced and genetically induced diabetic mice. The experimental description presents details for optimal staining of lipids using ORO, including tissue collection, sectioning, staining, imaging and measurements of tissue lipids, in a time frame of less than 2 d.

Functions of autophagy in normal and diseased liver

Autophagy has emerged as a critical lysosomal pathway that maintains cell function and survival through the degradation of cellular components such as organelles and proteins. Investigations specifically employing the liver or hepatocytes as experimental models have contributed significantly to our current knowledge of autophagic regulation and function. The diverse cellular functions of autophagy, along with unique features of the liver and its principal cell type the hepatocyte, suggest that the liver is highly dependent on autophagy for both normal function and to prevent the development of disease states. However, instances have also been identified in which autophagy promotes pathological changes such as the development of hepatic fibrosis. Considerable evidence has accumulated that alterations in autophagy are an underlying mechanism of a number of common hepatic diseases including toxin-, drug- and ischemia/reperfusion-induced liver injury, fatty liver, viral hepatitis and hepatocellular carcinoma. This review summarizes recent advances in understanding the roles that autophagy plays in normal hepatic physiology and pathophysiology with the intent of furthering the development of autophagy-based therapies for human liver diseases.

Making, baking, and breaking: the synthesis, storage, and hydrolysis of neutral lipids

The esterification of amphiphilic alcohols with fatty acids is a ubiquitous strategy implemented by eukaryotes and some prokaryotes to conserve energy and membrane progenitors and simultaneously detoxify fatty acids and other lipids. This key reaction is performed by at least four evolutionarily unrelated multigene families. The synthesis of this "neutral lipid" leads to the formation of a lipid droplet, which despite the clear selective advantage it confers is also a harbinger of cellular and organismal malaise. Neutral lipid deposition as a cytoplasmic lipid droplet may be thermodynamically favored but nevertheless is elaborately regulated. Optimal utilization of these resources by lipolysis is similarly multigenic in determination and regulation. We present here a perspective on these processes that originates from studies in model organisms, and we include our thoughts on interventions that target reductions in neutral lipids as therapeutics for human diseases such as obesity and diabetes.

Cholesterol ester droplets and steroidogenesis

Intracellular lipid droplets (LDs) are dynamic organelles that contain a number of associated proteins including perilipin (Plin) and vimentin. Cholesteryl ester (CE)-rich LDs normally accumulate in steroidogenic cells and their mobilization is the preferred initial source of cholesterol for steroidogenesis. Plin1a, 1b and 5 were found to preferentially associate with triacylglycerol-rich LDs and Plin1c and Plin4 to associate with CE-rich LDs, but the biological significance of this remains unanswered. Vimentin null mice were found to have decreased ACTH-stimulated corticosterone levels, and decreased progesterone levels in females, but normal hCG-stimulated testosterone levels in males. Smaller LDs were seen in null cells. Lipoprotein cholesterol delivery to adrenals and ovary was normal, as was the expression of steroidogenic genes; however, the movement of cholesterol to mitochondria was reduced in vimentin null mice. These results suggest that vimentin is important in the maintenance of CE-rich LDs and in the movement of cholesterol for steroidogenesis.

Small GTPase Rab40c associates with lipid droplets and modulates the biogenesis of lipid droplets

The subcellular location and cell biological function of small GTPase Rab40c in mammalian cells have not been investigated in detail. In this study, we demonstrated that the exogenously expressed GFP-Rab40c associates with lipid droplets marked by neutral lipid specific dye Oil red or Nile red, but not with the Golgi or endosomal markers. Further examination demonstrated that Rab40c is also associated with ERGIC-53 containing structures, especially under the serum starvation condition. Rab40c is increasingly recruited to the surface of lipid droplets during lipid droplets formation and maturation in HepG2 cells. Rab40c knockdown moderately decreases the size of lipid droplets, suggesting that Rab40c is involved in the biogenesis of lipid droplets. Stimulation for adipocyte differentiation increases the expression of Rab40c in 3T3-L1 cells. Rab40c interacts with TIP47, and is appositionally associated with TIP47-labeled lipid droplets. In addition, over-expression of Rab40c causes the clustering of lipid droplets independent of its GTPase activity, but completely dependent of the intact SOCS box domain of Rab40c. In addition, Rab40c displayed self-interaction as well as interaction with TIP47 and the SOCS box is essential for its ability to induce clustering of lipid droplets. Our results suggest that Rab40c is a novel Rab protein associated with lipid droplets, and is likely involved in modulating the biogenesis of lipid droplets.

The minor allele of the missense polymorphism Ser251Pro in perilipin 2 (PLIN2) disrupts an α-helix, affects lipolysis, and is associated with reduced plasma triglyceride concentration in humans

Perilipin 2 (PLIN2) is the most abundant lipid droplet (LD)-associated protein in nonadipose tissue, and its expression correlates with intracellular lipid accumulation. Here we identified a missense polymorphism, Ser251Pro, that has major effect on protein structure and function, along with an influence on human plasma triglyceride concentration. The evolutionarily conserved Ser251Pro polymorphism was identified with the ClustalW program. Structure modeling using 3D-JigSaw and the Chimera package revealed that the Pro251 allele disrupts a predicted α-helix in PLIN2. Analyses of macrophages from individuals carrying Ser251Pro variants and human embryonic kidney 293 (HEK293) cells stably transfected with either of the alleles demonstrated that the Pro251 variant causes increased lipid accumulation and decreased lipolysis. Analysis of LD size distribution in stably transfected cells showed that the minor Pro251 allele resulted in an increased number of small LDs per cell and increased perilipin 3 protein expression levels as compared with cells carrying the major Ser251 allele. Genotyping of 2113 individuals indicated that the Pro251 variant is associated with decreased plasma triglyceride and very low-density lipoprotein concentrations. Altogether, these data provide the first evidence of a polymorphism in PLIN2 that affects PLIN2 function and may influence the development of metabolic and cardiovascular diseases.

Genome-wide analysis of host mRNA translation during hepatitis C virus infection

In the model of Huh-7.5.1 hepatocyte cells infected by the JFH1 hepatitis C virus (HCV) strain, transcriptomic and proteomic studies have revealed modulations of pathways governing mainly apoptosis and cell cycling. Differences between transcriptomic and proteomic studies pointed to regulations occurring at the posttranscriptional level, including the control of mRNA translation. In this study, we investigated at the genome-wide level the translational regulation occurring during HCV infection. Sucrose gradient ultracentrifugation followed by microarray analysis was used to identify translationally regulated mRNAs (mRNAs associated with ribosomes) from JFH1-infected and uninfected Huh-7.5.1 cells. Translationally regulated mRNAs were found to correspond to genes enriched in specific pathways, including vesicular transport and posttranscriptional regulation. Interestingly, the strongest translational regulation was found for mRNAs encoding proteins involved in pre-mRNA splicing, mRNA translation, and protein folding. Strikingly, these pathways were not previously identified, through transcriptomic studies, as being modulated following HCV infection. Importantly, the observed changes in host mRNA translation were directly due to HCV replication rather than to HCV entry, since they were not observed in JFH1-infected Huh-7.5.1 cells treated with a potent HCV NS3 protease inhibitor. Overall, this study highlights the need to consider, beyond transcriptomic or proteomic studies, the modulation of host mRNA translation as an important aspect of HCV infection.

The evolutionarily conserved protein CG9186 is associated with lipid droplets, required for their positioning and for fat storage

Lipid droplets (LDs) are specialized cell organelles for the storage of energy-rich lipids. Although lipid storage is a conserved feature of all cells and organisms, little is known about fundamental aspects of the cell biology of LDs, including their biogenesis, structural assembly and subcellular positioning, and the regulation of organismic energy homeostasis. We identified a novel LD-associated protein family, represented by the Drosophila protein CG9186 and its murine homolog MGI:1916082. In the absence of LDs, both proteins localize at the endoplasmic reticulum (ER). Upon lipid storage induction, they translocate to LDs using an evolutionarily conserved targeting mechanism that acts through a 60-amino-acid targeting motif in the center of the CG9186 protein. Overexpression of CG9186, and MGI:1916082, causes clustering of LDs in both tissue culture and salivary gland cells, whereas RNAi knockdown of CG9186 results in a reduction of LDs. Organismal RNAi knockdown of CG9186 results in a reduction in lipid storage levels of the fly. The results indicate that we identified the first members of a novel and evolutionarily conserved family of lipid storage regulators, which are also required to properly position LDs within cells.

Trans-Golgi proteins participate in the control of lipid droplet and chylomicron formation

LDs (lipid droplets) carrying TAG (triacylglycerol) and cholesteryl esters are emerging as dynamic cellular organelles that are generated in nearly every cell. They play a key role in lipid and membrane homoeostasis. Abnormal LD dynamics are associated with the pathophysiology of many metabolic diseases, such as obesity, diabetes, atherosclerosis, fatty liver and even cancer. Chylomicrons, stable droplets also consisting of TAG and cholesterol are generated in the intestinal epithelium to transport exogenous (dietary) lipids after meals from the small intestine to tissues for degradation. Defective chylomicron formation is responsible for inherited lipoprotein deficiencies, including abetalipoproteinaemia, hypobetalipoproteinaemia and chylomicron retention disease. These are disorders sharing characteristics such as fat malabsorption, low levels of circulating lipids and fat-soluble vitamins, failure to thrive in early childhood, ataxic neuropathy and visual impairment. Thus understanding the molecular mechanisms governing the dynamics of LDs and chylomicrons, namely, their biogenesis, growth, maintenance and degradation, will not only clarify their molecular role, but might also provide additional indications to treatment of metabolic diseases. In this review, we highlight the role of two small GTPases [ARFRP1 (ADP-ribosylation factor related protein 1) and ARL1 (ADP-ribosylation factor-like 1)] and their downstream targets acting on the trans-Golgi (Golgins and Rab proteins) on LD and chylomicron formation.

Systems biology of adipose tissue metabolism: regulation of growth, signaling and inflammation

Adipose tissue (AT) depots actively regulate whole body energy homeostasis by orchestrating complex communications with other physiological systems as well as within the tissue. Adipocytes readily respond to hormonal and nutritional inputs to store excess nutrients as intracellular lipids or mobilize the stored fat for utilization. Co-ordinated regulation of metabolic pathways balancing uptake, esterification, and hydrolysis of lipids is accomplished through positive and negative feedback interactions of regulatory hubs comprising several pleiotropic protein kinases and nuclear receptors. Metabolic regulation in adipocytes encompasses biogenesis and remodeling of uniquely large lipid droplets (LDs). The regulatory hubs also function as energy and nutrient sensors, and integrate metabolic regulation with intercellular signaling. Over-nutrition causes hypertrophic expansion of adipocytes, which, through incompletely understood mechanisms, initiates a cascade of metabolic and signaling events leading to tissue remodeling and immune cell recruitment. Macrophage activation and polarization toward a pro-inflammatory phenotype drives a self-reinforcing cycle of pro-inflammatory signals in the AT, establishing an inflammatory state. Sustained inflammation accelerates lipolysis and elevates free fatty acids in circulation, which robustly correlates with development of obesity-related diseases. The adipose regulatory network coupling metabolism, growth, and signaling of multiple cell types is exceedingly complex. While components of the regulatory network have been individually studied in exquisite detail, systems approaches have rarely been utilized to comprehensively assess the relative engagements of the components. Thus, need and opportunity exist to develop quantitative models of metabolic and signaling networks to achieve a more complete understanding of AT biology in both health and disease.

FSP27 and PLIN1 interaction promotes the formation of large lipid droplets in human adipocytes

Human adipocytes express high levels of two distinct lipid droplet proteins, fat specific protein 27 (FSP27; also called CIDEC), a member of the CIDE family, and perilipin1 (PLIN1), a member of the PAT family. Both proteins play a role in fat metabolism in adipocytes, but how they interact is not known. Our present study demonstrates that FSP27 and PLIN1 co-localize and interact in cultured human primary adipocytes. We also found that the C-terminal domain of FSP27, aa 120-220, interacts with PLIN1. Individual expression of exogenous FSP27 or PLIN1 increased triglyceride content and decreased glycerol release (a measure of lipolysis), but co-expression of both proteins did not further increase triglyceride content or decrease lipolysis in human adipocytes. However, the combination of PLIN1 and FSP27 increased the average size of lipid droplets or caused the formation of unilocular adipocytes. Our data suggest that FSP27 interacts with PLIN1 to regulate lipid droplet size in human adipocytes in a concerted manner.