Lipid droplet growth: regulation of a dynamic organelle

Research progress of hydrogen sulfide fluorescent probes targeting organelles

Hydrogen sulfide (H2S) is implicated in numerous physiological and pathological processes in living organisms. Abnormal levels of H2S can result in various physiological disorders, highlighting the crucial need for effective identification and detection of H2S at the organellar level. Although numerous H2S fluorescent probes targeting organelles have been reported, a comprehensive review of these probes is required. This review focuses on the strategic selection of organelle-targeting groups and recognition sites for H2S fluorescent probes. This review examines H2S fluorescent probes that can specifically target lysosomes, mitochondria, endoplasmic reticulum, Golgi apparatus, and lipid droplets. These fluorescent probes have been meticulously classified and summarized based on their distinct targets, emphasizing their chemical structure, reaction mechanisms, and biological applications. We carefully designed fluorescent probes to efficiently enhance their ability to recognize target substances and exhibit significant fluorescence variations. Furthermore, we discuss the challenges inherent in the development of fluorescent probes and outline potential future directions for this exciting field.

Surface tension-driven sorting of human perilipins on lipid droplets

Perilipins (PLINs), the most abundant proteins on lipid droplets (LDs), display similar domain organization including amphipathic helices (AH). However, the five human PLINs bind different LDs, suggesting different modes of interaction. We established a minimal system whereby artificial LDs covered with defined polar lipids were transiently deformed to promote surface tension. Binding of purified PLIN3 and PLIN4 AH was strongly facilitated by tension but was poorly sensitive to phospholipid composition and to the presence of diacylglycerol. Accordingly, LD coverage by PLIN3 increased as phospholipid coverage decreased. In contrast, PLIN1 bound readily to LDs fully covered by phospholipids; PLIN2 showed an intermediate behavior between PLIN1 and PLIN3. In human adipocytes, PLIN3/4 were found in a soluble pool and relocated to LDs upon stimulation of fast triglyceride synthesis, whereas PLIN1 and PLIN2 localized to pre-existing LDs, consistent with the large difference in LD avidity observed in vitro. We conclude that the PLIN repertoire is adapted to handling LDs with different surface properties.

Clog P-Guided Development of Multi-Colored Buffering Fluorescent Probes for Super-Resolution Imaging of Lipid Droplet Dynamics

Super-resolution fluorescence imaging of live cells increasingly demands fluorescent probes capable of multi-color and long-term dynamic imaging. Understanding the mechanisms of probe-target recognition is essential for the engineered development of such probes. In this study, it is discovered that the molecular lipid solubility parameter, Clog P, determines the staining performance of fluorescent dyes on lipid droplets (LDs). Fluorescent dyes with Clog P values between 2.5 and 4 can form buffering pools outside LDs, replacing photobleached dyes within LDs to maintain constant fluorescence intensity in LDs, thereby enabling dynamic super-resolution imaging of LDs. Guided by Clog P, four different colored buffering LD probes spanning the visible light spectrum have been developed. Using Structured Illumination Microscopy (SIM), the role of LD dynamics have been tracked during cellular ferroptosis with the secretion, storage, and degradation of overexpressed ACSL3 proteins. It is found that LDs serve as storage sites for these proteins through membrane fusion, and further degrade overexpressed proteins via interactions with organelles like lysosomes or through lipophagy, thereby maintaining cellular homeostasis.

Label-free long-term measurements of adipocyte differentiation from patient-driven fibroblasts and quantitative analyses of in situ lipid droplet generation

The visualization and tracking of adipocytes and their lipid droplets (LDs) during differentiation are pivotal in developmental biology and regenerative medicine studies. Traditional staining or labeling methods, however, pose significant challenges due to their labor-intensive sample preparation, potential disruption of intrinsic cellular physiology, and limited observation timeframe. This study introduces a novel method for long-term visualization and quantification of biophysical parameters of LDs in unlabeled adipocytes, utilizing the refractive index (RI) distributions of LDs and cells. We employ low-coherence holotomography (HT) to systematically investigate and quantitatively analyze the 42-day redifferentiation process of fat cells into adipocytes. This technique yields three-dimensional, high-resolution refractive tomograms of adipocytes, enabling precise segmentation of LDs based on their elevated RI values. Subsequent automated analysis quantifies the mean concentration, volume, projected area, and dry mass of individual LDs, revealing a gradual increase corresponding with adipocyte maturation. Our findings demonstrate that HT is a potent tool for non-invasively monitoring live adipocyte differentiation and analyzing LD accumulation. This study, therefore, offers valuable insights into adipogenesis and lipid research, establishing HT and image-based analysis as a promising approach in these fields.

Targeting lipid droplets and lipid droplet-associated proteins: a new perspective on natural compounds against metabolic diseases

BACKGROUND

Lipid droplet (LD) is a metabolically active organelle, which changes dynamically with the metabolic state and energy requirements of cells. Proteins that either insert into the LD phospholipid monolayer or are present in the cytoplasm, playing a crucial role in lipid homeostasis and signaling regulation, are known as LD-associated proteins.

METHODS

The keywords "lipid droplets" and "metabolic diseases" were used to obtain literature on LD metabolism and pathological mechanism. After searching databases including Scopus, OVID, Web of Science, and PubMed from 2013 to 2024 using terms like "lipid droplets", "lipid droplet-associated proteins", "fatty liver disease", "diabetes", "diabetic kidney disease", "obesity", "atherosclerosis", "hyperlipidemia", "natural drug monomers" and "natural compounds", the most common natural compounds were identified in about 954 articles. Eventually, a total of 91 studies of 10 natural compounds reporting in vitro or in vivo studies were refined and summarized.

RESULTS

The most frequently used natural compounds include Berberine, Mangostin, Capsaicin, Caffeine, Genistein, Epigallocatechin-3-gallate, Chlorogenic acid, Betaine, Ginsenoside, Resveratrol. These natural compounds interact with LD-associated proteins and help ameliorate abnormal LDs in various metabolic diseases.

CONCLUSION

Natural compounds involved in the regulation of LDs and LD-associated proteins hold promise for treating metabolic diseases. Further research into these interactions may lead to new therapeutic applications.

The PPIase Activity of CypB Is Essential for the Activation of Both AKT/mTOR and XBP1s Signaling Pathways during the Differentiation of 3T3-L1 Preadipocytes

In this study, we undertook an extensive investigation to determine how CypB PPIase activity affects preadipocyte differentiation and lipid metabolism. Our findings revealed that inhibition of CypB's PPIase activity suppressed the expression of crucial proteins involved in adipocyte differentiation and induced changes in proteins regulating the cell cycle. Furthermore, we clarified the impact of CypB's PPIase activity on lipid metabolism via the AKT/mTOR signaling pathway. Additionally, we demonstrated the involvement of CypB's PPIase activity in lipid metabolism through the XBP1s pathway. These discoveries offer invaluable insights for devising innovative therapeutic strategies aimed at treating and averting obesity and its related health complications. Targeting CypB's PPIase activity may emerge as a promising avenue for addressing obesity-related conditions. Furthermore, our research opens up opportunities for creating new therapeutic strategies by enhancing our comprehension of the processes involved in cellular endoplasmic reticulum stress.

Role of ADAR1 on Proliferation and Differentiation in Porcine Preadipocytes

Recent research has identified ADAR1 as a participant in the regulation of lipid accumulation in mice. However, there are no reports on the roles of ADAR1 in proliferation, apoptosis and differentiation of porcine preadipocytes. In this study, we investigated the role of ADAR1 in differentiation, proliferation and apoptosis of porcine preadipocytes using CCK-8, EdU staining, cell cycle detection, RT-qPCR, Western blot, a triglyceride assay and Oil Red O staining. The over-expression of ADAR1 significantly promoted proliferation but inhibited the differentiation and apoptosis of porcine preadipocytes. The inhibition of ADAR1 had the opposite effect on the proliferation, differentiation and apoptosis of porcine preadipocytes with over-expressed ADAR1. Then, the regulation mechanisms of ADAR1 on preadipocyte proliferation were identified using RNA-seq, and 197 DEGs in response to ADAR1 knockdown were identified. The MAPK signaling pathway is significantly enriched, indicating its importance in mediating fat accumulation regulated by ADAR1. The study's findings will aid in uncovering the mechanisms that regulate fat accumulation through ADAR1.

Solvatochromic Buffering Fluorescent Probe Resolves the Lipid Transport and Morphological Changes during Lipid Droplet Fusion by Super-Resolution Imaging

The varied functions of lipid droplets, which encompass the regulation of lipid and energy homeostasis, as well as their association with the occurrence of various metabolic diseases, are intricately linked to their dynamic properties. Super-resolution imaging techniques have emerged to decipher physiological processes and molecular mechanisms on the nanoscale. However, achieving long-term dynamic super-resolution imaging faces challenges due to the need for fluorescent probes with high photostability. This paper introduces LD-CF, a "buffering probe" for imaging lipid droplet dynamics using structured illumination microscopy (SIM). The polarity-sensitive LD-CF eliminates background fluorescence with a "cyan filter" strategy, enabling wash-free imaging of lipid droplets. In the fluorescent "off" state outside droplets, the probes act as a "buffering pool", replacing photobleached probes inside droplets and enabling photostable long-term SIM imaging. With this probe, three modes of lipid droplet fusion were observed, including the discovery of fusion from large to small lipid droplets. Fluorescence intensity tracking also revealed the direction of lipid transport during the lipid droplet fusion.

p53 suppresses lipid droplet-fueled tumorigenesis through phosphatidylcholine

Choline deficiency causes disorders including hepatic abnormalities and is associated with an increased risk of multiple types of cancer. Here, by choline-free diet-associated RNA-Seq analyses, we found that the tumor suppressor p53 drives the Kennedy pathway via PCYT1B to control the growth of lipid droplets (LDs) and their fueling role in tumorigenesis. Mechanistically, through upregulation of PCYT1B, p53 channeled depleted choline stores to phosphatidylcholine (PC) biosynthesis during choline starvation, thus preventing LD coalescence. Cells lacking p53 failed to complete this response to choline depletion, leading to hepatic steatosis and tumorigenesis, and these effects could be reversed by enforcement of PCYT1B expression or restoration of PC abundance. Furthermore, loss of p53 or defects in the Kennedy pathway increased surface localization of hormone-sensitive lipase on LDs to release specific fatty acids that fueled tumor cells in vivo and in vitro. Thus, p53 loss leads to dysregulation of choline metabolism and LD growth and couples perturbed LD homeostasis to tumorigenesis.

Challenges in Pharmacological Intervention in Perilipins (PLINs) to Modulate Lipid Droplet Dynamics in Obesity and Cancer

Perilipins (PLINs) are the most abundant proteins in lipid droplets (LD). These LD-associated proteins are responsible for upgrading LD from inert lipid storage structures to fully functional organelles, fundamentally integrated in the lipid metabolism. There are five distinct perilipins (PLIN1-5), each with specific expression patterns and metabolic activation, but all capable of regulating the activity of lipases on LD. This plurality creates a complex orchestrated mechanism that is directly related to the healthy balance between lipogenesis and lipolysis. Given the essential role of PLINs in the modulation of the lipid metabolism, these proteins can become interesting targets for the treatment of lipid-associated diseases. Since reprogrammed lipid metabolism is a recognized cancer hallmark, and obesity is a known risk factor for cancer and other comorbidities, the modulation of PLINs could either improve existing treatments or create new opportunities for the treatment of these diseases. Even though PLINs have not been, so far, directly considered for pharmacological interventions, there are many established drugs that can modulate PLINs activity. Therefore, the aim of this study is to assess the involvement of PLINs in diseases related to lipid metabolism dysregulation and whether PLINs can be viewed as potential therapeutic targets for cancer and obesity.

Research progress on the mitochondrial mechanism of age-related non-alcoholic fatty liver

Non-alcoholic fatty liver disease (NAFLD) has become the most common chronic liver disease worldwide. Reduced activity and slower metabolism in the elderly affect the balance of lipid metabolism in the liver leading to the accumulation of lipids. This affects the mitochondrial respiratory chain and the efficiency of β-oxidation and induces the overproduction of reactive oxygen species. In addition, the dynamic balance of the mitochondria is disrupted during the ageing process, which inhibits its phagocytic function and further aggravates liver injury, leading to a higher incidence of NAFLD in the elderly population. The present study reviewed the manifestations, role and mechanism of mitochondrial dysfunction in the progression of NAFLD in the elderly. Based on the understanding of mitochondrial dysfunction and abnormal lipid metabolism, this study discusses the treatment strategies and the potential therapeutic targets for NAFLD, including lipid accumulation, antioxidation, mitophagy and liver-protecting drugs. The purpose is to provide new ideas for the development of innovative drugs for the prevention and treatment of NAFLD.

Delayed Supplementation Strategy of Extracellular Vesicles from Adipose-Derived Mesenchymal Stromal Cells with Improved Proregenerative Efficiency in a Fat Transplantation Model

Background

Mesenchymal stromal cells (MSCs) and their secreted extracellular vesicles (MSC-EVs) possess similar proregenerative effects when injected into defects immediately following trauma. However, MSC-EVs are superior to MSCs in terms of storage and rejection reflection, while immediate administration of MSC-EVs is related to several target cells for most donor cells die within few weeks. Besides, the inflammatory cascade is incited, providing an unfavorable environment for target cells. We hypothesized that delayed injection of MSC-EVs might have priority on tissue regeneration than instant injection.

Method

Extracellular vesicles isolated from adipose-derived mesenchymal stromal cells (ADSC-EVs) were administered into human umbilical vein endothelial cells (HUVECs) in vitro at different doses. The migration of HUVECs was assessed using the scratch wound healing assay, whereas the length of tubes and number of vessel-like structures formed by HUVECs were determined using tube formation assay. Next, 24 BALB/c nude mice were randomly divided into three groups (n = 8). For the EV-delayed group, ADSC-EVs were injected into transplanted fat a week later than the EV-immediate group. The volume and weight of grafts were measured at 3 months after fat transplantation. Further, the number of CD31-possitive vessels and CD206-possitive cells in the fat grafts was quantified.

Results

Compared with the EV-immediate group, the EV-delayed group had a higher fat tissue retention volume (0.11 ± 0.02 mL versus 0.08 ± 0.01 mL), more neovessels (31.00 ± 4.60 versus 24.20 ± 3.97), and fewer cysts. Furthermore, there were more Ki67-positive cells (25.40 ± 7.14 versus 16.20 ± 4.17) and CD206-positive M2 macrophages cells (23.60 ± 3.44 versus 14.00 ± 3.85) in the EV-delayed group than in the EV-immediate group.

Conclusion

Delayed injection of ADSC-EVs promotes fat graft volume retention by stimulating angiogenesis. These findings suggest that delayed supplementation might be a more effective strategy for the application of MSC-EVs in tissue regeneration.

Dynamic enlargement and mobilization of lipid droplets in pluripotent cells coordinate morphogenesis during mouse peri-implantation development

Mammalian pre-implantation embryos accumulate substantial lipids, which are stored in lipid droplets (LDs). Despite the fundamental roles of lipids in many cellular functions, the significance of building-up LDs for the developing embryo remains unclear. Here we report that the accumulation and mobilization of LDs upon implantation are causal in the morphogenesis of the pluripotent epiblast and generation of the pro-amniotic cavity in mouse embryos, a critical step for all subsequent development. We show that the CIDEA protein, found abundantly in adipocytes, enhances lipid storage in blastocysts and pluripotent stem cells by promoting LD enlargement through fusion. The LD-stored lipids are mobilized into lysosomes at the onset of lumenogenesis, but without CIDEA are prematurely degraded by cytosolic lipases. Loss of lipid storage or inactivation of lipophagy leads to the aberrant formation of multiple cavities within disorganised epithelial structures. Thus, our study reveals an unexpected role for LDs in orchestrating tissue remodelling and uncovers underappreciated facets of lipid metabolism in peri-implantation development.

Red to NIR-emissive anthracene-conjugated PMI dyes with dual functions: singlet-oxygen response and lipid-droplet imaging

The rich chemistry of solution-processable red and near-infrared (NIR) organic emitters has emerged as an attractive and progressive research field because of their particular applications in organic optoelectronics and bioimaging. Also, one can see that the research area of perylene monoimide-based red and NIR-emissive fluorophores is underexplored, which prompted us to design and synthesize three anthracene-conjugated PMI dyes exhibiting strong emission in the red and NIR window in solution. Three PMI-based fluorophores were synthesized via conjoining anthracene and donor moieties (-Ph, -N,N-PhNMe₂) with a PMI core via an acetylene linkage at the peri-position, which helped to attain extensive electronic conjugation, which was reflected in red and NIR-emission in solution. The key molecular features to be highlighted here are: all three dyes are strongly emissive in solution, as unveiled by the excellent absolute fluorescence QYs; and they possess tuneable emission properties, guided by the donor strength and a profound Stokes shift (100-200 nm). The three fluorescent dyes demonstrated appreciable singlet-oxygen (¹O₂) sensitivity when photoirradiated with methylene blue (MB) in solution, showing a substantial blue-shift in emission in a ratiometric manner. Further, the treatment of dye-MB solution with α-tocopherol (¹O₂ scavenger) validated the presence of ¹O₂ as the only oxidizing species generated by MB in solution. Computational investigations gave insight into the twisting of donor moieties in their ground-state optimized geometries, the modulation of the FMO energy gap, and the thermodynamic feasibility of the ¹O₂ reaction. Finally, via taking advantage of the red and NIR-emission, we successfully utilized one of the fluorophores as a lipid-droplet marker for bioimaging in HepG2 cells.

Novel Insights on Lipid Metabolism Alterations in Drug Resistance in Cancer

Chemotherapy is one of the primary treatments for most human cancers. Despite great progress in cancer therapeutics, chemotherapy continues to be important for improving the survival of cancer patients, especially for those who has unresectable metastatic tumors or fail to respond to immunotherapy. However, intrinsic or acquired chemoresistance results in tumor recurrence, which remains a major obstacle in anti-cancer treatment. The high prevalence of chemoresistant cancer makes it urgent to deepen our understanding on chemoresistance mechanisms and to develop novel therapeutic strategies. Multiple mechanisms, including drug efflux, enhanced DNA damage reparability, increased detoxifying enzymes levels, presence of cancer stem cells (CSCs), epithelial mesenchymal transition (EMT), autophagy, ferroptosis and resistance to apoptosis, underlie the development of chemoresistance. Recently, accumulating evidence suggests that lipid metabolism alteration is closely related to drug resistance in tumor. Targeting lipid metabolism in combination with traditional chemotherapeutic drugs is a promising strategy to overcome drug resistance. Therefore, this review compiles the current knowledge about aberrant lipid metabolism in chemoresistant cancer, mainly focusing on aberrant fatty acid metabolism, and presents novel therapeutic strategies targeting altered lipid metabolism to overcome chemoresistance in cancer.

Adaptive and maladaptive roles of lipid droplets in health and disease

Advances in the understanding of lipid droplet biology have revealed essential roles for these organelles in mediating proper cellular homeostasis and stress response. Lipid droplets were initially thought to play a passive role in energy storage. However, recent studies demonstrate that they have substantially broader functions, including protection from reactive oxygen species, endoplasmic reticulum stress, and lipotoxicity. Dysregulation of lipid droplet homeostasis is associated with various pathologies spanning neurological, metabolic, cardiovascular, oncological, and renal diseases. This review provides an overview of the current understanding of lipid droplet biology in both health and disease.

Zeaxanthin Remodels Cytoplasmic Lipid Droplets via β3-Adrenergic Receptor Signaling and Enhances Perilipin 5-Mediated Lipid Droplet–Mitochondria Interactions in Adipocytes

Cytoplasmic lipid droplets (LDs), which are remarkably dynamic, neutral lipid storage organelles, play fundamental roles in lipid metabolism and energy homeostasis. Both the dynamic remodeling of LDs and LD–mitochondria interactions...

Zeaxanthin ameliorates obesity by activating the β3-adrenergic receptor to stimulate inguinal fat thermogenesis and modulating the gut microbiota

The stimulation of fat thermogenesis and modulation of the gut microbiota are promising therapeutic strategies against obesity. Zeaxanthin (ZEA), a carotenoid plant pigment, has been shown to prevent various diseases; however, the therapeutic mechanism for obesity remains unclear. Herein, whether ZEA improves obesity by activating the β3-adrenergic receptor (β3-AR) to stimulate white adipose tissue (WAT) thermogenesis and modulating the gut microbiota was investigated. C57BL6/N mice were fed a high-fat diet (HFD) supplemented with ZEA for 22 weeks. ZEA treatment reduced body weight, fat weight, adipocyte hypertrophy, liver weight, and lipid deposition, and improved dyslipidaemia, serum GPT, GOT, leptin, and irisin levels, glucose intolerance, and insulin resistance in HFD-fed mice. Mechanistically, ZEA treatment induced the expression of β3-AR and thermogenic factors, such as PRDM16, PGC-1α, and UCP1, in inguinal WAT (iWAT) and brown adipose tissue. ZEA treatment stimulated iWAT thermogenesis through the synergistic cooperation of key organelles, which manifested as an increased expression of lipid droplet degradation factors (ATGL, CGI-58 and pHSL), mitochondrial biogenesis factors (Sirt1, Nrf2, Tfam, Nampt and Cyt-C), peroxisomal biogenesis factors (Pex16, Pex19 and Pmp70), and β-oxidation factors (Cpt1, Cpt2, Acadm and Acox1). The thermogenic effect of ZEA was abolished by β3-AR antagonist (SR59230A) treatment. Additionally, dietary supplementation with ZEA reversed gut microbiota dysbiosis by regulating the abundance of Firmicutes, Clostridia, Proteobacteria, and Desulfovibrio, which were associated with the thermogenesis- and obesity-associated indices by Spearman's correlation analysis. Functional analysis of the gut microbiota indicated that ZEA treatment significantly enriched the lipid metabolism pathways. These results demonstrate that ZEA is a promising multi-target functional food for the treatment of obesity by activating β3-AR to stimulate iWAT thermogenesis, and modulating the gut microbiota.

An insight on the future therapeutic application potential of Stevia rebaudiana Bertoni for atherosclerosis and cardiovascular diseases

Stevia rebaudiana Bertoni is a native plant to Paraguay. The extracts have been used as a famous sweetening agent, and the bioactive components derived from stevia possess a broad spectrum of therapeutical potential for various illnesses. Among its medicinal benefits are anti-hypertensive, anti-tumorigenic, anti-diabetic, and anti-hyperlipidemia. Statins (3-hydro-3-methylglutaryl-coenzyme A reductase inhibitor) are a class of drugs used to treat atherosclerosis. Statins are explicitly targeting the HMG-CoA reductase, an enzyme in the rate-limiting step of cholesterol biosynthesis. Despite being widely used in regulating plasma cholesterol levels, the adverse effects of the drug are a significant concern among clinicians and patients. Hence, steviol glycosides derived from stevia have been proposed as an alternative in replacing statins. Diterpene glycosides from stevia, such as stevioside and rebaudioside A have been evaluated for their efficacy in alleviating cholesterol levels. These glycosides are a potential candidate in treating and preventing atherosclerosis provoked by circulating lipid retention in the sub-endothelial lining of the artery. The present review is an effort to integrate the pathogenesis of atherosclerosis, involvement of lipid droplets biogenesis and its associated proteins in atherogenesis, current approaches to treat atherosclerosis, and pharmacological potential of stevia in treating the disease.

Lipid droplets form a network interconnected by the endoplasmic reticulum through which their proteins equilibrate

Lipid droplets (LDs) are globular intracellular structures dedicated to the storage of neutral lipids. They are closely associated with the endoplasmic reticulum (ER) and are delineated by a monolayer of phospholipids that is continuous with the cytoplasmic leaflet of the ER membrane. LDs contain a specific set of proteins, but how these proteins are targeted to the LD surface is not fully understood. Here, we devised a yeast mating-based microscopic readout to monitor the transfer of LD proteins upon zygote formation. The results of this analysis indicate that ER fusion between mating partners is required for transfer of LD proteins and that this transfer is continuous, bidirectional and affects most LDs simultaneously. These observations suggest that LDs do not fuse upon mating of yeast cells, but that they form a network that is interconnected through the ER membrane. Consistent with this, ER-localized LD proteins rapidly move onto LDs of a mating partner and this protein transfer is affected by seipin, a protein important for proper LD biogenesis and the functional connection of LDs with the ER membrane.

Lipid droplets throughout the evolutionary tree

Intracellular lipid droplets are utilized for lipid storage and metabolism in organisms as evolutionarily diverse as animals, fungi, plants, bacteria, and archaea. These lipid droplets demonstrate great diversity in biological functions and protein and lipid compositions, yet fundamentally share common molecular and ultrastructural characteristics. Lipid droplet research has been largely fragmented across the diversity of lipid droplet classes and sub-classes. However, we suggest that there is great potential benefit to the lipid community in better integrating the lipid droplet research fields. To facilitate such integration, we survey the protein and lipid compositions, functional roles, and mechanisms of biogenesis across the breadth of lipid droplets studied throughout the natural world. We depict the big picture of lipid droplet biology, emphasizing shared characteristics and unique differences seen between different classes. In presenting the known diversity of lipid droplets side-by-side it becomes necessary to offer for the first time a consistent system of categorization and nomenclature. We propose a division into three primary classes that reflect their sub-cellular location: i) cytoplasmic lipid droplets (CYTO-LDs), that are present in the eukaryotic cytoplasm, ii) prokaryotic lipid droplets (PRO-LDs), that exist in the prokaryotic cytoplasm, and iii) plastid lipid droplets (PL-LDs), that are found in plant plastids, organelles of photosynthetic eukaryotes. Within each class there is a remarkable array of sub-classes displaying various sizes, shapes and compositions. A more integrated lipid droplet research field will provide opportunities to better build on discoveries and accelerate the pace of research in ways that have not been possible.

Interaction of the Lysophospholipase PNPLA7 with Lipid Droplets through the Catalytic Region

Mammalian patatin-like phospholipase domain containing proteins (PNPLAs) play critical roles in triglyceride hydrolysis, phospholipids metabolism, and lipid droplet (LD) homeostasis. PNPLA7 is a lysophosphatidylcholine hydrolase anchored on the endoplasmic reticulum which associates with LDs through its catalytic region (PNPLA7-C) in response to increased cyclic nucleotide levels. However, the interaction of PNPLA7 with LDs through its catalytic region is unknown. Herein, we demonstrate that PNPLA7-C localizes to the mature LDs ex vivo and also colocalizes with pre-existing LDs. Localization of PNPLA7-C with LDs induces LDs clustering via non-enzymatic intermolecular associations, while PNPLA7 alone does not induce LD clustering. Residues 742-1016 contains four putative transmembrane domains which act as a LD targeting motif and are required for the localization of PNPLA7-C to LDs. Furthermore, the N-terminal flanking region of the LD targeting motif, residues 681-741, contributes to the LD targeting, whereas the C-terminal flanking region (1169-1326) has an anti-LD targeting effect. Interestingly, the LD targeting motif does not exhibit lysophosphatidylcholine hydrolase activity even though it associates with LDs phospholipid membranes. These findings characterize the specific functional domains of PNPLA7 mediating subcellular positioning and interactions with LDs, as wells as providing critical insights into the structure of this evolutionarily conserved phospholipid-metabolizing enzyme family.

Hepatic PPARα function and lipid metabolic pathways are dysregulated in polymicrobial sepsis

Despite intensive research and constant medical progress, sepsis remains one of the most urgent unmet medical needs of today. Most studies have been focused on the inflammatory component of the disease; however, recent advances support the notion that sepsis is accompanied by extensive metabolic perturbations. During times of limited caloric intake and high energy needs, the liver acts as the central metabolic hub in which PPARα is crucial to coordinate the breakdown of fatty acids. The role of hepatic PPARα in liver dysfunction during sepsis has hardly been explored. We demonstrate that sepsis leads to a starvation response that is hindered by the rapid decline of hepatic PPARα levels, causing excess free fatty acids, leading to lipotoxicity, and glycerol. In addition, treatment of mice with the PPARα agonist pemafibrate protects against bacterial sepsis by improving hepatic PPARα function, reducing lipotoxicity and tissue damage. Since lipolysis is also increased in sepsis patients and pemafibrate protects after the onset of sepsis, these findings may point toward new therapeutic leads in sepsis.

Image-based analysis of living mammalian cells using label-free 3D refractive index maps reveals new organelle dynamics and dry mass flux

Holo-tomographic microscopy (HTM) is a label-free microscopy method reporting the fine changes of a cell's refractive indices (RIs) in three dimensions at high spatial and temporal resolution. By combining HTM with epifluorescence, we demonstrate that mammalian cellular organelles such as lipid droplets (LDs) and mitochondria show specific RI 3D patterns. To go further, we developed a computer-vision strategy using FIJI, CellProfiler3 (CP3), and custom code that allows us to use the fine images obtained by HTM in quantitative approaches. We could observe the shape and dry mass dynamics of LDs, endocytic structures, and entire cells' division that have so far, to the best of our knowledge, been out of reach. We finally took advantage of the capacity of HTM to capture the motion of many organelles at the same time to report a multiorganelle spinning phenomenon and study its dynamic properties using pattern matching and homography analysis. This work demonstrates that HTM gives access to an uncharted field of biological dynamics and describes a unique set of simple computer-vision strategies that can be broadly used to quantify HTM images.

Organellar Calcium Handling in the Cellular Reticular Network

Ca²⁺ is an important intracellular messenger affecting diverse cellular processes. In eukaryotic cells, Ca²⁺ is handled by a myriad of Ca²⁺-binding proteins found in organelles that are organized into the cellular reticular network (CRN). The network is comprised of the endoplasmic reticulum, Golgi apparatus, lysosomes, membranous components of the endocytic and exocytic pathways, peroxisomes, and the nuclear envelope. Membrane contact sites between the different components of the CRN enable the rapid movement of Ca²⁺, and communication of Ca²⁺ status, within the network. Ca²⁺-handling proteins that reside in the CRN facilitate Ca²⁺ sensing, buffering, and cellular signaling to coordinate the many processes that operate within the cell.

A Glance at the Nuclear Envelope Spectrin Repeat Protein 3

Nuclear envelope spectrin repeat protein 3 (nesprin-3) is an evolutionarily-conserved structural protein, widely-expressed in vertebrate cells. Along with other nesprin family members, nesprin-3 acts as an essential component of the linker of nucleoskeleton and cytoskeleton (LINC) complex. Naturally, nesprin-3 shares many functions with LINC, including the localization of various cellular structures and bridging of the nucleoskeleton and cytoskeleton, observed in vitro. When nesprin-3 was knocked down in vivo, using zebrafish and mouse models, however, the animals were minimally affected. This paradoxical observation should not limit the physiological importance of nesprin-3, as recently, nesprin-3 has reignited the interest of the research community in studies on cancer cells migration. Moreover, nesprin-3 also plays an active role in certain developmental conditions such as adipogenesis and spermatogenesis, although more studies are needed. Meanwhile, the various protein binding partners of nesprin-3 should also be emphasized, as they are necessary for maintaining the structure of nesprin-3 and enabling it to carry out its various physiological and pathological functions. Nesprin-3 promises to further our understanding of these complex cellular events. Therefore, this review will focus on nesprin-3, examining it from a genetic, structural, and functional perspective. The final part of the review will in turn address the limitations of existing research and the future perspectives for the study of nesprin-3.

Comparison of the Effects of Browning-Inducing Capsaicin on Two Murine Adipocyte Models

The increasing prevalence of obesity and its associated comorbidities has gained attention in developing effective treatments and strategies that promote energy expenditure and the conversion of fat from a white to a brite phenotype. Capsaicin, bioactive component of chili peppers and a transient receptor potential channel vanilloid 1 (TRPV1) agonist, has been known to stimulate the process of thermogenesis. In this study, the effects of capsaicin were assessed on two murine cellular models by quantifying the dynamic of lipid droplets (LDs) and the expression of genes involved in adipocyte browning. Present findings demonstrated that treatment with norepinephrine or capsaicin combined with norepinephrine on 3T3-L1 cells and X9 cells significantly promoted the reduction of LDs area surface and size. The transcription of browning related genes such as uncoupling protein 1 (Ucp1), T-box transcription factor 1 (Tbx1), PR domain containing 16 (Prdm16), peroxisome proliferator-activated receptor γ coactivator 1α (Ppargc1a) and cell death-inducing DNA fragmentation factor A-like effector A (Cidea) was up-regulated by chronic capsaicin treatment on differentiated 3T3-L1 cells. Instead, X9 cells were significantly responsive only to the treatment with norepinephrine, used as positive control.

Lipid Droplets Mediate Salt Stress Tolerance in Parachlorella kessleri

Microalgae are known to respond to salinity stress via mechanisms that include accumulation of compatible solutes and synthesis of antioxidants. Here, we describe a salinity-tolerance mechanism mediated by lipid droplets (LDs). In the alga Parachlorella kessleri grown under salt-stress conditions, we observed significant increases in cell size and LD content. LDs that were closely grouped along the plasma membrane shrank as the plasma membrane expanded, and some LDs were engulfed by vacuoles. Transcriptome analysis showed that genes encoding lysophospholipid acyltransferases (LPLATs) and phospholipase A2 were significantly up-regulated following salt stress. Diacylglycerol kinase and LPLAT were identified in the proteome of salt-induced LDs, alongside vesicle trafficking and plastidial proteins and histone H2B. Analysis of fatty acid composition revealed an enrichment of C18:1 and C18:2 at the expense of C18:3 in response to salt stress. Pulse-chase experiments further suggested that variations of fatty acid composition were associated with LDs. Acetate stimulation research further confirmed a positive role of LDs in cell growth under salt stress. These results suggest that LDs play important roles in salt-stress tolerance, through harboring proteins, participating in cytoplasmic component recycling, and providing materials and enzymes for membrane modification and expansion.

CDP-DAG synthase 1 and 2 regulate lipid droplet growth through distinct mechanisms

Lipid droplets (LDs) are evolutionarily conserved organelles that play critical roles in mammalian lipid storage and metabolism. However, the molecular mechanisms governing the biogenesis and growth of LDs remain poorly understood. Phosphatidic acid (PA) is a precursor of phospholipids and triacylglycerols and substrate of CDP-diacylglycerol (CDP-DAG) synthase 1 (CDS1) and CDS2, which catalyze the formation of CDP-DAG. Here, using siRNA-based gene knockdowns and CRISPR/Cas9-mediated gene knockouts, along with immunological, molecular, and fluorescence microscopy approaches, we examined the role of CDS1 and CDS2 in LD biogenesis and growth. Knockdown of either CDS1 or CDS2 expression resulted in the formation of giant or supersized LDs in cultured mammalian cells. Interestingly, down-regulation of cell death-inducing DFF45-like effector C (CIDEC), encoding a prominent regulator of LD growth in adipocytes, restored LD size in CDS1- but not in CDS2-deficient cells. On the other hand, reducing expression of two enzymes responsible for triacylglycerol synthesis, diacylglycerol O-acyltransferase 2 (DGAT2) and glycerol-3-phosphate acyltransferase 4 (GPAT4), rescued the LD phenotype in CDS2-deficient, but not CDS1-deficient, cells. Moreover, CDS2 deficiency, but not CDS1 deficiency, promoted the LD association of DGAT2 and GPAT4 and impaired initial LD maturation. Finally, although both CDS1 and CDS2 appeared to regulate PA levels on the LD surface, CDS2 had a stronger effect. We conclude that CDS1 and CDS2 regulate LD dynamics through distinct mechanisms.

Differential Reliance on Lipid Metabolism as a Salvage Pathway Underlies Functional Differences of T Cell Subsets in Poor Nutrient Environments

T cells compete with malignant cells for limited nutrients within the solid tumor microenvironment. We found that effector memory CD4 T cells respond distinctly from other T cell subsets to limiting glucose and can maintain high levels of interferon-γ (IFN-γ) production in a nutrient-poor environment. Unlike naive (T_N) or central memory T (T_CM) cells, effector memory T (T_EM) cells fail to upregulate fatty acid synthesis, oxidative phosphorylation, and reductive glutaminolysis in limiting glucose. Interference of fatty acid synthesis in naive T cells dramatically upregulates IFN-γ, while increasing exogenous lipids in media inhibits production of IFN-γ by all subsets, suggesting that relative ratio of fatty acid metabolism to glycolysis is a direct predictor of T cell effector activity. Together, these data suggest that effector memory T cells are programmed to have limited ability to synthesize and metabolize fatty acids, which allows them to maintain T cell function in nutrient-depleted microenvironments.

Hhip inhibits proliferation and promotes differentiation of adipocytes through suppressing hedgehog signaling pathway

Adipogenesis, which directly control body fat mass, plays a crucial role in lipid metabolism and obesity-related diseases. Hedgehog interacting protein (Hhip) belongs to Hedgehog (Hh) signaling pathway. The Hh signaling pathway was already linked with adipogenesis in previous reports, however, the physiological functions of Hhip on lipid deposition are still poorly understood. In this study, the level of Hhip was down-regulated during the development of porcine adipose tissues. Recombinant Hedgehog interacting protein (rHhip) could down-regulate cell cycle related genes and cell numbers in S phage to inhibit cell proliferation. Moreover, rHhip could increase adipocytes differentiation by targeting canonical Hh signaling, indicated by the increase of lipid accumulation and up-regulation of Glut4 and PPARγ expression. Collectively, these findings illustrated the essential role of Hhip in the proliferation and differentiation of adipocytes, and provided a potential novel target for preventing obesity.

High Glucose Level Impairs Human Mature Bone Marrow Adipocyte Function Through Increased ROS Production

Bone marrow adipocytes (BMAds) accumulate in aging, menopause, and metabolic diseases such as Type 2 diabetes. These osteoporotic conditions are associated with oxidative stress and hyperglycemia which are both considered as critical factors underlying bone fragility. Glucose excess and reactive oxygen species (ROS) are known to favor adipogenesis over osteoblastogenesis. In this study, we investigated whether high glucose exposure could determine dysfunction of mature BMAds, specifically through ROS production. The effects of low (LG, 5 mM) or high glucose (HG, 25 mM) concentrations were examined using human bone mesenchymal stromal cells (hBMSCs) in the time course of differentiation, and, up to 21 days once adipocytes were mature. HG did not alter the adipocyte differentiation process of hBMSCs. Yet, after 21 days under HG exposure, PPARG, CEBPA, and adiponectin mRNA expressions were decreased. These alterations were also observed following adipogenic inducer withdrawal as well as in adipocytes fully differentiated in LG then cultured in HG for the last 11 days. Without inducers, HG condition also led to decreased leptin mRNA level. Importantly, intracellular and extracellular ROS concentrations measured using Amplex Red were significantly raised by 50% under HG exposure. This rise was observed once adipocytes ended differentiation and was reproduced within the different cell culture settings without any cytotoxicity. Among genes involved in ROS metabolism, the mRNA level of the H₂O₂ generating enzyme NOX4 was found upregulated in the presence of HG. Following cell separation, mature BMAds were shown to overproduce ROS and to display the gene alterations in contrast to non-lipid-laden cells. Finally, a non-lethal treatment with a pro-oxidant agent under LG condition reduces the mRNA levels of PPARG, adiponectin, and leptin as the HG condition does in the absence of inducers, and amplifies the effect of glucose excess on gene expression. HG concentration drives mature BMAds toward altered expression of the main adipokines and transcriptional factors. These perturbations are associated with a rise in ROS generation likely mediated through enhanced expression of NOX4. Mature BMAds are thus responsive to changes in glucose and ROS concentrations, which is relevant regarding with their phenotype and function in age- or metabolic disease-related osteoporosis.

Dynamic of lipid droplets and gene expression in response to β-aminoisobutyric acid treatment on 3T3-L1 cells

Research on adipobiology has recognized the browning process of white adipocytes as a potential therapeutic strategy for the treatment of obesity and related morbidities. Physical exercise stimulates the secretion of myokines, such as b-aminoisobutyric acid (BAIBA), which in turn promotes adaptive thermogenesis. White adipocyte conversion to brown cells involves dynamic changes in lipid droplet (LD) dimension and in the transcription of brown-specific marker genes. This study analyzes the effect of different doses of BAIBA and at different days of development on 3T3-L1 cells by evaluating morphological changes in LDs and the expression of browning gene markers. Results suggested that the highest concentration of BAIBA after 4 days of differentiation produced the most significant effects. The number of LDs per cell increased in comparison to control cells, whereas the surface area significantly decreased. Brown adipocyte markers were up-regulated, but the effect of treatment was lost at 10 days of differentiation. The thermogenic program induced by BAIBA may reflect a rapid adaptation of adipose tissue to physical exercise. This connection stresses the beneficial impact of physical exercise on metabolic health. The thermogenic program induced by BAIBA may reflect a rapid adaptation of adipose tissue to physical exercise. This connection stresses the beneficial impact of physical exercise on metabolic health.

Stress Coping Strategies in the Heart: An Integrated View

The heart is made up of an ordered amalgam of cardiac cell types that work together to coordinate four major processes, namely energy production, electrical conductance, mechanical work, and tissue remodeling. Over the last decade, a large body of information has been amassed regarding how different cardiac cell types respond to cellular stress that affect the functionality of their elaborate intracellular membrane networks, the cellular reticular network. In the context of the heart, the manifestations of stress coping strategies likely differ depending on the coping strategy outcomes of the different cardiac cell types, and thus may underlie the development of distinct cardiac disorders. It is not clear whether all cardiac cell types have similar sensitivity to cellular stress, how specific coping response strategies modify their unique roles, and how their metabolic status is communicated to other cells within the heart. Here we discuss our understanding of the roles of specialized cardiac cells that together make the heart function as an organ with the ability to pump blood continuously and follow a regular rhythm.

TFAP2 transcription factors are regulators of lipid droplet biogenesis

How trafficking pathways and organelle abundance adapt in response to metabolic and physiological changes is still mysterious, although a few transcriptional regulators of organellar biogenesis have been identified in recent years. We previously found that the Wnt signaling directly controls lipid droplet formation, linking the cell storage capacity to the established functions of Wnt in development and differentiation. In the present paper, we report that Wnt-induced lipid droplet biogenesis does not depend on the canonical TCF/LEF transcription factors. Instead, we find that TFAP2 family members mediate the pro-lipid droplet signal induced by Wnt3a, leading to the notion that the TFAP2 transcription factor may function as a 'master' regulator of lipid droplet biogenesis.

Lipid droplet and peroxisome biogenesis occur at the same ER subdomains

Nascent lipid droplet (LD) formation occurs in the endoplasmic reticulum (ER) membrane but it is not known how sites of biogenesis are determined. We previously identified ER domains in S. cerevisiae containing the reticulon homology domain (RHD) protein Pex30 that are regions where preperoxisomal vesicles (PPVs) form. Here, we show that Pex30 domains are also sites where most nascent LDs form. Mature LDs usually remain associated with Pex30 subdomains, and the same Pex30 subdomain can simultaneously associate with a LD and a PPV or peroxisome. We find that in higher eukaryotes multiple C2 domain containing transmembrane protein (MCTP2) is similar to Pex30: it contains an RHD and resides in ER domains where most nascent LD biogenesis occurs and that often associate with peroxisomes. Together, these findings indicate that most LDs and PPVs form and remain associated with conserved ER subdomains, and suggest a link between LD and peroxisome biogenesis.

Fat storage-inducing transmembrane protein 2 (FIT2) is less abundant in type 2 diabetes, and regulates triglyceride accumulation and insulin sensitivity in adipocytes

Fat storage-inducing transmembrane protein 2 (FIT2) aids in partitioning of cellular triacylglycerol into lipid droplets. A genome-wide association study reported FITM2-R3H domain containing like-HNF4A locus to be associated with type 2 diabetes (T2DM) in East Asian populations. Mice with adipose tissue (AT)-specific FIT2 knockout exhibited lipodystrophic features, with reduced AT mass, insulin resistance, and greater inflammation in AT when fed a high-fat diet. The role of FIT2 in regulating human adipocyte function is not known. Here, we found FIT2 protein abundance is lower in subcutaneous and omental AT obtained from patients with T2DM compared with nondiabetic control subjects. Partial loss of FIT2 protein in primary human adipocytes attenuated their lipid storage capacity and induced insulin resistance. After palmitate treatment, triacylglycerol accumulation, insulin-induced Akt (Ser-473) phosphorylation, and insulin-stimulated glucose uptake were significantly reduced in FIT2 knockdown adipocytes compared with control cells. Gene expression of proinflammatory cytokines IL-18 and IL-6 and phosphorylation of the endoplasmic reticulum stress marker inositol-requiring enzyme 1α were greater in FIT2 knockdown adipocytes than in control cells. Our results show for the first time that FIT2 is associated with T2DM in humans and plays an integral role in maintaining metabolically healthy AT function.-Agrawal, M., Yeo, C. R., Shabbir, A., Chhay, V., Silver, D. L., Magkos, F., Vidal-Puig, A., Toh, S.-A. Fat storage-inducing transmembrane protein 2 (FIT2) is less abundant in type 2 diabetes, and regulates triglyceride accumulation and insulin sensitivity in adipocytes.

Architecture of Lipid Droplets in Endoplasmic Reticulum Is Determined by Phospholipid Intrinsic Curvature

Lipid droplets (LDs) store fats and play critical roles in lipid and energy homeostasis. They form between the leaflets of the endoplasmic reticulum (ER) membrane and consist of a neutral lipid core wrapped in a phospholipid monolayer with proteins. Two types of ER-LD architecture are thought to exist and be essential for LD functioning. Maturing LDs either emerge from the ER into the cytoplasm, remaining attached to the ER by a narrow membrane neck, or stay embedded in the ER and are surrounded by ER membrane. Here, we identify a lipid-based mechanism that controls which of these two architectures is favored. Theoretical modeling indicated that the intrinsic molecular curvatures of ER phospholipids can determine whether LDs remain embedded in or emerge from the ER; lipids with negative intrinsic curvature such as diacylglycerol (DAG) and phosphatidylethanolamine favor LD embedding, while those with positive intrinsic curvature, like lysolipids, support LD emergence. This prediction was verified by altering the lipid composition of the ER in S. cerevisiae using mutants and the addition of exogenous lipids. We found that fat-storage-inducing transmembrane protein 2 (FIT2) homologs become enriched at sites of LD generation when biogenesis is induced. DAG accumulates at sites of LD biogenesis, and FIT2 proteins may promote LD emergence from the ER by reducing DAG levels at these sites. Altogether, our findings suggest that cells regulate LD integration in the ER by modulating ER lipid composition, particularly at sites of LD biogenesis and that FIT2 proteins may play a central role in this process.

Gold nanourchins and celastrol reorganize the nucleo- and cytoskeleton of glioblastoma cells

The physicochemical properties and cytotoxicity of diverse gold nanoparticle (AuNP) morphologies with smooth surfaces have been examined extensively. Much less is known about AuNPs with irregular surfaces. This study focuses on the effects of gold nanourchins in glioblastoma cells. With limited success of monotherapies for glioblastoma, multimodal treatment has become the preferred regimen. One possible example for such future therapeutic applications is the combination of AuNPs with the natural cytotoxic agent celastrol. Here, we used complementary physical, chemical and biological methods to characterize AuNPs and investigate their impact on glioblastoma cells. Our results show that gold nanourchins altered glioblastoma cell morphology and reorganized the nucleo- and cytoskeleton. These changes were dependent on gold nanourchin surface modification. PEGylated nanourchins had no significant effect on glioblastoma cell morphology or viability, unless they were combined with celastrol. By contrast, CTAB-nanourchins adversely affected the nuclear lamina, microtubules and filamentous actin. These alterations correlated with significant glioblastoma cell death. We identified several mechanisms that contributed to the impact of AuNPs on the cytoskeleton and cell survival. Specifically, CTAB-nanourchins caused a significant increase in the abundance of Rock1. This protein kinase is a key regulator of the cytoskeleton. In addition, CTAB-nanourchins led to a marked decline in pro-survival signaling via the PI3 kinase-Akt pathway. Taken together, our study provides new insights into the molecular pathways and structural components altered by gold nanourchins and their implications for multimodal glioblastoma therapy.

Arabidopsis lipid droplet-associated protein (LDAP) - interacting protein (LDIP) influences lipid droplet size and neutral lipid homeostasis in both leaves and seeds

Cytoplasmic lipid droplets (LDs) are found in all types of plant cells; they are derived from the endoplasmic reticulum and function as a repository for neutral lipids, as well as serving in lipid remodelling and signalling. However, the mechanisms underlying the formation, steady-state maintenance and turnover of plant LDs, particularly in non-seed tissues, are relatively unknown. Previously, we showed that the LD-associated proteins (LDAPs) are a family of plant-specific, LD surface-associated coat proteins that are required for proper biogenesis of LDs and neutral lipid homeostasis in vegetative tissues. Here, we screened a yeast two-hybrid library using the Arabidopsis LDAP3 isoform as 'bait' in an effort to identify other novel LD protein constituents. One of the candidate LDAP3-interacting proteins was Arabidopsis At5g16550, which is a plant-specific protein of unknown function that we termed LDIP (LDAP-interacting protein). Using a combination of biochemical and cellular approaches, we show that LDIP targets specifically to the LD surface, contains a discrete amphipathic α-helical targeting sequence, and participates in both homotypic and heterotypic associations with itself and LDAP3, respectively. Analysis of LDIP T-DNA knockdown and knockout mutants showed a decrease in LD abundance and an increase in variability of LD size in leaves, with concomitant increases in total neutral lipid content. Similar phenotypes were observed in plant seeds, which showed enlarged LDs and increases in total amounts of seed oil. Collectively, these data identify LDIP as a new player in LD biology that modulates both LD size and cellular neutral lipid homeostasis in both leaves and seeds.

Lipid Droplet Biogenesis

Lipid droplets (LDs) are ubiquitous organelles that store neutral lipids for energy or membrane synthesis and act as hubs for metabolic processes. Cells generate LDs de novo, converting cells to emulsions with LDs constituting the dispersed oil phase in the aqueous cytoplasm. Here we review our current view of LD biogenesis. We present a model of LD formation from the ER in distinct steps and highlight the biology of proteins that govern this biophysical process. Areas of incomplete knowledge are identified, as are connections with physiology and diseases linked to alterations in LD biology.

Turning Over a New Leaf in Lipid Droplet Biology

Lipid droplets (LDs) in plants have long been viewed as storage depots for neutral lipids that serve as sources of carbon, energy, and lipids for membrane biosynthesis. While much of our knowledge of LD function in plants comes from studies of oilseeds, a recent surge in research on LDs in non-seed cell types has led to an array of new discoveries. It is now clear that both evolutionarily conserved and kingdom-specific mechanisms underlie the biogenesis of LDs in eukaryotes, and proteomics and homology-based approaches have identified new protein players. This review highlights some of these recent discoveries and other new areas of plant LD research, including their role in stress responses and as targets of metabolic engineering strategies aimed at increasing oil content in bioenergy crops.

Lipids in Insect Oocytes: From the Storage Pathways to Their Multiple Functions

In insect physiology, the mechanisms involved in the buildup and regulation of yolk proteins in developing oocytes have been thoroughly researched during the last three decades. Comparatively, the study of lipid metabolism in oocytes had received less attention. The importance of this issue lies in the fact that lipids make up to 40% of the dry weight of an insect egg, being the most important supply of energy for the developing embryo. Since the oocyte has a very limited capacity to synthesize lipids de novo, most of the lipids in the mature eggs arise from the circulation. The main lipid carriers in the insect circulatory system are the lipoproteins lipophorin and vitellogenin. In some species, the endocytosis of lipophorin and vitellogenin may account for about 10% of the lipids present in mature eggs. Thus, most of the lipids are transferred by a lipophorin-mediated pathway, in which the lipoprotein unloads its lipid cargo at the surface of oocytes without internalization. This chapter recapitulates the current status on lipid storage and its utilization in insect oocytes and discusses the participation of key factors including lipoproteins, transfer proteins, lipolytic enzymes, and dynamic organelles such as lipid droplets. The new findings in the field of lipophorin receptors are presented in the context of lipid accumulation during egg maturation, and the roles of lipids beyond energy source are summarized from the perspective of oogenesis and embryogenesis. Finally, prospective and fruitful areas of future research are suggested.